GB2222271A

GB2222271A - Collinear acousto-optic modulator

Info

Publication number: GB2222271A
Application number: GB8916656A
Authority: GB
Inventors: Garry Joslin
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1988-08-26
Filing date: 1989-07-21
Publication date: 1990-02-28
Also published as: IT1233183B; IT8967724A0; KR900003673A; GB8916656D0; FR2635879A1; JPH0281036A; DE3927121A1

Description

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COLLINEAR ACOUSTO-OPTIC MODULATOR Background of the Invention

This invention relates generally to devices and techniques for modulating the frequency of a light beam. This invention relates particularly to the frequency modulation of light by the interaction of acoustic waves with the light. Still more particularly, this invention relates to the modulation of light in a Sagnac effect rotation sensor.

A fiber optic ring interferometer typically comprises a loop of fiber optic material that guides counter-propagating light waves. Afte r traversing the loop, the counter-propagating waves are combined so that they interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the interference, which is dependent upon the relative phase of the counter-propagating waves.

Fiber optic ring interferometers have proven to be particularly useful for rotation sensing. Rotation of the loop creates a relative phase difference between the counter-propagating waves in accordance with the well known Sagnac effect. The amount of phase difference is a function of the angular velocity of the loop. The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing it to determine the rotation rate.

A closed loop fiber optic rotation sensor may include a frequency shifter near where each of the counterpropagating waves is introduced into the sensing coil to null the rotation-induced phase difference between them. The amount that the waves must be adjusted in frequency to null the Sagnac phase shift indicates the rotation rate of the sensing loop. The amount of the frequency shift may be determined by measuring the electrical drive signal supplied to the frequency shifter.

The use of frequency shifters to null out the Sagnac phase shift greatly increases the dynamic range of the fiber optic rotation sensor.

A Bragg cell may be used to shift the frequency of an optical wave. A Bragg cell acoustooptic-modulator typically comprises a crystal that is "5 driven by an acoustic transducer to produce acoustic waves. The acoustic waves interact with a light beam that propagates through the crystal. Applying modulating signals to the acoustic transducer controls the frequency of the acoustic waves in the crystal. The acoustic wavefronts 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 frequency wo, then the portion of the beam reflected from the Bragg cell has frequency coo + com; and the transmitted portion of the beam has the original frequency coo.

Bragg cell optical frequency shifters have included crystals formed of PbMO04 or Te02. These crystals provide a satisfactory range over which the optical frequency may be shifted, but it has been found that the large birefringence of these crystals causes depolarization of light in the fiber optic rotation sensor. Depolarization is undesirable because only light waves of the same polarization will produce the interference patterns that are processed to determine the rotation of the sensing loop.

To correct for this depolarization, the polarization state of the light in the fiber must be manipulated to be the linear state that is aligned with the crystal axis as it passed through the crystal.- A fiber optic rotation sensor may include two acoustooptic frequency shifters. Both the clockwise beam and the counterclockwise beam enter the two frequency shifters, which there are in general four locations in the fiber where the polarization must be adjusted. Requiring four polarization control devices greatly complicates the structure and stable operation of a fiber optic rotation sensor.

The typical Bragg cell requires the input beam to be collimated and directed toward the crystal an an angle such that after the beam enters the crystal, the beam will be an a defined angle, called the Bragg angle, to the wave fronts of the acoustic field. At the Bragg angle the frequency of the acoustic field is added to the frequency of the light and the collimated optical beam is diffracted by the Bragg angle. The diffracted beam is thus shifted in frequency before it propagates out of the crystal. For prior art acoustooptic modulators with parallel end faces, the output beam is diverted from the input beam by twice the Bragg angle.

In constructing a fiber optic rotation sensing system including prior art acoustooptic modulators, the angular offset of the optical beam passing through the acoustooptic modulator must be accommodated for by collimating the optical signal passing through the acoustooptic modulator. This collimation is accomplished by the difficult and expensive task of machining the mounts for the collimating lenses at the proper angle.

It has also been found that the polished parallel end faces of the acoustooptic modulator are capable of supporting standing acoustic waves. Changes in the acoustic frequency vary the wavelength of the acoustic waves between the optical end faces by constructive and destructive interference. This interference varies the strength of the acoustic field, which alters the optical efficiency of the acoustooptic modulator and adds amplitude noise to the optical signal. Summary of the Invention

An acoustooptic modulator according to the present invention for shifting the frequency of an optical signal comprises a block formed of a dielectric material. The block has first, second and third surfaces and an acoustic transducer is mounted to the first surface for launching acoustic waves in the block. The second and third surfaces are oriented relative to the first surface such that an optical beam of a selected frequency incident upon the first surface at a selected angle refracts into the block and is diffracted by the acoustic waves to form an output beam from the block that is collinear with the incident beam and shifted in frequency therefrom.

