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Definition: the phenomenon that an input laser beam generates a beam with three times the optical frequency
Frequency tripling is a process of nonlinear frequency conversion where the resulting optical frequency is three times that of the input laser beam. In principle, this can be achieved with a χ(3) nonlinearity for direct third-harmonic generation [12–14], but this is difficult due to the small χ(3) nonlinearity of optical media and phase-matching constraints (except for tripling in gases). Therefore, frequency tripling is usually realized as a cascaded process, beginning with frequency doubling of the input beam and subsequent sum frequency generation of both waves, with both processes being based on nonlinear crystal materials with a χ(2) nonlinearity.
The main application of frequency tripling is the generation of ultraviolet light. Most common is the generation of 355-nm light by frequency tripling of a laser beam with 1064 nm, as obtained from a Nd:YAG or Nd:YVO4 laser. A common approach is to use two LBO crystals, the first being phase-matched for second-harmonic generation and the second for sum frequency generation. It is easy to make this process efficient when using pulses from a Q-switched or mode-locked laser, but also possible in continuous-wave operation e.g. with intracavity frequency doubling and resonant sum frequency generation.
Power Conversion Efficiency
Theoretically, the total power conversion efficiency of the frequency tripling process could be close to 100 %. For that, the frequency doubler should have a conversion efficiency of 2/3, so that the second-harmonic wave has twice the power of the remaining fundamental wave, and both have equal photon numbers. In practice, the efficiency of the frequency doubler is normally somewhat lower (often around 40 to 50 %), and in particular the sum frequency mixer is far from 100 % efficient. The latter problem can result from many effects, such as too low optical intensities, design limitations enforced by optical damage, effects of spatial walk-off, mismatch of pulse duration and/or temporal walk-off, etc. Tentatively, the conversion works best for high peak powers in not too short pulses, and when the beam quality is high and the optical bandwidth not too high.
With numerical modeling, the whole frequency tripling process can be simulated with fairly high accuracy and reliability of the results.
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