US2623165A - Modulated light communication system - Google Patents

Description

155-518 AU 233 EX PIP-8106 x2 2,623,165 X50 p Dec. 23, 1952 H. MUELLER ETAL 2,623,165

uonuumn LIGHT couuumcmxon sys'rsu i Filed Jan. 7, 1948 2 1 27 SCREEN lMA GE BRIGH T TRANSPARENT MEDIUM /3 /5 P/EZO-ELECTR/G 3 an YSTAL 3 l3 TRANSPARENT 3 22 LENS MED/UM n A LS I? Q3 L/GHT 5 7 j i L 5- 25 AMHJF/ER Sal/R65 P/EZ E i ELECTRIC CRYSTAL MODULA TOR OSGILLA r01? g 7 (28 za 2v F lg. 6'.

TRANSPARENT MEDIUM 1 AMPLIFIER I I 0-.- 5? 9 Fig. .9.

SGILLATOR OSCILLA TOR 'I'II WW I MODULATOR moouu ran in van tors I L Hans Muel ler F198. Ruben H. R/nes Attorney Dec. 23, 1952 H. MUELLER ETAL HODULATED ucm COMMUNICATION SYSTEM Filed Jan. 7, 1948 FIG.3.

FIG.4.

INVE N To s HANS MUELLE R065 BY 00 gr RINES ATTORNEY Patented Dec. 23, 1952 OFFICE MODULATED LIGHT COMIWUNICATION SYSTEM Hans Mueller, Belmont, and Robert H. Rimes. Brookline, Mass.

Application January 7, 1948, Serial No. 1,002

25 Claims. 1

The present invention relates to communication. and more particularly to the transmission and reception of signal intelligence.

An object of the invention is to provide a new and improved system for secret signaling.

Another object is to provide a new and improved system for communication employing light modulation.

A further object is to provide a new and improved system for transmitting.

A further object still is to provide a new and improved system for receiving.

Still another object is to provide a new improved system for transmitting and receiving.

In a copending application. serial No. 1,003, filed of even date herewith by Robert H. Rines. there is disclosed a novel light-modulation method and system operable to produce large light effects at ultrasonic frequencies, that is substantially independent of frequency, and that is nevertheless attended by little background noise.

A further object of the invention is to provide a new and improved system employing the lightmodulation principles described in the said copending application.

Other and further objects will be explained hereinafter and will be more particularly pointed out in the appended claims.

The invention will now be more fully described in connection with the accompanying drawings, in which Fig. 1 is a diagrammatic view of circuits and apparatus illustrating the ultrasonic lightmodulation method and system disclosed in the said application; Fig. 2 is an explanatory diagram; Fig. 3 is a reproduction of a photograph obtained by employing the light-modulation system of Fig. 1, illustrating the efiect produced upon a polarized beam or bundle of light in response to longitudinal compressional ultrasound waves imparted to a transparent solid medium through which the beam or bundle is passed; Fig. 4 is a reproduction of a similar photograph illustrating the effect of transverse shearing ultrasound waves, the light beam, however, being of different polarization; Fig. 5 is a reproduction of a similar photograph illustrating the effect produced by employing a medium the transverse dimension of which is comparable to the wavelength of the ultrasound waves in the medium; Fig. 6 is a diagrammatic view, similar to Fig. l, of a transmitting system constructed in accordance with a preferred embodiment of the present invention; Fig. 7 is a similar view of a receiving system; Fig. 8 is a view similar to Fig. 6 of a modified transmitting system; and Fig. 9 is 2 similarly a view similar to Fig. 7 of a modified receiving system.

As disclosed in the said application, a light source I, such, for example, as a mercury arc, is provided to produce high-intensity light rays. The present invention is not however restricted to use with visible light. On the contrary. it has particular use in connection with infra-red rays. An infra-red or even an ultra-violet filter 3 may. therefore, if desired. be employed. As explained in the said application, a filter 3, adapted to produce monochromatic visible light of any desired wavelength may also be employed. though the invention is operable with chromatic as well as monochromatic light.

The light waves are shown collimated by a lens 5 into a parallel beam or bundle of rays of crossdimension corresponding to the cross-dimension of the lens 5. The beam or bundle of light rays is caused to pass through a plane polarizer I such. for example, as a Nicol prism or a piece of polaroid, and thereafter to impinge upon a substantial area of the front surface 9 of a medium I3. The medium I3. of course, should be transparent to the light rays employed, whether visible, infrared or ultraviolet, along the direction of travel of the light through the medium l3 between the front surface 9 and the preferably parallel rear surface II. The transparent medium I3 is preferably of the same cross-dimension as the crossdimension of the light beam. Any other wellknown focusing system, such as a parabolic reflector, may be used to direct the rays upon the medium I3. The rear surface H of the transparent medium I3 is shown separated from the front surface 9 by a thickness T. The transparent medium I3 may be constituted of a glass or non-crystalline fused quartz block, or any other transparent solid or liquid. It may or may not be piezoelectric. For the present, it will be assumed. in the further description. that it is not piezoelectric but, on the contrary, that it is optically inactive, birefringent-free. and strain-free.

