JP2013229357A - Distribution feedback type semiconductor laser and otdr device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a distribution feedback type semiconductor laser which can prevent the occurrence of a kink in current vs. optical output characteristic.SOLUTION: A distribution feedback type semiconductor laser 100 includes a semiconductor laminate structure and reflective membranes 11 and 12. The reflective membrane 11, which is provided at an end face on one side, has a reflection rate of 3% to 6%, both ends inclusive. The reflective membrane 12, which is provided at an end face on the other side, has a reflection rate higher than that of the reflective membrane 11. The semiconductor laminate structure includes a semiconductor substrate 1, a diffraction lattice 21, an n-type guide layer 3, and an active layer 4. The diffraction lattice 21 includes a region 21a formed on the reflective membrane 11 side and having a short diffraction lattice cycle.

Description

  The present invention relates to a distributed feedback semiconductor laser and an OTDR (Optical Time Domain Reflectometer) apparatus, for example, a distributed feedback semiconductor laser used in OTDR and an OTDR apparatus using the distributed feedback semiconductor laser.

  Distributed feedback semiconductor lasers (hereinafter referred to as DFB (Distributed FeedBack) lasers) are excellent in single-axis mode stability and have been used for long-distance communications. In recent years, DFB lasers are also used for high-speed optical communication over short distances. Furthermore, as a new application of the DFB laser, introduction into a measuring instrument such as an OTDR (Optical Time Domain Reflectometer), a motion sensor, or the like is in progress. In such applications, importance is attached to higher wavelength controllability, narrower spectral width, and lower temperature dependence of wavelength than single-axis mode stability.

  For such OTDR applications, FP (Fabry-Perot) lasers have been used so far. However, in consideration of improvement in measurement accuracy in OTDR, it is desirable to use a DFB laser that has higher wavelength controllability, a narrower spectral width, and less wavelength temperature dependence than FP lasers. However, although the DFB laser is excellent in single wavelength characteristics, the optical output is smaller than that of the FP laser. Therefore, it is conceivable to increase the optical output by intentionally causing the DFB laser to oscillate several modes of several modes.

  As an example of a DFB laser that oscillates in multiple modes, for example, a DFB laser in which a plurality of phase shifters are provided in the vicinity of one end face has been proposed for the purpose of application to a fiber amplifier excitation light source (Patent Document 1). This DFB laser has a reflectance of 2% or less at both end faces, increases κL, and causes two-mode oscillation by spatial hole burning. Also, a plurality of phase shifters are provided in order not to change the electric field strength distribution greatly.

  For the purpose of generating terahertz waves, there has been proposed a DFB laser that oscillates a DFB laser in two modes by setting a transverse higher-order mode (Non-Patent Document 1). In this DFB laser, the ridge width is widened to 7.5 μm so that a transverse higher-order mode is established, and two-mode oscillation is realized by the difference in transmission refractive index.

  In addition, a coupled mode semiconductor laser in which a semiconductor laser operating at a high output and a semiconductor laser operating at a single wavelength are optically coupled has been proposed (Patent Document 2). This coupled mode type semiconductor laser can be stably controlled over a wide frequency range and light output range. Further, there has been proposed a semiconductor optical device that improves the yield by suppressing the variation in characteristics of a plurality of semiconductor optical devices manufactured from one wafer by devising the formation position of the diffraction grating (Patent Document 3). ).

JP-A-5-145194 JP-A-4-349682 JP 2011-142239 A

Andreas Klehr et al., "High-Power Monolithic Two-Mode DFB LaserDiodes for the Generation of THz Radiation", IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 2, MARCH / APRIL 2008, p.289- 294

  However, the inventor has found that the above-described method has the following problems. In the DFB laser according to Patent Document 1, since two-mode oscillation is realized by utilizing the effect of spatial hole burning, a kink occurs in the current-light output characteristics. Further, the DFB laser according to Non-Patent Document 1 realizes two-mode oscillation by oscillating the transverse higher-order mode. For this reason, the mode becomes unstable due to current injection, leading to the occurrence of kinks.

  However, in the OTDR measurement device, if a kink occurs in the current-light output characteristics, the measurement waveform is distorted, and an adverse effect such as erroneous detection of a measurement abnormality occurs.

  Other problems and novel features will become apparent from the description of the specification and the accompanying drawings.

