WO2005015700A2 - Optical cavity ring power laser source with high spectral finesse - Google Patents

Optical cavity ring power laser source with high spectral finesse

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Publication number
WO2005015700A2
WO2005015700A2 PCT/EP2004/051655 EP2004051655W WO2005015700A2 WO 2005015700 A2 WO2005015700 A2 WO 2005015700A2 EP 2004051655 W EP2004051655 W EP 2004051655W WO 2005015700 A2 WO2005015700 A2 WO 2005015700A2
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WO
WIPO (PCT)
Prior art keywords
laser
laser source
cavity
reflecting
source
Prior art date
Application number
PCT/EP2004/051655
Other languages
French (fr)
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WO2005015700A3 (en
Inventor
Jean-Pierre Huignard
Arnaud Brignon
Original Assignee
Thales
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Publication date
Application filed by Thales filed Critical Thales
Publication of WO2005015700A2 publication Critical patent/WO2005015700A2/en
Publication of WO2005015700A3 publication Critical patent/WO2005015700A3/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
    • H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
    • H01S5/4031—Edge-emitting structures
    • H01S5/4062—Edge-emitting structures with an external cavity or using internal filters, e.g. Talbot filters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
    • H01S3/083—Ring lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H01S3/109—Frequency multiplication, e.g. harmonic generation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14—External cavity lasers
    • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
    • H01S5/142—External cavity lasers using a wavelength selective device, e.g. a grating or etalon which comprises an additional resonator
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/2036—Broad area lasers

Definitions

  • the present invention relates to a laser power source with a ring optical cavity of great spectral fineness.
  • a laser power source (of the order of a hundred Watts). This source is shown diagrammatically in FIG. 1, and the main characteristics are recalled here.
  • This known source comprises the following elements: - An optical cavity 1 consisting of a ring delimited by four mirrors 2 to 5 and in the optical path of which are inserted: - A gain laser medium 6, for example an Nd crystal- YAG or Nd- YV0, pumped by diodes, serving both for amplification of the waves and for the recording of a dynamic hologram generating a conjugate wave by mixing with four waves. - A non-reciprocal element 7 ensuring the control of the respective intensity of the incident and conjugate counter propagating waves in the cavity. - An optical output path 8 from the gain medium 6 on which is placed an output mirror 9 partially reflecting and partially transmissive. The particular characteristics of this source are based on wave mixing and phase conjugation properties in the gain laser medium. This type of source ensures an adaptive correction of the aberrations of the amplifying medium and makes it possible to extract, at the output of the mirror
  • a continuous or pulse beam of excellent spectral and spatial quality a problem posed by this known source is that the gain medium 6 heats up strongly when it is desired to cause the cavity to produce a high power (100 Watts or more), because the optical efficiency of the laser medium (optical power delivered / pumping optical power) of the order of 50%, and an electrical / optical efficiency (optical power delivered / electric power applied to the pumping diodes) much lower, of the order of 25%, which requires the use of means of cooling the expensive and bulky laser medium 6.
  • the gain of the laser medium 6 may be insufficient in certain cases, and a second laser medium 10 must then be inserted into the ring of the cavity.
  • the subject of the present invention is a laser power source (in continuous mode, greater than 100 Watts, and even at 1 kW) which does not have the heating problems of the first source mentioned above, while having great finesse. spectral (for example of the order of 1 nm).
  • the laser source according to the invention of the type with semiconductor laser emitter and ring cavity, the cavity comprising at least two reflecting devices is characterized in that the emitter is partially reflective and partially transmissive and is included as a device reflecting from the ring cavity and as a medium for forming a dynamic hologram.
  • FIG. 1 cited above, is a simplified diagram of a known laser source, and • Figures 2 and 3 are simplified diagrams of two embodiments of a laser source according to the invention. • Figure 4 is a simplified diagram of a non-reciprocal element that can be used in the source of the invention.
  • the invention starts from the source of FIG. 1, because the oscillator formed by the ring cavity is capable of radiating a single-mode and single-frequency wave by exploiting properties of wave mixing and dynamic holography in the gain medium. . In this oscillator, two counter propagating waves are generated which are strictly conjugate in phase. Under these conditions, it is possible to obtain, at the output mirror, a single mode Gaussian wave which constitutes the wave emitted by the source.
  • the invention proposes to modify the cavity ring of FIG. 1, by removing the crystal 6, and by using for the laser amplifier function at least one semiconductor laser emitter element, and preferably a strip of such emitters.
