GB2119947A - Acousto-optic device - Google Patents
Acousto-optic device Download PDFInfo
- Publication number
- GB2119947A GB2119947A GB08309171A GB8309171A GB2119947A GB 2119947 A GB2119947 A GB 2119947A GB 08309171 A GB08309171 A GB 08309171A GB 8309171 A GB8309171 A GB 8309171A GB 2119947 A GB2119947 A GB 2119947A
- Authority
- GB
- United Kingdom
- Prior art keywords
- transducer
- bragg
- cut
- bragg cell
- block
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- GQYHUHYESMUTHG-UHFFFAOYSA-N lithium niobate Chemical compound [Li+].[O-][Nb](=O)=O GQYHUHYESMUTHG-UHFFFAOYSA-N 0.000 claims abstract description 16
- 239000013078 crystal Substances 0.000 claims abstract description 15
- 230000008878 coupling Effects 0.000 claims abstract description 11
- 238000010168 coupling process Methods 0.000 claims abstract description 11
- 238000005859 coupling reaction Methods 0.000 claims abstract description 11
- 229910052751 metal Inorganic materials 0.000 claims abstract description 9
- 239000002184 metal Substances 0.000 claims abstract description 9
- 239000010931 gold Substances 0.000 claims abstract description 7
- 229910052737 gold Inorganic materials 0.000 claims abstract description 7
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 claims abstract description 6
- 230000003287 optical effect Effects 0.000 claims description 16
- 229910052738 indium Inorganic materials 0.000 claims description 3
- APFVFJFRJDLVQX-UHFFFAOYSA-N indium atom Chemical compound [In] APFVFJFRJDLVQX-UHFFFAOYSA-N 0.000 claims description 3
- 238000001228 spectrum Methods 0.000 abstract description 9
- 239000000463 material Substances 0.000 description 3
- 238000010276 construction Methods 0.000 description 2
- 238000002044 microwave spectrum Methods 0.000 description 2
- 230000004044 response Effects 0.000 description 2
- 230000004075 alteration Effects 0.000 description 1
- 239000004411 aluminium Substances 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000000034 method Methods 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 230000000644 propagated effect Effects 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R23/00—Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
- G01R23/16—Spectrum analysis; Fourier analysis
- G01R23/17—Spectrum analysis; Fourier analysis with optical or acoustical auxiliary devices
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/11—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/11—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves
- G02F1/113—Circuit or control arrangements
Landscapes
- Physics & Mathematics (AREA)
- Nonlinear Science (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Mathematical Physics (AREA)
- Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
Abstract
A spectrum analyser incorporates a 36 DEG Y cut lithium niobate Bragg cell provided on one face with an interdigital transducer. The 36 DEG Y cut maximises the ratio of quasi-longitudinal to quasi- shear wave piezoelectric coupling with the interdigital transducer and thereby minimises the output of spurious signals from the analyser. In a preferred embodiment the Bragg cell comprises two lithium niobate crystals bonded by a metal (preferably gold) film, the crystal attached to the transducer being 36 DEG Y cut and the other crystal being cut so as to maximise acousto-optic coupling.
Description
SPECIFICATION
Bulk-wave acousto-optic device
This invention relates to bulk-wave acousto-optic devices and is particularly concerned with micro-wave spectrum analysers (such as those used in radar systems, for example) incorporating such devices.
Acousto-optic devices are known in which the microwave signal to be analysed is used to generate bulk or surface-propagated ultrasonic elastic waves (which will subsequently be referred to merely as acoustic waves) in piezoelectric material which are propagated across the path of a laser beam. The periodic alteration of refractive index within the piezoelectric material caused by the elastic waves causes Bragg deflection of the laser beam, which is split up into a multiplicity of beams whose intensity-deflection distribution corresponds to the spectrum of the microwave signal. Such acousto-optic devices are known as
Bragg cells.
An object of the present invention is to provide a Bragg cell microwave spectrum analyser having a higher bandwidth than previously used devices.
Atypical device in accordance with the invention has a centre frequency of 1 GHz and a 50% bandwidth.
A subsidiary object of the invention is to provide a microwave analyser of high piezoelectric efficiency and hence high sensitivity and or high signal-to-noise ratio.
