Cavity-Based Medium Resolution Spectroscopy (CBMRS) in the THz: A Bridge Between High- and Low-Resolution Techniques for Sensor and Spectroscopy Applications

System technologies and sensor applications in the THz largely bifurcate between high-resolution ( 1 GHz) methods. There is an extensive high-resolution literature and many approaches to low-resolution spectroscopy have been proposed and demonstrated. In this paper, we discuss and apply an approach, cavity-based medium resolution spectroscopy (CBMRS) that is applicable continuously across this intermediate resolution gap. CBMRS systems use high brightness and spectrally pure continuous wave electronic sources to determine absolute absorption coefficients by the measurement of 100–1000 mode widths of a high-finesse Fabry–Perot cavity. This eliminates the challenge of distinguishing broad spectral features from a complicated baseline that results from variations in source power, transmission efficiency, and detector response. CBMRS has many fewer data points than observed with high-resolution systems interrogating near Doppler-limited spectra. Thus, CBMRS sacrifices the “absolute” specificity of high resolution Doppler-limited systems, but still can retain excellent specificity. Indeed, this smaller sampling does not set the specificity limit, but rather it is set by whether or not the analyte has a specific THz signature. Results from pressure broadened (including atmospheric pressure) applications and molecules with dense and/or quasi-continuous spectra will be presented. Comparisons with related observations in the literature are discussed.