Laser amplifiers with high gain and small thermal aberrations

Abstract

The present invention discloses a laser amplifier with high gain and low thermally induced optical aberrations on the amplified laser beam. The amplifier designs allow simple multipass configurations to optimally extract the gain and reduce thermally induced index of refraction aberrations, making it possible to obtain an amplified laser beam of high quality combined with very high overall gains comparable to those achievable with expensive regenerative amplifiers. The amplifier includes a thin active laser solid to create the population inversion and associated heat generation within the thin laser active solid possible for the desired gain value. The system includes a cooling device in thermal contact with the thin active laser solid to provide good heat transport and high reflectivity coatings at the wavelengths of the pump and laser wavelengths. The pump light sources are laser diodes tuned to the maximum absorption of the laser active material. The amplifier also includes an optical system to transport the pump light to the laser active solid in such a way as to further confine the absorption of light along the two orthogonal directions in the plane of the laser active solid in order to get high population inversion and consequently high gains possible.

1 . A solid-state laser amplifier system, comprising:
 a) at least one laser-active solid having dimensions length L 1 , width W 1 , and thickness t 1 ;   b) a pumping light source;   c) light beam shaping optical system positioned adjacent to the pumping light source for shaping and directing a pump light beam from said pumping light source into a first surface of said at least one laser-active solid with an elliptical, round, or rectangular beam of light with a length or long axis L 1 ′ and a width W 1 ′ satisfying a condition L 1 ′/W 1 ′≧1, and L 1 ′<L 1  and width W 1 ′<W 1 , and wherein a region of said at least one laser-active solid illuminated by the beam of light produces a nonuniformly pumped gain region defined by dimensions L 1 ′×W 1 ′×t 1  of said at least one laser-active solid giving rise to nonparallel isotherms, and wherein t 1  is in a range from about 10 microns to about 1 millimetre so as to most strongly localize the absorbed light and ensuing pumped gain region that develops from pumping said at least one laser-active solid with said pump light;   d) a cooling device, wherein the laser-active solid is slab-shaped and is fixedly connected at a second surface thereof to the cooling device, and wherein a major portion of heat generated in the laser-active solid by the pump light is removed by the cooling device to cool the second surface of the laser-active solid; and   e) an optical system configured to bring the laser beam to be amplified into the laser active solid, so that it overlaps with the nonparallel isotherms that arise from nonuniform pumping, and is incident at an angle to a normal to the first surface of the laser active solid to substantially remove effects of the nonparallel isotherms that arise from nonuniform pumping of the laser active solid by the pump light beam and cooling requirements to achieve high gain conditions for the laser beam.   
     
     
         2 . The solid-state amplifier system according to  claim 1  wherein said optical system and said laser active material are configured to direct the laser beam to make at least two passes through the laser gain medium to further average the thermal aberrations and increase energy extraction from the said gain volume. 
     
     
         3 . The solid state amplifier system according to  claim 1  in which the cooling device is a cryogenic cooler that maintains the temperature of the laser active solid under pumped conditions at a temperature at which a differential change in index of refraction of the laser active solid with temperature is substantially close to zero (dn/dT=0) to further reduce thermal aberrations. 
     
     
         4 . The solid-state laser amplifier system according to  claim 1  wherein the slab-shaped laser active solid includes periodically disposed channels of length between about 0.1 micron to about 100 microns, said channels being filled with an absorbing material or scattering centers to introduce regions of absorption and scattering losses along a direction defined by L, and prevent accumulated stimulate emission along the L 1 ′ pumped region from depleting the gain in this direction and accompanying reduction in pulse quality for use in the amplification of laser pulses. 
     
     
         5 . The solid-state laser amplifier system according to  claim 4  wherein said channels are small cylinders or narrow lines that run parallel to a direction of W 1 ′ of the pumped gain medium. 
     
     
         6 . The solid-state laser amplifier system according to  claim 4  wherein the absorbing material are selected from the group consisting of semi-metals that strongly absorb at the laser wavelength, specifically tuned quantum dots tuned to act as saturable absorbers at the laser wavelength imbedded in an index matching polymer. 
     
     
         7 . The solid-state laser amplifier system according to  claim 4  wherein the scattering centers include air spaces or voids. 
     
     
         8 . The solid-state laser amplifier system according to  claim 1  including a slab-shaped nonactive laser material bonded to said first surface of said slab-shaped laser active solid to give a composite structure to provide mechanical strength for thermal contact of said slab-shaped laser active solid to the cooling device, said slab-shaped nonactive laser material having an index of refraction for matching an index of refraction of said laser active solid to avoid diffraction and Fresnel losses in coupling the laser beam to be amplified into and out of the pumped gain region. 
     
