JP2004356571A - Distributed feedback semiconductor laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a distributed feedback (DFB) semiconductor laser device which is not affected by reflected return light and includes a high single-wavelength oscillation probability. <P>SOLUTION: The distributed feedback semiconductor laser device comprises a substrate 1 and a multilayered structure formed on the substrate, the multilayered structure includes a first diffraction grating 2 which includes a light transmission area including at least an active layer 5 and a periodic structure in the resonant direction of the light transmission area and is formed approximately all over the light transmission area, and a second diffraction grating 9 which includes a periodic structure in the resonant direction of the light transmission area and is formed near the end face of the light transmission area for emitting laser light, and the first refraction grating 2 includes a phase shift part 18 in which phases of the periodic structure are partially discontinuous. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a distributed feedback semiconductor laser device.
[0002]
[Prior art]
2. Description of the Related Art Distributed feedback semiconductor laser devices (DFB laser devices) are widely used as light sources for long-distance optical fiber communication because they can oscillate at a single wavelength due to a diffraction grating formed therein.
[0003]
Here, in a general DFB laser device, the emitted laser light may be reflected at an external reflection point and enter the device as return light. As described above, when the return light is incident on the DFB laser device, the oscillation state of the laser light changes under the influence of the return light, and noise occurs in the DFB laser device.
[0004]
For this reason, usually, an optical isolator is inserted between the DFB laser device and the optical fiber in order to prevent the return light from entering the DFB laser device. The optical isolator has a property of transmitting only light traveling in the direction of emission from the DFB laser device and not transmitting light traveling in the direction of incidence on the DFB laser device. This prevents return light from being incident on the DFB laser device.
[0005]
However, when the above-mentioned optical isolator is applied, there is a problem that the cost of the optical module in which the DFB laser device and the optical fiber are mounted is increased correspondingly. Therefore, if a DFB laser device that hardly causes an increase in noise even when return light is incident, that is, a DFB laser device with strong reflection return light resistance can be realized, a low-cost optical module that does not require an optical isolator can be realized. become.
[0006]
In order to realize the above-described optical module, there is a DFB laser device in which a diffraction grating is provided only near an end surface from which laser light is emitted (hereinafter, referred to as a front end surface). Hereinafter, the DFB laser device will be described with reference to the drawings. FIG. 6 is a diagram showing a cross-sectional structure of the DFB laser device in a direction along a resonance direction of laser light.
[0007]
The DFB laser device shown in FIG. 6 includes a semiconductor substrate 101, a diffraction grating 102, a light guide layer 103, an active layer 104, a cladding layer 105, an anti-reflection (AR) coating film 106, and a high reflection (HR) coating film 107.
[0008]
First, an n-type semiconductor In (indium) P (phosphorus) substrate is formed on the semiconductor substrate 101. A diffraction grating 102 is formed in the vicinity of an end face (hereinafter, referred to as a front end face) of the semiconductor substrate 101 on the side where the laser is emitted. The diffraction grating 102 plays a role of causing a DFB laser device to oscillate laser light having a wavelength corresponding to the period. Further, since the diffraction grating 102 is formed near the front end face, the reflectance of the front end face with respect to the laser light incident on the DFB laser device from the outside is equivalently increased.
[0009]
Next, an optical guide layer 103, an active layer 104, and a cladding layer 105 are sequentially formed on the semiconductor substrate 101 on which the diffraction grating 102 is formed. The light guide layer 103 is an n-type semiconductor made of InGa (gallium) As (arsenic) P. The active layer 104 is formed of a semiconductor material having a smaller band gap than InP. The cladding layer 105 is formed of an InP p-type semiconductor. As described above, the semiconductor substrate 101, which is an InP n-type semiconductor having a bandgap larger than the active layer 104, and the cladding layer 105, which is an InP p-type semiconductor having a bandgap larger than the active layer 104, are activated. By sandwiching the layer 104, the DFB laser device has a so-called double hetero (DH) structure.
[0010]
An anti-reflection (AR) coating film 106 is formed on the front end face of the DFB laser device. Further, a high-reflection (HR) coating film 107 is formed on the opposite side (hereinafter referred to as a rear end face) of the DFB laser device from which the laser beam is emitted. The HR coating film 107 plays a role of reflecting the laser light generated in the DFB laser device toward the front end face and generating a standing wave of the oscillation laser light with the diffraction grating 102.
[0011]
The operation of the conventional DFB laser device configured as described above will be briefly described. First, the DFB laser device shown in FIG. 6 emits laser light from the front end face thereof due to resonance generated between the diffraction grating 102 and the HR coating film 107. Part of the laser light emitted from the DFB laser device is reflected at an external reflection point and enters the DFB laser device.
[0012]
On the other hand, a diffraction grating 102 is provided near the front end face of the DFB laser device. As described above, by providing the diffraction grating 102, the reflectance of the front end face with respect to the laser light is equivalently increased. Therefore, the return light is reflected on the front end face. As a result, the return light is suppressed from entering the inside of the DFB laser device, and the generation of noise in the DFB laser device is suppressed (for example, see Patent Document 1 or Non-Patent Document 1).
[0013]
[Patent Document 1]
JP-A-6-310806
[Non-patent document 1]
Huang et al., "Diffraction grating length dependence of PC-LD resistance to reflection return light", IEICE General Conference Program, 1998, p. 405
[0014]
[Problems to be solved by the invention]
However, since the above-mentioned conventional DFB laser device has the HR coating film 107, it has a problem that it is difficult to oscillate at a single wavelength (hereinafter, this is referred to as a low single-wavelength oscillation probability). The details will be described below.
[0015]
As shown in FIG. 7, in the DFB laser device including the diffraction grating 201 and the HR coating film 202 formed on the entire surface in the resonator length direction, the phase of the diffraction grating 201 in the portion 204 that contacts the HR coating film 202 changes. Then, the mode gain of the laser light generated in the DFB laser device changes. Then, depending on how the mode gain of the laser light changes, the DFB laser device cannot oscillate laser light of a single wavelength. Such a problem also exists in a DFB laser device in which the diffraction grating 102 is not in contact with the HR coating film 107 as shown in FIG. Specifically, the gain of the laser light generated by the DFB laser device is determined by the HR coating of the extended diffraction grating 102 when the diffraction grating 102 is extended to the HR coating film 107 of the DFB laser device. It depends on the phase in the film 107.
[0016]
Here, the period of the diffraction grating 102 is a very small value of about 200 nm. Therefore, it is very difficult to cleave the DFB laser device so that the phase of the diffraction grating 102 in the HR coating film 107 is always constant. Therefore, when the DFB laser device is manufactured, the phase of the diffraction grating 102 in the HR coating film 107 varies, and the gain of the laser light in the DFB laser device shown in FIG. 6 varies. As a result, the DFB laser device cannot always emit a single-wavelength laser beam.
[0017]
SUMMARY OF THE INVENTION It is an object of the present invention to provide a distributed feedback (DFB) semiconductor laser device that is not affected by reflected return light and has a high single-wavelength oscillation probability.
[0018]
Means for Solving the Problems and Effects of the Invention
A distributed feedback semiconductor laser device according to the present invention includes a substrate, and a multilayer structure formed on the substrate. The multilayer structure includes at least an optical waveguide region having an active layer, and a periodic structure in a resonance direction of the optical waveguide region. And a first diffraction grating formed on substantially the entire surface of the optical waveguide region, and an end surface having a periodic structure in a resonance direction of the optical waveguide region and emitting laser light from the optical waveguide region. A second diffraction grating formed in the vicinity, and the first diffraction grating has a phase shift portion in which the phase of the periodic structure is partially discontinuous. Since the second diffraction grating is formed near the end face of the optical waveguide region that outputs the laser light, the coupling coefficient near the end face that outputs the laser light increases. As a result, even if the reflected return light is input to the end face that outputs the laser light, the reflected return light is reflected on the end face, and the reflection return light resistance of the distributed feedback semiconductor laser device is increased. In addition, since the first diffraction grating is provided with the phase shift portion, the laser light of the Bragg wavelength easily satisfies the resonance condition in the device. As a result, the distributed feedback semiconductor laser device can oscillate laser light of a single wavelength with high probability.
[0019]
It is desirable that the amount of phase shift in the phase shift unit is an amount corresponding to a quarter of the oscillation wavelength with respect to the reference period of the first diffraction grating. With such a configuration of the phase shift unit, the Bragg wavelength laser light more easily satisfies the resonance condition, so that the distributed feedback semiconductor laser device oscillates a single wavelength laser light with a higher probability. Becomes possible.
