Optical device for beam splitting and recombining

Details Optical device for beam splitting and recombining Optical device for beam splitting and recombining

1993-05-19

1995-04-25

Ullah, Akm E.

Optical waveguides

With optical coupler

Particular coupling function

385130, G02B 612

Patent

active

054106257

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to an optical device. More particularly, it relates to devices for performing beamsplitting and/or recombination functions, and to devices employing such functions.
2. Discussion of Prior Art
The emerging field of integrated optical systems has generated a number of components analogous to those employed in electronic circuits. However, the optical signal division and recombination functions are difficult to implement in efficient, cheap and compact form. Patent Cooperation Treaty Application No PCT/GB 88/00124, (published as WO88/07179 on Sep. 22, 1988 corresponding to U.S. Pat. No. 4,975,237) relates to apparatus for light scattering measurements. It discloses use of fibre optic directional couplers for light beam division (beamsplitting) and recombination in a heterodyne light beating system. The arrangement can become complex in multiple simultaneous measurements, which require a cascaded arrangement of fibre optic couplers. It is neither inexpensive nor efficient in terms of light intensity transfer.
A particular need for multiway beamsplitting occurs in an electro-optic beamsteering device of the kind disclosed in Patent Cooperation Treaty Application No PCT/GB 88/00928 published as WO89/04988 on Jun. 1st, 1989 corresponding to U.S. Pat. No. 5,239,598. This device consists of an array of parallel waveguides of electro-optic material formed in a layered semiconductor structure. The waveguides have optical path lengths controlled by electrical bias applied to them. The array provides a common output beam which is steerable with appropriate applied bias conditions. Such a device requires light from a single source to be divided into a number of equal intensity beams (eg ten) for input to respective waveguides. This implies use of a multiway beamsplitter compatible with a layered semiconductor structure. Such a beamsplitter is not currently available.
In U.S. Pat. No. 3,832,029 and in "Image Formation Using Self-imaging Techniques", Journal of the Optical Society of America, Vol 63, No 4, Apr. 1973 pages 416-419, O Bryngdahl describes the use of a square cross-section optical tunnel to produce a self-image of a symmetrical object. The tunnel is of length L=n(2d.sup.2 /.lambda.) where n is an integer, d is the width of the tunnel and .lambda. is the wavelength of the light in the tunnel. In both references it is suggested that at lengths other than those given above multiple self images may be obtained. These lengths being L=(n+V/v)2d.sup.2 /.lambda. where V and v are integers with no common factor. However neither reference teaches what length of tunnel is necessary to achieve required numbers of self-images.
It is, however, known to employ a rectangular waveguide in a self-imaging role to provide beamsplitting, recombination and interferometric functions. This is disclosed by A Simon and R Ulrich in Applied Physics Letters, Vol 31, No. 2 Jul. 15, 1977, pages 77 to 79. The device comprises four optically polished glass blocks arranged as walls of a waveguide with rectangular cross-section. The aspect (width to height) ratio W.sub.y /W.sub.x of the waveguide is .sqroot.2; the waveguide has fully reflecting (mirror) side walls and is air cored. It has a length L=4W.sub.x.sup.2 /.lambda. (for a core refractive index of 1). The waveguide receives light from an illuminated test object at an off-centre position at one end, and generates two off-centre images at the other end for subsequent output. The off-centre input avoids output image overlap. This is the beamsplitter function. The beam combined function consists of two off-centre images at one end being transformed into two off-centre output images at the other. If the two input images are in phase then one of the two output images is dark. If the two input images are out of phase then both output images will have non zero light intensity. The beam combiner is equivalent to a beamsplitter with two input and two output ports. The interferometer function is produced by arranging tw

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