Planar lightwave circuit

Details Planar lightwave circuit Planar lightwave circuit

1999-09-03

2001-10-16

Lester, Evelyn A (Department: 2873)

Optical waveguides

Planar optical waveguide

C385S014000, C385S045000, C385S130000, C385S131000, C385S132000

Reexamination Certificate

active

06304706

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a planar lightwave circuit and, more particularly, to a planar lightwave circuit having regions sandwiched between a plurality of waveguides like a star or Y-branching waveguide.
Conventionally, a planar lightwave circuit formed on a planar substrate can have various functions such as multiplexing/demultiplexing, optical branching, and optical switching, and hence is expected as a practical optical device or component. A multi/demultiplexer and optical branching circuit, in particular, are expected as passive parts important for a wavelength multiplexing network system and access network.
FIGS. 8
to
10
show an arrayed-waveguide grating multi/demultiplexer using silica glass for a planar lightwave circuit.
FIG. 8
shows the arrayed-waveguide grating multi/demultiplexer.
FIG. 9
shows part of the arrayed-waveguide grating multi/demultiplexer.
FIG. 10
shows part of a cross section taken along a line B-B′ in FIG.
9
.
As shown in
FIG. 8
, in this arrayed-waveguide grating multi/demultiplexer, first of all, signal light incident from input waveguides
801
is expanded in an input-side slab waveguide
802
and strikes an arrayed waveguide
803
. In the arrayed waveguide
803
, since optical path length differences are set between the adjacent waveguides, the signal light which is guided through the arrayed waveguide
803
and incident on an output-side slab waveguide
804
has phase differences. The signal light is therefore focused and demultiplexed by different output waveguides
805
depending on the wavelengths satisfying diffraction conditions.
In the arrayed waveguide
803
, as shown in
FIGS. 9 and 10
, cores
803
a
are clearly separated from each other. In the connection portion between the arrayed waveguide
803
and the input-side slab waveguide
802
or output-side slab waveguide
804
, spacings on the &mgr;m order are formed between the cores
803
a.
As shown in
FIG. 10
, each core
803
a
is sandwiched between lower and upper clads
806
and
807
made of silica glass having a refractive index lower than that of the core
803
a,
thereby forming an optical waveguide.
As described above, a multi/demultiplexer and optical branching circuit are expected as passive components important for a wavelength multiplexing network system and access network. It is essential for these passive components that the propagation loss of light signals is as low as possible.
In the conventional arrayed-waveguide grating multi/demultiplexer shown in
FIG. 8
, however, there are spacings on the &mgr;m order are formed between the respective cores
803
a
at the connection region between the cores
803
a
constituting the arrayed waveguide
803
and the input-side slab waveguide
802
. For this reason, part of incident signal light from the input-side slab waveguide
802
to the arrayed waveguide
803
is scattered through the spacings of the &mgr;m order. The propagation loss of signal light due to this scattering is as large as 50% of the total loss.
As described above, in a conventional circuit in which signal light branches, such as an arrayed-waveguide grating multi/demultiplexer, signal light is scattered through the spacings between the branching cores. Hence, a propagation loss occurs.
According to a reference (C. van Dam, A.A.M. Staring et al., “Loss reduction for phased-array demultiplexers using a double etch technique” Integrated Photonics Research 1996 Boston, Mass., April 29-May 2, pp. 52-55), in an InGaAsP-based arrayed-waveguide grating multi/demultiplexer, a transition region is formed on the boundary between a slab waveguide and an arrayed waveguide by etching halves of cores so as to reduce the propagation loss of signal light. Even if, however, this structure is applied to glass-based waveguides, the propagation loss reducing effect is very small.
In addition, according to this technique, in a lithography process of transferring a circuit pattern, etching must be performed twice after a mask is accurately aligned, resulting in a complicated process.
According to another reference (Jerry C. Chen and C Dragone, “A Proposed Design for Ultralow-Loss Waveguide Grating Routers”, IEEE Photon. Technol. Lett., vol. 10, pp. 379-381, March, 1998), a simulation result is reported, which indicates that a reduction in loss can be attained by optimizing a circuit configuration. However, the above problem of scattering of signal light still remains unsolved.
As described above, when signal light is to be branched or demultiplexed from one waveguide or slab waveguide into a plurality of waveguides, the spacings between the respective waveguides at the branching point are ideally 0 in terms of the loss of light.
However, photolithography and etching techniques used in the process of forming waveguides have their own limits of resolution, and the spacings between the respective waveguides (cores), e.g., glass-based waveguides, at the branching point are about 1 &mgr;m or more. For this reason, in a conventional planar lightwave circuit, an excess waveguide loss occurs at such a branching portion or demultiplexing portion. Demands have therefore arisen for a reduction in loss at the portion.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to reduce the propagation loss of light at a branching point of waveguides constituting a planar lightwave circuit and a peripheral portion of the branching point.
In order to achieve the above object, according to an aspect of the present invention, there is provided a planar lightwave circuit comprising a plurality of nonparallel adjacent waveguides constituting a branching waveguide, and buried layers arranged between the adjacent waveguides, wherein the waveguides are made up of cores and surrounding clads, the buried layers are formed, in tight contact, between the cores to extend from a branching point from which the cores of the adjacent waveguides branch and to decrease in thickness as spacings between the cores of the adjacent waveguides increase with an increase in distance from the branching point, a refractive index of the buried layer is higher than that of the clad, and a refractive index of the core is not less than that of the buried layer.
With this arrangement, the refractive index of each of the portions between the adjacent cores branching from the branching point gradually decreases with an increase in distance from the branching point.


REFERENCES:
patent: 5127081 (1992-06-01), Koren et al.
patent: 6222966 (2001-04-01), Khan et al.
patent: 2-113209 (1990-04-01), None
patent: 4-70605 (1992-03-01), None
patent: 9-73021 (1997-03-01), None
patent: 10-48444 (1998-02-01), None

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