Biomedical applications of terahertz self-mixing interferometry

Abstract

A novel imaging and tissue analysis scheme in the terahertz frequency band has the benefits of simplicity, coherence, and high sensitivity.Terahertz radiation has an inherently low penetration depth in hydrated biological tissue, so skin is an ideal target for imaging at terahertz frequencies.Examining structures within the surface layers of skin is a mainstay of diagnosis in pathology, and using several different frequency regions to image these can provide complementary information about skin structure and function.The research community has successfully developed diagnostic methods based on the response of tissue in the terahertz spectral region. 1 However, to date, the absence of a compact, robust, inexpensive sensing solution has impeded general adoption of terahertz radiation for biological applications.The quantum cascade laser (QCL) is one of the most promising radiation sources for imaging at terahertz frequencies.As with other lasers, QCLs exhibit self-mixing (SM), whereby re-injection of emitted radiation into the laser cavity affects the laser operating parameters.We can exploit this phenomenon for sensing purposes, using it to measure displacement, velocity, and fluid flow, as well as for coherent and incoherent imaging.Here we present an imaging and tissue analysis scheme in the terahertz band that exploits the interferometric nature of coherent optical feedback in a terahertz QCL.SM interferometry is essentially a homodyne detection scheme, where the stable local oscillator signal (the QCL emission) is combined with the timedelayed version of itself.Figure 1 shows the basic structure and operating principles of our SM interferometer.The re-injected light interferes ('mixes') with the intra-cavity electric field, causing small variations in the fundamental laser parameters,