WO1999002975A1

WO1999002975A1 - Raman analysis device comprising a micro-laser

Info

Publication number: WO1999002975A1
Application number: PCT/FR1998/001440
Authority: WO
Inventors: Serge Gidon; Michel Dupuy
Original assignee: Commissariat A L'energie Atomique
Priority date: 1997-07-07
Filing date: 1998-07-06
Publication date: 1999-01-21
Also published as: FR2765687A1; FR2765687B1

Abstract

The invention concerns a Raman analysis device comprising: a micro-laser (20), a device (31) for detecting a Raman spectrum, an optical fibre (26) to bring a pump beam (24) towards the micro-laser and transmit a Raman signal towards the sensing device (31). A second optical fibre may be provided for sampling a Raman signal coming from the sample to be analysed.

Description

RAMAN ANALYSIS DEVICE INCLUDING A MICROLASER

Technical field and prior art

The invention relates to the field of Raman analysis. The Raman technique is a chemical analysis method that identifies the content of a compound by analyzing its molecular vibration spectrum

(spectral signature).

A detailed description of the method is given in the work entitled "Infrared and Raman spectroscopy", Chapter One: "Infrared absorption and Raman effect" (M. Peyron), coordinator of the publication: B.G. Mentzen; monograph of the Scientific and Technical Update Center; collection of work from advanced training sessions - INSA Lyon (Masson et Cie, Paris 1974).

This technique requires a laser source to "excite" the molecules. Very generally lasers (of the gas or "solid" type) are positioned near the analysis spectrograph within which the sample to be analyzed is placed: FIG. 1 represents a known device for Raman analysis.

The beam of a laser 2 (generally Argon) is focused on a sample 4 to be analyzed. The light scattered by the sample is collected and analyzed by a spectrometer 6. In order to avoid any parasitic Raman spectrum, the beam is propagated in the air.

For measurements in industrial or medical environments, it is advantageous to be able to offset the sample from the analysis means, by means of optical fibers, the excitation signal can be transported laser and, in return, convey the Raman signal from the sample.

It turns out that this technical solution requires the use of separate channels (laser and Raman signal), ie at least two fibers. Indeed, the optical fiber in which the laser excitation signal is conveyed generates a Raman spectrum (that of silica) which would be superimposed on the analysis signal and jam it. We therefore use a device shown diagrammatically in FIG. 2, where optical fibers are designated by the references 8, 10.

Using two fibers for a Raman probe complicates the design. An optical system 12 must be added to the probe to achieve the overlap of the illuminated and observed fields (analysis and excitation zones). The manufacturers adopt various solutions which are, in any event, bulky.

Statement of the invention

The invention aims to solve this problem. It relates to a Raman probe configuration which makes it possible to circumvent this difficulty and makes it possible to produce an extremely compact Raman probe. It uses a single optical fiber terminating in a chip laser, or microlaser, intended to be placed directly in front of the medium to be analyzed.

According to the invention, the Raman analysis device comprises:

- a microlaser,

Means for detecting a Raman spectrum, for example a spectrometer or spectrograph, - an optical fiber to bring a pumping beam to the microlaser and to transmit a Raman signal to the spectrometer or the spectrograph.

The analysis device according to the invention is efficient because the light energy transported by the fiber (which serves to "pump" the microlaser) certainly induces a Raman spectrum, but around the pump frequency (wavelength) which is fundamentally different from the emission of the microlaser. The microlaser acts as a wavelength converter at the fiber end: the Raman signal of the medium to be analyzed is around the wavelength of the laser beam, and the two Raman spectra are therefore offset one with respect to the other. The invention also relates to a Raman analysis device, comprising:

- a microlaser,

- a device for detecting a Raman spectrum,

- a first optical fiber for bringing a pumping beam towards the microlaser,

- a second optical fiber for transmitting, to the detection device, a Raman signal.

This avoids any overlap between the Raman spectrum of the first fiber and the Raman signal from the medium to be analyzed.

The microlaser can be of the triggered type. It can also operate continuously. In addition, means for doubling or tripling the frequency of the laser beam can be provided. The invention also relates to a Raman analysis method, using a device as above, the microlaser being directed towards the medium to be analyzed. Brief description of the figures

Anyway, ^• the characteristics and advantages of the invention will become more apparent in light of the following description. This description relates to the exemplary embodiments, given by way of explanation and without limitation, with reference to the appended drawings in which:

- Figures 1 and 2 show Raman analysis devices according to the prior art.

- Figure 3 illustrates an embodiment of the invention.

- Figure 4 shows the spread of the Raman spectrum of silica. - Figures 5A and 5B show another embodiment of the invention.

- Figure 6 shows a microlaser cavity.

- Figure 7 shows the assembly of a fiber and a microlaser.

Detailed description of embodiments of the invention

An embodiment of the invention is shown in Figure 3. The laser 20 constitutes the heart of the probe.

It is a laser chip, or microlaser. Microlasers which can be used in the context of the present invention are described for example in document EP-653 824 or else in the article by JJ Aubert, which appeared in Laser Focus World, June 95, entitled "Q-Switched microchip lasers bring new applications to light ". The chip laser is therefore a "solid" laser of very small dimensions (<mm ³ ). All the components of the laser are integrated to form a reliable monolithic block. Thus, the laser mirrors are directly affixed to the laser material.

Various emission wavelengths are possible, depending on the nature of the laser material. Thus, for YAG, the emission wavelength is 1064 nm, while the pump wavelength is 808 nm. Other materials, giving rise to emissions at other wavelengths, are given in document EP-653,824.

The frequency of the beam coming from the microlaser can be doubled or tripled, or multiplied by n (n> 3), using a device generating a non-linear effect, placed for example in the cavity of the microlaser or at proximity.

The laser chip is "pumped" by the radiation 24 of a diode, via an optical fiber 26 which transmits on the one hand the beam 24 of the pump (at the wavelength 808 nm in the case of the YAG) and of on the other hand, the Raman signal, back to a device 31 capable of receiving or detecting a Raman spectrum, such as for example a spectrograph. An optical system allowing the separation of the laser and Raman beams can be installed near the spectrograph. For example, an interference filter 28 makes it possible to reject the signal of the excitation laser, of the Raman spectrum. The Raman signal from the sample 22 passes through the microlaser before being transmitted by the fiber 26.

In the device and the method according to the invention, the microlaser is turned towards the medium to be analyzed: the laser radiation does not pass through an optical fiber before arriving at the sample 22. A micro-optical system 32 can optionally make it possible to focus the laser beam on a precise analysis zone of the object or of the medium 22 to be analyzed. This optical system also makes it possible to collect the Raman signal at an optimal optical opening angle.

The spectral purity of emission of a microlaser, or laser chip (single-mode aspect, inherent in the compactness of the optical cavity of this laser) makes this source well suited to Raman measurements, because the Raman spectrum is "calibrated" on the laser emission line.

In addition, the laser chip is capable of operating in pulses (recurrent at several KHz) by "passive triggering", which allows Raman measurements in "synchronous detection" mode. This detection mode eliminates the luminescence of the sample when it is illuminated by green or near UV light for which the Raman effect is optimum. The passive triggering technique of a microlaser is described in document EP-653,824.

The "offset" between the pump wavelength and the wavelength of the laser emission is taken advantage of in the present invention. The microlaser actually acts as a wavelength converter at the end of the fiber 26.

For example, the "pump" of a YAG: Nd laser is at 808 nm, its emission is at 1064 nm. This beam of "pump" can induce a Raman spectrum: that of the silica fiber (which extends approximately on 3000 cm ^'1 ), but this spectrum spreads from 808 nm and only overlaps partially with that of the sample to be analyzed. FIG. 4 shows the spreading 34 of the Raman spectrum due to the fiber ("excited" by the pump wavelength), relative to the Raman signal positioned either in the vicinity of the fundamental emission line of the YAG (line at 1064 nm), spreading represented by arrow 36, that is to say in the vicinity of the harmonic line of the doubled YAG (at 532 nm), spreading represented by arrow 38.

With a laser operating on the fundamental line, there may be an overlap of the Raman spectra of the silica of the fiber (excited by the pump) and of the sample (excited by the laser line). This difficulty can be circumvented by a structure with two fibers of the type of those illustrated in FIGS. 5A or 5B, on which numerical references identical to those of FIG. 3 designate identical or corresponding elements therein.

In FIG. 5A, a second fiber 27 transmits the Raman signal, which has passed through the microlaser, towards the analysis device 31, directly or through the filter 28 (as in the figure

5A).

In FIG. 5B, a second fiber 29 transmits the Raman signal directly from the sample 22 to the analysis device 31.

For "in-vivo" measurements, for which the fibers are introduced into a human or animal body, the embodiment of FIG. 3 is more convenient since a single fiber can be introduced with, at its end, the microlaser 20.

In general, and whatever the technique used, the measurement carried out and the analysis carried out, it is convenient to mount a microlaser to the end of an optical fiber, as illustrated in FIG. 7 where the references 26 and 20 respectively designate an optical fiber and a microlaser, a positioning device 25 fixing their relative position. The microlaser is then used as a source of Raman excitation at the end of the fiber.

It can then be introduced into a human or animal body, the pumping beam being introduced from the outside towards the microlaser; The fiber is for example a multimode fiber and the microlaser is produced so as to have lateral dimensions equal to or substantially equal to, or close to, the diameter of the fiber.

With a laser used in frequency doubling, there is no overlap of these spectra, as can be seen in FIG. 4 where the domains 38 and 34 are clearly separated.

Depending on the level of sensitivity sought for the Raman probe, one of these configurations will be chosen rather than the other. The frequency doubled chip laser is more efficient.

A microlaser for a device according to the invention is shown in FIG. 6. It comprises an active medium 50 and inlet 52 and outlet 54 mirrors delimiting the cavity. This can also contain trigger elements (passive Q-Switch), for example in the form described in

EP-653 824, or else non-linear elements

(frequency doubler, tripler), as described in application EP-742,615.

The "pump" beam transmitted to the microlaser 20 by the optical fiber 26 is transmitted by the mirror 52 of cavity while it is reflected by the mirror 54 of the laser output (double pass operation).

The laser beam 56 is blocked by the mirror 52 but is partly transmitted by the output mirror 54.

The Raman signal passes through the cavity of the microlaser and is not attenuated by the mirrors 52, 54.

To produce the mirrors 52, 54, it is possible to stack dielectric layers. In the case of a Raman probe operating on a harmonic wavelength (for example: doubling of frequency), the transmissions of the mirrors also satisfy the requirements for obtaining the harmonic signal (doubly intra or extra-cavity of the laser chip) .

The invention has been described in the context of Raman analysis, but can just as easily be applied, and with the same advantages, to the stimulated Raman analysis technique.

Claims

1. Raman analysis device comprising:

- a microlaser (20),

- an optical fiber (26) for bringing a pumping beam (24) towards the microlaser.

- an optical detection device (31), the microlaser transmitting a Raman signal to the detection device (31).

2. Raman analysis device, comprising: - a microlaser (20),

- a device (31) for optical detection,

- a first optical fiber (26) for bringing a pumping beam towards the microlaser,

- a second optical fiber (27, 28) for transmitting, to the detection device (31), a Raman signal, this second optical fiber (27) being arranged so as to collect a Raman signal, coming from a sample to be analyzed , after this signal has passed through the microlaser (20).

3. Device according to claim 2, the second optical fiber (29) being arranged so as to collect a Raman signal directly on the surface of a sample to be analyzed (22).

4. Device according to one of claims 1 to 3, the microlaser being a triggered microlaser.

5. Device according to one of claims 1 to 3, the microlaser being of the type operating continuously.

6. Device according to one of the preceding claims, further comprising means for multiplying the frequency of the laser beam.

7. Device according to one of the preceding claims, further comprising means (28) for filtering to reject, from the Raman spectrum, the signal of the excitation laser.

8. Device according to one of claims 1 to 7, the microlaser being mounted at the end of the fiber.

9. Raman analysis method of a sample (22), comprising the use of a device according to one of claims 1 to 8, the microlaser being directed towards the sample to be analyzed.

10. The method of claim 9, the microlaser being triggered passively, the Raman measurement being carried out in synchronous detection.