Optical mixing device

Details Optical mixing device Optical mixing device

1993-05-19

1995-12-12

Bovernick, Rodney B.

Optical waveguides

With optical coupler

Particular coupling function

385146, G02B 626

Patent

active

054757762

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to an optical mixing device.


DISCUSSION OF PRIOR ART

Optical devices for beam mixing are well known in the prior art. Beamsplitters are employed to mix two optical beams as described by A F Harvey in "Coherent Light" p1046, Wiley, London (1970). They may be used in free space or incorporated into waveguide systems.
Optical Y-junctions of various forms are known in the prior art for the production of mixed beams. Various passive symmetrical Y-junctions are discussed by Z. Weissman, A. Hardy and E. Marom in "Mode-Dependent Radiation Loss in Y-Junctions and Directional Couplers", IEEE Journal of Quantum Electronics Vol. 25, No. 6 (1989) pp. 1200-1208. Asymmetric Y-junctions are discussed by K. Shirafuji and S. Kurazono in "Transmission Characteristics of Optical Asymmetric Y Junction with a Gap Region", Journal of Lightwave Technology Vol 9, No 4 (1991) pp. 426-429. Active Y-junctions are also known, and examples are described by H. Sasaki and I. Anderson in "Theoretical and Experimental Studies on Active Y-junctions in Optical Waveguides", IEEE Journal of Quantum Electronics Vol. QE-14, No. 11 (1978) pp. 883-892. Each of these references discusses in detail the use of Y-junctions for beam splitting, but gives little detail on their use for beam combination or mixing. Indeed the symmetric Y-junctions, both active and passive, are inefficient splitters. Their transmission is heavily dependent on the angle of splitting; transmission is as low as 20% for splitting of a few degrees.
Prior art arrangements for mixing of more than two beams involve beamsplitter or Y-junction devices used in series. The losses of individual devices are therefore multiplied leading to very inefficient beam mixing.
If subsequent detection is required then the mixed beam, or beams, may be directed to appropriate detectors. However, in addition to their inefficiency, prior art mixing devices based on Y-junctions also suffer from the disadvantage of only having one output port. This redu the available information concerning the relative phases of the input beams. Therefore such prior art devices are of limited usefulness for applications such as heterodyne detection where comprehensive phase information is important.


SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative form of optical mixing device.
The present invention provides an optical mixing device for operation at a wavelength .gamma. and including: directed to the input region, waveguide to the output region, and characteristics of the input radiation beams are in combination arranged to provide for modal dispersion in the waveguide giving rise to input radiation mixing in the output region and mixed radiation detection by the detecting means.
The present invention provides the advantage that two input beams may be efficiently mixed. Theoretically the invention might provide mixing with 100% efficiency. In practice, efficiencies of 75% have been achieved in non-optimised embodiments of the invention. The invention provides the additional advantage that phase information contained in input radiation beams is not lost when the beams are mixed. This enables mixing to be carried out prior to detection. The waveguide incorporated in devices of the invention may be of rectangular cross-section, of height 2a and width 2b, where b is greater than a. The detecting means may be located centrally in the output region. Additional detecting means may be included. These may be two additional detecting means located within the output region distant b/2 to either side laterally of the centre of the output region, where b is as defined earlier. In such a device the waveguide may be of length L=2b.sup.2 /.gamma., where .gamma. is the wavelength of input radiation measured within the waveguide.
Mixing devices of the invention may incorporate first and second detecting means. Each of these detecting means may be located within the output region and distant b/2 laterally from and on a re

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