Semiconductor laser

Abstract

A semiconductor laser (10) according to the present invention comprises, in order, an n-type InP substrate (11), an active layer (13), and a p-type InP cladding (15). Current block layers (14) are periodically provided in the waveguide direction in the vicinity of the active layer in the p-type InP cladding. The current block layer is an n-type semiconductor that is substantially lattice-matched to InP, and has an oscillation wavelength of 1.6-2.4 μm.　Thus, the present invention can provide a semiconductor laser capable of sweeping across a wide wavelength range.

Description

semiconductor laser

[0001] The present invention relates to a wavelength-swept semiconductor laser.

[0002] Optical gas measurement systems are capable of measuring concentrations with high accuracy in real time, and are used in a variety of fields, such as analyzing the components of gases emitted from factories and gases in the atmosphere, inspecting for gas leaks from equipment and pipes, and testing for Helicobacter pylori in medical care.

[0003] Optical gas measurement systems basically utilize the phenomenon in which gas molecules exhibit significant optical absorption at wavelengths specific to each gas species. Gas absorption lines can be broadly divided into fundamental tones, which involve only one vibrational energy, and overtones and combination tones, which involve multiple vibrational energies. For many gas species, fundamental tones exhibit significant optical absorption in the wavelength range of 4 μm or longer, while overtones and combination tones exhibit significant optical absorption in the wavelength range of 3 μm or shorter.

[0004] Figure 8 shows the wavelength range and intensity of absorption lines for gas species that exhibit significant light absorption in the wavelength range from 1.6 μm to 2.4 μm. In the wavelength range around 2 μm, there are many absorption lines of gases that are closely related to the greenhouse effect and environmental pollution, such as methane, carbon dioxide, nitrous oxide, carbon monoxide, hydrochloric acid, water, and ammonia.

[0005] Gas absorption lines have narrow linewidths, with multiple absorption lines clustered in a narrow wavelength range for each gas species. Typical gas measurements use a single absorption line, which has a narrow linewidth, so a light source for gas measurement should have a single wavelength and a narrow linewidth. Semiconductor lasers are compact and relatively easy to use for emitting light at a single wavelength. Furthermore, they consume less power than gas lasers and solid-state lasers, and are inexpensive, making them widely used as light sources for gas measurement.

[0006] Regarding semiconductor lasers used for gas measurement in the wavelength region around 2 μm, research and development of semiconductor lasers on InP substrates that oscillate at a single wavelength (hereinafter referred to as "lasers in the wavelength region around 2 μm") is progressing, and some such lasers are commercially available (for example, Non-Patent Documents 1 to 3).

[0007] In semiconductor lasers on InP substrates that oscillate in the wavelength range around 2 μm, a multiple quantum well (MQW) structure with compressively strained InGaAs or InGaAsP well layers as the active layer enables oscillation at wavelengths longer than the optical fiber communication band. Furthermore, application of DFB and DBR laser structures, which have been proven in the fabrication of lasers for optical fiber communication, enables oscillation at a single wavelength. This laser can cover the wavelength range of gas absorption lines shown in Figure 8, from 1.6 μm to 2.1 μm. Furthermore, by using InAs for the well layers, a laser with an oscillation wavelength of 2.3 μm can be fabricated (see, for example, Non-Patent Document 4). Furthermore, by using InGaAsSb for the well layers, a wavelength range up to 2.4 μm can be covered. Below, an example of a semiconductor laser using InGaAsSb for the well layers is described.

[0008] As shown in FIG. 9 , a semiconductor laser 30 using InGaAsSb for the well layers comprises an n-type InP substrate 31, an n-type InP cladding layer 32, an InGaAsP guide layer 33, an active layer 34 using InGaAsSb for the well layers and barrier layers, an InGaAsP guide layer 35, a p-type InP cladding layer 36, a p-type InGaAs contact layer 37, a p-type electrode 38, and an n-type electrode 39.

[0009] Although a large compressive strain is applied to the InGaAsSb well layer in the active layer 34, the layer structure of the semiconductor laser 30 is substantially the same as that of lasers used in optical fiber communications, except for the MQW, and therefore this laser can be fabricated in the same manner as lasers for optical fiber communications.

[0010] As described above, the semiconductor laser 30 uses an MQW having a well layer to which a large compressive strain is applied, thereby increasing the emission wavelength. The emission wavelength of the MQW is basically close to the band gap wavelength of the well layer.

[0011] Figure 10 shows the change in bandgap wavelength when the thickness of the MQW well layer of a 2 μm band laser on an InP substrate is changed. Here, InGaAs or InGaAsSb (Sb molar composition ratio [Sb] = 0.1, 0.2, 0.3) is used for the well layer and barrier layer, respectively, and the compressive strain of the well layer is 2%. The barrier layer is lattice-matched to InP, and the Sb molar composition ratio of the barrier layer is equal to that of the well layer.

[0012] As shown in Fig. 10, by using InGaAsSb for the well layer, a bandgap wavelength of 2.2 μm to 2.4 μm can be obtained. By using an MQW including this InGaAsSb well layer for the active layer, even a laser on an InP substrate can cover the wavelength range from 2.2 μm to 2.4 μm.

