Christoph R. Englert

Doppler asymmetric spatial heterodyne spectroscopy (DASH): concept and experimental demonstration

We describe the concept of Doppler asymmetric spatial heterodyne spectroscopy (DASH) and present a laboratory Doppler-shift measurement using an infrared laser line. DASH is a modification of spatial heterodyne spectroscopy optimized for high precision, high accuracy Doppler-shift measurements of atmospheric emission lines either from the ground or a satellite. We discuss DASH design considerations, field widening, thermal stability and tracking, noise propagation, advantages, and trade-offs. DASH interferometers do not require moving optical parts and can be built in rugged, compact packages, making them suitable for space flight and mobile ground instrumentation.

PMCs and the water frost point in the Arctic summer mesosphere

In August, 1997 the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) obtained vertical profiles of OH number density and polar mesospheric cloud (PMC) brightness by scanning the limb up to 71° N while the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) obtained co-located vertical profiles of temperature. MAHRSI OH densities are converted to water vapor using a one-dimensional model that assumes photochemical equilibrium. By combining water vapor profiles with CRISTA temperatures we map the frost point both vertically and horizontally in the Arctic summer mesosphere. Our data show that supersaturation can exist between 80-87 km suggesting that growth of ice particles is limited to these altitudes. Additionally, we find that not only is supersaturation an insufficient condition for a PMC but also that PMCs can exist in apparently unsaturated air.

Flatfielding in spatial heterodyne spectroscopy

Spatial heterodyne spectroscopy (SHS) is a Fourier-transform spectroscopic technique that simultaneously records all path differences using a detector array. Compared to conventional Fourier-transform spectroscopy that measures interferogram samples sequentially in the time domain, SHS is insensitive to a changing scene; however, the effects caused by differences in the detector elements and/or the optics for each sample must be addressed with a flatfield correction. The flatfield correction is typically a characteristic of the instrument and does not change with the observed scene. We present three different flatfielding approaches. Each is based on different assumptions and is applicable depending on the instrumental effects dominating the flatfield.

Design and laboratory tests of a Doppler Asymmetric Spatial Heterodyne (DASH) interferometer for upper atmospheric wind and temperature observations

We describe the design, fabrication and laboratory tests of a Doppler Asymmetric Spatial Heterodyne (DASH) interferometer for upper atmospheric wind and temperature observations of the O[1D] 630 nm emission. The monolithic interferometer has no moving parts, a large étendue, relaxed fabrication and alignment tolerances and can measure multiple emission lines simultaneously. Laboratory measurements indicate that the design resolution and étendue were achieved and that thermal drifts can be determined with sufficient precision for geophysical applications.

Initial ground-based thermospheric wind measurements using Doppler asymmetric spatial heterodyne spectroscopy (DASH)

We present the first thermospheric wind measurements using a Doppler Asymmetric Spatial Heterodyne (DASH) spectrometer and the oxygen red-line nightglow emission. The ground-based observations were made from Washington, DC and include simultaneous calibration measurements to track and correct instrument drifts. Even though the measurements were made under challenging thermal and light pollution conditions, they are of good quality with photon statistics uncertainties between about three and twenty-nine meters per second, depending on the nightglow intensity. The wind data are commensurate with a representative set of Millstone Hill Fabry-Perot wind measurements selected for similar geomagnetic and solar cycle conditions.

Doppler Asymmetric Spatial Heterodyne Spectroscopy (DASH) : An innovative concept for measuring winds in planetary atmospheres

We introduce an innovative concept for inferring altitude profiles of horizontal wind in planetary atmospheres by measuring the Doppler shift of multiple emission lines versus altitude. Instruments using this approach will be especially well suited for interplanetary missions because they will be compact, rugged, and lightweight while minimizing power consumption and maximizing sensitivity, all without moving parts.

