EP2335330A1

EP2335330A1 - Laser device with high average power fibre

Info

Publication number: EP2335330A1
Application number: EP09736437A
Authority: EP
Inventors: Johan Boullet; François Salin; Eric Cormier
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2008-09-02
Filing date: 2009-09-02
Publication date: 2011-06-22
Also published as: WO2010026309A1; FR2935554A1; US20110292952A1; JP2012501532A; FR2935554B1

Abstract

The invention relates to a laser device (1) having a fibre emitting a single-mode transverse radiation controlled at a given wavelength, which comprises: at least one laser diode (2) capable of emitting a pump wave and a sheathed amplifying optic-fibre segment (6) having two ends, the amplifying optic fibre (2) including a core and a pumping sheath, the fibre being doped with a rare-earth dopant, characterised in that the core of the fibre has a diameter of between 12 µm and 200 µm, and in that the device (1) includes: a coupling means (4) for coupling the pump wave in the pumping sheath to at least one end of the fibre, and a resonator capable of re-injecting a laser beam at the given wavelength at the two ends of said segment (6), said resonator including an intra-cavity wavelength selective means (5, 13) capable of interaction with the injection means (4) so as to perform a filtration on the given wavelength and re-inject into the fibre (6) the pump wave which has not been absorbed after passing in the fibre (6).

Description

HIGH POWER FIBER LASER DEVICE

[001] The invention relates to an optical fiber laser device delivering a transverse monomode beam of high average power.

TECHNICAL FIELD OF THE INVENTION

The field of the invention is that of high power fiber lasers emitting in the spectral band 970 nm-985 nm.

STATE OF THE PRIOR ART

[003] In the field of optical fiber telecommunication networks, it is necessary to have laser sources for pumping efficient and high-brightness Erbium-ion doped amplifiers having an infrared wavelength close to 976 nm. This high pumping power density makes it possible to consider increasing the number of amplified spectral channels simultaneously and / or to increase the distance between two amplifiers on the long-distance networks.

[004] In these telecommunication networks, the necessary pump power must be as high as possible and the beam must remain at the diffraction limit (that is to say simultaneously present a high power, and a high brightness).

[005] The fact of having a fundamental laser source around 976 nm high brightness and high power allows, via a frequency conversion stage, to also have a blue harmonic source having the same characteristics (powerful and gloss).

[006] This harmonic wavelength in blue, about 488 nm is advantageously used in the fields of biology, surgery or medicine. The laser sources at substantially 488 nm are generally gas lasers operating with Argon. These sources sometimes include two lines at wavelengths of 514.5 nm and 488 nm.

[008] Laser sources with two lines at substantially 514.5 nm and 488 nm are produced in a gaseous medium, because there is no solid-state laser medium emitting directly at these wavelengths.

[009] In the fields of biology or medicine in which they find an application, these laser sources at substantially 488 nm should preferably have an excellent spatial quality in order to focus the beam on as small a volume as possible.

[010] Another application of these fiber laser sources emitting at substantially 976 nm is the possibility of high power pumping of lasers and amplifiers. The high pump intensities that can be reached make it possible to produce oscillators or short amplifiers with crystal amplifying media or fibers doped with Erbium or Ytterbium ions, in a continuous, tripped or ultra-short pulse mode.

[011] Today, this spectral band at substantially 976 nm in the infrared is obtained from semiconductor laser diode.

These laser diodes are preferably related to transverse single-mode laser diodes (low power) and / or laser diodes of power (multimode).

[013] The choice of the use of laser sources in the infrared wavelength range is justified by the advantages they confer, in particular: their compactness, their longevity, their high electrical efficiency or their low manufacturing cost. .

[014] However, the choice of these laser diodes can only be a compromise between power and beam quality, which is an undeniable disadvantage in situations where a high power density is required. [015] Indeed, the transverse monomode laser diodes deliver a power limited to a few hundred milliwatts, while the power laser diodes, emitting several watts, have a highly multimode beam, that is to say of low spatial quality. .

[016] To overcome these limitations, infrared laser sources described in the publication "Singlemode emitter array laser bars for high-brightness applications", from the "IEEE 19th" document, are known in the prior art.

International Semiconductor Laser Conference Digest, Th A6, pp. 45-6 (2004) ", of

, N.Lichtenstein, Y.Manz, P.Mauron, A.Fily and S.Arlt. This publication describes a solid state amplifier generating 976 nm radiation at a power of 1.4 W from a 1x3 μm laser diode.

[017] However, in this amplifier, the power generated is limited to a few watts because of the small emitting surface.

[018] A solution to increase the power, is then to use a laser diode with a funnel structure that provides higher power, but with larger spectral widths of the order of 4 to 5 nm .

[019] Such a solution is described in a publication entitled "Nearly-diffraction limited 980 nm tapered laser diode lasers with an output power of 6.7 W", published in the document "Conference Digest of 19th ISLC, IEEE, pp. 43-4 (2004) "and written by K.Paschke, B.Sumpf, F.Dittmar, G.Erbert, J.Fricke, A.Knauer, S.Schwertfeger, H.Wenzel and G.Trankle.

[020] But the major disadvantage of sources such as laser diodes with a funnel structure, is the strong dependence of their spectrum with the temperature, and thus with warming of the structure and the power of pumping. [021] Moreover, such a solution is a difficult implementation, expensive, a large footprint and suffers from a lack of reliability because it requires a delicate alignment.

[022] In addition, the maximum power that such a laser can deliver is limited to about 10 W.

[023] The production of radiation TEM00 at substantially 488 nm by doubling the beam at 976 nm is therefore also limited to 1 W. On the other hand, direct emission of the radiation at 488 nm by Ar technology is limited to a few W .

Also known in the prior art, laser architectures whose amplifying medium is a material doped with Yb ions used to produce laser sources substantially at 976 nm These laser architectures include optical pumping devices emitting the 910nm-940nm band

[025] Solutions based on materials doped with rare earth ions, for example diode pumped Ytterbium ions, use either crystals or optical fibers in which the doping ions are incorporated. The fiber solution has the advantage of being with a fully fiberized solution and therefore compact, ultrastable and reliable.

[026] In addition, the solution provided by the solid-state doped solid-donor amplifying media at substantially 976 nm makes it possible to carry out an opto-optical conversion between the power supplied by the pump and the power delivered by the laser that can go up to 80%.

[027] However, the emission of a solid amplifying medium (crystal or silica) doped with ytterbium ions at substantially the laser wavelength of 977 nm implies strong constraints on the geometry of the amplifier (transverse section and length amplifier medium), pump beam and pumping level. To achieve this emission at the wavelength of substantially 977 nm, the pumping intensity must exceed a limit value, called transparency intensity (~ 30kW / cm ² ) for silica doped with ytterbium ions when pumped to substantially 915nm and that it is desired to emit at substantially 977 nm ) necessary to perform the optical amplification. When the amplifying medium is a crystal doped with ytterbium ions, the necessary pump intensity is obtained by strongly focusing the pump laser, the useful crystal length being limited by the Rayleigh zone of the pump beam. The pump source must be chosen sufficiently bright to ensure transparency over the entire length of the crystal. The solution provided by the doped optical fiber lifts this constraint along the Rayleigh length, since the pump beam is guided by the fiber, which makes it possible to obtain high pumping intensities over long fiber lengths. To maximize the pump intensity in the doped zone (core of the fiber), it was first proposed to inject the pump wave directly into the monomode doped core of the fiber. However, this pumping technique requires the use of semiconductor laser diodes (or any other laser source) in the band 910 nm - 940 nm limited by diffraction and therefore low power (<1W). Another solution consists in guiding a more powerful multi-mode pump in a concentric guiding sheath with a doped core, called a pumping sheath. The latter, however, must have a limited section to ensure at all points of the fiber the local pump intensity condition greater than the intensity of transparency.

[028] Until now, the limited power of the pump diodes in the 910 nm-940 nm band have led to limiting the diameter of the pump ducts to ~ 25 microns, also making it possible to develop laser sources doped with optical fibers. rare earth ions delivering an average power of 3.5 W maximum at an infrared wavelength of substantially 977 nm, with strong constraints on the level of pumping employed and the geometry of the fiber used. This work, which until then was the record for the emission of a fiber laser at substantially 977nm, is reported in the publication [KH Ylla Jarkko, R.Selvas, DB Soh, JK Sahu, CA. Codemard, J. Nillson, SA Allam and AB Grudinin, "A 3.5W 977nm jacketed air clad fiber laser ytterbium doped fiber laser ", OSA Trends in Optics and Photonics, Advanced SoNd State Lasers Vol 34, (2000).]

[029] Today, the technical progress of semiconductor laser technology has led to the commercial availability of high power fiber diodes (10W -1 kW) in the 910 nm - 940 nm band whose multimode beam is delivered on fibers of diameter between 100 .mu.m and 800 .mu.m under a numerical aperture of substantially 0.22. By using these more powerful diodes, it is therefore possible to remove the constraint related to the level of pumping required to achieve the transparency of the material.

[030] However, the use of fibers having pump sheath diameters compatible with the use of these high power diodes leads to a new physical lock for laser emission at substantially 977 nm. Indeed, to emit a single-mode laser beam, the core of the doped fiber must have a core of limited diameter. The fiber used in the publication [K. H. Ylla Jarkko, R. Selvas, D. B. Soh, J. K. Sahu, CA. Codemard, J. Nillson, S. A. Allam and A. B. Grudinin, "A 3.5W 977nm Jacketed Air Clad Fiber Laser Ytterbium Doped Fiber Laser," OSA Trends in Optics and Photonics. Advanced Solid State Lasers Vol. 34, (2000).] Had a core 9 μm in diameter.

[031] Currently, conventional fiber technologies do not allow to exceed core diameters of the order of 12 microns to remain limited by diffraction. Thus, the overlap between the pump wave propagating in the pump sheath and the doped core is very small.

[032] Given this low overlap between pump wave and doped core, the lengths of fiber necessary to absorb the pump wave injected are typically several meters, or even tens of meters.

[033] However, when the length of the fiber is increased, the laser line of a wavelength in the band 1010 nm - 1100 nm, for which these fibers are usually used, has a gain much greater than that of the a line around which it is desired to obtain a laser radiation of substantially 977 nm. The study of the influence of this parasite gain and the resulting limitations are given in the publication [J.Nilsson, JDMinelly, R.Paschotta, AC Tropper, and AC Hanna, "Ring doped cladding pumped single-mode. level fiber laser ", Opt. Lett. 23, 355 (1998)].

[034] Thus, to obtain an infrared fiber laser source at about 977 nm, it is appropriate to use a short enough fiber to limit the parasitic gain in the spectral band 1010 nm - 1100 nm. However, a fiber fulfilling this condition, given the limited section of its core, will have the disadvantage of not absorbing the pump wave of the laser diode. The laser designed with such a fiber will thus be of low power and limited.

SUMMARY OF THE INVENTION

[035] The invention aims to remedy the problems related to the technical difficulties encountered in the generation of laser sources of a fundamental wavelength in the infrared at a wavelength of substantially 977 nm at the output of a system laser whose amplifying medium is an optical fiber doped with rare earth ions. This laser is compact, low cost, high power.

[036] The invention proposes to improve the high power fiber laser devices by the use of a doped fiber whose core diameter is greatly increased compared to the standard monomode fiber diameters and which is capable of providing a monomode optical guidance of the laser wave so as to increase the power of the laser beam.

[037] More precisely, the subject of the invention is a fiber laser device emitting transverse single-mode radiation controlled at a given wavelength comprising:

at least one laser diode able to emit a pump wave, and

a section of sheathed amplifying optical fiber having two ends, said amplifying optical fiber comprising a core of an included diameter; between 12 μm and 200 μm and a pump sheath, the fiber being doped with a rare earth dopant said device comprises:

means for coupling the pump wave in the pump casing to at least one end of the fiber, and

a resonator able to reinject a laser beam at the given wavelength at the two ends of said section,

said resonator comprising selective intra-cavity wavelength elements adapted to cooperate with the injection means so as to filter on the given wavelength and also to reinject into the fiber the unabsorbed pump wave after a passage in the fiber.

Advantageously, the cooperation of the selective means with the injection means makes it possible to inflict losses in the spectral band of the parasitic radiation 1010 nm-1100 nm.

According to particular embodiments:

the coupling means comprise two lenses, said lenses being chosen from at least one of the following lenses: microlens, cylindrical, elliptical, hyperbolic lenses, or aspherical capacitors;

the coupling means refer to a coupler comprising N input multimode fibers that can be soldered directly to the fiber-optic outputs of N pump diodes and an output fiber with or without a guiding core supporting propagation of a mode. substantially identical to that of the amplifying fiber, capable of being directly soldered to the broad-core amplifying fiber.

the coupling means relate to a wide mode optical fiber whose transverse section is progressively thinned so as to adopt a funnel structure. the broad-mode fiber has one end having the same diameter as the fiber delivering the pump wave and the other end having the diameter of the sheath of the amplifying fiber so that said funnel is welded at one end to the fiber delivering the pump beam and at the other end is soldered to the broad mode fiber.

the selective elements relate to an element chosen from at least one of the following elements: a dichroic mirror, an absorbing or interferometric filter, a curvature of the amplifying fiber, a doping element added in the constitution of the core of the fiber amplifier, an external solid network, a prism, a Bragg grating photoinscribed in the core of the amplifying fiber or a Bragg grating external to the amplifying fiber; ]

^' i

the fiber is a broad modal area fiber or an LMA fiber;

X

the diameter of the pump sheath is between 50 and 800 μm; -

- the heart and the sheath are concentric;

the heart has a diameter greater than 12 μm;

the fiber is doped with an element chosen from at least one of the following elements: ytterbium ions, germanium, phosphorus, boron or fluorine;

the fiber is capable of emitting a beam at the diffraction limit at the output of the core;

the fiber is intrinsically polarization-maintaining or maintained in a fixed position;

the broad-mode fiber is a micro-structured silica air fiber;

- The fiber is rigid because it is held in a pure silica rod with an external diameter greater than 1 mm ("rod-type fiber" fiber technology) - The fiber is flexible.

- The sheath of the fiber is a waveguide, adapted to perform the guiding of the pump wave, formed by a ring of air holes, called air duct, having a numerical aperture greater than 0.5;

the guidance of the wave at the wavelength of substantially 977 nm is achieved by an array of air holes parallel to the optical axis surrounding the doped core;

- The fiber appatient to the family of micro-structured air-silica fibers.

the given wavelength is in the infrared;

the amplifying fiber is of a reduced length so that the laser gain of the parasitic radiation in the 1010 nm-1100 nm band remains below 60 dB;

the laser diode delivers powers of 10 to 1000 W, and

the pump wave is coupled to a multimode fiber having a diameter of between 50 μm and 800 μm.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will emerge on reading the description which follows, with reference to the appended figures, which illustrate:

FIG. 1 illustrates a representation of the device according to one embodiment of the invention;

FIG. 2 represents the section of an optical fiber according to one embodiment of the invention;

FIG. 3A illustrates the absorption and emission spectra of ytterbium ions when they are inserted in a silica matrix, which is the case in an optical fiber; FIG. 3B illustrates the energy levels of the ytterbium ions;

FIG. 4 represents the power of the high power fiber laser at three energy levels as a function of the pump power, and

FIG. 5A and 5B represents the output spectrum of the laser, according to one embodiment of the invention.

For clarity, identical or similar elements are identified by identical reference signs throughout the figures.

DETAILED DESCRIPTION OF AN EMBODIMENT

[040] In an exemplary embodiment of the device, according to the invention, illustrated in FIG. 1, the device 1 comprises:

a laser diode 2;

an amplifying optical fiber 6;

coupling means 4;

a resonator including wavelength selective elements 5,13.

[041] The laser diode 2 used is a transverse multimode pump source at about 915 nm, whose power is 230W, the emitted radiation of which is delivered by an optical fiber having a diameter of 400 μm and a numerical aperture NA = 0, 22.

[042] The lens 10 having a numerical aperture of 0.5 and a focal length of 8 mm and the dichroic mirror 9 is totally reflective at the pumping wavelength at about 915 nm.

[043] The means for coupling the pump wave in the sheath of the doped fiber comprise two respective focal length lenses 18 mm and 8 mm 14 and 15. and a transparent dichroic mirror at the pump wavelength and reflecting at the laser wavelength 13.

[044] The selective optics relate to two dichroic mirrors 11 and 12.

[045] The optical fiber 6 is a photonic crystal fiber also called in English "rod-type photonic crystal fibers". This fiber is of reduced length which does not exceed 1.23 m.

[046] A section of this fiber is illustrated in Figure 2.

[047] Photonic fibers are not, like conventional fibers, entirely made of a transparent solid material such as doped silica; in section, a photonic fiber has a network of air holes.

[048] These holes are parallel to the axis of the fiber, and extend longitudinally along the fiber. Practically, these holes can be obtained by manufacturing the preform by assembling capillary tubes or silica cylinders, respecting the pattern of the holes to be obtained in the fiber. Stretching such a preform provides a fiber with holes corresponding to the capillary tubes.

[049] The presence of these holes in the material of the fiber creates variations in the average index of the material. These variations of the index can, as in a conventional optical fiber, be used for the total internal reflection guidance of light signals at suitable wavelengths.

[050] This fiber 6 is a constituent element of the device 1 which allows to implement a high power fiber laser at three energy levels.

[051] It may be noted that this type of fiber laser is more compact, more stable and does not need a cooling mode compared to semiconductor technologies. It also has a better beam quality, the beam quality being imposed by the guiding properties of the fiber, so it has a better resolution for marking applications. [052] This optical fiber is doped with a rare earth ion which is in this embodiment mainly ytterbium.

[053] It is therefore an optical fiber doped ytterbium (Yb) ultra wide heart type rod (otherwise known in English "rod-type fiber")

[054] Ytterbium belongs to the category of rare earth ions or metal ions which are commonly used to make laser sources. Of all the ions that can be used, only these ytterbium ions, which are part of the rare earth ions, have a transition towards 976 nm.

[055] Note that for the fiber used the ratio of the core / sheath surfaces, is substantially 6.5.

In another embodiment, this ratio is between 5 and 100.

3+

[056] Figure 3A illustrates the absorption and emission spectra of Yb ions

3+ in silica, materials doped with Yb ions have a very large transmitting cross section at 976 nm, associated with an absorption band around 915 nm.

[057] This FIG. 3A illustrates, in particular, that the absorption cross section of Tytterbium in the glasses is much larger than the emission cross section.

[058] Depending on the crystalline or amorphous matrix used, the transmitting cross sections towards 976 nm and the absorption cross sections around 915 nm are higher or lower. With an absorption band of about ten nanometers wide at mid-height around 915 nm, diode pumping can be envisaged. Nevertheless, this strong emission around 976 nm is accompanied by an equally intense absorption.

[059] In FIG. 3B, it can be seen that the pumping takes place between the sub-level

2 3 the lowest of the F7 / 2 byte and the highest sub-level of the F5 / 2 byte. 3

The emission takes place between the lowest sub-level of the F5 / 2 byte and the sub-level

2 lowest level of the F7 / 2 byte.

[060] Thus a true transition to three levels, which imposes significant constraints on the pumping of these materials to obtain the emission around 976 nm.

[061] The material doped with ytterbium ions absorbs any radiation around 976 nm in the absence of pumping. To obtain a transmission towards 976 nm, the pumping of the material will have to be intense enough to carry out the transparency population inversion at the laser wavelength 976 nm in the doped medium. In other words, it is necessary that the pumping intensity around 915 nm is sufficient to achieve transparency of the doped medium to 976 nm. This intensity, corresponding to the cancellation of the absorption at the 976 nm laser wavelength in the material, is called the transparency intensity.

3+

[062] The use of this silica fiber doped with Yb ions makes it possible to confine the pump in the doped medium, and to easily reach the intensity of transparency along the length of this doped medium. This fiber thus guarantees a strong interaction of the pump beam with the doping ion, over a great length thanks to the confinement of the light in the pumping sheath of the fiber.

[063] This ytterbium-doped fiber used for this transition is a transverse monomode fiber having record dimensions (diameter of 80 μm) for a single-mode fiber. Such extreme dimensions of the doped core are made possible by the network of very small air holes (smaller than

100nm, illustrated in Figure 2, element 19) decreasing the average index of the sheath surrounding the doped heart and to obtain a numerical aperture of the heart of the order of 0.01.

[064] The substantially 977 nm signal propagates in the 80 μm diameter doped core and the substantially 915 nm pump propagates inside the 200 μm diameter optical cladding with a large numerical aperture greater than 0.7. This sheath is defined by a microstructure filled with air (illustrated in Figure 2, item 18). This microstructure has holes much larger than those defining the core (> 2μm) in a pattern that preserves the symmetry of the fiber around its longitudinal axis.

[065] It is specified that the dimensions of the holes respectively defining the core and the sheath of the fiber can be adjusted according to the desired guiding characteristics: core diameter, numerical aperture of the sheath or core.

[066] In the pumping configuration by the optical cladding, this fiber has a pump absorption of 10 dB / m at 915 nm.

[067] It allows, thanks to a very efficient doped pump-core coating to absorb 230W of pump on very short lengths of 123cm. Over such a length, the pumping intensity remains uniformly along the fiber less than the transparency intensity and the parasitic gain in the spectral band 1010 nm - 1100 nm sufficiently small to be compensated by the set (5) of selective elements in wavelength

[068] In this device, the laser diode 2 emits radiation at a wavelength of between 910 and 940 nm.

[069] In a particular configuration, the pump laser diode used delivers powers of 10W to 1000W, the pump beam can be delivered directly in free space, or be coupled to a multimode fiber having diameters of 50 to 800μm.

[070] The light from the pump diode is coupled to a transport fiber and then injected into the amplifying fiber 6 through optical means 4. These coupling means 4 comprise in this particular configuration two lenses 14 and 15.

[071] The optical means 4 are designed to couple the light from the transport fiber into the laser fiber. In particular these optical means have a magnification which allows the image of the core of the transport fiber of the pump at the laser diode output to have a dimension substantially equal to or smaller than the diameter of the pump sheath of the laser fiber 6.

[072] Similarly, these optical means 4 have a numerical aperture equal to or greater than the product ON, / G where G is the magnification of the optical system 4 and ON ₁ is the numerical aperture of the transport fiber.

For example, if we consider the multimode diode 2 at 915 nm coupled in a fiber of diameter 400 μm and numerical aperture 0.22, and the laser fiber of FIG. 2, the pump sheath 18 has a diameter of 200 μm and a numerical aperture of 0.7, the optical means must have a magnification G ≤ 200/400 = 0.5 and an image numerical aperture of at least 0.5. In the example presented here, the optical means are composed of a pair of two aspherical lenses: a first lens 14 of 18 mm focal length and a second lens of focal length 8 mm.

The lens 15 of 8mm 15 has a numerical aperture of 0.5.

[073] In a particular configuration, these two lenses may be microlenses, cylindrical, elliptical, or hyperbolic, or aspherical condensers.

[074] In another configuration, the coupling means may be:

- A coupler comprising N multimode input fibers capable of being welded directly to the fibrated outputs of N pump diodes and an output fiber capable of being directly soldered to the broad-core amplifying fiber.

- A broad-mode optical fiber whose cross section is gradually thinned so as to adopt a funnel structure. This fiber has one end having the same diameter as the fiber delivering the pump wave and the other end having the diameter of the sheath of the amplifying fiber; [075] In the case where the coupling means relate to a fiber section, the two ends of this "funnel" fiber are then soldered respectively at the output of the transport fiber of the diode 2 and at the input of the fiber Amplifier with a big heart doped 6.

[076] In Figure 2, the fiber has a geometry that allows a single-mode propagation in the core 19 and multimode in the pump sheath. The ratio between the diameters of the core 19 and the pump sheath is less than 10. In this particular configuration, the transverse monomode fiber has significant dimensions with a diameter of the doped core 19 of 80 microns. Such dimensions of the doped core are made possible by the network of very small air holes (<100 nm in diameter) decreasing the average index of the cladding and making it possible to obtain a numerical aperture of the heart of the order of 0.01.

[077] The substantially 977 nm signal propagates in the 80 μm diameter doped core 19 and the substantially 915 nm pump propagates inside the 200 μm diameter optical cladding with a large numerical aperture greater than 0.7.

[078] This sheath is defined by a microstructure filled with air 18. This microstructure 18 has holes much larger than those defining the core (> 2μm) in a pattern that preserves the symmetry of the fiber around its longitudinal axis.

[079] For lower powers, it is possible to use smaller hearts. The ratio between the transverse surfaces of the doped core 19 of the fiber 6 and the pump sheath 18 must remain in the range 5-100 and preferably closer to 5. The sheath of the fiber 6 may have a sheath diameter 18 guiding the pump between 50 and 400 μm.

[080] The amplifying fiber has a monomode propagation of the beam in the heart doped at the wavelength of substantially 977 nm. The fiber is inherently polarizing or simply held in a fixed position. The core of the fiber may contain in addition to the rare earth doping ions one or more of the following chemical species: Germanium, Phosphorus, Boron, Fluorine.

[081] The doped core of the fiber 18 has a diameter greater than 12 microns. It is therefore a broad modal area fiber or LMA fiber (Large Mode Area).

[082] The broad mode fiber can be a micro-structured silica air fiber, rigid or flexible. The length of the fiber 6 is chosen so that the pumping intensity at the fiber exit is greater than the transparency intensity at the fiber outlet and the undesirable gain in the 1010 nm-1100 nm band is maintained. less than 6OdB.

[083] In this embodiment, for 230W pump injected, 63W of residual pump power is measured after a propagation length of 123cm, for a calculated transparency power of 11 W. This power corresponds to transparency intensities of 30kW / cm2 and a cross section of the pump casing 18 of 31500 / vm.

[084] In this embodiment, the gain at 1030nm is substantially 50 dB.

[085] The residual pump wave at the outlet of the pump casing as well as the laser wave coming out of the core of the amplifying fiber 6 are then collimated by an optical means 10.

[086] The laser wave is reflected by the mirror 9, totally reflecting at the laser wavelength of substantially 977 nm while the pump wave at substantially 915 nm is not reflected. This residual pump is then incident on a mirror 8 strongly reflecting the pump wavelength of 915 nm in this embodiment.

[087] The position of this mirror is calculated so that the pump beam reflected by the mirror is exactly re-injected into the pump sheath of the laser fiber. For a plane mirror this position corresponds to that of the image of the output face of the laser fiber through the optical means 10.

[088] The pump wave then makes a second trip in the laser fiber, which increases the pump's absorption, and increases the population inversion, as well as the efficiency of the laser.

[089] In another configuration, this pump recycling means may be a photo-inscribed Bragg grating in the core of the fiber, or a free-space massive Bragg grating, or a prism, or a grating.

[090] A dichroic mirror 13 is placed between the two optical means 14 and 15. This dichroic mirror is totally reflective around 977 nm and totally transparent at the pump wavelength.

[091] A second mirror 11 totally reflecting around 977 nm is placed on the path of the laser beam to form a resonator with the face of the fiber opposite the pump.

[092] In another configuration, the resonator corresponds to mirrors with high reflectivity (HR) or finite reflectivity at the wavelength of substantially 977 nm.

The fiber-end reinjection devices may be Bragg gratings photo-inscribed directly into the doped core of the reflective fiber at the wavelength of substantially 977 nm, or undoped fiber sections with Bragg gratings. reflecting the wavelength of substantially 977 nm inscribed in the core, these sections of fibers being welded to the amplifying fiber. The reinjection devices at the ends of the fiber may be massive Bragg gratings.

One or more of the elements constituting the resonator may be wavelength selective, i.e. reflective at the wavelength of substantially 977 nm and very low reflectivity in the band (1010 nm - 1100 nm). [093] The set of optical elements 5 comprises the mirror 12 which is totally transparent at the 977 nm laser wavelength and has a reflectivity> 99% in the 1010 nm-1100 nm band, and the cavity background mirror 11 which has a transmission> 99% in the 1010 nm - 1100 nm band.

[094] In total, this set inflicts sufficient losses in the 1010 nm - 1100 nm band for the laser to oscillate spontaneously around 977 nm.

[095] In another configuration, the means of exerting a wavelength selection relates to:

one or more dichroic mirrors able to reflect a signal of a defined wavelength;

one or more absorbing or interferometric filters;

a particular curvature of the amplifying fiber;

a doping element added in the constitution of the core of the absorbing fiber in the 1010 nm-1100 nm band, or

- a Bragg grating photo-inscribed in the heart of the fiber or a solid Bragg grating exterior to the fiber.

- a prism or a network

[096] This device 1 thus makes it possible to generate a power laser at around 976 nm making it possible to easily reach powers of the order of several hundred watts, compared to 10W in the state of the art, with a quality of excellent beam.

[097] The laser 7 delivered is a transverse single-mode beam 7 around 976 nm which is very powerful.

[098] The device, by the combination:

an ultra wide Yb doped fiber core of the bar type, spectrally selective optics making it possible to eliminate the parasitic laser effects on unwanted wavelengths, and

an optical device for recycling the unabsorbed pump,

allows to produce these levels of powers of more than 100W, even 1 kW to 977nm. according to the power of the pump diode at substantially 915 nm,

[099] In Figure 4, is shown the power of the laser at substantially 977 nm of high power at three energy levels depending on the pump power at substantially 915 nm.

The laser threshold is reached at the pump power value of the diode from 18W to 915nm. At the maximum pump power available at 230W, the laser produces power up to 94 W at 977 nm.

The efficiency slope of the laser between the pump power and the laser power is 48%.

The quality of the laser beam remains excellent at such power values, and the performance of the device is limited by the available pump power of the diode.

FIG. 5A represents the output spectrum of the laser measured at full output power from an optical spectrum analyzer with a resolution of 0.07nm.

As shown in this figure, the laser oscillates spontaneously over a spectral range of 6 nm centered at 977 nm.

Because of the efficient spectral filtering of the action of the combination of the second and third mirrors 12, 1, the parasitic emission at 1030 nm is 35 dB below the maximum laser signal at 977 nm.

More than 98% of the spectral power density is contained in the spectral range from 975 nm to 980 nm. By way of comparison, FIG. 5B represents the output spectrum of the laser as well as the amplified spontaneous spectrum of the emission obtained by suppressing the feedback of the mirror 11.

In addition, the doped fiber has a core diameter very greatly increased compared to standard monomode fiber diameters (that is to say having cores of diameter <12 .mu.m). The diameter of the core is chosen between 12 μm and

200 μm. To achieve the very high powers, the invention implements the use of special optical fibers with large modal area (LMA) index jump or micro-structured, may have record heart diameters> up to 80μm currently while providing single-mode optical guidance of the laser wave around 977 nm.

The device according to the invention also makes it possible to obtain a high-power laser at 488 nm since it has an excellent spatial quality making it possible to focus the beam on as small a volume as possible and a power sufficient to obtain efficiencies. important in the nonlinear stage.

This device constitutes a solid-state laser medium emitting directly at wavelengths at substantially 488 nm, which has the advantage of being less bulky, more reliable and less expensive than devices using a solid medium emitting between 800. and 1100 nm to which is added a non-linear optical stage for performing frequency mixing or doubling. These devices implement a method of producing radiation at 976 nm or 1029 nm and doubling it in frequency. Such a nonlinear frequency doubling stage imposes strong constraints on the characteristics of the fundamental beam at 976 or 1029 nm.

The invention is not limited to the embodiments described and illustrated. It is furthermore not limited to these exemplary embodiments and the variants described.

Indeed, in a variant, the photonic optical fiber may be doped with rare earth ions or metal ions other than Ytterbium ions.

Claims

A fiber laser device (1) emitting transverse monomode radiation controlled at a given wavelength comprising:

at least one laser diode (2) capable of emitting a pump wave, and

a sheathed amplifying optical fiber section (6) having two ends, said amplifying optical fiber (6) comprising a core and a pump sheath, the fiber being doped with a rare earth dopant,

characterized in that the core of the amplifying optical fiber (6) has a diameter of between 12 μm and 200 μm, and that the device (1) comprises:

means for coupling (4) the pump wave in the pump casing to at least one end of the fiber, and

a resonator able to reinject a laser beam at the given wavelength at the two ends of said section (6),

said resonator comprising selective elements (5, 13, 8, 9) in intra-cavity wavelength capable of cooperating with the injection means (4) so as to filter on the given wavelength and also to be reinjected in the fiber (6) the unabsorbed pump wave after passing through the fiber (6).

A fiber laser device according to claim 1, wherein the coupling means (4) comprises two lenses (14,15), said lenses being selected from any one or more of the following lenses: microlens, cylindrical lens, elliptical , hyperbolic, or aspherical capacitors.

Fiber laser device according to claim 1, in which the coupling means (4) relate to a coupler comprising N multimode input fibers that can be soldered directly to the fibrated outputs of N diodes. pump and an output fiber capable of being directly soldered to the amplifying fiber (6).

A fiber laser device according to claim 1, wherein the coupling means (4) relate to a wide mode optical fiber whose cross section is progressively thinned to adopt a funnel structure.

A fiber laser device according to the preceding claim, wherein said wide mode fiber has one end having the same diameter as the pump wave delivering fiber and the other end having the diameter of the sheath of the fiber amplifying fiber. so that said funnel is welded to one end of the fiber delivering the pump beam and at the other end to the broad mode fiber.

A fiber laser device according to any one of the preceding claims, wherein the selective elements relate to an element selected from any one or more of the following: a dichroic mirror, an absorbing or interferometric filter, a curvature of the amplifying fiber, a doping element added in the constitution of the core of the amplifying fiber, an external solid network, a prism, a Bragg grating photoinscribed in the core of the amplifying fiber or a Bragg grating external to the amplifying fiber.

A fiber laser device according to any one of the preceding claims, wherein the fiber is a broad modal area fiber or an LMA fiber

(acronym for Large Mode Area).

Fiber laser device according to any one of the preceding claims, wherein the diameter of the pump sheath is between 50 μm and 400 μm.

The fiber laser device according to any one of the preceding claims, wherein the core and the sheath are concentric.

A fiber laser device as claimed in any one of the preceding claims, wherein the core has a diameter greater than 12μm.

A fiber laser device according to any one of the preceding claims, wherein the fiber (6) is doped with an element selected from at least one of the following: ytterbium ions, neodymium ions, germanium, phosphorus, boron or fluorine.

12. Fiber laser device according to any one of the preceding claims, wherein the fiber is capable of emitting a beam at the diffraction limit at the output of the core.

A fiber laser device as claimed in any one of the preceding claims, wherein the fiber is intrinsically bias-maintaining or held in a fixed position.

A fiber laser device according to any one of claims 3 to 13, wherein the broad mode fiber is a micro structured silica air fiber.

A fiber laser device according to any one of the preceding claims, wherein the fiber (6) is rigid and is held in a pure silica bar of outer diameter greater than 1 mm.

The fiber laser device according to any one of claims 1 to 14, wherein the fiber (6) is flexible.

17. Fiber laser device according to any one of the preceding claims, wherein the sheath of the fiber (6) is a waveguide, adapted to perform the guiding of the pump wave, formed by a ring of holes. air having a crown of air holes, called air duct, having a numerical aperture greater than 0.5.

18. Fiber laser device according to the preceding claim, wherein the waveguide wavelength of substantially 977 nm is achieved by an array of air holes parallel to the optical axis surrounding the doped heart.

19. Fiber laser device according to any one of the preceding claims, wherein the fiber (6) belongs to the family of air-silica micro-structured fibers.

The fiber laser device according to any of the preceding claims, wherein the given wavelength is in the infrared.

A fiber laser device according to any one of the preceding claims, wherein the fiber (6) is of a reduced length so that the laser gain of the spurious radiation in the 1010 nm-1100 nm spectral band remains below 60 dB. .

A fiber laser device as claimed in any one of the preceding claims, wherein the laser diode (2) emits a pump wave in the spectral band of 910 nm to 940 nm.

23. A fiber laser device as claimed in any one of the preceding claims, wherein the laser diode delivers powers of 10 to 1000W.

24. A fiber laser device as claimed in any one of the preceding claims, wherein the pump wave is coupled to a multimode fiber having a diameter of between 50μιm and 800μm.