JPH11163464A - Distribution feedback type semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DFB laser which is easily manufactured, a threshold gain difference is kept satisfactory as compared to a λ/4 phase shifted DFB laser, without being affected by an end surface phase while an optical output equivalent to constant grating DFB laser is obtained. SOLUTION: For a λ/4 shifted DFB laser of an AR/AR coating, L1 /L2 , κ1 .L1 >κ2 .L2 , where L2 is length in resonator axis direction of a diffraction grating in front of a phase shift point, L1 is that of the diffraction grating rear the phase shift point, κ2 is an average value of coupling factor for the diffraction grating in front of phase shifted point, and κ1 is that of diffraction grating rearward of the phase shift point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser, and more particularly to a distributed feedback semiconductor laser in which a coupling coefficient changes in a resonator axis direction.

[0002]

2. Description of the Related Art A distributed feedback laser (hereinafter, referred to as a DFB laser) provided with a diffraction grating uses a Bragg reflection to obtain a single-wavelength laser oscillation operation even during modulation. Therefore, it is widely used for long-distance optical fiber communication. DFB lasers currently widely used include a uniform grating DFB laser and a λ / 4 phase shift DFB laser.
There is a laser.

The uniform grating DFB laser has a constant diffraction grating pitch and no phase jump, and has both end faces of the resonator with a non-reflective coating film (hereinafter, referred to as an AR coating film) and a high-reflection coating film (hereinafter, HR). (Referred to as a coating film). Further, the λ / 4 phase shift DFB laser has a λ / 4 phase shift point at the center in the resonator axis direction, and AR-AR coating is applied to both end faces of the resonator.

As described above, the uniform grating DFB laser has an AR-HR coating,
Side, a large optical output is obtained, but the λ / 4 phase shift D
Since the FB laser basically has the same light output from both end faces, the light output that can be extracted from the front is smaller than that of the uniform grating DFB laser. However, compared with a λ / 4 phase shift DFB laser, a uniform grating DFB laser is inferior in single mode performance and production yield, and is inferior in dynamic single mode when a laser is digitally modulated. However, there is a disadvantage that the signal error rate becomes large when used.

As a value that quantitatively handles the single mode property, there is a threshold gain difference usually represented by Δα. The laser is
When the gain of the optical waveguide exceeds a certain value, laser oscillation occurs. The gain of this laser oscillation is called a threshold gain. In a DFB laser, there is a threshold gain for each mode oscillating at a different wavelength. The difference in threshold gain between the mode having the lowest threshold gain and the mode having the next lowest threshold gain is referred to as a threshold gain difference Δα. Due to the structure, a large threshold gain difference is obtained with a λ / 4 phase shift DFB laser. However, with a uniform grating DFB laser, only a small threshold gain can be obtained.

[0006] Uniform grating DFB laser and λ / 4
The phase shift DFB laser, when used for optical communication,
Each has the above-mentioned conflicting advantages and disadvantages, and lasers that solve these disadvantages have been studied. For example, in the 1985 IEICE National Conference on Semiconductors and Materials, Paper No. 310 (1985), an AR-HR coating was applied to a λ / 4 phase shift DFB laser to shift the phase shift position to the rear HR side. Thus, there has been proposed a method of taking out a large forward light output while maintaining the threshold gain difference at a certain value.

However, DF with HR coating
In the B laser, the threshold gain difference generally fluctuates depending on the phase of the end face of the diffraction grating, so that a single mode failure logically occurs at a certain probability. Therefore, even if the asymmetric λ / 4 phase shift DFB laser with the AR-HR coating improves the single mode yield as compared with the uniform grating DFB laser, it is inevitable that defects occur at a certain rate.

[0008] Also, research has been conducted to make the forward optical output and the backward optical output different from each other in the λ / 4 phase shift DFB laser having the AR-AR coating. For example, "Asymmetric λ / 4-Shifet InGaAsP / InP DFB Laser
s "M. Usami et. (IEEE J. Quantum Electron., QE-23,
pp. 815-821, June 1987), by shifting the λ / 4 phase shift point forward from the center in the resonator axial direction,
A method for improving the forward light output without significantly impairing the single mode property is disclosed.

[0009] For example, Japanese Patent Publication No. 1-37872 discloses a method in which the height of a diffraction grating is changed before and after a phase shift point to change a front light output and a rear light output. Also, for an analysis in which the height of the diffraction grating is changed, for example, see “λ / 4-Shifet InGa
AsP / InP DFB Lasers "k.Utaka et. (IEEE J. QuantumEle
ctron., QE-22, pp. 1042-1051, July 1986).

A conventional distributed feedback semiconductor laser will be described below with reference to the drawings. FIG. 9 is a cross-sectional view showing an example of the configuration of a conventional distributed feedback semiconductor laser of this type. In FIG. 9, 1 is an n-type InP substrate, 2 is a diffraction grating, 3 is an n-InGaAsP guide layer, 4 is an SCH-strained MQW layer, 5 is a p-InP cladding layer, 6 is a p-contact layer, and 7 is a p-contact layer. A side electrode, 8 is an n-side electrode, and 9 is an AR coating film. N-type InP with (100) surface orientation
A diffraction grating 2 having a λ / 4 phase shift is engraved on a substrate 1 at a center position with respect to the resonator axis direction. Since the pitch of the diffraction grating 2 is half the wavelength of the light in the optical waveguide, this phase shift is obtained by shifting the phase of the diffraction grating by π.

The characteristic of the λ / 4 phase shift DFB laser shown in FIG. 9 is that the height of the peak of the diffraction grating 2 is lowered from the λ / 4 phase shift position on the front side and raised on the rear side. It is a point. On the substrate 1 on which the diffraction grating 2 is formed, a substrate having a thickness of 100 nm and a band gap wavelength of 1.15 μm is provided.
An n-InGaAsP guide layer 3 (having a composition of 1.15 μm), an active layer, and a p-InP clad layer 5 are laminated. In recent years, the SCH-strain MOW layer 4 is used for the active layer. SC
H is an abbreviation for Separate Confinement Heterostructure, in which an InGaAsP layer is provided on both sides of a strained MQW to have an optical confinement effect, and MOW is a Multi-Quan
An abbreviation for tum well, which is called a multiple quantum well structure.

In an optical waveguide having such a diffraction grating, the equivalent refractive index in the resonator axis direction is, for example, n = n ₀ + n ₁ · c
It fluctuates periodically like os (2β ₀ z + Ω). Here, assuming that the wave number of light in vacuum is k ₀ , κ defined as κ = k ₀ n _1/2 is called a coupling coefficient. The laser shown in FIG.
To change the coupling coefficient κ before and after the / 4 shift point,
In this example, the height of the diffraction grating is changed, but there is also a configuration in which the duty of the diffraction grating is changed in order to change the coupling coefficient κ.

