JPH02106083A - Device for constricting spectrum line width of semiconductor laser - Google Patents

Abstract

PURPOSE:To assure the stable spectrum line width constriction by a method wherein an optical path setting up means is provided so that a part of laser beams resonated in a photoresonator may be fed back to a semiconductor laser but the reflected beams on the incident part of the photoresonator may not be fed back to the semiconductor laser. CONSTITUTION:The incident beams emitted from a semiconductor laser 1 reach an incident end A of a photoresonator 2 through a condenser 3 so that the laser beams reflected on the incident end A may be obliquely entered into the incident end A not to be fed back to the semiconductor laser 1. When the optical path of resonator is around integer times of the wave length of laser beams entered into the photoresonator 2, a resonator will be caused to externally emit the resonant beams in multiple directions while the resonant beams in one direction b is fed back to the semiconductor laser 1 along the optical axis of the incident beams. These fed back beams furnished with the data only on the resonance characteristics of the photoresonator 2 is brightened only when the beams are resonated with the resonance frequency of the photoresonator 2. Through these procedures, when the optical path from the semiconductor laser 1 to the incident part of the photoresonator 2 is around integer times of laser wave length, the semiconductor laser 1 can bring about the self retracting effect thus assuring the stable spectrum line width constriction.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a semiconductor laser spectral linewidth narrowing device for generating narrow spectral linewidth laser light necessary for coherent optical communication and coherent optical measurement.

(Prior Art) Various proposals have been made regarding semiconductor laser spectral linewidth narrowing devices for narrowing the oscillation spectrum linewidth of a semiconductor laser.

As a method for narrowing the spectral linewidth of a semiconductor laser, experiments are being conducted on an optical feedback method in which light reflected from an external reflective surface of the semiconductor laser is reinjected into the semiconductor laser and a broadband electrical negative feedback method.

As a type of optical feedback method, a method is known in which resonant light from an optical resonator composed of two confocal concave mirrors is returned to a semiconductor laser.

For example, the paper by Damani et al. entitled "On frequency stabilization of semiconductor lasers by resonant optical feedback"("Freque
ency 5tabilization of sem
iconductor lasers by reson
ant optjcal feedback”: B,
Dahmani, L, IIollberg, l? ,
Drullinger.

0ptics 1etlers, si o1.12. No,
11 (1987), pp. 875-878).

Next, an optical feedback method using this confocal optical resonator will be explained with reference to FIG.

The light emitted from the semiconductor laser 120 is transmitted through a lens (121).
The parallel light is made into parallel light and is made incident on the beam splitter 122.

The light reflected by beam splitter 122 is separated into a spectral linewidth narrowing system.

The spectral line width narrowing system includes a lens 1 that focuses parallel light rays.
230 reflection 83024. It is composed of a confocal optical resonator 126 and a light receiving element 128.

The reflecting mirror 124 is supported by an electrostrictive element (PZT-φ), and the position of the reflecting mirror 124 is finely adjusted.

The confocal optical resonator 126 is formed by facing two concave reflectors, one of which is made of an electrostrictive element (PZT).
-C), and is configured so that the resonator length can be changed.

The laser beam focused by the lens 123 is made incident at an angle with respect to the center line of the two concave mirrors of the confocal optical resonator 126, and the reflected light at the incident point does not return to the semiconductor laser 120.

Optical resonator 1 is activated when the optical path length of the V-shaped optical path between the incident point of the concave mirror and two points of the other concave mirror is an integral multiple of the wavelength of the incident light.
Light incident within 26 is caused to resonate.

The resonant light passes through the concave mirror and exits the resonator in four directions.

One direction of the resonator-emitted light emitted in four directions returns along the optical path of the light incident on the resonator and returns to the semiconductor laser 120.

In this way, the light that returns from the optical resonator 126 to the semiconductor laser 120 has only information about the resonance characteristics of the optical resonator 126, and the returned light becomes large only when it resonates with the resonant frequency of the optical resonator 126. .

Therefore, the semiconductor laser 120 produces a self-pulling effect, and stable spectral line width narrowing is performed.

The electrostrictive element (PZT-φ) supporting the reflecting mirror 124 adjusts the optical path length between the semiconductor laser 120 and the optical resonator 126 to iJ! in order to optimize the return optical phase. It is used for the purpose of arranging it.

The electrostrictive element (PZT-C) that supports one of the two concave reflecting mirrors that make up the optical resonator 126 is used to adjust the resonator length as described above and make it resonate as an integral multiple of the wavelength of the incident light. be done.

The light receiving element 128 is used to monitor the output of the resonator 126.

(Problems to be Solved by the Invention) As described above, the conventional spectral linewidth narrowing device for a semiconductor laser mainly uses an optical resonator composed of two confocal concave mirrors.

To this end, there are issues to be solved, such as the following.

(1) Since the resonator cavity is an air medium, it is affected by air fluctuations and temperature changes, making the resonator optical path length unstable.
It is difficult to achieve stable spectral line width narrowing.

(2) Since two confocal concave mirrors are used in the optical resonator, a spacer for keeping the distance between them constant, an electrostrictive element for slightly moving one of the confocal mirrors, and parts for holding them are required.

Therefore, the optical resonator tends to be large and cannot be miniaturized.

(3) Since the configuration of the optical resonator is as described above, the optical resonator has a large number of parts, lacks robustness, and is easily affected by external vibrations and shocks.

(4) In order to keep the resonator length of the optical resonator constant against temperature fluctuations, it is necessary to use a spacer with a low coefficient of thermal expansion and to make fine adjustments using an electrostrictive element.

(5) As mentioned above, an optical resonator requires many parts.The main parts require special high-precision machining and difficult adjustment during assembly.

The main purpose of the present invention is to solve the above-mentioned problems, to reduce the size of the
An object of the present invention is to provide a robust, low-cost spectral line width narrowing device for a semiconductor laser.

Another object of the present invention is a spectral linewidth narrowing device for a semiconductor laser that can stably narrow the spectral linewidth of a semiconductor laser by stabilizing the resonator length of the optical resonator and the refractive index of the waveguide. Our goal is to provide the following.

Still another object of the present invention is to provide a spectral linewidth narrowing device for a semiconductor laser, which narrows the spectral linewidth of a semiconductor laser and enables external modulation.

(Means for Solving the Problems) In order to achieve the above object, a semiconductor laser spectral linewidth narrowing device according to the present invention uses an optical feedback method in which reflected light from an external reflective surface is reinjected into a semiconductor laser. A spectral line width narrowing device for a semiconductor laser includes a semiconductor laser, an optical resonator in which an optical resonator cavity is formed by a solid waveguide that propagates light with low loss, and an optical resonator between the semiconductor laser and the optical resonator entrance part. The optical path length is set to be approximately an integer multiple of the laser wavelength, a part of the laser light that resonates within the optical resonator is returned to the semiconductor laser, and the reflected light from the optical resonant input section is reflected from the semiconductor laser. The structure includes an optical path setting means that maintains a relationship in which no light is returned to the optical path.

As described above, the spectral linewidth narrowing device for a semiconductor laser according to the present invention propagates light with low loss, instead of using an optical resonator composed of two confocal concave mirrors, etc. used in the conventional technology. It uses an integrated optical resonator in which an optical resonator cavity is formed with a solid waveguide, and the following various integrated optical resonators can be used as components.

(1) An optical resonator that propagates light with low loss and has two or more solid end faces with high reflectance.

(2) An optical resonator having the structure of an optical directional coupler with two terminals on the input side and two terminals on the output side.

(3) An optical resonator using an optical directional coupler and a ring-shaped waveguide structure including the optical directional coupler.

In addition, the optical path length between the light incidence part of the optical resonator and the semiconductor laser can be adjusted to approximately an integral multiple of the laser wavelength, or an optical path length stabilizing element consisting of a photodetector, an oscillator, a phase sensitive detector, and an optical path length drive mechanism can be used. It is possible to have a configuration that maintains stability.

(Function) With such a configuration, the spectral line width of the semiconductor laser is narrowed by the following function.

Emitted light from the semiconductor laser is guided into the optical resonator and reaches the incident end face of the optical resonator.

In order to prevent the laser beam reflected by the incident end face from returning to the semiconductor laser, the laser beam is made to enter the incident end face obliquely.

Resonance occurs if the resonator optical path length is approximately an integral multiple of the wavelength of the laser light incident on the integrated optical resonator. The resonant light is emitted to the outside of the optical resonator.

The light within the optical resonator 2 is emitted in a plurality of directions, one of which returns along the incident optical axis and returns to the semiconductor laser. The returned light from this optical resonator has only information about the resonance characteristics of the optical resonator, and the returned light becomes large only when it resonates with the resonant frequency of the optical resonator.

