JP5540360B2 - Optical power monitor integrated DFB laser - Google Patents

Description

  The present invention relates to an optical power monitor integrated DFB laser used in an optical transmitter.

  A semiconductor laser is an essential device in various fields such as an optical communication field and an optical pickup device, and there is a strong demand for low power consumption and stable operation over a long period of time with low current and high efficiency. However, semiconductor lasers gradually deteriorate with the operating time, and phenomena such as a decrease in optical output and wavelength shift appear. When these deteriorations exceed a predetermined limit, it is considered a failure, but adjustment is performed by a drive current or the like so that the optical output and the wavelength are constant until the failure is reached.

  In order to keep the optical output and wavelength of the semiconductor laser constant, it is necessary to monitor the optical output of the semiconductor laser and the like. A general monitoring method is a method of monitoring light output from the semiconductor with a photodiode (PD). (For example, see Non-Patent Document 1). By performing feedback control of the driving current of the semiconductor laser so that the amount of light received by the power monitor PD is constant, the optical output and wavelength of the semiconductor laser can be kept constant over a long period of time.

  On the other hand, we propose a distributed reflector (DR) laser in which a distributed Bragg reflector (DBR) is integrated in a DFB laser with a thin active layer structure, and a three-region integrated laser in which a power monitor region is added to a DR laser. (For example, see Non-Patent Document 2 below). These semiconductor lasers are characterized in that a low threshold and high efficiency operation is possible because the active layer has a fine line structure.

Hiroo Yonezu, “Optical Communication Element Engineering -Issuance / Light-Receiving Element”, published by Engineering Books Co., Ltd., February 15, 1984, p. 259-260 Heiwaya, 6 others, "Distributed reflection (DR) laser with thin wire structure", IEICE technical report [Laser / Quantum electronics], published by The Institute of Electronics, Information and Communication Engineers, May 17, 2002, LQE2002-16 H. Kogelnik, C.I. V. Shank, “Coupled-Wave Theory of Distributed Feedback Lasers”, J. Am. Appl. Phys. 43, 5, 1972, p. 2327-2335

  The above-mentioned PD monitor requires optical components such as lenses and duplexers, which not only increases the manufacturing cost and the component cost, but also hinders downsizing and further decreases the yield due to an increase in manufacturing steps. There are challenges.

  Also, an integrated power monitor with a semiconductor laser generally requires a regrowth of a core layer having a composition different from that of the semiconductor laser in order to prevent deterioration of the characteristics of the semiconductor laser due to a large absorption loss. There is an inevitable problem in yield reduction.

  In light of the above, an object of the present invention is to provide an optical power monitor integrated DFB laser that solves the above-described problems and also has a function as an optical power monitor while maintaining a high reflectance.

An optical power monitor integrated DFB laser according to the first invention for solving the above-mentioned problems is
An optical power monitor integrated DFB laser in which a distributed Bragg reflection region and a distributed feedback laser region are integrated,
The active layer of the waveguide in the distributed black reflection region is a structure that is a quantum well thin line that is periodically arranged in the light traveling direction and is thinned to about several tens of nanometers .
The active layer of the waveguide of the distributed feedback laser region is a structure that is a discontinuous separated quantum well thin line with respect to the light traveling direction,
The output of the semiconductor laser is monitored by the absorption current in the distributed Bragg reflection region.

An optical power monitor integrated DFB laser according to a second invention for solving the above-described problems is an optical power monitor integrated DFB laser according to the first invention.
Wherein when the length and the length of the distributed Bragg reflector region of the distributed feedback laser region was normalized by using the coupling coefficient of the diffraction grating kappa, the length L _DFB of the distributed feedback lasers area is "3. 5 <κ L _DFB <8 ”, the length L _DBR of the distributed Bragg reflection region is“ 2 <κ L _DBR <4 ”, and the absorption coefficient α with respect to the oscillation wavelength of the active layer fine wire in the distributed Bragg reflection region is“ 1500 cm ^{−. 1} <α <2000 cm ⁻¹ ”.

