Kenneth D. Marr

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.

Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI): Monolithic Interferometer Design and Test

The design and laboratory tests of the interferometers for the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument which measures thermospheric wind and temperature for the NASA-sponsored Ionospheric Connection (ICON) Explorer mission are described. The monolithic interferometers use the Doppler Asymmetric Spatial Heterodyne (DASH) Spectroscopy technique for wind measurements and a multi-element photometer approach to measure thermospheric temperatures. The DASH technique and overall optical design of the MIGHTI instrument are described in an overview followed by details on the design, element fabrication, assembly, laboratory tests and thermal control of the interferometers that are the heart of MIGHTI.

High-efficiency echelle gratings for MIGHTI, the spatial heterodyne interferometers for the ICON mission

Development of a new generation of low-groove density-blazed echelle gratings optimized for MIGHTI, a space-borne spatial heterodyne interferometer operating in the visible and near infrared is described. Special demands are placed on the wavefront accuracy, groove profile, and efficiency of these gratings. These demands required a new ruling for this application, with significant improvements over existing gratings. Properties of a new generation of highly efficient, plane gratings with 64  grooves/mm blazed at 8.2° are reported.

The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI): Wind and Temperature Observations from the Ionospheric Connection Explorer (ICON)

We describe the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI), an instrument designed to measure thermospheric wind and temperature as part of the Ionospheric Connection (ICON) Explorer mission proposal to the NASA Explorer program.

Retrieval of Lower Thermospheric Temperatures from O 2 A Band Emission: The MIGHTI Experiment on ICON

The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) is a satellite experiment scheduled to launch on NASA’s Ionospheric Connection Explorer (ICON) in 2018. MIGHTI is designed to measure horizontal neutral winds and neutral temperatures in the terrestrial thermosphere. Temperatures will be inferred by imaging the molecular oxygen Atmospheric band (A band) on the limb in the lower thermosphere. MIGHTI will measure the spectral shape of the A band using discrete wavelength channels to infer the ambient temperature from the rotational envelope of the band. Here we present simulated temperature retrievals based on the as-built characteristics of the instrument and the expected emission rate profile of the A band for typical daytime and nighttime conditions. We find that for a spherically symmetric atmosphere, the measurement precision is 1 K between 90–105 km during the daytime whereas during the nighttime it increases from 1 K at 90 km to 3 K at 105 km. We also find that the accuracy is 2 K to 11 K for the same altitudes. The expected MIGHTI temperature precision is within the measurement requirements for the ICON mission.

MIGHTI: The spatial heterodyne instrument for thermospheric wind measurements on board the ICON mission

We describe the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI), an instrument currently being built to measure thermospheric wind and temperature as part of the NASA Ionospheric Connection (ICON) Explorer mission.

Design and Laboratory Tests of the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) on the Ionospheric Connection Explorer (ICON) Satellite

We describe the design and laboratory tests of the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI), designed to measure thermospheric wind and temperature for the NASA-sponsored Ionospheric Connection (ICON) Explorer mission.

Measurement and modeling of the thermal behavior of a laboratory DASH interferometer

A Doppler Asymmetric Spatial Heterodyne (DASH) interferometer is a device that is suited to making line-of-sight measurements of thermospheric wind speeds from either ground- or space-based platforms. However, DASH interferometer characteristics are sensitive to temperature changes. These instrument changes can be tracked with calibration sources and subsequently corrected during data analysis. Even though these thermal effects can be corrected, a quantitative understanding of the physics driving them is important for future instrument designs. A previous study of the thermal behavior of a monolithic DASH system [Harlander et al, Opt. Express, 2010] measured a thermal response that was not consistent with a simplified model. It was suggested that this discrepancy was a result of the rotation of various optical components caused by the thermoelastic distortion of the monolithic interferometer elements which were cemented together yet had different coefficients of thermal expansion. This distortion effect was not included in the simplified model. In this study we assemble an interferometer with separate optical components which are allowed to expand independently with changes in temperature and therefore eliminates any distortion due to stresses induced by different coefficients of thermal expansion. Thus, by measuring the thermally induced change to the interference pattern generated by this interferometer, we may characterize the thermal behavior of the system and verify whether all the relevant physics is included in the simplified model. We find that the thermal drift measured by the experimental interferometer closely matches that predicted by the model. This important result will help in the material selection and overall design of future monolithic interferometers.