The medium l3 may be vibrated molecularly in any desired way. According to the illustrated embodiment of the invention, the vibrations are produced by means of an ultrasonic vibrator. For the production of high-frequency ultrasonic waves. the vibrator may be constituted of a piezoelectric crystal l5. as of quartz. but it may also be of the magnetostrictive. magnetomotive or of any other suitable type.

The quartz crystal l5 may be vibrated at a predetermined frequency by connecting its two electrodes ll and I! to an oscillator 2 l. The

period of vibration of the crystal I5 may be relatively low, say, several hundred kilocycles, more or less, or as high as ten megacycles, more or less. The ultrasonic vibrations of the quartz crystal l5 adiacent, for example, the bottom edge of the medium, will therefore become transmitted or directed into the medium l3 toward the top edge. The medium i3 may be held in place on the crystal IS in any desired way, as by cement or even by a layer of oil to aid in this transmission or direction.

Let it be assumed, for the moment, that the quartz crystal vibrates along its thickness dimension, so that it alternately elongates and contracts vertically. If the height dimension of the medium I3 is equal to a whole multiple of the wavelength of the ultrasound waves in the medium, standing waves of ultrasonic frequency. as before stated, will theoretically be set up in the medium between its bottom and top edge surfaces. It is assumed that the medium I3 is of such a nature that. when it is vibrated molecularly to produce these theoretical standing waves therein, it becomes birefringent to the light passing therethrough along the direction of travel of the light through the medium which is substantially perpendicular to the direction of the ultrasound waves between the bottom and top edges.

As the cross-dimension of the parallel beam or bundle of the plane-polarized light rays impinging upon the front surface 9 of the medium l3 corresponds to the cross-dimension of the lens 5, it is large compared to the dimension of the standing waves produced in the medium 13. This is to be contrasted with the conditions obtaining in the prior-art diffraction methods and in the systems employing frequency sensitive crystalline media or shaped media, as discussed in the said copending application.

After passing through the medium l3. and emerging from its rear surface II, this large parallel beam or bundle of polarized light is shown in Fig. 1 focused by a lens 23 upon a screen 21. The lens 23 and the screen 21, of course, may be replaced by some other optical system, such as an ocular, a camera, a photocell 22, or any other suitable system for receiving and indicating the beam of light from the rear surface I I of the medium 13.

An analyzer 25 is shown interposed between the lens 23 and the screen 21. It may be constituted of a piece of polarizing material oriented at right angles to the orientation of the polarizer I. Under normal conditions, therefore, when the circuit of the oscillator 2! is open, and the crystal l5, therefore, is not vibrating, the polarized light passing through the medium I3 will be extinguished by the analyzer 25, to a degree depending only on the effectiveness of the polarizing and analyzing materials, with the result that the screen 21 will be dark.

The term extinguished, of course, is used herein not only in its ordinary sense, as employed ordinarily in connection with visible light, but also more generally to denote also the more general phenomenon of blocking any of the light waves employed, whether or not visible.

When, however, the circuit of the oscillator 2| is closed. to render it effective to vibrate the quartz crystal l5, standing waves of ultrasonic frequency, as already explained, will be set up in the medium l3, between its bottom and top surfaces. Alternate horizontally disposed equally spaced sectional portions of the medium 13 will become compressed and dilated by oppositely phased components of the vibration waves, in consequence. These compressed and dilated sectional portions will be separated by portions of the medium I3 wherethe standing sound waves in the medium l3 will produce nodes. Corresponding changes in the refractive index will occur in the dilated and compressed portions of the medium 13, but the refractive index will remain unchanged at the nodes, since these nodal sections are not vibrating. The refractive-index changes will occur periodically, in synchronism with the vibrations of the medium.

It has already been stated that the invention is not restricted to use with monochromatic light. In order to simplify the explanation, however, it will be assumed, for the present, that the light from the source I is actually monochromatic, of wavelength A.

Considering. for the moment, in the plane of the front surface 9 of the medium l3, any one of the horizontally disposed sectional portions of the medium 13, let it be assumed that a change dm has occurred in its refractive index along the vertical direction V. and that a corresponding change (in: has occurred in its refractive index along the horizontal direction H, at right angles thereto. Let it further be assumed, for simplicity, that the plane of polarization of the light passing through the polarizer 1, upon reaching the plane of the front surface 9, is at 45 degrees to the vertical, as indicated at P in Fig. 2. This light, of amplitude E0, may therefore be considered as having two equal in-phase polarized components of amplitude The vertical component Ev is polarized along the vertical direction V. and the horizontal component Ex is polarized along the horizontal direction H. The instantaneous values of these components, at any time t. may be represented by the following equations:

where w is the angular frequency of the light.

Since the change dm in the index of refraction alongthe vertical direction V is different from the change dn: in the index of refraction along the-horizontal direction H, these two polarized components will suffer difierent phase shifts durin their passage through the medium I 3. These phase shifts may respectively be represented by where r is the ratio of the circumference to the diameter of a circle.

