Europaisches Patentamt (19) European Patent Office Office europeenpeen des brevets EP 0 655 170 B1

(12) EUROPEAN PATENT SPECIFICATION

(45) Date of publication and mention (51) intci.6: H01S 3/07, H01S 3/094, of the grant of the patent: H01S3/16 18.12.1996 Bulletin 1996/51 (86) International application number: (21) Application number: 93918690.4 PCT/US93/07423 Date of 05.08.1993 (22) filing: (87) International publication number: WO 94/05062 (03.03.1994 Gazette 1994/06)

(54) SEGMENTED SOLID STATE GAIN MEDIA WITH GRADIENT LEVEL SEGMENTIERTES VERSTAERKUNGSMEDIUM FUER FESTKOERPERLASER MIT GRADIENTENFOERMIGER DOTIERUNGSSTAERKE MILIEUX DE GAIN SEGMENTES A LASER A SOLIDE PRESENTANT UN NIVEAU DE DOPAGE A GRADIENT

(84) Designated Contracting States: • SHAND, Michael, L. FR Morristown, NJ 07962 (US)

(30) Priority: 17.08.1992 US 930256 (74) Representative: Hucker, Charlotte Jane et al Gill Jennings & Every (43) Date of publication of application: Broadgate House, 31.05.1995 Bulletin 1995/22 7 Eldon Street London EC2M 7LH (GB) (73) Proprietor: AlliedSignal Inc. Morristown, New Jersey 07962-2245 (US) (56) References cited: EP-A- 0 454 865 US-A-3 626 318 (72) Inventors: US-A- 4 860 301 US-A- 5 105 434 • RAPOPORT, William, R. Bridgewater, NJ 08807 (US)

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lO CO Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice the Patent Office of the Notice of shall be filed in o to European opposition to European patent granted. opposition a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. a. 99(1) European Patent Convention). LU

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Description opposite exit end having the higher dopant concentra- tion. In accordance with the present invention, this is ac- Field of the Invention complished by providing a laser gain medium, e.g. a rod or slab, composed of aligned segments having increas- This invention relates to solid state laser gain me- 5 ing dopant concentration from segment to segment, dia, more particularly to solid state laser gain media such that the dopant concentration increases in the di- composed of segments having different doping levels. rection of the pump energy input. Within each individual segment, the dopant concentration may be level, or it Background of the Invention may vary within practical limits, the essence being that 10 average dopant concentration changes from segment Pumping - especially longitudinal pumping with a to segment provide for the desired gradient. The seg- single wavelength pump source - of uniformly doped la- ments are in crystallographic alignment along the same ser hosts tends to cause non-uniform heating, leading axis. to internal stress/strain within the host where the outer The segmented construction of laser gain media of surfaces are cooled except for the end faces. Because is the present invention in essence provides for stepwise of the exponential decay of the transmission function, approximation of a continuous dopant gradient profile. the intensity I of a beam transversing a crystal is This is particularly advantageous for laser gain media composed of which are difficult to grow with gra- dient dopant concentration, because of the dopant I = l0e(-a/) e.g. 20 distribution factor between melt and crystal grown from the melt. where l0 is the initial input, a is the absorption coefficient The invention is applicable to all solid state laser at a given pump wavelength and / is the distance along crystals, especially Cr+3 doped laser crystals including the crystal. Absorbed energy translates into heat. Thus, alexandrite, and particularly to the family of tunable, much more energy (heat) is deposited at the entrance 25 -doped fluoride crystal laser gain media of the end of the host than further into the crystal. For example, composition Cr+3:XYZF6 wherein X is an alkali in tests with 6.35 and 5 mm diameter uniformly doped ion; Y is an alkaline earth metal ion, Cd+2 or Mg+2; and Cr+3:LiSrAIF6 ("CrLiSAF") rods of approximately 6 cm Z is AI+3, Ga+3 or Sc+3. These include Cr+3:LiCaAIF6 length, the input end of the rods shattered at 15-18 ("Cr:LiCAF"), Cr+3:LiSrAIF6 ("CrLiSAF"), and Cr+3: Watts input levels when almost all of the pump energy 30 LiSrGaF6 ("CrLiSGaF"). These crystals are somewhat was absorbed. Stress/strain calculations and thermal frangible, and tend to fracture on unequal heating deposition profiles indicated large areas of stress/strain due to high thermal gradients due to exterior surface Brief Description of the Drawings cooling except for the endfaces, and poor thermo-me- chanical material properties. This can lead to cata- 35 In the annexed drawings, strophic failure of the crystal without prior warning as the input power is increased. Longitudinal pumping of a uni- Fig. 1 illustrates energy absorption in a longitudinal- formly doped host can also cause localized thermal aug- ly pumped CrLiSAF rod of level Cr+3 dopant con- mentation due to Excited State Absorption (ESA) and centration along the length of the rod; upconversion processes, both of which are exacerbated 40 Fig. 2 schematically shows construction of a laser by uneven pump energy absorption in accordance with gain medium of segmented construction composed the above-stated absorption formula. Further, the fluo- of segments having increasing dopant level con- rescence lifetime in these crystals is strongly tempera- centration from segment to segment; and ture dependent near room temperature, resulting in re- Fig. 3 illustrates energy absorption in a longitudinal- duced stored energy in higher temperature areas and 45 |y pumped CrLiSAF laser gain medium (here a rod) lower quantum efficiency, producing more localized of segmented construction of the type illustrated by heat. Fig. 2.

