JPH02262366A - Photodetector and semiconductor laser equipped with photodetector - Google Patents

Abstract

PURPOSE:To control light output and wavelength simultaneously by for example providing waveguides to guide detected light and three or more photodetectors that are optically combined with the waveguides and further providing wavelength selecting waveguide type optical component wave devices or distributed reflectors between two or more photodetectors. CONSTITUTION:This device is equipped with waveguides 1-5 which guide detected light and three or more photodetectors 8-10 which are optically combined with the waveguides 1-5 and then, wavelength selecting waveguide type optical component wave devices or distribution reflectors 6 and 7 are provided between two or more photodetectors 8-10 and waveguides 1-5; besides, the foregoing waveguides 1-5, the photodetectors 8-10, and the waveguide type optical component wave devices or the distributed refractors 6 and 7 are formed in a monolithic shape. For example, the foregoing waveguide type optical wave component devices or the distributed reflectors 6 and 7 consist of grating in which Bragg wavelengths become variable according to current injection; besides, they have current injection means 11 and 12. Then wavelengths of light to be detected are detected from differential signals outputted by the photodetectors 8, 10.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a detection device for the oscillation wavelength and optical output of a wavelength tunable dynamic single mode laser, etc., which can be used for optical communication, optical applied measurement, etc. It is something.

[Prior Art] In optical communication systems that use optical fibers for long-distance, high-capacity transmission, pulse width broadening due to wavelength dispersion of optical fibers poses a problem in terms of transmission bandwidth. For this reason,
Several dynamic single-mode lasers have been proposed that oscillate in a single longitudinal mode spectrum even during high-speed modulation. In addition, wavelength multiplexing and coherent communication methods have been proposed as methods to enable even higher capacity transmission.
To realize these, it is necessary to control and stabilize the oscillation wavelength.

Conventionally, in order to control the oscillation wavelength of a semiconductor laser to a constant value,
A common method was to keep the operating temperature constant. However, control based on temperature has the disadvantage that the response speed is slow and that it is easily influenced by the usage environment. Therefore, as a light source whose oscillation wavelength can be electrically controlled, a dynamic single mode laser that has a flag wavelength control region and a phase adjustment region and whose wavelength is tunable in the range of 10 ni has been reported (Electro
nicsLatters1988 Vol, 24 No.
,] OP, 577-579). This wavelength tunable laser changes the refractive index of the medium due to interband absorption and dispersion of plasma absorption by injecting current into the tunable distributed reflector electrode, which changes the propagation constant of the waveguide and controls the flag wavelength. In addition, the amount of current injected into the phase adjustment region is also controlled at the same time, resulting in stable flag wavelength oscillation.

Dynamic single mode lasers with such a wide variable wavelength range are suitable not only for wavelength stabilization but also for wavelength multiplexing and frequency modulation applications.

However, even in the above-mentioned conventional examples, wavelength control is very important, and its detection generally uses a large system such as a spectrometer or a Fapsobell interferometer, and there is a strong desire for miniaturization.

Therefore, recently, a monolithically formed distributed feedback semiconductor laser device capable of controlling the oscillation wavelength has been applied for and published. This invention is disclosed in Japanese Patent Application Laid-Open No. 63-160391,
As shown in FIG. 4, in a distributed feedback semiconductor laser that includes a gain region and one or more distributed reflectors having a grating in the traveling direction of light coupled to the gain region, one of the distributed reflectors is At least one is a variable distributed reflector having a variable light propagation constant, and a first one having a flag wavelength longer than the flag wavelength of the distributed reflector.
a distributed reflector, a first photodetector coupled to the first distributed reflector, a second distributed reflector having a flag wavelength shorter than a flag wavelength of the distributed reflector;
a second photodetector coupled to a distributed reflector; a region optically coupling the first distributed reflector and the second distributed reflector; This distributed feedback semiconductor laser device is characterized in that the light propagation constant of the variable distribution reflector is controlled by a difference signal between the outputs of the detector and the second photodetector.

With the above configuration, the output of the first photodetector and the output of the second photodetector both change in correspondence with the oscillation wavelength. Furthermore, since the difference signal between these two signals changes monotonically with respect to the oscillation wavelength, the oscillation wavelength can be measured using the difference signal.

