Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-12-26
2004-03-30
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S002000, C385S004000, C385S008000, C385S009000, C385S039000, C385S040000, C385S042000
Reexamination Certificate
active
06714706
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a waveguide-type optical control device and a process for producing the same, and more particularly to a waveguide-type optical control device, which has a directional coupler-type Mach-Zehnder construction and can improve the ratio of the minimum attenuation level to the maximum attenuation level (extinction ratio) without complicating the construction, and a process for producing the same.
BACKGROUND OF THE INVENTION
Waveguide-type optical control devices are suitable for integration and a reduction in power consumption, and, thus, studies have been made on the utilization of waveguide-type optical control devices in optical switches or optical modulators. Further, in recent years, the spread of dense wavelength division multiplexing (DWDM) has lead to an increasing demand for variable optical attenuators as means for making optical powers of respective wavelengths uniform at the time of wavelength multiplexing, or as optical parts of optical ADMs (add drop multiplexers) which select a desired wavelength and inserts/removes the wavelength in a transmission line. Among others, variable optical attenuators having a directional coupler-type Mach-Zehnder (MZ) construction comprising two directional couplers provided on an LiNbO
3
(lithium niobate; LN) substrate, which is advantageous from the viewpoints of a reduction in size, a reduction in voltage, and a reduction in power consumption, and a phase shifter provided between the two directional couplers are being put to practical use.
FIG. 1
shows the construction of a conventional waveguide-type optical control device having a directional coupler-type Mach-Zehnder construction. In
FIG. 1
, a variable optical attenuator is exemplified as the waveguide-type optical control device.
The variable optical attenuator having a directional coupler-type Mach-Zehnder construction comprises: optical waveguides
1
a
,
1
b
which are provided parallel to each other on an LN substrate (not shown); a first directional coupler
2
provided within the optical waveguides
1
a
,
1
b
; a phase shifter
3
provided adjacent to the first directional coupler
2
; and a second directional coupler
4
provided adjacent to the phase shifter
3
. The phase shifter
3
comprises a first electrode
3
a
, a second electrode
3
b
, and a third electrode
3
c
. The third electrode
3
c
is used as a common electrode. A negative (−) voltage is applied to this electrode from a direct current power supply
3
d
, and a positive (+) voltage is applied to the first electrode
3
a
and the second electrode
3
b
from the direct current power supply
3
d
to cause an electric field.
Next, the operation of the waveguide-type optical control device (variable optical attenuator) shown in
FIG. 1
will be explained. A signal light introduced from the end of the optical waveguide
1
a
is branched in the first directional coupler
2
into signal light parts which are to be traveled respectively through optical waveguides
1
a
and
1
b
(branching ratio=50:50), and the branched signal lights are then input into the phase shifter
3
. The phase shifter
3
operates according to the magnitude of an applied voltage
31
from the direct current power supply
3
d
. When the voltage
31
is not applied from the direct current power supply
3
d
, the branched signal lights introduced into the optical waveguides
1
a
and
1
b
are input in an identical phase into the second directional coupler
4
and the whole light is output from the output terminal of the optical waveguide
1
b
while no light is output from the optical waveguide
1
a.
Next, when the applied voltage
31
is increased from 0 (zero) volt, the refractive index of the optical waveguides
1
a
and
1
b
are changed and, consequently, the propagation speed of signal lights, which travel respectively through the optical waveguides
1
a
and
1
b
, is changed. Since the voltage applied to the optical waveguide
1
a
is opposite in direction to the voltage applied to the optical waveguide
1
b
, a difference occurs in propagation speed between signal light, which travels through the optical waveguide
1
a
, and signal light which travels through the optical waveguide
1
b
in the phase shifter
3
. As a result, the signal light in the optical waveguide
1
a
and the signal light in the optical waveguide
1
b
are input in a mutually different phase into the second directional coupler
4
. For this reason, the branching ratio (coupling rate) of the second directional coupler
4
is deviated from the original rate 50%, and, as a result, a part of signal light, which, up to this stage, has been entirely output from the optical waveguide
1
b
in the second directional coupler
4
, is also output from the optical waveguide
1
a
. When the applied voltage
31
is increased to about 30 to 50 V, the signal light is substantially entirely output from the optical waveguide
1
a
. That is, setting the applied voltage
31
to a suitable value permits the coupling length L in the phase shifter
3
to be equivalently changed and, consequently, permits optical output corresponding to the change to be obtained.
When the voltage
31
was not applied, or when a voltage of about 30 to 50 V was applied, in order to output the whole signal light from any one of the optical waveguide
1
a
and the optical waveguide
1
b
in the second directional coupler
4
, the branching ratio (coupling rate) of the first directional coupler
2
to the second directional coupler
4
should be accurately brought to 50:50 (50%). To this end, the length of a portion where the optical waveguides
1
a
and
1
b
approach each other (coupling length L=&pgr;/2&ggr; wherein &ggr; represents Pockels constant) should be accurately brought to the half of the complete coupling length Lc (=&pgr;/2&kgr; wherein &kgr; represents coupling coefficient). The deviation of the branching ratio (coupling rate) of the first directional coupler
2
to the second directional coupler
4
from 50:50 (50%) results in increased leakage of the light signal from one waveguide to the other waveguide at the output terminal of the second directional coupler
4
and thus deteriorates the ratio of the minimum attenuation level to the maximum attenuation level (extinction ratio).
FIG. 2
shows the relationship between the gap and the coupling length in a directional coupler.
The length of a portion, where the optical waveguides la and
1
b
approaches and are coupled to each other (coupling length L), and a gap G are important to the directional coupler. In order to bring the branching ratio (coupling rate) to 50:50 (50%), it is necessary to eliminate a variation in the gap G and to bring the coupling length L to [complete coupling length Lc÷2]. These two are important parameters for a production process of the directional coupler.
FIG. 3
shows that characteristics vary according to the production parameters. When there is no variation in gap G shown in
FIG. 2 and
, at the same time, when the coupling length L is equal to the half of the complete coupling length Lc, ideal characteristics
130
are obtained, that is, the crosstalk is minimized and, consequently, the extinction ratio is increased. On the other hand, when there is a variation in gap G or when the coupling length L is not equal to the half of the complete coupling length Lc, deteriorated characteristics
131
are obtained. It is known that a change in coupling rate only by several percents from 50% causes this state.
In order to solve this problem, Japanese Patent Publication No. 72964/1994 proposes a construction such that, separately from electrodes for the phase shifter, electrodes for directional couplers are provided in the directional couplers in the optical waveguides to control the refractive index in the optical waveguides, thereby equivalently regulating the coupling length L. This construction will be explained in conjunction with FIG.
4
.
FIG. 4
shows another conventional waveguide-type optical control device.
NEC Corporation
Young & Thompson
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