Summary of the Invention Detailed Description of the Invention, of the Preferred so Embodiments and of the Best Mode Presently We have found that the problems of unequal heat Contemplated for its Practice. build-up in laser gain media can be ameliorated by pro- viding a gradient doping level in the pumping direction For an exemplary listing of solid state laser hosts within the laser gain medium, with the lower dopant con- suitable for constructing the segmented laser gain me- centration at the entrance end of the gain medium, fol- ss dia of this invention, reference is made to Alexander A. lowed by sections of increasing dopant concentration Kaminskii, Laser Crystals, Springer Verlag, Berlin Hei- along the length of the gain medium, such that the input delberg New York, 1981. Growth and fabrication into beam is gradually absorbed as it propagates toward the segments for use in the segmented laser gain media of

2 3 EP0 655 170 B1 4 this invention follows conventional procedures. on Cr: LiSAF, Cr: LiCAF, Cr: LiSGaF and alexandrite, with Fig. 1 shows typical energy absorption condition for CrLiSAF and Cr: LiCAF being most preferred. a longitudinally pumped, 6 cm long CrLiSAF crystal with In the segmented laser gain media of the present 1.7 mole percent level Cr+3 doping, with an absorption invention, the individual segments are crystallographi- coefficient (a) of =0.4 at 746 nm. The term a is the ab- 5 cally aligned, and may be brought into direct, optical sorption coefficient at a given wavelength and is related contact with each other, or they may be spaced apart. to the material by a = oa(X)N where aa(k) is the absorp- If spaced apart, the interfaces between the segments tion cross-section and N is number of Cr+3 ions per cm3. are preferably provided with an anti-reflection coating. For CrLiSAF 100 mole % chromium doped material N Such coatings are conventional. Alternatively, an index- = 8.8 x 1 021 ions/cm3. The figure plots the energy/0. 1 cc 10 matching fluid may be interposed between the interfac- deposited down the length of the crystal, assuming a 1 es. Optical bonding or cementing may also be em- Joule heat input at an area of 1 cm2 at the entrance face. ployed, to avoid or minimize Fresnel losses. The seg- The bulk of the energy is deposited in the frontal portion mented laser gain media of this invention are composed of the crystal, causing large thermal gradients, which of at least two segments, which may be of the same or can lead to catastrophic failure of the crystal. is of different size. There is no limit on the number of seg- Fig. 3 shows the energy absorption in the segment- ments that may be employed, other than the practical ed laser gain medium illustrated by Fig. 2, which is com- limits imposed by the losses which are inevitably in- posed of 4 segments, each having increasing dopant curred at the interfaces. While the individual segments concentration in pump direction, as shown in Fig. 2. The will ordinarily be of the same crystal material and merely total length of the segmented gain medium is the same 20 differ in dopand concentration, it is possible to combine as that of the single crystal unitary rod of Fig. 1 . In the within the same lasing medium different crystalline ma- segmented gain medium the energy is deposited in terials having overlapping lasing wavelength, so as to more uniform manner overall, thereby reducing the ther- meet particular requirements. mal gradient to less than that obtained under the condi- The segmented construction with gradient doping tions illustrated in Fig. 1 at the same level of energy in- 25 level as here disclosed solves many problems, and pro- put. This would allow operation at higher pulse rate, or vides many advantages, as, for example: at higher input levels. Provision of gradient chromium dopant level has the laser gain medium can be pumped at a greater additional benefits. Additional heating terms such as Ex- level (higher pump energy and/or greater frequen- cited State Absorption (ESA) and upconversion can re- 30 cy) than is possible with a comparable uniformly sult from the spectroscopic properties of the material. doped host; The fluorescence lifetime is temperature dependent, so less uneven absorption profiles can be obtained, re- that the quantum efficiency decreases with increasing sulting in reduced stress/strain within the host; temperature. Since the gradient doping level will tend to the absorption profile can be tailored to meet de- lower the maximum temperature overall by avoiding for- 35 sired absorption-dependent performance criteria; mation of high temperature zones towards the energy additional heating terms due to Excited State Ab- input side, overall quantum efficiency degrades less. sorption, upconversion and thermally degraded The upconversion term is dependent on the excited quantum efficiency can be minimized; state density squared, where heat is generated by it is a relatively inexpensive and straight-forward ex- neighboring excited states interacting to produce one Cr 40 pedient for increasing power handling capability; in the ground state and one in the excited state after it permits the use of lower doped, hence less scatter decay back to the excited state. The Excited State Ab- loss material for a given length and absorption than sorption (ESA) in the lasing wavelengths absorbs an in- may be present in a uniformly doped gain medium; put photon in the excited state instead of the ground individual damaged segments can be replaced; state and generates heat in a non-radiative relaxation 45 - it allows use of shorter length crystals, which are back to the excited state. All of these effects produce more easily obtained, have lower scatter, and better localized heat. Distributing the excited states density by wavefront quality; tailoring the absorption profile will reduce the localized it allows for rapid change of the doping profile by thermal load from these processes. The location of high- substitution of differently doped segments, so that est pumping density will have the greatest local ESA so the gain medium can be tailored to meet pumping term. Reducing the excited state density will result in conditions; as well as others. more uniform spread of ESA through the material, and a more uniform heat deposition term. It will not affect the gain term (oN* , where o is the emission cross-section Claims and N* is the excited state ion density) since that term 55 is a sum over the length. The same holds true for up- 1. A solid state laser gain medium comprised of at conversion as a heat source. least two segments of a doped crystalline laser host Preferred embodiments of the invention are based arranged in crystallographic alignment along the

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same crystal axis wherein the dopant concentration lang der gleichen Kristallachse angeordnet sind, varies from segment to segment so as to provide a wobei die Dotierungskonzentration sich von Seg- gradient dopant level in the gain medium. ment zu Segment verandert, urn im Verstarkungs- medium ein Gradienten-Dotierungsniveau bereit- 2. The solid state laser gain medium of claim 1 where- 5 zustellen. in the crystalline laser host is Cr+3 doped. 2. Halbleiterlaser-Verstarkungsmedium nach An- 3. The solid state laser gain medium of claim 1 where- spruch 1 , bei dem der Laserwirtskristall mit Cr+3 do- in the laser host is alexandrite. tiert ist. 10 4. The solid state laser gain medium of claim 1 where- 3. Halbleiterlaser-Verstarkungsmedium nach An- in the laser host is a Cr+3 doped crystal of the com- spruch 1 , bei dem der Laserwirt Alexandrit ist. position XYZF6 wherein X is selected from Li+, Na+, K+ and Rb+; Y is selected from Ca2+, Sr2+, Ba2+, 4. Halbleiterlaser-Verstarkungsmedium nach An- Cd2+ and Mg2+; and Z is selected from Al3+- Ga3+ 15 spruch 1 , bei dem der Laserwirt ein mit Cr+3 dotier- and Sc3+. ter Kristall der Zusammensetzung XYZF6 ist, wobei X aus der Gruppe bestehend aus Li+, Na+, K+ und 5. The solid state laser gain medium of claim 4 where- Rb+ ausgewahlt ist; Y aus der Gruppe bestehend in the laser host has the composition Cr+3:LiSrAIF6. aus Ca2+, Sr2+, Ba2+, Cd2+ und Mg2+ ausgewahlt 20 ist; und Z aus der Gruppe bestehend aus Al3+, Ga3+ 6. The solid state laser gain medium of claim 4 where- und Sc3+ ausgewahlt ist. in the laser host has the composition Cr+3: LiCaAIF6. 5. Halbleiterlaser-Verstarkungsmedium nach An- 7. The solid state laser gain medium of claim 4 where- spruch 4, bei dem der Laserwirt die Zusammenset- in the laser host has the composition Cr*-3: 25 zung Cr+3:LiSrAIF6 aufweist. LiSrGaF6. 6. Halbleiterlaser-Verstarkungsmedium nach An- 8. A solid state laser, comprising, in combination: spruch 4, bei dem der Laserwirt die Zusammenset- zung Cr+3:LiCaAIF6 aufweist. (a) a solid state laser gain medium in accord- 30 ance with claim 1 ; 7. Halbleiterlaser-Verstarkungsmedium nach An- (b) excitation means associated with said laser spruch 4, bei dem der Laserwirt die Zusammenset- gain medium for pumping said laser gain medi- zung Cr+3:LiSrGaF6 aufweist. um; (c) means for defining a resonant laser cavity 35 8. Halbleiterlaser, der in Kombination folgendes um- surrounding said laser gain medium; and fal3t: (d) energy extraction means associated with said laser cavity for rembving laser energy from (a) ein Halbleiterlaser-Verstarkungsmedium said laser cavity; gemaB Anspruch 1 ; 40 (b) dem Halbleiterlaser-Verstarkungsmedium wherein said excitation means are arranged such zugeordnete Anregungsmittel zum Pumpen that the pump energy enters said laser gain medium des Halbleiterlaser-Verstarkungsmediums; at the side having lower dopant concentration and (c) Mittel zum Festlegen eines das Halbleiter- propagates along a path of increasing dopant con- laser-Verstarkungsmedium umgebenden La- centration. 45 serhohlraumresonators; und (d) dem Laserhohlraumresonator zugeordnete 9. The solid state laser of claim 8 wherein the laser Energieentnahmemittel zum Entfernen von La- gain medium is a longitudinally pumped laser rod. serenergie aus dem Laserhohlraumresonator;

