JP3450169B2 - Distributed feedback semiconductor laser - Google Patents

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 whose coupling coefficient changes in the cavity axis direction.

[0002]

2. Description of the Related Art A distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) having a diffraction grating can obtain a laser oscillation operation of a single wavelength even at the time of modulation by utilizing Bragg reflection. Therefore, it is widely used for long-distance optical fiber communication. The DFB lasers widely used at present 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 both end faces of the resonator have a non-reflective coating film (hereinafter referred to as AR coating film) and a high-reflection coating film (hereinafter referred to as HR). (Referred to as coating film). Further, the λ / 4 phase shift DFB laser has a λ / 4 phase shift point at the center in the axial direction of the resonator, and both end faces of the resonator are AR-AR coated.

As described above, the uniform grating DFB laser has an AR-HR coating and therefore has an AR.
A large optical output can be obtained from the side, but 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, the uniform grating DFB laser is inferior to the λ / 4 phase shift DFB laser in the single mode property and the manufacturing yield is poor, and the dynamic single mode property when the laser is digitally modulated is also inferior, so that the optical communication However, there is a drawback that the signal error rate becomes large when used for.

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

Uniform grating DFB laser and λ / 4
A phase shift DFB laser is used for optical communication.
Each of them has the above-mentioned contradictory advantages and disadvantages, and researches on lasers for solving these disadvantages have been conducted. For example, in the 1985 National Conference on Semiconductor and Materials Division of the Institute of Electronics and Communication Engineers, Article No. 310 (1985), AR-HR coating was applied to a λ / 4 phase shift DFB laser, and the phase shift position was shifted to the rear HR side. Proposes a method of extracting a large front 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, and therefore a single mode failure occurs with a certain probability. Therefore, the asymmetric λ / 4 phase shift DFB laser with the AR-HR coating inevitably has defects at a certain rate even if the single mode yield is improved as compared with the uniform grating DFB laser.

Also, in the AR-AR coated λ / 4 phase shift DFB laser, studies have been conducted to make the front light output and the rear light output different from each other. For example, "Asymmetric λ / 4-Shifet InGaAsP / InP DFB Laser
s "M. Usami et. (IEEE J. Quantum Electron., QE-23,
p.815-821, June 1987), by shifting the λ / 4 phase shift point forward from the center of the resonator axial direction,
A method for improving the front light output without deteriorating the single mode property is disclosed.

Further, for example, Japanese Patent Publication No. 1-37872 discloses a method of changing the height of the diffraction grating before and after the phase shift point to change the front light output and the rear light output. In addition, for the analysis when changing the height of such a diffraction grating, see, for example, "λ / 4-Shifet InGa
AsP / InP DFB Lasers "k.Utaka et. (IEEE J. Quantum E
lectron., QE-22, p.1042-1051, July 1986).

A conventional distributed feedback semiconductor laser will be described below with reference to the drawings. FIG. 8 is a sectional view showing an example of the configuration of a conventional distributed feedback semiconductor laser of this type. In FIG. 8 , 1 is an n-type InP substrate, 2 is a diffraction grating, 3 is an n-InGaAsP guide layer, 4 is a SCH-strained MQW layer, 5 is a p-InP cladding layer, 6 is a p-contact layer, and 7 is p. A side electrode, 8 is an n-side electrode, and 9 is an AR coating film. N-type InP with surface orientation of (100)
The substrate 1 is provided with a diffraction grating 2 having a λ / 4 phase shift 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 a phase shift of the diffraction grating by π.

The characteristic feature of the λ / 4 phase shift DFB laser shown in FIG . 8 is that the height of the peak of the diffraction grating 2 is lowered from the λ / 4 phase shift position to the front side and increased to the rear side. It is a point. On the substrate 1 on which the diffraction grating 2 is formed, the thickness is 100 nm and the band gap wavelength is 1.15 μm (1.
An n-InGaAsP guide layer 3 (having a composition of 15 μm), an active layer, and a p-InP clad layer 5 are laminated. in recent years,
The SCH-strained MQW layer 4 is used for the active layer. SCH is an abbreviation for Separate Confinement Heterostructure, and InGaAsP layers are provided on both sides of the strained MQW to have an optical confinement effect, and MQW is Multi-Quantu
Abbreviation for m well, which is called a multiple quantum well structure.

