UV chirped pulse electro-optic dual comb generation

Dual electro-optic frequency comb (DOFC) methods are being developed to enhance detection sensitivity and speed for gas sensing in ultraviolet (UV) region. The dual combs are generated from a continuous-wave output of a Titanium Sapphire laser (700 nm to 1000 nm) that is split into two legs of an interferometer, each containing an electro-optic and acoustic-optic modulator. Phase coherent chirped pulses with slightly different bandwidths are used to drive EOMs and a small difference in the driving frequencies of the acousto-optic modulators enable sideband separation at the detector. Two novel spectral interleaving schemes for UV comb generation are demonstrated by either separately up-converting each near-IR comb or by combining the two near-IR combs prior to sum frequency mixing in a non-linear crystal. In both cases, the dual UV combs are detected using photon counting technique where the UV spectral information is uniquely downconverted to the RF domain by coherent mixing at a photomultiplier tube.

For rubidium, the UV optical frequency combs obtained from the two chirped-pulse downconversion processes (i.e., the combined and separated beam approaches) lead to severely distorted line shape functions that arise from temporal magnification of the spectral response function in the frequency domain. However, the natural (unperturbed) line shapes are easily recovered through the back-transformation to the time domain. Methods for DOFC wavelength extension to the deeper UV region are currently being explored using cavity enhanced nonlinear mixing techniques for quantum applications that may enhance the coincidence counting rates of entangled photons.

Model predictions (top) and Rb spectra in the UV obtained using the combined beam method (middle) and separated beam method (bottom).