WO2001065648A2 - Self-pulsing multi-section complex-coupled distributed feedback (dfb) laser - Google Patents

Abstract

A multi-section complex-coupled distributed feedback (DFB) laser device includes first and second device sections. Selection of the configuration and material(s) composing the DFB laser device causes self-pulsation of the light generated by such device. A light mode of a reflectivity spectrum for the first device section coincides with a side lobe of the reflectivity spectrum for the second device section. In addition, a light mode of the reflectivity spectrum of the second device section is arranged to be within the stop band of reflectivity spectrum for the first device section. The gratings of the sections can be in contact with electrodes coupled to receive injection current or a radio-frequency signal. Related systems and methods are also disclosed.

Description

SELF-PULSING MULTI-SECTION COMPLEX-COUPLED DISTRIBUTED

FEEDBACK (DFB) LASER DEVICE AND RELATED SYSTEMS AND METHODS

Cross-Reference to Related Application

This nonprovisional patent application claims priority benefits under Title 35, United States Code §119(e) based upon the provisional application entitled Method of Generation and

Stabilization of Periodic Intensity Oscillations using Multi-Section Complex-Coupled DFB

Laser assigned U. S. Application No. 60/186,591 filed March 3, 2000 naming Xinhong Wang,

Guifang Li, Mohammed Al-Mumin, and Weiming Mao as inventors.

Field of the Invention 1. Background of the Invention

This invention is directed to distributed feedback (DFB) laser devices and related systems and methods of making and using DFB lasers. In particular, the invention is related to self-pulsing multi-section complex-coupled DFB lasers that generates a self-pulsed light signal.

The invention is also directed to methods of making such devices, as well as systems incorporating such devices.

2. Description of the Related Art Self-pulsation in semiconductor lasers has been a subject of intense theoretical and experimental study in recent years. In particular, two-section distributed feedback (DFB) lasers have been shown to exhibit low-frequency self-pulsation due to longitudinal spatial hole burning (<10 GHz), negative differential gain (<10 GHz), and dispersive self Q-switching ( < 20 GHz ). Recently it was also reported that two-section index-coupled DFB lasers exhibit very-high-frequency self-pulsation in excess of 60 GHz . This high-frequency self-pulsation makes two-section DFB lasers attractive for applications such as variable-rate all-optical clock recovery, fiber backbone for broadband wireless networks, and/or for the generation of high- bit-rate optical time-division-multiplexing sources. However, two-section index-coupled DFB lasers require complicated design and fabrication to achieve strong index coupling, and require a tuning section to sustain the self-pulsation.

U. S. Patent No. 5,936,994 issued August 10, 1999 to Jin Hong et al. discloses a two- section complex-coupled distributed feedback (DFB) laser as related to wavelength-tunable lasers, with a specific purpose of enhancing the wavelength tuning range. The active region has multiple quantum wells defined by a multi-layer structure that has alternating layers of P-type doped InP and P-type doped InGaAsP. These layers are etched to define two gratings with different periods in respective sections of the laser. Electrodes are formed over each section of the laser. The current injection in the first electrode with respect to that in the second electrode can be varied so that lasing occurs in a left or right Bragg mode across a laser stop band. The device can thus be controlled to lase at either the right or left Bragg mode. Current injection via the electrodes can be used to tune the left or right Bragg mode.

Summary of the Invention This invention is directed to a self-pulsing multi-section complex-coupled distributed feedback (DFB) laser device that generates a self-pulsed light signal. The invention is also directed to systems that use such laser device, and related methods.

The disclosed laser device can comprise two or more sections. The device sections can comprise respective active regions for generation of laser light, and light confinement regions on opposite sides of the active regions, to reduce escape of light from the active regions. The device sections can comprise respective electrodes for injecting electric currents into respective active regions to tune the frequency of self-pulsed laser light generated by the DFB laser device. In a gain-coupled configuration, the grating(s) of respective sections can be defined in respective active regions of the device sections of the DFB laser device. In a loss-coupled configuration of the DFB laser device, either or both of the gratings of the DFB laser device can be in contact with the electrodes, or situated between the active region and electrodes, of respective sections of the DFB laser device. The active regions of the DFB laser device can comprise layers of indium phosphide (InP) and indium gallium arsenide phosphide (InGaAsP), or gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). The light confinement regions can comprise p-doped and n-doped InGaAsP layers in the case of a InP/InGaAsP laser device, or one or more layers of n-AlGaAs and p-AlGaAs in the case of a GaAs/AlGaAs laser device, for example. In the DFB laser device, the respective grating lengths, grating spacings, effective refractive indices, coupling coefficients, center frequencies, gain coefficients, permittivity perturbations, guided mode wave profiles, and/or electric currents injected into the first and second sections, can be established so that: (1) a mode of a first reflectivity spectrum of the first device section coincides with a side lobe of a second reflectivity spectrum of the second device section; and

(2) a mode of a second reflectivity spectrum of the second device section is positioned within the stop band of a first reflectivity spectrum of the first device section. Satisfaction of these two criteria assures that laser light generated by the DFB laser device is self-pulsing. In a three-section version of the DFB laser device, similar parameters for a third device section can be determined so that a mode of the third reflectivity spectrum of the third device section is positioned between the modes of the first and second reflectivity spectra. This version of the device can be used to reduce jitter of self-pulsed light produced by the device.

The disclosed laser device can be coupled to receive electric injection currents for tuning the frequency of the self-pulsation to generate a self-pulsed light signal that has a relatively constant period. The laser device can be incorporated into a system with jitter- reduction capability to stabilize the frequency of the self-pulsed light signal. The laser device can also be applied to use in recovery of an optical clock signal from an optical data signal. Thus, the disclosed laser device and systems incorporating it have significant utility in the optical networking industry. The present invention features systems comprising the self-pulsing multi-section complex-coupled distributed feedback (DFB) laser device of the present invention. In addition to the laser device, one such system comprises a radio-frequency (RF)/microwave-modulated optical source that generates an RF/micro wave-modulated signal on an optical carrier signal. The modulated optical carrier signal is supplied to the multi-section DFB laser device that generates self-pulsed laser light based thereon. The self-pulsed laser light produced by this system has reduced jitter due to the use of the modulated optical carrier signal. The system can be configured to operate the DFB laser device in either transmission or reflection mode.

Another disclosed system includes a multi-section DFB laser device with at least two sections for generating self-pulsed laser light, and at least one additional section for generating laser light based on an RF signal from the system's RF source. The laser light resulting from the RF signal is used in the generation of the self-pulsed laser light from the other two sections of the DFB laser device. This self-pulsed laser light has reduced jitter due to the relatively constant period of the RF signal used in its generation.

