JP2784038B2 - Orthogonal polarization type optical frequency shifter - Google Patents

Description

The present invention relates to an orthogonal polarization type optical frequency shifter using an acousto-optic device.

[Background 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 orthogonal to each other. This is a device for converting into laser light having Laser light obtained by the orthogonal polarization type optical frequency shifter is generally
It is used to obtain a beat signal when performing the optical heterodyne method.

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

2, reference numeral 101 denotes a laser oscillation device, and reference numeral 102 denotes a laser oscillation device.
Is an acousto-optic medium, 103 is a transducer for forming an ultrasonic wavefront in the acousto-optic medium 102, 104 is a drive circuit for driving the transducer, 105 is a reflecting mirror, and 106 is a polarization beam splitter.

The laser oscillation device 101 emits a laser beam 107 (center optical frequency f ₀ ) which is linearly polarized light (P wave) having a plane of polarization parallel to the paper surface. This laser beam 107 is incident on the acousto-optic medium 103 at a predetermined incident angle. A part of the incident laser light 107 is diffracted by the ultrasonic wavefront 108 formed in the acousto-optic medium 103, and becomes a diffracted light 109 at a predetermined angle with the traveling direction of the laser light 107. The light travels in the direction of travel, and the other part goes straight as it is to become the transmitted light 110.

In this case, the center of the diffracted light 109 is shifted by f ₁ (center light frequency; f ₀ + f ₁ ) and the plane of polarization is changed by 90 ° under the action of the ultrasonic surface 108. It becomes light (S wave) having a plane of polarization perpendicular to the plane of the paper. On the other hand, the transmitted light 110 has the same optical frequency and the same polarization plane as the laser light 107 (P wave).

The diffracted light 109 is reflected by a reflecting mirror 105 and its traveling direction is changed to a direction orthogonal to the transmitted light 110, while the transmitted light 110 goes straight and enters the polarization beam splitter 106 together. The polarizing beam splitter 1
In FIG. 6, the diffracted light 109, which is an S-wave, is reflected at right angles in the polarization beam splitter 106, while the transmitted light 1
10 is transmitted as it is. As a result, the diffracted light 109 and the transmitted light 110 are combined into one light. As a result, a laser beam having an optical frequency slightly deviated from the optical frequency of the laser beam 107 and having two polarization components orthogonal to each other is obtained.

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

Laser light is once separated into diffracted light and transmitted light, and these are combined using a reflecting mirror and a polarizing beam splitter. It is difficult to miniaturize.

When the light obtained by this device is used in a device that performs the optical heterodyne method, the obtained beat signal is the same as the center frequency of the ultrasonic wave for forming the ultrasonic wavefront on the acousto-optic medium 102. For this reason, the same frequency component as the beat signal is included in the electromagnetic wave leaking as noise from the connection cable 104a or the like connecting the drive circuit 104 and the transducer 103 for obtaining the ultrasonic wave,
This causes the S / N ratio to be reduced by being mixed into the beat signal as noise.

When the optical heterodyne method is performed, the lower the center frequency of the beat signal used, the easier the signal processing becomes. However, in the above-described conventional orthogonal polarization type optical frequency shifter, a shift that determines the frequency of the beat signal is used. There has been a certain limit to lowering the frequency for the following reasons.

That is, to change the frequency shift and polarization plane,
Utilizing the fact that Bragg diffraction is performed on the ultrasonic wave front formed in the acousto-optic medium, the following conditions must be satisfied in order for this Bragg diffraction to occur.

L ≧ (2nV) / (λ ₀ f ₁ ) (1) where L: distance at which the interaction between the ultrasonic wave and the light is performed n; refractive index of the acousto-optic medium V; sound velocity λ ₀ ; Wavelength f ₁ ; frequency to be shifted.

As is apparent from the above equation (1), the shift frequency f ₁
When the attempts to lower, L increases, in the case of using a solid material for conventional acoustooptic medium, it has been extremely difficult to make f ₁ lower than 40 MHz.

The present invention has been made under the above-mentioned background,
The objective is to obtain an orthogonal polarization type optical frequency shifter that has a simple structure, allows easy optical axis adjustment, etc., and has a different shift frequency and beat signal frequency, and also enables an extremely low beat signal to be obtained. It is what it was.

