JPH08279651A - Semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE: To provide a method of manufacturing a semiconductor laser which is improved in analog modulation distortion characteristics and auxiliary mode suppression ratio and restrained from deteriorating in signal due to noises. CONSTITUTION: A low-reflectivity film 5 and a high-reflectivity film 6 are provided to the rear and front of a device which forms the resonator of a semiconductor laser respectively, and a diffraction grating 7 is formed on all the surface of the resonator and so set as to make its front region 7b larger than its rear region 7a in coupling coefficient. For instance, the diffraction grating 7 is structured in such a manner that its front side is higher than its rear side. As the diffraction grating 7 is formed on all the surface of the resonator, an auxiliary mode by a composite resonator is restrained. The front region 7b of the diffraction grating 7 is set higher in coupling coefficient than the rear region 7a, whereby an electrical field intensity distribution can be set even along the direction of a resonator, and an analog modulation distortion can be lessened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having excellent analog modulation distortion characteristics.

[0002]

2. Description of the Related Art A single-axis mode semiconductor laser with high efficiency and small intermodulation distortion is required for an analog modulation light source used in a subcarrier multiplex optical transmission system. For example, for mobile communication systems, third-order intermodulation distortion (third)
intermodulation distortio
n; IMD ₃₎ is sufficiently small semiconductor laser is required. Also, in the optical CATV system, the complex second-order distortion (c
instant second order dis
CSO, complex third-order distortion (compos)
ite triple beat distortion
An element having a low n; CTB) is required.

Distributed feedback semiconductor laser (DFB laser)
Has excellent single mode of oscillation and is used as a light source for analog modulation. However, in the conventional DFB laser, the electric field intensity distribution in the cavity direction is largely non-uniform, and therefore the linearity of the current-optical output characteristics is large. Was insufficient, and the intermodulation distortion characteristics were not so excellent. This is because in a DFB laser, the greater the nonuniformity of the electric field intensity distribution in the cavity direction, the greater the change in the electric field intensity distribution due to current injection, and this change is due to the non-linearity of the current-optical output (IL) characteristic. This is because it causes Therefore, in order to reduce the change in the electric field strength distribution, it is necessary to make the electric field strength distribution in the resonator direction uniform.

For such a problem, G. Morthi
er et al., for example, IEEE Photonics Te
chnology Letters vol. 2 n
o. 6 (1990) pp. 388-390 proposes a λ / 4 phase shift type DFB laser formed so that the coupling coefficient becomes high near the end face. According to this structure, it is described that the electric field strength distribution in the resonator direction is made uniform and analog modulation distortion is reduced. However, in addition to the non-uniformity of the electric field strength distribution, factors of analog modulation distortion of the DFB laser include influence of relaxation oscillation and influence of leakage current. In particular, in order to suppress the influence of leakage current, it is necessary to operate with a current as low as possible. On the other hand, since the λ / 4 phase shift laser has a structure in which both end faces are coated with a low reflectance, sufficient efficiency cannot be obtained. Therefore, when applied to analog optical transmission, there is a problem that a high drive current is required and the modulation distortion due to the leakage current becomes remarkable.

On the other hand, Japanese Patent Laid-Open No. 62-19684 and US Pat. No. 5,111,475 propose a semiconductor laser in which a diffraction grating is formed in a part of the cavity direction. Each of these structures has a low reflection film on the front surface and a high reflection film on the rear surface, and has a structure in which a diffraction grating is formed in a part of the resonator direction from the front surface toward the inside of the resonator.

The former structure has a substrate 2 as shown in FIG.
1, a light guide layer 22, an active layer 23, and a clad layer 24 are formed, a low light reflection film 25 on the front surface and a high light reflection film 26 on the rear surface.
And a diffraction grating (corrugation) 27A is formed at the interface between the light guide layer 22 and the substrate 21. The element length is 300 μm, the height of the diffraction grating is about 300 Å, and the length of the area where the diffraction grating 27A is formed is 50 to 1 from the front surface.
With a structure of 50 μm, the coupling coefficient (κ) of the diffraction grating 27
Is described as about 30 cm ^-1 .

On the other hand, the latter configuration, as shown in FIG.
A diffraction grating 27B is formed at the interface between the substrate 21 and the light guide layer 22, and the element length is 250 μm and the diffraction grating 27 is formed.
It is described that the formation region length of B is 175 μm, and the product of the coupling coefficient of the diffraction grating and the region length forming the diffraction grating is in the range of 1.6 to 2.5. The same parts as those in FIG. 3 are designated by the same reference numerals.

[0008]

[Problems to be Solved by the Invention] The former JP-A-62-2
The one described in Japanese Patent Publication No. 196884 is intended to increase the output of a DFB laser, and is designed to set the product of the coupling coefficient (κ) and the element length (L) to κL ≦ 0.5. Is. Therefore, in the semiconductor laser having this structure, the analog modulation distortion characteristic is considered to be insufficient because the electric field strength distribution in the cavity direction is not taken into consideration.

The latter US Pat. No. 5,111,47
The one described in No. 5 is designed on the assumption that the second-order distortion becomes small when operated at the point where the slope efficiency is maximum as the bias point, and the operating bias point is set to the highest possible bias value. It is a thing. However, in the present invention, it is considered that the yield is not significantly improved because of the structure in which the electric field strength distribution in the cavity direction is made non-uniform and the diffraction grating formation region length is longer than the element length.

As described above, since the conventional semiconductor laser does not take into consideration the electric field strength distribution in the cavity direction that causes the modulation distortion, it is considered that the modulation distortion characteristics cannot be sufficiently improved.

To solve such a problem, in order to flatten the electric field strength distribution in the resonator direction to reduce the modulation distortion and to improve the yield against the distortion standard, a diffraction grating is formed only in a part of the resonator direction. In addition, it is considered that the coupling coefficient of the diffraction grating and the length of the region forming the diffraction grating are appropriately set (Japanese Patent Application No. 5-93460). With this semiconductor laser, the modulation distortion is reduced and the yield is improved.

However, in each of the above-mentioned semiconductor lasers, since the diffraction grating is formed only in a part of the resonator, the compound resonator is formed in the area where the diffraction grating is formed and the area where the diffraction grating is not formed. Will be formed,
The mode due to this composite resonator is superimposed on the oscillation spectrum, which causes the deterioration of the submode suppression ratio. Further, it is considered that such instability of single-mode oscillation of the semiconductor laser causes noise and causes signal deterioration.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor laser which has improved analog modulation distortion characteristics, improved submode suppression ratio, and suppressed signal deterioration due to noise, and a manufacturing method thereof.

[0014]

A semiconductor laser of the present invention has a low-reflectivity film on the front surface and a high-reflectance film on the rear surface of the front and rear surfaces of the element that constitutes the resonator, and has the entire resonator length. A diffraction grating is formed over the entire area, and the diffraction grating is configured such that its coupling coefficient is larger in the front surface side region than in the rear surface side.

For example, the height of the diffraction grating is made higher on the front surface side than on the rear surface side. Further, when the element length is 300 μm, the height of the diffraction grating in the approximately half area on the front surface side is 220
Å, the height of the diffraction grating is about 100
Å.

The method of manufacturing a semiconductor laser according to the present invention comprises the steps of applying a first photoresist on a region of the substrate over the entire length of the resonator and exposing the diffraction grating pattern to form a first pattern mask. A step of etching the substrate using the first pattern mask to form a diffraction grating having a low height; and applying a second photoresist while leaving the first pattern mask, And exposing a part of the entire length of the
Forming a pattern mask of the above, and the first and second
Etching the substrate using the pattern mask described in 1. to form a high diffraction grating in this region, and removing the first and second pattern masks, and then forming an upper layer on the diffraction grating. Including the process.

[0017]

In the semiconductor laser of the present invention, since the diffraction grating is formed over the entire length of the resonator, the secondary mode due to the composite resonator is suppressed. Further, by making the coupling coefficient of the diffraction grating on the front surface side larger than that on the rear surface side, the electric field strength distribution in the resonator direction can be flattened and analog modulation distortion can be reduced. As a result, analog modulation distortion can be reduced while maintaining the single mode characteristic of oscillation equivalent to that of a normal DFB laser.

