Direct Frequency Comb Spectroscopy

There is a variety of different kinds of direct frequency comb spectroscopy methods. Examples include dual comb spectroscopy, cavity enhanced comb spectroscopy, mid-IR comb spectroscopy, coherent control or remote sensing, to name only a few of these techniques. All of these techniques are usually based on analyzing the light generated by a frequency comb after it has interacted with a sample.

Dual comb spectroscopy for example uses two frequency combs of slightly differing line spacing. From each pair of optical lines, one from each comb, a radio frequency beat note is generated on a detector. This way, optical frequencies are converted into radio frequencies such that amplitude and phase changes caused by the interaction of one of the combs with a sample can be detected^[1,2,3].

The mid-IR domain, with its strong molecular fingerprints, is of special interest for direct frequency comb spectroscopy which features short acquisition times, high sensitivity and high accuracy over a broad bandwidth^[4,5]. Frequency combs can be generated in the mid-IR via difference frequency generation, but these typically have low power. This precludes short acquisition times and high sensitivity. TOPTICA has demonstrated a simple technique whereby a continuous-wave optical parametric oscillator efficiently converts a near-IR dual electro-optic comb to the mid-IR. The mid-IR dual comb carries >1 W optical power, is widely tunable from 2.2 – 4.0 µm,  and the comb line spacing is widely adjustable from kHz to GHz^[3].

A particularly intriguing application of direct comb spectroscopy is coherent control of the quantum state of cold particles, for example of trapped ions. Entangling quantum logic gates with high fidelity have been performed and even operations faster than the trap frequency are feasible with direct comb spectroscopy^[5,6].