  • the laser amplifier function at least one semiconductor laser emitter element, and preferably a strip of such emitters.
  • the present invention proposes the structures described below with reference to FIGS. 2 and 3.
  • the structure of FIG. 2 comprises a ring optical path 11.
  • this "ring” is the simplest possible, and its structure is triangular, with a reflecting device at each vertex of the triangle, but it could include a higher number of reflecting devices, and therefore a polygonal structure with more than three sides.
  • These three reflecting devices are: two simple mirrors 12, 13 and a semiconductor laser emitter 14.
  • This laser emitter 14 can be either a single power semiconductor laser element (of a few tens of Watts at least), or, preferably, a strip formed of several such laser elements, or else a single semiconductor laser with a wide ribbon.
  • the optical path in the cavity delimited by the two mirrors 12, 13 and the reflecting face (laser emission face) of the emitter 11 is then triangular.
  • the emitting surface 14A of the emitter 14 is in the form of a very narrow elongated ribbon, in particular when it is a strip with several elements or a laser ribbon, the emitted laser beam has a very wide opening in a plane.
  • a cylindrical lens 15 is placed in front of the emitter 14, on the path of the beams 11 B and 11C.
  • a cylindrical lens 17 that is to say in front of the face 14B of the transmitter 14 (face opposite to face 14A), there is respectively: a cylindrical lens 17, a spatial filter 18, which is advantageously a diaphragm disposed at the common focus of two converging lenses, or else a mirror associated with a Fabry-Perrot type interferometer and an optical output and reflection and transmission device 19, which is advantageously a partially reflecting and partially transmissive network for a given wavelength.
  • the laser oscillator 14 is provided with a layer R m i n (with minimum reflection) on its two faces to avoid oscillation between them.
  • the source represented in FIG. 3 comprises, like that of FIG. 2, an optical path in a ring 20.
  • this “ring” is also as simple as possible, and its structure is triangular (comprising respectively the branches 20A, 20B, 20C), with a reflecting device at each vertex of the triangle.
  • a laser transmitter 21 which can be one of the devices fulfilling the function of the transmitter 14 of FIG. 2.
  • a special mirror 22 it comprises, on its face opposite to that carrying a reflective layer, an opaque coating 23 in which a hole 24 is made just at the place coinciding with the top of the triangular structure.
  • a selectively reflecting element 25 for a given wavelength.
  • This reflecting element is advantageously a network shaped as a selective filter.
  • a cylindrical lens 26 is placed in front of the laser element 21.
  • a cylindrical lens 27 is interposed focusing the beam coming from the source 21 at the center of the hole 24 (and, conversely, widening the counter-propagating beam coming from reflection from the mirror 22), and in the branch 20B, a cylindrical lens 28 is interposed widening the cylindrical beam reflected by the mirror 22 (and, inversely focusing on the hole 24 the widened beam from filter 25).
  • a non-reciprocal element 29 is inserted, which can be identical to the element 16 in FIG. 2.
  • a cylindrical lens 30 is placed on the path of the source output beam.
  • This cavity like that of FIG. 1, operates by phase conjugation with an appropriate spatial and spectral filtering of the modes for producing a source emitting a power beam whose opening is limited by diffraction.
  • the intensity of the two counter propagating waves in the cavity is controlled by a non-reciprocal element 29 adapted to the emission wavelength.
  • the laser emitter 21 is provided, on its face on the outlet side, with a layer
  • FIG. 4 shows an exemplary embodiment of the non-reciprocal element 16 (or 29), which is the same as that of FIG. 1.
  • This element comprises, in order, a blade - referenced 31, a Faraday rotator 2 32 and a polarizer 33. According to a variant of the invention, the positions of elements 31 and 32 are reversed.
  • the generation of a dynamic hologram in the gain medium ( 14 or 21) results from a four-wave interaction.
  • the network that registers is of the gain network and index network type, the latter component being the most important in the case where the spatial period of the network is large.
  • the oscillation can only take place thanks to the formation of a dynamic hologram in the active semiconductor medium (laser emitter).
  • This dynamic hologram largely ensures active control of the quality of the source beam.
  • conventional elements such as a spatial and spectral filter and a non-reciprocal optical element are available in the cavity.
  • the source of the invention has another important characteristic: taking into account the spatial and spectral filtering of the beam, it achieves a phasing of all the emitting elements of the strip. Under these conditions, the luminance of the source is imposed by the length of the emitting bar and not by the dimension of an elementary radiating element.
  • power semiconductor laser sources such as laser arrays or a single very wide ribbon laser are phased, having typical wavelengths of 810 nm or 980 nm (most currently in use).
  • the device of the invention therefore achieves from semiconductor amplifiers the equivalent of a power source whose beam opening is limited by diffraction and realizing all the elements of the strip in phase.

Abstract

The invention relates to an optical cavity ring laser source, characterized in that said cavity comprises a pump gain fiber optic with a first end whose optical axis is optically aligned with an output mirror and a second end which is directed toward the first, the optical axis thereof intersecting the optical axis of the first end on the front surface of the first end and being distinct from the optical axis of the first end, also comprising a beam rephasing device and a non-reciprocal element respectively disposed between the second and first ends of the fiber optic.

Description

SOURCE LASER DE PUISSANCE A CAVITE OPTIQUE EN ANNEAU A GRANDE FINESSE SPECTRALE HIGH SPECTRAL RING OPTICAL CAVITY POWER LASER SOURCE
La présente invention se rapporte à une source laser de puissance à cavité optique en anneau à grande finesse spectrale. On connaît, par exemple d'après la référence suivante : P. Sillard, A. Brignon and J.-P. Huignard « Grating analysis of a self-starting loop resonator with a self-pumped phase conjugate mirror in a Nd :YAG amplifier » IEEE J. Quantum Electron. 34, 465-472 (1998), une source laser de puissance (de l'ordre d'une centaine de Watts). Cette source est schématisée en figure 1 , et on en rappelle ici les caractéristiques principales. Cette source connue comporte les éléments suivants : - Une cavité optique 1 se composant d'un anneau délimité par quatre miroirs 2 à 5 et dans le trajet optique de laquelle sont insérés : - Un milieu laser à gain 6, par exemple un cristal Nd-YAG ou Nd- YV0 , pompé par diodes, servant à la fois à l'amplification des ondes et à l'inscription d'un hologramme dynamique générant une onde conjuguée par mélange à quatre ondes. - Un élément non réciproque 7 assurant le contrôle de l'intensité respective des ondes incidentes et conjuguées contra propagatives dans la cavité. - Un trajet optique de sortie 8 issu du milieu à gain 6 sur lequel est placé un miroir de sortie 9 partiellement réfléchissant et partiellement transmissif. Les caractéristiques particulières de cette source reposent sur des propriétés de mélange d'ondes et de conjugaison de phase dans le milieu laser à gain. Ce type de source assure une correction adaptative des aberrations du milieu amplificateur et permet d'extraire, à la sortie du miroirThe present invention relates to a laser power source with a ring optical cavity of great spectral fineness. We know, for example from the following reference: P. Sillard, A. Brignon and J.-P. Huignard "Grating analysis of a self-starting loop resonator with a self-pumped phase conjugate mirror in a Nd: YAG amplifier »IEEE J. Quantum Electron. 34, 465-472 (1998), a laser power source (of the order of a hundred Watts). This source is shown diagrammatically in FIG. 1, and the main characteristics are recalled here. This known source comprises the following elements: - An optical cavity 1 consisting of a ring delimited by four mirrors 2 to 5 and in the optical path of which are inserted: - A gain laser medium 6, for example an Nd crystal- YAG or Nd- YV0, pumped by diodes, serving both for amplification of the waves and for the recording of a dynamic hologram generating a conjugate wave by mixing with four waves. - A non-reciprocal element 7 ensuring the control of the respective intensity of the incident and conjugate counter propagating waves in the cavity. - An optical output path 8 from the gain medium 6 on which is placed an output mirror 9 partially reflecting and partially transmissive. The particular characteristics of this source are based on wave mixing and phase conjugation properties in the gain laser medium. This type of source ensures an adaptive correction of the aberrations of the amplifying medium and makes it possible to extract, at the output of the mirror
9, un faisceau continu ou impulsionnel d'excellente qualité spectrale et spatiale. Un problème posé par cette source connue est que le milieu à gain 6 s'échauffe fortement lorsque l'on veut faire produire à la cavité une puissance élevée (100 Watts ou plus), car le rendement optique du milieu laser (puissance optique délivrée/ puissance optique de pompage) de l'ordre de 50%, et un rendement électrique/optique (puissance optique délivrée/puissance électrique appliquée aux diodes de pompage) bien inférieur, de l'ordre de 25 %, ce qui impose l'utilisation de moyens de refroidissement du milieu laser 6 onéreux et encombrants. En outre, le gain du milieu laser 6 peut être insuffisant dans certains cas, et l'on doit alors insérer dans l'anneau de la cavité un deuxième milieu laser 10. On connaît par ailleurs d'après les documents US 4 656 641 , US 4 905 252, US 5 572 542 et l'article de Goldberg L et al. « Single lobe opération of an 40-element laser array in an external ring laser cavity » Applied Physics Letters, American Institute of Physics, New York, US, vol. 51 , n° 12, 21 Septembre 1987 (1987-09-21 ), pages 871-873, XP000706879, des sources lasers à cavité externe, c'est-à-dire dans lesquelles l'émetteur laser ne fait pas partie des éléments constitutifs de la cavité proprement dite. Les cavités décrites dans tous ces documents ne comportent que des éléments passifs tels que des miroirs et des filtres, qui ne permettent pas de contrôler de façon efficace et précise la qualité du faisceau laser produit. La présente invention a pour objet une source laser de puissance (en régime continu, supérieure à 100 Watts, et même à 1 kW) ne présentant pas les problèmes d'échauffement de la première source mentionnée ci- dessus, tout en ayant une grande finesse spectrale (par exemple de l'ordre de 1 nm). La source laser conforme à l'invention, du type à émetteur laser semiconducteur et cavité en anneau, la cavité comportant au moins deux dispositifs réfléchissants est caractérisée par le fait que l'émetteur est partiellement réfléchissant et partiellement transmissif et est inclus en tant que dispositif réfléchissant de la cavité en anneau et en tant que milieu de formation d'un hologramme dynamique. La présente invention sera mieux comprise à la lecture de la description détaillée d'un mode de réalisation, pris à titre d'exemple non limitatif et illustré par le dessin annexé, sur lequel : • la figure 1 , citée ci-dessus, est un schéma simplifié d'une source laser connue, et • les figures 2 et 3 sont des schémas simplifiés de deux modes de réalisation d'une source laser conforme à l'invention. • la figure 4 est un schéma simplifié d'un élément non réciproque pouvant être utilisé dans la source de l'invention. L'invention part de la source de la figure 1 , car l'oscillateur formé par la cavité en anneau est capable de rayonner une onde monomode et monofréquence en exploitant des propriétés de mélange d'ondes et d'holographie dynamique dans le milieu à gain. Dans cet oscillateur, on génère deux ondes contra- propagatives qui sont strictement conjuguées en phase. On peut dans ces conditions obtenir au niveau du miroir de sortie une onde gaussienne monomode qui constitue l'onde émise par la source. L'invention propose de modifier l'anneau de cavité de la figure 1 , en supprimant le cristal 6, et en utilisant pour la fonction amplificateur laser au moins un élément émetteur laser semiconducteur, et, de préférence une barrette de tels émetteurs. Cependant, on ne pourrait pas remplacer simplement le cristal laser par de tels émetteurs laser semiconducteurs, car bien qu'ils soient susceptibles d'émettre une puissance optique élevée, typiquement plusieurs centaines de Watts, voire quelques kW, ils ont une émission laser à trop large spectre et trop large angle d'émission. Pour résoudre ces problèmes, la présente invention propose les structures décrites ci-dessous en référence aux figures 2 et 3. La structure de la figure 2 comprend un chemin optique en anneau 11. Dans le cas présent, cet « anneau » est le plus simple possible, et sa structure est triangulaire, avec un dispositif réfléchissant à chaque sommet du triangle, mais il pourrait comporter un nombre supérieur de dispositifs réfléchissants, et donc une structure polygonale à plus de trois côtés. Ces trois dispositifs réfléchissants sont : deux miroirs simples 12, 13 et un émetteur laser semiconducteur 14. Cet émetteur laser 14 peut être soit un seul élément laser semiconducteur de puissance (de quelques dizaines de Watts au moins), soit, de préférence, une barrette formée de plusieurs tels éléments laser, ou bien un laser semiconducteur unique à ruban large. Le trajet optique dans la cavité délimitée par les deux miroirs 12, 13 et la face réfléchissante (face d'émission laser) de l'émetteur 11 est alors triangulaire. Il comporte les faisceaux 11A (entre 12 et 13), 11 B (entre 13 et 14) et 11C (entre 14 et 12). Etant donné que la surface d'émission 14A de l'émetteur 14 est en forme de ruban allongé très étroit, en particulier lorsqu'il s'agit d'une barrette à plusieurs éléments ou d'un ruban laser, le faisceau laser émis a une ouverture très large dans un plan. Pour transformer ce faisceau large en un fin faisceau, et aussi pour transformer les faisceaux fins arrivant sur l'émetteur 14 depuis les miroirs 12 et 13, on dispose devant l'émetteur 14 une lentille cylindrique 15, sur le trajet des faisceaux 11 B et 11C. En général, il suffit d'une seule lentille commune aux deux branches 11B et 11C, car l'angle formé par les faisceaux 11 B et 11C est assez faible (quelques degrés, par exemple). De façon avantageuse, on intercale dans la cavité 11 un élément non réciproque 16 identique ou similaire à l'élément 7 de la figure 1. Sur le trajet de sortie de la source de la figure 2, c'est-à-dire devant la face 14B de l'émetteur 14 (face opposée à la face 14A), on dispose respectivement : une lentille cylindrique 17, un filtre spatial 18, qui est avantageusement un diaphragme disposé au foyer commun de deux lentilles convergentes, ou bien un miroir associé à un interféromètre de type Fabry- Perrot et un dispositif optique de sortie 19 à réflexion et transmission, qui est avantageusement un réseau partiellement réfléchissant et partiellement transmissif pour une longueur d'onde donnée. L'oscillateur laser 14 est muni d'une couche Rmin (à réflexion minimale) sur ses deux faces pour éviter l'oscillation entre celles-ci. La source représentée en figure 3 comporte, comme celle de la figure 2, un chemin optique en anneau 20. Dans le cas présent, cet « anneau » est également le plus simple possible, et sa structure est triangulaire (comportant respectivement les branches 20A, 20B, 20C), avec un dispositif réfléchissant à chaque sommet du triangle. A l'un des sommets de ce triangle (entre les branches 20A et 20C), on dispose un émetteur laser 21 , qui peut être l'un des dispositifs remplissant la fonction de l'émetteur 14 de la figure 2. A un autre sommet (entre les branches 20A et 20B), on dispose un miroir 22 particulier : il comporte, sur sa face opposée à celle portant une couche réfléchissante, un revêtement opaque 23 dans lequel est pratiqué un trou 24 juste à l'endroit coïncidant avec le sommet de la structure triangulaire. Au troisième sommet, on dispose un élément 25 réfléchissant sélectivement (pour une longueur d'onde donnée). Cet élément réfléchissant est avantageusement un réseau conformé en filtre sélectif. Comme dans le cas de la source de la figure 2, on dispose devant l'élément laser 21 une lentille cylindrique 26. Dans la branche 20A, on intercale une lentille cylindrique 27 focalisant le faisceau issu de la source 21 au centre du trou 24 (et, inversement, élargissant le faisceau contra-propagatif issu par réflexion du miroir 22), et dans la branche 20B, on intercale une lentille cylindrique 28 élargissant le faisceau cylindrique réfléchi par le miroir 22 (et, inversement focalisant sur le trou 24 le faisceau élargi provenant du filtre 25). Dans la branche 20C, on intercale un élément non réciproque 29, qui peut être identique à l'élément 16 de la figure 2. A la sortie de l'émetteur 21 (en regard de sa face opposée à celle qui fait face aux éléments 22 et 25), on dispose une lentille cylindrique 30 sur le trajet du faisceau de sortie de la source. Cette cavité, comme celle de la figure 1 , fonctionne par conjugaison de phase avec un filtrage spatial et spectral approprié des modes pour réaliser une source émettant un faisceau de puissance dont l'ouverture est limitée par diffraction. Comme pour la source de la figure 2, le contrôle de l'intensité des deux ondes contra- propagatives dans la cavité est réalisé par un élément non réciproque 29 adapté à la longueur d'onde d'émission. L'émetteur laser 21 est muni, sur sa face côté sortie, d'une couche9, a continuous or pulse beam of excellent spectral and spatial quality. A problem posed by this known source is that the gain medium 6 heats up strongly when it is desired to cause the cavity to produce a high power (100 Watts or more), because the optical efficiency of the laser medium (optical power delivered / pumping optical power) of the order of 50%, and an electrical / optical efficiency (optical power delivered / electric power applied to the pumping diodes) much lower, of the order of 25%, which requires the use of means of cooling the expensive and bulky laser medium 6. In addition, the gain of the laser medium 6 may be insufficient in certain cases, and a second laser medium 10 must then be inserted into the ring of the cavity. We also know from documents US 4,656,641, US 4,905,252, US 5,572,542 and the article by Goldberg L et al. "Single lobe operation of an 40-element laser array in an external ring laser cavity" Applied Physics Letters, American Institute of Physics, New York, US, vol. 51, n ° 12, September 21, 1987 (1987-09-21), pages 871-873, XP000706879, laser sources with external cavity, that is to say in which the laser transmitter is not part of the elements constituting the proper cavity. The cavities described in all these documents only contain passive elements such as mirrors and filters, which do not allow the quality of the laser beam produced to be effectively and precisely controlled. The subject of the present invention is a laser power source (in continuous mode, greater than 100 Watts, and even at 1 kW) which does not have the heating problems of the first source mentioned above, while having great finesse. spectral (for example of the order of 1 nm). The laser source according to the invention, of the type with semiconductor laser emitter and ring cavity, the cavity comprising at least two reflecting devices is characterized in that the emitter is partially reflective and partially transmissive and is included as a device reflecting from the ring cavity and as a medium for forming a dynamic hologram. The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which: • FIG. 1, cited above, is a simplified diagram of a known laser source, and • Figures 2 and 3 are simplified diagrams of two embodiments of a laser source according to the invention. • Figure 4 is a simplified diagram of a non-reciprocal element that can be used in the source of the invention. The invention starts from the source of FIG. 1, because the oscillator formed by the ring cavity is capable of radiating a single-mode and single-frequency wave by exploiting properties of wave mixing and dynamic holography in the gain medium. . In this oscillator, two counter propagating waves are generated which are strictly conjugate in phase. Under these conditions, it is possible to obtain, at the output mirror, a single mode Gaussian wave which constitutes the wave emitted by the source. The invention proposes to modify the cavity ring of FIG. 1, by removing the crystal 6, and by using for the laser amplifier function at least one semiconductor laser emitter element, and preferably a strip of such emitters. However, one could not simply replace the laser crystal with such semiconductor laser emitters, because although they are capable of emitting a high optical power, typically several hundreds of Watts, even a few kW, they have a laser emission at too high broad spectrum and too wide angle of emission. To solve these problems, the present invention proposes the structures described below with reference to FIGS. 2 and 3. The structure of FIG. 2 comprises a ring optical path 11. In this case, this "ring" is the simplest possible, and its structure is triangular, with a reflecting device at each vertex of the triangle, but it could include a higher number of reflecting devices, and therefore a polygonal structure with more than three sides. These three reflecting devices are: two simple mirrors 12, 13 and a semiconductor laser emitter 14. This laser emitter 14 can be either a single power semiconductor laser element (of a few tens of Watts at least), or, preferably, a strip formed of several such laser elements, or else a single semiconductor laser with a wide ribbon. The optical path in the cavity delimited by the two mirrors 12, 13 and the reflecting face (laser emission face) of the emitter 11 is then triangular. It includes the beams 11A (between 12 and 13), 11 B (between 13 and 14) and 11C (between 14 and 12). Since the emitting surface 14A of the emitter 14 is in the form of a very narrow elongated ribbon, in particular when it is a strip with several elements or a laser ribbon, the emitted laser beam has a very wide opening in a plane. To transform this wide beam into a fine beam, and also to transform the fine beams arriving on the emitter 14 from the mirrors 12 and 13, a cylindrical lens 15 is placed in front of the emitter 14, on the path of the beams 11 B and 11C. In general, it is sufficient to have a single lens common to the two branches 11B and 11C, since the angle formed by the beams 11 B and 11C is quite small (a few degrees, for example). Advantageously, there is interposed in the cavity 11 a non-reciprocal element 16 identical or similar to the element 7 of FIG. 1. On the output path of the source of FIG. 2, that is to say in front of the face 14B of the transmitter 14 (face opposite to face 14A), there is respectively: a cylindrical lens 17, a spatial filter 18, which is advantageously a diaphragm disposed at the common focus of two converging lenses, or else a mirror associated with a Fabry-Perrot type interferometer and an optical output and reflection and transmission device 19, which is advantageously a partially reflecting and partially transmissive network for a given wavelength. The laser oscillator 14 is provided with a layer R m i n (with minimum reflection) on its two faces to avoid oscillation between them. The source represented in FIG. 3 comprises, like that of FIG. 2, an optical path in a ring 20. In the present case, this “ring” is also as simple as possible, and its structure is triangular (comprising respectively the branches 20A, 20B, 20C), with a reflecting device at each vertex of the triangle. At one of the vertices of this triangle (between the branches 20A and 20C), there is a laser transmitter 21, which can be one of the devices fulfilling the function of the transmitter 14 of FIG. 2. At another vertex (between branches 20A and 20B), there is a special mirror 22: it comprises, on its face opposite to that carrying a reflective layer, an opaque coating 23 in which a hole 24 is made just at the place coinciding with the top of the triangular structure. At the third vertex, there is a selectively reflecting element 25 (for a given wavelength). This reflecting element is advantageously a network shaped as a selective filter. As in the case of the source in FIG. 2, a cylindrical lens 26 is placed in front of the laser element 21. In the branch 20A, a cylindrical lens 27 is interposed focusing the beam coming from the source 21 at the center of the hole 24 (and, conversely, widening the counter-propagating beam coming from reflection from the mirror 22), and in the branch 20B, a cylindrical lens 28 is interposed widening the cylindrical beam reflected by the mirror 22 (and, inversely focusing on the hole 24 the widened beam from filter 25). In the branch 20C, a non-reciprocal element 29 is inserted, which can be identical to the element 16 in FIG. 2. At the outlet of the transmitter 21 (opposite its face opposite to that which faces the elements 22 and 25), a cylindrical lens 30 is placed on the path of the source output beam. This cavity, like that of FIG. 1, operates by phase conjugation with an appropriate spatial and spectral filtering of the modes for producing a source emitting a power beam whose opening is limited by diffraction. As for the source in FIG. 2, the intensity of the two counter propagating waves in the cavity is controlled by a non-reciprocal element 29 adapted to the emission wavelength. The laser emitter 21 is provided, on its face on the outlet side, with a layer
R, T (à réflexion et transmission) assurant le couplage extérieur de cavité et d'une couche Rmin sur l'autre face. On a représenté en figure 4 un exemple de réalisation de l'élément non réciproque 16 (ou 29), qui est le même que celui de la figure 1. λ Cet élément comprend, dans l'ordre, une lame — référencée 31 , un rotateur 2 de Faraday 32 et un polariseur 33. Selon une variante de l'invention, on inverse les positions des éléments 31 et 32. Dans les deux sources décrites ci-dessus, la génération d'un hologramme dynamique dans le milieu à gain (14 ou 21) résulte d'une interaction à quatre ondes. Le réseau qui s'inscrit est du type réseau de gain et réseau d'indice, cette dernière composante étant la plus importante dans le cas où la période spatiale du réseau est grande. Ainsi, dans la source de l'invention, l'oscillation ne peut se faire que grâce à la formation d'un hologramme dynamique dans le milieu actif à semiconducteur (émetteur laser). Cet hologramme dynamique permet d'assurer en grande partie un contrôle actif de la qualité du faisceau de la source. De plus, pour améliorer encore les propriétés de ce faisceau, on dispose dans la cavité des éléments classiques tels qu'un filtre spatial et spectral et un élément optique non réciproque. La source de l'invention présente une autre caractéristique importante : compte tenu du filtrage spatial et spectral du faisceau, elle réalise une mise en phase de tous les éléments émetteurs de la barrette. Dans ces conditions, la luminance de la source est imposée par la longueur de la barre émettrice et non par la dimension d'un élément rayonnant élémentaire. Selon une application, on réalise la mise en phase de sources laser à semiconducteur de puissance telles que des réseaux de lasers, ou un laser unique à ruban très large, ayant des longueurs d'ondes typiques de 810 nm ou 980 nm (valeurs les plus courantes actuellement). Le dispositif de l'invention réalise donc à partir d'amplificateurs à semiconducteurs l'équivalent d'une source de puissance dont l'ouverture du faisceau est limitée par diffraction et réalisant une mise en phase de tous les éléments de la barrette. R, T (reflection and transmission) ensuring the external coupling of cavity and a layer R min on the other face. FIG. 4 shows an exemplary embodiment of the non-reciprocal element 16 (or 29), which is the same as that of FIG. 1. λ This element comprises, in order, a blade - referenced 31, a Faraday rotator 2 32 and a polarizer 33. According to a variant of the invention, the positions of elements 31 and 32 are reversed. In the two sources described above, the generation of a dynamic hologram in the gain medium ( 14 or 21) results from a four-wave interaction. The network that registers is of the gain network and index network type, the latter component being the most important in the case where the spatial period of the network is large. Thus, in the source of the invention, the oscillation can only take place thanks to the formation of a dynamic hologram in the active semiconductor medium (laser emitter). This dynamic hologram largely ensures active control of the quality of the source beam. In addition, to further improve the properties of this beam, conventional elements such as a spatial and spectral filter and a non-reciprocal optical element are available in the cavity. The source of the invention has another important characteristic: taking into account the spatial and spectral filtering of the beam, it achieves a phasing of all the emitting elements of the strip. Under these conditions, the luminance of the source is imposed by the length of the emitting bar and not by the dimension of an elementary radiating element. Depending on one application, power semiconductor laser sources such as laser arrays or a single very wide ribbon laser are phased, having typical wavelengths of 810 nm or 980 nm (most currently in use). The device of the invention therefore achieves from semiconductor amplifiers the equivalent of a power source whose beam opening is limited by diffraction and realizing all the elements of the strip in phase.

Claims

REVENDICATIONS 1. Source laser, du type à émetteur laser semiconducteur (14, 21) et cavité en anneau (11 , 20), la cavité comportant au moins deux dispositifs réfléchissants (12-13, 22, 25), caractérisée par le fait que l'émetteur est partiellement réfléchissant et partiellement transmissif et est inclus en tant que dispositif réfléchissant de la cavité en anneau et en tant que milieu de formation d'un hologramme dynamique. 2. Source laser selon la revendication 1 , caractérisée par le fait que l'émetteur laser comporte une barrette de lasers semiconducteurs. 3. Dispositif selon la revendication 1 ou 2, caractérisé par le fait que la cavité comporte au moins deux dispositifs de collimation dans un planCLAIMS 1. Laser source, of the semiconductor laser emitter type (14, 21) and ring cavity (11, 20), the cavity comprising at least two reflecting devices (12-13, 22, 25), characterized in that the transmitter is partially reflecting and partially transmissive and is included as a ring cavity reflecting device and as a medium for the formation of a dynamic hologram. 2. Laser source according to claim 1, characterized in that the laser transmitter comprises an array of semiconductor lasers. 3. Device according to claim 1 or 2, characterized in that the cavity comprises at least two collimation devices in a plane
(15, 26), disposés de part et d'autre de l'émetteur dans l'anneau, et un autre élément de collimation dans un plan disposé face à la sortie de l'émetteur (17, 30). 4. Source laser selon l'une des revendications précédentes, caractérisée par le fait que au moins deux dispositifs réfléchissants autres que l'émetteur sont des miroirs (12, 13). 5. Source laser selon l'une des revendications 1 à 3, caractérisée par le fait que au moins deux dispositifs réfléchissants autres que l'émetteur sont un miroir (22) revêtu d'une couche opaque (23) dans laquelle est formé un trou (24), et un dispositif réfléchissant sélectif en longueur d'onde (25), une lentille cylindrique (27, 28) étant interposée sur les faisceaux incident et réfléchi par ce dispositif. 6. Source laser selon l'une des revendications précédentes, caractérisée par le fait que la cavité en anneau comporte un élément optique non réciproque (16, 29). 7. Source selon la revendication 6, caractérisée par le fait que l'élément optique non réciproque comporte une lame demi-onde (31), un rotateur de Faraday (32) et un polariseur (33). (15, 26), arranged on either side of the transmitter in the ring, and another collimating element in a plane arranged opposite the outlet of the transmitter (17, 30). 4. Laser source according to one of the preceding claims, characterized in that at least two reflecting devices other than the emitter are mirrors (12, 13). 5. Laser source according to one of claims 1 to 3, characterized in that at least two reflecting devices other than the emitter are a mirror (22) coated with an opaque layer (23) in which a hole is formed (24), and a wavelength selective reflecting device (25), a cylindrical lens (27, 28) being interposed on the incident beams and reflected by this device. 6. Laser source according to one of the preceding claims, characterized in that the ring cavity comprises a non-reciprocal optical element (16, 29). 7. Source according to claim 6, characterized in that the non-reciprocal optical element comprises a half-wave plate (31), a Faraday rotator (32) and a polarizer (33).
PCT/EP2004/051655 2003-07-29 2004-07-29 Optical cavity ring power laser source with high spectral finesse WO2005015700A2 (en)

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FR0309336A FR2858476A1 (en) 2003-07-29 2003-07-29 LARGE OPTICAL CAVITY LASER SOURCE IN LARGE FINESSE SPECTRAL RING
FR03/09336 2003-07-29

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Citations (3)

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