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

Takeo Minamikawa, Kenta Hayashi, Tatsuya Mizuguchi, Yi-Da Hsieh, Dahi Ghareab Abdelsalam, Yasuhiro Mizutani, Hirotsugu Yamamoto, Tetsuo Iwata, Takeshi Yasui,

http://link.springer.com/article/10.1007/s10762-015-0237-6

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 (f rep) 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 f rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f THz of CW-THz radiation. Using an f rep-free-running PC-THz comb, the f THz 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, f THz 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.

Abstract-Adaptive sampling dual terahertz comb spectroscopy using dual free-running femtosecond lasers

Takeshi Yasui,

Ryuji Ichikawa,

Yi-Da Hsieh,

Kenta Hayashi,

Harsono Cahyadi,

Francis Hindle,

Yoshiyuki Sakaguchi,

Tetsuo Iwata,

Yasuhiro Mizutani,

Hirotsugu Yamamoto,

Kaoru Minoshima

& Hajime Inaba

http://www.nature.com/srep/2015/150602/srep10786/full/srep10786.html

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use. To overcome this limitation, here we have demonstrated adaptive-sampling THz-DCS, allowing the use of free-running femtosecond lasers. To correct the fluctuation of the time and frequency scales caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing clock signal in a data acquisition board. The results not only indicated the successful implementation of THz-DCS with free-running lasers but also showed that this configuration outperforms standard THz-DCS with stabilized lasers due to the slight jitter remained in the stabilized lasers.