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The
Latest FDVI Mark IV and the New Mark IV-3000
The new M,F& A
Mark IV All Fiber Doppler Velocity Interferometer Systems offer enhanced
capabilities in the measurement and recording of instantaneous velocity
versus time histories. Two versions of the Mark IV Systems are now available.
The original Mark IV
has been in use all over the world and gives excellent results provided
that the target surface of interest is reasonably reflective. In the latest
version of the Mark IV we have up-graded to a new ultra-narrow linewidth
laser for enhanced velocity resolution.
These systems also incorporate the latest developments, such as a
new Nickel Metal Hydride power and charging system, new front panel
arrangement including a “Balance” knob to easily equalize the “sin” and
“cos” signal amplitudes and a
digital battery voltage monitor.
The new systems are built in a 1U Rack Mount chassis and can be
easily up-graded to the Mark IV-3000 by the addition of the EDFA at a later
date if desired.
The Mark IV-3000
features an entirely new system architecture based on the same ultra-narrow
linewidth seed laser but this version incorporates an EDFA with +30 dB gain
to boost the relatively weak return light from marginal target surfaces to
levels that give high signal to noise ratios at the detection end. This boost in the reflected return light
is the equivalent of putting 3000 mW of laser power on the target.
However, you won't
have to worry about safety issues since this optical amplification occurs within
the system. Rather than amplifying the seed laser and putting large amounts
of power onto the target, we amplify the reflected light returned from the
target. Following the lead of the telecomm industry again, we don't amplify
the signal until after it gets weak. The amplified return light is
then sent on to the interferometer as before. This approach allows us to
use an extremely narrow linewidth (<20kHz) source for velocity
resolution potentially down to 0.03 m/s. And with the EDFA boost of the
return light we get saturation level signals from surface reflectivities as
low as 0.1% with the standard probe.
While developing the Mark IV-3000 we experimented with higher power lasers
with 100mW and 500mW of output power but found that the higher power levels
in these lasers increased the micro-acoustic noise. This leads to wavelength and phase jitter,
commonly referred to as “linewidth,” which limits in turn, the ultimate
velocity resolution of the system.
So we abandoned that approach in favour of the EDFA post reflection
amplifier.
By putting the
amplifier after the target reflection, we can keep the power levels
and thus the noise levels extremely low.
Another advantage to this approach is that the laser power levels on the
target are kept low, so damage to sensitive surfaces is not an issue. And although the performance is
equivalent to having a 3 Watt laser
in your lab, the safety and protocol issues remain those for a Class
IIIb, less than 15mW, laser.
.
Upgrades of our older systems to these new Mark IV and Mark IV-3000 configuration
are available, see the side bar.
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Unique Features of the M,F&A FDVI System
No Mirrors,
Beamsplitters or Etalons!
No Free-space Beams!
No Fiddling with Optics!
No Fringe Hunting!
No Fiber Splicing!
No Aligning of Anything!
EVER!
Super
Accurate Measurement of the Velocity of Any Moving Surface
Sub-nanosecond Time Resolution
Better than 1% Absolute Accuracy
Full-Quadrature Push Pull Output Determines Direction as well as Magnitude
Built-in Visible Laser for Alignment
All Solid State, Laser Included
Runs on Rechargeable NiMH Battery, Recharger Included
All Laser Light Contained in Optical Fiber, NTT-FC Connectors Throughout
Four 10 GHz Fiber Coupled Photodiode Modules Included
Single Fiber "C" Compatible Experiment Probe Included
Operators Technical Manual and Analysis Software Included
Delivery Approximately 9-12 weeks After Acceptance of Order
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A Breakthrough in VISAR Technology
A VISAR (Velocity
Interferometer System for Any Reflector) determines the velocity of an
accelerating or vibrating surface by measuring the Doppler shift of laser
light reflected off the surface in a unique form of interferometer. Changes
in the velocity of the surface produce changes in the wavelength of the reflected
light due to the Doppler effect. The interferometer is sensitive to
wavelength and converts these changes in the wavelength of the input light
to changes in the intensity of several output light signals. These intensities are then recorded by
fast photodiodes in a data acquisition and analysis system. Velocities can
be determined with accuracy of ±1% or better, in the range from a fraction
of a meter/sec up to thousands of km/sec and with sub-nanosecond time
resolution. The surface observed does not need to be mirror polished and
changes in it's reflectivity or in background light have no effect on the
derivation of velocity. The system requires no calibration and measurements
made with it can be considered to be absolute.
With this technology there are no limitations on maximum velocity or
response time as in the class of devices known as "Vibrometers." And unlike the class of devices recently
called “Photonic Doppler Velocimeters,” which are actually nothing more
than conventional Michelson or Mach-Zender displacement interferometers,
the signals coming from an FDVI or “VISAR” are already differentiated
optically and represent velocity directly.
The signals from the above-mentioned devices are displacement
fringes and need to be differentiated, usually by Fourier analysis, which
results in a loss of time resolution.
Another disadvantage of the displacement interferometer approach is
that the data acquisition must be fast enough to resolve these displacement
fringes and for velocities of significant interest to many, this can require
extremely expensive digital oscilloscopes with bandwidths of 10 GHz or
more.
The VISAR is truly a
remarkable invention. The pioneers of this technology include Lynn Barker,
Will Hemsing, Bill Isbell, David Goosman, and many others. They led the way
and deserve the credit for the development of the concept principally for
shock physics and weapons research at Los Alamos,
Sandia, and Lawrence Livermore National Laboratories.
There are now only a
very few commercial sources for VISAR systems and the systems offered by
others are still based primarily on the technology of the era of it's
invention - water-cooled Argon ion lasers, high voltage vacuum
photomultiplier tubes, piezo-electric and Pockel's cell sub-systems and so
on.
At Martin,
Froeschner & Associates we have re-invented the system using a
wholly new approach. The tremendous advances in high purity lasers,
ultra-fast detectors, fiber
amplifiers and filters and so on developed by the telecommunications
industry and centered on the 1550nm wavelength regime where attenuation and
dispersion are extremely low, make this the obvious wavelength to operate
in.
By analyzing and
re-imagining the optical layout of the original VISAR we have developed an
architecture which preserves the principles of the original delay leg
interferometer. but realizes it
entirely in fiber. In the M,F&A
FDVI’s there are no free space beams to align, no beamsplitters to
position, no fiber to beam couplers to fiddle with, no moving parts, nothing
to tune, tweak, hunt for, optimize or adjust.
Nothing at all.
Ever.
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How It Works
In the VISAR or Delay Leg Interferometer, incoming light is
split into two paths, one of which is substantially longer than the other,
and then recombined. Optical interference occurs at the re-combination
point. For simplicity, assume that the wavelength of the incoming light is
such that the delay leg of length L, contains an integral number of
waves. Then at the interference point the waves will be in phase and the
signal intensity will be maximum. Now, if the wavelength of the incoming
light begins to change, the number of wavelengths in the delay leg will not
quite exactly match the delay leg length. There will now be a small phase
difference at the re-combination point due to the missing or excess
fraction of a wavelength. This phase difference results in a change in
intensity of the re-combined signals. As the wavelength of the incoming
light continues to change, the intensity at the interference point will go
through a complete sinusoidal cycle and come back to maximum intensity.
Continued change in velocity produces more and more of these cycles in
intensity of "fringes" in the output intensity.
Sensitivity and Accuracy
The basic sensitivity of the instrument is determined by the
length of the delay leg. The change in frequency of the incoming light
required to change the number of waves in the delay leg by exactly one, and
therefore to cause the signal intensity at the interference point to go
through one complete cycle from max to min to max again, is easily
calculated. The velocity of a moving surface which will produce this much
change in frequency by Doppler shift is called the "fringe
constant," and is:
Kf = c lambdao
/ (2 L nD)
Notice that the only uncertainty in this constant, and thus in
the absolute accuracy of the instrument is in the measurement of the length
of the delay leg, L, (0.1%), the illuminating laser wavelength, lambdao,
(0.05%) and the index of refraction of the delay leg fiber, nD,
(0.01%). These are each measured and certified for every system delivered.
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Velocity Resolution
The length of the delay leg fiber determines the sensitivity
of the instrument to changes in velocity and also the time resolution. In
general longer the delay leg lengths produce more sensitivity in velocity.
The resolution in velocity depends on how well we can measure small
portions of a fringe shift. This
depends principally on the wavelength and phase jitter in the laser itself
as well as on the noise level and linearity of the photodetectors and data
recording system. Our experience is
that in ideal circumstances shifts on the order of 1/1000th or 0.1% of a full
fringe are resolvable. This is shown
as vres in the table
below.
The intrinsic wavelength and
phase jitter of the laser is the primary limit to velocity resolution since
the noise levels in the photodetectors and data acquisition system are
generally negligible. This jitter in
wavelength is commonly measured and specified as “linewidth” and is given in the Specifications above.
For example the light
frequency at 1550nm is around 194 Terahertz and with a “linewidth” of 20
kHz we would expect to see the laser output frequency to jitter over the
range from 194.000000020 to 193.999999980 Terahertz. This jitter will appear as apparent
velocity fluctuations on the order of 0.03 m/s. In other words about 30 mm (about one
inch) per second, phenomenally small compared to any other system available.
The maximum velocity resolvable is limited only by the total number
of fringes which can be counted within the record length of the data
acquisition system. Typical systems
have record lengths of several kilo- to several megabytes. This leads to maximum resolvable
velocities on the order of a thousand times the fringe constant. The range in velocity resolvable in any
particular system therefore ranges from about 0.001 Kf to about 1,000 Kf.
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The intrinsic time resolution of the
interferometer may be taken as the transit time of light through the delay
leg and is shown as tres
in the table below. Note that the
time resolution of your data acquisition system will also be relevant.
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Data Acquisition
Each FDVI System consists of everything you need except for a
digital data acquisition system or oscilloscope and a simple PC. The Mark
IV FDVI System requires four channels of data acquisition.
The user will also need to provide a trigger to this data
acquisition system which is timed to the experimental event so that the
data acquisition system captures data at the appropriate time of interest.
Note that the FDVI system needs no trigger, it operates continuously.
Analysis and Software
The velocity of the observed surface at any time is computed
from the number of whole, m, and partial fringes
v(t) = v(0) + m Kf +
arctangent(y (t), x(t)) Kf
where v(0) is the velocity at the beginning of the experiment,
normally zero, m is the number of whole fringes which have been counted
since then - the software does this counting automatically and in both
directions - and the last term is the current fractional part of a fringe
derived from y(t) and x(t) which are the two time dependent signal intensities
from the interferometer.
A complete software package to do this analysis is included with every FDVI
system. The source code is provided in highly modular and extensively
commented Fortran and also compatible with most "C" compilers so
you can adapt it to your system and preferences. Other software systems to do this
analysis are available on the web and elsewhere from their authors.
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Full Quadrature and Push-Pull
A further detail is that our interferometer actually
propagates two polarizations of the incoming light. One of these is
retarded by a quarter wavelength relative to the other. As a result, the
interference maxima in the two signals are displaced from one another, i.e.
they are in quadrature; one varies as the sine of the velocity and the
other as the cosine. From these two signals both the magnitude and
direction of any velocity change can be determined, i.e., we can
unambiguously distinguish acceleration from deceleration.
Another improvement over our previous Mark III Systems
provides two additional signals which are negative polarity versions of the
normal "sin" and "cos" signals. This is the so-called
"push-pull" implementation. It was originally discovered by Will
Hemsing and has been the standard for years in "conventional"
VISARs, those implemented with free-space beams and all the lenses, mirrors
and other complex optics these old technologies require. We are pleased to announce that the
advantages of the "push-pull" configuration are available in our
all-fiber FDVI's as well.
The principal benefit of the "push-pull"
configuration is that the two complementary signals can be subtracted from
one another, either in the software or electronically by a differential
scope input. As a result the signal level is doubled and any background is
eliminated.
For example:
y = (sin + backround) - (-sin + backround) = 2 sin
x = (cos + background) - (-cos + background) = 2 cos
and the velocity is derived from arctan (y.x) as before.
In an experiment where backgrounds may be high and signal
levels varying, this "push-pull" methodology is very desirable as
the uncertainties in the analysis is much reduced.
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Changeable Delay Leg Length
The delay length, L, may be chosen may be chosen to
suit the need of your experiment. As a general rule, select a delay leg
length so that the fringe constant is smaller than or at most about the
same as the velocities of interest in your experiment. Then, hedge towards
shorter lengths if you need better time resolution. On the other hand, the
overall time resolution of the system will be determined by the
photodetectors and data acquisition system, so there is little point in
going below this. The lengths in the chart below are provided simply as
examples. The shortest built for a customer is 75 mm, the longest 10 m.
Delay legs of 200 mm, 1 m and 10 m are provided as standard
equipment with each FDVI System and should cover the needs of almost any
experimental situation. Delay legs of any length may be purchased
separately if your needs so require. The fiber delay leg is easy to
replace.
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L
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Kf
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vres
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tres
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Example Applications
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10 mm
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15 km/s/fr
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20 m/s
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50 ps
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High Energy Density, Laser Effects, Exploding Foils
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100 mm
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1.5 km/s/fr
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2 m/s
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0.5 ns
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Explosives, Shock Hydrodynamics, Space Debris
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1 m
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150 m/s/fr
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0.2 m/s
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5 ns
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Interior Ballistics, Impacts, Damage Propagation
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10 m
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15 m/s/fr
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0.02 m/s
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50 ns
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Acoustics, Machining, Materials Science
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100 m
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1.5 m/s/fr
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0.002 m/s
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0.5 micro s
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Seismic Studies, Vibration, Motions Stability
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Experiment Viewing Probe
 The standard system contains a
focusable variable focal length "CS" mount lens coupled to a
single mode optical fiber. This optical fiber carries light from the
laser(s) in the FDVI chassis to the lens systems rear focal point. From
there the light emerges, is transported through the lens and projected onto
the experimental target surface. The light reflected from the experimental
surface is focused back to the same point where it enters the same fiber
and is transported back to the FDVI System. An optical circulator within
the FDVI then separates this returning light from the outgoing light and
sends it on to the Interferometer section of the system. A built-in red
pilot laser is provided so that visible light can be sent through the
Experiment Viewing Probe to assist in aiming of the probe's focal spot onto
the experimental surface.
The standard probe
lens has a focal length that can be varied from 5 mm to 50 mm. The aperture
is 27 mm. It is mounted in a standard 1" kinetic optical mount that is
mounted on a 12" optical rail.
A wide range of other
lenses, especially those designed for video cameras, are compatible with
this standard "C" thread probe. Adapters for other lens systems
such as the various popular 35 mm reflex camera systems are available, as
are systems based on microscopes and large optical systems such as
Celestron and Meade telescopes. Whatever your needs might be, we can
provide the necessary optics.
We also provide a
series of inexpensive models for high-energy experiments in which the
optical probe must be expendable.
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Photodiode Modules
 The optical output signals from
the FDVI are converted to electrical signals by M, F & A Series G 10
GHz InGaAs photodetectors. These convert the signal at the last
possible moment, right at the input connectors of the scope or data
acquisition system, eliminating all of the disadvantages of co-ax cable.
They have a built-in load resistance chosen to maximize the sensitivity
while keeping the response time at least as fast as the inherent response
time of the interferometer itself.
High impedance
connections are recommended, since there is no co-ax cable. The traditional
50 ohm termination to suppress cable reflection is unnecessary and will
reduce signal levels substantially.
Other Series G modules
are available with a wide range of gain, bandwidth amplifier and connector
options. Contact us if your experiment requires upgraded photodetectors.
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Verification of Proper System Operation
Because of its unique
design, proper operation of the All Fiber Doppler Velocity Interferometer
System can be verified without having to set up and execute an experiment
with a moving surface. Instructions for ensuring that each component is
working correctly is described in the manual that accompanies the FDVI.
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Multiple Channel Systems
Viewing more than a
single spot in an experiment requires one complete FDVI All Fiber
Doppler Velocity Interferometer System as listed above and one Data
Acquisition System for each spot viewed. To view four spots, for
example, four FDVIs and four oscilloscopes are required. The experimentor
will need to arrange simultaneous triggering of these four oscilloscopes.
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Component Specifications: FDVI Mark IV
Lasers
Laser
Type:
Semiconductor Diode, Distributed Bragg Reflector Stabilized,
Thermo-electrically cooled
Laser Output Power: up to 10 mW continuous
Operating Wavelength: 1550 nm
Output Linewidth: not more than 20 kHz
Visible Laser Wavelength: 850 nm
Visible Laser Power: 3 mW
Interferometer
Delay Leg Length1:
200 mm, 1 m, and 10 m standard, interchangeable. Others available.
