JP2011228384A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser with high reliability and characteristic reproducibility capable of being manufactured at a low cost in a simple manufacturing process.SOLUTION: The semiconductor laser comprises a refractive index modulation region 18 capable of changing refractive index (or a gain modulation region 31 capable of changing a gain) and a vertical coupling region 19 where a Mach-zehnder coupler is formed in a resonator. The refractive index modulation region is formed at a position that meets "(resonator length L)/(integer of 2 or higher)" from one reflecting mirror end face 9A.

Description

  The present invention relates to a semiconductor laser that can be used in optical fiber communication technology.

  As a conventional single-mode semiconductor laser, a distributed feedback type (DFB: Distributed Feedback) semiconductor laser that oscillates at a specific wavelength corresponding to the period by providing a diffraction grating near the active layer or the passive optical waveguide layer, A distributed Bragg reflector (DBR) semiconductor laser is a typical system. These semiconductor lasers have a side mode suppression ratio (SMSR), which is a single mode index, of 30 dB or more, and mode jumps are unlikely to occur during high-speed direct modulation. Is suitable.

  Further, a technology relating to a Fabry-Perot (FP) semiconductor laser having a ridge waveguide shape already exists (see Patent Document 1).

  In the semiconductor laser according to Patent Document 1, a plurality of slots (etching grooves) deeply dug from the top of the ridge to the vicinity of the active layer are provided, and the single mode property is improved to the same level as the DFB laser. The semiconductor laser according to Patent Document 1 is hereinafter referred to as a discrete mode semiconductor laser.

  In the discrete mode semiconductor laser, the optical waveguide layer composed of the active layer and the optical confinement layer is sandwiched between the upper cladding layer and the lower cladding layer. In addition, a contact layer is formed immediately below the uppermost electrode of the ridge waveguide, and an etching groove called a slot is also formed. The oscillation of the discrete mode semiconductor laser is basically realized by feedback from the cleaved end faces at both ends, as in a normal FP semiconductor laser.

  In the FP type semiconductor laser, there are many longitudinal modes having an interval Δλ that is inversely proportional to the resonator length L. The light in the waveguide takes a non-perturbation mode in a portion without a slot, and changes into a perturbation mode affected by a decrease in refractive index in a portion with a slot. For this reason, in the slot portion, slight reflection due to mismatch between both modes occurs, and the longitudinal mode gain undergoes periodic modulation.

  For example, when the slot distance from the end face is the resonator length L / N (N: integer), the longitudinal mode is selected approximately every N lines. Therefore, by selecting and combining the number and position of slots appropriately, it is possible to obtain a semiconductor laser with high single mode characteristics, and the FP type semiconductor laser greatly simplifies the manufacturing process compared to the DFB laser. Can be

  However, in the FP type semiconductor laser, in order to ensure a good single mode property, it is necessary to perform a complicated design such as a large number (about 20 places) of aperiodic arrangement (non-patent). Reference 1).

International Publication No. 2006/024674

Simon Osbourne et al., "Design of Single-Mode and Two-Color Fabry-Perot Lasers With Pattered Reflexive Index, IEEE Journal of Selected.

  In a single mode semiconductor laser such as a conventional DFB laser or DBR laser, it is necessary to form a diffraction grating using electron beam exposure or interference exposure. For this reason, a complicated manufacturing process including a plurality of epitaxial growth processes is required, and the throughput of the manufacturing process of the DFB laser and the DBR laser is lower than that of a general FP semiconductor laser.

  In addition, the DFB laser and DBR laser also generate a yield related to the oscillation wavelength depending on the reflection phase from the end face. Therefore, it is difficult to reduce the cost of the DFB laser and the DBR laser.

  On the other hand, in the discrete mode semiconductor laser, the reflection amount and scattering loss in the slot region are determined by the etching depth. For this reason, it is necessary to precisely control the etching amount, and it is necessary to securely cover the etching side wall and bottom surface with a protective film, and the discrete mode semiconductor laser still has problems in the reproducibility and stability of the manufacturing process. (In other words, there was a problem in the reproducibility of the semiconductor laser having stable characteristics).

  In a discrete mode semiconductor laser, there are a large number of slots. For this reason, the discrete mode semiconductor laser has been concerned about a decrease in output due to the occurrence of a loss in the etching groove and reliability during long-term energization.

  Accordingly, the present invention has been made to solve the above-described problems, and provides a semiconductor laser that can be manufactured at low cost by a simple manufacturing process and has high reliability and reproducibility of characteristics. Objective.

  In order to achieve the above object, a semiconductor laser according to the present invention is formed across a resonator having a pair of reflecting mirror end faces formed by being spaced apart by the length of the resonator and the pair of reflecting mirror end faces. A first optical waveguide layer including an active layer; a refractive index modulation region formed in the resonator capable of changing a refractive index; and the refractive index modulation formed in the resonator. A vertical coupling region that does not overlap with the region, and the refractive index modulation region satisfies (the resonator length) / (an integer of 2 or more) from one end face of the reflecting mirror. The Mach-Zehnder coupler comprising the first optical waveguide layer and the second optical waveguide layer formed apart from the first optical waveguide layer is formed in the vertical coupling region. Is formed.

