Interferometry

The Original Water Vapor Interferometer

This is the original interferometer used for finding the water vapor absorption hole for the Strategic Defense Initiative in 1988.

At the time we thought that the heaviest possible table would isolate the system from external vibrations.

This was true for low frequencies but high frequency acoustic noise was coupled into the table surface which then made its way up through support rods to the Mach-Zehnder mirrors themselves.

Since we were looking for 10^-5 wave fringe shifts it didn't take much acoustic noise to show up on the signal.

Diamond Window Interferometer

If the exit beam from a Mach Zehnder interferometer on a rigid frame is collinear with the input beam then the relative path between the two arms of the interferometer is invariant to small translations and rotations of that frame. Later SRL interferometers were suspended on spring supports inside of a vacuum chamber to improve their acoustic isolation.

This new design was used to measure laser absorption in small diamond windows being considered for use with the very high-powered lasers being developed for SDIO at the time.

Ultra-stable Sagnac Interferometer

The Sagnac interferometer uses counter-propagating probe and reference beams that are both reflected from each internal mirror. This output signal from the interferometer is thus invariant to translations of individual mirrors. SRL used this design to show how the speed of light changes in flowing water as a teaching demonstration (Fresnel Drag).

 Electron Density Interferometer

SRL developed an interferometer that used two wavelengths, 532 nm and 1064 nm, to measure both electron density and neutral gas density in a high power current switch at the Naval Research Laboratory. This system was sensitive to electron density changes over four orders of magniture with a bandwidth of 100 Mhz. The photograph shows a test plasma used in the development of this system.