[0020]
It is desirable that the first diffraction grating and the second diffraction grating are formed so as to sandwich the active layer.
[0021]
Further, it is desirable that the period of the first diffraction grating and the period of the second diffraction grating are substantially the same. The first diffraction grating plays a role of selecting the wavelength of the oscillating laser light, and the second diffraction grating selects the wavelength of the reflected laser light. Here, the wavelength of the laser beam input as reflected return light to the distributed feedback semiconductor laser device and the wavelength of the laser beam oscillated from the distributed feedback semiconductor laser device are substantially the same. Therefore, when the laser light oscillated by the own device is input as reflected return light, the reflected return light can be reflected at the end face.
[0022]
Further, it is desirable that the phase of the first diffraction grating and the phase of the second diffraction grating are different from each other. This is because if the phases of the first diffraction grating and the second diffraction grating match, the oscillation wavelength of the distributed feedback semiconductor laser device is less likely to be a single wavelength. Specifically, when the distributed feedback semiconductor laser device is started, resonance occurs in the first diffraction grating and also occurs in the second diffraction grating. In this case, if the second diffraction grating is formed so as to be in phase with the first diffraction grating, the first diffraction grating and the second diffraction grating return light near the front end face. In regions other than the vicinity of the front end face, light feedback occurs only by the first diffraction grating. Therefore, the feedback amount of light on the front end face side becomes larger than the feedback amount of light on the rear end face side. As a result, laser light generated by the first diffraction grating is prevented from being output at a single wavelength.
[0023]
The active layer is preferably made of InGaAsP, and the layer sandwiching the substrate and the active layer is preferably made of InP.
[0024]
Note that a distributed feedback semiconductor laser device according to another aspect of the present invention includes a substrate, and a multilayer structure formed on the substrate. The multilayer structure includes at least an optical waveguide region having an active layer, and an optical waveguide region. A diffraction grating that has a periodic structure in the resonance direction, and is formed on substantially the entire surface of the optical waveguide region. The diffraction grating is located near an end surface of the optical waveguide region that emits laser light. A first region having a coupling coefficient, a second region having a coupling coefficient smaller than the first coupling coefficient, and a phase shift portion in which the phase of the periodic structure is partially discontinuous are partially provided. . Even with such a configuration, it is possible to provide a distributed feedback semiconductor device having high resistance to reflected return light and having a high single-wavelength oscillation probability.
[0025]
The diffraction grating may further include a third region which is located near an end surface of the optical waveguide region facing the laser light emitting end surface and has a coupling coefficient substantially equal to the first coupling coefficient. . By adopting such a configuration, the feedback amount of the laser light from the front end surface side and the feedback amount of the laser light from the rear end surface side can be made equal. As a result, the single-wavelength oscillation probability of the distributed feedback semiconductor laser device is further improved.
[0026]
Further, it is desirable that the coupling coefficient of the diffraction grating changes smoothly at the boundary of each region.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, components having substantially the same function are denoted by the same reference numeral for the sake of brevity. Note that the present invention is not limited to the following embodiments.
[0028]
(1st Embodiment)
Hereinafter, a distributed feedback semiconductor (DFB) laser device according to a first embodiment of the present invention will be described with reference to the drawings. The DFB laser device according to the present embodiment can emit laser light of a single wavelength with a high probability and has high resistance to reflected return light. FIG. 1 is a diagram showing a structure of a DFB laser device according to the present embodiment. Specifically, FIG. 1A is a vertical cross-sectional view of the DFB laser device in a cavity length direction. FIG. 1B is a cross-sectional view of the DFB laser device taken along a cavity length direction. The oscillation wavelength of the semiconductor laser device shown in FIG. 1 is around 1.3 μm used for a laser for optical communication. The resonator length is 300 μm.
[0029]
The semiconductor laser device shown in FIG. 1 includes a substrate 1, a first diffraction grating layer 2, an n-type buffer layer 3, a first optical confinement layer 4, an active layer 5, a second optical confinement layer 6, and a first p-type confinement layer. -Type cladding layer 7, second p-type cladding layer 8, second diffraction grating layer 9, p-type current blocking layer 10, n-type current blocking layer 11, p-type buried layer 12, p-type contact layer 13, n-type An electrode 14, an insulating film 15, a p-type electrode 16, a non-reflective coating film 17 and a λ / 4 phase shift unit 18 are provided.