An optical power monitor integrated DFB laser according to a third aspect of the present invention for solving the above problem is the optical power monitor integrated DFB laser according to the first aspect of the present invention.
A phase shift is provided in the distributed feedback laser region.

An optical power monitor integrated DFB laser according to a fourth aspect of the present invention for solving the above problem is the optical power monitor integrated DFB laser according to the third aspect of the present invention.
Wherein when the length and the length of the distributed Bragg reflector region of the distributed feedback laser region was normalized by using the coupling coefficient of the diffraction grating kappa, the length L _DFB of the distributed feedback lasers area is "3 < κ L _DFB <5 ”, the length L _DBR of the distributed Bragg reflection region is“ 2 <κ L _DBR <4 ”, and the absorption coefficient α with respect to the oscillation wavelength of the active layer thin wire in the distributed Bragg reflection region is“ 300 cm ⁻¹ < α <2000 cm ⁻¹ ”.

  According to the present invention, it is possible to provide an optical power monitor integrated DFB laser having a function as an optical power monitor while maintaining a high reflectance.

It is the schematic diagram which showed the structure of the optical power monitor integrated DFB laser based on the Example of this invention. It is the schematic diagram which showed the structure of the cross section of the waveguide in aa 'shown in FIG. 1 in the optical power monitor integrated DFB laser concerning the Example of this invention. It is the figure which showed the thin line | wire width dependence of the reflectance of the DBR area | region in the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the schematic diagram which showed the 1st process in the manufacturing process of the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the schematic diagram which showed the 2nd process in the manufacturing process of the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the schematic diagram which showed the 3rd process in the manufacturing process of the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the schematic diagram which showed the 4th process in the manufacturing process of the optical power monitor integrated DFB laser concerning the Example of this invention. It is the schematic diagram which showed the 5th process in the manufacturing process of the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the schematic diagram which showed the 6th process in the manufacturing process of the optical power monitor integrated DFB laser concerning the Example of this invention. It is the figure which showed the SEM image of the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the figure which showed the SEM image of InP reverse mesa stripe etching in the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the figure which showed the SEM image of the InP forward mesa direction groove | channel formation etching in the optical power monitor integrated DFB laser which concerns on a present Example. It is the figure which showed the calculation result of the light intensity distribution at the time of no bias application in the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the figure which showed the calculation result of the reflectance spectrum of the DBR area | region in the optical power monitor integrated DFB laser which concerns on the Example of this invention. It is the figure which showed the light intensity distribution at the time of a bias of (lambda) / 8 phase shift structure. It is the figure which showed the light intensity distribution at the time of no bias of a uniform diffraction grating structure. It is the figure which showed the DFB area | region length dependence of the front end surface output efficiency. It is the figure which showed DBR area | region length dependence of DBR reflectance. It is the figure which showed DBR area | region length dependence of DBR reflectance.

Hereinafter, embodiments for implementing the optical power monitor integrated DFB laser according to the present invention will be described.
The optical power monitor integrated DFB laser according to the present invention has a structure in which a region having both a function as a Bragg reflector and a function as a power monitor (DBR region) and a region of the DFB laser are integrated, and an active layer thereof. Has a thin line structure.

The optical power monitor integrated DFB laser according to the present invention has a large absorption loss while processing the quantum well active layer into a periodic thin line structure, while having the same active layer as the DFB laser region. It is possible to monitor without receiving.
In addition, in the case of optical monitoring, the optical monitor region (DBR region) also has a function as a DBR region that reflects most of the light incident on the power monitor to the laser resonator. Oscillation characteristics can be maintained.

Furthermore, the power monitor can be manufactured at a low cost because it can be manufactured by a process integrated with the DFB laser region without increasing the regrowth process.
Further, the period of the thin line active layer in the DBR region in the optical power monitor integrated DFB laser according to the present invention is substantially the same as that in the DFB laser region.

  Conventionally, in order to stabilize the light output in the DFB laser region, a feedback mechanism that generally monitors the power of the output light and reflects the light output fluctuation in the output of the light output control circuit is required. Among these, in the part that monitors the power of the output light, a part of the output light is taken out by an optical component such as a prism, or the light that leaks from the opposite side of the output end of the DFB laser region is used for the PD. The method used for observation was used. However, the former method requires a discrete component such as a prism, and the enlargement is unavoidable, and part of the light output is impaired. The latter method produces a PD on a semiconductor substrate separately from the DFB laser region. There was a problem such as necessity.

  On the other hand, in the optical power monitor integrated DFB laser according to the present invention, a DBR region is used as a reflecting mirror on the opposite side to the output end of the active layer separation type DFB laser, and a part of the light is absorbed at that portion. It is characterized in that it also serves as a PD by monitoring the photocurrent.

  As a result, the DBR region itself can be produced simultaneously with the production of the DFB laser region, and the addition of a function as a PD can basically be achieved simply by providing a photocurrent extraction electrode on the DBR region. it can. Further, the configuration of the optical power monitor integrated DFB laser according to the present invention has the function of monitoring the output light power in a compact form integrated in the active layer isolation type DFB laser without increasing the number of manufacturing steps. A type DFB laser can be provided.

Embodiments of an optical power monitor integrated DFB laser according to the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic diagram showing a configuration of an optical power monitor integrated DFB laser according to the present embodiment. FIG. 2 is a schematic diagram showing a cross-sectional configuration of the waveguide at aa ′ shown in FIG. 1 in the optical power monitor integrated DFB laser according to the present embodiment.

  As shown in FIGS. 1 and 2, the optical power monitor integrated DFB laser according to the present embodiment is composed of a DFB laser region and a DBR region, and light generated in the DFB laser region has a high reflectivity in the DBR region. At the same time as being reflected by the reflector, the light is absorbed as an optical power monitor and output from the end of the DFB laser region. In addition, the antireflection film and the high reflection film are not formed on both end faces.

An optical power monitor integrated DFB laser according to the present embodiment includes an InP lower cladding layer 10, a GaInAsP double quantum well active layer 11, a lower optical confinement layer (OCL) 12, and an upper optical confinement layer. (OCL) 13, upper InP cladding layer 14, benzocyclobutene (BCB) resin 15, contact layer 16, SiO ₂ insulating film 17, DFB laser electrode 18, optical power monitor electrode 19, and lower electrode 20. Yes.

In FIG. 2, L _DFB is the DFB laser region length, Λa is the DFB laser cycle, Wa is the DFB laser thin line width, L _DBR is the DBR region length, Λp is the DBR cycle, Wp is the DBR thin line width, and phase. -Shift indicates the λ / 8 phase shift position. The active layer in the DFB laser region has a discontinuous separated structure including a phase shift region, and the active layer in the DBR region has quantum well thin lines that are thinned to about several tens of nanometers arranged periodically in the light traveling direction. This is the structure.

Thinning not only reduces the volume of the active layer region, but also increases the transition energy associated with the quantum confinement effect, making it possible to produce a low-loss waveguide with little absorption with respect to the oscillation wavelength. Reflectance can be realized.
Further, since the loss of the waveguide is low, the DBR region becomes a high reflectivity region, so that it is possible to increase the output efficiency while maintaining the low threshold operation by concentrating the optical output on one end face.
Further, the optical output η _df from the DFB laser region side shown in FIG. 1 can be monitored by an absorption current obtained by applying a voltage to the optical power monitoring electrode 19 in the DBR region.

  The coupled wave theory (see Non-Patent Document 3 above) analyzes the propagation of light through a periodically distributed refractive index and gain structure such as a diffraction grating, and the refractive index distribution and gain of the diffraction grating structure. Is approximately expressed by a sinusoidal function, and light propagation in the structure can be analyzed.

  In the optical power monitor integrated DFB laser according to the present embodiment, the refractive index distribution or gain distribution of the diffraction grating structure is expressed by a sine wave function in each of the DFB laser region and the DBR region, and the coupled wave equation of light is obtained. Thus, the transfer function F matrix in each region can be derived. In the present embodiment, the intensity distribution, transmission, and reflection spectrum of light in the optical power monitor integrated DFB laser according to the present embodiment were obtained using the product of the F matrix of each region and end face.

