Array waveguide diffraction grating optical...

Details Array waveguide diffraction grating optical... Array waveguide diffraction grating optical...

2000-04-21

2001-02-27

Ullah, Akm E. (Department: 2874)

Optical waveguides

With optical coupler

Plural

C359S199200

Reexamination Certificate

active

06195481

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an arrayed waveguide grating type optical multiplexer/demultiplexer used in optical wavelength devision multiplexing communications, and more particularly, to an arrayed waveguide grating type optical multiplexer/demultiplexer of which the spectrum response at output waveguides shows satisfactory flatness in the vicinity of a center wavelength and which ensures high yield during manufacture.
BACKGROUND ART
Recently, in the field of optical communications, researches have been intensively made on optical frequency devision multiplexing communication techniques for transmitting through a single optical fiber a plurality of sets of information at different wavelengths of light, in order to greatly increase the information transmission capacity. To attain such multiplexing communications, an optical multiplexer/demultiplexer is needed which multiplexes and demultiplexes a large number of light waves used.
The optical multiplexer/demultiplexer used for such applications is required to have the performance stated below.
First, using a large number of light waves with the narrowest possible wavelength spacing is effective in increasing the information transmission capacity, and therefore, the optical multiplexer/demultiplexer should be able to multiplex and demultiplex such a large number of light waves. For example, the multiplexer/demultiplexer is required to multiplex and demultiplex light waves with 100 GHz frequency spacing, which is equivalent approximately to 0.8 nm wavelength spacing in 1.55 &mgr;m band.
The optical multiplexer/demultiplexer is also required to have satisfactory passband flatness in the vicinity of passing wavelength.
For example, where an inexpensive LD is used as a light source in constructing an optical frequency devision multiplexing communication system with a view to reducing costs, the oscillation wavelength of the light source is liable to change with time or due to variations in temperature or humidity of the environment in which the light source is used. If the oscillation wavelength of the light source undergoes such a change, a loss variation occurs when light is propagated through the optical multiplexer/demultiplexer in the system, by an amount corresponding to the wavelength change depending on the spectrum response of the multiplexer/demultiplexer. The loss variation not only deteriorates the loss uniformity among wavelengths to be multiplexed/demultiplexed but also the S/N ratio, and eventually increases the cost of constructing the system.
In view of the foregoing, the loss variation of the optical multiplexer/demultiplexer should preferably be as small as possible. The optical multiplexer/demultiplexer is therefore required to have a characteristic such that the loss variation is, for example, 1 dB or less, that is, 1 dB bandwidth is large.
As such optical multiplexer/demultiplexer, an arrayed waveguide grating type is disclosed in Unexamined Japanese Patent Publication (KOKAI) No. 8-122557.
FIG. 8
is a plan view schematically showing the optical multiplexer/demultiplexer. This multiplexer/demultiplexer has a substrate
1
on which are arranged one or a plurality of input waveguides
2
, an input-side slab waveguide
3
connected to the input waveguide(s)
2
, a diffraction grating
4
connected to the input-side slab waveguide
3
and comprising a plurality of arrayed channel waveguides
4
a,
an output-side slab waveguide
5
connected to the arrayed waveguide grating
4
, and a plurality of output waveguides
6
connected to the output-side slab waveguide
5
.
In this optical multiplexer/demultiplexer, the junction between the input waveguide
2
and the input-side slab waveguide
3
is formed as shown in FIG.
9
.
Specifically, the input waveguide
2
, which is surrounded by a cladding material
10
and has a path width W
1
, has a tapered end portion expanded in the width direction of the path, and a slit
7
is formed in the center of the tapered portion, thus defining two waveguide portions
2
a
and
2
b
of equal width. The input waveguide
2
is connected to the input-side slab waveguide
3
at the tapered portion, or the two waveguide portions
2
a
and
2
b.
In the input waveguide
2
constructed in this manner, light propagated through the input waveguide
2
enters the input-side slab waveguide via the tapered portion. At this time, the two waveguide portions
2
a
and
2
b
of the tapered portion equivalently function as a core. Consequently, at a location just in front of the input-side slab waveguide
3
, the electric field distribution of light is spread as a whole in the width direction and has a bimodal shape with two maximal values.
This optical multiplexer/demultiplexer is allegedly capable of attaining 3 dB bandwidth of about 0.8 nm with respect to about 1 nm wavelength spacing.
In the prior art device, however, almost no consideration is given to the passband flatness of light output from the output waveguides
6
, or more specifically, to 1 dB bandwidth which is an important characteristic when the optical multiplexer/demultiplexer is applied to an actual optical frequency devision multiplexing communication system.
The inventors hereof therefore actually fabricated an optical multiplexer/demultiplexer as shown in
FIGS. 8 and 9
and examined its spectrum response.
Specifically, an optical multiplexer/demultiplexer with silica-based waveguides was produced, wherein the input waveguide
2
had a path width W
1
of 6.5 &mgr;m, the connecting portion of the input-side slab waveguide
3
had a width W
2
of 15.0 &mgr;m, the trapezoidal slit
7
had a connection width CW of 1.0 &mgr;m on the input waveguide
2
side and a connection width SW of 2.0 &mgr;m on the slab waveguide
3
side, the tapered portion was tapered at an angle &thgr; of 0.4°, and the waveguides had a relative index difference of 0.8% and a path height of 6.5 &mgr;m, to derive light with 100 GHz wavelength spacing, that is, about 0.8 nm wavelength spacing in 1.55 &mgr;m band. With light of 1.55 &mgr;m band input to the input waveguide
2
, the spectrum response was examined.
FIG. 10
shows the electric field distribution of light observed at a location just in front of the input-side slab waveguide
3
, and
FIG. 11
shows the spectrum response at the output waveguide
6
.
In
FIG. 10
, the horizontal axis represents the width direction of the path at a location immediately in front of the input-side slab waveguide
3
, and the position “0” indicates the center along the width direction, that is, the center point of the width W
2
shown in FIG.
9
. In FIG.
11
, the horizontal axis represents wavelength of light propagated through the output waveguide
6
, and the position “0” indicates the center wavelength of the propagated light.
To actually measure the electric field distribution, the fabricated optical multiplexer/demultiplexer must be destroyed, but in the experimentation, the electric field distribution was estimated/calculated by means of simulation according to beam propagation method (BPM), instead of destroying the device.
As is clear from
FIG. 10
, the electric field distribution showed a bimodal shape having maximal values a and b and a minimal value c therebetween. The spacing between the two maximal values a and b was 7.0 &mgr;m and the ratio c/a was 0.59.
With regard to the spectrum response, 1 dB bandwidth, which is a wavelength range 1 dB higher than a minimum insertion loss, was found to be 0.37 nm, and 3 dB bandwidth was 0.50 nm.
In the aforementioned Unexamined Japanese Patent Publication No. 8-122557, it is stated that 3 dB bandwidth can be further increased by setting the ratio SW/W
2
of the junction between the input waveguide and the input-side slab waveguide shown in
FIG. 9
to 0.2 to 0.6.
The inventors therefore fabricated an optical multiplexer/demultiplexer with a junction having the same parameters as the aforesaid ones, except that the connection width SW of the junction shown in
FIG. 9
was set to 3.0 &mgr;m, and measured the electric field distribution and the spe

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