Optimum Optical Designs for Diffraction-Limited Terahertz Spectroscopy and Imaging Systems Using Off-Axis Parabolic Mirrors
Open accessAbstract
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: these make achieving diffraction-limited performance a challenge, and lower 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.
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The system was designed to have minimal wavefront distortion and aberration by staying within the tolerance limits for beams originating from off-axis points in the emitter plane.
“The system was designed to have minimal wavefront distortion and aberration, by staying within the tolerance limits for beams originating from off-axis points in the emitter plane.17”
From Dual-Beam THz Spectrometer with Low-Aberration Optics and Off-Axis Multipixel Photoconductive Emitters for Reduced Systematic Errors (2025) · cites this paper as [17]
The excellent beam quality even for off-axis beam points is a feature of the (a,a) orientation adopted for each OAPM pair, which has minimal aberrations.
“The excellent beam quality even for off-axis beam points is a feature of the (a,a) orientation adopted for each OAPM pair,17 which has minimal aberrations.”
From Dual-Beam THz Spectrometer with Low-Aberration Optics and Off-Axis Multipixel Photoconductive Emitters for Reduced Systematic Errors (2025) · cites this paper as [17]
The second pair of OAPMs propagates the two THz beams from the sample plane to the detector plane, where diffraction-limited foci with similar optical performance are expected, including some overlap of the two beams at lower frequencies.
“The second pair of OAPMs propagates the two THz beams from the sample plane to the detector plane (Figure 1), where diffraction-limited foci with similar optical performance are expected,17 including some overlap of the two beams at lower frequencies.”
From Dual-Beam THz Spectrometer with Low-Aberration Optics and Off-Axis Multipixel Photoconductive Emitters for Reduced Systematic Errors (2025) · cites this paper as [17]
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“…even occur during propagation in free space: broadband electromagnetic pulses change spectral phase as a result of the geometric aberrations of the optical system, which alter the path length [6]; the Gouy effect causes the carrier-envelope phase to evolve as a beam propagates through a focus [7].”
Self-citation
- Authors
- Chopra, N., & Lloyd-Hughes, J.
- Affiliations
- University of Warwick (GB)
- ORCID
- Chopra, Nishtha
- Venue
- Journal of Infrared, Millimeter, and Terahertz Waves
- Published
- 2023-11-21
- Volume
- 44
- Issue
- 11-12
- Pages
- 981-997
- ISSN
- 1866-6892 (ISSN-L)
- 1866-6906
- OpenAlex
- W4388865587
- Cited by
- 21
- References
- 22
- Citations by year
- 2026: 5 · 2025: 10 · 2024: 6
- FWCI
- 1.56
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Locations
- Journal of Infrared Millimeter and Terahertz Waves (publishedVersion, cc-by)
- arXiv (Cornell University) (submittedVersion, cc-by)
- Figshare (submittedVersion, cc-by)
- arXiv (Cornell University) (cc-by)
- Funders
- Engineering and Physical Sciences Research Council (EP/V047914/1)
- Topics
- Physical Sciences › Engineering › Electrical and Electronic Engineering › Terahertz technology and applications
- Physical Sciences › Engineering › Biomedical Engineering › Advanced optical system design
- Physical Sciences › Chemistry › Spectroscopy › Spectroscopy and Laser Applications
- Keywords
- off-axis parabolic mirror, terahertz spectroscopy, terahertz imaging, diffraction-limited performance, geometric aberrations, aspheric mirrors, misalignment robustness, focal spot size, electric field strength, spectrometer design, optical pump-THz probe spectroscopy, ray tracing
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