Therefore, when the optical path length between the semiconductor laser and the outside of the optical resonator is approximately an integral multiple of the laser wavelength, the semiconductor laser produces a self-pulling effect, resulting in stable spectral linewidth narrowing.

The above is the effect when using an integrated optical resonator made of a glass block, etc. However, when an integrated optical resonator is constructed from the optical fiber part of a directional coupler having a waveguide such as an optical fiber, A similar effect can be obtained in the case of

In this case, the laser beam incidence part into the optical resonator becomes the coupling part of the directional coupler.

Compared to the optical resonator constructed from two confocal concave mirrors used in the prior art, the integrated resonator of the present invention has the same or lower resonance finesse, and the amount of light returned to the semiconductor laser is also the same. However, semiconductor lasers can achieve spectral linewidth narrowing even with a small amount of returned light.

As described above, the present invention uses an integrated optical resonator to narrow the linewidth of a semiconductor laser, so it has good temperature stability, is small and robust, can be manufactured at low cost, and is easy to assemble and adjust. .

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings and the like.

FIG. 1 is a schematic diagram showing an embodiment of a spectral linewidth narrowing device for a semiconductor laser according to the present invention using a hemispherical optical resonator.

The rear light of the semiconductor laser 1 is used for narrowing the spectral line width, and the front light is used as output light.

The light from the semiconductor laser 1 is focused by the lens 3 and a part of the light is made to enter the optical resonator 2.

The hemispherical optical resonator 2 is integrally made of uniform optical glass.

A plane passing through the center point of the sphere of optical resonator 2 (plane A) and a spherical surface (8
The surface) is coated with a highly reflective film.

The laser light that is input from the semiconductor laser 1 to the optical resonator 2 via the lens 3 has a certain inclination to the normal to the center of the plane (plane A).

Therefore, of the laser light obliquely incident on this center point, the reflected light is reflected in the direction indicated by (al) and does not return to the semiconductor laser 1.

The laser light incident on the optical resonator 2 as described above is
First, it travels in the direction of (dl), is reflected by the spherical surface (8 surfaces), returns to the incident point, is reflected in the direction indicated by tc> at the incident point, and is reflected by the spherical surface (8 surfaces).

In this way, a V-shaped repeated reflection optical path is formed between one point on the A surface and two points on the 8th surface, and resonance occurs when the optical path length of this optical path becomes an integral multiple of the laser beam wavelength.

The laser light resonating inside the optical resonator 2 leaks out of the resonator, and the 4 (a), (b), fC) and (d)
It is propagated in the direction.

Among them, only the light emitted in the direction of bl returns to the semiconductor laser 1, which backlights onto the path of incidence into the optical resonator 2.

When the optical path length between the incident point of the semiconductor laser 1 and the optical resonator 2 is set to be approximately an integer multiple of the laser wavelength generated by the semiconductor laser 1, the semiconductor laser 1 produces a self-pulling effect and produces a stable spectral line. Width narrowing is performed.

In this embodiment, an optical resonator 2 using a hemisphere, which is the most easily available, is used. However, even if the curved surface of the optical resonator 2 is an ellipsoid of revolution or a paraboloid of revolution, the same figure-7 shape as described above can be used. It is possible to form a repeated reflection optical path, and similarly, an optical resonator 2 can be formed.

Furthermore, in this embodiment, the rear light of the semiconductor laser 1 is used for spectral line width narrowing, and the front light is used as the output light. The same thing can be done by using one as the output light and the other as the output light.

The apparatus shown in FIG. 2 represents another embodiment which operates on substantially the same principles as the previous embodiment.

This embodiment uses a semi-cylindrical optical resonator 22 instead of the hemispherical optical resonator 2.

The semi-cylindrical optical resonator 22 is made of optical glass or the like and has a semi-cylindrical shape.

The plane and columnar surfaces of the optical resonator 22 are coated with a reflective film with high reflectance.

Similar to the previous embodiment, a V-shaped repeating reflection optical path is formed between the two points of the incident plane and the columnar surface, and resonance occurs when the optical path length becomes an integral multiple of the laser beam wavelength.

When the optical path length between the incident point of the semiconductor laser 1 and the optical resonator 22 is set to be approximately an integral multiple of the laser wavelength generated by the semiconductor laser 1, the semiconductor laser 1 produces a self-pulling effect and becomes stable. Spectral linewidth narrowing is performed.

