CN1316696C - Adjustable extemal cavity laser - Google Patents

Abstract

Apparatus and methods that utilize dual, tunable elements to provide for selective wavelength tuning of a light beam. The apparatus comprises a first tunable wavelength selection element (24) positioned in a light beam and having a first adjustable free spectral range, a second tunable wavelength selection element (26) positioned in the light beam and having a second adjustable free spectral range, with the first and second tunable wavelength selection elements configured to define a joint transmission peak that is adjustable in phase according to tuning of the first and second tunable wavelength selection elements.

Description

Tunable External Cavity Lasers

technical field

The need for increased bandwidth in fiber optic communications is driving the development of advanced laser transmitters that can be used in Dense Wavelength Division Multiplexing (DWDM) systems where multiple separate data streams are propagated in a single fiber. Each data stream is produced by the modulated output of a semiconductor laser at a specific channel frequency or wavelength, and multiple modulated outputs are combined into a single fiber. The International Telecommunication Union (ITU) currently proposes a channel spacing requirement of approximately 0.4 nanometers, or about 50 GHz, to allow up to 128 channels to be carried by a single fiber within the bandwidth of currently available fibers and fiber amplifiers. In the future, greater bandwidth requirements will Most likely resulting in less channel isolation.

Background technique

Telecom DWDM systems are mainly based on distributed feedback (DFB) lasers. DFB lasers are stabilized by fabricating predetermined wavelength selective gratings at an early stage. Unfortunately, the statistical variation associated with individual DFB laser fabrication results in a spread of (wavelength) channel centers. Therefore, to meet the need for fixed grid (ITU grid) operation at telecom wavelengths, the DFB has been enhanced with an external reference etalon (etalon) and requires a feedback control loop. Variations in DFB operating temperature allow a range of operating wavelengths to enable servo control; however, inconsistent requirements for high optical power, long lifetime, and low electrical power dissipation have hindered use in applications requiring more than a single channel or a small number of adjacent channels.

Continuously tunable external cavity lasers have been developed to overcome the limitations of single DFB devices. Many laser tuning mechanisms have been developed to provide external cavity wavelength selection, such as mechanically tuned gratings for transmission and reflection. External-cavity laser tuning must be able to provide a stable single-mode output at the selected wavelength while effectively suppressing lasing effects associated with external-cavity modes within the cavity gain bandwidth,

Achieving these goals generally results in increased size, cost, complexity, and sensitivity of tunable external cavity lasers.

Therefore, there is a need for an external cavity laser and tuning mechanism that: avoids multimode lasing by effectively suppressing transmission peaks at wavelengths different from the selected wavelength; is simple and compact in design; can Direct implementation. The present invention fulfills these and other needs and overcomes the deficiencies identified in the background art.

Contents of the invention

The present invention relates to a laser device and method utilizing dual tunable elements to provide wavelength tuning of a light beam. The device of the present invention generally comprises: a first tunable wavelength selective element located in the light beam and having a first tunable free spectral range (spectral range); a second tunable wavelength selective element located in the light beam and having a second tunable A free spectral range, wherein the first and second tunable wavelength selective elements are configured to define a joint transmission peak adjustable in phase according to tuning of the first and second tunable elements.

More specifically, the first tunable wavelength selective element defines a first plurality of transmission peaks within the selected wavelength range, and the second tunable wavelength selective element defines a second plurality of transmission peaks within the selected wavelength range , the first and second plurality of transmission peaks are configured to jointly define a single joint transmission peak within a selected wavelength range adjustable by tuning of the dual tunable element. Tuning of the tunable wavelength selective element provides adjustment of the free spectral range of the element, thereby adjusting the two sets of transmission peaks to provide wavelength selection via the vernier effect. In some embodiments, a third tunable wavelength selective element having a third free spectral range may be positioned in the light beam.

The method of the present invention generally comprises: providing a first tunable wavelength selective element having a first tunable free spectral range and a second tunable wavelength selective element having a second tunable free spectral range; A selection element is positioned in the light beam; a joint free spectral range is defined in terms of first and second free spectral ranges of the two elements; and the joint free spectral range is adjusted by tuning the first and second tunable wavelength selective elements. The adjustment of the joint free spectral range may comprise adjusting the phase of the transmission peak defined by the joint free spectral range.

The present invention may be implemented in a laser device comprising a gain medium having first and second facets and emitting a beam from the first facet, and an end reflector positioned in the optical path and configured to define an external cavity with the second facet of the gain medium; a first tunable wavelength selective element positioned in the beam and having a first tunable free spectral range; positioned in the beam and having a second tunable wavelength selective element of a second tunable free spectral range; and the first and second tunable elements configured to define a joint transmission peak that can be adjusted according to tuning of the first and second tunable elements In-phase regulation.

The joint free spectral range defined by the dual tunable wavelength selective elements may in some embodiments be greater than the gain bandwidth of the gain medium. The gain medium facet may define a tunable wavelength selective element such that the gain medium has a free spectral range, and in some embodiments the first free spectral range of the first tunable wavelength selective element may be approximately equal to that of the free spectral range of the gain medium. multiple. In other embodiments, the second free spectral range of the second tunable wavelength selective element may also be approximately equal to a multiple of the free spectral range of the gain medium.

For example, as a non-limiting example, the first and second tunable wavelength selective elements can include: etalons, gratings, interference filters and/or other tunable devices, and can be tuned by thermo-optic, electro-optical, acoustic - Optical, piezo-optical, mechanical or other tuning mechanisms or effects to operate. Gain media may include light emitting diodes or flash lamp pumpable or electrically pumpable crystals, dyes, gases or other gain media.

In certain embodiments, the first and second tunable elements respectively comprise a thermo-optic tunable collimator such as a semiconductor substrate having first and second surfaces each having one or more thin films deposited thereon dielectric layer. The dielectric layers may include, for example, quarter wave dielectric layer pairs. As used herein, "thermo-optical" tuning refers to tuning by temperature-induced changes in the refractive index of the etalon material, temperature-induced changes in the etalon's physical thickness, or both. The etalon material may in some embodiments have a temperature dependent refractive index and a coefficient of thermal expansion, enabling thermo-optic tuning by selective heating or cooling with simultaneous thermal control of the etalon material refractive index and thermal control of the etalon physical thickness. The free spectral range of each etalon is selected such that thermal control of the etalon over a selected temperature range of the etalon provides tuning over a range substantially equal to the free spectral range via the thermo-optic effect.

In the operation of the external cavity laser using the dual tunable collimator of the present invention, the beam emitted by the gain medium passes through the dual tunable collimator, reflects from the end mirror, and returns to the gain medium through the collimator. The free spectral range of each etalon provides a different set of transmission peaks such that only one coincidence (overlap) or alignment of transmission peaks from two etalons occurs over a selected wavelength range such as the gain bandwidth of the gain medium. This provides a choice of only one wavelength in this wavelength range and avoids multimode lasing effects caused by external cavity lasers. Through selective variation of the temperature of the two etalons, the free spectral range of each etalon is varied through the thermo-optic effect to allow control of the transmission peak of the aligned etalons and thus the selection of the external cavity laser output wavelength. The difference between the finesse of the etalon and the free spectral range of the etalon is chosen to avoid multimode lasing on adjacent transmission peaks associated with the etalon. The full width half maximum of the dual etalon was chosen to avoid lasing effects of external cavity modes adjacent to the chosen wavelength.

The present invention provides a tuning system for external cavity lasers and other optical devices that is straightforward to implement, simple in design and provides fast, efficient tuning at selected wavelengths over a broad wavelength range. The use of dual tunable elements tuned to provide wavelength selection allows the use of shorter external laser cavities and effective side mode suppression, and provides a tuning mechanism for external cavity lasers that can be easily adapted or reconfigured, To meet the needs of different DWDM networks. These and other objects and advantages of the present invention will be apparent from the following detailed description.

Description of drawings

The present invention is more fully described below with reference to the accompanying drawings, which are for illustration purposes only.

Fig. 1 is the schematic diagram of the tunable external cavity laser of the present invention;

2A is a graph of first and second sets of transmission peaks provided by first and second tunable elements of the tunable external cavity laser device of FIG. 1;

Figure 2B is a graph of the joint transmission peak defined by the combined set of transmission peaks of Figure 2A;

Fig. 3 is a schematic cross-sectional view of an adjustable calibrator pair of the present invention;

Figure 4 is a graph of the transmission peaks defined by the etalon of Figure 3 tuned to provide a combined transmission peak at 1550 nm;

Figure 5 is a graph of the spectral linewidth of the joint transmission peak of Figure 4 provided by the combined effect of the etalon of Figure 3;

Figure 6 is a schematic diagram of a silicon etalon with thermal control elements;

Fig. 7 is the relational curve of frequency change of the present invention and the inclination degree of silicon etalon angle tuning;

8 is a schematic diagram of an external cavity laser device with three tunable elements of the present invention;

Fig. 9 is a schematic diagram of an external cavity laser device with double tunable gratings of the present invention;

Figure 10 is a schematic diagram of an external cavity laser device with grating and etalon tunable elements of the present invention;

11A is a schematic diagram of another external cavity laser device of the present invention, wherein the tunable collimator and tunable external cavity operate to provide fine tuning;

Figure 11B is a schematic diagram of the transmission peak defined by the adjustable etalon and adjustable external cavity of Figure 11A;

Figure 11C is a schematic illustration of the fine tuning provided by the tunable etalon and tunable external cavity of Figure 11A, shown as a graph of corresponding gain versus wavelength;

12A is a perspective view of another external cavity laser device of the present invention, wherein the adjustable collimator and adjustable external cavity are arranged in the MEMS device;

12B and 12C are top views of the external cavity laser device of FIG. 12A showing tuning of the tapered etalon of FIG. 12A;

Figure 13A is a schematic diagram of another embodiment of an external cavity laser device of the present invention utilizing dual tunable elements in a parallel configuration;

13B is a graph of the fine tuning provided by the external cavity laser device of FIG. 13A, shown as a function of corresponding gain versus wavelength;

Figure 14A is a top view of an adjustable air gap calibrator for the device of Figure 13A;

Figure 14B is a side view of the adjustable air gap calibrator of Figure 14A, shown in section along section line A-A;

Figure 15 is a schematic diagram of another embodiment of an external cavity laser device of the present invention, wherein a single birefringent etalon provides dual tunable orientations utilized in parallel;

Figure 16 is a schematic diagram of another embodiment of the external cavity laser device of the present invention, wherein a tunable external cavity is used to provide the reduced sharpness requirement of a tunable etalon;

17A is a perspective view of another external cavity laser device of the present invention, wherein the adjustable collimator and adjustable external cavity are arranged in the MEMS device;

17B and 17C are top views of the external cavity laser device of FIG. 12A showing tuning of the tapered etalon of FIG. 17A.

Detailed ways

Reference is made more particularly to the drawings for the purpose of illustrating the invention embodied in the apparatus shown in FIGS. 1 to 17 . It should be understood that the device may vary in configuration and detail of components, and the method may vary in detail and order of actions without departing from the basic concept disclosed. The invention is primarily disclosed in terms of the use of external cavity lasers. However, the invention can be used with various types of laser devices and optical systems. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not limiting, since the scope of the present invention is defined only by the claims. The relative sizes of components, as well as the distances between them, shown in the figures are of many examples exaggerated for clarity and therefore should not be considered limiting.

Referring to Figure 1, there is shown a laser device 10 of the present invention. Device 10 includes a gain medium 12 and a terminal or external reflective element 14 . The gain medium 12 may comprise a conventional Fabry-Perot diode emitter chip with an anti-reflective (AR) coated front facet 16 and a reflective or partially reflective rear facet 18 . Reflective element 14 may enclose a mirror or other reflective or retroreflective element. The outer laser cavity is represented by rear facet 18 and end mirror 14 . The gain medium 12 emits a coherent beam 19 from the front facet 16 which is collimated by a lens 20 defining an optical path 22 . A conventional output optical coupler (not shown) may be associated with the back facet 18 for coupling the output from the back facet into an optical fiber (not shown).

The first and second adjustable elements 24 , 26 are positioned within the outer cavity defined by the end mirror 14 and the facet 18 . Tunable elements 24 , 26 cooperate to preferentially feed selected wavelengths of light back to gain medium 12 during operation of laser device 10 . For exemplary purposes, the tunable elements 24, 26 are shown in the form of first and second tunable Fabry-Perot etalons, including parallel plate solid, liquid, or gas isolated etalons, and can be measured by optical thickness or path length The precise measurement of the tuned. In other embodiments, etalon 24 and/or etalon 26 may be replaced with a grating, tunable thin-film interference filter, or other tunable element, as described below. The first etalon 24 comprises surfaces 28 , 30 and has a first free spectral range FSR ₁ adapted to the spacing of the surfaces 28 , 30 and the refractive index of the material of the etalon 24 . The second etalon 26 includes surfaces 32 , 34 and has a second free spectral range FSR ₂ defined by the surfaces 32 , 34 and the refractive index of the material of the etalon 26 . The etalons 24, 26 may comprise the same material or different materials with different refractive indices.

Each of etalons 24, 26 are tuneable by adjusting their optical thicknesses to provide adjustment or tuning of FSR ₁ and FSR ₂ , which also provides selective wavelength tuning of laser device 10 as further described below. Tuning of etalons 24, 26 may include adjustment of the distance between faces 28, 30 and surfaces 32, 34 and/or adjustment of the refractive index of the etalon material, and may be accomplished using various techniques, including thermal- Optical, electro-optic, acousto-optic, and piezo-optic tuning to change the refractive index, as well as mechanical angular adjustment and/or thermal tuning to change the etalon surface spacing. Depending on the particular embodiment of the invention, more than one such tuning effect may be applied to one or both etalons 24, 26 simultaneously.

In the embodiment shown in FIG. 1, the first and second etalons 24, 26 are respectively tunable by the thermo-optic effect. The term "thermo-optic" tuning refers to tuning by temperature-induced changes in the refractive index of the etalon material, temperature-induced changes in the etalon's physical thickness, or both. The etalon material used in certain embodiments has a temperature dependent refractive index and thermal expansion coefficient such that thermo-optic tuning is accompanied by thermal control of the etalon material refractive index as well as thermal control of the etalon physical thickness by selective heating or cooling. The selection of etalon materials for efficient thermo-optic tuning is further illustrated below.

To provide thermo-optic tuning, thermal control element 36 is operatively coupled to etalon 24 and thermal control element 38 is operatively coupled to etalon 26 to provide heating and cooling (cooling) by means of thermal conduction. Thermal control elements 36 , 38 are also operatively coupled to controller 40 . Controller 40 may comprise a conventional data processor and provides tuning signals to thermal control elements 36, 38 for thermal adjustment or tuning of the etalon based on selectable wavelength information stored in a lookup table or other wavelength selection criteria. The etalons 24 , 26 also include temperature monitoring elements 37 , 39 operatively coupled to the controller 40 for monitoring etalon temperature and communicating etalon temperature information to the controller 40 during laser operation. Each thermal control element 36 , 38 includes a heating element (not shown) that allows the temperature of the calibrator to be adjusted in accordance with the commands of the controller 40 .

Thermal control of etalons 24, 26 by thermal control elements 36, 38 may be by conduction, convection, or both. In many embodiments, heat conduction is the primary means of heat flow and temperature regulation for etalons 24, 26, and convective effects that could cause unwanted or spurious thermal fluctuations in etalons 24, 26 should be suppressed. The external cavity laser device 10 may be designed or otherwise configured to allow or compensate for heat flow effects caused by heat convection across the operating temperature of the laser. For example, device 10 may be configured to restrict airflow in the vicinity of calibrators 24 , 26 . In other embodiments, etalons 24, 26 may be individually isolated in a low-conductivity environment or in a vacuum. Large gas paths to different temperature structures adjacent the etalon 24, and the use of thermal insulation for components proximate the etalon 24, 26 may also serve to inhibit heat transfer to or from the etalon. The design of device 10 may additionally be configured to provide stratified air and atmospheric flow proximate the calibrator, thereby avoiding potentially harmful thermal effects associated with turbulent flow.

Thermal control elements 36, 38 allow each etalon 24, 26 to be placed under independent thermal control. Thermal control elements 36, 38 may be used to provide common or parallel heating (heating and cooling of multiple etalons at substantially the same rate of temperature change) and differential heating (heating or cooling of multiple etalons at substantially different rates of temperature change) refrigeration) for wavelength tuning described below. As described below, thermal control elements 36, 38 may be integrated into one or more surfaces of each etalon 24, 26, the thermal control elements 36, 38 may correspond to heat sinks or thermal reservoirs, to allow rapid heating or cooling of the calibrator 24,26.

In certain embodiments, the etalons 24, 26 are constructed and arranged such that a single thermal controller or heat sink can provide effective tuning of both etalons 24, 26 simultaneously. Thermal sensors or monitors (not shown) are mounted on the etalons 24 , 26 or remotely located to monitor the calibrator temperature of the controller 40 . The etalons 24, 26 may be coupled or joined by an assembly (not shown) in which the etalons 24, 26 are positioned or angled to avoid unwanted optical coupling between the etalons 24, 26. Mounting the etalon 24, 26 with a material having suitable thermal properties can avoid unwanted thermal coupling between the etalon 24, 26 during tuning.

Facets 16, 18 of gain medium 12 define a Fabry-Perot etalon, thermal control element 42 is operatively coupled to gain medium 12 to maintain thermal stability of the distance between facets 16, 18 and to provide a stable output of gain medium 12. Thermal control element 42 is operatively coupled to controller 40 . The optical path length of the outer laser cavity defined by end mirror 14 and facet 18 may also be adjusted via a thermal control element (not shown) operatively coupled to end mirror 14 and controller 42, and may be adjusted according to an error from an error detection system or an operating Thermal control is performed with feedback from the system ground coupled to controller 42 to adjust end mirror 14 and/or gain medium 12 . Thermal control of external cavity path length is fully described in US Patent Application Serial No. 09/900,443, filed July 6, 2001, the disclosure of which is incorporated herein by reference.

The etalons 24, 26 provide selective wavelength tuning for the device 10 via the trimming effect. Referring also to FIG. 2A , the structure and configuration of the first etalon 24 defines a first set or sets of communication bands, modes or transmission peaks P ₁ (shown in solid lines), the maxima at a distance equal to FSR ₁ . Likewise the second etalon 26 defines a second or more sets of communication bands, modes or transmission peaks P ₂ (shown as dashed lines), the transmission maxima of peaks P ₂ at a distance equal to FSR ₂ . The first and second etalons 24, 26 are constructed and arranged in many embodiments such that the amplitudes of FSR ₁ and FSR ₂ are similar but not equal during laser operation.

The difference in free spectral range between FSR ₁ and FSR ₂ is achieved by providing each of the etalons 24, 26 with a difference in optical path length. The structure and configuration of the calibrators 24, 26 providing different free spectral ranges can be realized by different technical solutions. For example, a small net difference in the free spectral range of the two etalons can be obtained from a single parallel substrate that is machined and polished to a desired thickness and then divided. Additional operations are then performed on one half of the substrate, where material is extracted by grinding, polishing, or etching of reduced thickness, or additional layers of substrate material are added by common material deposition techniques of increased thickness. Thus, bisection of the original substrate will provide two etalons with slightly inconsistent optical path lengths and different free spectral ranges. It should be noted that for two calibrators of the same material and the same nominal thickness, the free spectral range can also be achieved by differences in temperature or angle between the two calibrators or other differences in tuning effects acting on the calibrators. small difference.

The difference δFSR of the free spectral ranges of the two calibrators 24, 26 is such that certain or selected peaks _P1 and _P2 of the two sets of transmission peaks overlap or align while leaving the remaining _P1 and _P2 non-overlapping or misaligned with each other . In FIG. 2A, the overlap or point of alignment of peaks _P1 and _P2 is at wavelength _λ0 . Additional overlapping points of peaks _P1 and _P2 occur outside the region shown in Figure 2A. The overlap of peaks _P1 and _P2 defines or results in the joint transmission peak _Pj (as shown in FIG. 2B ) of the two etalons 24, 26 with joint free spectral range or FSR _j splits the joint transmission peak P _j at the wavelength.

