Semiconductor laser

Abstract

This semiconductor laser (10) is a distributed-reflector semiconductor laser including: an active region (11) having optical gain in a direction in which light is waveguided; and a DBR region (12) having a diffraction grating. A carrier plasma DBR region includes, in the order given, an n-type semiconductor cladding layer (101), a carrier plasma diffraction grating (120), a core layer (121) in which carrier plasma light is waveguided, and a p-type semiconductor cladding layer (102).　Due to this configuration, the present invention can provide a semiconductor laser that has reduced optical loss and can operate at a low threshold value and high output.

Description

semiconductor laser

[0001] The present invention relates to a distributed Bragg reflector semiconductor laser including an active region having optical gain and a DBR region having a diffraction grating.

[0002] In optical communications, research and development of semiconductor lasers for use in the 1.3 μm and 1.55 μm wavelength bands is progressing. Furthermore, wavelength division multiplexing (WDM), which enables broadband transmission, is becoming increasingly important in order to support the ever-increasing volume of data communications in recent years. WDM is a method of transmitting signals using light of different wavelengths over the same fiber. WDM requires high-performance wavelength-tunable light sources. Wavelength-tunable light sources include DFB lasers, which change their oscillation wavelength by adjusting the temperature of the element, and wideband wavelength-selectable DFB laser arrays, in which multiple DFB lasers with different oscillation wavelengths are integrated into an array on the same substrate. In addition, wavelength-tunable DBR lasers, which utilize the carrier plasma effect to enable high-speed wavelength tuning, are also available.

[0003] Tunable semiconductor lasers are also expected to find applications in the medical field, such as in high-power semiconductor lasers used for surgical incisions and coagulation of bleeding areas, and as light sources for non-invasive biological monitoring devices used in optical coherence tomography (OCT).

[0004] Furthermore, tunable semiconductor lasers are used in the fields of environmental gas sensing and quality control in semiconductor processes. By continuously changing the oscillation wavelength using a tunable laser, it is possible to detect the absorption lines specific to the target gas and measure the gas concentration with high sensitivity. In these applications, wavelengths longer than the 1.6 μm band, where strong absorption lines of environmentally unfriendly gases exist, are primarily used, rather than the communication wavelength band.

[0005] Here, the operating principle of a wavelength-tunable distributed Bragg reflector (DBR) laser will be described. Figure 11 shows the cross-sectional structure of a typical DBR laser 40. It comprises an active region 41 that generates optical gain when current is injected, and two DBR regions 42_1 and 42_2, one at the front and one at the back, each having a diffraction grating 420 at both ends of the active region in the optical axis direction. The active region 41 comprises a lower SCH (Separated Confinement Heterostructure) layer 411, an active layer 412, and an upper SCH layer 413. The DBR regions 42_1 and 42_2 each comprise a core layer 421 and a diffraction grating 420.

[0006] 12 shows the reflection spectra of the two DBR regions 42_1 and 42_2. As shown in FIG. 12, the DBR regions 42_1 and 42_2 have a Bragg wavelength λ 2 determined by the period of the diffraction grating 420. Ｂｒａｇｇ It acts as a mirror that selectively reflects a specific wavelength range centered on the center. The Bragg wavelength is determined by the period of the diffraction grating, and normally the front and rear DBRs have the same diffraction grating period, resulting in the same Bragg wavelength. Therefore, only wavelengths within the reflection bands of both the front and rear DBRs are selectively confined within the cavity, and oscillation occurs due to the amplification effect in the active region. If the wavelength band confined within the cavity by the front and rear DBRs is sufficiently narrow, oscillation will occur in a single mode.

[0007] Furthermore, by designing the rear DBR to be longer than the front DBR, the optical output from the front and rear end faces can be adjusted. That is, by designing the reflectivity of the front DBR to be smaller than that of the rear DBR, the optical output from the rear end face can be suppressed and the optical output from the front end face can be increased. The diffraction gratings 420 in the front and rear DBR regions 42_1 and 42_2 generally have the same structure, but the reflectivity of the DBR regions 42_1 and 42_2 can be adjusted by the length of the DBR regions. The Bragg wavelength λ of the DBR regions 42_1 and 42_2 Ｂｒａｇｇ is λ Ｂｒａｇｇ = 2n ｅｑ where Λ is the period of the diffraction grating 420, n ｅｑ represents the equivalent refractive index. The equivalent refractive index n ｅｑBy changing the wavelength of the DBR laser 40, the oscillation wavelength of the DBR laser 40 can be changed.

[0008] In order to change the oscillation wavelength while maintaining oscillation of the DBR laser 40, the Bragg wavelengths of the front and rear DBR regions 42_1 and 42_2 are adjusted by simultaneously changing the Bragg wavelengths of both regions while matching the Bragg wavelengths of the two regions. Generally, refractive index modulation methods use temperature control or a method using the carrier plasma effect caused by current injection.

[0009] As a wavelength-tunable laser using the carrier plasma effect, a 1.5 μm band DBR laser using an InGaAsP / InP-based material has been reported. In recent years, wavelength-tunable lasers in various wavelength bands, such as the 1.3 μm and 2.0 μm wavelength bands, have been reported, all of which use the carrier plasma effect and operate on the same principle (Non-Patent Documents 1 and 2). In addition, a wavelength-tunable DBR laser has been reported that employs a diffraction grating structure, such as a sampled grating (SG) or a long-period grating (SSG), in the DBR region to widen the wavelength tuning range (Non-Patent Document 3).

