FR2635879A1 - ACOUSTO-OPTICAL MODULATOR AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

An acousto-optical modulator for shifting the frequency of an optical signal is disclosed. It comprises a block formed of a dielectric material 32 on a first surface 35 of which is mounted an acoustic transducer 34 intended to emit acoustic waves into the block. End faces 36, 38 of the block are oriented relative to the first surface so that an optical beam of a chosen frequency, arriving on one of the end faces at a chosen angle, is refracted in the block and diffracted by acoustic waves to form an output beam which is collinear with the incident beam and frequency shifted relative to it. Scope of application: interferometers, etc.

Description

The invention generally relates to devices and devices

  techniques for modulating the frequency of a light beam. In particular, the invention relates to the modulation of the frequency of light by the interaction of acoustic waves with light. The invention relates more particularly to the modulation of the

  light in a rotation sensor with Sagnac effect.

  An optical fiber ring interferometer typically includes a loop of fiber optic material that guides light waves propagating in opposite directions. After traversing the loop, waves propagating in opposite directions are combined to

  to interfere to form an optical output signal.

  The optical intensity of the output signal varies depending.

  interference, which depends on the relative phase of the

  waves propagating in opposite directions.

  Optical fiber ring interferometers have proved particularly useful for detecting rotation. A rotation of the loop generates a relative phase difference between the waves propagating in opposite directions, in accordance with the well-known effect of Sagnac. The amplitude of the phase difference is a function of the angular velocity of the loop. The intensity of the output optical signal produced by the interference of the waves propagating in opposite directions varies according to the speed of rotation of the loop. The detection of a rotation is performed by detecting the optical output signal and processing this signal to determine the

rotation speed.

  A fiber optic closed-loop rotation sensor may include a frequency shift element proximate the point where each of the counter-propagating waves is introduced into the responsive coil to cancel the rotational induced phase difference between they. The amplitude at which the waves must be adjusted in frequency to cancel the Sagnac phase shift

  indicates the speed of rotation of the sensitive loop.

  The magnitude of the frequency offset can be determined by measuring the electrical drive signal applied to the frequency shift element. The use of frequency shift elements to cancel the Sagnac phase shift significantly increases the dynamic range of the sensor.

optical fiber rotation.

  A Bragg cell can be used to shift the frequency of an optical wave. An acousto-optic Bragg cell modulator usually includes a crystal that is driven by an acoustic transducer to produce acoustic waves. Acoustic waves interact with a light beam that propagates through the crystal. The application of modulation signals to the acoustic transducer controls the frequency of the acoustic waves in the crystal. The acoustic wave fronts in the crystal function as a moving diffraction grating, which transmits a first portion of the incident optical beam and reflects a second portion. If the optical signal has a frequency W., the part of the beam reflected by the Bragg cell then has a frequency cOo + uWm, and the transmitted part of

  beam has the original frequency -Wo.

  The Bragg cell optical frequency shift elements comprise crystals formed of PbMoO4 or TeO2. These crystals have a sufficient range in which the optical frequency can be shifted, but it has been found that the strong birefringence of these crystals causes a depolarization of the light in the optical fiber rotation sensor. A depolarization is undesirable because only light waves of the same polarization produce the interference patterns that are

  processed to determine the rotation of the sensitive loop.

  To perform a correction preventing this depolarization, the polarization state of the fiber light must be processed to be the linear state that is aligned with the crystal axis as it passes through the crystal. An optical fiber rotation sensor may comprise two acousto-optical frequency shift elements. The clockwise rotating beam and the counter-rotating beam both enter the two frequency offset elements, where there are generally four locations in the fiber where the bias is to be adjusted. The need to use four polarization control devices greatly complicates the structure and operation

  stable of a fiber optic rotation sensor.

  The usual Bragg cell requires that the input beam be collimated and directed to the crystal at an angle so that the beam, after entering the crystal, forms a defined angle, called the Bragg angle, with the fronts. of the acoustic field. At the Bragg angle, the frequency of the acoustic field is added to the frequency of light and the collimated optical beam is diffracted from the Bragg angle. The diffracted beam is therefore shifted in frequency before leaving the crystal while propagating. In the case of acousto-optic modulators of the prior art with parallel extreme faces, the output beam is deflected from the

  input beam of double the Bragg angle.

  In the construction of a fiber optic rotation detection system comprising prior art acousto-optic modulators, the angular displacement of the optical beam passing through the modulator

  acousto-optics must be taken into account by collima-

  the optical signal passing through the acousto-optic modulator. This collimation is achieved by a difficult and expensive operation of machining the mounting brackets of the collimating lenses at the appropriate angle. It has also been found that the parallel and polished end faces of the acousto-optical modulator are able to withstand stationary acoustic waves. Variations in the acoustic frequency vary the length of the acoustic waves between the extreme faces

  optics by constructive and destructive interference.