Resolvable
Velocity Range:
from less than 0.1 meters/sec to essentially unlimited
Time Resolution1,2: as low as 0.05 ns
Power: 6 volt Latest Technology NiMH
rechargeable battery
Charging System: automatic recharger which runs from any 90 to 240v,
50 to 60 Hz, AC source
Compatibility: fitted with IEC standard entry module and power cord
to suit destination country
Dimensions: 1.6 by 7.3 by 11 inches
Weight: approximately 3 lbs
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Component Specifications Mark IV-3000
Lasers
Laser Type:
Semiconductor Diode, External Cavity Stabilized, Thermo-electrically cooled
Laser Output Power: up to 10 mW continuous
Operating Wavelength: 1550 nm
Output Linewidth: less than 20 kHz
Visible Laser Wavelength: 850 nm
Visible Laser Power: 3 mW
Optical Amplfier
Optical Amplifier
Type:Erbium Doped Fibre Amplfier (EDFA)
Pump Wavelength:980 nm
Optical Gain: +30 dB
Maximum Output Power: +15 dBM
Interferometer
Intrinsic
Built-In Delay Length: -0.2m
Addition Fiber Delay Lengths: 1.2 m and 10.2 m
Net Delay Leg Lengths: 0.2 m,
1.0 m, and 10.0 m
Delay Leg Index of Refraction: 1.4440
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Front Panel Controls:
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Keyswitch: Enables laser system
Digital Battery Voltage Monitor
“Charge” illuminated toggle for
charging system
“Pilot” illuminated toggle for
visible laser
“Laser” illuminated toggle for
main 1550 nm laser
“EDFA” illuminated toggle for
optical amplifier
“Ramp” illuminated toggle turns on
ramp generator to simulate velocity changes
Laser Power: 10 turn counting dial
Balance:
Quadrature: 10 turn counting dial
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Front Panel FC/APC Connectors:
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Laser Output to Experiment Probe and return
Output 1 - varies as sin of velocity
Output 2 - varies as -sin of velocity
Output 3 - varies as cosine of velocity
Output 4 - varies as -cosine of velocity
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Other
Power: internal 6 volt Advanced
Technology Nickel Metal Hydride rechargeable battery
Charging System: internal, 90 to 240 v, 50 to 60 Hz AC
Compatibility: IEC standard filtered
power entry module and power cord to suit destination country
Dimensions: approximately 1.6" x 17" x 12," Standard
1U Rack Mount
Weight: approximately 3 lbs
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FDVI System
Prices
Prices for purchase
directly from Martin, Froeschner & Associates follow. For orders with
international end users, see INTERNATIONAL ORDERS.
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FDVI
Mark IV All Fiber Doppler Velocity Interferometer System
All Fiber Delay
Leg Interferometer with 200 mm, 1 m and 10 m delay legs and +sin, - sin,
+cos, and -cos outputs
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10 mW Fiber Coupled
High Power, High Purity, Tunable Diode Laser System with Optical
Circulator and Visible Red Pilot Laser
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Fiber Coupled
Universal "C" Thread Experiment Probe
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Fiber Coupled Photodetectors,
Series G,10 GHz InGaAs
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Technical Manual
& Software including Source Code in "C" compatible Fortran
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Complete Mark IV
System Price
including all of the above components
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US$
45,995
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Special
Discounted Price on Mark IV System
to universities, educational institutions, and other nonprofit
organizations not engaged in development or production of weapons of mass
destruction
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US$
40,995
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FDVI Mark IV-3000 All Fiber Doppler Velocity Interferometer
Full Quadrature Push Pull System
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All Fiber Delay Leg
Interferometer with 200 mm, 1 m and 10 m delay legs and +sin, - sin,
+cos, and -cos outputs
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10 mW Fiber Coupled
High Power,Ultra-High Purity, Tunable Diode Laser System with Optical
Circulator and Visible Red Pilot Laser
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Erbium Doped Fibre
Amplifier with 30 dB gain
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Fiber Coupled
Universal "C" Thread Experiment Probe
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Fiber Coupled
Photodetectors, Series G,10 GHz InGaAs
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Technical Manual
& Software including Source Code in "C" compatible Fortran
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Complete Mark
IV-3000 System Price
including all of the above components
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US$
49,995
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Special
Discounted Price on Mark IV-3000 System
to universities, educational institutions, and other nonprofit
organizations not engaged in development or production of weapons of mass
destruction
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US$
44,995
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Specifications and prices subject to change without notice.
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An older FDVI Mark IV System