GB2139842A

GB2139842A - Acoustic optic device

Info

Publication number: GB2139842A
Application number: GB08312949A
Authority: GB
Inventors: J D Jackson; J Moroz
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-05-11
Filing date: 1983-05-11
Publication date: 1984-11-14
Also published as: DE3417140A1; JPS59212822A

Abstract

An acousto-optic device employs multiple deflection in order to achieve higher resolution than in conventional devices without increasing the electrical bandwidth. An incident optic beam (I0) is interacted with an acoustic beam produced by a first transducer (22) resulting in a once deflected optic beam (I1). The once deflected beam (I1) is then deflected again by an acoustic beam produced by a second transducer (23) resulting in a twice deflected optic beam (I2), and so on for each transducer of the device. The device can be used for optical scanning in, for example laser printers, where the increase in scan angle due to the multiple deflection provides an increase in the number of resolvable spots for a given electrical bandwidth and optic aperture. Other uses are in frequency agile radio or radar, and the transmission of high frequency r.f. signals, the latter making use of the frequency shifting of the optic beam produced at each acousto-optic interaction. <IMAGE>

Description

SPECIFICATION Acousto-optic device This invention relates to an acousto-optic device and in particular to a high resolution acousto-optic deflector and uses thereof.

It is well known that light propagating through a transparent material is diffracted by sound propagating in the material which modulates the refractive index of the material, thus producing a diffraction grating which may comprise a linearly controlled deflection grating or a travelling chirp grating, depending on the electrical input signal. This is known as acousto-optic interaction and lithium niobate LiNbO3 is a commonly employed transparent acousto-optic material. The sound wave may be either a bulk wave or a surface acoustic wave. The light may be either guided or unguided. The sound waves may be generated either by bulk plate or layer transducers in the case of bulk waves, or by interdigitated transducers in the case of surface acoustic waves. The way in which the device performance is specified will depend on the particular application.However, two parameters are generally important. One is the range of angles through which the light beam may be deflected, which is related to the allowable frequency range of the electrical signals that may be applied to the electric to acoustic transducer employed to produce the sound waves.

The second is the focussed spot size of the optic beam, which is related to the optical beam aperture in the acousto-optic interaction region. These two parameters combine to determine the resolution of the device, although in specific applications one may be interested in either of the two parameters independently. The optical resolution, that is the number of resolved spots, is given by the angle of scan divided by the angular spot size.

Two standard techniques for increasing the allowable range of angles through which the light beam may be deflected (optical angular range), by increasing the range of allowable electrical signals (bandwidth), and hence also the resolution of the device, are the multiple tilted transducer array and the phased transducer array, such as are described in "Guided-Wave Acousto-Optic Fundamentals and Wideband Applications" by Chen S. Tsai, SPIE Vol 139 Guided Wave Optical Systems and Devices (1978) pages 132-143.

In the standard multiple tilted transducer array light is subject to multiple deflections by means of a number of electric to acoustic transducers which are tilted with respect to one another, all of the sound waves produced thereby have respective frequencies and are designed to deflect energy from an incident optical beam. That is the sound wave produced by the second transducer interacts with the light of the incident optical beam which was undeflected by the first transducer; the sound wave produced by the third transducer interacts with the light of the incident optical beam which was undeflected by the first and second transducers, and so on.

In the standard phased transducer array a number of electric to acoustic transducer having the same frequency (centre frequency) and parallel propagation axes are arranged in a stepped configuration. As a result of the step height a phase shift is introduced between adjacent transducers as the frequency is varied.

According to one aspect of the present invention there is provided an acousto-optic device in which in use an optic beam is subjected to multiple deflection by interaction with each of a plurality of acoustic beams in turn, the first acoustic beam serving to deflect the optic beam, and each acoustic beam other than the first serving to further deflect the optic beam as deflected by the preceding acoustic beam.

According to a second aspect of the present invention there is provided an acousto-optic device in which in use an optic beam is subjected to multiple deflection by interaction with each of a plurality of acoustic beams in turn, the device including a plurality of acoustic beam sources, the sources being arranged relative to one another such that interaction of said optic beam with the acoustic beam provided by a first source produces a deflected optic beam, and such that each acoustic beam provided by the sources apart from the first source interacts with the optic beam as deflected by the acoustic beam provided by the preceding source, whereby to further deflect it.

According to another aspect of the present invention there is provided a method of multiply deflecting an optic beam by interaction with each of a plurality of acoustic beams, comprising the steps of interacting a first acoustic beam with said optic beam whereby to produce a deflected optic beam, and interacting each one in turn of the remaining acoustic beams of the plurality with the optic beam as deflected by interaction with the preceding acoustic beam whereby to further deflect it.

