Abstract
A single, free-running, dual-wavelength mode-locked, erbium-doped fibre laser was exploited to measure the absolute frequency of continuous-wave terahertz (CW-THz) radiation in real time using dual THz combs of photo-carriers (dual PC-THz combs). Two independent mode-locked laser beams with different wavelengths and different repetition frequencies were generated from this laser and were used to generate dual PC-THz combs having different frequency spacings in photoconductive antennae. Based on the dual PC-THz combs, the absolute frequency of CW-THz radiation was determined with a relative precision of 1.2 × 10−9 and a relative accuracy of 1.4 × 10−9 at a sampling rate of 100 Hz. Real-time determination of the absolute frequency of CW-THz radiation varying over a few tens of GHz was also demonstrated. Use of a single dual-wavelength mode-locked fibre laser, in place of dual mode-locked lasers, greatly reduced the size, complexity, and cost of the measurement system while maintaining the real-time capability and high measurement precision.
Highlights
A single, free-running, dual-wavelength mode-locked, erbium-doped fibre laser was exploited to measure the absolute frequency of continuous-wave terahertz (CW-THz) radiation in real time using dual THz combs of photo-carriers
We measured the absolute frequency of continuous-wave THz (CW-THz) radiation using dual polarization controller (PC)-THz combs induced by a dual-λ mode-locked fibre laser
This laser was operating in the free-running mode without stabilization of frep[1] and frep[2], a relative precision and accuracy of 1.2 × 10−9 and 1.4 × 10−9 were achieved at a measurement rate of 100 Hz due to the common-mode behaviour of frep[1] and frep[2], in addition to the fact that the interval between the PC-THz comb modes was kept equal regardless of the fluctuation in frep[1] and frep[2]
Summary
A single, free-running, dual-wavelength mode-locked, erbium-doped fibre laser was exploited to measure the absolute frequency of continuous-wave terahertz (CW-THz) radiation in real time using dual THz combs of photo-carriers (dual PC-THz combs). If dual PC-THz combs with different frep could be generated by a single free-running mode-locked laser, the real-time capability, precision, and practicability of THz frequency measurement would be enhanced. The use of ‘multiplexed’ mode-locked erbium-doped fibre (Er:fibre) lasers as dual-comb lasers has been demonstrated by multiplexing in the dimensions of centre wavelength, propagation direction, polarization state, or mode-locking mechanism[12,13,14,15,16,17] Among these schemes, use of a dual-wavelength (dual-λ) mode-locked Er:fibre laser is a promising way to generate a dual PC-THz comb because it emits two independent mode-locked pulsed light beams with different wavelengths, λ1 and λ2, from a single cavity, and their frep values are slightly detuned from each other due to dispersion in the fibre laser cavity[18]. Such characteristics in dual-λ mode-locked Er:fibre lasers have been successfully used in asynchronous optical sampling (ASOPS) pump-probe measurement[18], optical ranging[20], and optical spectroscopy[19]
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