Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Patent
1992-06-11
1996-12-17
Wieder, Kenneth A.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
364822, G06G 719
Patent
active
055857140
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to radio-frequency spectrum analysers using optical rf modulators for analysing signals and in particular though not exclusively to spectrum analysers employing Bragg cells in resonant cavities.
2. Discussion of Prior Art
One method for spectrum analysis of an FM 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 acoustic waves present in the Bragg 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 Apr. 1988), have shown that the diffraction efficiency of a stationary grating may be improved by placing the grating in a resonant cavity. The Bragg cell may be placed 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 monochromatic light cannot be based on this configuration. Another possible arrangement utilises a four mirror confocal cavity where the round trip path 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.
SUMMARY OF THE INVENTION
The object of the invention is to provide a wideband resonant cavity spectrum 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 so as to provide multiple channel, broadband frequency response.
In an advantageous arrangement the spatial light modulating cell is an acousto-optic 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 cavity provides narrow-band lasing which enhances the Bragg cell efficiency. 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 a linear output array on the opposed side of the Bragg cell.
Advantageously each of the optical fibres is passed around a piezo-electric cylinder to keep the cavities tuned to their respective wavelengths. A fibre optic copuling arrangement may be used to couple light from each resonant optical fibre cavity to an output optical fibre connected to an optical detector. Advantegeously the optical detectors may be photodiodes. In an alternative arrangement a single dynamic cavity is provided by locating the Bragg cell between a totally relflecting mirror and a partially reflecting mirror and vibrating means connected to one mirror such that the tuned cavity scans a range of wavelengths. Advantageously 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.
Advantageously white light is used with a collimating lens being provided on 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.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be
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Dawber William
Flynn Colin J.
French Herbert A.
Maitland Arthur
Shaw Andrew P.
Solis Jose M.
The Secretary of State for Defence in Her Britannic Majesty's Go
Wieder Kenneth A.
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