Method and apparatus for compensation of polarization-mode...

Optical communications – Transmitter and receiver system – Including optical waveguide

Reexamination Certificate

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C398S152000, C398S081000

Reexamination Certificate

active

06829440

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to techniques for compensating for influences on signal light due to polarization-mode dispersion of transmission lines. In particular, the invention relates to a method and apparatus for compensation of polarization-mode dispersion, which detects the strength of a specific frequency component in a baseband spectrum in signal light, to perform compensation of polarization-mode dispersion.
(2) Related Art
Presently, in Japan and overseas, the commercialization of optical transmission systems at transmission speeds of 10 Gb/s (gigabit/second) is advancing. Furthermore, in order to provide communication services of low cost and with high frequency efficiency to meet the rapid increase in transmission capacity demand due to the Internet and the like, there is a requirement to realize ultra high-speed optical transmission systems at transmission speeds of for example 40 Gb/s.
However, in such ultra high-speed optical transmission systems, the transmission waveform of the signal light deteriorates due to the influence of polarization-mode dispersion (referred to hereunder as PMD) and the like which occurs in the transmission line. Therefore, there is a problem that the transmission distance of the signal light is limited. This PMD is phenomenon that occurs due to a difference in propagation delay times of polarization components in the signal light (for example two mode light such as TE mode and TM mode), being inevitable phenomena for all optical fibers.
Consequently, in order to realize long distance optical transmission at ultra high-speed, application of PMD compensation technique is essential. Furthermore, since PMD also fluctuates with time due to changes in the transmission line environment, such as temperature or stress, automatic PMD compensation techniques are necessary for monitoring the condition of the PMD during system operation, and performing feedback control.
As a heretofore automatic PMD compensation technique, there is reported for example a compensation method in optical regions (refer, for example, to T. Takahashi et al., Electronics Letters Vol. 30, pp. 348-349, 1994, or F. Heismann et al., ECOC '98 Technical Digest, pp. 529-530, or Japanese Unexamined Patent Publication No. 11-196046) and a compensation method in electrical stages (refer, for example, to H. Bülow, NOC '97 Technical Digest, pp. 57-72).
Furthermore, the present inventors have, from the view point of simple configuration, and independence from modulation methods or other waveform deterioration factors (wavelength dispersion, non-linear effects), and high-speed benefits, proposed an automatic PMD compensation technique which adopts a compensation method in optical regions (refer, for example, to Japanese Patent Application No. 11-515959, or H. Ooi et al., OFC '99, Technical Digest WE5 pp. 86-88, 1999). This compensation technique is one which adopts a PMD monitor method of a simple configuration not requiring a large scale measuring instrument, to detect the strength of a specific frequency component in a baseband spectrum after transmission (for example, a strength of a 20 GHz component in a 40 Gb/s signal light), and then feedback controls an compensation amount so that the detection strength becomes a relative maximum. By applying this compensation technique, the transmission distance of the signal light is extended by at least four times.
However, in such a compensation technique, there is a problem in that the upper limit of the compensatable PMD is restricted to one time slot of the transmission light. That is to say, as shown in
FIG. 12
, the strength of the specific frequency component changes with respect to an optical delay amount &Dgr;&tgr;
T
(hereunder, PMD amount &Dgr;&tgr;
T
) between polarization-modes, due to the PMD in the transmission line, and when the PMD amount &Dgr;&tgr;
T
becomes the one time slot of the transmission light (for example 25 ps in the case of 40 Gb/s signal light), the strength of the specific frequency component becomes zero (or relative minimum). Therefore, in the case where the PMD amount &Dgr;&tgr;
T
exceeds the one time slot, when the compensation amount is feedback controlled so that the strength of the specific frequency component becomes relative maximum, the PMD amount &Dgr;&tgr;
T
after control increases, so that deterioration of the transmission light waveform becomes large.
With respect to this problem, as a technique for extending the range where PMD compensation is possible, a method has been proposed where for example a frequency B/2 Hz component, a frequency B/4 Hz component and a frequency B/8 Hz component in a baseband spectrum in a transmission light signal at transmission speed B b/s are extracted by a band pass filter (BPF), and the various strength are detected (refer, for example, to D. Sandel et al., Electronics Letters Vol. 34, pp. 2258-2259, 1998).
FIG. 13
is a view for explaining the aforementioned PMD compensation technique. The horizontal axis shows the value (&Dgr;&tgr;
T
/T) obtained by standardizing the PMD amount &Dgr;&tgr;
T
by one time slot T of the transmission light, while the vertical axis shows the strength of the frequency component extracted by each BPF. Here, the curve shown by (BPF 0.5/T) represents the strength of the B/2 Hz component, the curve shown by (BPF 0.25/T) represents the strength of the B/4 Hz component, and the curve shown by (BPF 0.125/T) represents the strength of the B/8 Hz component. Moreover, the curve shown by LPF represents the strength of the B/8 Hz component extracted by a low pass filter (LPF).
As shown in
FIG. 13
, the lower the frequency extracted by the BPF, the higher the PMD amount &Dgr;&tgr;
T
at which the strength of each component becomes zero. Therefore, the range where compensation is possible is extended. However, in the region where the PMD amount &Dgr;&tgr;
T
is small, the change in the detection strength (monitor strength) becomes small (each curve approaches a flat). Therefore, in the case where the PMD compensation amount is feedback controlled so that the monitor strength becomes relative maximum, convergence of the feedback control becomes poor. Furthermore, since there is an indefinite width in the sensitivity of the monitor system, there is also the possibility of time-wise unstable control. Therefore, with the aforementioned PMD compensation technique, three PMD monitors for detecting the strength of each frequency component are sequentially switched. With regards to the switching of the PMD monitors, the threshold values Th1, Th2 for the monitor strength are set beforehand. For example, in the case where the monitor strength increases, when the detection value (curve BPF 0.125/T) of the PMD monitor which detects the strength of the B/8 Hz component, increases up to the threshold value Th1 at the top of
FIG. 13
, the monitor is switched to the PMD monitor for detecting the strength of the B/4 Hz component. After this, when the detection value (the curve BPF 0.25/T) of this PMD monitor increases to the threshold value Th1, the monitor is switched to the PMD monitor for detecting the strength of B/2 Hz component. Furthermore, for example, in the case where the monitor strength reduces, the monitors are sequentially switched based on the threshold value Th2 at the bottom of
FIG. 13
, from the PMD monitor for detecting the strength of the B/2 Hz component, to the PMD monitor for detecting the strength of the B/4 Hz component, and then to the PMD monitor for detecting the strength of the B/8 Hz component.
In this way, by controlling so that the plurality of PMD monitors are sequentially switched in accordance with the threshold values Th1 and Th2 previously set with respect to the monitor strength, the range where PMD compensation is possible can be increased to one time slot or more.
However, with the abovementioned conventional PMD compensation technique which controls the switching of the plurality of PMD monitors, absolute values are used for the previously set thre

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