Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-08-09
2002-06-11
Kim, Ellen E. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S037000
Reexamination Certificate
active
06404946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer for use in optical wavelength (optical frequency) multiplex communication, and more specifically to an arrayed waveguide grating type optical multiplexer/demultiplexer having a sufficiently flat wavelength-dependent spectrum response in passing channel spacings or wavelength bands.
2. Discussion of the Background
In recent years, in the field of optical communication, researches have eagerly researched on an optical wavelength multiplex communication system that achieves a large increase in transmission capacity. In the multiplex communication system, the available transmission wavelength range is divided into a plurality of passing channel spacings, and pieces of information carried by a plurality of lightwave signals of different wavelengths are transmitted through a single optical fiber. For that purpose, an optical multiplexer/demultiplexer for multiplexing lightwaves of different wavelengths (frequencies) and for demultiplexing the wavelength-multiplexed lightwave into lightwaves of the original wavelengths is used in the optical wavelength multiplex communication.
To increase transmission capacity, it is effective to divide the transmission wavelength region into many passing channel spacings, or in other words, to use many lightwaves having narrow channel spacings. The optical multiplexer/demultiplexer is required to be capable of multiplexing and demultiplexing lightwaves having a frequency interval of 100 GHz, for instance, which corresponds to a wavelength interval of about 0.8 nm in the 1.55 &mgr;m region.
In the optical wavelength multiplex communication, sometimes a relatively inexpensive LD light source is used to reduce the costs of constructing the communication system. However, the oscillating wavelength in the LD light source can deviate from the designed wavelength due to variations in ambient conditions, such as temperature and humidity, and can vary with time. Therefore, when an LD light source is used, the wavelength of lightwave to be multiplexed or demultiplexed can vary. On the other hand, the spectrum response of the optical multiplexer/demultiplexer in a passing wavelength band (channel spacing) is wavelength-dependent (spectrum response will be hereinafter referred to also as wavelength-dependent spectrum response).
Therefore, when the oscillating wavelength of the light source varies as mentioned above, the loss of the light passing through the optical multiplexer/demultiplexer varies depending on the spectrum response of the optical multiplexer/demultiplexer by the amount corresponding to the variation of the oscillating wavelength of the light source. Such variation of loss makes the loss of multiplexed/demultiplexed light ununiform between lightwaves having different wavelengths. This ununiformity causes a deterioration in a signal-to-noise ratio in the transmission of pieces of information carried by lightwave signals. The less flat the wavelength-dependent spectrum response of the optical multiplexer/demultiplexer is, the more ununiform the loss becomes.
As described above, the optical multiplexer/demultiplexer for use in an optical wavelength multiplex communication system is not only required to be able to multiplex and demultiplex many lightwaves of narrow channel spacings, but is also required to have a sufficiently flat wavelength-dependent spectrum response in passing channel spacings in the wavelength range used for the multiplex communication.
To meet the first requirement of multiplexing and demultiplexing lightwaves of narrow channel spacings, an optical multiplexer/demultiplexer using an arrayed waveguide grating has been proposed.
An arrayed wavelength grating type optical multiplexer/demultiplexer shown as an example in
FIG. 8
has a plurality of input waveguides
2
formed on a substrate
1
. The input waveguides
2
are connected to an end face
3
a
of an input-side slab waveguide
3
having the other end face
3
b
thereof connected to ends of one-side of a plurality of channel waveguides
4
a
that constitute an arrayed waveguide grating
4
. The other ends of the channel waveguides
4
a
are connected to an end face
5
a
of an output-side slab waveguide
5
having the other end face
5
b
thereof connected to a plurality of output waveguides
6
.
The input-side slab waveguide
3
has opposite end faces
3
a,
3
b.
The end face
3
b
is formed to be a concave face that has a center of curvature positioned at the center of the other end face
3
a.
The end face
3
a
is formed to be a concave face having a center of curvature positioned at the middle point of a line connecting the centers of the end faces
3
a,
3
b.
Similarly, the end face
5
a
of the output-side slab waveguide
5
is formed to be a concave face whose center of curvature is positioned at the center of the other end face
5
b.
The end face
5
b
is formed to be a concave face having a center of curvature positioned at the middle point of a line connecting the centers of the end faces
5
a,
5
b.
As for the demultiplexer function of the optical multiplexer/demultiplexer, typically wavelength-multiplexed light is introduced through an input waveguide
2
connected to a central portion of, preferably, the center of the end face
3
a
of the input side-slab waveguide
3
. From the input waveguide
2
, the wavelength-multiplexed light is incident on the end face
3
a
of the input-side slab waveguide
3
, and diffracted in the slab waveguide
3
. Then, through the channel waveguides
4
a
that have different waveguide lengths, the light is incident on the end face
5
a
of the output-side slab waveguide
5
, undergoes interference in the slab waveguide
5
, and focuses on the other end face
5
b
of the slab waveguide
5
. Focusing positions are different according to wavelengths. For example, lightwaves each having a central wavelength in a corresponding one of the passing channel spacings for the optical multiplexer/demultiplexer, focus on their respective focusing positions on the slab waveguide end face
5
b
and are taken out through the output waveguides
6
connected to those focusing positions, respectively. As for the multiplexer function, signal light beams entering the input waveguides
2
or the output waveguides
6
and having different wavelengths are multiplexed, and the multiplexed signal light is taken out from the input waveguide
2
or the output waveguide
6
connected to the center of the input or output side slab waveguide end face
3
a
or
5
b.
In the optical multiplexer/demultiplexer described above, the angular dispersion on the end face
5
b
of the output-side slab waveguide
5
is expressed as follows:
d&thgr;/d&lgr;=m/
(
ns·d
) (1)
In equation (1), &thgr; denotes the angle of diffraction, m is the order of diffraction, &lgr; is the wavelength of an input lightwave, ns is the refractive index of the slab waveguides
3
and
5
, and d is the pitch between the channel waveguides
4
a.
When the focal length of the output-side slab waveguide
5
is denoted by F
1
, and the position on the slab waveguide end face
5
b
as viewed in the direction of the width of the slab waveguide
5
(typically, the distance from the center of the slab waveguide end face
5
b
to the focusing position) is denoted by x
1
, the linear dispersion on the end face
5
b
is expressed as follows:
dx
1
/
d&lgr;=
(
m·F
1
)/(
ns·d
) (2)
As mentioned above, since the input-side slab waveguide
3
and the output-side slab waveguide
5
have the same focal length F
1
, the linear dispersion on the end face
3
a
of the input-side slab waveguide
3
is the same as the linear dispersion on the end face
5
b
of the output-side slab waveguide
5
expressed by equation (2).
Therefore, the electric field distribution of the light that focuses on the focusing position on the end face
5
b
(places at which the slab waveguide end face
5
b
and the output waveguides
6
are connected) corresponds to the electri
Nakajima Takeshi
Nakamura Shiro
Ohta Toshihiko
Tanaka Kanji
Kim Ellen E.
The Furukawa Electric Co. Ltd.
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