FABRY-PEROT CAVITY DEMODULATOR

Abstract

The invention relates to a Fabry-Pérot cavity demodulation device (40), comprising: - at least two laser sources (12, 14), one with a first wavelength (λ1), the other with a second wavelength (λ2), different from the first wavelength; - optical means (22, 24, 30, 32) for directing the laser beams from these at least two sources towards a Fabry-Pérot cavity (40) and for capturing the laser beams from these at least two sources that are reflected by this cavity; - at least one detector (52, 54) for receiving the intensities of the laser beams from these at least two sources that are reflected by this cavity; - computing means (70) for calculating the ratio of said intensities. Figure 2

Description

[0063] - or one is positioned on a maximum and the other on a minimum of the spectrum (case of the [Fig.5]).

[0064] Other combinations are conceivable within the framework of the invention, each laser peak remaining at a fixed wavelength; thus, when the spectrum of the cavity translates, the reflected amplitude for each laser evolves as shown in [Fig.6] for a configuration such as that of [Fig.5] (wavelength Xi initially on a minimum of the spectrum and wavelength X2 initially on a maximum of the spectrum).

[0065] A ratiometric measurement can then be carried out, according to which the ratio between the amplitude 1 (= Al, amplitude of the radiation at wavelength Xi reflected by the cavity) and the amplitude 2 (= A2, amplitude of the radiation at wavelength X2 reflected by the cavity) is measured, for example using means 70, as a function of the position of the spectrum and therefore of the length of the cavity 40. A simulation (on Matlab) of the ratio A1 / A2 of the powers of two laser peaks located at the level of the flanks as a function of the spectral shift AX (on abscissa, in m) of the cavity is presented in [Fig.7]: this curve is simulated on Matlab, using the theoretical two-wave interference formula of a Fabry-Pérot cavity; it is clearly seen on this curve that a variation of the ratio amplitude 1 / amplitude 2 (=A1 / A2) makes it possible to identify a spectral shift. The minimum value of the ratio shown in [Fig.7] is obtained for Xi on a minimum and X2 on a maximum (see for example in [Fig.5]) and the maximum ratio, equal to 1, is obtained for the case where Xi and X2 are on the flanks on either side of an extremum, at the same amplitude (as for example in [Fig.3]). .

[0066] The spectral shift allowing to go from a maximum ratio to a minimum ratio is (Xi - X2) / 2 (this is in particular the case if Xi and X2 are on the same half period, as for example in the case of [Fig.6]).

[0067] This ratio measure compensates for power fluctuations that may occur along the optical fiber 32, or more generally along the optical path followed by the laser beams, given that the latter travel the same optical path and have very close or fairly close wavelengths (for example the wavelengths are on the same half-period, as in figures 6, 8 and 9, which makes it possible to preserve the single-mode aspect of the signal).

[0068] The invention can implement more than two laser peaks, for example three laser peaks as illustrated in [Fig.8] and [Fig.9], the first laser beam or peak (at wavelength XJ having a reflected intensity A1), the second laser beam or peak (at wavelength X2, different from X1) having a reflected intensity A2, the third laser beam or peak (at wavelength X3, different from X1 and different from X2) having a reflected intensity A3:

[0069] - in [Fig.8], a first laser beam has its wavelength Xi on a rising front of the FP cavity spectrum, the second laser peak has its wavelength ^ positioned on a maximum and the third laser peak has its wavelength X3 on a falling edge;

[0070] - in [Fig.9], a laser beam has its wavelength Xisur at a minimum of the spectrum of the FP cavity, a second laser peak has its wavelength ^ positioned on a rising edge and a third laser peak has its wavelength X3 on a maximum of the spectrum.

[0071] In this case, the diagram in [Fig.2] includes a third laser source at wavelength X3, with a detector associated with this third source, which allows the amplitude A3 of the radiation at wavelength X3 to be measured, which is reflected by the cavity 40, and a circulator.

[0072] In this case, at least three ratio measures are possible: A1 / A2; - and / or A1 / A3; - and / or A2 / A3;

[0073] The means 70 mentioned above can collect data from the detector associated with the third source, as it collects data from the other detectors 52, 54. These means can also process this measurement data and calculate at least two or at least three of the ratios above.

[0074] These ratios allow for a more precise estimation of the position of the spectrum (and therefore of the cavity length) via the calculation of the average, for example.

[0075] This three-laser method allows for a more precise measurement.

[0076] In another embodiment, illustrated in [Fig. 10], only one detector 20 is used to measure the signals reflected by the cavity 40, whether the measurement involves two or three lasers. A circulator 18 receives the radiation from the cavity 40 and directs it to the detector 20. Means 70', for example a microcomputer programmed to process the measurement data according to the invention, collect the data from this detector 20.

[0077] In this case, to discriminate between the signals, one can: 1. trigger the lasers sequentially, for example using an optical switch; in this case, the amplitudes are measured, and, using the switching frequency between the different lasers, it is possible to associate, for example using means 70', the measured amplitude to the corresponding laser; thus, it is possible to identify the contribution of each laser in order to make the ratiometric measurement; 2. and / or modulate each laser onto a carrier wave having a specific frequency, for example using a chopper (or oscillating diaphragm); as illustrated in Figures 1IA and 1IB, the 80° wheel of a chopper (for example a The Thorlabs chopper allows, from an incident beam 84 ([Fig. 11 A]), of continuous intensity 84' ([Fig. IIB]), the formation of an output beam 86 ([Fig. 11 A]) whose intensity 86' ([Fig. IIB]) is chopped in time. Thus, by isolating the frequency of each laser, it is possible to identify, for example using the means 70', the contribution of each in order to perform the ratiometric measurement.

[0078] The invention is distinguished from state solutions by its simplicity and robustness.

Claims

Demands

1. A Fabry-Pérot cavity demodulation device (40), comprising: - at least two laser sources (12, 14), one at a first wavelength (Xi), the other at a second wavelength (X2), different from the first wavelength; - optical means (22, 24, 30, 32) for sending the laser beams from these at least two sources to a Fabry-Pérot cavity (40) and for collecting the laser beams from these at least two sources which are reflected by this cavity; - at least one detector (52, 54, 60) for receiving the intensities of the laser beams from these at least two sources which are reflected by this cavity; - computing means (70, 70') for calculating the ratio of said intensities.

2. Device according to claim 1, comprising at least one third laser source, at a third wavelength (X3) different from the first wavelength and the second wavelength and means for sending the laser beams from these at least three sources to the Fabry-Pérot cavity (40), and at least one detector for receiving at least the intensities of the laser beam from which is reflected by this cavity.

3. 3. Device according to claim 2, said calculation means (70, 70') being capable of calculating at least two of the ratios A1 / A2, A2 / A3, A1 / A3, where Ai (i=l,2 or 3) is the amplitude of the radiation at wavelength Xi reflected by the Fabry-Pérot cavity (40).

4. 4. Device according to any one of claims 1 to 3, comprising a detector (52, 54) associated with each laser source, for detecting the beam from that source which is reflected into the cavity.

5. 5. A device according to any one of claims 1 to 3, comprising a single detector (20) for detecting the beams of the different laser sources that are reflected in the cavity, and further comprising: - means for sequentially triggering the laser sources; - and / or means for modulating each laser onto a carrier having a specific frequency.

6. A method for demodulating a Fabry-Pérot cavity (40), comprising: - send to the cavity the beam from each of at least two laser sources (12, 14), one at a first wavelength (Xi), the other at a second wavelength (X2), different from the first wavelength; - detect the intensities (Al, A2) of the laser beams from these at least two laser sources reflected in the cavity; - calculate the ratio of said intensities.

7. A method according to claim 6, wherein the beam from at least a third laser source, at a third wavelength (X3) different from the first wavelength and the second wavelength, is sent to the cavity (40) and the intensity (A3) of a beam from this third source reflected into the cavity is detected.

8. A method according to any one of claims 6 to 7, wherein at least two of the ratios A1 / A2, A2 / A3, A1 / A3 are calculated, where Ai (i=l,2 or 3) is the amplitude of the radiation at wavelength Xi reflected by the cavity.

9. A method according to any one of claims 6 to 8, wherein a detector (52 54) is associated with each laser source, to detect the beam from that source which is reflected into the cavity.

10. A method according to any one of claims 6 to 8, wherein a single detector (60) detects the beams of the different laser sources which are reflected in the cavity, and wherein: - a sequential triggering of the laser sources is carried out; - and / or a modulation of each laser on a carrier having a specific frequency.

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