Real-time absolute frequency measurement of continuous-wave terahertz wave based on dual terahertz combs of photocarriers with different frequency spacings

TL;DR

This paper introduces a real-time method for measuring the absolute frequency of continuous-wave terahertz waves using dual terahertz combs with different spacings, achieving high precision and monitoring large fluctuations.

Contribution

The paper presents a novel dual-comb-based technique for real-time, high-precision absolute frequency measurement of CW-THz waves, capable of tracking large frequency fluctuations.

Findings

Achieved frequency precision of 10^-11 at 100 Hz measurement rate

Successfully monitored large frequency fluctuations crossing multiple comb modes

Demonstrated robustness regardless of frequency control in the combs

Abstract

Real-time measurement of the absolute frequency of continuous-wave terahertz (CW-THz) waves is required for characterization and frequency calibration of practical CW-THz sources. We proposed a method for real-time monitoring of the absolute frequency of CW-THz waves involving temporally parallel, i.e., simultaneous, measurement of two pairs of beat frequencies and laser repetition frequencies based on dual THz combs of photocarriers (PC-THz combs) with different frequency spacings. To demonstrate the method, THz-comb-referenced spectrum analyzers were constructed with a dual configuration based on dual femtosecond lasers. Regardless of the presence or absence of frequency control in the PC-THz combs, a frequency precision of 10-11 was achieved at a measurement rate of 100 Hz. Furthermore, large fluctuation of the CW-THz frequencies, crossing several modes of the PC-THz combs, was correctly monitored in real time. The proposed method will be a powerful tool for the research and development of practical CW-THz sources, and other applications.