# 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
Additionally, a 20-dB spectral flatness was maintained in the wavelength range of 1.85-4.53 \(\upmu\)m without considering the residual pump peak at 1.95 \(\upmu\)m. The power conversion efficiency, determined to be 66.5%, was also remarkable. The authors calculated that only 3.3% of the SC power was contained in the spectral range of 1.94-1.97 \(\upmu\)m, corresponding to the residual pump peak. The developed SC source was characterized by high output power stability, as illustrated in Figure 11c. For a short-duration test (12 min) at a power of 10.17 W, the normalized  
Figure 11: SC spectrum measured for the average output power of 11.3 W (**a**); SC power as a function of launched pump power (**b**); and short-term output power stability (**c**). \(\Delta_{\text{rms}}\) refers to the normalized root mean square. Adopted with permission from [42]. Copyright 2019, Optica Publishing Group. Figure 10: SC spectrum measured for the average output power of 1.76 W. Inset shows the output power in different spectrum bands as a function of the coupled pulse energy. Adapted under the terms of the Creative Commons Attribution 4.0 License [114]. Copyright 2018, Optica Publishing Group. RMS of the SC power was 0.33%. Similar to the other cases, the spectrum LWE was restricted by the background loss of the fluoride fiber. It is also worth emphasizing that no photodegradation or temperature rise of the fusion splicing joint or fiber tip was observed during the experiment, which is of immense importance for practical applications. Mid-Infrared Supercontinuum Generation Using 2 \(\upmu\)m Pulses Emitted by Q-Switched/Gain-Switched and Mode-Locked Laser Systems  
High-power SCs can be generated using optical pulses on the picosecond or nanosecond scale, as stated in the Introduction. Q-switching of the fiber laser cavity [115; 116; 117] can simply lead to the emission of pulses with widths usually from several to tens of nanoseconds and repetition rates from tens to hundreds of kilohertz. Unfortunately, owing to their long duration, their peak powers are not considerably high; consequently, they are not suitable for broadband SC generation. Another method of pulse generation is mode-locking (ML), which can provide highly repetitive pulses with picosecond or sub-picosecond durations [118; 37; 119]. However, the average output power generated directly from single ML oscillators is also low; for high-power applications, it has to be boosted using power amplifiers that make the system complex, particularly if the chirped-pulse amplification (CPA) technique [120] is applied. A solution for the generation of short-kilowatt-level pulses is the simultaneous use of Q-switching and active mode-locking (QS-ML) techniques [121]. As a result, a series of ML sub-pulses, spaced exactly by the cavity round-trip time, are recorded within one Q-switch pulse envelope. The advantage of this method is the reduced complexity of the entire SC source owing to the single-oscillator approach. Based on this concept, the SC power of 7 W with a mid-IR spectrum cut at 4.7 \(\upmu\)m has been demonstrated [122]. The experimental setup of the pulsed Tm-doped fiber laser used in this experiment and the setups adopted to generate SC radiation are shown in Figure 12a,b, respectively. The laser resonator was formed by a tunable volume Bragg grating on one side, an active fiber, and an output coupler mirror on the other side. A 2.2-m-long double-clad TDF with a diameter core of 25 \(\upmu\)m (0.08 NA) was used as an active medium. It was pumped from both ends by two 792-nm laser diodes, each delivering a maximum output power of 150 W. In addition, two endcaps were fusion-spliced onto the tips of the active fiber to reduce the power density and, eventually, minimize the risk of thermal damage. Two free-space  
Figure 12: Schematic of the setup of the thulium-doped fiber laser (**a**); experimental setup used to generate SC radiation (**b**) and SC spectrum for an output power of 7 W (**c**).