# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 1 Introduction  
Mid-infrared (mid-IR) supercontinuum (SC) generation has attracted considerable interest in recent years, owing to its advantageous applications in many important fields of science and industry. First, this spectral range covers the characteristic absorption lines of several versatile materials and important molecules (e.g., CO\({}_{2}\), HCl, CH\({}_{4}\), C\({}_{2}\)H\({}_{6}\), O\({}_{3}\), NO, and NO\({}_{2}\)) [1, 2], leading to the adoption of mid-IR SC sources for imaging [3], spectroscopy [4, 5, 6], and remote sensing, including the remote detection of explosives and hazardous chemicals [7, 8]. The improvements in these sources have also paved the way toward new possible applications in defense [3, 9], utilizing directional infrared countermeasure (DIRCM) systems, where a collimated output beam can be used to blind an infrared detector of a heat-seeking missile [10, 11, 12]. Finally, their use in medicine (e.g., tissue ablation, coherent anti-Stokes Raman scattering (CARS) microscopy, and breath diagnostics) [13, 14, 15, 16, 17, 18] as well as in the generation of few-cycle optical pulses in the mid-IR region [19, 20, 21] or for the stabilization of frequency comb lasers [22] are also worth mentioning. Some of the aforementioned applications require laser sources that deliver high-quality, high-power beams with a broadband and flat spectrum covering the entire 2-5 um band. Therefore, research on such laser systems providing high spectral brightness, defined as the radiance per unit optical bandwidth, has been the primary focus in many laboratories. Supercontinuum generation is a phenomenon, first described in the 1970s [23, 24], in which high-intensity laser pulses launched into a nonlinear optical medium interact with it, leading to the emission of new light frequencies and, eventually, causing the output spectrum to be much broader than the spectrum of the pump radiation. The efficiency ofthis process depend on many factors, including the peak power of the irradiated pulses, nonlinearity, dispersion, transmission band, losses, and the interaction length of light with a nonlinear medium. Considering these issues, low-loss optical fibers are preferred for SC generation because they provide effective laser beam confinement in a small fiber core area over its entire length. A schematic of SC generation is shown in Figure 1.  
An SC is usually generated by pumping a nonlinear fiber with high-intensity femtosecond pulses delivered by mode-locked lasers. Spectral broadening of the propagation of light pulses is attributed to a combination of various third-order nonlinear effects, such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), dispersive-wave generation (DWG), and stimulated Raman scattering (SRS) [25]. Although this approach can lead to the generation of a wide continuum, the output power is usually limited to only the milliwatt level. To achieve higher output powers, laser systems that provide picosecond and nanosecond pulse trains with a high average output power must be employed. The dynamics of the SC generation process depends on the relationship between the duration of the pump pulse, the emission wavelength of the laser source, and the zero-dispersion wavelength (ZDW) of a nonlinear fiber [26]. When a fiber is pumped with femtosecond pulses in the anomalous dispersion region, such that the pump wavelength is longer than the ZDW of the nonlinear material, the spectrum broadening is determined mainly by soliton fission and soliton self-frequency shift (SSFS) [27; 28]. By pumping an optical fiber with ultrashort pulses in the normal dispersion region, the SC generation mechanism is dominated by SPM.