JPH0711648B2 - Cross polarization optical frequency shifter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention utilizes the effect of diffracting incident laser light at the wavefront of a traveling ultrasonic wave in an acousto-optic device and shifting the frequency of the laser light by the Doppler effect. And a device for generating light having a constant frequency difference with respect to the optical signal to be detected. In particular, it relates to a device for converting laser light having a certain optical frequency into laser light having two linearly polarized components having different optical frequencies and orthogonal to each other, and this device is called an orthogonal polarization type optical frequency shifter. Here, one of the two linearly polarized components orthogonal to each other is used as the optical signal to be detected, and the other is used as the optical signal of the local oscillation output.

In order to detect the optical frequency difference, for example, the output light beam of the orthogonal polarization type optical frequency shifter is transmitted through a 45 ° polarizer, and the optical frequency difference is detected.

Recently, high-precision, non-contact optical application measurement using the property of light has attracted attention, and in order to increase the measurement resolution of the fringe fraction (phase information of interference fringes) in the interference of light waves, and to automatically measure this, Optical heterodyne detection method is used. This optical heterodyne detection method, similar to radio heterodyne reception, mixes a signal to be detected with a local oscillation output signal to generate an intermediate wave signal (beat signal) having a difference frequency,
This is a method of performing signal processing. In telecommunications, a completely independent oscillator is used to obtain a local oscillation output signal, but in the case of light wave interference measurement, an independent optical oscillator with a stable intermediate wave signal frequency is manufactured. Is difficult. Therefore, laser light having a certain frequency difference with respect to the optical signal to be detected is generated and sent to the receiving end through the reference optical path, and this is used as a local oscillation output signal.

[Conventional technology]

Conventionally, there is an orthogonal polarization type optical frequency shifter having a configuration as shown in FIG. 2, and a He-Ne gas laser 1 (wavelength λ
= 632.8 nm, optical frequency f ₀ = 474.1T HZ) is laser light 2 which is a linearly polarized light (P-polarized light) having a plane of polarization parallel to the paper surface, and which has a polarization separation function.
The light enters the polarization beam splitter 3 and is separated into a P-polarized laser light beam 4 as transmitted light and an S-polarized laser light beam 5 as reflected light. Of these, the P-polarized laser light beam 4 enters the medium of the acoustooptic device 6 at a Bragg angle θ _B with respect to the wavefront of a longitudinal ultrasonic wave signal propagating by the excitation of the transducer 7. In addition, Linb
A high frequency signal f ₁ = 80MHZ is supplied from the drive circuit 8 to the transducer 7 made of an O ₃ 36 ° Y plate to excite it. As a result, the incident laser light beam 4 has an angle θ _{B with} respect to the 0th-order light (not shown) that travels straight and the wavefront of the ultrasonic signal.
The light is divided into a diffracted light 9 that is diffracted by and transmitted, and the diffracted light 9 is used here. The diffracted light 9 has a frequency f ₀ of the preceding optical signal shifted by the center frequency f ₁ MHZ of the acoustooptic device 6, that is, the optical frequency is (f ₀ + f ₁ ) MHZ. next,
The diffracted light 9 is incident on the second polarization beam splitter 10 having a polarization coupling function, and is transmitted as it is because it is P-polarized light. On the other hand, the S-polarized laser light beam 5 is reflected by the reflecting mirrors 11, 1.
The second polarization beam splitter 10 which is reflected by 2 and is described above
Is incident on, and is reflected by the S-polarized light, and is transmitted.

As a result, the light beam emitted from the second polarization beam splitter 10
13 is P-polarized light of the diffracted light 9 (optical frequency: (f ₀ + f ₁ ) MHZ)
And S-polarized light of reflected light 5 (optical frequency: f ₀ MHZ) are orthogonal to each other and combined on the same optical path, and two light beams with a frequency difference of f ₁ MHZ are obtained. .

