Stoyan I. Sargoytchev

Divided mirror technique for measuring Doppler shifts with a Michelson interferometer

Upper atmospheric winds have been measured for many year by the detection of the Doppler shifts of airglow emission lines using both Fabry-Perot and Michelson interferometers. The Michelson is usually used in a field-widened configuration with the path difference set at a large value. Filters isolate a single emission line and the Doppler shifts are manifested as phase shifts of the interferometric fringes. The phase shifts are measured by sampling the fringe at four points separated by about one-quarter wavelength in path difference. One problem with the technique is that the phase is subject to error if the source intensity varies while the four exposures are made. A solution is to divide one Michelson mirror into four quadrants, with a different coating on each quadrant, in order to provide the four fringe samples simultaneously. This method is proposed for WAMI, the Michelson instrument in the WAVES proposal. The technique of sampling a fringe at four points simultaneously while imaging is discussed and demonstrated. A laser source is Doppler shifted using a spinning disk and the velocities measured interferometrically agree with the known speed of the disk within statistical error.

A review of one-way and two-way experiments to test the isotropy of the speed of light

As we approach the 125th anniversary of the Michelson–Morley experiment in 2012, we have reviewed experiments that test the isotropy of the speed of light. Previous measurements are categorized into one-way (single-trip) and two-way (round-trip averaged or over closed paths) approaches and the level of experimental verification that these experiments provide is discussed. The isotropy of the speed of light is one of the postulates of the special theory of relativity and, consequently, this phenomenon has been subject to considerable experimental scrutiny. Here, we have tabulated significant experiments performed since 1881 and attempt to indicate a direction for future investigation.

Divided-mirror scanning technique for a small Michelson interferometer

A method is described for obtaining four simultaneous phase- stepped images from a Michelson interferometer in order to measure the Doppler shift of a spectral line. One of the Michelson mirrors is divided into quadrants and each quadrant coated separately, so the path difference varies by about (lambda) /4 from one quadrant to another. An image of the mirrors is formed outside the interferometer, where the light from the quadrants is diverted in different directions, and four separate images of the field of view are formed, one for each quadrant. For a given direction in the field of view, the fringe is sampled at four points on the interferogram separated by (lambda) /4 and from these four intensities, the phase of the fringe is calculated. Doppler shifts of the spectral line are seen as changes in the phase of the fringe. In earlier versions of the imaging Doppler Michelson technique, the sampling was done in sequence. Simultaneous sampling eliminates the errors caused by intensity variations during the measurement, making the technique useful for rapidly varying sources such as aurora.

Spectral Airglow Temperature Imager

A ground-based instrument for measurement of perturbations of the rotational temperature and vertical column emission rate of the O2 atmospheric nightglow layer at 94 km and the OH Meinel layer at 86 km is described with special emphasis on its suitability as a remote field instrument. Ground-based instruments are needed in the detailed study of planetary scale dynamic effects in the upper atmosphere because they show detailed perturbation development in both solar and universal time that is missed by satellite-borne instruments. Ground-based instruments must be stable, accessible to but not dependent upon operator interaction, and inexpensive. The technique of interference filter spectral imaging has shown itself to satisfy these requirements when embodied in the instrument MORTI, a mesopause oxygen rotational temperature imager. SATI represents a complete re-design of MORTI in order to make it more flexible for ground-based networks. In particular, the cryogenic cooling was replaced by thermo-electric cooling, removing the requirement for daily attention, an OH channel was added that will allow comparison of perturbation amplitudes at two significantly different altitudes, and real-time temperature and emission rate readout was incorporated into the revised software.

Narrowband etalon filters for stratospheric wind measurements

Proof of concept narrow-band etalon filters have been fabricated and characterized for the SWIFT instrument program. The Stratospheric Wind Interferometer For Transport studies is a limb viewing satellite instrument which is intended to measure stratospheric horizontal wind velocities in the altitude range of 20 to 40 km. In addition to providing the atmospheric research community with the first direct measurements of stratospheric dynamics on a global scale, continuous global SWIFT data is expected to improve long range weather forecasting in the troposphere. To isolate the single lines required for the Doppler measurement of the SWIFT instrument, two narrow-band germanium etalon filters centered near 9 micrometer and with 0.8 nm and 2.5 nm bandwidths were fabricated and tested. The SWIFT filter testbed consists of a cryogenic dewar and temperature controller for stabilizing and tuning the filters. The SWIFT filter requirements are discussed, as is the filter testbed design. The measured filter characteristics: transmittance as a function of wavelength, temperature and angle of incidence and tandem filter properties are discussed in the context of satellite instrument requirements.

Satellite instrument to measure stratospheric winds

The Stratospheric Wind Interferometer for Transport Studies (SWIFT) is a satellite-born limb-viewing instrument which will be capable of globally measuring horizontal winds at altitudes of between 20 and 40 km with a precision of < 5 m/s, a vertical resolution of 2 km and a horizontal resolution on the order of a hundred km. SWIFT will map stratospheric dynamics. The data from the instrument will be important input for models which seek to predict the global distribution of stratospheric ozone. In addition, the SWIFT data will provide observational input to tropospheric weather models, which are currently being extended to the stratosphere. With global stratospheric wind data, these enhanced models have the potential to significantly improve weather forecasting in the troposphere. The instrument will observe a thermal emission line of an abundant atmospheric constituent near 8 micrometers using a field widened Michelson interferometer. A doppler shift of the emission line is detected as a phase shift at the output of the interferometer. A 2D array detector monitors the phase both perpendicular to and along the limb, thus mapping the velocity field. The fundamental feasibility of the instrument will be shown. The basic instrument requirements are described and the instrument parameters are derived from them. The instrument will utilize radiatively cooled optics and Stirling cycle coolers for the detector and filters. This instrument will be suitable for inclusion on a medium to large satellite with multiple instruments. The lack of cryogens is consistent with its intended use on the operational weather satellites of the future.