Therefore, by controlling the flag wavelength of the variable distribution reflector using the difference signal between the outputs of the two photodetectors, it becomes possible to control the oscillation wavelength of the semiconductor laser. That is, the wavelength measurement means can be integrated into the semiconductor laser, and a semiconductor laser device capable of controlling the oscillation wavelength can be constructed compactly.

[Problems to be Solved by the Invention] However, in the above conventional example, the optical output can be detected as a sum signal of the outputs of the first and second photodetectors, but the wavelength dependence of the transmittance of the distributed reflector It is difficult to control the optical output simply by using the sum signal because it is asymmetric with respect to the center wavelength and is nonlinear.

[Means for Solving the Problems] The present invention includes a waveguide for guiding light to be detected, and three or more photodetectors optically coupled to the waveguide, and at least two photodetectors.
A waveguide type optical demultiplexer or a distributed reflector having wavelength selectivity is provided between the above photodetector and the waveguide, and the waveguide, the photodetector and the waveguide type optical demultiplexer are provided. A waveguide type optical demultiplexer or a distributed reflector is a photodetection device characterized by being monolithically formed on the same substrate. Consists of a grating whose wavelength is variable,
The above-mentioned photodetecting device is characterized in that it further includes current injection means.

The present invention also includes a semiconductor laser equipped with a photodetector in which the above-described photodetector and a wavelength tunable dynamic single mode semiconductor laser are monolithically formed on the same substrate.

According to the present invention, by using a demultiplexer from an optical waveguide that guides the light to be detected, the light is distributed, one of which is used for optical output control, and the other is used to pass through a distributed reflector or the like having a wavelength selection function. By providing an internal wavelength selection function, it is possible to control the optical output and wavelength without being affected by fluctuations in the optical output, and by feeding the obtained signal back to the light source, it can be stabilized. This makes it possible to realize a light source with a certain optical output and wavelength.

Further, by injecting carriers into a distributed reflector or the like having a wavelength selection function, it becomes possible to accurately and quickly set a desired wavelength using an electric signal.

(Example) FIG. 1 schematically shows an integrated photodetection device formed on a compound semiconductor substrate that best represents the features of the present invention.

The detected light incident on the main optical waveguide 1 is split into optical waveguides 3, 4.5 by a branching path 2, and the light propagating through the waveguide 4 is output as an electrical signal by a photodetector 9.

Since the optical output of the detected light is determined by the propagation loss of the optical waveguide 1 and the optical waveguide 4 and the radiation loss of the demultiplexer 2, it is constant.
Even if the wavelength shifts slightly, the amount of change is very small. Therefore, if the optical output is monitored using the photodetector 9, APC control can be performed by negative feedback based on the increase or decrease in the output signal.

Furthermore, for the light propagating through the optical waveguides 3 and 5, the distributed reflectors 6 and 7 act as filters with flag wavelengths λ, , λ6, respectively, so that the wavelength λ of the detected light is λA≦λ≦λ8.
If so, it is possible to know the wavelength of the detected light as a difference signal between the outputs of the photodetectors 8 and 9.

Here, the distributed reflectors 6 and 7 are each injected with a current controller 11.
The amount of decrease in the refractive index due to the 6 plasma effect, δ, which changes the flag wavelength, is proportional to the carrier density n, and in the AlGaAs system with λ0”0.85u, δ, *-1.3
x 10-2λ□J, δ in InGaAs system with 3 uI
n=-3.7X 10-". Since the refractive index of the carrier injection layer is changed in this way, the flag wavelength can be controlled. If you want to match the detected light to a desired wavelength λa, The amount of current injected into each of the photodetectors 8 and 10 is set in advance so that the outputs of the photodetectors 8 and 10 at wavelength λ are monitored by a spectrometer or the like.In this state, when the detected light is λ .

When the wavelength changes from λ to λ, a difference occurs between the output levels of the distributed reflectors 6 and 7, so that the wavelength of the incident light can be detected from the difference signal of the photodetector 8.10. If the direction and approximate amount of wavelength shift of the detected light can be detected in this way, it is possible to return it to the desired wavelength by feeding it back to the temperature, current, electric field, etc. of the light source of the detected light. Become.

In this embodiment, a symmetrical three-branch waveguide is used as a method of branching light from the main optical waveguide, but it is also effective to use a directional coupler with three IX branches with variable spacing as shown in FIG.

