# 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
AOM—acousto-optic modulator, ML—mode-locker; QS—Q-switch, TDF—thulium-doped fiber, OC—output coupler, LD—laser diode, DM—dichroic mirror. Adopted with permission from [122]. Copyright 2020, Optica Publishing Group. acousto-optic modulators (AOMs) that provide active ML and QS were used inside the cavity. The optical beam generated by the oscillator was first attenuated and subsequently coupled into a 15-m-long InP3 fiber with a core diameter of 7.5 \(\upmu\)m (NA of 0.3). To provide stable and reliable air-to-fiber coupling, an intermediate 1-m-long and 26-\(\upmu\)m-diameter core single-mode ZBLAN fiber was used. Radiation from the fluorozirconate fiber was launched into the InP3 fiber using two commercially available collimators. In the QS-ML regime, the fiber oscillator could generate a maximum average output power of 40 W at a Q-switch repetition rate of 150 kHz. However, for experiments with SC generation, an average output power of 15 W at a pulse repetition rate of 60 kHz was used. Simultaneous QS-ML provided a macro-QS pulse width of \(\sim\)90 ns with -10 ML sub-pulses recorded within one Q-switch pulse envelope, exactly spaced by 37.9 MHz, corresponding to the cavity round-trip time. The ML pulse width was \(\sim\)30 ps, and the most energetic QS-ML pulse featured an energy of 88 \(\upmu\)J and an estimated peak power of 60 kW. For the highest launched average pump power (10 W), an SC spectrum with a total output power of 7 W and an LWE at a wavelength of 4.7 \(\upmu\)m was obtained (Figure 12c). Although the spectrum was not extremely flat, as expected, this is the first experimental demonstration of high-power SC generation in an InP3 fiber pumped using a single-oscillator laser system. A possible reason for the lower spectral distribution in the mid-IR region is that the output spectrum is a superposition of different spectra generated by individual ML sub-pulses having different peak powers depending on the position in the QS envelope. A drawback of this source is that the pump fiber laser does not provide a compact all-fiber setup that is preferred in many applications. An alternative approach permitting high SC power distribution over a wide spectral range was proposed in ref. [123], where a resonantly pumped fast gain-switched (GS) and simultaneously mode-locked Tm-doped fiber laser and amplifier system was used to pump a fluoride fiber. In the gain-switching technique of a laser resonator, pulse operation is performed by an active medium gain on/off switching via pump-power modulation. In particular, a fast GS, along with resonant pumping, can provide stable 2-\(\upmu\)m pulses of short duration (<100 ns). In-band pumping ensures rapid population inversion that can be depleted by a single short-gain-switched pulse. Subsequently, the cross-relaxation and excited-state absorption processes can be reduced, leading to the generation of a stable, short 2-\(\upmu\)m pulse train [124]. Combining GS and ML can lead to the generation of a stable train of regular ML sub-pulses of the same width recorded within one GS pulse envelope [125]. The schematic of the setup of this pump source, including the setup for SC generation, is shown in Figure 13a. The oscillator was arranged in the linear cavity formed by a fiber Bragg grating (FBG) centered at 2000.1 nm on one side, a 30-cm-long single-mode gain TDF, in-core pumped by a 1.55-\(\upmu\)m MOPA, and SESAM on the other side. The 1.55-\(\upmu\)m MOPA delivered 100 ns pulses with a repetition rate of 50 kHz, thus providing in-band pumping and fast gain switching of the thulium oscillator. Mode-locking was performed using a commercially available SESAM (BATOP GmbH). It provided stable GS-ML pulses with a repetition rate of 50 kHz and an average output power of -20 mW. The GS pulse envelope had a full width at a half maximum (FWHM) of -120 ns. The repetition rate of the 100% modulated ML sub-pulses present within the GS pulse envelope was 111 MHz, consistent with the cavity round-trip time.