[Problems to be solved by the invention]

Since the configuration of the conventional orthogonal polarization type optical frequency shifter requires a plurality of polarization beam splitters and reflecting mirrors,
In the assembling process, the optical axis adjustment is complicated, and the characteristics of the light beam are deteriorated due to the influence of mechanical vibration such as disturbance, which further restricts the miniaturization of the device.

The present invention has been made to solve such a problem, and an object thereof is to obtain a polarization beam splitter and a reflecting mirror in order to obtain two light beams having orthogonal polarization planes and different frequencies. It is to reduce the number of required components and to downsize.

[Means for solving problems]

The present invention has been made to achieve the above object, by propagating an ultrasonic wave of an internal transverse wave, by the wavefront of this ultrasonic wave, the polarization plane and the optical frequency of the incident light that is linearly polarized light, respectively. The transmitted light having the same polarization plane and the same optical frequency, and the diffracted light that has a polarization plane that makes an angle of 90 degrees with the polarization plane of the incident light and that has the optical frequency of the incident light shifted An acousto-optical element that separates and emits, a reflecting unit that sends out the diffracted light emitted from the acousto-optical element to a beam splitter, and a transmitted light emitted from the acousto-optical element is incident to the transmitted light and the reflected light. A polarization beam splitter that emits the combined light beams is arranged on the same optical path.

[Action]

Incident light that is linearly polarized light is incident on an acousto-optic device that propagates transverse ultrasonic waves, so that the polarization planes are orthogonal to each other,
Since light beams whose optical frequencies differ by the central frequency of the acousto-optical element are obtained as diffracted light and transmitted light, the number of components, the polarization beam splitter and the reflection mirror, can be reduced.

〔Example〕

FIG. 1 is a configuration diagram showing an embodiment of an orthogonal polarization type optical frequency shifter according to the present invention. In the figure, the same components as those in FIG. 2 are designated by the same reference numerals. Reference numeral 14 is an acousto-optic device made of a single crystal of TeO ₂ . Reference numeral 15 denotes a transducer which forms electrodes on both main surfaces of a piezoelectric plate made of a LinbO ₃ X plate and propagates a transverse ultrasonic signal into the medium of the acoustooptic device 14. Reference numeral 22 is a drive circuit for supplying a high frequency signal having a center frequency f ₁ of 40 MHz to the transducer 15. Reference numeral 19 is a reflecting mirror which is a reflecting means, and 20 is a polarization beam splitter.

In the above-described configuration, the laser light 16 emitted from the He-Ne gas laser 1 (wavelength λ = 632.8 nm, optical frequency f ₀ = 474.1T HZ) and composed of linearly polarized light having an azimuth parallel to the paper surface is The Bragg angle is formed on the wavefront of the ultrasonic wave propagating to the acousto-optic element 14 and then enters the acousto-optic element 14. A high-frequency signal supplied from the drive circuit 22 causes the transducer 15 to excite the acousto-optic element 14,
Since the transverse ultrasonic wave signal is propagated, the refractive index changes depending on the propagation direction of light due to the photoelastic effect. Since the acousto-optic device 14 of the example has TeO ₂ having a tetragonal crystal structure, the tensor Δ (1 / ▲ n ² _i ▼) of the change in the refractive index is obtained.
Can be expressed by equation (1), where Pij is the photoelastic constant tensor and Sij is the strain tensor.

In this embodiment, the laser beam 16 is incident on the acousto-optic element 14 in the <001> direction, and the ultrasonic wave direction of the transverse wave propagating to the acousto-optic element 14 is <100>, and the transverse wave of the ultrasonic wave is Since the polarization direction is <010>, only S ₆ is generated in the acoustooptic element 14 as the distortion component of the distortion tensor Si in the equation (1). In addition, the principal axis of the index ellipsoid that represents the state of the change in the refractive index depending on the crystal direction based on the equation (1) is Z
It is obtained by rotating it about the axis <001> by 45 degrees, and its general formula is shown by formula (2). X ′ obtained when rotated further
The principal refractive indices nx, ny, nz on the axis and y'axis are expressed by equation (3).

