Design of a thermal imaging diagnostic using 90-degree off-axis parabolic mirrors

Abstract

Thermal imaging is an important, though challenging, diagnostic for shockwave experiments. Shock-compressed materials undergo transient temperature changes that cannot be recorded with standard (greater than ms response time) infrared detectors. A further complication arises when optical elements near the experiment are destroyed. We have designed a thermal-imaging system for studying shock temperatures produced inside a gas gun at Sandia National Laboratories. Inexpensive, diamond-turned, parabolic mirrors relay an image of the shocked target to the exterior of the gas gun chamber through a sapphire vacuum port. The 3000-5000-nm portion of this image is directed to an infrared camera which acquires a snapshot of the target with a minimum exposure time of 150 ns. A special mask is inserted at the last intermediate image plane, to provide dynamic thermal background recording during the event. Other wavelength bands of this image are split into high-speed detectors operating at 900-1700 nm, and at 1700-3000 nm for timeresolved pyrometry measurements. This system incorporates 90-degree, off-axis parabolic mirrors, which can collect low f/# light over a broad spectral range, for high-speed imaging. Matched mirror pairs must be used so that aberrations cancel. To eliminate image plane tilt, proper tip-to-tip orientation of the parabolic mirrors is required. If one parabolic mirror is rotated 180 degrees about the optical axis connecting the pair of parabolic mirrors, the resulting image is tilted by 60 degrees. Different focal-length mirrors cannot be used to magnify the image without substantially sacrificing image quality. This paper analyzes performance and aberrations of this imaging diagnostic.

Off-axis parabolic mirror relay microscope for experiments with ultra-cold matter

“…However, if one subsequently adjusts the xyz stage holding the microscope objective to optimise the resolution at each of these OAP mirrorobject distances, one can significantly increase the region over which high resolution can be achieved (and, thus, the volume in which ICCs can be imaged). The fully optimised OAP mirror imaging system can accommodate object displacements of ±500 µm -a range significantly greater than the anticipated ∼ 150 µm shift in mirror position due to thermal contraction of the copper holder as the temperature is lowered from 297 K to 4 K. The resolution also deteriorates if the relative orientations of the two OAP mirrors is sub-optimal, consistent with previous findings, 14 and so the relay must be aligned with precision.…”

“…A similar set-up was first reported by Malone et al, where a pair of OAP mirrors formed part of a thermal imaging diagnostic tool for shockwave experiments. 14 To the best of our knowledge, such an approach has never previously been applied to the imaging of features requiring sub-10 µm resolution. Here, we provide a robust report on the experimental performance of the system.…”

“…As explored in detail by Malone et al 14 , two identical OAP mirrors should be used in the mirror relay for optimal resolution. One must also consider the relative orientations of the OAP mirrors, else the image can be tilted (see Malone et al 14 the OAP mirrors adopted here, the NA is approximately 0.35. A NA of 0.35 is comparable to (and, in some cases, higher than) the values reported for many off-the-shelf reflective objectives, with a significantly reduced cost.…”

Analysis and optimization of THz imaging optics with off-axis parabolic mirrors

“…Symmetric orientation also automatically rectifies any rotation of the polarization vectors [4] back to original. Symmetric mirror configuration is realized by turning OAP3 around, making the output to have the same direction as the input (Figure 1, right).…”

Enhanced Terahertz Transmission Through Bullseye Plasmonics Lenses Fabricated Using Micromilling Techniques

“…At 330 GHz, the beam has a waist radius of 1.68 mm located 6.20 mm inside the feedhorn. Four diamond-turned, aluminum, 90 • off-axis parabolic (OAP) mirrors were orientated in a "tipto-tip" fashion such that rays from a single field point strike identical coordinates on each parabolic mirror [33]. A quartz quarter-wave plate (QWP) was placed in the collimated beam to act as an optical isolator.…”