JPH03116992A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To obtain an enough amount of optical feedback through a simple optical system and to enable an optical output to be easily monitored by a method wherein a lens is provided to condense the rear outgoing light of a laser element on an optical detector and to condense the reflected light from the optical detector on the light emitting end face of the laser element. CONSTITUTION:The light condensing function of a grating lens 20 composed of a diffraction grating and the like and the reflecting function of an optical detector 30 are utilized. That is, light emitted from the rear side of a semiconductor laser 10 is condensed on the optical detector 30, being diffracted by the grating lens 20, whereby the optical output of the semiconductor laser element 10 is monitored by the optical detector 30. The reflected light from the optical detector 30 is returned to the light emitting end face of the semiconductor laser element 10, being diffracted again by the grating lens 20. The wavelength of the oscillation light of the semiconductor laser element 10 is determined basing on the wavelength of the return light concerend. By this setup, an enough amount of an optical feedback can be obtained through a simple optical system and an optical output can easily be monitored.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a semiconductor laser device used for optical information processing, optical measurement, optical communication, etc., and particularly relates to an improvement of an external resonator structure. This invention relates to a semiconductor laser device.

(Prior Art) In recent years, for the purpose of application to optical information processing devices such as optical disk systems and optical scanners, optical meters, optical communication, etc.
Semiconductor lasers with various structures have been developed. These semiconductor lasers have various requirements for transverse mode characteristics, optical spectrum characteristics, noise characteristics, etc. depending on the application.

For example, semiconductor lasers used in hologram scanners and the like are required to have little wavelength fluctuation due to temperature or the like.

Various types of composite resonator structures including external resonators have been developed as means for realizing such wavelength-stabilized lasers. For example, as shown in FIG. 10, a semiconductor laser device in which a composite resonator is formed can be obtained by providing a reflecting mirror 92 behind a semiconductor laser element 91 and returning part of the rear emitted light to the semiconductor laser element 91. has been developed (1986 Spring Meeting of the Japan Society of Applied Physics, 4p-8).

However, this type of device has the following problems. That is, the light emitted from a semiconductor laser device generally has a beam spread (as shown by the broken line in FIG. 1O), and in a configuration in which this is fed back using a flat reflecting mirror, the amount of optical feedback is small. Therefore, in order to achieve wavelength stabilization by the composite resonator effect, the rear surface of the semiconductor laser element must be coated to have low reflection, which increases the oscillation threshold. Furthermore, with this structure, it is difficult to extract monitor light, and it is extremely difficult to perform constant light output (APC) operation.

On the other hand, in order to improve the coupling efficiency of the return light from the external resonator to the semiconductor laser probe, various methods have been considered to effectively return the reflected light to the laser element using a lens or the like.
All of these have complicated optical systems and are difficult to align.

(Problems to be Solved by the Invention) As described above, conventional composite resonator structures using reflecting mirrors have problems such as a small amount of optical feedback, difficulty in extracting monitor light, and difficulty in monitoring optical output. Also,
A method of improving the coupling efficiency of returned light to a semiconductor laser element using a lens or the like has a problem in that the optical system is complicated and alignment becomes difficult.

The present invention has been made in consideration of the above circumstances, and its purpose is to be able to obtain a sufficient amount of optical feedback with a simple optical system in an external resonator structure provided with an external resonator. Another object of the present invention is to provide a semiconductor laser device whose optical output can be easily monitored.

[Configuration of the Invention (Means for Solving the Problems) The gist of the present invention is to obtain a sufficient amount of optical feedback with a simple optical system by providing an optical feedback optical system including a wavelength dispersion element as an external resonator. The object of the present invention is to make it possible to do this, and also to make it easier to monitor the optical output.

That is, the present invention provides a semiconductor laser element, a lens whose focal length is wavelength-dependent, which is provided at a distance from one of the light emitting end faces of the laser element, and which transmits light through the lens or is reflected by the lens to focus the light. A semiconductor laser device comprising a photodetector for detecting emitted light, the lens condenses the rear emitted light from the laser element onto the photodetector, and collects the reflected light from the photodetector. The laser beam has a function of condensing the light onto the light emitting end face of the laser probe.

