LRS Micro-g LaCoste FG5, Manual, Page: 20 / 149

Design: Components and Function
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The test beam is then reflected back through the window to the interferometer
base, where it reflects off mirror #1, passes through the translator plate
(twiddler), reflects off mirror #2, and is recombined with the reference beam at
beamsplitter #2.
This interferometer is a Mach-Zender interferometer with a fixed (reference) arm
and a variable (test) arm. During a drop, the motion of the test mass/corner
cube affects the path length of the test beam. The interference fringes which
result from the recombination of the test beam and the reference beam provide
an accurate measure of the motion of the test mass relative to the mass
suspended on the superspring.
Two separate complementary, recombined beams are produced at beamsplitter
#2. The vertical recombined beam is focused by a lens to strike the detector
(APD), and the interference fringes are converted to a Continuous Wave (CW)
signal. The CW signal is then converted to a Transistor Transistor Logic (TTL)
signal and transmitted to the time interval analyzer.
The other recombined beam travels horizontally until it reaches the attenuator
plate (rattler). This beam is split and reflects "rattles" between the beamsplitter
coating and the uncoated side of the attenuator plate. Three beams of decreasing
intensity emerge from the coated side. The first and brightest of these beams
travels horizontally into the fringe viewer. The second and third beams are
deflected vertically by a mirror. A flag in front of the mirror blocks the second
beam, allowing the third (dimmest) beam to exit the interferometer where it is
reflected off mirror #3 and enters the collimating telescope. The collimating
telescope is used to compare this weak reference beam with another beam
reflected off of an alcohol pool to allow alignment of the laser beam with the
local vertical.
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