# 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 pump scheme was similar to that described above, except that the pump light was lens-coupled to the fluoride fiber. In this arrangement, 900 ps pulses at a wavelength of 1.55 \(\upmu\)m and a repetition frequency of 500 kHz, provided by a 4-GHz DFB laser, were amplified in a cascade of two EDFAs and an EYDFA providing over 2.1 W of average output power. Subsequently, the light propagating in a short piece of single-mode standard silica fiber was red-shifted, leading to a spectrum spanning from \(\sim\)1.4 to 2.4 \(\upmu\)m. Finally, it was amplified and further red-shifted in a single-stage TDFA, reaching a maximum average output power of 6.15 W and a spectrum spreading from \(\sim\)1.9 to 2.7 \(\upmu\)m. Owing to the precise optimization of the laser system, this approach led to efficient power distribution for wavelengths longer than 2.4 \(\upmu\)m. For wavelengths longer than 2, 2.4, and 2.5 \(\upmu\)m, powers of 5.09 (82.8% of the total output power), 2.02 (32.9% of the total output power), and 1.2 W (19.5% of the total output power) were measured, respectively. The radiation generated at the TDFA output was injected into a 12-m-long step-index InF\({}_{3}\) fiber (core/clad diameter of 7.5/125 \(\upmu\)m, NA of 0.3) by a telescope formed by two antireflection-coated lenses, allowing for a launch efficiency of \(\sim\)75%. Increasing the TDFA pump power corresponds to an increase in the output mid-infrared SC power, whereas the LWE of the output spectrum extends gradually toward the deep infrared wavelengths. After reaching an output power of 2.95 W, the spectrum spanned from \(\sim\)1.90 to 5.13 \(\upmu\)m (Figure 9c). The dips at \(\sim\)2.8 and \(\sim\)4.3 \(\upmu\)m correspond to OH\({}^{-}\) ions and the absorption of atmospheric CO\({}_{2}\) in the detection system, respectively. A spectrum flatness of 10 dB was demonstrated in the 1.96-4.97 \(\upmu\)m wavelength range. SC evolution was not examined at shorter wavelengths (<1.9 \(\upmu\)m) because of the limitations of the detection system used. Further, power upscaling was limited by the available pump power. A consecutive SC power increase was demonstrated by [PERSON] et al. [61]. Similar to the previous three cases, the SC source comprised a 1.55-\(\upmu\)m seed laser (PRF of 1-3 MHz, pulse duration of 1 ns), an amplification and frequency-shift module (EYDFA, SMF-28, and TDFA) with an output spectrum spanning the region of 1.9-2.6 \(\upmu\)m, a single-mode passive fiber, and an InF\({}_{3}\) fiber. The 11-m-long fluoride fiber featured a core diameter of 7.5 \(\upmu\)m, core NA of 0.3, and cut-off wavelength of 2.9 \(\upmu\)m. The output of the TDFA was first spliced into a short piece of passive single-mode silica fiber (functioning as a bridge between the TDF and fluoride fiber), and subsequently, the SMF was fusion-spliced into the InF\({}_{3}\) fiber. Furthermore, to protect the fiber end from photodegradation during high-power operation, an endcap made of multimode AlF\({}_{3}\) fiber was fabricated. The SC source was tested for both PRFs. At 3 MHz and a pump power of 17.5 W, the output power was 11.7 W, and the corresponding spectrum extended into the mid-infrared region with a cut-off wavelength at 4.6 \(\upmu\)m. Further power scaling up was possible, but it was not applied because the authors intended to avoid probable endcap damage under high-power operation. The power above 2.4 and 3.8 \(\upmu\)m was 7.26 (62.1% of the total output power) and 0.75 W (6.4% of the total output power), respectively. It was also noticed that the efficiency of power distribution toward the mid-infrared region (above 3.8 \(\upmu\)m) was limited by the inefficient peak power of the pump pulses. Therefore, in the following part of the study, the PRF was lowered to 1.5 MHz, and the maximum pump power was 18.3 W. As a result, an SC with a spectral coverage of 1.9-4.9 \(\upmu\)m and a maximal average power of 11.8 W was demonstrated (Figure 9d). The power conversion efficiency toward red wavelengths also significantly increased.