According to the present invention a spectrum analyser incorporates a bulk wave Bragg cell provided on one face with an interdigital transducer, the crystal orientation of the Bragg cell being such that in use, substantially zero Bragg reflection occurs from quasi-shear waves in the crystal. Preferably said Bragg cell is substantially cuboidal and composed of lithium niobate cut in the 36"Y plane with the interdigital transducer fixed to a 36"Y cut face of the cell. We have found that such a construction maximises the ratio of quasi-longitudinal : quasi-shear wave piezo-electric coupling of the interdigital transducer, and thereby minimisesthe output of spurious optical signals in use of the analyser.
The use of an interdigital transducer in a Bragg cell has particular advantages, as is shown in detail below, since an interdigital transducer may be constructed so as to generate a beam of acoustic waves in the Bragg cell with a well defined divergence or "spread" so that Bragg reflection of the optical beam occurs over a wide range of Bragg angles. Since the Bragg angle varies linearly with acoustic frequency, such a construction results in a wide bandwidth.
It is well known that the acousto-optic coupling factor, which determines the efficiency of Bragg reflection by an acoustic wave, varies with crystal orientation in non-isotropic materials such as lithium niobate. The crystal orientation for maximum acousto-optic coupling does not in general correspond to that for maximum piezo-electric efficiency. Thus in order to obtain maximum overall efficiency, i.e. maximum Bragg-deflected light for each component of the signal fed to the interdigitai transducer, it is desirable to independently maximise the piezo-electric and acousto-optic coupling factors of the Bragg cell without inroducing any attenuation of the acoustic or optical beams.We have found that such an optimisation in performance can be achieved in a hybrid Bragg cell for a spectrum analyser in accordance with the invention, comprising an interdigital transducer fixed to a first crystal cut so as to substantially maximise piezo-electric coupling between said transducer and said first crystal, and a second crystal bonded by a thin metal bonding layer to a face of said first crystal and cut so as to substantially maximise acousto-optic coupling between an acoustic beam from said transducer and an optical beam directed through said second crystal in use of the anlyser.
By a "thin" metal bonding layer is meant a metal bonding layer having a thickness not greater than one Fm.
Preferably said bonding layer is less than one gsm in thickness. We have found that gold is a particularly suitable metal for bonding lithium niobate crystals in a hybrid Bragg cell, although indium is also suitable.
The invention will be understood in more detail on consideration of Figures 1 to 6 ofthe accompanying drawings, of which:
Figure lisa schematic plan view showing by way of example one particular monolithic Bragg cell in accordance with the invention;
Figure2 is a plot showing the variation with acoustic frequency of the Bragg and beam-steering angles of the Bragg cell of Figure 1;
Figure 3 is a sketch perspective view illustrating an interdigital transducer mounted on a 36"Y cut lithium niobate crystal for use in a spectrum analyser in accordance with the invention;
Figure 4 is a schematic plan view showing by way of example one particular spectrum analyser in accordance with the invention;;
Figure 5 is an elevation showing the face of the Bragg cell of Figure 4 opposite the interdigital transducer, and
Figure 6 is a plot showing a typical frequency response of a monolithic lithium niobate Bragg cell in accordance with the invention.
Referring first to Figure 1, which shows the geometrical conditions for Bragg reflection, the Bragg cell shown comprises a polished 36"Y cut cuboidal lithium niobate block 1 with an interdigital transducer 2
mounted on one face and an array 3 of photoelectric detectors of known type mounted on an adjacent face.
In use a microwave or other signal to be analysed is fed to the transducer 2, which generates a beam 5 of bulk elastic waves, having the same frequency spectrum as the microwave signal in lithium niobate block 1.
An optical beam 4 from a laser (not shown)is refracted at an angle of entry C into the face of block 1 opposite that on which the photoelectric detector array is mounted and is Bragg-reflected by acoustic beam 5, undergoing a deflection of 2OBE The deflected beams (S) are detected by detector array 3 and displayed after being amplified. e5, the Bragg angle, is given by the formula
where: X0 is the optical wavelength in free space of beam 4,
n is the refractive index of lithium niobate, f is the frequency of the acoustic beam, and
va is the phase velocity of the acoustic beam.
Thus for a given orientation of the acoustic beam (defined by the beam steering angle eA) the angle C-OB and hence the linear deflection of beam 4 along array 3 will vary linearly with frequency when OB is small, as shown in Figure 2.
The beam steering angle Za for longitudinal bulk acoustic waves is given by the formula
where d is the period (as shown in Figure 1) of the interdigital transducer. It may be seen (Figure 2) that the beam steering angle will vary in a non-linear manner with frequency over a range of about 3:1 in frequency.
From Figure 1 it may be seen that the overall angle of deflection (d of the optical beam 4 is given by =C-2HB and that C = OB + OA- Thus Bragg reflection will only occur when C - = (3A. It may be seen from Figure 2 that this condition is satisfied at only two frequencies F+ and F-, assuming that the acoustic beam has zero width. These frequencies may be varied by adjusting the angle of entry and or the finger spacing d, since
The bandwidth of the Bragg cell may be arranged to extend somewhat outside the range F- to F+ by arranging F- and F+ to be sufficiently close together that the maximum angular discrepancy AD between
C - ()s and HA iS less than the divergence of the acoustic beam.This discrepancy is a maximum at the centre frequency Fc of the cell, and is determined by setting
to give
It will be seen that the required width of the acoustic beam in Bragg cells in accordance with the invention may be determined particularly easily. Furthermore the design of interdigital transducers to produce acoustic beams of specified divergence is a relatively simple matter to those skilled in the art, since the divergence increases as the number of finger pairs in the transducer is decreased.
Figure 3 shows a generally cuboidal 36"Y cut lithium niobate block with an interdigital transducer formed on one face. The transducer was made from 0.1 Fm thick aluminium deposited by a standard photolithographic process. The transducer is shown greatly enlarged relative to the size of the block for the sake of clarity, and comprises two sets 7 and 8 of twelve connected interdigitating fingers. The acoustic aperture D4 was 0.3 mm and the period of the transducer was 303plum. The dimensions D1, D2 and D3 of the block were respectively 5,2 and 8 mm.
As can be seen by reference to the illustrated X, Y and Z axes, a 36"Y cut block was used, acoustic and optical wedges W1 and W2 of approximately 1" being cut from faces 36"Y and X respectively. These suppress unwanted acoustic and optical reflections. The cell was then mounted in a gold plated "dural" box and connected electrically by gold wire bonding one set of fingers to a 50Q stripline, and the other side to earth. A laser beam of wavelength 633 nM was shone into an X face of the block and the optical frequency response (i.e. the relative amount of Bragg-deflected light as a function of the frequency of signal fed to the transducer) plotted at the optimum angle of entry C1 of the laser beam into the block.The results are shown in Figure 6 and the main parameters are as follows:
centre frequency = 1150 MHz
3dB bandwidth = 495 MHz (43%)
ripple = 24%(-1dB).
In the light of these results we recommend a 12 finger pair transducer having a period of 380,am in order to obtain a 500 MHz bandwidth at a centre frequency of 1 GHz.
Figures 4 and 5 show a preferred embodiment of the invention, Figure 4 being on elevation on face F. The spectrum analyser shown comprises a laser 9, a Bragg cell 10 on which is mounted a photoelectric detector array 11 and an interdigital transducer 12. The Bragg cell is provided with acoustic wedges 6AW and an optical wedge ow of approximately 1 degree. Optical beam 4 is Bragg deflected by acoustic beam 5 from the transducer and split up according to the spectrum of the microwave signal fed to the transducer. Signals detected by the array 11 are amplified by amplifier 12 and displayed on display 13. The dimensions D5, D6 and D7 are suitably 10,5 and 4 mm respectively. The period and aperture of transducer 12 are suitably 380 and 4.6 mm respectively, to give a centre frequency of approximately 1GHz and a bandwidth of approximately 500 MHz.
Bragg cell 10 differs from that shown in Figure 3 in that it comprises two accurately machined blocks 13 and 14 of lithium niobate bonded together by O.3 m thick gold bonding layer 15. Block 13 is 126"Y cut (transducer 12 being formed on the 126"Y face), this being the cut for lithium niobate which optimises the piezo-electric coupling factor for quasi-longitudinal waves, and block 14 is X cut, so as to maximise the acousto-optic coupling with optical beam 4.
