GB2183359A - Acousto-optic modulator - Google Patents

Abstract

An acousto-optic modulator for generating a modulated diffracted beam 14 from an incident laser beam 10 by Bragg diffraction from the refractive effect of an acoustic wave directed into a germanium block 21 by a transducer 2. Spurious delayed modulation signals caused by acoustic waves reflected from the far end 26 of the block are greatly reduced by providing several oblique saw cuts 27 which trap and dissipate the acoustic wave by multiple reflection. <IMAGE>

Description

SPECIFICATION Acousto-optic modulator The invention relates to an acousto-optic modulatorfor modulating a beam of optic radiation by interaction with acoustic waves in an acoustic medium in accordance with the Bragg relationship, said modulator comprising a block of material transparent to the optical radiation to be modulated and having respective opposite side faces of optical quality to provide input and output surfaces for a beam of said optical radiation, an end face provided with electroacoustic transducer means for directing a beam of acoustic waves into said blockto set up an interaction region for said beam of optical radiation between said input and output surfaces.

The operation of a modulator ofthe kind specified is discussed, for example, by E.l. Gordon in Proc. IEEE Vo. 54, October1966, pages 1391-1401. Figure 1 ofthe accompanying drawings is a diagram illustrating the principle of operation of such a modulator.A planar electroacoustictransducer 2, in theform of a piezoelectric wafer 3, formed for exampl e from a monocrystal of lithium niobate, with upper and lower metallised electrodes 4and 5, is mounted on one end face 6 of a block 1 of optically transparent material formed for example from a monocrystal ofgermanium.Thetransducer2 is energised ata suitable high frequency, for example several MHz, causing a corresponding regular succession of parallel acoustic wavefronts, indicated by parallel lines 7, to propagate in the block as, for example, a longitudinal wave disturbance with the velocity VL of a longitudinal acoustic wave in the direction indicated by the arrows 8. The associated local stress variations in the medium ofthe block will result in corresponding local variations in refractive index thus forming a corresponding diffraction structure which will propagate path 9 in the direction 8.

A beam 10 of optical radiation to be modulated, in the present case coherent radiation generated by a laser (notshown), is directed via a lens 11 and an optical side face 12 of the block 1, across the path ofthe propagating acoustic wave 7 in an interaction region 13 atthe Bragg angle 6 with respect to the propagating wave structure 7, causing a diffracted beam 14to be generated which is inclined at twice the Bragg angle 65to the direction ofthe input beam lOin the interaction region 13.The amplitudeofthe diffracted beam 14will depend on the amplitude of the acoustic wave 7, and therefore is used to form the modulated beam after passing out ofthe block 1 via the opposite optical side face 15. Is should be noted herein that such a modulator can function equally well when non-coherent optical radiation is employed provided thatthe Bragg diffraction conditions are satisfied.

A difficulty with this form of modulator is that when the acoustic wave 7 reaches the far end face 16 ofthe block it will tend to be reflected, and some ofthe acoustic energy may then follow a retroreflective path back towards the transducer 2, as indicated by the arrows 17. As this reflected wave passes in the reverse direction through the interaction region 13 crossed by the beam of optical radiation 10, it may generate a weak diffracted beam but the direction of motion of the corresponding acoustic diffraction structure will be reversed relative to the optical beam and the original Bragg angle relationship will not be properly met.

However, the reflected wave will continue to propagate until it reaches the transducer face 6 where some of the acoustic energy will be reflected as indicated bythearrows 18so as to travel back in the initial propagation direction forwhich the Bragg relationship will be correct, and as it passes again through the interaction region 13, a corresponding delayed modulation signal will be imposed in the modulated beam 14, whose amplitude will depend on the amplitude ofthe reflected acoustic wave. The presence of this delayed signal whose delay will be that ofthe round trip of the acoustic wave via the various points of reflection, is undesirable and will adversely affectthe performance ofthe modulatorespeciallyfor data transmission and ranging.

in the paper referred to above, a modulator is illustrated in whichthetransversefarend wall ofthe block has a layer of acoustic absorber to reduce reflection, and this is also indicated in Figure 1 by the reference 19.

Examples of a suitable acoustic absorbing material in the case of a germanium block, and indium and lead although neither have the same acoustic impedance as germanium and the resulting impedance mismatch will generate a significant reflected signal which will be greater in the case of indium. Because ofthis impedance mismatch, the reflected acoustic wave will be about -1 OdB relative to the acoustic wave incident on the end surface even when it is assumed that the acoustic absorbing material is a good acoustic absorber.

It is an object of the invention to provide an improved acousto-optic modulator in which the undesired reflected acoustic wave is significantly reduced in a manner that is relatively inexpensive to manufacture.

