Abstract-Unambiguous real-time terahertz frequency metrology using dual 10 GHz femtosecond frequency combs

Oliver Kliebisch, Dirk C. Heinecke, Stefano Barbieri, Giorgio Santarelli, Hua Li, Carlo Sirtori, Thomas Dekorsy,

Fig. 1. (a) Schematic overview of the dual-comb sampling setup. The two femtosecond lasers have the same wavelength but are depicted in red and orange for visual clarity. The terahertz beam path is indicated in gray. The dashed black line marks the common 10 MHz reference clock shared among all synthesizers. The dashed light blue path matches the stabilization feedback loop, which is shown in more detail in Fig. 1(b). A full description is given in the text. ZnTe, 2 mm thick ZnTe crystal; QWP, quarter-wave plate; PBSC, polarizing beam-splitter cube; BP, electronic bandpass filter; (b) detailed view of the signal conditioning of the first photodiode for frequency stabilization and in-loop characterization. After the longitudinal beat-mode spectrum is amplified, and a single mode is filtered, the signal is split into a feedback loop branch and an in-loop characterization branch [not shown in Fig. 1(a)].

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-11-1431

Terahertz frequency metrology by radio frequency downconversion using femtosecond optical sampling relies on the harmonic factor retrieval between the terahertz frequency and the optical sampling rate. At typical femtosecond laser repetition rates, this imposes an ambiguity for frequency metrology. We report on a dual-comb sampling system for the unambiguous frequency measurement of terahertz quantum cascade lasers with hertz-level precision. Two Ti:sapphire oscillators with 10 GHz repetition rate are used for the electro-optic sampling of terahertz radiation at 2.5 THz emitted by actively mode-locked terahertz quantum cascade lasers with 9.7 GHz and 19.6 GHz repetition rates. By coherent downconversion, the emitted terahertz waveforms are measured in the radio frequency domain. The terahertz frequency comb is stabilized by employing a phase-locked loop on a radio frequency beat-note signal. A second infrared sampling comb is used to measure the absolute frequencies of the terahertz radiation. This method, which is based on the detuning of the sampling repetition rates, allows the direct retrieval of the quantum cascade laser’s absolute frequency in real time without using additional optical frequency references for calibration. In order to demonstrate the feasibility of the stabilization and readout technique, a high-resolution spectroscopy measurement on gaseous methanol is presented.

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