A WIDE BANDWIDTH RF SPECTRUM ANALYSER

Abstract: A broadband electrical signal spectrum analyser comprises a spatial light modulator such as a Bragg cell (22) located within at least one resonant cavity and illuminated by a polychromatic source of light (20). The signal to be analysed is connected to the Bragg cell (22). The tuned cavity is arranged to lase the diffracted light to thereby enhance the signal before detection. In one arrangement the cavity is formed by opposed mirrors (23, 24) set at the Bragg angle and one of the mirrors is oscillated through a range of frequencies required by the broadband application. In a second arrangement a plurality of narrow band channels is formed by a fibre optic array. The fibres are arranged such that first ends of the fibres form a linear input array (41) and the second ends form a linear output array (42). Light at the Bragg angle passes into an appropriate one or more of the output array fibres, passes around the looped fibres to the input array where the light (zero order) is transmitted through the Bragg cell and then back to the fibre loop. A collimating lens (44) is provided on the input side of the Bragg cell and a Fourier transforming lens (45) is provided on the output side of the Bragg cell such that the position of the optical fibre in the array carrying the diffracted light is frequency dependent and the amount of light depends on the amplitude of that frequency in the electrical signal.

Description: &Hi Bgpwϊth EE Spggtrman&Lvser The invention relates to radio-frequency spectrumanalysersusingoptical rf modulators for analysing signals and inparticular thoughnot exclusively to spectrum analysers employing Bragg cells in resonant cavities. One method for spectrum analysis of an EM signal is to apply the signal to a Bragg cell and measure the optical diffraction effects produced by the interaction of laser light and the acousticwaves present intheBragg cell. The performance of Bragg cells in this application have the following performance limitations: a) limited frequency bandwidth at high diffraction efficiency; b) limited dynamic range; and c) significant material intermodulation effects. Steir et al. Applied Physics 27, No 8 (15 April 1988), have shown that the diffractionefficiencyof a stationary gratingmaybeimprovedbyplacingthe grating in a resonant cavity. The Bragg cell may beplaced in a two-mirror resonant cavity, with one mirror 100% reflecting and the other mirror partially reflecting and this results in enhanced Bragg cell diffraction. This arrangement can be used in a white light RF spectrum analyser. The inventor has shown, however, that a practical spectrum analyser using monochromaticlightcannotbebasedonthisconfiguration. Anotherpossible arrangement utilises a four mirror confocal cavitywhere the round trippath length for a ray of light remains constant for small changes in the Bragg angle. These arrangements have been shown to have limited bandwidth and resolution. Theobject of the invention is toprovideawideband resonantcavityspectrum analyser which overcomes problems associated with known arrangements. The invention provides a broadband electrical signal spectrum analyser comprising: a spatial light modulating cell; at least one resonant cavity in which the modulating cell is located; a source of light for providing a beam to illuminate the cavity; and means to connect the signal to the modulating cell; the arrangement being such that the cavity can be tuned to different wavelengths soas toprovidemultiplechannel, broadbandfrequencyresponse. In an advantageous arrangement the spatial light modulating cell is an acousto-σptic Bragg cell. Other arrangements are envisaged, however, in which equivalent diffraction occurs, as for example by changes of polarisation induced by the applied signal to be analysed. Advantageously an optical gain medium may be present such that each tuned cavityprovides narrow-bandlasingwhich enhancestheBraggcellefficiency. In one arrangement a plurality of fixed resonant cavities is formed by providing a fibre optic array. Advantageously the fibres of the array are arranged such that a linear array of input ends is formed on one side of the Bragg cell and the respective other ends of the optical fibres form alinear output array on the opposed side of the Bragg cell. Advantageously each of the optical fibres is passed around a piezo-electric cylinder tokeepthecavitiestunedtotheirrespectivewavelengths. Afibre optic coupling arrangement may be used to couple light from each resonant optical fibre cavity to an output optical fibre connected to an optical detector. Advantageously the optical detectors may be photodiodes. Inanalternativearrangementasingledynamiccavityisprovidedbylocating theBraggcell betweena totallyreflectingmirrorandapartiallyreflecting mirror andvibratingmeans connectedtoonemirror such that thetunedcavity scans a range ofwavelengths. Advantageouslyaslit isprovidedinthe image plane behind the partially transparentmirror and a detector is arranged to detect light passing through the slit. Advantageouslywhite light is usedwithacollimating lensbeingprovidedon one input side of the Bragg cell and a Fourier transforming lens being provided on the output side of the Bragg cell. Advantageously a high "Q" Bragg cell is used. The inventionwill nowbe describedbyway of example onlywith reference to the accompanying Drawings of which: Figure 1 shows a known laser RF signal spectrum analyser; Figure 2 shows a white light RF spectrum analyser using cavity enhanced diffraction; Figure 3 illustrates a four mirror confocal cavity configuration of the invention; Figure 4 shows a schematic diagram of an alternative RF spectrum analyser using a fibre optic array; and Figure 5 shows in greater detail one fibre optic channel of the Figure 3 arrangement. As shewn inFigure1, in a typical acouεto-optic spectrum analyser, light from a laser 10 is collimated (11) by an arrangement of lenses12,13 with a narrow slit (not shown) between toprovide a broadbeam incident onanacouεto-optic Bragg cell 14. Aradio frequency (RF) signal tobe analysed is coupled to the Bragg cell by a transducer 15, producing propagating acoustic waves within theBragg cell. Thequasi-monochromatic light incident on theBragg cell 14 interacts with the acoustic field therein to produce a diffraction pattern which is examined in the focal plane of a Fourier transforming lens 16 by an array of detectors 17. A stop 18 is provided to block undiffracted zero- order light. TheBragg cell 14 ispositioned such that theangleof incidence of the laser light is fixed. As the frequency of the acoustic wave in the Bragg cell changes, the position of the light focusedby lens16 alsochanges. Thus the outputs from the array17 canbeused toanalyse the F signal applied to the Bragg cell. " When used in thisway, there is poor frequency response and theperformance is limited by inteπt dulεtioneffects. Inte rodulation effects arise because of three effects: a) Hecht inteπrodulation due to light being diffracted more than once by the acoustic signal; b) material intermodulation due to the non-linear factors in the interaction equation and the breakdown of linearity at high acoustic pcwer densities; and c) amplifier intermodulations due to the large gains often required in practical applications. In order to improve thebandwidth of theRF spectrum analyser, polychromatic white light sourceshave been tried. In one arrangement collimated light is incident on the Bragg cell. The diffracted light at one output angle of diffraction (equal to the input angle) is monitored with an optical spectrometer. As the radio-frequency in the cell changes, the wavelengths of the light incident on the spectrometer changes. There is always an optical wavelength which satisfies the Bragg condition for diffraction at a given radio-frequency and hence the bandwidth is limited only by the transducer and polychromatic source bandwidths. However, in practice, the resolution of the spectrometer is limited by the degree of collimation of the incident light we have: where λ is thewavelength, θ is theBragg angle, Sλ is the optical bandwidth and S θ is the degree of collimation. When using an arc lampas thepolychromatic source, the beam is collimatedby focusing it onto a slit. The angular width subtended by the slit at the collimating lens system determines the degree of collimation. The resolution achievable is inversely related to the slit width, however the narrower the slit the lower the intensity of the light. So we seek a trade¬ off between resolution and intensity reaching the detectors. The inventors have shown that a high "G" transducer 15 is likely to reduce intermodulation effects (and thereby increase the dynamic range) but with a penalty of a reduction in the monochromatic frequency bandwidth, a penalty which may be overcome by, for example, using polychromatic illumination of the Bragg cell. Figure 2 shows an RF spectrum analyser according to the invention where a Bragg cell is placed in a tunable resonant cavitywhichnayhave gain. Light from an arc lamp 20 after collimation (21) is incident at angleθ on aBragg cell 22. The cell 22 is located in a lasing cavity defined by a 100% reflectingmirror 23 and apartially reflectingmirror24. Lightdiffracted at an angleθto thenormal is reflectedbackandforthbetween themirrors23 and 24. Light which is transmittedby thepartially reflecting mirror 24 is focused by alens25 toa slit in a screenwhere it isviewedby a detector 27. In this arrangement thewavelength of the diffracted light reaching the slit in the screen 26 is inversely proportional to the frequency applied to the Bragg cell 22. The mirror 23 is vibrated with an amplitude sufficient to change the cavity resonance frequency through the required spectral range. Thewavelengthwhich isdiffracted tothe slitwill thenbeon resonance in the cavity twice during each cycle. The light detected by the detector 27 is a series of pulses of amplitudeproportional to the amplitude of the F applied to the cell 22 for low diffraction efficiencies. It has been found that the cell 22 starts to saturate for cell efficiencies above about 5% and thus the usable dynamic range is 1imited to input RF powers