WO2013189811A1

WO2013189811A1 - Organic laser system and method for modifying the emission wavelength of the laser system

Info

Publication number: WO2013189811A1
Application number: PCT/EP2013/062194
Authority: WO
Inventors: Sébastien CHENAIS; Sébastien FORGET
Original assignee: Universite Paris 13
Priority date: 2012-06-18
Filing date: 2013-06-12
Publication date: 2013-12-27
Also published as: FR2992107A1; FR2992107B1

Abstract

The invention relates to a laser system (8) in which each amplification element (20) is a solid medium comprising at least a first layer (24) having a thickness (e1) lower than 100 μm, wherein the molecules suitable for emitting light radiation of each amplification element (20) form an organic gain medium (22) contained in the first layer (24) of the amplification element (20).

Description

An organic laser system and method for modifying the emission wavelength of the laser system

The present invention relates to a laser system comprising a laser cavity and a plurality of interchangeable organic gain media. The invention also relates to a method for modifying the laser emission wavelength of the laser system and also relates to the use of the laser system in the context of spectroscopy for example. The targeted applications are numerous and potentially cover biophotonics, spectroscopy, medical applications, metrology, research, this list is not restrictive.

In the field of biophotonics, measurements made on biological media are of several types. They are based on the absorption or on the fluorescence of these media. However, most of these environments absorb coherent light and have a wavelength in the visible or the ultraviolet.

The laser is therefore generally used for this type of application. A laser is a device emitting stimulated emission amplified light. The term comes from the acronym "Light Amplification by Stimulated Emission of Radiation", which translates into French by amplification of light by stimulated emission of radiation. Thus, the laser produces a spatially and temporally coherent light beam based on the laser effect.

In the same field of biophotonics, certain measures involve the use of a fluorophore grafted to the substance to be studied. A fluorophore is a compound that absorbs light whose wavelength is in the visible or ultraviolet. In laboratory practice, about ten fluorophores used absorb between 325 nm and 800 nm. There is therefore a need for multiple wavelengths.

In the field of medicine, laser treatment aims to precisely treat certain tissues and not others to avoid collateral damage. This is possible for chromophores of the human body, such as melanin (responsible for the coloring of hair, skin and eyes), hemoglobin or water.

A chromophore is a group of atoms having one or more double bonds, and forming with the rest of the molecule a sequence of conjugated double bonds, that is to say an alternation of double and single bonds. The existence of a sufficiently long sequence of conjugated double bonds in an organic molecule creates a delocalized electron cloud that can resonate with the incident radiation and thereby absorb it. Chromophores are therefore responsible for the colored appearance of certain organs in medicine. Each chromophore absorbs ranges of precise wavelengths. Thus, melanin absorbs for example light radiation whose wavelength is between 700 nm and 1100 nm. As a result, Alexandrite lasers emitting at 755 nm, AIGaAs diodes emitting at 810 nm or the Nd: Yag laser emitting at 1064 nm are used in the state of the art to target melanin. Hemoglobin has absorption bands around blue, green and yellow. But, hemoglobin absorbs very little at 660 nm which allows treatment in case of haemorrhage. In ophthalmology, the treatment of retinal detachment is done by photo-coagulating the blood to achieve a weld of the surface of the retina with a laser emitting at 577 nm. Thus, multiple different wavelengths covering the entire visible spectrum are involved in medical treatments because of the multiplicity of treated tissues. We can also mention flow cytometry.

In the field of spectroscopy, it is often essential to excite at different wavelengths the object to be characterized or to change the wavelength because the object studied is not always the same. Several wavelengths are thus used for the excitation light source (s).

This need also concerns LIDAR, when it is used to characterize diffusing media. Indeed, the LIDAR is a technique using the flight time of the light to determine the distances. But, in some configurations, it is used to characterize the nature of the molecules of the atmosphere or soils.

Thus, there is a need for multiple emission wavelengths of laser sources. This need is usually covered by a plurality of different lasers. Because of the cost and complexity involved, it is desirable to reduce the number of sources to be used to cover a wide wavelength range from ultraviolet to infrared.

To fill this need, it is known to use lasers having a wide range of tunability.

A known system for obtaining coherent coherent radiation in the visible is the optical parametric oscillator (also referred to as OPO). A parametric oscillator is a coherent and monochromatic light source. From a frequency pump laser wave ω _ρ , an optical parametric oscillator produces two waves of lower frequencies: the signal at the frequency oo _s is the complement (sometimes called IDLER) at frequency ω ₀ . The conservation of the energy imposes that the sum of the frequency of the signal and the complementary one is equal to the initial frequency of the pump, which is written mathematically like:

00p =) ₀ + 00 _s . The frequency conversion from the pump to the two generated waves is done using a non-linear optical interaction.

Such systems are bulky, very complex and very expensive.

Another type of coherent sources tunable in the visible are the sources called "Supercontinuum". These are sources based on the generation of a very broad spectrum by non-linear effects (for example Raman) in a microstructured optical fiber. These sources have a wide tunability in the visible and infrared (from 400 to 1800 nm, some manufacturers offer an extension up to 350 nm).

They have many disadvantages, especially for biophotonic applications. Indeed, since all the colors are produced simultaneously by non-linear effects in a microstructured fiber, the photons whose wavelengths are not desired must be filtered. Thus, on several Watts of total optical power, there are only a few milliwatts left in the spectral range of interest (a few picojoules per pulse) - which means that less than 0.1% of the total power is available to excite a specific fluorophore.

The most known and used tunable lasers are dye lasers. Indeed, the dyes of gain media provide a broad spectrum of emission. In the conventional dye laser system, a dye stream diluted in a solvent acts as a gain medium. The dye may be rhodamine, coumarin or any other known dye. As a solvent, methanol or ethylene glycol may be mentioned.

For a given dye, the tunability obtained is several tens of nanometers and, by dye change, a wide range of visible wavelengths becomes accessible.

However, these lasers pose many problems. The dye change is long and laborious, especially because of the many purges required to clean the old dye from the dye pumping system.

In addition, the products used are often toxic. This toxicity can be manifested by inhalation, skin contact or injection. It causes serious disorders such as blindness or cancer.

In addition, due to photobleaching of the organic molecules, a regular change of the dye solution is required for proper operation of the dye laser.

Finally, dye lasers are expensive and bulky.

There is therefore a need for a laser system for obtaining a broad spectral band, this laser system being easy to implement.

It is therefore desirable to be able to propose a laser system making it possible to obtain a broad spectral band, this laser system being easy to implement. According to the invention, this object is achieved by a laser system comprising a pumping source adapted to excite a transition to at least one pump wavelength. The laser system comprises a plurality of solid organic amplifier elements comprising a first layer having a thickness of less than 100 μηι and a gain medium contained in the first layer and adapted to emit light radiation at a stimulated emission wavelength. different under the effect of excitation at least one pump wavelength. At least two gain media of two amplifying elements of the plurality of amplifier elements are adapted to emit light at different stimulated emission wavelengths under at least one wavelength excitation. pump. The laser system includes an organic laser adapted to emit a laser beam at a plurality of emission wavelengths including a fixed output coupler adapted to transmit radiation to at least a portion of the or each wavelength of transmission, a support capable of accommodating an amplifier element among the plurality of amplifier elements in an identical position with respect to the output coupler for each amplifier element of the plurality of amplifier elements and such that for an amplifier element in the position, the laser emits a laser beam from the light radiation of the gain medium when the gain medium is excited by the pump source.

According to particular embodiments, the laser system comprises one or more of the following characteristics, taken separately or in any technically possible combination:

for all the amplifier elements in the position, the laser emits a laser beam having a factor M ^{2 of} less than 2.0.

for all the amplifying elements in the position, the laser emits a laser beam having a factor M ^{2 of} less than 1.2.

the materials of the gain media are chosen so that the laser emits laser beams at wavelengths between 380 nm and 800 nm.

each amplifier element of the plurality of amplifier elements is a consumable.

the laser has a cavity defining a laser emission direction and the pump source is an optical source emitting at least one pump wavelength in a non-perpendicular direction with the laser emission direction.

the support is a mobile disk in rotation comprising housings for several amplifying elements.

the support is a fixed base comprising a housing for an amplifier element. each amplifying element comprises a second layer serving to support the first layer, the second layer being in a transparent solid material at the or each emission wavelength, and

the amplifying element comprises a third layer placed on one face of the first layer, the other face of the first layer being in contact with the second layer, the third layer being airtight.

The invention also relates to a method for modifying the emission wavelength of a laser of the system as described above, the method comprising the steps of choosing at least one emission wavelength, choice an amplifier element among the plurality of amplifier elements as a function of the selected transmission wavelength and positioning of the amplifier element at the position.

The present invention also relates to the use of the laser system as described above to perform the spectroscopy of a molecule.

Other features and advantages of the invention will appear on reading the following detailed description of embodiments of the invention, given by way of example only and with reference to the drawings which show:

FIG. 1, a schematic view of a laser system according to the invention;

- Figure 2, a view of an amplifier element according to the invention in cross section along the axis II-ll shown in Figure 1;

FIG. 3, a view of a laser system according to another embodiment of the invention;

FIG. 4, a view of a laser system according to yet another embodiment of the invention, and

- Figure 5, a graph illustrating the tunability obtained by a laser system according to the invention.

The laser system 8 according to the invention comprises a laser 10. The laser 10 is adapted to emit a laser beam at a plurality of emission wavelengths λ _Ε . In what follows, by misnomer, the expressions "the laser 10 emits at λ _Ε " or "the laser 10 emits at one (or more) wavelengths λ _Ε " are used to signify that the laser 10 emits a laser beam at one (or more) emission wavelength (s) λ _Ε .

The laser 10 thus comprises a cavity 12 formed by an input mirror 14 and an output coupler 16. According to the example of FIG. 1, the mirror 14 and the coupler 16 appear as cylindrical mirrors for the convenience of three-dimensional representation. it being understood that they are spherical mirrors.

The output coupler 16 is fixed. In addition, it is adapted to transmit radiation to at least a part of the or each emission wavelength λ _Ε . The laser system 8 also includes a source 18 for pumping. The pump source 18 is adapted to excite molecules exhibiting non-zero absorption at at least one pump wavelength λ _Ρ .

In particular, the source 18 may be a laser. In particular, a doubled or tripled frequency Nd-YAG laser is particularly interesting. An Nd-YAG laser is a laser using an Nd-YAG crystal (acronym of the English name: neodymium-doped yttrium aluminum garnet) or neodymium-doped yttrium-aluminum garnet (chemical formula Nd: Y ₃ AI ₅ 0i ₂ ) . The dopant of the triply ionized neodymium typically replaces yttrium in the crystalline matrix, the two elements having a similar size. A doubled Nd-YAG laser emits at a wavelength of 532 nanometers (nm) while a tripled Nd-YAG laser emits at a wavelength of 355 nm.

In a variant, the source 18 is a laser diode emitting waves having a wavelength for example around 400 nm.

In another embodiment, the pump source 18 is a plurality of light-emitting diodes.

For the targeted applications, the pump wavelength (s) emitted by the source 18 are between 150 and 700 nm, preferably between 335 nm and 532 nm.

The laser system 8 also comprises an organic amplifier element 20, this amplifying element 20 comprising molecules adapted to emit light radiation at stimulated emission wavelengths under the effect of excitation at least one length of time. pump wave λ _Ρ .

According to the example of Figure 1, the amplifier element 20 is in the form of a cylinder whose base is a disk. This disk can be of so-called "standard" size, that is to say a disk one inch in diameter (2.54 cm (cm)).

The laser 10 comprises a support 21 capable of accommodating the amplifier element 20 in a position P with respect to the coupler 16 in which the laser 10 emits a laser beam.

In the P position, the pump source 18 thus serves to excite the molecules of the organic enhancer element. For the case of an optical type pumping source 18, the amplifying element is adapted to emit laser light at the wavelength _λ under the effect of pumping at the wavelengths of the pump. λ _Ρ .

In the particular case of FIG. 1, the support 21 is a disk 21 mobile in rotation about an axis X substantially parallel to the axis of the cavity 12.

The disc 21 comprises, for example, nine housings for the amplifying elements 20. In the example of FIG. 1, the disc 21 thus comprises nine amplifying elements 20A, 20B, 20C, 20D, 20E, 20F, 20G, 20H and 20I. . The nine amplifying elements 20A, 20B, 20C, 20D, 20E, 20F, 20G, 20H and 20I are organic and comprise molecules adapted to emit light radiation at stimulated emission wavelengths under the effect of excitation at at least one pump wavelength λ _Ρ . In addition, each amplifier element 20A, 20B, 20C, 20D, 20E, 20F, 20G, 20H and 201 is a solid medium having at least a first layer 24.

The section of the amplifying element 20A of FIG. 2 schematically illustrates that the molecules adapted to emit light radiation form an organic gain medium 22 contained in the first layer 24 of the amplifying element 20. In the context of this invention, the expression "content" means that the molecules adapted to emit radiation are in the first layer 24 and, if they are present in another layer, it is in the trace state.

By way of example, the four amplifying elements 20A, 20B, 20C and 20D are four distinct embodiments for the gain medium 22.

For the amplifier element 20A, the gain medium 22 is an organic semiconductor in the form of conjugated polymers. More specifically, the gain medium 22 of the amplifying element 20A is poly [2-methoxy-5- (2-ethylhexyloxy) -1,4-thenylenevinylene] (also referred to as MEH-PPV). ). This gain medium 22 has a tunability of between 560 and 650 nm.

Alternatively, the gain medium 22 of enhancer 20A is poly [(9,9-di-n-octylfluorenyl-2,7-diyl) -alt- (benzo [2,1, 3] thiadiazol-4 , 8-diyl) also referred to as F8BT. This example of a gain medium 22 with conjugated polymers has a tunability of between 530 and 600 nm.

In the case of the amplifier element 20B, the gain medium 22 is a pure dye. Specifically, the gain medium 22 of the enhancer element 20B is 4-di4'-tert-butylbiphenyl-4-yl amino-4'-dianovinylbenzene is used as a dye for the gain medium 22. This molecule is also known under the acronym FVIN. Under appropriate pumping, the gain medium 22 of the amplifier element 20B provides the laser 10 with a tuning range of between 590 nm and 630 nm.

According to the variant of the amplifier element 20C, the gain medium 22 comprises several small molecules. The first layer 24 of the amplifier element 20C is then a mixture of dyes. For example, the gain medium 22 of the enhancer element 20C is the mixture of the 4- (diacinomethylene) -2-methyl-6-p-di-methylaminostyryl) -4H-pian molecule (also referred to by the acronym DCM) which is doped in a molecule of tris (8-hydroxyquinoline) aluminum (III) (also referred to as Alq 3).

The amplifier element 20D comprises a first layer 24 according to yet another variant. In this case, the first layer 24 of the amplifier element 20D comprises a dye doped in a non-conductive polymer. According to the example of the amplifier element 20D, the non-conductive polymer is PMMA. PMMA is polymethyl methacrylate. PMMA has many advantages, mainly its transparency and resistance. PMMA thus replaces ordinary glass because of its better transmission of light (ie a theoretical transmission of 99.98% at 500 nm and for a first layer 24 of 1 cm long, this value does not take into account the reflections of Fresnel interfaces and possible defects that can lead to diffusion).

The dye employed in mixing with PMMA is rhodamine 6G in the case of the 20D amplifier element. Such a dye allows the laser 10 to emit a laser beam whose wavelengths are between 550 nm and 590 nm.

Other dyes are conceivable, such as coumarin 450, resulting in a tuning range between 430 nm and 480 nm, or pyromethene 597 enabling laser 10 to emit a plurality of waves whose emission wavelength is between 550 nm and 610 nm. The three dye molecules mentioned here are just a few of the hundreds of molecules that can exist commercially.

The thickness e1 of the first layer 24 is less than 100 micrometers (μηι). Such a thickness e1 makes the amplifier element 20 easy to achieve, which reduces its manufacturing cost.

Preferably, to further improve this effect, the thickness e1 of the first layer 24 is between 100 nm and 20 μηι.

In order to obtain such thin thicknesses e1, methods of manufacturing the first layer 24 which are known methods are used. By way of example, the centrifugal coating technique can be used. Centrifugal coating, also known as spin coating, is a widely used method of thin film deposition on a flat surface. This method consists in depositing a drop on a rotating plate, the drop then spreading by centrifugation, to form a layer. The thickness of the deposited layer depends on several factors. Some factors are related to how the method is implemented such as the angular velocity (the larger it is, the thinner the thickness will be), the acceleration (the bigger it is, the thinner the thickness will be) or the time of the operation (the longer the operation, the thinner the layer is to a lesser extent). Other factors are related to the deposited compound: the amount deposited (usually one or a few drops), its concentration in the solvent, its molar mass, its viscosity, etc.

Alternatively, a dip-coating method may be used to manufacture the first layer 24. This technique is based on a principle similar to centrifugal coating. But in this case, the substrate is soaked in the solution and is removed with controlled speed and angle. Alternatively, another method called "doctor blading" is employable. According to this method, a razor blade is translated at a defined distance from the substrate for the purpose of spreading the organic material. The translation speed and the height of the blade make it possible to define the final thickness of the first layer 24.

Other methods are also possible. Thus, printing techniques such as ink jet deposition can be used.

The methods cited are especially used for gain media which involve the use of polymers, whether conjugated or not.

When the gain medium 22 is a pure dye as in the case of the amplifier element 20B, different techniques than those in solution described above can be used. In particular, thermal evaporation is a feasible technique for small molecules. The evaporable molecules are electrically neutral molecules whose molar mass is sufficiently small so that they can be evaporated at a temperature below their degradation temperature. Typically, this concerns organic molecules of a few hundred g / mol, small oligomers, and excludes polymers which have molar masses of up to hundreds of thousands of g / mol. Thermal evaporation consists of putting the pure dye molecules in a crucible and heating them, which causes them to evaporate. This evaporation takes place in a vacuum chamber causing deposition on the substrate at the top of the enclosure.

The amplifier element 20 of FIG. 2 has, in the case of this example, a second layer 26 serving to support the first layer 24. The material M2 of the second layer 26 is a solid material transparent to the or each length of pump wave λ _ρ . It is also transparent to the or each emission wavelength λ _Ε of the laser 10 and to the or each emission wavelength A _s of the gain medium molecules 22. The material M2 of the second layer 26 is in particular a glass slide, quartz, plastic or any other material transparent to the emission wavelength λ _Ε of the laser 10.

The thickness e2 of the second layer 26 is less than 2 mm.

While remaining within the scope of the invention, the example shown in Figure 2 for the amplifier element 20 is capable of multiple modifications. Thus, according to a particular embodiment, the second layer 26 comprises the input mirror 14. This is the variant described in Figure 3. Such a cavity is called VECSOL by analogy with cavities VECSEL. A VECSEL cavity is a surface-emitting vertical external cavity laser diode which is a type of semiconductor laser diode emitting a laser beam perpendicular to the surface, in contrast to conventional wafer-emitting semiconductor lasers. It should be noted that, in each of the embodiments, the output coupler 16 is placed at a distance from the amplifier element 20. Thus, the cavity 12, in the sense of the invention, is not a cavity 12 made entirely by layers of the amplifier element 20.

It is also conceivable for the amplifier element 20 to comprise a third layer placed between one face of the first layer 24 and the air, the other face of the first layer 24 being in contact with the second layer 26. be in contact with the layer 24 or separated from it by an air space, vacuum or gas. The third layer is airtight and moisture-proof. Such a property makes it possible to greatly increase the lifetime of the layer comprising the organic gain medium.

The increase in the lifetime of the amplifier element 20 can also be obtained with a support 21 comprising a lateral translation means of the amplifier element 20, this translation allowing the amplifier element 20 to remain in the same plane but that the area impacted by the light rays flowing in the cavity 12 is different. Thus, several active areas of the first layer 24 are usable. This is particularly interesting when an active area of gain medium 22 is photoblanked. It is then sufficient to use the translation means to pump a different zone from the first layer 24.

If a standard size amplifier element 20 is used, namely a disk 1 inch in diameter (ie 2.54 cm) and an active zone is a circle with a diameter of less than 1 mm, then there are more than 100 Fifty active zones in the first layer 24. Thus, the use of a translation means makes it possible to increase the lifetime of the amplifier element 20 by a factor of 150.

In addition, since the transverse dimensions of the amplifier element 20 are not limited a priori, an amplifying element 20 of larger dimensions can be considered. Thus, for a square surface plate, for example 5 cm by 5 cm, eight hundred active zones are available.

According to the studies carried out by the applicant, the life of a laser 10 defined as the time after which the output energy of the laser has been divided by two varies according to the compounds and the pumping power between 500 and 25,000 seconds per active zone (at a rate of 10 Hertz (Hz)). This figure can even reach a million seconds in the presence of a protection vis-à-vis the oxygen of the air (at 10 Hz).

An amplifier element 20 in the form of a plate having a square area of 5 cm by 5 cm with a translation system therefore has a life of between 100 and 5500 hours. This is more than enough for most applications.

In addition, if a longer time is required, simply replace the used amplifier element 20 with another new amplifier element 20. Indeed, according to the invention, each amplifier element 20 is a consumable, that is to say a disposable object of easy replacement and low cost.

In fact, it is proposed in this laser system 8, interchangeable amplifier elements. In the context of this invention, it is understood by the term "interchangeable" that the amplifier elements 20 are not only substitutable but also that the replacement of an amplifier element 20 by another is easy.

This ease of replacement is notably enabled by the fact that the position P in which the support 21 positions the amplifier element 20 is identical for all the amplifier elements considered.

Thus, by way of example, according to FIG. 1, to replace the amplifying element 20 by the amplifying element 20A, it suffices to rotate the support 21 around its axis X.

Other forms of support 21 make it possible to obtain this interchangeability. According to the example of Figure 4, there is provided a support 21 in the form of a rectangular base. The support 21 has a circular hole for accommodating an amplifier element 20. This support 21 having an upper crosspiece movable in rotation about an axis 32 placed at one end of the cross member 30. In this variant, it suffices, to replace an amplifier element 20A with another amplifier element 20B, to rotate the cross member 30 about the axis 32 to access the amplifier element 20A, to recover the amplifier element 20A, to position the new amplifier element 20B and rotate the cross member 30 in the opposite direction.

The ease of replacing an amplifier element 20 with another is also permitted in that each amplifier element 20 is a solid medium comprising at least a first layer 24 having a thickness e1 less than 100 μηι.

With a plurality of amplifier elements 20 such that at least two of the plurality of amplifier elements 20 comprise molecules adapted to emit light radiation at stimulated emission wavelengths A _S i and A _{S 2} which are different, the laser 10 can emit a plurality of wavelengths when the pump source 18 operates.

However, the tunability of the laser 10 with a single amplifier element 20 is greater than 15 nm, preferably greater than 30 nm.

By combining these tunings for several amplifying elements 20 whose stimulated emission wavelengths are distinct, the emission range of the laser system 8 can become very extensive when the entirety of the amplifier elements 20 is taken into account. The laser 10 may in particular emit between 380 nm and 800 nm. This is not a "white" laser since the emission of a single amplifier element 20 is reduced to one spectral range of about fifty nanometers but, the laser 10 can emit continuously throughout the visible spectrum by replacing the gain media 22.

In order to obtain a controllable spectrum and an emission wavelength change for a given amplifier element 20, as can be seen in FIG. 1, the laser 10 further comprises a system 34 for selecting the length of the transmission. wave.

In the laser 10 of FIG. 1, the control of the wavelength is carried out thanks to the system 34 of selection of the wavelength. In particular, the system 34 is a dispersive system such as a prism, a grating (surface etched or a volume Bragg grating), a birefringent filter (such as a Lyot filter) or a Fabry-Perot etalon.

For example, in the case of a Fabry-Perot type filter inserted into the cavity 12, a rotation of this filter along an axis included in one of its planes varies its optical thickness.

As a variant, this Fabry-Perot effect is obtained thanks to the first layer 24 itself, provided that its thickness varies.

It is also possible to include in the cavity nonlinear crystals such as barium betaborate (3-BaB ₂ O ₄ ), lithium triborate (LiB ₃ 0 ₅ ) or any other crystal adapted to double or triple the frequency of a laser.

It is then possible to have access for example to a tunable source of a deep ultraviolet if the initial laser emits in the red (doubling of frequencies) or in another wavelength range by taking advantage of various mechanisms belonging to nonlinear optics. The frequency sum or the frequency difference are known examples.

In addition, the beam at the output of the laser 10 has a factor M ^{2 of} less than 2.0 and preferably less than 1.2.

The factor M ² , also known as beam quality factor or beam propagation factor, is a common measure of the beam quality of a laser beam. According to the ISO 11146 standard, it is defined as the product of the Gaussian collar size by the half-divergence of the measured far-field beam divided by λ / π where λ is the wavelength. This parameter corresponds to the product obtained if we consider a Gaussian beam of the laser limited by diffraction at the same wavelength. In other words, it can be written that the angular half-divergence of the light beam is given by the following mathematical expression:

Θ = M ² λ / (ττ ω ₀ ), where:

• Θ is the angular half-divergence of the light beam;

• ω o is the radius of the beam at the waist; and

• λ the wavelength of the laser beam. According to the ISO 1114 standard, the factor M ² is calculated by measuring the evolution of the beam radius along the direction of propagation (this evolution is also called caustic). Alternative methods based on wavefront measurements such as Shark-Hartmann measurements also exist but are not considered in the context of this invention, the M ² factor being defined according to the standard 1 1 146. The good quality of beam is made possible by a longitudinal pumping, that is to say that the pumping of the cavity 12 is not perpendicular to the direction of emission of the laser 10, and preferably with an angle less than 20 °, or substantially equal at 0 °. It can thus be obtained an output beam limited by diffraction.

In addition, the efficiency defined as the output energy on the pump energy is relatively high, of the order of several tens of percent. The traditional structure of the laser with an external cavity compared to the so-called DBR (Distributed Bragg Reflector) solutions as well as the longitudinal pumping of the cavity explain such an efficiency. Longitudinal pumping permits perfect overlap of the pump beam and the laser mode. This results in maximum efficiency.

Thus, the laser system 8 avoids the practical difficulties of using a liquid dye laser while retaining the advantage of great tunability. In addition, the toxic risks are limited because the gain media used are solid. In addition, the proposed techniques are relatively inexpensive and already well established in the industry. Thus, a series production would be easy to implement.

The laser system 8 is compact and can cover the spectrum from ultraviolet to near infrared. Such a range of wavelengths makes possible applications in various fields, such as biophotonics, medicine, spectroscopy or the use of LIDAR for the characterization of diffusing media. In each of these examples, the laser system 8 makes it possible to cover this need in different wavelengths with a single laser 10 instead of a plurality as in the state of the art.

The laser system 8 also makes it possible to provide the user with a laser 10 emitting a laser beam in the ultraviolet suitable for performing experiments based on the absorption or fluorescence of biological media.

Organic lasers are good candidates for use as gas sensors. In particular, it has been shown that the threshold of certain organic lasers depends on the quantity of molecules absorbed. These molecules can especially be explosive aromatic compounds, TNT. This makes them very sensitive sensors as has been shown in particular in the article A. Rose et al., Nature, Vol. 434, No. 7035, pages 876 to 879 (2005), or the article Yang et al, Advanced Functional Material, Vol. 20 pages 2093 to 2097 (2010). Their interest is however limited to the detection of a single species or group of species adapted to the emission of the laser. Being able to change wavelength and of emission enhancing elements for each measurement is a considerable advantage offered by the present system 8.

Example 1

An example of a laser system 8 has been implemented by the Applicant. This example is called "Example 1" in the following.

The pump source 18 in Example 1 is a pulsed laser. The duration of the transmitted pulses is 25 ns, the rate of 10 Hertz and the energy per pulse is 100 microjoules. The pump wavelength λ _Ρ chosen is 355 nm.

The cavity 12 comprises a plane mirror 14 allowing the pump light to pass with a transmission greater than 80% at 355 nm, for example, and reflecting the laser wavelength. For example, the reflection coefficient R is greater than 99% over the range 500 to 600 nm. This mirror 14 can also be treated with high reflectivity over the entire visible range, ie between 400 and 700 nm.

The second mirror 16 which is the output coupler is concave so that the cavity 12 is stable. For example, this radius of curvature is 200 mm. The second mirror 16 must be partially reflective to let out the laser beam. In the context of this example, its reflection coefficient R is 95% over the range 500 to 600 nm.

The gain medium 22 selected is PMMA for the second layer 26 and comprises pure coumarin 540, mixed with Pyromethene 567 and Rhodamine 640 and Pyromethene 597 for the first layer 24. Some dyes are used pure whereas others are mixed for the following reason: Pyromethene 567 for example does not absorb at 355 nm. By mixing it with Coumarin 540 ("donor"), the latter absorbs photons at 355 nm and transfers the energy thus acquired to Pyrromethene 567 ("emitter" or "acceptor") which then emits into the yolk. This energy transfer, called "de Forster", is very effective (up to 100%) for a suitable choice in the proportions of dyes (here 1% of each in PMMA). This applies only if the absorption spectrum of the transmitter covers the donor's emission spectrum, which is the case here.

In addition, the gain medium 22 is placed between the two mirrors 14, 16 at the Brewster angle relative to the axis of the cavity 12. The blade is placed at the Brewster angle with respect to the axis of the cavity so that the laser beam undergoes no loss (at the Brewster angle, the transmission is 100% for the polarization P). Incidentally, this also prevents the gain medium 22 behaves like a Fabry-Perot imposing its own modes.

When one is not perfectly at the angle of Brewster, the additional losses are induced in the cavity 12 which slightly decreases the performance of the laser 10. But, these losses are not crucial and a simple mechanical adjustment ensures positioning to one or two degrees which is largely sufficient.

In addition, to improve efficiency, the pump laser beam is focused on the amplifier element 20 with a diameter adapted to match the size of the laser mode. This diameter is furthermore calculated as a function of the size of the cavity of the radii of curvature of the mirrors 14, 16. In the case cited here by way of example, the cavity 12 is three centimeters long, which corresponds to a diameter of beam at the mirror 14 plane about 220 μηι.

The system 8 according to Example 1 further comprises a Fabry-Perot etalon 34 allowing the tunability. This standard 34 consists of a self-supporting thin film of 2 μηι thick. The material used is undoped PMMA (transparent throughout the visible) deposited by spin-coating on a substrate and then peeled off. The reflectivity is only a few percent on each face, but this is sufficient to modulate the losses in the cavity as a function of the wavelength and inclination of the standard.

The experimental results obtained are shown in FIG. 5. For a given material, by turning the standard a few degrees, the laser emission is tuned to about 50 nm. By changing medium to gain 22, it is placed on another range of wavelength. With four materials or mixtures of different materials (pure coumarin 540, mixed with Pyrromethene 567 and Rhodamine 640 and Pyromethene 597), a spectral range of more than 180 nm is covered in the visible by the laser of the system 8 .

Claims

1. Laser system (8) comprising:

a pump source (18) adapted to excite a transition at at least one pump wavelength (λ _Ρ ),

a plurality of solid organic amplifier elements (20) comprising:

A first layer (24) having a thickness (e1) of less than 100 μm and

A gain medium (22) contained in the first layer (24) and adapted to emit light radiation at a different stimulated emission wavelength (λ _Ε1 ) under the effect of at least one excitation; pump wavelength (λ _Ρ ),

at least two gain media (22) of two amplifying elements (20) among the plurality of amplifier elements (20) being adapted to emit light radiation at different stimulated emission wavelengths (λ _Ε1 , λ _Ε2 ) under the effect of excitation at least one pump wavelength (λ _Ρ ),

an organic laser (10) adapted to emit a laser beam at a plurality of emission wavelengths (λ _Ε ) comprising:

A fixed output coupler (16) adapted to transmit radiation to at least a part of the or each emission wavelength (λ _Ε ),

A support (32) capable of accommodating an amplifier element (20) among the plurality of amplifier elements (20) in an identical position (P) with respect to the output coupler (16) for each amplifier element (20) of the plurality of amplifier elements (20) and as for an amplifier element (20) in the position (P), the laser (10) emits a laser beam from the light radiation of the gain medium (22) when the gain medium ( 22) is excited by the pump source (18).

2. - A laser system according to claim 1, wherein for all the amplifier elements (20) in the position (P), the laser (10) emits a laser beam having a factor M ² less than 2.0.

The laser system of claim 1 or 2, wherein the gain media materials (22) are selected for the laser (10) to emit laser beams at wavelengths between 380 nm and 800 nm.

4. - laser system according to any one of claims 1 to 3, wherein each amplifier element (20) of the plurality of amplifier elements (20) is a consumable.

5. Laser system according to any one of claims 1 to 4, wherein the laser (10) has a cavity (12) defining a laser emission direction and the source (18) pumping is an optical source emitting at least one pump wavelength in a non-perpendicular direction with the laser emission direction.

6. - A laser system according to any one of claims 1 to 5, wherein the support (21) is a rotating disk having housing for a plurality of amplifier elements (20).

7. - laser system according to any one of claims 1 to 5, wherein the carrier (21) is a fixed base having a housing for an amplifier element (20).

8. - A laser system according to any one of claims 1 to 7, wherein each amplifier element (20) comprises a second layer (26) for supporting the first layer (24), the second layer (26) being in a solid material transparent to the or each emission wavelength.

9. - A laser system according to claim 8, wherein the amplifier element (20) comprises a third layer placed on one side of the first layer (24), the other side of the first layer (24) being in contact with the second layer (26), the third layer being airtight.

10. - A method for modifying the emission length of a laser (10) of a laser system (30) according to any one of claims 1 to 9, the method comprising the steps of:

choice of at least one emission wavelength,

selecting an amplifier element (20) from among the plurality of amplifier elements (20) as a function of the chosen emission wavelength,

positioning the amplifier element (20) at the position (P).

1 1. - Use of a laser system (8) according to any one of claims 1 to 9 for performing the spectroscopy of a molecule.