Coherent light generators – Raman laser
Patent
1993-08-11
1995-05-09
Davie, James W.
Coherent light generators
Raman laser
372 4, 372 51, H01S 330
Patent
active
054147239
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to laser systems for generating radiation at wavelengths of about 1.5 micrometers and wavelengths of about 2.8-2.9 micrometers and, more particularly, to laser systems for frequency shifting the light from a neodymium laser by stimulated Raman scattering.
BACKGROUND OF THE INVENTION
A desirable wavelength for laser radar (lidar) and over-the-horizon optical communications is 1.5 micrometers because this wavelength is considered to be eyesafe. Wavelengths in the range of about 2.8-2.9 micrometers have been found useful for medical applications such as laser surgery because these wavelengths are highly absorbed by the water in tissue and thus are effective for vaporizing tissue. However, reliable, low cost, high power lasers for directly generating such wavelengths are not commercially available.
Pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers are commercially available from a number of suppliers and are used in many fields due to their relatively low cost, large average and peak powers and high pulse repetition frequencies. A well-known method for frequency shifting a laser toward longer wavelengths is by stimulated Raman scattering. A laser beam is directed through a Raman cell containing a Raman active medium. When the intensity of the laser beam exceeds a threshold value, light is emitted by the Raman medium at a wavelength that is longer than the wavelength of the laser beam. The output of the Raman cell includes light at the laser wavelength and at the shifted wavelength. The frequency shift and the conversion efficiency are characteristics of the Raman medium.
The use of methane for Raman shifting of a neodymium YAG laser output at 1.06 micrometers to a wavelength of 2.8 micrometers is disclosed by Guntermann et al in Applied Optics, Vol. 28, No. 1, Jan. 1, 1989, pages 135-138. Medical applications are suggested. The generation of 1.54 micrometer radiation for laser radar using methane as a Raman active medium is disclosed by Patterson et al in Applied Optics, Vol. 28, No. 23, Dec. 1, 1989, pages 4978-4981. Deposition of soot-like particles on the Raman cell windows was reported to limit the operating life of the system.
Raman scattering using methyl, ethyl and isopropyl alcohol, acetone, trichloroethane and water as Raman active media is disclosed by Colles in Optics Communications, Vol. 1, No. 4, September/October 1969, pages 169-172. Picosecond pulses at 530 nanometers, which were provided by second harmonic generation of the output of a neodymium glass laser, were used as the pump pulses.
Efficient conversion of light from 30 picosecond pulses of a Nd:YAG laser at 1.064 micrometers to the first Stokes component at 1.53-1.56 micrometers in cyclohexane, acetone, 1,2-dichloroethane and 1,4-dioxane is disclosed by Krumin'sh et al in Soviet Journal of Quantum Electronics, Vol. 14, No. 7, July 1984, pages 1001-1002.
Raman conversion in acetonitrile and methane to the eye-safe wavelength near 1.5 micrometers from a Q-switched Nd:YAG laser is disclosed by Meadors and Poirier in IEEE Journal of Quantum Electronics, Vol. QE-8, No.4, April 1972, pages 427-428.
Stimulated Raman scattering of 100 picosecond pulses in hydrogen, deuterium and methane is disclosed by Hanna et al in IEEE Journal of Quantum Electronics, Vol. QE-22, No. 2, February 1986, pages 332-336. A mode-locked and Q-switched neodymium YAG laser followed by a second harmonic generator was used to generate pump pulses at 1.06 micrometers and 0.53 micrometers. The use of hydrogen, deuterium and methane as Raman active media are also disclosed by Lorre et al in IEEE Journal of Quantum Electronics, Vol. QE-15, No. 5, May 1979, pages 337-342 and by Ottusch et al in IEEE Journal of Quantum Electronics, Vol. 24, No. 10, October 1988, pages 2076-2080.
A Raman cell positioned inside a neodymium YAG laser resonant cavity is disclosed in U.S. Pat. No. 4,327,337, issued Apr. 27, 1982 to Liu. Deuterium is suggested as a Raman active medium. A 1.5 micron Raman laser is disclosed in U.S. Pat. No
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