| 摘要 |
1,087,822. Lasers. INTERNATIONAL BUSINESS MACHINES CORPORATION. April 30, 1965 [May 1, 1964], No. 18244/65. Heading H1C. [Also in Division G2] A laser emitting polarized light from an active medium 1, Fig. 1, has the stimulated emission restricted to a selected region of the active medium, the region of emission being determined by the potential applied to an electrooptic crystal 20 which operates in association with a Babinet compensator 8 in the laser optical cavity. The active medium may be either a gas the emitted light from which passes through windows 2 at the Brewster angle or a ruby. The optical cavity is defined by mirrors 4, 5 one of which passes an output beam 6. The electrooptic crystal 20 may be a potassium dihydrogen phosphate crystal having transparent electrodes 21 to which an adjustable and reversible control potential is applied from a source 24. In the absence of control potential the stimulated emission is restricted to a narrow region extending across the active medium and coplanar with its axis, the Babinet compensator causing circular or elliptical polarization of light emitted from all other regions. Only reflected plane polarized light is of sufficient intensity to effect lasing in the region where it passes through the active medium. The function of the electro-optic crystal is to effect a phase shift in the light passing through it which is opposite in effect to the phase shift produced at a corresponding point in the Babinet compensator, the amount of phase shift being determined by the magnitude and polarity of the control potential. When such a potential is applied, complete phase cancelling of the phase shift produced in light passing through the compensator is obtained in a planar region offset from the laser axis. By appropriate control the narrow lasing region in the active medium can thus be selected as required. In an alternative arrangement, Fig. 4 (not shown), and Fig. 6, a further Babinet compensator 31 and electrooptic crystal 32, together with a polarizer 30 are positioned in the optical cavity with a 90-degree optical relationship to the original components 8, 9. Consequently by suitable potential values from sources 24, 38, only a filament of the whole active medium 1 is able to lase. A modification, Fig. 6, uses a spherical mirror 40 in place of mirror 4, the mirror 40 and a lens 41 having a common focal point P within the active medium. Consequently the selected lasing region is in the form of a narrow double cone within the active medium. This arrangement includes a mask 44 with character-shaped portions through which the output beam may pass, the beam being then directed by a lens 45 on to the surface of a movable light-sensitive material 46. By suitably adjusting the potentials applied to the electro-optic crystals 9, 32, a selected cone-shaped portion of the active medium lases and a corresponding character in the mask is recorded on the film. |