According to the invention there is provided an acousto-optic modulator ofthe kind specified, characterised in that a plurality of adjacent saw cuts are formed in the block via the other end face thereof, said saw cuts being respectively inclined to the direction of propagation of the beam of acoustic waves directed into the block, and spaced from one anotherso that the acoustic waves launched by the transducer cannot reach the other end face of the block directly.

The saw cuts are preferably made parallel to one another and uniformly spaced, in the case of a block having a rectangular cross section transverse to the direction of propagation ofthe beam of acoustic waves, the saw cuts are preferably made parallel to the shorter cross sectional dimension.

The non-optical side faces ofthe block can have a layer of an acoustic absorber such as indium or lead applied thereto to increase the rate of dissipation of acoustic energy reflected thereby.

An acousto-optic modulator manufactured in accordance with the invention can be employed in an optical ranging system for surveying orfor radar, or as a modulatorfor optical communications.

Embodiments of the invention will now be described by way of example, with reference to Figure 2 ofthe accompanying drawings, which diagrammatically illustrates an acousto-optic laser modulator arrangement in accordance with the invention.

Referring to Figure 2 which illustrates an acousto-optic laser modulator arrangement in accordance with the invention, elements corresponding to those described with referenceto Figure 1 are given the same reference numerals. A C02 laser 20 provides a beam 10 of coherent optical radiation having a wavelength x = 10.6Xm, and a diameter of about 2mm. A germanium lens 11 is used to focusthe beam so that in the interaction region 13, the beam has a waist with a minimum diameter ofabout2ooum in orderto providethe modulatorwith a short rise time.Because the optical beam is focussed in the present example, the divergence of the optical beam is preferably matched by a corresponding divergence ofthe acoustic beam in order to provide an optimally high modulation efficiency. If it is notimportantto provide a short rise time, the optical radiation need not take the form ofafocussed beam but can, for example, comprise a normally collimated laser beam.

The modulator block 21 is formed from a monocrystal of germanium and in one example was ofwidth 20mm, thickness 5mm and overall length about 22mm. The transducer 2 comprised a wafer 3,35 degree Y-cutfrom a monocrystal of lithium niobate and operating in the fundamental thickness mode, which is pressure bonded to the end face 6 of the block 21 by the method described in U.K. Patent Application No.

8510700. One ofthe electrodes, namely 5, comprises a conductive film made up of layers of chromium, gold and indium applied priorto pressure bonding. The other electrode 4, whose dimensions determine the active region of the transducer 2 and hence the initial cross section ofthe acoustic beam 7, is applied after lapping the bonded wafer3 to the correct thickness for resonance at the required acoustic frequency. In the present examplethetransverse dimension ofthewafer3were 12mm in the plane of Figure 2 and 3mm inthe direction perpendicularthereto, the corresponding dimensions ofthe electrode 4 were 6mm and 0.3mm respectively.

The orientation ofthe block 21 relativeto the germanium crystal axes will depend on the application ofthe modulatorasfollows. if the highest modulation efficiency is required and the use of plane polarised light is permissible, the block 21 is cut so that the acoustic wave propagation path direction lies along the (111) crystal axis and the incident light must be polarised with the plane ofthe electricvector parallel to the acoustic wave propagation direction 9. if thins use of plane polarised light is not acceptable, the block 21 is cut sothatthe acousticwave propagation direction lies along the (100) germanium crystal axis.In this casethe polarisation plane direction is not critical and the device can operate with circularly polarised light, however the modulation sensitivity fortwo directions at right angles will, in general, be different and in the latter case the output will tend to become elliptically polarised.

in the case of the present modulator, the Bragg angle e8 is given by: -1 X eg = (-) 2A where X is the optical wavelength in the acoustic medium and A is the acoustic wavelength in the medium.

Thus in the case germanium,forwhich the refractive index n = 4,the lightfrom the CO2 laser20,whose free space wavelength Xc = 10.6m, will have a wavelength X in the medium of 2.65 > m. The acoustic wavelength will depend of course on the frequency and on the acoustic velocity which latter will depend on direction.Thus, for example, an acoustic wave having a frequency of 60M Hz directed along the (100) axis for whichthevelocityofa longitudinal wave = 4.72 x 103m/sec, will have a wavelength A = 78.7m giving a valueforthe Bragg angle = 0.96 degree. In a second example, an acoustic wave having a frequency of 100 MHz directed along the (111) directionforwhich VL = 5.5 x 103m/sec, will have A = 55m giving a valuefor the Bragg angle of 0 = 1.38 degrees.

In orderto reduce optical reflection from the optical faces 12 and 15 ofthe block2l,thefaces are each provided with an anti-reflection layer. In the present example both faces, although parallel to one another, are inclined by 2 degrees from the acoustic wave propagation directionS which is perpendicularto the end face 6. This arrangement was employed in orderto make the modulator block21 readily interchangeable in a mountwith other modulatorsfor otherfrequencies ororientations. In general, however, it is preferable for the avoidance of reflections, thatthe faces 12 and 15 should not be parallel to one another.Because ofthe smallness ofthe inclination angles and of other ray angles, and forthe sake of clarity of illustration, these angles are depicted in Figure 2withtheir magnitudes enlarged, especially within the block 21. Figure 2 is intended to represent the case forwhich the Bragg angle is 0.96 degrees.

