Multi-wavelength imaging of tissue phantoms by optical feedback interferometry

Optical Feedback Imaging is the most recent advancement of the traditional Laser Self-Mixing. It is a coherent scanning imaging technique with the unique feature of having the light source acting also as the signal detector. Optical Feedback Imaging is being increasingly applied to both inorganic and organic samples, especially in the infrared and Terahertz spectral region. The technique relies on the analysis of the back-scattered (or reflected) radiation, thus it is contactless, detectorless and suited to be applied also in-vivo. As a coherent technique in reflection, it is sensitive to both phase (index) contrast and amplitude (absorption) contrast. We demonstrate selective contrast imaging of different organic pigments at three different visible wavelengths allowing for unambiguous identification of absorption features in back-scattering geometry. However, a wealth of information about the chemical composition of tissues is more readily available at mid-infrared wavelengths, where molecular bonds have their spectral signatures. We also present Optical Feedback images collected in organic (agar-agar) tissue phantoms at mid infrared wavelengths. 1% sensitivity to potato starch concentration included in the agar-agar matrix was detected at 6.26 μm, in resonance to the amide-I band.