  Alternatively, the semiconductor laser according to the present invention includes a resonator having a pair of reflecting mirror end faces formed apart from each other by a cavity length, and an active layer formed between the pair of reflecting mirror end faces. An optical waveguide layer, a gain modulation region formed in the resonator capable of changing a gain, and a vertical coupling region formed in the resonator and not overlapping the gain modulation region; The gain modulation region is formed in the resonator satisfying (the resonator length) / (an integer of 2 or more) from one end face of the reflecting mirror, and the vertical coupling region includes A Mach-Zehnder coupler comprising the first optical waveguide layer and a second optical waveguide layer formed apart from the first optical waveguide layer is formed.

  The semiconductor laser according to the present invention has a refractive index modulation region formed in the resonator capable of changing the refractive index (or gain modulation formed in the resonator capable of changing the gain. Region) and a vertical coupling region in which a Mach-Zehnder coupler is formed. Here, the refractive index modulation region is formed at a position satisfying (resonator length L) / (integer of 2 or more) from one end face of the reflecting mirror.

  Therefore, it is possible to provide a semiconductor laser that can be manufactured at a low cost by a simple manufacturing process and has high reliability and reproducibility of characteristics.

It is a front view which shows a structure when the resonator of a semiconductor laser is seen from the light emission side. 2 is a cross-sectional view showing a configuration of a resonator in the semiconductor laser according to the first embodiment. FIG. FIG. 6 is a cross-sectional view showing another cross-sectional configuration of the resonator in the semiconductor laser according to the first embodiment. It is sectional drawing for demonstrating operation | movement of a vertical Mach-Zehnder coupler. It is a figure which shows the transmission spectrum of a vertical Mach-Zehnder coupler. It is a figure for demonstrating the mode modulation | alteration in a refractive index modulation area | region. It is a figure which shows the gain spectrum of an active layer, and the filter characteristic of a vertical Mach-Zehnder coupler. FIG. 6 is a diagram for explaining spectral characteristics in the whole resonator according to the first embodiment. FIG. 6 is a cross-sectional view showing a configuration of a resonator in a semiconductor laser according to a second embodiment. FIG. 6 is a cross-sectional view showing a configuration of a resonator in a semiconductor laser according to a second embodiment. FIG. 6 is a cross-sectional view showing a configuration of a resonator in a semiconductor laser according to a second embodiment. FIG. 6 is a cross-sectional view showing a configuration of a resonator in a semiconductor laser according to a third embodiment.

  Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.

<Embodiment 1>
FIG. 1 is a front view showing a schematic configuration of an element of a single mode semiconductor laser according to the present embodiment. FIG. 2 is a cross-sectional view showing the configuration of the semiconductor laser. Here, FIG. 1 is a front view seen from the light output side in FIG. FIG. 2 is a cross-sectional view taken along the line AA ′ of FIG. For simplification of the drawing, the upper and lower electrodes and wiring are not shown.

  As a material system of the semiconductor laser, InGaAsP (indium gallium arsenide phosphorus), InAlGaAs (indium aluminum gallium arsenide), or the like on an InP (indium phosphide) substrate, which is common in long wave optical communication, can be used. In this embodiment, an example of an InGaAsP optical waveguide layer on an n-type InP substrate will be described.

  As shown in FIG. 1, an optical confinement layer 2 is formed so as to sandwich the active layer 1. The active layer 1 and the optical confinement layer 2 constitute a first optical waveguide layer 15. Here, the first optical waveguide layer 15 is made of InGaAsP.

  As shown in FIG. 1, the first optical waveguide layer 15 is sandwiched between the upper cladding layer 4 and the lower cladding layer 5. Here, the upper cladding layer 4 is made of p-type InP, and the lower cladding layer 5 is made of n-type InP. In the present embodiment, the lower cladding layer 4 is handled as an integral part of the n-type InP substrate.

  As shown in FIG. 1, current blocking layers 14 combining p-type and n-type InP are formed on both side surfaces of the first optical waveguide layer 15 including the active layer 1. A contact layer 7 is formed on the upper surface of the upper cladding layer 4. In FIG. 2, the contact layer 7 is not shown.

  As can be seen from the configuration of FIG. 1, the semiconductor laser according to the present embodiment forms a buried waveguide structure.

  FIG. 2 can be understood as a cross-sectional view showing the configuration of the resonator of the semiconductor laser.

  As shown in FIG. 2, the resonator includes a pair of cleaved end faces (can be grasped as reflecting mirror end faces) 9 </ b> A and 9 </ b> B that are formed apart by the resonator length L. Here, the front side of the resonator (that is, the light output side) is only the cleavage end face 9A, but the rear side of the resonator (light non-output side) is a highly reflective coating on the cleavage end face 9B for the purpose of increasing the output. 16 is formed.

  Further, as shown in FIG. 2, in the resonator, the first optical waveguide layer 15 including the active layer 1 extends over the resonator length L (in other words, between the pair of cleavage end faces 9A and 9B). Is formed.

  Further, as shown in FIG. 2, the resonator includes a refractive index modulation region 18 that can change the refractive index, and a vertical coupling region 19 that does not overlap the refractive index modulation region 18. To do.

  A vertical Mach-Zehnder coupler is formed in the vertical coupling region 19. In other words, the region in the resonator where the vertical Mach-Zehnder coupler is formed is the vertical coupling region 19.

  The vertical Mach-Zehnder coupler includes a first optical waveguide layer 15 and a second optical waveguide layer 17 formed separately from the first optical waveguide layer 15. The second optical waveguide layer 17 does not have an active layer and is made of InGaAsP. The second optical waveguide layer 17 is formed in the upper cladding layer 4 in parallel with the extending direction of the first optical waveguide layer 15. A part of the upper cladding layer 4 made of InP exists between the first optical waveguide layer 15 and the second waveguide layer 17. That is, the Mach-Zehnder coupler in the vertical direction has a configuration in which the first optical waveguide layer 15, the upper cladding layer 4, and the second optical waveguide layer 17 are laminated in this order. The second optical waveguide layer 17 has a band gap wavelength having a composition that does not absorb light emitted from the active layer 1.

  Here, similarly to the first optical waveguide layer 15, the guided light travels through the second optical waveguide layer 17. That is, the length of the second optical waveguide layer 17 in the left-right direction in FIG. 2 needs to be long enough to allow the guided light to travel.

  A third optical waveguide layer 13 is formed in the refractive index modulation region 18. In other words, the region in the resonator where the third optical waveguide layer 13 is formed is the refractive index modulation region 18. Due to the presence of the third optical waveguide layer 13, the refractive index of the guided light in the resonator can be modulated.

  The third optical waveguide layer 13 is formed separately from the first optical waveguide layer 15. The third optical waveguide layer 13 does not have an active layer and is made of InGaAsP. The third optical waveguide layer 13 is formed in the upper cladding layer 4 in parallel with the extending direction of the first optical waveguide layer 15. In the configuration of FIG. 2, the second optical waveguide layer 17 and the third optical waveguide layer 13 are located in the same layer position in the upper cladding layer 4 (that is, the same depth position in the upper cladding layer 4). ). A part of the upper cladding layer 4 made of InP exists between the first optical waveguide layer 15 and the third waveguide layer 13. The third optical waveguide layer 13 has a band gap wavelength having a composition that does not absorb light emitted from the active layer 1.

  Further, in the present embodiment, the refractive index modulation region 18 is formed in the resonator satisfying (resonator length L) / (integer of 2 or more) from one cleavage end face. There may be only one refractive index modulation region 18, but when there are a plurality of refractive index modulation regions 18, each of the refractive index modulation regions 18 is (resonator length L) / (integer greater than or equal to 2) from one of the cleavage end faces, Separated.

  In the configuration illustrated in FIG. 2, the first refractive index modulation region 18 is formed at a position separated by L / N1 from one cleavage end face 9A. The second refractive index modulation region 18 is formed at a position separated from one cleavage end face 9A by L / N2. The third refractive index modulation region 18 is formed at a position separated from one cleavage end face 9A by L / N3. Here, N1, N2, and N3 are integers of two or more different values.

  The center of the length of each third optical waveguide layer 13 existing in each refractive index modulation region 18 is located at L / N1, L / N2, L / N3 from one cleavage end face 9A, respectively. . The L / N1, L / N2, and L / N3 are referred to as refractive index modulation positions. In other words, the center position of the width in the left-right direction in FIG. 2 of each refractive index modulation region 18 is located at the refractive index modulation position.

  Here, unlike the second optical waveguide layer 17, the propagation of the guided light is disturbed in the third optical waveguide layer 13. That is, it is necessary to set the length of the third optical waveguide layer 13 in the left-right direction in FIG. 2 to be sufficiently short so that the guided light does not come and go. As can be seen from this, the region length of the refractive index modulation region 18 in the left-right direction in FIG. 2 is sufficiently shorter than the region length of the vertical coupling region 19 in the left-right direction in FIG. In other words, the length of the third optical waveguide layer 13 in the left-right direction in FIG. 2 is sufficiently shorter than the length of the second waveguide layer 17 in the left-right direction in FIG. In the configuration of FIG. 2, the second waveguide layer 17 and the third waveguide layer 13 have the same configuration except for only the length.

  Note that at least one end of the vertical coupling region 19 (in other words, the second optical waveguide layer 17) satisfies (resonator length L) / (integer greater than or equal to 2) from one cleavage end face 9A. In addition, it is desirable to be formed. In the configuration example of FIG. 2, the second optical waveguide layer 17 is formed closer to the other cleaved end face 9 </ b> B than the third optical waveguide layer 13.

  The second optical waveguide layer 17 and the third optical waveguide layer 13 can be formed as follows.

  An InGaAsP optical waveguide layer is formed in a state in which a part of the upper cladding layer 4 made of InP is formed, and a predetermined portion of the optical waveguide layer is etched. Thereafter, the second optical waveguide layer 17 and the third optical waveguide layer 13 are formed in the same layer position in the upper cladding layer 4 by being embedded and grown with InP.

  As can be seen from the forming method, the distance between the first optical waveguide 15 and the optical waveguide layer (second optical waveguide layer 17 and third optical waveguide layer 13), and the optical waveguide layer (second optical waveguide layer). The thickness of the 17 and the third optical waveguide layer 13) is preset by epitaxial growth. Further, the length of the second optical waveguide layer 17 and the length of the third optical waveguide layer 13 are determined by photolithography.

  Therefore, in the semiconductor laser according to the present embodiment, the amount of reflection can be controlled with extremely high accuracy. Furthermore, since the etching outermost surface is not exposed to the atmosphere, the semiconductor laser according to the present embodiment is excellent in long-term reliability. That is, it is possible to provide a semiconductor laser that can be manufactured at low cost by a simple manufacturing process and has high reliability and reproducibility of characteristics.

  Further, by providing a plurality of refractive index modulation regions 18 at each refractive index modulation position, a stable single mode semiconductor laser can be provided.

  Further, the refractive index modulation region 18 is formed by providing the third optical waveguide layer 13 having a short length. Therefore, the refractive index modulation region 18 can be set in the resonator by a manufacturing process (etching process) that is simple and easy to control.

  The second optical waveguide layer 17 and the third optical waveguide layer 13 are formed in the same layer position in the upper cladding layer 4. Therefore, the second optical waveguide layer 17 and the third optical waveguide layer 13 can be simultaneously formed by etching the photoconductive layer into a predetermined pattern.

  FIG. 3 is a cross-sectional view showing a cross-sectional configuration of the vertical coupling region 19. Specifically, FIG. 3 is a cross-sectional view taken along the line B-B ′ of FIG. 2. In FIG. 3, the contact layer 7 not shown in FIG. 2 is shown.

  In FIG. 3, the film thickness of the first optical waveguide layer 15, the film thickness of the second optical waveguide layer 17, and the upper clad layer existing between the first optical waveguide layer 15 and the second optical waveguide layer 17. The film thickness 4 is designed to have a refractive index distribution in which mode coupling in the vertical direction in FIG. 3 exists. For example, the film thicknesses of the first optical waveguide layer 15 and the second optical waveguide layer 17 are each about 0.2 microns, and the widths of the first optical waveguide layer 15 and the second optical waveguide layer 17 (see FIG. 3) is about 1 micron. The film thickness of the upper cladding layer 4 existing between the first optical waveguide layer 15 and the second optical waveguide layer 17 is about 0.3 microns.

  Next, the operation of the vertical Mach-Zehnder coupler will be described with reference to FIGS.

  FIG. 4 shows the relationship between the waveguide modes in the vicinity of the vertical coupling region 19. FIG. 5 is a diagram for explaining the principle of wavelength selection by the vertical Mach-Zehnder coupler. Specifically, FIG. 5 is a diagram showing a transmission spectrum of the vertical Mach-Zehnder coupler.

  In a region where the second optical waveguide layer 17 is not formed in the resonator, the incident mode 20 exists only in the first optical waveguide layer 15. The incident mode 20 is decomposed into an even mode 22 (mode refractive index nR) and an odd mode 23 (mode refractive index nS) at the incident side boundary surface 21.

  When the difference between the two mode refractive indexes is Δn and the wavelength of light is λ, the region length Lf of the vertical coupling region 19 is selected to be M × λ / Δn. Here, M is an integer, and the proportionality coefficient is omitted in the above formula. When the region length Lf is selected as described above, the even mode 22 and the odd mode 23 are separated from each other on the outgoing side interface 25 after the interference operation between the even mode 22 and the odd mode 23 is performed by the vertical Mach-Zehnder coupler. Coupled with high efficiency, the emission mode 26 of the first optical waveguide 15 is formed.

  As shown in FIG. 5, the transmission spectrum 27 of the vertical Mach-Zehnder coupler has a periodic transmission peak on the wavelength axis as well known, and the transmission peak interval (FSR) 28 is It is inversely proportional to the region length Lf. In this embodiment, the region length Lf is selected so that the transmission peak interval 28 is approximately the same as the gain bandwidth of the active layer 1 of the semiconductor laser, and λ = 1.5 μm, M = 10, Δn = 0. When the region length is Lf = 150 μm, the transmission peak interval is 75 nm.

  Next, a mode modulation effect in the refractive index modulation region 18 will be described with reference to FIG.

  In the description of FIG. 6, the refractive index modulation positions L / N1, L / N2, and L / N3 are L / 8, L / 6, and L / 4, respectively. 6 is a diagram showing the modulation spectrum at the refractive index modulation position = L / 8, and the second stage in FIG. 6 is a diagram showing the modulation spectrum at the refractive index modulation position = L / 6. The third row of FIG. 6 is a diagram showing the modulation spectrum at the refractive index modulation position = L / 4, and the lowermost row of FIG. 6 is a modulation spectrum obtained by synthesizing the modulation spectra of the first, second, and third steps of FIG. FIG.