The modulated diffracted output beam 14, after refraction at the exitface 15 of the block, is directed along the modulator output axis and is collimated by a second germanium lens 23. An apertured diaphragm 25 is used to remove the undiffracted component ofthe emergent beam.

In order to reduce as far as possible any acoustic energy which can be reflected back along the acoustic propagation path 9,a plurality of saw cuts 27 are formed, in accordance with the invention, in the block 21 via the far end face 26. The saw cuts 27 are each inclined to the direction of propagation 9 of the beam of acoustic waves7 directed into the block 21 bythetransducer2. The saw cuts 27 are spaced from one another bya distance such that an acoustic wave propagating along the direction 9from thetransducer2 cannot reach the end face 26 directly but will be reflected by the inclined boundary of one ofthe saw cuts 27, afterwhichthe acoustic energy will be dissipated and dispersed within the block 21 by multiple reflection at the various boundary surfaces.The saw cuts 27 in the embodiment of Figure 2 are made obliquely parallel to one another ata uniform spacing one from another and are for manufacturing convenience made in a respective plane perpendiculartothe plane ofthe drawing, i.e. parallel to the shorter dimension of a cross section taken transverse to the acoustic wave propagation direction 9. The saw cuts 27 can be readily made using a diamond saw for example.

Although a relatively large number of, namely 10, slots 27 are shown in Figure 2, one example of a modulator in accordance with the invention was constructed in which onlyfour saw cuts were employed and wasfound, on measurement, to reducethe undesired reflection signalsto a level of -55dB.

It will be understood thatthe process of making foursawcuts in a block of germanium is quickerand simpler in manufacture and therefore less expensive than other alternative methods of reducing end reflections such as tapering thefarend ofthe block 21 and applying a surface layer of acousticabsorbent such as indium or lead.

It should be noted that, while Figure 2 illustrates a modulator block 21 provided with parallel uniformly spaced saw cuts 27, the saw cuts need not be parallel to one another and may be randomly spaced provided always that the spacing and inclination of adjacent saw cuts is such in relation to their extent as to preventthe acoustic beam from the transducer 2 from reaching the end face 26 directly, i.e. without interception bythe boundary of at least one saw cut. The plane ofthe saw cut need not be parallel to the shortercross-sectional dimension and that of respective cuts may be inclined differently to the plane ofthe drawing while remaining within the scope of the appended claims.

Itwill be apparentthat the saw cuts 27 need only be present in the region whose lateral extent relative tothe .

propagation direction S is that ofthe beam of acoustic waves7 in that region or is slightly greater, and arranged so that the acoustic energy is trapped in the spaces between adjacent cuts.

The non-optical side faces ofthe block 21, namely the upper and lowerfaces (not shown) lying above and below the plane of Figure 2, can have a layer of sound absorbantappliedthereto, as for example a layer of indium or lead about 200pm thick, in order to increase the rate of dissipation of acoustic energy.

Claims (7)

1. An acousto-optic modulatorfor modulating a beam of optical radiation by interaction with acoustic waves in an optical medium in accordance with the Bragg relationship, said modulatorcomprising a block of material transparent to the optical radiation to be modulated and having respective opposite side faces of optical quality to provide input and outputsurfaces for a beam of said optical radiation, an end face provided with electroacoustictransducer means for directing a beam of acoustic waves along a propagation axis in said block to set up an interaction region for said beam of optical radiation between said input and output surfaces, characterised in that a plurality of adjacent saw cuts are formed in the block via the other end face thereof, said saw cuts being respectively inclined to the direction of propagation ofthe beam of acoustic waves directed into the block, and spaced from one another so that the acoustic waves launched by the transducer cannot reach the other end face of the block directly.

2. An acousto-optic modulator as claimed in Claim 1, characterised in that said saw cuts are parallel to one another.

3. An acousto-optic modulatoras claimed in Claim 2, characterised in thatsaid saw cuts are uniformly spaced.

4. An acousto-optic modulator as claimed in any one ofthe preceding claims in which the block hasa rectangular cross section transverse to the direction of propagation of the beam of acoustic waves from the transducer, and the plane of each saw cut is parallel to the direction of the shorter cross-sectional dimension.

5. An acousto-optic modulator as claimed in any one ofthe preceding claims characterised inthatthe block is formed from a monocrystal of germanium.

6. An acousto-optic modulator substantially as herein described with reference to Figure 2 ofthe accompanying drawings.

7. An optical radar device characterised in that the emitted optical beam is modulated by an acousto-optic modulator as claimed in any one ofthe preceding claims.