(1 / ▲ n ² _o ▼) X ′ ² + P ₆₆ S ₆ x ′ ² y ′ ² + (1 / ▲ n
² _o ▼) y ′ ² + (1 / ▲ n ² _e ▼) z ² = 1 (2) In equations (2) and (3), n ₀ is the refractive index of ordinary light, ne
Is the refractive index of extraordinary light. As shown in the formula (3), the refractive index of the acousto-optic element 14 to which ultrasonic waves are applied decreases in the x'-axis direction and increases in the y'-axis direction, so that it enters the acousto-optic element 14. When the laser beam 16 is reflected by the ultrasonic wave front, it is separated into velocity components in the x'-axis and y'-axis directions having different propagation velocities.
The diffracted light 17 has a phase difference component obtained by combining the respective components of the x ′ axis and the y ′ axis in the <010> direction. In this way, the laser light 16 has a plane of polarization that makes an angle of 90 degrees with respect to the incident light, and has an optical frequency that is (f ₀ + f ₁ ) shifted by the central frequency f ₁ of the acoustooptic device 14. Diffracted light 17 and 0
Transmitted light that is P-polarized light of optical frequency f ₀ that is transmitted as the next light 18
And send it separately. At this time, the diffractive efficiency is adjusted so that the diffracted light 17 and the transmitted light 18 have substantially the same light amount.
It is desirable to adjust by the voltage supplied from the transducer 15 to the transducer 15.

The diffracted light 17 is reflected by the reflecting mirror 19 and enters the polarization beam splitter 20. Since this light is S-polarized light, it is reflected and transmitted. On the other hand, since the transmitted light 18 is P-polarized light, it passes through the polarization beam splitter 20. In this way, the outgoing light 21 of the polarization beam splitter 20 combines the S-polarized light having the optical frequency (f ₀ + f ₁ ) and the P-polarized light having the optical frequency f ₀ as components and sends them out on the same optical path. And the mutual frequency is f ₁
Two light beams with ＝ 40MHZ are obtained.

In the present embodiment, the light propagation direction is <001>, the ultrasonic wave traveling direction is <100>, and the transverse ultrasonic wave direction is <010>.
However, even if the direction of the anisotropic Bragg angle, that is, the light propagation direction is <001>, the ultrasonic wave traveling direction is <110>, and the ultrasonic wave polarizing direction is <10>, By rotating the principal axis of the refractive index ellipsoid of the optical element by 45 degrees, the plane of polarization of the incident light can be rotated by 90 degrees and diffracted, so that the same effect can be obtained.

〔The invention's effect〕

By reducing the required number of polarizing beam splitters and reflecting mirrors, it becomes easy to adjust the optical axis in the assembly process. Further, a smaller orthogonal polarization type optical frequency shifter can be obtained.
Further, since the influence of vibration due to disturbance or the like is reduced, the characteristics of light beams having orthogonal polarization planes and different frequencies can be improved.

[Brief description 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. 1 …… Laser oscillator, 6,14 …… Acousto-optic element, 7 …… Transducer (LiNbO ₃ 36 degree Y plate), 15 …… Transducer (LiNbO ₃ X plate), 3 …… First polarized beam splitter, 10… … Second polarization beam splitter, 20 …… Polarization beam splitter, 11,12 …… Reflector

Claims (1)

[Claims] 1. Propagating a transverse ultrasonic wave to the inside thereof, the transmitted wave having a polarization plane and an optical frequency which are respectively the same as the polarization plane and the optical frequency of the incident light, which is linearly polarized light, due to the wavefront of the ultrasonic wave. An acousto-optical element that has a plane of polarization orthogonal to the plane of polarization of the incident light and that separates the incident light into diffracted light in which the optical frequency of the incident light is shifted, and emits the acousto-optical element. A reflecting mirror for sending the diffracted light to a beam splitter, and a polarizing beam splitter for entering the transmitted light emitted from the acousto-optic device and emitting a light beam obtained by combining the transmitted light and the reflected light on the same optical path. An orthogonal polarization optical frequency shifter comprising: