U.S. patent application number 09/776772 was filed with the patent office on 2001-08-09 for wavelength multiplexing/demultiplexing unit, wavelength multiplexing/demultiplexing apparatus and wavelength multiplexing/demultiplexing method.
Invention is credited to Izawa, Tatsuo, Kato, Kuniharu, Oguchi, Taisuke, Yamada, Yasufumi.
Application Number | 20010012424 09/776772 |
Document ID | / |
Family ID | 18553916 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012424 |
Kind Code |
A1 |
Kato, Kuniharu ; et
al. |
August 9, 2001 |
Wavelength multiplexing/demultiplexing unit, wavelength
multiplexing/demultiplexing apparatus and wavelength
multiplexing/demultiplexing method
Abstract
An optical multiplexing/demultiplexing apparatus in which a
plurality of optical multiplexing/demultiplexing units which are
different from one another in operating wavelength band are
connected hierarchically. Each of the optical
multiplexing/demultiplexing units comprises an input waveguide for
receiving a wavelength multiplexed optical waves, a filter for
separating the wavelength multiplexed optical wave from the input
waveguide into a first optical wave in the corresponding operating
wavelength band and a second optical wave in the other wavelength
bands, an AWG optical multiplexer/demultiplexer for separating the
first optical wave from the filter into individual optical waves
each of a single wavelength, and a branch waveguide for directing
the second optical waves from the filter to the input waveguide of
a succeeding optical multiplexing/demultiplexing unit.
Inventors: |
Kato, Kuniharu;
(Ibaraki-ken, JP) ; Yamada, Yasufumi; (Mito-shi,
JP) ; Oguchi, Taisuke; (Mito-shi, JP) ; Izawa,
Tatsuo; (Tokyo, JP) |
Correspondence
Address: |
Pillsbury Winthrop LLP
East Tower, Ninth Floor
1100 New York Avenue, N. W.
Washington
DC
20005-3918
US
|
Family ID: |
18553916 |
Appl. No.: |
09/776772 |
Filed: |
February 6, 2001 |
Current U.S.
Class: |
385/24 ; 385/14;
385/37 |
Current CPC
Class: |
G02B 6/12007 20130101;
G02B 2006/12109 20130101; G02B 6/12021 20130101; G02B 6/12019
20130101 |
Class at
Publication: |
385/24 ; 385/37;
385/14 |
International
Class: |
G02B 006/293 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2000 |
JP |
2000-028528 |
Claims
What is claimed is:
1. An optical multiplexing/demultiplexing unit comprising: a first
waveguide for receiving a wavelength multiplexed optical wave;
filter means optically connected with said first waveguide for
separating the wavelength multiplexed optical wave from the first
waveguide into a first optical wave in a first wavelength band and
a second optical wave in a second wavelength band; means for
optical-demultiplexing the first optical wave in the first
wavelength band into optical waves each of a single wavelength; and
a second waveguide for directing the second optical wave in the
second wavelength band to a succeeding optical means.
2. The optical multiplexing/demultiplexing unit according to claim
1, wherein said means for optical-demultiplexing comprises a
plurality of optical multiplexing/demultiplexing circuits which are
different from one another in operating wavelength band.
3. The optical multiplexing/demultiplexing unit according to claim
1, wherein said means for optical-demultiplexing is an arrayed
waveguide grating circuit.
4. The optical multiplexing/demultiplexing unit according to claim
1, wherein said filter means is one of a highpass filter type of
optical multiplexer/demultiplexer, a lowpass filter type of optical
multiplexer/demultiplexer, and a bandpass filter type of optical
multiplexer/demultiplexer.
5. An optical multiplexing/demultiplexing unit comprising: a first
waveguide for receiving a wavelength multiplexed optical wave;
first filter means optically connected with the first waveguide for
separating the wavelength multiplexed optical wave from the first
waveguide into a first optical wave in a first wavelength band and
a second optical wave in a second wavelength band; second filter
means optically connected with the first filter means for
separating the first optical wave in the first wavelength band from
said first filter means into a third optical wave in a third
wavelength band and a fourth optical wave in a fourth wavelength
band; means for optical-demultiplexing the third optical wave in
the third wavelength band from said second filter means into
optical waves each of a single wavelength; a second waveguide for
outputting the second optical wave in the second wavelength band
from said first filter means to a succeeding optical means; and a
third waveguide for outputting the fourth optical wave in the
fourth wavelength band from said second filter means to a
succeeding optical means.
6. The optical multiplexing/demultiplexing unit according to claim
5, wherein said means for optical-demultiplexing is an arrayed
waveguide grating circuit.
7. The optical multiplexing/demultiplexing unit according to claim
5, wherein each of said first and second filter means is one of a
highpass filter type of optical multiplexer/demultiplexer, a
lowpass filter type of optical multiplexer/demultiplexer, and a
bandpass filter type of optical multiplexer/demultiplexer.
8. An optical multiplexing/demultiplexing unit comprising: a
substrate; a first cladding layer stacked on said substrate; a core
layer stacked on said first cladding layer; a second cladding layer
stacked on said core layer; and said core layer including: a first
waveguide for receiving a wavelength multiplexed optical wave;
filter means optically connected with said first waveguide for
separating the wavelength multiplexed optical wave from said first
waveguide into a first optical wave in a first wavelength band and
a second optical wave in a second wavelength band; means for
optical-demultiplexing the first optical wave in the first
wavelength band into optical waves each of a single wavelength; and
a second waveguide for directing the second optical wave in the
second wavelength band to a succeeding optical means.
9. An optical multiplexing/demultiplexing apparatus comprising: a
plurality of optical multiplexing/demultiplexing units which are
different from one another in operating wavelength band are
connected hierarchically, wherein each of the optical
multiplexing/demultiplexing units is according to one of said
optical multiplexing/demultiplexing units.
10. The optical multiplexing/demultiplexing apparatus according to
claim 9, wherein said means for optical-demultiplexing is an
arrayed waveguide grating circuit.
11. The optical multiplexing/demultiplexing apparatus according to
claim 9, wherein said first filter means is one of a highpass
filter type of optical multiplexer/demultiplexer, a lowpass filter
type of optical multiplexer/demultiplexer, and a bandpass filter
type of optical multiplexer/demultiplexer.
12. The optical multiplexing/demultiplexing apparatus according to
claim 9, wherein said optical multiplexing/demultiplexing units are
connected in the order of operating wavelength band beginning with
the shortest wavelength band.
13. The optical multiplexing/demultiplexing apparatus according to
claim 9, wherein said optical multiplexing/demultiplexing units are
connected in the order of operating wavelength band beginning with
the longest wavelength band.
14. The optical multiplexing/demultiplexing apparatus according to
claim 9, wherein said means for optical-demultiplexing comprises a
plurality of optical multiplexing/demultiplexing circuits which are
different from one another in operating wavelength band.
15. An optical multiplexing/de multiplexing apparatus in which a
plurality of optical multiplexing/demultiplexing units which are
different from one another in operating wavelength band are
connected hierarchically, each of said optical multiplexing/de
multiplexing units comprising: a first waveguide for receiving a
wavelength multiplexed optical wave; filter means optically
connected with the first waveguide for separating the wavelength
multiplexed optical wave from the first waveguide into a first
optical wave having the corresponding operating wavelength band and
a second optical wave having the other wavelength bands; means for
optical-demultiplexing the first optical wave from said filter
means into optical waves each of a single wavelength; and a second
waveguide for directing the second optical wave from said filter
means to a first waveguide of a succeeding optical
multiplexing/demultiplexing unit.