The .resultant of these two polarized components, upon emerging from the rear surface H of the medium B. will therefore no longer, in general, "be a plane-polarized wave. In general, the

wave will be elllptically polarized, and its components will be respectively represented by It is in this elliptically-polarized form that the components of the elliptically-polarized waves will pass through the crossed analyzer 25 on their way to the screen 21.

This operation is therefore not the same as that occurring with optically active crystals or other crystalline substances as before described. The operation occurring with the optically active crystals depends upon rotating the plane of plane polarization. The operation occurring with permanently doubly retracting crystalline substances depends upon modifying the amount of double refraction. The operation of the present invention. on the other hand, depends upon depolarizing plane-polarized waves into elliptically polarized waves.

It has been explained that the analyzer 25 may be so oriented that, when no ultrasonic vibrations whatever are propagated into the medium l3, the screen 21 is dark. When the oscillator 2! causes the crystal to set up standing ultrasonic waves in the medium I3, on the other hand, bright layers, striations, bands, regions or stripes 2 will appear on the screen 21. The layers, striations. bands, regions or stripes 4 between alternately disposed light layers 2 will remain dark.

With the analyzer adjusted in accordance with the above assumptions, so as to extinguish the light passing through the medium l3 at times when it is not vibrating, the dark stripes will correspond to the light passing through the nonvibrating or nodal portions of the medium IS. The light stripes 2, on the other hand, will correspond to the successive portions of the light beam which have become elliptically polarized during passage through the corresponding compressed and dilated portions or sections of the medium l3.

It is not, of course, essential that the analyzer 25 be so oriented as normally to extinguish the light passing through the medium l3. The analyzer 25 may be so oriented that, under normal conditions, when the medium is not vibrating, the screen 21 shall just be illuminated. The compressed and dilated sections of the medium I 3, produced in response to the vibration of the medium l3, may periodically produce elliptically polarized light, the major axis of which is normal to the orientation of the analyzer 25, so that most of the light passing through these sections, when they produce such elliptically polarized light, is extinguished. The light stripes 2 will then correspond to the nodes, and alternately disposed dark stripes 4 to the compressed and dilated sections of the medium 13, produced in response to the vibration of the medium 13.

Inaccordance with the present invention. therefore, the analyzer 25 may be adjusted initially so as normally either to extinguish the polarized light after its passage through the medium l3, or to permit the light to pass onto the screen 21. The analyzer 25 may be adjusted to a degree such as initially to produce extinction and such that a slight change in the analyzing process in one direction, resulting from the action of the birefringent medium I3 to change the state of polarization of the light, will provide bands or sections 2 of illumination alternately with dark bands or sections upon the screen 21. The analyzer 25 may, on the other hand, be adjusted to a degree such as initially almost, but not quite, to extinguish the analyzed light on the screen 21 and such that an equal change in the analyzing process in the opposite direction, resulting from the action of the birefringent medium on the light will produce dark bands or sections alternately with light sections 2 on the screen 21.

Since the light and dark layers 2 and 4 are produced periodically, in synchronisrn with the vibrations of the medium I3, the phenomenon, in reality, is produced stroboscopically. Because the frequency of the ultrasonic waves is many times greater than that of the flicker limit of the eye. however, the effect upon the observer will be the same as though the light layers 2 were produced with the aid of continuous light issuin from the analyzer 25.

The difference (mu-(1m) in the changes of the index of refraction along the vertical and horizontal directions is known as the birefringence of the medium. It is proportional to the total phase shift suffered by the light in passing through the compressed and dilated sections of the medium l3. The intensity of illumination of the light stripes 2 is proportional to the square of the sine of half the phase shift b sufiered in passing through the medium I3.

The stronger the vibrations of the sound waves, the greater will be the birefringence and the greater the intensity of illumination of the light stripes 2. It is accordingly possible to regulate the light intensity in accordance with the signal produced by the vibrating quartz l5, as determined by the oscillations of the oscillator 21. A linear relationship has been found to exist between the light intensity and the signal amplitude produced by the quartz IS.

The operation above described has been upon the assumption that the height dimension of the medium I3 is equal to a whole multiple of the wavelength of the ultrasonic waves propagated through the medium between its bottom and top surfaces. This, however, was for explanatory purposes only. It has been found that, particularly at the higher frequencies. a medium of any arbitrary dimension may be employed, irrespective of whether or not it is a multiple of the wavelength. provided only that it is large with respect to the wavelength of the ultrasound waves. It has also been found that such a photoelastic-shutter system is not frequency-sensitive, and the dimensions need bear no specific rela-- tionship to the wavelength of the ultrasound waves.

Though the flat area of the crystal I5 is shown. in Fig. l, as substantially equal to the cross-sectional area of the medium I3 into which it propagates the ultrasound waves, this is not essential. The medium I3 may have a cross-dimension many times the area of the crystal l5, though the birefringence effect resulting from the ultrasound waves will then not be produced strongly throughout the whole medium. To produce the birefringence effect throughout such a large medium, and thereby to obtain an unlimited light area, even several square feet, a plurality of vibrators, as shown in Fig. 8, may be employed in contact with successive portions of the medium l3. A single large flat area crystal may also be used. No slits or stops are necessary.

The reproduction in Fig. 3 of an actual photo graph obtained with the system of Fig. 1 will show, not merely the alternate horizontally-disposed light and dark layers or stripes 2 and 4, but rather a square-block-like appearance, produced by the addition of similar vertically disposed light and dark layers or stripes. These additional layers or stripes 2 and 4 are probably to be explained by the fact that, during the vibration, every elongation and contraction of the quartz crystal [5 in the vertical direction is automatically accompanied by a contraction and an elongation, respectively, in the horizontal direction. These, of course, are also transmitted into the medium I 3. In addition to the theoretical standing waves described above as set up in the medium [3 between the bottom and top surfaces in the vertical direction, therefore, standing waves appear to be set up also in the horizontal direction. Upon these standing waves at right angles to each other, moreover, there are doubtless superposed standing waves in still other directions, caused by reflection and other phenomena, the effects of which are not clearly shown in Fig. 3, though their existence appears to be betrayed in the photograph reproduced in Fig. 5. The result is not merely the before-described linear vibration of the medium l3, from top to bottom, but rather at least a two-dimensional vibration.

It is fortunate that, at least in the photograph reproduced in Fig. 3, the standing waves in the other directions do not interfere with the operation, according to the present invention, in the bottom-top direction. In this photograph, the standing waves at right angles to each other in the vertical and the horizontal directions are indicated as having very nearly equal effects upon the incident light. Since this incident light was described as polarized by the polarizer I at an angle of 45 degrees to the vertical, as represented at P, these standing waves appear to produce similar effects upon this type of polarized light, even though the vibrations of the two types of vibrations are not precisely the same. This may explain the block-like appearance of Fig. 3.

With the dimensions and materials used, the changes of refractive index caused by the transverse shearing strain have been found to be smaller than those produced by the longitudinal compressional strain in the vertical direction. The

bright striations Ii of Fig. 4 are therefore not so intense as those in the case illustrated in Fig. 3. Optimum results were found at substantially the 45 degree angle of polarization.

Instead of 45-degree polarization, the light issuing from the polarizer 1 may be polarized at any other desired angle. The effect of vertical polarization, for example, may be studied by considering the change of the refractive index resulting from the shearing strain produced in the medium It by its transverse vibrations as decomposed along two directions at right angles to each other along the plane of the surface 9. One of these may be an angle of 45 degrees with respect to the vertical, as may also be indicated at P, Fig. 2, and the other may be indicated at P. The components of the vertically-polarized waves along the 45-degree directions P and P, during their passage through the medium [3, will sufier diiferent phase shifts, depending on the changes in the refractive index along the two directions P and P. The nature of the compressed and dilated sectional portions and the nodal portions of the medium I3 produced by the standing waves, of course, will by unchanged by this change in the angle of polarization of the light waves passing through the medium. For the reasons already given, therefore, owing to the biaxial birefringence thus produced, light layers will still appear on the screen 21 corresponding to the sheared sectional portions of the medium. and these will still be separated by dark layers corresponding to the nodal sectional portions of the medium I3.

These light layers and dark layers are respectively shown at 6 and 8 on actual photograph. reproduced in Fig. 4, taken when employing light of vertical polarization in the system of Fig. 1.

It makes a difference, therefore, not only theoretically, but also in practice, whether the polarization of the incident waves lies in one plane or another plane. This again demonstrates that the operation, according to the present invention, is not the same as that with an optically active crystal, the operation of which depends upon rotation of the plane of plane polarization, and not the depolarizing of the plane-polarized waves into elliptically polarized waves.

The distances between the centers of the successively disposed light or dark striations or layers are respectively equal to one-half the wavelength A1 of the longitudinal compressional vibrations and one-half the wavelength M of the transverse compressional vibrations propagated vertically and horizontally, respectively, in the medium 13. The values of these wavelengths obtained by measurement may be used directly to find Young's modulus e and Poissons ratio 0'.

As an example, ultrasound waves of a frequency f=10 megacycles were propagated, as illustrated in Fig. 1, into a sample [3 of plate glass 3.1 x 1.4 x 1.0 centimeters, having a density p=2.61. Upon measurement, the wavelength A: of the longitudinal waves in the vertical direction was found to be 0.532 millimeter and the wavelength M of the transverse waves was found to be 0.312 millimeter. Young's modulus e was then calculated to be '7.t5 10 degrees/cm. from the formula 1-0 This value agrees with the values obtained for the same sample [3 by diffraction methods.

The ratio m of the longitudinal to the transverse wavelengths,

has been found to be in close agreement with the ratio of the displacements of the first diffraction orders produced by the transverse and the longitudinal waves. Poisson's ratio 0 may then be obtained from the equation light layers. intersecting patterns and scrolls I were found, probably resulting from numerous reflections within the medium.

Rough corners or edges have been found to introduce no uncertainties in the operation.

The invention has heretofore been explained in connection with birefringent-free and strainfree media l3. It is preferable to employ media [3 of this character, and to adjust the analyzer 25 so as to obtain complete extinction before the vibrations are initiated in the medium. Extremely intense stroboscopic-light layers have thus been observed, for example, with noncrystalline fused-quartz media.

Chrystalline substances having permanent birefringence, however, may also theoretically be employed. They also demonstrate light-intensity changes when subjected to ultrasonic waves, as described above. If an optically active medium 43. such as a piezo-electn'c crystal. is used. however, then. in order to detect the effect of the birefringence on the light emerging from the face I l of the medium. it is necessary to orient the analyzer 25 at right angles to the polarizer, and not so as to extinguish the light emerging from the medium, the plane of polarization of which has been rotated in passing through the medium. It is then only that the birefringence effect, in addition to the optical activity, may theoretically be detected. In practice. the much stronger effect of the change in optical activity produced by straining the medium may prevent the detection of the effects of birefringence.

The use, in the system of Fig. 1, of a strain free, optically-inactive, non-crystalline, isotropic medium 13 that is normally birefringent-free along the direction of travel of the light rays between the front surface 9 and the rear surface H of the medium l3, therefore, not only makes possible the measurements above-described and the observance of the effects of ultrasonics in such media, but it also provides a large continuous area of stroboscopic light which truly flashes from darkness to light of a high intensity. In the case of one crown-glass sample medium I3 of about the same dimensions as the sample previously discussed, the intensity change was found to be equal to one-tenth of the intensity of the mercury arc itself. Illumination of this order of intensity can be used, for example. to photograph scientific or other phenomena in motion at ultrasonic or other high frequencies. The frequency of the oscillator 2| needs merely to be adjusted until the moving object appears to stand still.

If a quarter-wave plate is inserted in front of each polarizing device 1 and 25, so that circularlypolarized. instead of plane-polarized, light is impinged upon the medium l3, and is analyzed after emerging therefrom, even stronger-intensity results occur. While some of the intensity resulting from the birefringence produced by the longitudinal waves is lost. this is apparently more than made up for by the light intensity resulting from the birefringence produced by the transverse waves. Waves having initially elliptical polarization may also be employed.

Continous-wave signals may be transmitted by the system of Fig. l or of Fig. 6 in many ways, as by interrupting the circuit of the oscillator 2| with a key. Modulated audio or video signals may be transmitted by modulating the carrier frequency of the oscillator 2|, as by means of a modulator 20. Since the thin high-frequency crystal I5 may be vibrated with wide side-bandwidths, the vibrations of the quartz crystal l5, oscillating at a high carrier frequency while in contact with the medium 13, are correspondingly modulated in accordance with the audio or video signal. The non-resonant, relatively largedimensioned medium [3 has been found to respond sufiiciently instantaneously to the modulated ultrasonic carrier, propagated thereinto from the crystal IE. to produce birefringence in response to the modulation signal.

At the receiving station. the lens 23 may therefore be caused to focus the birefringenceproduced elliptically-polarized light from the medium l3 on to some other light-receiving means than the screen 21. As illustrated in Fig. 7, for example. a photocell 22 may be employed to receive elliptically polarized light. In order to detect the modulation, the photocell 22 may be connected to an audio or video amplifier 28. Crystalline media I: may also be used for the photocell or photographic detection of this lightmodulation transmission, though with limitations of frequency sensitiveness, and accompanied by background noise or light.

The photocell 22 is itself sensitive to only a band of wavelengths. By designing it so that it shall respond with maximum effect to light having the wavelength of infrared or other monochromatic rays, most sensitive and most noiseless operation may then be obtained with monochromatic light of such wavelength.

It has been found possible to transmit as many as three different modulated signals through the system of Fig. 6 simultaneously, with good reproduction. at the receiving end, of all three signals. The transmission may be effected by feeding one or more of the modulating signals to the same quartz or other ultrasonic vibrator 15. Such procedure tends, however, to overload the mechanical parts of the system. The multi channels may also be obtained by employing a plurality of vibrators l5 and I5, as shown in Fig. 8, in sonic contact with the medium, and modulating the vibrators with different signals from the modulators 20 and 20. If the different modulations are of entirely different frequency ranges, such as correspond, for example. to an audio modulation and a video modulation, there will be very little distortion when received in the receiving system of Fig. 9 which is operated in a similar manner to the operation of the receiving system of Fig. 7. The photocell may feed tuned amplifiers. such as the amplifiers 2B and 28' which will separate out and amplify the intelligence of the different channels; for example, the amplifier 28 may separate out and amplify the audio channel while the amplifier 28' amplifies the video channel.

Audio frequencies ranging from 40 cycles to 15,000 cycles have been used to modulate a ten-megacycle ultrasonic carrier with noiseless and distortionless results.

The above-described system finds particular application also in television-projection systems such, for example, as the Scophony system, where liquid diffraction cells have heretofore been employed.

To control the volume and the performance of this light-modulation system, it is desirable: first. properly to orient the plane of polarization of the polarizer; secondly, to control the operation of the piezo-electric carrier on and off the resonant frequency of the -piezo-electric crystal; thirdly; to position the medium l3 so as to intercept more or less of the incident light; and fourthly, suitably to insert or remove a diaphragm, or to control the aperture of a diaphragm in the path of the light beam.

