coating for KTP crystal is important to reduce the reflection losses and increase the transmission efficiency of the crystal. The type of coating depends on the application and the wavelength of the laser.
For example, for SHG of 1064 nm, the standard coating is dual band AR (DBAR) at 1064/532 nm. This coating allows the input beam at 1064 nm to enter the crystal with minimal loss, and the output beam at 532 nm to exit the crystal with high transmission.
Another example is HR @ 1064 nm and HT @ 532 nm coating, which is used for compact laser-SHG devices. This coating reflects the input beam at 1064 nm back into the laser cavity, and transmits the output beam at 532 nm out of the crystal. This coating enhances the intracavity power and improves the SHG efficiency.
A third example is AR/AR@1064+1570+3300 nm coating, which is used for eye safe generation by OPO @ 1064 nm -> 1570 nm + 3300 nm. This coating reduces the reflection losses for both the pump beam at 1064 nm and the signal and idler beams at 1570 nm and 3300 nm, respectively. This coating increases the OPO conversion efficiency and output power.