US2449166A - Microwave-acoustic light valve - Google Patents

Description

Sept. i4, i948. w. D. HERSHBERGER 2,449,155

MICROWAVE-ACOUSTIC LIGHT VALVE original Filed June 15, 1944 IN V EN TOR.

Patented Sept. 174, 194g ear Nr fer-rca MrCRoWAvE-TACQUSTIC ltidlrrVALVEN William-RHrshbergei', Princeton; N EL, assigner# to Radio-*Corporatiom of America; a: corporation Originalapllicatinzun 1`5,'19'44;. SerialNo." 540,429.1' Dividediand this"applicationMay'14, p

isolaimsm (c1. nefs-masi..

1f, Thisfapplication-is .a divislorrroifmy,copengdihg: application Ser.- No. ,54Q,l29,;;;i1e.dz: June .15, :51944;- entitled Microwave acoustic1i wave-1 translatonfr" and assigned tOzthe samefassigneeas ltheinstant';

application. 54,

Thisini/entionerelatesggenerally;o-.:microwave#y transmission ;,and molegparticularly;tig-improved; mtl'lOds of and means or lgeneratinggacoustic: waves -v responseA to- ;microwavexenergy, absorp.:- tiQrlsn certainV eases.,

The invention#l utilizes .-5 the'rcharacterlstics gni.: various gases. which are substantially perfect :dielectrics at most radio; frequenciesibut'which. 'ab-i sorb considerable energy .;at. ,certai then-predeestermined microwave'gfrequencies:t; Flor.f,-eiiarnpler7 .15

in-an article byCleetoniandi-WilliamsdnPhysicatf: Review 45, 234 1934.2.;observationsfon microwar absprptioniingamnionia gasdndicateddzhat radi tionfhaving-a wavelength-o1;25 p.centimete1's1:will lose approximately; 63 percent pfiitszinitial'. energyfi 20 upon passing-through: 1.-1; meters;of ammoniacY gas in.V al'nonrinetalllc container atatmospheric pres-ia.,i sureg ItA was noted-further thatfihefabsorpti'ona frequency band is relatively wide sinceezthe:fab-s.`

sorption coeicient falls toapproximately one- 25;

half of its maximunnyalue at wavelengths of 1f` centimeterY and 1.5\centimeters.i The observations described in the article identied heretofore were inspired by earlier generaltheoretic workon` tionezof :faz ligli'tfbeamz-:directed through x said light valve; Thee-variablerefractivepropertiesv :ofl they gas.; prism Yeornprisingfthe' -liglitv valve 4are-*due fto" thefzvariations in vpressure within pre'deterininerlY regions Iof' ythe enclosed microwave .absorptive gasf inresponse-to the irradiating-zmicrowaveienergy-n It is believed? that the temperature and pressure -v changes :in 'the microwave absorptivegas; due: to f selective fdissipation Ytherei-n of thef irradiatedrrni-1 'l crowave energy,V sthe result of molecular res'oiif ancefeflfectsfincidental to excitation' ofthe` energy leveisfoff-the gas molecules. The= microwave energyrabsorption in the-fgas proper fincreases'zas the gas 'pressure isfincreased."`

The instant invention depends -upon :appli-- cants observationsf that microwave absorptive gasv ff when irradiated f by modulated* microwaves. provides sonic ori-supersonic wavescharacteristicm of the microwavemodulation; Therefore; aA source-loi microwaves modulatedeataudible 'or su; personic frequencies may '.beemployed `as-an1in=i dicating device forindicating thepresenceervan ious microwave absorptive-gases since'any appref ciable quanti-ty vof said Agas l present '-Wil'l generatel .fcorresponding .Y audible or supersonic -Wavess l Another'emllodinientv -offthe instant inventioncomprises a cavity resonator'includinglamiere-1*' wave absorptive v gas and havingfsa mcrowavee opaque, (acoustic Wave-permeable window forf` gether with observations on the infr'airedfspecf-R trum ofv this vVeras-butin" all `-sucliipri'or experi# ments no attemptwas -made lto determine, *ex* plain, for'utilize the veffect-uponth'e :gas off 'micr'oe coupled to a microwave transmission system, is 40.

heated by absorbed microwave energy'tdpijovidei direct indications. of transmitted finorowavepowb erf lasy a function of the' variation `in'fgas@pressurei*- providedby the absorbed microwave-energy; YIrilr said copending appli ':ation.=k the' microwaveL al1-:MiaA

iorptive gasalso comprisesthe thermometricfmea iium whereby Vthe `gas pressuremay -be-ascer=ii1 :aimed` The. present invention-:alsof-relates \to-`appli^ :ants invention described inlhi'scopending ap 50i".

