# 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
The seed laser was operated at a central wavelength of 2000 nm with a 3-dB bandwidth of 0.2 nm. In the next step, to increase the peak power of the generated pulses, the radiation was amplified in two TDFAs. The first TDFA delivered an average output power of \(\sim\)120 mW, with most of the pulse energy located in the spectral peak at \(\sim\)2 \(\upmu\)m. The second TDFA provided a maximum average output power of 2.45 W at a launched 790-nm pump power of 7 W. The output spectrum was moderately broadened compared to that of the oscillator, primarily owing to modulation instability and Raman scattering in the TDFA [30]. It spanned from \(\sim\)1.88 to 2.34 \(\upmu\)m; however, most of the power corresponded to wavelengths of approximately 2 \(\upmu\)m. More details of the laser system can be found in ref. [125]. Finally, the amplified 2-\(\upmu\)m pulse train was injected into an 11-m-long step-index \(\mathrm{InF_{3}}\) fiber with a core/clad diameter of 7.5/125 \(\upmu\)m and an NA of 0.3. As the launched average pump power was increased to 1.85 W, the average output SC power reached 1.14 W, and the corresponding optical-to-optical conversion efficiency was measured to be 61.6%. The output spectrum spanned from 1.9 to 4.75 \(\upmu\)m at the noise level (Figure 13b). Owing to the limitations of the detection system, the short-wavelength region of the spectrum was not investigated. Furthermore, the spectrum measured for the maximum output power was predominantly flat, except for the dips around \(\sim\)2.8 and 4.3 \(\upmu\)m corresponding to the absorption of \(\mathrm{OH^{-}}\) ions and atmospheric \(\mathrm{CO_{2}}\), respectively, during the free-space light propagation through the detection system. More specifically, the spectral bandwidth of 10 dB for the maximum output power, excluding the residual pump peak around a wavelength of 2 \(\upmu\)m, was 2500 nm, and the corresponding spectral range was from 1.94 to 4.44 \(\upmu\)m. Furthermore, the spectral integral shows that 88% and 30% of the output SC power are converted for wavelengths beyond 2.15 and 3.5 \(\upmu\)m, respectively. The presented GS-ML fiber laser system is relatively easy to develop, particularly because it is based on commercially available off-the-shelf components. The number of free-space optics was reduced to the minimum; therefore, the system was less susceptible to atmospheric conditions, such as dust and vibrations. It is believed that by applying a more powerful pump source and optimizing the \(\mathrm{InF_{3}}\) fiber length, it is possible to extend the LWE of the spectrum further into the mid-IR region while maintaining suitable spectral flatness. Figure 13: Experimental setup of SC generation (**a**). In order: semiconductor saturable absorber mirror (SESAM), fiber Bragg grating (FBG), wavelength division multiplexer (WDM), MOPA–Master Oscillator Power Amplifier, \(\mathrm{Er^{3+}Yb^{3+}}\)-doped fiber ring laser (EYDFL), laser diode (LD), \(\mathrm{Tm^{3+}}\)-doped fiber (TDF), pump and signal combiner (PSC), lenses (L1–L4), optical isolator (ISO), and dichroic mirror (DM). SC spectrum generated at the output power of 1.14 W (**b**). Adopted with permission from [123]. Copyright 2021, Optica Publishing Group.