Study on polarization properties and micro-Raman spectroscopy of femtosecond-laser-written KTP depressed-cladding waveguides with engineered cross-sections

We have fabricated a series of depressed-cladding waveguides with engineered cross-sections in potassium titanyl phosphate (KTiOPO₄ or KTP) crystal by femtosecond laser writing. Their polarization properties at both 633 nm and 1030 nm have been studied. The results show that polarization properties of depressed-cladding waveguides could be effectively modulated by tailoring the waveguide end-face. Notably, nearly polarization-independent guidance at 633 nm has been realized by cross-section engineering. In addition, micro-Raman spectroscopy indicates that a spectral blueshift (∼0.73 cm^-1) at around 691.68 cm^-1 Raman peak has been produced in femtosecond-laser-induced tracks. This study provides new insights for polarization control of depressed-cladding waveguides in KTP crystal, which is important to fabricate polarization-independent waveguide structures in integrated photonics.