EP1794556A1

EP1794556A1 - Coherence spectrometry device

Info

Publication number: EP1794556A1
Application number: EP05807814A
Authority: EP
Inventors: Lionel Canioni; Stéphane SANTRAN; Bruno Bousquet
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2004-10-01
Filing date: 2005-09-30
Publication date: 2007-06-13
Also published as: US7701583B2; US20080117435A1; FR2876182A1; JP2008514945A; FR2876182B1; WO2006037880A1

Abstract

The invention concerns the field of spectrometers. More particularly, the present invention concerns a spectrometer for distinguishing background noise from useful lines in spectrometric detection. It concerns a spectrometric device characterized in that it comprises at least one wavefront-dividing interferometer including at least two unbalanced arms and at least one air wedge, a device for imaging interference fringes, an imaging sensor of said fringes and means for processing a signal derived from said sensor. The invention also concerns a combination of such devices, the imbalances associated with the arms of each of the interferometers being all different.

Description

SPECTROMETRIC DEVICE FOR COHERENCE

The present invention relates to the field of spectrometers.

The present invention relates more particularly to a spectrometer for distinguishing background noise from lines useful in spectrometric detection.

One of the problems solved by the present invention can be found in the field of fluorescence detection created by a plasma. Indeed, a material, whether in solid, liquid or gaseous form, can, after excitation by a laser pulse or any other excitation system, be transformed into plasma (mixture of free electrons, ions, atoms and molecules). If the excitation of the material is important enough, other well-known physical phenomena come into play such as cascading ionizations and collisions between free electrons. These effects increase the temperature of the plasma produced. The braking radiation of electrons in motion (inverse Brehmsstrahlung effect) then gives a white light emitted by the plasma. The analysis of the radiative deexcitation of the atoms and ions then makes it possible to go back to the composition of these via a spectral analysis of the light emitted by the plasma.

The detection and collection of the fluorescence are carried out according to the prior art with an optical fiber placed at the level of the plasma plume. The spectrometers are then based on an optical design based on a diffraction ladder grating, an adjustable entry slot, and spherical mirrors for combining the object and image points on a CCD camera. A reading software then makes it possible to display the spectrum of light entering the device and this whatever the temporal or spatial coherence of the light. Thus, incoherent light, emitted for example by radiation from Brehmsstrahlung can distort the measurements and cause inaccuracies on the decisions. This method of "sorting" the emission spectrum among the incoherent white light is not satisfactory.

Several prior art documents disclose spectrometric methods by interferometry, such as the Canadian application CA 2 302 994, but none of these documents sought to solve the problem of incoherent white light.

The prior art also knows a NASA article published in the journal "Optics and Photonics News" in

January 2004. This article describes a Fourier Transform spectrometer based on an interferometer of

Michelson modified by replacing the mirrors by diffraction gratings. Such a spectrometer has a very high robustness to accelerations, but does not consider the problem of white light noise for the detection of atomic lines.

The present invention intends to overcome the drawbacks of the prior art by proposing a time coherence spectrometric device.

To do this, the present invention is remarkable, in its broadest sense, in that it relates to a spectrometry device characterized in that it comprises at least one wavefront separating interferometer comprising at least two unbalanced arms. and at least one air wedge, a fringe imaging device interference, an imaging sensor of said fringes and a signal processing means from said sensor.

Note that in general, such a spectrometer for separating the incoherent light from the coherent light will be called "coherence spectrometer" (15).

The imbalance introduced between the arms of the interferometer makes it possible to take into account the temporal coherence and therefore the elimination of the noise introduced by the incoherent white light.

Note that for some applications, large spectral resolutions are needed. A coherence spectrometer does not allow a sufficient spectral resolution, because of the number of pixels of the CCD, and the size of the optics. A particular combination of several coherence spectrometers according to the present invention makes it possible to increase the resolution.

The invention will be better understood by means of the description, given below for purely explanatory purposes, of an embodiment of the invention, with reference to the appended figures in which: FIG. spectrometer according to the invention;

FIG. 1B shows a second possible embodiment of the spectrometer according to the invention; FIG. 2 represents a third possible embodiment of the spectrometer according to the invention;

FIG. 3 represents an embodiment of the spectrometer according to the invention in a compacted form;

FIG. 4 represents a second embodiment of the spectrometer according to the invention in a compacted form; FIG. 5 represents a possible embodiment of the spectrometer according to the invention in a Mach-Zhender form; FIG. 6 represents the general diagram of the association of three spectrometers;

FIG. 7 represents an experimental example of the evolution of the resolution as a function of the number of coherence spectrometers on a line of the neon helium laser.

