Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
1999-12-02
2002-06-11
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S031000, C385S037000, C385S015000, C359S199200
Reexamination Certificate
active
06404950
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dispersion-compensating module which improves the transmission quality of large-capacity, high-speed optical transmission systems of WDM (Wavelength Division Multiplexing) type.
2. Related Background Art
A WDM type optical transmission system is a system which transmits a plurality of signal light components within a 1.55-&mgr;m wavelength band (1.53 &mgr;m to 1.57 &mgr;m) by way of an optical fiber transmission line network, and enables large-capacity, high-speed optical communications. This optical transmission system comprises an optical amplifier for optically amplifying a plurality of signal light components, collectively, and the like, in addition to an optical fiber line which is a transmission medium. In such WDM communications, various studies and developments are under way in order to enable communications with further larger capacity and higher speed.
One of important subjects to be studied concerning the optical transmission line is reduction of dispersion in a signal wavelength band. Namely, each signal light component has a certain bandwidth even though it is monochromatic. As a result, when dispersion occurs in the signal wavelength band in the optical transmission line, the signal light components having reached a receiving station by way of the optical transmission line after being sent out from a transmitting station deform their waveforms, thereby deteriorating their reception. Therefore, in the signal wavelength band, it is desirable that the dispersion in the optical transmission line be as small as possible.
However, standard single-mode optical fibers (hereinafter referred to as SMF), having a zero-dispersion wavelength in a 1.3-&mgr;m wavelength band, already installed as an optical transmission line have a dispersion of about 16 ps
m/km in a wavelength band of 1.53 &mgr;m to 1.57 &mgr;m which is used in WDM communications. Many of already installed optical transmission lines are constituted by such an SMF. Therefore, a dispersion-compensating module is disposed within a repeater in order to compensate for the dispersion of the optical transmission line, while making use of such an already installed optical transmission line.
With respect to the dispersion over the whole length of the optical transmission line to be compensated for, the dispersion-compensating module generates a dispersion having an opposite polarity with substantially the same absolute value. Specifically, the dispersion-compensating module comprises a dispersion-compensating optical fiber having a dispersion with a polarity opposite to that of the dispersion of the optical transmission line (including SMF), and compensates for the dispersion of the optical transmission line (including SMF) by adjusting the dispersion-compensating optical fiber to an appropriate length. Also, for reducing the size of the dispersion-compensating module, it is a common practice to wind the dispersion-compensating optical fiber into a coil having a small diameter.
SUMMARY OF THE INVENTION
The inventors have studied conventional dispersion-compensating modules and, as a result, have found out the following problems.
Recently, in optical amplifiers, the width of a signal wavelength band which can collectively be optically amplified has been expanded, and gain deviations in optically amplifiable signal wavelength bands have been reduced in order to improve the versatility thereof. On the other hand, inter-wavelength loss deviations (fluctuations in loss among signal wavelengths) occurring in the optical transmission line in the signal wavelength band are too large to neglect. Also, since inter-wavelength loss deviations in the signal wavelength band occur to a certain extent in a dispersion-compensating optical fiber as well, it is necessary to improve loss deviations in the whole transmission line including the conventional dispersion-compensating modules.
When a plurality of stages of conventional dispersion-compensating modules and optical amplifiers are disposed in such an optical transmission line; even if a plurality of signal light components sent out from a transmitting station exhibit no inter-wavelength optical power deviation at the time when sent out, they will generate an optical power deviation, due to inter-wavelength loss deviations, while they are propagating through the optical transmission line and conventional dispersion-compensating modules, and the optical power deviation is expanded by optical amplifiers having a small gain deviation. Since inter-wavelength optical power deviations are accumulated in the plurality of signal light components reaching the receiving station, a part of the signal light components may become weak, thereby generating reception errors.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide a dispersion-compensating module having a structure which compensates for the dispersion of optical transmission lines in a 1.55-&mgr;m wavelength band (1.53 &mgr;m to 1.57 &mgr;m) and adjusts loss fluctuations among signal wavelengths into an appropriate range.
The dispersion-compensating module according to the present invention is generally installed between repeaters together with an optical amplifier, whereas the object to be compensated for thereby is an SMF which is laid between a transmitting station and a receiving station, between repeater stations, between the transmitting station and a repeater station, or between a repeater station and the receiving station. For being installed on an already installed optical fiber transmission line and constituting a part of the line, the dispersion-compensating module comprises an input end for capturing signal lights propagating through the optical fiber transmission line and an output end for sending out the signal lights into the optical fiber transmission line, has a positive loss slope in a 1.55-&mgr;m wavelength band, and further comprises a structure for allowing the dispersion generated in a predetermined length of the optical fiber transmission line to be compensated for and loss deviations among individual signal wavelengths to be adjusted into an appropriate range.
Specifically, the dispersion-compensating module according to the present invention comprises a dispersion-compensating device and a loss-equalizing device. The dispersion-compensating device compensates for the dispersion of the above-mentioned optical fiber transmission line in the 1.55-&mgr;m wavelength band. Also, for compensating for the wavelength dependence of loss in at least the above-mentioned optical fiber transmission line and dispersion-compensating device, the loss-equalizing device adjusts the total loss slope of the optical fiber transmission line including the dispersion-compensating module such that the loss deviations among individual signal wavelengths in the 1.55-&mgr;m wavelength band caused by propagation in the optical fiber transmission line and dispersion-compensating device falls within the appropriate range.
In this specification, “loss slope” refers to the gradient of a graph indicating the wavelength dependence of transmission loss. Also, the above-mentioned optical fiber transmission line is an SMF having a zero-dispersion wavelength in a 1.3-&mgr;m wavelength band; and, letting L be the length of the above-mentioned SMF, and Cr be the absolute value of a permissible manufacturing error, the total loss slope (dB
m) of the dispersion-compensating module in the 1.55-&mgr;m wavelength band is greater than 0 but not greater than 0.000175×L+&agr;. In general, when an SMF having a zero-dispersion wavelength in a 1.3-&mgr;m wavelength band is employed as an optical fiber transmission line, the loss slope per unit length of the SMF is about −0.000175 (dB
rn/km=dB/(nm·km)). Therefore, the total loss slope of the dispersion-compensating module is ideally +0.000175×L when an SMF having a length of L is concerned. In practice, however, since the manufacturing err
Okuno Toshiaki
Sasaoka Eisuke
Tanaka Shigeru
Tsukitani Masao
Bovernick Rodney
Pak Sung
Sumitomo Electric Industries Ltd.
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