JPH02287521A - Orthogonal polarization type light frequency shifter - Google Patents

Abstract

PURPOSE:To obtain the orthogonal polarization type light frequency shifter which facilitates optical axis adjustment, etc., and obtains an extremely low beat signal by providing two light frequency shifter parts which differ in performance in an acoustooptic medium and providing them with mutually orthogonal planes of polarization, and obtaining two diffracted light beams which are slightly different in frequency. CONSTITUTION:When laser beam 7 is made incident on an ultrasonic wave front 8 at an angle thetaB, the 1st light frequency shifter 21 obtains the 1st diffracted light 9 and transmitted light 10 which propagates rectilinearly. This 1st diffracted light 9 is a P wave which has a frequency f0+fs shifting from the frequency f0 of the laser beam 7. Part of the transmitted light 10 of the 1st light frequency shift part 21 becomes the 2nd diffracted light 12 (S wave) and transmitted light 10 at the 2nd light frequency shift part 22. The frequency of the 2nd diffracted light 12 is f0+fl and the frequency of the beat signal obtained from the 1st and 2nd diffracted light beams 9 and 12 is the difference between f0+fl and f0+fs, i.e. the difference between fl and fs. Then fl and fs are both smaller than f0, so the difference between them is smaller. The beat signal of low frequency can, therefore, be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an orthogonal polarization type optical frequency shifter using an acousto-optic element.

[Prior Art] An orthogonal polarization type optical frequency shifter converts a laser beam having a certain optical frequency into two polarization components having an optical frequency slightly shifted from the optical frequency of the laser beam and which are orthogonal to each other. This is a device that converts laser light into a laser beam with The laser light obtained by this orthogonal polarization type optical frequency shifter is generally used to obtain a beat signal when implementing the optical heterodyne method.

FIG. 2 is a diagram showing a conventional example of an orthogonal polarization type optical frequency shifter.

In FIG. 2, reference numeral 101 indicates a laser oscillation device, and reference numeral 1
02 is an acousto-optic medium, and 103 is this acousto-optic medium 1
104 is a drive circuit for driving this transducer, 105 is a reflecting mirror, and 106 is a polarizing beam splitter.

The laser oscillation device 101 emits a laser beam 107 (center optical frequency f.) which is linearly polarized light (P wave) having a plane of polarization parallel to the plane of the paper. This laser beam 107 is incident on the acousto-optic medium 103 at a predetermined angle of incidence. A part of the incident laser beam 107 is subjected to the diffraction effect of the ultrasonic wavefront 108 formed in the acousto-optic medium 103 and becomes a diffracted beam 109 at a predetermined angle with respect to the traveling direction of the laser beam 107. The other part continues straight and becomes transmitted light 110.

In this case, the center optical frequency of the diffracted light 109 is shifted by f due to the action of the ultrasonic surface 108 (
The center optical frequency; fo+fl) and the polarization plane are 90'
The light is changed and becomes light (S wave) having a plane of polarization perpendicular to the plane of the paper. On the other hand, the transmitted light 110 maintains the same optical frequency and the same polarization plane as the laser light 107 (P wave).

The diffracted light 109 is reflected by the reflecting mirror 105 and its traveling direction is changed to a direction perpendicular to the transmitted light 110, while the transmitted light 110 travels straight and both enter the polarizing beam splitter 106. Within the polarizing beam splitter 106, the diffracted light 109, which is an S wave, is reflected in the right angle direction within the polarizing beam splitter 106, while the transmitted light 110 is transmitted as is. As a result, these diffracted light 109 and transmitted light 110 are combined into one light. As a result, a laser beam having an optical frequency slightly shifted from the optical frequency of the laser beam 107 and two leakage components orthogonal to each other is obtained.

[Problem M to be Solved by the Invention] However, the above-described conventional orthogonal polarization type optical frequency shifter has the following problems.

■ Laser light is first separated into diffracted light and transmitted light, and these are combined using a reflecting mirror and a polarizing beam splitter, which makes optical axis adjustment complicated and susceptible to external disturbances. It is difficult to downsize the device.

