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Laser Scanner Using Single-Mode Fiber and Cholesteric Liquid Crystal

IP.com Disclosure Number: IPCOM000060528D
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Goldburt, E: AUTHOR [+2]

Abstract

Using a single-mode fiber, mounted in a block and ground down to its core, together with a cholesteric liquid crystal layer as a helical output grating, and together with an electrostatic or electromagnetic modulation field, permits continuous variation of the out-coupled light angle r as a function of applied modulating field. This permits scan control for a laser beam without moving mechanical parts.

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Laser Scanner Using Single-Mode Fiber and Cholesteric Liquid Crystal

Using a single-mode fiber, mounted in a block and ground down to its core, together with a cholesteric liquid crystal layer as a helical output grating, and together with an electrostatic or electromagnetic modulation field, permits continuous variation of the out-coupled light angle r as a function of applied modulating field. This permits scan control for a laser beam without moving mechanical parts. The guided light in a single-mode fiber has associated with it a very accurately defined effective refractive index N (and hence effective wavelength g/N, where g is the wavelength of the guided light in vacuo) along the fiber axis, and the evanescent trailing fields may be reached, for example, by gluing the fiber into a curved groove in a quartz block, grinding away one side of the fiber to reach the core region, and highly polishing the resulting exposed surface. A single- mode fiber treated in this way might typically have a core diameter of about 7 mm and a coupling length (the length b over which the evanescent fields are exposed) of about 3 mm. A high resolution grating spectrometer can be constructed by placing a fine period grating on the polished fiber surface so as to interact with the evanescent fields of the light guided in the core region. In this device the grating acts as an output coupler for the guided light, causing it to radiate at a sharply defined angle into the free space outside the fiber (see figure). The angle at which the light escapes is given by r = arcsin (N - g/G) (1) where g is the grating period. If the device is seen as a spectrometer, one is interested in the variation of r with wavelength. The wavelength resolution of the device depends on the angular divergence of the diffracted beam in the xy plane. This is given by Wr = 2g/b (2) where b is the coupling length of the polished fiber core. It also has associated with it a divergence of about 15OE in the vertical plane (shaded in the figure). This divergence, which is exactly orthogonal to Wr, is caused by the core diameter being very small and can easily be eliminated using a cylindrical lens. In all devices that have so far been reported using this basic idea, the grating period has been assumed to be constant. Scanners could be made by varying the wavelength g of the light, but present-day semiconductor lasers do not offer sufficiently large wavelength ranges to make this idea worthwhile....