[0018]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of the semiconductor laser of the present invention,
FIG. 6 is a cross-sectional view of the main part of the structure cut along the resonator length direction. In the figure, n-InG is formed on the n-InP substrate 1.
aAsP optical guide layer 2, InGaAsP active layer 3, p-
The InP clad layer 4 is laminated. Although not shown, the active layer 3 and the cladding layer 4 have a mesa structure, and the p-InP current blocking layer and the n-type are formed so as to cover the mesa structure.
-InP current blocking layer, p-InP clad layer, p-
It goes without saying that the InGaAsP cap layer is formed, and the electrodes are further formed on the upper and lower surfaces, respectively.

The element length is set to 300 μm, and the low reflectance film 5 is formed on the front surface and the high reflectance film 6 is formed on the rear surface to form a resonator. Further, the interface between the substrate 1 and the light guide layer 2 has a diffraction grating 7 in which the interface is formed in a concavo-convex pattern with a fine period so as to form a stripe shape in the cavity direction, that is, in the direction orthogonal to the light traveling direction. Are formed over the entire length of the resonator. In the diffraction grating 7, the height of the front side diffraction grating 7a provided with the low reflectance film 5 is larger than the height of the rear side diffraction grating 7b provided with the high reflectance film 6. Is configured as follows. As a result, the diffraction grating 7
The coupling coefficient (κ) of (1) is large in the region 7a of 150 μm on the front surface side and is small in the region 7b of 150 μm on the rear surface side.

2A to 2D are sectional views showing a method of manufacturing the semiconductor laser shown in FIG. In this figure, only a part of the case where a plurality of semiconductor lasers are formed is shown. First, as shown in FIG. 2A, a negative photoresist 11 is applied on the n-InP substrate 1, a resist pattern having a period of 2025Å is exposed and developed by a two-beam interference exposure method. Next, as shown in FIG. 2B, the substrate 1 is shallowly etched by using the photoresist 11A having the pattern developed as a mask, thereby forming a diffraction grating 7b having a relatively low height and a height of 100Å.

Then, as shown in FIG. 2 (b), while leaving the negative photoresist 11A, a positive photoresist 12 is applied on the negative photoresist 11A, and half of the cavity length of the semiconductor laser to be finally formed is formed. Contact exposure is performed using a mask 13 having a mask pattern that exposes only a part of the area at an interval of 1/2 and over a half length area, and then developed. As a result, as shown in FIG. 2D, a resist pattern 14 in which the negative and positive photoresists 11A and 12A are superimposed is formed.

Then, the resist pattern 14 is used to further etch the substrate 11 and then the resist pattern 14 is removed to form a diffraction grating 7 having a height of 100 Å as shown in FIG. 2 (e). 1 mentioned above
The diffraction grating 7b having a height of 220 Å and a partially high height is formed with a period length of 2. In this step, a target pattern at the time of cleavage is also formed at the edge of the substrate (wafer).

Then, as shown in FIG. 1, an n-InGaAsP optical guide layer 2 of 1000 Å, a multiple quantum well (MQW) active layer 3, and an n-InGaAsP optical guide layer 2 were formed on the substrate on which the diffraction grating 7 was formed.
Next, the p-InP clad layer 4 is formed with a film thickness of about 0.5 μm by the MOVPE method.

Although not shown in the figure, after forming these layers, a positive photoresist is applied, and an active layer mesa stripe is formed by exposure and etching. After this, L
P-InP current blocking layer, n-InP current blocking layer, p-InP cladding layer, wavelength 1.4 μm by PE method
A p-InGaAsP cap layer having a composition is formed by ordinary buried growth. Next, electrodes are vapor-deposited on the upper and lower surfaces, and then cut into bars extending in a direction perpendicular to the drawing by cleavage. In the cleavage, it was cut out at the position of the mark provided when the diffraction grating pattern was formed. Then, after coating the front surface of the bar with a reflectance of 1% and the rear surface with a reflectance of 75%, the bar is cut into chips to complete a semiconductor laser whose main part is configured as shown in FIG.

According to this semiconductor laser, since the diffraction grating 7 is formed over the entire length of the resonator, deterioration of the submode suppression ratio due to the composite resonator mode does not occur. Further, the diffraction grating 7 has a height increased in the region 7a on the front surface side of the resonator to increase the coupling coefficient thereof, and a region 7a on the rear surface side.
Since the height is reduced at b to reduce the coupling coefficient, it is possible to flatten the electric field intensity distribution in the resonator direction, reduce analog modulation distortion, and improve the yield with respect to the distortion standard. Therefore, analog modulation distortion can be reduced while maintaining the single mode characteristic of oscillation equivalent to that of a normal DFB laser.