Bright polar mesospheric clouds formed by main engine exhaust from the space shuttle's final launch

The space shuttle launched for the last time on 8 July 2011. As with most shuttle launches, the three main engines injected about 350 t of water vapor between 100 and 115 km off the east coast of the United States during its ascent to orbit. We follow the motion of this exhaust with a variety of satellite and ground-based data sets and find that (1) the shuttle water vapor plume spread out horizontally in all directions over a distance of 3000 to 4000 km in 18 h, (2) a portion of the plume reached northern Europe in 21 h to form polar mesospheric clouds (PMCs) that are brighter than over 99% of all PMCs observed in that region, and (3) the observed altitude dependence of the particle size is reversed with larger particles above smaller particles. We use a one- dimensional cloud formation model initialized with predictions of a plume diffusion model to simulate the unusually bright PMCs. We find that eddy mixing can move the plume water vapor down to the mesopause near 90 km where ice particles can form. If the eddy diffusion coefficient is 400 to 1000 m(2)/s, the predicted integrated cloud brightness is in agreement with both satellite and ground-based observations of the shuttle PMCs. The propellant mass of the shuttle is about 20% of that from all vehicles launched during the northern 2011 PMC season. We suggest that the brightest PMC population near 70 degrees N is formed by space traffic exhaust.

Spatial heterodyne spectroscopy for long-wave infrared: first measurements of broadband spectra

We describe the design and implementation of a long-wave infrared (LWIR) spectrometer based on the spatial heterodyne spectroscopy (SHS) technique, and present the first measurements of broadband LWIR spectra taken with an SHS instrument. This work represents the first successful application of SHS to the field of LWIR spectroscopy, which is currently dominated by Fourier transform spectrometers, grating spectrometers, and Fabry-Perot interferometers. A unique combination of properties makes SHS a valuable addition to the existing types of LWIR spectrometers. Most notable are the interferometric throughput (no slit), lack of moving parts, and that the measured spectra are not contaminated by a changing scene, which makes SHS particularly suitable for applications in rugged environments and on moving platforms. The instrument discussed here is called the Spatial Heterodyne Imager for Chemicals and Atmospheric Detection (SHIMCAD), and is designed to cover the wavelength range between about 8.4 µm (1190 cm−1) and 11.2 µm (890 cm−1) with a spectral resolution of about 4 cm−1. First, laboratory SHS transmission spectra of methanol and polyimide (Kapton®) are presented. The instrument is built to be mobile, so that ultimately field measurements can be conducted.

Thermal sensitivity of DASH interferometers: the role of thermal effects during the calibration of an Echelle DASH interferometer

The use of a Doppler asymmetric spatial heterodyne (DASH) interferometer with an Echelle grating provides the ability to simultaneously image the 558 and 630 nm emission lines (e.g., at grating orders of n=8 and n=7, respectively) of atomic oxygen in the thermosphere. By measuring the Doppler shifts of these lines (expected relative change in wavelength on the order of 10⁻⁸), we are able to determine the thermospheric winds. Because the expected wavelength changes due to the Doppler shift are so small, understanding, monitoring, and accounting for thermal effects is expected to be important. Previously, the thermal behavior of a temperature-compensated monolithic DASH interferometer was found to have a higher thermal sensitivity than predicted by a simple model [Opt. Express 18, 26430, 2010]. A follow-up study [Opt. Express 20, 9535, 2012] suggested that this is due to thermal distortion of the interferometer, which consists of materials with different coefficients of thermal expansion. In this work, we characterize the thermal drift of a nonmonolithic Echelle DASH interferometer and discuss the implications of these results on the use of only a single wavelength source during calibration. Furthermore, we perform a finite element analysis of the earlier monolithic interferometer in order to determine how distortion would affect the thermal sensitivity of that device. Incorporating that data into the model, we find good agreement between the modified model and the measured thermal sensitivities. These findings emphasize the fact that distortion needs to be considered for the design of thermally compensated, monolithic DASH interferometers.

Spatial heterodyne spectroscopy at the Naval Research Laboratory

Spatial heterodyne spectroscopy (SHS) is based on traditional Michelson interferometry. However, instead of employing retro-reflectors in the interferometer arms, one or both of which are moving, it uses fixed, tilted diffraction gratings and an imaging detector to spatially sample the optical path differences. This concept allows high-resolution, high-throughput spectroscopy without moving interferometer parts, particularly suitable for problems that require compact, robust instrumentation. Here, we briefly review about 20 years of ground- and space-based SHS work performed at the U.S. Naval Research Laboratory (NRL), which started with a visit by Prof. Fred Roesler to NRL in 1993.