Illustrated in FIG. 1A, the spectrometer according to the invention is for example implemented at the output of an optical fiber (1) and a collimator mirror (2) for line detection comprising an incoherent white light noise.

For example, it uses a Michelson interferometer with an air wedge.

It comprises in known manner a separator (4) and a compensator (5), a slightly inclined cylindrical mirror (7) and a cylindrical mirror not inclined (6).

It should be understood that any interferometric device may be used for the present invention if it is possible to introduce an imbalance into the arms thereof. In particular, those skilled in the art may use Mach-Zender type devices.

More precisely, in the case of Michelson, the characteristics of the elements described above are as follows: the separator (4) is a 50/50 splitter over a very large spectral width. Note that this is possible using a metal treatment used for neutral metal filters that allow a transmission of (50% ± 5%) from 200nm to more than 1μm; the separator (4) and the compensator (5) have a very good surface quality, so as not to deteriorate the spatial quality of the beams, which would scramble the interference fringes; the thicknesses of the separators and compensators are as small as possible (for example of the order of a millimeter) in order to limit the variations in the shift of the beams passing through them as a function of the wavelength.

The two arms of the Michelson, consisting of segments (cylindrical mirror (β) -separator) and (cylindrical mirror (7) -separator) are unbalanced, that is to say that the lengths of these two segments are not equal. . This causes a non-zero difference in path between the rays emitted by the two arms. The Michelson used is therefore a wavefront separator.

This difference in operation makes it possible to eliminate the interference of any incoherent light (for example emitted by the Brehmsstrahlung effect). Indeed, the difference between the rays resulting from the delay between the two arms of the interferometer simulates two sources emitting incoherently: these sources do not interfere.

This imbalance is adjustable to be able to adapt the level of coherence to the measurement without damaging the interference.

The mirror (7) is inclined at about 0.2 degrees, forming a wedge of air between the two beams that interfere. Parallel interference fringes are then obtained and visualized on a large CCD detector (9).

The CCD sensor can be chosen in one direction, for example a line of 8000 pixels, and the representation of the interframes is obtained firstly thanks to a cylindrical doublet lens (8) in one direction to image the fringes, and cylindrical mirrors (6) and (7) in the other direction to focus the beam along a line.

The image obtained on the CCD sensor is then subjected to a mathematical treatment for the recovery of a fluorescence spectrum or more generally of a set of lines. Those skilled in the art know such algorithms in the field of Fourier transform spectroscopy. These algorithms use the size and contrast of the interference fringes obtained to determine the composition of the emission spectrum.

Note also that the quality of the CCD sensor is important so that it is not saturated by the incoherent white light received. The typical spectral resolution obtained using a spectrometer is of the order of 1000 with a 6000 pixel sensor.

At the output of the spectrometer according to the invention, and if the sensor does not saturate, it is then possible to determine the composition of the analyzed material.

Finally, it should be noted that the elimination of the incoherent light by the above device greatly facilitates the detection since all the incoherent white light is included in the peak of zero frequency. This is then eliminated by frequency processing. FIG. 1B shows an embodiment of the invention in which the cylindrical mirrors (6) and (7) for focussing the beam in the CCD sensor (9) are replaced in a cylindrical lens (8 '). In place of the mirrors (6) and (7), are then placed a non-inclined plane mirror (12) and an inclined plane mirror (13).

FIG. 2 represents an embodiment of the invention in which the separators and compensators are grouped into a single element (4 ').

Illustrated Figure 3, a coherence spectrometer in a particularly compact form is proposed. According to this embodiment, the light beam is separated and recombined in bonded fused silica prisms (IIa) and (IIb), which enhances the stability of the interferometric system. The plane mirrors (12) and (13) introduce the imbalance of the arms and the air wedge necessary for the invention and the cylindrical lenses (8) and (8 ') make it possible to image the interference fringes and the focus for example on a line for capturing on a CCD camera (9). The spectral analysis of the fringes then makes it possible to find the spectrum without being disturbed by the incoherent light.

FIG. 4 represents another mode of implementation of the invention with possibly two CCD sensors for the detection of interframes and a double separation of the beams at the level of the fused silica prism.

FIG. 5 illustrates an embodiment of the invention in the case of a Mach-Zender type interferometer. In this type of interferometer, the difference in between the two arms of the interferometer is obtained by the introduction of mirrors 13A, 13B, 13C and 13D. The separator 4 '' then acts as an air wedge for the observation of interference at the sensor 9.

It is understood that the embodiments presented above are only given by way of example and that many other industrializable embodiments of the invention are possible to achieve at least two unbalanced arms associated with a corner of the air.

Finally, note that this spectroscopic device can be very particularly used in fluorescence detection involving the inverse Brehmsstrahlung effect producing an incoherent white light by thermal agitation.