■ When the light obtained with this device is used in a device that implements the optical heterodyne method, the beat signal obtained is the same as the center frequency of the ultrasonic wave for forming an ultrasonic wavefront on the acousto-optic medium 102. . Therefore, the electromagnetic waves leaking as noise from the connection cable 104a and the like that connect the drive circuit 104 and the transducer 103 for obtaining this ultrasonic wave contain the same frequency component as the beat signal, and this This causes noise to be mixed into the signal and reduce the S/N ratio.

■ When implementing the optical heterodyne method, the lower the center frequency of the beat signal used, the easier the signal processing becomes. However, in the conventional orthogonal polarization type optical frequency shifter described above, There is a certain limit to how low the shift frequency can be determined for the following reasons.

That is, in order to shift the frequency and change the plane of polarization, we utilize the fact that Bragg diffraction occurs on the ultrasonic wavefront formed in the acousto-optic medium, but in order for this Bragg diffraction to occur, The following conditions must be met.

L≧(2nV)/(λof1>・(1), where L: Distance n where the interaction between ultrasound and light takes place; Refractive index of acousto-optic medium ■: Speed of sound λ0; Wavelength fl of incident laser light; Shift frequency.

As is clear from the above equation (1), the shift frequency f1
If an attempt is made to lower fl, L increases, and it is extremely difficult to reduce fl to below 40 MHz when using a normal solid material for an acousto-optic medium.

The present invention was made against the above-mentioned background, and has a simple structure and easy optical axis adjustment, etc. The shift frequency and the beat signal frequency are different, and the beat signal is extremely low. The purpose of the present invention is to obtain an orthogonal polarization type optical frequency shifter.

[Means for Solving the Problems] The present invention solves the above problems by having the following configuration.

A first ultrasonic wavefront is formed in an acousto-optic medium, and a linearly polarized laser beam is incident on the first ultrasonic wavefront from a direction that satisfies Bragg's diffraction condition to produce transmitted light that travels straight; A first optical frequency that is diffracted light that travels in a direction different from the traveling direction of the transmitted light, that has a polarization plane orthogonal to the polarization plane of the incident laser beam, and that is significantly shifted from the optical frequency of the incident laser beam. a first optical frequency shifter section that obtains a first diffracted light having
The transmitted light from the optical frequency shifter unit is incident on the second ultrasonic wavefront from a direction that satisfies Bragg's diffraction condition, and the diffracted light travels in the same direction as the traveling direction of the first diffracted light. obtaining a second diffracted light having a polarization plane parallel to the polarization plane of the transmitted light (incident laser light) and a second optical frequency significantly shifted from the optical frequency of the transmitted light; 2 optical frequency shifter section.

[Operation] According to the above configuration, the first diffracted light obtained from the first optical frequency shifter section and the second diffracted light obtained from the second optical frequency shifter section are orthogonal to each other. It is light that has a plane of polarization and a slightly different frequency. However, by appropriately arranging the first and second optical frequency shifters, it is extremely easy to bring these diffracted lights close together so that they practically appear to be on the same optical axis.

Therefore, these diffracted lights can be used in optical heterodyne methods and the like.

In this case, since there is no need to use a reflecting mirror, a polarizing beam splitter, etc. as in the conventional case, a simple configuration can be achieved, and optical axis adjustment etc. can be facilitated.

In addition, when using the light obtained with this device in the optical heterogain method, the beat signal obtained will be different from the center frequency of the ultrasonic wave used to form the ultrasonic wavefront in the acousto-optic medium. Electromagnetic waves leaked when creating sound waves do not mix into the beat signal as noise and cause a decrease in the S/N ratio.

Moreover, the frequency of the incident laser beam is f. The first frequency, which is the frequency of the first diffracted light, is set to (f.

+f, ), and the second frequency, which is the frequency of the second diffracted light, is (fo+f, l), then the frequencies of the beat signal obtained by these are (fo+f!J) and (fQ+f,
), i.e., f, and f.

