JPS61169820A - Orthogonal polarization type optical frequency shifter - Google Patents

Abstract

PURPOSE:To eliminate the need for a quarter-wavelength plate and facilitate an optical axis adjustment and to use laser light which has two polarized light components by transmitting the P polarized light component of the laser light and reflecting the S polarized light component, and diffracting the transmitted light by an acousto-optic element. CONSTITUTION:The laser light 30 is made incident on a polarization beam splitter 20, which transmits its P polarized beam 31 and reflects its S polarized beam 32. The beam 32 passes through the acousto-optic element 21 to become diffracted light 33, which is reflected by a mirror 24 and transmitted through the 2nd polarization beam splitter 25 as the P polarized light. The beam 32 becomes diffracted light 34 through the 2nd acousto-optic element 27 and the light is reflected by the beam splitter 25 as the S polarized light, thus obtaining two light beams 35 having a slight difference in frequency. Therefore, no quarter-wavelength plate is required and temperature dependency is eliminated; and the number of optical system parts is small, the optical path is easily adjusted, and the incident laser light having the P and S polarized light components is usable.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention diffracts an incident laser beam on the wavefront of a traveling ultrasound in an acousto-optic element (black cell), and shifts the frequency of the laser beam by the Doppler effect. The present invention relates to a device that generates light having a certain frequency difference with respect to an optical signal to be detected by utilizing the effect.

In particular, this device is referred to as an orthogonal polarization type optical frequency shifter. Here, one of the two orthogonal linearly polarized components is used as an optical signal to be detected, and the other is used as an optical signal of local oscillation output. Note that in order to detect an optical frequency difference (eg, 1 MHz), the output light beam of the orthogonal polarization type optical frequency shifter is transmitted through a 45° polarizer and then passed through a photodetector.

Recently, high-precision, non-contact optical measurement that takes advantage of the properties of light has attracted attention. A hetero gain detection method is used. This optical heterogain detection method is similar to radio heterogain reception, in which a local oscillation output signal is mixed with the signal to be detected, an intermediate wave signal (beat signal) having a difference frequency is generated, and signal processing is performed. . In N-air communication, a completely independent oscillator is used to obtain the local oscillation output signal, but in the case of optical interference measurement, an independent optical oscillator whose frequency of the intermediate wave signal is stable to the extent that there is no fluctuation is used. It is difficult to produce. Therefore, a laser beam having a certain frequency difference with respect to the optical signal to be detected is generated, sent to the receiving end through a reference optical path, and used as a local oscillation output signal.

[Conventional technology]

Conventionally, an orthogonal polarization type optical frequency shifter has a configuration as shown in Fig. 2, in which an ion gas laser 1 (wavelength: 647.1 nl, optical frequency f, = 463.61
The laser beam 2 emitted from H7) emits linearly polarized light (S-polarized light) having an electric field component perpendicular to the paper surface, and is converted into two light beams by the half mirror 3, namely a transmitted light beam 4 and a reflected light beam. It is separated into 5 parts. The transmitted light beam 4 is applied to the acousto-optic element 6 at an angle θB that meets the black condition.
(Black angle). This acousto-optic element 6 receives a high frequency signal (for example, a center frequency of 42M) from a drive circuit 7.
Hz) to excite the transducer 8 and propagate the ultrasonic signal within the medium body, so that the transmitted light beam 4 described above travels straight after entering the medium body.
The second light and the ultrasonic signal are sent out after being separated into diffracted light 9 that is diffracted at an angle θB with respect to the wavefront, and the diffracted light 9 is used here. This diffracted light 9 has a center frequency 42 of the acousto-optic element 6.
Shifting the frequency of the optical signal by 14Hz, the optical frequency is f
, +42MHz.

Next, this diffracted light 9 is reflected by a mirror 10 and transmitted through a 172-wave plate 11 whose optical axis is rotated by 45° around the traveling direction (optical axis) of the light beam.

This transmitted light beam 12 becomes linearly polarized light (P-polarized light) having an electric field component parallel to the plane of the paper, and is transmitted through the polarizing beam splitter 13.

On the other hand, the reflected light beam 5 from the half mirror 3 acts on the acousto-optic element 14 in the same way as the acousto-optic element 6 described above.
Of the zero-order light and the diffracted light 16 that are incident at an angle θB that meets the black condition and sent out, the latter diffracted light 16 is used. Note that the center frequency of the high-frequency signal supplied from the drive circuit 7 to the transducer 15 is 4314H2, which is higher by IMHz than the center frequency 42) IHz of the acousto-optic element 6 in this example, so that this diffracted light 16 The frequency of the optical signal is shifted by the center frequency of 43 MHz of the acousto-optic element 14, and the optical frequency becomes f(, +43 MHz. Next, this diffracted light 16 is reflected by the mirror 17 and enters the polarizing beam splitter 13 described above. Since this incident diffracted light 16 is originally S-polarized light, it is reflected by this polarizing beam splitter 13.

In this way, the P-polarized transmitted light beam 12 and the S-polarized diffracted light 16 are transmitted and reflected by the polarized beam splitter 13, so that the emitted light beam of the polarized beam splitter 13 has a light component as a component. Frequency @ (f
o+4214Hz > P polarization and optical frequency (f6 +4
3MHz) S-polarized light is placed on the open optical path. and,
Two optical beams with a mutual frequency difference of IMHz are obtained.

[Problem that the invention seeks to solve]

The conventional orthogonal polarization type optical frequency shifter requires a 172-wave plate 11 to be inserted on the optical path. This 172-wavelength plate is manufactured by polishing a birefringent material such as quartz to a predetermined thickness, but manufacturing is extremely difficult because the thickness accuracy requires submicron order. Furthermore, since the visual refraction changes depending on temperature, a temperature compensation means such as bonding together two quartz plates whose polarities are opposite to each other due to temperature changes in birefringence is required. Therefore, if a 172-wavelength plate were manufactured to be usable for practical use, it would be extremely expensive.

A first object of the present invention is to provide an orthogonal polarization type optical frequency shifter without using a 172-wave plate having the above-mentioned problems. The second object of the present invention is to
In conventional products, the incident laser beam was limited to S-polarized light, but
An object of the present invention is to provide an orthogonal polarization type optical frequency shifter that can use incident laser light having P-polarized light and S-polarized light components. A third object of the present invention is to provide an orthogonal polarization type optical frequency shifter that has fewer optical system components than conventional products and that facilitates optical axis adjustment work.

[Means for solving problems]

The present invention has been made to achieve the above object, and a first aspect of the present invention is to input a laser beam having each component of P-polarized light and S-polarized light, separate it into P-polarized light and S-polarized light, and emit the separated light. an acousto-optic element that receives one of the P-polarized and S-polarized beams emitted from the first polarized beam splitter and emits a diffracted light beam; A second polarization beam splitter or a non-polarization beam splitter that receives a diffracted light beam and outputs a combined light beam of P-polarized light and S-polarized light; The invention includes a first polarizing beam splitter that inputs a laser beam having each component of P-polarized light and S-polarized light, separates it into P-polarized light and S-polarized light, and outputs it, and P-polarized light that is emitted from the first polarized beam splitter. A first acousto-optic element and a second acoustic optical element which receive the beam and the S-polarized beam and respectively output the diffracted light beam; and a composite light of the P-polarized light and the S-polarized light which receives the two diffracted light beams. The present invention is an orthogonal polarization type optical frequency shifter characterized by comprising a second polarization beam splitter or a non-polarization beam splitter that emits a beam.

