Optimum optical designs for diffraction-limited terahertz spectroscopy and imaging systems using off-axis parabolic mirrors

TL;DR

This paper analyzes the optical design of off-axis parabolic mirrors for diffraction-limited terahertz spectroscopy and imaging, providing guidelines to optimize performance and robustness against misalignments.

Contribution

It introduces simple design rules based on marginal ray propagation to minimize aberrations and enhance diffraction-limited performance in THz systems.

Findings

Identified key geometric aberrations affecting THz mirror performance

Developed design guidelines for robust, low-aberration spectrometers

Achieved smaller focal spots and higher electric fields in simulations

Abstract

Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations that make achieving diffraction-limited performance a challenge, and which lowers the peak electric field strength in the focal plane. Here we quantify the impact of geometric aberrations on the performance of the most widely-used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy