1. BRIEF REVIEW OF HOW SPATIAL HETERODYNE SPECTROSCOPY WORKS

Spatial heterodyne spectroscopy (SHS) is a method of Fourier transform spectroscopy based on a Michelson interferometer modified by replacing the mirrors in each arm with fixed diffraction gratings, as shown in Fig. 1 [1–3]. For each wavelength in the wavefront entering the interferometer, two wavefronts with a wavelength-dependent crossing angle between them exit the interferometer. The resulting Fizeau fringes have wavelength-dependent spatial frequencies, are localized near the gratings, and are imaged by the exit optics on a position-sensitive detector. Each spectral element at the input is modulated by a unique spatial frequency at the output so that the fringe image is a constant plus the Fourier transform of the input spectrum about the heterodyne wavelength (the wavelength producing parallel output wavefronts). The Fourier transform of the fringe image produces the spectrum, with the zero of the transform corresponding to the heterodyne wavelength. The wavelength range of the recovered spectrum is fundamentally determined by the combination of the resolving power, which is equivalent to the number of illuminated grating grooves, and the number of detector samples, which limits the number of uniquely recoverable spatial frequencies.

 

Fig. 1. Schematic diagram of the basic SHS configuration. For each wavelength in the incident wavefront, two wavefronts with a wavelength-dependent crossing angle between them exit the interferometer. The resulting Fizeau fringes have wavelength-dependent spatial frequencies, are localized near the gratings, and are imaged by exit optics on a position-sensitive detector. The image is the Fourier transform of the input spectrum about the heterodyne wavelength (the wavelength producing parallel output wavefronts). The arrow indicates the typical location of the detector for FTS and Fabry–Perot spectrometry. At this position, the interferometer elements are completely out of focus [2].

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With SHS, high spectral resolution can be achieved with modest requirements on the spatial resolution of the detector. If the fringe pattern is imaged by N pixels in the horizontal dimension in Fig. 1, N/2 spectral elements can be obtained without aliasing. An interference filter can be used to eliminate aliasing and multiplex noise from out-of-band light. SHS achieves a maximum resolving power equal to the theoretical resolving power of a dispersive (grating and prism) system. At the same time, its field of view is characteristic of interferometric spectrometers. When expressed as a solid angle, the field of view of a nonfield-widened SHS is approximately 100 times larger than that of a conventional grating spectrometer of the same resolution.

Without degrading the resolving power, fixed field-widening prisms can be placed in the arms of the interferometer to increase the solid angle of the field of view by another factor of $\sim 100$. The prism angle is chosen so that the gratings, when viewed from the exit of the interferometer, appear, from a geometrical optics standpoint, to be coincident and perpendicular to the optical axis. Depending on the detector and the optics before the interferometer, zero, one, or two dimensions of spatial information can be recorded. Compared to scanning Fourier transform spectrometers (FTS) and Fabry–Perot spectrometers (FPS), SHS instruments have greatly relaxed alignment and surface figure tolerances. This is because in SHS, the elements of the interferometer are nearly imaged on the detector. As a result, each detector pixel integrates only over a small area in the interferometer and misalignments of a few wavelengths shift the heterodyne wavelength but do not reduce the fringe contrast. As long as the optical quality of the elements is good over the small area sampled by a detector pixel, flatness errors on the gratings, prisms, and beam splitter, or index of refraction inhomogeneities, distort the Fizeau fringe pattern but do not reduce its contrast. The measured response to monochromatic light sources provides a measure of the phase errors that can be used in software to correct broadband interferograms with minimal impact on the signal-to-noise ratio in the recovered spectrum [4]. In contrast, the typical location for the detector in a FPS and FTS is indicated in Fig. 1. At this location the interferometer elements are completely out of focus and optical misalignments and/or flatness errors of more than a fraction of a wavelength reduce the detected fringe contrast and the signal-to-noise ratio of the recovered spectrum.

Figure 1 shows an SHS implementation that includes transmissive elements, like the beam splitter. If only reflective, rather than transmissive, optical elements are preferred for a specific application, for example, because transparent, homogeneous materials are not available for a specific wavelength region, SHS can also be implemented in an all-reflective configuration. Measurements in the far-UV are an example for which an all-reflective SHS might be a good choice [1].

