1. Field of the Invention
This invention relates to an activation method of an optical communication system and a channel increasing/decreasing method. More particularly, this invention relates to an activation method of an optical communication system for executing gain slope compensation, output control and pre-emphasis control, and its channel increasing/decreasing method. This invention further relates to a computer-readable recording medium for causing a computer to execute a program of such an activation method and such a channel increasing/decreasing method.
2. Description of the Related Art
Multi-media communication typified by the Internet has spread drastically in recent years. Research and development of optical communication technologies for achieving ultra-long distance communication and larger capacity communication has been made intensively in the field of communication technologies so as to cope with the drastic increase of traffic due to the drastic spread. Attempts have been made also to improve the speed of time division multiplexing (hereinafter abbreviated as “TDM”) transmission and to attain higher density multiplexing of wavelength division multiplexing (hereinafter abbreviated as “WDM”) transmission to satisfy the requirement for a greater quantity of traffic.
In the optical communication system for transmitting WDM optical signals, in particular, the WDM optical signals are generated by multiplexing a plurality of optical signals with different wavelengths from each other. Therefore, it is necessary to control amplified spontaneous emission, induced Raman scattering (SRS), or the like, which occurs in the WDM optical signals and has wavelength dependence.
Optical communication systems according to the prior art technology generally include an optical transmission station for generating WDM optical signals for wavelength-division multiplexing a plurality of optical signals having mutually different wavelengths, an optical transmission line in which the WDM optical signals outputted from the optical transmission station are transmitted and an optical reception station for receiving the input WDM optical signals so transmitted, and processing these WDM optical signals. An optical repeater station is inserted into the optical transmission line. A plurality of optical repeater stations may be disposed, whenever necessary. The optical repeater station includes in some cases an optical amplifier for amplifying the WDM optical signals to a predetermined optical level to compensate for the transmission loss occurring in the optical transmission line, and an optical add/drop multiplexer (hereinafter abbreviated as “OADM”) in other cases for adding or dropping an optical signal corresponding to a predetermined channel to and from the WDM optical signals.
To elongate the distance between the stations in such an optical communication system, the optical amplifier controls its output optical level at the upper limit optical level at which four-wave mixing and mutual phase demodulation do not occur in the WDM optical signals in the optical transmission line. To control the output optical level, the optical amplifier generally detects the output optical level inclusive of amplifier spontaneous emission (hereinafter abbreviated as “ASE”). Therefore, the amplifier needs to perform peak power control for controlling the output with optical power of ASE taken into account.
There is a limit to transmission distance due to a gain tilt based on a gain as a function of wavelength of the optical amplifier. When optical amplifiers are cascaded to elongate the transmission distance, the gain tilt occurring in each optical amplifier is accumulated. In channels with low optical levels in the WDM optical signals, therefore, an optical signal-to-noise ratio (hereinafter abbreviated as “optical SNR”) gets deteriorated, whereas waveforms get deteriorated due to a non-linear optical effect in channels with higher optical levels. Therefore, the same applicant as the that of the present invention proposed a slope compensation method and an apparatus for the method for substantially leveling the gain tilt in Japanese Patent Application Nos. 11-074371 and 2000-051050 that have not yet been laid open. The gain as a function of wavelength is a characteristic curve representing a change of the gain relative to a change of the wavelength in the optical amplifier.
On the other hand, the optical communication system executes pre-emphasis for regulating the optical level in each optical signal in the optical transmission station to equalize the optical SNR in the optical signals (channels) in the WDM optical signals received by the optical reception station. In the optical communication system equipped with OADM, however, pre-emphasis must be executed since the number of optical repeater stations where each optical signal is repeated, is different. Therefore, the applicant of the present invention proposed in Japanese Patent Application No. 2000-009387, that has not yet been laid open, a pre-emphasis method and an apparatus therefor in an optical communication system equipped with OADM.
When the WDM optical signals are arranged in a plurality of wavelength bands, optical power of the WDM optical signals arranged on the shorter wavelength side shifts to the WDM optical signals arranged on the lower wavelength side.
Incidentally, a peak power control amount, a slope compensation amount and a pre-emphasis amount are determined in consideration of common physical phenomena such as ASE, induced Raman scattering, 4-optical wave mixing, self phase modulation, mutual phase modulation, and so forth. Since these amounts depend on one another, the optical communication system which adjusts all the amounts cannot separately determine and adjust them.
It is therefore an object of the present invention to provide a method of activating an optical communication system which executes slope compensation, peak power control, and pre-emphasis in an optimal procedure.
It is another object of the present invention to provide a method of increasing/decreasing the number of channels in an optimal procedure when the channels of WDM optical signals is increased or decreased in number in such an optical communication system.
It is still another object of the present invention to provide a recording medium where a program of activating an optical communication system and increasing/decreasing a channel is recorded.