(Function) According to the present invention, it is possible to obtain a sufficient amount of optical feedback with a simple optical system by utilizing the light focusing effect of a grating lens made of a diffraction grating, etc., and the reflection effect of a photodetector. . Furthermore, since a grating lens is used, alignment between the semiconductor laser element and the lens becomes easy. That is, when the relative positions of the semiconductor laser element and the lens shift, the amount of feedback of light of the wavelength that should originally be oscillated decreases, but the amount of feedback of other wavelengths increases.

In this case, the semiconductor laser element still oscillates, although the oscillation wavelength changes. Therefore, if the oscillation wavelength does not matter much, there is no need to precisely align the relative positions of the semiconductor laser element and the grating lens. Furthermore, since the configuration is such that the light transmitted through the grating lens is guided to the photodetector, it becomes easy to monitor the optical output.

(Example) Hereinafter, details of the present invention will be explained with reference to illustrated embodiments.

FIG. 1 is a schematic configuration diagram showing a semiconductor laser device according to a first embodiment of the present invention. In the figure, IO is a semiconductor laser element, and a grating lens 20 is arranged at a predetermined distance on the light emitting end face (rear surface) of this laser element IO, and a photodetector 30 is arranged further away from the grating lens 20. There is. Here, semiconductor laser element 1
0 is a heterostructure laser using a compound semiconductor material, and the photodetector 30 is a photodiode, a phototransistor, or the like. Further, the grating lens 20 is formed from a diffraction lens as described later.

With this configuration, light emitted from the rear surface of the semiconductor laser element 10 is diffracted by the grating lens 20 and focused on the photodetector 30. Thereby, the optical output of the semiconductor laser element 10 is monitored by the photodetector 30. Further, the light reflected by the photodetector 30 is diffracted again by the grating lens 20 and returns to the light emitting end face of the semiconductor laser element 10. The wavelength of this returned light determines the oscillation wavelength of the semiconductor laser device 10. In this arrangement, the photodetector 30 simultaneously serves as a light output monitor and a reflector for the external cavity.

In this embodiment, since the grating lens 20 functions as a wavelength dispersion element, the alignment accuracy of the optical system is relaxed. That is, even if the relative position of the grating lens 20 or the photodetector 30 with respect to the semiconductor laser probe 10 deviates from the designed value, the return light to the semiconductor laser probe 10 will be focused on the light emitting end facet, and the coupling efficiency will be improved. There will be no decline. This is a phenomenon called automatic focusing. This principle utilizes the wavelength dependence of the focal length of the grating lens, and uses the fact that positional deviations are corrected by wavelength deviations. Furthermore, this automatic focusing phenomenon can be observed even when a glass lens with large chromatic aberration is used instead of a grating lens (Appl, Opt, vol, 26).
゜No. 13, I) I), 2549-2533. ).

An example of the structure of the grating lens 20 is shown in FIG. In this grating lens 20, n from the center
When the radius of the th grid line is Rn, Ro is given by the following equation.

J 100] and JT"W' neck T -f, -f2 mnλ...■ Here, fl+f2 is as shown in FIG. Distance to 20, f2 is grating lens 20
It is the distance from to the photodetector 30. Further, λ is the oscillation wavelength of the semiconductor laser device 10. When the radius of the grating lens 20 is small, the above equation (2) can be approximated as follows.

Rn −2nλ/ (1/ f + +t7 f 2
) 111111■There is now a λ* 'l+
If the grating lens is designed so that Rn is given by the formula 0 for f2, then in the actual arrangement f
If l or f2 is slightly shifted, the oscillation wavelength will shift so as to satisfy equation (2), and the returned light will be focused on the light emitting end face of the semiconductor laser element In. This is because the oscillation gain is large for wavelengths with a large amount of light returning to the laser element. In addition, the size of the wavelength dispersion of the grating lens is
It varies depending on the focal length and the tilt angle of the grating lens.

Thus, according to this embodiment, in a semiconductor laser device having an external cavity structure, a simple optical system is sufficient by utilizing the light focusing action of the grating lens 20 made of a diffraction grating and the reflecting action of the photodetector 30. A large amount of optical feedback can be obtained. In this case, there is no need to strictly align the relative positions of the semiconductor laser element 10 and the grating lens 2°, and alignment of the semiconductor laser element 10 and the lens 20 becomes easy. Furthermore, since the configuration is such that the light transmitted through the grating lens 20 is guided to the photodetector 30, monitoring of the optical output becomes extremely easy, and APC operation becomes possible.

