Optical communications – Diagnostic testing – Determination of communication parameter
Reexamination Certificate
2000-05-02
2003-07-29
Pascal, Leslie (Department: 2633)
Optical communications
Diagnostic testing
Determination of communication parameter
C398S034000, C398S037000, C398S030000
Reexamination Certificate
active
06599039
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a monitoring apparatus for monitoring the transmission state in an optical transmission path through which light containing one or more light signals having different wavelengths propagates, a monitoring method, an optical amplification system including the monitoring apparatus, a method of controlling the optical amplification system, and an optical transmission system.
2. Related Background Art
An optical transmission system using a WDM (Wavelength Division Multiplexing) communication scheme includes an optical fiber transmission network through which WDM signals containing one or more light signals having different wavelengths propagates, and can perform large-capacity, high-speed communication. This optical transmission system includes an optical amplifier for amplifying WDM signals altogether, an optical ADM (Add-Drop Multiplexer) for dropping and adding some light signals contained in the WDM signals, and the like as well as an optical fiber transmission path as a transmission medium for light signals.
In the optical transmission system having the above structure, monitoring control of the optical amplifier is one of the important subjects. More specifically, the optical power of each amplified light (light signals) optically amplified by the optical amplifier is required to be controlled to a constant level in either of the following cases: a case wherein the number of light signals sent from the transmitter varies, a case wherein the number of light signals propagating a long an optical transmission path varies due to dropping/adding of light signals by the optical ADMs placed midway along the optical transmission path, and a case wherein a transmission loss in the optical transmission path or the like varies. In order to solve this problem, various proposals have been made.
For example, in “Control of Optical Output Level for WDM Optical Fiber Amplifier” (first prior art) proposed in the 1996 IEICE communication society conference B-1096, an acoustooptic filter whose passing wavelength characteristics change in accordance with a change in the frequency of ultrasonic waves is used as an optical transmission wavelength selection element. The acoustooptic filter is controlled by a sweep circuit, sweeps a band of 1,545 nm to 1,557 nm in a cycle of 400 &mgr;s, and converts WDM signals into a pulse string on the time axis. This pulse string is photoelectrically converted. A wave counter then detects the number of light signals contained in the WDM signals. Automatic level control (ALC) of the optical amplifier is performed on the basis of the information about the detected number of light signals. Note that the above acoustooptic filter is disclosed in “Wide Tunable Range and Low Sidelobe Level of Double-stage Polarization Independent Acousto-optic Tunable Filter” proposed in the 1996 IEICE general conference C-254.
According to the method proposed in “Characteristics of EDFA with Automatic Level Control for change of number of the wavelengths and input Signal level” proposed in the 1996 IEICE communication society conference B-1092 (second prior art), pilot light in the amplification band of an optical amplifier is dropped from an optical transmission path by a branching element placed on the output side of the optical amplifier, the optical power of the dropped pilot light is detected, and the optical amplifier is controlled to keep the optical power of this pilot light constant. As a technique similar to the second prior art, for example, the technique disclosed in Seo Yeon Park and Sang-Yung Shin, “Gain and power controlled EDFA and WDM optical networks”, 2nd Optoelectronics & Communications Conference (OECC'97) Technical Digest, July 1997, Seoul KOREA is known.
In addition, according to Takashi Ono, “Fiber grating wavelength monitor for optical amplifier control and administration in WDM transmission systems”, First Optoelectronics and Communications Conference (OECC'96) Technical Digest, July 1996, Makuhari Messe 17B3-2 (third prior art), part of WDM signals (some light signals) is dropped by a branching element connected to the output terminal of an optical amplifier and output as short pulses by an acoustooptic switch. Delays corresponding to the wavelengths of the respective light signals of the short pulses are given to the respective light signals through an optical circulator and optical fiber grating. The light signals having different wavelengths, to which the delays are given in this manner, are converted into pulses arranged along the time axis. The number of light signals contained in the WDM signals, operating wavelength band, the powers of the respective light signals are obtained from the number, positions, and optical powers of these pulses. In the third prior art, automatic level control (ALC) of the optical amplifier is performed on the basis of the obtained number of light signals contained in the WDM signals.
SUMMARY OF THE INVENTION
The present inventors found the following problems upon examination of the first and third prior arts. The first to third prior arts use special optical elements such as acoustooptic filters, acoustooptic switches, and optical circulators, and the second prior art uses pilot light. For this reason, these conventional systems need to have complicated arrangements, and become expensive systems.
In addition, since the first and third prior arts use acoustooptic filters and acoustooptic switches, variations in the power of a light signal of WDM signals which have specific wavelengths cannot be detected, although the number of light signals contained in the WDM signals can be detected. The second prior art using pilot light can neither detect the number of light signals contained in WDM signals nor detect variations in the power of light signals of the WDM signals which has a specific wavelength. The first to third prior arts cannot therefore discriminate whether a cause of a variation in the optical power of WDM signals is a variation in the optical power of a light signal having a specific wavelength or a loss variation in an optical transmission path. This problem is serious especially in an optical transmission system using an optical ADM.
The present invention has been made to solve the above problems, and has as its object to provide an optical transmission monitoring apparatus having a simple structure capable of identifying a cause of variations in the optical power of WDM signals, a monitoring method, an optical amplification system including the optical transmission monitoring apparatus, a method of controlling the optical amplification system, and an optical transmission system.
An optical transmission monitoring apparatus according to the present invention is an apparatus for monitoring a transmission state in an optical transmission path through which light belongs in a signal wavelength band and containing one or more light signals having different wavelengths propagates. This apparatus uses light in the signal wavelength band or light in a wavelength band different from the signal wavelength band as monitoring light signals.
More specifically, the optical transmission monitoring apparatus according to the present invention comprises first and second photodetectors and a monitoring section for monitoring a transmission state in an optical transmission path by using the detection results obtained by the first and second photodetectors.
The first photodetector detects at least one of the optical power of one or more monitoring light signals of light in the first wavelength band, which is included in a monitor wavelength band in which light propagating through an optical transmission path and containing one or more monitoring light signals having different wavelengths belongs, and the optical power of noise of the light in the first wavelength band. The second photodetector detects at least one of the optical power of one or more monitoring light signals of light in the second wavelength band included in a monitor wav
Li Shi K.
McDermott & Will & Emery
Pascal Leslie
Sumitomo Electric Industries Ltd.
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