Optical frequency combs have revolutionized many fields of physics over the past dozen years. They are produced by mode-locked femtosecond lasers, whose spectrum in the optical domain spans hundreds of terahertz and consists of hundreds of thousands of equidistant narrow modes (comb teeth). The frequency of each comb tooth is determined by two radio frequencies: repetition rate of the laser and carrier-envelope frequency. By establishing a direct link between the RF and optical domains, optical frequency combs have become the perfect tool for frequency metrology. The ability to serve as a precise frequency ruler has been utilized in conventional laser spectroscopy, enabling frequency-comb–assisted techniques.
A new approach employs optical frequency combs directly for spectroscopic measurements. It combines two distinct capabilities: extremely high spectral resolution of single comb component and broad spectral coverage of mode-locked femtosecond laser. As a result, the technique is virtually equivalent to a simultaneous measurement with tens of thousands of narrow linewidth lasers. Due to its very regular spectrum, the optical frequency comb can be efficiently coupled into an optical enhancement cavity, which improves vastly the interaction length with the sample. This approach allows to reach absorption sensitivities at a level of 10-10 cm-1 Hz-1/2 per spectral element. Thus it has the potential to become a real-time, highly sensitive, broad-bandwidth, high-resolution technique for molecular spectroscopy, as well as for applications such as breath analysis or atmospheric research.
In this talk, the principles of the technique and related experimental implementations will be discussed as well as examples of experimental setups will be presented.