JPH0136986B2 - - Google Patents

Description

Detailed Description of the Invention (a) Technical Field of the Invention The present invention relates to an optical frequency controller that uses a single frequency light source to generate two lights with a mutually stable frequency difference. The present invention relates to a light source device suitable for performing so-called heterodyne measurement.

(b) Background of the technology In recent years, methods of measuring various physical quantities using laser light have been widely used. Among these methods, measurement methods based on the wavelength (or frequency) of the laser light Because the frequency is as high as 10 to the 14th power or higher, it is impossible to convert it directly into an electrical signal.Usually, the light to be measured has a slightly different frequency compared to the frequency of this light. A so-called heterodyne measurement method is used in which the phase or frequency of the beat signal generated by this interference is detected. The magnitude of the above difference frequency is 7 out of 10, which can be processed using an electrical circuit.
or 10 to the 10th power (1/10 to 10 to the 8th power of the frequency of the light to be measured), and the two lights not only have stable frequencies, but also It is necessary that the difference frequency between them be kept stable.

(c) Prior Art and Problems Obtaining two lights having different frequencies as described above using two independent laser light sources is not practical in terms of frequency stability. Therefore, by using a single laser light source, for example by utilizing the Zeeman effect or the diffraction effect due to ultrasonic waves,
Methods have been proposed, such as generating light with a fixed difference frequency. However, in the former case, a large magnet is required to obtain the desired difference frequency, and two lights with difference frequencies can be obtained by separating clockwise circularly polarized light and counterclockwise circularly polarized light. Therefore, it has drawbacks such as not being applicable to semiconductor laser light whose oscillation light is linearly polarized light (one of two linearly polarized lights, TE waves and TM waves).

In addition, in the latter case, in order to obtain the desired difference frequency by changing the black reflection conditions that can be applied by ultrasonic waves, the ultrasonic modulator requires a large driving power, and the heat generated for this purpose is large. It has drawbacks such as the need for countermeasures.

On the other hand, as shown in FIG. 1, the light I (frequency ω) from the same light source is separated into two parts, and electro-optical elements 1a and 1b are used for each part, so that the phase is 1/4 of each other. The two lights Ia and Ib whose frequencies are ω + Δω and ω - Δω are further separated into two by modulation with an external signal (frequency Δω) whose period is shifted. By giving an appropriate optical path difference between the light beams Ia and Ib and combining them, two lights 0a (frequency ω + Δω) and 0b with different frequencies are generated.
A method of separating and extracting (frequency ω-Δω) has been proposed (patent pending).

According to the above method, two lights having a desired difference frequency can be obtained by using a relatively small external signal (electrical or magnetic signal), and the difference frequency can be given without using a magnetic field. It is also possible to use a semiconductor laser as a light source, which is effective in solving problems with conventional methods.However, when modulating light at the original frequency, amplitude modulation also As a result, this is superimposed on the beat signal, and it is difficult to separate them later using an electrical circuit, which has the disadvantage of restricting the scope of application.

(d) Purpose of the invention The present invention provides a light source device capable of generating two lights of different frequencies from a single light source.
It is an object of the present invention to provide a light source device that does not require a large magnetic field or electric field, can use a semiconductor laser as a light source, and can easily generate light of a constant intensity.

(e) Structure of the Invention The present invention comprises a pair of reflective surfaces installed opposite to each other on an optical axis, and an anisotropic optical amplification section installed on the optical axis between the pair of reflective surfaces. a laser that oscillates as an eigenmode two linearly polarized lights whose vibration planes are orthogonal to each other and has a resonator comprising a resonator; Two quarters, one each installed at a 45 degree angle to
It is characterized by comprising a wavelength plate and a Faraday rotator installed between one of the quarter-wave plates and one of the corresponding reflecting surfaces.

(f) Embodiments of the invention Examples of the invention will be described below with reference to the drawings.

FIG. 2 is a diagram for explaining the operating principle of the basic components of the present invention, in which the laser 2 has an external resonator consisting of reflecting surfaces (reflecting mirrors) 3 and 4;
A Faraday rotator 5 is provided in front of one of the reflecting surfaces (reflecting surface 4 in the figure).

In a gas laser or the like, two types of polarization modes exist in a degenerate state in the oscillated light. In the above configuration, attention is paid to the right-handed and left-handed circularly polarized light, which is passed through the Faraday rotator 5 and resonated. The Faraday rotator 5 exhibits different refractive indexes for clockwise and counterclockwise circularly polarized light under a magnetic field. The refractive index n of a medium and the speed v of light passing through the medium are nv=
const. Therefore, there is a difference in the time required for the right-handed circularly polarized light and the left-handed circularly polarized light to pass through the Faraday rotator 5 having a constant thickness. As a result, the distances between the reflective surfaces 3 and 4 for the respective polarized lights are apparently different. That is, the resonant frequencies are different between the clockwise rotation and the counterclockwise rotation.
In the above, if a Faraday rotator is not provided, or if a magnetic field is not applied even if a Faraday rotator is provided, the clockwise and counterclockwise circularly polarized light are in a time-reversal relationship with respect to each other. That is, it is in a degenerate state at that frequency. When a magnetic field is applied here, the degeneracy is broken and a two-frequency wave is created.

In this way, lights of different frequencies ω+Δω and ω−Δω are transmitted through the reflective surface 4 and output. In the above example, 2
Since the two lights are circularly polarized lights, they can be separated and extracted by passing through a λ/4 plate and a polarization separation element (not shown). Similarly, the above two lights can be extracted from the reflective surface 3 side.

The laser described above is not limited to a gas laser, and any laser whose oscillation light includes circularly polarized mode light can be used.

On the other hand, when using a laser that generates one of linearly polarized lights whose vibration directions are perpendicular to each other, a λ/4 plate is installed in front of each of the reflecting surface 3 and the Faraday rotator 5, as shown in FIG. 6 and 7 are provided. This will be explained using a case where a semiconductor laser is used as an example. That is, in a normal semiconductor laser, two mutually orthogonal linearly polarized lights called TE waves and TM waves become the eigenmodes of the resonator. In the semiconductor laser 21 as shown in FIG. 4, the TE wave is parallel to the surface of the active layer 22 (the hatched surface) which serves as the light emitting and waveguide region, and the TM wave is parallel to the surface (hatched surface). It has a vertical plane of polarization. Such TE waves and
The TM wave is transmitted to the end faces 23 and 24 of the semiconductor laser 21.
Consider the case where the light is emitted from the λ/4 plate and passes through the λ/4 plate.

In FIG. 3, the end face 23 of the semiconductor laser 21
The TM wave emitted from the λ/4 plate 6 becomes circularly polarized light by passing through the λ/4 plate 6, and after being reflected by the reflecting surface 3, it passes through the λ/4 plate 6 again and becomes linearly polarized light again. However, at this time, the plane of polarization is rotated by π/2, so it becomes a TE wave. Furthermore, this light is emitted from the end face 24, passes through the λ/4 plate 7, becomes circularly polarized light again, is reflected by the reflective surface 4, and then passes through the λ/4 plate 7 again, so that the plane of polarization changes to π. /2 rotations and returns to TM wave.

On the other hand, the TE wave emitted from the end facet 23 of the semiconductor laser 21 becomes circularly polarized light by the λ/4 plate 6 in the opposite direction to that in the above case, and is reflected by the reflecting surface 3 before being reflected by the λ/4 plate 6. This becomes a TM wave, which is further emitted from the end face 24 and becomes circularly polarized light in the opposite direction by the λ/4 plate 7. After being reflected by the reflective surface 4, it passes through the λ/4 plate 7 and becomes a TM wave. return.

In the above, the sign of the ±45 degree angle formed between the axis of the λ/4 plate and the surface of the active layer determines whether the TE wave and TM wave are clockwise or counterclockwise circularly polarized. It depends.

As described above, when a Faraday rotator is not provided or when a magnetic field is not applied even if a Faraday rotator is provided, the oscillation light of the semiconductor laser 21 is transmitted between the reflecting surfaces 3 and 4 as TE waves and
The TM wave resonates in a degenerate state. That is, by providing the λ/4 plates 6 and 7 on both sides of the semiconductor laser 21, it becomes impossible to distinguish between the TE mode and the TM mode of the resonator as a whole.

However, the light that passes through the Faraday rotator 5 has two states with different modes: right-handed circularly polarized light and left-handed circularly polarized light. As mentioned above, the Faraday rotator 5 exhibits different refractive indexes for each of these clockwise and counterclockwise circularly polarized lights, so that the refractive index between the reflecting surfaces 3 and 4 for each of the two states of light is different. The effective distances will be different and these lights will have different resonant frequencies.

