JPS59154086A - Frequency stabilized semiconductor laser - Google Patents

Abstract

PURPOSE:To contrive to stabilize the oscillation frequency in a simple constitution by providing a semiconductor laser oscillating by a nearly single axial mode, a specific detection circuit detecting the variation of oscillation frequency thereof, and a feedback means for stabilizing said frequency. CONSTITUTION:A sufficient gain is generated by injecting a current to a laser activating part 40 by impressing a voltage between the first electrode 52 and a negative side electrode 55, and then the semiconductor laser 1 oscillates by a single axial mode. The oscillated light thereof is coupled to an output optical wave guide 2 and utilized as a photo output 80. A part of said light couples with a ring-like optical wave guide 3, and only the oscillated light coincident with the resonance frequency of said light is coupled to a monitor optical wave guide 4 with hardly receiving attenuation and further converted into an electric signal by a photo detector 5. Signal processing is performed by a feedback amplifying circuit 6 so that the electric signal becomes maximum, and accordingly the voltage is impressed on the second electrode 53 of a laser reflection part 41. Thereby, the oscillation frequency can be stabilized in a simple constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser, and particularly to a semiconductor laser with a stabilized oscillation frequency.

Semiconductor lasers are beginning to be used as small, highly efficient light sources in various fields including optical fiber communications. Semiconductor lasers are also expected to be used in heterodyne communications and optical fiber gyros in the future. For these light sources, a semiconductor laser that oscillates at a stable single frequency is desired. As is well known, a simple Fabry-Perot cavity laser typically oscillates in three to four axial modes. Distributed feedback type and distributed reflection type semiconductor lasers are semiconductor lasers that have improved this point and made it possible to oscillate in a single axial mode. These devices achieve single-axis mode oscillation by using a diffraction grating made in a laser crystal as a laser resonator. However, from the left, even in these semiconductor lasers, the refractive index of the crystal changes with changes in the injection current and ambient temperature, so it is inevitable that the oscillation wavelength, or oscillation frequency, will fluctuate. This magnitude is, for example, approximately IX/°C.

This corresponds to 10-odd GHz/C in the 1.3 μm wavelength band, making it extremely difficult to use as a light source for heterogain communications or gyros, and it is desired to realize a semiconductor laser with a more stable oscillation frequency. Ta.

Conventionally, attempts have been made to stabilize the oscillation wavelength by detecting the wavelength of the output light of a semiconductor laser and applying electrical feedback to the semiconductor laser. The structure of the system was complicated and large, making it impractical.

An object of the present invention is to provide a semiconductor laser with a simple configuration and a stabilized oscillation frequency.

The semiconductor laser of the present invention is composed of a semiconductor laser that oscillates in a substantially single axial mode, a circuit that detects fluctuations in the oscillation frequency, and feedback means for stabilizing the oscillation frequency. The oscillation frequency detection circuit includes a leading waveguide connected to the output side of the semiconductor laser, a ring-shaped optical waveguide, a monitor optical waveguide, and a photodetector.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a block diagram for explaining the most basic embodiment of the present invention. An output optical waveguide 4 is connected to one output side of a semiconductor laser 1 that oscillates in a single-axis mode. A photodetector 5 is connected to the exit of the monitor optical waveguide 4 and detects the output light intensity.

Feedback amplifier circuit 6 applies an electric signal to semiconductor laser 1 so that the output of photodetector 5 is maximized.

With this configuration, the ring-shaped optical waveguide 3 forms a resonator, which is determined by f=mC/2・n−L, where its length is given, the effective refractive index is n, and the speed of light C9m is an arbitrary integer. Attenuation of the resonance frequency is selectively small.

Therefore, the output light from the semiconductor laser 1 leads to the output optical waveguide 2.
Of the light coupled from to the ring-shaped leading waveguide 3, the frequency f
Only the component is strongly coupled to the monitor leading wavepath 4. Therefore, if an electric signal is applied to the semiconductor laser 1 so that its output light intensity is maximized, the semiconductor laser 1 will oscillate at a stable single frequency if.

As described above, in the present invention, it is sufficient to simply apply feedback to maximize the output light intensity of the monitor optical waveguide 4, so that a semiconductor laser with a stabilized oscillation frequency can be realized with a simple configuration. At this time, the oscillation frequency of the semiconductor laser 1 that oscillates in a nearly single-axis mode needs to be approximately equal to one of the resonance frequencies f of the ring-shaped optical waveguide 3. Also,
When the ring-shaped optical waveguide 3 and the like are made of a semiconductor, the resonant frequency changes due to changes in the refractive index due to changes in ambient temperature, so it is desirable that at least the ring-shaped leading waveguide 3 is temperature stabilized.

Furthermore, if the temperature of the semiconductor laser 1 is also stabilized at the same time, the frequency can be stabilized better.

FIG. 2 shows a perspective view of an embodiment of the present invention, and FIG.
The figures show cross-sectional views along lines AA' and BB' of the semiconductor laser 1 and photodetector 5, respectively. This embodiment mainly consists of three parts. That is, an active optical element section 100 consisting of a semiconductor laser 1 and a photodetector 5 formed on a semiconductor substrate 10, and a dielectric-based optical waveguide 3. and an optical waveguide section 200 including a monitor optical waveguide 4, and a feedback amplification circuit 6.

First, the active optical element section 100 will be explained.

A buffer 7 of n-InP is deposited on a substrate 10 of (100) orientation n-InP by a conventionally well-known liquid phase growth method.
Layer 11, n Ino, asGaoasA8o, ax
Po, n7 optical waveguide layer 12 (composition wavelength 1.10 μm),
Undoped In(1,y2 Gao, 15 Ag3.
3, Pa, 6 g active layer 13 (composition wavelength 1.3 μr
A cladding layer 14 of n ) + p InP is continuously grown to produce a double heterojunction crystal. The thickness of each of these layers is approximately 5 μm, 1 μm, and 0.1 μm, respectively.
, 1 μm. Next, using photolithography and chemical etching, four parallel grooves 15a and 15b each having a width of about 7 μm are formed in the (011) direction.

16a and 16b are excavated until at least the buffer layer 11 is reached, and a laser stripe 17 with a width of 1.5 μm is formed.
Then, a photodetector stripe 18 having a width of 20 μm is formed. Thereafter, a portion of the active layer 13 in the longitudinal direction of the laser stripe 17 is removed, and a diffraction grating 30 with a period of 0.163 μm is formed by a combination of interference method and chemical etching method. Thereafter, the p-InP current blocking layer 19. n-InP current confinement layer 20. p
- InP buried layer 21. p-In. , 720ao,
ts 4soJx Po, 6+1 electrode layers 22 (composition wavelength 1.3 μm) are sequentially grown.

The thickness of each of these layers is approximately 0.8 μm as measured at the so-called flat portion between the grooves 15b and 16a.

The thicknesses were set to 0.5 μm, 1.5 μm, and 1 μm. In this buried growth, as the inventors of the present application have already clarified in Japanese Patent Application No. 56-166666, there are growth conditions in which the current confinement layer 20 of nInP does not grow on the upper part of the narrow mesa stripe. obtain. In this example, the current confinement layer 20 was not formed on the laser stripe 17, but the current blocking layer 19. The current confinement layer 20 had grown slightly thinner in both flat areas. Therefore, in order to form the photodetector 5, it is recommended to diffuse Zn through a 5in2 mask with an open window, so that the Zn diffusion layer 3 reaches at least the current blocking layer 19.
1 was formed. After that, the laser active section 40. Laser reflection section 41. Separation layer 5 for electrically isolating the light receiving section 42
After forming the separation layer 5 by the ion implantation method,
An insulating layer 51 of 5i02 was formed to cover the insulating layer 51, and first to third electrodes 52, 53, and 54 separated at the insulating layer 51 were formed by vapor deposition of an Au-Zn alloy and heat treatment. After this, the total thickness is about 100μ
After polishing the substrate 1o to a thickness of m, an n-side electrode 55 is formed and cleaved in a direction perpendicular to the above-mentioned stripes to complete the production of the active optical element section 100.

Next, the leading waveguide section 200 will be explained. This is an 8i film with a thickness of 1 μm that is vacuum-deposited on a glass substrate 7o.
0 film is formed, and it is connected to an output optical waveguide 2, a ring-shaped optical waveguide 3. A pattern with a width of 4 μm is left to serve as a monitor optical waveguide 4, and a 9Si02 film of approximately 5 μm is deposited on it by the OVD method.
It is covered with μm. The spacing between the above-mentioned leading waveguides was set to about 6 μm to provide weak coupling. The shape and dimensions of the ring-shaped optical waveguide 3 were set so that one of the resonance frequencies of the ring-shaped optical waveguide 3 was close to the oscillation frequency of the semiconductor laser 1. Precise frequency matching between the two is achieved by a Peltier element (
) and the temperature of the laser reflector 4 of the semiconductor laser l.
This was carried out using a bias electric field applied to 1.

The feedback amplification circuit 6 is a normal electric signal amplification circuit, mainly composed of an operational amplifier element, and operates to adjust the voltage applied to the laser reflection section 41 so that the output of the photodetector 5 is always maximized. .

Let's move on to the semiconductor laser device that combines the above three main components. First, the semiconductor laser 1 is operated by applying a voltage between the first electrode 52 and the negative electrode 55 to form the laser active part 4.
Sufficient gain is generated by injecting a current to zero, and the resonator formed by the cleavage plane 60 and the diffraction grating 30 of the laser reflection section 41 oscillates in a single axial mode. The oscillated light is coupled to the lead waveguide 2 and used as an optical output 80. A part of the oscillated light is coupled to the ring-shaped optical waveguide 3,
However, only the next oscillation that matches the resonant frequency is coupled to the monitor optical waveguide 4 with almost no attenuation, and is further converted into an electrical signal by the photodetector 5. The signal is processed by the feedback amplifier circuit 6 so that the electric signal is maximized, and a voltage is applied to the second electrode 53 of the laser reflection section 41.

This voltage changes the electric field near the diffraction grating 30 and changes its effective period through a change in the refractive index due to the electro-optic effect. As a result, the oscillation frequency changes. That is,
In this semiconductor laser device, the oscillation frequency of the output light can be fixed to the resonance frequency of the ring-shaped optical waveguide 3 by simply applying feedback so that the output of the photodetector 5 is maximized, and has a simple configuration. , the oscillation frequency could be stabilized.

This invention can be modified in several ways in addition to the embodiments described above. In the embodiment, the active optical element section and the optical waveguide section are made of different materials and later combined into a hybrid, but a so-called monolithic configuration in which the optical waveguide section is made of the same semiconductor material is also possible.

The semiconductor laser 1 is a distributed reflection type laser as an example, but it may also be a distributed feedback type laser in which the active layer and the diffraction grating are substantially aligned. In that case as well, it is possible to divide the electrode in the direction of the resonator and use a portion as a signal input section from the feedback circuit.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic structure of the present invention, FIG. 2 is a perspective view of an embodiment of the present invention, and FIGS. 3 and 4 are partial sectional views thereof. In the figure, 1 is a semiconductor, 2 is an output optical waveguide, 3 is a ring-shaped optical waveguide, 4 is a monitor optical waveguide, 5 is a photodetector, and 6 is a feedback optical amplifier. Representative Patent Attorney Uchihara Figure 1

Claims (1)

[Claims] A semiconductor laser which oscillates in a single axial mode and whose oscillation frequency changes depending on an applied electric signal, an output optical waveguide connected to at least one output side of the semiconductor laser, and a leading waveguide thereof. A ring-shaped waveguide having optical coupling, a monitor optical waveguide optically coupled to the ring-shaped waveguide, a photodetector connected to the monitor optical waveguide, and the output of the photodetector is maximized. and feedback means for sending an electrical signal back to the semiconductor laser.