When generating mid-infrared (mid-IR) supercontinuum light using a pump laser and a chalcogenide optical fiber, it is difficult to obtain broad wavelength coverage and high average power, primarily because of the low damage threshold of chalcogenide fibers as well as the peak power/average power tradeoff in available mid-IR pump sources. To improve upon current mid-IR supercontinuum performance that spans from 2 to around 15 μm with submilliwatt output powers by pumping chalcogenide large-core step-index fibers with optical parametric amplification (OPA)-based sources, researchers from the Technical University of Denmark, SelenOptics, the University of Rennes, and NKT Photonics demonstrated supercontinuum generation from 1 to 11.5 μm with a high average power of 35.4 mW from a tapered chalcogenide photonic-crystal fiber (PCF).
The large mode area of the untapered germanium arsenide selenide (Ge10As22Se68) PCF enables improved coupling efficiency and high damage threshold when accepting the pump light, while the tapered section offers strong nonlinear interaction and reduced dispersion for the pump wavelength, improving supercontinuum performance. The tapered PCF only allows single-mode propagation of the beam, improving beam quality and reducing losses because of high-order mode stripping. Experimental fiber optimization studies settled on ~31 cm total fiber length with a 15–20 cm long tapered waist with 6 μm core diameter and 2–3 cm long taper-transition regions, while a numerical study concluded that the waist section could be reduced to a few centimeters to reduce losses. Pumping this fiber with a 4 μm single-pass OPA with a 250 fs pulse duration, 21 MHz repetition rate, and 230 mW pump power produced supercontinuum generation at an average power of 57.3 mW from 1 to 8 μm. Reference: C. Rosenberg Petersen et al., Opt. Express, 25, 13, 15336–15348 (2017).