OSNR Measurement for Polarization-Multiplexed Signals
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Solution Overview
Problem
Current methods for measuring the noise level or OSNR of polarization-multiplexed optical signals in optical communication links are limited, especially when signals are subjected to non-linear effects, as they require service interruptions and make assumptions about noise variations, which are not valid for live traffic systems.
Innovation Solution
A method to determine quality parameters like ASE-only OSNR and signal-deformation OSNR by analyzing the power spectral density of the signal, using a reference optical spectrum trace to estimate spectral deformations and discriminate between signal and noise contributions, allowing non-intrusive measurement without service interruptions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional in-band OSNR measurement methods (polarization-nulling or differential polarization response) are used, then OSNR measurement is possible for highly polarized signals, but these methods fail for polarization-multiplexed signals which appear unpolarized
Solution Approach 1:
The patent uses a reference optical spectrum trace (copied from a known good signal or theoretical model) to represent the expected signal spectral shape. By comparing the actual signal spectrum against this reference copy, the method can identify and separate signal contributions from noise contributions even in polarization-multiplexed signals that appear unpolarized, thereby enabling accurate OSNR measurement where conventional methods fail
Solution Approach 2:
The patent changes the measurement approach from polarization-based parameters to spectral shape parameters. Instead of relying on polarization state (which is unpolarized in polarization-multiplexed signals), the method analyzes spectral density distribution and compares it against a reference spectrum, transforming the measurement basis to one that remains valid for polarization-multiplexed signals
2Measurement precision
If signal turn-off method is used to measure noise level, then OSNR can be determined in multi-channel WDM networks, but service interruption occurs which limits use to start of life characterization
Solution Approach 1:
The patent enables continuous OSNR measurement by analyzing the optical spectrum trace of the signal while it is actively transmitting. The method processes the spectral density information to separate signal and noise contributions mathematically, eliminating the need to turn off the signal and thereby maintaining continuous service availability while performing measurements
Solution Approach 2:
The patent replaces the mechanical approach of physically turning off the signal with a mathematical processing approach. By using spectral analysis and comparison with reference traces, the noise level is determined through computational methods rather than physical signal interruption, maintaining system continuity
3Productivity
If reference signal method is used to measure noise level, then non-intrusive measurement is possible, but spectral deformations from non-linear effects cause measurement errors
Solution Approach 1:
The patent creates an updated reference spectral shape that accounts for expected non-linear effects. By comparing the actual signal spectrum against this adjusted reference (which incorporates knowledge of typical spectral deformations from non-linear optical effects), the method can accurately separate signal and noise contributions even when non-linear effects are present, maintaining measurement accuracy in live traffic conditions
Data Source
AI summary
There is provided a method and an apparatus for determining quality parameters on a polarization-multiplexed optical signal based on an analysis of the power spectral density of the Signal-Under-Test (SUT). The method is predicated upon knowledge of the spectral shape of the signal in the absence of significant noise or spectral deformation. This knowledge is provided by a reference optical spectrum trace. Based on this knowledge and under the assumption that ASE noise level is approximately constant in wavelength over a given spectral range, the spectral deformation of the signal contribution of the SUT may be estimated using a comparison of the spectral variations of the optical spectrum trace of the SUT with that of the reference optical spectrum trace.