Coherent light generators – Particular beam control device – Modulation
Patent
1991-12-12
1995-01-17
Davie, James W.
Coherent light generators
Particular beam control device
Modulation
356 43, H01S 330, G01B 902
Patent
active
053832078
DESCRIPTION:
BRIEF SUMMARY
This invention relates to arrangements for producing and detecting signal frequency shifts produced by stimulated Brillouin scattering (SBS), and more particularly to such arrangements which may have practical applications in for example distributed temperature sensing.
It is proposed herein to utilise stimulated Brillouin scattering as a sensing mechanism for distributed temperature sensors in which a laser beam is directed into an optical fibre, acting as a remote temperature sensing element, to generate a back-scattered SBS signal. In principle the temperature of the remote sensing element can be determined from the frequency shift .nu..sub.B of the back-scattered Brillouin line where: ##EQU1## where n is the refractive index, V.sub.A the hypersonic velocity, .lambda. the wavelength of the incident beam and .THETA. the scattering angle=180.degree. for full back-scattering.
In a typical monomode optical fibre, .nu..sub.B =30 GHz and d.nu..sub.B /dT=5.5 MHz/.degree.C., hence optical resolutions of 5 MHz are required for a 1.degree. C. temperature resolution in a distributed temperature sensor. Although resolutions of this order of magnitude are achievable with high contrast confocal scanning Fabry-Perot interferometers, absolute optical frequency measurements with this degree of accuracy are difficult to achieve over protracted periods.
An alternative approach for signal recovery commonly used in light scattering experiments, particularly where the line width of the scattered light is narrow, is optical heterodyning where the scattered light signal is mixed with the unshifted input laser beam. Light scattered from spontaneous propagating density fluctuations in the medium will be incoherent and the line width of the heterodyne signal will be broadened by the dissipative mechanisms in the medium (hypersonic attenuation in the case of the Brillouin scattering). On the other hand, SBS is a coherent process such that the backscattered light is only subject to random phase fluctuations. Thus if this signal is heterodyned with the input laser beam the observed line width should be extremely narrow.
In principle this heterdyned signal at about 30 GHz could be directly observed using a photodiode with a frequency response in excess of 30 GHz, together with a very high frequency microwave spectrum analyser. However, such a solution would be extremely expensive, thus effectively eliminating this approach for distributed temperature sensing.
It is therefore an object of the present invention to provide an arrangement for detecting signal frequency shifts produced by SBS, which is more effective and relatively inexpensive so that it may be used in practical applications such as distributed temperature sensing.
According to the present invention there is provided an arrangement for producing signal frequency shifts by stimulated Brillouin scattering characterised in that it comprises optical fibre means adapted to be illuminated by a laser beam so as to generate two back-scattered signals by stimulated Brillouin scattering and means for combining the two back-scattered signals to product a beat frequency from the combined signals.
As the frequency shifts of the back-scattered signals vary with temperature, the beat frequency, which occurs at a considerably lower frequency than that of the back-scattered signals, will likewise be temperature-variable.
In one embodiment of the invention, the arrangement is adapted for use as a distributed temperature sensor. To this end, the optical fibre means preferably consists of two separate optical fibres, one of which is maintained at a reference temperature and the other of which is used as a distributed temperature sensing element. The laser source may comprise a single frequency pulsed laser diode for simultaneously illuminating the two optical fibres. The back-scattered signals generated within the two optical fibres are combined and the resultant optical beat frequency signal, which is indicative of the differential temperature of the two fibres, is detected by a photodetect
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Culverhouse et al, "Stimulated Brillouin Scattering: A means to realise tunable microwave generator or distributed temperature sensor", Electronics Letters, vol. 25, No. 14, Jul. 1989, pp. 915-916.
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Culverhouse David
Farahi Faramarz
Jackson David A.
Pannell Christopher N.
Tatam Ralph P.
British Technology Group Limited
Davie James W.
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