Patent GB-1040236-A (GB)
Title: Improvements in or relating to polarization rotation devices
IPC: G02F1/09
Filed: 1963-06-20
Abstract: 1,040,236. Light modulation; lasers. WESTERN ELECTRIC CO. June 20, 1963 [June 28, 1962], No. 24565/63. Headings H3B and H4F. A light rotation device, Fig. 1, utilizing the Faraday effect, comprises a transmitting and rotating element 30 of material consisting of the trivalent chromium compounds of chlorine, bromine and/or iodine, which are ferromagnetic below their Curie points and which exhibit a net magnetic moment in the presence of an applied magnetic biasing field, means 36, 32 for applying linearly polarized light waves to the medium, means 33, 37 for receiving the waves after traversing the medium, means 31 for maintaining the temperature of the medium below its Curie point, means H o  for magnetically saturating the medium and means H R  for providing a finite non-zero magnetic field component parallel to the direction of the light waves. 32 and 33 are polarizing prisms mutually aligned at 90 degrees so that in the absence of field H R  no light is transmitted through the device. A small percentage of the chromium ions may be replaced by paramagnetic ions as impurities. Field H 0  may be aligned in any direction in the yz plane; field H R  may be aligned in any direction outside the yz plane; field H R  may be aligned in any direction outside the yz plane; the two fields may be components of one field. The component parallel to the direction of light waves may be varied to modulate the rotation correspondingly. Reversing the direction of the field reverses the direction of light rotation. Element 30 may be oriented to lie in a plane other than that indicated. Means 31 may be a cryostat containing liquid helium. The chromium trihalides are capable of operation in a ferromagnetic resonance state and in accordance with another aspect of the invention, Fig. 6, element 51 is located in a short-circuited waveguide supporting a standing means pattern in the TE 10  mode. The element is positioned one halfwavelength from the short-circuit 53 and experiences a magnetic field oriented along the X axis. Light is transmitted through the element by way of crossed polarizer and analyzer 60, 61 and guides 56 and 59 which are dimensioned to be below cut-off for the microwave energy in the waveguide. A saturating direct field is applied in the direction H 0  and the element is maintained (by means not illustrated) at a temperature below its Curie point. The use of superconducting magnets to produce field H 0  is referred to. Details are given for an arrangement using a chromium tribromide crystal. Fig. 7 illustrates an application of the invention to an optical isolator which is stated to be of use in a Maser amplifier system for the purpose of preventing oscillation due to optical feedback to the source 81. The chromium trihalide element 71 is located between polarizer 79 and analyzer 80 which are mutually aligned at 45 degrees and is controlled to introduce a rotation of 45 degrees by a superconducting magnet 70 disposed within the low temperature chamber 72 provided for element 71. Magnet 70 saturates the crystal and is oriented to produce the appropriate component in the direction of the optical axis. The light is confined to the green part of the visible spectrum. Light passes freely from source 81 to utilization means 82, but reflected light is subjected to two rotations of 45 degrees and is therefore stopped by polarizer 79. The Specification includes curves of absorption versus wavelength and specific rotation versus wavelength for chromium tribromide.