)lication Ser. No. 540,428g1-ledf Jiine` 1152" 1941i' vherein the absorption Aof microwavefenergyo redetermined frequency-by certainfigases is 'eme loyed Vto provide an adjustable ylight valvewhicli-f closedgastoa surrounding medium; Y This'- emwf bodiment ofwthe invention provides' a convenient andefrelativelyefficient loud'fspeakerf or soundY generatore wherein-the -enclosed r`microwave rabsorptivelgasis irradiatedby microwaves modu-f tions?v in tlielight transmittlng characteristics omprises a agasprism providing variable:reira'c-VIV -55-"viele'-`v an gimproved method'iof Aandtneans'Lior 'em-ii ploying modulated microwave energy to provide acoustic waves of frequencies corresponding to the microwave modulation. Another object of the invention is to provide an improved method of and means for indicating the presence of predetermined gases. A further object of the invention is to provide an improved acoustic wave generator. Another object is to provide an improved method of and means for converting supersonically modulated microwaves to supersonic waves in a desired elasticY medium.

Other objects of the invention includev an improved method of and means for modulating a light beam as a function of the modulation characteristics of a source of modulated microwaves. An additional object of the invention is to'provide an improved method of and means for making oscillographic measurements. Another object is to provide an improved method of and means for measuring microwave power.

The invention will be described with reference to the accompanying drawings of which Figure 1 is a schematic block diagram of one embodiment thereof suitable for indicating the presence of predetermined gases, Figure 2 is a cross-sectional, elevational, partly-schematic view of a second embodiment of the invention providing an acoustic wave generatonrFigure 3 is a crosssectional, elevational, partly-schematic view of ay third embodiment of the invention providing means for modulating a light beam, and Figure 4 is a plan view of the light screen forming a portion of said third embodiment of the invention. Similar reference characters are applied to similar elements throughout the drawing.

Referring to Figure 1, an eiliciennvexible, and convenient detector for microwave absorptive gases comprises a microwave generator I having an output frequency corresponding tothe absorption frequency of the gas to be detected. The microwave generator I is'modulated by any conventional audible or supersonic signal source 3 coupled thereto. The modulated microwaves derived from the generator I are applied to a conventional microwave antenna such, for example, as ardipole l which may include, if desired, a wave reflector 9.

The modulated microwave energy radiated from the antenna i is directed toward a space in which the microwave absorptive gas I! is believed to be present. 1f any appreciable quantity of said microwave absorptive gas is subjected to the modulated microwave radiation, acoustic waves I3 will be generated, and may be detected either by ear or by any known acoustic wave detecting apparatus having the required sensitivity. It should be understood lthat the microwave generator, the modulating signal source, and the microwave radiating means may be combined in a suitably portable structure to provide convenient irradiation of any desired external region.

Referring to Figure 2, the modulated microwave output of the generator I is coupled to the input of a waveguide I'I, which includes reactive tuning plugs I9, 2i, for matching the surge impedance of the waveguide to the impedance of a cavity resonator 23 which is coupled to the output end of the waveguide. A flange 25 on the output end of the waveguide I'I is bolted or otherwise fastened to a complementary 4collar Z'I on one side of the cavity resonator 23. rIhe opening from the waveguide I'I into the cavity resonator 23 is sealed by a microwave-permeable, gas-tight window 29 which is maintained under pressure against a rubber gasket 3I by means of a threaded annular ring 33. The end of the cavity resonator remote from the window 29 includes a substantially microwave-opaque, acoustic wave-permeable screen 35 clamped in a supporting frame 31 secured to the ends of the walls of the cavity resonator 23.

Immediately adjacent to, parallel with, and externally of the screen 35, is secured a flexible diaphragm 39, of suitable plastic material for transmitting acoustic Waves from the gas enclosed within the cavity resonator 23 to a surrounding medium 4I. A convenient construction for supporting the diaphragm 39 comprises a second clamping frame 43 also supported by the ends of the cavity resonator walls.

