Patent GB-1175855-A (GB)
Title: Improvements in or relating to information processors
IPC: G02B6/04
Filed: 1967-06-30
Abstract: 1,175,855. Analog system for Fourier transformation. AMERICAN OPTICAL CO. 30 June, 1967 [25 Aug., 1966], No. 30277/67. Heading G4G. [Also in Divisions G2 and H4] In an information processor the optical path length of an optical fibre, e.g. a glass fibre, and thus the phase of a coherent light beam propagating through the fibre, is changed by varying the temperature or the stress applied to the fibre resulting in a variation in the refractive index, cross-section and length of the fibre. The temperature is varied by means of a heater (Fig. 2, not shown), and the stress by means of hydrostatic pressure, or piezoelectric or magneto-strictive expansion of a block of neighbouring material (Figs. 3, 4 and 5, not shown). The piezoelectric or magnetostrictive materials may be incorporated in the cladding of the fibre. The amplitude of the emerging light from the fibre may be changed by passing place polarized or circularly polarized coherent light through the fibre and twisting the fibre which provides rotation of the plane of polarization or applying a unidirectional transverse stress to the fibre which provides birefringence, the emerging light passing through an analysing polarizer (Fig. 6, not shown). Alternatively one plane of polarization may be variably absorbed by an absorber incorporated in the cladding of the fibre, a stress being applied to the device at 45 degrees to the line joining the absorber and fibre (Figs. 7 and 8, not shown). A Fourier transform of a set of incident signals may be obtained by utilizing an array of optical fibres 10, Fig. 9, whose optical path length and/or transmitted intensity are separately varied by appropriate influences as described above by passing the set of signals through appropriate transducers 62. A coherent light source, e.g. laser, illuminated the fibres at plane 50 and the light which emerges from the fibres at plane 52 is focussed on screen 56 by lens 58 which is at a distance from the screen equal to its focal length 60. The result is that the intensity distribution on screen 56 is the Fourier transform of the distribution at plane 52. The set of signals received may be from hydrophones (Fig. 10, not shown), the signals passing through filters where they are measured relative to an arbitrary phase and passed to transducers which produce the same phase displacements as measured on the array of fibres for the laser light propagated therethrough. The Fourier transform of the emergent light at the exit plane of the fibre array gives directly an image of the source of the sonic disturbances. If the disturbances are polychromatic each fibre of the array of fibres to which transduced signals are applied from the hydrophones, has a companion reference fibre to which no transduced signals are applied. A laser light is focused on to only one pair of fibres resulting in a Fourier transform radiation pattern, Fig. 11 (not shown), and by scanning the laser beam across the pairs of fibres the resulting Fourier transform radiation patterns can be compared. By sweeping the beam at various rates across the array of fibres and measuring the contrast ratio from the fringes in the time interval that it takes to sweep the beam, a correlation function is in effect measured, the function corresponding to the intensity of sonic disturbances originating from a given direction. Sources of different wavelengths may be scanned simultaneously.