Monitoring of optical transmission systems based on cross-correlation operation
AssigneeKorea Advanced Institute of Science and Technology
InventorsYun Chur Chung, Yuichi Takushima
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Abstract
In optical communication systems, an optical signal which is modulated with a data signal is transmitted from an optical transmitter, and is launched into a fiber-optic transmission link. The present invention provides a method and an apparatus for the measurement of the distribution of the reflectivity along an optical transmission line implemented by using the signal processing based on the cross-correlation function between the data signal with which the optical transmitter is modulated and the back-reflected signal returned to the optical transmitter to provide the in-service monitoring of the fiber-optic transmission link.
Claims13
A method for measuring a distribution of reflectivity along a fiber-optic transmission link connected to an optical transmitter, the method comprising the steps of:
- (a-1) filtering a data signal transmitted by the optical transmitter by using a low-pass filter;
- (a-2) detecting a back-reflected optical signal that returns to the optical transmitter and converting it to an electrical signal;
- (a-3) filtering the detected back-reflected optical signal by using a low-pass filter;
- (a-4) calculating a cross-correlation function, q(τ), between the filtered data signal and the filtered back-reflected optical signal, wherein τ is time and the cross-correlation function q(τ) is obtained by ensemble average of the filtered data signal and the filtered back-reflected optical signal;
- and
- (a-5) converting time axis of the calculated cross-correlation function q(τ) from time τ to distance z from the optical transmitter and obtaining the distribution of the reflectivity as R(z)=k·q(2z/νc), wherein νc is the group velocity of light in the fiber and the coefficient k is obtained based on an average input power to the fiber-optic transmission link and a conversion efficiency of an optical detection means.
An apparatus for monitoring a fault in a fiber-optic transmission link connected to an optical transmitter which is modulated with a data signal by measuring a distribution of reflectivity along the link, the apparatus comprising:
- optical directional coupling means for taking out a back-reflected signal returned from the fiber-optic transmission link to the optical transmitter, located between the optical transmitter and the fiber-optic transmission link;
- optical detection means for detecting the back-reflected signal as an electronic signal;
- a first low pass filter for extracting a low frequency component of the back-reflected signal with a cut-off frequency lower than the data signal;
- a second low pass filter for extracting the low frequency component of the data signal with a cut-off frequency lower than the data signal;
- data acquisition means for converting the filtered back-reflected signal and the filtered data signal;
- and
- signal processing means for calculating a cross-correlation function, q(τ), between the filtered data signal and the filtered back-reflected signal, wherein τ is time and the cross-correlation function q(τ) is obtained by ensemble average of the filtered data signal and the filtered back-reflected signal, and converting time axis of the calculated cross-correlation function q(τ) from the time τ to distance z from the optical transmitter and obtaining the distribution of the reflectivity as R(z)=k·q(2z/νc), wherein νc is the group velocity of light in the fiber and the coefficient k is obtained based on an average input power to the fiber-optic transmission link and a conversion efficiency of the optical detection means.
Dependencies are inferred from the claim text.