In a DFB laser, in order to make the field intensity of light as flat as possible in the resonator axial direction distribution, it is sometimes desirable to change the coupling coefficient κ in the resonator axial direction in design. For example, in Japanese Patent Application Laid-Open No. 8-255954,
In a / 4 shift DFB laser, the duty of the diffraction grating is changed to suppress spatial hole burning in the axial direction where the field increases and the carrier density decreases at the λ / 4 position. The coupling coefficient is reduced. In this case, since the height of the diffraction grating is not changed, the manufacture can be performed relatively easily.

For the same purpose, Japanese Patent Publication No. H8-17262 assumes that a higher-order diffraction grating is used.
However, the second-order diffraction grating, compared to the first-order diffraction grating,
Since distributed feedback due to Bragg reflection is significantly weakened, it is not an optimal design, and there is no product example that actually applies such a method.

[0015] Further, a DFB laser for analog use requires, in particular, a field flatness of light in the resonator axis direction in order to improve the linearity of light output characteristics with respect to an injection current. In order to realize such field flatness in a uniform grating DFB laser, the coupling coefficient on the AR side may be increased and the coupling coefficient on the HR side may be decreased. However, Japanese Patent Application Laid-Open No. 9-64456 discloses a selective growth. The coupling coefficient in the resonator axial direction is changed by utilizing the fact that the layer thickness of the epitaxial layer is changed from the growth inhibition mask width. An example having the same purpose of improving analog performance is disclosed in Japanese Patent Laid-Open No. 6-310.
In this case, a diffraction grating is provided only in a part on the AR side, and the diffraction grating in the region on the HR side is eliminated.

Introducing a phase shift into a diffraction grating has a disadvantage that it is more difficult to manufacture than a uniform grating. For example, in JP-A-61-222189,
Diffraction gratings are provided above and below the active layer, and the pitch of these diffraction gratings is shifted so that single mode oscillation is performed.

[0017]

The above-mentioned conventional distributed feedback semiconductor laser has the following problems. First, in the conventional asymmetric λ / 4 shift DFB laser, the forward optical output increases but the threshold gain difference is poor compared to the symmetric λ / 4 shift DFB laser. Failure to meet error rate criteria. In addition, there is a problem that the cost is increased because the single mode yield is reduced.

For example, in the technology disclosed in the article No. 310 of the 1985 IEICE National Conference on Semiconductors and Materials, which has been coated with an AR-HR coating, the HR side is affected by the end face phase. Failure occurs. Also, λ / 4 shift DF of AR-AR coating
For the B laser, when the length behind the phase shift position is L ₁ , its coupling coefficient is κ ₁ , the length before the phase shift position is L ₂ , and its coupling coefficient is κ ₂ , L ₁ >
L ₂ , κ ₁ = κ ₂ and asymmetric,
When L ₁ = L ₂ and κ ₁ > κ ₂ are asymmetric as in JP-A-7872, the threshold gain difference and the field flatness in the resonator axial direction are deteriorated as compared with the symmetric type.

The deterioration of the threshold gain difference is caused by the deterioration of the bit error rate,
Although the yield of the single mode is deteriorated, the deterioration of the field flatness also leads to the deterioration of the single mode property. That is, when a light field distribution occurs in the resonator axis direction, a carrier concentration distribution in the active layer occurs in response to the distribution, and the equivalent refractive index of the optical waveguide changes in the resonator axis direction. And the single mode property is worsened.

The second problem is that it is extremely difficult to change the coupling coefficient such that κ ₁ > κ _{2 in} the middle of the resonator, as disclosed in Japanese Patent Publication No. 1-37872. Today, strain MOWs are grown by metal organic chemical vapor deposition (hereinafter referred to as MO-VPE), and the diffraction grating formed on the substrate at this time is made of phosphine (PH ₃ ).
In addition, a desired height is obtained by breaking during heating in an arsine (AsH ₃ ) atmosphere. Therefore, sufficient manufacturing reproducibility of a diffraction grating whose height changes in the resonator axis direction cannot be ensured.

In JP-A-8-255954, the height of the diffraction grating is kept constant and the duty ratio is changed. However, in a DFB laser for long-wave optical communication, the pitch of the diffraction grating is about 200 nm to 250 nm. The length of a peak of one diffraction grating is about 100 nm to 120 nm. To modulate this length, a pattern forming accuracy of the order of 10 nm is required. It is also difficult to manufacture because it does not exist in the mass production technology.

Also, in the above-mentioned Japanese Patent Application Laid-Open No. 9-64456, the coupling coefficient in the resonator axial direction is changed by utilizing the fact that the thickness of the epitaxial layer is changed by the width of the mask for selective growth. Although the height of the diffraction grating can be formed with high accuracy by using this method, the change in the height of the diffraction grating is limited due to the dependence of the mask width on the selective growth. Shift DFB
It cannot be applied to the case where the height of the diffraction grating is to be changed by a considerable size like a laser.

Also, as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 61-222189, an attempt has been made to improve the threshold gain without using a phase shift which is difficult to manufacture by providing diffraction gratings above and below the active layer. Even in this case, a single-mode failure occurs stochastically. This is because the phase difference between the diffraction gratings cannot be controlled above and below the active layer, and the difference in the phase may significantly reduce the threshold gain difference.

The present invention has been made to solve such a problem, and can maintain a good threshold gain difference even when compared with a λ / 4 phase shift DFB laser, and is free from the influence of the end face phase and is uniform. It is an object of the present invention to provide a DFB laser that can be easily manufactured and has an optical output equivalent to that of a grating DFB laser.

[0025]

The end face phase dependence of the diffraction grating on the HR coating surface is a problem inherent in DFB lasers. In particular, if you look at the characteristic of the threshold gain difference,
If the end face phase of the HR coating surface changes, there is always a condition that the threshold gain difference becomes worse. Therefore, if the HR coating is applied, the single mode yield cannot be theoretically 100%. For the first time, the AR-AR coating can eliminate the problem of the end face phase.

Considering a λ / 4 phase shift DFB laser having an AR-AR coating, the length of the rear part from the phase shift point is L ₁ , the coupling coefficient is κ ₁ , and the length of the front part from the phase shift point is L. _2, the coupling coefficient as a kappa _2,
Consider changing these parameters. At this time, if κ ₁ L ₁ > κ ₂ L ₂ , the Bragg reflection in the rear part is large, and the front output becomes large. _{κ 1 L 1> κ 2 L} 2
To realize L ₁ > L ₂ , κ ₁ = κ ₂ and κ ₁
Considering the methods of setting> κ ₂ and L ₁ = L ₂ , in both cases, the threshold gain difference is worse than that of the symmetric λ / 4 shift DFB laser.
However, if the coupling coefficient is changed without moving the phase shift position, the deterioration amount of the threshold gain difference is smaller. It is better to change the field flatness by the coupling coefficient.

Thus, while satisfying κ ₁ L ₁ > κ ₂ L ₂ ,
Examination of a method of greatly changing the coupling coefficient, such as κ ₁ > κ ₂ and L ₁ <L ₂ , reveals that a condition exists in which the threshold gain difference is improved as compared with the symmetric λ / 4 shift DFB laser. . At this time, if only the phase shift position is changed,
The field flatness is also improved as compared with the case where only the coupling coefficient is changed.

Forming such a large change in κ in the resonator axial direction is also a major problem. Actual MO-VPE
Considering growth, changing the height of the diffraction grating
Quite difficult. Therefore, it is considered that the coupling coefficient κ is changed under the condition that the height of the diffraction grating is constant. The coupling coefficient is an amount that is proportional to a change width in which the equivalent refractive index changes by a trigonometric function in the waveguide direction. Therefore, the total coupling coefficient can be changed by forming a plurality of gratings in parallel with the waveguide by using the property of the sum of the trigonometric functions and changing the phase difference of these gratings for each region. .

In manufacturing such a plurality of diffraction gratings, using today's EB technology, it is possible to form diffraction gratings having different pitches and phases in the same plane. Further, by thinning out the diffraction gratings having the same height in some places, the coupling coefficient can be changed on average. In particular, if the repetition of the region with and without the diffraction grating is set obliquely with respect to the cavity axis direction, the change in the coupling coefficient in the cavity axis direction becomes gentle, and the active layer formation position moves in the cavity direction. Even if it deviates in the direction perpendicular to the direction, the average coupling coefficient does not change.

As a specific configuration, the distributed feedback semiconductor laser of the present invention is a distributed feedback semiconductor laser having a phase shift point of λ / 4 on a diffraction grating. The direction in which the reference laser light is emitted is backward, the length of the diffraction grating in the resonator axial direction ahead of the phase shift point is L ₂ , and the length of the diffraction grating in the resonator axial direction behind the phase shift point is L. ₁ , when the average value of the coupling coefficient of the diffraction grating before the phase shift point is κ ₂ , and the average value of the coupling coefficient of the diffraction grating behind the phase shift point is κ ₁ , L ₁ <L ₂ , κ ₁ · L ₁ > κ ₂ · L ₂

Further, κ ₁ · L ₁ -κ ₂ · L ₂ > 0.1 is characterized.

Further, κ ₁ · L ₁ -κ ₂ · L ₂ <1 is set.

[0033] Further, the present invention is characterized in that L ₁ / (L ₁ + L ₂ )> 0.1.

The direction in which the main laser beam is emitted is forward,
The direction in which the reference laser light is emitted is rearward, the length of the diffraction grating in the resonator axial direction ahead of the phase shift point is L ₂ , and the length of the diffraction grating in the resonator axial direction behind the phase shift point is L2.
When L ₁ , L ₁ <L ₂ , and the value obtained by integrating the coupling coefficient of the diffraction grating ahead of the phase shift point is calculated from the value obtained by integrating the coupling coefficient of the diffraction grating behind the phase shift point. It is characterized by being made smaller.

The length of the resonator is L, and the length of the diffraction grating provided forward from the rear end face of the resonator in the axial direction of the resonator is L.
L ′ (L ′ <L).

Further, the present invention is characterized in that the coupling coefficient of the diffraction grating before the phase shift point is made constant, and the coupling coefficient of the diffraction grating behind the phase shift point is made constant.

Further, the invention is characterized in that a diffraction grating whose coupling coefficient changes in the resonator axial direction by a combination of a plurality of gratings having different periods or phase differences is provided.

Further, each period of the plurality of gratings is fixed, the phase difference of each grating is changed for each predetermined region in the resonator axis direction, and the coupling coefficient of the diffraction grating is stepwisely changed in the resonator axis direction. It is characterized by changing to.

In a region where the coupling coefficient is large, the phases of the plurality of gratings are matched with each other, and in a region where the coupling coefficient is small, the phases of the plurality of gratings are mutually shifted.

Further, the invention is characterized in that a diffraction grating is provided in which the periods of the plurality of gatings are slightly different from each other, and the coupling coefficient changes continuously.

Further, the plurality of gratings are formed by two gratings which are separated in right and left in parallel in the resonator axis direction.

The two gratings have slightly different periods so that the number of periods differs from each other by one period within the resonator length L. The phases of the two gratings coincide with each other at the end faces of the resonators. And the phase is shifted by π.

Further, the two gratings modulate the width of the active layer for forming the optical waveguide by selective growth, and are formed on both end surfaces of the selective growth portion.

Further, the depth of the unevenness of the plurality of gratings is constant.

A region where a grating is formed and a region where a grating is not formed are repeatedly provided at intervals sufficiently smaller than the resonator length L. By changing the ratio of these regions, the average coupling coefficient can be reduced. It is characterized by having a diffraction grating for changing.

Further, the present invention is characterized in that a repetitive pattern of a region where the grating is formed and a region where the grating is not formed is provided obliquely with respect to the resonator axial direction.

Further, in some regions, the number of grating peaks is left at n / m, and in other regions, the number of grating peaks is q / p (m, n, p, q are all natural numbers of 1 or more). m> n, p> q). A diffraction grating is provided that changes the average coupling coefficient at a ratio of n / m: q / p by leaving.

[0048]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a structure of a distributed feedback semiconductor laser for explaining Embodiment 1 of the present invention, and is a view corresponding to FIG. 9 of a conventional distributed feedback semiconductor laser. In FIG. 1, 1 is an n-type I
nP substrate, 2 diffraction grating, 3 n-InGaAsP guide layer, 4 SCH-strained MQW layer (active layer), 5 p-I
An nP cladding layer, 6 is a p-contact layer, 7 is a p-side electrode, 8 is an n-side electrode, and 9 is an AR coating film.

As shown in FIG. 1, in Embodiment 1 of the present invention, a diffraction grating 2 is provided on an n-type InP substrate 1,
The n-InGaAsP guide layer 3, active layer 4, p
-An InP cladding layer 5 is provided. The diffraction grating 2 is provided with a λ / 4 phase shift point at a point where the resonator length L is divided into L ₁ and L ₂ . Here behind the L ₁ side, L
Assuming that the ₂ side is the front, the coupling coefficient on the rear side is κ ₁ , and the coupling coefficient on the front side is κ ₂ , the diffraction grating 2 has κ ₁ > κ ₂ ,
It is formed so as to satisfy the condition of L ₁ <L ₂ and κ ₁ L ₁ > κ ₂ L ₂ .

The active layer 4 may be bulk, but is preferably an SCH-strained MQW layer 4 having excellent light output characteristics. And n
Between the InGaAsP guide layer 3 and the SCH-strained MQW layer 4 in order to improve the crystallinity of the strained MQW.
An InP spacer may be interposed. The active layer 4 preferably has a BH (Buried Heterostructure) structure having a width of about 1.5 μm and current blocking layers provided on both sides thereof. Then, on the active layer 4 and the BH structure, p-
The contact layer 6 is laminated to form a p-side electrode 7, an n-side electrode 8 is formed on the back surface of the n-type InP substrate 1, and an AR coating film 9 having a reflectance of 2% or less is formed on both end surfaces. The structure is provided.

With such a structure, the front light output can be made larger than the rear light output, and a good threshold gain difference can be obtained. That is, when the front of the optical output P _f, the rear of the light output was P _r, becomes P _f / P _r is approximately _{exp (a (κ 1 L 1} -κ 2 L 2)). κL =
Considering the symmetrical λ / 4 shift laser of 1.8 as a reference, when asymmetrical, a in the above equation is about 2.2.

Even in a symmetrical λ / 4 shift DFB laser, the front / rear ratio is not 1 depending on the end face phase due to the residual reflection on the end face. It is necessary to perform a process in which the direction in which the higher power is output is measured, and the process of selecting chips is longer than that of the uniform grating DFB laser. Therefore, in a λ / 4 shift DFB laser,
The ability to increase the light output from one of the determined end faces is also effective in shortening the chip sorting process.

Therefore, in the asymmetric λ / 4 shift DFB laser as shown in FIG. 1, in order to exceed the variation of the front-to-back output ratio due to the end face phase and to always increase the output from one side, at least 1.2 times the design. It is necessary to have a front-to-back ratio. This requirement is κ ₁ L ₁ −κ ₂ L ₂ > 0.1
What is necessary is just to form a diffraction grating so that it may satisfy.

On the other hand, the front-back ratio cannot be increased without limit. Assuming that the AR-AR coating is used, the power density of light near both end faces differs depending on the ratio of the front-back ratio. If this ratio is large, kink occurs due to spatial hole burning. In order to prevent this, the front-to-back output ratio cannot be made 10 times or more. Therefore, it is necessary to satisfy κ ₁ L ₁ −κ ₂ L ₂ <1. In order to obtain a good threshold gain difference and a sufficient manufacturing tolerance, it is better to satisfy L ₁ / (L ₁ + L ₂ )> 0.1.

The influence of the end face phase due to the residual reflection is as follows.
Since the threshold gain difference and the front-to-back ratio are affected, the reflectivity at the end face is preferably suppressed to 0.5% or less if possible. By doing so, a good threshold gain difference can be obtained at all end face phases, and the single mode yield is theoretically 100%.

In the above description of the first embodiment, the lower grating structure having the diffraction grating 2 under the active layer 4 has been described. However, an upper grating or a structure having a grating beside the active layer may be used. Further, the first embodiment described above
In the above description, the diffraction grating is formed by waving the interface between InP and InGaAsP, but the diffraction grating may be formed by a method of forming InGaAsP intermittently in InP. In the above description of the first embodiment, the conductivity type of the substrate is n-type. However, it is needless to say that the substrate may be p-type, and the material may be a semiconductor material other than InGaAsP / InP, for example, InGaIAAs. Further, such a distributed feedback semiconductor laser can be integrated with a modulator or the like.

[0057] As the first _{requirement, κ 1> κ 2, L} 1 <L 2, κ 1 L 1> κ 2 L 2 has cited three conditions that, kappa _1> kappa ₂ Other 2
From the two conditions, the necessary condition is L ₁ <
_Two conditions of L ₂ and κ ₁ L ₁ > κ ₂ L ₂ are sufficient. Here, κ ₁ and κ ₂ are the average values of the coupling coefficients, respectively, and κ used in this specification means the coupling coefficient or the average value of the coupling coefficients. Extending to the case where κ changes a little more, the condition is that L ₁ <L ₂
If it can also be generalized from the integral value> L ₁ of κ between 0 and L ₁ integral value of κ to L, next, in integral form.

That is, when the front-back asymmetric structure is used for the purpose of improving the forward light output of the λ / 4 phase-shift DFB laser, regardless of whether κ is asymmetric or L is asymmetric, symmetric λ / 4 phase shift is performed. Single mode yield is worse than DFB laser. Specifically, the threshold gain difference Δ
α is worse than a symmetric λ / 4 phase shift DFB laser,
Since the field distribution of light also becomes large, kinks occur and the side mode suppression ratio (SMSR) deteriorates, so that noise cannot be used in communication applications. In the first embodiment, in order to improve the forward light output, κ ₁ L ₁ > κ ₂ L ₂ is satisfied. However, by satisfying L ₁ <L ₂ , a good threshold gain difference can be obtained. The point is that a threshold gain difference Δα larger than that of a symmetric λ / 4 phase shift DFB laser can be obtained, and the intensity distribution of the field is made uniform. As a result, the distributed feedback semiconductor laser of the present embodiment improves the side mode suppression ratio, the kink yield, and the single-axis mode yield as compared with the conventional λ / 4 phase shift DFB laser, and is about 30% as a whole in the related art. Yield can be improved to about 70%.

Embodiment 1 Next, a first embodiment of the present invention will be described with reference to FIG. As shown in FIG. 1, the distributed feedback semiconductor laser according to the first embodiment has an n-type In
It is assumed that a diffraction grating 2 waving in the [011] direction is formed on a P substrate 1. The pitch of the diffraction grating 2 is 203 nm for a 1.3 μm band DFB laser and 243 nm for a 1.55 μm band DFB laser. Resonator length L is 450 μm
A λ / 4 phase shift is provided at a position of L ₁ = 171 μm from the rear end face side. Therefore, the distance from this phase shift position to the front end face is L ₂ = l79 μm, and L ₁ / (L ₁ + L ₂ ) = L ₁
/L=0.38.

The height of the diffraction grating 2 is 45 nm on the rear side and 15 nm on the front side. On this diffraction grating 2, a thickness of 100 n
n-InGaAsP guide layer 3 having a composition of 1.15 μm in m;
An SCH-strained MQW layer 4 is formed, and a p-InP grading layer 5 is laminated. The SCH-strained MQW layer 4 includes five compression-strained InGaAsP well layers and a tensile strained InG
The strain MQW is a stress strain compensation type strain MQW made of an aAsP barrier layer. The thickness of the compression-strained InGaAsP well layer is 4.5 nm, and the thickness of the tensile-strained InGaAsP barrier layer is 10 nm.

In such a layer structure, the coupling coefficient is set to κ ₁ L = 3.3 and κ ₂ L =
Assuming 1.1, the front-to-back output ratio is P _f / P _r = 4,
The threshold gain difference ΔαL has a sufficiently large value of ΔαL = 0.76. Considering a symmetric λ / 4 shift DFB laser that can obtain the same threshold gain as the DFB laser having this structure, κL = 1.
In the case of 8, since ΔαL = 0.73, it is understood that a better threshold gain difference can be obtained than in the symmetric λ / 4 shift DFB laser.

Embodiment 2 Hereinafter, a second embodiment of the present invention will be described with reference to the drawings. FIG. 2 is a sectional view showing the structure of the distributed feedback semiconductor laser according to the second embodiment of the present invention. In the second embodiment, the diffraction grating 2 is provided only up to a certain distance from the rear end surface (P _r ), and a portion having a diffraction grating structure and a portion having no diffraction grating structure are formed. For example, when the length of the resonator is L, the length L ′ of the diffraction grating is L <L ′. Also, the active layer structure is the same for the part with and without the diffraction grating structure,
The p electrode and the n electrode are also common. That is, in the second embodiment, the diffraction grating 2 on the n-type InP substrate 1 has a rear end face (P
_{From the r} ) side, for example, only a region up to L ′ = 0.4 L is formed, and no diffraction grating is formed in the region at 0.6 L on the front side. The position of λ / 4 is provided at a position of 0.2 L from the rear end face. In the region where the diffraction grating 2 is present, the coupling coefficient is constant, for example, κL = 7.5.

The structure shown in FIG.
Is equivalent to a structure having a symmetrical λ / 4 shift DFB laser in the region of and an optical amplifier in a region of 0.6 L ahead of the symmetrical λ / 4 shift DFB laser.
The structure can provide a good threshold gain difference of the / 4 shift DFB laser and a large forward light output by the amplifier.

Specifically, with this structure, the threshold gain difference ΔαL
Is about 1.8, and the front-back ratio is about 2.7 times. The reason why ΔαL is very large is that ΔαL is considered to be substantially constant, so that if the effective L is reduced, Δα increases. Considering the length of a certain portion of the diffraction grating, Δα
(0.4L) is about 0.71, which is equivalent to a normal λ / 4 shift DFB laser. Further, in the case of this structure, the mode interval of the wavelength spectrum is widened and the threshold gain difference Δα is large as compared with the symmetric λ / 4 shift DFB laser having the same resonator length, so that the dynamic single mode property is very excellent. . Since the threshold gain difference Δα is large, optical transmission with a low error rate is possible in digital optical communication, and the forward optical output is large, so that the optical transmission can be made longer.

The λ / 4 shift DFB laser of the second embodiment is the same as the asymmetric λ / 4 shift DFB of the first embodiment.
It can be said that the laser and the same category satisfying the following conditions. L ₁ <L ₂ , (integral value of κ from 0 to L ₁ ) / L ₁ > (κ from L ₁ to L
Integral value) / L ₂ to, in this condition, there is lambda / 4 shifted to the position of z = L _1, 2 th conditions z <average value of the coupling coefficient of the region of L ₁ is, z> L It means that it is larger than the average value of the coupling coefficient of the area ₂ .

The asymmetric λ / 4 shift D of the first embodiment
The reason that the FB laser has a better threshold gain difference Δα than the symmetric λ / 4 shift DFB laser is obtained from the theoretical calculation of the mode-coupling method. This is because it can be understood that it is close to the partial λ / 4 shift DFB laser shown in FIG.

Embodiment 3 For example, in the distributed feedback semiconductor laser described in the first embodiment, the coupling coefficient κ of the diffraction grating
Needs to be changed in the axial direction. Since the coupling coefficient κ is almost proportional to the grating height of the diffraction grating, it may be changed. However, the actual grating storage height (depth of the concavities and convexities) is changed depending on each region in the actual buried growth of the grating. It is difficult. Therefore, in the third embodiment, the gratings are divided and arranged in parallel on the left and right above or below the optical waveguide, the phases of the left and right gratings are matched in the region where κ needs to be increased, and the left and right gratings in the region where κ needs to be reduced. The grating is shifted in phase. Hereinafter, the third embodiment will be described with reference to the drawings.

FIG. 3 is a perspective view showing a diffraction grating structure for explaining Embodiment 3 of the present invention. If the coupling coefficient can be changed in the resonator axis direction as described above,
Although good DFB laser characteristics can be obtained, it is difficult to change the coupling coefficient with the height of the diffraction grating in actual manufacturing. Therefore, the third embodiment has a structure in which the coupling coefficient is changed while keeping the height of the diffraction grating constant. As shown in FIG. 3, the distributed feedback DFB laser according to the third embodiment first places [0] on an n-type InP substrate 1 having a surface orientation of (100).
11], two pairs of silicon dioxide films 10 each having a width of 3 μm and extending in a direction of 1.8 μm are formed. This silicon dioxide film 10 becomes a growth inhibiting mask for forming an active layer by MO-VPE selective growth. The reason why the active layer of the DFB laser is formed by selective growth is to manufacture the active layer width with good control and to reduce the variation. As a result, the coupling coefficient and the oscillation wavelength can be precisely controlled, and the kink caused by the undulation of the active layer width. Can be eliminated. In this selective growth of MO-VPE, InP and InGaAsP are epitaxially grown, and the source gases are TMI, TMG, and AsH.
₃ , PH ₃ and organic metal is supplied by bubbling hydrogen. As for the doping, Si ₂ H ₆ ,
A gas obtained by diluting the DMZ _n hydrogen. The growth pressure is set to 75 Torr.

In the diffraction grating 2, two gratings are arranged in parallel in a gap of 1.8 μm between the silicon dioxide films 10. The change in the equivalent refractive index of the optical waveguide is the sum of the contributions of the two gratings. This is schematically shown in FIG.

