Optical Coherent Transponder Bias Error Correction
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Solution Overview
Problem
Existing methods for measuring and correcting skew and bias errors in optical coherent transponders are inaccurate and costly, especially for complex modulation formats like 16-QAM, and are affected by noise and instability, making it difficult to achieve optimal performance.
Innovation Solution
A method involving a de-skewing filter that introduces varying amounts of skew to measure optical power of positive and negative harmonics, detecting bias errors by analyzing non-orthogonality in power curves, and adjusting the bias voltage to correct these errors, using less expensive equipment like optical power meters and filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If expensive Optical Modulation Analyzers (OMAs) are used for skew measurement, then measurement accuracy is improved, but device cost increases
Solution Approach 1:
The patent replaces expensive, complex Optical Modulation Analyzers with simple, low-cost optical power meters and filters. The measurement system uses disposable or easily replaceable optical components (filters, power meters) instead of costly proprietary equipment, achieving the same measurement function at a fraction of the cost while maintaining sufficient accuracy for complex modulation formats like 16-QAM
Solution Approach 2:
The patent substitutes the complex electronic measurement system (OMA) with a simpler optical measurement approach using power meters and spectral filters. This replaces an electronically complex system with an optically-based system that is both cheaper and sufficiently accurate for the intended application of measuring I/Q skew in coherent optical modulators
2Adaptability or versatility
If existing skew measurement methods are used for complex modulation formats, then measurement capability is maintained, but measurement accuracy deteriorates
Solution Approach 1:
The patent changes the measurement parameters by using optical power measurements at specific wavelengths (corresponding to upper and lower sidebands) instead of traditional electrical field measurements. By measuring power at these specific spectral points and analyzing the difference, the system achieves accurate skew measurement for complex modulation formats like 16-QAM where traditional methods fail, without requiring the modulator to be operated at specific test points
3Ease of operation
If I/Q non-orthogonality is measured using traditional methods, then measurement capability is provided, but measurement accuracy deteriorates due to noise and instability
Solution Approach 1:
The patent extracts the essential measurement information by isolating specific spectral components (upper and lower sidebands) using optical filters, separating the measurement signal from noise and instability effects. By measuring only the power difference at these specific wavelengths rather than attempting to measure the complete I/Q signal, the system achieves high accuracy while being resistant to noise and environmental instabilities that plague traditional measurement methods
Data Source
AI summary
Systems and methods for detecting and correcting bias errors in optical coherent transponders are disclosed. An “outer” modulator in a transponder may, when properly biased, produce a phase offset of π/2 radians between in-phase and quadrature components of the optical signals transmitted in optical modulation formats by the transponder. The method may include providing input to a transponder to produce a periodic (and generally sinusoidal) output signal, measuring (using an optical power meter) the optical power of positive and negative harmonics of the signal while varying the amount of skew introduced by a de-skewing filter in the transponder, and determining that a curve representing the measurements performed on the positive harmonics and a curve representing the measurements performed on the negative harmonics are not orthogonal. The method may include adjusting the bias voltage of the modulator to make the two curves orthogonal, thus eliminating the bias error.