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Title: Secret communication system employing magnetic control of signal modulation on microwave or other electromagnetic carrier wave
Claims: What is claimed is: 1. A system for transmitting varying amplitude signals with a high degree of secrecy over a wave transmission medium, including a source of polarized electromagnetic waves of predetermined wavelength at the input to said medium, three transmission devices each adapted for transmitting electromagnetic waves in a given direction, and including a substance having a resonance spectrum with a strong, sharp absorption line therein including said predetermined wavelength within its frequency limits, and exhibiting strong anomalous dispersion in the immediate neighborhood of said line, a first and a second one of said devices being located at the input to said medium and the third being located at the output of said medium, means to apply an electromagnetic wave from said source to said first device in said given direction, means simultaneously to apply to said first device in the direction of wave propagation a weak magnetic field varying in accordance with the amplitude of the signals to be transmitted, in order to produce rotation effects in the applied wave proportional to the instantaneous signal amplitudes, representing signal modulation, means to apply the resulting modulated wave to the second of said devices in said given direction, means simultaneously to apply to said second device in the direction of wave propagation a strong magnetic field of such strength as effectively to cause the removal from the modulated wave in transmission through that device of the rotation effects representing signal modulation, and thus to render the signals carried by that wave secret to a high degree before the wave is transmitted over said medium, means at the output of said medium to apply the received wave to said third device in said given direction, means simultaneously to apply to said third device in the direction of wave propagation a strong magnetic field of the same strength as that applied to said second device but reversed in direction with respect thereto to reinsert effectively in the applied wave in transmission through said third device the rotation effects representing signal modulation removed by said second device, means to convert the rotation effects in the resulting wave into proportional amplitude modulation and means to detect the signals from the resulting amplitude modulated wave. 2. An optical system for transmitting varying amplitude signals with a high degree of secrecy over a wave transmission medium, comprising means to produce a linear polarized light beam of a predetermined wavelength, three Faraday cells each transparent to light in a given direction and including a substance having an atomic resonance spectrum with a strong, sharp absorption line including said predetermined wavelength within its frequency limits, and exhibiting strong anomalous dispersion in the immediate neighborhood of said line, two of said cells being connected in tandem in the input to said medium and the third of said cells being connected to the output of said medium, means to apply said linear polarized light beam to the first of said two cells in said given direction, means simultaneously to apply to said first cell in the direction of light propagation a weak magnetic field varying in accordance with the instantaneous amplitude of the signals to be transmitted so as to produce in that cell rotations of the plane of polarization of the applied polarized light beam in amounts proprotional to said signal amplitudes, representing signal modulation, means to apply the resulting modulated polarized light beam to the second of said two cells in said given direction, means simultaneously to apply to said second cell in the direction of light propagation a strong magnetic field of such strength as to convert the applied polarized light beam in said second cell into a substantially completely unpolarized beam in which the rotation effects representing signal modulation have been removed so as to render the transmitted signals secret to a high degree before the beam is transmitted over said medium, means at the output of said medium to apply the received converted light beam to said third cell in said given direction, means simultaneously to apply to said third cell a strong magnetic field of the same strength as that applied to said second cell but reversed in direction with respect thereto, to repolarize the light beam and insert therein the rotation effects representing signal modulation effetively removed by said second cell at the input to said medium, a polarization analyzer for converting the phase modulation represented by the rotation effects in the re-polarized beam into corresponding amplitude modulation and means to detect the signals from the resulting amplitude-modulated light beam. 3. The secret signal optical transmission system of claim 2, in which each of said Faraday cells comprises a glass container of a given length, containing a vapor of an alkaline metal at an optimum low pressure in the order of one-thousandth of a millimeter; and said means to produce a linear polarized light beam of said predetermined wavelength includes a high intensity discharge lamp containing the same alkaline metal vapor at approximately the same low pressure as used in each of said Faraday cells, a lens system for collimating the light from said lamp into a parallel beam of light rays and a Nicol prism polarizer for linearly polarizing said beam. 4. The secret signal optical transmission system of claim 2, in which each of said Faraday cells consists of a glass tube approximately 11 centimeters in length containing sodium vapor at an optimum pressure of about 2 .times. 10.sup.-.sup.3 millimeters obtained by heating the cell to approximately 250.degree. centigrade; said means for producing a linearly polarized light beam includes a sodium arc lamp used as a light source, a lens system for collimating the light from said lamp into a parallel beam of light rays and a Nicol prism for producing linear polarization of said beam; the weak signal modulating magnetic field applied to said one Faraday cell at the input to said medium is of the order of a few tenths of a gauss; and the magnitude of the strong magnetic field applied to said second Faraday cell at the input to said medium and to said third Faraday cell at the output of said medium is at least 150 gauss. 