Etalons 24, 26 may be tuned continuously so that etalon order _M1 and etalon order _M2 (of etalons 24, 26, respectively) comprising the joint transmission peak change frequency at the same rate. This may be achieved, for example, when the etalons 24, 26 heat or cool both etalons 24, 26 simultaneously at substantially the same rate of temperature change. In this case, the wavelength position of the joint peak _Pj is adjusted within a single joint free spectral range. Differential heating of the etalons 24, 26, applying a different rate of temperature change to each etalon, allows fine-tuned displacement of the beat between the modes so that the different peaks _P1 and _P2 are aligned so that in essence Discontinuously adjust the wavelength position of the joint transmission peak P _j over a range larger than FSR _j .

In this way, the wavelength of the joint transmission peak P _j corresponds to the selectable laser wavelength of the device 10 and can be controlled and selected according to the tuning of the etalon 24 , 26 . The respective transmission peaks _P1 and _P2 that are not aligned with each other are suppressed during laser operation. The wavelength of the joint transmission peak P _j may eg correspond to a specific transmission channel of a communication frequency band. The amplitude of FSR _j (and thus the amplitude of δFSR) can be chosen such that a single joint transmission peak P _j occurs only in the wavelength range of interest, such as within the gain bandwidth of gain medium 12 or within a selected portion thereof, to avoid multiple Multimode lasing occurs simultaneously on a joint transmission peak P _j . Alternatively, one or more suitable filters (not shown) may be used in device 10 to suppress etalon 24, 26 when more than one joint transmission peak P _j exists within the bandwidth of gain medium 12 Feedback to gain medium 12 at multiple wavelengths.

The sharpness of etalons 24, 26 and the amplitudes of FSR ₁ and FSR ₂ are chosen to avoid multimode laser action of misaligned peaks P ₁ and P ₂ adjacent to joint transmission peak P _j . The sharpness of the etalons 24, 26 are equivalent or virtually equal in some embodiments, but not in other embodiments of the invention.

The laser external cavity delineated by end mirror 14 and facet 18 of gain medium 12 defines a plurality of external cavity modes shown in FIG. 2A as external cavity mode peaks P _EC . The external cavity mode peak P _EC extends over the entire wavelength range of FIG. 2A , but only a portion of the external cavity mode peak P _EC is shown for clarity. The full-width half-maximum of the joint transmission peak P _j is chosen (according to the configuration of the etalon 24, 26) to prevent unwanted lasing at the cavity mode peak P _EC adjacent to the joint transmission peak P _j .

The etalons 24, 26 may be configured such that each of FSR ₁ and FSR ₂ is approximately equal to the free spectral range (not shown) of the gain medium 12 in order to maintain an approximate commensurate condition with respect to the facet reflectance of the gain medium 12 . However, such a condition is not required when the reflectance of the facets is sufficient to be suppressed by the antireflection coating. The absolute thickness of the etalons 24, 26 is chosen such that the thermal control elements 36, 38 provide the temperature range required to tune or adjust the position of the joint transmission peak P _j within the joint free spectral range FSR _j . The thickness of the etalon also depends on the choice of the etalon material described further below.

The difference in trim spacing provided by etalons 24 , 26 , ie the difference between FSR ₁ and FSR ₂ or the amplitude of delta FSR can vary depending on the desired level of wavelength selectivity, the wavelength range of interest and the particular use of device 10 . In many embodiments, calibrators 24, 26 are constructed and arranged such that FSR ₁ will fall within a few percent of FSR ₂ . Thus, for example, in some embodiments, FSR ₁ may be between approximately 99% and 101% of FSR ₂ , while in other embodiments, FSR ₁ may be between approximately 98% and 102% of FSR ₂ . In some embodiments, the difference in free spectral range of calibrators 24, 26 may be large such that FSR ₁ is between approximately 90% and 105% of FSR ₂ , and in some cases FSR ₁ may be FSR ₂ approximately between 90% and 110% or more.

FSR ₁ and FSR ₂ (and thus FSR _j ) of etalons 24, 26 will be chosen in some embodiments to avoid multimode lasing effects within the gain bandwidth of gain medium 12 or other wavelength ranges of interest, as described above stated. That is, etalons 24, 26 provide only a single joint transmission peak _Pj within the wavelength range such that FSRj is equal to or greater than the wavelength range of interest and provide suppression of transmission peaks adjacent to joint transmission peak _Pj . In some embodiments, the selected wavelength range may include a particular communications band, such as the "C" band, with selective tuning of etalons 24, 26 to locate the joint transmission peak Pj at a desired wavelength within the communications band.

The difference between FSR ₁ and FSR ₂ of etalons 24, 26 may also be defined in terms of the number of discrete transmission channels or wavelengths within the selected wavelength range. Therefore, FSR ₁ and FSR ₂ can have the following relationship:

FSR ₁ ≈(M/M±N)(FSR ₂ )

Where M is the total number of tunable wavelengths or transmission channels within the selected wavelength range, and N is a non-integer or integer selected according to different embodiments of the present invention. In other words, FSR ₁ may be approximately equal to the product of the quotient of the number of tunable wavelengths and the number of tunable wavelengths plus or minus the number N multiplied by FSR ₂ . The number N can be within a range, for example, between about 0.01 or less and about 10 or more. In general, any rational ratio of the free spectral range can provide a fine-tuning effect. In some embodiments, N may fall within a range between approximately 0.1 and approximately 5, and in some embodiments, N may fall within a range between approximately 1 and approximately 2.

The present invention also utilizes "broadband" fine-tuning, where the free spectral range of one of the etalons 24, 26 defines multiple transmission peaks generally corresponding to selectable wavelengths of the transmission grid, while the other etalon has a free spectral range, Let this spectral range only define a single transmission peak within the transmission grid. Tuning of the etalons 24, 26 allows selective alignment of a single transmission peak of one of the etalons with multiple transmission peaks defined by other etalons.

In operation of device 10 , beam 19 exits facet 16 of gain medium 12 , passes through collimators 24 , 26 , reflects from end mirror 14 , and returns to gain medium 12 via collimators 24 , 26 . The difference in the free spectral range 24, 26 results in the above-mentioned single joint transmission peak defined by the etalon 24, 26, and the wavelength of the joint transmission peak is returned from the etalon 24, 26 or back to the gain medium 12, so that in the joint The transmission peak wavelength provides the lasing action of device 10 . Simultaneously, parallel tuning of each etalon 24, 26 results in either an in-mode shift or tuning of the joint transmission peak _Pj in its free spectral range _FSRj . Differential tuning of each peak P _j results in a shift in fine beats between mode or transmission peaks P ₁ and P ₂ to provide a wavelength shift over a range substantially larger than FSR _j . In this way device 10 can provide fast shifting of tuning between widely separated wavelengths.

During operation of the laser device 10, tuning of the joint transmission peak of the etalons 24, 26 may be accomplished according to a particular set of communication channels such as the International Telecommunication Union (ITU) communication grid. A wavelength reference (not shown), such as a gridnerator, or other wavelength reference may be used in conjunction with device 10 and may be positioned within or outside the external cavity of device 10 . However, DWDM systems are actually increasingly dynamic or reconfigurable, and the operation of tunable external-cavity lasers with a fixed wavelength grid is less and less ideal. The laser device 10 of the present invention can provide continuous, selectable wavelength tuning over a wide range of wavelengths in a manner independent of a fixed, predetermined wavelength grid, thereby allowing rapid reconfiguration of DWDM systems.

The use of dual thermal-optical tunable etalons 24, 26 for wavelength selection of external cavity lasers eliminates the need for mechanical tuning presently in grating tunable external cavity lasers because. Thermo-optic tuning is practically solid-state, thus allowing more compact implementations than possible in grating-tunable lasers, with faster tuning and response times, better impedance to shock and vibration, and increased mode-coupling efficiency . Simultaneous tuning of dual tunable etalons provides better laser tuning than can be achieved with a single tunable etalon used with a static etalon.

The use of two simultaneously tunable etalons of the present invention provides substantial advantages over laser tuning mechanisms based on a single tunable etalon or other single tunable element. An important advantage is simplified calibrator manufacture. The use of wavelength selective dual (or more) tunable etalons allows for simpler construction of the reflective cover (further described below) and provides greater tolerance for substrate imperfections. This simple overlay is typically thinner than would be required for a single-tuned etalon, thus allowing a wider bandwidth of tuning operation.

Fine tuning with two or more tunable etalons of the present invention has the advantage of allowing thermo-optic tuning over a smaller operating temperature range than is possible with a single tunable etalon. This lower overall operating temperature reduces the undesirable convective effects described above, reduces power dissipation, and avoids the increased temperature dissipation that occurs in many etalon materials when the material is heated or cooled over a wide temperature range effect. This increase in dissipation may result from changes in the material's thermo-optic coefficient, changes in stress or strain induced by changes in the material coefficient of thermal expansion, and changes in the release of thermally excited free carriers as the temperature increases. For example, when the high temperature required for tuning results in excessive losses due to thermally excited free carriers in the etalon, thermo-optic tuning of a single etalon element can provide wavelength tuning only over a limited bandwidth range as required over a large temperature range.

Efficient thermo-optic tuning requires selection of etalon materials or materials that exhibit good thermo-optic effects (ie, materials that provide large refractive index changes with temperature changes). High refractive index and high temperature sensitivity materials will provide a wider tuning range with respect to the usable operating temperature range of the tunable etalon. High index materials also provide efficient angular tuning, and the high index of the etalon material helps suppress thermal gradients and allows faster tuning and better temperature control. Heating and cooling of the etalon material provides an increase or decrease in the physical thickness of the etalon with an appropriate thermal expansion coefficient, which also facilitates thermo-optic tuning.