16. A method of demultiplexing an incident wavelength multiplexed
optical wave in steps into different wavelength bands comprising: a
first step of separating the wavelength multiplexed optical wave
into a first optical wave in a first wavelength band and a second
optical wave in a second wavelength band and then demultiplexing
the first optical wave in the first wavelength band into individual
optical waves each of a single wavelength; and a second step, which
is repeated subsequent to the first step until all optical waves of
different wavelengths contained in the wavelength multiplexed
optical wave are separated, of separating the second optical wave
in the second wavelength band into a wavelength multiplexed optical
wave in a specific wavelength band and a optical wave in the
remaining wavelength band and then demultiplexing the wavelength
multiplexed optical wave in the specific wavelength band into
individual optical waves.
17. A method of demultiplexing an incident wavelength multiplexed
optical wave into different wavelength bands comprising: a first
step of separating the wavelength multiplexed optical wave into a
first optical wave in a first wavelength band and a second optical
wave in a second wavelength band by a first filter means, and then
outputting the first optical wave to succeeding optical means and
the second optical wave to a second filter means; a second step of
separating the first optical wave from said first filter means into
a third optical wave in a third wavelength band and a fourth
optical wave in a fourth wavelength band by a second filter means,
and then outputting the third optical wave to
multiplexing/demultiplexing means and the fourth optical wave to
succeeding optical means; and a third step of
optical-demultiplexing the third optical wave into optical waves
each of a single wavelength from said multiplexing/demultiplexing
means.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2000-028528, filed Feb. 7, 2000, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a wavelength
multiplexing/de multiplexing unit, a wavelength multiplexing/de
multiplexing apparatus and a wavelength multiplexing/de
multiplexing method which are available for optical communications
or the like.
[0003] Wavelength division multiplexing (WDM) is a technique of
combining a plurality of optical waves of different wavelengths for
transmission over a single optical fiber and used to expand the
transmission capacity of trunk lines of the Internet or the
like.
[0004] A wavelength multiplexing/demultiplexing circuit plays a key
role in WDM. That is, the wavelength multiplexing/demultiplexing
circuit is a device arranged to combine a plurality of optical
waves of different wavelengths at the transmitting end and a
separating wavelength multiplexed optical wave at the receiving
end.
[0005] For example, an arrayed waveguide grating (AWG) circuit,
which is a conventional wavelength multiplexing/demultiplexing
circuit, separates components of a multiplexed optical wave in the
following manner:
[0006] That is, a wavelength multiplexed optical wave is input to
the arrayed waveguide grating circuit through its input port and is
then caused to pass through a plurality of (say, m) waveguides
arranged in an array, whereby the multiplexed optical wave is
separated into n components (optical waves) each having a single
wavelength. Each of the optical waves with a different wavelength
is output from a corresponding respective one of n output ports.
The AWG circuit thus arranged is allowed to multiplex optical waves
of different wavelengths the number of which is usually 30 to
60.
[0007] Recently, demands for optical communications have been
remarkable and it is therefore required to further increase the
number of channels.
[0008] However, an attempt to increase the number of channels with
the conventional wavelength multiplexing/demultiplexing system
would require the development of a new wavelength
multiplexing/demultiplexing circuit having more arrayed waveguides.
The development and manufacture of such a wavelength
multiplexing/demultiplexing circuit require sophisticated
techniques, which will result in very costly devices. In addition,
the use of the new wavelength multiplexing/demultiplexing circuit
would force the exchange of existing facilities including
peripheral equipment, increasing the economical burden on
users.
[0009] On the other hand, an arrangement has been proposed which
increases the number of channels through the use of a number of
multiplexers/demultiplexers using dielectric interference filters
(for instance, DWDM components brochure by JDS FITEL, Dense
wavelength Division Multiplexers (DWDM) Modules brochure by OPLINK
COMMUNICATIONS). This arrangement is upgraded up to 16 or 40
channels with an eight-channel multiplexers/demultiplexers as one
unit. Each unit is constructed by connecting, in stages, three-port
multiplexers/demultiplex- ers using dielectric interference filters
(for instance, Optical Add Drop multiplexers (OADM) Modules
brochure by OPLINK COMMUNICATIONS). In this arrangement, a
three-port multiplexer/demultiplexer is used for each channel so
that the succeeding channel suffers reflection loss from the
preceding three-port multiplexer/demultiplexer cumulatively.
[0010] From this reason, each channel suffers more transmission
loss than the preceding one, thus, in the
multiplexing/demultiplexing unit, the transmission loss varies
greatly from channel to channel use is, therefore, at most 8 or 16.
Even in multichannel arrangement in which the units are connected
in stages the maximum increased number of channels is limited to at
most 40 or 64.
BRIEF SUMMARY OF THE INVENTION
[0011] It is an object of the present invention to provide a
wavelength multiplexing/demultiplexing unit, a wavelength
multiplexing/demultiplexin- g apparatus and method which permit the
number of channels to be increased readily at low cost.
[0012] To achieve the above object, according to the first aspect
of this invention, there is provided an optical
multiplexing/demultiplexing unit comprising a first waveguide for
receiving a wavelength multiplexed optical wave; filter means
optically connected with the first waveguide for separating the
wavelength multiplexed optical wave from the first waveguide into a
first optical wave in a first wavelength band and a second optical
wave in a second wavelength band; means for optical-demultiplexing
the first optical wave in the first wavelength band into optical
waves each of a single wavelength; and a second waveguide for
directing the second optical wave in the second wavelength band to
a succeeding optical means.
[0013] To achieve the above object, according to the second aspect
of this invention, there is provided an optical
multiplexing/demultiplexing unit comprising: a first waveguide for
receiving a wavelength multiplexed optical wave; first filter means
optically connected with the first waveguide for separating the
wavelength multiplexed optical wave from the first waveguide into a
first optical wave in a first wavelength band and a second optical
wave in a second wavelength band; second filter means optically
connected with the first filter means for separating the first
optical wave in the first wavelength band from the first filter
means into a third optical wave in a third wavelength band and a
fourth optical wave in a fourth wavelength band; means for
optical-demultiplexing the third optical wave in the third
wavelength band from the second filter means into optical waves
each of a single wavelength; a second waveguide for outputting the
second optical wave in the second wavelength band from the first
filter means to a succeeding optical means; and a third waveguide
for outputting the fourth optical wave in the fourth wavelength
band from the second filter means to a succeeding optical
means.
[0014] To achieve the above object, according to the third aspect
of this invention, there is provided an optical
multiplexing/demultiplexing unit comprising a substrate; a first
cladding layer stacked on the substrate;a core layer stacked on the
first cladding layer; a second cladding layer stacked on the core
layer; and the core layer including: a first waveguide for
receiving a wavelength multiplexed optical wave; filter means
optically connected with the first waveguide for separating the
wavelength multiplexed optical wave from the first waveguide into a
first optical wave in a first wavelength band and a second optical
wave in a second wavelength band; means for optical-demultiplexing
the first optical wave in the first wavelength band into optical
waves each of a single wavelength; and a second waveguide for
directing the second optical wave in the second wavelength band to
a succeeding optical means.
[0015] To achieve the above object, according to the fourth aspect
of this invention, there is provided an optical
multiplexing/demultiplexing apparatus comprising: a plurality of
optical multiplexing/demultiplexing units which are different from
one another in operating wavelength band are connected
hierarchically, wherein each of the optical
multiplexing/demultiplexing units is according to one of the
optical multiplexing/demultiplexing units.
[0016] To achieve the above object, according to the fifth of this
invention, there is provided an optical multiplexing/de
multiplexing apparatus in which a plurality of optical
multiplexing/de multiplexing units which are different from one
another in operating wavelength band are connected hierarchically,
each of the optical multiplexing/de multiplexing units comprising:
a first waveguide for receiving a wavelength multiplexed optical
wave; filter means optically connected with the first waveguide for
separating the wavelength multiplexed optical wave from the first
waveguide into a first optical wave having the corresponding
operating wavelength band and a second optical wave having the
other wavelength bands; means for optical-demultiplexing the first
optical wave from the filter means into optical waves each of a
single wavelength; and a second waveguide for directing the second
optical wave from the filter means to a first waveguide of a
succeeding optical multiplexing/demultiplexing unit.
[0017] To achieve the above object, according to the sixth of this
invention, there is provided a method of demultiplexing an incident
wavelength multiplexed optical wave in steps into different
wavelength bands comprising: a first step of separating the
wavelength multiplexed optical wave into a first optical wave in a
first wavelength band and a second optical wave in a second
wavelength band and then demultiplexing the first optical wave in
the first wavelength band into individual optical waves each of a
single wavelength; and a second step, which is repeated subsequent
to the first step until all optical waves of different wavelengths
contained in the wavelength multiplexed optical wave are separated,
of separating the second optical wave in the second wavelength band
into a wavelength multiplexed optical wave in a specific wavelength
band and a optical wave in the remaining wavelength band and then
demultiplexing the wavelength multiplexed optical wave in the
specific wavelength band into individual optical waves.
[0018] To achieve the above object, according to the seventh of
this invention, there is provided a method of demultiplexing an
incident wavelength multiplexed optical wave into different
wavelength bands comprising: a first step of separating the
wavelength multiplexed optical wave into a first optical wave in a
first wavelength band and a second optical wave in a second
wavelength band by a first filter means, and then outputting the
first optical wave to succeeding optical means and the second
optical wave to a second filter means; a second step of separating
the first optical wave from the first filter means into a third
optical wave in a third wavelength band and a fourth optical wave
in a fourth wavelength band by a second filter means, and then
outputting the third optical wave to multiplexing/demultiplexing
means and the fourth optical wave to succeeding optical means;
and
[0019] a third step of optical-demultiplexing the third optical
wave into optical waves each of a single wavelength from the
multiplexing/demultiplexing means.
[0020] With these configurations, a wavelength
multiplexing/demultiplexing unit, a wavelength
multiplexing/demultiplexing apparatus and method which permit the
number of channels to be increased readily at low cost can be
realized.
[0021] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0022] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0023] FIG. 1 is a schematic diagram of a optical
multiplexing/demultiplex- ing apparatus 10 according to a first
embodiment of the present invention;
[0024] FIG. 2A is an enlarged view of a optical
multiplexing/demultiplexin- g unit 11;
[0025] FIG. 2B shows the wavelength characteristic of the optical
multiplexer/demultiplexer of the AWG type 20;
[0026] FIG. 2C shows the wavelength characteristic of an
interference film used as the filter 30;
[0027] FIG. 2D shows the loss characteristic over the main
operating wavelength region at each stage when the optical
multiplexing/demultiplex- ing units 11 are connected in three
stages;
[0028] FIG. 3 is a diagram for use in explanation of the
arrangement of the optical multiplexer/demultiplexer of the AWG
type 20;
[0029] FIG. 4 is a sectional view taken along line C-C in FIG.
3;
[0030] FIG. 5 is a top view of a general directional coupler;
[0031] FIG. 6 is a sectional view taken along line C-C in FIG.
5;
[0032] FIG. 7 is a top view of a optical
multiplexing/demultiplexing circuit 32 of the Mach-Zehnder
interference type;
[0033] FIG. 8 is a sectional view taken along line C-C in FIG.
7;
[0034] FIG. 9 is a top view of a optical
multiplexing/demultiplexing circuit 33 of the interference filter
type;
[0035] FIG. 10 is a sectional view taken along line C-C in FIG.
9;
[0036] FIG. 11 is a diagram for use in explanation of a optical
multiplexing/demultiplexing unit 60 according to a second
embodiment;
[0037] FIG. 12A shows a first-stage optical
multiplexing/demultiplexing unit 100 of the optical
multiplexing/demultiplexing apparatus of the second embodiment;
[0038] FIG. 12B is a schematic diagram of the optical
multiplexing/demultiplexing apparatus of the second embodiment;
[0039] FIG. 13A shows the wavelength characteristic of optical
waves separated by the AWG optical multiplexer/demultiplexer
20;
[0040] FIG. 13B shows the wavelength characteristic of the filter
30 shown in FIG. 12B;
[0041] FIG. 13C shows the wavelength characteristic of the highpass
filter 35;
[0042] FIG. 14 shows an optical multiplexer/demultiplexer of the
AWG type 80 in which a first AWG waveguide 81 and a second AWG
waveguide 82 are cross-arranged;
[0043] FIG. 15 is a top view of a optical
multiplexing/demultiplexing unit 130 according to a fourth
embodiment; and
[0044] FIGS. 16A, 16B, 17A and 17B are diagrams for use in
explanation of the reason why lossy regions occur.
DETAILED DESCRIPTION OF THE INVENTION
[0045] Hereinafter, first through fourth embodiments of the present
invention will be described with reference to the accompanying
drawings. In the description below, like reference numerals and
characters are used to designate corresponding parts throughout
several views which have substantially the same function and
arrangement and repeated descriptions thereof are given only when
necessary.
[0046] Note that, in the description below, a optical wave of a
predetermined single wavelength corresponds to a channel to carry
the computer data or the like. In addition, a wavelength
multiplexed optical wave corresponds to multi-channels to carry the
computer data or the like.
[0047] (First Embodiment)
[0048] First, a description is given of a first embodiment of an
optical multiplexing/demultiplexing of the present invention.
[0049] An important point of the optical
multiplexing/demultiplexing apparatus of the present invention
resides in an idea of combining or separating multichannel optical
waves by hierarchically connecting a plurality of optical
multiplexing/demultiplexing units each of which combines or
separates optical waves in different wavelength bands.
[0050] FIG. 1 shows a schematic arrangement of the optical
multiplexing/demultiplexing apparatus 10 according to the first
embodiment of the present invention. First, the schematic
arrangement of the optical multiplexing/demultiplexing apparatus 10
will be described.
[0051] As shown in FIG. 1, the optical multiplexing/demultiplexing
apparatus 10 has a plurality of optical multiplexing/demultiplexing
units 11 connected in a hierarchical form by fiber 9.
[0052] FIG. 2A is an enlarged view of the optical
multiplexing/demultiplex- ing unit 11.
[0053] The optical multiplexing/demultiplexing unit 11 is
constructed such that an AWG optical multiplexer/demultiplexer 20,
a filter 30, an input waveguide 40, a coupling waveguide 50, output
waveguides 55 and a branch waveguide 70 are connected optically and
mounted on a separate board 12 made of plastic by way of example.
Each of these components will be described below.
[0054] [AWG Optical Multiplexer/Demultiplexer]
[0055] The optical multiplexer/demultiplexer 20 is a device that
wavelength-multiplexes optical waves by interference of light
passing through tens to hundreds of arrayed waveguides (AWG
waveguides) which are arranged in parallel and differ in optical
path length by predetermined amounts. In addition, the optical
multiplexer/demultiplexer 20 is also a device that demultiplexes a
wavelength multiplexed optical wave by passing the AWG waveguides
to optical waves each of which has a different wavelength. The
optical multiplexer/demultiplexer 20 will be described in detail
with reference to FIGS. 3 and 4.
[0056] FIG. 3 is a diagram for use in explanation of the schematic
arrangement of the AWG optical multiplexer/demultiplexer 20. FIG. 4
is a sectional view taken along line C-C of FIG. 3.
[0057] As shown in FIG. 4, the AWG optical
multiplexer/demultiplexer 20 has a stacked structure of a silicon
substrate 21, a lower cladding layer 22, a core layer 23, and an
upper cladding layer 24.
[0058] The lower cladding layer 22 and the upper cladding layer 24
are each made of silicon dioxide (SiO.sub.2) and surround the core
layer 23 to confine light.
[0059] The core layer 23 is a optical waveguide consisting of
germanium-doped silicon dioxide. Light can be confined in the core
layer 23 by making the index of refraction of the core layer 23
higher than those of the lower and upper cladding layers 22 and
24.
[0060] The core layer 23 is formed, as shown in FIG. 3, with an
input planar waveguide 23a, an input slab waveguide 23b, AWG
waveguides 23c, an output slab waveguide 23d, and output planar
waveguides 23e. Each waveguide is formed by means of etching
techniques.
[0061] The input planar waveguide 23a is a waveguide that
propagates input a wavelength multiplexed optical wave to the input
slab waveguide 23b.
[0062] The input slab waveguide 23b is a sector-shaped slab
waveguide and functions as a lens. That is, the wavelength
multiplexed optical wave directed to the input slab waveguide 23b
spreads radially in that waveguide and are then taken into the AWG
waveguides 23c.
[0063] The AWG waveguides 23c are arrayed waveguides in which tens
to hundreds of optical waveguides whose optical path lengths differ
by .DELTA.L are arranged in the form of an array. A phase
difference is produced between the optical waves that travel
through each of the AWG waveguides 23c and the optical wave that
travel through the adjacent one of the AWG waveguides, which is
given by n.multidot..DELTA.L where n is the refractive index of the
core layer 23. Thus, light rays directed from the AWG waveguides
23c to the output slab waveguide 23d are multiplexed optical waves
displaced in phase by n.multidot..DELTA.L.
[0064] The output slab waveguide 23d is an inversesector-shaped
waveguide. That is, light rays directed from the AWG waveguides 23c
to the output slab waveguide 23d interfere with one another in the
waveguide 23d, so that the light rays are separated into optical
waves each with a single wavelength. Each of the resulting optical
waves is directed to a corresponding one of the output waveguides
23e.
[0065] The output planar waveguides 23e direct the optical waves
each with a single wavelength to the succeeding device.
[0066] In contrast to the above description, assuming that the
optical waveguides 23e are on the light receiving side and the
optical waveguide 23a is on the light outputting side, the AWG
optical multiplexer/demultiplexer 20 will function as a multiplexer
that combines optical waves.
[0067] [Filter]
[0068] The filter 30 is one which separates a wavelength
multiplexed optical waves input from the input waveguide 40 into
wavelength components to be separated by the optical
multiplexer/demultiplexer in the corresponding optical
multiplexing/demultiplexing unit 11 and wavelength components to be
separated in the succeeding optical multiplexing/demultiplexing
unit. For the filter 30, any of a highpass filter type of optical
multiplexer/demultiplexer, a lowpass filter type of optical
multiplexer/demultiplexer and a bandpass filter type of optical
multiplexer/demultiplexer can be used as required.
[0069] As the highpass filter, the lowpass filter or the bandpass
filter type of optical multiplexer/demultiplexer, use may be made
of, for example, an interference film filter type of optical
multiplexer/demultiplexer. Also, as the bandpass filter type of
optical multiplexer/demultiplexer, use may be made of, for example,
a directional coupler or a Mach-Zehnder interference optical
multiplexer/demultiplexer. The directional coupler, the
Mach-Zehnder interference optical multiplexer/demultiplexer and the
interference film filter optical multiplexer/demultiplexer will be
described hereinafter.
[0070] [Directional Coupler]
[0071] The directional coupler is a device that separates optical
waves using evanescent coupling. FIG. 5 is a top view of a general
directional coupler, which is indicated generally at 31. FIG. 6 is
a sectional view taken along line C-C of FIG. 5.
[0072] In FIGS. 5 and 6, the directional coupler 31 comprises a
silicon substrate 31a, a lower cladding layer 31b, a core layer
31C, and an upper cladding layer 31d.
[0073] The lower and upper cladding layers 31b and 31d, consisting
of silicon dioxide, are formed to surround the core layer 31c to
serve the light confinement function.
[0074] The core layer 31c is a optical waveguide having first and
second core layers 31c-1 and 31c-2 consisting of germanium-doped
silicon dioxide. Light can be confined in the core layer 31c by
making the refractive index of the core layer 31c higher than those
of the lower and upper cladding layers 31b and 31d.
[0075] The directional coupler 31 has a coupling portion 3le in
which the first and second core layers 31c-1 and 31c-2 are situated
in close proximity to each other with a given spacing of, say, 10
.mu.m or less. In this coupling portion, the so-called evanescent
coupling occurs by which a optical wave that propagates through the
first core layer 31c-1 shifts to the second core layer 31c-2. The
length of the coupling portion 31e, i.e., the spacing between the
first and second core layers, is called the coupling length. A
sufficient coupling length to allow the shifting of optical waves
is ensured.
[0076] In the directional coupler 31 thus constructed, a wavelength
multiplexed optical wave input to the coupler 31 from the direction
indicated by arrow A in FIG. 5 is separated by means of the
evanescent coupling into an outgoing component that propagates
through the first core layer 31c-1 along the direction indicated by
arrow B and an outgoing component that propagates through the
second core layer 31c-2 along the direction indicated by arrow
D.
[0077] [Mach-Zehnder Interference Optical
Multiplexer/Demultiplexer]
[0078] The Mach-Zehnder interference optical
multiplexer/demultiplexer is a waveguide type of optical
multiplexer/demultiplexer that splits an incoming optical wave into
two optical waves in a directional coupler, then causes the optical
waves to propagate through waveguides of different lengths and
combines them again in the other directional coupler.
[0079] FIG. 7 is a top view of a Mach-Zehnder interference optical
multiplexer/demultiplexer, which is indicated generally at 32. FIG.
8 is a sectional view taken along line C-C of FIG. 7.
[0080] In FIGS. 7 and 8, the Mach-Zehnder interference optical
multiplexer/demultiplexer 32 comprises a silicon substrate 32a, a
lower cladding layer 32b, a core layer 32C, and an upper cladding
layer 32d.
[0081] The lower and upper cladding layers 32b and 32d, consisting
of silicon dioxide, are formed to surround the core layer 32c to
serve the light confinement function.
[0082] The core layer 32c is a optical waveguide having first and
second core layers 32c-1 and 32c-2 consisting of germanium-doped
silicon dioxide. Light can be confined in the core layer 32c by
making the refractive index of the core layer 32c higher than those
of the lower and upper cladding layers 32b and 32d.
[0083] The Mach-Zehnder interference optical
multiplexer/demultiplexer 32 has coupling portions 32e and 32f in
which the first and second core layers 32c-1 and 32c-2 are situated
in close proximity to each other with a given spacing of, say, 10
.mu.m or less. In this coupling portion, the shift of optical waves
between the first waveguides 32c-1 and 32c-2 occurs by the
evanescent coupling.
[0084] The distance L between the coupling portions 32e and 32f is
of the order of, say, 10 mm. The difference .DELTA.L in optical
path length between the first and second waveguides 32c-1 and 32c-2
over the distance L determines the spacing between wavelengths
separated by the Mach-Zehnder interference optical
multiplexer/demultiplexer 32. For example, by heating the second
waveguide 32c-2 using a heater formed above the waveguide to change
its refractive index and the optical path length difference
.DELTA.L, the wavelength spacing can be changed readily.
[0085] In the Mach-Zehnder interference optical
multiplexer/demultiplexer 32 thus arranged, when a wavelength
multiplexed optical wave is input to the directional coupler 31
from the direction of arrow A, a portion of the incoming signal
shifts to the second waveguide 32c-2 in the coupling portion 32e.
The optical path length difference causes the wavelength
multiplexed optical wave that propagates through the second
waveguide 32c-2 to have a phase difference with respect to the
wavelength multiplexed optical wave that propagates through the
first waveguide 32c-1. In the coupling portion 32f, the multiplexed
optical wave are combined and then separated with high precision.
As a result, each of separated waves is output as indicated by
arrows B and D respectively.
[0086] [Interference Film Filter Type of Optical
Multiplexer/Demultiplexer- ]
[0087] This type of optical multiplexer/demultiplexer utilizes the
selective light-transmitting property of a dielectric interference
film consisting of a composite film of, say, TiO.sub.2 and
SiO.sub.2.
[0088] FIG. 9 is a top view of an interference film filter type of
optical multiplexer/demultiplexer, which is indicated generally at
33, and FIG. 10 is a sectional view along line C-C of FIG. 9.
[0089] In FIGS. 9 and 10, the optical multiplexer/demultiplexer 33
comprises a silicon substrate 33a, a lower cladding layer 33b, a
core layer 33C, an upper cladding layer 32d, and a dielectric
interference film 33e.
[0090] The lower and upper cladding layers 33b and 33d, consisting
of silicon dioxide, are formed to surround the core layer 33c to
serve the light confinement function.
[0091] The core layer 33c is a optical waveguide consisting of
germanium-doped silicon dioxide. Light can be confined in the core
layer 33c by making the refractive index of the core layer 33c
higher than those of the lower and upper cladding layers 33b and
33d.
[0092] The dielectric interference film 33e has the property of
selectively transmitting components of light and consists of a
composite of a TiO.sub.2 film of, say, 0.17 .mu.m in thickness and
an SiO.sub.2 film of, say, 0.27 .mu.m in thickness. In order for
the dielectric interference film 33e to select light components
correctly, it is required that the film be placed vertically with
respect to the optical waveguide (i.e., the core layer 33c). For
this reason, the dielectric interference film 33e is fitted into a
groove formed in the silicon substrate 33a perpendicularly to the
core layer 33c.
[0093] In FIG. 9, a wavelength multiplexed optical wave directed to
the core layer 33c from the direction of arrow A is split by the
dielectric interference film 33e. That is, a portion of the
multiplexed optical wave is allowed to pass through the dielectric
interference film 33e in the direction of arrow B, but the
remaining portion is reflected back by the dielectric interference
film 33e in the direction of arrow D.
[0094] In the optical multiplexing/demultiplexing unit 11, when the
interference film type of optical multiplexing/demultiplexing
circuit 33 is used as the filter 30, the multiplexed components
allowed to pass through the dielectric interference film 33e are
directed to the AWG optical multiplexer/demultiplexer 20. On the
other hand, the multiplexed components reflected back by the
dielectric interference film 33e are directed to the succeeding
optical multiplexing/demultiplexing unit 11 via the branch
waveguide 70.
[0095] So far, the directional coupler, the Mach-Zehnder
interference optical multiplexing/demultiplexing circuit and the
interference film type of optical multiplexing/demultiplexing
circuit have been described in terms of the optical waveguide type.
However, this does not mean that the filter 30 is limited to the
optical waveguide type. For example, even if a WDM circuit
consisting of a lens, an interference film, and an optical fiber is
used as the filter, the object of the present invention can be
attained.
[0096] [Input Waveguide and Output Waveguides]
[0097] The input waveguide 40 is a waveguide that directs an
incident multiplexed optical wave to the filter 30.
[0098] The output waveguides 55 are waveguides of assembled type in
which optical fibers corresponding in number to optical waves to be
combined or separated (i.e., corresponding in number to channels)
are arranged in a plane. Each of the optical waves, each of a
single wavelength, separated in the AWG optical
multiplexer/demultiplexer 20 is directed through the output planar
waveguide 23e and a corresponding one of the output waveguides 55
to a corresponding one of light receiving modules (not shown),
which correspond in number to channels.
[0099] [Coupling Waveguide and Branch Waveguide]
[0100] Returning now to FIG. 2A, we describe the arrangement of the
optical multiplexing/demultiplexing unit 11.
[0101] The coupling waveguide 50 optically connects the filter 30
with the AWG optical multiplexer/demultiplexer 20. The components
of the multiplexed optical wave allowed to pass through the filter
30 is directed via the coupling waveguide 50 to the AWG optical
multiplexer/demultiplexer 20. The branch waveguide 70 optically
connects the filter with the succeeding optical
multiplexing/demultiplexing unit (i.e., the optical
multiplexing/demultiplexing unit as the second stage when the
optical multiplexing/demultiplexing unit 11 of FIG. 2A is taken as
the first stage). The components of the multiplexed optical wave
blocked in the filter 30 are directed via the coupling waveguide 70
to the succeeding optical multiplexing/demultiplexing unit 11.
[0102] Next, the operation of the optical
multiplexing/demultiplexing unit 11 will be described.
[0103] Suppose now that the AWG optical multiplexer/demultiplexer
20 separates four waves at center frequencies .lambda.1, .lambda.2,
.lambda.3, and .lambda.4 with a channel width of 2.DELTA..lambda.
and outputs them from the output planar waveguides 23e. The AWG
optical multiplexer/demultiplexer 20 has such a wavelength
characteristic as shown in FIG. 2B. The characteristic point of the
wavelength characteristic, shown in FIG. 2B, is that the loss of
each of the four waves, within in main operating wavelength band,
is substantially uniform. Accordingly, the loss of each of the four
waves in the optical multiplex/demultiplex apparatus 10 is also
substantially uniform since the optical multiplex/demultiplex unit
11 incorporates the AWG optical multiplexer/demultiplexer 20 as a
means for optical-demultiplexing.
[0104] Also, suppose that the filter 30 shown in FIG. 2A is an
interference film filter having such a wavelength characteristic as
shown in FIG. 2C. That is, the filter 30, upon receiving a
multiplexed signal containing four waves at center frequencies
.lambda.1, .lambda.2, .lambda.3, and .lambda.4, outputs a optical
wave having such a characteristic as shown in FIG. 2C to the
coupling waveguide 50. As shown in FIG. 2C, the passband of the
filter 30 is broader than the overall passband of the AWG optical
multiplexer/demultiplexer 20 (i.e., the passband from
.lambda.1-.DELTA..lambda. to .lambda.4+.DELTA..lambda.) and loss
within at least the overall passband of the AWG optical
multiplexer/demultiplexer 20 is substantially constant.
[0105] The characteristic point of the wavelength characteristic
shown in FIG. 2C is that optical waves separated by the AWG optical
multiplexer/demultiplexer 20 and output from the output waveguides
23e are low in loss. The reason is that the optical
multiplexing/demultiplexi- ng apparatus 10 of the present invention
is arranged such that optical waves to be output to the succeeding
stage by the branch waveguide 70 pass through only the filter 30
(without passing through the AWG optical multiplexer/demultiplexer
20).
[0106] Suppose that a total of twelve multiplexed optical waves at
central wavelengths .lambda.1, .lambda.2, .lambda.3, .lambda.4,
.lambda.5, . . . , .lambda.12 (arranged in the order of increasing
wavelength) is directed to the input waveguide 40 of the firststage
optical multiplexing/demultiplexing unit 11. Then, the filter 30
allows four signals at wavelengths .lambda.1, .lambda.2, .lambda.3,
and .lambda.4 to pass through and directs them to the AWG optical
multiplexer/demultiplexe- r 20. The AWG optical
multiplexer/demultiplexer 20 then separates the four signals at
wavelengths .lambda.1, .lambda.2, .lambda.3, and .lambda.4 and
outputs them from the output waveguides 23e.
[0107] On the other hand, the multiplexed optical waves at
wavelengths .lambda.5 to .lambda.12 blocked by the filter 30 are
directed via the branch waveguide 70 to the succeeding optical
multiplexing/demultiplexing unit.
[0108] Our experiments showed that the loss at the center
wavelength of each optical wave separated by the optical
multiplexing/demultiplexing unit 11 was of the order of 4.5 dB+0.2
dB for all the four signals, providing excellent wavelength
uniformity. Also, the loss of each optical wave output from the
branch waveguide 70 of one optical multiplexing/demultiplexing unit
could be reduced to a low value, of the order of 0.7 dB, at long
and short wavelength ends of the main operating wavelength
region.
[0109] FIG. 2D shows the loss characteristic over the main
operating wavelength region at each stage when the optical
multiplexing/demultiplex- ing units 11 are connected in three
stages as shown in FIG. 1.
[0110] As shown in FIG. 2D, the loss of each of the four waves
(.lambda.1 to .lambda.4) separated by the first-stage optical
multiplexing/demultiplexing unit 11 is 4.5 dB as in FIG. 2B. The
loss of each wave (.lambda.5 to .lambda.8) separated by the
second-stage optical multiplexing/demultiplexing unit 11 is 5.2 dB.
The loss of each wave (.lambda.9 to .lambda.12) separated by the
third-stage optical multiplexing/demultiplexing unit 11 is 5.9 dB.
That is, according to the optical multiplexing/demultiplexing
apparatus of the present embodiment, the difference in output
signal intensity between each stage can be limited to, at most, 1.4
dB between the first and third stages.
[0111] With the use of the AWG optical multiplexer/demultiplexer 20
capable of separating 125 channels, it becomes possible to
separating 375 wavelengths in the examples of FIG. 1 in which three
stage are connected. Insertion loss per stage is of the order of
4.5 dB and moreover the difference in light intensity level between
the first and third stages can be suppressed to about 1.4 dB which
is the same above.
[0112] Multiplexing /demultiplexing nearly 400 wavelengths with no
variation in transmission loss and with low transmission loss was
made feasible for the first time by present invention.
[0113] Of course, any AWG optical multiplexer/demultiplexer can be
selectively used. With the use of an AWG optical
multiplexer/demultiplexe- r capable of separating over 125
channels, more wavelengths could be handled. Conversely, it is also
possible to use an AWG optical multiplexer/demultiplexer adapted
for four channels by way of example. In addition, it is also
possible to use AWG optical multiplexers/demultiplex- ers each
adapted for a different number of channels in combination.
[0114] Next, the operation of the optical
multiplexing/demultiplexing apparatus 10 in which a plurality of
optical multiplexing/demultiplexing units 11 are connected in
stages will be described in terms of two examples.
EXAMPLE 1
[0115] The light sources of optical waves used for optical
communications are semiconductor lasers. In general, the
oscillating wavelength of semiconductor lasers exists in the
vicinity of the fundamental absorption end wavelength determined by
the forbidden band width (Eg) of a semiconductor material that
forms their active layer and ranges from the ultraviolet region to
the infrared region of the electromagnetic spectrum including the
visible region, depending on active layer materials (for example,
GaAs-, AlGaAs-, InP-, GaInP-, AlGaInP-, and HgCdTe-based
materials). However, the practicable wavelength range will be from
700 to 2000 nm taking the transmission efficiency (light
transmission factor) of optical fibers as optical wave transmitting
media into account.
[0116] The practicable wavelength range is from 1530 to 1610 nm (C
band+L band). In example 1, therefore, use is made of a 50 GHz AWG
whose wavelength range is 1530 to 1610 nm and channel spacing is
0.4 nm, and the total number of channels is set to 160 channels (40
channels.times.4 stages).
[0117] In this example, as the filter 30 use is made of a lowpass
filter type of optical multiplexer/demultiplexer which outputs low
frequency components from the input waveguide 40 the output
waveguide 50. In this example, a band pass filter type of optical
multiplexer/demultiplexer may be used instead.
[0118] In FIG. 1, multiplexed optical waves on 160-channels with
wavelengths in the range of 1530 to 1610 nm are directed in the
input waveguide 40 of the first-stage optical
multiplexer/demultiplexer 10. The multiplexed optical waves are
then directed to the filter 30 where the incoming optical waves are
separated into optical waves on 40-channels in the range of 1530 to
1550 nm and optical waves on 120-channel in the other wavelength
range (i.e., 1550 to 1610 nm in this example).
[0119] The optical waves on 40-channel in the wavelength range of
1530 to 1550 nm are directed through coupling waveguide 50 to AWG
optical multiplexer/demultiplexer 20 where they are split into 40
individual waves each of a single wavelength, which in turn are
directed trough output waveguides 55 to succeeding devices.
[0120] On the other hand, the optical waves on 120-channel in the
wavelength range of 1550 to 1610 nm are directed through branch
waveguide 70 and connecting fiber 9 to input waveguide 40 of
second-stage optical multiplexer/demultiplexer 10.
[0121] In example 1, 160 optical waves each of a single wavelength
are separated by four optical multiplexers/demultiplexers 10
adapted to separate wavelength ranges of 1530 to 1550 nm, 1550 to
1570 nm, 1570 nm 1590 nm, and 1590 to 1610 nm, respectively.
[0122] This example is arranged such that all the optical
multiplexers/demultiplexers are identical except the lowpass
filters and demultiplexres is performed in steps in units of 40
channels. However, the object of the present invention can also be
achieved by the use of an optical multiplexer/demultiplexer in
which different types of optical multiplexing/demultiplexing units
are connected in stages and demultiplexing is performed step by
step in different number of channels.
[0123] In this example, the 160-channel multiplexed optical waves
are entered into input waveguide 40 and 160 optical waves each of a
single wavelength are each output from a corresponding one of
output waveguide 55, it is also possible to enter 160 optical waves
of different wavelengths into output waveguides 55, then combine
them into multiplexed optical waves and output them from the input
waveguide 40.
EXAMPLE 2
[0124] The example 2 will be described next. This example is an
extension of 1550 nm-band circuit for one channel to the existing
1310 nm-band circuits for four channels.
[0125] FIG. 11 is a diagram for use in explanation of an optical
multiplexing/demultiplexing unit 60 of the example 2. The optical
multiplexing/demultiplexing unit 60 is identical in arrangement to
the optical multiplexing/demultiplexing unit 11 shown in FIG. 2A
except that its AWG optical multiplexer/demultiplexer 63 has four
output waveguides 63e, and four output waveguides 55 and the filter
30 selects channels in the 1310 nm band.
[0126] A wavelength multiplexed signal containing four channels in
the 1310 nm band and one channel in the 1550 nm band is directed to
the input waveguide 40 of the optical multiplexing/demultiplexing
unit 60. The multiplexed wave is separated by the filter 30 into a
optical wave containing four channels in the 1310 nm band and a
optical wave containing one channel in the 1550 nm band.
[0127] The optical wave containing four channels in the 1310 nm
band is directed from the filter 30 through the coupling waveguide
50 to the AWG optical multiplexer/demultiplexer 63 where it is
demultiplexed into individual waves (channels) each of a single
wavelength, which are then output from the output waveguides. On
the other hand, the optical wave containing one channel in the 1550
nm band is directed from the filter 30 through the branch waveguide
70 to a 1550-nm band optical receiver module not shown.
[0128] In the conventional optical multiplexing/demultiplexing
system, there is the need of using an AWG optical
multiplexer/demultiplexer capable of combining or separating
optical waves of five channels or more, which is very uneconomical.
However, the arrangement of FIG. 11 allows an economical channel
expansion through the use of the existing AWG optical
multiplexer/demultiplexer in which the number of output waveguides
63e is four. A further channel expansion can be carried out readily
by connecting one or more optical multiplexing/demultiplexing units
to the optical multiplexing/demultiplexing unit 60 shown in FIG.
11.
[0129] The optical multiplexing/demultiplexing unit 60 of the first
embodiment provides the following advantages:
[0130] First, the number of channels that are combined or separated
can be increased readily while ensuring inter-channel crosstalk and
insertion loss of substantially the same level as those in the
conventional system. As a result, efficient utilization can be
effected.
[0131] Second, the addition of some equipment allows the
communication capacity to be increased without discarding existing
facilities. That is, the communication scale can be expanded at low
cost.
[0132] Third, the system of the present invention is made up from
modules of the same configuration. Thus, the present system can be
manufactured readily at low cost.
[0133] Fourth, since the AWG optical multiplexer/demultiplexers are
used in the optical multiplexing/demultiplexing apparatus, 16 or
more optical waves can be combined or separated at one stage.
[0134] [Second Embodiment]
[0135] The second embodiment is effective in adding channels in a
wavelength band which is relatively greatly apart from a wavelength
band in current use. In such a case, it may be difficult to cause
the filter of each optical multiplexing/demultiplexing unit to
demultiplex optical waves with accuracy for transmission to the
succeeding unit. The reason is that the filter cannot have a
sufficiently great amount of attenuation in wavelength bands
greatly apart from its passband.
[0136] The second embodiment is directed to an optical
multiplexing/demultiplexing apparatus expandable up to a
sufficiently apart wavelength band. As a specific example, suppose
the case where a 1300-nm band is added to a 1550-nm band.
[0137] FIG. 12A shows the first-stage optical
multiplexing/demultiplexing unit, generally indicated at 100, in
the optical multiplexing/demultiplex- ing apparatus of the second
embodiment. The optical multiplexing/demultiplexing unit is
characterized in that the filter 35 is followed by a second filter
30.
[0138] In FIG. 12A, the AWG optical multiplexer/demultiplexer 20
combines or separates waves corresponding to 16 consecutive
channels in the 1550 nm band. The wavelength characteristic of
optical waves separated by the AWG optical
multiplexer/demultiplexer 20 is shown in FIG. 13A.
[0139] The second filter 30 has such a passband characteristic as
shown in FIG. 13B. The first filter 35 is a highpass filter having
the cutoff wavelength in the vicinity of 1400 nm as shown in FIG.
13C.
[0140] The operation of the optical multiplexing/demultiplexing
unit 100 will be described next with reference to FIG. 12B.
[0141] In FIG. 12B, first, a wavelength multiplexed wave which, in
addition to 16 waves (.lambda.1 to .lambda.16) corresponding to 16
consecutive channels in the 1550-nm band, further contains 16 waves
(.lambda.17 to .lambda.32) corresponding to 16 consecutive channels
in the 1550-nm band and eight waves in the 1300-nm band is input to
the filter 35 via the input waveguide 40.
[0142] In the filter 35, the multiplexed wave is split into a first
multiplexed optical wave containing 32 consecutive waves (.lambda.1
to .lambda.32) in the 1550-nm band and a second multiplexed optical
wave containing eight waves in the 1300-nm band. The first
multiplexed optical wave is allowed to pass through the filter 35
and then enter the second filter 30. On the other hand, the second
multiplexed optical wave is not allowed to pass through the filter
35 and is directed via a first branch waveguide 71 to an optical
multiplexing/demultiplexing unit 102 adapted for the 1300-nm
band.
[0143] The wavelength multiplexed wave directed to the second
filter 30 is separated into multiplexed wave containing 16 waves
(.lambda.1 to .lambda.16) corresponding to consecutive 16 channels
in the 1550-nm band and multiplexed wave containing 16 waves
(.lambda.17 to .lambda.32) corresponding to consecutive 16 channels
in the 1550-nm band. For example, the 16 waves (.lambda.1 to
.lambda.16) in the 1550-nm band are allowed to pass through the
second filter 30 and then enter the AWG optical
multiplexer/demultiplexer 20. On the other hand, the 16 consecutive
waves (.lambda.17 to .lambda.32) in the 1550-nm band are not
allowed to pass through the second filter 30 and are directed via a
second branch waveguide 70 to an optical
multiplexing/demultiplexing unit 101 adapted for the waves
(.lambda.17 to .lambda.32) in the 1550-nm band.
[0144] Such a configuration ensures easy expansion even if an
expansion wavelength band is greatly apart from that associated
with the existing device.
[0145] [Third Embodiment]
[0146] The inventive optical multiplexing/demultiplexing apparatus
implements expandable optical multiplexing/demultiplexing by
hierarchically connecting optical multiplexing/demultiplexing units
each using an AWG optical multiplexer/demultiplexer and a preceding
filter in combination. In expanding the available wavelength band
using multiple optical multiplexing/demultiplexing units, the AWG
optical multiplexer/demultiplexers used in each unit differ from
one another in their operating wavelength band. Thus, preparations
must be made for a plurality of AWG optical
multiplexer/demultiplexers that differ in operating wavelength
band, which may result in increased manufacturing cost.
[0147] In FIG. 14, there is shown an optical
multiplexing/demultiplexing apparatus 110 according to the third
embodiment which is intended to solve the above problem. Each
optical multiplexing/demultiplexing unit 120-123 included in the
optical multiplexing/demultiplexing apparatus 110 uses an AWG chip
formed with multiple AWGs differing in operating wavelength band.
In FIG. 14, there is illustrated an AWG optical
multiplexer/demultiplexer 80 in which a first AWG 81 and a second
AWG 82 are cross arranged. In this embodiment, it is assumed that
the operating wavelength band of the first AWG 81 is from .lambda.1
to .lambda.16 and the operating wavelength band of the second AWG
82 is from .lambda.17 to .lambda.32.
[0148] The use of such optical multiplexing/demultiplexing units,
each of which comprises AWG optical multiplexer/demultiplexer 80 in
which a first AWG 81 and a second AWG 82 are cross arranged,
provides great wavelength expandability using one kind of AWG
optical multiplexer/demultiplexer 80 as shown in FIG. 14. In FIG.
14, the first-stage AWG optical multiplexing/demultiplexing unit
120 is assumed to use 8-wave AWG whose main operating wavelength
band is, for example, .lambda.1 to .lambda.8. For this reason, the
filter 30 in the unit 120 is a bandpass filter that allows the main
operating wavelength band, .lambda.1 to .lambda.8, to pass through.
The second-stage AWG optical multiplexing/demultiplexing unit 121
uses 8-wave AWG whose main operating wavelength band ranges, for
example, from .lambda.9 to .lambda.16. Thus, the passband of the
filter 30 in the second-stage unit 121 ranges from .lambda.9 to
.lambda.16.
[0149] For further expansion, the second AWG 82 of the two AWGs
provided in the AWG optical multiplexer/demultiplexer 80 is simply
used. The optical multiplexing/demultiplexing apparatus 110 shown
in FIG. 14 is expanded to four stages through the use of branch
waveguides 70. The unit 122 uses AWG 82 whose main operating
wavelength range is .lambda.17 to .lambda.24 and the filter 30 is a
bandpass filter which allows .lambda.17 to .lambda.24 to pass
through. The unit 123 uses AWG 82 whose main operating wavelength
range is .lambda.25 to .lambda.32 and the filter is a bandpass
filters which allows .lambda.25 to .lambda.32 to pass through.
Further expansion is allowed by further connecting lower stages as
required.
[0150] According to the above-described configuration,
expandability over broad wavelength band can be realized at low
cost by making an optical multiplexing/demultiplexing apparatus
from multiple optical multiplexing/demultiplexing units each having
multiple AWGs different in operating wavelength band.
[0151] [Fourth Embodiment]
[0152] The fourth embodiment performs the same function as each of
the first to three embodiments, but differs in structure. That is,
each optical multiplexing/demultiplexing unit comprising an optical
multiplexer/demultiplexer of this embodiment is monolithically
integrated on a single silicon chip.
[0153] FIG. 15 is a top view of an optical
multiplexing/demultiplexing unit 130. To indicate the
correspondence with FIG. 2A, in FIG. 15, the filter 30 and the AWG
optical multiplexer/demultiplexer 20 are each shown encircled with
dotted lines. As described above, the optical
multiplexing/demultiplexing unit 130 is monolithically integrated
on a single silicon chip. The side view of the optical
multiplexing/demultiple- xing unit 130 remains unchanged from FIG.
4.
[0154] The optical multiplexing/demultiplexing unit 130 is
manufactured by the following process. A lower cladding layer 22
consisting of SiO.sub.2 is formed on a silicon substrate 21 and a
core layer consisting of germanium-doped SiO.sub.2 is formed on the
lower cladding layer. Naturally, the refractive index of the core
layer 23 is greater than that of the lower cladding layer 22.
[0155] After that, the core layer 23 is subjected to patterning as
shown in FIG. 15 and an upper cladding layer 24 is then formed
which consists of SiO.sub.2 as with the lower cladding layer
22.
[0156] In the optical multiplexing/demultiplexing unit 130 thus
formed, there is no optical connection point between each component
and each waveguide is formed in the continuous integrated form.
Therefore, optical waves can be propagated with low
attenuation.
[0157] To use an interference film filter optical
multiplexing/demultiplex- ing circuit for the filter 30, it is
required to provide a dielectric interference film perpendicularly
to the optical waveguide. In this case, the dielectric interference
film is guided to the waveguide using a jig while allowing it to
fall by its weight and then glued to the waveguide vertically with
very high accuracy. This embodiment is the same as the first
embodiment.
[0158] In this embodiment, the optical multiplexing/demultiplexing
unit 130 is configured to comprise the single core layer 23
integrating each components monolithically, shown in FIG. 15, but
is not limited to the configuration of the optical
multiplexing/demultiplexing unit 130. For example, the optical
multiplexing/demultiplexing unit 130 may be configured to comprise
a plurality of core layers each of which integrates the components
monolithically. Furthermore, it is not necessary to integrates the
all components in the single core layer 23. That is, a part of the
components, for example, the optical multiplexer/demultiplexer 20
or the like, may be integrated monolithically in the core layer 23
and each of the other components, for example the filter 30, the
branch waveguide 70 or the like, may be arranged as a individual
device.
[0159] The present invention has been described in terms of the
preferred embodiments, but is not limited to the embodiments. The
present invention may be carried out in other forms without
departing from the scope and sprit thereof.
[0160] For example, in the above embodiments, when optical
multiplexing/demultiplexing units are connected in stages to expand
the available operating wavelength region, lossy wavelength regions
may occur, depending on the filter or AWG optical
multiplexer/demultiplexer used. This will be described with
reference to FIGS. 16A, 16B, 17A and 17B taking, as an example, an
optical multiplexing/demultiplexing apparatus in which an optical
multiplexing/demultiplexing unit whose operating wavelength band is
from .lambda.1 to .lambda.4 is followed by an optical
multiplexing/demultiplexing unit whose operating wavelength band is
from .lambda.5 to .lambda.8.
[0161] FIG. 16A shows the wavelength characteristic of each AWG
optical multiplexer/demultiplexer in the preceding and succeeding
optical multiplexing/demultiplexing units. FIG. 16B shows the
wavelength characteristic (.lambda.1 to .lambda.4) of the filter in
the preceding optical multiplexing/demultiplexing unit and the
wavelength characteristic (.lambda.5 to .lambda.28) of the filter
in the succeeding optical multiplexing/demultiplexing unit.
[0162] As shown in FIG. 16B, the characteristic of each filter
changes sharply between the highest channel separated by the
preceding optical multiplexing/demultiplexing unit and the lowest
channel separated by the succeeding optical
multiplexing/demultiplexing unit (i.e., between .lambda.4 and
.lambda.5). In such a case, no lossy wavelength region occurs in
the two optical multiplexing/demultiplexing units, so that the
channels can be used consecutively.
[0163] When AWG optical multiplexer/demultiplexers having the
wavelength characteristic shown in FIG. 16A and filters having the
wavelength characteristic shown in FIG. 17B are combined to form
the preceding and succeeding optical multiplexing/demultiplexing
units, on the other hand, lossy wavelength regions will occur in
the two optical multiplexing/demultiplexing units.
[0164] That is, as shown in FIG. 17B, each of the preceding and
succeeding filters has a characteristic such that the transmission
factor changes in the region from .lambda.4 to .lambda.26. Thus, in
the preceding and succeeding optical multiplexing/demultiplexing
units, loss is high in the wavelength region in the vicinity of
.lambda.5, disabling proper demultiplexing.
[0165] In such a case, an additional wavelength is simply set by
keeping a given spacing between the operating wavelength bands of
the preceding and succeeding optical multiplexing/demultiplexing
units rather than consecutively adding the wavelength band of the
succeeding optical multiplexing/demultiplexing unit to the
wavelength band of the preceding optical
multiplexing/demultiplexing unit. For example, as shown in FIG.
17A, it is recommended that the wavelength .lambda.5 be skipped and
an optical multiplexing/demultiplexing unit having an operating
wavelength region of .lambda.6 to .lambda.9 be used anew.
[0166] According to the above concept, wavelength expansion can be
realized with more flexibility. For example, 16 wavelengths can be
selected from 1.51 .mu.m band for the preceding optical
multiplexing/demultiplexing unit, and eight wavelengths can be
selected from 1.55 .mu.m band for the succeeding optical
multiplexing/demultiplexi- ng unit. An optical
multiplexing/demultiplexing apparatus having such a configuration
will allow for expansion not only to a wavelength band contiguous
to the initially used wavelength band but also to a wavelength band
apart therefrom.
[0167] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *