Per-span NLI Measurement via VOA Dithering in Optical Links
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Solution Overview
Problem
Existing methods for in-service characterization of nonlinear interference (NLI) in optical fiber communication systems are limited, as they require commissioning and suffer from errors due to mis-provisioned fiber properties and complex modeling, making it challenging to optimize launch power and predict performance accurately.
Innovation Solution
The use of variable optical attenuators (VOAs) before and after each span in an optical link to isolate and measure NLI on a per-span basis, allowing for accurate estimation of noise performance and optimization of launch power through phase sensitive detection and dithering techniques, enabling in-service characterization and network-level optimization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If per-span NLI measurement is implemented at commissioning using the approach in U.S. Pat. No. 11,139,633, then per-span fiber nonlinear parameter measurement and modeling is enabled, but the measurement is not available once the optical link is in-service
Solution Approach 1:
The system dynamically adjusts VOA settings during in-service operation to enable per-span NLI measurements without disrupting traffic. The VOAs are dithered (small periodic adjustments) to modulate the signal power into each span, allowing real-time measurement of nonlinear interference while the link remains operational
Solution Approach 2:
The system changes operational parameters (VOA settings and amplifier gains) to isolate and measure NLI from individual spans. By adjusting these parameters and observing the resulting changes in total noise at the receiver, the system can extract per-span NLI contributions while maintaining service
2Ease of operation
If existing power control approaches use modeling with provisioned or measured parameters, then optimal launch power can be obtained, but errors from multiple sources reduce accuracy
Solution Approach 1:
The system implements feedback control by measuring actual per-span NLI contributions and using this information to adjust launch powers. The measured NLI data feeds back into the power optimization process, allowing the system to converge on optimal launch powers based on actual system performance rather than relying solely on theoretical models
Solution Approach 2:
The system uses its own measured performance data to optimize its operation. By measuring NLI from each span and using this information to adjust launch powers, the system performs self-optimization without requiring external intervention or complex external modeling
3Ease of operation
If modeling of system performance and optimal launch power is performed, then launch power optimization is achieved, but significant computing power is required
Solution Approach 1:
The system replaces complex computational modeling with direct physical measurement. Instead of using sophisticated software models to predict NLI and optimize power, the system directly measures NLI from each span using VOAs and receivers, then uses this measured data to guide power adjustments, significantly reducing computing requirements
4Measurement precision
If VOAs are dithered to determine linear and nonlinear noise contributions, then per-span NLI measurement is enabled, but the measurement process must not disrupt in-service channels
Solution Approach 1:
The system applies small partial dithering to the VOAs rather than large adjustments. These small periodic variations in VOA settings are sufficient to modulate the signal for measurement purposes while remaining below the threshold that would disrupt in-service channel performance or trigger error conditions
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 approach allows for accurate measurement of per-span NLI without disrupting in-service channels, optimizing operating conditions, validating noise modeling, and confirming restoration paths, thereby enhancing the capacity and reliability of optical communication systems.
Implementation Method 1
An enabler is to have a pair of variable optical attenuators (VOAs) before and optionally after the line fiber of each span, where the first VOA before the fiber is for changing power into the span under test
Implementation Method 2
observing total noise, at an optical receiver, from all of the plurality of spans
Data Source
AI summary
Systems and methods of optimizing launch power for each span in an optical system with a plurality of spans are provided. In an embodiment, a method includes varying power on a span under test of the plurality of spans; observing performance measurements related to of one or more channels, at corresponding optical receivers; and one of setting launch power for the span under test and repeating the varying and observing, responsive to the observed measurements.