[Means for Solving the Problems] The present invention has solved the above-mentioned problems by adopting the following configuration.

A first ultrasonic wave front is formed in an acousto-optic medium, and linearly polarized laser light is incident on the first ultrasonic wave front, and travels in a direction that is different from the traveling light that travels straight and the traveling direction of the transmitted light. A first diffracted light having a first optical frequency shifted from the optical frequency of the incident laser light while having a plane of polarization or a plane of parallel polarization orthogonal to the plane of polarization of the incident laser light, Forming a second ultrasonic wavefront in the acousto-optic medium, and transmitting at least light transmitted from the first optical frequency shifter to the second ultrasonic wavefront. A second diffracted light traveling in a direction different from the traveling direction of the transmitted light and having a second optical frequency shifted from the optical frequency of the transmitted light, the polarization plane of which is the first frequency shifter The plane of polarization of the first diffracted light obtained from When the plane of polarization of the laser light is parallel to the plane of polarization of the transmitted light, the plane of polarization is orthogonal to the plane of polarization of the first diffracted light obtained from the first frequency shifter. And a second optical frequency shifter for obtaining a second diffracted light which is parallel to the polarization plane of the transmitted light when the polarization plane of the incident laser light is orthogonal to the first plane. The second diffracted light and the second diffracted light are extracted as one light beam to obtain light including two types of laser light having a slight frequency difference from each other and having orthogonal polarization planes. .

[Operation] According to the above configuration, the first diffracted light obtained from the first optical frequency shifter and the second diffracted light obtained from the second optical frequency shifter are orthogonal to each other. This is light having a polarization plane and slightly different frequencies. In addition, by appropriately arranging the first and second optical frequency shifters, it is very easy to bring these diffracted lights close to each other so as to be regarded as substantially the same optical axis in practical use.

Therefore, these diffracted lights can be used for an optical heterodyne method or the like.

In this case, since it is not necessary to use a reflecting mirror, a polarizing beam splitter, or the like as in the related art, a simple configuration can be obtained, and the optical axis adjustment and the like are facilitated.

When the light obtained by this apparatus is used for the optical heterodyne method, the obtained beat signal differs from the center frequency of the ultrasonic wave for forming the ultrasonic wave front in the acousto-optic medium. Electromagnetic waves that leak when producing sound waves do not mix with the beat signal as noise and cause a reduction in the S / N ratio.

Moreover, the frequency of the incident laser beam and f _0, the first frequency is the frequency of the first diffraction light (f ₀ + f _s), the second frequency is the frequency of the second diffraction light (f ₀ + F _l ), the frequency of the beat signal obtained by them is
The difference between (f ₀ + f _l ) and (f ₀ + f _s ), that is, the frequency of the difference between _fl and f _s . In this case, the difference between them from significantly small compared to f _l and f ₀ originally and f _s is smaller.
Therefore, it is possible to obtain a beat signal having an extremely low frequency as compared with the related art, and it is possible to significantly facilitate beat signal processing.

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

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

The laser oscillation device 1 is a He-Ne gas laser device and oscillates a laser beam 7 (S wave) having a wavelength of 633 nm (optical frequency; f0 = 474.1 THz) and having a plane of polarization perpendicular to the plane of the drawing.

The acousto-optic medium 2 is formed in a substantially rectangular parallelepiped shape from fused quartz, and has a first surface 2a on one surface 2a.
The transducers 31 and 32 are mounted, and the face 2b opposed to the face 2a is inclined to prevent the ultrasonic waves from being reflected back and forth and causing harmful effects.

The first transducer 31 is a piezoelectric crystal
This is an X cut plate of LiNbO ₃ (the cut surface is cut so as to be orthogonal to the crystal axis [100]), and electrodes (not shown) are formed on the front and back cut surfaces (front and back surfaces).
Then, a high-frequency drive signal supplied from the first drive circuit 41 to these electrodes generates a transverse ultrasonic wave to form a transverse ultrasonic wave front in the acousto-optic medium 2.
In this case, the ultrasonic frequency f _s = 40 MHz of the shear wave. The size of the first transducer 41 is
The area is less than half the area of the surface 2a of the acousto-optic medium 2. The acousto-optic medium 2 is attached to the left of the drawing with a virtual surface 2f intersecting the surfaces 2a and 2b and substantially dividing the acousto-optic medium 2 into two.

