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Segmented Solid State Laser Gain Media with Gradient 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 LASER GAIN MEDIA WITH GRADIENT DOPING 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) DO o 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 Printed by Jouve, 75001 PARIS (FR) 1 EP0 655 170 B1 2 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 crystal 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 crystals 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 chromium-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 metal 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.
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