OTDR Fiber Characterization via Power Dither SRS Compensation
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Solution Overview
Problem
In dense wavelength division multiplexing (DWDM) optical links, Stimulated Raman Scattering (SRS) causes power transfer from optical pulses injected by OTDRs to WDM optical signals, leading to incorrect characterization of optical fibers.
Innovation Solution
Applying a power dither to data-bearing optical signals with high and low power levels, computing and averaging OTDR traces, and adjusting them based on differential SRS gain to remove its effect, while modulating the pump current of optical amplifiers and synchronizing OTDR measurements with the power dither.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If optical pulses are injected by OTDR to characterize optical fiber, then fiber length and attenuation can be estimated, but Stimulated Raman Scattering transfers power from optical pulses to WDM signals, leading to incorrect characterization
Solution Approach 1:
The patent applies periodic power dither modulation to the optical amplifier pump current at a specific frequency range (0.1-1000 Hz). This periodic modulation creates corresponding periodic variations in the SRS effect, allowing the OTDR to distinguish SRS-induced power transfers from actual fiber characteristics by analyzing the frequency-domain response. The periodic action enables separation of the harmful SRS effect from the measurement signal.
Solution Approach 2:
The patent changes the operating parameters of the optical amplifier by modulating the pump current with power dither at controlled amplitude and frequency. This parameter change induces corresponding changes in the SRS gain, which are then detected by the OTDR. By analyzing how the SRS gain varies with pump power, the system can compute and remove the SRS contribution from the OTDR traces, improving measurement accuracy.
2Measurement precision
If power dither is applied to data bearing optical signals, then SRS-induced distortion can be compensated, but the power dither must be removed from the signals after characterization
Solution Approach 1:
The patent extracts and removes the power dither component from the optical signals after the characterization measurement is complete. The system detects the power dither at the amplifier output and applies an opposite phase dither (180 degrees out of phase) to cancel the original dither signal. This extraction and removal process eliminates the artificial modulation from the data signals while preserving the measurement data that was obtained during the dithered state.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method accurately characterizes optical fibers by compensating for SRS-induced distortion, providing precise loss measurements and improving the accuracy of fiber characterization in DWDM optical links.
Implementation Method 1
Due to a common nonlinear phenomenon in the WDM-based optical communication networks known as Stimulated Raman Scattering (SRS), at least some power from the optical pulses is transferred to WDM optical signals
Implementation Method 2
The OTDR injects a series of optical pulses into the optical fiber under test and detects light that is Rayleigh scattered and reflected from different locations in the fiber
Data Source
AI summary
The disclosed systems and methods for characterizing an optical fiber in a dense wavelength division multiplexing (DWDM) optical link. The characterizing comprising: i) applying a power dither to data bearing optical signals propagating in the optical fiber, the power dither having a high-power level and a low-power level; ii) computing optical time-domain reflectometer (OTDR) traces corresponding to the high-power level and the low-power level of the power dither; iii) averaging the OTDR traces corresponding to the high-power level and the OTDR traces corresponding to the low-power level into average OTDR traces; computing a differential Stimulated Raman Scattering (SRS) gain from the OTDR traces; and iv) adjusting the average OTDR traces based on the differential SRS gain.