Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-12-05
2001-05-29
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06239889
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to a method for monitoring the performance of optical WDM systems and in particular to an optical signal power detection method using a signature bit pattern.
2. Background Art
In optical transmission systems, various parameters such as power are measured to obtain information on the operating conditions of the transmission link. The fiber optic cables are tested by measuring the power losses associated with the light transmission, and thus, determine potential transmission errors.
In a WDM system, because different wavelengths of light have different attenuation characteristics, it is important to determine the attenuation of the fiber optic cable with respect to a specific wavelength of light used for a particular transmission channel. It is important to detect accurately the optical power of individual optical signals for many reasons, such as improved control of optical amplifiers, signal tracking at the optical layer, monitoring the accumulation of optical noise in a link with cascaded amplifiers, etc.
It is known to monitor the input and output of an optical amplifier in order to control the gain. To this end, fractions of the input and output signals are coupled out by taps (couplers) and detected by photodiodes. The electrical signals, recovered after this detection, are then used by the power monitor as needed. Since the power of the coupled out signal is very low in the case of digital systems, the power monitors required to detect and process this low signal are rather complex.
To date, the only other method for detecting the optical power of signals without using expensive optical filters is to amplitude modulate the optical signal to a controlled modulation depth with a signal (dither) unique to the respective transmission system. This method, disclosed in U.S. Pat. No. 5,513,029 by Kim Roberts, issued on Apr. 30, 1996 and assigned to Northern Telecom Limited, requires additional optical components, such as an external modulator and optical attenuators, and also requires electronics and real time control software at the transmitter, to both apply the amplitude modulation and to detect it, in order to accurately control the modulation depth.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for detecting the optical power of a digital optical signal in the presence of other optical signals at different wavelengths, without using expensive fixed or tracking optical filters.
Accordingly, there is provided a method for measuring the power of an optical signal (s
1
) travelling on a first channel (&lgr;
1
) of a WDM transmission system, comprising the steps of, generating a signature bit pattern (s
BP1
), adjusting the power level of the signature bit pattern to a predetermined ratio (m) with the power of the optical signal, inserting the signature bit pattern (s
BP1
) into the frame of the optical signal (s
1
) and transmitting same along a span of transmission medium, measuring the power of the signature bit pattern (s
BP1
) at a point of interest on the span, and determining the optical power of the optical signal (s
1
) in the point of interest.
The advantage of this invention is that it provides a simple method for determining the optical power in a WDM system, whereby no additional electronics, expensive optical components and real time software are required at the transmitter side, resulting in considerable savings in circuit pack layout space, cost and development time.
REFERENCES:
patent: 5513029 (1996-04-01), Roberts
patent: 5654816 (1997-08-01), Fishman
patent: 5699081 (1997-12-01), Denkin et al.
patent: 6043915 (2000-03-01), Giles et al.
patent: 6111676 (2000-08-01), Lemus
patent: 54-105404 (1979-08-01), None
patent: 62-139150 (1987-08-01), None
patent: 63-85934 (1988-06-01), None
patent: 63-203027 (1988-08-01), None
patent: 4-66835 (1992-06-01), None
patent: 9224016 (1997-08-01), None
Habel Richard A.
Harley James
Bello Agustin
Chan Jason
Diaconescu Aprilia U.
Nortel Networks Limited
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