2-18 μm mid-infrared supercontinuum generation in a step-index chalcogenide fiber

During the last decade there has been a surge of interest in developing mid-infrared (mid-IR) fiber-based supercontinuum (SC) sources. Such broadband light sources take advantage of extreme spectral broadening of high-intensity laser pulses in infrared optical fibers usually made of soft glasses, such as chalcogenides that offer the widest transmission window and the highest nonlinearity. Beyond their spatial coherence and high brightness, mid-IR fiber SC sources are nowadays operating over some wavelength ranges of thermal sources with superior performances for spectroscopic applications. However, intrinsic limitations of current fibers or even integrated waveguides now appear to impose the long-wavelength edge of mid-IR SC sources around 13-15 μm. The current research has to focus now on extending the wavelength coverage over the entire mid-IR molecular fingerprint region (often defined as from 2 to 20 μm). We here overcome this limitation by the engineered nonlinear transformation of femtosecond pulses over the full transmission window of a step-index chalcogenide fiber. In contrast to previous works, we reach the long-wavelength transparency edge of Se-rich glass family near 18 μm, and without including arsenic and antimony compounds considered as toxic elements and pollutants. Our end-to-end control of both materials chemistry and nonlinear fiber optics, including glass synthesis and purification, fiber design and drawing, as well as engineering of SC generation, has allowed us to optimize each of these crucial steps in order to demonstrate coherent mid-IR SC generation spanning from 2 to 18 μm.