Enhanced nonlinear effects in femtosecond laser-inscribed depressed cladding waveguides in sapphire crystals.

High-performance depressed cladding waveguides can be fabricated in crystals using ultrafast laser inscription. The investigation of nonlinear phenomena, which manifest during the transmission of high peak power femtosecond pulses within waveguides, holds significant importance for their practical integration into waveguide lasers and waveguide-based components, among other pioneering applications. In this study, the depressed cladding waveguides were successfully prepared in sapphire crystal by a femtosecond laser. The nonlinear phenomena occurring in this waveguide were investigated. The experimental results show that the nonlinearity in the depressed cladding waveguides is significantly enhanced compared with that of the bulk. This enhancement notably manifests through augmented nonlinear losses (NLs) and the third harmonic (TH) blueshift increase. Meanwhile, we theoretically investigate the influence of nonlinear effects on the TH, such as self-phase modulation (SPM), cross-phase modulation (XPM), and group delay. Our results reveal that the phase mismatch between the TH and the pump pulses is the main reason for the asymmetric broadening and blueshift of the TH spectrum. Our study reveals the unique nonlinear properties of the waveguides and lays the foundation for further relevant studies and applications of such waveguides.