[0013] As mentioned above, the wavelength range from 1.6 μm to 2.4 μm can be covered by using InGaAs, InGaAsP, or InAs for the well layer, so the wavelength range from 1.6 μm to 2.4 μm of the absorption lines of the gas species shown in FIG. 8 can be covered by a laser on an InP substrate.

[0014] Figure 11 shows the oscillation spectrum of a DFB laser with an InGaAs / InGaAs MQW active layer fabricated on an InP substrate (injection current: 60 mA). The linewidth of the oscillation peak is approximately 0.1 to 0.2 nm, and the side mode suppression ratio (SMSR) is approximately 30 dB.

[0015] Gas absorption lines tend to be concentrated in a narrow wavelength range. Also, depending on the wavelength, the absorption lines of multiple gas species may be close to each other. For example, as shown in Figure 12, １２ C １６ O ２ ) absorption line (dotted line in the figure) and water ( １ H ２ １６The absorption lines of carbon dioxide (CO) and water (solid lines in the figure) are both located near a wavelength of 2.05 μm, with many absorption lines observed clustered in a narrow wavelength range. The absorption lines were acquired using HITRAN (High-resolution transmission molecular absorption database). The spacing between the absorption lines is approximately 0.5 nm for carbon dioxide and 2 to 3 nm for water.

[0016] In this way, in gas measurement, the line width of the semiconductor laser's oscillation spectrum is narrower than the wavelength interval of this absorption line, so in gas measurement using a semiconductor laser, one of the absorption lines is targeted, the laser's oscillation wavelength is swept near the absorption line of the gas, and the light transmission at that time is measured, thereby making it possible to measure the gas concentration.

[0017] The oscillation wavelength of a semiconductor laser changes depending on the temperature of the heat sink on which the semiconductor laser is mounted and the current injected into the semiconductor laser. The time response of the laser wavelength change is faster when the injection current is changed than when the heat sink temperature is changed. For this reason, in general gas measurements, the laser wavelength is swept by changing the injection current.

[0018] Figure 13 shows the change in oscillation wavelength due to the heat sink temperature and injection current of a DFB laser. The DFB laser has a cavity length of 900 μm, an InGaAs / InGaAs MQW active layer on an InP substrate, and an oscillation wavelength set to approximately 2.05 μm. At heat sink temperatures of 15°C, 25°C, 35°C, and 45°C, increasing the injection current to the laser from 50 mA to 150 mA shifts the oscillation wavelength to the longer wavelength side by approximately 0.2 nm.

[0019] Because both the linewidth of the gas absorption line and the linewidth of the laser oscillation spectrum are narrow, a wavelength sweep for one gas absorption line can be achieved by shifting the semiconductor laser by about 0.2 nm, and this change in transmittance allows the selection and measurement of only one absorption line of the target gas.

[0020] The change in the oscillation wavelength due to the injection current of a DFB laser is mainly due to a change in the effective refractive index caused by a rise in temperature near the active layer due to Joule heat generated by the current. In this case, since the heat capacity of the laser itself is small, the oscillation wavelength of the laser changes in a shorter time than when the temperature of the heat sink is changed. When measuring trace gas concentrations, methods such as f-detection and 2f-detection can be used, in which the light source is modulated at a frequency of several hundred MHz and the signal from the detector is phase-sensitively detected at a frequency that is an integer multiple of that frequency.

[0021] On the other hand, changing the temperature of the heat sink allows for a wider range of temperature changes than changing the injection current. Figure 14 shows the change in oscillation wavelength when the injection current to the DFB laser is kept constant and the temperature of the heat sink is changed. The oscillation wavelength of the laser changes by about 2 nm when the temperature of the heat sink is changed by 20°C, and the rate of change of the oscillation wavelength is about 0.1 nm / °C.

[0022] This change in the temperature of the heat sink allows the measurement of multiple gas absorption lines. Figure 15 shows the change in transmittance when light from a DFB laser is incident on a gas cell filled with carbon dioxide while changing the temperature of the heat sink. A comparison with the spectrum of gas absorption lines (dotted line in the figure) shows that the transmittance changes depending on the wavelength and intensity of the gas absorption line, indicating that multiple gas absorption lines can be measured.

[0023] As shown in Figure 15, if the laser light can be swept over a wide wavelength range, not only can multiple absorption lines of a single gas species be measured, but multiple gas species can also be measured with a single laser. For example, if the oscillation wavelength of the DFB laser is set to around 2.047 μm when the heat sink temperature is 15°C and the oscillation wavelength is changed by the heat sink temperature, the absorption lines of both carbon dioxide and water gases (Figure 12) can be measured.

[0024] However, in actual gas measurement systems, the method of sweeping the wavelength by changing the temperature of the heat sink is rarely used, for the reasons explained below.

[0025] In a typical laser module, the laser is mounted on a heat sink, which is mounted on a submount, and the submount is mounted on a Peltier element. The temperature of the laser is measured by a thermistor placed on the heat sink. When changing the temperature of the heat sink, the heat capacity of not only the heat sink but also the submount and Peltier element is added. Because the total heat capacity from the laser to the Peltier element is large, it is difficult to change the temperature of the laser in a short period of time.

