Second harmonic generation in a hollow-core fiber filled with GaSe nanosheets

Frequency conversions using various all-fiber nonlinear devices can benefit many applications, including communications, sensing, microscope, and imaging. In this study, we report in-fiber second harmonic generation (SHG) in an optical hollow-core fiber (HCF) that can be used for frequency conversion. The HCF filled with a dispersion of gallium selenide (GaSe) nanosheets in the ultraviolet-cured optical adhesive supports a well-propagating mode in the fiber core, which enables effective light interaction with dispersed GaSe nanosheets and a strong SHG process. Based on theoretical analysis, the optimal HCF length for maximizing SHG is approximately 0.41 mm. The broadband SHG of the GaSe-filled HCF device is demonstrated by tuning the pump wavelength from 1460 to 1600 nm. Moreover, a time-varied SHG in the device is revealed due to additional second-order susceptibility induced by the polarization of silica. The proposed device with a compact and robust structure has the potential to be connected to existing all-fiber telecom and sensing systems for numerous nonlinear engineering applications.