  This interference varies the strength of the acoustic field

  This alters the optical efficiency of the acousto-optic modulator and adds amplitude noise to the optical signal. An acoustooptic modulator according to the

  the present invention, for shifting the frequency of ur.

  optical signal, comprises a block formed of a dielectric material. The block has first, second and third surfaces and an acoustic transducer is mounted on the first surface to emit acoustic waves into the block. The second and third surfaces are oriented with respect to the first surface so that an optical beam of a selected frequency, arriving at the first surface at a selected angle, refracts in the block and is diffracted by the acoustic waves to form a output beam of the block which is co-linear with the incident beam and shifted in frequency by

report to him.

  The block of dielectric material forming part of the acoustooptic modulator according to the present invention is advantageously formed of arsenic trisulphide. The optical beam arriving on the acousto-optic modulator is preferably parallel to the first surface of the block; The second surface of the block is advantageously oriented with respect to the first surface so that the incident optical beam arrives on the acoustic waves under the Bragg angle. The second and third surfaces of the block are advantageously oriented with respect to the first surface so that an optical beam arriving on the second or the third surface and parallel to the first surface arrives on the acoustic waves at the Bragg angle. A method according to the invention for producing an acousto-optical modulator for shifting the frequency of an optical signal comprises the steps of forming first, second and third surfaces on a block of dielectric material and emitting acoustic waves. in the first surface of the block. The method also includes orienting the second and third surfaces with respect to the first surface so that an optical beam of a selected frequency, arriving at the first surface at a selected angle, refracts into the block and is diffracted by the waves. acoustic in order to form a block output beam that is co-linear

  with the incident beam and. which is shifted in frequency

accordingly.

  The method according to the invention preferably comprises

  the step of forming the arsenic trisulfide block. The method advantageously comprises the step of applying the incident optical beam

  parallel to the first surface of the block.

  The method advantageously comprises the step of orienting the second surface of the block relative to the first surface so that the incident optical beam arrives on the acoustic waves from the angle of

Bragg.

  The method advantageously comprises the step of orienting the second and third surfaces of the block relative to the first surface so that an optical beam arriving on the second or third surface and parallel to the first surface reaches the

  acoustic waves from the angle of Bragg.

  The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting example and in which: FIG. 1 is a schematic view of an acousto-optic modulator of the prior art comprising light beams of entry and exit; and

  FIG. 2 is a diagrammatic view

  polishing an acoustic modulator according to the invention.

  FIG. 1 represents an acoustic modulator

  prior art optics 10 which is constructed to include a crystal 12 and an acoustic transducer 14

  attached to a face 16 of the crystal 12. The acoustic transducer

  14 produces acoustic waves that move globally from the face 16 of the crystal to the opposite face 18. The acoustic wave fronts are indicated by

parallel lines 20.

  A light beam 22 arrives on a surface 24 of the crystal 12 under the Bragg half-angle I with respect to the normal to the surface 24. The majority of the incident beam is refracted at the surface 24 and enters the crystal 12. The light interacts, inside the crystal 12, with the acoustic waves 20 and is shifted in frequency. The light propagates through the crystal and exits at a surface 26 which is opposite to the surface 24 where light has entered the crystal 12. It can be seen in FIG. 1 that the diffracted beam is deviated from the angle of

  Bragg with respect to the incident beam.

  Figure 2 illustrates an acoustic modulator

  optical device 30 according to the invention. The acousto-optic modulator comprises a crystal 32 on a surface 35 of which is fixed an acoustic transducer 34. The crystal 32 has two surfaces 36 and 38 which are inclined? The surfaces 35, 36 and 38 are advantageously

  formed so as to be optically flat, without

  regularities or superficial striations.

  Referring again to FIG. 2, the incident beam is parallel to the surface 31, so that it reaches the surface 36 forming an angle I 'with the normal to the surface 36. The inclined surface 36 is precisely ground and uses the refractive index of the crystal 32 to compensate for the Bragg angle. Part of the incident light is diffracted in the crystal and interacts with acoustic wave fronts produced by the acoustic transducer 34. Part of the optical beam is diffracted in the crystal from the acoustic wavefront and is directed to the surface 38. The optical beam is refracted at the surface 38 and emerges from the

  crystal being co-linear with the incident beam.

  As the incident and output beams of the acousto-optic modulator 30 are co-linear, it becomes unnecessary to machine lens supports at the Bragg angle. The invention allows lens holders used to focus the optical beams to be machined in a co-linear manner, which is easier to perform with greater precision than their machining.

the angle of Bragg.

  The extreme faces of the acoustic modulator

  Prior art optics 30 must be machined at the angle I 'to produce the desired interaction with the acoustic wave fronts and the optical beam. In the case of the acousto-optic modulator 12 having extreme right angle surfaces, the Bragg half-angle I is given by: sinI = 2 = 2v (1)

2A (1)

  o is the wavelength of the incident light, A is the acoustic wavelength in the acousto-optical modulator 30, f is the frequency rf at which the acousto-optic modulator 30 is controlled and v is the

  acoustic velocity in the acousto-optical modulator 30.