This is the frequency of the difference between In this case, f, l! and fS originally meant f. The difference between them is even smaller. Therefore, it is possible to obtain a beat signal with an extremely low frequency compared to the conventional method, and beat signal processing can be made significantly easier.

[Embodiment] FIG. 1 is a diagram showing the configuration of an embodiment of the present invention. Hereinafter, one embodiment of the present invention will be described in detail with reference to FIG.

In FIG. 1, numeral 1 is a laser oscillation device, numeral 2 is an acousto-optic medium, numerals 31 and 32 are first and second transducers, respectively, and numerals 41 and 42 are first and second drive circuits, respectively.

The laser oscillation device I is a He-Ne gas laser device, and has a wavelength of 633 nm (optical frequency; fO = 474, 1
'1' Hz) and has a polarization plane perpendicular to the paper plane? (S wave).

The acousto-optic medium 2 is made of fused silica and is formed into a substantially rectangular parallelepiped shape, and one surface 2a thereof has the first and second
transducers 31 and 32 are mounted, coplanar 2
The surface 2b facing a is an inclined surface in order to prevent ultrasonic waves from being reflected back and forth and causing harmful effects.

The first transducer 31 is an X-cut plate (cut so that the cut plane is perpendicular to the crystal axis [100]) of L i N b O 3 which is a piezoelectric crystal, and has front and back cut planes (front and back Although not shown, electrodes are formed on the surface of the substrate. Then, transverse ultrasonic waves are generated by high-frequency drive signals supplied from the first drive circuit 41 to these electrodes, and an ultrasonic transverse wave front is formed in the acousto-optic medium 2 . In this case, the frequency of the ultrasonic transverse wave is f s =40 MHz. Note that this first
The size of the transducer 41 is less than half the area of the surface 2a of the acousto-optic medium 2, and
a virtual plane 2 that intersects b and roughly divides the acousto-optic medium 2 into two;
It is attached to the left side in the figure with f as the border.

Therefore, by the first transducer 41,
An ultrasonic wavefront of a transverse wave is formed in a region of the acousto-optic medium 2 to the left in the figure with the virtual surface 2f as a boundary, and this region constitutes the first optical frequency shift section 21.

That is, when the laser beam 7 is incident on the ultrasonic wavefront 8 from a direction forming an angle θ, a part of the laser beam is diffracted in a direction forming an angle 2θ8 with respect to the traveling direction of the laser beam 7. At the same time, the other part goes straight and becomes transmitted light 10. In this case, the diffracted light has a frequency f shifted by f from the center frequency fq of the laser beam 7. +f (first frequency), and has a polarization plane making an angle of 90° with respect to the polarization plane of the laser beam 7 (P wave).

The second transducer 32 is a 36° Y cut plate of L i N b O3 which is a piezoelectric crystal (the cut plane is cut at 36° with respect to a plane perpendicular to the crystal axis [010]). , electrodes are formed similarly to the first transducer 31, and the acousto-optic medium 2
Forms an ultrasonic wavefront within.

However, this second transducer 32 is attached to a part on the right side in the figure with respect to the virtual plane 2f, and generates longitudinal waves in a region on the acousto-optic medium 2 on the right side in the figure with the virtual plane 2f as a boundary. It forms an ultrasonic wavefront, and this region is used as the second optical frequency shift section 21. Note that as these first and second transducers 31 and 32, in addition to the above-mentioned examples, the following can be applied. In other words, the X-cut plate of S i 02 (for longitudinal wave generation), the Y-cut plate (for transverse wave generation) of S i 02, and the 47°Y of LiTaO2.
Cut plate (for longitudinal wave generation), also X-cut plate (for transverse wave generation), ZnO Z-cut plate (for longitudinal wave generation), same <39
°Y cut plate (for transverse wave generation), etc.

In this second optical frequency shift section 22, the first
Transmitted light 1 transmitted from optical frequency shift section 1 to section 21 of
0 is diffracted into second diffracted light 12 (S wave) having the same polarization plane as the transmitted light 10, and the other part is transmitted as is.

