Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-05-03
2002-11-19
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200, C359S341300, C359S341410, C359S341440
Reexamination Certificate
active
06483630
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical amplifier used in a multiplexed optical transmission path, a method of controlling a multiplexed light output delivered out of the optical amplifier, an optical transmission system using the optical amplifier and a method of monitoring and controlling the optical transmission path for transmission of the multiplexed light output.
In recent years, with the demand for reducing the cost of an optical communication system, a so-called wavelength multiplexing optical transmission scheme has been studied in which two or more kinds of signal light beams of different wavelengths are multiplexed and transmitted through a single optical transmission fiber. Since the optical amplifier has a wide amplifying wavelength band and can afford to perform amplification at low noise, it is suitable for use as an amplifier in the wavelength multiplexing optical transmission. A rare earth added optical fiber constituting the optical amplifier and a semiconductor optical amplifier have each the gain which has wavelength dependency and therefore, there occurs a difference in light output or gain between wavelengths after amplification. Specifically, the inter-wavelength difference is accumulated in the course of multi-stage relay based on optical amplifiers and after the relay, the difference in optical power between wavelengths is extended. As a result, the maximum relay transmission distance of the whole system is limited by a degraded S/N ratio of a wavelength of the multiplexed wavelengths which has the lowest power. Accordingly, it is of importance to provide an optical amplifier which does not cause the light output difference between wavelengths.
Thus, for example, a scheme described as “Flattening of Multi-wavelength Amplifying Characteristics in Optical Fiber Amplifier Using Fiber Mu-factor Control” in The Institute of Electronics and Information and Communication Engineers of Japan, Technical Report OCS94-66, OPE94-88 (1994-11) has been known as a conventional scheme.
The conventional scheme uses a fiber gain controller (AFGC) for monitoring the total output of four signal light beams subject to wavelength multiplexing and controlling the fiber gain such that the output level becomes constant. Through this, the inter-input wavelength difference is made to be 0 dB and the fiber gain is controlled to a constant value of 12 dB so as to minimize the difference between wavelengths. Further, by using an auto-power controller (APC) based on an optical attenuator 58, light loss is adjusted while keeping the fiber gain constant at 12 dB to make the fiber gain spectrum unchanged even when the relay mu-factor is changed.
Typically, in a practical system of wavelength multiplexing transmission, transmitting signal information pieces represented by wavelengths are often independent of each other. In this case, only necessary signals are transmitted and there is a possibility that unnecessary signals are stopped, that is, placed in standby condition.
However, in the conventional scheme in which the total output of multiplexed four signal light beams is monitored and controlled, when the number of multiplexed signal wavelengths is changed, the total output remains unchanged but outputs of signal light beams of individual wavelengths change greatly. With the outputs of signal light beams greatly changed in this manner, there arise problems that signal transmission is adversely affected and that when the outputs exceed a signal transmission distance limit, the signal transmission becomes impossible.
Further, in the conventional scheme in which the total light output is controlled, when the output of one signal light beam decreases, this decrease adversely affects the signal transmission.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical amplifier which can control individual wavelength outputs without greatly affecting signal transmission even when the number of signal wavelengths subject to multiplexing changes and an optical transmission system using the optical amplifier.
Another object of the present invention is to provide a method of monitoring a probe light beam included in a multiplexed light ray propagated through a transmission path in the optical transmission system and controlling an output of the probe light beam and outputs of the multiplexed light beams from the optical amplifier in accordance with a result of monitoring.
Another object of the present invention is to provide an optical transmission path monitoring and controlling method which can decide whether or not an optical transmission path trunk line, a probe light beam per se, multiplexed light beams per se, an optical amplifier and a transmitter are normal by monitoring the probe light beam and other multiplexed light beams included in the multiplexed light ray propagated through a transmission path in an optical transmission system and detecting what values the probe light beam and the other multiplexed light beams take in relation to predetermined values, respectively.
To accomplish the above objects, the present invention comprises an optical amplifying medium to which input multiplexed light beams are guided, an exciting source for exciting the optical amplifying medium, light branching means for branching part of the multiplexed light beams amplified by the optical amplifying medium, light separating means for separating a probe light beam from multiplexed light beams branched by the light branching means, light receiving means for detecting an output of the probe light beam separated by the light separating means and converting the detected output into an electric signal, and control means for controlling an amount of operation of the exciting source on the optical medium such that an output of the light receiving means becomes constant, and with the construction as above, the output of the probe light beam can be rendered to be constant, the outputs of the signal light beams can be rendered to be constant and even when the number of signal wavelengths subject to multiplexing changes at that time, the individual wavelengths can be controlled without greatly affecting the signal transmission.
Preferably, the above optical amplifier further comprises second light receiving means for detecting outputs of multiplexed light beams branched by the light branching means, second light branching means arranged to precede the optical amplifying medium and operative to branch part of the multiplexed light beams guided to the optical amplifying medium, second light separating means for separating the probe light beam from the multiplexed light beams branched by the second branching means, third and fourth light receiving means for detecting the output of the probe light beam branched by the second light separating means and the outputs of the multiplexed light beams branched by the second branching means, respectively, and input detecting means for delivering a command to the control means when the output of the probe light beam detected by the third light receiving means is lower than a predetermined value and the outputs of the multiplexed light beams detected by the fourth light receiving means are higher than a predetermined value, and the control means controls the amount of operation of the exciting source on the optical medium such that the output of the second light receiving unit becomes constant, so that with the above construction, event when the probe light beam becomes abnormal, all of the multiplexed light beams can be controlled.
Preferably, the optical amplifier further comprises alarm means for informing abnormality, the input detecting means delivers a signal to the alarm means when the output of the probe light beam and the outputs of the multiplexed light beams are lower than the predetermined values, respectively, so as to actuate the alarm means, and with the above construction, even when abnormality occurs, the abnormality can be informed easily.
Preferably, in the optical amplifier, the optical amplifying mediu
Antonelli Terry Stout & Kraus LLP
Moskowitz Nelson
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