JPH06338653A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To improve the uniformity of a field strength distribution in the interior of the resonator of a semiconductor laser by a method wherein diffraction gratings are provided at an angle of roughly 45 deg. to the axes of optical waveguides and at an angle of roughly 45 deg. to the end face of the resonator, interposing the optical waveguides between them. CONSTITUTION:An N-type InP buffer layer 11 is formed on an N-type InP substrate 10, a DBR layer 12, which is formed by forming alternately an N-type InGaAsP layer and an N-type InP layer in one period, is formed and an InGaAsP active layer 13, a P-type DBR layer 14 and a P-type InGaAs cap layer 15 are grown. Then, a photoresist 16 is applied and surfaces 17 at an angle of 45 deg. to the end face of a resonator are formed by exposure, developing and etching. Moreover, after a photoresist 18 is applied and the photoresist 18 is exposed and developed in such a way that it is left only on the parts of the surface at an angle of 45 deg., a P side electrode 19 and an N side electrode 20 are deposited. Then, parts, which are deposited on the surfaces at an angle of 45 deg., of the electrode 19 are removed by a lift-off method and diffraction gratings 22 are formed using again a photoresist 21 left on the surface at an angle of 45 deg..

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 intermodulation distortion characteristics.

[0002]

2. Description of the Related Art As a light source for analog modulation used in a subcarrier multiplex optical transmission system or the like, a single-axis mode semiconductor laser with high efficiency and small intermodulation distortion is required. For example, in a mobile communication system, third-order intermodulation distortion (3rd
intermodulation distortio
A device having a sufficiently small n; IMD ₃ ) is required.
The distributed feedback semiconductor laser (DFB laser) has a good single-mode property and is being used as a light source for analog modulation. However, in a normal DFB laser, the carrier and electric field strength distribution in the cavity direction are non-uniform, so -Light output (I-
L) The linearity of the characteristic was insufficient and the intermodulation distortion characteristic was not excellent either.

By the way, at the 53rd Autumn Meeting of Applied Physics of Japan, 1992, 16p-V-1, Ogawa et al. Announced a surface emitting laser using a reflecting surface of 45 ° and a semiconductor multilayer film. This is a structure in which two 45 ° reflecting surfaces, an active layer, and a distributed reflector are formed, and laser light is emitted from the substrate side.

[0004]

However, the conventional semiconductor laser is intended for surface emission, not for optical communication and analog modulation.

An object of the present invention is to improve the uniformity of the electric field intensity distribution inside the resonator of a semiconductor laser without impairing the uniaxial mode property, and to improve the linearity of the current-optical output characteristic and the modulation distortion characteristic. It is in.

[0006]

The semiconductor laser for analog modulation of the present invention comprises an optical waveguide comprising an active layer formed on a semiconductor substrate and a cladding layer sandwiching the active layer, and an axis of the optical waveguide. End face formed to form an angle of about 90 ° with respect to the axis of the optical waveguide.
And a diffraction grating which is provided so as to form an angle of 45 ° and is formed at an angle of approximately 45 ° on the end face with the optical waveguide interposed therebetween.

Further, it is characterized in that the electrode on the substrate side is divided into at least two parts.

Further, the clad layer is composed of a semiconductor multilayer film.

Further, it is characterized in that the end face is coated with a low reflectance.

[0010]

The principle of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing an example of the structure of a semiconductor laser according to the present invention. In FIG. 1, the resonator is composed of an end face 5, a diffraction grating 6 and an electrode 7, and laser light is output from the end face 5.

With such a structure, single-axis mode oscillation is performed at a wavelength that is reflected vertically from the diffraction grating 6 to the substrate-side electrode 7.

Further, since the diffraction grating is used for coupling the optical waveguide and the semiconductor multi-layer film, the spread of the reflected light becomes smaller than that in the case where the surface of 45 ° is used as in the conventional semiconductor laser. Therefore, in the semiconductor laser having the structure of the present invention, the loss of the resonator is reduced and the threshold current is reduced.

FIG. 2 shows a semiconductor laser of the present invention and a conventional DF.
6 shows a distribution of electric field strength inside a resonator of a B laser.

In the case of a normal DFB laser, the electric field strength in the cavity direction is non-uniform as shown in FIG. On the other hand, in the semiconductor laser of the present invention, the uniformity of the electric field intensity distribution inside the resonator is improved as shown in FIG. Therefore, in the semiconductor laser of the present invention, the linearity of the current-optical output characteristic is improved and the modulation distortion is reduced.

Further, if the electrode 7 is formed only on a part of the substrate 1, the oscillation wavelength selectivity becomes strong and the single mode property is improved.

Also, by making the cladding layer 2 on the substrate side 1 a semiconductor multilayer film layer, the wavelength selectivity is enhanced and the uniaxial mode property is improved.

Further, by forming the other clad layer 4 from a semiconductor multilayer film, the spontaneous emission light generated in the active layer 3 is returned to the active layer 3 (photon recycling effect) to reduce the oscillation threshold current. It

In addition, the end face 5 on which the diffraction grating 6 is not formed
Efficiency is improved by applying a low reflectance coating to the.

[0020]

EXAMPLE The following is an example of the present invention of 1.3 μm.
The band semiconductor laser will be described with reference to the drawings.

FIG. 3 is a diagram showing the manufacturing process of the first embodiment.

As shown in FIG. 3A, the n-InP buffer layer 11 is formed on the n-InP substrate 10 by MOVPE.
Is formed to a thickness of 2000Å, and n of 1.2 μm wavelength composition is formed.
-InGaAsP (thickness 1024Å) and n-InP (1
024Å) as one cycle, and the DBR layer 12 is formed to have a thickness of about 3 μm, and the InGaAsP active layer 1 having a 1.3 μm wavelength composition is formed.
3, and the p-type DBR layer 14 with a thickness of about 2 μm and p-InG
The aAs cap layer 15 is grown to 2000 Å.

Next, as shown in FIG. 3B, a photoresist 16 is applied and exposed to light, developed, and etched to 45.
Forming a 17 ° surface 17.

Further, as shown in FIG. 3C, a photoresist 18 is applied and exposed and developed so that the photoresist remains only on the surface of 45 °, and then the p-side electrode 19 and the n-side electrode 20 are formed. Vapor deposition.

Next, as shown in FIG. 3D, the electrodes on the 45 ° surface are removed by lift-off, and the photoresist 21 remaining on the 45 ° surface is used again to form a diffraction grating pattern by the interference exposure method. Form.

After this, as shown in FIG. 3E, the diffraction grating 22 is formed by etching. Further, the entire surface is exposed and developed, and the remaining photoresist is removed.

After that, the element is cleaved at the portion shown by the dotted line in FIG.

This device oscillates at 1.31 μm, the prototype device is modularized, and the third-order intermodulation distortion is measured with two signals. As a result, the average fiber output is 5 mW and the modulation degree is 20%.
It was possible to obtain a good strain characteristic of -80 dBc.

When the end face of this element is coated with a reflectance of 1% or less, the efficiency is 2 times that of the conventional semiconductor laser.
It was improved by 0% or more.

Next, a second embodiment will be described with reference to FIG.

FIG. 4 shows a structure in which the electrode 37 on the substrate 1 side is composed of two parts, the electrode 37 is formed only on a part of the substrate 1, and the part contributing to the reflection of light is narrowed. When such an element was manufactured in the same manner as in the case of the first embodiment shown in FIG. 3, in the case of the conventional semiconductor laser, the submode suppression ratio was 30 dBc, while it was 25 dBc, and the oscillation Unimodality was improved.

Furthermore, FIG. 5 shows a third embodiment of the present invention. This is a structure in which the clad layer 42 on the substrate 1 side of the active layer 3 is a semiconductor multilayer film. The submode suppression ratio of this device was measured to be 28 dBc, which was an improvement in single-mode property over the conventional semiconductor laser.

FIG. 6 shows a fourth embodiment of the present invention. This is a structure in which the cladding layers 42 and 43 on both sides of the active layer 3 are semiconductor multilayer films. When such a device was manufactured, the oscillation threshold current was reduced by 20%. Also,
The secondary mode suppression ratio was 30 dBc.

[0034]

According to the present invention, it is possible to provide a low distortion analog modulation semiconductor laser.

[Brief description of drawings]

FIG. 1 is a diagram showing a principle structure of a semiconductor laser according to the present invention.

FIG. 2 is a diagram showing electric field intensity distributions of a laser according to the present invention and a conventional laser.

FIG. 3 is a view for explaining the manufacturing process of the first embodiment of the present invention.

FIG. 4 is a sectional view schematically showing the structure of the second embodiment of the present invention.

FIG. 5 is a sectional view schematically showing the structure of the third embodiment of the present invention.

FIG. 6 is a sectional view schematically showing the structure of the fourth embodiment of the present invention.

[Explanation of symbols]

1 substrate 2 clad layer (substrate side) 3 active layer 4 clad layer (growth layer side) 5 end face perpendicular to the axis of the waveguide 6 diffraction grating formed on a surface forming an angle of 45 ° with the axis of the waveguide 7 substrate Side electrode 8 Growth side electrode 10 n-InP substrate 11 n-InP buffer layer 12 n-InP clad layer 13 InGaAsP active layer 14 p-InP clad layer 15 p-InGaAs cap layer 16 photoresist 17 45 ° surface 18 photoresist 19 electrode 20 Electrode 21 Photoresist 22 Diffraction Grating 31 n-InP Substrate 32 n-InP Clad Layer 33 InGaAsP Active Layer 34 p-InP Clad Layer 35 End Face 36 Diffraction Grating 37 Electrode 38 Electrode 42 n-type DBR Layer 43 p-type DBR Layer

Claims (4)

[Claims] 1. An optical waveguide comprising an active layer formed on a semiconductor substrate and a clad layer sandwiching the active layer, and an end face formed at an angle of about 90 ° with respect to the axis of the optical waveguide. And a diffraction grating which is provided so as to form an angle of approximately 45 ° with respect to the axis of the optical waveguide, and which is provided at the end face with an angle of approximately 45 ° with the optical waveguide interposed therebetween. A semiconductor laser characterized by being processed. 2. The semiconductor laser according to claim 1, wherein the electrode on the substrate side is divided into at least two parts. 3. The semiconductor laser according to claim 1, wherein the clad layer is composed of a semiconductor multilayer film. 4. The semiconductor laser according to claim 1, wherein the end face is coated with a low reflectance.