Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2000-12-21
2003-01-21
Epps, Georgia (Department: 2873)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S001000, C372S021000, C372S028000, C372S029010
Reexamination Certificate
active
06510255
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical transmission apparatuses and, more specifically, to an optical transmission apparatus using an external optical modulator. The apparatus is capable of controlling a bias voltage applied to the external optical modulator for making the bias voltage follow a shift in optimal bias voltage caused by DC drift.
2. Description of the Background Art
In optical transmission apparatuses used in conventional optical communications systems, conventionally adopted is a modulation scheme of directly modulating an electric current fed to a laser diode constructing a light source with an input signal and outputting an optical signal modulated with the input signal. In this modulation scheme, the current fed to the laser diode changes, and therefore, an optical signal from the laser diode changes in oscillation wavelength due to its chirp characteristics. If transmitted over a long distance, the optical signal outputted from such laser diode is deteriorated in waveform under the influence of chromatic dispersion within an optical fiber.
For future mobile communications, high frequencies, especially, extremely high frequencies (millimeter-wave band), will be used for substantially increasing transmission rate, because band reservation can be made easily in these frequencies. However, if signals in the millimeter-wave band are transmitted through a coaxial cable, a large loss will be caused. Thus, in this case, amplifiers are required at several tens-of-millimeter intervals. This requirement poses problems in view of cost and reliability for actual system configuration. Therefore, for millimeter-wave band signal transmission, use of optical fibers with less transmission loss is required. However, laser diodes now commercially available have frequency response characteristics of around 10 GHz, and thus cannot respond to signals of extremely high frequency. For this reason, such laser diodes cannot be directly modulated by the millimeter-wave signals.
Therefore, for long-distance transmission or transmission of signals of high frequency such as millimeter waves, an optical transmission apparatus having a Mach-Zehnder-type external optical modulator is, in principle, less prone to cause chirp and capable of responding to signals of higher frequency has been suggested for use.
FIG. 11
shows the structure of a Mach-Zehnder-type external optical modulator (hereinafter referred to as MZ-type optical modulator). An optical carrier outputted from a light source is provided to the MZ-type optical modulator, and branched to two optical waveguides. When a voltage is applied to electrodes provided on a crystal substrate to cause an electric field, the index of refraction in the waveguides is changed. As a result, the lights propagated through the waveguides are changed in phase. Note that shown in
FIG. 11
is such structure that only the light through one optical waveguide is phase-modulated. The lights from these optical waveguides are combined with each other, and outputted as an optical signal from a MZ-type optical modulator. The optical electric field of the outputted optical signal is represented by the following equation (1).
E
Ex
-
Mod
(
V
,
t
)
=
1
2
exp
(
ⅈ
ω
t
)
+
1
2
expⅈ
[
{
ω
t
+
φ
(
V
,
t
)
}
]
=
1
2
exp
{
ⅈ
(
ω
t
+
φ
(
V
,
t
)
2
)
}
exp
{
-
ⅈφ
(
V
,
t
)
2
}
+
1
2
exp
{
ⅈ
(
ω
t
+
φ
(
V
,
t
)
2
)
}
exp
{
ⅈ
φ
(
V
,
t
)
2
}
=
1
2
exp
{
ⅈ
(
ω
t
+
φ
(
V
,
t
)
2
)
}
[
exp
{
-
ⅈφ
(
V
,
t
)
2
}
+
exp
{
ⅈ
φ
(
V
,
t
)
2
}
]
=
2
cos
{
φ
(
V
,
t
)
/
2
}
exp
{
ⅈ
(
ω
t
+
φ
(
V
,
t
)
2
}
(
1
)
provided that
φ
(
V
,
t
)
=
φ
D
C
(
V
)
+
φ
RF
(
t
)
=
φ
D
C
(
V
)
+
m
cos
ω
f
t
(
2
)
where V is a bias voltage, m is a phase modulation factor, and &ohgr;f is angular frequency of an analog signal. By using this optical electric field, power of the optical signal outputted from the MZ-type optical modulator is given by the following equation (3).
P
out
=
E
Ex
-
Mod
(
V
,
t
)
×
E
Ex
-
Mod
*
(
V
,
t
)
=
2
cos
2
(
φ
(
V
,
t
)
/
2
)
=
S1
+
cos
φ
(
V
,
t
)
=
1
+
cos
(
φ
DC
(
V
)
+
m
cos
ω
f
t
)
=
1
+
cos
φ
DC
(
V
)
cos
(
m
cos
ω
f
t
)
-
sin
φ
(
V
)
sin
(
m
sin
ω
f
t
)
=
1
+
cos
φ
DC
(
V
)
{
J
0
(
m
)
+
2
J
2
(
m
)
cos
(
2
ω
f
t
)
+
…
}
-
sin
φ
DC
(
V
)
{
2
J
1
(
m
)
cos
(
ω
f
t
)
+
2
J
3
(
m
)
cos
(
3
ω
f
t
)
+
…
}
(
3
)
At this time, a relation between a bias voltage and an optical output is shown in FIG.
12
. In
FIG. 12
, the lateral axis represents the bias voltage, while the vertical axis represents optical output power. As such, the power of the optical signal outputted from the MZ-type optical modulator exhibits a sine-wave characteristic with respect to the bias voltage applied to the MZ-type optical modulator.
However, in the MZ-type optical modulator, the above relation between the bias voltage and the optical output may be varied with various conditions, such as time and temperature. Such phenomenon is called DC drift. This DC drift phenomenon is shown in FIG.
13
.
Such DC drift phenomenon as shown in
FIG. 13
causes a shift in phase state from an initial state. Accordingly, average power of the optical signal outputted from the MZ-type optical modulator is changed, thereby causing deterioration in signal characteristic. Note that the bias voltage at the initial phase state (where the amount of second order distortion caused by non-linearity of the external modulator is minimum) is hereinafter referred to as an optimal bias voltage (denoted by Vb in the drawing).
In the conventional optical transmission apparatus using the external optical modulator, the bias voltage applied to the external optical modulator is controlled based on the average power of the optical signal outputted from the external optical modulator so that the bias voltage can be made to follow the shift in the optimal bias voltage caused by DC drift. The average power can be calculated by converting the optical signal into an electrical signal and measuring DC components of the electrical signal, that is, electric power.
More specifically, in the initial state in which the bias voltage is optimally set, average power of the output optical signal is measured in advance, and stored as a reference value. Thereafter, the applied bias voltage is controlled so that the average power of the output optical signal agrees with the reference value.
However, compared with the shift in the applied bias voltage from the optimal bias voltage, a change in the average power of the output optical signal is extremely small. Therefore, it is very difficult to make the applied bias voltage accurately follow the shift in the optimal bias voltage based on the average power of the output optical signal.
Here, described is a relation between a shift in the applied bias voltage from the optimal
Masuda Kouichi
Yamamoto Hiroaki
Dinh Jack
Matsushita Electric - Industrial Co., Ltd.
Wenderoth , Lind & Ponack, L.L.P.
LandOfFree
Optical transmission apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical transmission apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical transmission apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3052649