1.  Field of the Invention
The present invention relates to high gain amplification of coherent light in a laser medium that has been cryogenically cooled.  In particular, the present invention relates to high gain ultrashort pulse amplification in cryogenically cooled amplifiers. 
2.  Description Of the Prior Art
Past work has demonstrated various schemes for amplification of pulsed (or continuous) laser light from pulse energies typical of mode-locked lasers (10xe2x88x926 to 10xe2x88x9210 Joules), up to energies of 10xe2x88x926 Joules or higher.  The higher energies are necessary for many applications, including wavelength conversion of light, micromachining and laser surgery. 
Past work by the current inventors has demonstrated that the use of cryogenically-cooled laser media (here defined as cooling the laser medium to a temperature below that which can be achieved using widely used water or antifreeze-water cooling systems, or through thermoelectric cooling schemes) can be effective in allowing pulse amplifiers to handle very high average powers of  greater than 3 watts, with high conversion efficiency of pump light to amplified laser light, and with simultaneously near-gaussian (TEM00) beam quality.  This is a result of the higher thermal conductivity of the laser medium (in this case titanium-doped sapphire) at low temperatures, combined with the reduced value of index change with temperature for the laser medium at low temperatures.  In the past work, the cryogenically-cooled crystal was used in a low-gain configuration, as a xe2x80x9cpower amplifierxe2x80x9d that followed a more-conventional non-cryogenically-cooled amplifier stage. 
The following two works describe past work by the present inventors in using cryogenically-cooled ti:sapphire for low-gain, high-power amplification of ultrashort pulses:
S. Backus, C. Durfee, M. M. Murnane, and H. C. Kapteyn, xe2x80x9cHigh Power Ultrafast Lasers,xe2x80x9d Review of Scientific Instruments, vol.  69, pp.  1207-1223, 1998. 
S. Backus, C. G. I. Durfee, G. A. Mourou, H. C. Kapteyn, and M. M. Murnane, xe2x80x9c0.2-TW laser system at 1 kHz,xe2x80x9d Optics Letters, vol.  22, pp.  1256-1258, 1997. 
Other prior art inventions focus on amplifying ultrashort pulses in color-center laser media or very similar f-centers media.  In these lasers, the active medium is a crystal (NaCl, KCl, and others) that can be temporarily xe2x80x9cdamagedxe2x80x9d using radiation.  The damage sites in the crystal (f-centers or color centers) can act as the xe2x80x9cdopantxe2x80x9d or active atom in the host crystal, and lasers and laser amplifiers can be made using these media.  However, these color centers will anneal-out of the crystal over time, and this usually necessitates cooling of the crystal to cryogenic temperatures to avoid fading-out of the lasing action.  Ultrafast and other amplifier systems using these media have been built.  However, the distinguishing characteristics of these systems from the present invention is that 1) the reason for cooling is primarily to preserve the laser medium, not to enhance its optical and thermal characteristics; 2) the power level of these lasers has generally been lower, not higher, than that of the prevailing ultrafast laser-amplifier technology; i.e. less than 100 milliwatts (as opposed to several watts); and 3) the total gain demonstrated in any of these systems has been limited to well under 106 (2.2xc3x97105).  In none of these works are the thermal or thermal-optic characteristics of the material even mentioned, since in general this is not a consideration for lasers emitting average powers of  less than  less than 1 watt. 
The following works describe past laser amplifier systems that utilized xe2x80x9ccolor-centerxe2x80x9d laser materials:
G. Lenz, W. Gellermann, D. J. Dougherty, K. Tamura, and E. P. Ippen, xe2x80x9cFemtosecond fiber laser pulses amplified by a KCl:TI+ color-center amplifier for continuum generation in the 1.5-mu m region,xe2x80x9d Optics Letters, vol.  21, pp.  137-139, 1996. 
G. Sucha, S. R. Bolton, and D. S. Chemla, xe2x80x9cGeneration of High-Power Femtosecond Pulses Near 1.5-Mu-M Using a Color-Center Laser System,xe2x80x9d Ieee Journal of Quantum Electronics, vol.  28, pp.  2163-2175, 1992. 
