JP2003152272A - Dispersed phase shift structure distributed feedback semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dispersed phase shift structure distributed feedback semiconductor laser (DFB-LD) which operates stably in a single mode, is high in slope efficiency, and has a large production tolerance. SOLUTION: This distributed feedback semiconductor laser carrying out optical feedback by a diffraction grating is equipped with an element which is divided into three regions 1, 2, and 3 in the lengthwise direction. The region 1 is set shorter than the region 3, and the diffraction grating is shifted in phase by λ/8 between the regions 1 and 2 and between the regions 2 and 3 respectively. With this setup, an internal electric field is dispersed to phase shifts at two spots, the phase shifts reside on the forward side, and light intensity concentrates in the forward side with asymmetrical distribution, so that front end face slope efficiency is improved. A distance between two phase shift spots is kept constant independently of a positional deviation that occurs when the element is cleaved, so that the characteristics of an element have a large tolerance for the deviation of a cleavage position, and a DFB-LD operating stably in a single mode and high in slope efficiency can be realized with high yield.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser used in an optical transmission system, and more particularly to a phase shift distributed feedback semiconductor laser having high single mode oscillation stability. Further, it relates to a semiconductor laser array in which a plurality of these lasers are arranged in an array.

[0002]

2. Description of the Related Art A semiconductor laser used in a general optical transmission system is required to operate in a single mode, and a distributed feedback semiconductor laser (hereinafter, referred to as DFB) integrated with a diffraction grating is required.
-LD) is widely used. In particular, as a structure with a high single longitudinal mode yield, a λ / 4 phase shift DF in which the phase of the diffraction grating is shifted by a half period in the center of the laser resonator.
B-LD is used. The λ / 4 phase shift structure is a known structure and is described, for example, in “1994, Ohmsha, Ltd., edited by Applied Physics Society, semiconductor laser, page 272, FIG. 12.12.”. The λ / 4 phase shift DFB-LD is characterized in that a single longitudinal mode operation can be obtained with a high yield by applying antireflection (hereinafter referred to as AR) coating on both end surfaces.

On the other hand, in order to suppress the fluctuation of the refractive index inside the semiconductor laser resonator when the phase shift DFB-LD is directly modulated or when the reflected light from the outside is increased, the phase shift amount is conventionally set. Λ / 4 shift from λ / 8
Invention of shifted DFB-LD (Patent No. 318670)
No. 5) and the invention of DFB-LD with 3λ / 8 shift to improve the stability of the single longitudinal mode at the time of current injection (JP-A-63-032988).

[0004]

In the conventional λ / 4 phase shift DFB-LD, stable single mode characteristics can be obtained with a high yield, but the phase shift position is in the center of the element and both end faces are non-reflective. Because of the coated symmetrical structure, the light output emitted from both end faces is basically equal, and the slope efficiency per one end face is lower than that of the uniform diffraction grating structure without phase shift. Therefore, in order to obtain a sufficient light output from the front facet on the emission side, high reflection (HR) is applied to the rear facet.
"Asymmetrical reflection structure" with coating, λ / 4
An “asymmetrical λ / 4 phase shift structure” has been proposed in which the phase shift position is shifted from the center to the front (Japanese Patent Laid-Open No. 05-2005).
-145194). However, the asymmetric reflection structure has a problem that the single-mode stability changes greatly depending on the phase of light on the end face. In addition, in the asymmetrical λ / 4 phase shift structure, the single mode stability and the slope efficiency strongly depend on the position of the phase shift portion, so that in the process of cutting out the device by cleavage, the device characteristics vary depending on the cleavage position. There has been a problem that the tolerances have changed due to large changes. Therefore, high slope efficiency can be obtained in a stable single mode, and D with a large manufacturing tolerance.
FB-LD is required.

An object of the present invention is to provide a stable single mode with high slope efficiency and a large manufacturing tolerance.
It is to realize D.

[0006]

The phase-shift structure distributed feedback semiconductor laser of the present invention is a distributed feedback semiconductor laser having at least an active layer and a diffraction grating, and optical feedback is performed by the diffraction grating. From the side in the length direction, the region 1, the region 2, ..., The region n are divided into n (n is an integer of 3 or more) regions in this order, and the diffraction grating is provided at the boundary of each of the (n-1) regions. Of λ / b ₁ and λ / b, respectively
₂ , ... λ / b _{n- 1} are shifted by (b ₁ , b ₂ , ...
b _n-1 is an integer), the total amount of phase shift (λ / b ₁ + λ /
b ₂ + ... + λ / b _n-1 ) is (m + 1/4) λ (m is an integer).

Alternatively, in a distributed feedback semiconductor laser having at least an active layer and a diffraction grating and performing optical feedback by the diffraction grating, the device is arranged in the order of region 1, region 2 and region 3 in the length direction from the front end face side. Is divided into three regions, and the periodic structure of the diffraction grating is λ / b ₁ at the boundary between region 1 and region 2,
The phase shift is λ / b ₂ at the boundary between the regions 2 and 3 (b ₁ and b ₂ are integers), and the total phase shift amount (λ / b ₁
+ Λ / b ₂ ) is (m + 1/4) λ (m is an integer).

Alternatively, in a distributed feedback semiconductor laser having at least an active layer and a diffraction grating and performing optical feedback by the diffraction grating, the device is arranged in the order of region 1, region 2 and region 3 in the length direction from the front end face side. Is divided into three regions, and the periodic structure of the diffraction grating is λ / 8 phase-shifted at the boundaries between the regions 1 and 2, and the regions 2 and 3, and the total phase shift amount is λ / 4. It is characterized by being.

Alternatively, in a distributed feedback type semiconductor laser having at least an active layer and a diffraction grating and performing optical feedback by the diffraction grating, the element is lengthwise from the front end face side in a region 1, a region 2, ..., A region n. In the order of n (n is an integer of 3 or more), and the phase of the diffraction grating is λ / b ₁ and the region 2 at the boundary of each of the (n-1) regions.
And the region 3 has a λ / b ₂ phase shift at the boundary (b ₁ and b ₂ are integers), and the total phase shift amount (λ / b ₁
+ Λ / b ₂ ) is (m + 1/4) λ (m is an integer).

Alternatively, at least the active layer and the diffraction grating
Distributed feedback semiconductor that has an element and performs optical feedback by a diffraction grating
In the body laser, the element is located in the longitudinal direction from the front end face side.
1, area 2, ..., Area n in the order n (n is 3 or more
(N-1) boundaries of each area
The phase of the diffraction grating is λ / b₁, Λ / b
₂, ... λ / b_n-1It is shifting each (b₁, B₂, ...
b_n-1Is an integer), and the total amount of phase shift (λ / b₁+ Λ /
b₂+ ... + λ / b_n-1) Is (m + 3/8) λ
(M is an integer).

The distributed feedback semiconductor laser described above is
The length of region 1 is shorter than the length of region n, or
Preferably, the length of region 1 is shorter than the length of region 3,
It is also preferred that the sum of the lengths of region 1 and region 2 is equal to the length of region 3.

The distributed feedback semiconductor laser described above is
It is preferable to have a gain coupling type diffraction grating structure in which the optical gain distribution of the active layer changes in the same direction as the diffraction grating in the length direction. Further, the above distributed feedback semiconductor laser has an antireflection coating on both end faces of the element. It is also preferred that a film is formed and the end face reflectance is lower than 0.2%.

Further, the distributed feedback semiconductor laser array is
A plurality of the distributed feedback semiconductor lasers are arranged in an array, and a means for condensing the front output light of the distributed feedback semiconductor lasers into one optical fiber is integrated.

(Operation) The principle of the present invention will be described below with reference to FIGS.

The differential quantum efficiency η _d of the DFB-LD is given by η _d = η _i · 2α _th / (2α _th + α ₀ ). Here, η _i is the internal quantum efficiency, α ₀ is the internal loss, and α _th is the DFB-LD reflecting mirror loss.

Since η _d corresponds to the sum of the light output from both end faces, the slope efficiency with respect to the light output from each end face is the ratio of the light output intensity at the front and back end faces ( It is necessary to allocate η _{d according} to ν). This is because the light intensity distribution in the cavity of the DFB-LD is not uniform as in a Fabry-Perot (FP) LD without a diffraction grating. Therefore, DFB-LD
In this case, the slope efficiency of the front end face can be improved by controlling the light intensity distribution inside the resonator by designing the diffraction grating structure and increasing the coefficient ν.