Therefore, by the first transducer 41,
In the acousto-optic medium 2, an ultrasonic wave front of a transverse wave is formed in a region on the left side of the drawing with respect to the virtual surface 2f,
This region constitutes the first optical frequency shift unit 21. That is, the laser beam 7 is angled θ _{B with} respect to the ultrasonic wavefront 8.
When made incident from the direction Nasu, together with a portion of the laser light becomes the first diffracted light 9 is the angle 2 [Theta] _B forms direction diffraction with respect to the traveling direction of the laser beam 7, another part go straight As a result, transmitted light 10 is obtained. In this case, the diffracted light 9
Has a frequency (first frequency) of the frequency f ₀ + f _s shifted by f _s from the center frequency f ₀ of the laser beam 7, and the angle 90゜Nasu relative polarization of the laser beam 7 This is light (P wave) having a plane of polarization.

The second transducer 32 is a piezoelectric crystal
36 ° Y-cut plate of LiNbO ₃ (cut so that the cut surface makes a 36 ° angle to the plane perpendicular to the crystal axis [010])
In the same manner as the first transducer 31, electrodes are formed, and an ultrasonic wave front is formed in the acousto-optic medium 2. However, the second transducer 32 is attached to a portion on the right side in the drawing with respect to the virtual surface 2f, and a longitudinal wave is formed in a region on the right side in the drawing with respect to the virtual surface 2f in the acousto-optic medium 2. This region forms an ultrasonic wave front, and this region is used as a second optical frequency shift unit 21. In addition, as the first and second transducers 31 and 32, the following can be applied in addition to the above-described example. That is, Si
O ₂ X-cut board (for generating longitudinal waves), Y-cut board (for generating transverse waves), LiTaO ₃ 47 ° Y-cut board (for generating longitudinal waves),
Also X-cut board (for shear wave generation), ZnO Z-cut board (for longitudinal wave generation), also 39 ゜ Y-cut board (for shear wave generation)
And so on.

In the second optical frequency shift unit 22, the first
A part of the transmitted light 10 transmitted from the optical frequency shift unit 21 is diffracted into a second diffracted light 12 (S wave) having the same polarization plane as the transmitted light 10, and the other part is left as it is. To Penetrate.

In this case, the traveling direction of the second diffracted light 12 is the first
As the same as the traveling direction of the diffracted light 9, ultrasonic frequency f _l of the transducer 32 is selected. F _l for this condition is satisfied is determined as follows.

Now, when the Bragg angle of the first optical frequency shifters 21 and theta _B, a _{θ B = (λf s) /} (2V s) ... (2).

Here, λ is the wavelength of the laser beam V _s ; the sound velocity of the transverse ultrasonic wave in the acousto-optic medium 2.

On the other hand, if the Bragg angle of the second optical frequency shift unit 22 is θ _B ′, then θ _B ′ = (λf _l ) / (2V _l ) (3)

However, lambda; the acoustic velocity in the longitudinal ultrasonic waves of the acoustic-optic medium within 2; transmitted light 10 (= the laser beam 7) Wavelength V _l of.

The traveling direction of the second diffracted light 12 is the first diffracted light 9
In this case, θ _B = θ _B ′ needs to be satisfied in order to make the traveling direction the same as the traveling direction of.

Therefore, from the equation (2) and (3), f _l = a _{(V l / V s) ·} f s ... (4).

Since f _s = 40 MHz V _s = 3750 m / sec V _l = 5950 m / sec, f _l = 63.5 MHz from the above equation (4).

That is, the frequency f _s of the first transducer 31
= 40 MHz (= shift frequency of the first optical frequency shift unit 21), and the frequency f _l of the second transducer 32 =
By setting the frequency to 63.5 MHz (= shift frequency of the second optical frequency shift unit 22), the light travels in the same direction, has (f ₀ + f _s ) and (f ₀ + f _l ) frequencies, and is orthogonal to each other. The first diffracted light 9 and the second diffracted light 9
Can be obtained.