[0010] T. Shindo, N. Fujiwara, Y. Ohiso, T. Sato, and H. Matsuzaki, “Quasi-continuous tuning of a 1.3-μm-wavelength superstructure grating distributed Bragg reflector laser by enhancing carrier-induced refractive index change,” Opt. Express, Vol. 29, pp. 232-243, 2021.T. Shindo et al., “2.0-μm Wavelength Superstructure-Grating-(SSG-) Distributed Bragg Reflector Laser With Tuning Range of Over 50 nm,” in IEEE Photonics Technology Letters, Vol. 33, no. 13, pp. 641-644, 1 July1, 2021.Y. Tohmori, Y. Yoshikuni, T. Tamamura, H. Ishii, Y. Kondo, and M. Yamamoto, “Broad-range wavelength tuning in DBR lasers with superstructure grating (SSG),”IEEE Photon. Technol. Lett., vol. 5, no. 2, pp.126-29, Feb. 1993.

[0011] In a typical wavelength-tunable DBR laser, the reflectivity of the DBR is determined by the design of the refractive index coupling coefficient of the diffraction grating and the DBR region length. The refractive index coupling coefficient is determined by the refractive index difference between the concave and convex portions of the diffraction grating. In a typical DBR laser, the refractive index coupling coefficient is set to 30 cm －１ 100cm from －１ In particular, in a diffraction grating having multiple reflection peaks, such as the above-mentioned SSG-DBR laser, increasing the number of reflection peaks reduces the reflectivity of each peak, and therefore a relatively high refractive index coupling coefficient is required.

[0012] As an example of a wavelength-tunable DBR laser, a DBR laser in the 2 μm wavelength band will be described. Fig. 13 shows a cross-sectional structure of the DBR region 42 in a typical DBR laser. The DBR region 42 has a structure in which an n-InP cladding layer 401, a core layer 421, a diffraction grating layer 422, and a p-InP cladding layer 402 are stacked in this order on an n-InP substrate. The wavelength λ is determined according to the period Λ of the concave and convex portions of the diffraction grating 420 formed on the diffraction grating layer 422. Ｂｒａｇｇ = 2n ｅｑ Bragg wavelength λ that satisfies Λ Ｂｒａｇｇ In order to improve the reflectivity of the DBR, it is necessary to increase the diffraction grating coupling coefficient κ.

[0013] 14 shows the calculation results of the dependence of the refractive index coupling coefficient κ on the depth of the diffraction grating in a DBR laser in the 2 μm wavelength band. The DBR structure shown in FIG. 13 was used for the calculation. InGaAsP was used for the diffraction grating layer 422 of the DBR structure, and its bandgap wavelengths were set to 1.1 μm (black circles in the figure), 1.3 μm (white circles in the figure), and 1.5 μm (white squares in the figure). InGaAs (white triangles in the figure) was also used for the diffraction grating layer 422 of the DBR structure.

[0014] In general, in a 2 μm band DBR laser, an InGaAsP layer having a band gap wavelength of 1.1 μm is used as the diffraction grating layer 422. In this case, ―１ To obtain a high refractive index coupling coefficient of about 100 nm, it is necessary to form a deep concave-convex pattern of about 100 nm, which is difficult to fabricate using a normal semiconductor processing process. Therefore, to obtain a high refractive index coupling coefficient without forming a deep concave-convex pattern, it is necessary to use a material with a high refractive index for the diffraction grating layer 422. As shown in Figure 14, by using a material with a large bandgap wavelength and a high refractive index, a high refractive index coupling can be obtained even with the same diffraction grating depth.

[0015] Next, a case where a material with a high refractive index is used for the diffraction grating layer 422 will be described. FIG. 15 shows the optical confinement factor in the core layer 421 when the refractive index of the diffraction grating layer 422 is changed. In a wavelength tunable DBR laser, a high optical confinement factor in the core layer 421 is desirable to efficiently change the wavelength by current injection. As shown in FIG. 15 , when the refractive index of the diffraction grating layer 422 is increased, a high optical confinement factor is obtained in an InGaAsP layer having a bandgap wavelength of 1.2 μm to 1.4 μm, with the highest optical confinement factor being near the 1.3 μm band. Materials with a high refractive index corresponding to bandgap wavelengths of 1.4 μm or more reduce the optical confinement, resulting in a degradation of the wavelength tunability of the DBR laser 40.

[0016] Next, FIG. 16 shows the optical confinement ratio in the p-InP layer 402 versus the refractive index of the diffraction grating layer 422. The p-InP layer 402 has a higher optical loss than the n-InP layer 401. This optical loss tends to increase as the oscillation wavelength becomes longer, and is large in the 2 μm wavelength band. Therefore, in a semiconductor laser, it is necessary to design the laser so that the proportion of guided light distributed in the p-InP layer 402 is low. In a typical DBR structure, the diffraction grating layer 422 is close to the p-InP layer 402, and therefore, as the refractive index of the diffraction grating layer 422 increases, the light distribution in the p-InP layer 402 increases, and optical loss increases, as shown in FIG. 16. As a result, the laser threshold increases and the optical output decreases.

[0017] As described above, in order to increase the DBR reflectivity in a DBR laser, it is necessary to use a material with a high refractive index for the diffraction grating layer, but this increases the optical loss inside the laser resonator, resulting in a deterioration of the laser characteristics.