FIG. 1A shows a conventional symmetrical λ / 4 phase shift structure DFB-LD, FIG. 1B shows a conventional asymmetric λ / 4 phase shift structure DFB-LD, and FIG. 1C shows the distributed phase shift structure of the present invention. The diffraction grating structure and internal light intensity distribution of DFB-LD are shown.

In the conventional symmetric λ / 4 phase shift structure, the λ / 4 phase shift portion is located at the center of the resonator, and the phase of the electric field is matched with the Bragg wavelength in the left and right regions as seen from the phase shift portion. Is strongly confined around the λ / 4 phase shift. In this case, since the light intensity distribution is almost symmetrical, the light output from both sides is almost equal. The optical output slope efficiency of one end surface is half of the differential quantum efficiency.

In the conventional asymmetric λ / 4 phase shift structure, since the λ / 4 phase shift position is moved from the center to the front, the light intensity is concentrated on the front end face in such a way that it is dragged to the λ / 4 phase shift position. However, the light emission intensity ratio ν of the front and rear end faces becomes larger than 1, and the light output slope efficiency of the front end face increases. However, in the asymmetric type, since the internal light intensity distribution is determined by the shift amount D from the element center of the phase shift section, the single mode stability and slope efficiency also strongly depend on the phase shift position D. Since D changes depending on the deviation of the cleavage position, the device characteristics are greatly affected.

On the other hand, in the dispersed phase shift structure of the present invention, the λ / 4 phase shift, which has been conventionally located at one center,
Instead, a λ / 8 phase shift is introduced at two points near the center and in the front, and conventionally, the electric field confined around the central λ / 4 phase shift is dispersed into the two λ / 8 phase shifts. Since the position of the phase shift is on average forward,
The light intensity distribution is concentrated on the front side, which has the effect of improving the front end face slope efficiency. On the other hand, since there are two phase shift parts, the distances D ₁ and D ₃ from the element end face to the phase shift parts are affected by the positional deviation at the time of cleavage, but the distance D ₂ between the phase shift parts is Since it is constant regardless of the variation, the tolerance for the variation in the cleavage position of the element characteristics becomes larger than that of the asymmetric λ / 4 phase shift structure.

FIG. 2 shows an end face cleavage position of slope efficiency (unit: W / A) and side mode suppression ratio (SMSR) (unit: dB) in the cases of FIGS. 1 (a), 1 (b) and 1 (c). The calculation result of dependence is shown.

The element length was fixed at 450 μm, the end face residual reflectance was 0.1%, and κL was 1.5. The conventional symmetric type (marked with ▲ in the figure) has a λ / 4 phase shift in the center of the resonator of the λ / 4 phase shift structure, and the conventional asymmetric type (marked with □ in the figure) is the position of the λ / 4 phase shift part. The structure was shifted from the center to the front by 35 μm. On the other hand, the dispersed structure (marked with ● in the figure) of the present invention has D ₁ / D ₂ / D ₃ = 100 μm / 1
It was set to 25 μm / 225 μm. With respect to these three types of structures, the cleavage position deviations of both end surfaces are −20 μm and 0, respectively.
(Design value), +20 μm was set in 3 ways, and calculation was made for 9 cases of 3 × 3 including the front and rear end faces. The distributed structure has a slightly lower SMSR than the conventional λ / 4 shift structure, but the slope efficiency is greatly improved. Further, the variation in characteristics is smaller than that of the conventional two structures. These results also show that the distributed structure has a slope efficiency and S with respect to the change of the cleavage position as compared with the asymmetrical structure and the conventional symmetrical structure.
It can be seen that the variation in MSR is small and the manufacturing tolerance is large. As described above, the distributed phase shift structure of the present invention is a stable single mode DFB-LD that operates with high slope efficiency.
And has a large manufacturing tolerance.

Slope efficiency in the conventional λ / 4 shift structure and the distributed structure DFB-LD of the present invention in the case of FIG. 3 (a) end face reflectance of 0.1% and (b) end face reflectance of 1.0%. And the relationship distribution of side mode suppression ratio and dependency on end face residual reflectance are shown. The calculation was performed by changing the phase at the end face. When the end face residual reflectance is 1.0%, the slope efficiency and the SMSR are greatly reduced with a certain probability in the distributed structure, whereas the reflectance is reduced to 0.1% to reduce the variation in characteristics. It was found that good characteristics can be obtained with high yield. Therefore, in the distributed structure, it is necessary to sufficiently reduce the end face reflectance by using an antireflection (AR) coating,
It is preferably 0.2% or less.

The phase shift position may be any number as long as it is two or more, but it is necessary to be positioned on the front end face side on average in order to improve the front slope efficiency. When the distance from the front end face to the first phase shift part is shorter than the distance from the rear end face to the first phase shift part, the light emission intensity ratio ν becomes larger than 1, and the forward slope efficiency is improved. If the total of the plurality of phase shifts is λ / 4, the conventional phase shift is λ / 4 phase shift D at one central point.
The single longitudinal mode stability equivalent to FB-LD is obtained. Similarly, the sum of the plurality of phase shifts is λ / 8 or 3λ /
If it is 8, the conventional phase shift is λ /
Performance equivalent to that of the 8 or 3λ / 8 phase shift DFB-LD can be obtained.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1A is a diagram showing the device structure and internal light intensity distribution of a conventional symmetrical λ / 4 phase shift distributed feedback semiconductor laser, and FIG. 1B is a conventional asymmetric λ / 4 phase shift distributed feedback. FIG. 1C is a diagram showing an element structure and an internal light intensity distribution of the type semiconductor laser, FIG. 2C is a diagram showing an element structure and an internal light intensity distribution of the dispersion λ / 4 phase shift DFB of the present invention, and FIG. ), (B) and (c), the distribution diagram showing the relationship results of the front end face slope efficiency and the side mode suppression ratio SMSR, and the diagram showing the device end face cleavage position deviation tolerance (assumed necessary amount), FIG. ) Is the front end face slope efficiency and the side mode suppression ratio SMS in the elements of FIGS. 1A and 1C when the end face reflectance is 0.1%.
A distribution chart showing a relational result of R, (b) shows a relational result of the front end face slope efficiency and the side mode suppression ratio in the elements of FIGS. 1 (a) and 1 (c) when the end face reflectance is 0.1%. FIG. 4 is a perspective view of one embodiment of the distributed feedback semiconductor laser of the present invention, and FIG. 5 is a perspective view of the second embodiment.

It should be noted that these referenced drawings are merely schematic representations of the size, shape and arrangement of each structural component to the extent that the present invention can be understood. Therefore, the present invention is not limited to the illustrated examples.

Since FIGS. 1 to 3 have already been described in the section of (Operation), the description thereof will be omitted here.

FIG. 4 shows the structure of the first embodiment of the present invention. The semiconductor laser of this embodiment has a cavity length of 450.
The diffraction grating 2 is formed on the semiconductor substrate 6 by the well-known electron beam exposure method and well-known lithography with a thickness of μm. The diffraction grating 2 has two λ / 8 phase shift regions 3 at 100 μm away from the center of the resonator and the front end face 5a.
An InGaAsP optical guide layer was grown to a thickness of 0.1 μm by well-known epitaxial growth, and seven layers of compressive strain quantum well active layer 1 (well: 7 × 6 nm, barrier layer: 5 × 10) were formed.
nm).

After that, the InGaAsP clad layer is set to 3 μm.
m, an InP cap layer is grown to a thickness of 0.2 μm. Well-known
On the InP cap layer and InP semiconductor by the electrode forming method of
The electrodes 4a and 4b are formed on the back surface of the body substrate 6. Also, semi-conductive
The front end face 5a and the rear end face 5b of the body laser have a reflectance of 0.1%.
A non-reflective coating AR is applied. Diffraction of the above
The etching depth when forming the grating 2 is determined by the distributed feedback
Coefficient κ is about 70 cm ^-10.03 μm so that
It was The period of the diffraction grating 2 is 240.0 nm.