The two gratings are in phase in the rear 0.38 L region. Then, a λ / 4 shift is introduced at a position of 0.38L from the rear. The λ / 4 shift is to shift the phase of the refractive index change by π, which is equivalent to inverting the sign of the refractive index. Therefore, from the λ / 4 position forward, the sign of the change in the grating is inverted with respect to the rear, but at the same time, the phase of one grating is set to cos ⁻¹ (1/3), and the phase of the other grating is set to −c
os ^-1 (1/3)

By shifting the phases in this way, the coupling coefficient on the front side decreases and becomes 1/3 of the rear side. This can be expressed by the following equation. If the phase shift position is z = 0, the rear side: -sin (z) -sin (z) =-2sin (z) The front side: sin (z + cos ^-1 (1/3)) + sin (z- cos ^-1 (1/3)) = 2 / 3sin (z)

Here, the case where the coupling coefficient is changed at a ratio of 3: 1 has been described, but it can be generalized as a matter of course. When two gratings are combined in this way, if the contributions of the two gratings are not exactly half in the width of the active layer, the sum will be out of order. However, today, since a grating can be formed by EB (electron beam processing), such a diffraction grating can be formed with sufficient alignment accuracy.

That is, in Embodiment 3, the phase difference between the two gratings is changed, and the total coupling coefficient κ is changed stepwise by interference. To obtain a large forward light output, κ before and after the λ / 4 shift position
As described above, it is effective to change .gamma .. However, the phase difference of each grating is set to 0 for a large κ portion, and an appropriate phase difference is provided for a small κ portion. With this structure, it is not necessary to control the magnitude of κ by the height of the grating or the duty ratio, etc., which enables easy and flexible control, greatly improves the yield of elements, and reduces the production cost. A semiconductor laser is obtained.

Embodiment 4 Next, a distributed feedback semiconductor laser according to a fourth embodiment of the present invention will be described. In a distributed feedback semiconductor laser in which a λ / 4 phase shift is introduced, there is a problem of hole burning in which the field increases at the λ / 4 position and the carrier density decreases. In order to solve this, the coupling coefficient of the grating may be reduced near the phase shift point. Conventionally, as a method of reducing the coupling coefficient of the grating, a method of controlling the depth of the grating or changing the duty ratio of the grating as disclosed in Japanese Patent Application Laid-Open No. 8-255954 has been attempted. Is difficult as described above, and the method of controlling the duty ratio is not practical because it is necessary to leave the InGaAsP thin film with a very small width (50 nm or less) at the center of the resonator.

Therefore, in the fourth embodiment, in a distributed feedback semiconductor laser in which a diffraction grating is formed on a side surface of an active layer by forming an optical waveguide by selective growth on a stripe whose width is modulated, two pairs of left and right sides are used. The pitch of the grating on the side surface is slightly changed so that the phase matches at both ends of the waveguide and is inverted at the center. By forming two gratings with slightly different pitches from EB, κ can be changed with high accuracy in the axial direction, and κ can be very smoothly changed so that κ is small at the center and large at the end. In addition, the effect of hole burning can be suppressed very well.

FIG. 5 is a view showing a structure of a diffraction grating for explaining the fourth embodiment. In the fourth embodiment, by modulating the distance between the two pairs of silicon dioxide films 10 described in the third embodiment, the width of the active layer formed by selective growth is modulated to modulate the equivalent refractive index. Accordingly, the diffraction grating is formed on the (111) B plane inclined surface of the selective growth. By making the phases and periods of the gratings on both slopes slightly different from each other, the coupling coefficient is changed in the resonator axial direction. The example of FIG.
This is a structure for reducing the coupling coefficient at the center of the resonator in order to solve spatial hole burning in the axial direction in the shift DFB laser.

In this diffraction grating structure, the heights of the two gratings are equal, and the phases do not change discontinuously on the way. Just make the pitch slightly different and change it by one period between the lengths of the resonators. At this time, at the end faces of both resonators, the phases of the two gratings are matched so that the phases are inverted at the center of the resonator. By doing so, the coupling coefficient gradually decreases toward the center, and considering the addition of trigonometric functions, the sign is reversed at the center, resulting in a λ / 4 phase shift. The formation of such two gratings is based on the silicon dioxide film 10
Is performed by EB patterning.

More specifically, the diffraction grating is composed of two gratings having slightly different pitches, and causes the following “beat” in the distribution of the equivalent refractive index in the axial direction z (pitch Λ << resonator Length L). From this equation, combining gratings with different pitches by Λ ² / L, κ changes in the axial direction as 2 cos (πz / L), κ is positive when z <L / 2, and κ is positive when z> L / 2. κ becomes negative and the absolute value of κ can be asymptotically approached to zero where the sign of κ is reversed. Since reversing the sign of κ is equivalent to introducing a λ / 4 phase shift,
This structure can be a grating structure that reduces κ at the λ / 4 point.

Embodiment 5 Next, a fifth embodiment of the distributed feedback semiconductor laser of the present invention will be described. In a distributed feedback semiconductor laser, there is a case where it is desired to change the coupling coefficient κ of the grating in the axial direction in order to control the field distribution of light in the axial direction. In such a case, conventionally, the height and the number of layers of the embedded diffraction grating are changed, but this structure requires a complicated manufacturing process and increases the cost. In the fifth embodiment, a region where a grating is formed is a plurality of stripe-like regions oblique to the waveguide direction, and a region with and without a grating is provided, and the ratio of these regions is changed. A diffraction grating structure that changes the coupling coefficient κ was used.

FIG. 6 is a diagram showing a structure of a diffraction grating for explaining the fifth embodiment. Although the case where the DFB laser is manufactured by the selective growth is described above, there is also a method of manufacturing the laser in which the lateral width of the active layer is formed by etching. In the case of this etching method, it is difficult to align the above-described method of dividing the diffraction grating into the left and right by bringing the left and right dividing lines to the center of the width of the active layer. Therefore, in the fifth embodiment, a structure having tolerance for the position of the active layer with respect to the grating formed by EB is used.

The magnitude of the coupling coefficient is controlled by changing the coupling coefficient on average by repeating the portion with and without the diffraction grating. The portions painted black in FIG. 6 are the portions exposed by EB. The DFB laser structure to be created has a λ / 4 shift at 0.38L from the rear,
In the rear part from the λ / 4 shift position, κ ₁ L = 3.3, and in the front part, κ ₂ L = 1.1.

In order to realize this, the grating portion is formed as usual in the rear portion, but the region in which the grating is formed in the front portion is 1 / of the active layer. this
The method of setting the 1/3 region is such that a portion with a grating is 0.4 μm and a portion without a diffraction grating is 0.8 μm in a cross section perpendicular to the resonator direction. The direction of the pattern area for the presence or absence of the grating is oblique to the resonator axis direction. By thus reducing the grating formation area to 1/3, the coupling coefficient becomes about 1/3 (strictly speaking, the coupling coefficient value undulates in the resonator axis direction, but on average it becomes 1/3. And does not significantly upset the DFB laser design). This EB pattern is formed in a region wider than the width of 1.5 μm which is to be an active layer region.

In this manner, in the photoresist step of cutting the active layer to a width of 1.5 μm, even if a lateral displacement occurs, the average value of the coupling coefficient does not change. If the repetition direction with and without the grating is perpendicular to the resonator axis direction, the coupling coefficient changes because the size of the field of the light applied to the grating changes with the lateral displacement. However, in the structure of the present embodiment, the average coupling coefficient may be considered, and the value does not change.

More specifically, when a diffraction grating is formed by EB on an InP substrate, in a portion where κ is reduced, a region for forming a grating is formed by a plurality of stripe-like regions oblique to the waveguide direction. And The magnitude of κ is controlled by the width of the stripe (0.4 μm). After this,
After growing the guide layer, the active layer, and the grading layer, mesa etching is performed so that the waveguide width becomes 1.5 μm. At this time, even if misalignment occurs, the magnitude of κ does not change. This is because the ratio of the grating region does not change even if the grating formation region has an oblique stripe shape, even if it is shifted laterally. Assuming that the formation region of the grating is a plurality of stripes that are not oblique but are parallel to the waveguide direction, in order to make the change of κ insensitive to the lateral displacement of the active layer, each stripe width is set to 0.2. It needs to be extremely short, less than μm. Since this is at the same level as the pitch, the grating almost becomes a dot, and the controllability of etching is reduced. Therefore, controllability of κ is not improved. In the fifth embodiment, with the above-described configuration, the etching for forming the diffraction grating and the etching for changing κ can be performed in the same step, and the rate of change of κ can be controlled by the ratio of the grating forming region. Therefore, extremely accurate control can be performed.

Embodiment 6 FIG. Next, a distributed feedback semiconductor laser according to a sixth embodiment of the present invention will be described. FIG. 7 is a sectional view of a distributed feedback semiconductor laser for explaining Embodiment 6 of the present invention. In the above-described fifth embodiment, the average coupling coefficient is changed depending on whether or not the area where the grating is formed has a stripe pattern. However, the coupling coefficient value is still slightly wavy. Therefore, in the sixth embodiment, the presence or absence of the peak of the grating is set at the level of several periods of the grating, so that a smooth coupling coefficient can be obtained.

In the example of FIG. 7, up to 0.38L at the rear,
The portion at the front 0.62L is an example in which the coupling coefficient is reduced to 1/3. The rear portion is a normal grating, but the front portion has a configuration in which only one of three peaks is left. The coupling coefficient is proportional to the coefficient of cos (2πZ / Λ + Ω) when Fourier expansion is performed on the modulation of the equivalent refractive index in the resonator axis direction, where the pitch is Λ. , The coupling coefficient is reduced to 1/3. That is, the contribution of the higher-order diffraction grating to the coupling coefficient is negligible as compared to the contribution of the first-order diffraction grating.
Although it is a third-order diffraction grating, it is 1 /
There are 3 effects. This is apparent in terms of Fourier coefficients.

In general, if one region is left with n out of m gratings and another region is left with q gratings in p gratings, the coupling coefficient changes at a ratio of n / m: q / p. Can be done. However, as shown in FIG. 7, if the peaks of the diffraction grating are left with a three-fold period, Bragg reflection also occurs at the upper and lower angles of the optical waveguide, and power loss occurs. Therefore, it is desirable to leave a diffraction grating at random so as not to have such periodicity as much as possible.

[0088]

As described above, the distributed feedback semiconductor laser of the present invention has the following effects by using the above-described configuration. First, as in the configurations described in the first and second embodiments, the symmetric λ / 4 shift DF
Since the threshold gain difference between the B laser and the FB-LD is about the same or more, and the front-to-back light output ratio is comparable to that of the FP-LD with a coating of 30% to 70%, the light output is about 40% as compared with the symmetric λ / 4 DFB laser. There is an effect that can be improved.

FIG. 8 is a diagram showing that the threshold gain difference and the field flatness are improved as compared with the conventional example. A
In an R-AR coated λ / 4 shift DFB laser, there are three parameters: κ ₁ , κ ₂ , L ₁ / (L ₁
Considering + L ₂ ), first, a condition for imposing a constant condition on the threshold gain of the mode that oscillates first is imposed so as to make a valid comparison. Then, comparison is made for a case where a certain front-to-back output ratio is obtained. FIG. 8 shows a case where P _f / P _r = 4. As can be seen from FIG. 8, a conventional kappa ₁
≧ κ ₂ , L ₁ ≧ L ₂ compared to the case of κ ₁ > κ ₂ , L ₁
<L ₂ is excellent in the threshold gain difference, field flatness becomes better.

Second, as described in the third and fourth embodiments, a structure utilizing the phase difference and the pitch difference between a plurality of diffraction gratings is used to accurately produce a coupling coefficient distribution in the resonator axis direction. become able to. The reason is that since the height of the diffraction grating may be constant, it is not necessary to control the height of the diffraction grating with an accuracy of several nm. The diffraction grating 1
Since each duty may be constant, it is not necessary to perform patterning with an accuracy of several tens of nm. Further, since such a plurality of diffraction gratings can be formed by EB in one photoresist process, it is possible to form a phase difference and a pitch difference between each other.

Third, the method of repeating the presence / absence of a diffraction grating at a shorter cycle than the length of the resonator and changing the ratio thereof to change the coupling coefficient in the resonator direction on average also makes the distribution of the coupling coefficient smaller. It can be created with high accuracy and reproducibility. One of the reasons is that, as described in the fifth embodiment, the repetition of the presence or absence of the diffraction grating is formed in a direction oblique to the resonator axis direction, so that the coupling coefficient changes with respect to the shift of the active layer position. This is because, at the same time, a smooth value can be obtained in which the coupling coefficient averages the presence or absence of the diffraction grating. In another method, as described in the sixth embodiment, the method of repeating the presence or absence of the peak of the diffraction grating at several levels can prevent the average value of the coupling coefficient from being uneven.