[0018] In order to solve the above-mentioned problems, the semiconductor laser according to the present invention is a distributed reflection type semiconductor laser including, in a light-guiding direction, an active region having optical gain and a DBR region having a diffraction grating, the DBR region including, in this order, an n-type semiconductor cladding layer, the diffraction grating, a core layer through which the light is guided, and a p-type semiconductor cladding layer.

[0019] According to the present invention, it is possible to provide a semiconductor laser that is a DBR laser and can operate with a low threshold and high output power while reducing optical loss.

[0020] FIG. 1 is a schematic cross-sectional view showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing the configuration of a DBR region in the semiconductor laser according to the first embodiment of the present invention. FIG. 3 is a diagram for explaining the operation of the semiconductor laser according to the first embodiment of the present invention. FIG. 4A is a diagram for explaining the effect of the semiconductor laser according to the first embodiment of the present invention. FIG. 4B is a diagram for explaining the effect of the semiconductor laser according to the first embodiment of the present invention. FIG. 5A is a diagram for explaining the effect of the semiconductor laser according to the first embodiment of the present invention. FIG. 5B is a diagram for explaining the effect of the semiconductor laser according to the first embodiment of the present invention. FIG. 6 is a schematic cross-sectional view showing the configuration of a DBR region in a semiconductor laser according to a second embodiment of the present invention. FIG. 7 is a diagram for explaining the operation of the semiconductor laser according to the second embodiment of the present invention. FIG. 8 is a diagram for explaining the operation of the semiconductor laser according to the third embodiment of the present invention. FIG. 9 is a diagram for explaining the operation of the semiconductor laser according to the third embodiment of the present invention. FIG. 10 is a diagram for explaining the operation of the semiconductor laser according to the third embodiment of the present invention. FIG. 11 is a schematic cross-sectional view showing the configuration of a conventional semiconductor laser. FIG. 12 is a diagram for explaining the operation of the conventional semiconductor laser. Fig. 13 is a cross-sectional schematic diagram showing the configuration of a DBR region in a conventional semiconductor laser. Fig. 14 is a diagram for explaining the operation of a conventional semiconductor laser. Fig. 15 is a diagram for explaining the operation of a conventional semiconductor laser. Fig. 16 is a diagram for explaining the operation of a conventional semiconductor laser.

[0021] First Embodiment A semiconductor laser according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 5B.

[0022] 1, the semiconductor laser 10 according to this embodiment has the configuration of a distributed Bragg reflector semiconductor laser (hereinafter also referred to as a "DBR laser"), and includes, on an n-type InP substrate (not shown), an active region 11, a first DBR region (front DBR region) 12_1, and a second DBR region (rear DBR region) 12_2, arranged in this order in the light guiding direction (x direction in the figure). The semiconductor laser 10 also includes electrodes 104 for injecting current into each region. The semiconductor laser 10 also has AR films 105 on both end facets.

[0023] The active region 11 includes, in order from the n-type InP substrate side in the stacking direction (z direction in the drawing), an n-type cladding layer 101, a lower SCH (Separated Confinement Heterostructure) layer 111, an active layer 112, an upper SCH layer 113, a p-type cladding layer 102, and a contact layer 103, and includes an electrode 104 on the contact layer.

[0024] The first DBR region (front DBR region) 12_1 and the second DBR region (rear DBR region) 12_2 each have the same layer structure. FIG. 2 shows an example of the layer structure of the DBR region 12 of the semiconductor laser 10.

[0025] The DBR region 12 includes, in order from the n-type InP substrate side, an n-type InP cladding layer 101, a diffraction grating layer 122, a core layer 121, and a p-type InP cladding layer 102. A diffraction grating 120 is formed at the boundary between the diffraction grating layer 122 and the n-type InP cladding layer 101. In the drawing, the depth of the diffraction grating 120 is denoted by d ｇ Here, an example is shown in which an n-type InP substrate is provided on the n-type InP cladding layer side as the substrate, but a p-type InP substrate may be provided on the p-type InP cladding layer side.

[0026] In the semiconductor laser 10, the active region 11, the front DBR region 12_1, and the rear DBR region 12_2 have lengths of 450 μm, 300 μm, and 600 μm, respectively. A phase adjustment region (100 μm in length, not shown) may be provided between the rear DBR region 12_2 and the active region 11. An SOA (300 μm in length, not shown) may be integrated between the front DBR region 12_1 and the light-emitting end facet.

[0027] An example of the semiconductor laser 10 according to this embodiment will be described below.

[0028] The active region 11 comprises, on an n-type InP substrate, an n-type InP cladding layer 101, an InGaAsP-lower SCH layer 111, an active layer 112, an InGaAsP-upper SCH layer 113, a p-type InP cladding layer 102, and a p-type InGaAs contact layer 103. The active layer 112 uses an InGaAs quantum well that has optical gain in the 2.0 μm oscillation wavelength band. Here, the 2.0 μm wavelength band refers to the wavelength band from 1.6 μm to 2.3 μm.

[0029] The DBR region 12 includes, on an n-type InP substrate, an n-type InP cladding layer 101, a diffraction grating layer 122, an InGaAs core layer 121, an InGaAsP (1.1Q) guide layer, a p-type InP cladding layer 102, and a p-type InGaAs contact layer 103. InGaAsP (1.3Q) is used for the diffraction grating layer 122. A diffraction grating (concave and convex) 120 is formed at the boundary between the diffraction grating layer 122 and the n-type InP cladding layer 101.

[0030] Here, as described above (FIG. 15), it is desirable that the band gap wavelength of InGaAsP of the diffraction grating layer 122 be 1.2 to 1.4 μm.

