Temperature-insensitive frequency conversion achieved by phase-matching outside principal planes and thermally induced phase-mismatch compensation

To enhance the thermal stability and scale up the output power of frequency conversion, a temperature-insensitive frequency conversion method is proposed, which is achieved by phase-matching outside principal planes and thermally induced phase-mismatch compensation. In the method, there are three cascaded crystals. Two crystals at the ends are phase matched with propagation directions outside principal planes. The other crystal, with an opposite sign of first temperature derivative of phase mismatch, is sandwiched in the middle and used for compensating the thermally induced phase mismatch generated in the first crystal. In a proof-of-principle study, two KTiOPO4 (KTP) crystals cut at (θ = 78.28 deg, φ = 40.99 deg) and a compensation crystal LiB3O5 (LBO) are employed for temperature-insensitive second harmonic generation of 1064 nm. As a result, the temperature bandwidth and maximum conversion efficiency, respectively, reach at 49.3°C and 56.8% with an interaction length of 14 mm. The proposed method is capable of increasing thermal stability for various frequency conversion processes across a broad spectral band.