Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
1999-06-18
2001-01-30
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S028500
Reexamination Certificate
active
06181412
ABSTRACT:
BACKGROUND OF THE INVENTION
In conventional manner, such apparatus comprises an emission laser and a receiver that includes an edge filter together with means for processing on the basis of the power P
T
transmitted through the filter. One particular known use lies in incoherent Doppler wind Lidar (IDWL) for detecting wind.
Incoherent Lidar type systems suffer from technical problems associated with:
the low level of power received which is due specifically to the size of the receive telescope, with the upper limit on said size being determined by considerations of mass and expense, and correspondingly by effective use of the power received;
the alignment constraints on the numerous solid components as required by conventional systems; and
constraints on the mechanical stability of such equipment, in particular when on board a satellite.
In the particular case of an IDWL type Lidar, two additional problems can arise:
the effect of the return signal being subject to dispersion in the atmosphere, known as “speckle”; and
the various spectral characteristics of backscattering by aerosols and by molecules.
IDWL type Lidar systems measure the frequency displacement to which laser radiation backscattered by the atmosphere is subjected, and it does so by means of frequency-selective means which process the backscattered optical signal prior to detection. The differences between the various IDWL type Lidar systems lie essentially in the nature of the frequency-selective means.
It is known to use a frequency discriminator that associates a Fabry-Perot etalon (FPE) and an edge filter. Such a system is described in particular: in the article by C. L. Korb, B. M. Gentry, and C. Y. Weng, entitled “Edge technique: theory and application to Lidar measurement of atmospheric wind”, published in Applied Optics No. 31, 1992, pp. 4202-4213; in U.S. Pat. No. 5,216,477 (Korb); and in the article by B. M. Gentry and C. L. Korb entitled “Edge technique for high-accuracy Doppler velocimetry”, published in Applied Optics No. 33, 1994, pp. 5770-5777.
In that edge filter technique or “edge technique”, a shift in the frequency of the backscattered laser radiation is converted into a variation in the amplitude of the light that passes through the Fabry-Perot interferometer.
That technique suffers from the drawback of using only a portion of the backscattered power that it receives, and in addition, another portion of said received power is used for normalizing the signal. Unfortunately, as mentioned above, the power received is limited by the size of the receiving telescope.
It has been suggested that this power can be increased by replacing a single telescope which is heavy and expensive with a plurality of smaller telescopes that are lighter and less expensive. Reference can be made in particular to the article by S. Ishii et al., entitled “Optical fiber coupled multi-telescope Lidar system: application for a Rayleigh Lidar”, published in “Review of Scientific Instruments”, No. 67, 1996, pp. 3270-3273. However, the amount of power received remains quite low and the problem of optimizing use of the received power remains and no solution is found for it.
In known systems which incorporate an edge filter, it will be observed that the photodetection devices implement PIN diodes, avalanche photodiodes (APD), or indeed photomultiplier tubes.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to provide a Lidar type incoherent laser telescope system that enables better use to be made of the power that is received.
The invention thus provides an incoherent Doppler laser detection and ranging system of the Lidar type comprising an emission laser and a receiver device including at least one telescope, an edge filter, and processor means responsive to the power P
T
transmitted by the filter, wherein the processor means has data inputs relating to the power P
T
and to the power P
R
and outputs a normalized signal &Dgr;P
N
, where:
Δ
P
N
=
P
T
-
P
R
P
T
+
P
R
P
R
designating the power reflected by the edge filter.
In the system the processor means comprise first detector means for generating a current i
T
corresponding to the power P
T
and second detector means for generating a current i
R
corresponding to the power P
R
, and:
Δ
P
N
=
Δ
i
N
=
i
T
-
i
R
i
T
+
i
R
It is particularly advantageous for the receiver device to comprise an array of N individual telescopes.
In which case, the receiver device can comprise N branches, each of which has an optical fiber, each of which is coupled to the outlet of one of said N individual telescopes.
In a first variant, the optical fibers are monomode, and each branch includes an optical circulator each having a first outlet applied to an edge filter with a transmission outlet, and each having a second outlet constituting a reflection outlet. The transmission outlet of each edge filter can then be coupled to one of the N inlets of a first photodetector which outputs a current i
T
proportional to the transmitted power P
T
, and the second outlet of each optical circulator can be applied to one of the N inlets of a second photodetector which outputs a current i
R
proportional to the reflected power P
R
. Each branch may include a prefilter for filtering at least the molecular component of backscattering, the filter being disposed upstream from the corresponding optical circulator. It is preferable for each of the first and second photodetectors to include such a prefilter, thereby making it possible to implement only two filters, instead of N filters.
In a preferred embodiment of this variant, the transmission outlet of each of the edge filters is coupled to one of the N inlets of the optical scanner device which has N′ outlets applied to N′ inlets of a charge coupled device outputting a current i
T
proportional to the transmitted power P
T
, and the second outlet of each of the circulators is coupled to one of the N inlets of an optical scanner device which has N′ outlets applied to N′ inlets of a charge coupled device outputting at least one current i
R
proportional to the reflected power P
R
.
In a preferred variant, said monomode or multimode fibers are coupled to the inlet of a multimode optical fiber.
The outlet of the multimode optical fiber can then be coupled to a common optical circulator having both a first outlet applied to a common edge filter with a transmission outlet, and a second outlet which constitutes a reflection outlet. In a first embodiment, said transmission outlet is coupled to one of the N inlets of a third photodetector which outputs a current i
T
proportional to the transmitted power P
T
, and said second outlet of the optical circulator is applied to one of N inlets of a fourth photodetector which outputs a current i
R
proportional to the reflected power P
R
. Each of the third and fourth photodetectors can include a prefilter for filtering the backscattered molecular component. In a second embodiment, the transmission outlet of the edge filter is coupled to an optical scanner device which has N′ outlets applied to N′ inlets of a charge coupled device outputting a current i
T
proportional to the transmitted power P
T
, and the second outlet of said circulator is coupled to an optical scanner device having N′ outlets applied to N′ inlets of a charge coupled device outputting at least a current i
R
proportional to the reflected power P
R
.
REFERENCES:
patent: 5216477 (1993-06-01), Korb
patent: 5446280 (1995-08-01), Wang et al.
patent: 5790242 (1998-08-01), Stern et al.
patent: 5905576 (1999-05-01), Takada et al.
Fischer K.W. et al.: “Visible Wavelength Doppler Lidar for Measurement of Wind and Aerosol Profiles During Day and Night Optical Engineering”, Feb. 1, 1995, vol;. 34, No. 2, pp. 499-511—XPOOO490738.
Korb C.L., et al.: “Edge Technique Doppler Lidar Wind Measurements with High Vertical Resolution Applied Optics”, Aug. 20, 1997, vol. 36, No. 24, pp. 5976-5983, XP000699715.
Ishii S. et al.: “Op
Popescu Alexandru Florin
Winzer Peter Johannes
Agence Spatiale Europeene
Buczinski Stephen C.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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