     
         9 . The solid-state laser amplifier system according to  claim 8  wherein said composite structure formed by the slab-shaped nonactive laser material bonded to said first surface of the slab-shaped laser active solid is shaped as a trapezoid or parallelepiped with a pre-selected angle at the ends of the composite structure into which the laser beam being amplified is directed and an output from which the amplified laser beam exits from the composite structure, and wherein said optical system is configured for redirecting the amplified laser beam back through the pumped gain volume to substantially average out thermal aberrations for further increasing gain of the laser beam. 
     
     
         10 . The solid-state laser amplifier system according to  claim 4  wherein said optical system is configured for directing the laser beam to be amplified into the pumped gain region at one or more angles to generally avoid the regions of absorption and scattering losses as the laser beam passes through the pumped gain volume. 
     
     
         11 . The solid-state laser amplifier system according to  claim 1  including a dielectric material located between the second surface of the laser active solid and the cooling device to give increased reflection of said laser beam to be amplified while simultaneously providing good thermal contact of said laser active solid to said cooling device. 
     
     
         12 . The solid-state laser amplifier system according to  claim 1  wherein said pumping light source is a laser diode array. 
     
     
         13 . The solid-state laser amplifier system according to  claim 1  wherein said pumping light source is a vertical stack of linear laser diode arrays. 
     
     
         14 . The solid-state laser amplifier system according to  claim 1  wherein said pumping light source is a fibre coupled laser diode array or vertical stack of linear diode arrays. 
     
     
         15 . The solid-state amplifier system according to  claim 1  wherein the pumping light source is a laser pump source. 
     
     
         16 . The solid-state laser amplifier system according to  claim 1  wherein said optical system is configured for directing the laser beam to be amplified through the surface of the laser-active solid along a direction which is substantially the same as the pump light beam direction in an angle multiplexed fashion so that the laser beam to be amplified makes two or more passes through the pumped gain volume. 
     
     
         17 . The solid-state laser amplifier system according to  claim 8  wherein said optical system is configured for directing the laser beam to be amplified through the surface of the non-laser-active solid along the pump light beam direction in an angle multiplexed fashion to substantially overlap with the pumped gain volume along the L 1 ′ direction primarily in a region where the temperature isotherms are most parallel to the cooled surface. 
     
     
         18 . The solid-state laser amplifier system according to  claim 8  wherein said optical system is configured for directing the laser beam to be amplified through the composite structure at an angle substantially parallel to the surface of the laser active material in contact with the cooler in such a way as to a zig-zag beam path through the pumped gain volume that effectively averages out nonparallel components of the thermal isotherms experienced by different parts of the laser beam to be amplified and thereby remove or greatly reduce thermal aberrations on said laser beam. 
     
     
         19 . The solid-state laser amplifier system according to  claim 8  wherein said optical system is configured for directing the laser beam to be amplified through the composite structure at an angle to the surface normal in such a way as to cancel out nonparallel components of the thermal isotherms along the surface normal experienced by the laser beam to be amplified and thereby remove or greatly reduce thermal aberrations on said laser beam. 
     
     
         20 . The solid-state laser amplifier system according to  claim 8  said optical system is configured for directing the laser beam to be amplified through the composite structure at a preselected angle to the surface normal and for producing a zig zag beam path in a plane of the first surface to average out nonparallel components to thermal gradients in both orthogonal directions to the surface normal along both directions of W 1  and t 1 . 
     
     
         21 . The solid-state laser amplifier system according to  claim 1  wherein the laser active solid is selected from the group consisting of YAG, YLF, YVO 4  and Sapphire host crystals containing laser active atoms selected from the group consisting of Ti, Nd, Er, Yb, Cr in YAG, YLF, YVO4 and Sapphire host crystals. 
     
     
         22 . The solid-state laser amplifier system according to  claim 8  in which the cooling device is a cryogenic cooler that maintains the temperature of the laser active solid under pumped conditions at a temperature at which a differential change in index of refraction of the laser active solid with temperature is substantially close to zero (dn/dT=0) to further reduce thermal aberrations. 
     
     
         23 . The solid-state laser amplifier system according to  claim 10  in which the cooling device is a cryogenic cooler that maintains the temperature of the laser active solid under pumped conditions at a temperature at which a differential change in index of refraction of the laser active solid with temperature is substantially close to zero (dn/dT=0) to further reduce thermal aberrations. 
     
     
         24 . The solid-state laser amplifier system according to  claim 20  in which the cooling device is a cryogenic cooler that maintains the temperature of the laser active solid under pumped conditions at a temperature at which a differential change in index of refraction of the laser active solid with temperature is substantially close to zero (dn/dT=0) to further reduce thermal aberrations. 
     
     
         25 . A solid-state laser amplifier system, comprising:
 a) at least one laser-active solid having dimensions length L 1 , width W 1 , and thickness t 1 ;   b) a pumping light source;   c) light beam shaping optical system positioned adjacent to the pumping light source for shaping and directing a pump light beam from said pumping light source into a first surface of said at least one laser-active solid with an elliptical, round, or rectangular beam of light with a length or long axis L 1 ′ and a width W 1 ′ satisfying a condition L 1 ′/W 1 ′≧1, and L 1 ′<L 1  and width W 1 ′<W 1 , and wherein a region of said at least one laser-active solid illuminated by the beam of light produces a nonuniformly pumped gain region defined by dimensions L 1 ′ ×W 1 ×t 1  of said at least one laser-active solid giving rise to nonparallel isotherms, and wherein t 1  is in a range from about 10 microns to about 1 millimetre so as to most strongly localize the absorbed light and ensuing pumped gain region that develops from pumping said at least one laser-active solid with said pump light;   d) a cryogenic cooling device, wherein the laser-active solid is slab-shaped and is fixedly connected at a second surface thereof to the cooling device, and wherein a major portion of heat generated in the laser-active solid by the pump light is removed by the cooling device to cool the second surface of the laser-active solid, and wherein said cryogenic cooling maintains the temperature of the laser active solid under pumped conditions at a temperature at which a differential change in index of refraction of the laser active solid with temperature is substantially close to zero (dn/dT=0) to further reduce thermal aberrations; and   e) an optical system configured to bring the laser beam to be amplified into the laser active solid at an angle to a normal to the first surface of the laser active solid, so that it overlaps with the nonparallel isotherms that arise from nonuniform pumping, and is incident at an angle to a normal to the first surface of the laser active solid to remove substantially nonparallel isotherms that arise from nonuniform pumping of the laser active solid by the pump light beam and cooling requirements to achieve high gain conditions for the laser beam.   
     
     
         26 . The solid-state amplifier system according to  claim 25  wherein said optical system and said laser active material are configured to direct the laser beam to make at least two passes through the laser gain medium to further average the thermal aberrations and increase energy extraction from the said gain volume. 
     
     
         27 . The solid-state laser amplifier system according to  claim 25  wherein the slab-shaped laser active solid includes periodically disposed channels of length between about 0.1 micron to about 100 microns, said channels being filled with an absorbing material or scattering centers to introduce regions of absorption and scattering losses along a direction defined by L 1  and prevent accumulated stimulate emission along the L 1 ′ pumped region from depleting the gain in this direction and accompanying reduction in pulse quality for use in the amplification of laser pulses. 
     
     
         28 . The solid-state laser amplifier system according to  claim 27  wherein said channels are small cylinders or narrow lines that run parallel to a direction of W 1 ′ of the pumped gain medium. 
     
     
         29 . The solid-state laser amplifier system according to  claim 27  wherein the absorbing material are selected from the group consisting of semi-metals that strongly absorb at the laser wavelength, specifically tuned quantum dots tuned to act as saturable absorbers at the laser wavelength imbedded in an index matching polymer. 
     
     
         30 . The solid-state laser amplifier system according to  claim 27  wherein the scattering centers include air spaces or voids. 
     
     
         31 . The solid-state laser amplifier system according to  claim 25  including a slab-shaped nonactive laser material bonded to said first surface of said slab-shaped laser active solid to give a composite structure to provide mechanical strength for thermal contact of said slab-shaped laser active solid to the cooling device, said slab-shaped nonactive laser material having an index of refraction for matching an index of refraction of said laser active solid to avoid diffraction and Fresnel losses in coupling the laser beam to be amplified into and out of the pumped gain region so that the thickness t 1  of the laser active solid can be made as small as possible for a maximum rate of heat transfer to the cooling device so as to produce minimum thermal aberrations and maximum gain per unit length for a given power of the pumping light. 
     
     
         32 . The solid-state laser amplifier system according to  claim 25  said optical system is configured for directing the laser beam to be amplified through the composite structure at a preselected angle to the surface normal and for producing a zig zag beam path in a plane of the first surface to average out nonparallel components to thermal gradients in both orthogonal directions to the surface normal along both directions of W 1  and t 1 .