In the late 1980s, SHS became practical due to the increasing availability of suitable array detectors, such as charge coupled devices (CCDs) and computers to perform fast Fourier transforms. Since then, the science community has used SHS in several adaptations, addressing measurement challenges for which SHS offers advantages over other spectroscopic techniques. Examples are given in the following, together with references that include both technical details and scientific results.

SHS was first utilized at the University of Wisconsin-Madison (UW-M) for high-spectral-resolution measurements of diffuse sources in the UV [1,3]. For such applications, when compared to other conventional spectroscopic techniques, the main advantages of SHS are the high interferometric throughput, high spectral resolution, compact and rugged package, and relaxed alignment and fabrication tolerances. SHS instruments for the UV were also flown on sounding rockets by UW-M and other institutions (e.g., [5,6]).

The basic SHS configuration also lends itself to modifications to meet other challenging measurement requirements. The passband of SHS can, for example, be increased by using echelle gratings rather than first-order gratings [7]. Another example is a tunable, all reflection SHS, developed at the University of California–Davis, which allows the rapid change of the spectrometer’s passband [8]. A third type of modified SHS, developed by the St. Cloud State University and Naval Research Laboratory (NRL), is a concept optimized for measuring Doppler shifts for remote sensing measurements of atmospheric winds, called Doppler Asymmetric Spatial Heterodyne (DASH) spectroscopy.

In the following sections, we review the SHS work performed at NRL primarily from a historical perspective. Technical details of the instruments, hardware challenges and solutions, and science results from the measurements are included predominantly by reference.

2. BEGINNINGS OF SHS AT NRL

The first SHS instrument built at NRL, the Spatial Heterodyne IMager for Mesospheric Radicals (SHIMMER), was conceived of as a result of conversations between Prof. Fred L. Roesler (UW-M) and Dr. Robert (Bob) R. Conway (NRL) in 1993. At that time, Conway was working on the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI), which was measuring middle atmospheric hydroxyl (OH) and nitric oxide (NO) density profiles using near-UV solar resonance fluorescence as observed from the Space Shuttle Cryogenic Infrared Spectrometer and Telescope for the Atmosphere-Shuttle Pallet Satellite (CRISTA-SPAS) platform [9]. A key observational requirement for measuring OH was high spectral resolution within a narrow spectral range centered around 309 nm to isolate the solar resonance fluorescence OH lines against a bright Rayleigh scattered background. At that time, Roesler was working on the innovative SHS technique with his former graduate student Dr. John M. Harlander [1], and he realized that it offered a means to achieve the required spectral resolution using a dramatically smaller and lighter instrument than MAHRSI. Drs. Roesler and Conway agreed to collaborate on developing the SHS concept for the near-UV.

The following two sections cover two space flight SHS experiments called SHIMMER-MIDDECK and SHIMMER on Space Test Program Satellite-1 (STPSat-1) that directly resulted from these initial discussions. Subsequently, we describe the Michelson Interferometer for High-resolution Thermospheric Imaging (MIGHTI), which is an instrument that is currently being built at NRL for the NASA Ionospheric Connection (ICON) Explorer Satellite. MIGHTI will measure thermospheric neutral winds and temperature. Finally, we will conclude with an outlook for potential future SHS satellite instruments.

3. SHIMMER-MIDDECK EXPERIMENT

After the two successful MAHRSI flights and the construction of a successful proof of concept SHS OH instrument in the laboratory, the SHIMMER team worked on an instrument for flight in the Space Shuttle mid-deck, where the SHIMMER-MIDDECK instrument, shown in Fig. 2, would view the Earth’s limb through the orbiter’s side-hatch window in order to measure the middle atmospheric OH profile.

 

Fig. 2. View into the SHIMMER-MIDDECK Spectrometer. From the top left counterclockwise: telescope, SHS interferometer components mounted in a VascoMax steel cage, relay optics, CCD camera [10].

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Under the leadership of Dr. Conway, the instrument was built and after he retired in 2001, Joel G. Cardon (NRL), a longtime colleague of Conway’s who previously had been in charge of the instrument characterization and testing, took over as the principal investigator for the MIDDECK flight. Under the leadership of Mr. Cardon, the SHIMMER-MIDDECK flight on STS-112 (October 2002) was successfully executed [10]. The data were taken from about 330 km orbit altitude for a total of about 35 min., distributed over a short checkout period before the Orbiter docked onto the International Space Station and two data taking periods that occurred after undocking and before returning to Earth. The core team for this effort was composed of Dr. Michael H. Stevens (NRL), Dr. Charles M. Brown (NRL), Ronen Feldman (Artep, Inc.), and John F. Moser (Artep, Inc.), all of whom were part of the MAHRSI team, Drs. Roesler and Harlander, and Dr. Christoph R. Englert (NRL), who joined Conway’s group in 1999.

Even though the instrument performed well for this flight and both solar spectra and the characteristic OH signature were properly resolved in the measured spectra, no OH altitude profiles could be retrieved, most likely due to the fact that the Orbiter side hatch window was contaminated while docked to the International Space Station, resulting in significant scattering. In spite of this difficulty, the data gathered during this flight demonstrated that SHS was indeed a very capable and suitable technique for this measurement.

4. SHIMMER ON STPSAT-1

In parallel to the SHIMMER-MIDDECK effort, the SHIMMER team developed an improved SHS interferometer for the measurement of OH using NASA funding. The interferometer, shown in Fig. 3, was no longer composed of individually mounted optical components but was a monolithic design in which all components were optically contacted. This interferometer was virtually impossible to misalign, presenting a major advantage, considering the harsh vibration environment of space launches. Moreover, the monolithic design allowed for significant mass savings [2].

 

Fig. 3. Monolithic SHS interferometer for the near UV, flown as part of SHIMMER on STPSat-1. All elements are made of fused silica and are optically contacted [2].

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In January 2002, Dr. Englert received an offer from the DoD Space Test Program (STP) to fly a SHIMMER payload, using the monolithic interferometer, on the upcoming STPSat-1 mission. The team concluded that by using major parts of the MAHRSI electronics, the “SHIMMER on STPSat-1” could be built within the given schedule and budget constraints. SHIMMER thus became the primary payload of STPSat-1, a spacecraft built by AeroAstro in Ashburn, Virginia. The SHIMMER on STPSat-1 instrument was built, calibrated, and tested at NRL under the leadership of the principal investigator, Dr. Englert, and his team, which was predominantly composed of the SHIMMER-MIDDECK team members. The interferometer and optics were designed by Prof. Harlander and the interferometer was built by LightMachinery Inc. (Ottawa, Canada).

STPSat-1, shown in Fig. 4, was launched on 7 March 2007 from Cape Canaveral on board an ATLAS V launch vehicle as part of STP-1, the first mission using an Evolved Expendable Launch Vehicle Secondary Payload Adapter for launching multiple spacecraft. STPSat-1 was injected into a 560 km altitude and 35.4° inclination circular orbit and was operated for the first year from Kirtland Air Force Base in New Mexico. Subsequently, for an additional 1.5 years, it was operated by NRL from the Blossom Point Satellite Control and Tracking Station (Maryland) in collaboration with Tiger Innovations, LLC [11]. SHIMMER data were archived and analyzed at the NRL Space Science Division. The STPSat-1 and SHIMMER missions ended on 7 October 2009. Portions of the SHIMMER project were supported by the DoD Space Test Program, ONR, and NASA.

 

Fig. 4. Artist’s conception of STPSat-1 on orbit (courtesy of AeroAstro).

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The SHIMMER on STPSat-1 mission met its two primary mission objectives: (1) demonstrate that Spatial Heterodyne Spectroscopy is a technique that is suitable and offers advantages for long-duration space flight applications and (2) measure middle atmospheric OH density profiles at low to mid latitudes and at all daytime local solar times [12,13]. In addition, SHIMMER measured the diurnal variation of polar mesospheric clouds (PMCs) at the southward limit of their high latitude occurrence regions [14,15].

Major results from SHIMMER on STPSat-1 include

The successful flight of SHIMMER on STPSat-1 also led to the development of other applications for SHS including an SHS prototype for the long wave infrared [19].

5. MIGHTI ON ICON

As mentioned above, a major result from the successful SHIMMER missions was that it established space flight heritage for further SHS space instrumentation. In 2013, the NRL MIGHTI instrument was selected by NASA for flight on the ICON Explorer mission. ICON is led by the University of California, Berkeley by the principal investigator, Dr. Thomas Immel.

