in o 0‘ p v“.- .oa N.» a h.“ “.5. . - u. \ r . n a \ .... . . . \ ... c. O . . . . . . Q g .. a. wQ-p‘o or... A w; u \ S .. .. .u to n o o‘. ‘ o n ‘0‘ Au ‘ I »‘ ‘ o . Pr “w ~ -!o I? . f \ a . .U I J 1" z , .44- .. ’4 2’: '3: ‘4. I .‘-'.n M! ' .- ‘6 o- c 's' INIUJHINHHIHIINWINIHWWIIIJNIUIHHIHIIW 301764 0107 r A METHOD FOR INCREASING ThE ACCURACY OF INTENSITY MEASUREMENTS OF LIGHT DIFFRACTED BY ULTRASONIC WAVES BY John Hente Eggle A THESIS Submitted to the College of Science and Arts of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Physics 1956 Fifi'SICSn-i :1 ; L; .1 THESIS ACKNOWLEDGEMENT I wish to express my sincere thanks to Professor E. A. Hiedemann for suggesting this problem and for his guidance in developing it. {3“ w W A METHOD FOR INCHEASING THE ACCURACY OF INTENSITY MEASUdEMENTS OF LIGHT DIFFRACTED BY ULTRASONIC WAVES By John Hente Nagle AN ABSTRACT A THESIS Submitted to the College of Science and Arts of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Physics 19 56 Approved 3 A gig-MW A Method for Increasing the Accuracy of Intensity Measurements of Light Diffracted by Ultrasonic Waves. John W. Nagle Abstract Optical methods based on ultrasonic effects may be used to make quantitative measurements of elestic censtants and losses for transparent materials. Light passing through ultrasonic sound fields set up within the material under investigation will undergo diff- raction. Intensity and position measurements of the re- sulting diffraction pattern will yield information which makes it possible to determine the properties mentioned above. Errors caused by light scattered or refracted into the intensity measuring device are avoided by modulating the ultrasonic wave within the material, and then receiving selectively by means of a photo multiplier, only the compoa nent of the diffraction pattern which is modulated. TABLE OF CONTENTS IntrOdUCtion coco...oooeooeoooooooooooooocoooooooooooooe 1 Experimental Apparatus ecooooooeoooooooopoeeo00000000000 3 COHCIUS10n coco-o00.0000000000000000.0000000000000000... 16 Bibliography OOOOOOOOOOOOOOIOOOOOOOOOOOOOOOOOOCCOOOOCOOC 19 11 TABLE OF ILLUSTRATIONS Block Diagram of Ultrasonic Generator .................. 3 Power Amplifier ........................................ 6 High Voltage Power Supply .............................. 7 Modulator .............................................. 10 Photomultiplier and Pro-Amplifier ...................... 12 SBlGCtive Amplifier 00.00000oo0000000000000000000000000. 15 iii INTRODUCTION In transparent materials, optical methods based on ultrasonic effects may be used to make quantitative measure- ments of such properties as elastic constants and losses. By means of an ultrasonic generator, standing or pro- gressive waves of sound may be produced in a substance under investigation. Light passing through these sound fields will suffer diffraction in a manner similar to light dif- fracted by an optical grating. Measurements of the inten- sities and positions of the resulting diffraction patterns will yield information which will lead to the determination of the propertities mentioned previously. For measurements of the intensity of the light dif- fracted by ultrasonic waves, one must avoid errors caused by light scattered or refracted into the light measuring device. An effective way to exclude all other effects than those produced by the ultrasonic waves is to modulate the ultrasonic frequency and to receive selectively only the output of the photocell detector which is at the frequency of modulation. The development of an experimental set-up appropriate for such measurements is the object of this thesis. By means of an amplitude modulated ultrasonic generator, the amplitude of the sound field within the material under investigation is varied between zero intensity and a maximum intensity at a frequency in the audio range. This in turn modulates the light intensity in the resulting diffraction pattern with the same frequency. The portion of the dif- fraction pattern which is to be measured is then allowed to fall upon the face of a photomultiplier which will con- vert the light signal into an electrical signal, for which the voltage output will be proportional to the light inten- sity incident upon the photomultiplier tube. At this stage, the signal will have a component modulated with the fre- quency of modulation, this being the desired signal. In addition, the signal may have other components which cor- respond to the optical background. This composite signal is then fed through an ampli- fier which is tuned to pass only frequencies in the im- mediate neighborhood of the original modulation frequency and to reject all other frequencies. Thus, only the signal produced by the modulated ultrasonic wave field will be passed and those frequencies caused by the various forms of noise will be rejected. Direct measurements of the signal passed are then made with the aid of an alternating current vacuum tube voltmeter. EXPERIMENTAL APPARATUS In the following discussion, the equipment used in the unit mentioned in the introduction will be described in detail after a discussion of the more important factors which had to be taken into consideration in the design and construction of the apparatus. The block diagram shown below is presented to help illustrate the relationship between the respective com- ponents as they are incorporated in the production of a signal produced by the low frequency modulation of a high frequency generator. So-‘n‘J Powcv Match:- . Trims Auscv 7 Geauflov Ah'hiscv Cure.» H 1 Madonna? P. w” The High Frequency Power Amplifier. A very important part of the apparatus is the high frequency power amplifier which, in conjunction with an imp pedance matching device, delivers an alternating current sig- nal to the transducer. The power amplifier used in this circuit consists of two stages: a buffer stage, followed by a power amplification stage. The buffer stage is excited at the desired frequency by means of a Heathkit Radio Frequency Signal Generator which has a frequency range of from .160 to 220 megacycles. The buffer stage itself is a plate tuned voltage amplifier which incorporates a 6V6 pentode vacuum tube. This stage is coupled to the power amplification stage by means of a coupling condenser connecting the plate tank circuit of the eve to the grid circuit of the power amplifier stage. The power amplification stage incorporates a single blhbl power pentode vacuum tube which is likewise tuned in the plate circuit by means of a L/C tank circuit. The inductance coils used in both tank circuits are of the plug-in type which are used in most amateur radio transmitters and their values may be interchanged in order that the proper frequencies of oscil- lation may be secured. The club, when used in such an application as mentioned above, is found to have the following recommended zero exci- tation voltages and currents: for a plate voltage of 750 volts, screen voltage of 120 volts and a grid bias voltage. of - 45 volts, the plate current should be of the order of 120 milliamperes. Operating a this plate voltage the amplifier may be operated for intermittent periods only. When operated continuously or when it is plate modulated, which it is in its present application, the plate voltage must be reduced to a minimum of 600 volts under which condition the plate current will be reduced to 110 milli- amperes. The impedance matching circuit which is used in con- junction with the power amplifier is tYFical of hose used in such matching applications.2 By varying the inductance and capacitance of the matching circuit, the output imped- ance of the power amplifier may be matched to the input im- pedance of the transducer and hence a maximum of power trans- mission to the transducer is accomplished. Filtered voltage is supplied to the amplifier by means of a direct current power supply which is capable of de- veloping a peak voltage of 750 volts with a current rating of 200 milliamperes. Output voltage is controlled by means of a V-2 Variac in the primary circuit of the power trans- former. Plate voltage for the buffer stage is regulated at 300 volts by means of two voltage regulator tubes. Amplitude Modulation and the Modulator. Amplitude modulation3 is produced by varying the ampli- mmijmzd. «M39... 4: 85¢- or .054! ct _O.Ifil 91m m>m v5 .[ [ .I'-| ‘ “T , 0"l'fl ‘ 14¢ teams e352: _II........IJ_ F [iii] 3233:. _o w _ «208:0 .. - a, H H .. V cannon _ w e . a. a. xoml—l Eel—l a _ at or 524282 :05 ._ uo>oom m0 IE: >Jmn—am union mes—.49, :9...- 0003 EFL—hug coo} % (SOON o<.¢<> <2 00 oo>o mam >o¢mm w__ml\| +0. 00> och <2 CON :0. 0mm»; oo>o~l mSm _ 4 LHO. m; Um ._ 21', 4| 1! = 05V o o ' | 1 <29... Ioé - NOD D-IOO I It incorporates two 807 pentode power amplifier tubes oper- ating in class ABl. A 60h driver stage is excited by means of an audio frequency oscillator, and it is coupled to the negatively biased grids of the 807’s by means of an inter- stage audio transformer of turns ratio 3:1, total secondary to primary. Such a modulator unit is capable of delivering ho watts of undistorted power to the high frequency power amplifier. A universal modulation output transformer was used in the circuit in order that the voltage at the plate of the modu- lated amplifier could be adjusted to vary between zero and twice the D.C. operating voltage. Photomultiplier Circuit and Pre-Amplifier. As was mentioned in the introduction, a photomultiplier circuit is used to measure the relative intensities of the diffraction patterns produced by passing light from a slit source through the ultrasonic wave field. For the circuit dlosen, either a 931-a or a IP21 phOtomultiplier tube may be used, with the latter being the more sensitive of the two light detectors.5 The high voltage power supply used to supply voltage to the dynodes of the photomultiplier tube is capable of pro; ducing a potential drop across the tube of 900 volts for maxi- mum sensitivity. By means of a one megohm variable resistance which is in series with the photomultiplier tube, the negative 11 mesh mm_u_.._n_2ooml A . ms. 39% Man. .3 N. .3 oo>om I. no. u 3H J, ‘ ‘ 'voltage across the tube may be reduced by 200 volts, thereby decreasing the sensitivity of the photomultiplier circuit. The signal from the photomultiplier circuit is fed directly through a coupling condenser to a high gain pre- amplifier stage. A 5879 pentode voltage amplifier vacuum tube was used in this stage