G. Sucha and D. S. Chemla, xe2x80x9cKilohertz-Rate Continuum Generation By Amplification of Femtosecond Pulses Near 1.5-Mu-M,xe2x80x9d Optics Letters, vol.  16, pp.  1177-1179, 1991. 
Schneider and C. L. Marquardt, xe2x80x9cBroadly Tunable Oscillator-Amplifier System Using Lithium (F-2+)a Centers in Kcl,xe2x80x9d Optics Letters, vol.  10, pp.  13-15, 1985. 
A need remains in the art for single stage high power amplification of coherent light. 
An object of the present invention is to provide single stage high power amplification of coherent light. 
A relatively simple laser amplifier system amplifies pulses in a single xe2x80x9cstagexe2x80x9d from xcx9c10xe2x88x929 joules to more than 10xe2x88x923 joules, with unprecedented average power (1-10 watts, and in future hundreds of watts) and exceptional beam quality (M2 less than 2).  Thus, very high gain and high output power is achieved simultaneously in a simple, single-stage amplifier system, as a result of cryogenic cooling of the laser medium. 
The laser medium is cooled substantially below room temperature, as a means to improve the optical and thermal characteristics of the medium.  This is done with the medium inside a sealed, evacuated or purged cell to avoid moisture or other materials condensing on the surface.  A xe2x80x9cseedxe2x80x9d pulse from a separate laser is passed through the laser medium, one or more times, in any of a variety of configurations including single-pass, multiple-pass, and regenerative amplifier configurations.  The energy of the input pulse is amplified by a factor of more than 250,000 times using the single gain medium. 
As a result, the amplifier can start with a very low energy pulse, and efficiently amplify the pulse to high energy, with high conversion efficiency of xe2x80x9cpumpxe2x80x9d energy into energy of the amplified light pulse, in a simple, single-stage amplifier.  The resulting output pulse energy will often reach the xe2x80x9csaturation fluencexe2x80x9d where the total extracted power approaches the power injected into the laser amplifier.  In our implementation of this scheme, a total optical-to-optical conversion efficiency of  greater than 25% was achieved in some cases.  This compares with a maximum of 15% obtained using the same optical configuration but without cryogenic cooling. 
The amplifying medium might have a host selected from the following list: Sapphire (Al2O3), Yttrium-Aluminum Garnett (Y2Al5O12), Yttrium-Lithium Flouride (LiYF4), LiSAF (LiSrAlF4), LiCAF (LiCaAlF4), KY(WO4)2), YVO4, or YAlO3.  The amplifying dopant might be selected from the following list: Titanium (Ti3+), Neodymium (Nd3+), Chromium (Cr3+), Holmium (Ho3+), Erbium (Er3+), Thulium (Tm3+), Praseodymium (Pr3+), Ytterbium (Yb3+), Europium (Eu3+), Dysprosium (Dy3+), or Terbium (Tb3+). 
Hence, the amplifying medium might be selected from the following list: Nd3+:Y3Al5O12, Nd3+:YAlO3, Ti3+:Al2O3, Ce3+:LiCaAlF4, Ce3+:LiSrAlF4, Nd3+:LiYF4, Yb3+:Y3Al5O12, Cr3+:Al2O3, Cr3+:LiCaAlF4, Cr3+:LiSrAlF4, Pr3+:LiYF4, Nd3+:KY(WO4)2, Ho3+:YAlO3, Ho3+:Y3Al5O12, Ho3+:LiYF4, Er3+:LiYF4, Er3+:Y3Al5O12, Er3+:YAlO3, Tm3+:YAlO3, or Tm3+:Y3Al5O12 
The pump laser might be selected from the following list: diode-pumped frequency doubled Nd:YAG, lamp-pumped frequency doubled Nd:YAG, semiconductor diode laser, ruby laser, diode-pumped Nd:Vanadate, or diode-pumped Nd:YLf. 
The amplifying medium might comprise titanium doped sapphire.  The amplifying medium might be a non-linear parametric amplification medium, allowing a single pass, high gain configuration.  Alternatively, the amplifier might have a regenerative amplifier configuration or a multipass amplifier configuration.  Fiber optics could be used for transmitting light between the pump laser and the amplifier.