[0030]
The substrate 1 is an n-type semiconductor substrate made of In (indium) P (phosphorus). On the substrate 1, a stripe structure (3 to 7) extending in the resonator length direction is formed. The stripe structure includes an n-type buffer layer 3, a first optical confinement layer 4, an active layer 5, a second optical confinement layer 6, and a first p-type cladding layer 7 in this order from the bottom of FIG. Are laminated. On both sides of the stripe structure, a p-type current block layer 10 and an n-type current block layer 11 are formed. Further, a second p-type cladding layer 8 is formed on the stripe structure, and a p-type buried layer 12 and the like are formed thereon. Hereinafter, the DFB laser device according to the present embodiment will be described in more detail.
[0031]
First, an n-type buffer layer 3 is formed on a substrate 1. The n-type buffer layer 3 is an n-type semiconductor made of InP and has a thickness of about 100 nm. The first diffraction grating layer 2 is formed between the substrate 1 and the n-type buffer layer 3. The first diffraction grating layer 2 is made of InGa (gallium) As (arsenic) P and is formed to have a thickness of about 70 nm, a λg of about 1.2 μm, and a period of about 203 nm. The first diffraction grating layer 2 plays a role in selecting the wavelength of the laser light oscillated by the DFB laser device. A λ / 4 phase shift unit 18 is formed near the center of the first diffraction grating layer 2 as shown in FIG. The λ / 4 phase shift unit 18 is one in which the phase of the periodic structure is partially discontinuous, and the amount of phase shift is ４ of the oscillation wavelength with respect to the reference period of the first diffraction grating. And plays a role in causing oscillation at the Bragg wavelength.
[0032]
On the n-type buffer layer 3, a first optical confinement layer 4 is formed. The first optical confinement layer 4 is made of InGaAsP, and is formed so that λg is about 1.05 μm and the thickness is 60 nm. An active layer 5 is formed on the first light confinement layer 4. The active layer 5 is a multiple quantum well active layer, and includes 10 pairs of well layers and barrier layers. The well layer is an InGaAsP well layer having a thickness of about 6 nm in which a compressive strain is introduced in a range of about 0.6%, and the barrier layer has a thickness of about 10 nm in which a strain is not intentionally introduced and a λg of about 10 nm. It is an InGaAsP barrier layer of about 1.05 μm.
[0033]
On the active layer 5, a second optical confinement layer 6 is formed. The second optical confinement layer 6 is made of InGaAsP, and is formed so that λg is about 1.05 μm and the thickness is about 60 nm. On the second optical confinement layer 6, a first p-type cladding layer 7 is formed. The first p-type cladding layer 7 is a p-type semiconductor made of InP, and is formed to have a thickness of about 100 nm.
[0034]
A second p-type cladding layer 8 is formed on the stripe structure. The p-type cladding layer 8 is made of InP and formed to have a thickness of about 300 nm. Thus, the DH structure is formed by sandwiching the active layer 5 between the n-type buffer layer 3 and the first p-type clad layer 7 made of a material having a band gap larger than that of the active layer 5.
[0035]
In addition, at the boundary between the first p-type cladding layer 7 and the second p-type cladding layer 8, which are the uppermost layers of the stripe structure, and about 50 μm from the front end face of the DFB laser device, FIG. As shown in (b), a second diffraction grating layer 9 is formed. The second diffraction grating layer 9 is made of InGaAsP and is formed to have a thickness of about 100 nm, a λg of about 1.2 μm, and a period of about 203 nm.
[0036]
Here, the second diffraction grating layer 9 will be described in detail. The second diffraction grating layer 9 increases the coupling coefficient near the front end face of the DFB laser device, equivalently increases the reflectance at the front end face, and reflects the laser light input from the outside. Fulfill. Since the laser light input from the outside to the front end face is a reflection return light of the laser light output from the DFB laser device, the laser light has substantially the same wavelength as the output laser light. Therefore, the period of the second diffraction grating layer 9 is the same or substantially the same as the period of the first diffraction grating layer 2 provided for generating the output laser light. Note that the substantially same width here is about the width of the spectral line width of the oscillation wavelength of the DFB laser device.