In an optical waveguide having such a diffraction grating, the equivalent refractive index in the cavity axis direction is, for example, n = n ₀ + n ₁ · c
It fluctuates periodically like os (2β ₀ z + Ω). Here, let k0 be the wave number of light in a vacuum, and κ defined as κ = k ₀ n _1/2 is called the coupling coefficient. The laser shown in FIG.
In order to change the coupling coefficient κ before and after the / 4 shift point,
This is an example in which 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 the DFB laser, in order to make the field intensity of light as flat as possible in the axial direction of the resonator, it may be preferable to change the coupling coefficient κ in the axial direction of the resonator in terms of design. For example, in Japanese Patent Laid-Open No. 8-255954, λ
In the / 4 shift DFB laser, the duty of the diffraction grating is changed in order to suppress the spatial hole burning in the axial direction in which the field becomes large and the carrier density becomes small at the λ / 4 position. The coupling coefficient is reduced. In this case, since the height of the diffraction grating is not changed, the manufacturing can be performed relatively easily.

For the same purpose, Japanese Patent Publication No. 8-17262 presupposes that a high-order diffraction grating is used.
However, the second-order diffraction grating is
Since distributed feedback due to Bragg reflection is significantly weakened, the design is not optimal, and there are no product examples that actually apply such a method.

Further, in the DFB laser for analog use, in order to improve the linearity of the light output characteristic with respect to the injection current, the field flatness of light in the axial direction of the cavity is particularly required. In order to realize such field flatness in the 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, in Japanese Patent Laid-Open No. 9-64456, selective growth is performed. The coupling coefficient in the cavity axial direction is changed by utilizing the fact that the thickness of the epitaxial layer changes depending on the width of the growth inhibition mask. As an example having the same purpose of improving analog performance, Japanese Patent Laid-Open No. 6-310
There is a Japanese Patent No. 806 publication, and 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.

Further, compared to the uniform grating, the introduction of the phase shift into the diffraction grating has a drawback that it is difficult to manufacture. For example, in Japanese Patent Laid-Open No. 61-222189,
Diffraction gratings are provided above and below the active layer, and the pitches of the diffraction gratings are shifted so that single mode oscillation is achieved.

[0017]

The conventional distributed feedback semiconductor laser described above has the following problems. First, in the conventional asymmetric λ / 4 shift DFB laser, the forward light output is increased as compared with the symmetric λ / 4 shift DFB laser, but the threshold gain difference is poor, and the code required for recent optical digital communication is The error rate criteria cannot be met. Further, there is also a problem that the single mode yield is lowered and the cost is increased.

[0018] For example, in the technology disclosed in Article No. 310 of the National Conference on Semiconductor and Materials Division of the Institute of Electronics and Communication Engineers of 1985, which has been coated with AR-HR, there is a certain probability that the technology is influenced by the end face phase on the HR side. Defects occur. In addition, λ / 4 shift DF of AR-AR coating
With respect to the B laser, when the length from the phase shift position to the rear side is L ₁ , its coupling coefficient is κ ₁ , the length from the phase shift position to the front side is L ₂ , and its coupling coefficient is κ ₂ , ₁ >
When L ₂ and κ ₁ = κ ₂ are asymmetrical,
When L ₁ = L ₂ and κ ₁ > κ ₂ are asymmetrical as in Japanese Patent No. 7872, the threshold gain difference and the field flatness in the cavity axis direction are worse than in the symmetrical type.

The deterioration of the threshold gain difference is caused by the deterioration of the code error rate,
Although the yield of single mode is deteriorated, the deterioration of field flatness is also deteriorated of single mode property. That is, when a field distribution of light occurs in the cavity axis direction, a carrier concentration distribution of the active layer occurs accordingly, and the equivalent refractive index of the light waveguide changes in the cavity axis direction. Associated with the deterioration of the single mode.

Second, there is a problem in that it is very difficult to change the coupling coefficient such that κ ₁ > κ _{2 in} the middle of the resonator, as in Japanese Patent Publication No. 1-37872. In today's strained MOW, crystals are grown by metalorganic vapor phase epitaxy (hereinafter referred to as MO-VPE method). At this time, the diffraction grating formed on the substrate is
H _3), arsine (in breaking during heating in AsH ₃₎ atmosphere, to obtain the desired height. Therefore, the manufacturing reproducibility of the diffraction grating whose height changes in the cavity axis direction cannot be sufficiently ensured.

In the above-mentioned Japanese Patent Laid-Open No. 8-255954, the height of the diffraction grating is kept constant and the duty ratio is changed. However, in the DFB laser for long-wave optical communication, the pitch of the diffraction grating is about 200 nm to 250 nm. The length of one crest of one diffraction grating is about 100 nm to 120 nm, and if it is attempted to modulate this length, pattern forming accuracy of the order of 10 nm is required. Similarly, it is difficult to manufacture because it does not exist in the mass production technology.

