Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1999-07-15
2000-08-08
Allen, Stephone B.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
356 285, G01B 902
Patent
active
061005169
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an optical device using a coherent detection, more particularly applied to velocity measurement and laser rangefinding. Other applications of the invention are e.g. the chemical analysis of gaseous constituents, the analysis of velocity ranges, contactfree metrology, laser imaging and medical instrumentation.
PRIOR ART
At optical wavelengths, coherent detection encounters various problems, particularly the problems of matching fields to the detector (phase matching and amplitude matching), as well as intensity and phase fluctuations of the optical signal caused by the interaction of the beam with disturbed media.
Various solutions are known for solving these first two problems.
Thus, for bringing about an optimum interference of the fields on the detector, various conditions have been established in the articles by A. E. Siegman entitled "The antenna properties of optical heterodyne receivers", published in Applied Optics, vol. 5, No. 10, October 1966, pp 1588-1594, S. C. Cohen, entitled "Heterodyne detection: phase front alignment, beam spot size, and detector uniformity", published in Applied Optics, vol. 14, No. 8, August 1975, pp 1953-1959 and St. Fowler et al, entitled "Analysis of heterodyne efficiency for coherent laser radars", published in SPIE, vol. 1936, 1993, pp 137-146. In particular, the alignment tolerances are given by the Siegman's theorem in the article by this author referred to hereinbefore:
A.sub.R is the equivalent reception surface and .OMEGA..sub.R is the solid detection angle. In general, the above relation is proved for extremely low values of .OMEGA..sub.R. This leads to particularly tight alignment tolerances, which partly explain the difficulties of implementing coherent detection at short wavelengths.
In order to get round the alignment problems, various procedures have been proposed in EP-164 181, U.S. Pat. No. 5,114,226 and U.S. Pat. No. 4,611,912.
In EP-164 181, use is made of an optical fibre device and a coupler for mixing the two beams. In U.S. Pat. No. 5,114,226 use is made of a modified form of the Michelson interferometer, which incorporates a reflex reflector, a polarizer cube and a .lambda./4 plate. In U.S. Pat. No. 4,611,912, an interferometer having a single measuring arm is implemented by introducing a partly reflecting plate into the measuring beam.
These complex arrangements have made it possible to produce industrial devices, which remain fragile, onerous and dimensions incompatible with a microsystem approach for large scale or collective manufacture.
Various proposals have been made for minimizing the effects of speckle. The theorem of Van Cittert Zernike supplies a criterion for dimensioning the reception pupil by expressing the coherence radius .rho.c at the pupil, as a function of the wavelength .lambda. and the apparent diameter .theta..sub.s of the laser spot on the target: ##EQU1##
This formula indicates that the optimum detection conditions are obtained when the emission and reception pupil have the same dimension and when the beam is focussed on the target. This approach is described in EP-164 181 and U.S. Pat. No. 5,114,226, the focussing distance being fixed by construction, which gives the device a field depth of a few metres around the focussing point. In order to increase the field depth, an alternative is to use a dynamic focussing system. The latter approach is prejudicial for two reasons, namely the low speed of the measurements and the complexity of the device. Mode filtering can be obtained by using monomode filters, but a major problem is constituted by the coupling difficulties. In U.S. Pat. No. 4,611,912, the measuring beam is collimated and a spatial filtering is used in the focal plane of the reception optics. This procedure is difficult to implement, it being difficult to install a very small diaphragm of approximately 10 .mu.m in the focus of the reception optics and significantly attenuates the signal.
DESCRIPTION OF THE INVENTION
The present invention proposes a device making it possi
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Analysis of Heterodyne Efficiency For Coherent Laser Radars, Stuart Fowler; SPIE vol. 1936 Applied Laser Radar Technology (1993) pp. 137-146.
Heterodyne Detection: phase front alignment, beam spot size, and detector uniformity, Steven C. Cohen; Applied Optics, vol. 14, No. 8 Aug. 1975, pp. 1953-1959.
The Antenna Properties of Optical Heterodyne Receivers, A. E. Siegman, Applied Optics vol. 5, No. 10, Oct. 1966 pp. 1588-1594.
Frequency-Modulated ND: Yag Micro Chip Lasers, J.J. Zayhowski and A. Mooradian, Optics Letters vol. 14, No. 12, Jun. 15, 1989.
Besesty Pascal
Chartier Germain
Molva Engin
Nerin Philippe
Allen Stephone B.
Commissariat a l'Eneriqie Atomigue
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