According to a further aspect of the present invention there is provided a method of obtaining a scanning optic beam comprising the steps of multiply deflecting an optic beam by interaction with each of a plurality of acoustic beams in turn, wherein a first acoustic beam serves to produce a once deflected beam from said optic beam and wherein each acoustic beam other than the first serves to further deflect the deflected optic beam as produced upon interaction of the preceding acoustic beam, the acoustic beams being provided by respective electric to acoustic transducers; and applying the appropriate r.f. signals to the transducers in the appropriate sequence to achieve the desired scanning optic beam.

According to yet another aspect of the present invention there is provided a method of determining the frequency of an electrical signal comprising the steps of applying the electrical signal to each of a plurality of electric to acoustic transducers which are disposed relative to one another whereby to cause multiple deflection of an optic beam by interaction with a respective acoustic beam launched from each transducer in turn, wherein a first transducer of the plurality serves to provide a first acoustic beam which produces a once deflected optic beam from said optic beam, and wherein each transducer of the plurality other than the first serves to further deflect the deflected optic beam as produced upon interaction of the acoustic beam, of the preceding transducer; detecting the direction of the deflected optic beam as produced by interaction with the acoustic beam of the last transducer of the plurality, and determining the frequency ofthe electrical signal therefrom.

According to yet another aspect of the present invention there is provided a method of generating a high frequency r.f. signal for transmission on an optical carrier comprising the steps of generating an optic beam of a first frequency; apply an r.f. signal to each of a plurality of electric to acoustic transducers which are disposed relative to one another whereby to cause multiple deflection of the optic beam by interaction with a respective acoustic beam launched from each transducer in turn, wherein a first transducer of the plurality serves to provide a first acoustic beam which produces a once deflected optic beam from the optic beam, and wherein each transducer other than the first serves to further deflect the deflected optic beam as produced upon interaction of the acoustic beam of the preceding transducer, the deflected optic beam as produced by interaction with the acoustic beam of the last transducer of the plurality thus having a frequency which has been shifted in frequency from the first frequency by the sum of the radio frequencies applied to the transducers.

According to another aspect of the present invention there is provided a method of transferring information comprising modulating an optic beam with the information; applying the modulated optic beam to an acousto-optic device having a plurality of transducers for causing acoustic beams in response to electrical signals applied thereto; applying deflection control signals to the transducers so as to scan the optical beam; and applying the scanning modulated optic beam to a surface whilst causing relative movement between the surface and the optic beam transversely of the scan direction, said surface being sensitive to record or display the information thereon, wherein the optic beam is multiply deflected by each of the acoustic beams output from the transducers in turn, whereby to increase its resolution, a first transducer of the plurality providing an acoustic beam serving to produce a once deflected beam upon interaction with the modulated optic beam input thereto and each transducer other than the first serving to further deflect the deflected beam as produced by the acoustic beam of the preceding transducer.

According to a further aspect of the present invention there is provided apparatus for transferring information including an optic beam source, means for modulating the optic beam with the information; an acousto-optic device having a plurality of transducers for causing acoustic waves; means for coupling the modulated optic beam to the acousto-optic device; means for generating an optic beam deflection control signal to be applied to the transducers in use of the apparatus whereby to scan the coupled optical beam; means to couple the scanned optic beam to a light sensitive surface for recordal or display of information thereon; means for causing relative movement between the light sensitive surface and the optic beam transverse to the direction of scanning; and wherein the transducers are arranged relative to one another whereby interaction of the modulated optic beam with the acoustic beam provided by a first transducers of the plurality produces a once deflected modulated optic beam and whereby each transducer other than the first serves to provide a respective acoustic beam which further deflects the deflected optic beam as produced by the acoustic beam of the preceding transducer.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 illustrates, somewhat schematically, an acousto-optic device and indicates the affect of an incident optical beam; Figure 2 illustrates a known multiple tilted transducer array; Figure 3 illustrates, schematically, multiple deflection of a light beam in accordance with the present invention; Figure 4 illustrates, schematically, a multiple tilted transducer array according to the present invention; Figure 5 illustrates, schematically, a signal channelliser employing multiple deflection of a light beam in accordance with the present invention, and Figure 6 illustrates a high frequency r.f. generator employing multiple deflection of a light beam in accordance with the present invention.

