Laser-based Mid-infrared Sources and Applications

6Supercontinuum and Frequency Comb Sources

6.1 Supercontinuum Sources

Broadband and spatially coherent sources in the mid‐IR, especially in the molecular fingerprint region, 3–20 μm, attract enormous interest due to the plethora of applications they are enabling, including spectroscopy, multispecies trace‐gas detection, hyperspectral imaging, and nano‐IR imaging, just to name a few. Supercontinuum (SC) generation occurs when relatively narrow‐band input pulses undergo extreme nonlinear spectral broadening to yield a broadband, spectrally continuous “white light” output. SC sources typically use a single pump laser with nano‐, pico‐, or femtosecond pulse durations and take advantage of nonlinear effects in a wide variety of media including solids, liquids, and gases, but typically in waveguiding structures. In the mid‐IR, SC sources can provide spectral coverage spanning up to several octaves and reach as far as >20 μm in wavelength. Unlike conventional broadband blackbody sources such as globars, SC laser sources are spatially coherent; they combine the brightness of lasers with extremely wide spectral coverage of blackbody radiation. Hence, SC radiation can be focused down to a diffraction‐limited spot size and, due to the dramatically increased radiance (expressed in watts per steradian per square meter), they are very beneficial for such applications as remote sensing, time‐resolved spectroscopy, the study of metamaterials, and nano‐IR spectroscopy.

Alfano and Shapiro were ...