In this case, the diffraction angles θ _B of these diffracted lights are θ _B =
Since θ _B ′ = 3.38 mrad, the center distance between the first optical frequency shift unit 21 and the second optical frequency shift unit 22 is P
, The distance d between the beams of these diffracted lights is d = 0.
It is 14 mm, and it can be seen that these diffracted lights are practically on the same optical axis. Moreover, these two diffracted lights are lights having orthogonal polarization planes with frequencies (f ₀ + f _s ) and (f ₀ + f _l ), respectively. Thus, this example, by using the optical heterodyne method, the beat _{signal, (f 0 + f l)} - be a _{(f 0 + f s) =} f l -f s = 23.5MHz extremely low frequency it can.

A part of the first diffracted light 9 generated in the first optical frequency shift unit 21 is diffracted again in the second optical frequency shift unit 22 to generate a third diffracted light 13. This third
The diffracted light 13 has a frequency (f ₀ + f _s −f _l ) and a polarization plane orthogonal to the laser light 7. If the diffraction efficiency for generating the third diffracted light 13 is large, most of the first diffracted light 9 becomes the third diffracted light 13 and the diffracted light 9 having a required intensity is obtained. Can not be taken out to the outside.

However, in diffraction by longitudinal ultrasonic waves, the figure of merit M (tensor amount) generally has anisotropy with respect to the polarization azimuth. Figure of merit M of light having a plane of polarization parallel to the direction of travel
Is M = 0.32 × 10 ⁻¹⁵ sec ³ / Kg, whereas the figure of merit M of light having a plane of polarization perpendicular to the traveling direction of longitudinal ultrasonic waves is M = 1.56 × 10 ⁻¹⁵ sec. ³ / Kg. That is, the figure of merit of light having a parallel polarization plane is about 1/5 that of light having a perpendicular polarization plane.
Here, it is known that the figure of merit M is substantially proportional to the diffraction efficiency when the diffraction efficiency is not so large. In the case of this embodiment, since it can be said that the diffraction efficiency is not so large, the diffraction efficiency can be said to be 1/5. That is, since the diffraction efficiency of the third diffracted light 13 is small, the amount by which the intensity of the first diffracted light 9 is reduced is small. On the other hand, since the diffraction efficiency for generating the second diffracted light 12 is high, the second diffracted light 12 can also be extracted efficiently. As a result,
In this embodiment, theoretically, an optical power utilization efficiency of 80% or more can be obtained. As a glass material having the same anisotropy as this fused quartz, a trade name SF sold by Shot Corporation
-8 and SF-14, and as a crystalline material, there is one using a TeO ² single crystal and transmitting ultrasonic waves in the [001] axis direction of the crystal.

The embodiment described above has the following advantages.

Unlike the conventional example described above, there is no need to use a reflecting mirror, a polarizing beam splitter, etc., so that the structure is simple, the manufacturing is easy, the optical axis adjustment is simple, and vibration or other Less susceptible to disturbances.

When the light obtained by this device is used in a device that performs the optical heterodyne method, the obtained beat signal differs from the center frequency of the ultrasonic wave for forming the ultrasonic wave front on the acousto-optic medium 2. Therefore, the electromagnetic waves leaking from the driving circuits 41 and 42 for obtaining the ultrasonic waves do not mix with the beat signal as noise and cause a reduction in the S / N ratio.

The center frequency of the beat signal used in implementing the optical heterodyne method can be significantly reduced, and the beat signal processing can be significantly facilitated.

In the above-described embodiment, the laser beam is first diffracted by the ultrasonic wavefront of the transverse wave, and then diffracted by the ultrasonic wavefront of the longitudinal wave. Good. However, in this case, since the anisotropy of the diffraction efficiency cannot be used as in the above-described embodiment,
It cannot be denied that the utilization efficiency of the optical power is reduced as compared with the one embodiment.

In the above-described embodiment, one acousto-optic medium has two
Although the example in which the two optical frequency shifters are formed has been described, these two optical frequency shifters may be formed in separate acousto-optic media. In this case, the material of each acousto-optic medium may be different, or two acousto-optic media may be joined by optical bonding.