less than thosewhich give greater than 5% cell efficiencies. Figure 3 shews an alternative arrangement inwhich aBragg cell 30 is located within a four mirror (31 - 34) confocal cavity. Incident radiation Ei n , iε diffracted by the Bragg cell (E α ) and is then partially transmitted by the firstmirror31 toprovideoutput radiationE^.. RadiationE3, reflectedat the mirror 31, is reflected via mirrors 32 - 34 back to mirror 31 where it partially supplements the initially transmitted radiation. For small changes in the Bragg angle the round trip path length remains constant in this cavity. If gain is introduced into the cavity then laεing occurs when the round trip gain equals the round trip loss. The usable bandwidth of the combined cavity andBraggcell islimited, formonochromatic systems, by the bandwidth of the cavity with the cell in it. This is given, approximately, by: /Ntf = c 1 / L where f is the round trip cavity loss andL is the optical path length for the round trip. For small diffraction efficiencies the output intensity, lout, varies linearly with the diffraction from the cell 30 and the dynamic range of the cell/cavity combination is similar to that of the cell alone. The angular resolution of the cavity/cell combination is approximately the same as that of the cell alone. Toavoidtheproblemsoflimitedbandwidthand resolution imposedby the above monochromatic systems, a multiple cavity system could be used. For each frequency channel, a different cavity is used, tuned to its respective wavelength. Figure 4 illustrateε an arrangement of the invention having a single Bragg cell 40 located within a plurality of fixed resonant cavities formed by a fibre optic array; each fibre forming oneoptical cavity. Fibres 1 to n of a fibre optic array are arranged such that one set of ends forms a linear input array41 at the optical input side of theBragg cell 40 while the other ends of the fibres are arranged to form a linear output array 42 on the optical output side of theBragg cell. Light 43 froma monochromatic source is incident on theBragg cell 40 after transmission throughacollimating lens 44. AFourier transformlens45 focuses diffracted light towards the output array of optical fibres 1 to n. After Fourier transformation the spatial position in the image plane of theFTlens 45 corresponds to the frequency of thelight emerging fromtheBragg cell 40. As in thepreviousarrangementan electrical signal tobe analysed is connected to theBragg cell such that the acoustic field formed therein interactswiththe incidentlight43 toproduce diffracted light dependent on the amplitude and frequency of the spectral composition of the electrical signal. Figure 5 shows one monccrode fibre optic channel 50 in greater detail. Input light impingeson theBragg cell 40 at an angle of incidenceθand thelinear input and output arrays 41 and42 arearranged toreceive/transmit diffracted light at a range of diffraction anglesό. As shown, diffractedlight enters the end 51 of the optical fibre 50 in the output array 42. It is then transmitted through the optical fibre via a fibre optic coupling 52 to the opposed end 53 in the input array41. Diffracted light emerging fromtheend 53 of the optical fibre is incident on theBragg cell and thezeroth order is transmitted linearly through the Bragg cell to enhance the diffracted radiation entering the end 42 of the fibre 50. In this manner diffracted light iscirculated repeatedlyaroundthefibre50 andthroughtheBraggcell. Aproportion of the diffracted light is coupled to an outputmonomodeoptical fibre 54 in the coupling 52. The repeated transmission of the diffracted light through theBragg cell enhances the signal receivedby theoutputfibre 54. A photodiode 55 is provided to measure light in the output fibre 54. The optical fibre 50 thus forms one fibre ring cavitywhere the cavity length can be tuned by appropriate selection of the length of the optical fibre50. The optical fibre is passed around apiezo-electric cylinder 56 toenablethe length andhence the tuning of the ringtobemaintainedwithinnarrowlimits. In like manner each optical fibre in the array is tuned so as to provide n frequency channels with appropriately tuned resonant cavities. The full width of theBragg cell 40 can be illuminated, so that with this arrangement the resolution and the bandwidth are maintained whilst still retaining the diffraction efficiency of the cavity. The use of enhanced Bragg cell diffraction should allow large reductions in inter odulation by reducing the acoustic power densities requiredwithin the cell. In addition, the invention provides a practical white light system with a very high resolution and large bandwidthwithout the usual problems associated with extremely low light levels. Although the invention has been described in relation to the acousto-optic effect in a Bragg cell to produce diffraction of transmitted light other spatiallightmodulators τaa be usedinthemannertaughtbythisinventionso astoproducebroadbandperformance. Onesuchpossibilityistomakeuseofa cell in which the polarisation of a transmitted beam can bemodulated by an applied rf signal.

Claims: Claims 1. A broadband electrical signal spectrum analyser comprising: a spatial light modulating cell, a source of light providing a beam to illuminate the modulating cell, and means to connect the signal to the modulating cell characterised in that: there is providedatleast one resonantcavityinwhichthemodulatingcell is located; the arrangement being such that the cavity can be tuned to different wavelengths soastoprovidemultiplechannel, broadbandfrequencyresponse. 2. A broadband electrical signal spectrum analyser as claimed in claim 1 characterised in that the spatial light modulating cell is an acousto-optic Bragg cell. 3. Abroadbandelectrical signal spectrumanalyser asclaimedinclaim1 or2 characterised in thatan optical gainmedium is present such that each tuned cavity provides narrow-band lasing which enhances the modulating cell efficiency. . Abroadband electrical signal spectrumanalyser asclaimed inanyone of claims 1 to3 characterised in that apluralityof fixed resonant cavities is formed fcy providing a fibre optic array. 5. A broadband electrical signal spectrum analyser as claimed in claim 4 characterised in that thefibresof thearray arearranged such thatalinear array of input ads is formed on one side of the modulating cell and the respective other ends of theoptical fibres formalinear output array on the opposed side of the modulating cell. 6. A broadband electrical signal spectrum analyser as claimed in claim 5 characterised in that each of the optical fibres is passed around a piezo¬ electriccylinder tokeepthecavitiestunedtotheir respectivewavelengths. 7. A broadband electrical signal spectrum analyser as claimed in claim 6 characterised in that a fibre optic coupling arrangement is used to couple light from each resonant optical fibre cavity to an output optical fibre connected to an optical detector. 8. A broadband electrical signal spectrum analyser as claimed in claim 7 characterised in that the optical detectors are photodiodeε. 9. A broadband electrical signal spectrum analyser as claimed in claim 1 characterised in that a single dynamic cavily is provided by locating the modulating cell between a totally reflecting mirror and a partially reflecting mirror and avibrating means is connected toonemirror such that the tuned cavity scans a range of wavelengths. 10. A broadband electrical signal spectrum analyser as claimed in claim 9 characterised in that a slit is provided in the image plane behind the partially transparent mirror and a detector is arranged to detect light passing through the slit. 11. A broadband electrical signal spectrum analyser as claimed in claim 10 characterised in that white light is used with a cdlimating lens being provided on one input side of themodulating cell andaFourier transforming lens being provided on the output side of the modulating cell. 12. A broadband electrical signal spectrum analyser as claimed in claim 11 characterised in that the modulating cell has a high "Q" value.

Inventors: DAWBER WILLIAM, FLYNN COLIN JAMES, FRENCH HERBERT ALAN, MAITLAND ARTHUR, SHAW ANDREW PETER, DAWBER, WILLIAM, FLYNN, COLIN, JAMES, FRENCH, HERBERT, ALAN, MAITLAND, ARTHUR, SHAW, ANDREW, PETER
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1990
Main classification: G01R23
IPC codes: G01R23