5. The secret signal optical transmission system of claim 2, in which the strong magnetic field applied to said second Faraday cell at the input to said medium to convert the signal-modulated polarized light beam transmitted through that cell to a substantially completely unpolarized light beam and the oppositely directed magnetic field of the same strength applied to said third Faraday cell at the output of said medium is a direct current magnetic field obtained by the provision of a suitably proportioned magnet coil respectively surrounding each of these cells, supplied with direct current from an individual voltage source of suitable value connected across the terminals of the coil in reverse manner for the two coils; and to increase the degree of secrecy in the transmitted signals an alternating current of the same frequency, amplitude and phase is superposed on the direct current produced in each of these coils by the applied direct current magnetic field, the selected amplitude of the superposed alternating current with respect to that of the direct current in each magnet coil providing one independently chooseable parameter and the selected frequency of the superposed alternating current providing a second independently chooseable parameter for the secret system. 6. The secret signal optical transmission system of claim 2, in which the strong magnetic field applied to said second Faraday cell at the input to said medium and the strong magnetic field of the same magnitude but in the opposite direction applied to said third Faraday cell at the output of said medium, is a direct current magnetic field obtained by the use of a suitably proportioned magnet coil surrounding the respective cells and an individual d-c voltage source of suitable value connected across the terminals of each coil; and to increase the degree of secrecy in the transmitted signals, an alternating current of the same adjustable frequency and amplitude is superposed on the direct current produced in the magnet coil surrounding each of said second and third Faraday cells by the applied direct current magnetic field, and an adjustable phase shifter is inserted in one of the two alternating current circuits so formed to provide an additional parameter for the secret system. 7. A secret microwave signal transmission system comprising a signal transmitting station and a signal receiving station connected by an air wave transmission medium; said transmitting station including means for generating a microwave of predetermined wavelength for use as a carrier wave, a source of varying signals to be transmitted over the system with secrecy, a straight section of elongated waveguide line for propagating the generated microwave in the direction of the longitudinal axis of that section towards said medium, two transmission devices connected in tandem in said line section in the path of the propagated microwave, each of said devices having an atomic resonance spectrum with a strong, sharp absorption line including said predetermined wavelength within its frequency limits, and exhibiting strong anomalous dispersion in the immediate neighborhood of said line, means controlled by said source of signals for applying to the first of said devices in said line section a weak magnetic field varying in accordance with the instantaneous amplitudes of said signals, said one device operating in response to the applied weak magnetic field, in a manner analogous to the operation of a Faraday cell on light waves, to rotate the microwave in variable amounts proportional to said instantaneous signal amplitudes, the resulting rotation effects in the microwave emerging from said one transmission device and entering the second transmission device in said line section representing signal modulation, means for applying to said second transmission device a sufficiently strong magnetic field to convert the signal-modulated microwave in transmission through that device into a wave in which the rotation effects representing signal modualtion have been effectively eliminated so as to render the transmitted signals relatively secret and means to impress the converted microwave on said medium for transmission thereover to said receiving station; and said receiving station including means for picking up the received converted microwave from said medium, a second elongated waveguide line section for propagating the received microwave longitudinally thereover, a third transmission device identical with said second transmission device at the transmitting station, inserted in said second line section in the path of the propagated microwave, means for applying to said third transmission device a strong magnetic field of the same magnitude as that applied to said second transmission device but opposite in direction thereto, said third device operating in response to the applied oppositely-directed strong magnetic field to re-insert in the microwave transmitted therethrough the rotation effects effectively eliminated from that wave by said second transmission device at the transmitting station, thereby restoring the signal modulation thereto and reproducing the signal-modulated microwave substantially as it appeared at the input to said second transmission device at the transmitting station, means to convert the signal phase modulation on the resulting wave into equivalent amplitude modulation and means to detect the signals from the resulting amplitude-modulated microwave. 8. The secret microwave signal transmission system of claim 7, in which said first transmission device used as a modulator at the transmitting station comprises a block of compressed, finely-divided ferrite crystals, centrally located within said one waveguide line section, the means for applying the weak signal modulating field to that device consists of a suitably proportioned magnet coil surrounding the portion of the waveguide line section adjacent to that device, connected to said source of signals; each of said second and said third devices used at the transmitting station and at the receiving station, respectively, comprises a container centrally located within the waveguide line section at that station containing a thick layer of a solution of finely-divided particles of an alkaline metal, such as sodium, in liguid ammonia; and the means for respectively applying the strong magnetic field to said second and said third device is an individual suitably proportioned magnet coil surrounding the portion of the waveguide line section adjacent the device, and an individual d-c voltage source of suitable valve connected across the terminals of each coil. 9. A secret signal transmission system comprising a signal transmitting station and a signal receiving station connected by a wave transmission medium, said