FIG. 3 is a schematic configuration diagram showing a second embodiment of the present invention. This embodiment differs from the previously described embodiments in that the grating lens is tilted to increase wavelength dispersion and focus tolerance. Incidentally, the grating lens 21 in this case has grooves in the upper half of the grating lens 2o shown in FIG. 2 above. Even with such a configuration, it is possible to obtain a sufficient amount of optical feedback as in the first embodiment, and it is possible to achieve alignment and to make the monitor more compact.

FIG. 4 is a schematic diagram showing a third embodiment of the present invention. In this embodiment, a photodetector 30 is placed in contact with the end face of a grating lens. The grating lens 22 in this case is formed thicker than in the first embodiment. This arrangement is effective in shortening the optical path length of the external resonator.

FIG. 5 is a schematic configuration diagram showing a fourth embodiment of the present invention. In this embodiment, a rod lens 40 having a large chromatic aberration is used as a wavelength dispersive element (an element whose focal length is highly dependent on wavelength) instead of a grating lens. Even when such a rod lens 40 is used, the automatic focusing phenomenon described above occurs, and the same effects as in the first embodiment can be obtained.

FIG. 6 is a schematic configuration diagram showing a fifth embodiment of the present invention. In this embodiment, a reflective grating lens 23 is used to condense light emitted from the rear surface of the semiconductor laser probe 10 onto a photodetector 30. The only difference from the first embodiment is that the grating lens is of a reflective type.
The principle is exactly the same. In this embodiment, the photodetector is placed side by side with the semiconductor laser, but it does not necessarily have to be in this position and can be placed in any desired position.

In the above embodiments, a case has been shown in which the photodetector 30 is used as a reflecting mirror of an external resonator, but it is not necessarily necessary to use the photodetector 30 as a reflecting mirror.

FIG. 7 is a schematic configuration diagram showing a sixth embodiment of the present invention, in which a reflective grating lens 24 is used as a reflective mirror of an external resonator. The surface of the grating lens 24 is coated to increase its reflectance so that more light returns to the semiconductor laser element 10, but a portion of the diffracted light is transmitted and enters the photodetector 30. It has become.

In the case of this embodiment, unlike the case of the first embodiment, the photodetector 30 only performs light monitoring, so there is no need for precise positioning. In this case, the oscillation wavelength is determined only by the position of the grating lens 24. The radius Rn of the n-th grating line from the center of the grating lens 24 is given by the following equation.

2J "y" starvation ("--2f 1 mnλ...■ Said■
Similar to the equation, the above equation (2) can be approximated as follows.

R1J . . . ■ FIG. 8 is a schematic configuration diagram showing a seventh embodiment of the present invention. In this embodiment, a reflective grating lens 25 is used as the grating lens in the second embodiment. That is, the grating lens 25 has the sixth
The surface is coated as in the example above.

Of course, even with such a configuration, the same effects as in the sixth embodiment can be obtained.

FIG. 9 is a schematic diagram showing the eighth embodiment of the present invention. In this embodiment, instead of the transmitted light of the grating lens 25, reflected light (for example, zero-order reflected light) is used as the monitor light.

With such a configuration, it is possible to arrange the photodetector on the same side of the grating lens 25 as the laser element 10.

Note that the present invention is not limited to the embodiments described above, and can be implemented with various modifications without departing from the gist thereof.

[Effects of the Invention] As detailed above, according to the present invention, by using a grating lens made of diffracted photons, it is possible to realize an external cavity semiconductor laser device that can obtain a sufficient amount of optical feedback with a simple optical system. This also makes it easier to monitor the optical output.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing a semiconductor device according to a first embodiment of the present invention, FIG. 2E is a diagram showing a groove configuration of a grating lens used in the above embodiment, and FIGS. FIG. 10 is a schematic configuration diagram showing another embodiment of the invention, and FIG. 10 is a schematic configuration diagram showing a conventional device. 10... Semiconductor laser element, 20°25... Grating lens, 0... Photodetector, 40... Rod lens.

Claims (1)

[Claims] A semiconductor laser element, a lens whose focal length is wavelength-dependent, which is provided facing away from one of the light emitting end faces of the laser element, and light transmitted through or reflected by this lens to be condensed. and a photodetector for detecting the
The semiconductor device characterized in that the lens focuses rear emitted light from the laser element onto a photodetector, and also focuses reflected light from the photodetector onto a light emitting end face of the laser element. laser equipment.