LRS Micro-g LaCoste FG5 User manual

Table of Figures
1. CONCEPT AND HISTORY OF THE FG5.................................................. 1-1
1.1. The FG5 Absolute Gravimeter................................................................................................... 1-1
1.2. HISTORY...................................................................................................................................... 1-1
1.3. FG5 Design Features .................................................................................................................... 1-2
2. DESIGN: COMPONENTS AND FUNCTION ............................................. 2-1
2.1. The Dropping Chamber .............................................................................................................. 2-2
2.1.1. CART/DRAG-FREE CHAMBER........................................................................................... 2-4
2.1.2. TEST MASS ............................................................................................................................ 2-5
2.1.3. DRIVE MECHANISM ............................................................................................................ 2-6
2.1.4. SERVICE RING ...................................................................................................................... 2-6
2.1.5. VIEWING PORT..................................................................................................................... 2-7
2.1.6. THE DROP .............................................................................................................................. 2-7
2.2. The Interferometer Base.............................................................................................................. 2-9
2.2.1. LASER..................................................................................................................................... 2-9
2.2.2. OPTICS AND BEAM PATH .................................................................................................. 2-9
2.3. The Superspring......................................................................................................................... 2-12
2.3.1. SUPERSPRING MASS ......................................................................................................... 2-12
2.3.2. SPHERE DETECTOR SYSTEM .......................................................................................... 2-12
2.4. The System Controller ............................................................................................................... 2-14
2.4.1. REQUIRED HARDWARE ................................................................................................... 2-15
2.4.2. OPTIONAL HARDWARE.................................................................................................... 2-15
2.4.3. SOFTWARE .......................................................................................................................... 2-15
2.5. Electronics................................................................................................................................... 2-15
2.5.1. TIMING SYSTEM ................................................................................................................ 2-15
2.5.2. DROPPER CONTROLLER .................................................................................................. 2-17
2.5.3. SUPERSPRING CONTROLLER.......................................................................................... 2-18
2.5.4. LASER CONTROLLER........................................................................................................ 2-18
2.5.5. POWER SUPPLIES............................................................................................................... 2-19
2.5.6. OPTIONAL SYSTEMS......................................................................................................... 2-19
2.5.6.1. ENVIRONMENTAL SENSORS....................................................................................... 2-19
2.5.6.2. ROTATION MONITOR.................................................................................................... 2-20
3. HOW TO SET UP AND RUN THE FG5..................................................... 3-1
3.1. Setting Up the FG5 ....................................................................................................................... 3-1
3.1.1. ELECTRONICS CASE ........................................................................................................... 3-1
3.1.1.1. Model ML-1 Laser:.............................................................................................................. 3-2
3.1.1.1.1. Warm Up........................................................................................................................ 3-2
3.1.1.1.2. Operation .......................................................................................................................3-2
3.1.1.2. WEO Model 100 Laser ........................................................................................................ 3-3
3.1.2. SUPERSPRING ....................................................................................................................... 3-5
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Table of Figures
3.1.3. INTERFEROMETER...............................................................................................................3-6
3.1.4. DROPPING CHAMBER TRIPOD ..........................................................................................3-6
3.1.5. ION PUMP ...............................................................................................................................3-7
3.1.6. LEVELING THE DROPPING CHAMBER ............................................................................3-8
3.1.7. CABLE CONNECTIONS ........................................................................................................3-9
3.1.8. THE SUPERSPRING.............................................................................................................3-11
3.1.8.1. TRAVEL LOCK.................................................................................................................3-12
3.1.8.2. SUPERSPRING ZERO-POSITIONING............................................................................3-12
3.1.9. BEAM VERTICALITY .........................................................................................................3-13
3.1.10. FRINGE OPTIMIZING..........................................................................................................3-14
3.2. Running the FG5.........................................................................................................................3-17
3.2.1. SYSTEM CONTROLLER .....................................................................................................3-17
3.2.2. DROPPER CONTROLLER...................................................................................................3-17
3.2.3. PROGRAM SETUP ...............................................................................................................3-17
3.3. Shutting Down the FG5 ..............................................................................................................3-18
3.3.1. COMPUTER ..........................................................................................................................3-18
3.3.2. SUPERSPRING......................................................................................................................3-18
3.3.3. INTERFEROMETER.............................................................................................................3-19
3.3.3.1. OPTION 1: Model ML-1 Laser. ........................................................................................ 3-19
3.3.3.2. OPTION 2: WEO Model 100 Laser ...................................................................................3-19
3.3.4. DROPPING CHAMBER........................................................................................................3-19
3.3.5. POWER ..................................................................................................................................3-19
3.4. Disassembling and Packing the FG5.........................................................................................3-20
3.4.1. ELECTRONICS .....................................................................................................................3-20
3.4.2. SYSTEM CONTROLLER .....................................................................................................3-20
3.4.3. ROTATION MONITOR (IF INCLUDED)...........................................................................3-21
3.4.4. DROPPING CHAMBER........................................................................................................3-22
3.4.5. DROPPING CHAMBER TRIPOD ........................................................................................3-22
3.4.6. INTERFEROMETER BASE.................................................................................................. 3-22
3.4.7. SUPERSPRING......................................................................................................................3-23
3.4.8. TURBO PUMP (IF USED) ....................................................................................................3-23
4. ADJUSTMENT AND MAINTENANCE........................................................4-1
4.1. The Dropping Chamber...............................................................................................................4-1
4.1.1. REPLACEMENTS AND ADJUSTMENTS ............................................................................4-1
4.1.1.1. Removing The Dropping Chamber Cover............................................................................4-1
4.1.1.2. Replacing the Dropping Chamber Cover..............................................................................4-2
4.1.1.3. Replacing the Drive Belt ......................................................................................................4-4
4.1.1.4. Adjusting the Drive Belt Tension .........................................................................................4-5
4.1.1.5. Replacing the Ferrofluidic Vacuum Feedthrough.................................................................4-5
4.1.1.6. Replacing the V-Plate ...........................................................................................................4-6
4.1.1.7. Replacing the Linear Bearings..............................................................................................4-7
4.1.1.8. Replacing the Shaft Bearings—Drive Pulley........................................................................4-8
4.1.1.9. Replacing the Shaft Bearings—Top Pulley ..........................................................................4-9
4.1.1.10. Replacing the Rotary Shaft Encoder...................................................................................4-9
4.1.1.11. Pumping Down the Dropping Chamber ...........................................................................4-10
4.2. The Interferometer .....................................................................................................................4-10
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Table of Figures
4.2.1. ALIGNMENT........................................................................................................................ 4-10
4.3. The Superspring......................................................................................................................... 4-13
4.3.1. REPLACEMENTS AND ADJUSTMENTS.......................................................................... 4-13
4.3.1.1. Removing the Superspring Cover ...................................................................................... 4-13
4.3.1.2. Removing the Service Ring................................................................................................ 4-13
4.3.1.3. Replacing the Coil (Linear Actuator)................................................................................. 4-14
4.3.1.4. The Mass Mainspring/Hanger............................................................................................ 4-15
4.3.1.5. Replacing the Flexures....................................................................................................... 4-15
4.3.1.6. Assembling the Superspring............................................................................................... 4-18
4.3.1.7. Replacing the Focus Lever Motor...................................................................................... 4-19
4.3.1.8. Adjusting the Micro-Switches............................................................................................ 4-19
4.3.1.9. The Aneroid Wafer Assembly............................................................................................ 4-19
4.3.1.10. The Delta Rods................................................................................................................. 4-20
4.4. Timing System and Data Acquisition....................................................................................... 4-21
4.4.1. TIMING ................................................................................................................................. 4-21
4.4.1.1. Avalanche Photo Diode Printed Circuit Board .................................................................. 4-23
4.4.1.2. APD Board and Photo Diode Supply Module.................................................................... 4-23
4.4.1.3. Rubidium Oscillator ........................................................................................................... 4-24
4.4.2. DATA ACQUISITION.......................................................................................................... 4-25
4.4.2.1. Computer Interface Cards .................................................................................................. 4-25
4.5. DROPPER CONTROLLER ..................................................................................................... 4-25