[0026] Therefore, when using a method such as f-detection or 2f-detection, which performs phase-sensitive detection of a signal from a detector at a frequency that is an integer multiple of the detector frequency, if a long time is required for the temperature of the active layer to stabilize after changing the temperature of the heat sink, the light source cannot be modulated at a high frequency, and measurement takes a long time. Thus, while changing the temperature of the heat sink can expand the wavelength range that the laser can sweep, it is difficult to increase the modulation frequency, making it difficult to apply this method to an actual gas measurement system.

[0027] As described above, while gas measurement systems using lasers are capable of high-precision concentration measurement in real time, it is difficult to handle multiple absorption lines or gas species with a single laser. Therefore, in gas measurement systems using lasers, if you want to significantly change the oscillation wavelength by injecting current, you need to increase the temperature near the laser active layer by injecting current.

[0028] M. Mitsuhara and M. Oishi, “Chapter 2: 2 μm wavelength lasers employing InP-based strained-layer quantum wells,” in “Long-wavelength infrared semiconductor lasers (ed. H.K. Choi,),” Wiley, New Jersey, 2004. T. Sato et al., “2.1-μm-wavelength InGaAs multiple-quantum-well distributed feedback lasers grown by MOVPE,” IEEE Journal of Selected Topics in Quantum Electronics, VOL. 13, NO. 5, 2007, pp. 1079-1082. NTT Electronics Corporation, “Sensing Components.”<URL: https: / / www.ntt-electronics.com / product / gas_sensing / gas_sensing.html> T. Sato et al., “2.33-μm-wavelength distributed feedback lasers with InAs-In0.53Ga0.47As multiple-quantum wells on InP substrates,” IEEE Photonics Technology Letters, VOL. 20, NO. 12, 2008, 1045-1047.

[0029] The change in the oscillation wavelength due to the temperature of the laser active layer and the injection current is caused by the Joule heat, which increases the temperature near the active layer and changes the effective refractive index, as mentioned above. Joule heat is basically proportional to the resistance. Therefore, increasing the resistance near the active layer is an effective way to increase the oscillation wavelength due to the injection current.

[0030] 16A is a diagram illustrating the resistance near the active layer of an edge-emitting semiconductor laser. In FIG. 16A, when the stripe width is W and the cavity length is L, the resistance R at a layer thickness t of the layer structure consisting of an n-type semiconductor layer 41, an active layer 42, and a p-type semiconductor layer 43 is proportional to the length through which current flows (=film thickness t), and can be expressed by equation (1).

[0031] R=(ρV×t) / (W×L) (1)

[0032] Here, ρV is called volume resistivity (unit: Ω cm), a physical quantity determined by the material and not by the size of the element. In the structure of Figure 16A, the stripe width is difficult to change significantly due to limitations imposed by the need to obtain a single transverse mode. On the other hand, the cavity length can be set to approximately 200 μm to 1 mm in a DFB laser, allowing for a high degree of freedom. Therefore, a configuration that shortens the cavity length is effective in increasing the resistance R near the active layer. However, shortening the cavity length reduces the volume of the active layer that contributes to light emission, making it difficult to obtain a large optical output.

[0033] 16B shows the effect on the laser characteristics when the cavity length is shortened. By shortening the cavity length, the wavelength shift of the oscillation wavelength due to the injection current can be increased, but it becomes difficult to obtain a large optical output.

[0034] Shortening the cavity length also affects the fabrication of the laser, as explained in detail below. Semiconductor lasers used in gas measurement are required to oscillate at a single wavelength. As a semiconductor laser having a diffraction grating on an InP substrate, a distributed feedback (DFB) laser, in which the diffraction grating located near the active layer is embedded by regrowth of InP, is actually used in lasers for gas measurement.

[0035] The light intensity distribution in the waveguide direction of the DFB laser is expressed as the coupling constant of the diffraction grating, κ (cm －１), the coupling constant κ depends on κL (called the normalized coupling constant), which is the product of κ and the cavity length L, and κL is often set to about 1 to achieve a uniform light intensity distribution within the waveguide. The coupling constant κ depends on the height of the diffraction grating, and generally, the deeper the diffraction grating, the larger κ becomes. In order to prevent the normalized coupling constant κL from becoming smaller even when the cavity length is shortened, the coupling constant κ must be increased. To increase the coupling constant, the diffraction grating must be made deeper. However, as the diffraction grating becomes deeper, the shape of the diffraction grating is more likely to change when regrowing in InP, and crystal defects are more likely to occur.

[0036] As described above, in a semiconductor laser for gas sensing, shortening the cavity length makes it easy to change the oscillation wavelength by changing the injection current of the DFB laser, but on the other hand, it is difficult to obtain a large optical output, and the fabrication of this semiconductor laser is also difficult, which is a problem.

[0037] In addition, the wavelength scanning range can be expanded by using two or more lasers with different oscillation wavelengths. However, when two or more lasers are used, the number of devices and parts to be controlled increases, making the measurement system more complex, and this leads to problems such as increased power consumption and costs for the measurement system.

[0038] In order to solve the above-mentioned problems, a semiconductor laser according to the present invention comprises, in order, an n-type InP substrate, an active layer, and a p-type InP cladding, and current blocking layers are provided in the p-type InP cladding near the active layer and periodically arranged in the waveguide direction, the current blocking layers being n-type semiconductors that are approximately lattice-matched to InP, and having an oscillation wavelength of 1.6 μm or more and 2.4 μm or less.

