Optical: systems and elements – Optical frequency converter – Harmonic generator
Reexamination Certificate
2001-02-23
2003-09-02
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Harmonic generator
C372S022000
Reexamination Certificate
active
06614584
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a nonlinear frequency converter apparatus particularly for diode-pumped solid state lasers, and more particularly to an automatic phase matching adjustment method and apparatus for incident and converted wavelength components.
2. Discussion of the Related Art
Diode pumped solid state lasers are efficient, compact and reliable sources of high beam quality optical radiation. The group of solid state lasers includes most commonly the neodymium laser, and also ruby lasers, but there are many others. Triply ionized neodymium is the active material of neodymium lasers. In a crystal, the neodymium is a substitutional dopant (most commonly for yttrium). Neodymium may also be incorporated into a glassy matrix. Neodymium may further form part of a crystal, such as with neodymium pentaphosphate NdP5O14. The most common host for neodymium is yttrium aluminum garnet (YAG), or Y3AI5O12. Other common neodymium hosts include yttrium lithium fluoride (YLF), or YLiF4; gadolinium scandium gallium garnet (GSGG), or Gd3Sc2Ga3O12; yttrium aluminate (YALO or YAP), or YALO3; and yttrium vanadate (YVO), or YVO4. Neodymium may also be hosted by phosphate and silicate glasses. Some more recently discovered neodymium host materials holding promise in the solid state laser field include gadolinium vanadate (GdVO), or GdVO4; and yttrium vanadate (YVO), or YVO4. Ytterbium (Yb) is also being doped into such crystals as YAG, YLF and YVO. Gadolinium vanadate crystals may also be doped with thulium (Tm) or thulium-holmium (Tm,Ho), rather than with neodymium. Titanium doped sapphire (Ti:Al2O3) and erbium doped YAG (Er:YAG) are also coming into vogue in the solid state laser field.
The principal wavelengths of lasing action for most solid state lasers is in the infrared (IR) spectral range. However, it is desirable to convert solid state lasers to lase in the ultraviolet (UV) spectral range. This frequency conversion is achieved with high efficiency by means of nonlinear optical conversion using nonlinear optical crystals. These crystals are normally arranged in the laser setup within the laser resonator for CW systems and outside the laser resonator for pulsed systems.
Commonly employed nonlinear conversion processes are harmonic generation, such as second, third and fourth harmonic generation (SHG, THG and FHG, respectively), and sum frequency generation (SFG). Other techniques include Raman shifting, sum and difference frequency mixing and parametric conversion. Harmonic generators may be packaged with the laser. Other techniques such as the above-mentioned ones are normally done using separate accessories. Many nonlinear optical crystals are available for doubling of the light frequency of solid state lasers thus converting the light into the visible range. However, efficient quadrupling and quintupling of laser radiation present significant challenges due to a very limited selection of nonlinear crystals and a necessity for special operating conditions for efficient and long-lasting operation.
Very few nonlinear crystals are available for nonlinear conversions of solid state laser light below 300 nanometers (nm) due to their transparency, non-linear coefficients and adequate birefringence. Of these, beta barium borate (BBO), or _—BaB2O4, lithium borate (LBO), or LiB3O5, and Cesium Lithium Borate (CLBO), or CsLiB6O10, each allow efficient conversion to the shortest wavelengths.
Laser systems using frequency conversion are being used in increasingly demanding industrial, medical and scientific applications. For example, it is desired for micro-machining of various materials in electronics and medical device manufacturing to have a laser with a short output wavelength and that has high stability and may be operated over a long term in a hands-off capacity. Minimizing the number and/or duration between scheduled maintenance of the laser system advantageously results in a reduction of running cost and an increase of production throughput. Conventional laser frequency converters require periodic re-adjustment of the phase-matching angle of the crystal during which time production is interrupted. For maximizing conversion efficiency, it is desired that a phase matching condition be continuously fulfilled, wherein the phase velocities for both an incident and a converted wavelength are made equal.
A common method utilizes a natural birefringency of the nonlinear optical crystal. When this method is applied, for example, to second harmonic generation, one of the waves is ordinary and the other is extraordinary. The direction of propagation of the two waves in the crystal is chosen so that refractive indices for the two waves are equal to each other (see e.g. J. E. Midwinter, J. Warner, Brit. J. Applied Physics, v. 16, p. 1135 (1965); and G. C. Bhar, D. C. Hanna, B. Luther-Davies, R. C. Smith, Optics Communications, v.6, p.323 (1972), each of which is hereby incorporated by reference).
This direction is characterized by a certain angle, called the “phase-matching” angle, between the direction of propagation of the waves and an optical axis of the crystal. It is desired to have an accuracy of maintaining the phase-matching angle that is substantially better than 1 mrad, such as 0.1 mrad. Additionally, since the ordinary and extra-ordinary refractive indexes are temperature dependent, the phase-matching angle is a function of crystal temperature. Therefore, it is desired to maintain the crystal at a constant temperature, in addition to maintaining the direction of the two waves at the phase-matching angle. Likewise, the phase matching condition can be attained by either varying the angle at constant temperature, or varying the crystal temperature at a fixed beam propagation angle with respect to the crystallographic axes.
Therefore, it is desired to have a frequency converter undergo periodic re-alignment of the angle of the non-linear optical crystal angle and/or the temperature of the crystal, in order to maintain an optimal conversion efficiency. It is recognized herein that a possible way of reducing the frequency of realignments is to provide a mechanically highly stable beam direction with respect to the optical axis of the crystal, and to stabilize the temperature of the crystal in absolute terms, e.g., by means of high-accuracy temperature controller. This approach may be difficult to implement in practice, particularly since the local temperature inside the crystal may be different from the temperature of the crystal holder, which would typically be what is stabilized by the temperature controller. The reason for this difference is most commonly local heating due to absorption of the laser beam at defect and impurity sites within the crystal. Since the amount of heat depends on the laser power and the condition of the crystal, such local temperature variations lead to instability and a hysteresis of the output power of the converter.
Therefore, it is desired to have means for adjusting the phase-matching in the crystal based on parameters of the converted beam, in a closed-loop arrangement, rather than relying on the stability of environmental parameters. We can propose several approaches. For example, the power of the converted beam can be optimized by means of some algorithm that scans a range of temperatures/angles to determine an optimum position, The main problem here is that based on a single measurement of the output power alone, the direction of adjustment of the phase-matching angle or temperature in order to restore perfect alignment is not determined. The algorithm might includes varying the beam angle (or crystal temperature) and simultaneously monitoring changes in the output power, for determining the optimum angle (or temperature). A disadvantage of this algorithm is that it leads to variations of the power of converted beam, thus rendering on-line adjustment using this technique undesirable.
U.S. Pat. No. 3,962,576, which is hereby incorporated by reference, discloses an automatic phase matching apparatus and method
Govorkov Sergei
Slobodchikov Evgueni
Lambda Physik AG
Stallman & Pollock LLP
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