  For example, when 300 μm is adopted as a typical resonator length L, the longitudinal mode interval in the long wave band is about 1 nm. When three spectra at the refractive index modulation positions = L / 8, L / 6, and L / 4 are combined, the mode gain is modulated at 8 times, 6 times, and 4 times the original longitudinal mode interval. Here, the length Ld of each refractive index modulation region 18 may be selected to be Ld = 15 μm or less (for example, 10 μm) corresponding to M = 1 in the vertical Mach-Zehnder coupler. When these effects are combined with a certain reference wavelength, a modulation spectrum as shown in the lowermost part of FIG. 6 is obtained, and the modulation spectrum is strongly modulated every modulation peak interval 29 (= about 24 nm) and is likely to oscillate. There will be a mode.

  On the other hand, as shown in the upper part of FIG. 7, the gain bandwidth 30 of the active layer 1 is typically 50 to 100 nm, and simultaneously oscillates at these plural modulation peak wavelengths in addition to the focused center wavelength. there is a possibility. Even in the case of single mode oscillation, the wavelength stability may not be sufficient, such as the SMSR does not reach 30 dB, or the oscillation wavelength varies greatly with temperature variation.

  Therefore, as described above, the Lf is selected so that the FSR of the vertical coupling region 19 is approximately the same as the gain bandwidth of the active layer 1 of the semiconductor laser as shown in the lower part of FIG. Thus, the entire resonator can oscillate completely in only one mode due to the filter effect, as shown in FIG.

  As described above, an SMSR and wavelength stability comparable to those of the DFB-LD can be obtained with a normal resonator length element. Further, compared with a conventional discrete mode semiconductor laser that requires a large number of slot regions, the number of necessary refractive index modulation regions 18 can be reduced, so there is little loss and it is suitable for high output.

  According to the semiconductor laser according to the present embodiment, the longitudinal mode gain in the resonator is periodically modulated by the action of a small number of refractive index modulation regions 18, and the FSR is approximately equal to the gain bandwidth of the active layer 1. It is possible to perform wavelength selection in the vertical coupling region 19 selected in (1). Thereby, it is not necessary to form a diffraction grating, and a semiconductor laser excellent in single mode property and reliability can be realized at low cost.

  Note that at least one end of the vertical coupling region 19 (second optical waveguide layer 17) is located at a position (for example, L / 2) (resonator length L) / (integer greater than or equal to 2) from one cleavage end face 9A. ), It is possible to add a refractive index modulation effect.

<Embodiment 2>
In the first embodiment, in order to simplify the manufacturing process as much as possible, the third optical waveguide layer 13 in the refractive index modulation region 18 and the second optical waveguide layer 17 in the vertical coupling region 19 are simultaneously processed and manufactured. is doing. However, the third optical waveguide layer 13 and the second optical waveguide layer 17 are not limited to be formed at the same time and formed in the same layer position in the upper cladding layer 3.

  For example, the third optical waveguide layer 13 and the second optical waveguide layer 17 may be formed in separate steps. When the separate process is adopted, as shown in FIG. 9, the formation position of the third optical waveguide layer 13 in the upper cladding layer 4 and the formation position of the second optical waveguide layer 17 in the upper cladding layer 4 , Not necessarily the same layer.

  In the first embodiment, the third optical waveguide layer 13 is formed in the refractive index modulation region 18 in order to modulate the refractive index. Here, the center position of each refractive index modulation region 18 is a position of (resonator length L) / (integer of 2 or more).

  In contrast, a configuration in which the third optical waveguide layer 13 is omitted and the film thickness of the first optical waveguide layer 15 at a position corresponding to the refractive index modulation region 18 is changed may be employed. .

  In the configuration of FIG. 10, the film thickness of the first optical waveguide layer 15 in the refractive index modulation region 18 is thicker than the film thickness of the first optical waveguide layer 15 in other regions (for example, the vertical coupling region 19). It has become.

  In the configuration of FIG. 11, the thickness of the first optical waveguide layer 15 in the refractive index modulation region 18 is thinner than the thickness of the first optical waveguide layer 15 in other regions (for example, the vertical coupling region 19). It has become.

  10 and 11, the center position of the region where the film thickness of the first optical waveguide layer 15 in the refractive index modulation region 18 is changing (the width of the film thickness changing region in the horizontal direction in FIGS. 10 and 11). Is (resonator length L) / (integer of 2 or more).

  10 and 11, only the thickness of the first optical waveguide layer 15 is changed, and the thickness of the active layer 1 is not changed.

  Thus, even if the third optical waveguide layer 13 is omitted by changing the film thickness of the first optical waveguide layer 15 in the refractive index modulation region 18, the refractive index of light in the refractive index modulation region 18. Can be modulated.

<Embodiment 3>
In the first and second embodiments, the refractive index modulation region 18 capable of changing the refractive index is provided in the resonator. In this embodiment, instead of the refractive index modulation region 18, a gain modulation region capable of changing the gain is provided in the resonator. The configuration except that the refractive index modulation region 18 is changed to the gain modulation region is the same as the configuration of the first embodiment. Specifically, it is as follows.