  According to Snell's law, the angle of Bragg B in the modula-

  The acousto-optic sensor 30 is given by: sinai sin B = (2) where n is the index of refraction of the crystal 32 at the wavelength of the incident light. The refraction angle of the input beam should be equal to the Bragg angle so that the input and output beams of the modulator

  acousto-optics 30 are co-linear.

  With reference to FIG. 2, in the case of the crystal 32, the end face 36 of which forms an angle I 'with the plane of the acoustic transducer 34, an input beam parallel to the acoustic transducer 34 reaches the end face 36 at the angle I 'with the normal at the extreme face 36. In accordance with' Snell's law, the input beam is refracted at an angle R with respect to the normal, which is given by: sin I '= nsin R (3 ) The desired condition is that I '= B + R; therefore, by substitution, we find that: sin B tg R = (4) il-cos B For arsenic trisulfide, AS2S3, at a wavelength = 840 nm, the refractive index is n = 2 51. When the acousto-optical modulator 30 is controlled at an acoustic frequency f = 80 MHz, the acbustic speed being 2600 m / s, the angle B is equal to 15 mrads and the angle R is equal to 3.41 mrads. . These angles give an angle of the extreme face of 8.56 mrads, which

  can be easily formed on crystal 32.

  Arsenic trisulfide has a low birefringence and a high quality coefficient. The low birefringence prevents depolarization that takes place in materials such as PbMoO4 and TeO2 normally

  used for the manufacture of acoustical modulators

  optics. The high coefficient of quality allows the acoustooptic modulator 30 to be of small size and minimal energy consumption while presenting a

high optical efficiency.

  The operating characteristics of the acousto-optic modulator 30 were tested at frequencies close to f = 78 MHz. The overall optical efficiency is about 69% and the sideband suppression is about 57 dB. By varying the frequency, it appeared that the 3 dB frequency peaks were about 73.9 and 82.7 MHz, giving a band

approximately 8.8 MHz.

  The results of the test showed that the acousto-optic modulator 30 produced a dB reduction of the amplitude noise in the optical transmission, caused by standing waves in prior art designs. The inclined end faces of the crystal 32 reflect the waves inside the crystal at increasingly higher angles, which does not sustain stationary acoustic waves between the end faces 36 and 38. The frequency rf has been modulated with a frequency deviation of about 180 kHz around the center frequency rf, and the optical output signal was measured by means of a fast photodetector and analyzed by means of a spectrum analyzer. By varying the center frequency rf between 77 and 79 MHz, it was found that the amplitude modulation of the fundamental frequency was

less than 25 dB.

  It goes without saying that many modifications can be made to the acousto-optical modulator described

  and shown without departing from the scope of the invention.

Claims (10)

  1. Acousto-optical modulator for shifting the frequency of an optical signal, characterized in that it comprises a block formed of a dielectric material (32) and having first, second and third surfaces (35, 36, 38) an acoustic transducer (34) mounted on the first surface for emitting acoustic waves into the block, the second and third surfaces being oriented with respect to the first surface so that an optical beam of a selected frequency, arriving at the second surface at a chosen angle, either refracted in the block and diffracted by the acoustic waves to form an output beam of the block which is co-linear with the beam   incident and which is shifted in frequency.   Acousto-optical modulator according to claim 1, characterized in that the block is formed of   to understand arsenic trisulfide.   3. Acousto-optical modulator according to claim 1, characterized in that the optical beam   incident is parallel to the first surface of the block.   4. Acousto-optical modulator according to claim 3, characterized in that the second surface of the block is oriented relative to the first surface so that the incident optical beam reaches the waves. acoustic from the angle of Bragg.   5. Acousto-optical modulator according to claim 3, characterized in that the second and third surfaces of the block are oriented relative to the first surface so that an optical beam arriving on the second or the third surface and parallel to the first surface reaches the acoustic waves under the angle of Bragg.   6. Process for producing an acoustical modulator   optical device for shifting the frequency of an optical signal, characterized by the steps of forming first, second and third surfaces (35, 36, 38) on a block of dielectric material (32), for emitting acoustic waves in the first surface of the block, to orient the second and third surfaces with respect to the first surface so that an optical beam of a chosen frequency, arriving at the second surface at a selected angle, is refracted in the block and diffracted by the acoustic waves to form a block output beam that is co-linear with the incident beam and which is shifted in frequency.   7. The method of claim 6, wherein   terized in that it consists in forming the block so that it includes arsenic trisulfide.   8. The method of claim 6, wherein   in that it consists in applying the optical beam   incident parallel to the first surface of the block.   9. The method of claim 8, wherein   characterized by orienting the second surface of the block relative to the first surface so that the incident optical beam reaches the acoustic waves from the angle of Bragg.   10. The method of claim 8, wherein   characterized in that it consists in orienting the second and third surfaces of the block relative to the first surface so that an optical beam arriving on the second or on the third surface and parallel to the first surface reaches the acoustic waves under the angle of Bragg.