Methods of the invention can be used to determine the configuration and/or materials of a DFB laser device. These methods can comprise determining the configuration and/or material(s) for a multi-section complex-coupled distributed feedback (DFB) laser device, and determining reflectivity spectra for the first and second device sections of the laser device. The methods comprise determining whether the modes of the reflectivity spectra are arranged so as to cause self-pulsation. If the result of this step is negative, the methods can comprise modifying at least one of the configuration and material(s) of the laser device, and repeating the above steps. On the other hand, if the modes of the reflectivity are arranged so as to cause self- pulsation, the methods can comprise forming the laser device with the configuration and materials(s) determined to cause self-pulsation of the laser device. One of these methods can be used to form a three-section laser device by determining whether the mode of the reflectivity spectrum for a third section is between the modes of the reflectivity spectra of the first and second sections. If this determination is negative, this method comprises modifying at least one of the configuration and/or material(s) of the laser device and repeating previous steps. On the other hand, if this determination is affirmative, this method comprises forming the laser device with the configuration and material(s) determined to cause self-pulsation of the laser device. Another method of the invention comprises receiving an optical data signal with a multi-section complex-coupled distributed feedback (DFB) laser device, receiving electric currents with the DFB laser device, and generating a recovered clock signal with the DFB laser device, based on the optical data signal and the electric currents.

These together with other features and advantages, which will become subsequently apparent, reside in the details of construction and operation of the invention as more fully hereinafter described and claimed, reference being made to the accompanying drawings, forming a part hereof wherein like numerals refer to like elements or features throughout the several views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Brief Description of the Drawings Fig. 1 is a schematic cross-sectional view of a two-section complex-coupled distributed feedback (DFB) laser device.

Fig. 2 is a flowchart of a method for making a self-pulsing two-section complex- coupled distributed feedback laser device.

Fig. 3 is a graph of representative reflectivity spectra ri, r₂ versus variable Δβ for a two- section complex-coupled DFB laser device.

Fig. 4 is a relatively general cross-sectional view of a gain-coupled distributed feedback DFB laser device.

Fig. 5 is a relatively detailed cross-sectional view of a gain-coupled DFB laser device. Fig. 6 is a relatively general cross-sectional view of a loss-coupled DFB laser device with gratings formed in contact with electrodes of respective device sections.

Fig. 7 is a relatively detailed cross-sectional view of a loss-coupled DFB laser device with gratings formed in contact with electrodes of respective device sections.

Fig. 8 is a relatively general cross-sectional view of a loss-coupled DFB laser device with gratings spaced apart from electrodes of respective device sections. Fig. 9 is a relatively detailed cross-sectional view of a loss-coupled DFB laser device with gratings spaced apart from electrodes of respective device sections.

Figs. 10A, 10B, and IOC are cross-sectional views of square-step, saw-tooth, and undulating configurations for gratings of DFB laser devices in accordance with the invention.

Fig. 11 is a system for reducing jitter of laser light generated by a multi-section complex-coupled distributed feedback (DFB) laser device.

Fig. 12 is a system comprising a radio-frequency (RF)/microwave-modulated optical source, a circulator, a control unit, and a multi-section complex-coupled DFB laser device, for reducing jitter in the self-pulsed optical signal generated by the DFB laser device.

Fig. 13 is a relatively detailed exemplary embodiment of the system of Fig. 12. Fig. 14 is a cross-sectional view of a three-section complex-coupled distributed feedback (DFB) laser device. Fig. 15 shows graphs of representative reflectivity spectra , r₂, r₃ versus variable Δβ for a three-section complex-coupled DFB laser device.

Fig. 16 is a flowchart of a method for making a three-section complex-coupled DFB laser device. Fig. 17 is a view of a DFB laser device used for recovery of an optical clock signal from an optical data signal.

Description of the Preferred Embodiments As used herein, the following terms have the following definitions: "And/or" means "either or both". "Coupled" in an electronic sense refers to joining electronic components together with a conductive line such as a wire or cable, or by transmission of signals through air or other media, or space, for example, whether directly or through intermediate device or medium. "Coupled" in an optical sense means joining optical, electro-optical, or opto-electrical devices together so as to permit passing of light from one to another. Optical coupling can be done through any transmissive media, including optical fibers, optical waveguides, air, water, space, optical adhesive, or other media, whether directly or through intermediate device or medium. "Coupled" as used in this sense should not be confused with "complex-coupled", "gain- coupled", "loss-coupled", or "index-coupled" which terms refer to the manner in which optical field is exchanged between the forward- and backward-propagating light waves in a device section.

"Complex-coupled" means that index-coupling, in addition to gain- or loss-coupling, are vehicles for coupling of light between the forward- and backward-propagating light modes within the section(s) of the laser device. Complex-coupled devices have a coupling coefficient that is a complex number. "Index-coupled" means that differences in refractive index defining the grating(s) of a distributed feedback (DFB) laser device, are the primary vehicle for coupling of light between forward-propagating and backward-propagating light modes within the device section(s) of the laser device. Index-coupled devices have a coupling coefficient K that is a real number;

"Gain-coupled" means that differences in gain of the active region(s) associated with grating(s) of a distributed feedback (DFB) laser device, are the primary vehicle for coupling of light between forward-propagating and backward-propagating light modes within the section(s) of the laser device. Gain-coupled devices have a coupling coefficient K that is an imaginary or complex number;

"Gain" is a measure of the amount of photons generated by the active region of the DFB laser device per unit energy input for their generation; "Grating period" refers to the length along the optical propagation direction through which the material and/or structure undergoes one complete cyclic change.

"Loss-coupled" means that differences in loss of light in the gratings of the sections of the DFB laser device are the primary vehicle for coupling light between forward-propagating and backward-propagating light modes within the section(s) of the DFB laser device. Loss- coupled devices have a negative gain, i.e., the coupling coefficient is a negative imaginary or complex number.

"N-type " refers to a semiconductor material doped with donor atoms. The donor atoms can be Si, Se in the case of gallium arsenide (GaAs)/aluminum gallium arsenide (AlGaAs) semiconductor materials, or Si in the case of the indium phosphide (InP)/indium gallium arsenide phosphide (InGaAsP).

"Primary mode" refers to the mode in a device section that has the lowest threshold. Threshold is the minimum electrical injection current required for lasing of the device.

"P-type" refers to a semiconductor material doped with acceptor atoms. The acceptor atoms can be beryllium (Be), magnesium (Mg), zinc (Zn), cadmium (Cd), silicon (Si), carbon

(C), or copper (Cu) in the case of gallium arsenide (GaAs)/aluminum gallium arsenide

(AlGaAs) semiconductor materials, or Zn, Be, Mg in the case of the indium phosphide

(InP)/indium gallium arsenide phosphide (InGaAsP).

"Radio-frequency (RF)/microwave-modulated signal" refers to a signal modulated in the radio-frequency or microwave range.

"Self-pulsing" refers to a phenomenon in which a laser device, supplied with direct current (DC) bias or injection, generates a signal with a periodic intensity variation. "Side lobe" refers to a mode other than the primary mode.

"(s)" means "one or more" of the preceding thing. Thus, "electrode(s)" means "one or more electrode."