[0031] 3 shows the calculation results of the refractive index coupling coefficient and the light distribution ratio to the p-InP layer with respect to the diffraction grating depth of the DBR region 12 in the DBR laser 10. The calculation was performed using software "Fimmwave" (manufactured by Photon Design).

[0032] In a conventional DBR laser, an InGaAsP layer with a band gap wavelength of 1.1 μm is used as the diffraction grating layer 122. From FIG. 14, the refractive index coupling coefficient is 40 cm －１ In order to obtain this level, a diffraction grating depth of about 55 nm is required. Also, as can be seen from Fig. 16, the light distribution ratio to the p-InP layer in the conventional configuration is about 0.101.

[0033] On the other hand, in the DBR laser 10, InGaAsP (1.3Q) is used for the diffraction grating layer 122, and as shown in FIG. 3, the refractive index coupling coefficient is 40 cm －１The diffraction grating depth required to obtain this is about 25 nm, which is smaller than the diffraction grating depth (55 nm) of a conventional configuration. This allows the core layer 121 to be crystal-grown on a surface with a small diffraction grating depth, thereby improving the crystal quality of the core layer 121.

[0034] Furthermore, the light distribution ratio to the p-InP layer is 0.088, which is a reduction of 10% or more compared to the conventional configuration, thereby reducing the light loss in the p-InP layer.

[0035] The operating characteristics of the DBR laser 10 will be described. The oscillation threshold of the DBR laser 10 is approximately 25 mA at room temperature (25°C). This value is higher than that of a conventional DBR laser (refractive index coupling coefficient: 40 cm －１ ) is lower than the oscillation threshold (32 mA).

[0036] When the current injected into the active region 11 is 100 mA, the optical output intensity of the DBR laser 10 is 11 mW, which is 25% higher than that of a DBR laser with a conventional structure.

[0037] In this way, in the DBR laser 10, by forming a diffraction grating on the n-type semiconductor side, the optical loss inside the resonator can be reduced, and the laser characteristics can be made lower threshold and higher output.

[0038] The wavelength tunability of the DBR laser 10 will now be described. The amount of change in refractive index due to the carrier plasma effect caused by current injection is evaluated from the amount of wavelength change when current is injected into the DBR regions 12 before and after the DBR laser 10.

[0039] When a total current of 100 mA is injected into the front and rear DBRs, the oscillation wavelength changes by 11 nm compared to the wavelength when the current is 0 mA. The refractive index change rate obtained from this value is approximately 0.51%, which is equivalent to the refractive index change rate (0.50%) in DBR lasers with conventional structures.

[0040] As described above, the semiconductor laser (DBR laser) according to this embodiment can maintain wavelength tunability and operate at a low threshold and high output power, compared to DBR lasers with conventional structures.

[0041] <Effects> The effects of the semiconductor laser 10 according to this embodiment will be described with reference to FIGS. 4A and 4B.

[0042] Fig. 4A shows the layer structure of the DBR region 12 in the DBR laser 10. For comparison, Fig. 4B shows the layer structure of the DBR region 12 in a conventional DBR laser.

[0043] In a conventional DBR laser, as shown in Figure 4B, the diffraction grating layer 122 is formed on the p-InP 102 side, so the optical field is distributed on the p-InP 102 side. A large portion 134 (dotted line) of the optical field distribution 133 is distributed in the p-InP 102. As a result, light is absorbed in the p-InP 102, resulting in large optical waveguide loss.

[0044] 4A, in the DBR laser 10, even if a high-refractive-index material is used for the diffraction grating layer 122 to improve the reflectivity of the DBR, the diffraction grating layer 122 is formed on the n-InP 101 side, so the optical field is distributed on the n-InP 101 side. Of the optical field distribution 131, the portion 132 (dotted line) distributed in the p-InP 102 is small. As a result, the optical loss due to the p-InP 102 can be suppressed.

[0045] Furthermore, in the DBR laser 10, the active region 11 having a quantum well structure with optical gain has the same structure as that of a conventional DBR laser, so that the manufacturing uniformity and long-term reliability of the quantum well structure itself can be guaranteed to be of the same quality as that of a conventional DBR laser.

[0046] As a configuration in which a high refractive index material is used for the diffraction grating layer while avoiding optical loss in the p-InP layer, a configuration in which a diffraction grating layer 522 having a diffraction grating 520 in a DFB laser is formed on the n-InP 501 side of core layers 511 to 513 including an active layer, as shown in FIG. 5A, can be considered.

[0047] In a conventional DFB structure (FIG. 5B), after the quantum well layer 512, which is the active layer, is formed, the diffraction grating 520 is formed on top of it. As a result, the quality of the quantum well layer 512 is not affected by the formation of the diffraction grating, and the conventional DFB structure is easy to manufacture and has excellent long-term reliability and manufacturing stability. However, a large portion 534 (dotted line) of the optical field distribution 533 is distributed in the p-InP 502. As a result, light is absorbed in the p-InP 502, resulting in large optical waveguide loss.

[0048] As shown in FIG. 5A, in a configuration in which the diffraction grating 520 is formed on the n-InP substrate 501 side of the active layer 512, an increase in optical loss in the p-InP 502 can be suppressed. The portion 532 (dotted line) of the optical field distribution 531 that is distributed in the p-InP 502 is small. As a result, optical loss due to the p-InP 502 can be suppressed. However, it is difficult to grow a high-quality active layer (e.g., a quantum well layer) 512 on the unevenness of the diffraction grating 520. As a result, the quality of the active layer 512 deteriorates, and the uniformity and long-term reliability of the device characteristics deteriorate.

[0049] On the other hand, the semiconductor laser 10 according to the present embodiment has a DBR laser configuration, and the diffraction grating 120 is formed on the substrate side of the DBR region 12. This makes it possible to improve the uniformity of the device characteristics and long-term reliability without deteriorating the quality of the active layer 112.

[0050] According to the semiconductor laser of this embodiment, the optical loss in the DBR laser can be reduced, and it can operate at a low threshold and high output.

[0051] In particular, in the 2.0 μm wavelength band, where InP-based semiconductors have a larger optical loss than the 1.3 to 1.55 μm wavelength band, the semiconductor laser according to this embodiment can suppress optical loss even when a material having a high refractive index such as InGaAs is used for the diffraction grating layer, and can operate at a low threshold and high output with high refractive index coupling. －１ ) the grating depth can be reduced, thereby improving the quality of the crystal grown on the grating.

[0052] <Method for manufacturing a semiconductor laser> A crystal was used in which an n-type InP cladding layer 101, a lower SCH layer 111, a multiple quantum well active layer (MQW) 112, and an upper SCH layer 113 were grown in this order on an n-InP substrate. The multiple quantum well layer has optical gain in the 2.0 μm oscillation wavelength band.

[0053] First, the active region 11 of the DBR laser 10 and the portion corresponding to the SOA region are left, and the upper SCH layer 113 and the active layer 112 in the other portions are selectively etched and removed.

[0054] Next, a uniform diffraction grating 120 having irregularities corresponding to an oscillation wavelength band of 2.0 μm is formed (processed) on the surface of the n-type InP cladding layer 101 in a portion corresponding to the DBR region 12 .

[0055] Next, the semiconductor layer structure of the DBR region 12 and the phase adjustment region is grown by butt-joint regrowth. The layer structure of the DBR region 12 is an InGaAsP (1.3Q) diffraction grating layer 122 on a diffraction grating, an InGaAs core layer 121, an InGaAsP (1.1Q), and a p-InP cladding layer 102, each of which is lattice-matched to InP. This results in a structure in which the diffraction grating 120 is embedded in the boundary between the n-type InP cladding layer 101 and the InGaAsP (1.3Q) diffraction grating layer 122.

[0056] Next, a p-InP cladding layer 102 and a p-InGaAs contact layer 103 are grown over the entire surface of the device by regrowth. The thickness of the p-InP cladding layer 102 is set to 2.0 μm so that the light field does not reach the electrode region.

[0057] Next, a mesa structure is formed by etching, and semi-insulating InP layers doped with Fe are formed on both sides of the mesa structure by burying and regrowth.

[0058] Next, a p-type electrode 104 is formed on the surface of the layer structure including the p-type contact layer 103 of the mesa structure.

[0059] Next, the InP substrate is polished to about 150 μm, and an n-type electrode is formed on the back surface of the substrate.

[0060] This completes the wafer processing steps for the semiconductor laser 10. The waveguide structure after wafer processing has a buried heterostructure with InP layers formed on both sides of the mesa in the horizontal direction. The stripe width of the mesa structure is 2.5 μm, and the diffraction grating 120 formed in the DBR region 12 allows operation at a single wavelength.

[0061] In the semiconductor laser 10, the SOA portion has the same layer structure as the active region 11 of the DBR laser 10. The DBR region and the phase control region have the same layer structure formed by butt-joint growth. This allows a structure in which multiple regions are integrated to be manufactured at low cost by reducing the number of regrowths.

[0062] Second Embodiment A semiconductor laser according to a second embodiment of the present invention will be described with reference to FIGS.

[0063] <Configuration of Semiconductor Laser> The semiconductor laser 20 according to this embodiment is a wavelength tunable DBR laser.

[0064] 6, the DBR region 22 of the semiconductor laser 20 includes, in order, an n-type InP cladding layer 101, a diffraction grating layer 122, a diffraction grating cover layer 123, a core layer 121, and a p-InP cladding layer 102. In this manner, the diffraction grating cover layer 123 is formed between the diffraction grating layer 122 and the core layer 121 of the DBR region 22.

[0065] In the DBR laser according to the first embodiment, the unevenness of the diffraction grating 120 is formed at the boundary between the n-InP cladding layer 101 and the diffraction grating layer 122. This configuration is formed by forming the unevenness on the surface of the n-InP cladding layer 101 and then regrowing InGaAsP (having a bandgap wavelength of 1.3 μm, for example) of the diffraction grating layer 122. Therefore, because the InGaAsP quaternary mixed crystal layer is grown on the surface having the unevenness, compositional fluctuations in the mixed crystal are likely to occur, resulting in a deterioration in crystal quality.

[0066] In addition, in conventional DBR structures, after forming the irregularities in the InGaAsP layer, the InP layer is regrown. In this configuration, the InP is grown on the irregular surface, so composition fluctuations are unlikely to occur, and degradation of crystal quality can be suppressed.

[0067] On the other hand, in the DBR laser 20, a diffraction grating layer 122 made of InGaAsP is formed on the n-InP cladding layer 101, and the irregularities of the diffraction grating 120 are formed on the upper surface of the grating layer 122. InP (diffraction grating cover layer) 123 is regrown on the irregularities formed on the InGaAsP layer. In other words, the irregularities of the InGaAsP layer are filled with InP.