  Also in this embodiment, as a material system of the semiconductor laser, InGaAsP, InAlGaAs, or the like on an InP substrate generally used in long wave optical communication can be employed. In this embodiment, an example of an InGaAsP optical waveguide layer on an n-type InP substrate will be described.

  As shown in FIG. 1 (FIG. 1 is also a front view seen from the light output side of FIG. 12 to be described later), in this embodiment as well, the light confinement layer 2 is formed so as to sandwich the active layer 1. Is formed. The active layer 1 and the optical confinement layer 2 constitute a first optical waveguide layer 15. Here, the first optical waveguide layer 15 is made of InGaAsP.

  As shown in FIG. 1, the first optical waveguide layer 15 is sandwiched between the upper cladding layer 4 and the lower cladding layer 5. Here, the upper cladding layer 4 is made of p-type InP, and the lower cladding layer 5 is made of n-type InP. In the present embodiment, the lower cladding layer 4 is handled as an integral part of the n-type InP substrate.

  Further, as shown in FIG. 1, current blocking layers 14 in which p-type and n-type InP are combined are formed on both side surfaces of the first optical waveguide layer 15 including the active layer 1. A contact layer 7 is formed on the upper surface of the upper cladding layer 4. In addition, in FIG. 12 mentioned later, illustration of the said contact layer 7 is abbreviate | omitted.

  As can be seen from the above configuration, the semiconductor laser according to the present embodiment forms an embedded waveguide structure.

  FIG. 12 is a cross-sectional view showing the configuration of the resonator of the semiconductor laser according to the present embodiment.

  As shown in FIG. 12, the resonator has a pair of cleaved end faces (which can be grasped as reflecting mirror end faces) 9 </ b> A and 9 </ b> B formed to be separated by the resonator length L. Here, the front side of the resonator (that is, the light output side) is only the cleavage end face 9A, but the rear side of the resonator (light non-output side) is a highly reflective coating on the cleavage end face 9B for the purpose of increasing the output. 16 is formed.

  As shown in FIG. 12, the first optical waveguide layer 15 including the active layer 1 is provided in the resonator over the resonator length L (in other words, between the pair of cleaved end faces 9A and 9B). Is formed.

  In the present embodiment, as shown in FIG. 12, a gain modulation region 31 that is a region where the gain can be changed and a vertical coupling region that does not overlap with the gain modulation region 31 are included in the resonator. 19 exists.

  A vertical Mach-Zehnder coupler is formed in the vertical coupling region 19 as in the first embodiment. In other words, the region in the resonator where the vertical Mach-Zehnder coupler is formed is the vertical coupling region 19.

  The vertical Mach-Zehnder coupler includes a first optical waveguide layer 15 and a second optical waveguide layer 17 formed separately from the first optical waveguide layer 15. The second optical waveguide layer 17 does not have an active layer and is made of InGaAsP. The second optical waveguide layer 17 is formed in the upper cladding layer 4 in parallel with the extending direction of the first optical waveguide layer 15. A part of the upper cladding layer 4 made of InP exists between the first optical waveguide layer 15 and the second waveguide layer 17. That is, the Mach-Zehnder coupler in the vertical direction has a configuration in which the first optical waveguide layer 15, the upper cladding layer 4, and the second optical waveguide layer 17 are laminated in this order. The second optical waveguide layer 17 has a band gap wavelength having a composition that does not absorb light emitted from the active layer 1.

  Here, similarly to the first optical waveguide layer 15, the guided light travels through the second optical waveguide layer 17. That is, the length of the second optical waveguide layer 17 in the left-right direction in FIG. 12 needs to be long enough to allow the guided light to travel.

  On the other hand, a third optical waveguide layer 33 is formed in the gain modulation region 31. In other words, the region in the resonator where the third optical waveguide layer 33 is formed is the gain modulation region 31. The presence of the third optical waveguide layer 33 can modulate the gain of guided light in the resonator.

  The third optical waveguide layer 33 is formed separately from the first optical waveguide layer 15. The third optical waveguide layer 33 has a composition that absorbs the guided light, and adds a loss in the third optical waveguide layer 33. For example, the third optical waveguide layer 33 according to the present embodiment has the same configuration as the first optical waveguide layer 15. That is, the third optical waveguide layer 33 includes an active layer and is made of InGaAsP.

  The third optical waveguide layer 33 is formed in the upper cladding layer 4 in parallel with the extending direction of the first optical waveguide layer 15. In the configuration of FIG. 12, the second optical waveguide layer 17 and the third optical waveguide layer 33 have different depth positions in the upper cladding layer 4 (that is, different layers in the upper cladding layer 4). Position). A part of the upper cladding layer 4 made of InP exists between the first optical waveguide layer 15 and the third waveguide layer 33.