[0037]
Further, it is preferable that the phase of the second diffraction grating layer 9 does not coincide with that of the first diffraction grating layer 2. This is because if the phases of the first diffraction grating layer 2 and the second diffraction grating layer 9 match, the oscillation wavelength of the DFB laser device is unlikely to be a single wavelength. Specifically, when the DFB laser device is started, resonance occurs in the first diffraction grating layer 2 and also occurs in the second diffraction grating layer 9. In this case, if the second diffraction grating layer 9 is formed so as to be in phase with the first diffraction grating layer 2, the first diffraction grating layer 2 and the second diffraction grating layer near the front end face. 9 causes light feedback, and in a region other than the vicinity of the front end surface, light feedback occurs only by the first diffraction grating layer 2. Therefore, the feedback amount of light on the front end face side becomes larger than the feedback amount of light on the rear end face side. As a result, the laser light generated in the first diffraction grating layer 2 is prevented from being output at a single wavelength.
[0038]
On both sides of the stripe structure and the second p-type cladding layer 8, a p-type current blocking layer 10 and an n-type current blocking layer 11 are formed. The p-type current block layer 10 is a p-type semiconductor made of InP. Further, the n-type current block layer 11 is an n-type semiconductor made of InP. The p-type current blocking layer 10 and the n-type current blocking layer 11 play a role of confining current in the active layer 5.
[0039]
A p-type buried layer 12 is formed on the n-type current block layer 11 and the second p-type cladding layer 8, and a p-type contact layer 13 is formed on the p-type buried layer 12. The p-type buried layer 12 is made of InP and is a p-type semiconductor. The p-type contact layer 13 is made of InGaAs and is a p-type semiconductor.
[0040]
An insulating film 15 is formed on the p-type contact layer 13. The insulating film 15 is formed of a silicon oxide film, and has an opening in a region above the stripe structure. Then, a p-side electrode 16 made of a Ti / Pt / Au alloy is formed in the window. The p-side electrode 16 is in contact with the p-type contact layer 13 through the window. On the other hand, below the substrate 1, an n-type electrode 14 made of an Au / Sn alloy is formed.
[0041]
Further, a non-reflective coating film 17 having a reflectance of 1% or less is formed on each of the front end face and the rear end face of the DFB laser device according to the present embodiment. This is because, when the reflectance exceeds 1%, a problem of a variation in the phase of the diffraction grating between the front end face and the rear end face occurs, and the single wavelength oscillation probability is significantly reduced.
[0042]
Hereinafter, the single-wavelength oscillation probability and the reflected return light resistance of the DFB laser device according to the present embodiment will be described with reference to the drawings. FIG. 2 is a diagram illustrating an effect of the DFB laser device according to the present embodiment.
[0043]
First, the single-wavelength oscillation probability will be described. In the DFB laser device according to the present embodiment, since the non-reflection coating films 17 are formed on both end surfaces, there is no high reflection coating film. Therefore, in the DFB laser device according to the present embodiment, the problem that the phase of the diffraction grating in the high reflection coating film fluctuates and the oscillation of the laser light of a single wavelength is prevented is eliminated. Further, the DFB laser device according to the present embodiment includes a λ / 4 phase shift unit 18. The λ / 4 phase shift section 18 plays a role of satisfying the resonance condition of the Bragg wavelength laser light in the DFB laser device. Therefore, the DFB laser device according to the present embodiment can oscillate a single-wavelength laser beam at the Bragg wavelength with a high probability.
[0044]
Next, the reflected return light resistance will be described. As shown in FIG. 2, emitted light is output from the front end face of the DFB laser device. A part of the emitted light is reflected at a reflection end outside the DFB laser device, and is input to the front end face of the DFB laser device as reflected return light. Here, a second diffraction grating layer 9 is formed near the front end face of the DFB laser device. Since the second diffraction grating layer 9 is formed near the front end face, it plays a role of equivalently increasing the reflectance at the front end face. Therefore, the reflected return light input to the front end face is reflected as shown in FIG. 2, and does not enter the inside of the DFB laser device. As a result, the DFB laser device can have high reflection return resistance, and the occurrence of noise in the DFB laser device can be prevented.
[0045]
Here, a conventional DFB laser device and the DFB laser device according to the present embodiment will be compared. As a conventional DFB laser device, in addition to the diffraction grating formed only in the vicinity of the front end face described in the related art, a diffraction grating is formed on the entire surface in the cavity length direction, and furthermore, at a substantially central portion of the diffraction grating. There is a phase shift DFB laser device provided with a λ / 4 phase shift unit. Since the phase shift DFB laser device is provided with the λ / 4 phase shift unit, it can oscillate laser light of a single wavelength with high probability.
[0046]
However, the phase-shift DFB laser device has a lower effective reflectance at the front end face than the DFB laser device according to the present embodiment. Therefore, the reflected return light reflected from the reflection point enters the DFB laser device and generates noise in the DFB laser device.
[0047]
On the other hand, according to the DFB laser device according to the present embodiment, since the reflectance of the front end surface is relatively high, the reflected return light is reflected by the front end surface and is input into the DFB laser device. Nothing. As a result, generation of noise in the DFB laser device is suppressed. Specifically, the DFB laser device according to the present embodiment can reduce the relative intensity noise by about 10 dB as compared with the conventional phase shift DFB laser device.
[0048]
Although the embedded type is used for the first diffraction grating layer 2 and the second diffraction grating layer 9, other configurations may be employed.
[0049]
Further, in the present embodiment, the first diffraction grating layer 2 is formed on the entire surface of the substrate 1 in the resonator length direction, but the first diffraction grating layer 2 is not necessarily formed on the entire surface. There is no. Specifically, there may be areas near the front end face and near the rear end face where the first diffraction grating layer 2 is not formed. However, also in this case, the first diffraction grating layer 2 needs to be formed to have a symmetric length in the front-rear direction when viewed from the center in the cavity length direction of the DFB laser device.
[0050]
In the present embodiment, the first diffraction grating layer 2 and the second diffraction grating layer 9 are formed, but the diffraction grating layers to be formed are not limited to this. That is, another diffraction grating layer other than these diffraction grating layers may be formed.
[0051]
In the present embodiment, the first diffraction grating layer 2 and the second diffraction grating layer 9 are arranged so as to sandwich the active layer 5, but the positional relationship between these diffraction grating layers is not limited to this. . For example, the first diffraction grating layer 2 and the second diffraction grating layer 9 may both be formed in the n-type buffer layer 3 so as to be vertically parallel, or both may be formed in the first p-type cladding layer 7 and the first p-type cladding layer 7. It may be formed in the second p-type cladding layer 8 in parallel up and down.
[0052]
(Second embodiment)
Hereinafter, a DFB laser device according to a second embodiment of the present invention will be described with reference to the drawings. The DFB laser device according to the present embodiment has a difference in the diffraction grating layer from the DFB laser device according to the first embodiment. Specifically, in the first embodiment, the first diffraction grating layer 2 and the second diffraction grating layer 9 are provided, but in the present embodiment, as shown in FIG. There is one grid. Except for the arrangement of the diffraction grating, the configuration is the same as that of the first embodiment. Here, FIG. 3 is a diagram showing the structure of the DFB laser device according to the present embodiment. Specifically, FIG. 3A is a cross-sectional view perpendicular to the cavity length direction of the DFB laser device. FIG. 3B is a cross-sectional view of the DFB laser device taken along a cavity length direction. The oscillation wavelength of the semiconductor laser device shown in FIG. 3 is around 1.3 μm used for a laser for optical communication. The resonator length is 300 μm.
[0053]
The DFB laser device according to this embodiment includes a substrate 1, an n-type buffer layer 3, a first optical confinement layer 4, an active layer 5, a second optical confinement layer 6, a p-type cladding layer 28, and a p-type current blocking layer. 10, n-type current block layer 11, p-type buried layer 12, p-type contact layer 13, n-type electrode 14, insulating film 15, p-type electrode 16, non-reflective coating film 17, λ / 4 phase shift portion 18, and diffraction A grating layer 20 is provided.
[0054]
Here, other than the p-type cladding layer 28 and the diffraction grating layer 20 are the same as those of the first embodiment, and therefore the description is omitted. Therefore, hereinafter, the p-type cladding layer 28 and the diffraction grating layer 20, which are the differences between the present embodiment and the first embodiment, will be described in detail with reference to FIG.
[0055]
First, the p-type cladding layer 28 is a p-type semiconductor film having a combined thickness of the first p-type cladding layer 7 and the second p-type cladding layer 8 in FIG.