The acousto-optic device 1 illustrated in Figure 1 comprises a planar substrate of acousto-optic mate rial, typically lithium niobate (LiNbOB) which may have a surface layer of higher refractive index than the substrate comprising a waveguide on the substrate. The surface layer may be produced by diffusion or ion implantation. An acoustic wave 2 is generated by means of an electrical to acoustical transducer 3 having electrical input E, for example a SAW (surface acoustic wave) transducer comprising an interdigitated metal electrode pattern formed on the surface of the substrate, or the surface layer. The acoustic wave 2 propagates across the device to an acoustic absorber 4.An incident optical beam 5, comprising a collimated planar beam 1o whose width is the optic aperture, is directed towards the device 1 such as to be incident at an angle. Around certain critical angles of incident +0B,the incident beam has energy coupled into a deflected beam whose direction is dependent on the acoustic and optic wavelengths. The angle BB is called the Bragg angle and is given by OB~X1/(2A2) where Aq is the incident light wavelength and A2 is the acoustic wavelength.

In dependence on the frequency of the electrical signal applied to the transducer 3, the acoustic frequency and thus the direction of the deflected beam will vary, two possibilities being indicated at 6; the undeflected beam is indicated at 7.

The known tilted transducer array illustrated in Figure 2 includes four interdigitated SAW transducers 8, 9, 10, 11 of frequency f1 ,f2,f3 and f4 producing acoustic signals, of respective frequencies, directed towards an acoustic absorber 12, the transducers and absorber being disposed at a surface waveguide 13 through which an incident optical beam 14 is directed. Upon interaction of the incident beam 14 with the each of the acoustic signals respective deflected light beams 15,16,17 and 18 are produced.

In contrast to this known deflection arrangement in which all of the acoustic signals are designed to deflect energy from the incident optical beam the present invention involves deflection of a deflected beam. Figure 3 illustrates the principle of the present invention. An incident optical beam 1o is interacted with a first acoustic beam 20, producing a deflected optical beam 11. This once deflected optical beam Ii is interacted with a second acoustic beam 21 producing a deflected optical beam 12 comprising a twice deflected optical beam.Thus is the present arrangement the second and subsequent acoustic beams are launched in the appropriate direction to interact with the light deflected by the previous acoustic beam, rather than with the remaining undeflected light of the incident beam as is the known arrangement. Thus the present arrangement allows a further increase of the optical angular range over and above that obtained by the standard techniques, and an increase in resolution is obtained without requiring an increase in electrical bandwidth.

Figure 4 shows schematically a deflector according to an embodiment of the present invention.

Blocks 22, 23 and 24 each represent one or more transducers to which respective electrical signals E1, E2, E3 are input. Since the invention is particularly, but not exclusively applicable to SAW devices Figure 4 is drawn in terms thereof. The transducer blocks 23 to 24 are arranged together with an acoustic absorber 25 at a surface waveguiding region 26. An incident optical beam lo is deflected by block 22, the thus produced deflected beam Ii is then deflected by block 23 and the thus produced deflected beam 12 is then deflected by block 24 to produce deflected beam 13.

Thus by use ofn sets of standard transducers, each launching acoustic waves in the appropriate direction to interact with the light deflected by the preceding set of transducers, uptown times the resolution of a single set of standard transducers may be achieved. An additional advantage of this technique is that the access time, that is the time taken for the acoustic wave to travel across the optic aperture, is only increased slightly for n sets of transducers over that for a single transducer.

The high resolution acousto-optic device provided by the present invention has various applications.

For example, it may be used as a laser deflector in a solid state deflector for a laser printer or copier. A laser printer is a high speed printer which basically comprises means to convert the characters etc. of input data to be printed to a suitable serial data stream form, a deflection control signal, for example a line scan voltage ramp, being generated in synchronism with the serial data stream; a laser beam source; laser modulation drive circuits which convery the logic pulses in the serial data stream to suitable voltage and current levels for either driving a modulator for a CW (continuous wave) laser or driving the modulation of a semiconductor laser directly; a line scan laser deflector driven in accordance with the line scan voltage ramp referred to above, for example; and optics to focus the deflected laser beam to write on a photo-sensitive surface, such as a rotating selerium drum, to produce an electrostatic pattern of the text character, for example. The drum picks up powdered ink on the electrostatic pattern and deposits it on to plain paper. The ink is set in, for example, a pressure process to produce the printed copy.

The optical waveguide of the acousto-optic device is interfaced to the laser source and the photosensitive surface by appropriate optics and coupling assemblies. The multiple deflection technique of the present invention allows an increase in scan angle to be achieved in comparison with known acoustooptic devices, and hence an increase in the number of resolvable spots for a given device electrical bandwidth and optical aperture which would previously have determined the maximum resolution.

Appropriate r.f. signals must be applied to the relevant transducers in correct sequence to achieve the desired scanning beam. The provision of an increased number of resolvable spots may alternatively be employed in optical scanner arrangements other than the laser printer/copier application specifically mentioned above.

Acousto-optic devices are also used to determine the frequency of the electrical signal applied to their electrical input. This is achieved by detecting the direction of the deflected optical beam using some form of detector, typically a photodiode array. The multiple deflection technique of the present invention allows the frequency resolution to be increased in comparison with that obtainable by standard techniques. It has the additional advantage that only a slight increase in access time, which may be less than one per cent, is required to achieve a frequency resolution that is several hundred per cent improved. Figure 5 illustrates schematically a signal channelliser employing the multiple deflection technique of the present invention.The channelliser comprises a laser 30 producing an optical beam which is applied to a multiple deflection acoustooptic device 32 via expansion optics 31 which serve to provide a collimated beam therefor. The device 32 has transducers 33 to which a signal to be channellised can be applied via input 34. The deflected optical signal output from device 32 is applied to optics 35 which serve to focus it onto a corresponding photodiode of a photodiode array 36 having a plurality of outputs 37. There may be one or more photodiodes per channel. This system cannot, however, resolve two simultaneously present frequencies which are separated by the frequency resolution of the system. It would resolve them if they were presented sequentially. Such channellising receivers may be employed in the fields of frequency agile radio communication and in frequency agile radar, and the multiple deflection technique of the present invention provides increased r.f. frequency resolution for a given access time in comparison with standard techniques.

Another application for the multiple deflection technique of the present invention is concerned with the transmission of high frequency r.f. signals on an optical carrier. In an acousto-optic interaction the deflected optical beam is frequency shifted from its original frequency by the frequency of the acoustic beam. Thus, for example, an optical beam that undergoes ten deflections by acoustic beams of 500 MHz frequency is then shifted in frequency by 5 GHz.

Figure 6 illustrates a high frequency r.f. generator employing this technique. The optical output of a laser 41 is split in two at 42 by an optical splitter. One part is then frequency shifted, by means of an acousto-optic device 43, by 10 times 500 MHz. The frequency shifted optical beam is combined at 44 with the non frequency shifted part of the output of laser 41, and applied to a photodetector 45, which may be at the other end of a transmission medium, for example on optical fibre link 46. The output of the photodiode 45 is a 5 GHz signal, the difference between the two optical frequencies. Thus a 5 GHz signal has been generated without using electronics of frequency greater than 500 MHz in the generation.

Claims

1. An acousto-optic device in which in use an optic beam is subjected to multiple deflection by interaction with each of a plurality of acoustic beams in turn, the first acoustic beam serving to deflect the optic beam, and each acoustic beam other than the first serving to further deflect the optic beam as deflected by the preceding acoustic beam.

2. An acousto-optic device in which in use an optic beam is subjected to mulitple deflection by interaction with each of a plurality of acoustic beams in turn, the device including a plurality of acoustic beam sources, the sources being arranged relative to one another such that interaction of said optic beam with the acoustic beam provided by a first source produces a deflected optic beam, and such that each acoustic beam provided by the sources apart from the first source interacts with the optic beam as deflected by the acoustic beam provided by the preceding source, whereby to further deflect it.

3. An acousto-optic device as claimed in claim 2, wherein the acoustic beam sources comprise electric to acoustic transducers.

4. an acousto-optic device as claimed in claim 3, including a body of piezo-electric material having a surface in which acoustic and optic beams may be propagated, the electric to acoustic transducers being disposed whereby to launch acoustic beams in the surface and across the path of said optic beam.

5. A device as claimed in claim 4, wherein the transducers comprise interdigitated transducers, for providing surface acoustic waves, disposed on the surface on a first side of said optic beam path, and including acousto absorbing means disposed on the surface on the opposite side of said optic beam path.

6. A method of multiply deflecting an optic beam by interaction with each of a plurality of acoustic beams, comprising the steps of interacting a first acoustic beam with said optic beam whereby to produce a deflected optic beam, and interacting each one in turn of the remaining acoustic beams of the plurality with the optic beam as deflected by interaction with the preceding acoustic beam whereby to further deflect it.

7. A method as claimed in claim 6, wherein said optical beam is propagated in a surface of a body of piezo-electric material and the acoustic beams are launched into the surface from respective electric to acoustic transducers.

8. A method as claimed in claim 7, wherein the transducers comprise interdigitated transducers for producing surface acoustic waves.