  According to one embodiment, a distributed feedback semiconductor laser is provided on a semiconductor multilayer structure and a first end face in a waveguide direction, and has a reflectivity of 3% or more and 6% or less with respect to the guided light. And a higher reflectivity than the first reflective film with respect to the guided light, which is formed on the second end surface opposite to the first end surface in the waveguide direction. The semiconductor multilayer structure includes an active layer formed on a semiconductor substrate, a first semiconductor layer stacked adjacent to the active layer, and A second semiconductor layer stacked adjacent to the first semiconductor layer; and a first diffraction grating formed on a part of the first end face side.

  According to one embodiment, it is possible to provide a distributed feedback semiconductor laser that can prevent the occurrence of kinks in current-light output characteristics.

1 is a perspective view schematically showing a configuration of a DFB laser 100 according to a first embodiment. It is sectional drawing which shows typically the structure of the DFB laser 100 in the II-II line cross section of FIG. 2 is a spectrum diagram showing an oscillation spectrum of the DFB laser 100 according to the first embodiment. FIG. 3 is a spectrum diagram showing a reflectance spectrum of the DFB laser 100 according to the first embodiment. FIG. 6 is a graph showing a result of theoretical calculation of the relationship between the reflectance of the reflective film 11 of the DFB laser 100 according to the first embodiment and the ratio (yield) of elements that oscillate multimode. 3 is a cross-sectional view schematically showing a configuration of a DFB laser 200 according to a second embodiment. FIG. It is a block diagram which shows typically the structure of the OTDR measuring device 300 concerning Embodiment 3. FIG. 10 is a graph showing a measurement waveform in the OTDR measurement apparatus 300 according to the third embodiment.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted as necessary.

Embodiment 1
First, the distributed feedback (Distributed-FeedBack, hereinafter referred to as DFB) laser 100 according to the first embodiment will be described. FIG. 1 is a perspective view schematically showing the configuration of the DFB laser 100 according to the first embodiment. FIG. 2 is a cross-sectional view schematically showing the configuration of the DFB laser 100 taken along the line II-II in FIG. The DFB laser has a semiconductor multilayer structure including an n-type semiconductor substrate 1, a diffraction grating 21, an n-type guide layer 3, an active layer 4, a p-type cladding layer 5, and a p-type contact layer 6. A diffraction grating 21 is formed on the upper surface of the n-type semiconductor substrate 1. An n-type guide layer 3 is formed on the upper part of the diffraction grating 21. An active layer 4 is formed on the n-type guide layer 3. The active layer 4 has, for example, a quantum well (Multi Quantum Well, MQW) structure including a stacked body of a well layer and a barrier layer. A p-type cladding layer 5 is formed on the active layer 4. A p-type contact layer 6 is formed on the p-type cladding layer 5. A p-electrode 7 is formed on the p-type contact layer 6. An n electrode 8 is formed on the lower surface of the n-type semiconductor substrate 1.

  The n-type semiconductor substrate 1, the diffraction grating 21, the n-type guide layer 3, the active layer 4, the p-type cladding layer 5, and the p-type contact layer 6 are made of, for example, InP or InGaAsP. A part of the n-type semiconductor substrate 1, the diffraction grating 21, the n-type guide layer 3, the active layer 4, the p-type cladding layer 5, the p-type contact layer 6 and the p-electrode 7 are formed by, for example, etching. Thus, the mesa 9 is configured.

  Both sides of the mesa 9 are embedded by the embedded layer 10. The buried layer 10 is made of, for example, Fe-doped InP so as to have a high resistance and exhibit semi-insulating properties. Therefore, the DFB laser 100 has a so-called buried hetero (hereinafter referred to as BH) structure.

  A reflective film 11 is formed on one end face of the DFB laser 100 in the waveguide direction. A reflective film 12 is formed on the other end face of the DFB laser 100 in the waveguide direction. The reflective film 11 is configured to have a reflectance that is smaller than that of the reflective film 12. In the present embodiment, a silicon nitride (SiN) film is used as the reflective film 11, and the reflectance is 4%. The reflective film 12 uses a multilayer film of silicon (Si) and amorphous silicon, and its reflectance is 90%.