[0056]
The diffraction grating layer 20 is made of InGaAsP and is formed on the entire surface of the substrate 1 in the resonator length direction such that λg is about 1.2 μm and the period is about 203 m. As shown in FIG. 3B, the diffraction grating layer 20 has two regions A and B having different thicknesses. The region A is formed in a region of about 50 μm from the front end face, and has a layer thickness of about 100 nm. Further, the region B is formed from the terminal end to the rear end surface of the region A, and has a layer thickness of about 70 nm. At the boundary between the regions A and B, the layer thickness changes smoothly (not shown in the figure). It should be noted that a λ / 4 phase shift unit 18 is provided substantially at the center of the diffraction grating layer 20, as in the first embodiment.
[0057]
The single-wavelength oscillation probability and the reflected return light resistance of the DFB laser device according to the present embodiment configured as described above will be described with reference to the drawings. FIG. 4 is a diagram illustrating an effect of the DFB laser device according to the present embodiment.
[0058]
First, the single-wavelength oscillation probability will be described. In the DFB laser device according to the present embodiment, the antireflection coating films 17 are formed on both end surfaces, and the λ / 4 phase shift unit 18 is provided. Thus, it becomes possible to oscillate a single-wavelength laser beam at the Bragg wavelength.
[0059]
Next, the reflected return light resistance will be described. As shown in FIG. 4, emitted light is output from the front end face of the DFB laser device. Part of the emitted light is reflected at a reflection point outside the DFB laser device, and is input as reflected return light to the front end face of the DFB laser device.
[0060]
Here, a region A having a large layer thickness exists near the front end face of the DFB laser device. As described above, by increasing the thickness of the diffraction grating layer 20 near the front end face, the coupling coefficient in the region A can be increased. When the coupling coefficient increases near the front end face, it can be considered that the reflectance at the front end face is equivalently increased. Therefore, the DFB laser device according to the present embodiment can reflect the input reflected return light at the front end face, as in the first embodiment. As a result, the reflection feedback light resistance of the DFB laser device is improved, and generation of noise is prevented. Specifically, the DFB laser device according to the present embodiment can reduce relative intensity noise by about 10 dB as compared with the conventional phase shift DFB laser device, similarly to the DFB laser device according to the first embodiment.
[0061]
Further, in the DFB laser device according to the present embodiment, only one diffraction grating is formed, and therefore, compared with the DFB laser device in which a plurality of diffraction gratings are formed as in the DFB laser device according to the first embodiment, The number of steps for masking the diffraction grating can be reduced. Here, since the masking process of the diffraction grating is a complicated process, the DFB laser device can be easily manufactured by reducing the number of processes in such a process.
[0062]
Here, in order to improve the reflection effect of the return reflected light in the phase shift DFB laser device, it is conceivable that the layer thickness of all the diffraction gratings is the same as the region A (about 100 nm). According to such a method, it is possible to obtain the same effect of reflecting return light as in the present embodiment.
[0063]
However, in the above-described phase shift DFB laser device, the coupling coefficient in the entire resonator becomes too large, and the amount of light feedback inside the DFB laser device becomes excessive. As a result, the light intensity at the central portion in the cavity length direction increases remarkably, a phenomenon called axial hole burning occurs, and there is a problem that the probability of single wavelength oscillation decreases.
[0064]
On the other hand, in the DFB laser device according to the present embodiment, the coupling coefficient is such that axial hole burning does not occur in the region C occupying 50% or more of the cavity length. Therefore, the DFB laser device according to the present embodiment can have high resistance to reflected return light while maintaining a high single-wavelength oscillation probability.
[0065]
In this embodiment, as shown in FIG. 3, the diffraction grating layer has the regions A and B, but the shape of the diffraction grating layer is not limited to this. Specifically, as shown in FIG. 5, the diffraction grating layer may have a region C in addition to the regions A and B. Hereinafter, the DFB laser device shown in FIG. 5 will be described.
[0066]
The DFB laser device shown in FIG. 5 has a difference in that the diffraction grating layer of the DFB laser device shown in FIG. 3 is divided into three regions. Except that the diffraction grating layer is divided into three regions, the DFB laser device shown in FIG. 5 is the same as the laser device shown in FIG.
[0067]
Here, the diffraction grating layer 21 of the DFB laser device shown in FIG. 5 includes regions A, B, and C. The region A is formed in a region of about 50 μm from the front end face, and has a layer thickness of about 100 nm. The region C is formed in a region of about 50 μm from the rear end face, and has a layer thickness of about 100 nm. The region B is provided between the region A and the region C, and has a layer thickness of about 70 nm.