The block of dielectric material included in the acoustooptic modulator according to the present invention preferably is formed to comprise arsenic trisulphide. The optical beam incident upon the acoustooptic modulator preferably is parallel to the first surface of the block.

The second surface of the block preferably is oriented relative to the first surface such that the incident optical beam impinges upon the acoustic waves at the Bragg angle. Both the second and third surfaces of the block preferably are oriented relative to the first surface such that an optical -beam incident upon either the second or third surface and parallel to the first surface impinges upon the acoustic waves at the Bragg angle.

A method according to the present invention for forming an acoustooptic modulator for shifting the frequency of an optical signal comprises the steps of forming first, second and third surfaces on a block of a dielectric material and launching acoustic waves into the first surface of the block. The method also includes orienting the second and third surfaces relative to the first surface such that an optical beam of a selected frequency incident upon the first surface at a selected angle refracts into the block and is diffracted by the acoustic waves to form an output beam from the block that is collinear with the incident beam and shifted in frequency therefrom.

The method according to the invention preferably includes the step of forming the block to comprise arsenic trisulphide.

The method preferably includes the step of applying the incident optical beam parallel to thd first surface of the block.

The method preferably includes the step of orienting the second surface of the block relative to the first surface such that the incident optical beam impinges upon the acoustic waves at the Bragg angle.

The method preferably includes the step of orienting both the second and third surfaces of the block relative to the first surface such that an optical beam incident upon either the second or third surface and parallel to the first surface impinges upon the acoustic waves at the Bragg angle.

Brief Description of the Drawings

Figure 1 illustrates a prior art acoustooptic modulator having input and output light beams; and

Figure 2 illustrates an acoustooptic modulator according to the present invention.

Description of the Preferred Embodiment

Figure 1 illustrates a prior art acoustooptic modulator 10 that formed to include a crystal 12 and an acoustic transducer 14 attached to a side 16 of the crystal 12. The acoustic transducer 14 produces acoustic waves that travel generally from the side 16 of the crystal toward the opposite side 18. The acoustic wavefronts are indicated by the parallel lines 20.

A light beam 22 impinges upon a surface 24 of the crystal 12 at the Bragg half angle I with respect to the normal to the surface 24. Most of the incident beam refracts at the surface 24 and enters the crystal 12.

The light in the crystal 12 interacts with the acoustic waves 20 and is shifted in frequency. The light propagates through the crystal and exits at a surface 26 that is opposite the surface 24 where the light enters the crystal 12. It may be seen from Figure 1 that the diffracted beam is deflected by the Bragg angle from the incident beam.

Figure 2 illustrates an acoustooptic modulator 30 according to the present invention. The acoustooptic modulator 30 includes a crystal 32 having an acoustic transducer 34 attached to a surface 35 thereof. The crystal 32 includes a pair of surfaces 36 and 38 that are angled with respect to the surface 35. The surfaces 32, 36 and 38 preferably are formed to be optically flat without surface irregularities or striations.

Still referring to Figure 2, the incident beam is parallel with the surface 32 so that the beam strikes the surface 36 at an angle I'with respect to the normal to the surface 36. The angled surface 36 is precision ground and employs the refractive index of the crystal 32 to compensate for the Bragg angle. A portion of the incident light diffracts into the crystal and interacts with acoustic wavefronts produced by the acoustic transducer 34. Part of the optical beam in the crystal diffracts from the acoustic wavefront and is directed toward the surface 38. The optical beam refracts at the surface 38 and emerges from the crystal collinear with the incident beam.

Having the incident and output beams of the acoustooptic modulator 30 collinear removes the requirement of machining lens is mounts to the Bragg angle. The present invention permits lens mounts for lenses used to focus the optical beams to be machined collinearly, which is easier to accomplish with great precision than machining them at the Bragg angle.

The end faces of the prior art acoustooptic modulator 30 must be machined at the angle I' in order to produce the desired interaction with the acoustic wavefronts and the optical beam. For the acoustooptic modulator 12 having right angled end surfaces the Bragg half angle I is given by

X fX sin I = 2A = -V (1) where X is the wavelength of the incident light, A is the acoustic wavelength in the acoustooptic modulator 30, f is the rf frequency at which the acoustooptic modulator 30 is driven and v is the acoustic velocity in the acoustooptic modulator 30. By Snell's law, the Bragg angle B in the acoustooptic modulator 30 is given by sin B = sinDI 11 (2) where il is the refractive index of the crystal 32 at the wavelength of the incident light. The angle of refraction of the input beam must equal theBragg angle so that the input and output beams of the acoustooptic modulator 30 are collinear.

Referring to Figure 2, for the crystal 32 having the end face 36 making an angle FwIth the plane of the acoustic transducer 34, an input beam parallel to the acoustic transducer 34 will strike the end face 36 at the angle No the normal to the end face 36. According to Snell's law, the input beam is refracted to an angle R from the normal given by sin I' = 7isin R. (3) The desired condition is that I' = B + R; therefore by substitution it is found that tan R sin B Tj - cos B (4) For arsenic trisulphide, AS2S3, at a wavelength 1 = 840 rim the refractive index is TI = 2.51. When the acoustooptic modulator 30 is operated at an acoustic frequency f = 80 MHz with the acoustic velocity being 2600 m/sec, the angle B = 5.15 mrad and the angle R = 3.41 mrad.

These angles give an end face angle of 8.56 mrad, which may be readily formed on the crystal 32.

Arsenic trisulphide has low birefringence and a high figure of merit. The low birefringence prevents depolarization that occurs in materials such as PbMO04 and Te02 that are normally used to manufacture acoustooptic modulators. The high figure of merit permits the acoustooptic modulator 30 to have small size and minimal power consumption while providing high optical efficiency.

The operational characteristics of the acoustooptic modulator 30 were tested at frequencies near f = 78 MHz. The overall optical efficiency is about 69%, and sideband suppression is about 57 dB. By varying the frequency it was found that the 3 dB frequency points were about 73.9 and 82.7 MHz, giving a bandwidth of about 8.8 MHz.

The test results showed that the acoustooptic modulator 30 provides a 15 dB reduction in optical transmission amplitude noise caused by standing waves in prior art designs. The angled end faces of the crystal 30 reflect waves inside the crystal at progressively higher angles which does no not support acoustic standing waves between the end faces 36 and 38. The rf frequency was modulated with a frequency excursion of about 180 KHz about the rf center frequency, and the optical output was measured with a fast photodetector and analyzed by a spectrum analyzer. As the rf center frequency varied between 77 and 79 MHz, amplitude modulation of the fundamental frequency was less than 25 dB.

Claims

What is claimed is:

1. An acoustooptic modulator for shifting the frequency of an optical signal, characterised by:

a block formed of a dielectric material, the block having first, second and third surfaces; an acoustic transducer mounted to the first surface for launching acoustic waves in the block, the second and third surfaces being oriented relative to the first surface such that an optical beam of a selected frequency incident upon the second surface at a selected angle refracts into the block and is diffracted by the acoustic waves to form an output beam from the block that is collinear with the incident beam and shifted in frequency therefrom.

2. The acoustooptic modulator of claim 1 wherein the block is formed to comprise arsenic trisulphide.

3. The acoustooptic modulator of claim I wherein the incident optical beam is parallel to the first surface of the block.

4. The acoustooptic modulator of claim 3 wherein the second surface of the block is oriented relative to the first surface such that the incident optical beam impinges upon the acoustic waves at the Bragg angle.

5. The acoustooptic modulator of claim 3 wherein the both the second and third surfaces of the block are oriented relative to the first surface such that an optical beam incident upon either the second or third surface and parallel to the first surface impinges upon the acoustic waves at the Bragg angle.

6. A method for forming an acoustooptic modulator for shifting the frequency of an optical signal, characterised by the steps of:

forming first, second and third surfaces on a block of a dielectric material; launching acoustic waves into the first surface of the block; orienting the second and third surfaces relative to the first surface such that an optical beam of a selected frequency incident upon the second surface at a selected angle refracts into the block and is diffracted gby the acoustic waves to form an output beam from the block that is collinear with the incident beam and shifted in frequency therefrom.

7. The method of claim 6 including the step of forming the block to comprise arsenic trisulphide.

8. The method of claim 6 including the step of applying the incident optical beam parallel to the first surface of the block.

9. The method of claim 8 including the step of orienting the second surface of the block relative to the first surface such that the incident optical beam impinges upon the acoustic waves at the Bragg 10 angle.

10. The method of claim 8 including the step of orienting both the second and third surfaces of the block relative to the first surface such that an optical beam incident upon either the second or third surface and parallel to the first surface impinges upon the acoustic waves at the 15 Bragg angle.

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