Optical waveguides – Polarization without modulation
Reexamination Certificate
2002-07-29
2004-08-10
Healy, Brian M. (Department: 2874)
Optical waveguides
Polarization without modulation
C385S014000, C385S129000, C385S130000, C385S131000
Reexamination Certificate
active
06775426
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarization mode dispersion compensating device, which is a technology applied to an optical transmission system for high speed optical communications using optical fibers, optical switching, optical information processing, etc., and which is particularly useful in compensating the polarization mode dispersion.
The present invention also relates to a technique for effectively compensating the polarization mode dispersion even when a difference between the optical powers of the TE polarization component and the TM polarization component is large.
2. Description of the Related Art
In conjunction with the increase of the transmission capacity due to the advance of the IT field, the bit rate of the optical signals has a tendency of increasing from 2.5 Gb/s to 10 Gb/s, and further to 40 Gb/s. Here, the polarization mode dispersion poses a problem.
FIG. 1
is for explaining the polarization mode dispersion, and showing an optical fiber
1001
, an input optical pulse
1002
, a TE or TM polarization component
1003
of the input optical pulse
1002
, a TM or TE polarization component
1004
of the input optical pulse
1002
, an output optical pulse
1007
, a TE or TM polarization component
1005
of the output optical pulse
1007
, and a TM or TE polarization component
1006
of the output optical pulse
1007
.
In general, the optical fiber has the polarization mode dispersion due to its birefringence, so that the optical signals propagating through the optical fiber will be propagated by being divided into a fast propagation component and a slow propagation component depending on the polarization planes. In
FIG. 1
, the polarization component
1003
is the fast propagation component, which becomes the polarization component
1005
at the output end. On the other hand, the polarization component
1004
is the slow propagation component, which becomes the polarization component
1006
that arrives later than the polarization component
1005
at the output end. The output optical pulse
1007
is a sum of the polarization component
1005
and the polarization component
1006
, so that the waveform of the output optical pulse
1007
will be distorted as a result.
The amount of the polarization mode dispersion is about 0.2×L
1/2
(ps) to 2×L
1/2
(ps) for a fiber length of L (Km), for example. Namely, assuming the optical fiber of 100 Km long, the polarization mode dispersion of 20 ps can occur at worst. This value is not a serious problem for 2.5 Gb/s (pulse width of 400 ps) or 10 Gb/s (pulse width of 100 ps), but it can cause a fatal waveform distortion for 40 Gb/s (pulse width 25 ps), which in turn can degrade the bit error rate largely.
In order to resolve this problem, conventionally, the polarization mode dispersion has been compensated by a configuration as shown in
FIG. 2
, which has an input optical fiber
1101
, an input optical pulse
1102
, a TE or TM polarization component
1103
of the input optical pulse
1102
, a TM or TE polarization component
1104
of the input optical pulse
1102
, a polarization controller
1105
, an optical fiber
1106
with a particularly large polarization mode dispersion such as a polarization maintaining fiber, a TE or TM polarization component
1107
, a TM or TE polarization component
1108
, an optical coupler
1109
, a photodetector
1110
, an electric band-pass filter
1111
, a control system
1112
of the polarization controller
1105
, an output optical fiber
1113
, a waveform reshaped optical pulse
1116
, a TE or TM polarization component
1114
of the optical pulse
1116
, and a TM or TE polarization component
1115
of the optical pulse
1116
(see, George Ishikawa, Hiroki Ooi, and Yuichi Akiyama, APCC/OECC '99, pp. 424-428).
The configuration of
FIG. 2
uses a scheme for compensating the polarization mode dispersion by adjusting the polarization state of the input optical pulse
1102
by the polarization controller
1105
such that the delayed polarization component
1103
will be entered into a fast propagation direction of the optical fiber
1106
while the advancing polarization component
1104
will be entered into a slow propagation direction of the optical fiber
1106
. As the optical fiber
1106
, one with a particularly large polarization mode dispersion such as the polarization maintaining fiber is used. The polarization maintaining fiber has the polarization dispersion of about 1 ps per 1 m, for example.
According to the configuration of
FIG. 2
disclosed in the above mentioned reference, a part of the optical signal is split at the optical coupler
1109
and detected at the photodetector
1110
, and an electric signal obtained by the photoelectric conversion of the detected light at the photodetector
1110
is sent to the control system
1112
through the electric band-pass filter
1111
with a bandwidth equal to one half of the transmission speed. The control system
1112
controls the polarization controller
1105
to maximize the intensity of the electric signal (i.e., the intensity of the detected light), so as to minimize the polarization mode dispersion, i.e., to minimize a difference between the differential group delays of the polarization component
1114
and the polarization component
1115
, such that the waveform reshaped optical pulse
1116
can be obtained.
However, the conventional art shown in
FIG. 2
has the following drawbacks. The first drawback is a limitation on the bit rate of the optical signals. The configuration of
FIG. 2
requires the electric band-pass filter
1111
with a bandwidth equal to exactly one half of the bit rate, so that the bit rate cannot be changed. The second drawback is that, when “10” codes appear consecutively as in “10101010 . . . ”, for example, the higher harmonic component at one half of the bit rate increases so that there is a problem of affecting the electric feedback. The third drawback is that it requires the photodetector
1110
with a speed equal to one half of the bit rate so that there is a problem of making the photodetector
1110
expensive.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a polarization mode dispersion compensating device which is capable of changing the bit rate, which does not affect the feedback system even when “10” codes appear consecutively, and which can be formed by using a low speed photodetector.
It is another object of the present invention to provide a polarization mode dispersion compensating device which is capable of effectively compensating the polarization mode dispersion even when a difference between the optical powers of the TE polarization component and the TM polarization component is large.
According to one aspect of the present invention there is provided a polarization mode dispersion compensating device, comprising: a polarization mode dispersion equalizer configured to receive an input optical signal propagated through an optical fiber, and output a polarization mode dispersion compensated optical signal by compensating a polarization mode dispersion of the input optical signal such that a difference between transmission delays of a TE polarization component and a TM polarization component of the input optical signal becomes minimum; a polarization component splitting unit configured to receive the polarization mode dispersion compensated optical signal outputted from the polarization mode dispersion equalizer, and split a part of the polarization mode dispersion compensated optical signal into the TE polarization component and the TM polarization component; an optical XOR circuit configured to receive the TE polarization component and the TM polarization component split by the polarization component splitting unit separately at two input ports through an identical optical path length, and output a logical operation result of an optical XOR operation on the TE polarization component and the TM polarization component entered at the two input ports; and a control sys
Ito Toshio
Sato Rieko
Suzuki Yasuhiro
Healy Brian M.
Kilpatrick & Stockton LLP
Nippon Telegraph and Telephone Corporation
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