Third harmonic generation scheme

Figure 5 presents the scheme of the 18 MeV S-band linac at the University of Tokyo NERL, with a 0.3 TW femtosecond Ti Sapphire laser beam providing 795 nm, 100 fs (FWHM), 30 mj/shot pulses, which are split to provide photocathode illumination through third harmonic generation into a 265-nm pulse with 4 to 6 ps (FWHM) duration, as well as probe laser beams. 

Figure 1 Main nonlinear frequency conversion schemes. (A) Second harmonic, (B) sum frequency, (C) parametric emission and (D) third harmonic generation.

Here we find that the third harmonic (4wi) is almost located in the middle of a notch of the transfer function. Thus it will enter the generation of the control force with a high spectral weight so that the control scheme accidentally stabilizes its fundamental frequency u>i. This effect can be overcome by an additional low-pass filter as shown by the dashed curve and the circles. Again the third harmonic is located near a minimum of the transfer function, but since the notch is not so steep due to the low-pass filter, the spectral weight of the harmonic is smaller. Therefore the component of the control force that supports the fundamental frequency uji is reduced. 

---