Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-01-26
2001-05-29
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S491000, C356S517000
Reexamination Certificate
active
06239877
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and to a device provided for interferential measurement of the phase shift between two light beams coming from the same polarized source by fine measurement of the displacement of the fringes of an interference pattern between two light beams, one of the beams undergoing phase variations due to variations in the refractive index thereof, such as applied refractometry.
2. Description of the Prior Art
Among the various known refractometer types, there is the working with a monochromatic source whose beam is divided so as to pass concurrently through two tanks, one for the reference liquid and the other for the medium to be analysed, and with means for gathering the two beams together and for lighting a photodetector. Interferences, occuring because of the variation of the optical path resulting from the index variation and the intensity variations due to the displacement of the interference fringes, are measured.
French Patent 2,596,526 illustrates an example of refractometric detector in which each one of the (reference and measuring) tanks is independently part of an interferometry system, both tanks being supplied with light by the same source. Detection is achieved by two independent photodetectors which thus receive each a luminous intensity which sinusoidally varies as a function of the difference in the refractive indices between the reference tank or the measuring tank on the one hand and the air on the other. Individual calibration of each photometer is thus necessary. This optical system comprises a piezoelectric element designed to vibrate a mirror on which part of the light beam coming from the source reflects.
French Patent 2,697,336 describes a modulated phase type refractometer in which two beams that have passed respectively through a reference cell and a cell containing a medium whose refractive index undergoes variations are caused to interfere with each other. The two beams are formed from a single beam emitted by a laser after passing through a Pockels cell to which a fast alternating voltage suited to oscillate the interference pattern is applied, such as a sawtooth signal (
FIG. 1
) whose frequency is of the order of several KHz and whose amplitude is sufficient to obtain a displacement greater than 1.5 fringe (FIG.
2
). The result on the interference pattern is the superposition of a slow variation linked with the composition variation of the medium and of a faster oscillating variation at the frequency of the alternating voltage. The intensity variations due to the fringe displacements are measured by a photometer and applied to electronics which converts the variations to logical signals and measurement of the phase shift between the cell control signal and the modulated intensity signals by counting by means of a clock signal. When the refractive index of the medium does not change, the output signal of the measuring phototransistor is that shown in FIG.
2
.
The previous system eliminates many sources of inaccuracy of the prior systems insofar as the two interfering beams both come from the Pockels cell and therefore have the same optical <<past>>. However, for several reasons notably linked with the method used for measuring time lags between signal fronts, it appears that the accuracy that can be obtained (several %) can be considered insufficient for certain applications, notably in the field of chromatography.
SUMMARY OF THE INVENTION
The interferential method according to the invention greatly improves the accuracy that can be obtained when measuring the phase shift between two light beams coming from the same polarized source, when one of the beams undergoes a relatively slow phase variation and one of the two beams is subjected to a relatively fast periodic phase modulation by a modulating signal. The method comprises fine measurement of the displacement of the fringes of an interference pattern formed by causing the two beams to interfere with each other, picked up by a photodetector sensitive to the luminous intensity variation resulting from the displacement of the interference fringes.
The method can be used in many fields, notably in order to determine for example refractive index variations concomitant with variations in the composition of a medium studied, in relation to a reference medium.
The method can be used notably in the field of high-performance liquid chromatography, referred to as HPLC, either analytical or preparative, where it is important to know the composition of mixtures with high precision.
The method comprises determining the frequency spectrum of the signal coming from the detector and measuring the phase shift (by calculating the complex argument thereof for example) that affects the fundamental frequency of this frequency spectrum.
The method can be advantageously applied to interferential refractometry operations where the slow phase modulation is the result of a refractive index variation of a medium.
According to an embodiment, the frequency spectrum is formed from a signal portion coming from the photodetector and included in a measuring window substantially equal (and preferably really equal) to at least one period of the modulating signal, and the period of the modulating signal is therefore preferably controlled by the duration selected for the measuring window.
The amplitude of the signal detected by the photodetector is also preferably adjusted so that the analog-to-digital converter always works at full scale despite possible transparency variations of the zell containing the medium studied.
In order to improve the accuracy of the results even further, the phase shift measurements can be performed separately on distinct fractions of the fast modulation signal.
The device according to the invention comprises an optical instrument including an optical system which generates two beams from the same light source, phase shifters which modulate one of the two beams where a relatively slow phase variation and which modulates one of the two beams with a relatively fast periodic phase modulation, a device which causes the two beams to interfere with each other and a photodetector for detecting the displacement of the fringes of the interference pattern formed from the two beams. The device comprises a system which determines the frequency spectrum of the signal coming from the photodetector and which measures the phase shift that affects the fundamental frequency of this frequency spectrum.
The device according to the invention can be applied to interferential refractometry operations. The phase-shifter which modulates one of the beams to a relatively slow phase modulation in relation to that of the other beam comprises at least one cell containing a medium whose refractive index varies (in correlation with variations in the composition thereof for example), the phase shifter for modulating one of the two beams with to a relatively fast phase modulation comprises a cell suited to dephase the light under the action of a periodic modulation voltage applied thereto, and the electronic system comprises a computer programmed to carry out a fast Fourier transform (FFIT) on the signals coming from the detector during a fixed time interval and to determine the phase shift that affects the fundamental component of the spectrum.
The device preferably comprises a means for permanently adjusting the period of the modulation voltage to the time interval duration, this means being for example a computers combining the respective signs of the harmonic phases and the sign of the fundamental frequency in order to generate a correction signal.
The device preferably comprises an automatic gain control which controls the amplification gain applied to the signals from the photodetector.
According to a particular embodiment, the device comprises an amplifier which amplifies the signal coming from the photodetector, a the amplified signal during an acquisition time interval, a signal generator which periodically controlsthe second
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Turner Samuel A.
LandOfFree
Polarized interferential measurement wherein the modulation... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polarized interferential measurement wherein the modulation..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarized interferential measurement wherein the modulation... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2470928