US2707749A

US2707749A - System of light beam communication

Info

Publication number: US2707749A
Application number: US100360A
Authority: US
Inventors: Mueller Hans
Original assignee: Individual
Current assignee: Individual
Priority date: 1949-06-21
Filing date: 1949-06-21
Publication date: 1955-05-03
Anticipated expiration: 1972-05-03
Also published as: DE1119414B

Description

May 3, 1955 H. MUELLER SYSTEM OF LIGHT BEAM COMMUNICATION Filed June 21', 1949 5 Sheets-Sheet l Modulating Device l I 3 9 Analyzer I 7 Device Signal Source I 2 J 3 5 1 1 /MaduIat/'ngDev/'ce F] Amplifier Polarizing Device and Detector I max. Intensity Am ift/er 59 @174 Detector M d t e 11 a or Ose/l/atar an 0 //er Amp/tf/er SM .and Phase W Sh/ffer Attorneys May 3, 1955 H. MUELLER 2,707,749

SYSTEM OF LIGHT BEAM COMMUNICATION Filed June 21, I949 3 Sheets-Sheet 2 Output Signal Avera e Intensify 2 2. a '1'. J93; law-32".

Phase Shift Doubled Carr/er Modular/an Input Signal Carrier Carrier Intensify I M0 du/af/an Oufpuf Slgna/ Phase Sir/ff 3 Natural L lghf Carr/er Modular/an F I g. 2

Input Slgna/ ln van/or Hans Mueller by mmuaud A Horney;

May 3, 1955 H. MUELLER SYSTEM OF LIGHT BEAM COMMUNICATION Filed June 21, 1949 5 Sheets-Sheet 3 .219 a a d Fig. 8.

Modulator Oscillator Ma du/afor Oscf/lafar In venfor Hans Mueller y m a! m Attorneys United States SYSTEM OF L'KGHT BEAM CQIVZR EUNECATEGN Hans Mueller, Belmont, Mass., assignor of one-half to Robert H. Rimes, Belmont, Mass.

Application lane 21, 1949, Serial No. 1Gtl,36tl

34 Claims. (Cl. 256-7) The present invention relates to communication and more particularly to the transmission and reception of signal intelligence with the aid of electromagnetic waves.

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 signal transmission and reception.

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

In the copending application of Hans Mueller and Robert H. Rines, Serial No. 1,002, filed January 7, 1948, issued December 23, 1952, as United States Letters Patent No. 2,623,165, there is disclosed a novel light-modulation method and system for producing large light effects at ultrasonic frequencies substantially independent of frequency and with high signal-to-noise ratio.

A further object of the present invention is to provide a new and improved system employing the light-modulation 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.

In summary, the present invention involves producing a beam of electromagnetic waves having at least adjacent first and second polarized portions. portions are preferably complementarily plane-polarized. The first polarized portion of the beam is passed through a first transparent medium or medium portion and the second polarized portion of the beam is passed through a second transparent medium or medium portion adjacent the first medium or medium portion. The signal to be transmitted is produced, and a first component of the signal is fed to the first medium or medium portion to alter the state of polarization of the first portion of the beam emerging from the first medium or medium tion of the beam emerging from the second medium or I medium portion. The first and second beam portions emerging from the respective first and second media or medium portions are then directed along a common direction incoherently to superpose the beam portions and to form a partially polarized beam comprising a natural electromagnetic-wave component and a signal-varying resultant polarized component of the superposed said altered states of polarization of the first and second portions of the beam of electromagnetic waves. In accordance with a preferred embodiment of the invention, the media or medium portions are rendered birefringent by vibrational waves, adjacent media or medium portions being controlled by adjacent half-wavelengths or out-of-phase components of the vibrational-wave signal. A plurality of alternate medium portions may thus produce, in response to the vibrations in the medium, elliptically polarized waves from the corresponding incident alternate The adjacent beam "ice plane-polarized light beam portions, and adjacent alternate medium portions may produce complementary elliptically polarized waves from the corresponding complementary plane-polarized adjacent alternate incident beam portions. The resultant elliptically polarized component of the partially elliptically polarized beam produced by the incoherent superposition of the above-described elliptically polarized waves may then be received. By converting the resultant elliptically polarized component into a plane-polarized wave, the received beam may be analyzed and the signal intelligence detected. Preferred systems for carrying out the present invention are hereinafter described in detail.

The invention will now be more fully described in connection with the accompanying drawings, Fig. 1 of which is a block diagram schematically illustrating an apparatus operating in accordance with the broad principles underlying the present invention; Figs. 2, 3 and 4 are wave-form diagrams explanatory of the operation of the apparatus of Fig. 1; Fig. 5 is a perspective view of a combined transmitting and receiving communication system embodying the present invention in preferred apparatus form; Figs. 6 and 7 are explanatory wave-form diagrams illustrating the operation of the system of Fig. 5; Fig. 8 is a schematic side-elevation of a modified transmitter constructed in accordance with the present invention embodying successive layers of phase-shifting material; Fig. 9 is a View similar to Fig. 8 of another modification embodying a phase-shifting material and a plurality of differently oriented polarizing members; Fig. 10 is a view similar to Fig. 8 of a further modification in which a modulating signal is introduced by mechanically vibrating polarizing members.

In order that the terminology hereinafter employed to describe the various states of polarization of electromagnetic waves may be unambiguous and clear, the following definitions are presented.

Natural or unpolarized light, such as light from an ordinary light source, is of equal intensity along all directions lying in any plane normal to the direction of propagation of the light. Such light may be considered as comprising an electric vector, the amplitude or magnitude of which varies sinusoidally with time. The electric vector assumes all directions and phases of oscillation with the same probability. No matter what type of analyzer may be rotated in the path of a beam of light having such a natural state of polarization, no direction will be found in which the light intensity is greater than in any other direction.

Plane-polarized light, sometimes called linearly polarized light, may be considered as comprising an electric vector, the amplitude of which sinusoidally varies with time and the direction of which is always parallel to a particular predetermined direction in some plane normal to the direction of propagation of the light. Such planepolarized light may be produced, for example, with the aid of Nichol prism, a polaroid sheet, or a similar doubly refracting device inserted in the path of a beam of natural light. The plane formed by the direction of the electric vector and the direction of propagation is often termed the plane of polarization. The orientation or polarization of the electric vector depends upon the angular orientation of the plane-polarizing Nichol prism or similar device. If a second plane-polarizing device, frequently termed an analyzer, is rotated in the path of a planepolarized wave, a maximum intensity of light will be received through the analyzer when it is oriented parallel to the direction of orientation of the electric vector of the plane-polarized light. When the analyzer is oriented at right angles to the direction of polarization of the planepolarized wave, however, the light becomes extinguished. For all other orientations of the analyzer, more or less light intensity will penetrate the analyzer depending upon whether its angle of orientation is nearer to parallelism with or normality to the orientation of the electric vector of the light.

Circularly polarized light may be considered as comprising horizontal and vertical plane-polarized components of equal peak amplitude, and equal light frequency, but phase-displaced ninety degrees apart so that the resultant electric vector described a circle. Like natural light, the intensity of circularly polarized light, when viewed through a rotating plane-polarizing analyzer, for example, appears the same in all directions lying in any plane normal to the direction of propagation. circularly polarized light may be differentiated from natural light, however, by employing the combination of a quarterwave phase-shifting plate, hereinafter described, and a plane-polarizing analyzer. The quarter-wave plate will relatively shift the phase of the horizontal and vertical plane-polarized components another ninety degrees so that the resultant electric vector becomes once more a plane-polarized electric vector which may, of course,

be extinguished, as before explained, by proper orientation of the plane-polarizing analyzer. No possible combination or orientation of a quarter-wave plate and a plane-polarizing analyzer, however, will produce extinction of a beam of naturally polarized light. The quarterwave plate, previously referred to, may comprise a doubly retracting crystal in which the index of refraction presented to one component of an incident polarized light beam is sufficiently different from the index of refraction presented to the complementary component of the planepolarized light to cause the two components to be relatively phase-shifted ninety degrees or a quarter wavelength in passing through the plate. When the resultant electric vector of the phase-shifted components describes a circle by rotating clockwise, the polarization is said to be right-circularly polarized. When, on the other hand, the electric vector describes a circle in rotating counterclockwise, left-circularly polarized light is produced.

