Contains research objectives and reports on three research projects.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-0300U. S. Air Force Office of Scientific Research (Contract F44620-71-C-0051)Naval Air Systems Comman

Dobbels, M. A.

Ezekiel, Shaoul

Hackel, Lloyd A.

Ryan, T. S.

Stolle, E. M.

Sullivan, J. P.

Youmans, Douglas G.

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XII. LASER APPLICATIONSAcademic Research StaffProf. S. EzekielGraduate StudentsL. A. Hackel J. A. Monjes J. P. SullivanP. D. Henshaw T. J. Ryan D. G. YoumansJ. W. StafurikRESEARCH OBJECTIVESOur interest is primarily in the application of lasers to a variety of measurementproblems. In certain cases, the available lasers are not too suitable and considerableresearch has to be done to improve or modify the laser performance. Several projectsare in progress.1. Laser Frequency StabilizationThe motivation for this work stems from the need for a long-term frequency stabi-lized laser in the precision measurement length, i.e., length standards. Such a laserwould find applications in earth strain seismometry, optical communication, and fun-damental measurements, in particular, those related to experimental relativity.The task, at present, is to stabilize the frequency of the 5145 A argon ion laser.The stabilization scheme employs a resonance absorption line, observed in a molecularbeam of iodine, which we have recently uncovered, as an absolute frequency reference.The absorption is measured by the resonance fluorescence induced by the laser in themolecular beam. Such an absorption line is an ideal reference element, because of theisolated and unperturbed conditions in the beam. Moreover, since the molecular beamcan be oriented at right angles to the laser beam, the width of the iodine resonance islimited to its natural width which is inferred to be 50 kHz from lifetime measurements.Preliminary experiments indicate that it should be possible to stabilize the frequencyof the laser to one part in 1013Our major effort thus far has been the improvement of the short-term stability ofthe argon laser by suppression of plasma oscillations and noise in the laser plasmatube. This has enabled us to get a high-resolution absorption spectrum of 12 in amolecular beam. The measured width of the individual 12 lines is less than 10 MHz[Av/vZ 10-8], which includes approximately 6 MHz of Doppler broadening, because ofthe geometrical angular width of the molecular beam. Work is in progress to narrowthe observed I2 linewidths and also to lock the laser frequency to one of the iodineresonances.2. Pulsed Ion Laser HolographyThe purpose of this research is to investigate possibilities for generating pulsed,high-power, multicolor laser output by using a mixture of noble gases, such as argon,krypton and xenon, for interferometric and holographic applications.This work is supported by the Joint Services Electronics Programs (U. S. Army,U.S. Navy, and U.S. Air Force) under Contract DAAB07-71-C-0300), and by theU. S. Air Force Office of Scientific Research (Contract F44620-71-C-0051).QPR No. 104 139(XII. LASER APPLICATIONS)For pulsed color holography, a laser is needed that simultaneously puts out threeprimary colors of equal intensity in one short pulse. This work involves the study ofexcitation mechanisms in the presence of a mixture of noble gases to yield the desiredlaser output.Aside from color holography, a hologram that is recorded with two colors (or fre-quencies) and reconstructed with only one color displays contour lines (lines of equaldistance from the hologram plate) superposed over the surface of the holographic image.The spacing between contour lines, which represents surface changes, is proportionalto the frequency separation of the two colors used in the recording of the hologram.For this type of application, an investigation is being conducted into schemes for gen-erating two-frequency laser output where the frequency separation can be varied. Ulti-mately, we would like to use this surface contour generation method, which requiresonly a single short exposure, to study dynamic behavior of structures and materials andalso for growth measurement in biological subjects.A pulsed ion laser has been constructed with sufficient energy (~ 10 mJ) in one pulse,(single frequency and single color)to make a good quality hologram. By use of two dis-persion prisms, a scheme for selecting various laser colors with the minimum of losseshas been developed. Multicolor holography is in progress.3. Flow MeasurementsLaser Doppler techniques are being developed