Optical waveguides – Integrated optical circuit
Reexamination Certificate
1998-03-23
2001-01-16
Lee, John D. (Department: 2874)
Optical waveguides
Integrated optical circuit
C359S199200, C385S024000, C385S037000, C385S047000
Reexamination Certificate
active
06175670
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to planar lightguide circuits, in particular those employed in multiplexing and demultiplexing of signals
2. Discussion of the Related Art
Dense Wavelength Division Multiplexing (DWDM) is the terminology applied to the latest efforts to improve the effectiveness of optical signal multiplexing and demultiplexing. DWDM, particularly that provided on a Planar Lightguide Circuit (PLC), is very attractive for mass-production of passive components for optical fiber communication systems.
Presently, many attempts are being made to use UV laser-induced fiber gratings in sensitized optical fibers to produce wavelength-selective passive components. Nevertheless, to use such gratings, one must either use a 3 dB splitter-combiner to handle the selected and back-reflected wavelength, and thereby incur the disadvantage of a 6 dB minimum insertion loss, or use a circulator, which is complex, expensive, and bulky. It is desirable to avoid these disadvantages.
SUMMARY OF THE INVENTION
According to the invention, a planar lightguide circuit has in close proximity on a substrate first and second planar lightguides having differing effective indices of refraction in a first region. Preferably, the lightguide having the higher effective index of refraction parallels in close proximity in the first region the lightguide having the lower effective index of refraction and propagating a plurality of wavelength-multiplexed signals. A first planar grating in coupling proximity to said first and second planar lightguides in the first region backward couples a first signal of a selected wavelength from one of said first and second lightguides to the other of said first and second lightguides. The coupling will work in either direction to provide either demultiplexing or multiplexing of optical signals of differing wavelengths. A multiplicity of lightguides like the second lightguide will provide dense demultiplexing or multiplexing of the differing wavelengths. Advantageously, all signal-bearing inputs and outputs can appear at a common edge of the substrate.
Typically, the above-described relationship of different effective indices of refraction in the first region, the backward-coupling region, is provided by differences in widths of the lightguides rather than by differences in material composition or differences in layer thicknesses. Nevertheless, the differences in effective indices could be supplemented by differences in layer thickness or even by differences in composition. The effective index of refraction n
e
of each lightguide will be intermediate between n
0
and n
c
, where n
0
is the effective index of refraction of the medium surrounding the lightguide and n
c
is the effective index of refraction of material in the interior of the lightguide. The exact value of n
e
will depend on the exact shape of the lightguide and upon the wavelength of light propagating in the waveguide. Fatter, e.g., wider, lightguides of a given material will tend to have the relative higher values of n
e
. Preferably, the lightguide having the higher effective index of refraction bends away from the lightguide having the lower effective index of refraction toward a second region of larger separation such that no interaction between the lightguides occurs. In a portion of the second region, at least one of the lightguides tapers toward a nominal guide width, so that the planar lightguide circuit can interface with its surroundings, e.g., a fiber array. It is a subsidiary feature of the invention that the lightguides have like effective indices of refraction and like common widths at least in that portion of the second region where interfacing may be desired. This feature also facilitates the appearance of all signal-bearing inputs and outputs substantially uniformly at a common edge of the substrate.
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Lee John D.
Lucent Technologies - Inc.
Wisner Wilford L.
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