Self-normalizing phase measurement in multimode terahertz spectroscopy based on photomixing of three lasers

TL;DR

This paper introduces a self-normalizing phase measurement technique in multimode terahertz spectroscopy using three lasers, enabling high-accuracy phase detection and drift compensation in nonideal environments.

Contribution

The authors develop a novel three-laser photomixing method that normalizes phase measurements to a reference frequency, improving accuracy and stability in terahertz spectroscopy.

Findings

Achieved phase measurement accuracy of 1-2 microns or 10^{-8} of optical path length.

Demonstrated drift compensation using a fixed reference frequency.

Applicable in non-ideal environmental conditions outside laboratories.

Abstract

Photomixing of two near-infrared lasers is well established for continuous-wave terahertz spectroscopy. Photomixing of three lasers allows us to measure at three terahertz frequencies simultaneously. Similar to Fourier spectroscopy, the spectral information is contained in an nterferogram, which is equivalent to the waveform in time-domain spectroscopy. We use one fixed terahertz frequency \nu_ref to monitor temporal drifts of the setup, i.e., of the optical path-length difference. The other two frequencies are scanned for broadband high-resolution spectroscopy. The frequency dependence of the phase is obtained with high accuracy by normalizing it to the data obtained at \nu_ref, which eliminates drifts of the optical path-length difference. We achieve an accuracy of about 1-2 microns or 10^{-8} of the optical path length. This method is particularly suitable for applications in nonideal environmental conditions outside of an air-conditioned laboratory.