<title>Dual parallel-cavity Fabry-Perot interferometer</title>

Abstract

Laser Doppler shift velocimetry operating in the fringe mode is widely used to record the velocity history of optically reflective solid objects. When the object is rapidly accelerated, the time resolution of the Fabry-Perot Interferometer (FPI)/streak camera system is frequently inadequate to unambiguously record the data. That is, there is a 'dead time' during which fringes form and expand without producing a resolvable record. To remedy this, we have developed a dual, parallel cavity FPI which produces two sets of fringes at the slit of the recording streak camera. The input mirror of the FPI is a conventional flat while the output mirror is stepped so there are two mirror spacings. The input light is divided by the respective areas of the stepped mirror to form two sets of fringes; a higher quality set of fringes 'A' is used to determine the velocity history while a second set of fringes 'B' is used as a 'referee' to determine the number of fringe jumps during the 'dead time'. Since the two sets of fringes are necessarily interleaved, there is a possibility of fringe overlap, i.e., fringes of set A can spatially overlap fringes of set B. To date, two dual FPI's have been successfully constructed and characterized. The theory of operation, details of construction, and the results are described.