As mentioned above, if a Faraday rotator is not provided or if a magnetic field is not applied even if a Faraday rotator is provided, these two modes are in a time-reversal relationship with each other, so the magnetic field is applied to the Faraday rotator. When the frequency of right-handed circularly polarized light is given by ω+Δω, the frequency of left-handed circularly polarized light is given by ω−Δω.

As described above, for example, two lights with frequencies ω+Δω and ω-Δω are emitted from the reflecting surface 4 as clockwise and counterclockwise circularly polarized light, and these are polarized and separated using a λ/4 plate in the same manner as described above. It becomes possible to separate and take out using the element. That is, according to the above configuration, it is possible to use a semiconductor laser as a light source equivalent to a gas laser. Note that in the semiconductor laser used above, the end face 23 as in a normal semiconductor laser is
and 24 reflective coatings are not required;
Rather, it is preferable that an anti-reflection coating be applied.

According to the method of the present invention, the magnetic field applied to the Faraday rotator in order to provide the required difference frequency (Δω) may be relatively small, so the structure of the light source device can be downsized, and the difference frequency It also becomes easier to control. Further, there is an advantage that the intensities of the two obtained lights are constant. When the frequency difference Δω is fixed to a constant value, a permanent magnet is used as the magnetic field, but it is also possible to modulate Δω using an alternating magnetic field.

FIG. 5 is a diagram showing an example of a practical configuration of a laser light source device according to the present invention, in which a semiconductor laser is used as a light source. That is, a semiconductor laser 21 is mounted on a heat sink 8, and for example, ball lenses 9 and 10 for condensing light are provided at both light output ends of the semiconductor laser 21, and on both sides of the ball lenses 9 and 10,
λ/4 plates 61 and 71 are provided, respectively. λ/
One side of the four plates 61 is coated with a reflective coating 31 having a reflectance of 99%, for example. On the other hand, a Faraday rotator 5 is provided on one side of the λ/4 plate 71, for example, so as to be in contact with it. The Faraday rotator 5
A reflective coating 41 with a reflectance of 90%, for example, is applied to one side of the .

In the above configuration, the distance between the reflective coating 31 and the reflective coating 41 is about 1 mm,
The length between both end faces of the semiconductor laser 21 is about 0.3 mm, and the diameter of the ball lenses 9 and 10 is about 0.2 mm, and is configured as an extremely miniaturized two-frequency laser light source device.

(g) Effects of the Invention According to the present invention, an extremely compact and highly efficient laser light source device suitable for a light source device for heterodyne measurement can be realized, and as a result, the heterodyne measurement method using light can be used in a wide range of applications. This has the effect of making it applicable to various fields.

[Brief explanation of drawings]

Fig. 1 is a diagram for explaining an example of a conventional light source device for generating two lights of different frequencies from a single light source, and Fig. 2 is a diagram for explaining the basic principle of the present invention. , 3 and 4 are diagrams for explaining the configuration principle when a semiconductor laser is used in the laser light source device of the present invention, and FIG. 5 is a diagram showing a practical configuration example of the laser light source device according to the present invention. FIG. In the figure, 1a and 1b are modulators, 2 is a laser, 3 and 4 are reflective surfaces, 5 is a Faraday rotator, 6 and 7, 61 and 71 are λ/4 plates, 8 is a heat sink, 9 and 10 are ball lenses , 21 is a semiconductor laser, 22 is an active layer, 23 and 24 are end faces, and 31 and 41 are reflective coatings.

Claims (1)

[Claims] 1. Comprising a pair of reflective surfaces installed opposite to each other on the optical axis, and an anisotropic optical amplification section installed on the optical axis between the pair of reflective surfaces. a laser that has a resonator and oscillates two linearly polarized lights whose vibration planes are orthogonal to each other as eigenmodes; Two 1/4 wavelength plates each installed at an angle of 45 degrees, and a Faraday rotator installed between one of the 1/4 wavelength plates and the corresponding reflecting surface. A laser light source device comprising: 2. The optical amplifying section is a semiconductor laser with anti-reflection coating applied to both end faces thereof, and the surface of the active layer thereof forms an angle of 45 degrees with respect to the main axis of the quarter-wave plate. A laser light source device according to claim 1, characterized in that the laser light source device is arranged as follows.