[0068]
As described above, since the region A and the region C are provided symmetrically with respect to the center of the DFB laser device, the feedback amount of the laser light from the front end surface side and the feedback amount of the laser light from the rear end surface side are reduced. Can be made equal. As a result, the single-wavelength oscillation probability of the DFB laser device is further improved.
[0069]
The oscillation wavelength of the DFB laser device according to the first and second embodiments is in the 1.3 μm band, but may be in the 1.55 μm band or another oscillation wavelength.
[0070]
In the second embodiment, the diffraction grating layers 20 and 21 are of an embedded type, but other configurations may be employed.
[0071]
In the first and second embodiments, the semiconductor laser device has a so-called buried hetero (BH) structure, but may have another structure such as a ridge structure.
[Brief description of the drawings]
FIG. 1 is a diagram showing a cross-sectional structure of a DFB laser device according to a first embodiment of the present invention.
FIG. 2 is a diagram for explaining an effect of the DFB laser device according to the first embodiment of the present invention.
FIG. 3 is a diagram showing a cross-sectional structure of a DFB laser device according to a second embodiment of the present invention.
FIG. 4 is a diagram for explaining an effect of a DFB laser device according to a second embodiment of the present invention.
FIG. 5 is a diagram showing a cross-sectional structure of a DFB laser device according to another example of the second embodiment of the present invention.
FIG. 6 is a diagram showing a cross-sectional structure of a conventional DFB laser device.
FIG. 7 is a diagram showing a cross-sectional structure of a conventional DFB laser device.
[Explanation of symbols]
1 substrate
2 First diffraction grating layer
3 n-type buffer layer
4 First light confinement layer
5 Active layer
6 Second light confinement layer
7. First p-type cladding layer
8 Second p-type cladding layer
9 Second diffraction grating layer
10 p-type current block layer
11 n-type current block layer
12 p-type buried layer
13 p-type contact layer
14 n-type electrode
15 Insulating film
16 p-type electrode
17 Non-reflective coating film
18 λ / 4 phase shift unit
20, 21 Diffraction grating layer
28 p-type cladding layer

Claims (9)

Board and
Comprising a multilayer structure formed on the substrate,
The multilayer structure is
An optical waveguide region having at least an active layer,
A first diffraction grating having a periodic structure in the resonance direction of the optical waveguide region, and formed on substantially the entire surface of the optical waveguide region;
A second diffraction grating that has a periodic structure in the resonance direction of the optical waveguide region, and is formed near an end face of the optical waveguide region that emits laser light;
The distributed feedback semiconductor laser device, wherein the first diffraction grating has a phase shift portion in which the phase of the periodic structure is partially discontinuous. 2. The distributed feedback semiconductor according to claim 1, wherein the amount of phase shift in the phase shift unit is an amount corresponding to a quarter of an oscillation wavelength with respect to a reference period of the first diffraction grating. Laser device. 2. The distributed feedback semiconductor laser device according to claim 1, wherein the first diffraction grating and the second diffraction grating are formed so as to sandwich the active layer. 2. The distributed feedback semiconductor laser device according to claim 1, wherein a period of the first diffraction grating and a period of the second diffraction grating are substantially the same. 5. The distributed feedback semiconductor laser device according to claim 4, wherein a phase of said first diffraction grating and a phase of said second diffraction grating are different from each other. The active layer is made of InGaAsP;
The distributed feedback semiconductor laser device according to claim 1, wherein the layer sandwiching the substrate and the active layer is made of InP. Board and
Comprising a multilayer structure formed on the substrate,
The multilayer structure is
An optical waveguide region having at least an active layer,
Having a periodic structure in the resonance direction of the optical waveguide region, including a diffraction grating formed on substantially the entire surface of the optical waveguide region,
The diffraction grating includes:
A first region having a first coupling coefficient and located near an end surface of the optical waveguide region that emits laser light;
A second region having a coupling coefficient smaller than the first coupling coefficient;
A distributed feedback semiconductor laser device having a phase shift portion in which the phase of a periodic structure is partially discontinuous. The diffraction grating further includes a third region located near an end surface of the optical waveguide region facing the end surface from which the laser light is emitted, and having a coupling coefficient substantially equal to the first coupling coefficient. The distributed feedback semiconductor laser device according to claim 7, wherein: The distributed feedback semiconductor laser device according to claim 7, wherein a coupling coefficient of the diffraction grating changes smoothly at a boundary between the regions.

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