Last decade of laser spectroscopy has marked a remarkable progress in the fields of ultrafast optics and optical frequency metrology, based on the possibility of precise radiation control from the femtosecond lasers. The spectrum of these lasers makes a large number of discrete, properly spaced, sharp lines of different wavelengths/frequencies – hence the name frequent comb. Thus the frequency comb generated by the femtosecond laser can be used as a frequency ruler for measuring optical frequencies in the same way that the regular ruler is used to measure the length.
The frequency comb is the ideal light source for a broadband spectroscopy with an optical resonator. Its spectrum extends to several hundreds of nanometers in wavelength, allowing multiple different compounds to be explored at the same time, while the highly defined mode structure of the frequency comb enables high-resolution spectroscopy. The frequency comb can be very effectively coupled with the resonant modes of the optical resonator/cavity, which adds to ultrahigh sensitivity. Such characteristics of Cavity-Enhanced Frequency Comb Spectroscopy (CEFCS) makes it the ideal detection technique of trace species (atoms, molecules, etc.), with a wide range of applications such as:
- Medical analysis of human breath – disease diagnosis by detecting the presence of biomarker molecules in the patient’s breath;
- Detection of explosive materials and biologically hazardous or harmful substances;
- Air quality control.
The CEFCS technique couples the optical frequency comb with a high-quality optical resonator that contains unknown atoms and molecules. Unknown compounds within the resonator absorb the light at precisely defined wavelengths, characteristic and unique for particular atoms and molecules. The comb-resonator coupling allows for a dramatic increase of the optical path of the light in the sample, resulting in ultra-high detection sensitivity. The light going out of the resonator is then spectrally analyzed and the absorption lines are identified, thus achieving unambiguous analysis of unknown atoms and molecules within the resonator.