4.5.1. DROPPER CONTROL MODES ........................................................................................... 4-27
4.5.1.1. STANDBY.........................................................................................................................4-27
4.5.1.1.1. To Select ...................................................................................................................... 4-27
4.5.1.1.2. Function ....................................................................................................................... 4-27
4.5.1.1.3. To Deselect .................................................................................................................. 4-27
4.5.1.2. MANUAL ..........................................................................................................................4-27
4.5.1.2.1. To Select: ..................................................................................................................... 4-27
4.5.1.2.2. Function: ...................................................................................................................... 4-27
4.5.1.3. DROP................................................................................................................................. 4-27
4.5.1.3.1. To Select: ..................................................................................................................... 4-27
4.5.1.3.2. Function: ...................................................................................................................... 4-28
4.5.1.4. OSCILLATE...................................................................................................................... 4-28
4.5.1.4.1. To Select: ..................................................................................................................... 4-28
4.5.1.4.2. Function: ...................................................................................................................... 4-28
4.5.2. Analog Servo.......................................................................................................................... 4-28
4.5.2.1. Cart-Position ...................................................................................................................... 4-29
4.5.2.2. Cart Velocity ...................................................................................................................... 4-29
4.5.2.3. Sphere Servo ...................................................................................................................... 4-30
4.5.2.4. Active Sphere Servo........................................................................................................... 4-30
4.5.2.5. Feedforward Sphere Servo................................................................................................. 4-31
4.6. Superspring Controller .............................................................................................................. 4-31
4.6.1.1. SUPERSPRING CONNECTIONS .................................................................................... 4-31
4.6.1.2. MOTOR DRIVE SELECTION ......................................................................................... 4-33
4.6.1.2.1. OFF .............................................................................................................................. 4-33
4.6.1.2.2. AUTO .......................................................................................................................... 4-33
4.6.1.2.3. WINDOW.................................................................................................................... 4-33
4.6.1.2.4. REMOTE ..................................................................................................................... 4-33
4.6.1.2.5. MANUAL.................................................................................................................... 4-33
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Table of Figures
4.6.1.3. Active Servo .......................................................................................................................4-33
4.6.1.4. Coil Drive (Current Driver) ................................................................................................4-34
4.6.2. Maintenance Schedule............................................................................................................4-34
4.6.2.1. Annually .............................................................................................................................4-34
4.6.2.2. Semi-annually..................................................................................................................... 4-34
5. TROUBLESHOOTING ...............................................................................5-1
5.1. The FG5 System ............................................................................................................................5-1
5.2. System Problems ...........................................................................................................................5-2
5.2.1. DROPPING CHAMBER..........................................................................................................5-2
5.2.2. INTERFEROMETER AND LASER........................................................................................5-5
5.2.3. SUPERSPRING........................................................................................................................5-8
5.2.4. SYSTEM CONTROLLER .....................................................................................................5-10
5.2.5. ELECTRONICS .....................................................................................................................5-11
5.2.6. ENVIRONMENTAL SENSORS PACKAGE .......................................................................5-12
5.2.7. ROTATION MONITOR ........................................................................................................5-13
5.3. Gravity Site Selection..................................................................................................................5-14
5.3.1. GEOLOGIC STABILITY ......................................................................................................5-14
5.3.2. SITE STABILITY ..................................................................................................................5-14
5.3.3. ENVIRONMENTAL NOISE.................................................................................................5-14
5.3.4. TEMPERATURE STABILITY..............................................................................................5-14
5.4. AC POWER.................................................................................................................................5-15
6. SWITCHING THE AC POWER ..................................................................6-1
6.1. LASER ...........................................................................................................................................6-1
6.1.1. WEO Model 100 Laser.............................................................................................................6-1
6.1.2. ML-1 Laser...............................................................................................................................6-2
6.2. Portable Ion Pump Power Supply ...............................................................................................6-2
6.3. Computer.......................................................................................................................................6-2
7. PUMPING/BAKING THE CHAMBER ........................................................7-1
7.1. Choosing the Pumping Method....................................................................................................7-1
7.1.1. Cold Pumping the Chamber...................................................................................................... 7-1
7.1.2. Baking Out the Chamber ..........................................................................................................7-1
7.2. Connecting the Turbo Pump to the Dropping Chamber...........................................................7-2
7.3. Heating the Chamber....................................................................................................................7-3
7.4. Starting the Ion Pump ..................................................................................................................7-4
8. THE ROTATION MONITOR.......................................................................8-1
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Table of Figures
8.1. INSTALLATION ......................................................................................................................... 8-1
9. CHECKLISTS AND LOGS ........................................................................ 9-1
9.1. Switching the AC Power .............................................................................................................. 9-1
9.2. FG5 Setup...................................................................................................................................... 9-2
9.3. Dropping Chamber Pump Down and Bake-Out ....................................................................... 9-5
9.4. Dropping Chamber Maintenance ............................................................................................... 9-7
9.4.1. Removing Dropping Chamber Cover....................................................................................... 9-7
9.4.2. Replacing Dropping Chamber Cover....................................................................................... 9-7
9.4.3. Replacing Drive Belt................................................................................................................ 9-8
9.4.4. Adjusting Belt Tension ............................................................................................................ 9-9
9.4.5. Replacing Ferrofluidic Vacuum Feedthrough.......................................................................... 9-9
9.4.6. Replacing V-plate .................................................................................................................. 9-10
9.4.7. Replacing the Linear Bearings ............................................................................................... 9-11
9.4.8. Replacing Shaft Bearings (Drive Pulley) ............................................................................... 9-13
9.4.9. Replacing Shaft Bearings (Top Pulley).................................................................................. 9-14
9.4.10. Replacing Rotary Shaft Encoder ............................................................................................ 9-15
9.5. Interferometer Alignment.......................................................................................................... 9-16
9.6. Superspring Maintenance.......................................................................................................... 9-18
9.6.1. Removing Superspring Cover ................................................................................................ 9-18
9.6.2. Replacing Superspring Cover................................................................................................. 9-18
9.6.3. Removing Superspring Service Ring ..................................................................................... 9-19
9.6.4. Replacing Superspring Coil ................................................................................................... 9-19
9.6.5. Replacing Superspring Flexures............................................................................................. 9-20
9.6.6. Assembling Superspring ........................................................................................................ 9-22
9.6.7. Replacing Superspring Focus Lever Motor............................................................................ 9-25
9.6.8. Adjusting Superspring Micro-Switches ................................................................................. 9-26
9.7. Packing the FG5 ......................................................................................................................... 9-26
10. INDEX ........................................................................................................... 10-1
Figure 2-1 The FG5 System......................................................................................... 2-1
Figure 2-2 Front view of the dropping chamber ...................................................... 2-2
Figure 2-3 Side view of the dropping chamber ........................................................2-3
Figure 2-4 Front view of the cart/drag-free chamber..............................................2-4
Figure 2-5 Side view of the cart/drag-free chamber................................................2-5
Figure 2-6. Side view of the service ring................................................................... 2-6
Figure 2-7 Top view of the service ring. .................................................................... 2-7
Figure 2-8 Side view of interferometer optics and beam path .............................2-11
Figure 2-9 The Superspring. ...................................................................................... 2-13
Figure 2-10 The Superspring sphere detection system..........................................2-14
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Table of Figures
Figure 2-11 Timing diagram...................................................................................... 2-16
Figure 2-12 Rotation Monitor....................................................................................2-21
Figure 3-1: Superspring / Interferometer Setup...................................................... 3-4
Figure 4-1 Drive Pulley Assembly............................................................................. 4-7
Figure 4-2: Interferometer Top Lid.......................................................................... 4-11
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Table of Figures
This Page is Intentionally Blank
vii