Semiconductor materials such as Si, Ge, and GaAs exhibit high refractive index, high temperature sensitivity of refractive index, and high thermal diffusivity, thereby providing good etalon materials for thermo-optically tunable embodiments of the present invention. Many microfabrication techniques are available for semiconductor materials, so the use of semiconductor calibrator materials allows thermal control and other electrical functions to be integrated directly into the calibrator, thereby providing higher tuning accuracy, reduced power consumption, less Component manipulation and a more compact implementation. Silicon has significant advantages as an etalon material, having a refractive index of approximately 3.478 and a coefficient of thermal expansion (CTE) of approximately 2.62 x 10 ^-6 /°K at ambient temperature. Silicon is dispersed and has a group index of refraction Ng=3.607. There are also a number of silicon processing technologies that allow thermal control elements to be integrated directly on or into the silicon calibrator, as described below.

Temperature tuning of a silicon etalon via a free spectral range requires an increase or decrease in the etalon optical path length OPL by an amount equal to λ/2. The optical path length OPL is equal to the product of the refractive index of the etalon material and the physical thickness or distance across the etalon, ie OPL=nL, where n is the refractive index of the etalon material and L is the physical distance across the etalon. The change in optical path length with respect to temperature can be expressed as:

$\frac{\mathrm{<span\; class="notranslate">\; PPML</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; no</span>}\frac{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; L</span>}\frac{\mathrm{<span\; class="notranslate">\; dn</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}$

where T is temperature (°K), α = coefficient of thermal expansion (1/°K), and

$\frac{\mathrm{<span\; class="notranslate">\; PPML</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; dT</span>}}{\mathrm{<span\; class="notranslate">\; OPL</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}\frac{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}\frac{\mathrm{<span\; class="notranslate">\; dn</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}\frac{\mathrm{<span\; class="notranslate">\; dn</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}$

Therefore, the change of the optical path length ΔOPL can be expressed by the following formula:

$\mathrm{<span\; class="notranslate">\; \&Delta;OPL</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; PPML</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; OPL</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}\frac{\mathrm{<span\; class="notranslate">\; dn</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}$

Silicon has a refractive index of approximately 3.48 at a temperature of 25°C, so

$\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}\frac{\mathrm{<span\; class="notranslate">\; dn</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 49.2</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 6</span>}}$

Tuning of a silicon etalon of thickness 100 micrometers in the wavelength range λ/2 = 750 nanometers at 1500 nanometers,

$\mathrm{<span\; class="notranslate">\; \&Delta;OPL</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; OPL</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}\frac{\mathrm{<span\; class="notranslate">\; dn</span>}}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}$

This is equivalent to (3.48)(0.0001m)(49.2×10 ^-6 /°K)(ΔT), or ΔT≈43.8°K. This in turn corresponds to a temperature range of about 20°C - 64°C, which can be readily provided using commercial thermal control elements.

Referring to Fig. 3, a thermo-optically tunable etalon pair 44 is configured for use in tuning a telecommunications transmitter external cavity laser. The collimator pair 44 includes a first collimator 46 and a second collimator 48 positioned in the light beam 50 . Etalons 46, 48 are used in association with laser external cavities (not shown), as described above and shown in FIG. The respective thicknesses of the various layers of the etalon 46, 48 shown in FIG. 3 are exaggerated for clarity and are not necessarily shown to scale.

The first etalon 46 includes a 100 micron thick silicon layer 52 disposed between wavelength (λ/4) dielectric layer mirrors 54,56. Dielectric mirror 54 is shown to include a pair of high and low index λ/4 layers 58 , 60 , with low index layer 60 adjacent silicon layer 52 . Dielectric mirror 56 also includes a pair of high and low index λ/4 layers 62 , 64 , with low index layer 62 adjacent silicon layer 52 . Silicon layer 52 may comprise, for example, a conventional commercially available 100 micron thick polished silicon wafer upon which quarter wave layers 58, 60, 62, 64 are deposited by evaporation, sputtering, or other conventional layer formation techniques.

In the embodiment depicted in FIG. 3, dielectric mirrors 54, 56 each include only a single pair of λ/4 layers, although in other embodiments a larger number of such quarter wave layers may be used. The low index layers 60, 62 comprise silicon dioxide ( _SiO2 ), while the high index layers 58, 64 comprise silicon. Various other high and low index materials are known in the art and may be selected for the quarter wave layers 58-64. Silicon has a high index of refraction and allows the fabrication of relatively sharp thermo-optic tunable collimators using only a single pair of quarter-wave layers in each dielectric mirror 54,56.

The second etalon 48 includes a 102 micron thick silicon layer 66 disposed between quarter wave (λ/4) layer dielectric mirrors 68 , 70 . Dielectric mirror 68 includes high index λ/4 layer 72 and low index λ/4 layer 74 , wherein low index layer 74 is adjacent to silicon layer 52 . Dielectric mirror 56 also includes a pair of high and low index λ/4 layers 76 , 78 , with low index layer 62 adjacent silicon layer 66 . In etalon 48 , silicon layer 66 is shown comprising a 100 micron thick silicon wafer 80 with a two micron capping layer of silicon 82 deposited thereon, giving layer 66 an overall thickness of 102 microns. Silicon layer 82 and quarter wave layers 72, 74, 76, 78 may be deposited via various conventional layer formation techniques, as described above. The low index quarter wave layers 74, 76 are shown comprising _SiO2 , while the high index layers 72, 78 are shown as silicon, but other high and low index materials may be substituted or interchanged. A greater number of high and low index wave layers than the single layer pairs shown may be used in the mirrors 68,70. As noted above, the use of silicon as a high index quarter wave layer allows for higher etalon sharpness using only a single pair of quarter wave layers.

The different thicknesses (100 microns vs. 102 microns) of the central silicon layers 52, 66 of the etalons 46, 48 provide a 2% difference in the free spectral range of the etalons. Etalons 46, 48 are constructed and arranged to provide wavelength tuning within the telecommunications band between approximately 1528 nanometers and approximately 1561 nanometers. Referring to Figure 4, etalons 46, 48 define a series of transmission peaks or sequences shown as Series 1 and Series 2 respectively, in the graph of Figure 4, wavelength is shown on the horizontal axis and the corresponding gain is shown on the vertical axis. The free spectral ranges of the etalons 46, 48 differ from each other by δFSR to establish the fine-tuning distance such that the single point of overlap of the transmission peaks of series 1 and series 2 occurs within the wavelength range between about 1525 and about 1575 nanometers (surrounding the above telecommunications band). The overlapping points of the peaks of series 1 and series 2 are shown at a wavelength of 1550 nm. The relationship of the series 1 and 2 peaks between 1550 nm and 1575 nm (not shown in FIG. 4 ) is approximately a mirror image of that shown in FIG. 4 between 1525 and 1550 nm.

Thus, etalons 46, 48 provide transmission at a single wavelength within the aforementioned telecommunications band between 1531 nm and 1563.5 nm, while etalons 46, 48 provide a joint free spectral range beyond this wavelength range. The International Telecommunication Union (ITU) currently requires a channel isolation of approximately 0.4 nanometers or about 50 GHz, which is easily achieved by thermal tuning of the etalons 46,48. This channel isolation allows up to 128 channels to be carried by a single fiber within the bandwidth of currently available fibers and fiber amplifiers.

In thermal tuning of the etalons 46, 48 over a broad wavelength range, various additional considerations should be made to provide accurate tuning. Silicon has the aforementioned higher dispersion in the telecommunications infrared band, both in terms of refractive index and temperature sensitivity of the refractive index. The properties of silicon are however well known, so the dispersion present in the temperature tuning of the etalon 46, 48 for any particular wavelength range can be mapped and identified, and appropriate tuning adjustments made to compensate for this dispersion. Furthermore, the refractive index of the deposited silicon layer is slightly different from that of the bulk silicon wafer, so in some embodiments, this difference in refractive index must be accounted for in the etalon design.

Figure 5 graphically shows the single-pass plane wave transmission or response for a single etalon 46 and aligned etalons 46, 48, with wavelength shown on the horizontal axis and transmission shown on the vertical axis. As shown in FIG. 5, the joint transmission band established at 1550 nm by the overlapping of series 1 and series 2 of etalons 46, 48 (as shown in FIG. 4) has narrower spectral lines than the corresponding single transmission band of etalon 46 Width.

As shown above, microfabrication techniques can allow the tuning function to be integrated directly onto the calibrator. Referring to FIG. 6 , there is shown a silicon etalon 84 with a thermal control element 86 located on a surface 88 of the etalon 84 . The thermal control element 86 is shown as an annular ring that modulates the passage of a light beam (not shown) through the thermal control element 86 . Thermal control element 86 comprises a conductive material that is heated by the application of an electrical current to provide etalon 84 with the temperature change required for the thermo-optic tuning described above. Conductors 90, 92 electrically connect thermal control element 86 to a current source (not shown). A temperature monitoring device such as a thermistor (not shown) may be integrated on the calibrator 86 or located remotely from the calibrator 86 .

Thermal control element 86 and conductors 90, 92 may be formed on etalon surface 88 by photolithography and material deposition techniques. For example, photoresist (not shown) may be applied to surface 88 to be patterned according to the configuration of control elements 86 and conductors 90, 92 and then developed to remove the exposed photoresist. Conductor material may then be deposited in the developed pattern and the remaining photoresist stripped from the surface to provide thermal control element 86 and conductors 90 , 92 as shown in FIG. 6 . Alternatively, surface 88 may be etched in a pattern corresponding to thermal control element 86 and conductors 90 , 92 , the conductor material being deposited in grooves to provide thermal control element 86 and conductor 90 that are recessed relative to surface 88 . In another embodiment, thermal control element 88 may include a transparent conductive layer made of indium tin oxide (ITO). The traces of the diffused resistors and various configurations are formed directly to the silicon etalon 84 using known techniques.

Calibrator 86 may be mounted in frame 94 . Frame 94 is also micromachined from silicon so that the coefficients of thermal expansion of frame 94 and etalon 86 are matched. The high thermal diffusivity in frame 94 promotes or enhances the symmetry of the temperature of etalon 86 and prevents uneven heating and cooling of etalon 86 during tuning. Frame 94 and etalon 86 may be obtained from the same batch of silicon material or from different silicon materials. Frame 94 facilitates handling and installation of calibrator 86 . The frame 94 also facilitates the positioning of a temperature probe 95 that monitors the temperature of the etalon frame 94 , which acts as a thermal reservoir for the thermal control of the etalon 86 .

