| The development of field-widening made it possible to use Michelson interferometers with high spectral-resolving power to analyze light from extended, weak sources. One application is the measurement of high-altitude winds. The wind-induced Doppler shift of airglow emissions provides one way to measure wind speeds near the mesopause remotely, both from the ground and from space. Because of the faintness and extended nature of the source and the small size of the wavelength shift, field-widened Michelson interferometers are ideal instruments for making these measurements.; Doppler-induced wavelength shifts are converted by the Michelson into displacements of the interference fringe pattern. Unlike Michelsons used for spectroscopic analysis, the wind-measuring Michelson is scanned just enough to determine the change in position of one fringe. Many methods of performing this scan have been developed, none of them simple. It is not easy to produce an instrument of interferometric quality that is at the same time stable and adjustable. That leads to the great disadvantage of wind-measuring Michelsons: they are expensive.; A new scanning method is presented in which both output beams from the field-widened Michelson are used. The Michelson is adjusted so that at zero wind speed the intensity of the outputs is balanced. Wind-induced Doppler shifts will upset the balance, and the intensity difference between the two outputs is used to determine wind speed. The end result is a process that can yield the same resolution as present wind-Michelsons, but using as little as one quarter the amount of light. This could lead to smaller, lighter and cheaper instruments.; Test results of a bench experiment are shown, and comparisons between the differential method and other scanning methods are made. The strengths and weaknesses of the differential approach are discussed, and a proposal is made for a field instrument that would take advantage of those strengths. |
