Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2001-12-26
2003-12-09
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S341430
Reexamination Certificate
active
06661570
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier applicable to wavelength division multiplexing (WDM) transmission systems, an optical communication system including the optical amplifier, and an optical fiber module constituting a part of the optical amplifier.
2. Related Background Art
In WDM transmission systems including optical amplifiers as optical communication systems, OADM (Optical Add/Drop Multiplexer) and OXC (Optical Cross Connect) have recently been incorporated as structures for separating a part of multiplexed signals by signal channel units or sending out a predetermined wavelength of signals or multiplexed signals to transmission lines by signal channel units in order to improve the reliability and operating efficiency of each system as a whole. In such a WDM system, attention is paid to transitional output signal power fluctuations in optical amplifiers occurring due to changes in the number of channels of inputted signals. Therefore, it is required for optical amplifiers to be controlled such that the output signal power (output signal level) per channel becomes constant even if the input power changes.
As means for suppressing the transitional output signal power fluctuations caused by input signal power fluctuations occurring due to changes in the number of signal channels, high-speed automatic gain control (AGC) has been proposed.
Here, the response speed of AGC is required to become faster as the speed at which the number of signal channels changes is faster, and as the number of the added or dropped signal increases.
As a technique for speeding up AGC, Seo Yeon Park, et al., “Dynamic Gain and Output Power Control in a Gain-Flattened Erbium-Doped Fiber Amplifier,” IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 10, No. 6, JUNE 1998, for example, proposes feed-forward control by detecting only the power of input signal (input signal level), and a pumping light power corresponding to the power of input signal is calculated by use of a linear expression, so as to maintain a constant gain. This AGC technique can achieve higher speed more easily as compared with conventional feedback control, whereby a response time of 650 ns is realized in actually developed control circuits.
SUMMARY OF THE INVENTION
The inventor has studied the prior art and, as a result, has found a problem as follows. Namely, even if feed-forward control is carried out in a conventional optical amplifier by detecting power fluctuations in input signals, the control starting time will be delayed due to a delay in a control circuit, whereby overshoot may occur by the amount of delay in the control circuit.
If the response time of the control circuit controlling a pumping light source is substantially zero, the power of pumping light outputted from the pumping light source can be regulated at substantially the same time when a power fluctuation in input signals is detected in the vicinity of an input end of an optical amplifier (the fluctuation is detected before amplification), whereby transient power fluctuations in output signals will hardly occur. However, actual control circuits have a response time to a certain extent. Therefore, even if a power fluctuation in input signals is detected in the vicinity of an input end of an optical amplifier (at the point of time indicated by arrow A in FIG.
1
), a transient power fluctuation (dynamic gain fluctuation PD) in output signals will occur in an optical amplifier for the duration of response time t, of the control circuit after the actual power fluctuation in input signals is generated as shown in
FIG. 1
(see A. K. Srivastava, et al., “EDFA Transient Response to Channel Loss in WDM Transmission System,” IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 3, MARCH 1997). Also, Y. Sun et al., “Fast power transients in WDM optical networks with cascaded EDFAs,” ELECTRONIC LETTERS, Feb. 13, 1997, Vol. 3, No. 4 indicate that transient gain fluctuations accumulate in a system in which optical amplifiers (EDFAs) are connected in a multistage fashion.
However, the change delay time of the pumping light power evaluated in the above-mentioned Srivastava reference with respect to the power fluctuation in input signals is 7 &mgr;s or more, whereby it is unknown whether the transient power fluctuation in output signals can fully be suppressed or not if the delay time further approaches zero. Also, it does not mention any difference in response of the optical amplifier depending on the wavelength of pumping light.
In order to overcome the above-mentioned problem, it is an object of the present invention to provide an optical amplifier comprising a structure which can effectively suppress transient output signal power fluctuations (gain fluctuations) occurring due to delays in control, an optical fiber module included in the same, and an optical communication system comprising a structure for effectively restraining the power fluctuations from accumulating.
The optical amplifier according to the present invention is an optical device, employed in a WDM transmission system for transmitting a plurality of channels of signals included in a predetermined signal wavelength band, for amplifying the signals propagating through an optical transmission line included in the WDM transmission system.
In order to overcome the above-mentioned problem, the optical amplifier according to the present invention comprises, at least, a branching device, a light-receiving device, an amplification optical device (amplification optical fiber), a pumping light source, a delay medium, and a control system. The branching device taps part of light including the signals propagating through the optical transmission line. The light-receiving device detects a power fluctuation (level fluctuation) of the part of light tapped by the branching device. The amplification optical fiber is an optical fiber doped with a rare-earth element or the like for amplifying the signals. The pumping light source launches pumping light of a predetermined wavelength into the amplification optical fiber. The delay medium is an optical device, arranged between the branching device and the amplification optical fiber, for delaying the light reaching the input end of the amplification optical fiber from the branching device. The control system monitors, by way of the light-receiving device, the power of the part of light tapped by the branching device, and controls the pumping light source.
Transient power fluctuations in output signals can fully be suppressed if the pumping light power can intentionally be changed at the same time when the fluctuation in input signal level occurring in the vicinity of the input end of the optical amplifier is detected (the amount of change in pumping light power being adjusted according to the amount of fluctuation in input signal power). However, it is unrealistic for the response time from the detection of fluctuation to the power control of pumping light to become zero. Therefore, the present invention utilizes the delay medium, so as to delay the signal input to the amplification optical fiber, thus making it possible to detect the input signal power before amplification.
Preferably, in the optical amplifier according to the present invention, the delay medium has an insertion loss of 3 dB or less. This is because of the fact that it can effectively restrain transient or dynamic power fluctuations from occurring due to changes in the number of signal channels while suppressing an excess degradation of noise figure.
Specifically, it is preferred that the signal delay time given by the delay medium be at least 1 &mgr;s but not longer than 50 &mgr;s. Preferably, the delay medium includes a single-mode optical fiber having a length of at least 0.2 km but not longer than 10 km. Since the transmission loss of a single-mode optical fiber is usually about 0.2 dB/km, it can minimize the degradation of noise figure in the optical amplifier, and can delay input signals to the amplification optical fiber by a time within the range of 1
Hellner Mark
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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