〔Example〕

FIG. 1 is a configuration diagram showing an embodiment of an orthogonal polarization type optical frequency shifter according to the present invention, and in the same figure, 19 is a He-M
e-gas laser (wavelength: 632.8 layers m, optical frequency f
, = 474. ITHz), 20 and 25 are a first polarizing beam splitter and a second polarizing beam splitter, respectively, and 21 and 27 are a first acousto-optic element (center frequency: 8-layer MHz) and a second acousto-optic element (center frequency: 81 MHz), respectively. , 22 and 28 are transducers installed in each acousto-optic medium of the first acousto-optic element 21 and the second acousto-optic element 27, respectively, and 23 and 29 supply high frequency signals (80H1lz and 81HH7) to the transducers 22 and 28, respectively. A drive circuit, and 24 and 26 are mirrors.

Next, the operation of this example will be explained in detail. The laser beam 30 emitted from the He-Ne gas laser 19 is a linearly polarized beam having an orientation of 45 degrees with respect to the optical axis on the paper, and this is incident on the first polarizing beam splitter 20 having a polarization separation function. A P-polarized laser beam 31 is transmitted, and an S-polarized laser beam 32 is reflected and separated into two light beams. The first polarizing beam splitter 20 has a cubic overall shape, and is made by joining the bottom surfaces (squares) of two triangular prism-shaped glass substrates to each other via a dielectric multilayer film. Zinc sulfide (refractive index;
A plurality of layers (H layer) of a high refractive index material such as 2.29') and It (1 layer) of a low refractive index material such as cryolite (refractive index; 1.25') are alternately formed (in this example; 23 layers). Next, the P-polarized laser beam 31 enters the wavefront of the ultrasonic signal of the first acousto-optic element 21 while making a Black angle θB. This acousto-optic element 21 is mounted on the side surface of an acousto-optic medium made of tellurite glass (manufactured by HOY^ ■: AOT-5).
(Limb 0336° Electrodes are formed on the amphibious surface of a piezoelectric plate made of a Y plate. Resonance frequency: 80HH2
), and a high frequency signal (
80 MHz) is supplied to the transducer 22 to excite it, and the ultrasonic signal is propagated within the medium of the acousto-optic element 21. As a result, the incident laser beam 31 is
An angle θ with respect to the wavefront of the 0th-order light traveling straight and the ultrasonic signal
The beam is divided into diffracted light 33 that is diffracted at B, and sent out, and the diffracted light 33 is used here.

This diffracted light 33 has a center frequency of 80M of the acousto-optic element 21.
Shifting the frequency of the optical signal by H2, the optical frequency is f,
+80MHz. Next, this diffracted light 33 is reflected by a mirror 24, enters a second polarizing beam splitter (the structure is the same as the first polarizing beam splitter 20) having a coupling function, and is transmitted as it is since it is P-polarized light. do.

On the other hand, the S-polarized laser beam 32 is reflected by the mirror 26 and enters the wavefront of the ultrasonic signal of the second acousto-optic element 27 at a Black angle θB. This second acousto-optic element 27 has basically the same structure as the first acoustic optical element 21, and is equipped with a transducer 28 on the side of the medium.
A high frequency signal (81 MHz) is supplied to this from a drive circuit 29. Here again, the incident laser beam 3
2 separates and sends out zero-order light that travels straight and diffracted light 34 that is diffracted at an angle θB with respect to the wavefront of the ultrasound signal, and utilizes the diffracted light 34.

This diffracted light 34 also has a center frequency 818 of the acousto-optic element 21.
Shift the optical signal by H2, the optical frequency is f, +81H1
It becomes lz. This diffracted light 34 also enters the second polarization beam splitter 25 described above, and since it is S-polarized light, it is reflected and sent out. At this time, the components of the light beam 35 emitted from the second polarizing beam splitter 25 include the P-polarized light (light frequency: f1+80MHz) of the diffracted light 33 and the S-polarized light (light frequency: f1+81HH2) of the diffracted light 34 on the same optical path. and the mutual frequency difference is I MHz2
You will get a book light beam.

