Apparatus and Method for Suppressing Parasitic Lasing and Applications Thereof

Abstract

Apparatus and methods that enable the suppression of amplified spontaneous emission (ASE) and prevention against parasitic lasing in cryogenically-cooled laser amplifier systems, thus allowing sustainable extraction efficiency when increasing the pump power and suitable for large-scale, high average-power laser systems employing large-aperture gain media. A gain medium having a known index of refraction for operation in an evacuated, cryogenic environment includes an ASE-absorbing epoxy composition on the perimetrical edge of the gain medium, wherein the epoxy composition has an index of refraction that substantially matches the index of refraction of the gain medium.

Claims

1. A method to reduce and/or eliminate gain clamping in a solid state laser/amplifier apparatus, comprising: providing a solid state laser/amplifier apparatus including a gain medium having a known index of refraction for operation in an evacuated cryogenic environment; providing the gain medium having a perimetrical edge including an amplified spontaneous emission (ASE)-absorbing epoxy composition disposed on at least a portion of the perimetrical edge of the gain medium, wherein the epoxy composition has an index of refraction that substantially matches the index of refraction of the gain medium; and coupling out from the gain medium at least a portion of transversely propagating ASE so as to prevent a build-up of parasitic oscillations in the gain medium.

2. The method of claim 1, wherein the perimetrical edge of the gain medium is a diffuse surface.

3. The method of claim 1, wherein the ASE-absorbing epoxy composition has a thickness equal to or greater than 10 microns (m) and equal to or less than 1 centimeter (cm).

4. The method of claim 1, further comprising disposing the gain medium including the ASE-absorbing epoxy composition disposed on at least a portion of the perimetrical edge of the gain medium in a cryorefrigerator.

5. The method of claim 1, comprising providing a Ti:Sapphire gain medium.

6. The method of claim 1, wherein the ASE-absorbing epoxy composition is a mixture of Stycast 1266 and Stycast 2850 epoxy resins.

7. The method of claim 1, wherein the ASE-absorbing epoxy composition comprises at least two epoxy components each having a different index of refraction and combining the at least two epoxy components in a predetermined proportion to achieve the epoxy composition index of refraction that substantially matches the index of refraction of the gain medium.

8. The method of claim 7, wherein the epoxy composition index of refraction matches the index of refraction of the gain medium to within five percent.

9. The method of claim 7, wherein the at least two epoxy components are low temperature-compatible epoxy components.

10. The method of claim 9, wherein the low temperature is equal to or less than zero degrees centigrade.

11. The method of claim 9, wherein the low temperature is equal to or less than 200 Kelvin.

12. The method of claim 7, wherein the ASE-absorbing epoxy composition includes an ASE-absorbing optical dopant.

13. The method of claim 12, wherein the ASE-absorbing optical dopant consists of metal particles.

14. The method of claim 13, wherein the metal particles have sizes that are equal to or less than 10 m.

15. The method of claim 12, wherein the ASE-absorbing optical dopant is selected from a group consisting of graphite, graphene, iron and powders absorbing near a gain crystal's ASE wavelength.

16. The method of claim 1, further comprising providing a bonding base intermediate the perimetrical edge of the gain medium and the ASE-absorbing epoxy composition, wherein the bonding base comprises a low-temperature epoxy.

17. The method of claim 16, wherein the bonding base further includes an optical dopant that enables at least one of refractive index tuning and ASE absorption.

18. The method of claim 17, comprising adjusting the index-tuning optical dopant level in the bonding base to tune the refractive index and adjusting the ASE-absorbing optical dopant level to control a thermal profile and ASE absorption profile of the solid state laser/amplifier apparatus.

19. The method of claim 16, wherein the optical dopant is a powdered optical material.

20. The method of claim 16, wherein the optical dopant has a refractive index that is greater than a refractive index of the transparent, low-temperature epoxy of the bonding base.

21. The method of claim 16, wherein the bonding base is Stycast 1266 and the optical dopant is SrTiO.sub.3.

22. A solid state laser/amplifier apparatus designed for operation in an evacuated, cryogenic environment, comprising: a gain medium having a known index of refraction, wherein the gain medium has a perimetrical edge; and an amplified spontaneous emission (ASE)-absorbing epoxy composition disposed on at least a portion of the perimetrical edge of the gain medium, wherein the epoxy composition has an index of refraction that substantially matches the index of refraction of the gain medium, wherein the gain medium is characterized by a gain clamping parameter that is greater than a gain clamping parameter of a similar gain medium of a laser/amplifier apparatus designed for operation in an evacuated, cryogenic environment that does not include an ASE-absorbing epoxy composition disposed on at least a portion of the perimetrical edge of the gain medium, wherein the epoxy composition has an index of refraction that substantially matches the index of refraction of the gain medium.

23. The solid state laser/amplifier apparatus of claim 22, wherein the perimetrical edge of the gain medium is a diffuse surface.

24. The solid state laser/amplifier apparatus of claim 22, wherein the ASE-absorbing epoxy composition has a thickness equal to or greater than 10 microns (m) and equal to or less than 1 centimeter (cm).

25. The solid state laser/amplifier apparatus of claim 22, wherein the gain medium including the ASE-absorbing epoxy composition is disposed on a cold finger of a cryorefrigerator.

26. The solid state laser/amplifier apparatus of claim 22, wherein the gain medium is a Ti:Sapphire crystal.

27. The solid state laser/amplifier apparatus of claim 22, wherein the ASE-absorbing epoxy composition is a mixture of Stycast 1266 and Stycast 2850 epoxy resins.

28. The solid state laser/amplifier apparatus of claim 22, wherein the ASE-absorbing epoxy composition comprises at least two epoxy components each having a different index of refraction, in a predetermined proportion to achieve the epoxy composition index of refraction that substantially matches the index of refraction of the gain medium.

29. The solid state laser/amplifier apparatus of claim 28, wherein the at least two epoxy components are low temperature-compatible epoxy components.

30. The solid state laser/amplifier apparatus of claim 28, wherein the ASE-absorbing epoxy composition includes an ASE-absorbing optical dopant.

31. The solid state laser/amplifier apparatus of claim 30, wherein the ASE-absorbing optical dopant consists of metal particles.

32. The solid state laser/amplifier apparatus of claim 31, wherein the metal particles have sizes that are equal to or less than 10 m.

33. The solid state laser/amplifier apparatus of claim 31, wherein the ASE-absorbing optical dopant is selected from a group consisting of graphite, graphene, iron and powders absorbing near a gain crystal's ASE wavelength.

34. The solid state laser/amplifier apparatus of claim 22, further comprising a bonding base disposed intermediate the perimetrical edge of the gain medium and the ASE-absorbing epoxy composition, wherein the bonding base a low-temperature epoxy.

35. The solid state laser/amplifier apparatus of claim 34, wherein the bonding base further includes an optical dopant that enables at least one of refractive index tuning and ASE absorption.

36. The solid state laser/amplifier apparatus of claim 34, wherein an amount of the optical dopant level in the bonding base is sufficient to control a thermal profile and ASE absorption profile of the solid state laser/amplifier apparatus.

37. The solid state laser/amplifier apparatus of claim 34, wherein the optical dopant is a powdered optical material.

38. The solid state laser/amplifier apparatus of claim 34, wherein the optical dopant has a refractive index that is greater than a refractive index of the transparent, low-temperature epoxy of the bonding base.

39. The solid state laser/amplifier apparatus of claim 34, wherein the bonding base is Stycast 1266 and the index-tuning optical dopant is SrTiO.sub.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] FIG. 1 is a schematic drawing of a cylindrical disc laser gain medium having a perimetrical edge entirely coated with an ASE-absorbing epoxy composition, disposed in a cryocooler, which assembly would be placed in an evacuated environment within a laser cavity, according to an illustrative, exemplary embodiment of the invention.

[0060] FIG. 2 is a schematic drawing of a rectangular laser gain medium having a perimetrical edge partially coated with an ASE-absorbing epoxy composition, disposed in a cryocooler, which assembly would be placed in an evacuated environment within a laser cavity, according to an illustrative, exemplary embodiment of the invention.

DETAILED DESCRIPTION OF NON-LIMITING, EXEMPLARY EMBODIMENTS

[0061] The disclosed embodiments describe apparatus and methods to avoid gain clamping in solid state lasers operating at cryogenic temperatures and/or in high vacuum environments by suppressing parasitic optical amplification. This is accomplished by bonding, or otherwise bringing into contact without a gap, a light-absorbing, refractive index-matched material (hereinafter ASE absorption material) to/with at least a portion of the edge or circumferential surface (hereinafter perimetrical edge) of the laser gain medium (see FIG. 2). The perimetrical edge of the gain medium will advantageously be ground and/or optically diffuse. The gain medium, typically in the form of a disc (although not necessarily round or of circular geometry) having such an ASE absorption material on at least a portion of its perimetrical edge allows the amplified spontaneous emission in the transverse direction to be coupled out to prevent parasitic oscillations from building up. The ASE absorption material reduces the strength of the Fresnel reflections and adds high absorption losses, both of which serve to increase the losses in the transverse direction, which prolongs the onset of parasitic lasing by increasing the threshold gain required to sustain transverse laser action.

[0062] The ASE absorption material is prepared with low temperature-compatible epoxies. In order to achieve an optical refractive index that closely matches the laser gain medium, different epoxies with high and low refractive indices are mixed together in predetermined proportions. Super-fine metal particles with absorption at ASE wavelengths may be added to the epoxy base to enhance the absorption capability.

[0063] For some high power applications, abrupt heat build-up at the boundary between the gain medium and the ASE absorption material can be a problem. To avoid this, a transparent, low-temperature epoxy mixed with a powdered optical material having a higher refractive index (for example, SrTiO.sub.3) can be used as a bonding base. The thermal profile can thus be controlled by adjusting the ASE-absorbing dopant level in the bonding base.

Illustrative Example

[0064] In an exemplary embodiment, the ASE absorption material was a mixture of Stycast 1266 epoxy and Stycast 2850 epoxy, which was bonded to the ground/diffuse perimetrical edge surface of a Ti:Sapphire crystal (laser gain medium). These two cryogenic epoxies were combined in a 1:1 mix ratio resulting in a refractive index substantially equal to that of sapphire. The epoxy mixture layer thickness was on the order of 1 mm to avoid possible cracking caused by a mismatch between material thermal coefficients of the composite epoxy and sapphire.

[0065] As is generally illustrated in FIG. 1, the ground edge of a cylindrical disc Ti: Sapphire crystal was in optical contact with the ASE absorption material made from 50:50 mixed Stycast 1266 and 2850. The thickness of the epoxy was controlled by placing the crystal inside a copper ring whose inner radius was larger than the crystal radius by an amount equal to the desired epoxy thickness. The outer circumferential edge of the copper ring was wrapped in indium foil before it was clamped in a copper mount, which attached to the cold finger of a pulse tube cryorefrigerator. The heat deposited by the pump laser was conducted out through the copper crystal mount. The entire assembly was cooled to a temperature of approximately 70 K and maintained under high vacuum during operation.

[0066] The ASE absorption treatment on the Ti:sapphire crystal edge is carried out as follows:

[0067] i. Carefully clean the ground edge of the crystal with isopropyl alcohol to remove oil and other contaminants;

[0068] ii. Prepare Stycast 1266 and Stycast 2850 according to the procedure recommended by the manufacturer;

[0069] iii. Mix Stycast 1266 and Stycast 2850 with a 1:1 ratio. Shake the mixture with a vortex mixer for over 5 min;

[0070] iv. Apply the mixture to the perimetrical edge of the crystal. A cavity mold (for example, a copper ring fitting the size of crystal) can be used to confine the mixture thickness applied around the crystal. A small amount of alcohol can be used to wet the side of the crystal before applying the epoxy mixture in order to realize good optical contact;

[0071] v. Wait 48 hours for the epoxy mixture to harden.

[0072] The embodied method enables (perfect) index matching for a broad range of materials because the index of refraction of the absorption layer can be effectively tuned (by using different proportions of two different types of epoxy, using a high-index powder in a low-index base, etc.). This differs from other index matching solutions offered in the past, as the lack of tunability excluded the possibility of exactly matching the index of refraction of the material of interest and/or the adaptability of the cladding to multiple materials. The embodied invention thus allows for index matching in cryogenic, high-vacuum environmentsconditions that are too extreme for other index matching solutions. This is a crucial requirement for many high power and high energy laser systems.

[0073] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0074] The indefinite articles a and an, as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean at least one.

[0075] The phrase and/or, as used herein in the specification and in the claims, should be understood to mean either or both of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with and/or should be construed in the same fashion, i.e., one or more of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the and/or clause, whether related or unrelated to those elements specifically identified.

[0076] As used herein in the specification and in the claims, or should be understood to have the same meaning as and/or as defined above. For example, when separating items in a list, or or and/or shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as only one of or exactly one of, or, when used in the claims, consisting of, will refer to the inclusion of exactly one element of a number or list of elements. In general, the term or as used herein shall only be interpreted as indicating exclusive alternatives (i.e. one or the other but not both) when preceded by terms of exclusivity, such as either, one of, only one of, or exactly one of.

[0077] As used herein in the specification and in the claims, the phrase at least one, in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase at least one refers, whether related or unrelated to those elements specifically identified.

[0078] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

[0079] In the claims, as well as in the specification above, all transitional phrases such as comprising, including, carrying, having, containing, involving, holding, composed of, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases consisting of and consisting essentially of shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

[0080] While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.