WO2017148858A1

WO2017148858A1 - Optical excitation device for generating stimulated raman processes, apparatus for measuring stimulated raman processes and method of optical excitation for generating stimulated raman processes

Info

Publication number: WO2017148858A1
Application number: PCT/EP2017/054494
Authority: WO
Inventors: Charles-Henri HAGE; Laurent Heliot
Original assignee: Centre National De La Recherche Scientifique; Universite Lille 1 Sciences Et Technologies
Priority date: 2016-03-01
Filing date: 2017-02-27
Publication date: 2017-09-08
Also published as: FR3048503A1

Abstract

The invention relates to an optical excitation device (20) for generating stimulated Raman processes, said optical device (20) being intended to receive the laser beam (11) of a pulsed laser source (10). The optical device (20) includes a beam splitter for splitting the laser beam into first and second channels (210, 220). The first channel (210) includes a first optical fibre (211) and a system for pre-shifting frequency, said system being suitable for applying a first time-dependent frequency shift. The optical device (20) furthermore comprises a second optical fibre (233) that is arranged to collect the first and second sub-beams (21, 22) exiting from the first and second channels (210, 220) and to apply thereto a second time-dependent frequency shift, the second optical fibre (233) and the system for pre-shifting frequency being configured so that the first and second sub-beams (21, 22) have an identical frequency shift. The invention furthermore relates to a measuring system and an excitation method.

Description

OPTICAL EXCITATION DEVICE FOR GENERATING STIMULATED RAMAN PROCESSES, STIMULATED RAMAN PROCESS MEASUREMENT ASSEMBLY, AND EXCITATION METHOD

OPTICAL TO GENERATE STIMULATED RAMAN PROCESSES

DESCRIPTION

TECHNICAL AREA

The invention relates to the field of non-linear optics and more precisely to optics based on stimulated Raman processes. More specifically, it relates to an excitation optical device for generating stimulated Raman processes, a stimulated Raman process measurement set, and an optical excitation method for generating Raman Stimulated processes.

STATE OF THE PRIOR ART

Non-linear analysis techniques, such as stimulated Raman microscopy (called "CARS" for Coherent Raman anti-Stokes Scattering or "SRS" for Stimulated Raman Scattering), require optical excitation devices adapted to provide at least two optical radiations having a difference in wavelengths (or "optical frequencies") defined and, if possible, adjustable.

In the context of this document, the notions of optical frequency, wavelength, wave number and energy of electromagnetic radiation are interchangeable. Indeed, the skilled person is able to move from one to another of these quantities by means of simple calculations perfectly within reach. Thus, if it is preferentially referred to the optical frequency throughout this document, the skilled person is able to transpose these same lessons from the frequency domain to the time domain, that is to say, transposed to the lengths wave. It should also be noted that to evoke certain values, such as the emission wavelength of the laser source, the notions of wavelength and wave number have been preferred for convenience, since these are the quantities generally used by those skilled in the art.

The following paragraph also applies to the magnitudes observed in stimulated Raman measurements. Indeed, these measurements allow the observation of molecular bonds by exciting one of their resonant frequencies. The resonance frequencies of these molecular bonds can be expressed in terms of vibration frequency as well as in wavelength, wave number, or in terms of energy. The skilled person being able to move from one size to another, it was chosen, for the sake of brevity mainly to mention the vibration frequencies. Of course, in the same way as for the quantities related to electromagnetic radiation, the person skilled in the art is able to easily move from one size to another and is able to transpose all the teachings of this document of the invention. frequency domain to the spatial domain, that is to say transposed to wavelengths.

Indeed, if we take the example of stimulated Raman microscopy mentioned above, such an optical device is necessary to provide a first electromagnetic radiation, said pump, and a second electromagnetic radiation, said Stokes, these two radiations having a difference optical frequency corresponding to the vibration frequency of a molecular link to be imaged. This same device, for increasing the efficiency of the stimulated Raman process and lowering the average power of the radiation to limit the alteration of samples that are often biological, is generally configured to provide the first and second pulsed radiation.

Nevertheless, the use of such pulsed radiation generates an expansion of the spectral content, or content of optical frequencies, of these radiations. As a result, as shown in FIG. 1A, a measurement that has a resolution of the vibration frequencies, also called spectral resolution, is degraded because, because of this spectral broadening, it is not possible to distinguish between two molecular bonds. with close vibration frequencies.

In order to solve this problem, it is conceivable to use pulses having the necessary spectral width, corresponding to a pulse duration. about 2 picoseconds in a configuration called "simplex", but the systems for generating these pulses are bulky and impractical use. It is also conceivable to use a broad Stokes radiation and a fine pump radiation in order to acquire all the necessary vibration frequencies, but the acquisition time of this information is incompatible with the speed required for the analysis of the living. These solutions are therefore not satisfactory.

In order to improve the spectral resolution of such non-linear measurements while retaining the necessary speed, it is known from document WO 2010/106376 to use a linear and constant frequency time drift according to the technique known as "spectral focusing". .

The term frequency drift or frequency drift, also known by the English name "chirp", a time distribution of optical frequencies of electromagnetic radiation. Such drifting, or stretching, is generally based on the propagation of radiation in a medium whose refractive index, and therefore the speed of propagation of said radiation, depends significantly on its optical frequency. This phenomenon is well known as chromatic dispersion. In such a medium, the different optical frequencies of a radiation do not propagate at the same speed, which induces a temporal stretching of the latter, the pulse duration of which can pass, for example, a pulse duration of 100 femtoseconds at a pulse duration of 5 to 10 picoseconds.

The principle of such a time frequency drift is to impose on both pump and Stokes radiation a drift of constant and equal frequency (for example of 2 terahertz per picosecond). The temporal overlap of these two radiations then makes it possible to create "pairs" of optical frequencies pump and Stokes, whose difference, corresponding to a vibration frequency to be analyzed, will be constant over the entire spectral range and the entire pulse duration of the pump and Stokes radiation, it is this principle which is illustrated in FIGS. 1B. This configuration therefore allows a significant improvement in the spectral resolution (generally an order of magnitude). Moreover, contrary to a spectral filtering of the pump and Stokes radiations, all the pairs of frequencies participate in the excitation. vibration for increased efficiency. Finally, by modifying the relative temporal position of the two radiations, for example by delaying or advancing the Stokes radiation with respect to the pump radiation, it is possible to shift the optical frequency difference of the pairs, and therefore the vibration frequency analyzed, this in designing a frequency difference between pump and constant Stokes radiation over the entire pulse duration. It is thus possible to analyze different vibration frequencies simply by modifying the relative delay of one radiation with respect to the other.

For this purpose, the optical device described in document WO 2010/106376 is intended to receive a so-called broadband laser beam and has, with reference to FIG.

A dichroic splitter plate 18, for separating the laser beam into two components, one high frequency, corresponding to the pump radiation, the other low frequency, corresponding to the Stokes radiation, to a first and a second optical channel respectively,

each of the first and second optical channels comprises a dedicated frequency drift system 30 and 24 for applying a frequency drift to each of the high and low frequency components,

a common frequency drift system arranged to intercept the two low and high frequency components at the output of their respective channels and to apply a time frequency drift to both components.

Thus, with such an optical device, the pump and Stokes radiation are stretched temporally and the resulting nonlinear excitation is focused in the frequency space. This excitation is thus resolved in frequency-and it is possible to map, within a sample, with such an optical device a precise molecular bond.

Nevertheless, such an optical device has a number of disadvantages. Indeed, the use of three frequency drifts and the need to obtain the same temporal spread for each of the components, requires a great stability and finesse of the adjustments and imposes a strong constraint on frequency drifting systems which complicates all the use of such a device. It will be added to this fact that the pump and stored radiation provided at the output of such an optical device are those supplied at the output of the glass block 28 and that it is therefore necessary to provide a system for injecting these radiations into the microscope and / or spectrometer to which said optical device will be associated.

STATEMENT OF THE INVENTION

The object of the invention is to overcome these drawbacks and is thus intended to provide a non-linear excitation optical device for generating stimulated Raman processes that is capable of providing a focused non-linear excitation in the frequency space without present the size of the devices of the prior art and which has a simplicity of adjustment and connection.

To this end, the invention relates to an excitation optical device for generating stimulated Raman processes, said optical device being intended to receive the laser beam of a pulsed laser source,

the optical device comprising:

an optical splitter for separating the laser beam at first and second sub-beams into first and second optical paths of the optical device,

the first optical channel comprising:

a first optical fiber, called Stokes, configured to provide an optical frequency shift of the first sub-beam,

a frequency pre-drift system adapted to apply a first frequency drift to the first sub-beam,

the optical device further comprising a second optical fiber, called common frequency drift, arranged to recover the first and second sub-beam at the output of the first and second channels and to apply to them a second time frequency drift the second optical fiber and the frequency preventer system being configured so that the first and second sub-beams have a substantially identical frequency drift at the output of the second fiber optical, the second optical fiber having a mode diameter greater than 5 μιη and preferably 10 μιη,

at least one of the first and second channels has a time delay system for adjusting the relative delay between the first and second sub-beams at the output of the second optical fiber.

The inventors have surprisingly discovered and in contradiction with the knowledge of those skilled in the art, that the use of a second optical fiber having a mode diameter greater than 5 μιη and a delay between the first and the second sub-beam allowed to limit or even eliminate the non-linear effects between the first and the second sub-beam.

Indeed, as the work of M. Cleff et al. published in 2011 in the scientific journal "Applied Physics B" Volume 105 number 4 pages 801-805, when performing measurements in stimulated Raman microscopy it is necessary to limit the non-linear effects which would have the effect of deforming the drift in frequency of the pulses and to cause interactions between the two sub-beams, of the type of cross-phase modulation (also known as XPM), inducing a detrimental measurement noise. Those skilled in the art are therefore urged, in accordance with the work of M. Cleff et al., When using optical fibers, to separate the first and second sub-beams using two separate multimode optical fibers. Thus the use of a second optical fiber according to the configuration proposed by the invention goes against the knowledge of the skilled person.

Thus, such a device does not require an ultra-wideband source which strongly relaxes the constraints on the source laser source and on the design of the frequency drift elements. Moreover, the frequency drift is largely provided by the frequency drift fiber. Congestion related to the pre-stretching system is thus greatly reduced in a device according to the invention vis-à-vis a device of the prior art and it is therefore possible to provide a compact optical device.

In addition, with such a device, the first and second sub-beams have a time shift when introduced into the frequency drift fiber. common which vanishes in output of this same fiber. Indeed, the phenomenon of frequency drift induces propagation velocities, inside this fiber, different for pump and Stokes radiation. The first and second sub-beams therefore meet only in the last centimeters of the second optical fiber, that is to say the common frequency drift optical fiber. This low interaction length, potentially associated with the particular properties of this second fiber, makes it possible to limit the nonlinear interactions between the first and second sub-beams which could reduce the spectral resolution of the measurement or introduce noise into the measurement.

Moreover, since the optical fiber of frequency drift provides the output of the optical device, the optical device according to the invention can easily be connected to a microscope and / or spectrometer for making measurements. This same output in the form of an optical fiber also allows applications to endoscopy to allow non-linear measurements in vivo.

Finally, it should be noted that the use of a common frequency drift optical fiber for Stokes and pump radiation, as well as some elements to perform a frequency pre-drift of the Stokes radiation, go against the customary practice of those skilled in the art and use what is generally seen by the latter as limitations or disadvantages as strategic elements or advantages.

In summary, the invention is based in particular on the counter intuitive use of a second optical fiber common to the first and second sub-beams for:

apply frequency drift to the first and second sub-beams,

- To obtain a spatial superposition of the first and second beams, to facilitate the integration of the optical device.

All of these advantages are achieved without observing non-linear effects that would be expected to be achieved by those skilled in the art with the configuration of the invention.

By substantially identical frequency drift at the output of the second optical fiber, it must be understood that the difference in frequency drift between the first and the second sub-beam is small enough to result in an improvement in the spectral resolution sufficient to achieve the desired resolution.

The first optical fiber may be a microstructured optical fiber shaped to obtain, from non-linear effects, a frequency shift either in the form of a super continuum or in the form of a single offset wave, so as to allow a variable frequency offset as a function of the power of the first sub-beam at the input of the first fiber.

Such a first optical fiber has the advantage of being able to finely adjust the frequency shift of the first sub-beam in order to provide the Stokes radiation and therefore the difference in frequency between the pump radiation, ie the second sub-beam. , and Stokes radiation.

The first optical fiber is preferably shaped to obtain, from non-linear effects, a frequency shift in the form of a single offset wave.

In this way, the frequency shift efficiency is optimized and the filtering operation of the first sub-beam at the output of the first optical fiber is lightened, since it is necessary to filter only the frequency of the laser beam at the origin of the first sub-beam.

The frequency pre-drift system may be adaptable according to the wavelength of the first and second sub-beam output of the second optical fiber.

Such adaptability of the frequency pre-drift system is particularly advantageous in the case where the excitation optical device would be used to study a wide frequency range of vibration, such as for example in the context of stimulated Raman spectroscopy measurements. Indeed, with such adaptability, it is not necessary to change the pre-drift system during significant changes in the measured vibration frequency.

The frequency pre-drift system may include an optical amplifier adapted to amplify the first sub-beam. Such an amplifier, being used according to a parabolic amplification process, makes it possible to amplify the first sub-beam while imposing on it a perfectly linear time drift of frequency with the gain of amplification. This temporal frequency drift can therefore easily and precisely be adjusted by modifying the gain of amplification.

The frequency pre-drift system may comprise at least one block of glass or at least one third optical fiber.

With such glass blocks, the adjustment of the frequency drift applied to the first sub-beam can be done simply by substituting a block of glass or by adding / removing one of the glass blocks.

The second optical fiber may be a microstructured optical fiber whose core is undoped.

With such a second optical fiber it is possible to obtain a good frequency drift of the second sub-beam while limiting or even eliminating the non-linear interactions between the first and second sub-beams in the second optical fiber. Such a limitation, or even elimination, makes it possible to avoid the degradation of the resolution of Raman stimulated measurements that such nonlinear interactions could generate.

The second optical fiber may have a length greater than 1.5 m, preferably greater than 3 m.

The invention further relates to a stimulated Raman process measurement set comprising a pulsed laser source, an excitation optical device for generating stimulated Raman processes and an optical measuring device, such as a microscope or spectrometer,

the optical device being an optical device according to the invention.

Such an assembly benefits from the advantages associated with the use of an optical device according to the invention.

The laser source and the optical device can be configured to allow parameterizable nonlinear excitation. Such an assembly is adapted to allow a stimulated Raman measurement to observe several types of molecular bonds, since the frequency of the observed vibration can be parameterized, or to make Stimulated Raman spectroscopy.

The invention further provides an optical excitation method for performing optical measurements using stimulated Raman processes such as Raman microscopy or Raman spectroscopy measurements, the method comprising the steps of:

recovering a laser beam from a pulsed laser source,

separating the laser beam into a first and a second sub-beam,

applying a frequency shift to the first sub-beam by means of a first optical fiber, called Stokes,

application to the first sub-beam at the output of the first optical fiber of a frequency time drift,

combining the first and the second sub-beam with the means in a second optical fiber, called frequency drift,

applying a temporal frequency drift to the first and second combined sub-beams, this by means of the second optical fiber so that the first and second sub-beams have a substantially identical frequency time drift at the output of the second fiber optical,

wherein it also provided before the combination of the first and second sub-beams, applying a time delay to one of the first and second sub-beams to adjust the relative delay between the first and second sub-beams in output of the second optical fiber.

Such a method makes it possible to obtain stimulated Raman excitation having the same advantages as that provided by a device according to the invention.

In addition, the following steps can be provided: modifying the frequency offset applied to the first sub-beam by means of a first optical fiber, by modifying the power of the laser beam transmitted in the first optical channel, and so as to modify the wavelength shift between the first and the second sub-beam at the output of the second optical fiber,

correction of at least one of the first and second time frequency drift, so as to correct the frequency drift variation generated by the change in the frequency offset and to provide the same time frequency drift between the first and second frequency drift the second sub-beam at the output of the second optical fiber,

correction of the applied time delay so as to synchronize the first and the second sub-beam at the output of the second optical fiber.

With such additional steps, it is possible to modify the excitation at the origin of the stimulated Raman processes and to make observations of several molecular bonds or Raman Stimulated spectroscopy.

In addition, the following step may be provided:

before the step of modifying the frequency offset applied to the first sub-beam, modifying the emission wavelength of the laser source, the step of modifying the applied frequency offset can be adapted to compensate for the modification in emission wavelength of the laser source so that the frequency change applied to the first sub-beam does not lead to a modification of the wavelength of the first sub-beam at the output of the second optical fiber .

Such a method has the advantage of allowing the use of a frequency derivative by means of parabolic amplification processes. Indeed, the first sub-beam having a substantially constant wavelength, it can be amplified by means of an optical amplifier and thus benefit from the advantages related to parabolic amplification. The invention further relates to the use of an optical device according to the invention for performing stimulated Raman measurements such as stimulated Raman imaging or spectroscopy.

Such use benefits from the advantages of the device according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments, given purely by way of indication and in no way limiting, with reference to the appended drawings in which:

FIGS. 1A and 1B schematically illustrate the interest of the spectral focusing method, FIG. 1A showing that in the absence of chromatic time drift, many vibration frequencies are excited and it is not possible to solve them. spectrally, FIG. 1B illustrating the principle of spectral focusing, that is to say that a constant and identical chromatic time drift makes it possible both to spectrally resolve the different vibration frequencies and to acquire them one by one by a simple temporal shift from one radiation to the other,

FIG. 2 illustrates a schematic representation of a stimulated Raman measurement system equipped with an optical excitation device according to the invention,

FIGS. 3A and 3B illustrate the operation of a Stockes fiber equipping the device according to the invention, with FIG. 3A the output spectrum of such an optical fiber for different powers of the beam injected at the input and FIG. 3B a simulation. the spectral shift occurring in such an optical fiber for a given injected power as a function of the length of the optical fiber,

FIG. 4 illustrates the principle of frequency time drift implemented in the context of the invention by showing the temporal intensity profile for two beams of different wavelengths before and after frequency drift, FIG. 5 makes it possible to compare the spectral resolutions obtained with a Raman measurement stimulated without frequency drift and with frequency drift obtained according to the principle of the invention, a stimulated Raman spectrum also being provided for reference,

FIGS. 6A to 6C illustrate experimentally the result of a stimulated Raman measurement obtained by means of an optical device according to the invention, with FIG. 6A illustrating the ratio of the specific signal (called "resonant" on the non-signal specific (so-called "non-resonant") as a function of the frequency offset, here expressed in wavenumber, for three different vibration frequencies, FIGS. 6B and 6C showing stimulated Raman images for an offset between the pump radiation and the radiation Stokes corresponding to the vibration frequencies of 1003 and 1040 cm, these vibration frequencies being provided in wave numbers, which is the quantity usually used for frequencies in spectroscopy,

FIG. 7 schematically illustrates an optical device according to a second embodiment of the invention.

Identical, similar or equivalent parts of the different figures bear the same numerical references so as to facilitate the passage from one figure to another.

The different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

The different possibilities (variants and embodiments) must be understood as not being exclusive of each other and can be combined with one another.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Figure 2 schematically illustrates a nonlinear measurement system 1, such as a Raman Stimulated microscopy measurement system.

Such a measurement system 1 comprises:

a laser source 10 for supplying a laser beam 11 whose emission wavelength corresponds to the pump radiation,

an optical excitation device 20 according to a first embodiment of the invention, a microscope 30 adapted to allow a non-linear measurement.

The laser source 10 is a pulsed titanium-sapphire laser source with a variable wavelength emission over a wavelength range of 790 to 960 nm. The duration of the pulses of the laser source can be between 50 and 200 femtoseconds typically substantially equal to 100 femtoseconds. The laser source has a pulse repetition rate of between 8 MHz and 160 MHz, preferably between 30 MHz and 100 MHz, and advantageously equal to 80 MHz. The laser emission power is greater than 250 mW for low vibration frequency analysis (i.e., less than 1500 cm) and 400 mW for high frequency analysis (ie that is, greater than 2500 cm ^"1 ).

In this first embodiment, the laser source 10 is used for the emission of a laser beam 11 whose wavelength is fixed for example at a wavelength of 800 nm.

The optical device 20 comprises:

a first so-called "separator" dichroic plate 231 for separating the laser beam 11 into a first and a second sub-beam 21, 22 on respectively a first and a second optical channel 210, 220 of the optical device 20,

the first optical channel 210, which comprises itself:

a first optical fiber 211 called Stokes configured to provide a frequency shift of the first sub-beam 21,

a glass block 212 acting as a frequency pre-drifting system,

a delay line 213 for authorizing synchronization of the first and second sub-beams 21, 22 at the output of the optical device 20,

the second optical channel 220 which comprises a delay system 221 of the second sub-beam 22 to compensate in part for the delay of the first radiation induced by the passage in the first optical fiber 211,

a so-called "combinatrice" dichroic plate 232 arranged to intercept the first and second optical channels and to recombine the first and second sub-beams 21, 22, a second optical fiber 233 called frequency drift arranged vis-à-vis the second splitter plate 232 to recover the first and the second sub-beam 21, 22 at the output of the first and second lanes 210, 220 and for their apply a temporal drift of frequency.

It is recalled here that the energy of a given radiation can be qualified both by the energy of this same radiation itself, by its wavelength or its optical frequency, or its number of wave. Thus, when mentioned below and in the remainder of this document, a wavelength shift between two radiations, such as between the pump radiation and the radiation Storees, that can be described as a shift in optical frequency between these two radiations as their energy shift. These notions of offsets are well known to those skilled in the art, it is able to make, by a simple calculation perfectly within reach, the transition from one offset value to another.

The first splitter plate 231 makes it possible to separate the laser beam 11 into a first and a second sub-beam 21, 22 into the leaders respectively on the first and the second optical path 210, 220. The reflection ratio of the splitter blade 231 can be adapted according to the type of non-linear measurements to be made.

In a usual configuration, the first splitter plate 231 is chosen to provide the power required for the maximum spectral shift of the first sub-beam 21, allocating the sub-beam 22 the remainder of the available power. In the conformation illustrated in FIG. 2, in which the first optical channel 210 corresponds to the reflected radiation, the second optical channel corresponding to the transmitted radiation, the separating plate 231 can be chosen to have a reflection ratio satisfying this condition.

Thus to give an example in which the laser source 10 can provide a laser beam 11 with a power of 800 mW, the separator blade 231 can be configured to provide a first sub-beam 21 with a power of 200 mW, the remaining power, that is to say 600 mW being supplied through the second sub-beam 22. Similarly, for a laser source 10 can provide a laser beam 11 with a power of 3 W , the splitter blade can also be configured to provide a first sub-beam 21 with a power of 200 mW, the rest, that is to say 2.8 W being supplied through the second sub-beam 22.

After separation by the first splitter plate 231, the first sub-beam 21 is injected into the first optical fiber 211 to obtain a frequency shift. Such a first optical fiber 211 is a microstructured optical fiber shaped to obtain, from nonlinear optical effects, a frequency shift either in the form of a super continuum or in the form of a single offset wave.

Such optical fibers are known, for those which make it possible to obtain a frequency shift in the form of a supercontinuum from the wavelength of a laser beam 11, the works of Ortigosa-Blanch and its coauthors published in 2002 in the journal "Journal of the Optical Society of America B" Volume 19 number 11 pages 2567-2572. These optical fibers use a dispersive wave phenomenon to transfer a portion of the energy of the first injected sub-beam in shorter wavelengths and phase modulation and pulse shift phenomena by internal Raman scattering for transfer the rest of the energy to the longer wavelengths. These optical fibers output a sub-beam dispersed over a relatively wide frequency range in which the required Stokes wavelength can be selected by means of a matched filter. Since the generation dynamics of these supercontinua are complex, before the chosen spectral band can be used, it must be ensured that the imposed frequency time drift results in the desired frequency drift, for example by propagating it in a first system. chromatic dispersion.

Optical fibers to obtain the frequency shift in the form of a single shifted wave, at wavelengths and for shifts of interest, are known from the work of Cormack and his coauthors published in 2002 in the scientific journal These optical fibers use a Raman shift of a soliton to provide a frequency offset of the first sub-beam 21. This Raman shift varies with the power of the beam introduced into the fiber. optical and with the length of the optical fiber. These variations are illustrated in Figures 3A and 3B. FIG. 3A thus illustrates four normalized wavelength spectra that can be obtained for an increasing power of the input beam of the first optical fiber 211, the first, second, third and fourth spectra 412, 413, 414, 415A, B corresponding respectively to a fiber input power of 26 mW, 46 mW, 107 mW and 206 mW for an injection rate of the order of 40 to 50%. It can thus be seen in this figure that the peak 411 corresponds to the portion of the first sub-beam 21 not shifted in frequency, and that the peaks to which the references 412, 413, 414, 415A, 415B have been attached correspond to the part of the first sub-beam 21 which has been shifted in frequency. Thus, for a power of the first relatively small sub-beam 21, 26 mW, the wavelength shift is less than 12 nm, a frequency shift of less than 5.4 THz or 180 cm ¹ . This same wavelength shift, when the power of the first sub-beam is increased to 107 mW, becomes greater than 167 nm, ie a frequency shift greater than 63 THz or 2100 cm ¹ . It will also be noted that the exceeding of a power threshold of the first sub-beam 21 makes it possible to generate the appearance of an offset of a second soliton at a lower wavelength. It is this phenomenon which is illustrated for the power of 206 mW, the first peak 415A being centered at a wavelength of approximately 1062 nm while the second peak 415B is centered at a wavelength of 918 nm.

In the same way, FIG. 3B illustrates the wavelength distribution as a function of the length of the first optical fiber 211, the first sub-beam 11 having two peaks 421, 422 wavelength 421 to 421. the original wavelength and the other 422 which has the wavelength shift. It can thus be seen in this figure that the shift in wavelength, and therefore in frequency, also increases with the length of the first optical fiber 211.

In this way, with a first optical fiber 211 based on a Raman shift of a soliton, it is possible to have a major part of the first sub-beam 21 shifted in frequency so as to obtain the radiation Stockes having an optimized power this with adjustable frequency offset. Indeed, by modifying the power of the first sub-beam 21 at the input of the first optical fiber 211, it is possible to modify the wavelength of the first sub-beam 21 at the output of the first optical fiber 211 and therefore the frequency difference between the pump radiation and the Stokes radiation.

Advantageously and in order to optimize the intensity ratio between the part of the first sub-beam 21 at the output of the first optical fiber 211 having the frequency offset and the part remaining at the original frequency, the first optical fiber may be in accordance with the optical fibers disclosed by Hage and his co-authors published in 2011 as part of Proceedings of SPIE, Volume 8071 pages 807101-1 to -7, 2011, and the article by Bendahmane and his co-authors published in 2013 as part of the conference "OSA Workshop on Specialty Optical Fibers and their Applications", reference W3.36.

The first optical fiber 211 is equipped, at its output, with a filter, not shown, adapted to reduce the intensity of, or even eliminate, the portion of the first sub-beam 21 that has not undergone the frequency shift.

The first sub-beam 21 is intercepted, as illustrated in FIG. 2, at the output of the first optical fiber 211 by the glass block 212. The glass block is configured to apply a frequency drift to the first sub-beam 21 Such a frequency pre-drift makes it possible to compensate for the difference in time frequency drift between the first and second sub-beams 21, 22. In a usual configuration of the invention, namely a wavelength shift included between 70 and 270 nm, the frequency pre-drift of the first sub-beam 21 makes it possible to provide the major part of the total frequency time drift of the first sub-beam 21. Thus, the glass block 212 is preferentially configured to allow a frequency drift of the first sub-beam 21 making it possible to reach a pulse duration after frequency drift of between 1 and 10 picoseconds.

The glass block 212 forms a frequency pre-drift system adapted to apply a first frequency drift to the first sub-beam 21 after it leaves the first optical fiber 211, The first sub-beam 21 is then intercepted by the delay line 213 to allow synchronization between the first and second sub-beams 21, 22 at the output of the optical device 20. The delay line 213 comprises:

a polarization splitter cube 213A,

a quarter-wave plate 213B,

a mirror 213C.

Such a delay line benefits from the fact that, according to the use of the Raman shift of a soliton, the sub-beam 21 is injected on one of the neutral axes of the Stokes fiber 211 and the resulting offset soliton has a linear polarization. In this way, the first sub-beam 21 is reflected by the polarizing splitter cube 213A towards the mirror 213C through the quarter-wave plate 213B. The first sub-beam 21 is then reflected by the mirror towards the polarizing splitter cube 213A by passing through the quarter-wave plate 213B. The first sub-beam is thus 90 ° out of phase and is then transmitted by the separator cube 213A towards the combiator blade 232 in order to exit the first optical path 210 and to be recombined by this combiator blade 232 with the second sub-beam. -Beam 22.

With such a configuration, the power of the sub-beam 21 reflected by the separator cube, and therefore the power transmitted to the combina- tor blade 232, is a function, according to the Malus law, of the angle between the polarization of the sub-beam 21. at the output of the fiber 211 and the vertical. It is therefore possible to add a half-wave plate, not shown, to thereby provide a suitable power adjustment system.

In the delay line 213, the mirror 213C is movably mounted so as to allow an adjustment of the distance between the polarizing splitter cube 213A and the mirror 213C and thus allow by this displacement an adjustable delay of the first sub-beam 21 vis-à-vis of the second sub-beam 22.

The second optical channel 220 comprises only the delay system 221 which makes it possible to roughly adjust the length of the optical path to that of the sub-beam 21. In the configuration of the optical device 20 of the invention illustrated on FIG. 2, the delay system 221 is fixed, the synchronization between the first and the second sub-beam at the output of the second optical fiber is making by adjusting the distance between the polarizing splitter cube 213A and the mirror 213C in the delay line 213.

The delay system 221 has a plurality of mirrors arranged to lengthen the optical path of the second optical sub-beam 22. In this manner, the optical device 20 may be designed with first and second sub-beams 21, 22 "roughly" synchronized , the delay line 213 of the first channel 210 for finely adjusting the relative delay between the first and the second sub-beam 21, 22 at the output of the second optical fiber 233. In this way, the delay system 221 and the delay line 213 together form a time delay system for adjusting the relative delay between the first and the second sub-beam 21, 22 at the output of the second optical fiber 233.

It may nevertheless be noted that such a delay line configuration 213 / delay system 221 to allow a fine adjustment of the relative delay between the first and the second sub-beam 21, 22 is only given by way of example . Other configurations are perfectly conceivable without departing from the scope of the invention. Thus, for example, the second channel 22 may be equipped with an adjustable delay line while the first channel 21 has or not a fixed delay system. Similarly, the delay line 213 may be of another type. Thus, it can be envisaged to provide an optical device 20 having a better compactness by using, for example, a fiber-delayed line.

The first and second sub-beams 21, 22 are intercepted by the combina- tion blade 232 at the output of the first and second optical channels 210, 220. The combina- tion blade 232 then makes it possible to inject the first and second sub-beams 21, 22 into the second optical fiber 233.

The second optical fiber 233 is adapted to apply to the first and second sub-beams 21, 22 a frequency time drift. As indicated above, because of the differences in wavelength of the first and second sub-beams 21, 22 at the entrance of the second optical fiber 233, the time frequency drift to which the second sub-beam 22 is subjected is significantly greater than that to which the first sub-beam 21 is subjected. This phenomenon is illustrated in FIG. 4.

In this FIG. 4 is illustrated the temporal spreading of two beams of different wavelengths before and after passage in a frequency drift fiber 1.9 m in length. Thus, the curves 431 and 432 correspond to beams of 1 m W of power whose respective wavelength is 800 nm and 960 nm, the pulse duration of these two beams being 170 femtoseconds. After time stretching, the 800 nm beam, whose curve is referenced 433, has a pulse duration of 1.8 picoseconds, a temporal stretching factor greater than 10, while the 960 nm beam, whose curve is referenced 434, has a pulse duration of 620 femtoseconds, a time stretching factor of about 3.6. Thus, it can be considered that, according to the wavelengths at the output of the first and second optical channels 210, 220 of the first and second sub-beams, the temporal stretch, and therefore the frequency drift, of the second sub-beam. beam 22 by the second optical fiber 233 is 2 to 6 times greater than that of the first sub-beam 21.

The temporal stretching of the second sub-beam 22, corresponding to the pump radiation, is preferably adapted to make it possible to obtain a pulse duration of the second sub-beam 22 between 1 and 10 picoseconds.

In order to obtain such a time stretching, and therefore the corresponding frequency drift, the second optical fiber 233 can be sized from the following equation

(1

Lfib corresponding to the length of the optical fiber, c the speed of light, ΔΤ, ηί the pulse duration at the input of the optical fiber, λ the wavelength of the second sub-beam, D the chromatic dispersion value of the optical fiber at the wavelength of the second sub-beam, ATéti e the desired pulse duration.

Thus, considering a light speed of 3.10 ⁵ nm.ps ^-1 , an initial pulse duration of 0.1 picosecond, a pump wavelength of 800 nm and a dispersion D of 100 ps.km ^_1 . nm ^_1 , it is possible to obtain a pulse duration of second sub-beam 22 of 1.477 picoseconds with a length of second optical fiber 233 of 1.6 m.

The second optical fiber, in order to limit the interactions between the first and second sub-beams 21, 22, is preferably configured to limit the deleterious effects, such as the non-linear effects crossed between the two sub-beams, such as the effects of non-degenerate four-wave mixing, and the effects produced by each single sub-beam such as fluorescence. Such a configuration can be obtained by a pure silica optical fiber core, without a doping element, as is the case for an air / silica microstructured optical fiber with an undoped core. In the same way, in order to limit the interactions between the first and the second sub-beam 21, 22 and the non-linear phenomena, the second optical fiber has a large modal area, typically characterized by a diameter greater than 5 μm, or even 10 μιη.

Thus, in a conventional configuration of the invention, the second optical fiber 233 is an air / silica microstructured optical fiber made of undoped silica, in other words of the undoped silica photonic crystal type, and which has a greater or equal modal diameter. at 10 μιη.

The second optical fiber 233 has its output connected to the microscope 30. The second optical fiber 233, in order to optimize its connection to the microscope 30, may comprise a suitable output, such as an output equipped with a lens or provided with a suitable connector complementary to a connector equipping said microscope 30.

The second optical fiber 233 is connected to the microscope so as to allow illumination of a sample by the first and second sub-beams 21, 22. The first and the second sub-beams 21, 22 being synchronized and having a temporal drift of substantially identical frequency, the stimulated Raman optical excitation thus obtained benefits, as schematically shown in FIGS. 1A and 1B, from the frequency focusing related to the time frequency drift.

Indeed, FIG. 1A illustrates the expansion in the frequency space of pulsed pump and Stokes radiation 441, 442 that has not been subjected to a frequency drift. This same figure also shows, on the bottom, the beach of 443 vibration frequencies that can potentially be excited from these two radiations. The stimulated Raman measurement involving non-linear interactions at 4 waves, the spectral width of the pump and Stokes radiations add up to give a very wide frequency range of vibration 443 and the stimulated Raman spectrum 444 that can be obtained is therefore scrambled. .

On the other hand, in the context of the invention, as shown in FIG. 1B, and as explained in the context of the prior art and partially indicated in the brief description of FIGS. 1A and 1B, the time drift of identical frequency for the radiations Pump and Stokes 451 and 452 amounts to "coupling" the pump and Stokes optical frequencies so that all these couples excite the same frequency of molecular vibration, which can now be identified. This coupling therefore leads to a significant increase in the spectral resolution. Moreover, with such an identical frequency drift, a simple modification of the relative delay between the radiations makes it possible to obtain a variation of the optical frequency difference of the pairs of pump and Stokes optical frequencies in order to analyze another vibration frequency.

Of course, since the optical device 20 according to the invention is adapted to provide an excitation adapted to generate stimulated Raman processes, it can be connected by its second optical fiber 233 to another type of optical measurement device making it possible to perform non-linear measurements without departing from the scope of the invention. Among the possibilities of application of an optical device 20 according to the invention, mention may be made, for example, of stimulated Raman spectroscopy and its variants (polarization analysis "P-CARS" and of correlation "CARS-CS"), microscopies of two-photon absorption and second harmonic generation fluorescence and their variants (stimulated fluorescence, "FLIM" life time measurement and energy transfer between "FRET" fluorophores, fluorescence correlation spectroscopy "FCS", analysis in polarization of the generated signals).

Such an optical device 20 makes it possible to implement an optical excitation method for making nonlinear measurements such as Raman microscopy, the method comprising the following steps: recovery of the laser beam 11 from the pulse laser source 10,

separating the laser beam 11 into first and second sub-beams 21, 22,

applying a frequency offset to the first sub-beam 21 by means of the first optical fiber 211,

application to the first sub-beam 21 at the output of the first optical fiber of a first frequency drift,

applying a time delay to the first and second sub-beams to synchronize the first and second sub-beams at the output of the second optical fiber,

combining the first and second sub-beams by means of the combiator blade 232 and injecting these sub-beams into the second optical fiber 233,

application to the first and second sub-beams 21, 22 combined a second time frequency drift by means of the second optical fiber 233, so that the first and the second sub-beam 21, 22 has a substantially identical frequency time drift in output of the second optical fiber.

The optical device 20 according to this first embodiment can operate according to a first variant in imaging to provide a given excitation. In this embodiment, the optical device 20 is configured to provide pump and Stokes radiation whose wavelength is fixed and therefore with a previously fixed offset between them.

According to a second variant, the optical device 20 according to this first embodiment can operate in spectroscopy or an adjustable excitation imaging. According to this second variant, the shift in wavelength, and therefore in frequency, between the pump radiation and the Stokes radiation is provided, for a low frequency change (that is to say less than a frequency corresponding to the number 300 cm) by a variation of the delay between the Stokes radiation and pump radiation, and for a significant frequency change (that is to say greater than a frequency corresponding to the wavenumber 300 cm), by a modification of the power of the second sub-beam 21. This last modification of the power of the second sub-beam 21 may be performed either by modifying the portion of the laser beam 11 transmitted in the second channel 210, or by modifying the power emitted by the laser source 10. In the same way, during the variation of the Stokes wavelength, and therefore the wavelength shift of the second sub- beam 21, the frequency drift transmitted by the glass block 212 and the time shift of the delay line are also varied.

In this second variant, the conservation of frequency drift by the glass block 212, for a significant frequency change of the first sub-beam 21, can be obtained by a substitution of the glass block 212 by another glass block. Thus, it is possible to provide the optical device 20 with a batch of glass blocks, each adapted to provide a frequency drift in a given Stokes radiation wavelength range. According to this same variant, this same preservation of the frequency drift can be done by adding an additional block of glass, the latter making it possible to apply an additional frequency time drift to the first sub-beam 21.

In the context of this second variant and still for a significant frequency change of the first sub-beam 21, the optical excitation method that allows to implement the optical device 20 further comprises the following steps:

modifying the frequency offset applied to the first sub-beam 21 by means of a first optical fiber 211, this by modifying the power of the laser beam 10 transmitted in the first optical channel, and so as to modify the wavelength shift between the first and the second sub-beam at the output of the second optical fiber 234,

correction of the first time frequency drift, so as to correct the temporal frequency drift variation generated by the modification of the frequency offset and to provide the same time drift of frequency between the first and the second sub-beam at the output of the second optical fiber 233,

correction of the applied time delay so as to synchronize the first and second sub-beams 21, 22 at the output of the second optical fiber 233.

In the context of this method, the modification of the frequency offset applied to the first sub-beam 21 by means of a first optical fiber 211 can be done either by modifying the power of the laser beam 11 emitted by the laser source 10 or by modifying the portion of the beam transmitted in the first optical path 210 this by changing the configuration of the first separator plate 231, or by the addition before the fiber 211 of a polarizer preceded by a half wave plate.

FIG. 5 illustrates the resolution benefit obtained by the use of frequency drift in the context of the invention. Indeed, this figure compares the spectral resolution 461 obtained by means of an optical device 20 according to the invention, this taking advantage of the possibilities offered by the frequency drift, with the spectral resolution 462 obtained by means of a device according to the prior art not implementing a frequency drift. To put into perspective and illustrate the needs of the spectral resolution required for Raman Stimulated measurements, these two spectral resolutions 461, 462 are put in parallel with a Raman spectrum Stimulated 463 can not be more conventional. It can thus be seen that, in view of the fineness of the observable Raman Stimulated peaks, only the resolution allowed by the optical device 20 according to the invention should make it possible to reproduce the details of the stimulated Raman spectrum.

FIGS. 6A to 6C show the result of stimulated Raman measurements made by means of an optical device 20 according to the invention on polystyrene beads for three different non-linear excitations, the optical frequency differences between the first and the second under beam 21, 22 corresponding to the respective wave numbers of 1003 cm ¹ , 1040 cm ¹ and 1080 cm ¹ .

The experimental conditions for performing these measurements are as follows: a laser beam 11 whose wavelength is set at 800 nm for a pulse duration of 120 femtoseconds and a rate of 80 MHz for a power of 800 mW,

a separator plate transmitting 5% of the laser beam towards the second optical channel 220, ie 40 mW, the remainder being transmitted to the first channel, ie 95% of the laser beam 11 or 760 mW,

a Stokes fiber having a length of about 2 m, a sub-beam injection rate of about 50% to obtain an offset of 170 nm for an injected power of 53 mW and a shift of 250 nm for an injected power of 103 mW, during the measurements presented in FIGS. 6A to 6C, the Stokes beam being centered on 870 nm and delivering a power of 2.5 mW,

the application to the sub-beam 21 of a first time frequency drift by a BBO type glass block with a length of 8 cm inducing by internal reflections an optical path of about 15 cm,

a polarization splitter cube 213A inducing an optical path of 5 cm of glass and therefore an additional frequency time drift in addition to being a component of the delay line,

a combi blade consisting of a high-pass filter with a cut-off wavelength of 950 nm, the properties of this filter, inclined at 45 °, for transmitting the Stokes beam and for reflecting the pump beam,

a half-wave plate on each beam in order to control the power delivered by the microscope 30,

a second optical fiber 233 of length 1.8m, microstructured, having a mode diameter of 12.5 μιη, the two beams being collimated at the output of this second optical fiber by a lens of 35 mm focal length in order to adapt their mode at the rear pupil of the objective of the microscope 30,

pump power of 20 mW and Stokes beam of 1 mW. The polarizations of the two beams are collinear.

FIG. 6A illustrates the spectral resolution capacity of the invention by showing the specific signal I / IN ratio (called "resonant" and whose intensity is noted I) on the non-specific signal (called "non-resonant" and whose intensity is noted I NR) obtained for each of the measurements. It can be seen in FIG. 6A that, since this ratio is directly related to the intensity of the Raman spectrum, the Raman spectrum would not be accessible in the case of pulses having no frequency drift. It is thus more precisely observed that the ratio IR / I NR decreases very rapidly when the vibration frequency moves away from the wave name is 1003 cm ¹ which corresponds to a strong resonance of polystyrene to reach at 1060 cm ¹ a value low. This shows the spectral selectivity of the stimulated Raman measurements performed by means of an optical device 20 according to the invention.

Figures 6B to 6C illustrate this phenomenon. Indeed, the image of the figure

6B corresponding to the non-linear excitation whose wave number is 1003 cm ¹ corresponds to the resonance of the polystyrene mentioned above, while the 6C image corresponds to the non-linear excitation excluding resonance for a number of wave of 1040 cm ¹ . It can thus be seen that the images obtained on the resonance and off resonance are significantly different, this for a near excitation difference since the excitation difference is only 37 cm ¹ between the image of FIG. 6B and FIG. 6C. Thus, the frequency focusing allowed by the device 10 according to the invention makes it possible to significantly increase the spectral resolution for the non-linear measurements.

FIG. 7 illustrates an optical device 20 according to a second embodiment in which the first optical channel 210 comprises an amplification system implementing a parabolic amplification process which makes it possible to amplify the first sub-beam 21 while at the same time imposing a temporal drift of perfectly linear frequency. An optical device 20 according to this second embodiment differs from an optical device 20 according to the first embodiment in that it does not comprise a glass block, the majority of the time frequency drift applied to the first sub-unit. beam being provided by means of an optical amplifier 214.

As shown in FIG. 7, a device according to this second embodiment differs only in its second optical channel 210. Indeed, the latter comprises:

the first optical fiber 211, the delay line 213 disposed at the output of the first optical fiber

211,

the optical amplifier 214 adapted to amplify the first sub-beam by applying a frequency drift time.

The optical amplifier 214 comprises a pump source 214A and a third optical fiber 214B adapted to allow the optical amplification of the first sub-beam 21. The optical amplifier 214 may as well be a rare-earth doped optical amplifier that Raman-type optical amplifier, or even a rare-earth-rare / Raman hybrid optical amplifier or a parametric amplifier. Such optical amplifiers have the advantage, in the context of the invention, of providing a frequency drift that varies with the amplification gain applied to the optical signal. In this way, the optical device 20 may have, with such an optical amplifier 214, a time drift of adaptable frequency this by adjusting the gain from the power emitted by the pump source.

In the case where the optical amplifier is a rare-earth type optical amplifier, the third optical fiber 214B is an earth-rare doped optical fiber, such as Er erbium, Ytterbium Yb, Thullium Tm, Neodymium Nd and Praseodyne Pr. Of course, the doping type of the third optical fiber and the wavelength of the pump source are chosen as a function of the wavelength of the desired pump radiation. Thus, for example, for a Stokes beam centered on 1060 nm, it is possible to use a configuration like that used in the works of Fermann and his coauthors published in 1999 in the scientific journal "Optics Letters" Volume 24 number 20 pages 1428 1430, in which a ytterbium-doped 4.6 m fiber 214B is used, together with a 2W pump source at 976 nm, which makes it possible to provide a gain of more than 30 dB. This type of optical amplifier is quite accessible design or purchase for the skilled person.

In the same case of an optical amplifier 214 of the rare earth doped type, the wavelength of the second beam must be fixed to allow optimized amplification. Thus, for the variants of the invention in which the frequency shift between the pump radiation and the radiation Stockes must be variable, as the applications to spectroscopy or multi-energy imaging, it is necessary to operate at wavelength of the first sub-beam 21, and thus the fixed Stokes radiation, the setting of the frequency shift between the pump and Stokes radiation is by modifying the wavelength of the laser beam 11 and thus the pump radiation. This configuration is described more precisely later in this document.

The optical amplifier 214 may also be a Raman amplifier. This type of optical amplifier has the advantage of allowing an amplification of a radiation over a relatively wide range that depends on the wavelength of the pump source 214A and using silica optical fibers. For example, for a pump source 214A having a transmission wavelength 1455 nm, it is possible to obtain a gain for a wavelength range from 1500 nm to more than 1600 nm, the gain being particularly significant for a wavelength range of 1530 to 1570 nm. In addition, the range of wavelengths in which it is possible to obtain a gain depending solely on the emission wavelength of the pump source 214A, it is possible, with a pump source whose length of emission wave is parameterizable to obtain amplification over a range of wavelengths of the second relatively wide sub-beam. With such an optical amplifier, it is therefore possible to envisage a configuration similar to that of the optical device 20 according to the second variant of the first embodiment.

Nevertheless, in order to allow the provision of an optical device 20 according to the invention relatively compact, in a usual configuration of the invention, whatever the type of optical amplifier, the pump source is a laser diode having a length of fixed emission wave. According to this possibility and as explained above for the amplification of the rare-earth doped type, only the wavelength of the laser beam 11, and therefore of the second sub-beam 22 at the output of the second optical channel 220 is modified, the wavelength of the first sub-beam 21 at the output of the first optical channel 210 being fixed by the optical amplifier 214.

In this conventional configuration and according to a first variant of the same type as that of the optical device 20 according to the first embodiment, the optical device 20 can be configured to function in imaging and to provide a excitation given. The device according to this second embodiment and according to this first variant makes it possible to implement an excitation method identical to that of the optical device 20 according to the first variant of the first embodiment.

In the same way according to a second variant and in this same conventional configuration, the optical device 20 can also be configured to operate in spectroscopy or to operate in adjustable excitation imaging. According to this possibility, the optical device 20 is necessarily intended to equip a measuring system 1 comprising a laser source 10 whose emission wavelength is parameterizable. Indeed, a large variation (that is to say greater than a frequency corresponding to the wavenumber 300 cm) of the non-linear excitation provided by the optical device 20 requires to vary the wavelength of the laser beam 11.

Thus according to this second variant, during a significant modification of the non-linear excitation provided by the optical device 20, that is to say greater than an optical frequency corresponding to a wave number of 300 cm, the wavelength of the laser source 10 is varied so as to have the required wavelength shift vis-à-vis the wavelength of the pump radiation, that is to say the first sub-beam 21. The power of the laser source and / or the configuration of the first splitter plate 231 are modified so as to output the first optical fiber with the wavelength shift required of the first sub-beam 21 corresponding to the length. amplification waveform of the optical amplifier 214. The power of the pump source 214A is then adjusted to provide the frequency time pre-drift necessary for the frequency drift of the first sub-beam 21 output of the second the optical fiber 233 coincides with that of the second sub-beam 21 after modification of the wavelength of the laser beam 11.

In this way, and contrary to that of the first variation mode, it is possible with an optical device 20 according to this second embodiment to significantly modify the nonlinear excitation allowed by the optical device 20 without having to intervene directly. in the optical device. Indeed, it is not necessary, as is the case in the first embodiment, to physically intervene on a pre-drift system of the second channel, since simple modification of the power of the pump source makes it possible to adjust the time drift frequency.

It will be noted that according to this second variant and this second embodiment, the wavelength of the second sub-beam being intended to be varied over a relatively wide frequency range, the sizing of the second optical fiber 233 to allow drifting. time-of-frequency is configured on the basis of the lowest frequency of this given frequency range, so as to provide a suitable frequency drift over the entire width of the frequency range. Thus, while a second optical fiber 234 with a length of 1.6 m is sufficient in the first embodiment, this same optical fiber preferably has a length greater than 3 m.

The optical device 20 according to this second variant and this second embodiment makes it possible to implement an excitation method which is different from that of the optical device 20 according to the second variant of the first embodiment in that it is in addition planned the next step:

Before the step of modifying the frequency offset applied to the first sub-beam 21, modification of the emission wavelength of the laser source 10,

And in that the step of modifying the frequency offset applied to the first sub-beam 21 is adapted to compensate for the change in emission wavelength of the laser source so that it does not lead to for modifying the wavelength of the first sub-beam 21 at the output of the second optical fiber 234.

It will be noted that such an adaptation can easily be obtained by means of an optical amplifier, it is also possible to obtain it by means of a frequency shift obtained by means of blocks of glass and / or third optical fibers, their combination being modified during the modification of the emission wavelength of the laser source in order to adapt the frequency pre-drift applied to the first beam 21.

It may be noted that it is also possible in this same configuration and in the case where the optical amplifier 214 is an amplifier of the type Raman, to work with a laser beam 11 whose wavelength is fixed, the wavelength of the first sub-beam 21 being varied in a reduced range of wavelengths contained in the range of wavelengths in which the gain of the optical amplifier 214 is particularly significant. According to this possibility, the modification of the excitation is carried out by varying the frequency offset supplied by the first optical fiber 211 by modulating the power of the first sub-beam at the input of the first optical channel 10.

Claims

An excitation optical device (20) for generating stimulated Raman processes, said optical device (20) is adapted to receive the laser beam (11) of a pulsed laser source (10),

the optical device (20) comprising:

an optical splitter for separating the laser beam into a first and a second sub-beam (21, 22) on a first and a second optical path (210, 220) of the optical device (20),

the optical device (20) being characterized in that the first optical channel (210) comprises:

a first optical fiber (211), called Stokes, configured to provide an optical frequency shift of the first sub-beam (21),

a frequency pre-drift system adapted to apply a first frequency drift to the first sub-beam (21),

the optical device (20) further comprising a second optical fiber (233), called a frequency drift, arranged to recover the first and second sub-beams (21, 22) at the output of the first and second channels ( 210, 220) and for applying thereto a second time frequency drift, the second optical fiber (233) and the frequency pre-drift system being configured so that the first and second sub-beams (21, 22) have a frequency drift. time drift of substantially identical frequency at the output of the second optical fiber (233), the second optical fiber having a mode diameter greater than 5 μιη and preferably 10 μιη,

at least one of the first and second channels (210, 220) includes a time delay system for adjusting the relative delay between the first and second sub-beams (21, 22) at the output of the second optical fiber ( 233).

An optical device (20) according to claim 1, wherein the first optical fiber (211) is a microstructured optical fiber shaped to obtain from a nonlinear effect a frequency shift in the form of a super continuum, either in the form of a single offset wave, so as to allow a variable frequency offset depending on the power of the first sub-beam (21) at the input of the first fiber (211).

3. Optical device (20) according to claim 1 or 2, wherein the frequency pre-drift system is adaptable according to the wavelength of the first and second sub-beam (21, 22) at the output of the second optical fiber.

An optical device (20) according to any one of claims 1 to

3, wherein the frequency prediction system comprises an optical amplifier (214) adapted to amplify the first sub-beam (21).

An optical device (20) according to any one of claims 1 to

4, wherein the frequency pre-drift system comprises at least one glass block (212) or at least one third optical fiber.

An optical device (20) according to any one of claims 1 to

Wherein the second optical fiber (233) is a microstructured optical fiber whose core is undoped.

7. Optical device (20) according to claims 1 to 6 wherein the second optical fiber (233) has a length greater than 1.5 m preferably greater than 3 m.

8. A stimulated Raman process measurement set (1) comprising a pulse laser source (10), an excitation optical device (20) for generating stimulated Raman processes and an optical measuring device (30), such as a microscope or a spectrometer, the measuring assembly (1) being characterized in that the optical device

(20) is an optical device according to any one of claims 1 to 7.

The measurement assembly (1) according to claim 8, wherein the laser source (10) and the optical device (20) are configured to allow a parameterizable nonlinear excitation.

An optical excitation method for performing optical measurements using stimulated Raman processes such as Raman microscopy or Raman spectroscopy measurements, the method characterized by comprising the steps of:

recovering a laser beam (11) originating from a pulsed laser source (10),

separating the laser beam (11) into a first and a second sub-beam (21, 22),

application of a frequency shift to the first sub-beam

(21) by means of a first optical fiber (211), called Stokes,

applying to the first sub-beam (21) at the output of the first optical fiber (211) a frequency time drift,

combination of the first and second sub-beams in a second optical fiber (233), said frequency drift, said second optical fiber (233) having a mode diameter greater than 5 μιη and preferably 10 μιη applying a drift temporal frequency to the combined first and second sub-beams (21, 22) by means of the second optical fiber (233) so that the first and second sub-beams (21, 22) exhibit frequency time drift substantially identical output of the second optical fiber, wherein it also provided before the combination of the first and second sub-beam (21, 22), an application of a time delay to at least one of the first and second sub-beam (21, 22) -beams (21, 22) for adjusting the relative delay between the first and the second sub-beam (21, 22) at the output of the second optical fiber (233).

The optical excitation method according to claim 10, wherein the following steps are further provided:

modifying the frequency offset applied to the first sub-beam (21) by means of a first optical fiber (211), this by modifying the power of the laser beam (10) transmitted in the first optical channel, and so as to modify the wavelength shift between the first and second sub-beams at the output of the second optical fiber (234)

correction of at least one of the first and second time frequency drift, so as to correct the frequency drift variation generated by the change in the frequency offset and to provide the same time frequency drift between the first and second frequency drift the second sub-beam at the output of the second optical fiber (234),

correction of the applied time delay so as to synchronize the first and second sub-beams (21, 22) at the output of the second optical fiber (233).

The optical excitation method according to claim 11, wherein the following step is further provided:

before the step of modifying the frequency offset applied to the first sub-beam (21), modifying the emission wavelength of the laser source,

wherein the step of modifying the applied frequency offset is adapted to compensate for the emission wavelength change of the laser source so that the frequency change applied to the first sub-beam (21) does not occur. causes no change in the wavelength of the first sub-beam (21) at the output of the second optical fiber (233).

13. The method of claim 11 or 12, wherein the step of applying to the first sub-beam (21) at the output of the first optical fiber (211). a temporal frequency drift is implemented by means in particular of an optical amplifier (214) adapted to amplify the first sub-beam (21).

14. A method according to any one of claims 11 to 13, wherein the step of applying to the first sub-beam (21) at the output of the first optical fiber (211) of a frequency drift is implemented. in particular by means of at least one glass block (212) or at least one third optical fiber.

15. Use of an optical device (20) according to any of claims 1 to 7 for performing stimulated Raman measurements such as stimulated Raman imaging or spectroscopy.