  In the optical power monitor integrated DFB laser according to the present embodiment, the thin line structure in the DBR region is designed to operate with the lowest threshold and high efficiency as a DFB laser. Even in this case, the DBR region is sufficient as a monitor. Light absorption is obtained.

  In general, when the active layer in the DBR region is processed into a quantum wire structure, a lateral quantum confinement effect is produced, and the absorption loss of light can be reduced due to the band gap expansion (blue shift effect) associated therewith. On the other hand, the coupling coefficient of the DBR region tends to decrease as the active layer is thinned. As described above, since the waveguide loss and the coupling coefficient due to the thinning of the active layer are in a trade-off relationship, the necessary absorption rate and reflectivity are first set at the time of design, and the fine line width corresponding to that is determined. It becomes a procedure.

FIG. 3 is a diagram showing the thin line width dependence of the reflectivity of the DBR region in the optical power monitor integrated DFB laser according to the present embodiment.
As shown in FIG. 3, in the optical power monitor integrated DFB laser according to the present embodiment, the highest reflectivity can be obtained when the DBR fine line width W _DBR is processed to about 40 nm, and the performance of the laser is improved. It was found from the results of light propagation analysis that this was possible.

The calculation by the program using the coupled wave theory is as follows: DFB laser region length L _DFB = 100 μm, DFB laser cycle Λa = 241.25 nm, DFB laser wire width Wa = 90 nm, DBR region length L _DBR = 200 μm, DBR cycle Λp = 242.5 nm, DBR fine line width Wp = 40 nm, and a structure having a λ / 8 phase shift region at a position of 80 μm from the front end face on the DFB laser side.

4 to 9 are schematic views showing a manufacturing process of the optical power monitor integrated DFB laser according to the present embodiment.
As shown in FIG. 4, in the first step, a p-InP substrate 10 having a main surface orientation of (001), which is also used as an InP lower cladding layer 10 in a later step, is used as a growth semiconductor substrate. A GaInAsP lower optical confinement layer (OCL) 12 and a GaInAsP double quantum well structure (1% compression strain quantum well: 6 nm, 0.15% tensile strain barrier layer: 10 nm by metal organic vapor phase epitaxy (OMVPE). ) 11 and GaInAsP upper optical confinement layer (OCL) 13 are sequentially laminated to produce an epitaxial substrate for producing a laser device. Next, a SiO ₂ mask 30 serving as a dry etching mask is deposited by about 20 nm by a CVD method, and a diffraction grating whose line width is modulated using an electron beam resist 31 is drawn by an electron beam (EB) exposure method.

As shown in FIG. 5, in the second step, the electron beam resist pattern 31 is transferred to the SiO ₂ mask 30 by CF ₄ reactive ion etching and used as a dry etching mask, and then CH ₄ / H ₂ reactive ion etching. The active layer is directly processed by (RIE) to form a diffraction grating having excellent perpendicularity.

  As shown in FIG. 6, in the third step, after the damaged layer by dry etching is removed by wet etching, the groove portion is filled with i-InP32 by OMVPE, the upper optical confinement layer 13, the upper InP cladding layer 14, and the contact Layer 16 is laminated.

As shown in FIG. 7, in the fourth step, a groove 33 for electrode separation is formed by dry etching and wet etching to form a stripe structure.
As shown in FIG. 8, in the fifth step, the SiO ₂ insulating film 17 is formed by flattening with a benzocyclobutene (BCB) resin 15.
As shown in FIG. 9, in the sixth step, after opening the electrode window, metal deposition (Ti / Au) is performed in each of the DFB region, the DBR region, and the power monitor (PM) region, and the electrode 18 for the DFB laser. Then, the optical power monitoring electrode 19 and the lower electrode 20 are formed.

Since the DFB region and the DBR region in the optical power monitor integrated DFB laser according to the present embodiment can be manufactured by simply changing the EB exposure pattern with a thin line width, the active region and the passive region can be formed in a lump without performing an extra process. There is an advantage.
Further, since the waveguide structures of the active region and the passive region are almost the same, there is an advantage that the coupling loss between the regions can be made almost zero.
In addition, there is an advantage that the active region and the passive region can be manufactured collectively without degrading the crystal quality of the epitaxial substrate for manufacturing the laser device by a low damage etching process.

