Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1992-03-10
1994-02-15
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
250214R, H01J 4014
Patent
active
052869687
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and a device for multichannel analog detection.
Multichannel detectors such as arrays or matrices of photodiodes are developing very rapidly and are now in current use.
The methods of detection frequently used with single-channel detectors in order to improve their signal
oise ratio are often difficult to transpose directly to multichannel detectors. For this reason the development of a method and of a device for multichannel analog detection assumes great significance.
In particular, the method of synchronous detection is currently used with single-channel detectors. It gives complete satisfaction and its significance needs no further justification.
According to this method, which can be used equally well for spectrometers, dichrometers, polarimeters, ellipsometers, optical or scanning electron microscopes, interferential microscopes, etc., a source, most often a luminous source, is modulated. After detection, the modulated signal is amplified then multiplied by a reference whose mean value is zero. A low-pass filter followed by a DC amplifier makes it possible to obtain the information with a good noise rejection. A rejection rate of more than 90 decibels is thus frequently obtained.
Having regard to the significance of this method, an attempt has been made to transpose it for use in multichannel detectors.
To this end, a first solution consists in coupling as many synchronous detectors as there are elements in the multichannel detector. This solution requires the parallel reading of the specialized circuit and rapidly becomes impossible to employ for detectors comprising a large number of elements.
As an indication, multichannel detectors, which are currently widespread, are arrays or matrices of diodes incorporating from 256 to several million channels.
A second solution is to read each of the elements of the multichannel detector in series at a frequency which is higher than the modulation frequency, to digitize the information, to store it in various memories, and to process the signal thus derived using a programmed ADP device.
This solution is limited at present by the processing time of the digital memories and can only be employed with very low frequencies of modulation of the signal (a few Hertz). Moreover, the mean being found digitally, it is necessary to use analog-digital converters of very high resolution (greater than or equal to 12 bits) in order to obtain a good dynamic range and a reasonable speed of measurement. Such converters are very expensive.
Similar difficulties are encountered in the employment of heterodyne detections applied to the multichannel detectors.
The objective of the invention is thus to propose a method and a device for multichannel analog detection which are simple to employ, which improve the signal
oise ratio of the measured signal and exhibit a good dynamic range.
To this end, there is proposed a method for multichannel analog detection of a signal to be detected in which: of frequency f accompanied by noise; .PHI. producing an attenuated modulated signal; produces, for each point, a primary analog electrical signal; point P, a value V(P,.PHI.); .PHI., are read, digitized, stored; each of these values the operations which make it possible to store the values V(P,.PHI.) are repeated; digitally processed in such a way as to produce a data item representative of the amplitude and of the phase of S(P).
The invention also relates to a device for multichannel analog detection of a signal to be detected comprising: attenuated modulated signal; producing for each point a primary analog signal; the integration over N periods of the primary analog signal; given .PHI. value; successively where i is an integer varying from 1 to n; of S(P) from the values V(P,.PHI.).
The invention will be described in more detail with reference to the attached Figures in which: according to the invention. invention. the invention. of an application. device according to the invention. invention. light with which the detection device of the invention
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"Quantitative Surface topography determination by Nomarski reflection microscopy. I. Theory", Lessor et al., Opt Soc. America, vol. 69, No. 2. pp. 357-366.
Boccara Albert-Claude
Charbonnier Francois
Fournier Daniele
Gleyzes Philippe
Allen S. B.
Centre National de la Recherche Scientifique "CNRS"
Nelms David C.
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