Further, in the above-mentioned Japanese Patent Laid-Open No. 9-64456, the coupling coefficient in the axial direction of the resonator is changed by utilizing the fact that the layer thickness of the epitaxial layer changes depending on the growth inhibition mask width of the selective growth. Although this method can be used to form the height of the diffraction grating with high accuracy, there is a limit to the change in the height of the diffraction grating because the mask width dependence of the selective growth is used, and therefore the asymmetric λ / 4 Shift DFB
This is not applicable when it is desired to change the height of the diffraction grating with a considerable size, such as a laser.

Further, as in the above-mentioned Japanese Patent Application Laid-Open No. 61-222189, an attempt is made to improve the threshold gain by providing diffraction gratings above and below the active layer without using a phase shift which is difficult to manufacture. Even in this case, a single mode defect is stochastically generated. This is because it is not possible to control the phase difference between the diffraction gratings above and below the active layer, and the difference in the thresholds may cause a significant decrease in the threshold gain difference.

The present invention has been made to solve the above problems, and can maintain a favorable threshold gain difference even when compared with a λ / 4 phase shift DFB laser, and is not affected by the end face phase and is uniform. It is an object of the present invention to provide a DFB laser that can obtain an optical output comparable to that of a grating DFB laser and that can be easily manufactured.

[0025]

The end face phase dependence of the diffraction grating on the HR coated surface is a problem inherent in DFB lasers. Especially in terms of the characteristic of threshold gain difference,
If the phase of the end face of the HR coated surface changes, the condition that the threshold gain difference deteriorates always appears. Therefore, if the HR coat is applied, the single mode yield cannot theoretically be 100%. Only with AR-AR coating can the problem of end face phase be eliminated.

Considering an AR-AR coated λ / 4 phase shift DFB laser, the length of the rear part is L ₁ from the point of phase shift, the coupling coefficient is κ ₁ , and the length of the front part from the phase shift point is L. ₂ , the coupling coefficient is κ ₂ ,
Consider varying these parameters. If κ ₁ L ₁ > κ ₂ L _{2 at} this time, the Bragg reflection in the rear part is large, and thus the front output is large. κ ₁ L ₁ ＞ κ ₂ L ₂
To achieve L ₁ > L ₂ , κ ₁ = κ ₂ and κ ₁
Considering the method of> κ ₂ and L ₁ = L ₂ , the threshold gain difference becomes worse than the symmetrical λ / 4 shift DFB laser. However, the amount of deterioration of the threshold gain difference is smaller when the phase shift position is not moved and only the coupling coefficient is changed. Regarding the field flatness, it is better to change only the coupling coefficient.

Therefore, while satisfying κ ₁ L ₁ > κ ₂ L ₂ ,
Examining the method of greatly changing the coupling coefficient such that κ ₁ > κ ₂ and L ₁ <L ₂ , it was found that there are conditions for improving the threshold gain difference as compared with the symmetrical λ / 4 shift DFB laser. . At this time, when changing only the phase shift position,
The field flatness is also improved compared to the case where only the coupling coefficient is changed.

It is also a big problem to form such a large change of κ in the cavity axis direction. Actual MO-VPE
Considering growth, changing the height of the diffraction grating is
Quite difficult. Therefore, let us consider changing the coupling coefficient κ under the condition that the height of the diffraction grating is constant. The coupling coefficient is an amount in which the equivalent refractive index in the waveguide direction is proportional to the width of change in a trigonometric function. Therefore, the total coupling coefficient can be changed by utilizing the property of addition of trigonometric functions to form multiple gratings in parallel with the waveguide and changing the phase difference of these gratings for each region. .

Further, in the manufacture of such a plurality of diffraction gratings, it is possible to form diffraction gratings having different pitches and phases in the same plane by using the EB technology of today. In addition, the coupling coefficient can be changed on average by thinning out diffraction gratings having the same height. In particular, if 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 will be gentle, and the active layer formation position will be in the cavity direction. On the other hand, even if it shifts in the vertical direction, the average coupling coefficient does not change.

As a specific structure, the distributed feedback semiconductor laser of the present invention has a phase shift point of λ / 4 on the diffraction grating and has distributed reflection feedback (AR) coating on both end faces. Type semiconductor laser, the direction of emitting the main laser beam is forward, the direction of emitting the reference laser beam is backward, the length in the resonator axial direction of the diffraction grating in front of the phase shift point is L ₂ , and the phase shift point is The length of the further rear diffraction grating in the cavity axis direction is L ₁ , the coupling coefficient of the diffraction grating forward of the phase shift point is a constant value κ ₂ , and the coupling coefficient of the diffraction grating rear of the phase shift point is a constant value. If κ ₁ ,
0.3 ≦ L ₁ / (L ₁ + L ₂ ) ≦ 0.45, κ ₁ · L ₁
It is characterized in that> κ ₂ · L ₂ .

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

Further, it is characterized in that κ ₁ · L ₁ −κ ₂ · L ₂ <1.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1. Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 is a sectional view showing a structure of a distributed feedback semiconductor laser for explaining a first embodiment of the present invention, and is a view corresponding to FIG . 8 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
nP clad 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, according to the first embodiment of the present invention, a diffraction grating 2 is provided on an n-type InP substrate 1,
On top of that, n-InGaAsP guide layer 3, active layer 4, p
-InP clad layer 5 is provided. The diffraction grating 2 is provided with a λ / 4 phase shift point at the point where the resonator length L is divided into L ₁ and L ₂ . Here, the L ₁ side is backward, L
Assuming that the _second side is the front side, the rear side coupling coefficient is κ ₁ , and the front side coupling coefficient is κ ₂ , the diffraction grating 2 has κ ₁ > κ ₂ ,
It is formed so that L ₁ <L ₂ and κ ₁ L ₁ > κ ₂ L ₂ are satisfied.

The active layer 4 may be bulk, but is preferably a SCH-strained MQW layer 4 having excellent light output characteristics. Also n
Between the InGaAsP guide layer 3 and the SCH-strained MQW layer 4 in order to improve the crystallinity of the strained MQW.
InP spacers may be sandwiched. The active layer 4 preferably has a lateral width of about 1.5 μm and a BH (Buried Heterostructure) structure in which a current blocking layer is provided on both sides thereof. Then, p- is formed on the active layer 4 and the BH structure.
The contact layer 6 is laminated to form the p-side electrode 7, the n-side electrode 8 is formed on the back surface of the n-type InP substrate 1, and the 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 favorable threshold gain difference can be obtained. That is, when the front light output is P _f and the rear light output is P _r , P _f / P _r is approximately exp (a (κ ₁ L ₁ −κ ₂ L ₂ )). κL =
Considering the symmetric λ / 4 shift laser of 1.8 as a reference, when asymmetrical, a in the above formula is about 2.2.

Even in the symmetrical λ / 4 shift DFB laser, the front / rear ratio is not 1 depending on the phase of the end face due to the residual reflection on the end face, so that the light output from both end faces in the chip selection process during actual manufacturing. It is necessary to perform a process in which the one having a larger power in measurement is the front, and the chip selection process is longer than that of the uniform grating DFB laser. Therefore, in the λ / 4 shift DFB laser,
The ability to increase the light output from one fixed end surface is effective in shortening the chip selection process.

Therefore, in the asymmetrical λ / 4 shift DFB laser as shown in FIG. 1, in order to exceed the variation in the front-rear output ratio due to the end face phase and to surely increase the output from one side, at least 1.2 times is designed. It is necessary to have a front-rear ratio of. This requirement is κ ₁ L ₁ -κ ₂ L ₂ > 0.1
The diffraction grating may be formed so as to satisfy the above condition.

On the other hand, the front / rear ratio cannot be increased without limit. Assuming that AR-AR coating is used, the power density of light near both end faces varies depending on the ratio of front-rear ratio. If this ratio is large, a kink occurs due to the spatial hole burning. In order to prevent this, the front / rear output ratio cannot be set to 10 times or more. Therefore, it is necessary to satisfy κ ₁ L ₁ −κ ₂ L ₂ <1. Further, 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
Since it reaches the threshold gain difference and the front / rear ratio, the end face reflectance should be kept to 0.5% or less if possible. Then, good threshold gain differences can be obtained at all end face phases, and the theoretical single-mode yield is 100%.

In the above description of the first embodiment, the lower grating structure having the diffraction grating 2 below the active layer 4 has been described, but an upper grating or a structure having a grating beside the active layer may be used. In addition, 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 in InP in discrete steps. Further, in the above description of the first embodiment, the conductivity type of the substrate is n-type, but of course it may be p-type, and the material thereof may be a semiconductor material other than InGaAsP / InP-based, such as InGaAIAs-based. It is also possible to integrate such a distributed feedback semiconductor laser with a modulator or the like.

