Tunable Optical Modulator Pre-Compensation for Distortion
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Solution Overview
Problem
Current optical transmitters using Mach Zehnder modulators (MZMs) face challenges with non-linear voltage/optical power characteristic responses, leading to distortion in multi-level modulation schemes, which complicates the transmitter architecture and reduces performance, especially in high bit rate interfaces.
Innovation Solution
An optical transmitter with a tunable modulator and a feedback system that measures distortion at the receiver and adjusts the modulation to pre-compensate for it, using a coherent detector to measure distortion and feed it back to the transmitter, allowing for improved transmission capacity without the need for complex transmitter configurations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single MZM is used with multilevel electrical driving signals, then the transmitter architecture is simplified, but the non-linear voltage/optical power characteristic response causes distortion in the output constellation
Solution Approach 1:
The patent applies preliminary action by pre-distorting the electrical driving signals before they are applied to the MZM. The distortion compensation unit calculates pre-distortion values based on the desired constellation points and the known non-linear transfer function of the MZM, then applies these pre-distorted values to counteract the expected non-linear distortion, resulting in accurate output constellation points.
Solution Approach 2:
The patent implements feedback by measuring the actual output constellation points at the receiver and using this information to adjust the pre-distortion parameters. The system continuously monitors the distortion and updates the pre-distortion values to maintain accurate constellation points despite variations in MZM characteristics or operating conditions.
2Manufacturing precision
If multiple MZMs are combined in series or parallel, then the distortion is reduced, but the transmitter complexity and cost increase
Solution Approach 1:
The patent changes the parameters of the electrical driving signals by applying pre-distortion to compensate for the MZM's non-linear transfer function. Instead of changing the physical structure (adding multiple MZMs), the system modifies the electrical parameters (voltage levels and timing) to achieve linearized output characteristics, thereby maintaining constellation accuracy with a single MZM.
3Manufacturing precision
If the MZM is modulated with reduced amplitude RF signal to use only the linear part of the dynamic range, then the distortion is reduced, but the extinction ratio and performance are reduced
Solution Approach 1:
The patent applies preliminary pre-distortion to the electrical driving signals, allowing the MZM to operate across its full dynamic range including the non-linear regions. The pre-distortion pre-compensates for the expected non-linear effects, enabling the system to use the complete dynamic range of the MZM while maintaining accurate constellation points, thus preserving both the extinction ratio and the output accuracy.
Data Source
AI summary
An optical transmitter has a modulator for modulating data onto an optical signal for transmission to the receiver, the modulated signal having components at one or more constellations of points of different amplitudes and phases. The modulator is tunable such that distortions of the points of the one or more constellation can be tuned, and a tuning controller is provided for receiving a feedback signal from the receiver indicating a distortion measured at the receiver, and for tuning automatically the modulator to adjust the modulation based on the received feedback signal to pre-compensate for the measured distortion. Such pre-compensation can reduce the amount of distortion in the transmission system and thus enable more transmission capacity, without the need for a complex transmitter.