As a result of actually manufacturing the semiconductor device of FIG. 1 by the method of the embodiment described above, IMD ₃ ≦
The ratio (yield) of elements that satisfy −80 dBc is 25%
Met. Moreover, when the submode suppression ratio (SMSR) was measured, 40 dB or more was obtained. The yield of the conventional semiconductor laser manufactured in the same manner with respect to the above strain standard is 1
2% and the SMSR was about 35 dB. Thus, it was confirmed that the semiconductor laser of the present invention improved the yield and the single axis mode stability.

Although the multi-quantum well active layer is used in this embodiment, the same effect can be obtained when the bulk active layer is used. Needless to say, the same result can be obtained in the case of a 1.5 μm band laser or a laser of that wavelength band.

[0028]

As described above, the present invention has the low reflectance film on the front surface and the high reflectance film on the rear surface of the front and rear surfaces of the semiconductor laser constituting the resonator, and the resonator has the entire length. Since the diffraction grating is formed and the coupling coefficient of the diffraction grating is larger in the region on the front surface side than on the rear surface side, the secondary mode due to the composite resonator can be suppressed and the electric field in the resonator direction can be suppressed. The intensity distribution can be flattened to reduce the analog modulation distortion, and the analog modulation distortion can be reduced while maintaining the single mode of oscillation, and the yield can be improved.

In this case, by making the height of the diffraction grating higher on the front surface side than on the rear surface side, it is possible to form a structure in which the coupling coefficient of the diffraction grating is increased on the front surface side.

In the manufacturing method of the present invention, the first pattern mask is formed on the substrate over the entire length of the resonator, and then the substrate is etched to form a diffraction grating having a low height. Of a semiconductor laser having a diffraction grating having a different coupling coefficient by forming a second pattern mask on a part of the entire length of the container and then etching the substrate to form a diffraction grating having a high height. Can be realized.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of an embodiment of a semiconductor laser of the present invention.

2A to 2D are cross-sectional views showing a method of manufacturing the semiconductor laser of FIG. 1 in process order.

FIG. 3 is a sectional view of an example of a conventional semiconductor laser.

FIG. 4 is a sectional view of another example of a conventional semiconductor laser.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 n-InP substrate 2 Guide layer 3 Active layer 4 Cladding layer 5 Low reflectance film 6 High reflectance film 7 Diffraction grating 7a High diffraction grating 7b Low height diffraction grating

Claims (5)

[Claims] 1. A front and rear surface of a semiconductor laser device forming an optical resonator has a low-reflectance film on the front surface and a high-reflectance film on the rear surface, respectively, to form a diffraction grating over the entire length of the resonator, A semiconductor laser characterized in that this diffraction grating is configured such that its coupling coefficient is larger in the front surface side region than in the rear surface side. 2. The semiconductor laser according to claim 1, wherein the height of the diffraction grating is higher on the front surface side than on the rear surface side. 3. When the element length is 300 μm, the height of the diffraction grating in the approximately half area on the front surface side is 220 Å, and the height of the diffraction grating in the approximately half area on the rear surface side is 100 Å. The semiconductor laser according to claim 2. 4. The semiconductor laser according to claim 1, wherein an optical guide layer, an active layer, and a clad layer are laminated and formed on a substrate, and a diffraction grating is formed at an interface between the substrate and the optical guide layer. 5. A step of forming a first pattern mask by applying a first photoresist on a substrate for forming a semiconductor laser in a region over the entire length of a resonator and exposing a diffraction grating pattern to form a first pattern mask, and The step of etching the substrate using the first pattern mask to form a diffraction grating having a low height, and applying the second photoresist while leaving the first pattern mask, to reduce the total length of the resonator. Exposing a part of the area to form a second pattern mask covering the part of the area; and etching the substrate using the first and second pattern masks to form a height in this area. A method of manufacturing a semiconductor laser, comprising: a step of forming a high diffraction grating; and a step of forming an upper layer on the diffraction grating after removing the first and second pattern masks.