The apparatus shown in FIG. 3 also shows another embodiment which operates on substantially the same principle as the embodiment shown in FIGS. 1 and 2.

In this embodiment, an optical glass prism having an isosceles triangular cross section is used as the optical resonator 23.

Three planes of the optical resonator 23 are coated with a reflective film with high reflectance.

7 between the two points on the incident plane and the other plane as in the previous embodiment.
A repeatedly reflected optical path in the shape of a letter is formed, and resonance occurs when the optical path length becomes an integral multiple of the laser beam wavelength.

When the optical path length between the semiconductor laser l and the incident point of the optical resonator 23 is set to be approximately an integral multiple of the laser wavelength generated by the semiconductor laser 1, the semiconductor laser 1 produces a self-pulling effect and becomes stable. Spectral linewidth narrowing is performed.

FIG. 4 shows still another embodiment in which an etalon is used as the optical resonator 24.

The etalon used here is an optical element in which a metal film or dielectric layer is coated on both sides of a parallel flat plate made of glass or crystal with extremely good flatness to increase the reflectance. the plane of incidence;
A repeatedly reflected optical path is formed on the other surface, and resonance occurs when the optical path length becomes an integral multiple of the laser beam aX.

Then, the optical path length of semiconductor laser 1 → lens 3 - beam splitter 4 - reflecting mirror 5 - optical resonator 24 and the optical path length of semiconductor laser 1 → lens 3 → beam splinter 4 - reflecting mirror 6 - optical resonator 24 are The wavelength is set to be approximately an integral multiple of the laser wavelength generated by the semiconductor laser 1.

The emitted light from the semiconductor laser 1 is converted into parallel light by the lens 3, split into two parts by the beam splitter 4, and each reflected by the reflecting mirror 5.6, forming an optical resonator 2 formed by an etalon.
4. At this time, the normal line of the reflecting surface of the etalon forming the optical resonator 24 is installed with a slight inclination with respect to the laser beam optical axis, so that the reflected light from the incident surface does not return to the semiconductor laser 1. .

When the light incident on the etalon forming the optical resonator 24 has such a small angle of incidence, there is a substantial finesse.
esse) decreases, but the spectral line width narrowing effect of the semiconductor laser 1 occurs as in the previous example.

The device shown in FIG. 5 is a schematic diagram showing still another embodiment of a semiconductor laser spectral line width narrowing device using an optical directional coupler.

In this embodiment, the optical resonator 25 is incorporated into an optical fiber directional coupler having two terminals at the input end and two terminals at the output end.

Input end face f of optical fiber 25 of optical fiber directional coupler
c) A high-reflectance reflective film is formed on the output end face (d), and the optical path between the two end faces is used as an optical resonator 25.

The emitted laser light from the semiconductor laser 1 enters the incident end face (a) of the optical fiber 7 of the optical fiber directional coupler, and at the coupling part 8, a part of the propagating light enters the optical resonator 25, and the remaining propagating light enters the optical resonator 25. The Ill light is emitted from the emission end face (b).

A portion of the laser light resonated in the optical resonator 25 travels backward through the incident end face (a) of the optical fiber 7 from the coupling portion 8 and returns to the semiconductor laser 1, thereby narrowing the spectral line width.

At this time, the optical path length between the semiconductor laser 1 and the coupling portion 8 of the directional coupler is set to approximately an integral multiple of the laser wavelength.

The embodiment shown in FIG. 6 is yet another embodiment having many common configurations with the embodiment shown in FIG.

In this embodiment, instead of the optical resonator 24 using the etalon used in the embodiment shown in FIG.
is formed.

A condenser lens 10 is provided at both ends of an optical resonator 26 formed by an optical fiber.
．． 11 are arranged respectively.

Both end faces of the optical resonator 26 made of an optical fiber are set obliquely with respect to the incident optical axis to prevent light reflected from the end faces from directly returning to the semiconductor laser l.

FIG. 7 is a schematic diagram showing an embodiment of a spectral line width narrowing device for a semiconductor laser in which the optical resonator 27 is formed using an optical fiber ring resonator.

Coupling section 12, 13.1 of three optical fiber directional couplers
4 are bonded together as shown in FIG. 7 to form a spectral line width narrowing device for a semiconductor laser.