Figure 3 shows an optical waveguide 3 as a filter with wavelength selectivity.
A waveguide type optical demultiplexer 32 in which a grating having a vector oblique to the direction of the light beam is formed on the optical demultiplexer 32 is used, and a part of the incident light is diffracted by the optical demultiplexer 32. It is detected by photodetectors 34 and 35 installed at predetermined positions. The light that is not affected by the optical demultiplexer 32 travels straight and is detected by the photodetector 36.

The direction of the light diffracted by the optical demultiplexer 32 of the diffraction grating is
The light is diffracted at an angle that satisfies the flag condition, and is determined by the wavelength λ of the light to be detected, the period Δ of the diffraction grating, the incident angle θ of the light onto the diffraction grating, etc.

Now the desired wavelength λ. The diffracted light is detected by the photodetector 34.
If the design is such that the same amount of light is irradiated to 35 and 35, the diffracted light will be
After passing through the transparent characteristic region 33, the photodetectors 34 and 3
5 will be detected. When the wavelength of the detected light changes, the flag condition by the diffraction grating 32 changes,
The conditions of incidence on the photodetectors 34, 35 will change.

In other words, the amount of change in wavelength can be detected by monitoring the outputs of the photodetectors arranged in parallel at a certain interval and obtaining a difference signal from the subtractor 38. Further, the light that is not diffracted by the optical demultiplexer 32 travels straight and is detected as an output by the photodetector 36, but it has a wavelength λ. Since the output of the photodetector 36 does not change even if it changes from λ to λ, it is suitable for APC of a light source if used as a monitor of optical output.

The optical demultiplexer 32 is configured to be able to inject a current, but as the carrier concentration increases, the refractive index of the medium in the leading waveguide decreases and the flag wavelength of the diffraction grating 32 changes. Within the variable range, it is possible to monitor the detected light for a desired wavelength within the variable range. Furthermore, it is also effective for setting the balance between the amounts of light incident on the photodetectors 34 and 35.

The integrated photodetector described above can be integrated on the same substrate as a wavelength tunable dynamic single mode laser, and the present invention is very useful in controlling the optical output and oscillation wavelength of the laser. It is effective for

Note that in this example, a configuration using compound semiconductor materials such as GaAs-based and InP-based materials was explained, but the present invention can also be applied to other materials, such as semiconductor materials such as Si and ferroelectric materials such as LiNb0:+. Needless to say.

[Effects of the Invention] As explained above, according to the present invention, light is distributed from the waveguide that guides the light to be detected, and one side is used as a first output monitor and the other side is passed through a filter having a wavelength selection function. By using it for wavelength monitoring, it is possible to control the optical output and wavelength of the detected light at the same time.

Furthermore, by monolithically forming the laser on the same substrate as the wavelength tunable dynamic single mode laser, a light source for optical communication or optical applied measurement with stable oscillation wavelength and optical output can be obtained.

[Brief explanation of drawings]

FIG. 1 shows an integrated photodetection device having a symmetrical three-branch dynamic waveguide embodying the present invention. FIG. 2 shows an integrated photodetection device having a three-branch directional coupling waveguide with variable spacing embodying the present invention. Fig. 3 shows an integrated photodetection device having a waveguide type optical demultiplexer according to the present invention, and Fig. 4 shows a wavelength variable type photodetection device having a conventional two-branch waveguide formed on the same substrate. This is a DBP laser device. 1.3, 4, 5.31 are optical waveguides, 2 is a symmetrical three-branch waveguide, 6.7.32 is a distributed black reflector with a diffraction grating, 8.9, 10.34, 35.36 is an optical waveguide. Detector, 33 is a transparent waveguide layer corresponding to the light diffraction path, 11, 12, 37 is an injection current controller, and 13, 38 is a subtractor. Patent applicant Canon Co., Ltd.

Claims (1)

[Claims] 1. A waveguide for guiding light to be detected, and three or more photodetectors optically coupled to the waveguide, and at least two or more photodetectors and the guide. A waveguide type optical demultiplexer or distributed reflector having wavelength selectivity is provided between the waveguide, and the waveguide, the photodetector, and the waveguide type optical demultiplexer or distributed reflector are the same. A photodetection device characterized by being monolithically formed on a substrate. 2. The optical detection according to claim 1, wherein the waveguide type optical demultiplexer or distributed reflector is composed of a grating whose flag wavelength is variable by current injection, and has current injection means. Device. 3. A semiconductor laser comprising a photodetecting device in which the photodetecting device according to claim 2 and a wavelength tunable dynamic single mode semiconductor laser are monolithically formed on the same substrate.