V`If it is desired to generate supersonic vibrations of a single frequency in response to corresponding supersonic modulation of the microwave source, the cavity resonator 23 may be proportioned to resonate at said supersonic frequency in Yany manner known in the art. Alternatively, the cavity resonator 23 may be proportioned to resonate to the microwave carrier frequency to provide a substantially perfect termination for the waveguide transmission system thereby providing maximum absorption of microwave energy in the microwave absorptive gas enclosed within said resonator. Two tuning plugs I9, 2|, coupled to the waveguide I'Iv, may be adjusted to match the terminating impedance of the cavity resonator 23 to the surge impedance of the waveguide Il in order substantially to eliminate wave reflections from the cavity resonator. A further modification of the device comprises a cavity resonator proportioned so that `a v plurality of overlapping microwave modes are excited therein, and also proportioned-so that no serious resonance effects are obtained at the desiredV acoustic modulation frequency.y

The cavity resonator may be rectangular, or of any other convenient shape, and if rectangular, may be excited so that its resonant frequency lfem Will be *ULS n 2 c. 2 n.: 2 a fm" 2 c +Qu where @NH3 lis the speed of electromagnetic waves in the gas enclosed within the cavity resonator, nx, ny and nz are the modal indices defining the modes of oscillation, and 1.x, ly, and Zz 'are the linear dimensions of the rectangular enclosure.

For the same rectangular cavity resonator, the frequencies fs of the supersonic modes of oscillation are given by Y VNHB ma: 2 m11 2 m: 2

(2) f 2 t t l where VNH3 is the speed of sound in the gas enclosed in the rectangular enclosure, mx,`my and mz are the modal indices defining the acoustic mode of operation, and lx, ly, and lz have the same significance as in Formula 1.

Since the microwave frequency in `the enclosed gas is ofthe order of 106 times as large as the acoustic frequency in said gas, the acoustic4 frequency may be conveniently selected so that` the cavity resonator resonates at both the microwave and acoustic frequencies. Forexample, a carrier frequency of 2.3 times '101 cycles (corresponding to a wavelength ofA 1.3 centimeters) may, by reference to Formulas l and 2, be modulated by an acoustic frequency of the order of twentythree fthousand 'cycles' both types "of waves `are eXct'ed in the 1, 1, Omode. It shouldbe'-under--' stood that a-la'rg'evariety of other combinations of exciting frequenciesandV resonant characteristics are possible-by proper choiceof 'rimini-andate'. Mg, my andmz.

Thus it will loe-seen -that the oavityfresonator 23, coniiningami-crowave absorptive gasfsuchfafs ammonia, may be -resonated eitlier -tof the 'irradiating microwave frequency, to' 'the Jrn-odulating. supersonic Vor' audible frequencies, or' bbth,V there# by providing an-extremely'flexible sourc'ef audible or supersonic-acoustic waves having a'n single acoustic frequency, or extending over any desired band of acousticfrequenciesasl is desirable in a loud speaker. Y Y

Furthermore, the' eXible membrane 39 may comprisel either an acoustic wave-permeable material which does not vibrate substantially 4with the exciting acoustic waves, or it may comprise a vibrating diaphragm which'vibratessympathetically.l with the waves generated. in the cav-ityfresonator`A t-ogenerate corresponding acoustic waves in thesurrounding medium.

Figure 3 comprises an embodiment oftheinvention"y which may bev employed' for modulating a source of light for oscillographic or acousticwave recording purposes. The output of the modulated generator i is coupled to the input of the waveguide I1 which includes branches I8 and 2G.

The branch i8 transmits modulated microwave energy to a first cavity resonator 23 and the branch 2) transmits the remainder of the microwave energy to a second similar cavity resonator 24. Both cavity resonators may be proportioned and designed substantially as described heretofore in Figure 2, whereby they resonate simultaneously to both the microwave carrier frequencies and a supersonic modulated frequency.

A source of desired signal intelligence 5 is connected to modulate a supersonic signal source 3 which in turn modulates the microwave generator I. The diaphragms 39, Ml, of the cavity resonators 23, 24 respectively, preferably are located any integral number of wavelengths apart at the operating supersonic modulating frequency. These diaphragms form the end walls of a supersonic trough 45 enclosing a supersonic-responsive fluid such as, for example, any type of Xylol. Thus, standing, or if desired, travelling supersonic waves will be excited in the xylol confined within the supersonic trough 45 which will provide therein regions of varying fluid density.

Fluid-tight windows lll, 49 in opposite side walls of the supersonic trough 45 provide means whereby a light beam derived from a light source 5| may be directed through the Xylol to an image screen 53.

If standing waves are established within the xylol in the supersonic trough 45, the light beam directed through the Xylol medium will provide a pattern of varying density on the light sensitive screen 53 as indicated in Figure 4.

