Molecular Spectroscopy with Optical Frequency Combs

5 Molecular Spectroscopy with Optical Frequency Combs 

The application of frequency combs to molecular spectroscopy opens the way to new and very promising techniques, which are based on a combination of laser spectroscopy and Fourier-transform spectroscopy [1342], 

At first we will discuss the use of a single optical frequency comb where the probe is placed inside an optical resonator with resonance frequencies matching the modes of the comb. Therefore all absorption frequencies of the molecule under investigation contribute simultaneously to the absorption of the comb. Outside the resonator a prism or grating disperses the different frequencies and a CCD array can detect simultaneously all absorption lines inside the spectral range A v of the comb. For a measuring time Af the signal-to-noise ratio is increased by a factor VAr Av. 

The sensitivity can be further enhanced if a second frequency comb with a slightly different mode spacing is applied where the difference amounts to about 200-600 Hz. While the output of the first comb passes through the absorption cell that of the second comb does not (Fig. 9.93). The output beams of the two combs are superimposed and the resulting difference frequency spectrum is monitored with a time resolving detector. 

This technique is quite similar to Fourier-transform spectroscopy, where the phase difference between the two beams in a Michelson interferometer are generated by changing the path difference as a function of time. In the dual comb spectroscopy the phase difference it generated by the different repetition frequencies of the two 

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