FIG. 10 is a diagram showing an SEM image of the optical power monitor integrated DFB laser according to this example.
As shown in FIG. 10, in the optical power monitor integrated DFB laser according to the present example, it can be seen that thin-line active layers are periodically formed.

  In addition, in the optical power monitor integrated DFB laser according to the present embodiment, when an electrode is formed in the DBR region, the DFB laser electrode 18 and the optical power monitor are used in order to obtain electrical insulation from the front DFB laser region. It is necessary to form the electrode separation groove 33 between the electrode 19 and the optical power monitor integrated DFB laser according to the present embodiment, but the electrode separation groove 33 in the DFB laser region and the DBR region is formed. There is an advantage that batch fabrication is possible when forming a high mesa waveguide.

  Incidentally, the waveguide structure of an optical semiconductor element such as a semiconductor laser is generally a high mesa type, a low mesa type, or a buried type. The high mesa structure is characterized in that the mesa is formed up to the active layer portion, and since both sides of the active layer are air layers, the light confinement effect is high. The low mesa waveguide structure is a structure in which a mesa is formed up to the upper layer of the active layer, and the embedded structure refers to a structure in which both sides of the high mesa or the low mesa are embedded with a semiconductor layer, and has good heat dissipation, etc. It has the characteristics of.

  In the optical power monitor integrated DFB laser according to the present embodiment, a layer structure for laser element fabrication is fabricated on a p-type growth semiconductor substrate (p-InP substrate). Since it is an n-type semiconductor with high mobility, a certain amount of deep groove is required to obtain electrical insulation, but a groove that is too deep also causes light loss.

Further, in the formation of the high mesa waveguide structure in the fourth step, hydrochloric acid-based wet etching was used for the InP portion, and reactive ion etching (RIE) of CH ₄ / H ₂ was used for the GaInAsP OCL and the active layer. A stripe pattern is formed in the direction 110 with respect to the growth semiconductor substrate, and the InP high mesa structure is an inverted mesa structure by etching the InP layer using a mixed solution of HCl / CH ₃ COOH = 1: 4. It can be.

  On the other hand, since the electrode separation groove 33 is perpendicular to the stripe direction, the InP layer has a shallow forward mesa direction of about 53 ° on the 111 plane. Therefore, by adjusting the distance between the electrode separation grooves 33, it is possible to form the electrode separation grooves 33 having an arbitrary depth simultaneously with the formation of the high mesa structure.

FIG. 11 is a diagram showing an SEM image of InP reverse mesa stripe etching in the optical power monitor integrated DFB laser according to this example. FIG. 12 is a view showing an SEM image of InP forward mesa groove formation etching in the optical power monitor integrated DFB laser according to the present example.
As shown in FIG. 12, the groove 33 for electrode separation having an arbitrary depth can be formed by adjusting and etching the groove width W at the boundary between the DFB laser region and the DBR region.

13 and 14 and Table 1 to be described later are calculation results by a program using the coupled wave theory of the power monitor integrated DFB laser according to the present embodiment.
FIG. 13 is a diagram showing the calculation result of the light intensity distribution when no bias is applied in the optical power monitor integrated DFB laser according to the present embodiment. FIG. 14 is a diagram showing the calculation result of the reflectance spectrum of the DBR region in the optical power monitor integrated DFB laser according to this example.

  The reflectivity of the DBR region depends on the active layer width (ridge width), the number of quantum well wires, and the length of the DBR region. For example, when the active layer width is reduced, the loss is reduced, but the coupling coefficient is also reduced. become. Although the reflectivity can be increased by increasing the number of thin wires in order to increase the coupling coefficient or by increasing the DBR region length, there is a problem that the loss also increases. The configuration of the optical power monitor integrated DFB laser according to the present embodiment is determined in consideration of these points and the ease of actual fabrication.