As a modification, if plane-polarized light is used, for example, the analyzer 25 may be provided with suitable phase-shifting plates. The elliptically polarized waves emerging from the compressed and dilated regions of the medium l3 will thus be properly analyzed, whil permitting the plane-polarized light passing through the nodal-region portions of the medium to penetrate the analyzer 25. The use of a quarter-wave plate with the analyzer 25 may be particularly desirable, for example, where the thickness T of the medium is such as to produce exactly a ninety-degree phase shift between the components V and H of the incident light, circularly polarizing the light emerging from the compressed and dilated regions.

The description above has been simplified on the assumption that monochromatic light is used. Monochromatic light has its practical applications. The signalling system of Fig. 6, for example, could be used with infra-red or other invisible rays to provide added secrecy. The discussion above is equally applicable, however. to all wavelengths, even when employed simultaneously, that is, to chromatic light.

Further modifications will occur to persons skilled in the art, and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A communication system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through the medium along the predetermined direction a bundle of light of cross-dimension large compared to the wavelength of the standing waves, a polarizer for polarizing the light prior to its pas sage through the medium, an analyzer for analyzing the light after its passage through the medium, the analyzer being adjusted so that a change in the analyzing produced thereby in one direction will provide extinction of the analyzed light and a substantially equal change in the analyzing produced thereby in the opposite direction will permit the passage of the analyzed light, means for roducing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, means for varying the molecular vibrations, and means for receiving the analyzed light to detect the variation.

2. Apparatus as set forth in claim 1 the vibrating means of which comprises means for propagating ultrasonic waves into the medium.

3. Apparatus as set forth in claim 1 the'vibrating means of which comprises piezo-electric means.

4. Apparatus as set forth in claim 1 all of the dimensions of the medium of which are large 12 compared to the wave-length of the standing waves.

5. A communication system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through the medium along the predetermined direction a bundle of light of cross-dimension large compared to the wavelength of the standing waves, a polarizer for polarizing the light prior to its passage through the medium, an analyzer for extinguishing the light after its passage through the medium, means for producing molecular vibrations of th said predetermined wavelength at one of the said edges of the medium. means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves. means for modulating the molecular vibrations, and means for receiving the analyzed light to detect the modulations.

6. Apparatus as set forth in claim 5 the vibrating means of which comprises means for pr pagating ultrasonic waves into the medium.

A communication system having, in comination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to th light passing therethrough along the predetermined direction, means for passing through the medium along the predetermined direction a bundle of light of cross-dimension large compared to the wavelength of the standing waves, a polarizer for polarizing the light prior to its passage through the medium, an analyzer for extinguishing the light after its passage through the medium, means for producing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves. means for modulating the molecular vibrations with a plurality of independent sigfl'afl'f'fifefiii sfor receiving the analyzed light,;and me'afifcoopFa e tive with the receivingf eansfor sepiTratfig'aiid amplifying the independent signals.

8.Acoiis m amg,incom bination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined ultrasonic wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through u u i.- HUM-m 13 the medium along the predetermined direction a bundle of light of cross-dimension large compared to the wavelength of the standing waves, a polarizer for polarizing the light prior to its passag through the medium, an analyzer for analyzing the light after its passage through the medium, the analyzer being adjusted so that the analyzed light is almost but not quite extinguished, means for producing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, means for modulating the molecular vibrations with an audio signal, and means for receiving the analyzed light to detect the signal.

9. A transmitter having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times th said wavelength, becomes birefringent to light passing therethrough along the predetermined direction, means for passing light through the medium along the predetermined direction, a polarizer for polarizing th light prior to its passage through the medium, means for producing molecular vibrations of the said predetermined wavelength at one of the said dges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, and means for modulating the molecular vibrations of the medium.

10. A communication system having, in combination, a medium having front and rear surfaces and that is transparent to light along the direction between the front and rear surfaces,

the medium being normally birefringent-free to light propagated along the said direction, means for passing a bundle of light toward a substantial area of the front surface, a polarizer interposed in the path of the light, an analyzer positioned in the path of the light emerging from the rear surface after its passage through the medium, means for producing ultrasonic vibrations at an edge of the medium of wavelength small compared with the distance between the said edge and a second edge of the medium, means for directing the vibrations into the medium toward the said second edge in order that the vibrations may be reflected therefrom to set up standing waves that render th medium birefringent to the light, means for modulating the molecular vibrations with an audio signal, and means positioned beyond the analyzer for rendering th signal detectable.

11. A communication system having, in combination, a medium having front and rear surfaces and that is transparent to light, means for passing a bundle of light toward a substantial area of the front surface, a plane polarizer interposed in the path of the light and oriented along a dimension of the medium, an analyzer positioned in the path of the light emerging from the rear surface after its passage through the medium and oriented at right angles to the polarizer, means for producing carrier-wave vibrations at an edge of the medium of wavelength small compared with the distance between the said edge and a second edge of the medium from which the Vibrations may be reflected, means for modulating the amplitude of the said carrier-wave vibrations, and means positioned beyond the analyzer for rendering the modulations detectable.