The reason why such good controllability of the coupling coefficient can be obtained is that the structure is such that the coupling coefficient is changed while the height of the diffraction grating is kept constant, and at the same time, the patterning of the diffraction grating is performed at a dose amount. Can be formed by EB exposure in which is modulated.

[Brief description of the drawings]

FIG. 1 is a diagram for describing Embodiment 1 of the present invention.

FIG. 2 is a diagram for explaining Embodiment 2 of the present invention.

FIG. 3 is a diagram for explaining a third embodiment of the present invention.

FIG. 4 is a diagram for explaining Embodiment 3 of the present invention.

FIG. 5 is a diagram for explaining Embodiment 4 of the present invention.

FIG. 6 is a diagram for explaining Embodiment 5 of the present invention.

FIG. 7 is a diagram for explaining Embodiment 6 of the present invention.

FIG. 8 is a diagram showing an example of the effect of the present invention.

FIG. 9 is a diagram illustrating an example of a conventional distributed feedback semiconductor laser.

[Explanation of symbols]

 Reference Signs List 1 n-type InP substrate 2 diffraction grating 3 n-InGaAsP guide layer 4 SCH-strained MQW layer (active layer) 5 p-InP clad layer 6 p-contact layer 7 p-side electrode 8 n-side electrode 9 AR coating film 10 silicon dioxide film

Claims (18)

[Claims] 1. A distributed feedback semiconductor laser having a λ / 4 phase shift point on a diffraction grating, wherein a main laser beam is emitted in a forward direction, a reference laser beam is emitted in a rear direction, and the phase shift point is shifted from the phase shift point. The length of the front diffraction grating in the resonator axis direction is L ₂ , the length of the diffraction grating behind the phase shift point in the resonator axis direction is L ₁ , and the coupling coefficient of the diffraction grating before the phase shift point is L ₂ . When the average value is κ ₂ , and the average value of the coupling coefficient of the diffraction grating behind the phase shift point is κ ₁ , L ₁ <L ₂ , κ ₁ · L ₁ > κ ₂ · L _2. Distributed feedback semiconductor laser. 2. The distributed feedback semiconductor laser according to claim 1, wherein κ ₁ · L ₁ −κ ₂ · L ₂ > 0.1. 3. The distributed feedback semiconductor laser according to claim 1, wherein κ ₁ · L ₁ -κ ₂ · L ₂ <1. . 4. The distributed feedback semiconductor laser according to claim 1, wherein L ₁ / (L ₁ + L ₂ )> 0.1. Semiconductor laser. 5. In a distributed feedback semiconductor laser having a phase shift point of λ / 4 on a diffraction grating, a direction in which a main laser beam is emitted is forward, a direction in which a reference laser beam is emitted is backward, and When the length of the front diffraction grating in the resonator axis direction is L ₂ , and the length of the diffraction grating behind the phase shift point in the resonator axis direction is L ₁ , L ₁ <L ₂ ; A distributed feedback semiconductor laser, wherein a value obtained by integrating a coupling coefficient of a diffraction grating ahead of a phase shift point is smaller than a value obtained by integrating a coupling coefficient of a diffraction grating behind the phase shift point. 6. The distributed feedback semiconductor laser according to claim 1, wherein the length of the resonator is L and the distance from the rear end face of the resonator is L. When the length of the diffraction grating provided forward in the resonator axial direction is L ′, L ′ <
A distributed feedback semiconductor laser characterized by L. 7. The distributed feedback semiconductor laser according to claim 1, wherein the diffraction grating is located forward of the phase shift point. Wherein the coupling coefficient of the diffraction grating behind the phase shift point is constant. 8. A distributed feedback semiconductor laser having a λ / 4 phase shift point on a diffraction grating, wherein a coupling coefficient changes in the resonator axial direction by a combination of a plurality of gratings having different periods or phase differences. A distributed feedback semiconductor laser comprising a diffraction grating. 9. The distributed feedback semiconductor laser according to claim 8, wherein each period of the plurality of gratings is constant, and a phase difference of each grating is changed for each predetermined region in the resonator axial direction, and the diffraction is performed. A distributed feedback semiconductor laser, wherein a coupling coefficient of a lattice is changed stepwise in the resonator axis direction. 10. The distributed feedback semiconductor laser according to claim 9, wherein the phases of the plurality of gratings are matched with each other in a region where the coupling coefficient is large, and the phases of the plurality of gratings are in a region where the coupling coefficient is small. A distributed feedback semiconductor laser characterized by being shifted from each other. 11. The distributed feedback semiconductor laser according to claim 8, further comprising a diffraction grating in which the periods of the plurality of gatings are slightly different from each other, and a coupling coefficient changes continuously in the resonator axial direction. And a distributed feedback semiconductor laser. 12. The distributed feedback semiconductor laser according to claim 8, wherein the plurality of gatings are arranged in parallel in the resonator axial direction and are left and right. A distributed feedback semiconductor laser, comprising two separate gratings. 13. The distributed feedback semiconductor laser according to claim 12, wherein the two gratings have slightly different periods so that the number of periods differs from each other by one period within the resonator length L. A distributed feedback semiconductor laser characterized in that the phase is matched at the resonator end face and the phase is shifted by π at the center of the resonator. 14. The distributed feedback semiconductor laser according to claim 12, wherein the two gratings modulate an active layer width for forming an optical waveguide by selective growth, and provide a selective growth portion. A distributed feedback semiconductor laser formed on both end faces. 15. The distributed feedback semiconductor laser according to any one of claims 8, 9, 10, 10, 11, 12 and 13, wherein the plurality of gratings are A distributed feedback semiconductor laser characterized in that the depth of the unevenness is fixed. 16. A distributed feedback semiconductor laser having a phase shift point of λ / 4 on a diffraction grating, wherein a pattern in which a grating is formed at an interval sufficiently smaller than the resonator length L and a region in which no grating is formed are repeated. And a diffraction grating for changing an average coupling coefficient in the resonator axial direction by changing a ratio of these regions. 17. The distributed feedback semiconductor laser according to claim 16, wherein a repetitive pattern of a region where the grating is formed and a region where the grating is not formed is provided obliquely with respect to the resonator axis direction. Distributed feedback semiconductor laser. 18. A distributed feedback semiconductor laser having a phase shift point of λ / 4 on a diffraction grating, wherein the grating peaks are left at a ratio of n out of m in a certain region, and the grating peaks are left in another region. q of p (m, n, p, q are all integers of 1 or more, m> n, p>
q) By leaving, the average coupling coefficient is n / m: q
A distributed feedback semiconductor laser comprising a diffraction grating that changes in the resonator axis direction at a ratio of / p.