# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 3 Mid-Infrared Supercontinuum Generation Using Femtosecond Pulses  
The optimal pump source for SC generation is a mode-locked laser that emits the high-peak-power pulses necessary to trigger the nonlinear processes responsible for continuum evolution. Such pulses can be generated directly by single oscillators as well as optical parametric generators (OPGs) and amplifiers. Therefore, it is not surprising that the first demonstration of SC generation in an InF\({}_{3}\) fiber was reported using ultrashort laser pulses. In 2013, [PERSON] et al. [101] performed an experiment in which an optical parametric amplifier providing 70-fs optical pulses at a wavelength of 3.4 \(\upmu\)m was used to pump a 9.5-m-long InF\({}_{3}\) fiber. The nonlinear fiber was characterized by a core/clad diameter of  
Figure 3: Dispersion profile of single-mode fluoroindate fibers manufactured by Thorlabs (**a**) [94] and Le Verre Fluore (**b**). (**b**) Reproduced with permission from [59]. Copyright 2018, Chinese Laser Press. 16/120 \(\upmu\)m, NA of 0.14, ZDW at a wavelength of 1.83 \(\upmu\)m, and a cut-off wavelength of 2.8 \(\upmu\)m.  
For the pump pulse energy of 120 nJ (peak power of 1.7 MW), the output SC spectrum spanned from \(\sim\)2.4 to 4.95 \(\upmu\)m with a 20-dB spectral flatness from 2.7 to 4.7 \(\upmu\)m (Figure 4). The authors expected further spectral broadening, as the fluoride fiber losses at longer wavelengths were not the dominant factor limiting spectrum extension. The losses at \(\sim\)4.7 \(\upmu\)m were determined to be 0.5 dB/m, and the spectrum decrease was not extremely sharp, as shown in Figure 4. According to the authors, the achieved spectrum width was probably limited by the dispersion of the extremely broad supercontinuum laser pulse generated in the nonlinear fiber. Although the spectrum spanned over 2000 nm in the mid-IR window, the authors did not mention the average output SC power. However, considering that such pump laser systems usually operate at a repetition rate of up to several kHz, the output power was at the milliwatt level. It is worth mentioning that the solid-state femtosecond pump source used was relatively large and expensive. The alternative for OPGs is femtosecond mode-locked Tm-doped fiber lasers. Considering that the ZDW of fluorohate fibers is usually located below 2000 nm, they could be suitable candidates for pumping fluoride fibers. This architecture also allows easy application of an amplifier, thus offering scaling up of the output SC power. This solution has been demonstrated by [PERSON] et al. [102]. A commercial femtosecond fiber laser emitting at 1560 nm with a 50 MHz pulse repetition frequency (PRF) was used as the seed. After amplification in an erbium-doped fiber amplifier, the pulse train was launched into a highly nonlinear fiber with the aim of shifting more than half of the pulse energy (as a result of SSFS) to approximately 1960 nm. The final part of the pump source was a cladding-pumped thulium-doped fiber amplifier used to increase the average power at wavelengths of approximately 2 \(\upmu\)m to 570 mW. A dispersion-compensating fiber (DCF) was used to compensate for the overall dispersion of the active fiber. Assuming a sech\({}^{2}\) intensity profile, the output pulse width was calculated to be 97.5 fs. The output spectrum, extended from \(\sim\)1.9 to 2.25 \(\upmu\)m with two intensity peaks around 1.96 and 2.12 \(\upmu\)m, is presented in the inset of Figure 5. Furthermore, more than 90% of the pulse energy was red-shifted to approximately 2125 nm. Such a pump signal was launched into a dispersion-engineered indium fluoride fiber having a core diameter of 7 \(\upmu\)m, ZDW at 1.9 \(\upmu\)m with a small anomalous  
Figure 4: Supercontinuum generation in a 9.5-m-long fluorohate fiber. Adopted with permission from [101]. Copyright 2013, Optica Publishing Group. dispersion at 2.1 \(\upmu\)m and over the mid-infrared region to 4.5 \(\upmu\)m (\(<\)7 ps/nm/km).