[0039] According to the present invention, it is possible to provide a semiconductor laser capable of sweeping a wide wavelength range.

[0040] FIG. 1 is a schematic cross-sectional view showing a portion of the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 2 is a diagram for explaining the operation of the semiconductor laser according to the first embodiment of the present invention. FIG. 3A is a diagram for explaining the operation of the semiconductor laser according to the first embodiment of the present invention. FIG. 3B 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 operation of the semiconductor laser according to the first embodiment of the present invention. FIG. 4B is a diagram for explaining the operation of the semiconductor laser according to the first embodiment of the present invention. FIG. 4C is a diagram for explaining the operation of the semiconductor laser according to the first embodiment of the present invention. FIG. 4D is a diagram for explaining the operation of the semiconductor laser according to the first embodiment of the present invention. FIG. 5 is a diagram for explaining the operation 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 semiconductor laser according to a first example of the present invention. FIG. 7A is a diagram for explaining a method for manufacturing the semiconductor laser according to the first example of the present invention. FIG. 7B is a diagram for explaining a method for manufacturing the semiconductor laser according to the first example of the present invention. FIG. 7C is a diagram for explaining a method for manufacturing the semiconductor laser according to the first example of the present invention. FIG. 7D is a diagram for explaining a method for manufacturing the semiconductor laser according to the first example of the present invention. FIG. 7E is a diagram for explaining a method for manufacturing a semiconductor laser according to a first embodiment of the present invention. It is a diagram showing the main gas species that have strong absorption lines in the wavelength region around 2 μm and the wavelength region of their absorption lines. FIG. 9 is a cross-sectional schematic diagram showing the configuration of a conventional semiconductor laser. FIG. 10 is a diagram showing the change in band gap wavelength depending on the Sb composition ratio of the well layer and the film thickness of the well layer in a quantum well structure using a well layer with a compressive strain of 2%. The Sb composition ratio of the barrier layer is set equal to that of the well layer. FIG. 11 is a diagram showing the oscillation spectrum of a DFB laser with an InGaAs / InGaAs MQW active layer fabricated on an InP substrate. FIG. 12 is a diagram showing the oscillation spectrum of a DFB laser with an active layer of carbon dioxide ( １２ C １６ O ２ ) and water ( １ H ２ １６FIG. 13 shows the wavelength and intensity of the absorption line in the wavelength region around 2.05 μm of SiO2 (O). FIG. 13 shows the change in oscillation wavelength with heat sink temperature and injection current in a DFB laser with an InGaAs / InGaAs MQW structure as the active layer fabricated on an InP substrate. FIG. 14 shows the change in oscillation wavelength with a constant injection current and a change in heat sink temperature in a DFB laser with an InGaAs / InGaAs MQW structure as the active layer fabricated on an InP substrate. FIG. 15 shows the change in transmittance when light from a DFB laser with different heat sink temperatures is incident on a gas cell filled with carbon dioxide. The bottom of the figure shows the intensity of the carbon dioxide absorption line measured by HITRAN. FIG. 16A is a diagram for explaining a conventional semiconductor laser. FIG. 16B is a table showing the effect of cavity length on the laser in FIG. 16A.

[0041] First Embodiment A semiconductor laser according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 5. FIG.

[0042] <Configuration of Semiconductor Laser> FIG. 1 shows a schematic cross-sectional view of a portion of a semiconductor laser 10 according to the present embodiment.

[0043] The semiconductor laser 10 has a complex-coupled DFB (Complex-Coupled-Distributed Feedback) laser configuration, and has an oscillation wavelength of 1.6 μm or more and 2.4 μm or less.

[0044] The semiconductor laser 10 includes, in order, an n-type InP substrate 11, an n-type InP 12, an active layer 13, and a p-type InP clad 15, and includes a current blocking layer 14 in the p-type InP clad 15 near the active layer 13. The n-type InP 12 does not necessarily have to be disposed.

[0045] The active layer 13 is made of a semiconductor that corresponds to a laser oscillation wavelength of 1.6 μm or more and 2.4 μm or less, and is, for example, a multiple quantum well layer, and the quantum well layer may be made of InGaAs, InGaAsP, InAs, or InGaAsSb.

[0046] The current blocking layers 14 are periodically arranged in the waveguide direction (x direction in the drawing). The current blocking layers 14 comprise, in order from the active layer 13 side, n-type InP and n-type InGaAsP. The current blocking layers 14 are not limited to this, and may be made of other n-type semiconductors that are approximately lattice-matched to InP.

[0047] The thickness of the region between the current blocking layer 14 and the active layer 13 in the p-type InP cladding 15 is preferably 10 nm or more and 30 nm or less.

[0048] 2 is a diagram for explaining a flow 1 of holes (indicated by an arrow in the figure) in the vicinity of the active layer 13 in the semiconductor laser according to this embodiment (a complex-coupled DFB laser having a current blocking layer 14). In the semiconductor laser 10, as shown in FIG. 2, a current blocking layer 14 made of n-InGaAsP 141 and n-InP 142 is buried in the p-InP cladding 15 in the vicinity of the active layer 13.

[0049] The p-InP cladding 15 is a path for holes to move (inject) into the active layer 13, but the current blocking layer 14 acts as a barrier to the injection of holes (making the injection of holes difficult), so the holes are injected into the active layer 13 avoiding the current blocking layer 14.

[0050] 3A shows the flow of holes in the semiconductor laser 10 according to this embodiment (a complex-coupled DFB laser having a current blocking layer 14). For comparison, FIG. 3B shows the flow of holes in a conventional index-coupled DFB laser 20. In the figure, solid arrows indicate hole flow 1, and dotted arrows indicate electron flow 2. The conventional index-coupled DFB laser 20 has a diffraction grating 24.

[0051] In the conventional index-coupled DFB laser 20, as shown in FIG. 3B, there is no region in the p-InP cladding 25 that acts as a barrier to hole injection, so holes are injected uniformly into the active layer 23.

[0052] On the other hand, in the semiconductor laser 10 according to this embodiment, the current blocking layers 14 are periodically arranged, in other words, regions that serve as hole injection paths and regions that serve as hole injection barriers are periodically arranged in the p-InP cladding 15. Therefore, holes are injected non-uniformly into the active layer 13.

[0053] Due to this difference in hole injection, the semiconductor laser 10 according to this embodiment can increase the electrical resistance of the element and generate more Joule heat than the conventional index-coupled DFB laser 20. As a result, the temperature in the vicinity of the active layer increases due to the injected current.

[0054] The resistance of the semiconductor laser 10 according to this embodiment can be changed by changing the proportion of the current blocking layer 14 in the p-InP cladding 15. The proportion of the current blocking layer 14 is the ratio of the area of ​​the region where the current blocking layer 14 is disposed to the area of ​​the region where the current blocking layer 14 is not disposed in a horizontal cross section (a cross section parallel to the substrate surface including the dashed dotted line in the figure), and is the ratio of the lengths (waveguide direction, x direction) of the regions when the widths of the regions are equal.

[0055] 4A shows a schematic diagram of the flow of holes in a structure without the current blocking layer 14. The holes are injected uniformly into the active layer 13.

[0056] 4B to 4D show schematic diagrams illustrating the change in hole flow when the ratio of the current blocking layer 14 is changed. The film thicknesses of the n-InGaAsP 141 and n-InP 142, which are the current blocking layers 14, are constant, and the area ratio of a portion of the p-InP 15 (the portion sandwiched between the current blocking layers 14) to the current blocking layer 14 (for example, the n-InGaAsP 141) is 2:1 (FIG. 4B), 1:1 (FIG. 4C), and 1:2 (FIG. 4D).

[0057] As shown in FIGS. 4B to 4D, as the area ratio of the current blocking layer 14 increases, a part of the p-InP 15 (the part sandwiched between the current blocking layers 14), i.e., the hole injection region, becomes narrower, and therefore the resistance near the active layer increases.

[0058] In the semiconductor laser 10 according to this embodiment, holes are injected non-uniformly into the active layer 13 due to the current blocking layer 14 being disposed.

[0059] On the other hand, electrons have a higher mobility than holes and are electrically attracted to holes. For this reason, electrons are distributed at high concentrations in regions of the active layer 13 where holes are distributed at high concentrations. Since laser gain is due to the recombination of electrons and holes, if holes and electrons are distributed non-uniformly in the active layer 13, the distribution of gain in the active layer 13 will also be non-uniform.

[0060] FIG. 5 shows the flow of holes 1 and the flow of electrons 2 (101 in the figure) in the waveguide direction (x direction) of the semiconductor laser 10 according to this embodiment, the amount of holes injected into the active layer 13 (102 in the figure), the gain of the active layer 13 (103 in the figure), and the equivalent refractive index (104 in the figure).

[0061] When the current blocking layers 14 and p-InP layers 15 are periodically arranged in the waveguide direction (x direction) of the semiconductor laser, the gain of the active layer region 13 is modulated in accordance with the amount of injected holes (102 in the figure), and is modulated in the waveguide direction (103 in the figure). This gain modulation structure functions as a gain-coupled DFB laser structure.

[0062] On the other hand, since the current blocking layer 14 contains InGaAsP, which has a larger refractive index than InP, the equivalent refractive index is also modulated in the waveguide direction (see 104 in the figure). This equivalent refractive index modulation structure functions as an index-coupled DFB laser structure.

[0063] As described above, the characteristics of the semiconductor laser according to this embodiment reflect the characteristics of both a gain-coupled DFB laser and an index-coupled DFB laser. In this laser structure, the gain and the equivalent refractive index of the active layer 13 increase and decrease in opposite phases.

[0064] The semiconductor laser according to this embodiment can increase the element resistance and generate more Joule heat than a general DFB laser, so that the oscillation wavelength can be significantly shifted to the long wavelength side by increasing the injection current. If the semiconductor laser according to this embodiment is used in a gas measurement system with an oscillation wavelength of 1.6 μm to 2.4 μm, the oscillation wavelength can be swept over a wider wavelength range by injection current alone than with a conventional index-coupled DFB laser.

[0065] The semiconductor laser according to this embodiment differs from conventional complex-coupled DFB lasers having current blocking layers in that it utilizes heat generation in the laser structure.

[0066] Conventional complex-coupled DFB lasers with current blocking layers are lasers formed on an InP substrate that oscillate in the 1.3 μm or 1.55 μm wavelength band used in optical fiber communications. Therefore, heat generation in the active layer deteriorates the threshold current and output characteristics, and is therefore a problem that needs to be suppressed.