  Here, attention is paid to the diffraction grating 21. The diffraction grating 21 has a region 21a with a short diffraction grating period and a region 21b with a long diffraction grating period. In the present embodiment, the diffraction grating period of the region 21a having a short diffraction grating period is 241 nm, and the diffraction grating period of the region 21b having a long diffraction grating period is 242 nm. The region 21a having a short diffraction grating period is formed on the reflective film 11 side having a low reflectance. The region 21b having a long diffraction grating period is formed on the reflective film 12 side having a high reflectance.

  Next, the operation of the DFB laser 100 will be described. FIG. 3 is a spectrum diagram showing an oscillation spectrum of the DFB laser 100 according to the first embodiment. FIG. 3 shows a case where the temperature is 25 ° C. and the injection current is 100 mA (pulse current). As shown in FIG. 3, it can be understood that the DFB laser 100 oscillates stably at about 1550 nm. In the DFB laser 100, the period of the diffraction grating 21 is modulated inside the resonator, so that the reflection spectrum of the diffraction grating 21 is wider than that of a diffraction grating having a single period. Therefore, stable two-mode oscillation as shown in FIG. 3 is possible.

  FIG. 4 is a spectrum diagram showing the reflectance spectrum of the DFB laser 100 according to the first embodiment. In FIG. 4, the reflectance spectrum of the DFB laser 100 is indicated by a solid line L1. As a comparative example, a reflectance spectrum of a general semiconductor laser having a diffraction grating with a uniform period is indicated by a broken line L2.

In general, the DFB laser oscillates at a point where the reflectance spectrum of the diffraction grating and the FP mode position λ _FP determined by the resonator end face coincide. In a general DFB laser, since the reflectance spectrum of the diffraction grating is narrow, an appropriate difference in reflectance is produced between the sub mode and the main mode. As a result, good single axis mode oscillation can be obtained.

  On the other hand, the DFB laser 100 has a lower peak reflectance than a general semiconductor laser, and at the same time, the reflectance spectrum is broadened due to the effect that the Bragg wavelength is modulated. Thereby, in the DFB laser 100, the difference in reflectance of the diffraction grating between the main mode and the sub mode is reduced, and the sub mode is more likely to oscillate than a general semiconductor laser.

  In the DFB laser 100, the reflectance of the reflective film 11 is designed to be as high as 4% in order to further promote submode oscillation. Thereby, the sub mode and the main mode are more likely to oscillate at the same time, and stable multi-mode oscillation is possible. FIG. 5 is a graph showing the result of theoretical calculation of the relationship between the reflectance of the reflective film 11 of the DFB laser 100 according to the first embodiment and the ratio (yield) of elements that oscillate multimode.

  As shown in FIG. 5, when the reflectance of the reflective film 11 is low, oscillation occurs in a single axis mode, so that the yield of elements that oscillate in multiple modes is reduced. On the other hand, when the reflectance of the reflective film 11 is increased, the FP mode is liable to oscillate. Therefore, as the reflectance of the reflective film 11 increases, the single-axis mode yield decreases and the probability of oscillation in the sub mode increases.

  In general DFB lasers, single axis mode stability is required, and thus the reflectance of the reflective film 11 is typically set to a reflectance of 2% or less. On the other hand, since the DFB laser 100 needs to oscillate in multiple modes, the reflectance of the reflective film 11 is desirably 3% or more. By setting the reflectance of the reflective film 11 to 3%, the yield of the element that oscillates in multimode is 50%. By further increasing the reflectivity of the reflective film 11, the yield of elements that oscillate in multiple modes can be increased. However, the yield of an element that oscillates in a multimode is saturated at about 90%.

  In consideration of the output of light emitted from the reflective film 11, it is not desirable to make the reflectance of the reflective film 11 excessively high in order to ensure sufficient light output in high output applications. From the calculation results, from the viewpoint of optical output, it is estimated that the upper limit of the design is a reflectance of 6% at which the yield of an element that oscillates in multiple modes is saturated. Furthermore, considering the balance between the yield of elements that oscillate in multiple modes and the light output, it is considered that the reflectance of the reflective film 11 is preferably 4%.

