Wavelength modulation imaging with tunable mid-infrared semiconductor laser: spectroscopic and geometrical effects

Wavelength modulation imaging (WMI) is capable of determining both spectroscopic and geometrical properties of a target, but the latter is often ignored in spectroscopic studies. This work theoretically and experimentally demonstrates the importance of both in WMI applications. Experiments were performed with an all-digital signal processing approach employing a tunable mid-infrared laser capable of digital wavelength modulation. All three orders of wavelength-derivative images, 0th, 1st, and 2nd are generated simultaneously. Higher order images can reveal or enhance features that are not evident in the 0th order. An example shows a synthetic imaging approach that combines the 2nd order WMI of CO gas with a focal plane array image to allow chemical visualization with minimal background clutter. In another example, fine geometrical features were revealed for a target that has little intrinsic spectroscopic signatures.