# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 6 Mid-Infrared Supercontinuum Generation Using Fiber MOPA Seeded with 2 \(\upmu\)m Pulses  
Broadband and high spectral flatness SC generation can also be achieved by using ps-scale optical pulses with a central wavelength of \(\sim\)2 \(\upmu\)m. Recently, semiconductor fiber-pigtailed lasers operating at approximately 2 \(\upmu\)m have become available that have paved the way for the development of fiber-based MOPA systems. InF\({}_{3}\) fibers exhibit favorable dispersion properties for SC generation when pumped at this wavelength; thus, these pump sources are extremely useful for efficient SC generation. This configuration of the pump laser system was successfully used for SC generation by [PERSON] et al. [114], who demonstrated a 2.5-octave SC spectrum, extending up to 5 \(\upmu\)m, generated by an InF\({}_{3}\) fiber. The MOPA system was composed of a 1953-nm discrete-mode laser diode gain-switched at a PRF of 1 MHz, delivering pulses with a duration of \(\sim\)100 ps. First, the pump pulses were pre-amplified in two TDFAs core-pumped by 1.56 \(\upmu\)m fiber lasers. The next two fiber amplifiers were developed using a double-clad fiber (core/clad diameter of 11/127 \(\upmu\)m and 25/250 \(\upmu\)m), cladding-pumped at a wavelength of 790 nm. A detailed description of MOPA optimization is presented in [114]. An average output power of 10.34 W and a pulse width of 35 ps were detected at the end of the final TDFA. The maximum pulse energy and corresponding pulse peak power were estimated to be 10.34 \(\upmu\)J and 295 kW, respectively. It was also noticed that the optical pulse was compressed along the fiber amplifier chain (from \(\sim\)100 to 35 ps) that resulted from the interplay between the dispersion and SPM. This radiation, applying free-space coupling, was launched into a commercially available 10-m-long fluoroindate fiber with a core/cladding diameter of 9/125 \(\upmu\)m and core NA of 0.26 ([PERSON] [94]). At the maximum pump power, the total average SC output power was measured to be 1.76 W and the spectrum extended from \(\sim\)1 to over 5 \(\upmu\)m (Figure 10). The power for wavelengths longer than 2.9 and 3.5 \(\upmu\)m was determined to be 0.56 (32% of the total output power) and 0.33 W (19% of the total output power), respectively. Furthermore, the authors demonstrated remarkable spectrum flatness. The spectrum was maintained with spectral fluctuation within 3 dB over a span of 1870 nm, from 2.5 to 4.37 \(\upmu\)m.  
A 2-\(\upmu\)m laser diode can be replaced by another pulsed laser source operating in this wavelength region, such as a mode-locked fiber laser, as reported by [PERSON] et al. [42]. In the experiment, the authors demonstrated 10-W-level SC generation in the InF\({}_{3}\) fiber. The MOPA was seeded with a 1956-nm SESAM mode-locked fiber laser delivering 60 ps pulses at a PRF of 33 MHz with an average output power of 10 mW. Two fiber amplifiers were used to amplify the pulses. The average power of the pump pulse train was boosted by the first TDFA to 1.65 W and by the second amplifier to 17 W under the maximal available pump power. The output of the final TDFA was spliced to a 7-\(\upmu\)m core (NA of 0.2) mode-field adaptor, allowing easy pump-radiation launching into the fluoride fiber. The InF\({}_{3}\) fiber featured a core diameter of 7.5 \(\upmu\)m, NA of 0.3, and length of 11 m. The geometrical parameters of the fluoroindate and silica (MFA) fibers permitted efficient fusion splicing with a splicing loss of 0.12 dB. The output end of the InF\({}_{3}\) fiber was also terminated with a short piece of AlF\({}_{3}\) fiber that improved the fiber performance during operation at high powers. The output SC spectrum, at the maximum pump power, spanned the wavelengths of 0.78-4.7 \(\upmu\)m, and the maximum average output SC power was 11.3 W (Figure 11a,b).