Embodiment 2
Next, the DFB laser 200 according to the second embodiment will be described. FIG. 6 is a cross-sectional view schematically showing the configuration of the DFB laser 200 according to the second embodiment. In the DFB laser 200, unlike the DFB laser 100 according to the first embodiment, the diffraction grating 22 is formed only on a part of the reflection film 11 side in the light guiding direction. That is, a region 23 where the diffraction grating 22 is not formed exists at the boundary surface between the n-type semiconductor substrate 1 and the n-type guide layer 3. In the present embodiment, the resonator book (distance between the reflective film 11 and the reflective film 12) is 600 μm, and the diffraction grating 22 is formed in a range of 150 μm from the end face on the reflective film 11 side. Since the other structure of the DFB laser 200 is the same as that of the DFB laser 100, description thereof is omitted.

  In general, in the DFB laser, the length of the region where the diffraction grating is formed and the reflection spectrum width are in an inversely proportional relationship. Therefore, when the length of the region where the diffraction grating is formed is shortened, the reflection spectrum is widened. That is, it can be understood that a reflection spectrum can be broadened as in the DFB laser 100 even in a structure in which a diffraction grating is formed only in a part of the waveguide direction as in the DFB laser 200.

  In the DFB laser 200, the length of the region where the diffraction grating is formed is set to 1/4 of the resonator length. When this configuration is compared with a general DFB laser in which the diffraction grating is formed over the entire resonator, in this configuration, the reflectance spectrum by the diffraction grating spreads four times. As a result, in the first embodiment, a reflectance spectrum similar to that shown in FIG. 4 can be obtained. Further, in order to cause laser oscillation in three modes, the length of the region where the diffraction grating is formed must be 1/3 or less of the resonator length.

  In this configuration, since the diffraction grating is formed only in part of the waveguide direction of the resonator, the drawing time when drawing a diffraction grating pattern such as an electron beam exposure method can be shortened. Therefore, it is possible to improve the manufacturing throughput and manufacture a DFB laser that stably oscillates in multiple modes at a low cost.

Embodiment 3
Next, an OTDR measurement apparatus 300 according to the third embodiment will be described. FIG. 7 is a block diagram schematically illustrating a configuration of the OTDR measurement apparatus 300 according to the third embodiment. The OTDR measurement apparatus 300 is a specific example of an application apparatus in which the above-described DFB laser 100 or 200 is incorporated as a module. The OTDR measurement apparatus 300 includes a semiconductor laser module 31, an optical circulator 32, a light receiving module 33, an amplifier 34, and a signal processing unit 35.

  The semiconductor laser module 31 incorporates the DFB laser 100 or 200 described above. The semiconductor laser module 31 outputs inspection light 37 to the optical fiber 36 to be inspected via the optical circulator 32.

  The reflected light 38 returning from the optical fiber enters the light receiving module 33 through the optical circulator 32. The light receiving module 33 outputs a signal obtained by photoelectrically converting the reflected light 38 to the amplifier 34. The light receiving module 33 is configured by incorporating an avalanche photodiode, for example.

  The amplifier 34 amplifies the signal from the light receiving module 33 and outputs the amplified signal to the signal processing unit 35. The signal processing unit 35 processes the signal from the amplifier 34 and detects the intensity of the reflected light 38.

  FIG. 8 is a graph showing a measurement waveform in the OTDR measurement apparatus 300 according to the third embodiment. The vertical axis of the graph in FIG. 8 indicates the light intensity of the reflected light 38, and the horizontal axis indicates the length of the optical fiber 36. A solid line L3 in FIG. 8 shows a measurement waveform in the OTDR measurement apparatus 300, and a broken line L4 shows a measurement waveform in a comparative example described later. In the OTDR measurement apparatus 300, the noise can be reduced to 0.2 dBp-p. This is because the DFB laser 100 or 200 oscillates in a multi-mode, so that the coherence of light is weak and noise is hardly generated even when multiple reflection occurs. Further, since the reflectance of the reflective film 11 is designed to be as high as several times that of a general DFB laser, there is also an effect that the reflective film 11 is reflected when return light is incident from the outside.

  As a comparative example, an OTDR apparatus using a DFB laser in which a uniform diffraction grating is formed in the cavity direction was also evaluated. In the comparative example, the noise of the measurement waveform was relatively high at 0.8 dBp-p, and the measurement sensitivity was insufficient. This is because a general DFB laser has high single-axis mode characteristics and strong coherence, so that noise generation due to multiple reflection in the fiber increases. Another reason is that the DFB laser itself is sensitive to reflection resistance.

  As described above, by configuring the OTDR measurement apparatus using the DFB laser 100 or 200, it is possible to reduce noise components included in the measurement waveform of the OTDR measurement apparatus.