Concept and History of the FG5 1
1. Concept and History of the FG5
1.1. The FG5 Absolute Gravimeter
The FG5 absolute gravimeter is a high precision, high accuracy, transportable
instrument that measures the vertical acceleration of gravity (g). The operation
of the FG5 is simple in concept. A test mass is dropped vertically by a
mechanical device inside a vacuum chamber, and then allowed to fall a distance
of about 20cm. The FG5 uses a laser interferometer to accurately determine the
position of the free-falling test mass as it accelerates due to gravity. The
acceleration of the test mass is calculated directly from the measured trajectory.
The laser interferometer generates optical interference fringes as the test mass
falls. The fringes are counted and timed with an atomic clock to obtain precise
time and distance pairs. These data are fit to a parabolic trajectory to give a
measured value for g. This method of measuring gravity is absolute because the
determination is purely metrological and relies on standards of length and time.
The distance scale is given by a frequency stabilized helium neon (HeNe) laser
used in the interferometer. A rubidium atomic time-base provides the time scale
used for the accurate timing. The value of gravity obtained with the FG5 can be
used without the loop reductions and drift corrections normally required when
using relative instrumentation.
1.2. HISTORY
The FG5 is a new generation of absolute gravimeter based on technology
developed over the last thirty years by Dr. James Faller of the National Institute
of Standards and Technology (NIST), and his colleagues. Beginning with a
white-light-fringe interferometric system built in 1962, Faller and coworkers have
continuously improved the designs of the instruments. The most recent
predecessors of the FG5 was the series of six JILAg gravimeters, built in 1985 at
the Joint Institute of Laboratory Astrophysics (JILA), with support from NIST,
the Defense Mapping Agency (DMA), the National Oceanographic and
Atmospheric Administration (NOAA), the Canadian Geophysical Survey (GSC),
the University of Hanover Institute for Earth Measurement, Germany, the
Finnish Geodetic Institute, Finland, and the University of Vienna Institute for
Metrology and Geophysics, Austria.
1-1

Concept and History of the FG5 1
1.3. FG5 Design Features
The FG5 incorporates a number of significant advancements in design which
reduce or eliminate systematic errors identified in the earlier versions, and which
make the FG5 easier to use. These improvements are:
• An inline interferometer beam path which eliminates systematic errors
from tilt-induced path length changes.
• Complete redesign of the Superspring, a device for providing an inertial
mass that contains a retroreflective corner cube. The new Superspring has
improved performance, and at the same time greatly reduced size. The drift
problems of earlier designs have been reduced substantially.
• Completely new tripod design, which supports the test chamber, for extra
stability. The tripod is now built symmetrically with respect to the drop line.
• Improvements to the electronics reflect new technology and make the
instrument smaller and easier to use.
• This absolute gravimeter is designed to work with a new rugged iodine-
stabilized laser system (WEO model 100) traceable to the BIPM.
• The system controller has been updated to an Intel-based personal
computer with a standard language interface. The decision to use standard
PC technology has allowed the FG5 to offer more computing power while
reducing the size of the instrument.
• “g”, a user-friendly, Windows©-based, full-featured software program
takes interferometer data and environmental data. This software provides an
immediate value for the local gravity in real-time. The program is also a full-
featured post-processing software program that allows complete ability to
vary data analysis procedures and to vary environmental corrections.
1-2

