US 2187908 A
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Examiner AU Z 33 E K U R Z y l 8 7 90 8 H. J. MCCREARY BLECTROIAGNETIC WAVE TRANSIISSION FIPSIU() Patented Jan. 23, 1940 UNITED STATES L Wimmer PATENT OFFICE 2,187,908 ELECTROMAGNETIC WAVE TRANSMISSION Harold J. McCreary, Genoa, Ill. Application June 15, 1936, Serial No. 85,260
This invention relates to the transmission of sound or other forms of signals by electromagnetic waves and in its illustrated form includes means for separating beams of light into their various colors and utilizing each color separately for two-way communication or utilizing only the invisible infra-red rays, if preferred. Accord'- ing to another aspect it relates broadly to the transmission of a plurality of messages or the like by a single super-carrier wave, or any forrn of high frequency super-carrier by means of modulation of the super-carrier with a plurality of independent lower frequencies, each modulated by an independent message.
In the illustrated form the electromagnetic waves used are those known as light waves, but in many of its aspects the invention will be applicable to other forms of electromagnetic waves. In fact, inasmuch as some electromagnetic waves of frequencies beyond the visible spectrum are called light waves and inasmuch as the waves of frequencies still further removed from the visible spectrum have some of the qualities associated with light waves, the term light may be taken as a convenient term including all of the electromagnetic phenomena having whatever quality of light may be necessary for the particular purpose ln mind.
This invention utilizes many of the principles of radio, some of which have already been applied to light beam transmission. Particularly, it utilizes the principle of transmitting a modulated carrier wave in accordance with the signal to be transmitted and translating this modulation at the receiving station to produce the intended signal. Such modulations may be in accordance with sound, code, television, or perhaps other forms of signals, the term signal being used to include all of these.
One object of the present invention is to transmit signals over an electromagnetic wave through a conduit. In the illustrated form this is accomplished by utilizing light waves, the conduit being such as to conserve these waves, protect them from adverse atmosphericv conditions, and from interception, and to guide them as may be desired.
Another object of the invention is to utilize such a conduit (or a single path without a conduit) for two way communication by electromagnetic waves. In the illustrated form this is accomplished by providing a semitransparent mirror at both stations, each alined with the conduit or path so that the transmitter can send light through the mirror into the path or conduit and (Cl. Z50-7) the receiver can receive light reflected by the mirror from the conduit or path.
Another object of the invention is to transmit messages by invisible light. This can be done very easily with the proposed construction, simply by utilizing only the invisible color paths, such as the infra-red paths. It is not even necessary to use a color filter, since white light transmitted to the prism will be divided up, and all the separated visible rays can be blocked out.
Another object of the invention is to utilize such a conduit or path for a plurality of independent carrier waves, each transmitting at least one separate set of signals. In the illustrated form this is accomplished by utilizing separate colors (including the invisible rays) for the carrier Waves and merging and separating these colors by refraction prisms. Each color may be used for two way communication by means of a semitransparent mirror.
An additional object of the invention is to transmit a large number of messages by a single carrier wave, which for convenience may be called a supercarrier wave. This is accomplished by modulating the supercarrier wave, which is of extremely high frequency, by a plurality of minor carrier waves of lower frequencies and separating these lower frequencies by suitable filters, each minor carrier to be translated separately. Of course, the minor carrier waves may be intermediate carrier` waves and each modulated with a plurality of carrier waves of still lower frequencies. Light waves are especially suitable for the supercarrier waves, because of their extremely high frequency and because of their ability to be separated by optical means, such as refraction prisms, which facilitates the use of a plurality of supercarrier waves having closely spaced frequencies without danger of insufficient separation thereof.
Although the novel features by which these objects are accomplished all co-act with one another and mutually contribute to make up the preferred form of the invention, yet it may be possible to use each of the features without the other novel features. Thus, the principle of utilizing a supercarrier wave for transmitting a large nels available over a single path. These various features together contribute toward making the wave carrying tube practical since its construction would not ordinarily be commercially warranted except for a large number of channels.
Other objects of the invention will be apparent from the foregoing description and from the drawing, in which:
Fig. l is a largely diagrammatic illustration of a system embodying this invention.
Fig. 2 is a more or less diagrammatic illustration of the bending of a light beam carried in a closed path.
Fig. 3 is an illustration of a modified arrangement, carrying the light beam around a curve in a closed path.
Fig. 4 illustrates a repeater station for use with this invention.
Fig. 5 is a more or less diagrammatic illustration of a new form of light valve which may be used with this invention.
Fig. 6 shows a modication of Fig. 5.
Although this invention may take numerous forms, only one system has been illustrated together with a few modications of details thereof. One station for this system has been diagrammatically illustrated in Fig. l. 'I'his station includes any number of combination translating and amplifying units, one such combination suitable for two way telephony, being illustrated at A together with leads to indicate the possible addition of other translating units. Theprinciples of systems including the translating and amplifying units A are already well known and need not be described in detail. One such system was described in an article by Colpits and Blackwell published in the Transactions of the American Institute of `Electrical Engineers, February 17, 1921, pages 205-300. Ampliers il may be connected in place of the carrier line connections there disclosed. To the input side of receiving amplier Il of such a combination unit a photoelectric cell I2 is connected, the amplifier Il of course being properly designed and coupled thereto and to the lters 1| according to known principles. Any desired type of cell may be used.
such for example as that illustrated in Kunzr patent, No. 1,381,474, or such cells as may be developed tobe responsive to still lower wave frequencies. To the output side of a transmitting amplier i3 is connected a light valve Il. This light valve may be any suitable type of light valve, such as the system using a Kerr cell between two polarizing units having their axes at right angles to one another, or the novel form of light valve disclosed herein, depending upon which is best or most readily available for any particular purpose. The light valve Il modulates a beam of light issuing in the form of parallel rays from a light source system which may include an arc light i6, parabolic reflector I1, and a lens system I8 for bringing the rays into parallelism. Or a modulated light source such as that of Kuchenmeister patent, No. 1,835,226, may be used as the equivalent of a constant source and valve.
A semitransparent mirror 2| is positioned as shown with respect to the photo-electric cell i2 and the light valve Il so that approximately half of the light striking the back side of this mirror from the light valve will pass through it along the transmission path 22, likewise half of the incoming light in the transmission path 22 which strikes the front side of the photo-electric cell i2. It follows that a message may be transmitted in one direction along this path by the light valve I4 and may be received over this path from the other direction by the photo-electric cell I2, thus setting up two way communication along this path. Such semitransparent mirrors are already well known and may be in the form of a plate or in the form of a pair of prisms secured together with the semitransparent reiecting surfaces between them. It should be noted that by semitransparent reflector is meant any device which will reflect some waves of the frequency in question while permitting the passage therethrough of other waves moving in the opposite direction. If subvisible waves are used, the device might conceivably not have this quality as to visible rays.
In the path 22 is provided a refraction prism 26 which will separate the various assistiti: colors ransmitted y e g va n Send the 1i Bt a@ rough e path 25 to the other station. The
co ors may be `s`rbdibl`lid by any opaque screening means positioned in the direction toward which said other colors will be transmitted by the refraction prism 26. In the illustrated form of the `invention the path 25 is included in a tube 21, the portion 28 of which closest to the prism 26 is blackened on its surface to absorb and block out all rays except the rays going in the desired direction. 'I'he remainder of the tube 21 except a similar darkened portion at the other station may have a highly reflective lished or silveregiwinner surface o conserve sirdlllitrays vas much as possible.
It may be assumed that the only rays from the path 22 which the refraction prism 26 would transmit in the proper direction to pass through tube 21 would be the infra-red rays, and that all other rays reaching the prism 26 from the path 22 would be absorbed and blocked by the blackened portion 28 of the tube. The path 22, therefore, may be considered the infra-red path. It should be noted that if white light is transmitted in the opposite direction through the tube 21 it would likewise be divided by the refraction prism 26, and the infra-red rays would be transmitted along the path 22 to the semitransparent mirror 2l, which would reflect some of these rays to the photo-electric cell I2. All other rays received by the prism 26 from the tube 21 (assuming that they are parallel) would be refracted to a different degree by the prism and would therefore take dierent light paths. Thus, all the red light would take the path labeled red; all of the orange light would take the path labeled orange"; all of the yellow light, the path labeled yellow; all of the green light, the path labeled green; all of the blue light, the path labeled b1ue"; and all of the violet light, the path labeled violet. Of course, these various paths have been spread out more in an angular sense than would actually be the case, but by providing greater length than has been illustrated between ,the prism 26 and the various semitransparent mirrors 2i, of which one is positioned in each path, adequate separation of the paths would be obtained, and ofcourse the spectrum could be divided into much more narrow zones, or even individual lines of frequencies, to greatly increase the number of paths.
Just as the incoming light would be separated into the plurality of color paths, 'in like manner each of these paths could be utilized for transmission. In other words, the complete receiving and transmitting unit illustrated in Fig. 1 could be provided for each path, the amplifying and translating portion A, the receiving cell I2, amplier II, transmitting portion I4, I6, I1, I8, and the semitransparent mirror 2|. In the case of the yellow band, for example, the white light from the arc light modulated by the light valve would be directed onto the prism 26 over the path marked yellow. Only the yellow light, however, would be refracted by the prism through the tube 21, all other light received along this yellow path being refracted by the prism onto the black screen indicated at 28. Likewise, of the various parallel rays thrown onto the prism 26 through the tube 21, only the yellow ray would be refracted by the prism into the path marked yellow to be reected by the semitransparent mirror in this path to the corresponding photoelectric cell similar to the cell I2, illustrated.
The use of the tube 21 is not necessary nor is the use of such a screen as that indicated at 28, since without these elements only the desired colors would be transmitted to the prism at the other station corresponding to the prism 26. The use of the tube 21 and screen 28 have the advantage, however, of excluding dust and fog and preventing interception of the messages or signals by an outside party, and, if the inner surface of the tube 21 is highly polished, it will also be eifective to conserve some of the light which though not exactly parallel is suiiiciently near parallelism to be effectively utilized. It should be noted that the screen 28 would probably be much longer than illustrated in order to block out al1 of the rays which are not sufficiently near parallelism. Whether the tube 21 is used or not the path of light need not be straight, since it can be reected by a mirror 4I or a reecting prism.
For some or all purposes the light could be car- I ried around bends without the use of such a mirror as the mirror 4I by bending the tube gradually as shown in Fig. 3. Of course, the more gradual the bend the more nearly the rays will remain in parallelism and the less will be the loss due to reection. Since communication of this invention to Bell Telephone Laboratories, Inc., that organization has done considerable experimentation with guided waves using a frequency of two billion cycles. See for example 50' Telephony, May 9, 1936, page 12.
Instead of a hollow tube the light may be carried by a solid glass bar which may be silvered on its outer surface. Although such a bar would not be as perfectly transparent as clean, dry air or a vacuum, its transmitting ability would be constant without care, and it might, therefore, be preferred for some purposes.
A very important advantage of the use of a closed tube for the light path is that it can be kept entirely free of dust or fog so that the transmission of light will never be impaired by climatic conditions. Of course, if waves of frequencies below those seriously affected by light and fog are used, this advantage will not be so important, and the tube may be omitted. It should be understood, of course, that the ends of the tube 21 would be closed by optically fiat glass or by the prism itself, and that at all points the tube 21 would be effectively sealed. If desired. de..hiimidiixins .cr srafaaitinaannara could begonncted td the time, st sealing tHe tube, clean, dry air could be blown through it, in which case the ends of the tube would not need to be closed, or if the tube is suiciently air tight, it may be evacuated.
Of course, in-v 'f'xamnel If desired, two way repeater stations may be used when messages or other signals are to be transmitted over a long distance. Such a repeater system would include at each end thereof a refracting prism for separating 4the various colors. For each color it would include at each end a semitransparent mirror 42|, as seen in Fig. 4. There would also be two repeater units, each of which would receive the incoming light from one mirror, translating it by a photo-electric cell 4I2, amplifying it by amplifier 4I I, and by light valve 4I4 modifying a beam of light transmitted as in Fig. 1 through the other mirror 42| to the related prism.
In Fig. a novel form of light valve has been illustrated which can be used in place of light valve I4. 'This valve includes two transparent piezoelectric crystals 5I. The characteristic of such crystals is that they expand and contract in one direction as they are subjected to an electrostatic field. In other words, an electrostatic eld causes them to elongate in proportion to the intensity of the eld within certain limits. Some quartz crystals cut in a certain way already known have this piezoelectric characteristic and are fully transparent. 'I'hey are, therefore, ideally suited for use as the crystals 5I. 'Ihese crystals may be mounted in any suitable way as by being enclosed in a glass or quartz housing 52 and cemented to the xed walls 53 thereof with Canada balsam or any other transparent ce' ment. The crystals are triangular in shape and are so proportioned that their inner surfaces 54 are slightly out of contact when there is no electrostatic field present. At this time light entering the crystal in the direction indicated by the arrow 56 at an angle greater than the critical angle of the prism with respect to the inner surfaces 54 will be substantially totally reected by the surface 54 of the prism which it enters. However, if the crystals are subjected to an electrostatic field as by means of electrodes 51 and 58 on opposite sides of said crystals, the crystals will expand and if they were properly placed originally their inner surfaces 54 will come together in optical contact so that light entering in the direction of arrow 56 will pass through both crystals and through the envelope 52. The electrodes 51 and 58 may be darkened so that they will absorb most of the light striking them, but if not, the light which is reected to them when the crystals are separated will be reected back to the inner surface 54 and by it back toward the arrow 56. Of course, the electrodes 51 and 58 may be positioned on opposite sides of the two prisms instead of each at the end of one prism as illustrated. and there may be a bank of such light valves as are made up by the two prisms so as to have a larger cross-section for the passage of light. The prisms should, of course, be so cut and the electrodes so positioned as to cause the crystals to expand in the proper direction.
It will be noted that in the illustrated form the light valve changes from the condition of passing substantially all of the light to a condition of passing substantially none of the light. This sharp valve operation will be very desirable in some instances as in sending messages by telegraphic code, but it might be entirely unsatisfactory when a gradual modulation is desired, as for voice transmission. For the purpose of obtaining modulation, the spacing of the two inner walls 54 may be tapered so that lt-is slightly greater at one end than at the other. If the light then endeavors to pass through the entire area, it will pass only through that portion vof the area in which the surfaces are in contact. As a heavier electrostatic field is applied, the crystals will expand more and put more of their surfaces in contact, thus passing more light and producing the desired modulation. A similar result may be obtained by cutting one oi the crystals so that it will not expand. Since the other crystal will expand more on its long side than its short side, the area of the crystals in contact will vary with the field intensity.
In Fig. 6 is shown a modified form of valve in which the angle of reflection is changed to shift the reflected beam more or less into registry with an opening in a screen a considerable distance away. The crystal is cut to permit placing the electrodes on the parallel faces as shown.
It should be recognized that the use of this general type of light valve is not necessary to the practice of the remaining aspects of the invention, since other light valves already known may be used.
Operation of the system For the sake of clarity the operation of the system may now be reviewed, assuming that it is used for two-way voice communication and that to this end the wires I5 are connected to a suitable telephone instrument or circuit. In a manner already well known, the translating and amplifying unit A will cause the light valve I4 to modulate the light passing therethrough from arc light I6 in accordance with the voice modulations. Since there is no negative light, the valve may be biased to pass a medium amount of light normally. The useful portion of this modulating light will pass through the semitransparent mirror 2| to the refracting prism 26, by which the light will be separated into different colors, one of which (in this instance, infra-red) will be transmitted through the tube 21 or through the air to the other station. The rest of the colors will be absorbed by the blackened screen 28.
We may now assume that Fig. 1 illustrates the other station at which infra-red light is being received through tube 21. This light will pass through the prism and will be refracted slightly by it to fall on the particular semitransparent mirror 2i, which is placed at the proper position to receive the infra-red light. Approximately half of the light received by said mirror will be reflected to a photo-electric cell l2, which by means already well known will be amplified and translated into a voice current which will be carried by the wires i5 to the telephone instrument. The words of the party receiving this telephone message will be transmitted by the light valve il and through the semitransparent mirror 2l back over the same path in the manner already described.
In case a plurality of different conversations are to be carried over the infra-red path, each one by a carrier channel of a different frequency from the others, the frequency belonging to the telephone line l5 will be separated from the others by band pass filters 1I, as described in the Colpits and Blackwell article previously referred to. The amplifiers li will be untuned or tuned broadly enough to amplify all of the frequencies properly. Furthermore, the system can be duplicated for each of the various colors so that there is hardly any limit to the number of messages that can be transmitted through a single light tube 21. Of course the same system of a supercarrier wave having a plurality of channels therein of tuff.ff-'ser,alg-ate-` Y rts of.-. the spectrumraif still more channels should be desired, they may be obtained by polarization of 15 the light along different axes, in which case an entire set of waves may be transmitted along each of two perpendicular planes or perhaps in a still larger number of planes. In this connection it may be noted that the Kerr cell light valve sys- N tem transmits only polarized light and is, therefore, particularly adaptable to the present invention if different channels along different axes of polarization are desired.
When a fairly large number of colors are used, it may be preferred to avoid the expense of having a separate semitransparent mirror for each color, in which case a single semitransparent mirror may be used positioned between the light tube and the prism. In this case two prisms will $0 be provided, one for the outgoing waves and one for the incoming waves, one of the prisms being positioned to receive the light which strikes the mirror from the tube and the other being positioned to have the desired light passing therethrough, strike the mirror and pass into the tube. For example, the receiving prism might be in a position such that the light from the tube would be reflected onto it, while the transmitting prism would be alined with the tube and the semi- 0 transparent mirror. It should be observed that wherever the semitransparent mirror is used, the 's relationship of the receiving and transmitting ele- 5' ments may be reversed with respect thereto. It f should a1so be noted that if the mirror l1 has a noticeable effect in retransmitting incoming signals, this mirror l1 may be omitted.
'I'he disclosures of this application are illustrative, and the invention is not to be limited by them. In fact, if modifications or improvements are not at once obvious, they may be devised in the course of time to make additional use of the broad ideas taught and covered by this application. The claims are intended to point out novel features and not to limit the invention except as may be required by prior art.
1. Apparatus for two-way simultaneous transmission of signals over a single path including means for producing a plurality of beams of light including waves of different frequencies at each end of the path, prism means at each end of the path for transmitting through the path from each outgoing beam of light a frequency dierent from that transmitted from the other outgoing beams and for separating the incoming beams of light of different frequencies, photo-electric cell means at each end of the path for translating the received light beams of different frequencies into 70 signals, and partially reflecting means for the beams at each end of the path for transmitting part of each incoming beam to an associated photo-electric cell means and for transmitting the outgoing beams into the path while keeping Il them away from the associated photo-electric cell means.
2. Apparatus for simultaneous transmission and reception of signals over a single path including means for producing a plurality of light beams including waves of different frequencies, each modulated in accordance with a diierent set of signals, refraction means for directing a different frequency of light wave from each light beam along the path and for separating the incoming light waves of different frequencies into a plurality of beams corresponding to the rst-mentioned plurality of beams, partially reflecting means effective as to each of the beams and adapted to separate the incoming waves from the path of outgoing waves, a photo-electric cell adapted to be actuated by each of the separated Waves, and a translating device actuated by each cell for translating the light waves into signals.
3. Apparatus for simultaneous two-way communication including a substantially uniform semitransparent mirror, means for directing a light wave modulated in accordance with outgoing signals against one side of said mirror. and means positioned opposite the other side of the mirror and out of alinement with the mirror and the first named means for translating incoming light waves from a remote signal source, said mirror being so positioned with respect to the two means that the incoming waves approaching the mirror over the same path as that followed by the outgoing light waves in passing from the mirror will pass from the mirror to the translating means, the useful portion of one set of waves passing through the mirror and the useful portion of the other set of waves being reflected by the mirror.
4. Apparatus for two-way simultaneous transmission and amplification of signals over a succession of single paths, including means at each end of the succession of paths for transmitting along the paths a plurality of beams of light of I Examine different frequencies having signal modulations imposed thereon and amplifying equipment at a point between successive paths including for each of the paths on the two sides of the amplifying system a refraction unit for angularly separating the different frequencies of incoming waves and for merging different frequencies of outgoing waves converging from angularly spaced points, a translating device for each incoming frequency, and means at each of the angularly spaced points for producing a modulated light beam, amplifying apparatus for amplifying the translated signals of each translating device for one path and causing said signals to be impressed upon an outgoing beam for the other path, and means for preventing conict between incoming and outgoing signals consisting of a partially reflective device positioned in the path of each beam for diverting light of one direction from the path While permitting light of the other direction to remain in the path.
5. Apparatus for simultaneous transmission and reception of signals over a single path, including means for producing a plurality of beams of electromagnetic waves including waves of different frequencies in a range of directional waves capable of refraction, each of said beams being modulated in accordance with a different set of signals, refraction means for directing a different frequency of wave from each beam along the path and for angularly separating the incoming waves of diiferent frequencies into a plurality of beams corresponding to the first mentioned plurality of beams, partially reecting means effective as to each of the beams and adapted to separate the incoming waves from the path of outgoing waves, and translating apparatus for translating the electromagnetic waves of each of the separated beams into signals including a translating device for each of the separated beams responsive to the waves thereof.
HAROLD J. MCCREARY.