12. A communication system having, in combination, a medium having front and rear surfaces and that is transparent to light, means for passing a bundle of light toward a substantial area of the front surface, a plane polarizer interposed in the path of the light and oriented at forty-five degrees to a dimension of the medium. an analyzer positioned in the path of the light emerging from the rear surface after its passage through the medium and oriented at right angles to the polarizer, means for producing carrierwave vibrations at an edge of the medium of wavelength small compared with the distance between the said edge and a second edge of the medium from which the vibrations may be reflected, means for modulating the amplitude of the said carrier-wave vibrations, and means positioned beyond the analyzer for rendering the modulations detectable.

13. A communication system having, in combination, a medium having front and rear surfaces and that is transparent to light, means for passing a bundle of light toward a substantial area of the front surface, means interposed in the path of the bundle of light for circularly polarizing the light, means positioned in the path of the light emerging from the rear surface after its passage through the medium for analyzing the light, means for producing carrier-wave vibrations at an edge of the medium of wavelength small compared with the distance between the said edge and a second edge of the medium from which the vibrations may be reflected, means for varying a property of the wave, and means positioned beyond the analyzer for rendering the variation detectable.

14. A communication system having, in combination, a medium having substantially parallel front and rear surfaces, that is transparent to light. and that is of a predetermined refractive index, means for passing a bundle of light toward a substantial area of the front surface, means for polarizing the light prior to its reaching the front surface, means oriented normally to the polarizing means for analyzing the light emerging from the rear surface after its passage through the medium, means for periodically straining the medium at equally spaced intervals along a direction substantially parallel to the front and rear surfaces in order to alter the refractive index of the medium at the said intervals by a predetermined amount along one direction substantially parallel to the said surfaces and by a diflferent amount along a direction at an angle to the said one direction, thereby to render the medium periodically birefringent, means for varying the periodic birefringence in accordance with a signal, and means positioned beyond the analyzing means for rendering the signal detectable.

15. Apparatus as set forth in claim 14 the medium of which is initially strain-free and the straining means of which comprises means for propagating ultrasound waves into the medium.

16. A communication system having, in com bination, a medium having substantially parallel front and rear surfaces separated by a thickness T. that is transparent to light, and that is of refractive index 11, means for passing a bundle of light having a predetermined wavelength 1 toward a substantial area of the front surface, means for plane-polarizing the light prior to its reaching the front surface, means for analyzing the light emerging from the rear surface after its passage through the medium in the direction of the thickness '1, means for periodically straining the medium at equally spaced intervals along a direction substantially parallel to the front and rear surfaces in order to alter the refractive index of the medium at the said intervals by an amount dni along one direction at an angle to the plane of polarization of the light and by a different amaunt dm along a second direction at right angles to the said one direction, thereby to shift the phase of the component of the polarized light substantially parallel to the said one direction by an amount and -to shift the phase of the component of the polarized light substantially parallel to the said second direction by an amount whereby the light emerging from the rear surface of the medium becomes periodically converted into elliptically polarized light prior to its becoming analyzed by the analyzing means, and means for altering the periodic straining in accordance with a signal of lower periodicity.

17. A communication system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at vibrational wavelengths less than a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through the medium along the predetermined direction a bundle of light of cross-dimension large compared to the dimension of the standing waves, a polarizer for polarizing the light prior to its passage through the medium, an analyzer for extinguishing the light after its passage through the medium, a plurality of means disposed adjacent one of the said edges of the medium for producing molecular vibrations of the said vibrational wavelengths and for directing the vibrations into the medium toward the said other edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, means connected to the vibrating means for modulating the molecular vibrations, and means for receiving the analyzed light to detect the modulations.

18. A communication system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at vibrational wavelengths less than a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through the medium along the predetermined direction a bundle of light of cross-dimension large compared to the dimension of the standing waves, a polarizer for polarizing the light prior to its passage through the medium, an analyzer for extinguishing the light after its passage through the medium, a plurality of piezoelectric crystals disposed adjacent one of the said edges of the medium for producing molecular vibrations of the said vibrational wavelengths and for directing the vibrations into the medium toward the said other edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, means connected to the piezoelectric crystals for modulating the molecular vibrations, and means 10: receiving the analyzed light to detect the modulations.

19. A transmitter having, in combination, a medium that is transparent to light along a Predetermined direction and that, when molecularly vibrated at vibrational wavelengths less than a predetermined vibrational wavelength to produce standing waves therein between a pair of edges or tne medium spaced from each other in a direction substantially prependicular to the said predetermined direction a distance corresponding to severaltimes the said wavelength, becomes birefringent to light passing therethrough along the predetermined direction, means for passing light through the medium along the predetermined direction, a polarizer for polarizing the light prior to its passage through the medium, a plurality of means disposed adjacent one of the said edges of the medium for producing molecular vibrations of the said vibrational wavelengths and for directing the vibrations into the medium toward the said other edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, and means connected to the vibrating means for modulating the molecular vibrations of the medium.