FIG. 13 shows a case where no voltage is applied to the DBR region, which shows the characteristics of the optical power monitor integrated DFB laser according to this embodiment as a laser, the light intensity of the λ / 8 phase shift unit is large, and the end face on the DFB laser side It shows that the light output of is large. For example, in a general DFB laser having a symmetric λ / 4 phase shift structure in which the λ / 4 phase shift unit is at the center of the resonator, the phase of the electric field matches the Bragg wavelength in the front and rear regions viewed from the phase shift unit. Is strongly confined around the λ / 4 phase shift, the light intensity has a substantially symmetric distribution, and the light outputs from both the front end face and the rear end face are substantially equal.

  Further, in the DFB laser having an asymmetric λ / 4 phase shift structure in which the λ / 4 phase shift position is forward from the center, the light intensity is increased at the λ / 4 phase shift position, so that the light intensity on the front end face side is the rear end face. The light emission intensity at the front end surface is increased. In general, a DFB laser with an asymmetric λ / 4 phase shift structure has an internal light intensity distribution determined by the amount of shift from the center of the element of the phase shift unit, so single-mode stability and slope efficiency are also strongly dependent on the phase shift position. To do. However, in the optical power monitor integrated DFB laser according to the present embodiment, the light is concentrated on the λ / 8 phase shift unit, and the stability of the single mode is high.

FIGS. 17, 18 and 19 show the DFB region length dependence of the front end face output efficiency and the cavity length dependence of the threshold current value in each case of the DFB laser with phase shift and the DFB laser without phase shift. The calculation result is shown.
17 and 18, the output efficiency η _df from the front end face is about 50% or more and the reflectivity R from the power monitor DBR region is also about 90% or more with or without phase shift. It can be seen that there is a coupling coefficient κL between the DFB region and the DBR region that is compatible.
Further, FIG. 19 shows that the threshold value increases when κL is reduced (the region length L _DFB is shortened). The threshold current value on the vertical axis indicates a value normalized to a stripe width of 1 μm.

In FIG. 17, the value of the coupling coefficient κL at which the output efficiency η _df from the front end face is about 50% or more is κL _DFB <5 in the case of a DFB laser having a phase shift, and in the case of a DFB laser having no phase shift. Then, κL _DFB <8. Here, although depending on the required specifications of the device, if about 3 times the minimum threshold is set as the allowable range (lower limit), in the case of a DFB laser with a phase shift, 3 <κL _DFB is required, and the phase shift In the case of a DFB laser without a gap, 3.5 <κL _DFB is required.

Further, from FIG. 18, the coupling coefficient κL of the power monitor DBR region in which the reflectance of about 90% or more is obtained is about 2 <κL _DBR in both the case of the DFB laser with phase shift and the case of no phase shift. . The upper limit of κL _DBR depends on the size of the device, but the DFB region length is 400 μm and the DBR region length is 400 μm (upper limit of κL _DFB is 4) so that the device size does not become too large.

Figure 14 is a calculation result of the reflectivity of the DBR region when you are monitoring the optical power voltage is applied to the DBR region, even if you -1V bias is applied to the DBR region, even if you -5V is applied, It can be seen that a high reflectance can be maintained in the vicinity of 1545 nm.

  The characteristic structure of the optical power monitor integrated DFB laser according to the present embodiment is the DFB laser region length and the phase shift position in the above parameters. In the optical power monitor integrated DFB laser according to the present embodiment, in order to make the DBR region function as a power monitor, a phase shift is introduced into the DFB laser region so that the light intensity distribution in the DBR region is increased to some extent. A short resonator having a region length of about 100 μm was designed.

FIG. 15 is a diagram showing a light intensity distribution at the time of no bias of the λ / 8 phase shift structure. FIG. 16 is a diagram showing a light intensity distribution at the time of no bias of a uniform diffraction grating structure without a phase shift.
The results of FIG. 15 show that, in the DFB laser with phase shift introduced, when the DBR region is designed optimally (L _DBR > 200 μm), high PM quantum efficiency (in the DBR region as a power monitor) even when no bias is applied. (Quantum efficiency) is obtained. On the other hand, in the DFB laser without phase shift, even when the DBR region is optimally designed (L _DBR > 200 μm), the light intensity distribution in the DBR region is about 0.6% of the entire light intensity distribution from FIG. Thus, it can be seen that a sufficient value cannot be obtained when there is no bias.

