Optical multiplexer/demultiplexer

Details Optical multiplexer/demultiplexer Optical multiplexer/demultiplexer

2000-07-06

2002-07-09

Bovernick, Rodney (Department: 2874)

Optical waveguides

With optical coupler

Plural

C385S027000, C385S037000, C385S039000

Reexamination Certificate

active

06418249

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical multiplexer/demultiplexer having an arrayed waveguide grating, and, more particularly, to an optical multiplexer/demultiplexer capable of correcting a deviation of the wavelength of multiplexed/demultiplexed optical signal.
BACKGROUND ART
In optical wavelength (optical frequency) division multiplexing systems simultaneously transmitting signal beams having different wavelengths (frequencies) set at narrow wavelength intervals to thereby significantly increase the transmission capacity of optical transmission line, a plurality of optical signals are multiplexed by means of the multiplexing function of an optical multiplexer/demultiplexer on the transmitter side and wavelength-multiplexed light signal beam is demultiplexed to original optical signals by the demultiplexing function of an optical multiplexer/demultiplexer on the receiver side. This type of optical multiplexer/demultiplexer uses a grating to demultiplex wavelength-multiplexed light signal beams. Due to the limited diffraction order, however, the conventional grating can not sufficiently disperse wavelength-multiplexed light signal beam and has a difficulty in demultiplexing wavelength-multiplexed light signal beam into a plurality of optical signals when the wavelength intervals of the optical signals are narrow. In this respect, for example, an optical multiplexer/demultiplexer using an arrayed waveguide grating is used in the multiplexing of optical signals and demultiplexing of multiplexed light signal beam.
As shown in
FIG. 4
, this type of optical multiplexer/demultiplexer has slab waveguides
110
and
112
connected to both ends of an arrayed waveguide grating
107
, so that wavelength-multiplexed light signal beam
200
input from an input waveguide
103
is demultiplexed into a plurality of optical signals which are taken out of a plurality of output waveguides
105
. Reference numeral
101
indicates a substrate.
Specifically, the input-side slab waveguide
110
is comprised of a two-dimensional waveguide which has an effect of optical confinement in the vertical direction, and has an input end face (end face on the input waveguide side) and an output end face (end face on the grating side) thereof formed in arc shapes as viewed in a horizontal plane. The center of curvature
110
y
of the output end face of the input-side slab waveguide
110
coincides with the center
110
y
of the input end face of the slab waveguide
110
. The input waveguide
103
is connected to the input end face of the input-side slab waveguide
110
at the center of curvature
110
y.
The input ends of a plurality of channel waveguides
107
a
which constitute the arrayed waveguide grating
107
are connected to the output end face of the slab waveguide
110
at intervals in the widthwise direction of the output end face.
The wavelength-multiplexed signal beam introduced into the input-side slab waveguide
110
from the input waveguide
103
is dispersed in the input-side slab waveguide. Dispersed optical signal waves reach the output end face of the slab waveguide
110
at the equal phase. In
FIG. 4
, reference numeral
210
indicates the equi-phase plane.
The output-side slab waveguide
112
is comprised of a two-dimensional waveguide which demonstrates optical confinement in the vertical direction, and has an input end face (end face on the grating side) and an output end face (end face on the output waveguide side) thereof formed in arc shapes as viewed in a horizontal plane. The center of curvature O of the input end face coincides with the center O of the output end face. The further outward a waveguide in the plurality of channel waveguides
107
a
is located, the longer the waveguide length becomes. The output ends of the channel waveguides
107
a
are connected to the input end face of the output-side slab waveguide
112
at intervals in the widthwise direction of the input end face.
As the waveguide lengths of the channel waveguides
107
a
of the grating
107
differ from one another, the optical signal waves that have propagated in the respective channel waveguides
107
a
have different phases from one another at the output of the grating
117
. These optical signal waves are dispersed in the output-side slab waveguide
112
and are focused on the widthwise positions different according to the wavelengths on the output end face of the output-side slab waveguide.
The arrayed waveguide grating type optical multiplexer/demultiplexer satisfies the following equation (1).
n
s
·D·
sin &phgr;
112
+n
c
·&Dgr;L=m&lgr;
  (1)
where the symbol &phgr;
112
represents the diffraction angle of light in the output-side slab waveguide
112
, &lgr; is the wavelength, n
s
is the refractive index of the slab waveguide
112
, n
c
is the refractive index of the channel waveguides
107
a,
&Dgr;L is the difference between waveguide lengths of the adjacent channel waveguides
107
a,
D is the interval of the channel waveguides
107
a
on the input end face of the slab waveguide
112
, and m (integer) is the diffraction order.
The optical signal wave having the wavelength (center wavelength &lgr;
M
) thereof exhibiting the diffraction angle &phgr;
112
of zero has an equi-phase plane
220
a
which extends along the input end face of the output-side slab waveguide
112
. Because the input end face of the slab waveguide
112
is formed in an arc whose center of curvature is located at the center O of the output end face of the slab waveguide
112
, the optical signal having the center wavelength &lgr;
M
is focused on the center of the output end face. The optical signal having the wavelength (center wavelength &lgr;
M
) thereof exhibiting the diffraction angle &phgr;
112
(≠
0
) has an equi-phase plane
220
inclined by an angle &phgr;
112
counterclockwise in
FIG. 4
with respect to the equi-phase plane
220
a associated with the center wavelength &lgr;
M
. Therefore, this optical signal is focused on a position P on the output end face of the output-side slab waveguide
112
, which position is shifted in the widthwise direction from the center O of the output end face. That is, the optical signal focusing position P changes depending on the diffraction angle &phgr;
112
. In other words, the distance X between the center O of the output end face of the output-side slab waveguide
112
and the focusing position P differs according to the wavelength &lgr;.
When the diffraction angle &phgr;
112
(hereinafter referred to as “&phgr;”) is small, sin &phgr;≈&phgr;, so that modifying the equation (1) yields the following equation.
n
s
·D·&phgr;+n
c
·&Dgr;L=m·&lgr;
Solving this equation with respect to &phgr;, we obtain
&phgr;=(
m·&lgr;−n
c
·&Dgr;L
)/(
n
s
·D
).
Differentiating both sides of the above equation with respect to &lgr; yields
(
d&phgr;/d&lgr;
)=[{
m−
(
dn
c
/d&lgr;
)·&Dgr;
L}·n
s
−{m·&lgr;−n
c
·&Dgr;L
)·(
dn
s
/d&lgr;
)}]/{(
n
s
)
2
·D}.
In the vicinity of the center wavelength &lgr;
M
, the following is satisfied.
&lgr;=&lgr;
M
=(
n
c
·&Dgr;L
)/
m
Thus, the following relationship is met.
&Dgr;
L=
(
m·&lgr;
M
)/
n
c
Substituting the above relationship in the right-hand side of the above equation about (d&phgr;/d&lgr;), yielding
(
d&phgr;/d&lgr;
)=(&Dgr;
L

s
·D·&lgr;
M
)×{
n
c−&lgr;
M
·(
dn
c
/d&lgr;
)}
By expressing the radius of curvature of the input end face of the output-side slab waveguide as f
o
, the dispersion dX/d&lgr; of the distance X with respect to the wavelength &lgr; is given by the following equation (2).
ⅆ
X
/
ⅆ
λ
=
f
O
·
(
ⅆ
φ
/
ⅆ
λ
)
=
{
(
f
O
·
Δ
⁢
 
⁢
L
)
/
(
n
S
·
D
·
λ
M
)
}
×
{
n
C
-
λ
M
·
(
ⅆ
n
C
/
ⅆ
λ
)
}
(
2
)
The above equation (2) indicates that the demultiplexed optical signal whose wavelength differs from the center wavelength &lgr;
M
by d&lgr;

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