Mid-infrared (MIR) light in the range of 2-20 µm is useful for chemical and biological identification due to the presence of many molecular characteristic absorption lines in this spectral region . A coherent, few-cycle source with a simultaneous coverage of the broad MIR range can further enable new applications such as mirco-spectroscopy , femtosecond pump-probe spectroscopy , and high-dynamic-range sensitive measurements Until now numerous schemes have
been developed to generate coherent MIR radiation, such as synchrotron beam lines, quantum cascade lasers, supercontinuum sources , optical parametric oscillators (OPO) and optical parametric amplifiers (OPA). These schemes all have their own strengths and weaknesses in terms of complexity, bandwidth, power, efficiency, and pulse durations. Among them, intra-pulse difference frequency generation (IDFG) is attracting growing attention thanks to the development of high-power femtosecond 2 µm lasers that can effectively pump small-bandgap non-oxide nonlinear crystals to generate high-power broadband coherent MIR light. Compared to the normally used OPOs and OPAs, IDFG allows a reduction in system complexity and enhancement of reliability, as the need to align two separate beams or cavities at high precision is removed. Besides, the MIR output is intrinsically carrier-envelope-phase (CEP) stable with IDFG .