  Further, in the present embodiment, the gain modulation region 31 is formed in the resonator satisfying (resonator length L) / (integer of 2 or more) from one cleavage end face. There may be only one gain modulation region 31, but when there are a plurality of gain modulation regions 31, each gain modulation region 31 is separated from one cleavage end surface by (resonator length L) / (integer of 2 or more). ing.

  In the configuration illustrated in FIG. 12, the first gain modulation region 31 is formed at a position separated by L / N1 from one cleavage end surface 9A. The second gain modulation region 31 is formed at a position away from one cleavage end face 9A by L / N2. The third gain modulation region 31 is formed at a position separated from one cleavage end face 9A by L / N3. Here, N1, N2, and N3 are integers of two or more different values.

  The center of the length of each third optical waveguide layer 33 in each gain modulation region 31 is located at L / N1, L / N2, L / N3 from one cleavage end face 9A. The L / N1, L / N2, and L / N3 are referred to as gain modulation positions. In other words, the center position of the width of each gain modulation region 31 in the left-right direction in FIG. 12 is located at the gain modulation position.

  Here, unlike the second optical waveguide layer 17, the propagation of guided light is disturbed in the third optical waveguide layer 33. That is, it is necessary to set the length of the third optical waveguide layer 33 in the left-right direction in FIG. 12 to be sufficiently short so that the guided light does not come and go. As can be seen from this, the region length of the gain modulation region 31 in the left-right direction in FIG. 12 is sufficiently shorter than the region length of the vertical coupling region 19 in the left-right direction in FIG. In other words, the length of the third optical waveguide layer 33 in the left-right direction in FIG. 12 is sufficiently shorter than the length of the second waveguide layer 17 in the left-right direction in FIG.

  Also in the present embodiment, at least one end of the vertical coupling region 19 (in other words, the second optical waveguide layer 17) extends from one cleavage end face 9A to (resonator length L) / (2 It is desirable to be formed at a position satisfying the above integer). In the configuration example of FIG. 12, the second optical waveguide layer 17 is formed closer to the other cleaved end face 9 </ b> B than the third optical waveguide layer 33.

  The second waveguide layer 17 and the third waveguide layer 33 shown in FIG. 12 are formed by the following process.

  In a state in which a part of the upper cladding layer 4 made of InP is formed, a first layer to be the third optical waveguide layer 33 is formed, and one photolithography process is performed on the first layer ( Production of third optical waveguide layer 33). Thereafter, InP burying growth is performed, and then a second layer to be the second optical waveguide layer 17 is formed, and the photolithography process is performed twice on the second layer (second conductive layer). Wave layer 17 creation). After that, buried growth of InP is performed.

  As can be seen from the forming method, the distance between the optical waveguides 15, 17, 33, the thickness of the second optical waveguide layer 17, and the thickness of the third optical waveguide layer 33 are set in advance by epitaxial growth. Further, the length of the second optical waveguide layer 17 and the length of the third optical waveguide layer 33 are determined by photolithography.

  Therefore, in the semiconductor laser according to the present embodiment, the amount of reflection can be controlled with extremely high accuracy. Furthermore, since the etching outermost surface is not exposed to the atmosphere, the semiconductor laser according to the present embodiment is excellent in long-term reliability. That is, it is possible to provide a semiconductor laser that can be manufactured at low cost by a simple manufacturing process and has high reliability and reproducibility of characteristics.

  Further, by providing a plurality of gain modulation regions 31 at each gain modulation position, a stable single mode semiconductor laser can be provided.

  Further, the gain modulation region 31 is formed by providing the third optical waveguide layer 33 having a short length. Therefore, the gain modulation region 31 can be set in the resonator by a manufacturing process (etching process) that is simple and easy to control.

  Also in the present embodiment, the film thickness of the first optical waveguide layer 15, the film thickness of the second optical waveguide layer 17, and the gap between the first optical waveguide layer 15 and the second optical waveguide layer 17. The film thickness of the upper cladding layer 4 is designed to have a refractive index distribution in which mode coupling in the vertical direction exists.

  In the present embodiment, Lf is selected so that the FSR of the vertical coupling region 19 is approximately the same as the gain bandwidth of the active layer 1 of the semiconductor laser. As a result, the entire resonator can oscillate completely in only one mode due to the filter effect. As described above, an SMSR and wavelength stability comparable to those of the DFB-LD can be obtained with a normal resonator length element. Further, compared with a conventional discrete mode semiconductor laser that requires a large number of slot regions, the number of necessary gain modulation regions 31 can be reduced, so there is little loss and it is suitable for high output.

  According to the semiconductor laser according to the present embodiment, the longitudinal mode gain in the resonator is periodically modulated by the action of a small number of gain modulation regions 31, and the FSR is about the same as the gain bandwidth of the active layer 1. Wavelength selection in the selected vertical coupling region 19 can be performed. Thereby, it is not necessary to form a diffraction grating, and a semiconductor laser excellent in single mode property and reliability can be realized at low cost.

  Note that at least one end of the vertical coupling region 19 (second optical waveguide layer 17) is located at a position (for example, L / 2) (resonator length L) / (integer greater than or equal to 2) from one cleavage end face 9A. ), It is possible to add a refractive index modulation effect.

<Embodiment 4>
In the third embodiment, the third optical waveguide layer 33 is formed in the gain modulation region 31 in order to modulate the gain. Here, the center position of each gain modulation region 31 is a position of (resonator length L) / (integer of 2 or more).

  In contrast, the third optical waveguide layer 33 is omitted, and the thickness of the active layer 1 in the first optical waveguide layer 15 at the position corresponding to the gain modulation region 31 is changed. May be.

  For example, the film thickness of the active layer 1 in the first optical waveguide layer 15 in the gain modulation region 31 is set to be equal to that of the active layer 1 in the first optical waveguide layer 15 in the other region (for example, the vertical coupling region 19). Make it thicker than the film thickness. Alternatively, the thickness of the active layer 1 in the first optical waveguide layer 15 in the gain modulation region 31 is set to be equal to that of the active layer 1 in the first optical waveguide layer 15 in another region (for example, the vertical coupling region 19). Make it thinner than the film thickness.

  Also in the present embodiment, the center position of the region where the film thickness of the active layer 1 in the gain modulation region 31 is changed is (resonator length L) / (integer of 2 or more).

  In the present embodiment, unlike the second embodiment, it is necessary to change the film thickness of the active layer 1 in the first optical waveguide layer 15. The increase / decrease in the thickness of the active layer 1 can be regarded as the increase / decrease in the volume of the active layer 1, can change the absorption of the guided light in the active layer 1, and can change the gain.

  In this way, by changing the film thickness of the active layer 1 in the gain modulation region 31, it is possible to modulate the gain of light in the gain modulation region 31 even if the third optical waveguide layer 33 is omitted. Become.

  1 active layer, 2 optical confinement layer, 4 upper cladding layer, 5 lower cladding layer, 7 contact layer, 9A one cleaved end face, 9B other cleaved end face, 13, 33 third optical waveguide layer, 14 current blocking area, 15 First optical waveguide layer, 16 High reflection coating, 17 Second optical waveguide layer, 18 Refractive index modulation region, 19 Vertical coupling region, 20 Incident mode, 21 Incident side interface, 22 Even mode, 23 Odd mode, 25 emission side interface, 26 emission mode, 27 transmission spectrum, 28 transmission peak interval, 29 modulation peak interval, 30 gain bandwidth, 31 gain modulation region, L resonator length, Lf coupling region length.

Claims (10)

A resonator having a pair of reflecting mirror end faces formed apart by the resonator length;
A first optical waveguide layer including an active layer formed between the pair of reflecting mirror end faces;
A refractive index modulation region formed in the resonator capable of changing a refractive index;
A vertical coupling region formed in the resonator and not overlapping with the refractive index modulation region,
The refractive index modulation region is
From one end face of the reflecting mirror, it is formed in the resonator satisfying (the resonator length) / (an integer of 2 or more),
The vertical coupling region includes
A Mach-Zehnder coupler comprising the first optical waveguide layer and a second optical waveguide layer formed apart from the first optical waveguide layer is formed.
A semiconductor laser characterized by the above. The refractive index modulation region is
Multiple,
The semiconductor laser according to claim 1. In the refractive index modulation region,
A third optical waveguide layer shorter than the second optical waveguide layer is formed apart from the first optical waveguide layer.
The semiconductor laser according to claim 1. The second optical waveguide layer and the third optical waveguide layer are:
Formed in the same layer position,
The semiconductor laser according to claim 3. The film thickness of the first optical waveguide layer in the refractive index modulation region is
Different from the film thickness of the first optical waveguide layer in the vertical coupling region,
The semiconductor laser according to claim 1. A resonator having a pair of reflecting mirror end faces formed apart by the resonator length;
A first optical waveguide layer including an active layer formed between the pair of reflecting mirror end faces;
A gain modulation region formed in the resonator capable of changing the gain;
A vertical coupling region formed in the resonator and not overlapping with the gain modulation region,
The gain modulation region is
From one end face of the reflecting mirror, it is formed in the resonator satisfying (the resonator length) / (an integer of 2 or more),
The vertical coupling region includes
A Mach-Zehnder coupler comprising the first optical waveguide layer and a second optical waveguide layer formed apart from the first optical waveguide layer is formed.
A semiconductor laser characterized by the above. The gain modulation region is
Multiple,
The semiconductor laser according to claim 6. In the gain modulation region,
A third optical waveguide layer having a composition that is shorter than the second optical waveguide layer and absorbs guided light is formed apart from the first optical waveguide layer.
The semiconductor laser according to claim 6. The film thickness of the active layer in the first optical waveguide layer in the gain modulation region is
A thickness of the active layer in the first optical waveguide layer in the vertical coupling region is different,
The semiconductor laser according to claim 6. At least one end of the second optical waveguide layer is
From one end face of the reflecting mirror, it is formed at a position satisfying (the resonator length) / (an integer of 2 or more),
7. The semiconductor laser according to claim 1, wherein the semiconductor laser is characterized in that:

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