In some situations indium may be used instead of gold for the bonding layer, even though this metal has a much higher acoustic attenuation than gold.
Claims (9)
1. A bulk wave Bragg cell provided on one face with an interdigital transducer, the crystal orientation of said cell relative to said transducer being such that in use, substantially zero Bragg reflection from quasi shear waves occurs in the cell.
2. A substantially cuboidal bulk wave Bragg cell according to Claim 1 composed of lithium niobate cut in the 36 Y plane with the interdigital transducer fixed to a 36 Y cut face of the cell.
3. A bulk wave Bragg cell according to Claim 1 or Claim 2 wherein in use the divergence of the acoustic beam from said transducer is approximately equal to the difference between the angle of entry of the optical beam into the cell and the sum of the beam-steering and Bragg angles at the centre frequency of the ceil.
4. A hybrid Bragg cell according to any preceding Claim comprising a first substantially cuboidal block of lithium niobate on one face of which an acousto optic transducer is mounted and a second substantially cuboidal block of lithium niobate bonded with a metal bonding layer to said first block at a face opposite said transducer, wherein in use, an optical beam enters said second block, said first block being cut at an angle of approximately 36 Y with the transducer mounted on the 36 Y face and said second block being cut at such an angle that said optical beam exhibits maximum opto-acoustic coupling with the acoustic beam from said transducer.
5. A hybrid Bragg cell according to Claim 4wherein said metal is indium.
6. A hybrid Bragg cell according to Claim 4wherein said metal is gold.
7. A hybrid Bragg cell according to any of Claims 4, 5 or 6 wherein said layer is less than 1 am thick.
8. A Bragg cell substantially as described hereinabove with reference to Figure 3 of the accompanying drawings.
9. A Bragg cell substantially as described hereinabove with reference to Figures 4 and 5 of the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08309171A GB2119947B (en) | 1982-04-01 | 1983-04-05 | A cousto-optic device |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8209660 | 1982-04-01 | ||
| GB08309171A GB2119947B (en) | 1982-04-01 | 1983-04-05 | A cousto-optic device |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2119947A true GB2119947A (en) | 1983-11-23 |
| GB2119947B GB2119947B (en) | 1985-07-31 |
Family
ID=26282442
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08309171A Expired GB2119947B (en) | 1982-04-01 | 1983-04-05 | A cousto-optic device |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2119947B (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4759613A (en) * | 1985-08-19 | 1988-07-26 | U.S. Philips Corp. | Acousto-optic modulator |
| RU2193793C1 (en) * | 2002-05-23 | 2002-11-27 | ЗАО "Русское технологическое агентство" | Dual-coordinate scanner for coherent radiation in ultraviolet range (alternatives) |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1532250A (en) * | 1974-12-09 | 1978-11-15 | Matsushita Electric Industrial Co Ltd | Light deflector of acousto-optic interaction type |
| GB2014327A (en) * | 1978-02-03 | 1979-08-22 | Ceskoslovenska Akademie Ved | Acousto-optic device |
| GB2017954A (en) * | 1978-04-01 | 1979-10-10 | Ceskoslovenska Akademie Ved | Acousto-optic device |
-
1983
- 1983-04-05 GB GB08309171A patent/GB2119947B/en not_active Expired
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1532250A (en) * | 1974-12-09 | 1978-11-15 | Matsushita Electric Industrial Co Ltd | Light deflector of acousto-optic interaction type |
| GB2014327A (en) * | 1978-02-03 | 1979-08-22 | Ceskoslovenska Akademie Ved | Acousto-optic device |
| GB2017954A (en) * | 1978-04-01 | 1979-10-10 | Ceskoslovenska Akademie Ved | Acousto-optic device |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4759613A (en) * | 1985-08-19 | 1988-07-26 | U.S. Philips Corp. | Acousto-optic modulator |
| RU2193793C1 (en) * | 2002-05-23 | 2002-11-27 | ЗАО "Русское технологическое агентство" | Dual-coordinate scanner for coherent radiation in ultraviolet range (alternatives) |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2119947B (en) | 1985-07-31 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |