Optical: systems and elements – Optical modulator – Light wave temporal modulation
Patent
1992-11-17
1995-12-12
Dzierzynski, Paul M.
Optical: systems and elements
Optical modulator
Light wave temporal modulation
359259, 359301, 372700, 25022712, G02F 103, G02B 2608
Patent
active
054755255
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a transverse electrical filter operating optically and more particularly to a programmable transverse filter using optical architecture.
This programmable transverse filter is more particularly applicable to the matched filtering of wideband signals.
The principle of the transverse filter is the summation of signals extracted with various delays and various weights, along a transmission line that is excited by the signal to be filtered. It is sought to determine, with the aid of such a linear filter, the time of appearance of a signal p(t) known a priori. This transient signal p(t) of finite duration T is in reality mixed with noise b(t) which is assumed to be independent of p(t). Thus, it is the signal x(t)=p(t)+b(t) which it is necessary to filter. When such a filter is produced and maximizes the signal-to-noise ratio at the instant T, it is said to be matched. In the case of ideal white noise, the impulse response h(t) of the matched filter is h(t)=p(-t). When the noise is not white, this filter is no longer optimal, however it nevertheless still enables the time of arrival of p(t) to be determined in most cases.
Matched filters are produced in the majority of cases by the weighting method such as described in the document: "J. Max--"Signal processing methods and techniques and applications to physical measurements"published by Masson, 1987. The signal x(t) is permanently injected into the input of a delay line consisting of N elements each giving a delay .tau. as shown in FIG. 1. It is furthermore assumed that there is sampling on N+1 points of the signal p(t):p(o), p(.tau.), p(N.tau.). The output from each element constituting the delay line is weighted by a coefficient .lambda..sub.k such that: p(t). We thus have -1<.lambda..sub.k <1. When at a given instant t.sub.o we form the sum of the N+1 weighted outputs, we obtain a signal Y(t.sub.o) such that: ##EQU1##
This is in fact the output of the matched filter at the instant t.sub.o -T.
The present technique makes it possible to produce such a device by numerical means but it is then limited to low and middle frequencies. Other solutions, based on optical fibers, have been proposed such as that described in the document K. P. Jackson--H. J. Shaw --"Fiber-optic delay-line signal processors" in Optical Signal Processing, edited by J. L. Horner, Academic Press, but they run up against the difficulty of producing a large number of coupling points.
The device according to the invention makes it possible to resolve these two problems, namely, it allows operation at high frequencies with a large number of coupling points and it therefore permits a large number of samples on signals at very high frequencies, typically N=1024 from 0 to 20 GHz.
The invention therefore relates to a transverse filter characterised in that it includes: signal to be filtered and retransmitting the optical wave modulated by the electrical signal, this retransmitted optical wave being linearly polarized; elementary channels; the elementary channels, each unit including as many delay creation circuits as there are elementary channels and each placed on an elementary channel; them onto a light intensity detector.
The various subjects and characteristics of the invention will emerge more clearly in the description which follows and in the attached figures which show:
FIG. 1, a system according to the known art already described;
FIG. 2, a general illustrative embodiment of a transverse filter according to the invention;
FIG. 3, a detailed illustrative embodiment of a transverse filter according to the invention;
FIG. 4, an illustrative embodiment of a unit for creating delays;
FIG. 5, a variant embodiment of a unit for creating delays;
FIG. 6, a variant embodiment of the means of detecting light intensity.
A general illustrative embodiment of a transverse filter according to the invention will therefore now be described with reference to FIG. 2.
This filter includes: electrical control signal x(t) which enables the beam to be modulated so as to provide a bea
REFERENCES:
patent: 3272988 (1966-09-01), Bloom et al.
patent: 4882235 (1988-03-01), Resler
patent: 5117237 (1992-05-01), Riza
patent: 5329118 (1994-07-01), Riza
Proceedings of the Society of Photo-Optical Instrumentation Engineers, vol. 176, 1979, pp. 17-27, Taylor, "Fiber and Integrated Optical Devices for Signal Processing".
Dolfi Daniel
Huignard Jean-Pierre
Tournois Pierre
"Thomson-CSF"
Collins Darryl S.
Dzierzynski Paul M.
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