Abstract

Over the past decade high-resolution and broadband spectroscopy has received a major boost from the advent of optical frequency combs (OFCs) [1, 2]. Direct frequency comb spectroscopy (DFCS) exploits high-speed multiplexing approaches in order to simultaneously detect a massive set of extremely accurate channels, allowing for ultra-broadband and high speed spectroscopic investigations [3, 4]. Here we present a different DFCS approach based on a scanning Fabry-Pérot (FP) micro-cavity resonator, SMART DFCS (Scanning Micro-cAvity ResonaTor DCS), that is able to detect the spectrum of the OFC with a resolution much better than the comb-mode spacing, limited by the micro-resonator resonance linewidth. The SMART DFCS significantly reduces the system complexity and allows in principle direct implementation of this method to any spectral region from THz to UV. Figure 1 shows the operating scheme of the SMART technique. An OFC is coupled to a FP micro-resonator with a free-spectral-range much larger and a linewidth narrower than the comb repetition rate, corresponding to the case where only one comb mode is resonant with the cavity at a time. To avoid spectral aliasing between the adjacent transmission orders of the FP, an optical-tunable-filter (OTF), either before or after the FP micro-resonator, has to be used in order to select the appropriate spectral portion of the OFC to be detected. The absolute frequency calibration of the optical axis and the compensation of any micro-cavity temporal jitter (or frequency dispersion) are obtained by combining to the OFC a free-running CW laser acting as frequency marker and by taking advantage of the clearly resolved comb modes which act as a reference ruler for frequency remapping. Therefore, effective coherent averaging of the recorded spectra can be performed to boost the signal-to-noise ratio (SNR) of the detection system and to improve the sensitivity of the technique.

© 2017 IEEE