Optical waveguides – With optical coupler – Particular coupling structure
Patent
1993-05-20
1995-01-03
Healy, Brian
Optical waveguides
With optical coupler
Particular coupling structure
385 24, 385 27, 385 28, 385 39, 385 48, G02B 626
Patent
active
053793544
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an intensity dividing device for use in division of radiation.
2. Discussion of Prior Art
Radiation intensity dividing devices are known, such as for example optical fibre Y-junctions. Y-junctions may be symmetrical, for division of one input beam into two substantially equal intensity beams. Such devices 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. Active symmetric Y-junctions which employ electro-optic effects to achieve asymmetric splitting are also known. An example is 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. However, symmetrical Y-junctions, both active and passive, suffer from high losses, particularly for split angles greater than a few degrees.
Asymmetric Y-junctions capable of dividing an input beam into two beams of differing intensities are also known. One such device is described by K. Shirafuji and S. Karazono in "Transmission Characteristics of Optical Asymmetric Y Junction with a Gap Region", Journal of Lightwave Technology, Vol 9, No 4 (1991) pp 426-429. It is considerably more efficient than more conventional Y-junctions since it uses total internal reflection to redirect radiation to one of the two output parts. Radiation reaches the other output port by coupling across a gap. This radiation is not deviated from the input direction of propagation. The power splitting ratio is determined by the width of the gap.
All Y-junctions, however, suffer from the disadvantage that they can only provide two way splitting. Therefore, to achieve higher order splitting Y-junctions are used in series, thus multiplying the losses incurred at each stage.
Many other forms of intensity dividing device are also known. In International Patent Application No. PCT/US89/00190, published under International Publication No. WO 89/06813 E. Kapon describes optical waveguide junctions. One incorporates a single input waveguide with four single mode output waveguides of differing widths and/or differing refractive indices, radiating from an end. The output waveguides are therefore characterised by different propagation constants. For a given input wavelength, different modes of the input waveguide will couple to different output waveguides, as a result of the different propagation constants. However, this is an inefficient device with high transmission losses, since energy from each mode will in general enter each output waveguide but will be lost from those with unfavourable propagation constants.
An alternative device described by E. Kapon incorporates four single mode input waveguides of differing widths and/or refractive indices, converging into an area from which three single mode output waveguides radiate. The modes excited in the common area are dependent on which input waveguides are providing radiation beams. The output waveguides operate as described for the single input devices. These devices enable radiation to be divided according to the ratio of excitation of modes in the waveguides feeding the output waveguides. However, as previously stated they are highly inefficient.
In U.S. Pat. No. 4,693,546 J. P. Lorenzo and R. A. Soref describe a "Guided Wave Optical Power Divider". It is in the form of an X-junction. Two input waveguides converge on an input end of a crossover region and two output waveguides diverge from an output end of the region. The input and output waveguides are single mode and of width W. The crossover region supports two modes, one odd and one even, and is of width 2 W. The device is formed from crystalline silicon and the crossover region is doped. In an undoped device radiation passes through the crossover region substantially undeviated, and enters the first output waveguide. In a doped device waveg
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Healy Brian
The Secretary of State for Defence in Her Britannic Majesty's Go
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