In addition to the thermo-optic tuning mentioned above, the wavelength selection of the dual tunable etalon can also be achieved by mechanical angle tuning. Angular tuning involves rotating or tilting the etalon relative to the passing beam to increase or decrease the phase difference of successive reflections in the etalon, thereby increasing and decreasing the free spectral range. Referring to Figure 7, where the effect of tilt (miradians) on frequency change (hertz) is shown graphically for a 50 GHz thick silicon etalon, the wavelength of zero tilt is at approximately 1550 nanometers. Angular tuning of the etalon can be accomplished using various conventional fine micropositioning or translation devices, providing precise angular positioning of the etalon for wavelength tuning.

Continuous tuning of the dual etalon of FIG. 1 allows tuning of the joint transmission peaks of the etalons within the joint free spectral range specified by the etalons by providing the same angular tuning to each of the two etalons substantially simultaneously. Differential angular tuning provides displacement within fine tuning distances for tuning over a range greater than the joint free spectral range. The high refractive index of silicon provides good margin for angular tuning of etalons, while tuning of dual silicon etalons provides efficient wavelength tuning over a wide wavelength range. Other materials with high refractive indices can also be used for the angle-tunable etalon. As mentioned above, angular tuning of etalons can also be used in conjunction with thermo-optic tuning in one or both etalons. Tuning of the etalon may additionally or alternatively be accomplished by applying strain to change the etalon refractive index, injecting a current into the etalon material, or applying a potential to the etalon surface to change the etalon refractive index, or by Any other phenomenon or effect that provides a change in etalon optical path length to allow tuning of the etalon of the present invention.

To avoid the phenomenon of multimode lasing with a dual tunable etalon, the etalon can be configured such that only a single joint transmission peak occurs within the gain bandwidth of the gain medium used by the etalon, as described above. This requirement, while easy to implement, imposes design constraints on both calibrators. In some embodiments it is advantageous to have a third tunable etalon which can be used to control the wavelength range when performing tuning by the first and second tunable etalons. Referring to Figure 8, there is shown a laser device 96 wherein like reference numerals are used to designate like parts. The device includes a third adjustable etalon 98 which, together with the first and second etalons 24, 26, is used to select the output wavelength of the device 10 through a triple trimming effect. The etalon 98 includes planes 100, 102 and, similar to the etalons 24, 26, includes materials that allow thermo-optic tuning of the etalon 98, which, like the etalons 24, 26, may alternatively or additionally be adjusted by angle or tilt, Strain, electro-optic effect or other tuning mechanisms or effects for tuning.

The etalon 98 is configured to suppress one or more joint transmission peaks established or defined by the etalon pair 24 , 26 . Like etalons 24, 26, etalon 98 is configured to establish multiple transmission peaks within the gain bandwidth of the cavity. Used in conjunction, the three etalons 24, 26, 98 allow only one joint transmission peak established by the three etalons to occur within the gain bandwidth of the cavity. In some embodiments, the adjustable etalon 98 may be replaced with a conventional static interference filter, such as a bandpass filter, to allow use of a portion of the gain bandwidth. However, such an embodiment using a bandpass filter does not allow the option to use different parts of the gain bandwidth as provided by device 96 .

In the apparatus 96 of FIG. 8 , each of the three etalons 24 , 26 , 98 is operatively coupled to a common heat sink or reservoir 104 . The etalons 24, 26, 28 include thermal control elements 106, 108, 110, respectively, which are integrated into the etalon material in the manner described in connection with FIG. The thermal control elements 106, 108, 110 are respectively operatively coupled to a controller 40 which provides selective wavelength control via the elements 106, 108, 110 by selective heating or cooling. Control elements 106, 108, 110 provide independent temperature control of calibrators 24, 26, 98, respectively, while thermal reservoir 104 allows rapid readjustment of calibrators 24, 26, 98 by conduction to the base temperature defined by thermal reservoir 104 .

The use of dual tunable elements for the fine tuning of the present invention can be accomplished with various tunable elements other than calibrators. Referring to Fig. 9, there is shown a dual grating tunable laser device 112, wherein like reference numerals are used to designate like parts. The device 112 includes first and second gratings 114, 116 disposed on the optical path 22 and operating reflectively. The gratings 114, 116 provide diffraction to the light beam 119, the combined diffractive effect with the gratings 114, 116 resulting in a joint transmission peak in the manner described above. The wavelength of the combined transmission peak and thus the output wavelength of the device 112 can be adjusted by rotational and/or translational positioning of the gratings 114 , 116 .

Sequential or parallel tuning of the gratings 114, 116 (where the gratings 114, 116 are simultaneously placed at substantially the same rate of rotation and/or translation) provides a shift in the combined transmission peak of the gratings 114, 116 over the combined spectral range of the gratings 114, 116 or tuning. Differential rotation and/or translation of the grating provides tuning over a wavelength range substantially larger than the combined free spectral range. Various conventional positioning devices and systems are known and used to provide precise rotational and translational tuning of the gratings 114, 116 for wavelength tuning of the device 112.

The gratings 114, 116 are shown in FIG. 9 as two reflective operations. In other embodiments, one or both of the gratings 114, 116 may be used for transmission rather than reflection. In embodiments where both gratings are transmissive, an end reflector (not shown) is positioned in the optical path 22 after the gratings.

Referring now to FIG. 10, there is shown another laser apparatus 118 of the present invention, wherein like reference numerals refer to like parts. A dual tuning element is provided in device 118 as etalon 24 and grating 114 . Etalon 24 may be tuned by thermo-optical tuning, angular tuning, and/or other tuning effects, as described above. The grating 114 can be rotated for tuning; or it can be fixed while moving the end mirror 14 for tuning through the grating 114 in a conventional Littman-Metcalf arrangement. The etalon 24 and grating 114 together define the above-mentioned joint transmission peak, which is tuned within the joint free spectral range defined by the etalon 24 and grating 114 via successive tunings of the etalon 24 and grating 114 . Differential tuning of etalon 24 and grating 114 provides fine-tuning displacement for wavelength tuning over a larger range, as described above.

Various other tunable wavelength selective elements can be selected for fine tuning. For example, tapered collimators and tapered interference filters that are adjusted by translation relative to the center of the beam are also useful in the present invention, as disclosed in U.S. Patent Application Serial No. 09/814,464, filed March 21, 2001, which Publicly cited as reference. The use of such other adjustable elements for the fine tuning of the present invention is also contemplated within the scope of this disclosure.

Referring now to FIG. 11A, there is shown another external cavity laser device 120 of the present invention, wherein like reference numerals represent like components. The device 120 includes a gain medium 12 that emits a beam 19 from an anti-reflective coating facet 16 to an end mirror 14 with an adjustable collimator 12 positioned in the beam 19 behind the end mirror 14 . The end mirror 14 and the facet 18 of the gain medium 12 together define the outer laser cavity 124 described above. The etalon 122 is tuned by the electro-optic effect to vary its optical path length OPL and thus the free spectral range, and comprises a region or layer 126 of electro-optic material. Application of a potential to the surface of layer 126 via a transparent electrode (not shown) will cause the refractive index of layer 126 to change, thereby changing the optical path length and free spectral range of etalon 122 . The end mirror 14 is shown as being partially reflective, so the light beam 19 exits the mirror 14 and can be coupled into an optical fiber (not shown).

In device 120, external cavity 124 acts as a second tunable element which, together with tunable etalon 122, provides the dual tuning element for fine tuning of the laser of the present invention. In other words, end mirror 14 and gain medium facet 18 define an etalon. In this regard, the end mirror 14 or external cavity can generally be tuned by mechanical, thermal, electro-optic or other mechanisms that control the length of the cavity. A voltage applied to layer 128 changes the optical path length of external cavity 124 . The combined feedback from tunable etalon 122 and external cavity 124 to gain medium 12 provides tunable wavelength selection for device 120 .

Referring also to FIG. 11B , the laser external cavity 124 has a free spectral range FSR _Cavity defining multiple modes or transmission peaks 130 . As shown, every fourth external cavity transmission peak corresponds to, or is aligned with, a selectable transmission channel 132, 134, 136, 138, 140, 142. Channels 132-142 may correspond to the wavelength channels of the ITU grid described above. The free spectral range FSR _Cavity of the external cavity 124 and the free spectral range FSR _Etalon of the calibrator can have the following relationship:

K(FSR _Etalon )≈(M/M±N)(FSR _Cavity )

Where K is a rational fraction, M is the total number of tunable wavelengths or transmission channels within the selected wavelength range, and N is a non-integer or integer that can be selected according to different embodiments of the present invention. Referring to FIG. 11C , fine tuning of the external cavity laser 120 is shown for an ITU channel grid of K=3, providing a tuning shift on each third external cavity mode peak for each displacement of the fine tuning pitch.

As shown in FIG. 11C , etalon 122 defines a plurality of transmission peaks P _Et separated by FSR _Etalon . Figure 11C shows six different tuning configurations A, B, C, D, E, and F for etalon 122 and external cavity 124, each corresponding to wavelength selection on each of channels 132-142, respectively. Additional optional channels (not shown) exist between each channel 132-142. Thus, at tuning adjustment A, the single etalon transmission peak _PEt is aligned with cavity mode peak 130 on channel 142, which corresponds to a wavelength of 1563.5 nm, as shown in FIG. 11C. The other etalon transmission peak P _Et on tune A is not aligned with any cavity mode 130 . At tuning adjustment B, the single etalon transmission peak P _Et is aligned with cavity mode peak 130 on channel 140 . Likewise, on tuning configurations C, D, E, and F, the etalon transmission peak _PEt is aligned with the cavity mode peak corresponding to channels 138, 136, 134, and 132, respectively. Differential tuning of the external cavity 124 and etalon provides displacement of the fine pitch to provide the tuned AF shown in Figure 11C. Parallel tuning as described above allows for finer tuning, enabling channel selection in the range between the tuning AFs shown in FIG. 11C.