[0067] On the other hand, the semiconductor laser according to this embodiment actively utilizes the heat generated in the active layer, and has the effect of being able to sweep the oscillation wavelength only by injection current.

[0068] Furthermore, despite the semiconductor laser according to this embodiment having a structure in which heat is easily generated near the active layer, it has laser characteristics (lasing threshold current, injection efficiency, reliability, etc.) comparable to DFB lasers without current blocking layers, similar to conventional complex-coupled DFB lasers with current blocking layers (e.g., H. Shaio et al., “High performance and high reliability of 1.55-μm current-blocking grating complex-coupled DFB lasers,” IEEE Photonics Technology Letters, VOL. 10, NO. 9, 1998, pp. 1238-1240; F. Lee et al., “High-reliable and high-speed 1.3-μm complex-coupled distributed feedback buried-heterostructure laser diodes with Fe-doped InGaAsP / InP hybrid grating layers grown by MOCVD,” IEEE Transactions on Electron Devices, VOL. 55, NO. 2, 2008, pp. 540-546; Z. Chuang et al., “Very-low-threshold highly efficient, and low-chirp 1.55-μm complex-coupled DFB lasers with a current-blocking grating,” IEEE Photonics Technology Letters, VOL. 8, NO. 11, 1996, 1438-1440, C. Hu et al., “Uncooled 1.3-μm complex-coupled DFB BH laser diodes with the Fe-doped InGaAsP-InP hybrid current-blocking grating,” IEEE Photonics Technology Letters, VOL. 18, NO. 14, 2006, 1551-1553.)This is believed to be because the complex-coupled DFB laser having a current blocking layer has the characteristics of both a gain-coupled DFB laser and an index-coupled DFB laser, as described above.

[0069] In a semiconductor laser for gas sensing fabricated on an InP substrate, the desired oscillation wavelength is 1.6 μm to 2.4 μm. As shown in FIG. 9, the basic layer configuration is the same as that of semiconductor lasers used in optical fiber communications. Therefore, the growth and process technologies required for fabricating the laser can be the same as those for semiconductor lasers used in optical fiber communications, except for the active layer. As mentioned above, the laser characteristics are hardly degraded by a current blocking layer in a complex-coupled DFB laser in the optical fiber communications wavelength band. Therefore, even if the oscillation wavelength band is extended to 1.6 to 2.4 μm, a complex-coupled DFB laser with a current blocking layer can maintain good laser characteristics.

[0070] As shown in Figures 12 and 15, gas absorption lines are very narrow, so semiconductor lasers for gas sensing must oscillate at a single wavelength. DFB lasers that do not have a phase shift region in the diffraction grating generally tend to oscillate at two wavelengths (two modes).

[0071] On the other hand, it is known that complex-coupled DFB lasers with current blocking layers tend to oscillate at the shorter wavelength of the two modes (see the aforementioned papers by H. Shaio et al. and F. Lee et al.). Therefore, complex-coupled DFB lasers with current blocking layers can easily obtain oscillation at a single wavelength, which is required for semiconductor lasers used as gas measurement light sources, even though they do not have a phase shift region in the diffraction grating.

[0072] The semiconductor laser according to this embodiment has a complex-coupled DFB laser structure having a current blocking layer, which allows the oscillation wavelength to be changed by an injected current. In addition, the semiconductor laser can have the laser characteristics required for a light source of a gas measurement system.

[0073] In addition, in lasers used for gas measurement, the wavelength of the gas absorption line is determined by the gas species, so the oscillation wavelength must be adjusted to match the wavelength of the gas absorption line. Since the oscillation wavelength of a semiconductor laser changes with temperature, temperature control of the laser is necessary to stabilize the laser oscillation wavelength. This temperature control is generally performed using electrical temperature control with a Peltier element.

[0074] On the other hand, with the semiconductor laser according to this embodiment, the oscillation wavelength can be changed in a short time by injecting current into the laser under conditions where the temperature of the heat sink on which the laser is mounted is kept constant by a Peltier element.When the oscillation wavelength of the laser is to be changed significantly, it is effective to generate a large amount of Joule heat near the active layer.

[0075] First Example A semiconductor laser according to a first example of the present invention will be described with reference to FIGS. 6 to 7E.

[0076] <Configuration of Semiconductor Laser> As shown in FIG. 6 , the semiconductor laser 10 according to this embodiment has a layer structure of a complex-coupled DFB laser having a current blocking layer, and includes, on an n-type InP substrate 11, an n-InP 12, an InGaAsP optical confinement layer 131 having a thickness of 100 nm and a bandgap wavelength of 1.3 μm, four periods of MQW 132 having compressively strained InGaAs well layers and tensilely strained InGaAs barrier layers, an InGaAsP optical confinement layer 133 having a thickness of 80 nm and a bandgap wavelength of 1.3 μm, p-InP (part of the p-InP cladding 15) having a thickness of 15 nm, n-InGaAsP 141 having a bandgap wavelength of 1.2 μm and a thickness of 30 nm, n-InP 142 having a thickness of 30 nm, p-InP 15, and p-InGaAs 16. The layer structure up to n-InP 142 is grown by metalorganic molecular beam epitaxy, and p-InP 15 and p-InGaAs 16 are grown by metalorganic vapor phase epitaxy.

