# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 3 Mid-Infrared Supercontinuum Generation Using Femtosecond Pulses
The output spectrum generated from the nonlinear fiber is depicted in Figure 5.  
Indium fluoride fibers of two lengths of 30 and 55 cm were used in the experiment. The shortest fiber generated an average output power of 258 mW with a 20-dB bandwidth of 2250 nm, whereas the longer fiber generated an output power of 247 mW with a 20-dB bandwidth of 2980 nm. In both cases, the spectrum covered nearly two octaves of bandwidth and extended up to 4.6 \(\upmu\)m. It was also observed that increasing the length of the nonlinear fiber further did not increase the generated bandwidth. The ratio of the SC power outside the pump spectral region (shown in the inset of Figure 5) to the total output SC power was calculated to be 70%. Mid-Infrared Supercontinuum Generation Using Optical Parametric Oscillators and Amplifiers Delivering Picosecond Pulses  
Continuing the discussion in Section 3, a similar experiment employing an optical parametric oscillator and amplifier system was performed by [PERSON] et al. [103]. However, in this case, they used longer picosecond-scale pulses with the possibility of output wavelength tunability; thus, the pump radiation could be adjusted in relation to the ZDW of a nonlinear medium. The pump laser system delivered \(\sim\)70 ps optical pulses at a repetition frequency of 1 kHz and a maximum pulse energy of 400 \(\upmu\)J at 1.7 \(\upmu\)m. The wavelength of the output pulses could be tuned from 1.5 to 16 \(\upmu\)m. The step-index, single-mode 9-m-long fluoroindate fiber used as a nonlinear medium, featured a core/clad diameter of 9/125 \(\upmu\)m, an NA of 0.26, and a cut-off wavelength of 3.2 \(\upmu\)m. The fiber exhibited losses below 0.5 dB/m for all wavelengths between 1.7 and 4.8 \(\upmu\)m, a ZDW at 1.72 \(\upmu\)m, and a flattened dispersion profile (\(<\)14 ps\(\cdot\)nm\({}^{-1}\cdot\)km\({}^{-1}\)) over the range of 2 to 4.4 \(\upmu\)m.  
Supercontinuum generation was investigated as a function of the pump wavelength and pulse energy/peak power launched into the fiber. This allowed the optimal pumping conditions to be determined for the nonlinear medium used. First, the pump wavelength was varied to maintain the pump pulse energy launched into the fiber at a constant level. This allowed the authors to determine the optimal pump wavelength for the nonlinear medium being used, providing efficient SC spectrum extension toward the mid-IR. The broadest spectrum was obtained when pumping at 2.02 \(\upmu\)m. In the second part of the study,  
Figure 5: SC spectrum recorded for the average output power of 247 mW. Adopted with permission from [102]. Copyright 2015, Optica Publishing Group. SC evolution as a function of pump pulse energy was examined while maintaining the pump wavelength at 2020 nm. Figure 6 shows the resulting SC spectra emitted from the InF\({}_{3}\) fiber, recorded for four different pump pulse energies (peak powers). As expected, the growth of the long-wavelength edge can be observed as a result of the increase in the pump energy. At the highest coupled pulse energy of 8.3 \(\upmu\)J, the spectrum covered the entire 2-5 \(\upmu\)m interval with a cut-off wavelength of 5.25 \(\upmu\)m, representing an octave of optical bandwidth. The signal drop at \(\sim\)4.2 \(\upmu\)m corresponds to absorption by CO\({}_{2}\) molecules in the detection system. The 5 dB flatness of the spectral intensity, achieved with 8.3 \(\upmu\)J pump pulses, was maintained in the wavelength interval from 2 to 5 \(\upmu\)m (a span of 3000 nm). The main limitation of further spectrum extension was the damage to the InF\({}_{3}\) fiber at higher pump pulse energies; the damage threshold was experimentally determined to be \(\sim\)200 GW/cm\({}^{2}\). Similar to the case of ultrashort (femtosecond) pump lasers, the average output power generated from OPGs is generally at the mW level. A simpler boosting method is to employ a power amplifier. This approach was tested by [PERSON] et al.