If the phase shift introduced between complementary plane-polarized components of an incident light beam is not exactly ninety degrees, but has a value ranging from between zero degrees and ninety degrees, or between ninety degrees and one hundred eighty degrees, and so on,

the resultant electric vector describes an ellipse and not a circle, and, such light is commonly termed elliptically polarized light. If the phase shift is ninety degrees but the amplitudes of the complementary plane-polarized components are not equal, elliptically polarized light is also produced. The magnitude of the electric vector and hence the intensity of the light will not be the same in all directions, as in the case of circularly polarized light, but will be less along the direction of one of the complementary plane-polarized components of the incident 7 light, termed the minor axis of the ellipse, than along the direction of the other complementary component, termed the major axis of the ellipse. When viewed through a rotating plane-polarizing analyzer, therefore, when the direction of orientation of the plane-polarizing analyzer is parallel to the major axis of the ellipse, a maximum intensity of light is received. When, on the other hand, the analyzer is oriented parallel to the minor axis of the ellipse, a minimum intensity of light is received, but there can be no complete extinction of light in any direction of orientation of the plane-polarizing analyzer. If a composite analyzer comprising a quarter-wave plate and a plane-polarizing analyzer is employed, there is a direction of orientation along which complete extinction is possible. This may be understood from the following considerations. Since the components of the elliptically polarized light along the major and minor axes of the ellipse are initially phase-displaced by ninety degrees, they will produce plane-polarized light when passed through the quarter-wave plate. A plane-polarizing analyzer, po-

sitioned beyond the quarter-wave plate, can thus extinguish the light. If, instead of the quarter-wave plate, a phase plate is employed that introduces a phase shift corresponding to the initial phase displacement between any other complementary component electric vectors than those parallel to the major and minor axes of the ellipse, then the other component electric vectors may be brought into phase to produce a plane-polarized wave. This plane-polarized wave may be extinguished by a properly oriented plane-polarizing analyzer. In right-elliptically polarized light, the resultant electric vector of the component electric vectors parallel to the major and minor axes of the ellipse rotates clockwise. Left-elliptically polarized light is produced by a counterclockwise rotation of the resultant electric vector in describing the ellipse. Circularly polarized light, before discussed, is a special case of elliptically polarized light in which the eccentricity of the ellipse is zero.

When there is a mixture of natural light and some other type of polarized light, the mixture is termed partially polarized light. A mixture of natural light and elliptically polarized light, for example, is termed partially elliptically polarized light.

When two light beams have different light frequencies or wavelengths, or differing phase-varying relationships, the beams are said to be incoherent. Light beams produced by two different light sources, for example, are incoherent, since they by necessity have different frequency or wavelength distributions and phase relationships. Light beams from a common source that travel over vastly different paths are similarly incoherent because of incoherent phase relationships. Light beams may even be transmitted from a common source along equal paths and may still be incoherent if one beam, for example, suffers a frequency shift, such as a Doppler shift, in passing along its path. Incoherent light beams, unlike coherent light beams, do not interfere to produce resultant effects such as diffraction patterns. Incoherent beams remain, instead, distinct and separate beams, one superimposed upon the other, but not interfering with the one other.

Referring to Fig. 1, adjacent light beams, 51 and 53 are shown passing through respective light-modulating

devices

1 and 3 which may, for example, comprise mechanical shutters, Kerr cells, birefringent media and the like. The light beams S1 and S3 may each have a particular predetermined state of polarization. The two beams may comprise adjacent portions of a single light source or may emanate from separate light sources. The beam S3 is shown passing through a polarizing device 5 of any desired character before reaching the modulating device 3. Even if, therefore, the beams S1 and S3 originate from the same source, they are incoherent by the time they reach the modulators l and 3.

It has previously been proposed to superpose adjacent incoherent beams such as, for example, rightand leftcircularly polarized light beams intermittently transmitted by shutter or other light-modulation systems. At a receiving station unequipped with a proper analyzer, the combination of rightand left-circularly polarized light would appear to be natural light and there would be no hint that signal intelligence was being transmitted along the beams. With the aid of a right or left quarterwave analyzer that will respectively produce a plus or minus ninety degree phase-shift between complementary plane-polarized components, however, one of the two beams, either the rightor left-circularly polarized beam, may be converted into a plane-polarized wave and the intelligence may be obtained therefrom.

In accordance with the present invention, however, a much more secret transmission system is provided by producing out-of-phase modulation upon adjacent preferably elliptically or circularly polarized beams.

Returning to the simple example of Fig. l, a source of signals, such as, for example, a sine-wave generator 7,

is shown operating the

modulating devices

1 and 3. The

source 7 preferably feeds one signal component to operate the modulating device 1 and a preferably out-ofphase signal component to operate the modulating device 3. The modulated light emerging from the device 1 is therefore out of phase with the modulated light emerging from the device 3. If the modulator 7 is a sine-wave generator, as before suggested, the intensity I1 of the light beam emerging from the device 1, may at a time i=0, Fig. 2, pass through a minimum value I1 minimum. One-half cycle later, at time t=t1, the light intensity I1 may have sinusoidally varied to a maximum value 11 maximum. At a later time t=r2, one-half cycle ,later, the light intensity I may again reach its minimum value I1 minimum. The intensity I3 of the difierently polarized incoherent beam emerging from the modulating device 3, on the other hand, at time t=0, has a maximum value I3 maximum, since the device 3 is operated in anti-phase with the device 1. At time t ri, onehal'r cycle later, the light intensity I3 passes through a mini- ;mum value I3 minimum. the light intensity I3 again passes through its maximum value I maximum. Assuming that the light intensities from the source S1 and the source S3 that impinge upon the

devices

1 and 3 are initially the same, and assuming that the

modulators

1 and 3 are identical in structure and performance, the maximum and minimum values of the emerging light intensities I and I3 will be the same. 11 maximum Will then equal I3 maximum, and I1 minimum will equal 13 minimum.

The two incoherent and diiferently polarized beams of intensity I1 and I3. having the same maximum and minimum intensities, are then directed into space along a common direction toward, for example, a photocell receiver or detector 9, a human eye, a camera, a lightsensitive mosaic, or some other light receiver. Since the receiver 9 does not differentiate between the different states of polarization of the light reaching it, it receives the incoherently superposed differently polarized beams of intensity I3 and ii. The resultant intensity received by the receiver 9, therefore, is a constant value Ir, as shown in Fig. 4, since the beams I1 and I rise and fall in intensity in anti-phase. The photocell or other receiver 9 cannot, therefore, detect the sine-wave or other modulating intelligence carried by the beams. The beams would appear, rather, as a single constant-intensity beam upon which no intelligence is super-imposed.

If, on the other hand, a proper type of analyzer it is inserted before the photocell 9 so that the state of polarization of one of the beams I1 and 13 may be eliminated, the receiver may receive only the other state of polarization of the other beam and the modulating signal intelligence may be recovered in the photocell 9. As a specific example, if the resultant beam In is a partially polarized beam comprising, as an illustration, a natural-light component and an elliptically polarized component, when this beam is focused upon the photocell 9 by, for example, a lens 8, the photocell 9 will detect no signal modulation. Even the use of a conventional plane-polarizing analyzer 11, would be of no avail in detecting the modulation intelligence. Only if a particular combination of an appropriate phase-shifting plate and a properly oriented plane-polarizing analyzer is used, that is capable of selecting, for example, the major axis of the elliptically polarized component in preference to the other components of the mixed natural and elliptically polarized beam, can the photocell 9 detect the signal modulation.

A practical system with the aid of which the present invention may be practiced is illustrated in preferred form in Fig. 5. An oscillator 13 is shown connected between the

electrodes

15 and 17 of a plurality of piezoelectric crystals 19, to vibrate the crystals in response to the oscillations of the oscillator 13. The electrode 15 is shown adjacent a preferably strain-free transparent medium 21 as of glass, fuzed quartz, or any similar transparent material. The vibrations of the crystals 19 One-half cycle later, at t=t2,

will cause molecular-vibration waves to be set up in the medium 21. It is to be understood, of course, that any other vibrating means, such as a magnetostrictive vibrator, a magnetomotive element or any other vibrating device, may equally well be employed.

As described in the said copending application, standing waves will be set up in the medium 21 as the vibration Wave travels to the top of the medium and reflects back down again. There will be nodal regions 23 of the medium 21 which remain unaifected by the vibration wave. The

medium portions

25, 27, 29, 31, etc. in between these nodal regions, however, do not remain un affected by the vibrations. .At any one instant of time, the medium portion 25, for example, may be compressed While the adjacent medium portion 27 is being dilated. The medium portion 2?, similarly to the medium portion 25, will also be at this time compressed, and the adjacent medium portion 31 will be dilated corresponding to the dilation of the medium portion 27. Each

medium portion

25, 27, 29, 31, etc., will pass from compression to dilation at twice the frequency of the vibrational wave propagated into the medium from the crystals 19 as a result of the reflection of the waves from the top of the medium. The

medium portions

25, 27, '29, 31, etc., thus act as separate media forming shutters to t.e light passing thereth-rough as they change from conditions of compression to conditions of dilation at double the frequency of the oscillations of the oscillator 13.

The distance M2 between successive nodal regions 23 of the medium 21, or the distance between corresponding portions of the

medium portions

25, 27, 29, 31, etc., corresponds to half the wave-length of the vibrational waves propagated into the medium .21 by the crystals 19. There is a time delay produced between the shutter operation of each successive section .25, 27, 29, 31, etc., that is dependent upon the period between successive oscillations of the oscillator 13.