If the supersonic signal source 3 is unmoduiated by signal intelligence, the pattern on the light sensitive screen 53 will comprise stationary vertical lines which vary gradually from black to white. However, if the supersonic signal source 3 is modulated by signal intelligence such, for example, as audible sound waves, the pattern on the light sensitive screen 53 will comprise lines which rapidly vary in intensity. The signal intelligence modulation characteristics, however, may be recorded upon a moving photographic lm 55 disposed behind the image screen 53, and i1- laminated throughanfaperture Si :at .s ome predetermined pint'on the screen'ze:

'Thus'.thezthird' embodiment 'of the :invention may be' employed for l recording sound, for other signal-intelligence, on a movirig' photogra'phic nlrn' byv Vappl'vin'g the signal intelligence to modul-ate thesupersonirc signais'ource lwhichifurther modulates the microwavecarrier source.

Itfshouldl-Lbe'.understood-thatvarious'modifications of the irivei'itic'in disclosed may be' .employed inialccordanceivith known microwave'a'nd acoustic wavetechnique-and' that theinvention4 vmay .be utilized iform any desired type-of Ho'scillographic, television-or'other 'light -mo'dula'ting apparatus. For XamplaA since the acoustic wave 'amplitudes uirctionj` of 'theniif :row'a've'rnodulaticnn chartics', ani 'oustif `wai/earnpiitu'de'measuring device-natfibe'empioyed:l in cmbihationwith the device :er Figure tp indicate directlyl Lmiemvvalvetpwi' or microwave modulation i percentage characteristics. fSi'inilarly, suchV measurements may be derivedj'byl-pticalor electri-calafnalysis of-V th :modulated `light beam" of the" deviceof iFig'lr 3". n i Y fvano ptnerin'i-icrowave absorstivefgasesj have been' ts'tedaridffoundtolbe satisfactory for microwave -applicationsin apparatus of the type described heretofore. The following table indicates the microwave frequencies at which some of these various gases have been found to abs-orb considerable microwave energy as indicated by the absorption coecients wh-ich have been measured:

Thus the invention discloses several modifications of an improved method of and means for irradiating predetermined microwave absorptive gases by means of modulated microwaves of a predetermined frequency t-o generate acoustic waves in said irradiated gases. The invention may be employed, as described heretofore, for indicating the presence of `certain gases, for generating acoustic waves characteristic of microwave modulation, or for modulating a light beam in accordance with said microwave modulation.

I claim as my invention:

1. A light valve for indicating microwave modulation characteristics including a chamber for confining a microwave absorptive gas, means for irradiating said gas by modulated microwaves of a frequency absorbed by said gas to provide pressure variations in said gas characteristic of said modulation, a uid medium having light transmission characteristics dependent upon pressure variations therein, and means for transmitting said pressure variations in said gas to said medium.

2. Apparatus for indicating microwave modulation characteristics including at least one chamber for conning a microwave absorptive gas, means for generating microwaves of a frequency absorbed by said gas, means for generating waves of ultrasonic frequency, means for modulating said ultrasonic waves in accordance with said signal intelligence, means for modulating said microwaves with said modulated ultrasonic waves, means'for irradiating said gas Vby gas, means for generating microwaves of a ire"k quency absorbed by said gas, means for generating Waves of ultrasonic frequency, means for modulating said rultrasonic waves in accordance with signal'intelligence, means for modulating said microwaves with said modulated ultrasonic waves, means for irradiating said gas by said modulated microwaves to provide pressure variations in said gas characteristic of said modulated ultrasonic Waves, an ultrasonic resonator enclosing an ultrasonic responsive medium having light transmission characteristics dependent upon pressure variations therein, and means for transmitting said ultrasonic pressure variations in said gas to said medium to establish ulttrasonic standing waves therein.

Y Number 4. Apparatus for detecting signal intelligence including'a chamber enclosing a microwave absorptive gas, a source of microwaves modulated by said signal intelligence, means for irradlating said gas by saidmodulated microwaves to provide pressure variations therein characteristic of said signal intelligence, a iiuid medium having light transmission characteristics dependent upon pressure variations therein, means for transmitting said pressure variations in said gas to said medium, and means for directing a light beam through said liquid medium to vary said light as a function of said signalintelligence.

WILLIAM D. HERSHBERGER.

REFERENCES CITED The following references are of record in the rile of this patent:

UNITED STATES PATENTS Name Date 2,155,659 Jeiree Apr. 25, 1939 2,155,661 Jeiree Apr. 25, 1939 2,308,360 Fair Jan. 12,1943 Willard Mar. 28, 1944

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