Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-04-15
2004-12-28
Lee, Diane I. (Department: 2876)
Optical waveguides
With optical coupler
Input/output coupler
C385S043000, C398S084000
Reexamination Certificate
active
06836594
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims benefit of Japanese Patent Application No. 2001-116749 filed on Apr. 16, 2001, the contents of which are incorporated by the reference.
The present invention relates to array waveguide gratings used as light wavelength multiplexing/demultiplexing elements for optical communication, array waveguide grating modules, optical communication units and optical communication systems using the same array wavelength lattices. More specifically, the present invention concerns array waveguide gratings with improved light signal frequency characteristics, array waveguide modules, optical communication units and optical communication systems using the same array waveguide gratings.
With processes of usual time internet connection and communication data capacity increase, demands for large capacity data transfer are increasing. In the optical communication using light signals, it is very important for large capacity data transfer to improve the degree of wavelength multiplexing. In this respect, the role of array waveguide gratings as multiplexing/demultiplexing elements for multiplexing and demultiplexing light wavelengths is important, and the array waveguide gratings are thought to be one of key devices. The array waveguide grating has a passive structure, and also has a narrow light wavelength transmission width and a high extinction ratio. The array waveguide grating also has such features as that it can multiplex and demultiplex a number of light signals in correspondence to the number of waveguides.
Such array waveguide grating is desirably free from sudden changes of its output level or loss level with variations of the laser output light signal frequency from the center optical frequency of each optical waveguide. Also, where multiple stages of array waveguide gratings are connected, the modulation components of the light signal are cut off outside a bandwidth, in which the individual array waveguide gratings commonly transmit the light signal. Thus, it is important from the standpoint of improving the light signal transmission efficiency as well to realize a transmission characteristic with a flat peak level with respect to optical frequency.
FIG. 33
shows an example of such array waveguide grating. The illustrated array waveguide grating
10
has a substrate
11
, on which one or more first channel waveguides (i.e., input channel waveguides)
12
, a plurality of second channel waveguides (i.e., output channel waveguides)
13
, a channel waveguide array
14
with a plurality of component channel waveguides bent in a predetermined direction with different radii of curvature, a first sector-shape slab waveguide
15
connecting the first channel waveguides
12
and the channel waveguide array
14
to one another and a second sector-shape slab waveguide
16
connecting the channel waveguide array
14
and the second channel waveguides
13
to one another, are formed. Multiplexed light signals with wavelengths &lgr;
1
to &lgr;
n
, are incident from the first channel waveguides
12
on the first sector-shape slab waveguide
15
, then proceed with their paths expanded therethough and are then incident on the channel waveguide array
14
.
In the channel waveguide array
14
, the component array waveguides have progressively increasing or reducing optical path lengths with a predetermined optical path length difference provided between adjacent ones of them. Thus, the light beams proceeding through the individual array waveguides reach the second sector-shape slab waveguide
16
with a predetermined phase difference provided between adjacent ones of them. Actually, wavelength dispersion takes place, and the in-phase plane is inclined in dependence on the wavelength. Consequently, the light beams are focused (i.e., converged) on the boundary surface between the second sector-shape slab waveguide
16
and the plurality of second channel waveguides
13
at positions different with wavelengths. The second channel waveguides
13
are disposed at positions corresponding to their respective wavelengths. Given wavelength components &lgr;
1
to &lgr;
n
thus can be taken out independently from the second channel waveguides
13
.
FIG. 34
shows, to an enlarged scale, a boundary part between the first channel waveguides and the first sector-shape slab waveguide in the array waveguide grating shown in FIG.
33
. The first channel waveguides
121
to
123
, which are shown in a first boundary part
18
shown in
FIG. 33
as well, have optical waveguides
211
to
213
having a rectangular shape with a width Wp and length L
2
and terminating in the first sector-shape slab waveguide
15
.
FIG. 35
shows a boundary part in the case of using parabolic or second degree function shape waveguides disclosed in Japanese Patent Laid-Open No. 9-297228. In this case, the first channel waveguides
121
to
123
shown in the first boundary part
18
have optical waveguides
221
to
223
having a second degree function shape with a length L
2
and terminating with a width Wp in the sector-shape slab waveguide
15
.
Insertion loss and transmission width are usually in a trade-off relation to each other. However, where rectangular optical waveguides
211
to
213
shown in
FIG. 34
are used in lieu of the second degree function shape light waveguides
221
to
223
shown in
FIG. 35
, the transmission width can be improved without sacrifice in the insertion loss. It is thus a great merit to use the rectangular optical waveguides
211
to
213
shown in
FIG. 34
for realizing a flat transmitted light frequency characteristic.
The above description has concerned with the shapes of the optical waveguides, which are disposed in the first boundary part
18
between the first channel waveguide
12
and the first sector-shape slab waveguide
15
shown in FIG.
33
. Such optical waveguides
21
and
22
are provided for the purpose of providing for harmonic mode of input at their locality of contact with the slab waveguide to make the Gaussian waveform peak part as flat as possible.
In lieu of providing the above contrivance with respect to the optical waveguides
21
and
22
, the same effects are obtainable by providing optical waveguides of the same shapes in the second boundary part
19
as the boundary between the second channel waveguides
13
and the second sector-shape slab waveguide
16
. Here, for the sake of the simplicity of description, only the shapes of the optical waveguides in the first boundary part
18
will be considered.
Where the rectangular optical waveguides
211
to
213
as shown in
FIG. 34
are used, the variable shape parameters are only the width Wp and the length L
2
of the rectangular part. Therefore, if the width Wp and the length L
2
can assume only values limited on the design, it is possible to change the characteristics in such ranges. In other words, in this case a problem is posed that the degree of freedom in fine adjustment and fine design for realizing various properties is very low. For example, the problem may concern the transmission width and the stroke in the trade-off relation to each other. These problems will be discussed in detail in the following.
FIG. 36
shows an ideal characteristic of wavelength multiplexed light signals. In the graph, the ordinate is taken for the transmitted light signal power level, and the abscissa is taken for the wavelength. The individual light signals
311
,
312
,
313
have a rectangular waveform and also have a maximum transmission width. Thus, signal components of other light signals are not mixed with the signal components of the intrinsic light signals. Where such ideal light signals
311
to
313
are multiplexed, by connecting multiple stages of array waveguide gratings or array waveguide grating modules the bandwidth of the individual light signals is not reduced. The center wavelength of the light signals
311
to
313
may be deviated, but the signal level is not varied. However, no light signal transmitted through such array waveguide grating has such ideal rectangular wavefor
Lee Diane I.
McGinn & Gibb PLLC
NEC Corporation
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