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All Fiber Doppler Velocity Interferometer Systems, a new kind of VISAR |
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Martin, Froeschner & Associates All Fiber
Doppler Velocity Interferometer (VISAR) Systems
updated: March 2010
Papers by Physicists Using our FDVI
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The new M, F & A Mark 4 All Fiber Doppler Velocity Interferometer System offers enhanced capabilities in the measurement and recording of instantaneous velocity versus time histories. Accelerations and vibrations of almost any surface can be tracked with unprecedented ease and accuracy. Operating on the same principles as the VISAR, but with no requirement for alignment and maintenance of discrete optical elements, the FDVI Mark 4 delivers turn-key reliability, sub-nanosecond time resolution, and with delay leg lengths of up to 30 meters, can resolve velocity changes as small as 0.1 metres per second. New features include full quadrature, push-pull configuration for unambiguous analysis even with dynamically changing background light and a co-axially fiber coupled visible laser source for ease in target spotting. Light from the systems internal high purity laser is focused onto the target by a 5 to 50 mm efl variable focus lens and the reflected Doppler-shifted light collected by the same lens and returned by the same fibre to the interferometer for analysis. Other optical probes are available, including microscopic, telescopic and inexpensive expendable versions optimized for a range or working distances. The system is all solid-state, requires no external laser or associated systems and is powered by internal long-life batteries, maintained by a universal international compatible charging system.
Super Accurate Measurement of of the Velocity of Any Moving Surface Interferometer / Laser Specifications 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 Laser Type: Semiconductor Diode, Distributed Bragg Reflector Stabilized, Thermo-electrically cooled Laser Output Power: up to 15 mW continuous Operating Wavelength: 1550 nm Wavelength Stability: better than 2 parts per billion Output Linewidth: less than 100 kHz
Charging System: automatic recharger which runs from any 90 to 240v, 50 tor 60 Hz, AC source Compatibility: fitted with interchangeable US, European, English and other connectors Dimensions: 1.6 by 7.3 by 11 inches Weight: approximately 3 lbs Note 1. The Delay Leg Length directly determines the fringe constant, time resolution, and velocity resolution. See Resolution, Applications, and Options. Note 2. The time resolution will also be affected by the temporal resolution of the data acquisition system.
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. The instantaneous frequency of the light, and thus the velocity of the observed surface, is derived from changes in quadrature of the relative intensity of two optical signals within the interferometer. These intensities are recorded by fast photodiodes in a data acquisition and analysis system. Velocities can be determined with accuracy of ±1% (plus or minue1%) or better, in the range from a fraction of a meter/sec up to thousands of km/sec. With this technology there are no limitations on maximum velocity or response time as in the class of devices known as "vibrometers". Changes in velocity over thousands of m/s can be recorded 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. 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 which preserves the principles of the original delay leg interferometer but realizes it entirely in fiber. There are no free space beams to align, no beamsplitters to position, no fiber to beam couplers to fiddle with. Nothing to tune, tweak, hunt for, optimize or adjust. Nothing at all. Ever. Our Full Quadrature FDVI Mark 4 Push Pull System is completely realized in fiber, inside and out. Prices for purchase by US customers follow. For international orders, prices, and payment, see INTERNATIONAL ORDERS. FDVI Mark 4 All Fiber Doppler Velocity Interferometer
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. 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 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. 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. 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(S(t), C(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 S(t) and C(t) which are the two time dependent signal intensities from the interferometer, properly corrected for offset and background and normalized to the unmodulated light intensity reflected from the experiment and recorded by the third Reference signal. 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. Each FDVI System consists of everything you need except for a digital data acquisition system or oscilloscope and a simple PC. The Mark 4 FDVI Push Pull 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. Our new FDVI Mark 4 Systems are now in full- quadrature, push pull. This 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 now available in our all-fiber FDVI's as well. The benefits of the "push-pull" configuration are: The principal benefit 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" + b.g.) - ("-sin" + b.g.) = 2 sin x = ("cos" + b.g.) - ("-cos" + b.g.) = 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. The only difference in operation is that the new push-pull FDVI system has four output channels and four Series "G" photodetector modules, rather than the three of the standard system. Resolution, Applications, and Options 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 and shorter lengths produce higher time resolution. This is convenient since experiments involving high velocities generally occur on short time scales. The resolution in velocity depends on how well we can measure small portions of a fringe shift which depends principally on the noise level and linearity of the data recording system. Our experience is that shifts on the order of 1/1000th or 0.1% of a full fringe are achievable. This is shown as vres in the table below. The maximum velocity resolvable is limited only by the number of fringes which can be resolved within the record length of the data acquisition system. Typical systems have record lengths of several kilo- to several megabytes. This leads to the 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. 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. 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.
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.
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. 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. |
Martin, Froeschner & Associates
14300 Mines Road, Livermore, California, 94550
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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 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.