# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 3 Mid-Infrared Supercontinuum Generation Using Femtosecond Pulses
who developed a 2.4-5.4 \(\upmu\)m SC source based on a fluoroindate fiber pumped by an Er\({}^{3+}\):ZrF\({}_{4}\) fiber amplifier, seeded with 400 ps pulses at 2.75 \(\upmu\)m [58; 104]. The setup and output-measured spectra are shown in Figure 7.  
The setup consisted of (1) a 2.75 \(\upmu\)m OPG providing a 400-ps pulse train with an average output power of 2 mW generated at a pulse repetition frequency (PRF) of 2 kHz, (2) an erbium-doped fluoride fiber amplifier pumped by a 970-nm laser diode, and (3) a low-loss fluoroindate fiber that was fusion-spliced to the fluoride amplifier. The amplifier was made of a 1.25-m-long Er\({}^{3+}\):ZrF\({}_{4}\) double-clad fiber. The fluoride amplifier end was subsequently spliced directly into an InF\({}_{3}\) fiber with a core diameter of 11-12 \(\upmu\)m or 12.5-14.5 \(\upmu\)m. The attenuation was significantly low, equaling 30 dB/km for all wavelengths between 2.2 and 4.2 \(\upmu\)m, with a minimum value of 12 dB/km at 3.8 \(\upmu\)m.  
The amplified signal generated at the maximum available pump power was broadened to \(\sim\)3.2 \(\upmu\)m when measured at the noise level. Different lengths of InF\({}_{3}\) fiber were tested during the course of the experiment. The broadest spectrum was recorded for the 15-m  
Figure 6: SC spectra obtained from the InF\({}_{3}\) fiber pumped at 2.02 \(\upmu\)m, recorded for injected pump pulse energies of \(\sim\)2.7 \(\upmu\)J (36.3 kW), 3.9 \(\upmu\)J (52.4 kW), 5.5 \(\upmu\)J (73.9 kW), and 8.3 \(\upmu\)J (111.5 kW). Reproduced under the terms of the Creative Commons CC BY license [103]. Copyright 2016, The Authors, published by Springer Nature. long fluoride fiber. It spanned more than one octave, from 2.4 to 5.4 \(\upmu\)m (\(-\)20 dB level), with 82.3% of its energy beyond 3 \(\upmu\)m (Figure 7, lower (blue) curve). As can be seen, for wavelengths longer than 5.2 \(\upmu\)m, the steep decrease in the spectral density resulted from the InF\({}_{3}\) fiber transparency limit. The average output SC power was measured to be 8 mW. The main limitation of the output SC power was the onset of the parasitic lasing threshold of the erbium fiber amplifier that most probably resulted from the slight reflections at the fusion splice. However, to further increase the output power, the research group used the same setup configuration, and the seed OPG was replaced with one operating at ten times higher PRF (20 kHz) and delivering an average output power of 7 mW [104]. The authors also used an InF\({}_{3}\) fiber, 14 m in length, with a core diameter of 12.5-14.5 \(\upmu\)m. Therefore, they achieved 145 mW SC power with a spectral width similar to that obtained for the seed operating at 2 kHz (Figure 7, upper (red) curve). Similar to that in the previous case, the parasitic lasing threshold limited the maximum achievable output power. It is worth emphasizing that OPG-based SC sources can mainly be used in a laboratory environment. Although OPGs can provide high peak power pump pulses, also with wavelength tuning possibility, they are more complex than classical laser systems (and thus more expensive), require constant maintenance, and are susceptible to environmental conditions (vibrations, air humidity). For this reason, so far, they have found only limited use in commercial products and have not been used for commercial InF\({}_{3}\) fiber-based supercontinuum sources. Therefore, alternative pump schemes have been searched for and examined over the last years. Examples of such laser systems are described in the following Sections.