MIGHTI uses the DASH technique, which is a slightly modified SHS technique. It was developed at NRL in collaboration with St. Cloud State University starting in 2005 [20]. The development started with successful ground-based demonstrations [21–24] and was motivated by the fact that thermospheric winds and temperatures are critically important for understanding and characterizing the Earth’s ionosphere/thermosphere region and its coupling to lower atmospheric dynamics and solar forcing. This region is also of increasing national importance, since it influences medium- and high-frequency wave propagation in the upper atmosphere.

To determine the neutral wind, MIGHTI measures the Doppler shift of naturally occurring thermospheric emission lines. Since most of the high-spectral-resolution information about the line center of an atmospheric emission line is contained in a part of the SHS interferogram that is offset from the zero path position (center of the detector array in Fig. 1), one can reduce mass, size, and power requirements of the experiment by only measuring the offset part of the interferogram [21]. The minor modification to the basic SHS concept that is necessary to accomplish this is to lengthen one interferometer arm with respect to the other. With this modification, the sampled optical path difference interval is no longer symmetric about zero, hence the name DASH spectroscopy. This approach is very similar to the stepped Michelson technique, which was applied very successfully by the WINDII experiment on the NASA UARS satellite [25]. Key advantages over the stepped Michelson method are that DASH has no need for moving interferometer parts, it provides many more interferogram samples measured simultaneously, and it can observe several emission lines simultaneously.

MIGHTI was designed to measure winds using the Doppler shifts of the atomic oxygen red ($\lambda =630\mathrm{nm}$) and green ($\lambda =557.7\mathrm{nm}$) emission lines and to measure temperature using the band shape of the molecular oxygen A-band around $\lambda =762\mathrm{nm}$ using a multichannel photometer approach. Figure 5 shows a three-dimensional (3D) model of one of the two identical optical sensors of the MIGHTI instrument. Two sensors are needed to determine horizontal wind vectors as a function of altitude from the limb images. The fields of view of the two sensors are pointed 45° and 135° from the spacecraft ram direction. Light enters the sensors through baffles that minimize stray signal contributions from the sun and the illuminated Earth during the day. The entrance pupil at the end of the baffle can be stopped down in daytime to 15% of its full size to achieve improved stray light rejection. The Earth limb scene is imaged onto the interferometer gratings by the entrance optics, which defines the field of view using a field stop and a Lyot stop. The entrance optics also superimposes calibration lamp signals to track thermal drifts of the interferometer. The interferometer, which is located in the upper left of Fig. 5 in a temperature controlled enclosure, superimposes Fizeau fringes onto the scene, which are then imaged onto the CCD camera shown on the upper right. The exit optics include a dichroic wedge, which separates the red and green signals from the atmosphere to minimize multiplex noise. Narrowband interference filters are used to isolate the red and green lines and to measure three in-band and two off-band channels of the oxygen A-band. Figure 6 shows an image taken with the MIGHTI Engineering unit.

 

Fig. 5. 3D model of one of the two identical optical sensors of the MIGHTI instrument. The components are described in the text.

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Fig. 6. False color image taken with MIGHTI Engineering Unit Camera. Laser lines with wavelengths close to the oxygen red and green airglow lines were used to illuminate instrument field of view in the laboratory. The dichroic wedge separates the colors on the CCD so they appear side by side. The fringes are created by the SHS interferometer and the periodic structure on the top is an image of a notch pattern on one grating, which allows for the assessment of image drift on the CCD. Below the red image are five near-infrared filter elements that provide a photometric measurement of the oxygen A-band shape. This section appears dark here, since no in-band light entered the instrument. The approximate positions of the filters are indicated by white lines.

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ICON is scheduled to launch in 2017 for a nominal two year mission in a circular, low inclination, low Earth orbit.

6. FUTURE SHS INSTRUMENTS AT NRL

Our successful efforts of applying SHS and DASH to space borne remote sensing challenges that require the measurement of diffuse sources with high spectral resolution in a limited spectral band motivate us to continue the development of this technique. Specifically, future scientific discovery and environmental monitoring missions will benefit from even smaller instruments that can be deployed on nano-sats, on cubesats, or as secondary payloads on host spacecraft.

Ideas and designs for future SHS and DASH instruments continue to develop, as indicated by presentations by multiple groups, including NRL (e.g., [26–28]).