Fig. 1. State-of-the-art mid-IR sources¹⁰.

Only a few concepts, such as quantum-cascade lasers^{11, 12} (QCL, green area) or CO₂ lasers¹³, provide direct mid-IR generation. However, these sources emit rather narrow spectra. Supercontinuum generation (supercontinuum, orange area) in nonlinear fibers^14–16, waveguides¹⁷ and crystals¹³ represent extensively explored routes towards broad mid-IR spectra that are limited to average powers between 10 µW and 50 mW. Frequency downconversion in nonlinear crystals is a prominent alternative technique to obtain high power mid-IR emission. Operating in the transparency window for oxide nonlinear crystals (blue-colored background), optical parametric oscillators (OPO, black line)¹⁸ have attained watt-level and optical parametric amplifiers (OPA, blue lines)^{19, 20} and have attained 10 Watt-level average output power at a wavelength of up to 5 µm. OPAs and OPCPAs with longer wavelength emission have been realized by utilizing the transparency properties of nonoxide nonlinear crystals (light orange colored background), which provide an extended transparency window^{21, 22}. Steinle et al. demonstrated the advantage of a long wavelength driver in conjunction with nonoxide nonlinear crystals with a narrow-linewidth, cascaded OPA (narrow-linewidth OPA, dashed blue line) that attain average powers in the 1−100 mW range and tunability between 4.5 and 20 µm²³. Direct pumping at longer wavelengths for broadband difference frequency generation (DFG, brown line) or intrapulse DFG (IPDFG, brown line), however, has so far been relatively unattractive because the used pump lasers either had poor average-power scalability^{24, 25} or could only attain high pulse energies at the expense of relatively low repetition rates in the kHz range²⁶. Utilizing a short wavelength driver for IPDFG in a nonoxide nonlinear crystal has resulted in a 100 mW average power broadband frequency comb that is spectrally centered at 11.5 µm²⁷. In addition to the ongoing development of high average power Ho:YAG thin-disk oscillators²⁸ and mid-IR Cr:ZnSe-bulk lasers²⁹ (Ho:YAG / Cr:ZnS, violet line), the results presented in this work (red line) are based on the recent development of high-power Tm-doped fiber laser emitting few-cycle pulses that are centered at 1.9 µm wavelength³⁰ as a driving source for IPDFG