RTP may also be used in a single-crystal configuration for phase-modulation, taking advantage of the high r33 electro-optic coefficient. In this case, the laser emission is polarised along the crystal's Z-axis and the optical length of the crystal can be precisely adjusted by the voltage applied between the Z-sides. The following features make RTP a good material for phase-modulation: No water-sensitiveness High electro-optic constant No piezo or pyro-electric effects It is often used as an OPO / OPA gain medium for emission in the 3µm range as well as an OPO crystal for eye-safe emission at high average power.