signal transmitting station comprising means for generating an electromagnetic wave of predetermined wavelength for use as a carrier wave, a source of varying signals to be transmitted over the system as modulations of said electromagnetic carrier wave with secrecy, two transmission devices each adapted for transmitting said electromagnetic wave in a given direction, each of said devices including a substance having an atomic resonance spectrum with a strong, sharp absorption line including said predetermined wavelength within its frequency limits, and exhibiting strong anomalous dispersion in the immediate neighborhood of said line, means for applying the generated electromagnetic wave to one of said devices in said given direction, means for simultaneously applying to said one device a weak modulating magnetic field varying in accordance with the instantaneous amplitudes of the signals from said source, said one device operating in response to the applied weak magnetic field to signal modulate the applied electromagnetic wave in transmission therethrough by causing variable rotation of that wave in amounts proportional to said instantaneous signal amplitudes, means for applying the resulting signal-modulated electromagnetic wave to the second of said devices in said given direction, means for simultaneously applying to said second device a strong magnetic field, said second device being responsive to the applied strong magnetic field when its magnitude exceeds a given value to render the transmitted signals substantially secret, by converting the applied electromagnetic wave into a similar wave in which the rotation effects are effectively removed therefrom and means for impressing the resulting converted wave on said transmission medium for transmission thereover to said receiving station; and said signal receiving station includes means for receiving the converted electromagnetic wave from said medium, a third transmission device identical with said second transmission device at the transmitting station, supplied with the received converted wave in said given direction, means for applying simultaneously to said third device a strong magnetic field of the same magnitude as that applied to said second transmission device at said transmitting station but opposite in direction to that field, said third device being responsive to the applied strong magnetic field applied thereto to effectively remove its secrecy from the signal modulation on the converted electromagnetic wave in transmission through that device by effectively re-inserting the rotation effects therein removed by said second device at the transmitting station and thus reproducing the original signal-modulated electromagnetic wave substantially as it appeared at the input of said second device, means for converting the phase modulation represented by the rotation effects in the resulting wave into proportional amplitude modulation and means to detect the signals from the resulting amplitude-modulated wave. 10. A secret optical signal transmission system comprising a signal transmitting station and a signal receiving station connected by a wave transmission medium; said signal transmitting station including means for producing a linear polarized light beam of a predetermined wavelength for use as a carrier wave, a source of varying signals to be transmitted with secrecy over the system as modulations of said carrier wave, two Faraday cells each transparent to light in a given direction and containing a substance having an atomic resonance spectrum with a strong, sharp absorption line including said predetermined wavelength within its frequency limits, and exhibiting strong anomalous dispersion in the immediate neighborhood of said line, means for applying the linear polarized light beam to one of said cells in said given direction, means for simultaneously applying thereto a weak magnetic field in the direction of light propagation, varying in accordance with the instantaneous amplitudes of the signals from said source, said one device being responsive to the applied weak magnetic field to signal modulate the linear polarized light beam in transmission therethrough by producing variable rotations of its plane of polarization proportional to said instantaneous signal amplitudes, means for applying the resulting signal-modulated polarized light beam to the second of said cells in said given direction, means for simultaneously applying to said second cell a strong magnetic field in the direction of light propagation, said second cell being responsive to the applied strong magnetic field when its magnitude exceeds a predetermined value to effectively conceal the signal modulation on the polarized light beam by converting that beam into a substantially completely unpolarized light beam in which the rotation effects representing signal modulation have been effectively removed and means to impress the resulting substantially unpolarized light beam on said transmission medium for transmission thereover to said receiving station; and said receiving station includes means for receiving the converted unpolarized light beam, a third Faraday cell identical with said one and said second Faraday cells at the transmitting station, supplied with the received converted unpolarized light beam in said given direction, means for applying to said third Faraday cell a strong magnetic field of the same magnitude as that applied to said second cell but in the opposite direction, said third cell being responsive to the applied strong magnetic field in said opposite direction to effectively restore the signal modulation to the light beam in transmission therethrough by effectively re-inserting therein rotation effects substantially indentical with those effectively removed therefrom by said second cell and representing signal modulation and thus reproducing the signal-modulated polarized light beam substantially as it appeared at the input of said second device in said transmitting station, a polarization analyser for converting the phase modulation represented by the rotation effects in said reproduced light beam into equivalent amplitude modulation, and means to detect the signals from the resulting amplitude-modulated light beam. Other info: Inventors: Bomke, Hans A. (Sea Girt, NJ, US) Application Number: 610445 Filing Date: 1956-09-10 Publication_date: 1976-02-03 Assignee: The United States of America as represented by the Secretary of the Army (Washington, DC) Primary Class(es): 380/59 359/281, 398/183 Other Classes: US Patent Ref:
Other Refs: Other References: The Bell System Technical Journal Vol. 31 No. 1 Jan. 1952, pp. 1-31. | |