Patent GB-1121540-A (GB)
Title: A system for deflecting a coherent quasi-monochromatic light beam
IPC: G01B9/02
Filed: 1966-04-15
Abstract: 1,121,540. Light deflectors. TECHNICAL OPERATIONS Inc. 15 April, 1966 [24 May, 1965], No. 16740/66. Heading H4F. [Also in Division H1] A system for deflecting a coherent, quasimonochromatic light beam comprises an optical cavity bounded by two mirrors for multiplepass propagation of the light beam to produce a localized interference pattern at one of the mirrors, and means for varying the optical path length between the mirrors causing the localized. interference pattern to move across the plane of the mirror. The light beam must have a coherence length many times the distance between the two mirrors of the enclosure. The angle between the two mirrors is so adjusted that the fringe separation obtained is equal to the diameter of the mirror (output aperture), i.e. only one fringe is visible at any time in the field of view. This fringe is imaged into a point on a screen by means of cylindrical optics. The optical path length is varied, and the fringe thus moved, by changing the refractive indek inside the cavity by subjecting electrooptic materials which exhibit the Kerr or Pockels effect to an electric field, or by moving an end mirror of the cavity along the optical axis of the system. In a first embodiment, Fig. 1, not shown, an electro-optic crystal (10) operates as a multiplepass optical cavity, i.e. a Fabry-Perot type cavity, one of the mirror surfaces of the crystal being cut at a slight angle to the other mirror surface of the crystal. The laser producing the coherent, monochromatic light beam may be directly coupled to one mirror surface of the crystal so that one end mirror serves both as an end mirror for the laser and an end mirror for the Fabry-Perot enclosure defined by the crystal. Alternatively, Fig. 2, not shown, the laser (12) may be separated from the electrooptic crystal (10), a telescopic system (22), (23) adjusting the aperture of the beam entering the optical elements. The thickness of the crystal is such that a change of at least one half-wave length in optical path-length is achieved by application of a transverse electric field so that the one-fringe pattern may be moved across the entire output aperture of the system. The cylindrical lens (25) which causes the fringe to be imaged to a point on the screen (21) may be positioned on either side of the crystal which forms the optical cavity, its alignment being less critical when it is positioned before the electro-optic crystal, Fig. 2, not shown. In a third embodiment, Fig. 3, not shown, the optical path length is varied between two mirrors (30) and (31) slightly tilted with respect to each other which form the Fabry- Perot enclosure, by means of a Kerr cell located between the mirrors at any position where the light beams are parallel, i.e. a telescopic system is also. positioned between the mirrors in the optical cavity. In a fourth embodiment, Fig. 4, not shown, two telescopic lens systems with the Kerr cell positioned in the collimated beam between them, are located in the enclosure, and by arrangement of the lenses in the two telescopic systems the aperture of the collimated light beam may be increased or decreased for optimum utilization of the Kerr cell. The light source may be a laser directly coupled to the Fabry-Perot enclosure with a single reflective coated surface acting as a common mirror for the laser and for the enclosure. In a fifth embodiment, Fig. 5, not shown, an output fringe is caused to shift by mechanically moving one end mirror of the enclosure back and forth with respect to the other end mirror of the enclosure. For this the mirror to be moved is attached to the end of a piezo-electric ceramic crystal length expander in the shape of a cylinder to which an electric field is applied. In another embodiment the optical path length is varied by an optical flat (110), Fig. 10, not shown, positioned between the two mirrors which form the enclosure, and oscillatable about an axis crossing the optical axis of the system. For this the optical flat is attached at each end to piezo-electric expander crystals which are driven in phase by a suitable voltage source. Alternatively the optical path length may be varied by employing heavy flint glass, its refractive index being varied by means of a strong magnetic field. The light spot may be moved over a twodimensional area transverse to the optical axis of the system by setting up two Fabry- Perot enclosures in line along the optical axis to produce fringes at right angles to each other. The Fabry-Perot enclosures may comprise enclosures in which one mirror is moved, i.e. piezo-electric enclosures, Fig. 6, not shown, electro-optic crystal enclosures, Figs. 7 and 8, not shown, or an electro-optic enclosure and a piezo-electric enclosure, Fig. 9, not shown.