In this case, the traveling direction of the second diffracted light 12 is the same as that of the first diffracted light 12.
The ultrasonic frequency f! of the transducer 32 is set so that it is the same as the traveling direction of the diffracted light 9. 2 is selected. fβ to satisfy this condition is determined as follows.

Now, if the Bragg angle of the first optical frequency shift section 21 is θ, then θ,=(λf3)/(2V,”) (2).

however, λ; wavelength of laser beam 7 ■、；Sound velocity in the acousto-optic medium 2 of transverse ultrasonic waves shall be.

On the other hand, if the Bragg angle of the second optical frequency shift section 22 is θ, ゛, then θ8-=(λf3)/(2VJ!>...(3)
It is.

However, λ: wavelength V of transmitted light 10 (=laser light 7),! is the sound velocity in the acousto-optic medium 2 of longitudinal ultrasound.

The traveling direction of the second diffracted light 12 is the same as that of the first diffracted light 9.
In order for the traveling direction to be the same as that of , it is sufficient that θ8=θ8'.

Therefore, from the above equations (2) and (3), f 1
” (Vl/V)・f, (4).

here, f　s　240MHz　Hz Vs = 3750 m / SeC V B = 5950m / SeC Therefore, from the above equation (4), f J = 63.5MHz becomes.

That is, the frequency f of the first transducer 31
S = 40 MH2 (=shift frequency of the first optical frequency shift section 21), and the frequency f of the second transducer 32 is set to -63,51 VIHz (-shift frequency of the second optical frequency shift section 22). As a result, they proceed in the same direction, and (fo+f, ) and (
It is possible to obtain the first diffracted light and the second diffracted light 12 having a frequency of (fo+fl) and polarization planes orthogonal to each other.

In this case, the diffraction angle θB of these diffracted lights is θB=θB
” = 3.38 m r a d, so if the distance between the centers of the first optical frequency shift section 21 and the second optical frequency shift section 22 is P, the distance d between the beams of these diffracted lights is , d=0.14mm, and these diffracted lights can be seen as being on the same optical axis in practical terms.Furthermore, these two diffracted lights each have a frequency of (f
+f > and (fo+s f, is light with l orthogonal polarization planes. Therefore, if this is used, for example, in the optical heterodyne method, the beat signal is (fo+f,) - (fo + f
s ) = f Of s = 23-5 MHz, which can be an extremely low frequency.

Note that a part of the first diffracted light 9 generated in the first optical frequency shift section 21 is diffracted again in the second optical frequency shift section 22 to generate a third diffracted light 13. This third diffracted light 13 has a frequency (fo+fs-f,) and a polarization plane orthogonal to the laser light 7. If the diffraction efficiency for producing the third diffracted light 13 is large, most of the first diffracted light 9 becomes the third diffracted light 13, and the diffracted light with the necessary intensity cannot be obtained. It is also conceivable that it will not be possible to get it out.

However, in diffraction by longitudinal ultrasound, the figure of merit M (tensor amount) generally has anisotropy with respect to the polarization direction, and in the fused silica used in this example, the The figure of merit M for light with a plane of polarization parallel to the direction of propagation is M = 0.32 The figure of merit M of the light is M=1.56X 10'''15
SeC3/K g. That is, the figure of merit of light with parallel polarization planes is about 175 that of light with perpendicular polarization planes. Here, it is known that this figure of merit M is approximately proportional to the diffraction efficiency when the diffraction efficiency is not very large. In the case of this example, it can be said that the diffraction efficiency is not very high, so the diffraction efficiency can also be said to be 115. That is, since the diffraction efficiency of the third diffracted light 13 is low, the amount by which the intensity of the first diffracted light is reduced is small. On the other hand, since the diffraction efficiency for producing the second diffracted light 12 is high, the second diffracted light 12 can also be efficiently extracted. As a result, in this example, theoretically, 8
Optical power utilization efficiency of 0% or more can be obtained. Glass materials having the same anisotropy as this fused silica include the product names 5F-8 and 5F-14 sold by Schott. There is one in which ultrasonic waves are propagated in the [0011 axial direction of .