The laser light from the semiconductor laser 1 that rotates clockwise through the ring-shaped waveguide is transmitted to the first coupling portion 13. The counterclockwise direction enters the ring-shaped waveguide from the second coupling part 14 and causes ring resonance, and a part of the clockwise resonant light enters the second coupling part 1.
4. A portion of the counterclockwise resonant light passes through the first coupling portion 13 and becomes return light to the semiconductor laser 1 .

The emitted laser light from the semiconductor laser 1 enters the optical fiber 15 and is split into two at the coupling part 12 of the first directional coupler.

The branched laser beams are then connected to the optical fiber ring resonator forming the optical resonator 27, to the coupling portion 13 of the second optical fiber directional coupler, and to the coupling portion 14 of the third optical fiber directional coupler. It is made incident from

Laser light that has not entered the ring resonator, which is the optical resonator 27, is emitted from the output end 19.18 of the coupling portion 13.14 of the optical fiber directional coupler and does not return to the semiconductor laser l.

The laser light resonated in the optical resonator 27 passes through the coupling portions 13, 14 and 12 of the directional coupler, becomes a return light to the semiconductor laser 1, and undergoes spectral linewidth narrowing.

The device shown in FIG. 8 is a schematic diagram showing an embodiment of a more specific spectral linewidth narrowing device for a semiconductor laser by incorporating a long-term stabilization control system into the basic embodiment shown in FIG. It is.

The semiconductor laser 1 is supplied with current from the injection current control circuit 31 via the inductance L, and is caused to oscillate, thereby emitting laser light.

Laser light from a semiconductor laser 1 is focused by a lens 3 and is made incident on an optical resonator 2 having the shape described in connection with FIG.

As described above, the angle of incidence is set so that the reflected light from the incident surface of the optical resonator 2 does not return to the semiconductor laser 1.

A part of the light emitted from the optical resonator 2 is detected by the light receiving element 34 and fed back to the injection current control circuit 31.

As a result, the injection current control circuit 31 controls the semiconductor laser 1
The injection current is controlled so that the oscillated light has a wavelength that resonates in the optical resonator 2.

Furthermore, constant frequency modulation is applied to the optical path length variable drive mechanism 32 by the oscillator 36 to modulate the optical path length between the incident parts of the semiconductor laser 1 and the optical resonator 2, and the output signal of the light receiving element 34 and the output signal of the oscillator 36 are adjusted in phase. The phase is detected by the sensitive detector 35, and the phase shift output is fed back to the electrostrictive element 33 of the variable optical path length drive mechanism, so that the optical path length between the incident parts of the semiconductor laser 1 and the optical resonator 2 becomes approximately an integral multiple of the laser wavelength. control.

With this configuration, it is possible to realize a semiconductor laser line width narrowing device that is stable over a long period of time and against changes in environmental conditions.

Furthermore, in this configuration, by superimposing an external modulation input on the current injected into the semiconductor laser 1, it is possible to change the laser light frequency once.

At this time, a reactance (L) is incorporated to prevent external modulation input from flowing into the injection control circuit 31.

When the external modulation frequency fm satisfies the following relationship with respect to the free spectral range (FSR) of the optical resonator 2, high modulation efficiency can be obtained.

fm=n-FsR However, n is a rational number.

The device shown in FIG. 9 is a schematic diagram showing an embodiment of a more specific spectral line width narrowing device for a semiconductor laser by incorporating a long-term stabilization control system into the basic embodiment shown in FIG. It is.

The long-term stabilization control system has almost the same configuration as that shown in FIG. 8, and common elements are denoted by common numbers and their explanation will be omitted.

In this embodiment, as a variable optical path length driving mechanism, a variable optical path length element (PZTI) 41 of an optical fiber using an electrostrictive element is used.
is used.

This configuration can accommodate a lower frequency external modulation input compared to the external modulation input shown in FIG.

A second variable optical path length element (PZT2) 42 using an electrostrictive element is incorporated into the optical fiber 25 portion forming the optical resonator, and the optical path length of the optical fiber is modulated by external modulation input.

In other words, it is possible to modulate the optical resonator length.

At this time, the external modulation input is simultaneously input to the injection current control circuit 31, and the injection current of the semiconductor laser 1 is also modulated in the same phase.

Thereby, the optical path length of the laser beam can be maintained at approximately an integral multiple of the laser wavelength.

Next, an example of experimental data on spectral line width narrowing by the spectral line width narrowing device for a semiconductor laser according to the present invention will be shown.