[0077] The photoluminescence emission wavelength of the MQW 132 is about 2.05 μm, which is designed to match the absorption line of carbon dioxide shown in FIG.

[0078] The semiconductor laser 10 has a p-type electrode 171 on the surface of the p-InGaAs 16, and an n-type electrode 172 on the back surface of the n-type InP substrate 11. It also has a high-reflection film 181 on one end face in the waveguide direction (x direction in the drawing), and a low-reflection film 182 on the other end face.

[0079] <Method for Manufacturing Semiconductor Laser> An example of a method for manufacturing a semiconductor laser according to this embodiment will be described with reference to FIGS. 7A to 7E.

[0080] First, n-InP 12, InGaAsP optical confinement layer 131, MQW 132, InGaAsP optical confinement layer 133, p-InP 15_1, n-InGaAsP 141_1, and n-InP 142_1 are grown in this order on an InP substrate 11 (FIG. 7A).

[0081] Next, a mask 3 is formed on the n-InP 107 using an electron beam exposure system, and then the n-InGaAsP 141_1 and the n-InP 142_1 are periodically removed by etching. As a result, the n-InGaAsP 141 and the n-InP 142 that constitute the current blocking layer 14 are formed (FIG. 7B). At this time, an etchant is selected that etches the n-InGaAsP 141_1 and the n-InP 142_1 in the regions where the mask 3 is not formed, but hardly etches the p-InP cladding 15_1.

[0082] Next, after removing the mask 3, p-InP 15 is regrown by metal organic vapor phase epitaxy so as to bury the current blocking layer 14 (FIG. 7C).

[0083] Next, when a ridge waveguide type DFB laser is to be fabricated, p-InP 15 (overcladding portion), p-InGaAs 16 and an InP protective layer (not shown) are grown in this order over the entire surface of the wafer.

[0084] Next, a waveguide for the semiconductor laser is formed by dry etching and wet etching, and then an insulating film (not shown) is deposited over the entire surface of the wafer.

[0085] Next, after removing the insulating film and the InP protective layer on the p-InGaAs 16, the metal of the p-type electrode 171 is evaporated onto the p-InGaAs 16 by resistance heating.

[0086] Next, after polishing the n-InP substrate 11, an n-type electrode 172 is evaporated and the wafer is annealed.

[0087] Next, a ridge waveguide laser with a cavity length of 500 μm is formed by cleavage.

[0088] Finally, a high-reflection film 181 is deposited on one of the cleaved facets, and a low-reflection film 182 is deposited on the other facet.

[0089] <Effects> The effects of the semiconductor laser 10 according to this embodiment will be described.

[0090] For comparison, a conventional index-coupled DFB laser without a current blocking layer was prepared. The fabrication method was the same, with the layer structure from the n-type InP substrate to the MQWs being the same, and InGaAsP with a thickness of 100 nm and a bandgap wavelength of 1.3 μm was grown on top of it. After forming a diffraction grating on this InGaAsP, p-InP, p-InGaAs, and InP protective layers were grown over the entire wafer. The subsequent processes were the same as for the complex-coupled DFB laser with the blocking layer described above.

[0091] In the semiconductor laser according to this embodiment (a complex-coupled DFB laser having a current blocking layer) and the DFB laser without the current blocking layer 14, the oscillation threshold current at a heat sink set temperature of 25° C. is 30 mA in both lasers. The oscillation wavelength at an injection current of 33 mA is also 2.05 μm, which is almost the same.

[0092] When the heat sink temperature is fixed at 25°C and the injection current is increased from 30 mA to 110 mA, the wavelength of the conventional index-coupled DFB laser changes from 2.05 μm to 2.0506 μm. The rate of change of the oscillation wavelength with respect to the injection current is 0.0075 nm / mA.

[0093] On the other hand, in the semiconductor laser according to this example (a complex-coupled DFB laser having a current blocking layer), the oscillation wavelength changes from 2.05 μm to 2.052 μm. The rate of change of the oscillation wavelength with respect to the injected current is 0.025 nm / mA.

[0094] From this, the change in oscillation wavelength due to the injection current of the semiconductor laser according to this embodiment is 2 nm, and when measuring carbon dioxide gas as shown in FIG. 15, two or more absorption lines can be measured.

[0095] In the case of a conventional index-coupled DFB laser, the change in oscillation wavelength due to the injection current is less than 1 nm, and at most one absorption line can be measured.

[0096] According to the semiconductor laser of this embodiment, by using a complex-coupled DFB laser having the current blocking layer 14, it is possible to measure the absorption lines of a plurality of gases using a single laser only by changing the injection current.

[0097] In this example, the method for manufacturing a semiconductor laser was described by growing p-InP simultaneously on the side and top surfaces of the current blocking layer 14. However, this is limited to cases where the thickness of the current blocking layer 14 is relatively thin, approximately 100 nm or less. To increase the current blocking efficiency or the coupling constant of the diffraction grating, it is necessary to thicken the current blocking layer 14. In this case, the method of simultaneously growing p-InP on the side and top surfaces of the current blocking layer 14 increases the possibility of voids (hollow regions) occurring near the side surfaces of the current blocking layer 14 where no p-InP is embedded, or of large undulations occurring on the surface after embedding with p-InP.

