LRS Micro-g LaCoste FG5, Manual, Page: 19 / 149

Design: Components and Function
2
2.2. The Interferometer Base
The interferometer base is an aluminum housing which supports the
optics for
splitting, directing, and recombining the laser beams.
2.2.1.LASER
The FG5 employs a stabilized helium-neon laser to provide an accurate and
stable wavelength used in the interferometric measurement system. There are
two lasers which are currently available for the FG5.
•
The Winters Electro-Optics Model 100 iodine stabilized laser. This laser is a
primary standard for the definition of the meter at the Bureau International
des Poids et Measures (BIPM) in Sevres, France. It is a highly stabilized
distance standard having an absolute frequency accuracy of 1 part in 10
10
(50
kHz).
•
The Micro-g Solutions Model ML-1 frequency/intensity stabilized HeNe laser
is characterized by a slow, linear drift. Unlike the WEO Model 100 Iodine
Laser, it must be periodically calibrated to achieve the best accuracy.
However, it is more rugged than the iodine laser.
2.2.2.OPTICS AND BEAM PATH
Refer to Figure 2-1 and Figure 2-8 for the following description of the beam path.
The optical fiber directs the laser beam from the laser head to the interferometer
base. At the input of the interferometer, a lens collimates the light from the
optical fiber. It is then directed to
beamsplitter #1 , where it is split into the test
beam and the reference beam. The reference beam is split again at beamsplitter
#2 and travels to the Avalanche Photo Diode (APD) and the fringe viewer. The
path length of the reference beam remains constant.
The
test beam is reflected vertically at beamsplitter #1 , and passes through a
compensator plate and a window in the bottom of the Dropping Chamber. It is
then reflected back down by the corner cube in the test mass. The test beam
returns through the window, the compensator plate, and passes down through
the interferometer base to the superspring. The test beam passes through the top
window of the superspring chamber to a corner cube in the superspring mass.
2-9