Frequency conversion from near-infrared to mid-infrared in highly nonlinear optical fibres

Abstract

Chalcogenide or heavy metal oxide glasses are well known for their good transparency in the mid-infrared (MIR) domain as well as their high nonlinear refractive index (n₂) tens to hundreds times higher than that of silica. We have investigated the nonlinear frequency conversion processes, based upon either stimulated Raman scattering (SRS) or soliton fission and soliton self-frequency shift (SSFS) in fibres made up with such highly nonlinear infrared transmitting glasses. First, SRS has been investigated in a chalcogenide As2S3 step index fibre. In the single pass configuration, under quasi continuous wave 1550 nm pumping, Raman cascade up to the forth Stokes order has been obtained in a 3 m long piece of fibre. The possibility to build a Raman laser thanks to in-fibre written Bragg gratings has also been investigated. A 5 dB Bragg grating has been written successfully in the core. Then, nonlinear frequency conversion in ultra-short pulse regime has been studied in a heavy metal oxide (lead-bismuth-gallium ternary system) glass photonic crystal fibre. Broadband radiation, from 800 nm up to 2.8 μm, has been obtained by pumping an 8 cm long piece of fibre at 1600 nm in sub-picosecond pulsed regime. The nonlinear frequency conversion process was assessed by numerical modelling taking into account the actual fibre cross-section as well as the measured linear and nonlinear parameters and was found to be due to soliton fission and Raman-induced SSFS.