# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 5 Mid-Infrared Supercontinuum Generation Using Fiber MOPA Seeded with 1.55 \(\upmu\)m Pulses
The first report on this concept was presented in 2006 [113], but the first practical implementation using InF3 fibers was conducted by [PERSON] et al. in 2018 [59]. The pump source consisted of a 1.55-\(\upmu\)m distributed feedback (DFB) fiber-pigtailed laser (50 ps, 1 MHz), a double-clad EDFA pumped at 980 nm, and two TDFAs, of which the first was core-pumped by the EDFA and the second was cladding-pumped at 791 nm. The amplified  
Figure 8: Schematic of the setup for SC generation based on a MOPA seeded with \(\sim\)1.55 \(\upmu\)m pulses. EDFA: erbium-doped fiber amplifier, EYDFA: erbium:ytterbium-doped fiber amplifier, SMF: single-mode (silica) fiber, TDFA: thulium-doped fiber amplifier. and initially broadened signal with an output power of 2.3 W was subsequently launched using butt-coupling into a 20-m-long InP3 fiber (core diameter of 9.5 \(\upmu\)m, clad diameter of 100 \(\upmu\)m, NA of 0.3, ZDW of 1.9 \(\upmu\)m, cut-off wavelength of 3.7 \(\upmu\)m). After optimization, the authors demonstrated the generation of 1-W SC spanning over 2.25 octaves, from 1 to 5 \(\upmu\)m (Figure 9a). The SC spectrum exhibited high spectral flatness with a 6-dB spectral bandwidth from 1.91 to 4.77 \(\upmu\)m.  
Using a MOPA seeded by a 1550-nm laser diode as a pump source enables the linear scaling up of the average SC output power, as stated above. This can be achieved simply by increasing the repetition frequency of the pulses generated by the seed. Such SC power scaling has been demonstrated by [PERSON] et al. [60]. The pump laser system involved a 1.55-\(\upmu\)m seed laser, a dual-stage EYDFA, a 10-m-long single-mode passive silica fiber (SMF-28e), and a single-mode TDFA followed by a mode field adapter (MFA). The DFB laser was electrically modulated and emitted a train of 1.6-ns pulses with a variable PRF of 100-1000 kHz. The pump laser system generated an average power of 2.27 and 6.41 W for PRFs of 100 and 1 MHz, respectively. The output signal spectrum covered a spectral range of approximately 2-2.7 \(\upmu\)m. The output pigtail of the MFA was fusion-spliced with a 10-m-long InP3 fiber with a core diameter of 7.5 \(\upmu\)m and an NA of 0.3. The fusion splicing loss was determined to be 0.07 dB; thus, it provided an extremely high coupling efficiency,  
Figure 9: SC spectra generated from fluoroindate fibers, recorded for different average output powers: 1 W (**a**), 1.35 and 4.06 W (**b**), 2.95 W (**c**), and 11.8 W (**d**). Corresponding pulse repetition frequencies are shown in parentheses. (**a**) Reproduced with permission from [59], Copyright 2018, Chinese Laser Press. (**b**) Adopted with permission from [60], Copyright 2018, Optica Publishing Group. (**c**) Reproduced from [77]. (**d**) Reproduced under the terms of the Creative Commons BY license [61]. minimizing the heat accumulation at the pump-end of the fluoride fiber. Moreover, to reduce the power intensity at the end facet of the InF\({}_{3}\) fiber and improve the power handling performance of the SC source, an endcap made of a piece of multimode ZBLAN fiber of \(\sim\)200 \(\upmu\)m in length was spliced. By gradually increasing the pump power, the SC spectrum becomes increasingly flatter with the simultaneous extension of the LWE. For a maximal output power of 1.35 W (PRF of 100 kHz), the LWE reached 5.2 \(\upmu\)m, and a 1.5-dB spectral flatness was maintained in the range of 2.48-4.75 \(\upmu\)m. The power conversion efficiency was determined to be 59.5%. When the PRF was 1 MHz, an SC power of 4.06 W (power conversion efficiency of 63.3%) and a spectrum that extended to 5.1 \(\upmu\)m were demonstrated (Figure 9b). The developed SC source was also characterized by high output power stability with a normalized root mean square (RMS) of 0.37%, determined when the source generated an output power of 4 W at a PRF of 1 MHz. Furthermore, no power degradation was observed in the experimental study. The spectrum spanning the entire 2-5 \(\upmu\)m wavelength band with Watt-level output power was also demonstrated by [PERSON] et al. [77].