Optical Modulator Bias Selection Using Phase Sweeping
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Solution Overview
Problem
Next-generation optical modems, particularly those using InP technology, face challenges in achieving optimal modulator bias points due to a small phase adjustment range, leading to performance degradation and increased costs if poor hardware and algorithms are used, whereas lithium niobate modulators have more flexible bias points but require a generalized search that is not always optimal.
Innovation Solution
A method for selecting minimum and half-power bias points in optical modulators using a local algorithm that sweeps phase adjusters to find optimal settings, employing detectors to determine bias points and potentially using data inversion for correct constellation generation, applicable to both lithium niobate and InP modulators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If InP modulators are used to reduce size and cost, then device size and manufacturing cost are reduced, but the phase adjustment range becomes limited making it difficult to find optimal bias points
Solution Approach 1:
The patent performs preliminary bias point selection during manufacturing by sweeping phase adjusters to identify minimum and half-power bias points before the modulator is deployed. This preliminary characterization stores optimal bias settings that compensate for the limited phase adjustment range of InP modulators, enabling them to achieve optimal performance despite their restricted tuning capability.
Solution Approach 2:
The patent systematically varies phase adjuster parameters through sweeping operations to map out the performance characteristics of the modulator. By changing phase parameters and measuring optical output, the system identifies optimal bias points that maximize performance within the constrained phase adjustment range of InP technology.
2Ease of operation
If a generalized search algorithm is used for bias point selection, then flexibility in finding bias points is improved, but the performance degradation at non-optimum points is not minimized
Solution Approach 1:
The patent replaces the conventional generalized search algorithm with a deterministic two-point selection method that identifies minimum and half-power bias points through systematic sweeping. This substitution eliminates the performance degradation associated with non-optimum bias selection by ensuring that only carefully selected optimal points are used, while maintaining operational flexibility through the ability to choose between different bias point pairs.
Solution Approach 2:
The patent employs feedback through optical detection during the bias point selection process. Detectors monitor the optical output while phase adjusters are swept, providing real-time feedback that enables precise identification of minimum and half-power bias points. This feedback mechanism ensures optimal performance by continuously guiding the selection process toward the best bias settings.
3Ease of manufacture
If the phase adjustment range is limited in InP modulators, then manufacturing cost is reduced, but the search space for bias points becomes much larger requiring more complex algorithms
Solution Approach 1:
The patent segments the bias point selection process into distinct phases: first identifying minimum bias points by sweeping inner phase adjusters, then identifying half-power bias points by sweeping outer phase adjusters. This segmentation breaks down the complex 6-dimensional search space into manageable sequential steps, reducing algorithmic complexity while maintaining accuracy in bias point selection for InP modulators.
Data Source
AI summary
A modulator bias selection method, a coherent optical transmitter, and optical modulator solve the problem of generating a correct constellation using the bias points with the minimum phase adjustment range. The optimum modulator bias systems and methods include a coherent optical transmitter with control of four (XI, XQ, YI, YQ) quadrature data signals via a transmitter (Tx) application specific integrated circuit (ASIC), with a modulator bias controller which implements an algorithm to find the optimum bias points. The optimum bias points yield a correct constellation with minimum phase/bias adjustment. An algorithm is used to find the optimum bias solution using fast, simple method, adjusting only one quadrature at a time and exploiting a control feature of the Tx ASIC. This algorithm is significantly simpler than a generalized search, is a local algorithm, and uses only DC power measurement at the transmitter.