In addition to the embodiments described above, the present invention provides a method in which each center frequency of the first and second acousto-optic systems is arbitrarily selected relative to the frequency of a desired intermediate wave signal (e.g. 0.IMHz). ;60HH7 and 60.1)4Hz). Since the frequency of the intermediate wave signal and the center frequency difference between the first and second acousto-optic elements are equal, the center frequency of the acousto-optic element may be selected depending on the individual purpose. Although two acousto-optic elements 21 and 21 are used in the embodiment, only one of them may be used and the center frequency of the acousto-optic element may be set to the frequency of the intermediate wave signal. The first and second polarizing beam splitters may be configured with a Wollaston prism, a Rochon prism, etc., and the second polarizing beam splitter may maintain a substantially constant polarization state if optical loss is acceptable. A non-polarizing beam splitter that emits light while maintaining the same polarization may also be used. This non-polarizing beam splitter has a cubic overall shape, with a multilayer film described later being deposited on the bottom surface (square) of one triangular prism-shaped glass substrate, and the bottom surface (square) of the other triangular prism-shaped glass substrate. This multilayer film has equal energy transmittance for P-polarized light and S-polarized light, and equal energy reflectance for P-polarized light and S-polarized light, regardless of the polarization state of the incident light. Example: Energy transmittance: 41%, Energy reflectance: 4
6%), specifically zinc sulfide (refractive index: 2.
29) a high refractive index material layer (H layer), a low refractive index material layer (1 layer) such as cryolite (refractive index; 1.25), the H layer, the silver thin film layer, the Hw! It is formed by sequentially stacking the above layer J1 and the above layer H. As for the laser light, in addition to a gas laser, a semiconductor laser, a dye laser, a solid-state laser, etc. having each component of P-polarized light and S-polarized light may be used.

Further, the drive circuit may be shared as shown in FIG.

〔Effect of the invention〕

As described above, according to the present invention, the 172-wavelength plate, which was an essential optical component of conventional products, is no longer necessary, so temperature dependence is eliminated, and a relatively inexpensive polarizing beam splitter and mirror are used. Thus, an orthogonal polarization type optical frequency shifter can be realized. Furthermore, according to the configuration of the present invention, the number of optical system components is reduced by one compared to conventional products, which not only facilitates optical axis adjustment but also reduces the influence of disturbances such as vibration. be able to. Furthermore, since the components of the incident laser beam need only have at least each of P-polarized light and S-polarized light (the light intensity of each component does not matter), the field of use can be expanded.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an orthogonal polarization type optical frequency shifter according to the present invention, and FIG. 2 is a configuration diagram of a conventional orthogonal polarization type optical frequency shifter. 19... Laser light, 20... First polarizing beam splitter, 21... First acousto-optic element, 21... Second acousto-optic element, 25... Second polarizing beam splitter

Claims (2)

[Claims] (1) A first polarizing beam splitter that inputs a laser beam having each component of P-polarized light and S-polarized light, separates it into P-polarized light and S-polarized light, and outputs it; an acousto-optic element that receives one of the S-polarized beams and emits a diffracted light beam; and an acousto-optic element that receives the other beam and the diffracted light beam and emits a combined light beam of P-polarized light and S-polarized light. An orthogonal polarization type optical frequency shifter comprising a two-polarization beam splitter or a non-polarization beam splitter. (2) A first polarizing beam splitter that inputs a laser beam having each component of P-polarized light and S-polarized light, separates it into P-polarized light and S-polarized light, and emits it, and a P-polarized beam that is emitted from the first polarized beam splitter. A first acousto-optic element and a second acousto-optic element each receive a diffracted light beam and emit a diffracted light beam upon receiving the two diffracted light beams, respectively, and a combined light beam of P-polarized light and S-polarized light is produced by receiving the two diffracted light beams. An orthogonal polarization type optical frequency shifter comprising a second polarization beam splitter or a non-polarization beam splitter that emits light.