[0068] As a result, the crystal (InP) grown on the irregularities does not experience compositional fluctuations during growth, and degradation of crystal quality is suppressed, as in conventional DBR lasers. This improves the crystal quality of the core layer 121 and other layers grown on the diffraction grating cover layer 123. In this way, a uniform, high-quality regrown layer can be formed on the irregularities of the diffraction grating layer 122 in the DBR region 22.

[0069] An example of a DBR laser will be described below as the semiconductor laser 20 according to this embodiment.

[0070] As in the first embodiment, the DBR laser 20 has an oscillation wavelength in the 2 μm wavelength band. The DBR laser 20 includes a front DBR region (length: 300 μm) 22_1, an active region (length: 450 μm) 11, a phase adjustment region (length: 100 μm), and a rear DBR region (length: 600 μm) 22_2. An SOA (length: 300 μm) is integrated between the front DBR and the output facet of the DBR laser 20.

[0071] The active region 11 is the same as that in the first embodiment.

[0072] The DBR region 22 comprises, on an n-type InP substrate, an n-type InP cladding layer 101, a diffraction grating layer 122, a diffraction grating cover layer 123, a core layer 121, and a p-InP cladding layer 102 in this order.

[0073] The diffraction grating layer 122 is made of InGaAsP (1.3Q). The diffraction grating cover layer 123 is made of InP. The thickness of the diffraction grating layer 122 is 50 nm. The thickness of the diffraction grating layer 122 may be 50 to 200 nm or may be another thickness. The thickness of the diffraction grating cover layer 123 is 50 nm. The thickness of the diffraction grating cover layer 123 may be 50 to 200 nm or may be another thickness. A diffraction grating (concave and convex) 120 is formed at the boundary between the diffraction grating layer 122 and the diffraction grating cover layer 123.

[0074] 7 shows the calculation results of the relationship between the diffraction grating depth, the refractive index coupling coefficient, and the light distribution ratio to the p-InP layer in the DBR laser 20. When the diffraction grating depth changes from 0 to 50 nm, the refractive index coupling coefficient changes from 0 to 65 cm －１ Compared to the diffraction grating structure in the first embodiment, a deeper diffraction grating needs to be formed to obtain the same refractive index coupling coefficient, but a sufficiently larger refractive index coupling coefficient can be obtained compared to a general DBR structure.

[0075] In the DBR laser 20, the refractive index coupling coefficient (κ=40 cm －１ When a diffraction grating structure (with a diffraction grating depth of about 30 nm) is used that can obtain a .DELTA..DBR, the light distribution in the p-InP layer is about 0.09, which is a sufficiently small value compared to conventional DBR lasers. Therefore, a DBR with a high refractive index coupling coefficient and low optical loss can be realized.

[0076] Next, the operating characteristics of the DBR laser 20 will be described. The oscillation threshold of the DBR laser 20 is approximately 28 mA at room temperature (25°C), providing low-threshold operation. Furthermore, when the injection current into the active region 11 is 100 mA, the optical output intensity is 10 mW, providing an optical output comparable to that of the first embodiment.

[0077] Next, the wavelength tunability of the DBR laser 20 will be described. The amount of change in refractive index due to the carrier plasma effect caused by current injection can be obtained from the amount of wavelength change when current is injected into the front and rear DBR regions 22_1 and 22_2. When a total current of 100 mA is injected into the front and rear DBRs, the oscillation wavelength changes by 13 nm compared to the oscillation wavelength when the DBR current is 0 mA. In this way, the wavelength tunability can be improved compared to the first embodiment. This is thought to be due to the improved crystal quality when the diffraction grating is embedded in the DBR region 22.

[0078] According to the semiconductor laser of the present embodiment, the DBR laser can reduce optical loss and operate at a low threshold and high output. Furthermore, the crystal quality of the core layer in the DBR region can be improved, thereby improving the laser characteristics.

[0079] <Method for Manufacturing Semiconductor Laser> A method for manufacturing the semiconductor laser 20 according to this embodiment will be described below.

[0080] First, an n-InP cladding layer 101, a diffraction grating layer (1.3Q) 122, and a diffraction grating cover layer (InP) 123 are formed on an n-InP substrate, and then a lower SCH layer 111, a multiple quantum well active layer (MQW) 112, and an upper SCH layer 113 are grown in this order.

[0081] Next, during butt-joint regrowth of the DBR region 22, the portions that will become the active region 11 and SOA region of the DBR laser 20 are left, and the other regions are selectively etched. At this time, the diffraction grating cover layer (InP) 123 is exposed by selective wet etching.

[0082] Next, the diffraction grating cover layer 123 is removed only from the portion corresponding to the DBR region 22 to expose the diffraction grating layer (InGaAsP, 1.3Q) 122. A diffraction grating pattern is written on the exposed diffraction grating layer (InGaAsP, 1.3Q) 122 to form irregularities with a depth of about 30 nm.

[0083] Next, a diffraction grating cover layer (InP) 123 is regrown on the irregularities of the diffraction grating layer 122 to re-bury the irregularities of the diffraction grating, and then an InGaAs core layer 121, InGaAsP (1.1Q), and p-InP are butt-joint regrown, thereby forming a structure in which the diffraction grating 120 is buried at the boundary between the InGaAsP (1.3Q) layer and the n-InP layer.

[0084] Other steps, such as the p-InP cladding layer and the burying and regrowth step using semi-insulating InP, are the same as those in the first embodiment.