20. A transmitting system having, in combination, a. medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined directlon a distance corresponding to several times the said wavelength. becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successive portions of the medium disposed between the said edges. each of width substantially equal to the half-wavelength of the standing waves, along the predetermined direction, a beam of light having a plurality of successive portions corresponding to the plurality of successive half-wavelength portions of the medium, plane-polarizing means disposed to pclarize the successive beam portions prior to their passage through the corresponding medium portions, means for producing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, thereby rendering the successive medium portions birefringent in antiphase in response to the resulting standing waves,

17 and means for modifying the vibrations in accordance with a signal.

21. A transmitting system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined ultrasonic vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successive portions of the medium disposed between the said edges, each of width substantially equal to the halfwavelength of the standing waves, along the predetermined direction, a beam of light having a plurality of successive portions corresponding to the plurality of successive half-wavelength portions of the medium, plane-polarizing means disposed to polarize the successive beam portions prior to their passage through the corresponding medium portions, means for producing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, thereby rendering the successive medium portions birefringent in anti-phase in response to the resulting standing waves, and mean for modifying the vibrations in accordance with a signal of greater wavelength.

22. A transmitting system having, in combination, a medium that is transparent to light along a predetermined direction and that. when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successive portions of the medium disposed between the said edges, each of width substan tially equal to the half-wavelength of the standing waves, along the predetermined direction, a beam of light having a plurality of successive portions corresponding to the plurality of successive half-wavelength portions of the medium, plane-polarizing means disposed to polarize the successive beam portions prior to their passage through the corresponding medium portions at forty-five degrees with respect to the direction of propagation of the standing waves, means for' producing molecular vibrations oi. the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, thereby rendering the successive medium portions birefringent in anti-phase in response to the resulting standing waves, and means for modifying the vibrations in accordance with a signal.

23. A transmitting system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a. direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successive portions of the medium disposed between the said edges. each of width substantially equal to the half-wavelength of the standing waves. along the predetermined direction, a beam of light having a plurality of successive portions corresponding to the plurality of successive half-wavelengh portions of the medium, plane-polarizing means disposed to polarize the successive beam portions prior to their passage through the corresponding medium portions in a plane parallel to the said predetermined direction, means for producing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, thereby rendering the successive medium portions birefringent in antiphase in response to the resulting standing waves, and means for modifying the vibrations in accordance with a signal.

24. A transmitting system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined vibrational wavelength to produce standing waves therein between a pair or edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successive portions of the medium disposed between the said edges. each of width substantially equal to the half-wavelength of the standing waves, along the predetermined direction, a beam of light having a plurality oi successive portions corresponding to the plurality of successive half-wavelength portions of the medium, circular-polarinng means disposed to polarize the successive beam portions prior to their passage through the correspondlng medium portions. means for producing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium. means for directing the vibrations into the medium toward the other said edge of the medium in order that the vibrations may be reflected therefrom to set up the said standing waves, thereby rendering the successive medium portions birefringent in antiphase in response to the resulting standing waves, and means for modifying the vibrations in accordance with a signal.

25. A communication system having, in combination, a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined ultrasonic carrier vibrational wavelength to produce standing waves therein between a pair of edges of the medium spaced from each other in a direction substantially perpendicular to the said predetermined direction a distance corresponding to several times the said wavelength, becomes bir f n t0 he li h passing therethrough along the predetermined direction, means for determined direction, a beam of light having a plurality of successive portions correspondnig to the plurality oi successive half-wavelength portions of the medium. plane-polarizing means disposed to polarize the successive beam ortions prior to their passage through the corresponding medium portions, means for producing molecular vibrations of the said predetermined wavelength at one of the said edges of the medium, means for directing the vibrations into the medium toward the other said edge oi the medium in order that the vibrations may be reflected therefrom to set up the said standing waves. thereby rendering the successive medium portions birefringent in anti-phase, means for modulating the amplitude of the carrier-wave vibrations in accordance with a signal of greater wavelength, means for receiving the light passed through the medium, means for converting the received light into planepolarized light, and means for detecting the modulation of greater wavelength upon the ultrasonic carrier wavelength as variations in the converted light to reproduce the signal of greater wavelength.

EANS MUELLER. ROBERT E. RINES.

namnmvcns crran The iollowing references are oi. record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,565,566 Hartley Dec. 15, 1925 1,642,011 Chubb Sept. 13, 1927 1,694,661 Meissner Dec. 11, 1928 1,742,912 Hartley Jan. 7, 1930 1,849,488 Hanna Mar. 15. 1932 1,880,102 Meissner Sept. 27, 1932 1,885,604 Karolus Nov. 1, 1932 1,954,947 Pajes Apr. 17, 1934 1,997,371 Loiseau Apr. 9, 1935 1,997,628 Chubb Apr. 16, 1935 2,084,201 Karolus June 15, 1937 2,099,694 Land Nov. 23, 1937 2,234,329 Wolfi Mar. 11, 1941 2,418,964 Arenberg Apr. 15, 1947 2,531,951 Shamos et a1 Nov. 28, 1950 FOREIGN rii'rmws Number Country Date 132,858 Great Britain Sept. 23, 1919

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