Design: Components and Function 2
2. Design: Components and Function
The FG5 System (Figure 2-1) consists of a: Dropping Chamber, Interferometer,
Superspring, System Controller, and Electronics. A test mass is allowed to free-
fall inside the evacuated Dropping Chamber. The Interferometer is used to
monitor the position of the freely-falling test mass. The Superspring is an active
long-period isolation device used to provide an inertial reference for the gravity
measurement. The System Controller (computer) allows a flexible user
interface, controls the system, acquires data, analyzes data, and stores the
results. The Electronics provides high accuracy timing necessary for the
measurement and provides system servo control.
Figure 2-1 The FG5 System
2-1

Design: Components and Function 2
2.1. The Dropping Chamber
The Dropping Chamber (Figure 2-2 and Figure 2-3) is an evacuated
chamber which contains the Cart/Drag-Free Chamber which houses the Test
Mass. A Drive Mechanism is used to drop, track, and catch the test mass inside
the drag-free chamber. Laser light (Figure 2-1) passes through a window in the
bottom of the Dropping Chamber to the corner cube (inside the test mass), then
is reflected back down through the window to the interferometer.
Figure 2-2 Front view of the dropping chamber
2-2

Design: Components and Function 2
Figure 2-3 Side view of the dropping chamber
2-3

Design: Components and Function 2
2.1.1.CART/DRAG-FREE CHAMBER
The cart /drag-free chamber (Figure 2-4 and Figure 2-5) houses the test mass.
The purpose of the drag-free chamber is to reduce the residual air drag inside the
evacuated dropping chamber. The chamber also reduces magnetic and
electrostatic forces on the test mass, and provides a convenient method for
dropping and catching the test mass, as well as returning it to the top of the
chamber for the next drop. A Light Emitting Diode (LED) , located on the cart,
directs light through an optical glass sphere attached to the test mass. The
sphere focuses the light onto a linear detector, also mounted on the cart. This
system senses the position of the cart with respect to the test mass. A servo-
motor/drive belt system (Figure 2-2) moves the cart inside the Dropping
Chamber, using active feedback from the position sensor to maintain the cart in a
constant position relative to the test mass during free-fall. Since there is
essentially no relative motion between the test mass and the drag-free chamber,
the effects of residual air drag are eliminated.
Figure 2-4 Front view of the cart/drag-free chamber
2-4

