# 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 SC architectures presented in the previous two Sections exhibited immense potential for broadband mid-IR SC generation in InF\({}_{3}\) fibers; however, owing to the pump laser sources used, the output SC power was limited to milliwatts. To achieve a higher output SC power, more powerful pump sources are required. One successful approach is to use a 1.55-\(\upmu\)m nanosecond or sub-nanosecond seed laser in tandem with Er3+-doped  
Figure 7: SC spectra measured for the average output power of 8 mW (fiber core diameter of 11–12 \(\upmu\)m) and 145 mW (fiber core diameter of 12.5–14.5 \(\upmu\)m). Inset, in-amplifier setup for SC generation in the InF\({}_{3}\) fiber. SC spectra adopted with permission from [104]. Copyright 2018, Springer Nature. (or/and Er3+:Yb3+-doped) fiber amplifiers and Tm3+-doped fiber amplifiers (TDFAs) as pump sources [33; 59; 105]. Such a laser system configuration is commonly known as a master oscillator power amplifier (MOPA) [106; 107; 108]. In this approach, seed pulses (of picosecond or nanosecond duration and the desired wavelength) can be delivered by Q-switched/gain-switched oscillators (including fiber lasers) or pulsed semiconductor lasers. The optical pulses are subsequently amplified to the required peak power level in an appropriately designed one-stage or multi-stage fiber amplifier. Finally, the amplified pulses are launched into a nonlinear medium. This pump setup enables scaling up the average output SC power linearly while maintaining a relatively constant spectral width. This can be achieved by increasing the repetition frequency of the pulses generated by the seed with a simultaneous increase in the average pump power that can be easily realized by providing a suitable gain in the final power amplifier of the MOPA system. Provided that the peak power of the pumping pulses is at the same constant level, the width of the generated SC spectrum should be kept constant. An exemplary setup for SC generation based on a MOPA is shown in Figure 8.  
A train of seed pulses is first pre-amplified in an erbium-doped fiber amplifier (EDFA) and erbium:ytterbium-doped fiber amplifier (EYDFA). Subsequently, resulting from MI and Raman scattering, the pulses propagating in a short piece of single-mode standard silica fiber split into shorter sub-pulses and are red-shifted [109], leading to a spectrum spanning from \(\sim\)1.4 to 2.4 \(\upmu\)m. Finally, the spectral components from the \(\sim\)1.9 to 2.1 \(\upmu\)m range are amplified and undergo further SSFS to longer wavelengths in a single-stage or dual-stage TDFA that acts as a nonlinear and active medium. This mechanism is well-described in refs. [110; 111; 112]. The spectral components from \(\sim\)1.5-1.85 \(\upmu\)m are absorbed by the TDF that acts as an isolator, preventing back-reflection-induced damage to the EDFAs and EYDFA, while wavelengths longer than 1.85 \(\upmu\)m are amplified and further red-shifted during propagation through the TDF. In the fluoride fiber, the initially broadened pump radiation is extended further into the mid-infrared region, reaching the LWE theoretically determined by material losses. Note that in this approach, the pump pulse breaks up, forming a series of femtosecond-scale sub-pulses, which, being red-shifted, give rise to light components at a wavelength of \(\sim\)2 \(\upmu\)m. This technique can eliminate the use of high-peak-power mode-locked lasers operating in this wavelength region. Additionally, such a laser system is built with the use of widely available active fibers and standard fiber-coupled passive components. This allows for high flexibility in construction and reduction of development costs while providing good optical parameters for the generated pump pulse train. Based on this concept, multi-Watt-level SC generation with a spectrum extended over 5 \(\upmu\)m was demonstrated. The most interesting results are discussed below.