High power laser

Details High power laser High power laser

2000-01-21

2002-04-02

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular resonant cavity

C372S099000, C372S071000, C372S035000, C372S022000, C372S093000, C372S039000

Reexamination Certificate

active

06366596

ABSTRACT:

FIELD OF THE INVENTION
The field of the invention relates to high power diode pumped solid state lasers.
BACKGROUND OF THE INVENTION
In an end pumped laser, the pumping radiation from a pumping laser, a laser diode diode array, or fiber coupled laser diodes is directed through a lens or lens system into the laser medium. Prior art end pumped lasers have been developed using active ions doped crystals such as Nd:YVO4, Nd:YLF and Nd:YAG crystals which can be end pumped from both ends. However, thermal lensing, thermal and stress related birefringence and distortion and crystal fracture have been problems which result in limiting the laser output power from the laser. In order to achieve higher power output from Nd:YAG lasers especially in polarized TEM00 mode beam profile, for example, an external amplifier often was needed(see product offer from Light Solutions and Q-Peak). There have been attempts to address pumping induced thermal and stress caused problems. See U.S. Pat. No. 5,577,060. End pump lasers with undoped ends have been provided, see U.S. Pat. No. 5,936,984. However the power output is still limited. The conventional view has been that the best conversion efficiency and better mode quality is achieved through high brightness pumping. And for high efficiency, compactness and good mode quality, typical laser resonator is limited to 30 cm or less in length with the beam spot size in diameter of 0.6 mm or less. Also, prevailing Nd atomic percentage doping level is about 1%. The same is true for Nd:YLF and Nd:YVO4. Typical diode end pumped laser produces 10 watts or less in polarized TEM00 mode in the case such as Nd:YAG laser. There is still a need for better, more reliable high powered lasers.
Diode pumped solid state lasers have gained popularity over the lamp pumped lasers with high efficiency and compactness, particularly in those applications requiring TEM
00
mode laser beam. Diode end pumping laser crystals such as Nd:YAG, Nd:YLF and Nd:YV04 have produced the best mode quality and highest efficiency TEM
00
mode output. Recent advancement of non-linear harmonic generation technologies and harmonic laser applications, such as via hole drilling and laser direct writing, demand high power polarized TEM
00
solid state lasers with high quality mode profile. In the end pumped configuration, conventional heat removal from the laser crystal pumped by the diodes are called “conductive” cooling (laser crystal is directly mounted to a heat sink). Since only a percentage of the applied pump power is converted into laser radiation, the remaining power is transferred by competing mechanisms as radiationless transition and absorption to the host lattice, resulting in nonuniform heating of the laser medium. To counteract the thermal effects generated by the pump radiation, laser media have to be cooled during operation. Also, mounting the crystal to the heat sink can cause extra mechanical stress on the crystal that is sensitive to environmental and temperature changes. In turn, that will cause the long term instability of the laser operation in a industrial environment. The ensuing nonuniform temperature distribution, stress induced birefringence, thermal and stress induced distortion place a serious limit on the maximum output power at TEM00 mode and beam mode quality. Comprehensive discussions of thermal effects are presented in W. Koechner, Solid State Laser Engineering (Springer Verlag, pp. 393-451). For example, polarized TEM
00
mode output power of diode end pumped Nd:YAG laser typically is limited to about 10 watts. Higher output power, reliable, stable operation of the laser is critically important for its industrial applications.
SUMMARY OF THE INVENTION
According to the invention, a diode pumped solid state laser is provided which produces improved power output and improved long term stability as well as an improved efficiency of pumping power conversion.
According to the invention, a high power diode pumped solid state laser is provided. The laser includes a first and second reflective surfaces which form an optical resonator cavity. A laser medium, particularly a Nd doped laser medium for example: a Nd:YAG, a Nd:YLF, or a Nd:YVO
4
crystal is provided within the laser cavity. A fundamental frequency laser beam propagates from the front and back ends of the laser medium. The first reflective surface is highly reflective for fundamental beam. The second reflective surface is at least partially reflective for fundamental beam. The laser medium is end pumped by at least one diode pumping apparatus for example, a laser diode, or diode array, or fiber coupled laser diodes, whose wavelength matches at least one laser medium absorption band. The diode pumping apparatus is located adjacent either the front end or the back end of the laser medium, or both. The optical resonator cavity is configured to provide a laser beam diameter in the laser medium from about 0.8 mm to 2 mm in diameter for the flndamental beam. Desirably, the laser medium has a diameter of about 1.6 to 5 times the fundamental beam diameter in the laser medium. A cooling jacket is desirably provided surrounding the laser medium so that the laser medium is directly liquid cooled. Desirably, the resonator cavity is also configured so that the fundamental beam is operated at TEM00 mode. Preferably, a polarization discriminator for example, a polarizer or one or more Brewster plate is located within the resonator cavity to discriminate one polarization over the other in favor of a particular polarization.
In another aspect of the invention, Nd doped crystals are used. In particular, low Nd doped Nd:YAG, Nd:YLF, and Nd:YVO
4
crystals are used. Multidoped Nd zone crystals can be used or low Nd doped crystal with no or lower doped ends can be used. In a further aspect of the invention, an harmonic generator or nonlinear OPO generator, for example, a second, third and/or fourth harmonic generator or generators, for example a LBO, KTP or BBO crystal or any other suitable nonlinear crystals are provided within the resonator cavity to generate a second, third and/or fourth harmonic beams according to the invention. Such a harmonic laser lasers or intracavity OPO have high power and an improved efficiency and reliability.
Useful in the harmonic generation aspect of this invention are a type I or II phase matching nonlinear crystal for second harmonic generation, type I or type II phase matching crystal for third harmonic generation. In a type I phase matching crystal for second harmonic generation, the fundamental beam is polarized orthogonal to that of resulting second harmonic beam. In a type I matching crystal for third harmonic generation, the fundamental beam and second harmonic beam incident on the type I crystal in parallel polarization to one another and produce a third harmonic beam with perpendicular polarization to the first and second harmonic beams. In a type II phase matching crystal for third harmonic generation, the fundamental beam and second harmonic beam are orthogonally polarized and produce a third harmonic beam with its polarization parallel to the polarization of one of the input beams (for example in a type II LBO crystal the polarization of the fundamental beam (1064 nm) and the third harmonic beam (355 nm) will be parallel). Also useful are 4th harmonic generation crystals. Examples of suitable crystals for harmonic generations include LiNB
o
3, BaNa (Nb03); LiO3, KDP, KTiOPO4, CsLiB6010, BBO and LBO, preferably KTP, BBO and LBO and CLBO.
It is an object of the invention to provide improved power output of TEM
00
mode polarized diode end pumped solid state lasers.
It is an object of the invention to provide an efficient end pumped solid state laser with beam spot size in the laser medium from 0.8 mm to 2 mm in diameter and laser crystal diameter of 1.6x to 4x laser beam diameter.
It is an object of the invention to provide a stable and reliable end pumped solid state laser by using improved cooling method.
It is an object of the invention to provide an efficient, reliable end pumped harmon

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