[0085] The waveguide structure of the DBR laser 20 has a buried heterostructure in which Fe-doped semi-insulating InP layers are formed on both sides of the mesa in the horizontal direction, as in the first embodiment. The stripe width is 2.5 μm, and the laser operates at a single wavelength due to the diffraction grating formed in the DBR region 22.

[0086] Third Embodiment A semiconductor laser according to a third embodiment of the present invention will be described with reference to FIGS.

[0087] <Configuration of Semiconductor Laser> The semiconductor laser 30 according to this embodiment is an SSG-DBR laser having a super structure grating (SSG) in the grating region of the DBR region.

[0088] In a typical DBR laser, a uniform diffraction grating is used in the DBR region to change the wavelength in one reflection band. On the other hand, in the SSG-DBR laser 30, the DBR region has multiple reflection bands due to a special diffraction grating structure called a long-period diffraction grating, which enables a wide wavelength tunable range.

[0089] 8 shows a schematic diagram of the reflection band of the DBR region in the SSG-DBR laser 30. The diagram schematically shows a reflection spectrum 31 of the rear DBR, a reflection spectrum 32 of the front DBR, and a composite reflectance spectrum 33 obtained by multiplying the reflection spectra of the front and rear DBRs.

[0090] In the reflection spectra 31 and 32, the front and rear SSG-DBRs have reflection spectra in which multiple reflection peaks are arranged at equal intervals, and each has seven reflection peaks. The reflection peak intervals of the front and rear DBRs are respectively Δλ ｆｒｏｎｔ , Δλ ｒｅａｒ is shown.

[0091] The reflection peak interval between the front and rear DBRs is Δλ ｆｒｏｎｔ is Δλ ｒｅａｒ The SSG-DBR laser 30 is designed to be slightly larger than the reflection peak spacing. This difference in the reflection peak spacing enables the SSG-DBR laser 30 to achieve a wide wavelength tunable range while maintaining single-mode operation. This is because the reflection peak spacing between the front and rear DBRs is slightly different, so the reflection peaks of the front and rear DBRs match perfectly only at one wavelength, while the other reflection peaks do not match, resulting in a composite reflection spectrum with only one reflection peak. Therefore, in the composite reflection spectrum 33, only one reflection peak is observed, where the reflection peaks of the front and rear DBRs match.

[0092] 9 shows an example of the wavelength tuning operation of the SSG-DBR laser 30. In the SSG-DBR laser 30, a current is injected into the rear DBR of the front and rear DBR regions, causing refractive index modulation due to the carrier plasma effect, and shifting the reflection spectrum 34 of the rear DBR to the shorter wavelength side.

[0093] In this case, the peak where the reflection spectra 34 and 35 of the front and rear DBRs coincide is the second peak from the short wavelength side in the figure, and the peak of the composite reflection spectrum 36 can also be significantly shifted to the short wavelength side. Furthermore, since the reflection peaks of the front and rear DBRs coincide at only one wavelength, single-mode operation is possible. In this way, by adjusting the amount of current injected into each of the front and rear DBRs and individually adjusting the spectra of the front and rear DBRs, laser oscillation can be achieved using multiple reflection peaks, thereby achieving a wide wavelength tunable range.

[0094] The relationship between the wavelength shift caused by the carrier plasma effect and the spacing between reflection peaks of the SSG will now be explained. As shown in the wavelength tunable operation of the SSG described above, the oscillation wavelength can be significantly changed by selectively oscillating one of the multiple reflection peaks by adjusting the amount of current injected into the front and rear DBR regions.

[0095] In order to oscillate at a wavelength between the reflection peaks, it is necessary to inject current into the front and rear DBR regions simultaneously, and shift the reflection peaks of the front and rear DBRs simultaneously, as in a DBR laser with a general uniform diffraction grating. In other words, to realize an SSG-DBR laser that can oscillate at all wavelengths between the reflection peaks, it is necessary to ｒｅａｒ or Δλ ｆｒｏｎｔ In other words, the SSG-DBR requires that the wavelength tunable range obtained by the carrier plasma effect is larger than the larger of the reflection peak interval Δλ ｆｒｏｎｔ , Δλ ｒｅａｒ must be designed so that both of these are smaller than the wavelength tunable range obtained by the carrier plasma effect.

[0096] The structure of the SSG-DBR laser 30 will be described. The SSG-DBR laser 30 has, integrated in this order along the optical axis, a rear DBR region, a phase adjustment region, an active region, a front DBR region, and an SOA region. The front and rear DBR regions include an InGaAs core layer, an InGaAsP (1.3Q) diffraction grating layer, and an InP diffraction grating cover layer, with a diffraction grating formed at the boundary between the diffraction grating layer and the InP diffraction grating cover layer. The structure of the SSG-DBR laser 30 and its manufacturing method are the same as those of the second embodiment, except for the SSG-DBR structure of the diffraction grating.

[0097] The wavelength tunable range in the SSG-DBR laser 30 is approximately 13 nm, which is the same as that in the second embodiment. Therefore, the front and rear SSG-DBRs of the SSG-DBR laser 30 must both be designed so that the reflection peak interval is 13 nm or less. Here, the reflection peak interval Δλ of the front DBR is ｆｒｏｎｔ is 12 nm, and the reflection peak Δλ of the rear DBR is ｒｅａｒ is set to 11 nm.