As explained in the said copending application, when the

medium portions

25, 27, 29, 31, etc., become compressed or dilated, they become birefringent to polarized light entering the medium 21. A portion of the beam from a parallel-ray light source 33-53 may pass through a plane-polarizer 35 having an orientation of, for example, forty-five degrees with respect to the vertical, and may then penetrate the corresponding medium portion 25 of the medium 21. As the medium portion 25 is compressed by the vibrational waves, the indices of refraction of the medium portion 25 for the vertical and horizontal electric vector components of the forty-five degree incident plane-polarized light become unequal. The horizontal and vertical electric vector components becomes thus phase displaced so that the resultant light emerging from the medium portion 25 is, in general, elliptically polarized. The difference in the indices of refraction of the medium portion 25 presented to the vertical and horizontal electric vector components of the incident light by the vibrational straining of the medium, is called the birefringence of the medium. It is this birefringence which produces the phase displacement or shift A between the vertical and horizontal electric vector components of the incident light as given by the equation where T is the thickness of the medium from the front to its rear face, in is the index of refraction of the medium presented to the vertically polarized component, F111 is the refractive index presented to the horizontally polarized component, 7\ is the wavelength of the sound waves or other vibrational waves in the medium 21, and 1r is the ratio of the circumference to the diameter of a circle.

As can be seen from the above equation, increased thickness of the medium shutter 21 increases its birefringence-producing efiiciency. The power required to obtain a given birefringent effect decreases as the medium thickness increases, the relation being that of the inverse square, but the larger the medium the more difficult it is to manufacture it strain-free and the more serious heating and other undesirable effects.

If the phase shift A has a value between zero and ninety degrees, the forty-five degree polarized light incident upon the medium portion 25 will become elliptically polarized, as shown schematically at 45, with the major axis of the ellipse at a forty-five degree angle with respect to the vertical. The eccentricity of the ellipse depends upon the amount of the phase shift Aqs. When, of course, this phase shift is exactly ninety degrees, the emerging light is circularly polarized.

When, on the other hand, the medium portion 25 becomes dilated by the ultrasonic waves, the refractive index lZv again becomes different than the index in}, but in the opposite sense from the difference produced by the compression of the medium 25, so that the phaseshift A has the same magnitude but the opposite sign. If the birefringence produced by compression is referred to as positive birefringence, and the phase-shift is re ferred to as a positive phase shift +IA then when the medium is dilated, an equal amount of negative bifringence is produced and an equal phase shift --[A| results. The light beam emerging from the dilated medium portion 25 will again be elliptically polarized, but the major axis of the ellipse will be in the minus forty-five degree direction with respect to the vertical, complementary to the major axis of the elliptically polarized waves previously discussed for the case of compression.

Neglecting, for the moment, the time difference between the instants that each vibration Wave reaches the successive portions of the medium 21, at the time that the medium portion 25 is being compressed, the medium portion 29 is also compressed and the medium portion 27 and the medium portion 31 are dilated. The light beams passing through the

medium portions

25 and 29 will thus be elliptically polarized as shown at 45 and 49, while the light beams passing through the

medium portions

27 and 31 will be complementarily elliptically polarized as shown at 47 and 51. The alternate

medium portions

25 and 29 are thus operated in anti-phase with the adjacent alternate

medium portions

27 and 31, outof-phase components of the vibrational signal produced by the crystals 19, controlling adjacent medium portions. Actually, of course, the light emerging from the various medium portions is rapidly passing from a plane-polarized state to various states of elliptically polarized light, 7

back to plane-polarized light, and back to other states of elliptically polarized light, as the amplitude of the signal vibration-wave sinusoidally increases and decreases to produce its effects upon the medium 21. In general,

however, the light becomes elliptically polarized, and it is convenient, for purposes of explanation, to consider the elliptically polarized states 45 and 49 produced by the action of the

medium portions

25 and 29 on the incident light beam at a particular time during, for example, compression, and the elliptically polarized states 47 and 51 produced at a corresponding time by the action of the

medium portions

27 and 31 during dilation.

The electric vector describing the elliptically polarized beams 45 or 49 produced during compression of the respective

medium portions

25 and 29, will rotate in the opposite direction to the direction of rotation of the electric vectors describing the complementary elliptically polarized beams 47 and 51, produced during dilation of the respective

medium portions

27 and 31. This occurs because the phase shift produced by the medium portions 26 and 29 during compression is of opposite sign to the phase shift produced in the

medium portions

27 and 31 during dilation, as before explained.

The light emerging from the medium 21 comprises, therefore, a partially polarized beam having a natural wave-component and a resultant elliptically polarized wave-component corresponding to the resultant of the elliptically polarized beams 45, 47, 49 and 51. Partial cancellation may, of course, take place between the adjacent light beams 45, 47, 49 and 51-because of the oppositely rotating electric vectors. A principal resultant direction of polarization may thus be produced. Any receiving station having at least a plane analyzer will thus be informed that the light beam contains signal intelligence which it may then intercept.

if the plane-polarized light impinged upon the medium portion 25 is polarized at an angle of plus fortyfive degrees from the vertical by the adjacent polarizer 35, and the light impinged upon the next adjacent medium portion 27 is polarized at an angle of minus fortyfive degrees from the vertical by its adjacent planepolarizer 37, shown oriented at right angles to the polarizer 35, and the next medium portion 29 receives light from its adjacent polarizer 39 polarized at plus fortyfive degrees, and the next medium portion 31 receives light from its adjacent polarizer 41 polarized at minus forty-five degrees, then the electric vectors of the elliptically polarized beams 45, 47, 49, 51, etc., emerging from the respective

medium portions

25, 27, 29, 31, etc., will all rotate in the same direction. Again a combination of naturally polarized light and a different resultant elliptically polarized light is formed. Since, however, the electric vector of the light emerging from all the medium portions is at any time always rotating in the same direction, there is no cancellation of the light intensity emerging from successive medium portions, as in the previously discussed case where the electric vectors of alternate medium portions rotate in opposite directions.

The use of the successive strips of differently oriented

polarizers

35, 37, 39, 41, etc., furthermore, which will be hereinafter referred to as a stripped polarized, produces more efficient operation than can be produced with a system of the type described in the said copending application in which each medium portion of the medium 21 receives light of the same polarization. The reasons for this improved efliciency will be subsequently explained.

The beams of elliptically polarized waves emerging from the

medium portions

25, 27, 29, 31, etc., are directed along a common direction, indicated by the arrow in Fig. 5, by means of the directing action of a parabolic reflector 53. The reflector 53 will direct substantially parallel rays of light from the source 33 through the medium 21. A filter 55 may be used if it is desired to transmit monochromatic electromagnetic waves. If an infra-red ray filter 55 is employed, the transmitted light waves will, of course, be invisible to the eye. This is desirable in secret signaling applications.

The cumulative light beam formed by the adjacent incoherent beams emerging from the

medium portions

25', 27, 29, 3.1, etc., appears indistinguishable from natural light at a. distant station. The incoherent superposition of these beams produces, as before mentioned, a partially polarized beam comprising, in general, a natural-light component and an elliptically polarized component that is the resultant of the superposition or mixture of the elliptically polarized waves 45, 47, 49, 51, etc., emerging from the adjacent

medium portions

25, 27, 29, 31, etc. If, for example, a photocell 9 at the focus of a parabolic j light-receiving reflector 57, receives this cumulative beam,

it will merely indicate a constant intensity of light in an amplifier or other circuit connected thereto, without indicating the presence of any modulation. The photocell 9 may, of course, be replaced by any other light detector such, for example, as a light-sensitive mosaic, or a photographic film of either the stationary or moving-picture type, but still no modulation or intelligence will be detected. if an analyzer, such as the plane-polarizing analyzer 58, is inserted before the photocell or other detector 9, furthermore, no possible orientation of the plane-polarizing analyzer 58 will provide detection of a principal direction in which the light intensity is greatest, and it is thus impossible to tistinguish the received light beam from an unmodulated constant-intensity light beam of natural light. No one receiving the light beam with conventional equipment, would even know that polarized light was being employed, and the presence of signal intelligence in the light beam would not be detected.

If, however, an appropriate birefringent medium or phase-shifting plate ti is used in conjunction with the plane-polarizer analyzer to comprise a composite appropriately oriented analyz r, and if the phase-shifting plate so produces a sufiicient phase shift between the complementary electric vector components of the resultant elliptically polarized-wave component of the received partially elliptically polarized beam to bring back into phase or into antiphase the electric vector components parallel to and normal to the major axis of the el iptically polarized resultant wave, then, and then only, will the waves emerging from the phase shifter 6% be planepolarized so that they may be analyzed by the planepola-rizing analyzer 53. At this time, assuming that the photocell 9 is sensitive to the light-beam frequency and that its amplifier system is tuned to the carrier frequency of the oscillator 13, the modulation signal superimposed thereon may be detected.

For a particular medium 21 having a particular thickness T and a particular amplitude of compressional wave, the phase shift introduced by the periodic birefringence of the medium in response to the ultrasonic waves may produce circularly polarized light emerging from the medium portions 25, 2'7, 29, Si, etc. The rotation of the electric vector of the circularly polarized light emerging from the

various portions

25', 27, 29, 31, etc. of the medium 31 will always rotate together in a common direction if the

complementary polarizers

35, 37, etc. are employed as above described. The electric vectors will rotate first clockwise, then counterclockwise, then clockwise again in response to the positive and negative birefringence periodically produced at the frequency of the transmitted and reflected vibrational waves in the medium Zll. Similarly, in the general case where an elliptically polarized component of the mixture of natural and elliptically polarized light is produced, the electric vector will rotate first clockwise, then counterclockwise, then clockwise, again, at the radio-frequency of the oscillator 13. The transmitted beam thus appears indistinguishable from natural light at conventional lightreceiving stations.

Unless, therefore, the receiving station is equipped first, with a particular com osite analyzer comprising a phase-shifting plate 6i) of proper phase-shifting properties and a plane-polarizing analyzer 53 appropriately relatively orientated; second, with a photocell 9 of characteristics such that it responds to the wavelength of the light wave; and third, with an amplifier system tuned to the carrier frequency of the oscillator 13, it is not possible to detect that polar zation of light is being employed, let alone to detect the signal intelligence being transmitted along the light beam. Extremely secure signalling may thus be effected.

The oscillator 13 preferably comprises an ultrasonic or radio-frequency oscillator for causing the piezoelectric crystals 19 to produce standing waves of high frequency in the medium 21. l have successfully employed oscillating frequencies ranging from audio frequencies up to and including high radio frequencies. The standing waves produced by tie oscillator 13 driving the crystals 19, moreover, as discussed in the said copending application, may be employed as a carrier-frequency wave and in this connection may be modulated by a modulator 59 such as a source of audio or video signals, or a source of pulses for keying the oscillator on and off, or any other modulating signal source.

I have, for example, constructed and successfully operated systems of the type illustrated in Fig. 5 employing square media 21 of optical crown glass and pyrex glass six inches long, six inches wide and two inches thick. Such media require about 0.1 watt of driving energy per cubic inch of volume to operate satisfactorily as lightmodulation shutters. The opposite surfaces of the media were parallel to 0.001 inch. A similar medium of fused quartz was found to yield greater birefringent effects and hence more intense results than the optical glass with 10 to 50 times lower driving power requirement because of low acoustic damping, but it was difficult to maintain uniform nodal portions 23 when the ultrasound waves were propagated into the medium 21, and sharp resonance points of the shutter rendered it satisfactory only for dotdash communication and not for audio or video modulation. Two parallel rows of six X-cut quartz crystals of crystal dimensions one by one by 0.283 inch were attached to one of the six-inch by two-inch surfaces of the medium 21 by soldering their upper electrodes to the medium. The common electrode 15 of Fig. 5, for example, may be so secured to the bottom surface of the medium 21. These crystals were driven by a conventional well-shielded Hallicrafter crystal oscillator circuit 13 a a radio-frequency of about 0.4 megacycle. A large number of mechanical resonant frequencies of the medium 21 can be found, usually about 10 kilocycles apart. For the previously described media 21, a sharp set of nodes and a maximum birefringence effect was produced for the mechanical resonance resulting at 0.4 megacycle vibrational frequency. Vibrational waves of this frequency produced in the medium 21 about nine adjacent periodically compressed and dilated portions 2-5, 27, 29, 31, etc. Successive strips of

polaroid

35, 37, 39, 41, etc., one corresponding to each

respective medium portion

25, 27, 29, 31, etc., and each oriented to planepolarize light at right angles to the plane of polarization of the next adjacent strip, where placed adjacent the medium 21. The width of each strip was substantially the same as the width of each of the medium portions, which, in turn, was substantially the same as the half wavelength of the standing waves produced in the medium. The alternate

polarizing strips

35 and 39 were placed adjacent the

alternate indium portions

25 and 29; the adjacent

alternate strips

37 and 41 were placed near the adjacent alternate

medium portions

27 and 31; and so on.

The birefringence produced by longitudinal strains set up in the medium 21 by the ultrasonic vibrations was employed to polarize elliptically the incident li ht waves, by polarizing the incident waves along directions plus or minus forty-five degrees from the vertical, as before discussed. When the polarizing strips 35, 3f, etc. were oriented at plus forty-live degrees, with respect to the vertical or the direction of propagation of the vibra tional waves, and the

strips

37, 41, etc. were oriented at minus forty-five degrees, the before-described results were obtained in response to the birefringent action of these longitudinal strains. The birefringence produced by transverse stresses, also set up in the medium 21 by the vibrations, was also employed by polarizing the incident light vertically or horizontally. The alternate

polarizing strips

37, 41, etc. were, for example, oriented vertically while the alternate intermediate strips 3:1, 39, etc. were oriented horizontally to employ the birefringence produced by the transverse stresses. These transverse modes of vibration, however, have been found less preferable for the purposes of the present invention than the longitudinal vibrational modes because of poor vibrational patterns in the medium 21 and because of relatively narrow signal band widths obtainable therewith. Orientations of the polarizing strips between fortyiive degrees and the vertical or horizontal produced results partaking of the no. ture of both transverse-and longitudinal-mode birefringence, the transverse-node effect predominating the nearer the orientation to the vertical or horizontal, and

the longitudinal-mode effect predominating closer to plus or minus forty-five degrees.

An automatic-frequency control was incorporated at the transmitter by employing a feedback crystal 2 similar to the crystals 19 at the top surface of the medium 21 opposite the driving crystals 19. A plurality of feedback crystals may, if desired, be employed. The vibrations transmitted through the medium are received by and will vibrate the crystal 2 and the resulting oscillations may be amplified in an amplifier 4 and fed back, for example, betwen the grid and cathode of the oscillator tube of the crystal oscillator 13 to control the frequency of the oscillator 13. The shutter medium 21, therefore, automatically controls the frequency of the oscillator 13 as small shifts occur in the mechanical vibration or resonance frequency of the shutter resulting from temperature changes during operation. The amplifier 4 preferably contains a conventional phase-shifter to insure proper phase feedback from the crystal, as is well known in the art.

Employing the before-described two-inch thick medium 21 of optical glass about six to eight inches in front of a 450-watt airplane landing lamp-reflector system 33-53, having a beam width of about 7 to 10 degrees, an infrared filter 55 having a peak sensitivity of from between 8000 and 10,000 Angstroms, an infra-red-light-type polaroid stripped

polarizer

35, 37, 39, 41, etc., I successfully transmitted and received speech modulation over distances of up to seven miles. At the receiving end, I employed thin cellophane sheets 60, which are substantially quarter-Wave plates for light of 9000 Angstroms wavelength, an infra-red-type polaroid plane-analyzer 58, a Farnsworth type 6?EA six-stage photo-tube multiplier 9 having substantial sensitivity for wavelengths of the order of 10,000 Angstroms and a maximum sensitivity at about 8000 Angstroms, and an associated receiving circuit responsive to the 0.4 megacycle radiofrequency carrier of the oscillator 13 and provided with a detector for detecting the modulation superimposed on the carrier.

In the systems described in the said copending application the receiving system, even if it comprises a quarter-wave plate 60 in addition to the plane-polarizing analyzer 58, produces a signal at twice the frequency of the carrier-wave propagated into the medium 21 by the oscillator 13. The photocell receiver circuit must thus be tuned either to the modulation envelope of the carrier wave or to the second harmonic of the frequency of the oscillator 13. In accordance with the principles of the present invention, however, the receiver can be tuned only to the frequency of the oscillator 13, as will now be explained.

The intensity I of the light received through the analyzer 58 by the photocell 9 depends upon the square of the sine of half the phase shift produced by the irefringence in the medium 21. This relationship between intensity I and phase-shift 4) is in part plotted in curve A of Fig. 6. The system disclosed in the said copending application operates so that each portion of the medium 21 between nodes, such as the portion 25, for example, goes through compression strain, zero strain, and dilation strain, twice each cycle of the vibrational waves propagated into the medium, because the medium responds to both the directly propagated and reflected Waves. The light penetrating the analyzer 58, therefore, becomes completely extinguished twice for each cycle of the sound waves. This is equivalent, therefore, to operating from a point 0 on the curve A, Fig. 6, first swinging positively up on the right-hand side of curve A from point 0 to point P, then swinging back through point 0 to point -P on the left-hand side of the curve A. Consider, therefore, an input signal consisting of a carrier wave and a modulation signal superimposed thereon, fed to the medium 21 by the vibrating crystal 19. As shown graphically in Fig. 6, the modulated carrier oscillates between limits of P and -P of the curve A as each section of the medium 21 periodically becomes compressed and dilated in response thereto. The output signal, appearing as an intensity-modulated light beam, is also plotted in Fig. 6, varying between the horizontal base line drawn through the point 0 and the parallel horizontal line drawn through the points P and --P. The modulation envelope is shown produced as an output signal with, however, double the carrier frequency supporting the modulation, as above described. The shading indicates that each of the double carrier impulses produces, in general, elliptically polarized light beams from adjacent portions of the medium, the electric vectors of adjacent beams rotating in opposite directions. The average light intensity that may be detected with a plane analyzer and a photocell receiver is shown in dotted lines, labelled Average intensity.

With the system of the present invention, on the other hand, operation is effected, not about point 0, but about a point 0 on, for example, the right-hand portion of the intensity-phase-shift curve A plotted in Fig. 7. In effect, the present system is optically biased so that the light intensity does not periodically pass through a zero point as in Fig. 6, but passes from point 0' upward to a point Q and downward through 0 to a point R, all on one side of the zero point 0. In this manner there is never any complete extinction produced by the analyzing system 58-60, but, as shown, the modulated carrier-wave input signal operates between points R and Q to produce a modulated output carrier signal of the same frequency, and not double the frequency of the input carrier signal. The shading of the carrier in the output signal indicates the presence of elliptically polarized light. In the first cycle all of the portions of the medium have electric vectors rotating, for example, clockwise, as shown by the clockwise curved arrow, and in the next cycle all of the elliptically polarized light rotates counter-clockwise, as indicated by the counter-clockwise curve arrow, and so on. The average intensity of the output signal, however, is not low, as in the case of the system of the copending application, the performance of which is shown in Fig. 6. The average intensity is, on the contrary, high. There is, moreover, a constant presence of a natural light component in the output signal, labelled Natural light and shown cross-hatched. By means of operating at an optically biased point 0, furthermore, on the substantially linear portion of the intensity-phase-shift curve A, a greatly increased amplification or gain is produced over the signal gain of the systems that operate, as shown in Fig. 6. The combination in the output signal of the natural light component and the elliptically polarized resultant light component, the direction of rotation of the resultant electric vector of which periodically varies from clockwise to counterclockwise at the frequency of the carrier wave produced by the oscillator 13, is in itself indistinguishable from natural light without special equipment, as before described, and permits great security because of the large number of factors that must necessarily coincide to detect the modulated elliptically polarized resultant component.

If security is not desired, of course, the phase-shifting plate 60 may be employed at the transmitter side near the medium 21 instead of in the receiver. The stripped

polarizers

35, 37, 39, 41, etc., may, if desired, furthermore, be applied to the medium as by cement or any other means.

If desired, moreover, as shown in Fig. 10, the oscillator 13 may continuously oscillate the crystal 19 with a continuous-wave carrier. The modulation of that carrier may be effected by vibrating the stripped polarizers. The stripped

polarizers

35, 37, 39, 41, etc. may be carried by a holder 62, which is yieldingly mounted between supports 64, so that in their normal positions,

the polarizers are adjacent the nodes 23 in the medium 21. A portion of the support 62, shown at 66, may be of ferromagnetic material in order that, in response to the modulation signal of the modulator 59, as manifest in a coil 5'8 wound about the portion 66, the carrier 62 may be periodically moved up and down periodically to bring the

strip polarizers

35, 37, 39, 4-1, etc., in front of the respective

medium portions

25, 27, 29, 31, etc. In this manner the modulation signal is applied by means of the vibrating of the stripped polarizers, the carrier signal being applied by the oscillator 13, driving the crystals 19 to vibrate the medium 21.

Again, where secrecy is of less importance, an optically biased system may be provided by employing a single constant polarizer 65, Fig. 8, for polarizing all the incident light impinging on the medium 21 in the same plane of polarization. Adjacent the medium 21, however, are stripped phase-shifting plates shown, for example, as quarter-wave plates 75, 77, 79 and 81. Assuming that the thickness of the medium 21 is such that substantially circularly polarized waves emerge from the medium, the plate 75, for example, may produce a positive ninety-degree phase shift in the circularly polarized light emerging from its adjacent medium portion 25; the strip of phase-shifting material 77, on the other hand, adjacent the medium portion 27 will produce a negative ninety-degree phase shift; the phase-shifting plate 79, adjacent the medium portion 29, may produce a positive ninety-degree phase shift and the strip 81, adjacent the medium portion 31, a negative ninety-degree phase shift. The array of phase-shifting plates 75', 77, 79, 81, etc.,

acteristic, Fig. 7, is produced and light comprising a natural-light component and, in general, a resultant elliptically polarized component resulting from the incoherent superposition of the elliptically polarized waves emerging from adjacent portions of the medium shutter 21 are produced.

Alternately, the medium 21 may itself be initially stressed as by putting it under a constant pressure from a vise, not shown, between, for example, its top and bottom surfaces so that, in its initial state, even in the absence of vibrations produced by the crystals 19, the medium constitutes a birefringent half-wave phase-shifter in and of itself. Similarly, piezoelectric and other crystals having a permanent berefringent stress may also be employed.

The system of Fig. 5, in which a stripped

polarizer

35, 37, 39, 41, etc., is used in conjunction with a uniform phase-shifting plate 60, will hereinafter be called system (a). The system of Fig. 8, in which a uniform polarizer 66 is employed in conjunction with a stripped phaseshifting plate 75, 77, 79, 81, etc., will hereinafter be called system (b). Assume that the birefringence produced in the medium 21 is suificient to produce a ninetydegree phase shift between horizontal and vertical components of incident plane-polarized light. A comparison of the results then produced by systems (a) and (b) for incident light of different types of polarization when the systems are quiescent, as when, for example, the oscillator 13 is inoperative, and when the systems are operative, as when the ultrasound waves are propagated therein, follows:

Plane Polarized Circularly Polarized Elliptically Polarized Incident Light System System System System System System Quiescent Operating Quiescent Operating Quiescent Operating System (a) Natural- Partially Circular. Natural Partially Circulan Natural Partially Elliptical. System (1)) Plane..." Partially Plane do Partially Planem. Partially Plunc Partially Plane.

may comprise alternate right and left quartz quarterwave plates, and may be placed either between the plane polarizer 66 and the medium 21 or directly after the medium 21, as shown in Fig. 8. But in either case it is preferably positioned very close to the medium 21 in order to obtain close correspondence between the phaseshifting plates and the respective medium portions of the medium 21.

Again, if security in transmission is not of the prime essence, optically biased performance may be obtained by positioning between the plane polarizer 66 and the medium 21 a phase-shifting plate such as, for example, a quarter-wave plate 68, Fig. 9, for a medium 21 of such thickness as to produce substantially a ninety-degree birefringent phase shift. In this manner, circularly polarized light impinges upon all of the sections in the medium 21. Circularly polarized light penetrates the medium 21 and becomes plane-polarized therein as a result of the ninety-degree phase shifts produced by the medium. The strips of

polarizers

35, 37, 39, 41, etc., similar to those used in connection with the embodiment of Fig. 5, may then be placed to receive the emerging plane-polarized waves from their respective

corresponding regions

25, 27, 29, 31, etc., of the medium 21. The plate 68 need not be a quarter-wave plate if the medium produces less than or more than substantially a ninety-degree phase shift, but, in general, will produce a phase shift corresponding to the phase shift produced by the medium when birefringent. It is to be understood that even if the phase shift produced in the medium does not exactly equal the phase shift of the system 66-68, elliptically polarized waves of large eccentricity, almost plane-polarized, will result which can be almost extinguished by the

polarizers

35, 37, 39, 41, etc.

In all cases, however, a biased operation on the substantially linear portion of the intensity-phase-shift char- I have shown both theoretically and in practice that system (:2) provides a more eflicient system than system (b). System (a), moreover, provides secrecy which is unobtainable with system (b) since the use of merely a plane polarizer will detect that the transmission from system (b) involves the polarization of light. This may be seen from the above table. System (b) will always produce partially plane-polarized light, when ultrasound Waves are propagated in the medium 21, whereas system (a) produces partially circular or partially elliptically polarized light. If, furthermore, the phase-shifting plate employed in system (b) is not exactly of the right design, an observer with a plane polarizer will actually be able to observe dot-dash communication if it is being employed, or with the aid of a photocell and audio amplifier, will detect voice or other modulation intelligence. In system ((1), however, the periodically alternating right and left elliptically polarized beams, emerging from the

medium portions

25, 27, 29, 31, etc., form a partially elliptically polarized beam that is practically indistinguishable from natural light. An observer requires not only a very accurate proper phase plate and polarizing analyzer and a proper radio-frequency tuned photocell receiver, but also a proper relative orientation of the phase plate and polarizing analyzer in order to detect any intensity fluctuations or to find out that the transmission even involves the polarization of light.

Systems constructed in accordance with the present invention are particularly insensitive to jamming or interference. This is true since the use of the proper phase plate 6%) permits distinguishing between even much stronger interfering natural, plane or other polarized light and the periodically changing left and right elliptically or circularly polarized light emerging from the medium The medium 21 may be vibrated in compressional,

transversal, flexural, torsional and other modes depend ing upon the coupling of the crystals or other vibrators to the medium, the degree of homogeneity of the medium, and the transverse and longitudinal reflections of the Waves from the boundaries of the medium, as well as other factors.

If the incident light rays pass obliquely through the medium 21, penetrating adjacent layers or portions of the medium, they Will emerge with a phase shift that is a resultant of the phase shifts produced by the positive and negative birefringence of the medium portions through which the rays pass. On integration, the intensities of some oblique rays will therefore cancel the intensities of other rays so that the light intensity will increase and decrease with angular position of the axis of the transmitter, passing through several maxima and minima. Instead of reducing the angular aperture of the light beam to overcome this effect, it is desirable to reduce the sound-vibration frequency to keep the size of the layers as large as possible, and to place the stripped polarizers of Figs. 5, 9 and 10 and the stripped phase plates of Fig. 8 as close to the medium shutter 21 as possible. If, however, it is desired to obtain a larger angular aperture well-known spreading lenses, one corresponding to each

portion

25, 27, 29, 31, etc., may be placed between the light source 33 and the corresponding medium portions to converge the light rays to a focus within the corresponding

portions

25, 27, 29, 31, etc. of the medium, the rays then diverging from each medium portion into beams of Wider angular aperture.

For any given frequency, the stripped shutter system cannot thus be used for as large an angular aperture as the uniform shutter disclosed in the said copending application. The stripped shutter system of the present invention, furthermore, while producing more efficiency and greater secrecy, requires special stripped polarizing or phase-shifting elements for each carrier frequency.

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

What is claimed is:

1. An apparatus for signal transmission that comprises means for producing a beam of electromagnetic light Waves having adjacent first and second portions, means for polarizing the first portion of the beam, means for simultaneously polarizing the second portion of the beam, means for passing the first polarized portion of the beam through a first light transparent medium, means for passing the second polarized portion of the beam through a second light transparent medium adjacent the first medium, means for rendering the first and second portions of the beam incoherent, means for producing the signal to be transmitted, means for rendering the first medium birefringent in response to a first component of the signal thereby to alter the state of polarization of the first portion of the beam emerging from the first medium and for rendering the second medium birefringent in response to a second component of the signal thereby to alter the state of polarization of the second portion of the beam emerging from the second medium, and means for directing the incoherent first and second portions of the beam emerging from the respective first and second media along a common direction incoherently to superpose the beam portions and thereby to form a partially polarized beam comprising a natural electromagnetic-Wave component and a signalvarying resultant polarized component of the superposed said altered states of polarization of the first and second portions of the beam of electromagnetic Waves.

2. An apparatus for signal transmission that comprises means for producing a beam of electromagnetic light Waves having adjacent first and second portions, means for plane-polarizing the first portion of the beam along a predetermined plane, means for plane-polarizing the second portion of the beam along a different predetermined plane to render the second beam portion incoherent with the first plane-polarized portion of the beam, means for passing the first plane-polarized portion of the beam through a first light transparent medium, means for passing the second plane-polarized portion of the beam through a second light transparent medium adjacent the first medium, means for producing the signal to be transmitted, means for feeding a first component of the signal to the first medium to render the first medium birefringent thereby elliptically to polarize the first portion of the beam emerging from the first medium and for feeding a second component of the signal to the second medium to render the second medium birefringent, thereby elliptically to polarize the second portion of the beam emerging from the second medium, and means for directing the incoherent first and second elliptically-polarized beam portions emerging from the respective first and second media along a common direction incoherently to superpose the beam portions and thereby to form a partially elliptically polarized beam comprising a natural electromagnetic-Wave component and a signal-varying resultant elliptically polarized component of the superposed said elliptically polarized first and second portions of the beam of electromagnetic waves.

3. An apparatus for signal transmission that comprises means for producing a beam of electromagnetic light Waves having adjacent first and second portions, means for plane-polarizing the first portion of the beam along a predetermined plane, means for plane-polarizing the second portion of the beam along a different predetermined plane to render the second beam portion incoherent with the first plane-polarized portion of the beam, means for passing the first plane-polarized portion of the beam through a first light transparent medium, means for passing the second plane-polarized portion of the beam through a second light transparent medium adjacent the first medium, means for producing the signal to be transmitted, means for feeding a first component of the signal to the first medium to render the first medium birefringent thereby circularly to polarize the first portion of the beam emerging from the first medium and for feeding a second component of the signal to the second medium to render the second medium birefringent, thereby circularly to polarize the second portion of the beam emerging from the second medium, and means for directing the incoherent first and second circularly-polarized beam portions emerging from the respective first and second media along a common direction incoherently to superpose the beam portions and thereby to form a partially circularly polarized beam comprising a natural electromagnetic-Wave component and a signal-varying resultant circularly polarized component of the superposed said circularly polarized first and second portions of the beam of electromagnetic waves.

4. An apparatus for signal transmission that comprises means for producing a beam of electromagnetic light waves having adjacent first and second portions of substantially the same intensity, means for plane-polarizing the first portion of the beam along a predetermined plane, means for plane-polarizing the second portion of the beam along a plane substantially perpendicular to the said predetermined plane, thereby rendering the first and second polarized beam portions incoherent, means for passing the first plane-polarized portion of the beam through a first light transparent medium, means for passing the second plane-polarized portion of the beam through a second light transparent medium similar to and adjacent the first medium, means for producing the signal to be transmitted, feeding a first component of the signal to the first medium to render the first medium birefringent thereby elliptically to polarize the first portion of the 17 beam emerging from the first medium with the major axis of the ellipse disposed along a predetermined direction and for feeding a second component of' the signal subtantially identical with but out of phase with the first signal component to the second medium to render the second medium birefringent thereby elliptically to polarize the second portion of the beam emerging from the second medium with the major axis of the ellipse substantially perpendicular to the said predetermined direction, and means for directing the incoherent first and second elliptically polarized beam portions emerging from the respective first and second media along a common direction incoherently to superpose the beam portions and thereby to form a partially elliptically polarized beam comprising a natural electromagnetic-wave component and a signal-varying elliptically polarized resultant component of the superposed elliptically polarized first and second portions of the beam of electromagnetic waves.

5. The apparatus described in claim 4 and in which the first plane-polarizing means is oriented so that the first portion of the beam of electromagnetic Waves is plane-polarized at forty-five degrees with respect to a dimension of the firstmedium.

6. The apparatus described in claim 4 and in which the firstplane-polarizing means is oriented so that the first portion of the beam of electromagnetic waves is plane-polarized along a plane substantially parallel to a dimension of the first medium.

7. An apparatus for signal transmission that comprises means for producing a beam of electromagnetic light waves having adjacent first and second portions of substantially the same intensity, means for plane-polarizing the first portion of the beam along a predetermined plane, means for plane-polarizing the second portion of the beam along a plane substantially perpendicular to the said predetermined plane, thereby rendering the first and second polarized beam portions incoherent, means for passing the first plane-polarized portion'of the beam through a first light transparent medium, means for passing the second plane-polarized portion of the beam through a second light transparent medium similar to and adjacent the first medium, means for producing the signal to be transmitted, means for feeding a first component of the signal to the first medium to render the first medium birefringent thereby circularly to polarize the first portion of the beam emerging from the first medium and for feeding a second component of the signal substantially identical with but out of phase with the first signal component to the second medium to render the second medium birefringent thereby circularly to polarize the second portion of the beam emerging from the second medium, and means for directing the incoherent first and second circularly polarized beam portions emerging from the respective first and second media along a common direction incoherently to superpose the beam portions and thereby to form a partially circularly polarized beam comprising a natural electromagnetic-wave component and a signal-varying circularly polarized resultant component of the superposed circularly polarized first and second potrions of the beam of electro-magnetic waves.

8. An apparatus for signal communication that comprises means for producing a beam of electromagnetic light waves having adjacent first and second portions of substantially the same intensity, means for plane-polarizing the first portion of the beam along a predetermined plane, means for plane-polarizing the second portion of the beam along a plane substantially perpendicular to the said predetermined plane thereby rendering the first and second polarized beam portions incoherent, means for passing the first plane-polarized portion of the beam through a first light transparent medium, means for passing the second plane-polarized portion of the beam through a second light transparent medium similar to and adjacent the first medium, means for producing the signal to be transmitted, means for feeding a first com ponent of the signal to the first medium to-render the first medium birefringent thereby elliptically to polarize the .first portion of the beam emerging from the first medium with the major axis of the ellipse disposedalong a predetermined direction and for feeding a second component of the signal substantially identical with but out of phase with the first signal component to the second medium to render the second medium birefringent thereby elliptically to polarize the second portion of the beam emerging from the second medium with the'major axis of the ellipse substantially perpendicular to the said predetermined direction, means for directing the incoherent first and second elliptically polarized beam portions emerging from the respective first and second media along a common direction incoherently to superpose the beam portions and thereby to form a partially elliptically polarized beam comprising a natural electromagnetic-wave component and a signal-varyingelliptically polarized resultant component of the superposed elliptically polarized first and second portions of the beam of electromagnetic waves, means for analyzing the signal-varying elliptically polarized resultant component of the partially elliptically polarized beam and means for detecting the signal from the analyzed beam.

9. An apparatus for signal communication that comprises means for producing a beam of electromagnetic light waves having adjacent first and second portions of substantially the same intensity, means for plane-polarizing the first portion of the beam along a predetermined plane, means for plane-polarizing the'second portion of the beam along a plane substantiallyperpendicular to the said predetermined plane, thereby-rendering thefirst and second polarized beam portions incoherent, means for'passing thefirst plane-polarized portion of the beam through a first light transparent medium, means for passing the second" plane-polarized portion of the beam through a second light transparent medium similarto and adjacent the first medium, means'for producing a periodic signal to be transmitted comprising a carrier waveof predetermined frequency modulated by a-modulating signal, means for producing birefringence in the first medium in response to a first component of the periodic signal'to render the first medium birefringent, thereby periodically elliptically to polarize the first portion of the beam emerging from the first medium with the majoraxis of the ellipse disposed along a predetermined direction, and for producing birefringence in the second medium in responseto a second component-of the periodic signal one-hundred eightydegrees phase-displaced from thefirst periodic signal component to render the second medium birefringent, thereby periodically elliptically to polarize 'the' second portion'of-the'beam emerging from the second medium'with the major axis of the ellipse substantially perpendicular tothe said predetermined direction,' means for directing the incoherent first and second elliptically polarized beam portions emerging from the respective first'and second media along a'common direction incoherently to superpose'the beam portions and to form a partially elliptically polarized beam comprising a' natural electromagnetic-wave component and a signal-varying elliptically polarized resultant component of the superposed elliptically"polarized first and second portions or the beam of electromagnetic waves, means for analyzing the signal-varying elliptically polarized resultant component of the partially elliptically pola'rized'beam, means for receiving and-amplifying the modulated carrier-frequency'signal contained 'in the analyzed beam and means for reproducing-the modulating signal. l V

"10.Ari' apparatus for signal transmission that comprises means for' producing'abeam of electromagnetic light waves having adjacent first-and second portions of substantially the same intensity, means for plane-polarizing the first portion of the beam-alonga predetermined plane, means for plane-polarizingthe second portion of the beam along a plane substantially perpendicular to the said predetermined plane thereby rendering the first and second polarized beam portions incoherent, means for passing the first plane-polarized portion of the beam through a first portion of a light transparent medium, means for passing the second plane-polarized portion of the beam through a second portion of the light transparent medium adjacent the first portion of the medium, means for producing the signal to be transmitted, means for propagating the signal as a vibrational wave into the transparent medium to render the first and second portions of the medium birefringent in response to the action of successive oppositely-phased components of the vibration wave thereby elliptically to polarize the first and second portions of the beam emerging from the respective first and second portions of the medium with the major axis of the ellipse describing the first elliptically polarized beam portion substantially perpendicular to the major axis of the ellipse describing the second elliptically polarized beam portion, and means for directing the incoherent first and second elliptically polarized beam portions emerging from the respective first and second portions of the medium along a common direction incoherently to superpose the beam portions and thereby to form a partially elliptically polarized beam comprising a natural electromagnetic-wave component and a signalvarying elliptically polarized resultant component of the superposed elliptically polarized first and second portions of the beam of electromagnetic waves.

11. An apparatus for signal communication that comprises means for producing a beam of electromagnetic light waves having adjacent first and second portions of substantially the same intensity, means for plane-polarizing the first portion of the beam along a predetermined plane, means for plane-polarizing the second portion of the beam along a plane substantially perpendicular to the said predetermined plane, thereby rendering the first and second polarized beam portions incoherent, means for passing the first plane-polarized portion of the beam through a first light transparent medium, means for passing the second plane-polarized portion of the beam through a second light transparent medium similar to and adjacent the first medium, means for producing a periodic signal to be transmitted comprising a carrierwave of predetermined frequency modulated by a modulating signal, means for producing birefringence in the first medium in response to a first component of the periodic signal to render the first medium birefringent, thereby periodically elliptically to polarize the first portion of the beam emerging from the first medium with the major axis of the ellipse disposed along a predetermined direction and for producing birefringence in the second medium in response to a second component of the periodic signal one-hundred eighty degrees phase-dis.

placed from the first periodic signal component to render the second medium birefringent, thereby periodically elliptically to polarize the second portion of the beam emerging from the second medium with the major axis of the ellipse substantially perpendicular to the said predetermined direction, means for directing the incoherent first and second elliptically polarized beam portions emerging from the respective first and second media along a common direction incoherently to superpose the beam portions and to form a partially elliptically polarized beam comprising a natural electromagnetic-wave 12. An apparatus for signal transmission that comprises means for producing a beam of electromagnetic light Waves having a plurality of successively disposed portions of substantially the same intensity, means for plane-polarizing the alternate portions of the beam along a predetermined plane, means for plane-polarizing the remaining portions of the beam along a plane substantially perpendicular to the said predetermined plane, thereby rendering the successively disposed polarized beam portions incoherent, means for passing the alternate planepolarized portions of the beam through corresponding alternate portions of a light transparent medium, means for passing the remaining plane-polarized portions of the beam through corresponding remaining portions of the transparent medium, means for producing the signal to be transmitted, means for propagating the signal as a vibrational wave into the transparent medium to render the portions of the medium birefringent in response to the action of successive oppositely phased components of the vibration wave thereby elliptically to polarize the alternate and the remaining portions of the beam emerging from the respective alternate and remaining portions of the medium with the major axis of the ellipse describing the alternate elliptically polarized beam portions substantially perpendicular to the major axis of the ellipse describing the remaining elliptically polarized beam portions, and means for directing the incoherent elliptically polarized beam portions emerging from the respective portions of the medium along a common direction incoherently to superpose the beam portions and thereby to form a partially elliptically polarized beam comprising a natural electromagnetic-wave component and a signalvarying elliptically polarized resultant component of the superposed elliptically polarized portions of the beam of electromagnetic waves.

13. In a system having a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined frequency to produce standing waves therein, becomes birefringent to the light passing therethrough along the predetermined direction, communication apparatus that comprises means for passing through a plurality of successively disposed portions of the medium, 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 successively disposed portions respectively corresponding to the plurality of successively disposed half-wavelength portions of the medium, means for polarizing in a predetermined state of polarization the light in alternate beam portions prior to its passage through the corresponding alternate portions of the medium, means for polarizing in a state of polarization differing by an oddinteger multiple of ninety degrees from the said predetermined state of polarization the light in the remaining beam portions prior to its passage through corresponding remaining portions of the medium, means for molecularly vibrating the medium at the predetermined frequency,

means for modulating the molecular vibrations of the medium in accordance with a signal, means for analyzing the light transmitted through the medium and means for detecting the variations in the analyzed light resulting from the signal modulations.

14. The apparatus of claim 13 and in which the molecular vibration of the medium is effected by means for propagating ultrasonic waves into the medium.

15. 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 frequency to produce standing waves therein, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successively disposed portions of the medium, each of which is substantially equal to the vhalfwavelength of the standing waves, along the predetermined direction a beam of light having a plurality of successively disposed portions corresponding to the plurality of successively disposed half-wavelength portions of the medium,

a pluralityof similar polarizers, one disposed in each alternate beam portion neareachcorresponding alternate portion of the medium and having substantially'the same width as the medium portion for similarly polarizing the light prior to its passage through the alternate medium portions, a further plurality of "similar polarizers, each disposed to polarize at an angle diifering by an odd-integer multiple of ninety degrees from the polarization of the first-named plurality of similar polarizers and one disposed in each of the remaining beam portion near each corresponding remaining portion of the medium and having substantially the same width as the medium portion for similarly complementarily polarizing the light prior to its passage through the remaining medium portions, and means for molecularly vibrating the medium at the predetermined frequency to render the successively disposed medium portions birefringent in anti-phase.

16. Apparatus as set forth in claim 15 the vibrating means of which comprises means for propagating ultrasonic waves into the medium.

17. Apparatus as set forth in claim 15 the vibrating means of Which comprises a plurality of piezoelectric means.

l8. Apparatus as set forth in claim 15 in which the polarizers disposed in successively disposed beam portions near corresponding successively disposed medium portions are plane-polarizers oriented, respectively, at plus and minus forty-five degrees with respect to the direction of propagation of the molecular vibrations. l9. Apparatus as set forth in claim 15 in which the polarizersdisposed in successively disposed beam portions near corresponding successively disposed medium portions are plane-polarizers oriented," respectively, parallel or normal to the direction of propagation of the molecular vibrations.

' 20. A communication system having, in combination, a medium that is transparent to light along a predeter mined direction and that, when molecularly vibrated at a predetermined frequency to produce standing Waves therein, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successively disposed portions of the medium, each'of which is substantially equal to the half-wavelength of the standing Waves, along the predetermined direction a beam of light having a plurality of successively disposed portions corresponding to the plurality of successively disposed half-wavelength portions 'of the medium, a plurality of similar polarizers, one disposed in each alternate beamportion near each corresponding alternate portion of the medium and having substantially the same width as the medium portion for similarly polarizing the light prior to its passage through the alternate medium portions, a further plurality of similar polarizers, each disposed to polarize at an angle diifering by an oddinteger multiple of ninety degrees from the polarization of the first-named plurality of similar polar izers and one disposed in each remaining beam portion near each corresponding remaining portion of the medium and having substantially the same width as the medium portion for similarly complementarily polarizing the light prior to its passage through the remaining medium portions, and means for molecularly vibrating the medium at the predetermined frequencyto render the successively disposed medium portions birefringent in anti-phase, means for varying the molecular vibrator in accordance with a signal, means for receiving the light passed through the medium, means for converting the received light into plane-polarized light, means for analyzing the converted plane-polarized light, and means for detecting the variations in the analyzed light to reproduce the signal.

21. A transmitting system having, in combination, a medium that is transparent to light along a predetermined directiomand that, when molecularly vibrated at a predetermined frequency to produce standing waves therein, becomes birefringent to the lightpassin'g therethrough "a along the predetermined direction, means for passing through a plurality of successively disposed portions ofthe medium, each of width substantially equal to the half-wavelength of the standing waves, along the predetermined direction a beam of light having a pluralityof succes sively disposed portions corresponding to the plurality of successively disposed half-wave portions of the medium, a plurality of similar polarizers one disposed in each alternate beam portion near each corresponding alternate portion of the medium and having substantially the same width as the medium portion for similarly polarizing the light prior to its passage through the alternate medium portions, a further plurality of similar polarizers, each disposed to polarize at an angle differing by an odd-integer multiple of ninety degrees from the polarization of the first-named plurality of similar polarizers and one disposed in each remaining beam portion near each corresponding remaining portion of the medium and having substantially the same width as the medium portion for similarly complementarily polarizing the light prior to its passage through the remaining medium portions, means for molecularly vibrating the medium at the predetermined frequency thereby to render the successively disposed medium portions birefringent in anti-phase and to form complementary elliptically polarized beams having a common direction of electric vector rotation emerging from successively disposed medium portions, and means for directing the emerging beams along a common direction incoherently to superpose the beams and to form a partially elliptically polarized beam comprising a natural light-wave component and an elliptically polarized resultant component, the electric vector of which reverses direction of rotation at the said predetermined frequency.

22. 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 frequency to produce standing waves therein, becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successively disposed portions of the medium, 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 successively disposed portions corresponding to the plurality of successively disposed half-wave portions of the medium, a plurality of similar polarizers one disposed in each alternate beam portion near each corresponding alternate portion of the medium and having substantially the same width as the medium portion for similarly polarizing the light prior to its passage through the alternate medium portions, a further plurality of similar polarizers, each disposed to polarize at an angle differing by an odd-integer multiple of ninety degrees from the polarization of the first-named plurality of similar polarizers and one disposed in each remaining beam portion near each corresponding remaining portion of the medium and having substantially the same Width as the medium portion for similarly complementarily polarizing the light prior to its passage through the remaining medium portions, means for molecularlyvibrating the medium at the predetermined frequency thereby to render the successively disposed medium portions birefringent in anti-phase and to form complementary elliptically polarized beams having a common direction of electric vector rotation emerging from successively disposed medium portions, means for directing the emerging beams along a common direction incoherently to superpose the beams and to from a partially elliptically polarized beam comprising a natural lightwave and an elliptically polarized resultant component the electric vector of which reverses direction of rotation at the said predeterminedfrequency, means for receiving the directed partially elliptically polarized beam, means for phase-shifting the complementary plane-polarized components of the elliptically polarized resultant component of the received beam sufficient to produce plane-polarized light, means for analyzing the planepolarized light, and means tuned to the said predetermined frequency for receiving the analyzed light.

23. A communication system as claimed in claim 22 in which the eccentricity of the ellipses of the elliptically polarized waves is substantially zero and the phase-shifting means comprises a quarter-wave plate.

24. In a system having a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined frequency to produce standing waves therein, becomes birefringent to the light passing therethrough along the predetermined direction, communication apparatus that comprises means for passing through a plurality of successively disposed portions of the medium, 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 successively disposed portions respectively corresponding to the plurality of successively disposed half- I wavelength portions of the medium, means for similarly polarizing the light in alternate beam portions, means for similarly polarizing the light in the remaining beam portions but with a different polarization than that of the said alternate beam portions, means for molecularly vibrating the medium at the predetermined frequency, means for modulating the molecular vibrations of the medium in accordance with a signal, means for analyzing the light transmitted through the medium and means for detecting the variations in the analyzed light resulting from the signal modulations.

25. In a system having a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined frequency to produce standing waves therein, becomes birefringent to the light passing therethrough along the predetermined direction, transmitter apparatus that comprises means for passing through a plurality of successively disposed portions of the medium, 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 succesively disposed portions respectively corresponding to the plurality of successively disposed halfwavelength portions of the medium, means for similarly polarizing the light in alternate beam portions, means for similarly polarizing the light in the remaining beam portions but with a different polarization than that of the said alternate beam portions, means for molecularly vibrating the medium at the predetermined frequency, and

means for modulating the molecular vibrations of the medium in accordance with a signal.

26. In a system having a medium that is transparent to light along a predetermined direction and that, when molecularly vibrated at a predetermined frequency to produce standing waves therein, becomes birefringent to the light passing therethrough along the predetermined direction, transmitter apparatus that comprises means for passing through a plurality of successively disposed portions of the medium, 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 successively disposed portions respectively corresponding to the plurality of successively disposed half-wavelength portions of the medium, means for similarly polarizing the light in alternate beam portions, means for similarly polarizing the light in the remaining beam portions but with a complementary polarization to that of the said alternate beam portions, means for molecularly vibrating the medium at the predetermined frequency, and means for modulating the molecular vibrations of the medium in accordance with a signal.

27. 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 frequency to produce standing waves therein,

becomes birefringent to the light passing therethrough along the predetermined direction, means for passing through a plurality of successively disposed portions of the medium, 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 successively disposed portions corresponding to the plurality of successively disposed half-wave portions of the medium, a plurality of similar polarizers one disposed in each alternate beam portion near each corresponding alternate portion of the medium and having substantially the width of the medium portion for similarly polarizing the alternate beam portions, a further plurality of similar polarizers, each disposed to polarize at an angle differing by an odd-integer multiple of ninety degrees from the polarization of the first-named plurality of similar polarizers and one disposed in each remaining beam portion near each corresponding remaining portion of the medium and having substantially the width of the medium portion for similarly complementarily polarizing the remaining beam portions, means for molecularly vibrating the medium at the predetermined frequency thereby to render the successively disposed medium portions birefringent in anti-phase and to form complementary elliptically polarized beams having a common direction of electric vector rotation emerging from successively disposed medium portions, and means for directing the emerging beams along a common direction incoherently to superpose the beams and to form a partially elliptically polarized beam comprising a natural light-wave component and an elliptically polarized resultant component, the electric vector of which reverses direction of rotation at the said predetermined frequency.

28. An apparatus as claimed in claim 1 and in which the said means for rendering the first and second portions of the beam incoherent is disposed between the said beamproducing means and the said light-transparent media.

29. An apparatus as claimed in claim 1 and in which the said means for rendering the first and second portions of the beam incoherent is disposed in the path of the said first and second portions of the beam emerging from the said first and second media.

30. An apparatus as claimed in claim 1 and in which the said polarizing means are plane-polarizing means and the said means for rendering the first and second portions of the beam incoherent comprises first and second phaseshifting plates, one disposed in the path of each of the said first and second portions of the beam emerging from the said first and second media, the phase-shifting plates producing phase shifts of opposite sign.

31. An apparatus as claimed in claim 30 and in which the phase-shifting plates comprises right and left quarterwave plates, and the said first and second media, when birefringent, produce substantially ninety-degree phase Shifts.

32. An apparatus as claimed in claim 1 and in which the said polarizing means are means for producing elliptically polarized light; the said first and second media are of such thickness that, when rendered birefringent, they convert the light emerging therefrom into substantially plane-polarized light; and the said means for rendering the first and second portions of the beam incoherent comprises first and second plane polarizers, one disposed in the path of each of the said first and second portions of the beam emerging from the said first and second media, the plane polarizers producing polarizations that differ by an odd multiple of ninety degrees.

33. An apparatus as claimed in claim 32 and in which the means for producing elliptically polarized light is adjusted so that the eccentricity of the ellipse of the elliptically polarized light is substantially zero, and the said media are of such thickness that, when rendered birefringent, they produce substantially a ninety-degree phase-shift.

34. Apparatus as claimed in claim 10 and in which the said plane-polarizing means are vibrated periodically 25 into and out of alignment with the said portions of the medium that are rendered birefringent, thereby to modulate the said superposed beam portions in accordance with the vibration of the said plane-polarizing means.

26 Nicolson Nov. 30, 1937 Salinger June 6, 1944 Hammond July 16, 1946 Young Sept. 3, 1946 ODea Oct. 25, 1949 Shamos et al. Nov. 28, 1950 Rines Dec. 23, 1952 Mueller et al. Dec. 23, 1952 FOREIGN PATENTS Great Britain Sept. 23, 1919 Great Britain Aug. 12, 1936