The embodiment described in detail above has the following advantages.

■Unlike the conventional example mentioned above, there is no need to use a reflector, polarizing beam splitter, etc., so the YI structure is simple and easy to manufacture, and the optical axis adjustment is easy, and there is no vibration during use. Less susceptible to other disturbances.

(2) When the light obtained with this device is used in a device that implements the optical heterodyne method, the beat signal obtained is different from the center frequency of the ultrasonic wave for forming an ultrasonic wavefront on the acousto-optic medium 2. Therefore, electromagnetic waves leaking from the drive circuits 41 and 42 for obtaining the ultrasonic waves do not mix into the beat signal as noise and cause a decrease in the S/N ratio.

(2) It is possible to significantly lower the center frequency of the beat signal used when implementing the optical heterodyne method, and beat signal processing can be significantly facilitated.

In the above-mentioned embodiment, the laser beam is first diffracted by the ultrasonic wavefront of the transverse wave, and then by the ultrasonic wavefront of the longitudinal wave. good. However, in that case, it is not possible to utilize the anisotropy of diffraction efficiency as in the above-mentioned embodiment, so it is undeniable that the efficiency of use of optical power decreases in proportion to that in the above-mentioned embodiment. .

In addition, in the above-mentioned embodiment, two optical frequency shifters are formed in one acousto-optic medium, but these two
The two optical frequency shifters may be formed in separate acousto-optic media. In this case, the materials of the respective acousto-optic media may be different, or the two acousto-optic media may be joined by optical adhesion.

[Effects of the Invention] As detailed above, the present invention provides a first
to form an ultrasonic wavefront and direct the linearly polarized laser beam to the first
Transmitted light that enters from a direction that satisfies Bragg's diffraction condition with respect to the ultrasonic wavefront of a first optical frequency shifter section that obtains a first diffracted light having a polarization plane orthogonal to the optical frequency and a first optical frequency that is sharply shifted from the optical frequency of the incident laser beam; 2 ultrasonic wavefronts are formed, and the first
The transmitted light from the optical frequency shifter unit is incident on the second ultrasonic wavefront from a direction that satisfies Bragg's diffraction condition, and the diffracted light travels in the same direction as the traveling direction of the first diffracted light. and obtaining a second diffracted light having a polarization plane parallel to the polarization plane of the transmitted light (incident laser light) and a second optical frequency that is significantly shifted from the optical frequency of the transmitted light. By having a configuration including the optical frequency shifter section 2, the structure is simple and optical axis adjustment etc. are easy.
Moreover, an orthogonal polarization type optical frequency shifter is obtained in which the shift frequency and the frequency of the beat signal are different, and it is possible to obtain an extremely low beat signal.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is a diagram showing a conventional example. 1... Laser oscillation device, 2... Acousto-optic medium, 7.
... Laser light, 8... Ultrasonic wavefront, 9... First diffracted light, 10... Transmitted light, 12... Second diffracted light, 3
1.32...first and second transducer, 41
．． 42...First and second drive circuits.

Claims (1)

[Claims] A first ultrasonic wavefront is formed in an acousto-optic medium, and linearly polarized laser light is incident on the first ultrasonic wavefront to produce transmitted light that travels straight and the progress of this transmitted light. a first diffraction beam that is diffracted light that travels in a direction different from that of the incident laser beam, and has a polarization plane perpendicular to the polarization plane of the incident laser beam and a first optical frequency that is shifted from the optical frequency of the incident laser beam; a first optical frequency shifter section for obtaining light;
The transmitted light from the optical frequency shifter section is incident on the second ultrasonic wavefront, and is diffracted light that travels in a direction different from the traveling direction of the transmitted light, which is a polarized light of the transmitted light (incident laser light). an orthogonal polarization type optical frequency shifter comprising: a second optical frequency shifter section that obtains a second diffracted light having a polarization plane parallel to the wave front and a second optical frequency shifted from the optical frequency of the transmitted light; .