Next, FIG. 5 shows the structure of the second embodiment of the present invention. The semiconductor laser of the present embodiment example has a cavity length of 450 μm and is formed by well-known epitaxial growth.
InGaAsP clad layer with a layer thickness of 1 μm on the P substrate,
The multiple quantum well active layer 1 having a layer thickness of 0.2 μm and the layer thickness of 0.
A 1 μm InGaAsP optical guide layer is grown, and the active layer 1 is formed in a diffraction grating shape by a known electron beam exposure method and a known lithography. The diffraction grating 2 is provided with two λ / 8 phase shift regions 3 at 100 μm away from the center of the resonator and the front end face. After that, InG with a layer thickness of 2 μm
An aAsP clad layer and an InP cap layer having a layer thickness of 0.2 μm are formed, and electrodes are formed on the InP cap and the back surface of the InP semiconductor substrate 6 by a well-known electrode forming method.
Further, both end surfaces 5a and 5b of the semiconductor laser are coated with a non-reflection coating AR having a reflectance of 0.1%. The etching depth for forming the diffraction grating 2 is 0.03 so that the distributed feedback coupling coefficient κ is about 70 cm ^−1.
μm. The period of the diffraction grating 2 is 202.7 nm.

In the structure of the diffraction grating 7 as in the first and second embodiments, the phase shift region 3 is dispersed at two places, so that the conventional λ / 4 phase shift DFB-LD concentrates on the center phase shift portion. The light intensity distribution is dispersed in two λ / 8 phase shift parts. Since there is a λ / 8 phase shift on the front side, an asymmetric distribution in which the light intensity is concentrated on the front side is obtained, and there is an effect of improving the front end face slope efficiency.

Further, in the first and second embodiments, the active layer 1 is the compressive strained multilayer quantum well, but the present invention is not limited to this. That is, it is possible to use a tensile strain quantum well, a bulk, a quantum wire, a quantum dot, or the like.

(Other Examples) FIGS. 6 (a) to 6 (e) show examples of various diffraction gratings in the element structure in the case of the embodiment of FIG. 4, and FIGS. ) Shows examples of various diffraction gratings in the element structure in the case of the embodiment example of FIG.

The structures shown in FIGS. 6 (a) and 7 (a) are common to each other.
Two places at the center of the shaker and 100 μm away from the front end face
The phase shift region is provided. The amount of phase shift is λ
/ B ₁, Λ / b₂Where, λ / b₁+ Λ / b₂= (M
+1/4) · λ (m is an integer) is satisfied.
In this way, the two phase shift regions are dispersed.
The center phase shift of the conventional λ / 4 phase shift DFB-LD
The electric field distribution concentrated in the
Scattered. b₁<B₂In the case of, λ / b on the front side₁Rank
Phase shift is λ / b₂Since it is larger than the phase shift,
Layered light intensity can be concentrated forward, front end slope
It has the effect of further improving efficiency. Also, b₁> N₂Place
If there is a large amount of phase shift in the center of the resonator,
At the same time as improving the edge slope efficiency,
It is possible to suppress deterioration of the suppression ratio (SMSR).

In the structure shown in FIGS. 6B and 7B, three phase shift regions are formed at positions 75 μm and 150 μm away from the center and front end face of the resonator. The phase shift amounts are λ / b ₁ , λ / b ₂ , and λ / b ₃ , respectively, where λ / b ₁ + λ / b ₂ + λ / b ₃ = (m + 1/4) ·
The condition of λ (m is an integer) is satisfied. In this way, by dispersing the phase shift regions at three locations, the conventional λ /
Four-phase shift The electric field distribution concentrated in the central phase shift part of the DFB-LD is dispersed into three phase shifts. Since there are three phase shift regions, λ / b ₁ on the front side
A phase shift is formed near the front end face, which has the effect of improving the front end face slope efficiency.

In the structure shown in FIGS. 6C and 7C, several phase shift regions are provided and λ / b ₁ + λ / b ₂ + ...
The condition of + λ / b _n = (m + 1/4) · λ (m is an integer) is satisfied. The distance from the rear end face to the phase shift region is longer than the distance from the front end face to the phase shift region. By thus distributing the phase shift regions in several places, the electric field distribution concentrated in the central phase shift portion of the conventional λ / 4 phase shift DFB-LD is dispersed in the several phase shift regions. Since many phase shift regions are distributed on the front side, the light intensity is concentrated on the front side, which has the effect of improving the front end face slope efficiency.

The structure shown in FIGS. 6 (d) and 7 (d) is similar to the structure shown in FIGS. 6 (c) and 7 (c), except that
λ / b ₁ + λ / b ₂ + ... + λ / b _n = (m + 1/8) ・
The condition of λ (m is an integer) is satisfied. In such a diffraction grating structure, the front end face slope efficiency is improved, and at the same time, since the total phase shift amount is smaller than λ / 4, the reflection loss loss effect (FEML), (Japanese Patent No. 318670).
According to 5), the fluctuation of the refractive index due to the direct modulation signal or the externally reflected return light is suppressed, and there is an effect of improving the low chirping characteristic and the reflected return light resistance.

The structure shown in FIGS. 6 (e) and 7 (e) is also shown in FIG.
(C), the same as the structure of FIG. 7 (c) except that λ / b
₁ + λ / b ₂ + ... + λ / b _n = (m + 3/8) · λ
The condition (m is an integer) is satisfied. In such a diffraction grating structure, the front end face slope efficiency is improved, and at the same time, since the total phase shift amount is larger than λ / 4, the single mode stability at the time of high injection current operation is improved.

[0040]

As described above, according to the present invention, a phase shift DFB semiconductor laser has a conventional λ / 4 phase shift section instead of the center λ / 4 phase shift section.
A phase shift is introduced to disperse the electric field near the two phase shifts. Further, since there is a phase shift on the front side, an asymmetric distribution in which the light intensity is concentrated on the front side is obtained, and there is an effect of improving the front end face slope efficiency. In addition, since the phase shift parts are located at the center and the front part, the distance between the phase shift parts is always constant without depending on the variation of the cleavage position, and the cleavage position variation of the element characteristics is different from that of the asymmetric phase shift structure. Has a large tolerance to. Therefore, there is an effect that it is possible to provide a distributed phase shift distributed feedback type semiconductor laser capable of achieving a high slope efficiency in a stable single mode and realizing a DFB-LD having a large manufacturing tolerance.

[Brief description of drawings]

FIG. 1A is a diagram showing the element structure and internal light intensity distribution of a conventional symmetric λ / 4 phase shift distributed feedback semiconductor laser, and FIG. 1B is a conventional asymmetric λ / 4 phase shift distributed feedback type. FIG. 3C is a diagram showing the device structure and the internal light intensity distribution of the semiconductor laser, and FIG. 7C is a diagram showing the device structure and the internal light intensity distribution of the distributed λ / 4 phase shift distributed feedback semiconductor laser of the present invention.

FIG. 2 is a distribution diagram showing a relationship actual result between front end face slope efficiency and side mode suppression ratio SMSR in the elements of FIGS. 1 (a), 1 (b) and 1 (c), and element end face cleavage position deviation tolerance (estimated required amount). FIG.

FIG. 3A shows a case where the end face reflectance is 0.1, FIG.
FIG. 1B is a distribution chart showing the relationship between the front end face slope efficiency and the side mode suppression ratio SMSR in the elements of FIGS. 1A and 1C, and FIG.
It is a distribution diagram which shows the relationship record of the front end surface slope efficiency and the side mode suppression ratio in the element of (a) and (c).

FIG. 4 is a perspective view of an embodiment of a distributed feedback semiconductor laser of the present invention.

FIG. 5 is a perspective view of a second embodiment example.

6 (a) to 6 (e) show examples (5 types) of various diffraction gratings in the element structure in the case of FIG.

7 (a) to 7 (e) show examples (5 types) of various diffraction gratings in the element structure in the case of FIG.

[Explanation of symbols]

1 Multi-layer compressive strained quantum well active layer 2 diffraction grating 3 λ / 8 phase shift region 4a Upper electrode 4b Lower electrode 5a Front end face 5b rear end face 6 InP semiconductor substrate

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Claims (11)

[Claims] 1. A distributed feedback semiconductor laser having at least an active layer and a diffraction grating, wherein optical feedback is performed by the diffraction grating, wherein an element is provided with a region 1, a region 2 in a length direction from a front end face side,
.., n in the order of region n (n is an integer of 3 or more), and the phases of the diffraction gratings are λ / b ₁ and λ / b _{2 at} the boundary (n-1) of each region. , ... λ / b _n-1 are shifted by each (b ₁ , b ₂ , ... b _n-1 are integers), and the total phase shift amount (λ / b ₁ + λ / b ₂ + ... + λ / b
_n−1 ) is (m + 1/4) λ (m is an integer), which is a distributed feedback semiconductor laser. 2. A distributed feedback semiconductor laser having at least an active layer and a diffraction grating, wherein optical feedback is performed by the diffraction grating, wherein the element is a region 1, a region 2, and a length direction from the front end face side.
It is divided into three regions in the order of region 3, and the periodic structure of the diffraction grating is phase-shifted by λ / b ₁ at the boundary between regions 1 and 2 and λ / b ₂ at the boundary between regions 2 and 3. Cage (b ₁ , b ₂
Is an integer), and the total amount of phase shift (λ / b ₁ + λ / b ₂ )
Is (m + 1/4) λ (m is an integer). A distributed feedback semiconductor laser. 3. A distributed feedback semiconductor laser having at least an active layer and a diffraction grating, wherein optical feedback is performed by the diffraction grating, wherein the element is a region 1, a region 2, in the length direction from the front end face side.
The diffraction grating is divided into three regions in this order, and the periodic structure of the diffraction grating has a λ / 8 phase shift at the boundaries between the regions 1 and 2, and between the regions 2 and 3, and the total amount of phase shift is λ /
4. A distributed feedback semiconductor laser, characterized in that 4. A distributed feedback semiconductor laser having at least an active layer and a diffraction grating, wherein optical feedback is performed by the diffraction grating, wherein the element is a region 1, a region 2, in the length direction from the front end face side.
.. are divided into n regions (n is an integer of 3 or more) in the order of region n, and the phase of the diffraction grating is λ / b ₁ and region 2 at the boundary of each of the (n-1) regions. There is a λ / b ₂ phase shift at the boundary of the region 3 (b ₁ and b ₂ are integers), and the total phase shift amount (λ / b ₁ + λ / b ₂ ) is (m + 1/4) λ ( A distributed feedback semiconductor laser, wherein m is an integer. 5. A distributed feedback semiconductor laser having at least an active layer and a diffraction grating, wherein optical feedback is performed by the diffraction grating, wherein the element is a region 1, a region 2, in the length direction from the front end face side.
..., n (n is an integer of 3 or more) regions in this order, and the phases of the diffraction gratings are λ / b ₁ and λ / b at the boundaries of the (n-1) regions, respectively. ₂ , ... λ / b
_{n-1 is} shifted by _n-1 (b ₁ , b ₂ , ... B _n-1 is an integer), and the total phase shift amount (λ / b ₁ + λ / b ₂ + ...
+ Λ / b _n-1 ) is (m + 3/8) λ (m is an integer)
A distributed feedback semiconductor laser characterized by the following. 6. The length of region 1 is shorter than the length of region n,
The distributed feedback semiconductor laser according to claim 1, 4, or 5. 7. The length of region 1 is shorter than the length of region 3,
The distributed feedback semiconductor laser according to claim 2. 8. The distributed feedback semiconductor laser according to claim 2, wherein the total length of the regions 1 and 2 is equal to the length of the region 3. 9. The distributed feedback semiconductor laser according to claim 1, having a gain coupling type diffraction grating structure in which the optical gain distribution of the active layer changes in the same direction as the diffraction grating in the length direction. . 10. A non-reflective coating film is formed on both end faces of the device, and the end face reflectance is lower than 0.2%.
10. The distributed feedback semiconductor laser according to any one of items 1 to 9. 11. A distributed feedback semiconductor laser according to claim 1, wherein a plurality of distributed feedback semiconductor lasers are arranged in an array, and a means for condensing the forward output light of them into a single optical fiber is integrated. A distributed feedback semiconductor laser array characterized by the above.