Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-08-08
2003-04-15
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S037000, C385S046000, C359S199200
Reexamination Certificate
active
06549696
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical wavelength multiplexer/demultiplexer, and more particularly, relates to a temperature-unreliable, optical wavelength multiplexer/demultiplexer using an arrayed-waveguide diffraction grating (hereinafter, called as “channel waveguide array”) composed of a plurality of channel waveguides having a silica glass as a core and having different length from each other, in which loss resulted of grooves that are formed on slab waveguide is reduced and of which spectrum response is optimized, and also relates to a channel waveguide array which is used for a wavelength-division multiplex transmission system.
BACKGROUND OF THE INVENTION
In the field of optical communication, a wavelength-division multiplexing transmission system that a plurality of signals are put on light having a plurality of wavelengths and the light loaded with the plurality of signals are transmitted through one optical fiber to increase optical communication capacity has been investigated and has been partially implemented in products. In the system, an optical wavelength-division multiplexer/demultiplexer for multiplexing or demultiplexing a plurality of signal lights plays an important role. Among others, an optical wavelength multiplexer/demultiplexer using a channel waveguide array can implement multiplexing/demultiplexing at a narrow wavelength spacing, and hence, can increase the number of multiplexing in communication capacity.
In
FIGS. 1 and 2
, the optical wave length multiplexer/dimultiplexer comprises a silica substrate
1
, a channel waveguide array
3
a
composed of a plurality of the channel waveguides
3
provided on the substrate
1
in certain pattern, each channel waveguide being composed of cores
2
made of silica glass, and a cladding
4
made of a pure silica glass and provided on the substrate
1
so that the core
2
and the substrate
1
may be covered with the cladding. To the core, titanium oxide (TiO
2
) is added. The channel waveguide
3
is formed by the core
2
and the cladding
4
and the channel waveguide array
3
has, for example, twenty-two channel waveguides
3
. Each channel waveguide
3
of the channel waveguide array
3
a
has different length from each other so that it becomes longer from one end to the other end (longer side). The channel waveguide array
3
a
is expected as a key device for the wavelength multiplex transmission system in case that the number of channel is increased, because it can be manufactured by same process and steps regardless of the number of the channel and because deterioration of its characteristics such as loss is less in principle. With respect to a transmitting wavelength, a channel interval and a transmitting center wavelength can be generally set by 100 GHz (approximately, 0.8 nm) or its multiple in accordance with the international standard.
To both sides of the channel waveguide array
3
a,
a fan-shaped input slab waveguide
5
which may be called, hereinafter, as “input waveguide” and an fan-shaped output slab waveguide
6
which may be called, hereinafter, as “output waveguide” are connected. One input channel waveguide
7
is connected to the fan-shaped input slab waveguide
5
and a plurality of output channel waveguides
8
-
1
~
8
-
8
.
In the above structure, signal lights including various wavelengths input in the input channel waveguide
7
are input through the fan-shaped input slab waveguide
5
in each core
2
of the channel waveguide array
3
a.
The signal lights input in the channel waveguide array
3
a
propagate through each core
2
to the fan-shaped output slab waveguide
6
, in which a light-collecting position is shifted in the contact surface of the fan-shaped output slab waveguide
6
and the output channel waveguides
8
-
1
~
8
-
8
because in-phase plane is declined depending on the wavelengths. As a result, the output signal lights in the fan-shaped output slab waveguide
6
are selectively output to the output channel waveguides
8
-
1
~
8
-
8
in accordance with the shift condition of the in-phase plane, whereby signal lights having different wavelengths are output from the eight waveguides.
A length “L” of each channel waveguide
3
in the channel waveguide array
3
a
changes by thermal expansion and a refractive index of silica glass constituting the core
2
changes with a temperature change. Accordingly, if a temperature changes, for example, from 0° C. to 60° C., the in-phase plane
9
changes to the in-phase plane
10
as shown in FIG.
1
. As a result, a light-collecting position is shifted in accordance with the temperature change, and wavelengths to be demultiplexed change.
In
FIG. 3
, “d” is a pitch of the channel waveguide in the channel waveguide array
3
a
and “&thgr;” is an emerging angle of signal light from the channel waveguide to the output slab waveguide
6
. If it is required to keep the in-phase plane shown in
FIG. 1
to be continuous with respect to certain wavelength, the following equation has to be satisfied.
(2&pgr;/&lgr;)
N
eff
·&Dgr;L
+(2&pgr;/&lgr;)
n
s
·d
sin &thgr;=2 &pgr;
m
(1)
wherein “&lgr;” is a wavelength, “m” is the number of degree (m=1, 2, 3, . . . ), &Dgr;L is a difference of length in the channel waveguide of the channel waveguide array
3
a,
N
eff
is an effective refractive index of the channel waveguide array
3
a,
and n
s
is an effective refractive index n of the fan-shaped output slab waveguide
6
. The effective refractive indexes N
eff
and n
s
are equal to the refractive index of silica glass which is used in the channel waveguide array
3
a
as the core
2
, and therefore, n is nearly equal to N
eff
and n
s
, respectively. Thus, the following formula can be derived from the formula (1).
&Dgr;&thgr;/&Dgr;
T
=(1/
n
)×(&dgr;
n/&dgr;T
)×&Dgr;
L/d
(2)
wherein “T” is a temperature, &Dgr;&thgr; is change of light beam angle (i.e. change of in-phase plane) when change of the temperature is &Dgr;T, and &dgr;n/&dgr;T is change of refractive index of the waveguide, and the influences of the thermal expansion are ignored because they are smaller than the change of refractive index. The change of wavelengths to be demultiplexed in accordance with the temperature change is represented by the following formula.
Δλ
/
Δ
T
=
(
λ
×
n
×
d
/
n
Δ
L
)
×
(
Δθ
/
Δ
T
)
=
(
λ
/
n
)
×
(
δ
n
/
δ
T
)
(
3
)
For example, a value of &Dgr;&lgr;/&Dgr;T in silica glass to which titanium oxide (TiO
2
) is added is 0.01 (nm/°C.), when n≈1.45, &dgr;n/&dgr;T≈1×10
−5
, &lgr;=1550 nm. The optical part materials using such the channel waveguide array
3
a
are used under the temperature such as 0° C. to 60° C., in general.
As a result, the channel waveguide array
3
a
can not be used in practical system, because the wavelengths to be demultiplexed is shifted by 0.6 nm at maximum in case that the temperature changes from 0° C. to 60° C. In order to reduce the change of center wavelength due to temperature-reliability, it has been proposed that a wedge-shaped groove is provided in a part of the channel waveguide array
3
a
and an optical resin material is inserted in the groove.
A conventional optical wavelength multiplexer/dimultiplexer in which a wedge-shaped groove having an optical resin material is provided is shown in
FIG. 4. A
value represented by the formula (2) has to be smaller than a value represented by the formula (3) to reduce the change of center wavelength due to the temperature-reliability. For the purpose, the wedge-shaped groove having a maximum width W is provided in a part of the channel waveguide array
3
a
and the optical resin material is inserted in the groove. As a result, the shift of demultiplexed wavelengths due to the temperature-reliability in in-phase plane is canceled. This situation is represented by the fo
Maru Koichi
Nounen Hideki
Okawa Masahiro
Teraoka Tatsuo
Uetsuka Hisato
Antonelli Terry Stout & Kraus LLP
Hitachi Cable Ltd.
Knauss Scott
Sanghavi Hemang
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