[Effects of the Invention] As described in detail above, the present invention forms a first ultrasonic wave front in an acousto-optic medium, and converts a linearly polarized laser beam into Bragg diffraction with respect to the first ultrasonic wave front. Transmitted light that enters from a direction that satisfies the condition, is transmitted light that travels straight, and diffracted light that travels in a direction different from the traveling direction of the transmitted light, and has a plane of polarization orthogonal to the plane of polarization of the incident laser light. A first optical frequency shifter for obtaining a first diffracted light having a first optical frequency slightly shifted from an optical frequency of the laser light; and a second ultrasonic wavefront formed in an acousto-optic medium, Diffraction in which light transmitted from the first optical frequency shifter is incident on the second ultrasonic wavefront from a direction satisfying the Bragg diffraction condition, and travels in the same direction as the travel direction of the first diffracted light. Light, the transmitted light (incident laser light) A second optical frequency shifter having a plane of polarization parallel to the plane of polarization and obtaining second diffracted light having a second optical frequency slightly shifted from the optical frequency of the transmitted light; Thus, an orthogonal polarization type optical frequency shifter having a simple structure, easy optical axis adjustment, etc., and having a shift frequency and a beat signal frequency different from each other and capable of obtaining an extremely low beat signal is obtained. Is what it is.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of one embodiment of the present invention, and FIG. 2 is a diagram showing a conventional example. DESCRIPTION OF SYMBOLS 1 ... Laser oscillation device, 2 ... Acousto-optic medium, 7 ... Laser light, 8 ... Ultrasonic wave front, 9 ... First diffracted light, 10 ...
... Transmitted light, 12 second diffracted light, 31, 32 first and second transducers, 41, 42 first and second drive circuits.

Claims (3)

(57) [Claims] 1. A first ultrasonic wave front is formed in an acousto-optic medium, and a linearly polarized laser beam is incident on the first ultrasonic wave front, and the transmitted light travels straight and the traveling direction of the transmitted light. A diffracted light that travels in a direction different from that of the incident laser light, has a polarization plane orthogonal to or parallel to the polarization plane of the incident laser light, and has a first optical frequency shifted from the optical frequency of the incident laser light. A first optical frequency shifter unit for obtaining a diffracted light beam; and a second ultrasonic wavefront formed in the acousto-optic medium, and transmitting at least the transmitted light from the first optical frequency shifter unit to the second optical frequency shifter unit.
Of the transmitted light, traveling in a direction different from the traveling direction of the transmitted light, and being shifted from the optical frequency of the transmitted light.
When the plane of polarization of the second diffracted light has an optical frequency of: and the plane of polarization of the first diffracted light obtained from the first frequency shifter is parallel to the plane of polarization of the incident laser light. Is perpendicular to the plane of polarization of the transmitted light, and the plane of polarization is orthogonal to the plane of polarization of the first diffracted light obtained from the first frequency shifter and the plane of polarization of the incident laser light. A second optical frequency shifter unit that obtains a second diffracted light that is parallel to the polarization plane of the transmitted light when the first diffracted light and the second diffracted light are parallel to each other. An orthogonal polarization type optical frequency shifter characterized by obtaining light including two types of laser light having a slight frequency difference from each other and having orthogonal polarization planes by extracting as one light beam. 2. In the case where diffracted light is obtained from the incident laser light or transmitted light by a first ultrasonic wave front or a second ultrasonic wave front formed on the acousto-optic medium, the plane of polarization of the diffracted light is the incident light. When obtaining diffracted light having a plane of polarization orthogonal to the plane of polarization of laser light or transmitted light, a transverse ultrasonic wave front is used as the ultrasonic wave front, and the plane of polarization of the diffracted light is the incident laser light or transmitted light. 2. The orthogonal polarization type optical frequency shifter according to claim 1, wherein when obtaining diffracted light having a polarization plane parallel to the polarization plane, a longitudinal ultrasonic wave front is used as the ultrasonic wave front. 3. The acousto-optic medium forming the longitudinal wave ultrasonic wave front is a glass material having anisotropy, and the acousto-optical medium forming the longitudinal wave ultrasonic wave front is anisotropic crystalline material. 3. The orthogonal polarization type optical frequency shifter according to claim 2, wherein the optical frequency shifter is a material.