Real-Time Determination of Absolute Frequency in Continuous-Wave Terahertz Radiation with a Photocarrier Terahertz Frequency Comb Induced by an Unstabilized Femtosecond Laser

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (frep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in frep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency fTHz of CW-THz radiation. Using an frep-free-running PC-THz comb, the fTHz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2 × 10−13 and a relative precision of 8.8 × 10−12 to a rubidium frequency standard. Furthermore, fTHz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.