FIG. 10 is an optical path diagram of an experimental apparatus for obtaining experimental data of the apparatus of the type shown in FIG.

FIG. 11 is a graph showing the measurement results.

SL is the semiconductor laser 1. in FIG. FPR is also an optical resonator 2. L1 and L2 also correspond to lens 2 in FIG. Ll is a collimating lens, L2 is a condensing lens, B
S is a beam splitter for extracting measurement light.

The specifications of each element are as follows.

SL...Semiconductor laser (Hitachi type) HLP1400L1
...Collimating lens (manufactured by Olympus) V8030 L2 ... Condensing lens (Olympus!! Ri V1815 BS ... Beam splitter (manufactured by HOYA) 5mm square FPR ... Optical glass material, semi-15mm, reflection of reflective surface Linewidth measurement system set at a rate of 90% and tilted approximately 5 degrees to the optical axis...delayed self-hetero gain method.

Savings on decomposition 100 K HZ The curve (■) shown in FIG. 11 is a measured value of a single semiconductor laser, and the line width (full width at half maximum) is 40 MH2.

(2) is the post-stenosis curve obtained in the above experiment, but it cannot be deciphered from this drawing. A value of approximately 100KH2 was obtained from the enlarged data.

(Effects of the Invention) Since the semiconductor laser spectral line width narrowing device according to the present invention is configured as described above, it has the following effects.

The optical resonator is made of optical glass material and has a hemispherical shape, so that deformation due to temperature changes is small or can be easily controlled to be small.

Moreover, it is small, lightweight, and robust, and can be manufactured at low cost.

The resonator cavity is a solid optical waveguide medium, and unlike air media, it is not affected by air fluctuations or temperature changes, and the resonator optical path length does not become unstable.

These features also apply to optical resonators formed from optical fiber directional couplers.

In the spectral linewidth narrowing device for a semiconductor laser according to the present invention, as described above, by returning only the laser light that resonates with the resonant frequency of the optical resonator to the semiconductor laser, the semiconductor laser produces a self-pulling effect and has a stable spectrum. Line width narrowing is performed.

This configuration has an optical resonator and a semiconductor laser as its main components, and can create a stable spectral line width narrowing device with a small number of parts and simple optical axis adjustment.

Taking advantage of the stability of the integrated optical resonator against disturbances,
A feedback system that controls the injection current so that the resonant frequency of the optical resonator is approximately an integer multiple of the oscillation frequency of the semiconductor laser, and an optical path control system that maintains the optical path length between the optical resonator and the semiconductor laser at approximately an integer multiple of the laser wavelength. The device of the present invention, which includes a feedback system for controlling the length, can stably narrow the spectral line width of a semiconductor laser over a long period of time.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a basic embodiment of a spectral linewidth narrowing device for a semiconductor laser according to the present invention using a hemispherical optical resonator. FIG. 2 is an optical path diagram showing an embodiment of the spectral line width narrowing device for a semiconductor laser according to the present invention using a semi-cylindrical optical resonator. FIG. 3 is an optical path diagram showing an embodiment of the spectral line width narrowing device for a semiconductor laser according to the present invention using an optical resonator having a prismatic cross section. FIG. 4 is an optical path diagram showing an embodiment of a spectral line width narrowing device for a semiconductor laser using an etalon as an optical resonator. FIG. 5 is an optical path diagram showing an embodiment of a spectral line width narrowing device for a semiconductor laser using an optical directional coupler as an optical resonator. FIG. 6 is an optical path diagram showing an embodiment of a spectral line width narrowing device for a semiconductor laser using an optical fiber as an optical resonator. FIG. 7 is a schematic diagram showing an embodiment of a spectral line width narrowing device for a semiconductor laser in which an optical resonator is formed using an optical fiber ring resonator. The device shown in FIG. 8 is a schematic diagram showing an embodiment of a more specific spectral linewidth narrowing device for a semiconductor laser by incorporating a long-term stabilization control system into the basic embodiment shown in FIG. It is. The device shown in FIG. 9 is a schematic diagram showing an embodiment of a more specific spectral line width narrowing device for a semiconductor laser by incorporating a long-term stabilization control system into the basic embodiment shown in FIG. It is. FIG. 10 is a schematic diagram showing an experimental apparatus for obtaining data on spectral line width narrowing of a semiconductor laser spectral line width narrowing device. FIG. 11 is a graph showing measurement results obtained by the experimental apparatus. FIG. 12 is a schematic diagram showing a spectral line width narrowing device for a semiconductor laser that has already been proposed. ■...Semiconductor laser 2.21-26...Optical resonator 3...Semiconductor laser condenser lens 4...Beam splinter 5.6...Reflector 7...Optical fiber 8...・Coupling part of optical directional coupler...Optical fiber 0.11...Lens 2.13.14...Coupling part 5 to 19 of optical directional coupler
...Optical fiber...Injection current control circuit 2...Variable optical path length drive mechanism 3...PZT of variable optical path length drive mechanism 4...Light receiving element 5...Phase sensitive detector 6...Oscillator 1... Optical fiber variable optical path length element (PZTI) 2.
...Second optical path length variable element (PZT2) Patent applicant Tokyo Air Instruments Co., Ltd. Representative Patent attorney Inoro ) Figure Figure 1゛a7= J'l';&4! ' To figure