  That is, as in the optical power monitor integrated DFB laser according to the present embodiment, a λ / 8 phase shift is introduced slightly behind the center of the DFB laser region of about 80 μm from the front end surface in the DFB laser region. By designing a short resonator with a laser region length of 100 μm, the light intensity distribution in the DBR region can be increased to 18% of the entire light intensity as shown in FIG. 15, which is sufficient for use as a power monitor. Quantum quantum efficiency of 8.8% can be realized. Further, as designed, the oscillation characteristics of the DFB laser can also realize a low threshold and high efficiency operation.

  Note that the optical power monitor integrated DFB laser according to the present embodiment has a thin line active layer, and due to the gain matching effect, a phase smaller by about λ / 8 than a general λ / 4 phase shift. The shift amount can oscillate at a lower threshold and is a more suitable structure, but a structure of λ / 4 phase shift may be used.

When the DBR region of an optical power monitor integrated DFB laser is used as an optical power monitor, the characteristics of the DFB laser region are also affected by the application of voltage to the DBR region, so the structures of the DFB laser region and the DBR region are limited. Can be controlled by structural parameters such as resonator length, fine line width, and period.

Table 1 is a table showing calculation results of the DBR region and the DFB region when a voltage is applied to the DBR region and the optical power is monitored.

For example, the light absorption characteristics of the DBR region can be controlled by the oscillation wavelength of the DFB laser and the thin line width of the DBR region. In the optical power monitor integrated DFB laser according to this example, the DFB laser region length is 100 μm, the DBR region length is 200 μm, the phase shift between both regions is λ / 8, and the distance between the active layer thin lines in the DBR region is 40 nm. Table 1 shows the calculation results of the characteristic change between the DFB laser region and the DBR region when a power is applied to the power monitor integrated DFB laser and the DBR region is used as the optical power monitor.

  Although Table 1 shows the results calculated for the above structure, the lengths of the DFB laser region length, the DBR region length, the distance between the active layer fine lines in the DBR region, etc. are not limited to the above numerical values. As can be seen from each figure, the front end face output efficiency, DBR reflectivity, and threshold current value change smoothly and continuously with respect to the length parameter, and are derived from FIGS. 15 to 19 and Table 1 and the like. In addition, the κL range and the value of the active layer absorption loss in the PM region give suitable ranges and values for some changes in the length parameter and the like.

When a voltage is applied to the DBR region to make an optical power monitor, destabilization of the oscillation mode of the DFB laser and a decrease in optical output are serious problems. That is, the refractive index of the DBR region is changed by applying a voltage, and the active layer absorption loss and the reflectance are changed. The optical power monitor is configured to monitor the optical output by the optical absorption current corresponding to the optical output of the DFB laser, and the high quantum efficiency, the low active layer absorption loss, and the reflectance are in a contradictory relationship.

When voltage of −1 V was applied , the PM quantum efficiency increased by about 7% compared to the case of no bias, and it was confirmed that the quantum efficiency was 15.4%, which was sufficient as a monitor. On the other hand, the decrease in reflectance with the DFB laser was only about 2%, and it was confirmed that the negative influence of voltage application was small.

At an applied voltage of −5 V, the quantum efficiency in the DBR region increased by about 13%, whereas the DFB laser and reflectance decreased by 4% and 3%, respectively. However, compared with the voltage -1V applied voltage -5 V is applied, although the reflectance is the remains to a reduction of 1%, despite five times the voltage application, it is the increase in the quantum efficiency of the DBR region smaller From the calculation results, it was found that the monitoring by applying the voltage of about −1 V is suitable for the configuration of the optical power monitor integrated DFB laser according to the present embodiment.

