Patent GB-1171411-A (GB)
Title: Optical Modulator
IPC: G02F1/09
Filed: 1968-03-25
Abstract: 1171411 Light deflectors INTERNATIONAL BUSINESS MACHINES CORP 25 March 1968 [9 May 1967] 14257/68 Headings H3U U21 U22 and U28  In an optical modulator comprising a transparent magnetostrictive medium, a light beam incident thereon at the Bragg angle is diffracted by means of the spin wave component of magnetoelastic waves which are produced in the medium, a magnetic field being also produced in the medium. In a first embodiment, Fig.1, a monochromatic and collimated incident light beam 12 from an He-Ne laser 14 is diffracted by a magnetoelastic rod 16 to impinge on target 19. Rod 19 may comprise of ferrimagnet yttrium iron garnet having rectangular cross-section or a ferromagnetic material. Bias field magnet 28, 30 produces the main bias magnetic field Hm in medium 16 parallel to the direction of propagation 22 of the magnetoelastic waves 24 established by means of transducer 48 (shear vibration source) energized by radiofrequency source 44 or by means of an antenna coil 25 energized by radio-frequency source 27. The applied magnetic field is changed by means of control field Hc produced by windings 32 and 34. The radio-frequency generator 44 is established at a fixed frequency, and the internal magnetic field Hi in rod 16 is established at a particular value for a selected Bragg angle # B  by means of variable voltage source 38 connected to control windings 32 and 34. A small change in the magnetic control signal produces a large change in the diffraction angle for the transmitted beam. The variation of the internal magnetic field Hi is in the form of a curve which increases to a maximum near the centre portion of rod 16, Fig.2 (not shown). The shape of the curve may be altered by altering the shape of rod 16 and the gradient of the applied magnetic field. The magnetoelastic wave consists of an elastic wave component and a spin wave component and due to divergent or convergent wave fronts. The magnetoelastic wave may be focused by using a curved transducer or by suitable gradients of the internal magnetic field. In a second embodiment, Fig.5, the magnetoelastic medium 106 which is a good laser host is situated in an angularly degenerate laser cavity including spherical mirrors 102 and 104 and pumping light 108 (the laser medium may be independent of the magnetoelastic medium). A transducer element 110, e.g. a crystal of zinc oxide or cadmium sulphide, is fixed to the magnetoelastic and active laser material 106, and high frequency electrical fields are applied to the transducer from radio-frequency generator 112. An absorbing material 115 for elastic waves, e.g. absorbing glass, is situated on the other side of medium 106. A magnetic field is applied in medium 106 by coil 116 and the manner in which it is varied to alter the Bragg angle for diffraction is similar to the description of the effect of varying the internal magnetic field Hi in Fig. 1. Although it is possible to scan the laser anodes only one lasing anode is present in the cavity. The lasing mode which is incident at the Bragg angle for the magnetoelastic waves is diffracted from them at the Bragg angle to form scanning beam 126. Medium 106 may have the coupling between these components the dispersion curves are separated into two branches, Fig.3 (not shown), and as a result the variation in Bragg angle as the internal magnetic field is varied is also representable as two curves, Fig. 4 (not shown). For selective diffraction of incident light beam 12 the magnetoelastic rod 16 may be suitably contoured to establish a preferred distribution of internal magnetic field, e.g. for a uniform distribution the magnetoelastic medium is ellipsoidal in contour. The distribution of magnetic field may be selectively varied by varying the orientation of both the main bias field Hm and the control field Hc. For magnetoelastic waves 24 travelling parallel to field Hm + He it is desirable to provide a circularly polarized shear wave transducer 48 with the direction of rotation such as to interact with the spin system of the magnetostrictive medium. For magnetoelastic waves 24 travelling perpendicular to the magnetic field, a linearly polarized shear wave transducer is desirable, with the direction of polarization parallel to the magnetic field. To attain a high diffracted intensity, laser beam 12 is plane polarized parallel to the propagation direction 22 of the magnetoelastic wave 24 and the magnetoelastic or optical waves are focused to have curved surfaces 128 and 130 to assist in focusing the laser light in the cavity on to the spherical mirrors 102 and 104.