| Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) combines broad spectral bandwidth, high spectral resolution, precise frequency calibration, and ultra-high detection sensitivity, all in one experimental platform based on an optical frequency comb interacting with a high-finesse optical cavity. Precise control of the optical frequency comb allows highly efficient, coherent coupling of individual comb components with corresponding resonant modes of the high-finesse cavity. The use of low dispersion mirrors permits almost the entire spectral bandwidth of the frequency comb to be employed for detection, covering a range of > 10% of the actual optical frequency. The long cavity lifetime dramatically enhances the effective interaction between the light field and intra-cavity matter, increasing the sensitivity for measurement of optical losses by a factor that is on the order of the cavity finesse. Finally, the light transmitted from the cavity is spectrally resolved to provide a multitude of detection channels with spectral resolutions ranging from several gigahertz to hundreds of kilohertz.;We begin with a discussion of the principles of cavity-enhanced direct frequency comb spectroscopy including the properties of frequency combs and optical cavities that are relevant to CE-DFCS systems. Methods for characterizing cavity-comb interactions and achieving simultaneous broad bandwidth and high resolution detection of the cavity transmitted beam are discussed. Measurements are presented that fully characterize the detection sensitivity, resolution, and spectral coverage of several CE-DFCS systems. We also outline several types of UV, optical, and IR frequency comb sources and optical cavity designs that can be used for specific spectroscopic applications. Finally, we present a series of experimental measurements on trace gas detections, human breath analysis, and characterization of cold molecular beams. These results demonstrate clearly that the wide bandwidth and ultrasensitive nature of the femtosecond enhancement cavity enables powerful real-time detection and identification of many molecular species including their quantum state distributions in a massively parallel fashion. |