Further, when the absorption coefficient in the DBR region is lowered, the PM quantum efficiency is lowered, so that the quantum efficiency operable as a power monitor becomes the lower limit. Assuming that the lower limit capable of operating as a power monitor is about 1%, a device without a phase shift has a small light intensity distribution in the DBR region (see FIG. 16), so the lower limit of the absorption coefficient needs to be about 1000 cm ^−1. Become. On the other hand, a device having a phase shift has a high light intensity distribution in the DBR region (see FIG. 15), and can maintain a PM quantum efficiency of 1% or more even at about 300 cm ⁻¹ .

That is, in a device without a phase shift, the absorption coefficient α is in a range of 1500 cm ⁻¹ <α <2000 cm ⁻¹ or less, and in a device with a phase shift, the absorption coefficient α is 300 cm ⁻¹ <α <2000 cm ⁻¹ or less. .

  As described above, according to the configuration of the optical power monitor integrated DFB laser according to the present embodiment, the characteristics of the semiconductor laser are hardly deteriorated even during the optical monitor operation, and it is not necessary to separately use an optical monitor such as a PD. In addition, the optical output of the DFB laser can be monitored without complicating the DFB laser manufacturing process.

  As described above, according to the optical power monitor integrated DFB laser according to the present invention, the structure in which the DFB laser region and the DBR region are separated can increase the difference in refractive index between the DFB laser region and the DBR region. Even if light is absorbed as an optical power monitor, the reflectivity of the DBR region is hardly lowered, and the function of the high light reflectivity as a Bragg reflector is not lowered, so the high output and low threshold of the DFB laser are maintained. As a result, the optical power can be monitored.

  Furthermore, according to the optical power monitor integrated DFB laser according to the present invention, it is not necessary to manufacture the optical power monitor region separately from the high reflection region, and the period of the DBR region can be made substantially the same as that of the DFB laser region. Therefore, it is possible to reduce the cost and improve the yield.

  The present invention can be used for an optical power monitor integrated DFB laser used in an optical transmitter.

10 InP lower clad layer (p-InP substrate)
11 GaInAsP double quantum well active layer 12 Lower optical confinement layer 13 Upper optical confinement layer 14 Upper InP clad layer 15 Benzocyclobutene (BCB) resin 16 Contact layer 17 SiO ₂ insulating film 18 DFB laser electrode 19 Optical power monitor electrode 20 Lower electrode 30 SiO ₂ mask 31 Electron beam resist 32 i-InP
33 groove

Claims (4)

An optical power monitor integrated DFB laser in which a distributed Bragg reflection region and a distributed feedback laser region are integrated,
The active layer of the waveguide in the distributed black reflection region is a structure that is a quantum well thin line that is periodically arranged in the light traveling direction and is thinned to about several tens of nanometers .
The active layer of the waveguide of the distributed feedback laser region is a structure that is a discontinuous separated quantum well thin line with respect to the light traveling direction,
An optical power monitor integrated DFB laser, wherein the output of the semiconductor laser is monitored by an absorption current in the distributed Bragg reflection region. Wherein when the length and the length of the distributed Bragg reflector region of the distributed feedback laser region was normalized by using the coupling coefficient of the diffraction grating kappa, the length L _DFB of the distributed feedback lasers area is "3. 5 <κ L _DFB <8 ”, the length L _DBR of the distributed Bragg reflection region is“ 2 <κ L _DBR <4 ”, and the absorption coefficient α with respect to the oscillation wavelength of the active layer fine wire in the distributed Bragg reflection region is“ 1500 cm ^−. The optical power monitor integrated DFB laser according to claim 1, wherein ¹ <α <2000 cm ⁻¹ . The optical power monitor integrated DFB laser according to claim 1, wherein a phase shift is provided in the distributed feedback laser region. Wherein when the length and the length of the distributed Bragg reflector region of the distributed feedback laser region was normalized by using the coupling coefficient of the diffraction grating kappa, the length L _DFB of the distributed feedback lasers area is "3 < κ L _DFB <5 ”, the length L _DBR of the distributed Bragg reflection region is“ 2 <κ L _DBR <4 ”, and the absorption coefficient α with respect to the oscillation wavelength of the active layer thin wire in the distributed Bragg reflection region is“ 300 cm ⁻¹ < 4. The optical power monitor integrated DFB laser according to claim 3, wherein α <2000 cm ⁻¹ .