Abstract

In recent years quantum cascade lasers have dominated the mid IR spectroscopy market. However, their limited tuning range, the presence of mode hops and their relatively slow tuning speed continue to spawn interest in alternative mid IR sources. Potentially, continuum generation in mid IR transmitting fibers provides an alternative[1], though the limited temporal coherence can be an issue. On the other hand frequency combs can deliver precision wavelength calibration while at the same time offering almost unlimited temporal coherence and high output powers. To cover the mid-IR spectral range, sometimes called the fingerprint region from 3 – 15 μm, with fiber frequency combs (see Fig. 1 below), difference frequency generation (DFG), optical parametric oscillators (OPOs) as well as optical parametric amplifiers (OPAs) can be implemented. DFG has the additional advantage that the carrier envelope offset frequency of such combs is automatically zero. DFG combs can cover almost the whole fingerprint region with appropriate tuning elements at power levels in the mW range[2]. Power levels up to 14 mW have recently been obtained, albeit with broadband non-tunable DFG sources[3]. OPOs and OPAs pumped with 10 W level fs fiber pump sources can provide power levels in the Watt range and beyond. With doubly resonant designs[4] pump power requirements at least for OPOs can be reduced below 100 mW, enabling a relatively compact system. PPLN, GaSe, AgGaSe₂, CSP as well as OPGaAs have served as popular nonlinear crystals. Other approaches based on mid IR transmitting waveguides or micro-combs have also been investigated but so far cannot offer comparable spectral coverage nor output power.

© 2015 IEEE