Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2000-12-29
2003-06-03
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S037000, C359S199200
Reexamination Certificate
active
06574397
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an arrayed waveguide grating (AWG) type optical multiplexer/demultiplexer which is employable as a wavelength-selecting device in a wavelength division multiplexing (WDM) transmission system.
2. Related Background Art
AWG type optical multiplexer/demultiplexers (hereinafter referred to as AWG circuits) are widely in use as a wavelength filter, which can take out or insert a specific wavelength upon interference, for a wavelength-selecting device in WDM transmission systems. Also, since the AWG circuits can be realized by general fine processing procedures such as lithography or etching without necessitating the machining as precise as that of diffraction gratings or the forming of multilayer films as precise as that of interference films, they are expected to develop as a main optical device in future WDM transmission systems together with their capability of assembling with other optical waveguide devices.
Such an AWG circuit has a structure in which an input waveguide, an input slab waveguide, a plurality of channel waveguides having respective lengths different from each other (phased array), an output slab waveguide, and a plurality of output waveguides are integrally formed on a single substrate and are covered with cladding glass.
In particular, U.S. Pat. No. 5,002,350 discloses an optical multiplexer/demultiplexer in which, among a plurality of channel waveguides, those adjacent each other have a constant optical path length difference therebetween. At respective portions where the channel waveguides connect with the input and output slab waveguides, in order to improve the wavelength multi/demultiplexing performances, the end parts of channel waveguides are arranged at equally spaced intervals along an arc having a predetermined radius or each of the end faces of input and output slab waveguides to which the end parts of channel waveguides are connected at equally spaced intervals is processed so as to match an arc having the predetermined radius.
SUMMARY OF THE INVENTION
The inventors have studied the conventional optical multiplexer/demultiplexers and, as a result, have found problems as follows. Namely, if the end parts of channel waveguides are arranged like an arc, then the convergence of light outputted from those located in peripheral portions to the slab waveguides may deteriorate under the influence of aberration and the like, while the convergence of light outputted from those located near the center thereof is fully maintained. In addition, if the end parts of channel waveguides are arranged at equally spaced intervals along an arc, then the convergence of light outputted from the channel waveguides located in peripheral portions in particular to the slab waveguides will further deteriorate due to interactions between adjacent channel waveguide. As a result, among the output waveguides provided so as to correspond to respective signal channels in the conventional optical multiplexer/demultiplexers, crosstalk between adjacent signal channels may be remarkable in the output waveguides located in the periphery in particular, whereas there has been a technical limit to lowering the crosstalk.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical multiplexer/demultiplexer comprising a structure which can effectively lower the crosstalk between adjacent signal channels among output waveguides provided so as to correspond to respective signal channels, in the output waveguides located in the periphery in particular, thereby realizing excellent wavelength multi/demultiplexing characteristics.
The optical multiplexer/demultiplexer according to the present invention is an AWG type optical multiplexer/demultiplexer, employable as a wavelength-selecting device in a WDM transmission system, comprising a substrate, and at least one input waveguide, a first slab waveguide, n (≧3) channel waveguides, a second slab waveguide, and a plurality of output waveguides provided for respective signal channels, which are disposed on the substrate.
In the optical multiplexer/demultiplexer according to the present invention, the first and second slab waveguides have respective predetermined slab lengths. In general, a slab length corresponds to the focal length of the optical input end functioning as the lens surface of the respective slab waveguide. The input waveguide is a waveguide for guiding to the first slab waveguide individual signals having respective channel wavelengths set at predetermined wavelength intervals as signal channels, and has an output end optically connected to an optical input end face of the first slab waveguide. The n channel waveguides are waveguides having lengths different from each other, and are two-dimensionally arranged on the substrate while in a state where an optical input end of each channel waveguide is optically connected to an optical output end face of the first slab waveguide so as to sandwich the first slab waveguide together with the input waveguide whereas an optical output end of each channel waveguide is optically connected to an optical input end face of the second slab waveguide so as to sandwich the second slab waveguide together with the output waveguides. The output waveguides are waveguides two-dimensionally arranged on the substrate while in a state where respective optical input ends thereof are optically connected to an optical output end face of the second slab waveguide, and are used for separately taking out signals having respective channel wavelengths set at predetermined wavelength intervals.
In particular, in the optical multiplexer/demultiplexer according to the present invention, at least one of the optical output end face of the first slab waveguide and the optical input end face of the second slab waveguide each connected to the n channel waveguides is processed flat so as to extend along a line intersecting the n channel waveguides. As a consequence, among the n channel waveguides, those adjacent each other have optical path length differences different from each other.
Specifically, with respect to the average value obtained from respective optical path length differences between all adjacent channel waveguides in the n channel waveguides, it is preferred that the maximum deviation of optical path length difference between adjacent channel waveguides in the n channel waveguides be set to 3% or more. It means that, letting &Dgr;L
k
(k=1 to (n−1)) be each optical path length difference between adjacent channel waveguides, &Dgr;L
MAX
be the maximum optical path length difference (or minimum optical path length difference) between adjacent channel waveguides, and &Dgr;L
AVE
be the average value of optical path length difference, at least the deviation &eegr; (maximum deviation) of maximum optical path length difference &Dgr;L
MAX
with respect to the average value &Dgr;L
AVE
satisfies the following condition:
η
=
|
Δ
L
A
V
E
-
Δ
L
MAX
|
Δ
L
A
V
E
≥
0.03
where
Δ
L
A
V
E
=
∑
k
=
1
n
-
1
Δ
L
k
n
-
1
.
Here, the optical multiplexer/demultiplexer according to the present invention is designed such that, though adjacent channel waveguides have optical path length differences different from each other, effective optical path length differences become constant between optical paths extending from the center of optical input end face of the first slab waveguide to the center of optical output end face of the second slab waveguide by way of the channel waveguides adjacent each other in order to realize wavelength multi/demultiplexing functions as a whole. Namely, as shown in
FIG. 4
, letting L(m) be the physical optical path length from the center P
1
of optical input end face of the first slab waveguide to the ce
Katayama Makoto
Nishimura Masayuki
Tanaka Shigeru
McDermott & Will & Emery
Rahll Jerry T
Sumitomo Electric Industries Ltd.
Ullah Akm E.
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