Generation of Broadband Mid-Infrared Supercontinuum Radiation in Cascaded Soft-Glass Fibers

The generation of mid-infrared (mid-IR) radiation, ranging from 2 - 5 mu m, is getting much attention in recent years thanks to many applications it can be used for, e.g. in free space optical communication, range finding, counter measures and remote chemical sensing systems. It also plays an increasing role in medicine, for instance in optical tissue ablation or optical coherence tomography, owing to the high water absorption in that wavelength range. In this research study, a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fluoride fiber is pumped by a Q-switched mode-locked (QML) thulium (Tm3+)doped double-clad silica fiber laser, emitting at around 2 mu m, to generate mid-IR supercontinuum (SC). Further spectral broadening of this SC radiation is achieved by coupling it into a chalcogenide arsenide-selenide (AsSe) photonic crystal fiber (PCF). An output power of 24 W at 2 mu m has been achieved in QML operation for the Tm3+-doped fiber laser. The SC output power from the ZBLAN fiber has been 7.8 W with a spectrum extending to approximately 4.1 mu m. For further wavelength broadening experiments, a long-wave-pass filter with a 3 dB edge around 3.6 mu m has been implemented between the ZBLAN and the AsSe fiber to cut out the residual pump light at 2 mu m and the radiation between 2 mu m and 3.5 mu m. The pump power was approximately 120 mW with a spectrum from 3.5 mu m to 3.9 mu m. First proof of principal experiments has been performed with 20 mW of averaged output power and a spectrum extending to 4.9 mu m. The coupling efficiency of the SC radiation from the ZBLAN fiber into the AsSe fiber has been around 30%.