for measurement of both low-speedand high-speed fluid flow. We have a two-dimensional heterodyne scheme for mea-suring velocity distribution in vortex rings. Later this scheme will be used in the studyof helicopter rotor-tip vortices. For high-speed flow greater than 103 cm/s a Fabry-Perot cavity is used as a frequency discriminator.4. Closed-Loop Holographic InterferometryWe are investigating a new type of holographic interferometer. A diffuse object hasbeen locked (within tens of angstroms) to its holographically stored virtual image bymeans of a servo loop. This technique is being explored for application to sensitivedisplacement detection using diffuse surfaces, and for measurement of monolayers.S. EzekielA. HIGH-RESOLUTION LASER-EXCITED SPECTRUM OF 12MOLECULAR BEAMA high-resolution spectrum of 12 has been excited by a tunable single-frequency5145 A argon ion laser. This is part of our effort at long-term stabilization of thefrequency of the 5145 A argon laser by locking it to an I1 resonance observed in a1, 2molecular beam.Figure XII-1 is a schematic diagram of the apparatus. A single-frequency cw argonion laser oscillating at 5145 A excites a molecular beam of iodine at right angles. Thelaser-induced resonance fluorescence of 12 is monitored on a photomultiplier. The fre-quency of the laser is tuned across the bandwidth of the laser gain medium to revealthe 12 structure shown in Fig. XII-2. The width of the individual iodine lines is lessthan 10 MHz, which includes at least 6 MHz of Doppler broadening because of theQPR No. 104 140-0-SLITSSBEAMOSCILLATING BEAM FLAGSINGLE FREQUENCY/" 5145 A ARGONION LASERFig. XII-1. Diagram of apparatus.FREQUENCY- - 40 MHzFig. XII-2. Resonance fluorescence of 12 molecular beam inducedby tunable single-frequency 5145 A argon laser.QPR No. 104_ .__ _ __C-141(XII. LASER APPLICATIONS)geometrical angular width of the molecular beam. The natural width of the 12 lines is~100 kHz, inferred from a lifetime measurement (3 is) previously performed in thebeam.1 The discrepancy between measured width and natural width, aside from geo-metrical Doppler broadening of the beam, is being studied. At present, the major cause offrequency jitter of the laser is acoustic noise. Coherent plasma oscillations have beensuppressed.3 An analysis of the 12 spectrum will be the subject of a future report.Work is now under way to lock the laser to the center of one of the 12 resonances.An effort is also being made to observe the "Lamb dip" in the fluorescence at the centerof the line when the molecular beam is subjected to a standing-wave field instead of atraveling-wave field. The width of the dip is approximately that of natural width, andit would be extremely useful for frequency stabilization.T. J. Ryan, D. G. Youmans, L. A. Hackel, S. EzekielReferences1. S. Ezekiel and R. Weiss, "Laser-Induced Fluorescence in a Molecular Beam ofIodine," Phys. Rev. Letters 20, 91 (1968).2. S. Ezekiel and R. Weiss, Quarterly Progress Report No. 88, Research Laboratoryof Electronics, M.I.T., January 15, 1968, pp. 61-66.3. D. C. Galehouse, U. Ingard, T. J. Ryan, and S. Ezekiel, "Investigations of CoherentOscillations in an Argon Ion Laser Plasma Tube, " Appl. Phys. Letters 18, 13 (1971).B. TWO-DIMENSIONAL LASER DOPPLER VELOCIMETER FORFLUID FLOWTraditionally, the velocity of a time-variant flow field has been measured by usinga hot-wire anemometer. The use of a hot wire in the wake of a helicopter poses twoSPECTRUMANALYSERPERIODICALLY PRODUCEDVORTEX RINGSAMPLIFIERMEASURING POINTLASER i PHOTO-15 mW He Ne LASER MULTIPLIERPRISM TYPE PINHOLE APERATURESEAM SPLITTER MASKS TOBLOCK UNSCATTEREDBEAMS FROM PMLOUD SPEAKER AMPLIFIER G NAFig. XII-3. One-dimensional dual scatter LDV system.This work was supported in part by the Naval Air Systems Command.QPR No. 104 142Fig. XII-4. Two-dimensional LDV system.Fig. XII-5. Vortex ring.Fig. XII-6. Velocity distribution through vortex ring.QPR No. 104- -- ~ -~t-y - --143(XII. LASER APPLICATIONS)serious problems: probe interference, and calibration in a flow field where the flowdirection is constantly changing. In order to avoid these problems a laser Dopplervelocimeter (LDV) has been constructed.The LDV, introduced by Yeh and Cummings measures the Doppler shift of laserlight scattered from small particles (smoke, dust, polyethylene spheres) which followthe flow field.The actual LDV system constructed is a dual scatter or fringe system. 