Referring to Figures 12A-12C, another embodiment of an external cavity laser device 144 is shown, wherein like numerals represent like parts. Device 144 is a microelectromechanical or microelectronic machined (MEMS) device fabricated from a monolithic semiconductor substrate 146 . Substrate 146 includes a recess 148 to receive gain medium 12, a recess 150 to receive tapered etalon 152, and a recess 153 to receive end mirror 14 with electro-optic element 128 thereon. The material of the substrate 146 is transparent over the entire gain bandwidth of the gain medium 12 and the light beam 19 emitted from the facet 18 of the gain medium 12 passes through the substrate 146 .

A tapered or wedge-shaped etalon 152 is used to channel select between multiple communication channels by varying the thickness of the etalon 152 presented to the optical path 22 or the distance between partially reflective surfaces of the etalon 152 . This can be accomplished by translating or driving collimator 152 in a direction perpendicular or substantially perpendicular to optical path 22 . As etalon 152 advances or translates into optical path 22, beam 19 propagating along the optical path passes through a thicker portion of etalon 152 that supports constructive interference between opposing faces 154, 156 on the longer wavelength channel . As the etalon 152 moves away from the optical path 22, the light beam 16 will pass through the tapered portion of the etalon 152 and expose the passband or transmission peak to the optical path 22 to support progressively shorter wavelength channels.

The etalon 152 is attached to the base 146 by flexible arms or hinges 158 , 160 . The hinges 158 , 169 movably support the collimator 152 within the recess 150 of the base 146 . The calibrator 152 is also connected to a plurality of electrode pieces 162 . A plurality of electrode members 164 connected to the substrate 146 are interleaved between the electrodes 162 . The electrodes 162 , 164 are received within the recess 150 of the substrate 146 . Electrical contact 166 is electrically connected to electrode 162 via conductor path 168 , and electrical contact 170 is electrically connected to electrode 164 via conductor path 172 . Changing the voltage on electrodes 162 , 164 drives or translates electrode 162 relative to electrode 164 . This action drives or translates collimator 152 relative to optical path 22 accordingly.

In operation of the apparatus 144, the free spectral range is changed by tuning the etalon 152 relative to the translation of the optical path 22, thereby changing the relationship of the transmission peak defined by the etalon 152. FIG. 12B shows etalon 152 positioned in such a way that the thinnest portion of etalon 152 between planes 154, 156 is positioned within optical path 22, while FIG. The etalon 152 is partially positioned within the optical path 22 . Positioning of etalon 152 relative to optical path 22 is achieved by selectively applying voltage to interleaved electrodes 162,164 via contacts 166,170 and conductor paths 168,172, as described above. The hinges 158 , 160 flex or bend depending on the orientation of the aligner 152 .

The surfaces 174, 176 of the recess 150 adjacent to the etalon 152 may be coated with an anti-reflection layer so that the entire tuning effect of the etalon 152 is provided by the etalon 152 itself. In other embodiments, the surfaces 174, 176 may be partially reflective such that tuning results from combined feedback of the surfaces 154, 156 of the etalon 152 and the surfaces 174, 176 of the groove 150 to achieve wavelength selection. The electro-optic element 128 may also be tuned during tuning of the etalon 152 to vary the external cavity path optical path length 124 by selectively applying a voltage to it. Thus, fine-tuning using both the transmission peak defined by the etalon 152 and the transmission peak corresponding to the external cavity mode can be achieved in the manner described above.

MEMS device 144 may be fabricated using known semiconductor material processing techniques. The calibrator 152, hinged parts 158, 160 and electrode parts 162, 164 are made of the same material as the calibrator 152 and the base 146, and are defined from the base 146 by micromachining of the semiconductor material of the base 144. pieces 158,160 and pole pieces 162,164. The use of interleaved electrodes for the operation of MEMS components is a known technique and is disclosed in "Introduction to Microelectromechanical Systems Engineering" by Nadim Maluf, Artech House, Inc. (Norwood MA (2000)), which is incorporated by reference . The gain medium 12 may include a separation device mounted in a recess 148 formed by machining the substrate 146 .

The various external cavity laser devices described above utilize dual tuning elements positioned or arranged in series. It is also possible to tune an external cavity laser according to the invention, where the tuning elements are used in parallel rather than in series. Referring now to FIG. 13A, an external cavity laser device 178 is shown, wherein like reference numerals are used to refer to like components. Device 178 includes gain medium 12 that emits light beam 19 from antireflective coating facet 16 along optical path 22 . Polarizing beam splitter/beam combiner 180 is positioned in optical path 22 such that partially reflective layer 182 of beam splitter 180 splits beam 19 and propagates beam 19a along optical path 22a to the end reflector 14a, and reflect beam 19b along optical path 22b toward end reflector 14b. End reflector 14a may be partially reflective to pass a portion of light beam 19a and may collect it as the light output of device 178 . The rear facet 18 of the gain medium may additionally or instead be partially reflective so that an optical fiber 184 may be positioned to receive light output therefrom.

A first tunable wavelength selective element 186 is positioned in the optical path 22a between the beam splitter 180 and the end mirror 14a, and a second tunable wavelength selective element 188 is positioned in the optical path between the beam splitter 180 and the end mirror 14b 22b. The tunable elements 186, 188 may comprise gratings, etalons, interference filters, or other tunable wavelength selective devices as described above, and may also be MEMS actuated or tuned according to other mechanisms as described above. In the embodiment shown in FIG. 13A, the tunable element 186 is a solid-state etalon that can be tuned based on thermo-optic, electro-optic, mechanical actuation, or other tuning mechanisms described above. The second adjustable element 188 is an adjustable air gap calibrator, described in detail below. Combined feedback from collimators 186, 188 along optical paths 22a, 22b to beam splitter/beam combiner 180, and thus feedback from beam splitter/beam combiner 180 along optical path 22, provides tunable (tunable) Feedback to provide wavelength selection.

The calibrators 186, 188 are operable to provide fine tuning in accordance with the present invention. FIG. 13B graphically shows such a form of fine-tuning in which a first etalon 186 has a first free spectral range FSR ₁ defining a plurality of transmission peaks P ₁ , and a second etalon 188 has a definition of a plurality of transmission peaks P ₂ The second free spectral range FSR ₂ . The second free spectral range FSR ₂ differs in amplitude from the first free spectral range by an amount selectable according to the configuration of the etalon 186 , 188 . Transmission peak _P2 is shown centered on selectable wavelength channels 190, 192, 194, 196, 198 and 200, which may correspond to the channels of the ITU grid described above. Tuning configurations A, B, C, D, E, F of Figure 13B correspond to different etalon tuning configurations, each of which is selected for one of the channel wavelengths 190-200. Thus, for example at tuning A, adjusting the etalons 186, 188 to align peak _P1 with peak _P2 provides transmission on selectable wavelength channel 200, which is shown at 1563.5 in the particular example of FIG. 13B. Nano. No other alignment of peaks _P1 and _P2 occurs within the range of selectable wavelength channels 190-200. Tuning relationship B achieves a single alignment of peaks P ₁ and P ₂ at wavelength 198 . Likewise, tunings C, D, E, F show the alignment of peak _P1 and peak _P2 at channel wavelengths 196, 194, 192, and 190, respectively.

Differential tuning of the etalons 186, 188 provides fine-tuned ground displacement to provide the tuned AF shown in Figure 13B. Parallel tuning as described above allows finer tuning to achieve channel selection in the range between the individual tuning AFs shown in FIG. 13B. In the embodiment shown in FIGS. 13A and 13B , end mirrors 14a, 14b are positioned such that external cavity mode peaks P _EC appear on each of the selectable wavelength channels 190-200, and that one external cavity mode peak P _ECs are positioned between each of the selectable wavelength channels 190-200. The external cavity defined by end mirror 14a and/or end mirror 14b can be tuned using the techniques described above to change the relationship of external cavity mode peak P _EC to selectable wavelength channels as desired.

Referring to Figures 14A and 14B, there is shown an adjustable air gap calibration device 188 of the present invention. Calibration device 188 is shown fabricated from a single bulk semiconductor substrate 202 . The solid state calibration element 204 is connected to four flexible arms 206 which in turn are connected to a base 208 . The base 208 is mounted on the base 202 . The substrate 202 also supports a planar electrode section 210 via a support 211 (as shown in FIG. 14B ), which is separate from and parallel to the calibration element 204 . The calibration element 204 is separated from the electrode portion 210 by a first air gap 212 , and is separated from the substrate 202 by a second air gap 214 . The electrode portion 210 and the calibration element 204 are configured such that a potential difference can be applied to the electrode portion 210 and the calibration element 204, respectively. Changing the potential difference can control the position of the calibration element 204 relative to the electrode element 210 , thereby controlling the dimensions of the air gaps 212 , 214 . The surface 216 of the calibration element 204 and the surface 218 of the substrate 202 may be mirror images such that the air gap 214 acts as a tunable air gap etalon capable of providing feedback to the gain medium 12 for wavelength selection.

In other embodiments, a conductor (not shown) may be included on the surface 218 of the substrate 202 to allow potential adjustment and positional control of the calibration element 204 relative to the substrate 202 . Furthermore, the surfaces 220, 222 of the calibration element 204 and the electrode portion 210, respectively, may be locally mirrored, such that the air gap 212 acts as an adjustable air gap calibrator.

Parallel use of tunable elements can also be achieved in single-birefringent wavelength selective elements, as illustrated in more detail in FIG. 15, which shows yet another example of an external cavity laser device 224 of the present invention, where like reference numerals represent similar parts. Device 24 includes gain medium 12 having a first anti-reflective coating facet 16 and a second reflective or partially reflective facet 18 . The facet 16 emits a light beam 19 along the optical path towards the end mirror 14, and the end mirror 14 together with the facet 18 defines the aforementioned external cavity. The end mirror 14 is shown as partially reflective, so a portion of the light beam 19 may exit the end mirror 14 and be collected into an optical fiber (not shown) as output.