Design: Components and Function 2
Figure 2-5 Side view of the cart/drag-free chamber
2.1.2.TEST MASS
The Test Mass (Figure 2-4 and Figure 2-5) is a retroreflective corner cube
surrounded by a support structure and balanced at the optical center of the
corner cube. The corner cube is a three-surface mirror which has the special
optical property that the reflected beam is always parallel to the incident beam.
In addition, the phase shift of the reflected beam is virtually constant with
respect to any slight rotation or translation of the corner cube around its optical
center1.
1Peck, Edson, J. Opt. Soc. Amer., 38, (1948)
2-5

Design: Components and Function 2
2.1.3.DRIVE MECHANISM
The drive mechanism (Figure 2-2) is a support structure inside the dropping
chamber on which the cart/drag-free chamber travels up and down, driven by a
DC servo motor.
2.1.4.SERVICE RING
The Service Ring (Figure 2-6 and Figure 2-7) is the base of the Dropping
Chamber. It provides connection and mounting for the following:
• A bellows-type vacuum valve for the initial evacuation of the vacuum
system
• A Ferrofluidic rotary vacuum feedthrough which connects the motor
shaft to the cart drive mechanism
• A servo motor/rotary shaft encoder assembly which moves the cart and
senses its position
• An electrical vacuum feedthrough which allows connection of the test
mass tracking electronics to the controller
• An ion pump, mounted on a 2¾” Conflat flange, which maintains the
vacuum once the chamber has been evacuated by the roughing pump
• Spare 2¾” Conflat and Mini-Conflat flanges are blanked off, and can be
used for additional vacuum accessories
Figure 2-6. Side view of the service ring.
2-6

Design: Components and Function 2
Figure 2-7 Top view of the service ring.
2.1.5.VIEWING PORT
The viewing port (Figure 2-2 and Figure 2-3) is located in the top flange of the
dropping chamber. It allows visual observation of the dropping chamber
interior when the rotation monitor is not fitted to the system. The rotation
monitor (when fitted to the system) is mounted to the top flange of the dropping
chamber, directly above the viewing port. When the rotation monitor is not
mounted, a cover for the port is used to exclude ambient light from the interior
of the dropping chamber during measurements.
2.1.6.THE DROP
In drop mode, a signal from the computer to the dropper controller initiates the
drop sequence. The cart drag-free chamber is driven slowly from its bottom
position to the “hold” position at the top of the drop. A second pulse initiates
the drop, and the cart accelerates downward at more than 1 g, leaving the test
mass in free-fall.
2-7

Design: Components and Function 2
When the cart has traveled about 5 mm downward from the hold position (as
measured by the shaft encoder) a separation of about 3 mm between the cart and
test mass has been achieved. The dropper controller then uses feedback from the
linear detector to maintain this separation for the remainder of the drop.
The free-falling test mass generates an interference fringe for each half-
wavelength (λ/2) of its movement. As the mass accelerates downward, the
fringes occur more and more closely in time. The resulting signal from the
avalanche photo diode (APD) is a “chirped” sine wave (Figure 2-11) whose
frequency is proportional to the free-falling test mass’s velocity.
Approximately a million fringes are generated during a single drop. A zero-
crossing discriminator (comparator) transforms the sinusoidal fringe signals
from the APD into a series of square Transistor-Transistor Logic (TTL) pulses.
The pulses are scaled (i.e., divided) by a user-defined factor which is set in the
software (typically 1000). A Time Interval Analyzer (TIA) measures the time
interval between each scaled pulse. The g-program fits each time and distance
pair to a parabolic trajectory to determine the value of g.
When the cart and test mass have descended past the catch point, the controller
signals the cart to reduce acceleration and then come to a stop. The falling mass
catches up to the descending cart and is brought gently to rest.
The system resets for the next drop. The entire sequence takes about 2 seconds
and can repeated up to thirty times per minute.
2-8

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 1010 (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

Design: Components and Function 2
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.
2-10