  Note that the present invention is not limited to the above-described embodiment, and can be changed as appropriate without departing from the spirit of the present invention. For example, in the above-described embodiment, the DFB lasers 100 and 200 are described as being composed of InP, but this is merely an example. For example, a DFB laser can be configured using another compound semiconductor mixed crystal such as GaAs or GaN.

  The semiconductor stack structure in the DFB lasers 100 and 200 is merely an example. For example, it goes without saying that other layers applied to the semiconductor laser, such as other cladding layers and current confinement layers, can be added as necessary.

  Although the DFB lasers 100 and 200 have been described as having a BH structure in which a mesa is embedded with a semi-insulating material, this is merely an example. For example, other semiconductor laser structures such as a ridge structure can be applied.

  Further, the conductivity type of each semiconductor layer of the semiconductor stacked structure in the DFB lasers 100 and 200 is merely an example. That is, a configuration in which the p-type and the n-type are appropriately exchanged is also possible.

  As mentioned above, the invention made by the present inventor has been specifically described based on the embodiments. However, the present invention is not limited to the embodiments already described, and various modifications can be made without departing from the scope of the invention. It goes without saying that it is possible.

1 n-type semiconductor substrate 3 n-type guide layer 4 active layer 5 p-type cladding layer 6 p-type contact layer 7 p-electrode 8 n-electrode 9 mesa 10 buried layer 11 reflective film 12 reflective films 21 and 22 diffraction grating 21a Short region 21b Long diffraction grating region 23 Region where diffraction grating 22 is not formed 31 Semiconductor laser module 32 Optical circulator 33 Light receiving module 34 Amplifier 35 Signal processing unit 36 Optical fiber 37 Inspection light 38 Reflected light 100, 200 DFB laser 300 OTDR measuring device

Claims (11)

A semiconductor laminated structure;
A first reflective film provided on a first end face in the waveguide direction of the semiconductor multilayer structure and having a reflectance of 3% or more and 6% or less with respect to the guided light;
A second end surface of the semiconductor multilayer structure opposite to the first end surface in the waveguide direction and having a higher reflectance than the first reflective film with respect to the guided light; Two reflective films,
The semiconductor laminated structure is
An active layer formed on top of a semiconductor substrate;
A first semiconductor layer stacked adjacent to the active layer on the semiconductor substrate;
A second semiconductor layer stacked adjacent to the first semiconductor layer;
A first diffraction grating formed on a part of the first end face side of the boundary surface between the first semiconductor layer and the second semiconductor layer,
Distributed feedback semiconductor laser. A second diffraction formed at a part of the boundary surface between the first semiconductor layer and the second semiconductor layer on the second end face side and having a grating period longer than that of the first diffraction grating; Further comprising a lattice,
The distributed feedback semiconductor laser according to claim 1. The first diffraction grating and the second diffraction grating are formed continuously.
The distributed feedback semiconductor laser according to claim 2. No diffraction grating is formed on the boundary surface between the first semiconductor layer and the second semiconductor layer other than the region where the first diffraction grating is formed,
The distributed feedback semiconductor laser according to claim 1. The first diffraction grating is:
The first end face is formed in a region of 1/3 or less of the length of the resonator between the first end face and the second end face.
The distributed feedback semiconductor laser according to claim 4. The first diffraction grating is:
The first end face is formed in a region of ¼ of the length of the resonator between the first end face and the second end face.
The distributed feedback semiconductor laser according to claim 5. The reflectance of the first reflective film with respect to the guided light is 4%.
The distributed feedback semiconductor laser according to claim 1. The reflectance of the second reflective film with respect to the guided light is 90%.
The distributed feedback semiconductor laser according to claim 1. The second semiconductor layer, the semiconductor substrate;
The first semiconductor layer is formed on the second semiconductor layer;
The active layer is formed on the first semiconductor layer;
The distributed feedback semiconductor laser according to claim 1. A cladding layer formed on the active layer;
A contact layer formed on the cladding layer;
A first electrode formed on the contact layer;
A surface of the semiconductor substrate on which the second semiconductor layer is formed, and a second electrode formed on the opposite surface.
The distributed feedback semiconductor laser according to claim 9. The distributed feedback semiconductor laser according to claim 1 is used as a light source.
OTDR measuring device.

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