1. General Description of Self-Pulsing Two-Section Complex-Coupled Distributed Feedback

(DFB) Laser Device

Fig. 1 is a schematic diagram of a two-section complex-coupled distributed feedback

(DFB) laser device 1 in accordance with the invention. The two-section DFB laser device 1 comprises device sections 2, 3. The device sections 2, 3 comprise respective gratings 4, 5 defined by one or more of respective repetitive units 6, 7. For simplicity, only one of each of the repetitive units 6, 7 is specifically indicated in Fig. 1, but it should be appreciated that each has several repetitive units, and in general many more than shown in the Figures. As is understood by those of ordinary skill in this technology, the gratings 4, 5 may each have from ten to many thousand repetitive units 6, 7. The repetitive units 6, 7 can be defined by periodic variation in refractive index, gain, or loss of the material(s) composing such gratings, along the z-axis direction in Fig. 1. In general, the configuration and/or material(s) used to form the two- section DFB laser device 1 is determined so as to cause self-pulsation of laser light produced by such device. In general operation of the laser device 1, electric current I₍, I₂ is injected into the device sections 4, 5, respectively. The electric current Ii, I₂ causes generation of photons in the active regions of the device sections 4, 5. These photons travel in the laser device 1 as forward- propagating light wave Fi and backward-propagating light wave Bi in the first device section 2, and as forward-propagating light wave F₂ and backward-propagating light wave B₂ in the second device section 3. The forward-propagating light waves Fi, F₂ and backward- propagating light waves B_b B₂ are related to one another in the following way. A portion of the light wave Fi travels along the positive z-axis direction in Fig. 1 and passes into the second section 3. This portion of the light Fi combines with light generated within the second section 3, to form the light wave F₂. The light wave F₂ travels in the positive z-axis direction to output interface 8 defined as the boundary between the second device section 3 and external medium 9. The external medium 9 can be air, a vacuum, an optically-transmissive medium such as an optical fiber or waveguide. The external medium 9 can alternatively be an optically- transmissive adhesive layer attached to an optical fiber, for example. A portion of the light wave F₂ travels in the positive z-axis direction through the interface 8 and into the external medium 9 as the light wave F_0UTPUT- Another portion of the light wave F₂ reflects from the repetitive units 6 and interface between the second section 3 and the external medium 9, and combines with a portion of the light generated in the second device section 3 to form the light wave B₂. A portion of the light wave B₂ travels in the negative z-axis direction and into the first device section 2. This portion of the light wave B₂ combines with a portion of the light generated or reflected in the device section 2, to form the backward-propagating light wave Bi. At the reflective end face 10 of the laser device 1, the light wave B₂ reflects to propagate in the positive z-axis direction to combine with light generated or reflected within the first device section 2, to form the forward-propagating light wave Fi. The ratio of the backward- propagating light wave Bi and the forward-propagating light wave Fi is directly proportional to the reflectivity ri of the laser device 1. Similarly, for the second device section, the ratio of the backward-propagating light wave B₂ and the forward-propagating light wave F₂ is proportional to the reflectivity r₂ of the second device section 3. The selection of the configuration and/or material(s) defines the reflectivities τ r₂ whose relationship can be made such as to cause self- pulsating behavior, as described in more detail below.

2. Method of Making Self-Pulsing Two-Section Complex-Coupled Distributed Feedback

(DFB) Laser Device The general method for determining the configuration and material(s) composing the device sections 2, 3 of the laser device 1 is now described with reference to Fig. 2. In step SI the method of Fig. 3 begins. In step S2 the configuration and/or material(s) are determined for the two-section distributed feedback (DFB) laser device 1 based on approximate analytical methods. In step S3 the reflectivity spectra are determined for the two device sections 2, 3 of the laser device 1 using the specification of the configuration and material(s). In step S4 a determination is made to establish whether the modes of the reflectivity spectra are arranged so as to cause self-pulsation. If not, in step S5, the configuration and/or material(s) of the laser device are modified and the method returns to step S3. On the other hand, if the determination in step S4 is affirmative, in step S6 the laser device 1 is formed with the configuration and material(s) determined to produce self-pulsation in such laser device. In step S7 the method of Fig. 2 ends. The method of Fig. 2 is thus an iterative process in which configuration and material(s) of the device sections 4, 5 of the two-section DFB laser device 1 can be corrected to generate reflectivity spectra that are analyzed for proper disposition of modes in the device sections 2, 3 to cause self-pulsation of the laser device 1.

In the case of a complex-coupled laser device 1, the configuration and/or material(s) defining the laser device 1 can be selected by the grating lengths Lj, L₂ and/or the spacings A_!5 Λ₂ of the repetitive units 6, 7. In addition, the structure of the gratings 4, 5 can be varied by defining the grating to have a square-step, saw-tooth, or undulating, wave-like profile, for example. The material(s) and dopant atoms or other chemical additives for the laser device 1 can be selected to modify the effective refractive indices ri_efn, ri_efc, the coupling coefficients κι, κ₂, the center frequencies ω₀ι, ω₀₂, gain coefficients γ^ γ₂, the permittivity perturbations Δεi, Δε₂, and guided mode wave profiles Δει_y(Λ:), Δε_2y(jc) of the respective device sections 2, 3. The DC currents injected into respective device sections 2, 3 of the device 1 influence the charge carrier concentrations in such sections, and thereby also the effective refractive indices n_effl, ri_efc. As shown by the exemplary reflectivity spectra r r₂ of Fig. 3, the above parameters can be selected for the complex-coupled laser device 1 so that:

(1) a mode of a first reflectivity spectrum of the first device section coincides with a side lobe of a second reflectivity spectrum of the second device section; and

(2) a mode of a second reflectivity spectrum of the second device section is positioned within the stop band of a first reflectivity spectrum of the first device section. Attainment of these criteria means that DFB laser device will generate self-pulsed laser light.

In Fig. 3, the modes A and B of the first and second device sections 2, 3 are primary modes.

However, it is possible that side lobes can also be used for the modes if defined as indicated in the above-stated criteria (1) and (2).

To determine the reflectivity r of a device section, the following series of equations can be used. The first equation expresses the permittivity perturbation Δε(x,z), which is the difference between the permittivity of the device section in the presence of the grating and the permittivity of the device in the absence of the grating. Since the permittivity perturbation Aε(x,z) is periodic in z, it can be expressed by a Fourier series expansion as follows:

(1) Aε(x, z) = A ε (x) ^ a_qe i(2 q π I Λ ) ^« q = —oo in which Δε(x) represents the profile of permitivity perturbation in the x-direction, a_q is a constant representing the strength of the index perturbation in the qth order of the grating, and A is the period of the grating. Given the permitivity perturbation profile in equation (1), the coupling coefficient between the forward and backward waves in the grating region is given by the following equation:

in which i is equal to V— 1 , ω is a variable representing the frequency of light generated by the laser device 1, ε₀ is the permittivity of free space, α is the order of the DFB grating, and z_y(x) is the guided wave profile of the light mode in the grating region, that can be determined from the eigenvalue equation:

where ε(x) represents the geometry and index profiles of the unperturbed waveguide, n_eff is the effective refractive index of the guided mode. The grating period together with the effective refractive index of the guided mode determine the center frequency ω₀ of a frequency band of light that will be reflected by the grating according to:

, „ - 2 π c 2 π c

( 3 ) ω ₀ = -₁- = — λ> / 2 a A

/ « ^■ » eff in which c is the speed of light in free space, q is the order of the device section, and Λ is the grating period. To simplify the expression of the reflectivity r, a series of variables are defined as follows. The variable Δω is defined as:

(4) A ω = ω - ω ₀

The variable Δβ is defined as: (₅) _{Δ /}J ₌ A^_ . „ c

The variable s is defined as:

( 6 ) s ² = \κ I² + (γ - i β in which γ is a vaπable indicating the gain coetticient. for tne purpose 01 computing tne reflectivity spectra, the gain coefficient γ can be assumed to be a fraction of the cavity loss.

Using the coupling coefficient K, the variable s, the gain coefficient γ, the variable Δβ, and the length L of the device section, the reflectivity r for a section can be determined from the following equation:

, _ -. K sinh sL

(7 ) r = .

(γ - iA β ) sinh sL - s cosh sL

3. Gain-Coupled Embodiment of Laser Device Fig. 4 is a view of a two-section distributed feedback (DFB) laser device 1 of the invention. The laser device 1 includes first and second device sections 2, 3 with respective gratings 4, 5. The gratings 4, 5 can be defined by respective repetitive units 6, 7. The laser device 1 comprises a substrate 12, a first light confinement region 14, an active region 16, and a second light confinement region 18. The gratings 4, 5 are formed in the active region 16. Hence, the laser device 1 of Fig. 4 is complex-coupled. The substrate 12 can be composed of a material such as n-doped InP or GaAs. The active region 16 can be composed of alternating layers of undoped InGaAsP and undoped InP. Alternatively, the active region 16 can be composed of one or more layers of GaAs positioned between layers of AlGaAs and AlGaAs. The light confinement regions 14, 18 are positioned on opposite sides of the active region 16, The light confinement regions 14, 18 can be formed so that they contact the active region 16, The light confinement regions 14, 18 have a lower refractive index than the active region 1 and thus, as positioned on opposite sides of the active region 14, tend to confine light within the active region to prevent its loss. The light confinement layers 14, 18 can each be composed ol one or more layers of p-doped and n-doped InGaAsP layers respectively in the case of the InP/InGaAsP laser device, or one or more layers of n-AlGaAs and p- AlGaAs in the case of the GaAs/AlGaAs laser device. Electrodes 20, 22 are formed over respective gratings 4, 5 for use in injecting electric currents into the gratings to influence the concentration of charge carriers The concentration of charge carriers affects the refractive indices of the material(s) composing the gratings 4, 5, and hence the frequency of self-pulsation of the output laser light F_OUTPUT o the laser device 1. The control unit 24 is connected to electrodes 20, 22 and generates th( electric currents supplied to the electrodes for tuning the frequency of the output laser ligh F_OUTPUT- The control unit 24 is also coupled to the substrate 12 to ground the control unit 2^ relative to the laser device 1. The control unit 24 can be a commercially-available device sucl as a 3900 Mainframe Modular Laser Diode Controller produced by ILX Lightwav< Corporation, PO Box 6310, Bozeman, MT 59771. The configuration and materials of the lase device 1 of Figure 4 are determined to produce self-pulsation. More specifically, the gratinj material(s) interacting with the light, the coupling coefficients κ_{h 2}, the center frequencies ω₀ι, ω₀₂, gain coefficients y γ₂, the permittivity perturbations Δεi, Δε₂, guided mode wave profiles Δεi_y(t), Δε_2y(x), and DC currents injected into respective gratings sections 2, 3, are selected to define the modes A, B of the respective reflectivity spectra r,, r₂ of respective sections 2, 3 according to equations (1) - (7) above. Selection of the materials of the DFB laser device 1 and the DC injection currents largely defines effective refractive indices r_efτι, r_efn of the material(s) interacting with the light. Selection of the configuration of the DFB laser device 1 largely defines the grating lengths Li, L₂, and the grating spacings Λi, Λ₂. Both configuration and material(s) composing the DFB laser device 1 impact the coupling coefficients Ki, κ , the center frequencies ω₀ι, ω₀₂, the gain coefficients γi, γ₂, the permittivity perturbations Δεi, Δε₂, and guided mode wave profiles ε_ly(x), Δε_2y(; ).

Fig. 5 is an relatively detailed exemplary embodiment of a laser device 1 of Fig. 4.The structure of the laser device 1 of Fig. 5 is similar to the laser device described in U. S. Patent No. 5,936,994, but of course differs therefrom in that configuration and materials have been determined using reflectivity spectra τ r₂ to produce self-pulsation of laser light generated by the laser device 1. The laser device 1 of Fig. 5 is a gain-coupled laser device due to the configuration of the active region that produces a varied gain along the z-axis and x-axis directions. The DFB laser device 1 may also optionally be configured to vary along the y-axis direction as well, although this feature is not shown in Fig. 5. In Fig. 5, the laser device 1 includes substrate 12, confinement region 14, active region 16, and confinement region 18. The substrate 12 supports the active region 16 and confinement regions 14, 18. The active region 16 generates laser light that is self-pulsed as defined by the configuration and material(s) of the laser device 1. The confinement regions 14, 18 are situated on opposite sides of the DFB laser device and help to confine the laser light within the active region 16. Electrodes 20, 22 are formed over respective device sections 2, 3, and are coupled to control unit 24 that injects current of charge carriers (i.e., electrons or holes) into the electrodes. The electrodes introduce the charge carriers into the active region to tune the frequency of self-pulsation of the laser light resulting from the laser device 1. In the case of an InP/InGaAsP device, the substrate 12 is a heavily n-doped InP substrate generally of hundreds of microns in thickness, and thus proportionally much larger than shown in Fig. 5. In the case of a GaAs/ AlGaAs device, the substrate 12 is an n-type GaAs substrate. An n-type doped InP or GaAs buffer layer 28 of typically one (1) to five (5) micron thickness is formed over the substrate 12. The confinement region 14 can include four confinement layers 30, 32, 34, 36 that are composed of n-type doped InGaAsP in the case of an InP InGaAsP device or AlGaAs in the case of a GaAs/AlGaAs. These layers have successively increasing refractive indices relative to one another from layer

30, 32, 34, to layer 36. The layers 30, 32, 34, 36 are each ten (10) to one-hundred (100) nanometers (nm) in thickness. The active region 16 overlies the confinement region 14. The active region 16 has a multiple quantum well structure that includes eight 1% compressively strained undoped InGaAsP or AlGaAs quantum wells 38 each five (5) nm thick, separated by seven undoped InGaAsP or AlGaAs unstrained barriers 40 with band gap corresponding to a wavelength of approximately 1.25 microns, each barrier being approximately ten (10) nm thick. In general, increasing the number of quantum wells provides higher gain per unit length of the laser cavity. Although seven layers 38, 40 are shown in Fig. 5, it is possible to use typically from five (5) to fifteen (15) alternating layers 38, 40 to form the active region 16. The band gap of the quantum well structure described above provides a lasing wavelength of the device at about 1.55 microns. The laser device 1 includes two undoped InGaAsP or AlGaAs buffer layers 42, 44 with successively decreasing refractive indices, provided over the active region 14. The layers 42, 44 each have a thickness of ten (10) to one-hundred (100) nm. The gain of gratings 4, 6 is varied along the z-axis and x-axis directions, and optionally also along the y- axis direction, by periodically etching grooves through the active region 14. More specifically, a resist layer can be formed over layer 44, exposed with light patterned with a mask in a photolithography system or an electron beam in an e-beam lithography system, developed, and baked for hardening. Etching of the exposed areas of the layers 38, 40 of the active region 16, i.e., those not covered by the resist layer, can be performed with a reactive ion etching (REE) technique, for example, to form grooves in the active region 16. Layer 46 composed of undoped InP or AlGaAs with band gap larger than the quantum well band gap energy, is deposited in the etched grooves of the active region 14 to form the gratings 6, 7. The confinement region 18 including layers 48, 50, 52, 54, 56 is formed on the layer 46. The layer 48 includes P-type doped InP or AlGaAs of one-hundred (100) microns in thickness deposited over the layer 46. An etch stop layer 50 composed of P-type doped InGaAsP or AlGaAs of one (1) to ten (10) nm thickness is deposited on layer 48. A layer 52 composed of P-doped InP or GaAs formed to one- to five-hundred (100-500) microns in thickness is formed over the etch stop layer 50. An upper cladding layer 54 composed of P-type InP or GaAs is formed over the layer 56 to a thickness of one- to two-thousand (1000-2000) nm. A highly doped P-type InGaAs or GaAs capping layer 56 is formed over the layer 60 to a thickness of one- to five- hundred (100-500) A. All of the layers 28-56 can be formed by an epitaxial crystal growth technique such as metalorganic chemical vapor deposition (MOCND). A conductive metal layer can be formed over the capping layer 56. The metal layer can be composed of chromium (Cr), titanium (Ti), gold (Au), titanium-gold (Ti-Au) or other metal or alloy. The layers 52, 54, 56 and the metal layer, are etched via REE, for example, to etch stop layer 50, to form electrodes 20, 22 on respective islands composed of layers 52, 54, 56. The control unit 24 is coupled to the electrodes 20, 22 via wires soldered to the electrodes 20, 22, for example, to control current injection into the device sections 2, 3. In addition, a metal electrode 26 can be formed by evaporation or sputtering, for example, on the back surface of the substrate 12. The metal electrode 26 can be used to electrically ground the laser device 1 relative to the control unit 24. 4. Loss-Coupled Embodiment of Laser Device

Fig. 6 is a view of a loss-coupled DFB laser device 1. The loss-coupled laser device 1 includes a substrate 12, light confinement region 14, an active region 16, and light confinement region 18. The configuration and materials of the regions 12, 14, 16, 18, and the electrodes 20, 22, are selected so that the modes of reflectivity spectra τ r₂ for the two device sections 2, 3 cause self-pulsation of laser light generated by the laser device 1. The electrodes 20, 22 are in contact with loss-coupled gratings 2, 3. In particular, the lossy elements, which are absorptive to light, of the grating units 6, 7 are of the same electrically conductive material as that of the electrodes. As a result, this embodiment allows a single-step process for the deposition of the grating elements and the electrodes. The manufacture of the laser device 1 is greatly simplified in the embodiment of Fig. 6. The control unit 24 is coupled to the electrodes 20, 22 to supply current to the device sections 2, 3 to tune the laser device 1 of Fig. 6. The control unit 24 is also coupled to the substrate 12 so that the laser device 1 is grounded relative to the control unit 24.

A relatively detailed embodiment of the loss-coupled laser device 1 is shown in Fig. 7. The laser device 1 includes a substrate 12, an active region 16, and light confinement regions 14, 18. The substrate 12 can be composed of InP or GaAs, for example. The substrate 12 can be from one-hundred (100) to one-thousand (1,000) microns in thickness, for example, five- hundred (500) microns being typical. The laser device 1 can include a buffer layer 28 of n-type doped InP or GaAs. The buffer layer 28 can typically be from one (1) to five (5) microns in thickness. The confinement layer 14 can be composed of contiguous layers 30, 32, 34, 36 composed of n-type doped InGaAsP in the case of a GaAs/TnGaAsP device or an AlGaAs in the case of a GaAs/AlGaAs device. The doping or formulation of the layers 30, 32, 34, 36 can be made so that the refractive indices of these layers grade between the relatively low refractive index of the substrate 12, to the relatively high index of the active region 16. The layers 30, 32, 34, 36 can from five (5) to one-hundred (100) nanometers in thickness, for example, twenty (20) nanometers being a typical thickness. The active region 16 overlies confinement region 14. The active region 16 includes alternating layers 38, 40. In Fig. 7, the active region includes eight (8) multiple quantum well layers, but it should be understood that from five (5) to fifteen (15) of such layers can be used in the laser device 1. Increasing the number of multiple quantum well layers 38 increases the gain of the laser device 1. The multiple quantum well layers 38 can include undoped InGaAsP or AlGaAs layers. The layers 38 can typically be from one (1) to one-hundred (100) nanometers thick, ten (10) nm being a typical thickness for each layer. The layers 38 can be one-percent (1%) compressively-strained so that internal stress in the active region 16 does not result in damage. The layers 40 can be unstrained InGaAsP or AlGaAs layers. The layer 48 includes P-type doped InP or AlGaAs of one-hundred (100) microns in thickness deposited over the active region 16. Etch stop layer 50 composed of undoped InGaAsP or AlGaAs layer can be formed over the layer 48. The etch stop layer 50 can also provide a diffusion barrier to prevent impurity atoms from the upper cladding layer 18 from diffusing into the active region 16 and undesirably modifying the electro-optical performance of such region. A layer 52 composed of P-type doped InP formed from typically one- to five-hundred (100-500) nanometers in thickness is deposited over the etch stop layer 50. An upper cladding layer 54 composed of P-type InP is formed over the layer 52 to a typical thickness of one- to two-thousand (1,000-2,000) A. A highly-doped P-type InGaAs capping layer 56 is formed over the layer 54 to a thickness of one- to five-hundred (100-500) A. Layers 28-56 can be formed by MOCND processes, for example. A conductive metal layer is formed over the capping layer 56. The metal layer can be composed of chromium (Cr), titanium (Ti), gold (Au), titanium-gold (Ti-Au) or other metal or alloy. The metal layer can be formed by evaporation or sputtering, for example. The layers 52, 54, 56 and the metal layer, are etched via RIE, for example, to etch stop layer 50, to form electrodes 20, 22 on respective islands composed of layers 52, 54, 56. The control unit 24 is coupled to the electrodes 20, 22 via wires, for example, soldered to the electrodes 20, 22 to control current injection into the device sections 2, 3. In addition, a metal electrode 26 can be formed on the back surface of the substrate 12. The metal electrode 26 is coupled to the control unit 24 to electrically ground the laser device 1 relative to such control unit. This too can be accomplished by soldering a wire to the metal electrode 26 and coupling such wire to the control unit 24. Fig. 8 is an alternative embodiment of a self-pulsing loss-coupled complex-coupled

DFB laser device 1. The DFB laser device 1 of Fig. 8 is similar in configuration and operation to the device of Fig. 6, with the exception that the repetitive units 6, 7 of respective gratings 4, 5 are formed in the light confinement region 18 spaced apart from respective electrodes 6, 7 and the active region 16. The repetitive units 6, 7 are made of a material absorptive to the laser light generated in the active region 16, to define respective loss-coupled gratings 4, 5.

Fig. 9 is a specific implementation of the device 1 of Fig. 8. The device 1 of Fig. 9 comprises substrate 12, light confinement region 14, and active region 16 the same as or similar to previously-described configurations. However, the light confinement region 18 and the gratings 4, 5 are defined somewhat differently from previously-described configurations. In Fig. 9, layers 48, 50 and 52 are formed over the active region 16, as previously described. The layer 52 is etched to form periodic openings using suitable etching techniques. Optically- absorptive material such as chromium (Cr), titanium (Ti), or other metal or alloy, is deposited in the openings in layer 52 to form the repetitive elements 6, 7 of respective gratings 4, 5. Layers 54 and 56 are formed over the layer 52 and the grating elements 6, 7. A metal layer is deposited over layer 56 and is patterned to form electrodes 20, 22 that are electrically coupled to the coupled unit 24 to receive respective DC currents from the control unit 24. Etching can be selectively performed between the electrodes 20, 22 to the layer 50 to electrically-isolate the electrodes 20, 22 from one another.

5. Alternative Grating Configurations As shown in Figs. 10 A- 10C, the gratings 4, 5 defined in respective sections 2, 3 of the laser device 1 can be formed with different configurations. Fig. 10A indicates a square-wavelike grating cross-section. Fig. 10B shows the gratings 4, 5 defined in respective sections 2, 3 with a saw-tooth configuration. Fig. 10C shows the gratings 4, 5 defined in the sections 2, 3 with an undulating or wave-like pattern. Through one or more deposition and/or etching steps, the gratings 4, 5 can be formed to have any of these and other configurations. The number of gratings in each section 2, 3 can be from tens to several thousands of repetitive units 6, 7, and thus many more than shown in the simplified configurations of Figs. 10 A- 10C and other Figures of this document.

6. Operation of the Laser Device The operation of the laser devices 1 of Figs. 1, 4-9 is similar. When currents are injected into the respective device sections of the device 1, the carrier number densities in active region of the respective sections will change. As a result, both the gain/loss and the index of refraction of the active regions can be tuned by the injection currents. Since the side lobe and the stop band of the DFB grating have finite width, the condition for self-pulsation is robust. Furthermore, relatively slight, with respect to optical frequency, shifts of the reflectivity spectra of the two device sections will only quantitatively change the nature of self- pulsation, such as its frequency and amplitude. Thus, as the injection currents are changed in the two sections of the gratings, the frequency of self pulsation will change accordingly. 7. Systems and Methods for Reducing Jitter in Laser Device Fig. 11 is a system for reducing jitter of a multi-section complex-coupled DFB laser device 1. The system 64 comprises the DFB laser device 1, a control unit 24, and a radio- frequency(RF)/microwave-modulated optical source 66. The source 66 generates an RF/micro wave-modulated optical signal on an optical carrier signal. The RF/microwave modulation is either at approximately the same frequency as that of the self-pulsation or a sub- harmonic thereof. The source 66 is coupled to supply the RF/microwave-modulated signal on the optical carrier signal, to the multi-section complex-coupled DFB laser device 1. The control unit 24 generates DC injection currents and is coupled to supply such currents to the DFB laser device 1. The device 1 generates self-pulsed light based on the RF/microwave- modulated signal on the optical carrier signal, and the DC currents from the control unit 24. The self-pulsed laser light is output from the laser device 1. Due to the relatively low jitter of signals produced by the source 66, the laser device 1 generates self-pulsed light with correspondingly reduced jitter.

In Fig. 12, an alternative configuration of the system 62 is shown. The system 64 of Fig. 12 is similar to that of Fig. 11, with the addition of circulator 68. The circulator 68 is coupled to receive the RF/microwave modulated signal on the optical carrier signal, from the source 66. The RF/microwave modulation is either at approximately the same frequency as that of the self-pulsation or a sub-harmonic thereof. The circulator is coupled to supply these signals to the laser device 1. The laser device It receives DC injection currents from the control unit 24. The laser device 1 generates self-pulsed light based on the RF/microwave- modulated signal on the optical carrier signal, and the DC injection currents. The device 1 is coupled to supply the self-pulsed laser light to the circulator 68, and the circulator outputs the self-pulsed laser light. By using the relatively low-jitter signals from the source 66 as the basis for generation of self-pulsed light, the laser device 1 produces the self-pulsed light with correspondingly low jitter.

The system 64 of Fig. 13 incorporates the multi-section complex-coupled DFB laser device 1 of any of the previous embodiments. In Fig. 13, the system 64 comprises DFB laser device 1, control unit 24, radio-frequency (RF)/microwave-modulated optical source 66, and circulator 68. The source 66 comprises a laser diode (LD) driver 70, a standard DFB laser device 72, a thermo-electric (TE) controller 74, optic fiber 76, polarization controller 78, Mach- Zender modulator 80, radio-frequency (RF) signal source 82, radio-frequency (RF) amplifier 84, and direct current (DC) voltage source 86. The LD driver 70 supplies a bias current for the DFB laser 72.. The DFB laser device 72 is also coupled to the TE controller 74 that controls the temperature of such device. Accordingly, the TE controller 74 can be used to maintain the temperature of the laser device 72 to make the wavelength of light generated by such device relatively constant. The wavelength of light generated by the laser device 72 can be tuned slightly by temperature. The DFB laser device 72 is coupled to supply its light signal via the optical fiber 76 to the polarization controller 78. The unit 78 controls polarization of the mode traveling in the light generated by DFB laser device 72. The polarization controller 78 is coupled to supply the polarization-controlled light to the Mach-Zender modulator 80. The RF signal source 82 generates an RF signal that is either at the frequency of self-pulsation of laser light from the DFB laser device 1 or a sub-harmonic thereof. Thus, for example, if the self- pulsation frequency of laser light from DFB laser device 1 is 40GHz, then the RF signal source 82 should have a frequency at 40GHz, or a sub-harmonic thereof, 20GHz, 10GHz, 5GHz, etc. The source 82 is coupled to supply the RF signal to the amplifier 84. Based on this RF signal, the amplifier 84 generates an amplified signal. The amplifier 84 is coupled to supply the amplified signal to the Mach-Zender modulator 80. The Mach-Zender modulator 80 is also coupled to receive a DC voltage from the source 86. The Mach-Zender modulator 80 generates an RF/microwave-modulated optical signal on an optical carrier signal, based on the polarization-controlled light, the amplified RF signal, and the DC voltage received from units 78, 82, 86, respectively. The Mach-Zender modulator 80 is optically-coupled to supply the modulated signal on the optical carrier signal to the circulator 68. The circulator 68 is in turn coupled to supply the RF/microwave-modulated light to the DFB laser device 1 of the invention. The device 1 receives DC injection currents from the control unit 24. The device 1 generates self-pulsed laser light based on the modulated signal on the optical carrier signal, and the DC injection currents from the control unit 24. Electric currents supplied to the respective device sections 2, 3 by the control unit 24 can be used to tune the frequency of pulsation of the light generated by the device 1. The self-pulsed light generated by the system 64 can be supplied to downstream elements) such as a modulator for modulating a data signal via return- to-zero (RZ), non-return-to-zero (NRZ), or other modulation scheme.