Claims (6)

[Claims] (1) In a semiconductor laser spectral linewidth narrowing device that uses an optical feedback method in which light reflected from an external reflective surface is re-injected into a semiconductor laser, a semiconductor laser and a solid waveguide that propagates light with low loss are used. An optical resonator forming an optical resonator cavity, and an optical path length between the semiconductor laser and the optical resonator entrance part are set to be approximately an integral multiple of the laser wavelength, and a part of the laser beam resonating within the optical resonator of the laser beam is set. Spectral linewidth narrowing of a semiconductor laser, characterized in that it is configured by providing an optical path setting means for maintaining a relationship in which the part is returned to the semiconductor laser, and the reflected light from the optical resonator entrance part is not returned to the semiconductor laser. Device. (2) The optical resonator has two or more solid interfaces with high reflectance through which light propagates with low loss, and the reflected light of the light incident on the first surface is not returned to the semiconductor laser. 2. The spectral line width narrowing device for a semiconductor laser according to claim 1, wherein a part of the light that resonates inside is fed back. (3) The optical resonator is formed using an optical directional coupler with two terminals on the input side and two terminals on the output side, and one input waveguide end face and one output waveguide end face of the directional coupler are connected to each other. Claim 1, wherein an optical resonator is formed with a high reflectance, a laser beam from a semiconductor laser is made incident on the remaining one input end, and the input and output parts of the optical resonator are the coupling parts of a directional coupler. spectral line width narrowing device for semiconductor lasers. (4) The optical resonator is formed by a ring-shaped waveguide incorporating a plurality of optical directional couplers, and the laser light from the semiconductor laser that rotates clockwise through the ring-shaped waveguide is connected to a first coupling portion. , the counterclockwise light enters the ring-shaped waveguide from the second coupling part and causes ring resonance, a part of the clockwise light of the resonant light enters the second coupling part, and a counterclockwise light enters the first coupling part. Claim 1: The light returns to the semiconductor laser through the coupling portion.
A spectral linewidth narrowing device for a semiconductor laser as described above. (5) A part of the light emitted from the optical resonator is detected by a light receiving element, and the output of the light receiving element is fed back to the injection current control circuit to control the injection current of the semiconductor laser, and a laser beam having a wavelength that resonates in the optical resonator is generated. Light is emitted, the optical path length between the semiconductor laser and the incident part of the optical resonator is modulated at a constant frequency by an optical path length drive mechanism, the light receiving element detection output and the modulated signal are phase detected by a phase sensitive detector, and the output is 2. The spectral line width narrowing device for a semiconductor laser according to claim 1, wherein the optical path length is controlled to be maintained at approximately an integral multiple of the laser wavelength by feedback to the optical path length driving mechanism. (6) The waveguide of the optical directional coupler is an optical fiber, a first optical path length modulation element is incorporated in the optical fiber waveguide forming the optical resonator, and the optical path length is modulated by modulation input from outside the device. The device and the semiconductor laser are modulated in synchronization to modulate the semiconductor laser oscillation wavelength, and the optical path length between the semiconductor laser and the optical resonator entrance part is incorporated into the optical fiber input waveguide into which the laser light from the semiconductor laser enters. A part of the light emitted from the optical resonator is detected by a light receiving element, and a phase sensitive detector outputs the output of the light receiving element and the modulated signal from the oscillator. 4. The spectral line width narrowing device for a semiconductor laser according to claim 3, wherein the optical path length is controlled to be maintained at approximately an integral multiple of the laser wavelength by performing detection and feeding back the output to the second optical path length modulation element.