[0098] Therefore, as shown in Fig. 7D, the side surfaces of the current blocking layer 14 are filled with p-InP 15_2 while the mask 3 remains, and then the mask 3 is removed and p-InP 15 is regrown as shown in Fig. 7E. In the laser structure of this example, the regrown surface excluding the side walls of the current blocking layer 14 is made of n-InP and p-InP, and further, the current blocking layer 14 has a nearly rectangular shape, so that it can be easily fabricated by this process (Figs. 7D and 7E).

[0099] The choice between the process of removing the mask and regrowing p-InP over the entire surface (FIG. 7C) and the process of leaving the mask and regrowing p-InP, and then removing the mask and regrowing p-InP (FIGS. 7D and 7E) may be made in consideration of the thickness of the current blocking layer 14 and the ease of regrowth.

[0100] In this embodiment, the thickness of the p-InP between the InGaAsP above the active layer 13 and the n-InGaAsP that will become the current blocking layer 14 is set to 15 nm. However, the thickness of this p-InP may be determined taking into consideration the effect on current blocking and the controllability of the process.

[0101] In detail, if the p-InP is made thicker, the distribution of holes will spread below the current blocking layer 14, thereby reducing the current blocking effect. On the other hand, if the p-InP is made thin, there is a high possibility that the p-InP will be completely etched when etching the n-InGaAsP that will become the current blocking layer 14. For these reasons, it is desirable that the thickness of the region between the current blocking layer 14 and the active layer 13 in the p-type InP cladding be 10 nm or more and 30 nm or less.

[0102] In this example, an example was shown in which InGaAs was used for the well layer because the oscillation wavelength was around 2 μm, but other materials than InGaAs, such as InGaAsP, InAs, or InGaAsSb, may also be used for the well layer to set the oscillation wavelength in the wavelength range of 1.6 μm to 2.4 μm. Since the effect of the current blocking layer is effective even if the oscillation wavelength changes, it goes without saying that it is effective whether the well layer is InGaAsP, InAs, or InGaAsSb.

[0103] In this example, an example of growing a laser structure by combining metalorganic molecular beam epitaxy and metalorganic vapor phase epitaxy has been shown, but the structure shown in FIG. 6 can also be grown by other growth methods. Specifically, only metalorganic vapor phase epitaxy may be used, or other growth methods such as molecular beam epitaxy using only a solid source or gas source molecular beam epitaxy may also be used.

[0104] In this embodiment, an example in which the current blocking layer is applied to a ridge waveguide laser has been shown, but the current blocking layer may also be applied to other laser structures, such as a buried structure laser.

[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 semiconductor laser comprising, in order, an n-type InP substrate, an active layer, and a p-type InP cladding, and current blocking layers are provided periodically in the waveguide direction in the vicinity of the active layer within the p-type InP cladding, the current blocking layers being n-type semiconductors that are approximately lattice-matched to InP, and having an oscillation wavelength of 1.6 μm or more and 2.4 μm or less.

[0109] (Appendix 2) The semiconductor laser according to appendix 1, wherein a thickness of a region between the current blocking layer and the active layer in the p-type InP cladding is 10 nm or more and 30 nm or less.

[0110] (Supplementary Note 3) The semiconductor laser according to Supplementary Note 1 or Supplementary Note 2, wherein the current blocking layer comprises n-type InP and n-type InGaAsP.

[0111] (Appendix 4) The semiconductor laser according to any one of Appendices 1 to 3, wherein the temperature of the active layer is increased by the current blocking layer, thereby shifting the oscillation wavelength to a longer wavelength.

[0112] (Supplementary Note 5) The semiconductor laser according to any one of Supplementary Notes 1 to 4, wherein the active layer comprises, in order from the InP substrate side, an InGaAsP layer, a multiple quantum well layer, and an InGaAsP layer, and the quantum well layer of the multiple quantum well layer is any one of InGaAs, InGaAsP, InAs, and InGaAsSb.

[0113] The present invention relates to a semiconductor laser and can be applied to a gas measurement system.

[0114] 10 Semiconductor laser 11 n-type InP substrate 13 Active layer 14 Current blocking layer 15 P-type InP cladding

Claims

1. A semiconductor laser comprising, in order, an n-type InP substrate, an active layer, and a p-type InP cladding, current blocking layers are provided periodically in the waveguide direction near the active layer within the p-type InP cladding, the current blocking layers being n-type semiconductors that are approximately lattice-matched to InP, and the oscillation wavelength is 1.6 μm or more and 2.4 μm or less.

2. The semiconductor laser according to claim 1, wherein the thickness of a region in said p-type InP cladding between said current blocking layer and said active layer is not less than 10 nm and not more than 30 nm.

3. The semiconductor laser according to claim 1 or 2, wherein the current blocking layer comprises n-type InP and n-type InGaAsP.

4. The semiconductor laser according to claim 1 or 2, wherein the temperature of the active layer is increased by the current blocking layer, thereby shifting the oscillation wavelength to a longer wavelength.

5. The semiconductor laser according to claim 1 or 2, wherein the active layer comprises, in this order from the n-type InP substrate side, an InGaAsP layer, a multiple quantum well layer, and an InGaAsP layer, and the quantum well layer of the multiple quantum well layer is any one of InGaAs, InGaAsP, InAs, and InGaAsSb.