9. A method of obtaining a scanning optic beam comprising the steps of multiply deflecting an optic beam by interaction with each of a plurality of acoustic beams in turn, wherein a first acoustic beam serves to produce a once deflected beam from said optic beam and wherein each acoustic beam other than the first serves to further deflect the deflected optic beam as produced upon interaction of the preceding acoustic beam, the acoustic beams being provided by respective electric to acoustic transducers; and applying the appropriate r.f. signals to the transducers in the appropriate sequence to achieve the desired scanning optic beam.

10. A method of determining the frequency of an electrical signal comprising the steps of applying the electrical signal to each of a plurality of electric to acoustic transducers which are disposed relative to one another whereby to cause multiple deflection of an optic beam by interaction with a respective acoustic beam launched from each transducer in turn, wherein a first transducer of the plurality serves to provide a first acoustic beam which produces a once deflected optic beam from said optic beam, and wherein each transducer of the plurality other than the first serves to further deflect the deflected optic beam as produced upon interaction of the acoustic beam of the preceding transducer; detecting the direction of the deflected optic beam as produced by interaction with the acoustic beam of the last transducer of the plurality, and determining the frequency of the electrical signal therefrom.

11. A method as claimed in claim 10, wherein said direction is detected by means of a photodiode array.

12. A method of generating a high frequency r.f.

signal for transmission on an optical carrier comprising the steps of generating an optic beam of a first frequency; apply an r.f. signal to each of a plurality of electric to acoustic transducers which are disposed relative to one another whereby to cause multiple deflection of the optic beam by interaction with a respective acoustic beam launched from each transducer in turn, wherein a first transducer of the plurality serves to provide a first acoustic beam which produces a once deflected optic beam from the optic beam, and wherein each transducer other than the first serves to further deflect the deflected optic beam as produced upon interaction of the acoustic beam of the preceding transducer, the deflected optic beam as produced by interaction with the acoustic beam of the last transducer of the plurality thus having a frequency which has been shifted in frequency from the first frequency by the sum of the radio frequencies applied to the transducers.

13. A method of transmitting a high frequency r.f. signal on an optical carrier comprising generating a high frequency r.f. signal by a method as claimed in claim 12 on one portion of the generated optic beam, combining the frequency shifted portion with the remaining non-frequency shifted portion of the generated optic beam, transmitting the combined beams along an optical path and detecting the two beam portions by a photodetector at the output end of the optical path, the output of the photodetector comprising a signal whose frequency comprises the sum of the radio frequencies applied to the transducers.

14. A method of transferring information comprising modulating an optic beam with the information; applying the modulated optic beam to an acousto-optic device having a plurality of transducers for causing acoustic beams in response to electrical signals applied thereto; applying deflection control signals to the transducers so as to scan the optical beam; and applying the scanning modulated optic beam to a surface whilst causing relative movement between the surface and the optic beam transversely of the scan direction, said surface being sensitive to record or display the information thereon, wherein the optic beam is multiply deflected by each of the acoustic beams output from the transducers in turn, whereby to increase its resolution, a first transducer of the plurality providing an acoustic beam serving to produce a once deflected beam upon interaction with the modulated optic beam input thereto and each transducer other than the first serving to further deflect the deflected beam as produced by the acoustic beam of the preceding transducer.

15. Apparatus for transferring information including an optic beam source, means for modulating the optic beam with the information; an acoustooptic device having a plurality of transducers for causing acoustic waves; means for coupling the modulated optic beam to the acousto-optic device; means for generating an optic beam deflection control signal to be applied to the transducers in use of the apparatus whereby to scan the coupled optical beam; means to couple the scanned optic beam to a light sensitive surface for recordal or display of information thereon; means for causing relative movement between the light sensitive surface and the optic beam transverse to the direction of scanning; and wherein the transducers are arranged relative to one another whereby interaction of the modulated optic beam with the acoustic beam provided by a first transducers of the plurality produces a once deflected modulated optic beam and whereby each transducer other than the first serves to provide a respective acoustic beam which further deflects the deflected optic beam as produced by the acoustic beam of the preceding transducer.

16. A multiple deflection acousto-optic device substantially as herein described with reference to Figure 3 or Figure 4 of the accompanying drawings.

17. A method of multiply deflecting an optic beam by interaction with each of a plurality of acoustic beams substantially as herein described with reference to Figure 3 or Figure 4 of the accompanying drawings.

18. A signal channelliser substantially as herein described with reference to Figure 5 of the accompanying drawings.

19. A high frequency r.f. generator substantially as herein described with reference to Figure 6 of the accompanying drawings.