Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
2001-05-18
2004-03-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Nonlinear device
C372S034000
Reexamination Certificate
active
06711184
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority of German Application No. 199 46 176.7, filed Sep. 21, 1999 and International Application No. PCT/EP00/08826, filed Sep. 9, 2000, the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a diode-pumped laser with internal frequency doubling in which a solid-state laser crystal with a temperature gradient directed substantially parallel to the cavity axis and a nonlinear optical crystal following in the direction of the beam path for converting the laser radiation with a fundamental laser wavelength into laser radiation of another wavelength are provided inside a laser cavity as active medium.
b) Description of the Related Art
A laser of the type mentioned above is known, for example, from S. Erhard et al., Trends in Optics and Photonics, Vol. 26, Advanced Solid-State Lasers, Martin M. Fejer, Hagop Injeyan, and Ursula Keller, eds. (Optical Society of America, Washington, D.C. 1999), pp. 38-44.
For various basic reasons, the conversion of laser radiation of a fundamental laser wavelength into laser radiation of a different wavelength, e.g., generation of the second harmonic by means of a nonlinear optical crystal, is burdened by problems of nonlinear dynamics resulting in an unstable output power behavior. The causes of this instability vary in nature.
The nonlinear coupling of laser modes through nonlinear optical processes in the frequency-doubling crystal leads to intensity fluctuations in the laser, or “green problem”, as it is called (T. Baer, J. Opt. Soc. Am. B3, 1175 (1986)).
The formation of thermal lenses in laser crystal and in frequency doubling crystal (KTP crystal) leads to a complex reaction of the output power as a function of the diode current, since the thermal lens influences the laser mode in the cavity and, therefore, the effectiveness of the frequency doubling which depends on the intensity of the laser field in the frequency doubling crystal. This prevents adjustment of power via the diode current.
The technical solutions offered by the prior art solve only partial problems without meeting the requirements as a whole.
For generating stable laser output powers, it is known to let laser cavities operate either in multimode (>100) operation or in monomode operation.
U.S. Pat. No. 5,446,749 provides a laser arrangement with a particularly long cavity. Operation with stable amplitude is achieved by means of exciting many longitudinal modes. The output power of the laser is not adjustable via the diode current because of the strong thermal lens formed in the utilized laser crystal rod, since a power fluctuation occurs in the generated second harmonic at the start of power adjustment. The particularly long cavity results in large dimensions of the laser produced in this way and entails high costs.
For monomode operation, it is known [S. Erhard et al., Trends in Optics and Photonics, vol. 26, Advanced Solid-State Lasers, Martin M. Fejer, Hagop Injeyan, and Ursula Keller, eds. (Optical Society of America, Washington, D.C. 1999), pp. 38-44] to generate the second harmonic through internal frequency doubling in a cavity of a Yb:YAG disk laser, as it is called, in which the utilized laser-active solid state medium is shaped like a thin disk whose dimension in the propagation direction of the laser radiation (laser axis) is sharply reduced in relation to the other dimensions. According to DE 43 44 227 A1, a laser crystal of this kind is fastened to a sturdy cooling element by its surface which is directed at right angles to the laser propagation direction. Accordingly, a temperature gradient predominantly parallel to the laser axis is formed in the crystal, so that the formation of an interfering thermal lens is sharply reduced. The described laser uses a long cavity (approximately 1 m) with a noncritical temperature phase-matched LBO crystal as nonlinear optical crystal. Monomode operation is compelled in a known manner in that etalons and birefringent filters are placed in the cavity to reduce the number of longitudinal modes. These are expensive, require very sensitive adjustment and cause intracavity losses, so that the effectiveness of the laser is reduced to the indicated 15.5%.
This object is met by a diode-pumped laser with internal frequency doubling in which a solid-state laser crystal with a temperature gradient directed substantially parallel to the cavity axis and a non-linear optical crystal following in the direction of the beam path for converting the laser radiation with a fundamental laser wavelength into laser radiation of another wavelength are provided inside a laser cavity as active medium, wherein the conversion of the laser radiation with fundamental laser wavelength into laser radiation of another wavelength is carried out with a lower non-linear conversion than required for achieving maximum power of the converted laser radiation.
U.S. Pat. No. 5,511,085 discloses another solution in the form of a microchip laser with small cavity length and intracavity nonlinear crystal. While the number of transverse modes is already limited by end-pumping, intracavity etalon effects further reduce the longitudinal mode spectrum through additional coatings on the laser crystal or nonlinear crystal. Strong thermal effects put increased stress on the optical elements, limit the laser output power and accordingly restrict the uses of the laser. This also makes adjustment of the arrangement more difficult.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to prevent interfering power fluctuations in the laser radiation generated by means of the nonlinear crystal at the start of the power adjustment in a simple and, therefore, inexpensive construction without power-reducing elements in the cavity or thermal effects having a negative impact on the laser behavior.
This object is met by a diode-pumped laser with internal frequency doubling in which a solid-state laser crystal with a temperature gradient directed substantially parallel to the cavity axis and a nonlinear optical crystal following in the direction of the beam path for converting the laser radiation with a fundamental laser wavelength into laser radiation of another wavelength are provided inside a laser cavity as active medium, wherein the conversion of the laser radiation with fundamental laser wavelength into laser radiation of another wavelength is carried out with a lower effectiveness than required for achieving maximum power of the converted laser radiation.
The effectiveness of the conversion can be adjusted substantially through the longitudinal dimensions of the nonlinear optical crustal along the cavity axis and should be in a range of 50% to 90% of the effectiveness at which the maximum power of the converted laser radiation can be achieved.
In contrast to the known solutions, a compact, short cavity is used in which a few modes are generated and in which additional mode-selective elements are dispensed with. As is well known, a step of this kind would not lead to a stable output power with intracavity frequency doubling. This stability is first achieved by the low conversion effectiveness by means of the nonlinear crystal which is constructed in a particularly way.
As a result of the construction of the laser-active medium as a disk-shaped solid-state laser crystal with low amplification and a substantial temperature gradient in the direction of the cavity axis, the disruptive effect of a thermal lens on the mode distribution is already sharply reduced. A residual action of a thermal lens forming in the laser-active medium in spite of the temperature gradient directed predominantly parallel to the cavity axis is eliminated in that its focal length is adjusted so as to be greater than the cavity length.
The principle of the disk laser is not used in the present invention to achieve the highest beam qualities for high-power lasers in continuous wave mode. The invention makes use of the
Beilschmidt René
Hollemann Guenter
Mendler Wolfram
Ip Paul
JENOPTIK Laser, Optik, Systeme GmbH
Reed Smith LLP
Vy Hung
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