## 1 Introduction  
The search for supercontinuum (SC) sources reaching into the mid-infrared (mid-IR) region (which covers the 2-20 \(\upmu\)m electromagnetic spectral range) has drawn much attention in recent years. This is mainly because the atmospheric transparent windows (3-5 \(\upmu\)m and 8-12 \(\upmu\)m) and the molecular fingerprint region are included in this range. Thanks to these features, Mid-IR SC is considered to be a new source highly applicable in the fields of spectroscopy, sensing, biology, metrology, and defense [1; 2]. Limited in terms of infrared absorption, silica-based SC generation could merely extend their spectral domain above \(\sim\)2.7 \(\upmu\)m [1; 3; 4; 5]. Meanwhile, several other alternative materials were selected for the realization of Mid-IR SC generation, some of which are soft glasses such as tellurite [6; 7], fluoride glasses [8; 9], and chalcogenide glasses (ChG) [10; 11; 12]. As an example, [PERSON] et al. reported an all-fiber configuration SC generation with a continuous spectrum from 1.6 to \(>\)11 \(\upmu\)m and 417 mW on-time average output power, by pumping a cascade of Zr-Ba-La-Al-Na fluoride (ZBLAN) fiber, arsenic sulfide (As\({}_{2}\)S\({}_{3}\)) fiber, and arsenic selenide (As\({}_{2}\)Se\({}_{3}\)) fiber with a master oscillator power amplifier (MOPA) [13]. Considering the optical and chemical properties of these materials, chalcogenide glass with its wide transparency in the Mid-IR range and several orders of magnitude higher nonlinearity than standard fused silica [14] surpasses the other glasses. Furthermore, by combining the characteristics of single-mode propagating and more flexible chromatic dispersion modification of microstructured optical fibers (MOF), chalcogenide MOFs would be optimal for Mid-IR SC generation. In this paper, optical transmissions of different fibers are compared, and the advantages of chalcogenide photonic crystal fiber (PCF) for generating Mid-IR supercontinua are introduced and discussed. Fabrication methods of chalcogenide fibers and chalcogenide PCF are described, and finally, examples of Mid-IR supercontinua generated from these fibers and applications are presented. 2 Why Choose Chalcogenide Microstructured Optical Fibers for Mid-Infrared Supercontinuum Generation? For the generation of supercontinua in the Mid-IR region, the gain medium or the conversion medium is of great importance, besides the pump source. Generally, there are two sorts of Mid-IR fibers, which are passive fibers and active fibers, respectively. Passive fibers are applied to transmit infrared information, whereas active fibers generate Mid-IR light by using nonlinear effects or rare-earth doping [15]. Figure 1 lists and compares the transmission coverage of several different fibers. As can be seen, among glass fibers, chalcogenide glass is the only one with a transparent domain paving the Mid-IR region. The transmission varies with the constituting chalcogen elements, such as sulfur, selenium, and tellurium [16], as shown in Figure 2. Thus, As-S fiber can transmit from 1 to 6.5 \(\upmu\)m [17], As-Se fiber from 1.5 to 10 \(\upmu\)m [18], and Te-based fibers, due to heavy atomic weight, can transmit further than 14 \(\upmu\)m. In terms of optical nonlinearity, chalcogenide glasses possess a nonlinear refractive index that is 2 to 3 orders of magnitude higher than that of silica [16]. These nonlinear optical properties are even enhanced in small-core single-mode fibers. Figure 1: Mid-infrared transmissions of different types of optical fibers. Reproduced with permission from [15], copyright Informa PLC, 2017. Figure 2: Mid-infrared attenuation of chalcogenide optical fibers in comparison to silica attenuation. In microstructured optical fibers (MOF), the mode of the light propagating in the fiber is determined by the diameter of the air holes (d) and the distance between air holes (\(\Lambda\)), as depicted in Figure 3. Thus, single-mode guiding regardless of the wavelength can be achieved in MOF, as long as the d/\(\Lambda\) ratio is less than 0.42 [19; 20].