Fig. 14 is a system that includes a three-section distributed feedback (DFB) laser device 1, control unit 24, a radio-frequency (RF) signal source 82, and an RF amplifier 84. The system of Fig. 14 accomplishes similar objectives to those of the system of Fig. 13, namely, production of a self-pulsation light signal with relatively reduced jitter. The three-section DFB laser device 1 has device sections 2, 3, and 86. These device sections can be the same or similar to the embodiments previously disclosed. The device section 86 serves the purpose of reducing jitter of the pulsed light signal F_OUTPUT produced by the device sections 2, 3 of the laser device 1. The device section 86 has a grating 87 defined by repetitive elements 88. The spacing Λ₃ of elements 88, the length Li of the grating 87, the effective refractive index n^, coupling coefficient κ₃, center frequency ω₀₃, gain coefficient γ₃, permittivity perturbation Δε₃, and guided mode wave profile Δε_3y(x), are selected to produce a reflectivity spectrum r₃ having a mode positioned strategically with respect to the modes of the reflectivity spectra ri, r₂ of the device sections 2, 3, respectively. More specifically, by selecting and/or modifying one or more of these parameters, the device section 86 is configured and composed of materials that produce a mode of reflectivity r₃ that is between the modes defined by reflectivities rj, r₂.

The RF signal source 82 generates an RF signal at the self-pulsation frequency of the output light F_OUTPUT) or a sub-harmonic thereof. The RF signal source 82 is coupled to supply the RF signal to the amplifier 84. The amplifier 84 generates an amplified RF signal based on the received RF signal. The third device section 86 includes an electrode 90. The amplifier 84 is coupled to supply the amplified RF signal to the electrode 90, generally with the aid of an impedance matching circuit (not shown). In addition, the electrode 90 is coupled to receive DC injection current I₃ from the control unit 24. Based on the amplified RF signal and the DC injection current, the third device section 86 generates forward-propagating light wave F₃ and backward-propagating light wave B₃. A portion of the light wave F₃ travels in the laser device 1 along the positive z-axis direction and passes from the third device section 86 into the second device section 2 as a portion of the forward-propagating light Fi. Conversely, a portion of the light wave Bi passing from the second device section 2 to the third device section 86 combines with light generated in the third device section 86 to form the light wave B₃. Rather than having the reflective interface 10 at the end of the first device section 2 as it is in the two- section DFB laser device, the three-section device 86 has an end at which the reflective face 10 is positioned. The backward-propagating light wave B₃ reflects from the end face 10 of the third device section 86 to combine with light traveling in the positive z-axis direction to form the forward-propagating light wave F₃. Propagation of forward- and backward-propagating light waves F^ B^ F₂, B₂ in the gratings sections 2, 3 is similar to that previously described with respect to Fig. 1, and results in generation of the self-pulsed laser light F_OUTPUT output from the three-section device 1. Use of the RF signal generated by the source 82 in the three- section DFB laser device 1 of Fig. 14 results in a self-pulsed laser light F_OUTPUT with a relatively low jitter.

Fig. 15 is a view of exemplary reflectivity spectra r r₂, r₃ versus Δβ for a self-pulsing three-section complex-coupled distributed feedback (DFB) laser device 1. As shown in Fig. 15, the third section 86 of the laser device 1 has a primary mode C in its reflectivity spectrum r₃ occurring between the modes A, B and equally spaced from the modes A, B of the reflectivity spectra r_b r₂. With modes A, B, C so arranged, the reflectivity spectra r,, r₂, r₃ can be used to generate self-pulsed laser light F_OUTPUT that is relatively free from jitter. Although modes A, B, C are primary modes in Figs. 15 it is possible that they could as well be other modes and produce self-pulsed light.

Fig. 16 is a flowchart of a method for making a three-section distributed feedback (DFB) laser device 1. In step SI the method begins. In step S2 the configuration and/or material(s) composing device sections 2, 3 are determined. This determination can be made by selection of the grating lengths L_b L₂, grating spacings Λi, Λ₂, effective refractive indices ri_em, ri_efc, coupling coefficients Ki, κ₂, center frequencies ω₀ι, ω₀₂, gain coefficients γi, γ₂, permittivity perturbations Δεi _» Δε₂ , guided mode wave profiles Δε^x), Δε_2y(x), and DC injection currents Ii, I₂. In step S3 the reflectivity spectra ri, r₂ for the sections 2, 3 of the three- section DFB laser device 1 are determined. In step S4 a determination is made to establish whether modes A, B of the reflectivity spectra rj, r₂ are arranged to cause self-pulsation of laser light generated by the device 1. If the determination in step S4 is negative, in step S5 the configuration and or material(s) used to form the laser device 1 are modified, and the method returns to step S3. On the other hand, if the determination in step S4 is affirmative, in step S6 the reflectivity spectrum r₃ for the third section of the laser device 1 is determined. In step S7 a determination is made to establish whether the mode C of reflectivity r₃ of the third section 86 is between the modes A, B of the reflectivity spectra τ_u r₂. If not, in step S5, the configuration and/or material(s) of the three-section DFB laser device 1 are modified and the method returns to step S3. This can be done by modifying the grating length L₃, grating spacing Λ₃, effective refractive index rie_fβ, coupling coefficient ₃, center frequency ω₀₃₎ gain coefficient γ₃, permittivity perturbation Δε₃, and/or the guided mode wave profile Δε_3y(x). On the other hand, if the determination in step S7 is affirmative, in step S8 the three-section DFB laser device 1 is formed according to the configuration and material(s) established in the preceding steps. In step S9 the method of Fig. 14 ends. 8. System Application of Multi-Section Complex-Coupled Two-Section DFB Laser Device Fig. 17 is a view of the multi-section DFB laser device 1 applied to recovery of a clock signal from an optical data signal FΓ_NPUT. The optical data signal includes optical data modulated on an optical carrier signal. For example, the optical data signal can be a return-to- zero (RZ), non-return-to-zero (NRZ) or other format. The optical data signal FΓ_NPUT travels into the device 1 that is configured and tuned via DC injection currents for self-pulsation as previously described, at approximately the frequency of the clock signal used in generation of the optical data signal. The device 1 extracts and outputs the optical clock signal F_OUTPUT from the optical data signal. The recovered clock signal F_OUTPUT can be used in the recovery of data from the optical data signal.

The many features and advantages of the present invention are apparent from the detailed specification and thus, it is intended by the appended claims to cover all such features and advantages of the described device, systems, and methods which follow in the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those of ordinary skill in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Accordingly, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.

Claims

Claims

1. A self-pulsing multi-section complex-coupled distributed feedback (DFB) laser device.

2. A multi-section complex-coupled distributed feedback (DFB) laser device receiving DC currents, the device self-pulsing to generate an optical signal with periodic intensity variation based on the DC currents.