Birefringent collimating element 226 is positioned in optical path 22 before end mirror 14 . The collimating element 226 may comprise a liquid crystal material that provides a high level of birefringence. Various uniform and non-uniform nematic and smectic liquid crystals can be used. Alignment layers 228, 230, which may comprise lapped polyimide flakes, are included on the surface of the collimator 226 as shown to facilitate alignment of individual liquid crystal molecules (not shown). Transparent electrodes 232 , 234 are adjacent to alignment layers 228 , 230 to allow a voltage to be applied across etalon 226 .

Birefringent collimating element 226 has different indices of refraction along different optical axes. In Fig. 15, the first ordinary optical axis 236 (dashed line) has a refractive index _no and the second special optical axis 238 (solid line) has a refractive index _ne , where _ne > _no . These optical axes are rotated by 40 degrees relative to the polarization axis of the laser light. The polarized light is efficiently split into multiple split beams along the optical axis of the birefringent etalon. The optical thickness of the etalon 226 on the first optical axis 236 has a free spectral range FSR _e , while the optical thickness of the etalon 226 on the second optical axis 238 has a free spectral range FSR _o , due to the difference in birefringence present in the collimating material Refractive index, so FSR _e is not equal to FSR _o . In other words, the "e" and "o" ray paths 236, 238 respectively experience different optical path lengths over the same physical thickness of the etalon 226 because of the difference between the refractive indices _ne and _no .

Selective application of voltage on electrodes 232, 234 and/or directional tilt adjustment of etalon 226 allows independent adjustment of FSR _e and FSR _o in a manner that provides fine tuning of the present invention. That is, the different free spectral ranges FSR _e and FSR _o result in two sets of transmission peaks P _e and _Po (not shown) that can be adjusted to selectively control where the peaks overlap, thereby providing fine-tuning in the manner described above. tuning. Differential tuning of FSR _e and FSR _o , which provides fine pitch displacement, and parallel tuning of FSR _e and FSR _o , which provides wavelength tuning in the free spectral range, can be achieved by applying a voltage across etalon 226 to change the refractive index n _e . Additionally, or alternatively, FSR _o can be varied by tilting or thermally adjusting the calibrator 226 to provide differential and parallel tuning. Thus, a single birefringent etalon 226 provides the same effect as using the dual tunable etalon of the above-described embodiment.

Referring now to FIG. 16, there is shown another external cavity laser device 240, wherein like reference numerals refer to like parts. Apparatus 240 includes gain medium 12 that emits beam 19 along optical path 22 from antireflection coated facet 16 to end mirror 14 which together with facet 18 defines outer laser cavity 124 as described above. First and second adjustable etalons 242 , 244 are positioned within outer cavity 124 between gain medium 12 and end mirror 14 . In this embodiment, etalon 242 includes the thermo-optic material described above and is configured for thermo-optic tuning by a thermo-optic control element (not shown) of the type described above. Calibrator 242 is operatively connected to controller or tuner 40 . The second etalon 244 comprises an electro-optic collimator material and has transparent electrodes 246, 248 on its surface for selectively controlling the refractive index of the etalon 244 (and thus controlling the etalon optical path length) according to the voltage applied across the etalon 244. ). Calibrator 244 may also be operatively coupled to tuner 40 .

External cavity 124 defined by facet 18 and end mirror 14 provides a third adjustable element in device 240 . That is, end mirror 14 and gain medium facet 18 define a tunable etalon, and a portion or layer 128 of electro-optic material having a voltage-dependent refractive index is positioned within cavity 124 . Applying a voltage across layer 128 through electrodes (not shown) changes the optical path length of external cavity 124 . The combined feedback to gain medium 12 from tunable etalons 242, 244 and tunable external cavity 124 provides tunable wavelength selection to device 240 via the triple trimming effect described in connection with device 96 of FIG.

The use of the tunable outer cavity 124 in conjunction with the dual tunable etalons 242, 244 reduces the sharpness requirements of the transmission peaks of the etalons 242, 244 for suppressing transmission peaks corresponding to unselected wavelengths. This in turn increases the tolerance of the etalon 242, 244 and simplifies the nature of the partially reflective coating on the surface of the etalon 242. 244, which is highly desirable for certain embodiments of the invention. Thus, in device 240, the joint transmission peak (not shown) defined by the set of transmission peaks of etalons 242, 244 may not be sharp enough to provide effective rejection of unselected channel wavelengths during laser operation. However, the additional filtering provided by the triple trimming effect of the external cavity 124 via the electro-optical element 128 allows the external cavity mode (not shown) to be aligned with the selected channel, thereby providing a tunable etalon with a relatively simple configuration. Effective fine-tuning.

An external cavity laser device with three tunable wavelength selective elements can be implemented in a MEMS device as shown in device 250 of FIGS. 17A-17C , wherein like reference numerals represent like components. Device 250 is fabricated using monolithic semiconductor substrate 146 . Base 146 includes recess 148 for gain medium 12, recess 252 for first tunable etalon 242, recess 150 for second tunable etalon 152, and recess 153 for end mirror 14 , the end mirror has an electro-optic element 128 . The material of the substrate 146 is transparent across the gain bandwidth of the gain medium 12 and the light beam 19 emitted from the gain medium 12 passes through the substrate 146 .

The etalon 152 forms the aforementioned wedge or tapered shape and serves to vary the thickness of the etalon 152 presented to the optical path or the distance between the partially reflective surfaces 154, 156 of the etalon 152 in the manner described above for multiple communications. Choice between channels. The collimator 152 is connected to the base 146 by flexible arms or hinges 158 , 160 that movably support the collimator 152 within a recess 150 of the base 146 . According to the above-mentioned voltage introduced via the electrical contacts 166, 170 and conductors 168 and 172, the plurality of electrode members 162 connected to the calibrator 152 interact with the plurality of electrode members 164 connected to the substrate 146 as described above, so that the electrodes 162 The selective change of the voltage on , 164 drives or translates the electrode 162 so that it moves relative to the electrode 164, so the etalon 152 is driven or translated relative to the optical path.

17B shows etalon 152 positioned in such a way that the thinnest portion of etalon 152 between planes 154 and 156 is positioned on optical path 22, while FIG. The etalon 152 is positioned in such a way that the thicker portion of the etalon 152 between them is positioned on the optical path 22 . The surfaces 174 , 176 of the groove 150 adjacent to the etalon 152 may have an anti-reflection coating, so that the etalon 152 itself provides the entire tuning effect of the etalon 152 . In other embodiments, the surfaces 174, 176 may be partially reflective such that tuning results from combined feedback of the surfaces 154, 156 of the etalon 152 and the surfaces 174, 176 of the groove 150 to achieve wavelength selection. During tuning of the etalon 152, the electro-optical element 128 can also be tuned in the manner described above by selectively applying a voltage to it via the contacts to change the outer cavity diameter optical path length 124 for a triple fine tuning using The transmission peaks defined by etalons 242 and 152 and the transmission peaks corresponding to external cavity modes are shown.

While the present invention has been described in conjunction with specific embodiments, those skilled in the art will understand that various modifications and equivalents may be substituted without departing from the spirit and scope of the invention. In addition, various modifications may be made to adapt a particular situation, material, composition of materials, treatment, procedure to the subject, spirit and scope of the invention. All such modifications are intended to come within the scope of the appended claims.

Claims (62)

