Long-term operation of CsLIB6O10 crystal in fourth-harmonic generation of Nd:YAG laser
K.Deki, Y.Kagebayashi, N.Kitatochi, Y.Ohsako, M.Horiguchi, Y.K.Yap,*Y.Mori,* T.Sasaki,* K.Yoshida,** Tsukuba R&D Laboratory, Ushio Research Institute of Technology Inc., 5-2-4 Tokodai, Tsukuba 300-26; Email: email@example.com
Long-term operation of fourth-harmonic generation by CLBO was evaluated by two types of lasers. One type of laser produces a high peak power beam at low repetition rate, and the other type of laser produces low peak power but can run at a high repetition rate.
Using the high peak power laser, more than 1000 h of stable operation was observed as shown in Fig. 1. A group of microdamages resembling the incident beam was observed at the back surface of the CLBO, and the small decrease in output was attributed to it. The Microdamages study showed that ion beam etching of CLBO surface was effective in avoiding the damage. When the etching depth was, e.g., 250 Åno power decrease, and no group of microdamages was recognized at the end of test (after 160h operation).
In the experiment using the low peak powerlaser, strong focusing was required for effective harmonic generation. When a 532-nm incident beam was focused with an f=100 mm spherical lens to a CLBO, the output power decreased drastically to <50% of the initial power during 15 h of irradiation at room temperature. Although no surface damage was observed in this case, the output beam profile was distorted as the output power decreased. These results lead to speculation that the refractive-index change occurred in the focused area of the CLBO. As Fig. 2 shows, we observed the abrupt refractive-index change using an optical heterodyne method with a frequencystabilized transverse Zeeman laser as a probe beam.
Because the optical heterodyne signal is proportional to the difference of the refractive index between the ordinary and extraordinary waves at the phase-matching direction, the refractive-index change can be the phase-retardation change in the focused region. Therefore, this is considered to be direct evidence of refractive-index change. When the angle of incidence was detuned from the phase-matching angle to eliminate the fourth-harmonic generation, such retardation change was not observed even though the same amount of focused beam at 532 nm was input to the crystal more than 15 h. Therefore, we can conclude that this refractive-index change was caused by selfheating of CLBO by absorbing 266-nm light
Figure 3 shows the methodology of CLBO for fourth-harmonic generation. Although reported earlier about the necessity of using CLBO at an elevated temperature ( > 130°C, it is also important to use CLBO at reduced 266-nm average power density below 2KW/ cm2 in order to avoid drastic decrease of output power. Below this density, no refractive-index change was observed