because of its low noise charac- teristics and its capacity for high gain amplification with a minimum of distortion. Selective Amplifier. The amplified signal from the 5879 pre-amplifier tube is fed directly into a selective amplifier which has the characteristic of passing only a narrow band of frequencies in the neighborhood of the modulation frequency. Frequencies resulting from background Optical ”noise” in the ultrasonic sound field and from noise in the electronic components are thus rejected. The circuit used in this application is quite similar to what is known as the "Selectoject.”u Although this amplifier does not have as narrow a band pass (it has a band width of approxbmately 50 cycles for a resonant frequency of 1000 cycles) as does a narrow band amplifier using a twin T network, it does have the great advantage of being capable of being tuned to any desired frequency between 80 and 10,000 cps. The output of the selective amplifier is fed directly in- to an A.C. vacuum tube voltmeter with which measurements of 13 the output voltage can be made and hence magnitudes of the light intensities of the diffraction determined with respect to a standard. 11+ a 4. 6:25 3.53% when”... 5:2 oo¢\e. .... one}: -u h; 03 .. £6 v-N.N ED. X@.D ZN v: 2- _ 2N1 In L FII— WL .3 .lllllllL 'I'llll ll'nl I'UI Tl.” l_.| :22 Jl. :22 In. 5. ..~ooo. .. r ._. _ «eon—H a xem xeuw QQNCLUSION The apparatus discussed in the previous section was used in the investigation of diffraction patterns resulting from ultrasonic waves in a liquid and in a solid medium. working with a liquid, Xylene, it was found that the modulation of the ultrasonic waves could be produced with little difficulty, and that as many as ten diffraction orders of good intensity could be observed when working with waves of three megacycle frequency. When the modulated signal from the photomultiplier was passed through the narrow band ampli- fier, it was found that the amplifier essentially passed only the component of the signal at the modulation frequency. Also, it was found that fliere was a 120 cycle component of a lesser magnitude which was also passed. The presence of this 120 cycle component is attributed to the Mercury Vapor light source which was being operated on 110 volts alternating cur- rent. Hence, the light source was in effect being modulated at 120 cycles. Although the narrow band amplifier was tuned at 1000 cps, it was found that the 120 cps modulation of the light source was too great to be completely blocked by the amplifier and thus part of it was passed, with the result that a signal to noise ratio of the order of ten to one was present. Working with glass, it was found that a modulated dif- fraction pattern could again be produced with standing waves of ten megacycle frequency, although only two diffraction 16 orders of very low intensity could be observed. This may be partially attributed to the use of a much.weaker light source than had been used with the liquid. In an effort to eliminate the 120 cycle component which was encountered in the observa- tion on a liquid, a 500 watt filament projection lamp oper- ating on 110 volts direct current was used as a light source. Upon filtering this light to pass a single frequency, it was found that the intensity transmitted was extremely low. This resulted, in turn, in a very low level signal output from the photomultiplier and once again the signal to noise ratio from the amplifier was not as good as could be desired. In an effort to produce a better signal to noise ratio in the measurements, the following additions to the present experimental apparatus will be incorporated: A direct cur- rent Mqury Vapor lamp will be used instead of the present light source. If power is supplied to this lamp from a bat- tery, an intense monochromatic light source of constant in- tensity will be available with the result that there will no longer be a noise component due to the modulated light source, which caused the reported difficulties. In addition, it will be necessary to replace the narrow band amplifier now being used in the detection circuit by another which will be tuned to a single frequency and which will have a much narrower frequency band pass than that exhibited by the present ampli- fier. l7 With the addition of these two pieces of equipment to the present experimental apparatus, the signal to noise ratio should be improved so much that a higher accuracy in the measurements of the intensity of light diffracted by ultra- sonic waves will be obtained. It is expected that this gain in accuracy will make it possible to study such problems of the theory of the diffract- ion of light by ultrasonic waves, for which the previous methods were insufficient. 18 1. 2. 3. h. BIBLIOGRAPHY ,'RCA Tube HandbOOk”, HB‘B, v01. 7‘80, Radio Corp- oration of America, Harrison, New Jersey (195k). Hueter, T. F., Bolt, R. H., "Sonics”, John Wiley and Sons, New York (1955). Seely, 8., ”Electron-Tube Circuits”, McGraw-Hill, New York (1950). ”Radio Amateur’s Handbook9’, 33rd ed., ARRL, West Hartford, Conn. (1956). Gault, R. L., ”Polarization Effects in Optical Diffraction” Master’s Thesis, Michigan State College (1953) 19 "IfiflqlijfijlgfifliflgiytfiimnuIlium “ 40107