1. An optical tuning device, comprising: A first tunable wavelength selective element configured to define a first plurality of tunable transmission peaks separated by a first tunable free spectral range, the first plurality of tunable transmission peaks being optically coupled to the optical tuning device within the gain bandwidth of the gain medium; and A second tunable wavelength selective element configured to define a second plurality of tunable transmission peaks separated by a second tunable free spectral range, the second plurality of tunable transmission peaks at a gain of the gain medium within the bandwidth; and a controller operatively coupled to each of the first and second tunable wavelength selective elements to adjust the first and second tunable free spectral ranges to produce at least one tunable joint transmission peak, wherein the at least one can Tuning each of the joint transmission peaks includes aligned corresponding pairs of transmission peaks, one from each of the first and second plurality of tunable transmission peaks, and tuning the at least one tunable transmission peak using a fine tuning effect. transmission peak. 2. The optical tuning device of claim 1, wherein the at least one joint transmission peak is adjustable upon tuning of the first and second tunable wavelength selective elements. 3. The optical tuning device of claim 1, wherein the first and second tunable wavelength selective elements comprise at least one etalon. 4. The optical tuning device of claim 1, wherein the first and second tunable wavelength selective elements comprise at least one grating. 5. The optical tuning device of claim 1, wherein the first and second tunable wavelength selective elements comprise first and second etalons. 6. The optical tuning apparatus of claim 5, wherein at least one of the first and second etalons is an adjustable air gap etalon. 7. The optical tuning device of claim 1, wherein the first and second tunable wavelength selective elements are configured in a birefringent etalon. 8. The light tuning apparatus of claim 5, wherein at least one etalon among the first and second etalons is angularly tuned. 9. The optical tuning device of claim 5, wherein at least one of the first and second etalons comprises a wedge-shaped etalon positioned by a MEMS actuator. 10. The optical tuning device of claim 5, wherein at least one etalon among the first and second etalons includes first and second surfaces, each of the surfaces having at least one collimator disposed thereon. quarter-wave dielectric pair layer. 11. The optical tuning device of claim 1 , further comprising a beam splitter located in the beam generated by the gain medium, the beam splitter located in front of the first and second tunable wavelength selective elements, the beam splitter passing the first beam to the first tunable wavelength selective element and transmits the second light beam to the second tunable wavelength selective element. 12. The optical tuning device of claim 5, wherein the controller is operatively coupled to the first and second etalons through thermal control elements, wherein the first and second etalons are thermo-optically tunable. 13. The optical tuning device of claim 1 , wherein the rear facet of the gain medium and the mirror optically coupleable to the gain medium define an outer laser cavity of the optical tuning device, wherein the outer laser cavity serves as a second tunable wavelength selective element. 14. The optical tuning device of claim 1, further comprising a third tunable wavelength selective element to provide a triple fine tuning effect. 15. The light tuning device according to claim 1, wherein the first tunable free spectral range FSR ₁ is related to the second tunable free spectral range FSR ₂ by the following formula: FSR ₁ ≈(M/M±N)(FSR ₂ ) where M is the total number of tunable wavelengths in the selected wavelength range, and N is an optional non-integer or integer. 16. The light tuning device of claim 1, wherein each of the first and second tunable free spectral ranges is greater than a wavelength channel spacing in a communications grid to which the light tuning device can be tuned. 17. The optical tuning device according to claim 1, wherein the optical tuning device is capable of continuous selective wavelength tuning over a wide wavelength range in a manner independent of a fixed predetermined wavelength grid. 18. A laser device comprising: base; a gain medium configured to emit a light beam in response to an electrical input; A first tunable wavelength selective element, operatively coupled to the substrate and positioned in the light beam, configured to define a first plurality of tunable transmission peaks having a first tunable free spectral range, the first plurality of The tunable transmission peak is within the gain bandwidth of the gain medium; A second tunable wavelength selective element, operatively coupled to the substrate and positioned in the light beam, is configured to define a second plurality of tunable transmission peaks having a second tunable free spectral range, the second plurality of the tunable transmission peak is within the gain bandwidth of the gain medium; and a controller operatively coupled to each of the first and second tunable wavelength selective elements for tuning the laser device produced by simultaneously adjusting the first and second free spectral ranges of the first and second tunable wavelength selective elements The wavelength of the resulting light output thereby defines a single joint transmission peak over a selectable wavelength range and is adjustable in phase upon tuning of said first and second tunable wavelength selective elements. 19. The laser device of claim 18, wherein the gain medium comprises a laser diode having first and second facets defining a cavity and emitting a beam of light from the first facet. 20. The laser device of claim 19 , further comprising a reflective element positioned in the beam after the first and second tunable wavelength selective elements, the reflective element and the second facet of the gain medium An outer cavity is defined. 21. The laser device of claim 19, wherein the first tunable wavelength selective element has a first free spectral range approximately equal to a multiple of the free spectral range of the gain medium. 22. The laser device of claim 19, wherein the second tunable wavelength selective element has a second free spectral range approximately equal to a multiple of the free spectral range of the gain medium. 23. The laser device of claim 19, wherein the selectable wavelength range is at least as large as the gain bandwidth of the gain medium. 24. The laser apparatus of claim 18, wherein said first and second tunable wavelength selective elements comprise at least one etalon. 25. The laser apparatus of claim 18, wherein said first and second tunable wavelength selective elements comprise at least one grating. 26. The laser apparatus of claim 18, wherein the first and second tunable wavelength selective elements comprise first and second tunable etalons. 27. The laser apparatus of claim 26, wherein at least one of the first and second tunable etalons is thermo-optically tunable. 28. The laser apparatus of claim 26, wherein at least one of the first and second tunable etalons is electro-optical tunable. 29. The laser apparatus of claim 26, wherein at least one of the first and second tunable etalons is angularly tuneable. 30. The laser apparatus of claim 26, wherein at least one of said tunable etalons comprises a semiconductor material. 31. The laser apparatus of claim 26, wherein at least one of said tunable etalons comprises first and second surfaces, each said surface having at least one quarter-wave dielectric pair disposed thereon layer. 32. The laser apparatus of claim 30, wherein the tunable etalon includes a thermal control element integrated thereon. 33. The laser apparatus of claim 27, wherein the tunable etalon is operatively coupled to a thermal controller. 34. The laser apparatus of claim 27, wherein the tunable etalon is operatively coupled to a thermal reservoir. 35. The laser apparatus of claim 18, further comprising a third tunable wavelength selective element positioned in said beam to provide a triple fine tuning effect. 36. The laser device according to claim 18, wherein the first tunable free spectral range FSR ₁ is related to the second tunable free spectral range FSR ₂ by the following formula: FSR ₁ ≈(M/M±N)(FSR ₂ ) where M is the total number of tunable wavelengths in the selected wavelength range, and N is an optional non-integer or integer. 37. The laser device of claim 18, wherein each of the first and second tunable free spectral ranges is greater than a wavelength channel spacing in a communications grid to which the laser device can be tuned. 38. The laser device according to claim 18, wherein the laser device is capable of continuous selective wavelength tuning over a wide wavelength range in a manner independent of a fixed predetermined wavelength grid. 39. The laser device of claim 18, wherein the second tunable wavelength selective element comprises a wedge etalon connected to the hinge element and the electrode element for positioning the wedge etalon in the beam. 40. The laser apparatus of claim 18 , further comprising a beam splitter positioned in the beam prior to the first and second tunable wavelength selective elements, the beam splitter passing the first beam to the first tunable wavelength selective element. The wavelength selective element passes the second light beam to the second tunable wavelength selective element. 41. The laser apparatus of claim 18, wherein at least one of the first and second tunable wavelength selective elements comprises a tunable air gap etalon. 42. The laser device of claim 41, wherein a tunable air gap etalon provides feedback to the gain medium for wavelength selection. 43. The laser apparatus of claim 18, wherein the first and second tunable wavelength selective elements are included in a birefringent etalon. 44. The laser device of claim 20, wherein the external cavity is used as a second tunable wavelength selective element. 45. A method for tuning an optical beam comprising: positioning a first tunable wavelength selective element in the light beam produced by the gain medium, the first tunable wavelength selective element configured to define a first plurality of tunable transmission peaks having a first tunable free spectral range, the said first plurality of tunable transmission peaks are within a gain bandwidth of said gain medium; positioning a second tunable wavelength selective element in the light beam, the second tunable wavelength selective element configured to define a second plurality of tunable transmission peaks having a second tunable free spectral range, the second a plurality of tunable transmission peaks within the gain bandwidth of the gain medium; and The first and second tunable wavelength selective elements are simultaneously tuned to align one of the first plurality of transmission peaks with one of the second plurality of transmission peaks to define a single joint transmission peak by fine-tuning the tuning effect. 46. The method of claim 45, further comprising: emitting a light beam from the first facet of the gain medium; and A reflective element is positioned in the beam after the first and second tunable wavelength selective elements, the reflective element and the second facet of the gain medium defining an outer laser cavity. 47. The method of claim 46, wherein a second tunable wavelength selective element is defined by the outer laser cavity. 48. The method of claim 45, further comprising splitting the light beam into a first light beam passing through the first tunable wavelength selective element and a second light beam passing through the second tunable wavelength selective element . 49. The method of claim 45, further comprising positioning a third tunable wavelength selective element in said light beam, defining a third plurality of tunable transmission peaks to provide a three-fold fine-tuned tuning effect, thereby defining a single combined transmission peak. 50. The method of claim 45, wherein: positioning the first tunable wavelength selective element includes positioning a first tunable etalon in the light beam; and Positioning the second tunable wavelength selective element includes positioning a second tunable etalon in the light beam. 51. The method of claim 50, wherein simultaneously tuning the first and second tunable wavelength selective elements comprises thermo-optically tuning the first and second tunable etalons. 52. The method of claim 51, wherein said thermo-optic tuning comprises: thermally adjusting the refractive index of the first tunable etalon; and The refractive index of the second tunable etalon is thermally adjusted. 53. The method of claim 52, wherein said thermo-optic tuning further comprises: thermally adjusting the physical thickness of the first tunable etalon; and Thermally adjusts the physical thickness of the second tunable calibrator. 54. A laser device comprising: base; Reflector; A gain medium having a first facet from which a light beam is emitted in response to an electrical input and a second facet opposite the first facet, the second facet and mirror defining an outer laser cavity having a first adjustable free spectral range, and providing a plurality of lasing modes having a first plurality of transmission peaks within the gain bandwidth of the gain medium; and a tunable wavelength selective element, operatively coupled to the substrate and positioned between the first facet and the mirror, configured to define a second plurality of tunable transmission peaks having a second tunable free spectral range, the said second plurality of tunable transmission peaks are within a gain bandwidth of said gain medium; Wherein the first tunable free spectral range is related to the second tunable free spectral range such that the first and second plurality of transmission peaks can be tuned by fine-tuning the tuning effect to produce a tunable joint transmission peak. 55. The laser apparatus of claim 54, wherein the tunable wavelength selective element comprises an etalon. 56. The laser device according to claim 54, wherein the first tunable free spectral range FSR ₁ is related to the second tunable free spectral range FSR ₂ by the following formula: FSR ₁ ≈(M/M±N)(FSR ₂ ) where M is the total number of tunable wavelengths in the selected wavelength range, and N is an optional non-integer or integer. 57. The laser device of claim 54, wherein the tunable wavelength selective element comprises a wedge etalon positioned by a MEMS actuator. 58. A laser device comprising: Reflector; a gain medium comprising a first facet from which a light beam is emitted in response to an electrical input and a second facet opposite the first facet, the second facet and mirrors defining an outer laser cavity; A first tunable wavelength selective element, positioned between the first facet and the mirror, is configured to define a first plurality of tunable transmission peaks having a first tunable free spectral range, the first plurality of tunable the transmission peak is within the gain bandwidth of the gain medium; A second tunable wavelength selective element, positioned between the first tunable wavelength selective element and the mirror, is configured to define a second plurality of tunable transmission peaks having a second tunable free spectral range, the second plurality of a tunable transmission peak within the gain bandwidth of the gain medium; and A third tunable wavelength selective element, positioned between the second tunable wavelength selective element and the mirror, is configured to define a third plurality of tunable transmission peaks having a third tunable free spectral range, the third plurality of a tunable transmission peak within the gain bandwidth of the gain medium; Wherein said first, second and third plurality of transmission peaks can be adjusted to produce a tunable joint transmission peak by a three-fold fine-tuning tuning effect. 59. The laser apparatus of claim 58, wherein the second wavelength selective element comprises an etalon. 60. The laser device of claim 58, wherein the second wavelength selective element comprises a wedge etalon positioned by a MEMS actuator. 61. The laser apparatus of claim 58, wherein the third wavelength selective element comprises an etalon. 62. The laser device of claim 58, wherein the third wavelength selective element is tuneable using the electro-optical effect.

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