3. A device as claimed in claim 2 wherein the device comprises at least two device sections coupled to receive respective DC currents, the device sections generating the self- pulsed optical signal.

4. A device as claimed in claim 3 wherein at least one of the device sections of the laser device is gain-coupled.

5. A device as claimed in claim 3 wherein at least one of the device sections of the laser device is loss-coupled.

6. A device as claimed in claim 2 wherein the device comprises at least two device sections, and at least two electrodes for injecting respective DC currents into associated device sections, the grating for at least one of the sections formed in contact with the electrode for the one section.

7. A device as claimed in claim 2 wherein the device comprises at least three device sections.

8. A device as claimed in claim 2 wherein the device comprises a multi-quantum well active region with alternating layers of indium phosphide (InP) and indium gallium arsenide phosphide (InGaAsP).

9. A device as claimed in claim 2 wherein the device comprises a multi-quantum well active region with layers of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs).

10. A distributed feedback (DFB) laser device having first and second device sections, and first and second electrodes, the first and second electrodes formed in contact with the gratings of the first and second device sections, respectively.

11. A distributed feedback (DFB) laser device as claimed in claim 10 wherein the laser device has a third device section and a third electrode, the third electrode formed in contact with the third device section.

12. A self-pulsing multi-section complex-coupled distributed feedback (DFB) laser device having first and second sections having respective gratings, the respective grating lengths, grating spacings, effective refractive indices, coupling coefficients, center frequencies, gain coefficients, permittivity perturbations, guided mode wave profiles, and/or electric currents injected into respective first and second device sections, established so that: (1) a mode of a first reflectivity spectrum of the first device section coincides with a side lobe of a second reflectivity spectrum of the second device section; and

(2) a mode of a second reflectivity spectrum of the second device section is positioned within the stop band of a first reflectivity spectrum of the first device section.

13. A device as claimed in claim 12 wherein the laser device has a third section with a respective grating, the respective grating length, grating spacing, effective refractive index, coupling coefficient, center frequency, gain coefficient, permittivity perturbation, guided mode wave profile, and/or electric current injected into the third device section, established so that:

(3) a mode of the third reflectivity spectrum of the third device section is positioned between the modes of the first and second reflectivity spectra.

14. A system comprising: a radio-frequency (RF)/microwave-modulated optical source generating an RF/microwave-modulated optical signal on an optical carrier signal; and a self-pulsing multi-section complex-coupled distributed feedback (DFB) laser device coupled to receive the RF/microwave-modulated optical signal on the optical carrier signal, the device generating self-pulsed light based on the received signals.

15. A system as claimed in claim 14 further comprising: a control unit generating DC currents, and coupled to supply the DC currents to respective sections of the device, the device generating the self-pulsed light further based on the DC currents.

16. A system as claimed in claim 14 wherein the source comprises: an RF signal source generating a RF/microwave-modulated electric signal; a DFB laser device generating an optical carrier signal; and a Mach-Zender modulator coupled to receive the RF/microwave-modulated electric signal on the optical carrier signal, the Mach-Zender modulator generating the RF/microwave-modulated optical signal on the optical carrier signal, based on the RF/microwave-modulated electric signal from the RF signal source and the optical carrier signal from the DFB laser device.

17. A system as claimed in claim 16 wherein the source further comprises a thermo- electric (TE) controller coupled to the DFB laser device, for controlling the temperature of the

DFB laser device to reduce jitter in the optical carrier signal generated by the DFB laser device.

18. A system as claimed in claim 16 wherein the source further comprises a laser diode (LD) driver generating a pump laser light, the LD driver coupled to supply the pump laser light to the DFB laser device, the DFB laser device generating the optical carrier signal based on the pump laser light.

19. A system as claimed in claim 16 wherein the source comprises a polarization controller coupled to receive the optical carrier signal from the DFB laser device, the polarization controller controlling polarization of the optical carrier signal, the polarization controller coupled to supply the polarization-controlled optical carrier signal to the Mach- Zender modulator.

20. A system as claimed in claim 16 wherein the source further comprises an amplifier coupled to receive the RF signal from the RF signal source, the amplifier generating an amplified RF signal based on the received RF signal, the amplifier coupled to supply the amplified RF signal to the Mach-Zender modulator for use in generating the RF/microwave- modulated optical signal on the optical carrier signal.

21. A system as claimed in claim 16 wherein the source further comprises DC voltage source coupled to supply DC voltage to the Mach-Zender modulator, the Mach-Zender modulator generating the RF/microwave-modulated optical signal on the optical carrier signal further based on the DC voltage.

22. A system as claimed in claim 16, further comprising: a circulator coupled to receive the RF/microwave-modulated optical signal on the optical carrier signal, and coupled to supply the RF/microwave-modulated optical signal on the optical carrier signal to the self-pulsing DFB laser device, the circulator coupled to receive and output the self-pulsed light from the self-pulsing DFB laser device.

23. A system comprising: a radio-frequency (RF)/microwave-modulated optical source generating an RF/microwave-modulated electric signal; a control unit generating DC currents; and a self-pulsing three-section distributed feedback (DFB) laser device having first, second, and third device sections, and first, second, and third electrodes positioned in association with respective first, second, and third device sections, the first and second electrodes coupled to receive respective DC currents from the control unit, and the third electrode coupled to receive the RF/microwave-modulated signal from the source, the device generating self-pulsed light based on the RF/microwave-modulated electric signal and the DC currents.

24. A method comprising the step of determining whether modes of reflectivity spectra of gratings sections of a distributed feedback (DFB) laser device are arranged so as to cause self-pulsation.

25. A method comprising the steps of: a) determining at least one of the configuration and material(s) for a distributed feedback (DFB) laser device; b) determining reflectivity spectra for first and second sections of the laser device; c) determining whether the modes of the reflectivity spectra are arranged so as to cause self-pulsation; if the result in the step (c) is negative, d) modifying at least one of the configuration and material(s) of the laser device; and e) repeating the steps (a) -(c); and if the result in the step (c) is affirmative, f) forming the laser device with the configuration and materials(s) determined to cause self-pulsation of the laser device.

26. A method comprising the steps of: a) determining at least one of the configuration and material(s) for a distributed feedback (DFB) laser device; b) determining reflectivity spectra for first and second sections of the laser device; c) determining whether the modes of the reflectivity spectra are arranged so as to cause self-pulsation; if the result in the step (c) is negative, d) modifying at least one of the configuration and material(s) of the laser device; and e) repeating the steps (a) - (c); and if the result in the step (c) is affirmative, f) determining reflectivity spectra for a third section of the laser device; g) determining whether the mode of the reflectivity spectrum for a third section is between the modes of the reflectivity spectra of the first and second sections; and if the determination in step (g) is negative, h) performing the steps (d) and (e); and if the determination in the step (f) is affirmative, i) forming the laser device with the configuration and materials(s) determined to cause self-pulsation of the laser device.

27. A method comprising the steps of: a) receiving an optical data signal with a self-pulsing multi-section distributed feedback (DFB) laser device; b) receiving electric currents with the DFB laser device; and c) generating a recovered optical clock signal with the DFB laser device, based on the optical data signal and the electric currents.