Gray-tracking of KTP crystal under high rep-rate laser pulses

The mechanism of gray-tracking and the influence of color center on gray-tracking are studied for KTP crystal, with a Q-switched laser whose wavelength and pulse width are 1 064 nm/532 nm and 6 ns respectively. The transmittance of crystal represents color center density. According to the relation between the transmittance and color center and the decisive role of color center density on gray-tracking, the critical gray-tracking density is defined, above which safe operation can be ensured. The experimental results show that gray-tracking absorbs not only the energy of ultraviolet and visible lights, but also that of near infrared light, giving a method for monitoring gray-tracking on line.