Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-07-28
2004-04-20
Hellner, Mark (Department: 2633)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337120
Reexamination Certificate
active
06724526
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inter-wavelength light power deviation monitoring method for monitoring, by a simple procedure with good precision, the wavelength characteristics of light power of wavelength-division multiplexed (WDM) signal light utilized in various optical communications, and to an optical equalizer and an optical amplifier for controlling the equalizing treatment for the wavelength characteristics of light power of WDM signal light by such a monitoring method.
2. Related Art
In optical communications, research and development have been vigorously done on a wavelength-division multiplexing (WDM) transmission system to increase a communication capacity by transmitting light having a plurality of wavelengths through a single optical fiber. Such a WDM transmission system is expected to become the means for realizing a future multi-media based world, because of such advantages that the system can utilize an existing optical fiber, leading to a lower introduction cost, and that the transmission path of the system can be used in a bit-rate free manner by adopting an optical amplifier having a wider amplification band, leading to easiness of future upgrade.
Generally, in a WDM optical communication system adopting optical amplifiers, in order to obtain predetermined transmission characteristics, it is required to transmit light by suppressing the deviation (tilt) of light power between channels to a few dB or less at each amplification repeating stage. This is because the upper limit of transmittable light power is restricted by the waveform degradation due to a nonlinear effect of an optical transmission path, and the lower limit is restricted by the degradation of a received S/N ratio.
It is known that wavelength characteristics are caused in the loss of an optical fiber transmission path such as due to stimulated Raman scattering or Rayleigh scattering as one of nonlinear effects. Particularly, the loss wavelength characteristics due to stimulated Raman scattering are caused in different magnitudes depending on various transmission conditions such as a length of an optical transmission path, the number of channels (i.e., the number of wavelengths of signal light), channel intervals (wavelength intervals of signal light), and a light power level. Typically, such as the number of channels and the wavelengths to be used in a WDM optical transmission system can be suitably set by a system user, so that the values of them are not always fixed. For example, in a lightwave network, such as the number of signal light and wavelength positions to be input into an optical ADM (Optical Add and Drop Multiplexer) device change dynamically, so that loss wavelength characteristics having different magnitudes are caused in optical transmission paths, leading to a serious problem of affection on transmission characteristics.
With respect further to optical amplifiers and dispersion compensators constituting various optical transmission systems, it is known that wavelength characteristics are included in gains and losses, and it is a problem that such wavelength characteristics in gains and losses cause light power deviations among respective channels, largely affecting on the transmission characteristics.
Thus, it is required to take a countermeasure to monitor light power deviations among respective channels caused in a WDM optical communication system, to thereby reduce such deviations. To this end, the present inventors have proposed such a technique to apply an active gain equalizer to a WDM optical communication system so as to control the characteristics of the gain equalizer such that monitored results of the light power deviations among respective channels are reduced (see Japanese Patent Application Nos. 11-54374 and 11-115971).
Conventional monitoring methods for monitoring the light power deviations among respective channels such as applied to the aforementioned patent applications include a method for calculating the light power deviations among channels based on amplified spontaneous emission (ASE) light generated by optical amplifiers (see Japanese Patent Application No. 11-054374). According to such a method, the spontaneous emission light power near signal light bands are monitored so that the dispersions of light power among respective channels can be monitored independently of the number of input signals or the variation of channels. Further, there is also known a method to directly measure the light power of respective channels such as by a general optical spectrum analyzer to thereby calculate light power deviations among respective channels.
However, those monitoring methods utilizing spontaneous emission light among the conventional methods for monitoring the inter-wavelength light power deviations are ones to indirectly monitor spontaneous emission light rather than directly monitoring signal light itself, leading to the possibility of monitoring error. Also, in those monitoring methods for directly measuring the respective signal light power by using such as an optical spectrum analyzer, since the respective signal light power should be accurately measured even when signal light are input into signal light bands in the densest manner, the measurement of signal light power should be conducted by using such as the high grade optical spectrum analyzer having a superior wavelength resolution, problematically resulting in a large-sized apparatus with higher cost in an apparatus adopting such a monitoring method.
SUMMARY OF THE INVENTION
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a monitoring method capable of directly measuring the signal light power to thereby detect with high precision the light power deviations among channels without requiring a higher wavelength resolution, and to provide an optical equalizer and an optical amplifier adopting such a monitoring method to thereby attain downsizing and cost reduction.
To achieve the above object, the present invention provides a method for monitoring inter-wavelength light power deviations, comprising the steps of: obtaining channel information concerning WDM signal light including a plurality of wavelength-division multiplexed channel light having wavelengths different from one another, setting, based on the channel information, a plurality of measuring wavelength regions each including at least one distinctive channel light; measuring channel light power of the WDM signal light, for only the respective measuring wavelength regions, and obtaining light power ratio for the respective measuring wavelength regions making use of measured values of said light power, to thereby judge the inter-wavelength light power deviations of the WDM signal light.
According to such a monitoring method: there are set at least two measuring wavelength regions suitable for light power measurement based on the channel information concerning such as the wavelength positions and transmission rates of respective channel light included in the WDM signal light, channel light power for only the respective measuring wavelength regions are measured, and the inter-wavelength light power deviations are judged from the light power ratio for the respective measuring wavelength regions, based on the measured results. In this way, it becomes possible to monitor the wavelength characteristics of WDM signal light power with higher precision while directly measuring the signal light power at a relatively low resolution of wavelength.
Concerning the plurality of measuring wavelength regions each including at least one distinctive channel light, it is possible to arrange such that each of the measuring wavelength regions includes a singular distinctive channel light, and these distinctive channel light are different from one another, or, each of the measuring wavelength regions includes a plurality of channel light, and at least one of the plurality of channel light is di
Kinoshita Susumu
Onaka Miki
Fujitsu Limited
Hellner Mark
Staas & Halsey , LLP
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