Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-01-08
2001-11-06
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06313934
ABSTRACT:
BACKGROUND OF THE INVENTION
Chromatic dispersion within optical systems causes wavelength-dependent group delay variations that limit data rates, increase bit error rates and otherwise corrupt transmission of optical signals. Measuring chromatic dispersion of an optical system enables performance limitations of the optical system to be predicted. Additionally, once the chromatic dispersion is characterized it can be compensated for to improve system performance.
Typically, chromatic dispersion measurements are performed by characterizing the group delay of optical test signals at various optical wavelengths measured relative to a reference signal. Chromatic dispersion is readily measured when the measurement access points of the optical system are available to a single chromatic dispersion measurement instrument. However, optical systems such as installed fiber networks and other types of communication systems have access points that are physically remote from each other, making chromatic dispersion measurements using a single measurement instrument unfeasible. One approach to chromatic dispersion measurements of such optical systems transmits the reference signal from a local access point to a remote access point of the system using a separate optical fiber. Temperature variations, mechanical stress and other environmental effects on the fiber induce variations in the reference signal that reduce the accuracy and repeatability of the chromatic dispersion measurements. An alternate approach multiplexes the reference signal and the optical test signals within the optical system being characterized. While the multiplexing substantially reduces the impact of environmental effects on the chromatic dispersion measurements, this approach involves additional modulators and detection elements which increase the cost and complexity of the measurements. Accordingly, there is a need for a scheme for measuring chromatic dispersion of optical systems having remote access points, that is independent of environment effects and that has low cost and complexity.
SUMMARY OF THE INVENTION
A chromatic dispersion measurement scheme constructed according to the preferred embodiment of the present invention has low cost and complexity and enables chromatic dispersion of optical systems having physically separated access points to be characterized independent of environmental effects. The scheme generates a modulation signal at a local access point of the optical system and generates a reference signal at a remote access point of the optical system. The modulation signal and the reference signal are synchronized via timing signals derived from global positioning satellites (GPS). The modulation signal modulates optical test signals having predetermined optical wavelengths and the modulated optical test signals are applied to the local access point of the optical system. Modulated optical test signals are transmitted through the optical system from the local access point to the remote access point where the signals are demodulated. The time delay of the demodulated signal relative to the reference signal is measured at the remote access point using an oscilloscope or other measurement instrument. Alternatively, the relative time delay is determined by a phase comparison of the demodulated signal and the reference signal that produces a phase difference signal. The phase difference signal at each optical wavelength corresponds to the relative delay of the optical test signal through the optical system at the optical wavelength. The relative delay through the optical system as a function of optical wavelength provides a measure of chromatic dispersion of the optical system between the local and remote access points.
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Agilent Technologie,s Inc.
Chan Jason
Imperato John L.
Payne David C
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