Optical waveguide crossings

Details Optical waveguide crossings Optical waveguide crossings

1998-09-22

2001-03-06

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling function

C385S027000, C385S001000, C385S024000, C385S039000, C385S040000, C385S050000

Reexamination Certificate

active

06198860

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to the field of optical waveguide crossings.
In constructing integrated optical circuits, space constraints and the desire to operate on multiple input waveguides necessitate waveguide crossings. It is crucial that the crossings be as efficient as possible. A typical application is optical switching, where a large number of inputs are directed to as many outputs, and crossing is necessary in order for each input to connect to every output. Simplicity of fabrication on small lengthscales means that the waveguides must actually intersect, and cannot simply pass over one another. Any additional three-dimensional structure adds considerable manufacturing difficulty.
FIG. 1
is a block diagram of a conventional crossing arrangement
100
. In a perfect crossing arrangement, optical modes
102
propagate with 100% transmission (throughput) from an input waveguide
104
to an output waveguide
106
on the opposite side of a crossing intersection
108
, with no reflection and with 0% transmission (crosstalk) to the crossing waveguide
110
. The attainment of low crosstalk is especially important since it is generally difficult to separate two signals that have been mixed, whereas low transmission can be remedied by a simple amplifier.
Previous works on waveguide crossings have dealt with waveguides based on index confinement (sometimes thought of as total internal reflection, the most familiar example of which is the fiber-optic cable). High (but not perfect) throughput can be attained in such devices, but only in the limit of very long or short wavelengths compared to the waveguide size. It is desirable to achieve perfect crossings for any wavelength, including the case where the waveguide width is of the same order as the wavelength, which allows maximum miniaturization. The design of good crossings in conventional devices has been a matter of trial and error.
SUMMARY OF THE INVENTION
The invention provides for perfect crossings in which optical modes propagate with 100% transmission (throughput) from an input waveguide to the output waveguide on the opposite side of a crossing, with no reflection and with 0% transmission (crosstalk) to the crossing waveguide. Moreover, the crossing is analyzed by a simple theory that enables the design of perfect crossings in any photonic crystal waveguide, and good crossings in other settings as well. In this theory, the intersection is treated as a resonant cavity, and perfect crossing is achieved when the cavity modes satisfy a simple symmetry constraint.
In one embodiment of the invention, photonic crystal waveguides are utilized, which provide an ideal setting for the application of this theory. Photonic crystals are materials with band gaps that restrict the propagation of light in certain frequency ranges. Their discovery in recent years has caused a rethinking of conventional methods for manipulating light, and has led to proposals for many novel optical devices. A linear defect in a photonic crystal is the basis for a new kind of waveguide, which relies on the band gap restriction instead of index confinement to prevent light from escaping. Similarly, a defect at a single location (a point defect) creates a resonant cavity, which traps light in a small region. The waveguide crossing design of the invention makes uses of both of these phenomena.
The high losses associated with bends in conventional waveguides, together with the typical situation of parallel inputs, has pushed conventional designers to work with shallow crossing angles that make it even more difficult to achieve low crosstalk. In photonic crystals, however, it is possible to make sharp waveguide bends with 100% transmission, obviating the need for shallow-angle crossings. In the invention, therefore, the designs can be restricted to perpendicular intersections, which simplify the attainment of perfect crossings. The same principles could be applied to slightly non-perpendicular crossings, with some loss of efficiency and simplicity.
Accordingly, in accordance with one embodiment of the invention there is provided an optical waveguide structure including a first waveguide, a second waveguide that intersects with the first waveguide, and a photonic crystal resonator system at the intersection of the first and second waveguides.
In accordance with another embodiment there is provided an optical waveguide crossing structure including a first waveguide that propagates signals in a first direction, a second waveguide that intersects with the first waveguide and propagates signals in a second direction, and a photonic crystal crossing region at the intersection of the first and second waveguides that prevents crosstalk between the signals of the first and second waveguides.
In accordance with another embodiment of the invention there is provided An optical waveguide structure including a first waveguide, a second waveguide, and a resonator system at the intersection of the first and second waveguides, the intersection possessing a first mirror plane that is parallel to the first waveguide, the resonator system supporting a first resonant mode that includes different symmetry with guided modes in the first waveguide with respect to the first mirror plane, the resonator system substantially reduces crosstalk from the second waveguide to said first waveguide.


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patent: 5784400 (1998-07-01), Joannopoulos et al.
patent: 6101300 (2000-08-01), Fan et al.
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Aretz K. et al.: “Reduction of Crosstalk and Losses of Intersecting Waveguide”, Electronics Letters, GB IEE Stevenage, vol. 25, No. 11, May 25, 1999, pp. 730-731.

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