Temperature-insensitive frequency conversion by phase mismatch self-compensation in the same type of crystals.

Aiming at high-power laser frequency conversion, we present a new scheme that can self-compensate for the thermally induced phase mismatch. The basic design of the scheme is that three crystals with the same type are cascaded, of which the crystals at both ends are used for frequency conversion and the middle crystal is used for compensating phase mismatch. By configuring the polarization states of the interacting waves in the middle crystal, the sign of the first temperature derivative of the phase mismatch is opposite to that of the frequency conversion crystals. The thermally induced phase mismatch in the first crystal can thus be self-compensated in the middle crystal. To verify the utility of the proposed scheme, we experimentally demonstrated temperature-insensitive second and third harmonic generation using KH2PO4 crystals. The results show that the temperature acceptance bandwidth is about two times larger than that of using a single crystal. Since the crystals used are of the same type, this scheme has excellent universal applicability and is almost completely free from the limitations of the laser wavelength, crystal and phase-matching type. Therefore, the scheme can be widely applied to various frequency conversion processes and is scarcely any limitations.