10. The solid state laser of claim 8 wherein the laser so wobei die Anregungsmittel derart angeordnet gain medium is transversely pumped. sind, dal3 die Pumpenergie an der Seite mit der niedrigeren Dotierungskonzentration in das Halb- leiterlaser-Verstarkungsmedium eintritt und sich Patentanspriiche entlang eines Weges mit zunehmender Dotierungs- 55 konzentration ausbreitet. 1. Halbleiterlaser-Verstarkungsmedium, das aus min- destens zwei Segmenten eines dotierten Laser- 9. Halbleiterlaser nach Anspruch 8, wobei es sich bei wirtskristalls besteht, die in Kristallausrichtung ent- dem Laserverstarkungsmedium urn einen in Langs-

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richtung gepumpten Laserstab handelt. concentration en dopant plus faible et se propage le long d'une trajectoire de concentration en dopant 10. Halbleiterlaser nach Anspruch 8, bei dem das La- croissante. serverstarkungsmedium in Querrichtung gepumpt wird. 9. Laser a solide selon la revendication 8, dans lequel le milieu de gain a laser est un barreau laser pompe longitudinalement. Revendications 1 0. Laser a solide selon la revendication 8, dans lequel 1. Milieu de gain a laser a solide constitue d'au moins 10 le milieu de gain a laser est pompe transversale- deux segments d'un hote laser cristallin dope dis- ment. poses dans un alignement cristallographique le long du meme axe d'un cristal, dans lequel la con- centration en dopant varie d'un segment a I'autre de maniere a fournir un niveau de dopage a gra- 15 dient dans le milieu de gain.

2. Milieu de gain a laser a solide selon la revendication 1 , dans lequel I'hote laser cristallin est dope au Cr*-3. 20 3. Milieu de gain a laser a solide selon la revendication 1, dans lequel I'hote laser est de I'alexandrite.

4. Milieu de gain a laser a solide selon la revendication 1 , dans lequel I'hote laser est un cristal dope au Cr+3 25 de la composition XYZF6, ou X est choisi parmi Li+, Na+, K+ et Rb+; Y est choisi parmi Ca2+, Sr2+, Ba2+, Cd2+ et Mg2+; et Z est choisi parmi Al3+, Ga3+ et Sc3+. 30 5. Milieu de gain a laser a solide selon la revendication 4, dans lequel I'hote laser a la composition Cr*-3: LiSrAIF6.

6. Milieu de gain a laser a solide selon la revendication 35 4, dans lequel I'hote laser a la composition Cr+3: LiCaAIF6.

7. Milieu de gain a laser a solide selon la revendication 4, dans lequel I'hote laser a la composition Cr+3: 40 LiSrGaF6.

8. Laser a solide comprenant, en combinaison:

(a) un milieu de gain a laser a solide conforme- 45 ment a la revendication 1 ; (b) des moyens d'excitation associes audit mi- lieu de gain a laser pour pomper ledit milieu de gain a laser; (c) des moyens pour definir une cavite laser re- so sonante entourant ledit milieu de gain a laser; (d) des moyens d'extraction d'energie associes a ladite cavite laser pour evacuer I'energie laser de ladite cavite laser; 55 dans lequel lesdits moyens d'excitation sont dispo- ses de telle sorte que I'energie de pompage entre dans le milieu de gain a laser du cote ayant une

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EP0 655 170 B1

FIG. 2

2.76 1.7 1.06 0.64 MOL % MOL % MOL % MOL % PUMP Cr Cr Cr Cr DIRECTION

FIG. 3

Segmented Crystals

0.7

0.6

0.5

0.4

alpha

h0.3

MM,rt*1 ita-ir* * m\j v hO.2

ho.i

0.0 1 2 3 4 5 CRYSTAL LENGTH (cm)

delta E alpha