2 ' 3 A one-dimensional dual scatter system is shown in Fig. XII-3. The laser beam is split intotwo parallel beams and then focused together. A particle traveling through the inter-section of the two beams scatters light of frequency fl from one beam, and of f2 fromthe other. The photomultiplier detects the difference or beat frequency of the scatteredlight, which is2U sin 0/2f =where U is the velocity component perpendicular to the bisector of the two beams (seeFig. XII-3); Xo laser frequency = 6328 A; and 0 is the angle between the two beams.The system is also called a fringe system, since a set of interference fringes is formedat the intersection of the two beams. The particle scatters from a bright fringe, movesinto a dark fringe, then scatters from a bright fringe, and so forth.A two-dimensional system has been constructed by adding another beam splitter toform 4 parallel beams. A photograph of the actual system is shown in Fig. XII-4. Thissystem has been used to measure the velocity distribution in periodically produced vor-tex rings (smoke rings). The rings are similar to a helicopter rotor wake becauseregions of time-variant concentrated vorticity are produced yet the flow field is simpler,since it is axisymmetric. A picture of a vortex ring is shown in Fig. XII-5 and the cor-responding velocity distribution in Fig. XII-6. The velocity curves are the U and Vcomponents (Fig. XII-5) of velocity vs time as the vortex ring travels by the fixedmeasuring point. There is a 1800 ambiguity in the velocity vector because frequencyis always positive. Flow visualization indicates that the U component is always posi-tive, while the V component is positive to the left of the central zero and negative tothe right.The measurement of the velocity distribution in the vortex rings demonstrates thecapability of measuring fluid velocities in flows containing time-variant concentratedvorticity. In the future the two-dimensional LDV system will be used to make veloc-ity measurements in the wake of helicopter rotors.J. P. Sullivan, S. EzekielQPR No. 104 144(XII. LASER APPLICATIONS)References1. Y. Yeh and H. Cummins, Appl. Phys. Letters 4, 176-178 (1964).2. M. J. Rudd, J. Phys. E. 2, 55-58 (1969).3. A. E. Lennert, E. B. Brayton, and F. L. Crossay, Summary Report of the Develop-ment of a Laser Velocimeter to be Used in Arnold Engineering Development CenterWind Tunnels, AEDC-TR-70-101, July 1970.C. CLOSED-LOOP, REAL-TIME, HOLOGRAPHIC INTERFEROMETRYUsing a real-time holographic interferometric arrangement as shown in Fig. XII-7,we have been successful in locking the real-time holographic image to the real objectby means of a feedback loop. In this way, a displacement of the surface of the realobject may be monitored with extreme sensitivity. Our preliminary results indicateFig. XII-7. Real-time holographic arrangement.that it is possible to detect displacements of the order of 10 A, and it is not unrea-sonable to expect that this number can be reduced by several orders of magnitude byfurther refinements of our scheme.The real-time holographic interference pattern is imaged by suitable optics onto thephotocathode of a photomultiplier. Any displacement of the object (an aluminum plate)with respect to the holographically stored image moves the interference pattern. ThisQPR No. 104 145(XII. LASER APPLICATIONS)fringe-pattern movement is detected and used in a feedback loop so as to null the originalfringe displacement. The feedback signal needed to maintain the fringe pattern at nullgives a measure of the object displacement.LOOP LOCKED ON MAXIMUMLOOP LOCKED ON MINIMUM CONTROL LOOP OPEN(a) (b) (C)Fig. XII-8. Closed-loop performance of the preliminaryfeedback loop.Figure XII-8 shows the performance of the preliminary feedback loop. Figure XII-8aand XII-8b shows the outputs of the photomultiplier when the feedback loop is lockedto the maximum and the minimum fringe intensities, respectively. Figure XII-8c showsthe output of the photomultiplier without the feedback loop, indicating the relative driftbetween the object and its stored image. The control element was a piezoelectric crys-tal mounted either behind the object or behind one of the mirrors in the reference beamas shown in Fig. XII-7.Applications of this technique are being investigated for (i) measurement of thin-filmdeposition, (ii) gas-surface interaction studies, (iii) sensitive displacement detectionwith diffuse objects, and (iv) direct measurement of vibrational amplitude and frequency.Extension of this technique for measurement of small rotation and for 3-dimensionaldisplacement is also being explored.M. A. Dobbels, E. M. Stolle, S. EzekielQPR No. 104 146

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