In this case, the device is installed as in Figure IA output of an optical fiber (1) for the transport of fluorescence, after a collimator mirror (2).

In order to improve the resolution of the measuring instrument, it is also possible, according to a second aspect of the invention, to associate a plurality of time coherence spectrometers as previously described.

Indeed, the use of a coherence spectrometer as described above only makes it possible to acquire a part of the interferogram corresponding to the interference fringes recorded on the CCD sensor. The combination of several spectrometers of different delays judiciously chosen makes it possible to acquire several parts of the interferogram, and increase as much as desired the resolution of the instrument.

Illustrated in FIG. 6, the spectrometer according to the invention consists for example of a divisible optical fiber (1 ') placed at the level of the plasma emission (10) which illuminates in parallel a plurality of coherence spectrometers (15) having imbalance of the optical paths between the arms different from each other.

He uses, for example, a Michelson interferometer with an air wedge for weak imbalances, and a Mach-Zhender interferometer for large imbalances.

Indeed, large arm length imbalances can cause problems on a Michelson interferometer, because of the depth of field limitation on imaging of the interference fringes.

The method of obtaining the spectrum from the acquisition of several interferograms can be explained in the following manner.

The algorithm for obtaining the spectrum is based on the Fourier transform. For a simple interferogram obtained using a single coherence interferometer, the calculation of the spectrum is well known from the prior art, and is done by simple Fourier transform followed by calculation of the magnitude. In the case of two coherence coherence spectrometers 15A, 15B, the interferograms obtained are SC (τ) and

SC (2τ), these interferograms corresponding to two coherence spectrometers of respective imbalances τ and

2τ. We first calculate the Fourier transform complex of the first interferogram FFT (SC (τ)), then the complex Fourier transform of the sum of the two interferograms FFT (SC (τ) + SC (2τ)). The spectrum resulting from the combination of the two coherence interferometers is finally obtained by calculating the magnitude of the multiplication of the preceding Fourier transforms: ABS [FFT (SC (T)) * FFT (SC (τ) + SC (2t) )].

For a number of N coherence spectrometers (15A, 15B, 15C), the procedure is recursive. The imbalance between the arms of each added coherence spectrometer is double the previous one. Thus, the interferogram of the third interferometer is written: SC (4τ). For example, the spectrum for three coherence spectrometers will be obtained using the formula:

ABS [FFT (SC (t)) * FFT (SC (t) + SC (2t)) * FFT (SC (T) + SC (2t) + SC (4τ))].

As an indication, this system of spectrometer combinations can be compared to the aperture synthesis used in astronomy. In this case, the spectrometers cover different ranges of optical delays, while in astronomy, the telescope mirrors cover different spatial openings.

In order to improve the spectroscopic results, it is preferable, even when combining several coherence spectrometers to have the CCD sensors with the most pixels possible. This also makes it possible to limit the number of coherence spectrometers.

FIG. 7 represents an experimental example of the evolution of the resolution as a function of the number of coherence spectrometers on a helium laser line. neon. As can be seen, the resolution increases almost linearly according to the number of spectrometer. This curve was obtained using a CCD sensor of only 1024 pixels. For the configuration with nine spectrometers, the measured linewidth is of the order of 9 picometers. A larger 6000 pixel sensor achieves an equivalent resolution of over 70000 with only 5 coherence spectrometers instead of 9.

Claims

Spectrometry device characterized in that it comprises at least one wavefront separating interferometer comprising at least two unbalanced arms and at least one air wedge, an interference fringe imaging device, a sensor imager of said fringes and a signal processing means from said sensor.

2. Spectrometry device according to claim 1, characterized in that said imbalance between the at least two arms is adjustable.

3. Spectrometry device according to claim 1, characterized in that said interferometer is of the type

Michelson.

4. Spectrometry device according to claim 1, characterized in that said interferometer is Mach-Zhender type.

Spectrometry device according to claim 1, characterized in that said signal processing means uses Fourier transform analysis.

Spectrometry device according to claim 1, characterized in that said fringe imaging device comprises at least one cylindrical doublet lens for imaging said fringes.

7. Spectrometry device according to claim 1, characterized in that said imaging sensor is a CCD camera.

8. Spectrometry device according to claim 1, characterized in that it further comprises a means for focusing said fringes to said sensor.

9. Spectrometry device characterized in that it comprises a plurality of interferometers (15A, 15B, 15C) wavefront separators, each of said interferometer comprising two unbalanced arms and at least one air wedge interference fring imaging, an imaging sensor of said fringes and a signal processing means from said sensor, the imbalances between the two arms of each of said interferometer being different.