Coherent light generators – Particular beam control device – Optical output stabilization
Patent
1999-04-07
2000-09-26
Scott, Jr., Leon
Coherent light generators
Particular beam control device
Optical output stabilization
372 34, 372 22, H01S 313
Patent
active
061251305
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention pertains to a process and device for minimizing the amplitude noise of solid lasers with frequency doubling within a cavity and more particularly to a process and device wherein a solid laser material and a frequency doubler crystal are located in a common cavity.
BACKGROUND OF THE INVENTION
Solid lasers (using mostly rare earth-doped crystals or glasses, e.g., Nd:YAG, Nd:YV0.sub.4, Nd:YAlO, Nd:YLF, Nd:glass or other, similar solid materials) with frequency doubling within the cavity have been known for a long time and are used for many applications in laser technology. What is used here is the generation of the second or higher harmonic vibrations in materials (mostly crystals, which have no inversion center, e.g., KTP, LBO, BBO, KNbO.sub.3, LiNbO.sub.3 or other) with a high nonlinear coefficient, which generates light of double (or multiple) frequency of the irradiated light wave by anharmonic vibrations of the lattice atoms, excited by an incident light wave. The process of generating higher harmonics strongly depends on the power density (cf., e.g., Kochner, Solid-State Laser Engineering), so that to generate frequency-doubled laser radiation of high efficiency, the nonlinear crystal is often introduced (at least in the case of continuously working (cw) lasers) either into the cavity of the laser itself or into a separate cavity (see above or, e.g., Yariv, Quantum Electronics, 3rd ed., p. 402). Even though the latter case of a separate cavity for the frequency doubler offers the fundamental advantage of low amplitude variations, this arrangement is characterized by a considerable complication due to the fact that the cavity belonging to the frequency doubler crystal must be actively stabilized to the frequency of the laser cavity and the laser radiation should be possibly a single-frequency radiation to achieve a high efficiency. The first case of introducing the frequency doubler crystal into the laser cavity is substantially less complicated compared with this; it is possible to work in this case with lasers which emit in longitudinal modes ranging in number from a few to many; the cavity mirrors are usually selected to be highly reflective mirrors for the laser wavelength in order to achieve a maximum increase in power in the cavity and thus the highest possible efficiency of doubling; at the same time, the output mirror is highly transmittent for the frequency-doubled radiation in order to be able to properly decouple it from the cavity.
However, this arrangement has a loud amplitude noise, which is an inherent feature of the system and which was first described, to the best of our knowledge, by T. Baer in J. Opt. Soc. Am. B, Vol. 3, No. 9, September 1986, p. 1175. There are many different approaches to explain this noise. Baer explains this by a competition of different modes (since the actually most intense mode is doubled best, it is attenuated most by the decoupling from the laser cavity, and another longitudinal mode will now become the most intense one, etc.). Other explanations are based on the total frequency generation or on the competition between modes of different polarization (cf., e.g., EP 0 457 590 A2). However, all these mechanisms are probably involved in the noise process at the same time.
The fact that the laser may have a very low-frequency noise, which is manifested by a "flickering" of the laser beam, whose degree of modulation may reach up to 100%, is especially disturbing for many applications. This noise is highly chaotic because of the nonlinear relationship of the doubling efficiency (see Koechner, see above); stable states may become temporarily established, which may be abruptly followed by loud noise. This phenomenon has been investigated in the literature in detail (see, e.g., Phys. Rev. A, Vol. 41, No. 5, March 1990, p. 2778, or Opt. Comm., Vol. 118 (1995), p. 289). Preliminary control models which are to eliminate this chaotic noise have also been designed, but the control bandwidth of these controllers is currently t
REFERENCES:
patent: 5383209 (1995-01-01), Hwang
patent: 5432807 (1995-07-01), Okazaki et al.
patent: 5854802 (1998-12-01), Jin et al.
Nikolov Susanne
Schalk Josef
Schmitt Nikolaus
Toesko Gunter
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