[0098] The wavelength tunability of the SSG-DBR laser 30 will now be described. The injection currents for the active region 11 and the SOA region are set to 90 mA and 100 mA, respectively. Different current sources are used for the front and rear DBRs, and the oscillation spectrum is measured as the current is varied from 0 to 100 mA in 1 mA increments. The phase adjustment region is also adjusted in the range of 0 to 10 mA so that the SMSR is minimized under each condition.

[0099] FIG. 10 shows the results of plotting all the peak wavelengths of the oscillation spectrum.

[0100] Each curve indicates a reflection peak possessed by the SSG-DBR. In the SSG-DBR laser 30, the front and rear DBRs are both designed to have seven reflection peaks, so seven reflection peaks are obtained. Furthermore, each curve shifts to the shorter wavelength side as current is injected. The wavelength tunable width of each reflection peak indicates the refractive index fluctuation width due to the carrier plasma effect. In the SSG-DBR laser 30, a wavelength tunable width of approximately 13 nm is obtained, as in the second embodiment. In this way, all SSG modes can be selectively oscillated, and oscillation can be performed at any wavelength in the range from 1954 nm to 2020 nm, resulting in a wavelength tunable range of 77 nm.

[0101] In the embodiment of the present invention, the core layer, diffraction grating layer, guide layer, and contact layer may be made of InGaAsP or InGaAs, which are lattice-matched to InP. Here, "lattice-matched" includes lattice matching and also includes a range in which the crystal quality is not degraded by strain due to lattice mismatch.

[0102] Furthermore, when the semiconductor layer structure of the semiconductor laser is formed by crystal growth on a semiconductor substrate other than an InP substrate, the core layer, diffraction grating layer, guide layer, and contact layer may be made of a material that lattice matches the substrate material.

[0103] Although the embodiment of the present invention has been described using an InP substrate as the substrate, other semiconductor substrates may be used. Also, substrates made of other materials, such as Si substrates or glass substrates, may be used by a wafer bonding process or the like.

[0104] In the embodiment of the present invention, a semiconductor laser in the 2.0 μm wavelength band is shown as an example, but the present invention is not limited to this. A semiconductor laser in a long wavelength band of 1.3 μm to 1.55 μm or a semiconductor laser in another wavelength band may also be used.

[0105] In the embodiments of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration and manufacturing method of the semiconductor laser are shown, but the present invention is not limited to these examples. Anything that can exhibit the functions and effects of the semiconductor laser can be used.

[0106] It should be noted that the present invention is not limited to the above-described embodiments, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0107] A part or all of the above-described embodiment or an example thereof can be described as, but is not limited to, the following supplementary notes.

[0108] (Supplementary Note 1) A distributed reflection semiconductor laser comprising, in a light-guiding direction, an active region having optical gain and a DBR region having a diffraction grating, wherein the DBR region comprises, in order, an n-type semiconductor cladding layer, the diffraction grating, a core layer through which the light is guided, and a p-type semiconductor cladding layer.

[0109] (Supplementary Note 2) The semiconductor laser according to Supplementary Note 1, comprising a diffraction grating layer having the diffraction grating, wherein the n-type semiconductor cladding layer is n-type InP, the p-type semiconductor cladding layer is p-type InP, the diffraction grating layer is InGaAsP lattice-matched to InP, and the core layer has a refractive index higher than that of the diffraction grating layer and is InGaAsP or InGaAs lattice-matched to InP.

[0110] (Supplementary Note 3) The semiconductor laser according to Supplementary Note 1 or Supplementary Note 2, further comprising a diffraction grating cover layer between the diffraction grating and the core layer.

[0111] (Appendix 4) The semiconductor laser according to Appendix 3, wherein the diffraction grating cover layer is InP.

[0112] (Supplementary Note 5) The semiconductor laser according to any one of Supplementary Note 2 to Supplementary Note 4, wherein the active region has the optical gain at a wavelength of 1.6 μm or more and 2.3 μm or less, the core layer is made of InGaAs lattice-matched to InP, and the band gap wavelength of the InGaAsP in the diffraction grating layer is 1.2 μm or more and 1.4 μm or less.

[0113] (Supplementary Note 6) The semiconductor laser according to any one of Supplementary Notes 1 to 5, wherein the diffraction grating is a super-period structure diffraction grating.

[0114] The present invention relates to a semiconductor laser and can be applied to optical communication systems and optical communication devices.

[0115] REFERENCE SIGNS LIST 10 Semiconductor laser (distributed reflection type semiconductor laser) 11 Active region 12 DBR region 101 N-type semiconductor cladding layer 102 P-type semiconductor cladding layer 120 Diffraction grating 121 Core layer 122 Diffraction grating layer

Claims

1. A distributed reflection semiconductor laser comprising, in the light-guiding direction, an active region having optical gain and a DBR region having a diffraction grating, wherein the DBR region comprises, in order, an n-type semiconductor cladding layer, the diffraction grating, a core layer through which the light is guided, and a p-type semiconductor cladding layer.

2. The semiconductor laser according to claim 1, comprising a diffraction grating layer having the diffraction grating, wherein the n-type semiconductor cladding layer is n-type InP, the p-type semiconductor cladding layer is p-type InP, the diffraction grating layer is InGaAsP that is lattice-matched to InP, and the core layer has a refractive index higher than that of the diffraction grating layer and is InGaAsP or InGaAs that is lattice-matched to InP.

3. The semiconductor laser according to claim 1 or 2, further comprising a diffraction grating cover layer between the diffraction grating and the core layer.

4. The semiconductor laser of claim 3, wherein said grating cover layer is InP.

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