Note that the first necessary condition is κ ₁ > κ
₂ , L ₁ <L ₂ , κ ₁ L ₁ > κ ₂ L ₂ are listed, but since κ ₁ > κ ₂ can be derived from the other two conditions, the necessary condition is L ₁ <L ₂ , κ ₁ L ₁ > κ ₂ L ₂
The following two conditions are sufficient. However, here κ ₁ , κ
Each ₂ is an average value of the coupling coefficient, and κ used in the present specification means a coupling coefficient or an average value of the coupling coefficients. Extending when κ changes slightly more finely, the condition becomes L ₁ <L ₂ and the integral value of κ from 0 to L ₁ > the integral value of κ from L ₁ to L. Can also be generalized with.

That is, when a forward / backward asymmetric structure is used for the purpose of improving the forward light output of a λ / 4 phase shift DFB laser, symmetric λ / 4 phase shift is performed regardless of whether κ is asymmetrical or L is asymmetrical. Single-mode yield is worse than DFB laser. Specifically, the threshold gain difference Δ
is worse than the λ / 4 phase-shifted DFB laser in which α is symmetrical,
Since the field distribution of light also becomes large, kinks occur and the side mode suppression ratio (SMSR) deteriorates, making noise unusable in communication applications. In the first embodiment, in order to improve the forward light output, κ ₁ L ₁ > κ ₂ L ₂ is set. However, by setting L ₁ <L ₂ , a good threshold gain difference can be obtained, which is optimal. The point is that it is possible to obtain a larger threshold gain difference Δα than the symmetrical λ / 4 phase shift DFB laser and to make the field intensity distribution uniform. As a result, the distributed feedback semiconductor laser of the present embodiment has improved side mode suppression ratio, kink yield, and single axis mode yield as compared with the conventional λ / 4 phase shift DFB laser, which is about 30% as a whole. The yield can be improved to about 70%.

Example 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 of Example 1 has an (100) plane orientation of n-type In.
It is assumed that the diffraction grating 2 corrugated in the [011] direction is formed on the P substrate 1. The pitch of the diffraction grating 2 is 203 nm for the 1.3 μm band DFB laser and 243 nm for the 1.55 μm band DFB laser. Resonator length L is 450 μm
Then, a λ / 4 phase shift is provided at a position 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 = 279 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 100n
an n-InGaAsP guide layer 3 having a composition of 1.15 μm at m,
The SCH-strained MQW layer 4 is formed and the p-InP glad layer 5 is laminated. The SCH-strained MQW layer 4 includes five compressive strained InGaAsP well layers and tensile strained InG.
The strain MQW is a stress strain compensation type strain MQW composed of an aAsP barrier layer. The thickness of the compressive strained InGaAsP well layer is 4.5 nm, and the thickness of the tensile strained InGaAsP barrier layer is 10 nm.

In such a layered structure, the coupling coefficient is κ ₁ L = 3.3 and κ ₂ L = in the rear part and the front part, respectively.
1.1, the front-rear output ratio is P _f / P _r = 4,
As the normalized threshold gain difference ΔαL, a sufficiently large value of ΔαL = 0.76 is obtained. Considering a symmetrical λ / 4 shift DFB laser that can obtain the same threshold gain as the DFB laser of this structure,
When κL = 1.8, ΔαL = 0.73, so symmetric type λ
It can be seen that a better threshold gain difference can be obtained than the / 4 shift DFB laser.

Embodiment 2 . For example, in the distributed feedback semiconductor laser according to the first embodiment described above, the coupling coefficient κ of the diffraction grating is
Needs to be changed in the axial direction. Since this coupling coefficient κ is almost proportional to the grating height of the diffraction grating, it suffices to change this, but the grating storage height (depth of concavities and convexities) is changed by each region in the actual grating embedded growth. Is difficult. Therefore, in the second embodiment , the gratings are divided into left and right in parallel above and below the optical waveguide, the phases of the left and right gratings are aligned in the region where κ needs to be increased, and the left and right gratings are arranged in the region where κ needs to be lowered. The structure is to shift the phase of the grating. Hereinafter, the second embodiment will be described with reference to the drawings.

FIG . 2 is a perspective view showing a diffraction grating structure for explaining the second embodiment of the present invention. If the coupling coefficient can be changed in the resonator axial direction as described above,
Although good DFB laser characteristics can be obtained, it is difficult to change the change of the coupling coefficient with the height of the diffraction grating in the actual manufacturing. Thus in the second embodiment, the height of the diffraction grating structure of changing the constant <br/> and while the coupling coefficient. This embodiment
The distributed feedback DFB laser of state 2 is, as shown in FIG.
First, on the n-type InP substrate 1 whose surface plane orientation is (100),
Silicon dioxide film 1 with a width of 3 μm extending in the [011] direction
Two 0s are formed with a spacing of 1.8 μm. The silicon dioxide film 10 serves as a growth blocking mask for forming the active layer by MO-VPE selective growth. The reason why the active layer of the DFB laser is formed by selective growth is that the width of the active layer can be well controlled and manufactured with little variation. As a result, the coupling coefficient and the oscillation wavelength can be precisely controlled, and the kink due to the waviness of the active layer width can be obtained. Can be eliminated. This MO-VP
InP and InGaAsP are epitaxially grown by the selective growth of E, but the source gas is TMI, TMG, As.
H ₃ and PH ₃ are used, and the organic metal is supplied by bubbling hydrogen. Regarding doping, Si ₂ H
₆ , a gas obtained by diluting DMZn with hydrogen is used. The growth pressure is 75 Torr.

In the diffraction grating 2, two gratings are arranged in parallel in the 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 shown schematically
It is a diagram 3.

The two gratings are in phase with each other in the rear 0.38 L region. And λ / 4 shift is introduced at 0.38L from the rear. The λ / 4 shift is to shift the phase of the change in the refractive index 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 cos ^-1 (1/3) and the phase of the other grating is -c.
os ^-1 (1/3) Move it.

By thus shifting the phases from each other, the coupling coefficient on the front side is reduced to 1/3 on the rear side. This can be expressed by the following formula. When 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 / 3s
in (z)

Here, the case where the coupling coefficient is changed by 3: 1 has been described, but of course it can be generalized. When the two gratings are combined in this way, the addition of the two gratings will be messed up unless the contributions of the two gratings are halved within the active layer width. 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 the second embodiment , the phase difference between the two gratings is changed and the total coupling coefficient κ is changed stepwise due to interference. If you want to obtain a large forward light output, κ before and after the λ / 4 shift position
It has been described above that it is effective to change the value of .gamma., But the phase difference between the gratings is set to 0 for the large .kappa. This structure eliminates the need to control the size of κ based on the grating height, duty ratio, etc., enabling easy and flexible control, greatly improving the device yield, and reducing the manufacturing cost. Type semiconductor laser can be obtained.

Third embodiment . Next, a third embodiment of the distributed feedback semiconductor laser of the present invention will be described. The distributed feedback type semiconductor laser having the λ / 4 phase shift has a problem of hole burning in which the field becomes large and the carrier density becomes small at the λ / 4 position. 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 in the above-mentioned Japanese Patent Laid-Open No. 8-255954 has been tried. The method of controlling the duty ratio is difficult as described above, and the method of controlling the duty ratio is also practically impossible because it is necessary to leave the InGaAsP thin film with an extremely small width (50 nm or less) at the center of the resonator.

Therefore, in the third embodiment , in the distributed feedback semiconductor laser in which the diffraction grating is formed on the side surface of the active layer by forming the optical waveguide by selective growth on the stripe whose width is modulated, two pairs of right and left are provided. The grating pitch on the side surface is slightly changed so that the phases match at both ends of the waveguide and are inverted at the center. By forming two gratings with slightly different pitches by EB, κ can be changed with high accuracy in the axial direction, and κ can be changed very gently at the center and at the edges. The effect of hole burning can be suppressed very well.

FIG . 4 is a diagram showing the structure of a diffraction grating for explaining the third embodiment . The third embodiment has a structure in which the width of the active layer formed by selective growth is modulated by modulating the distance between the two pairs of silicon dioxide films 10 described in the second embodiment to modulate the equivalent refractive index. Therefore, the diffraction grating comes to be formed on the (111) B plane slope of the selective growth. Then, the coupling coefficient is changed in the resonator axial direction by making the phases and periods of the gratings on both slopes slightly different from each other. In the example of FIG. 4 , the symmetrical type λ / 4
In the shift DFB laser, this is a structure for reducing the coupling coefficient in the central portion of the resonator in order to solve spatial hole burning in the axial direction.

In this diffraction grating structure, the heights of the two gratings are equal and the phase does not change discontinuously in the middle. However, the pitch is slightly different, and the length of the resonator can be changed by one period. At this time, the phases of the two gratings are made to match on the end faces of both the resonators, and the phases are inverted at the center of the resonator. In this way, 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 performed by the silicon dioxide film 10
It is possible by patterning EB by EB.

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 Long L). Combining gratings whose pitch differs by Λ ² / L from this equation,
When κ changes in the axial direction like 2cos (πz / L), κ becomes positive when z <L / 2 and becomes negative when z> L / 2. The value can be asymptotic to zero. Since inverting 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.

Fourth Embodiment Next, a fourth embodiment of the distributed feedback semiconductor laser of the present invention will be described. In the distributed feedback semiconductor laser, it may be 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, a structure in which the height and the number of layers of the embedded diffraction grating are changed is used, but this structure requires a complicated manufacturing process and is costly. this
In the fourth embodiment , a region forming a grating is made into a plurality of stripe-shaped regions oblique to the waveguide direction, a region with a grating and a region without a grating are provided, and the ratio of these regions is changed to combine the regions. A diffraction grating structure that changes the coefficient κ is used.

FIG . 5 is a diagram showing the structure of a diffraction grating for explaining the fourth embodiment . Although the case of manufacturing the DFB laser by selective growth has been described above, there is also a method of forming the lateral width of the active layer by etching as a laser manufacturing method. In the case of this etching method, in the above-described method of dividing the diffraction grating into left and right, it becomes difficult to align the line for dividing left and right with the center of the lateral width of the active layer. Therefore, in the fourth embodiment , the grating formed by the EB has a structure with a tolerance of the position of the active layer.

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 portion painted in black in FIG. 5 is the portion exposed by EB. The DFB laser structure to be created has a λ / 4 shift at a position of 0.38L from the rear,
From the λ / 4 shift position, κ ₁ L = 3.3 in the rear part and κ ₂ L = 1.1 in the front part.

In order to realize this, the rear part forms a grating region as usual, while the front part defines a region forming the grating as 1/3 of the active layer. this
The setting method for the 1/3 region is 0.4 μm for the part with the grating and 0.8 μm for the part without the diffraction grating as seen in the cross section perpendicular to the cavity direction. Then, the direction of the pattern region with or without the grating is made oblique with respect to the resonator axis direction. By setting the grating formation region to 1/3 in this way, the coupling coefficient becomes about 1/3 (strictly speaking, the coupling coefficient value is wavy in the cavity axis direction, but becomes 1/3 on average). Therefore, the DFB laser design is not seriously disturbed). This EB pattern is formed in a region wider than the width of 1.5 μm which becomes the active layer region.

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

More specifically, when the diffraction grating is formed on the InP substrate by EB, the region where the grating is formed is a plurality of stripe-shaped regions oblique to the waveguide direction in the portion where κ is reduced. And The width of this stripe (0.4 μm) controls the size of κ. After this,
After growing the guide layer, the active layer, and the glad layer, mesa etching is performed so that the waveguide width becomes 1.5 μm. At this time, the magnitude of κ does not change even if misalignment occurs. This is because the grating formation region has an oblique stripe shape, and the ratio of the grating region does not change even if the grating formation region is laterally displaced. If the region where the grating is formed is a plurality of stripes that are not oblique and are parallel to the waveguide direction, in order to make the variation of κ insensitive to lateral displacement of the active layer, each stripe width is 0.2 It will be necessary to make it extremely short, below μm. Since this is at the same level as the pitch, the grating is almost a dot, and the controllability of etching is reduced. Therefore, the 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 formation region. Therefore, extremely accurate control can be performed.

Embodiment 5 . Next, a fifth embodiment of the distributed feedback semiconductor laser of the present invention will be described. FIG. 6 is a sectional view of a distributed feedback semiconductor laser for explaining the fifth embodiment of the present invention. In the above-described fourth embodiment , the average coupling coefficient is changed depending on whether or not the area forming the grating has a striped pattern. However, the coupling coefficient value is slightly wavy. So this implementation
In state 5 , the structure with or without the peaks of the grating is set at the level of several cycles of the grating to obtain a smooth coupling coefficient.

In the example of FIG . 6 , for the rear 0.38L,
The front 0.62L is an example where the coupling coefficient is 1/3. The rear part is a normal grating, but only one mountain is left in three parts in the front part. The coupling coefficient is proportional to the coefficient of cos (2 πZ / Λ + Ω) when the pitch of Λ is used and the modulation of the equivalent refractive index in the cavity axis direction is Fourier expanded. If you leave, the coupling coefficient becomes 1/3. That is, the contribution of the higher-order diffraction grating to the coupling coefficient is negligible compared to the contribution of the first-order diffraction grating, and there is one peak in three.
It is also a 3rd-order diffraction grating, but as a 1st-order diffraction grating 1 /
It has 3 effects. This is clear in terms of Fourier coefficients.

Generally, if the number of peaks of the diffraction grating is left in n in m in one area and q in p in another area, the coupling coefficient changes at the ratio of n / m: q / p. Can be made. However, as shown in FIG. 6 , if the peaks of the diffraction grating are left in a triple cycle, Bragg reflection also occurs at the upper and lower angles of the optical waveguide, resulting in power loss. Therefore, it is desirable to leave the diffraction grating randomly so that such periodicity is avoided as much as possible.

[0068]

As described above, the distributed feedback semiconductor laser of the present invention having the above-described structure produces the following effects. First, as in the configurations described in the first and second embodiments, the symmetrical λ / 4 shift DF
The threshold gain difference is about the same as or more than that of the B laser, and the front-to-back optical output ratio comparable to that of the FP-LD with 30% to 70% coating can be obtained. There is an effect that can improve.

FIG . 7 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 shifted DFB laser, there are three parameters: κ ₁ , κ ₂ , L ₁ / (L ₁
Considering + L ₂ ), the condition that the threshold gain of the mode that oscillates first becomes a constant condition is imposed so that a valid comparison can be made. Then, comparison will be made for the case where a certain constant front-rear output ratio is obtained. FIG. 7 shows the case of P _f / P _r = 4. As can be seen from FIG. 7 , the conventional κ ₁
Compared to the case of ≧ κ ₂ and L ₁ ≧ L ₂ , κ ₁ > κ ₂ and L _{1 of the} present invention
<L ₂ is excellent in the threshold gain difference and the field flatness is also improved.

Secondly, as described in the second and third embodiments , the structure utilizing the phase difference and pitch difference of a plurality of diffraction gratings is used to accurately manufacture the coupling coefficient distribution in the resonator axial direction. become able to. The reason is that the height of the diffraction grating does not have to be constant and therefore the height of the diffraction grating does not have to be controlled with an accuracy of several nm. 1 of the diffraction grating
Since each duty may be constant, it is not necessary to perform patterning with an accuracy of several 10 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 or a pitch difference between them.

Thirdly, even in the method of repeating the presence / absence of the diffraction grating in a cycle shorter than the resonator length and changing the ratio thereof on average to change the coupling coefficient in the resonator direction, the coupling coefficient distribution is You will be able to create with high accuracy and reproducibility. The reason is that, as described in the fourth embodiment , the coupling coefficient changes with respect to the displacement of the active layer position by forming the presence or absence of the diffraction grating in a direction oblique to the cavity axis direction. This is because, at the same time, a smooth coupling coefficient can be obtained by averaging the presence or absence of the diffraction grating. As another one, as described in the fifth embodiment , it is possible to prevent unevenness in the average value of the coupling coefficient by repeating the presence or absence of peaks of the diffraction grating at several levels.

The reason why such a good controllability of the coupling coefficient is obtained is that the coupling coefficient is changed while the height of the crests of the diffraction grating is kept constant. At the same time, the patterning of the diffraction grating is performed at a dose amount. This is because it can be formed by EB exposure that is modulated.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a first embodiment of the present invention.

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

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

FIG. 4 is a diagram for explaining a third embodiment of the present invention.

FIG. 5 is a diagram for explaining a fourth embodiment of the present invention.

FIG. 6 is a diagram for explaining a fifth embodiment of the present invention.

FIG. 7 is a diagram showing an example of effects of the present invention.

FIG. 8 is a diagram showing an example of a conventional distributed feedback semiconductor laser.

[Explanation of symbols]

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

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-3-110885 (JP, A) JP-A-62-155584 (JP, A) JP-A-61-47685 (JP, A) JP-A-5- 48197 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01S 5/00-5/50 JISST file (JOIS)

Claims (3)

(57) [Claims] 1. A distributed feedback semiconductor laser having a phase shift point of λ / 4 on the diffraction grating and antireflection (AR) coating on both end faces, the main laser light emitting direction is forward, The length in the resonator axial direction of the diffraction grating in front of the phase shift point is L ₂ , and the length in the resonator axial direction of the diffraction grating behind the phase shift point is L. _{1. If} the coupling coefficient of the diffraction grating before the phase shift point is a constant value κ ₂ and the coupling coefficient of the diffraction grating behind the phase shift point is a constant value κ ₁ , 0.3 ≦ L ₁ / (L ₁ + L ₂ ) ≦ 0.45, κ ₁ · L ₁
A distributed feedback semiconductor laser having a characteristic of> κ ₂ · L ₂ . 2. The distributed feedback semiconductor laser according to claim 1, wherein κ ₁ · L ₁ −κ ₂ · L ₂ > 0.1. 3. A distributed feedback semiconductor laser according to claim 1, wherein κ ₁ · L ₁ −κ ₂ · L ₂ <1. .