Digital Pre-Distortion for IMDD Optical Transceivers
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Solution Overview
Problem
Intensity-modulation direct-detection (IMDD) systems face performance degradation due to chromatic dispersion, leading to limited transmission distances, and existing solutions like dispersion compensation modules increase system cost and complexity, while methods to compensate for nonlinearity in complex modulators are either impractical or result in reduced gain and poor optical signal-to-noise ratio (OSNR).
Innovation Solution
A digital signal processing (DSP) module performs a bias-related linear transform to align optical carrier and modulated signal vectors and applies digital pre-distortion to compensate for modulator nonlinearity, optimizing performance by enhancing the cosine of the angle between these vectors and mitigating nonlinearity effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If dispersion compensation module (DCM) is used to compensate for chromatic dispersion, then transmission distance is extended, but system cost and size increase and implementation becomes difficult
Solution Approach 1:
The patent replaces the physical dispersion compensation module (DCM) with a digital signal processing approach. Specifically, it uses a digital pre-distortion module that applies inverse chromatic dispersion compensation in the digital domain before modulation, and a digital post-compensation module that applies additional CD compensation after modulation. This substitutes the mechanical/optical DCM system with electronic DSP systems, reducing hardware complexity while extending transmission distance.
2Manufacturing precision
If transfer function inversion method is used to compensate for nonlinearity, then nonlinearity and distortions are fully compensated, but gain is reduced or removed due to bandwidth filtering effects
Solution Approach 1:
The patent applies pre-distortion to the drive signals before they are applied to the modulator. The digital pre-distortion module calculates corrected drive signals that pre-compensate for the modulator's nonlinearity and bandwidth filtering effects. By applying the compensation in advance rather than attempting to invert the transfer function after the fact, the system maintains signal gain while achieving nonlinearity compensation.
3Stability of the object's composition
If small signal condition is used to operate at quadrature bias point, then maximum signal excursion is achieved, but optical signal-to-noise ratio and sensitivity deteriorate
Solution Approach 1:
The patent changes the operating parameters by applying DC bias voltages to the modulator arms and using digital pre-distortion to adjust the drive signals. This allows the system to operate at the quadrature bias point for stability while the pre-distortion compensation maintains the optical signal-to-noise ratio by correcting for nonlinear effects that would otherwise degrade performance.
Data Source
AI summary
The present invention relates to a digital signal processing module in a transmitter of an optical transceiver. The digital signal processing module performs a bias-related linear transform in order to align, in a vector plane, the vector of an optical carrier signal with the vector of a modulated optical signal that is modulated by a modulator implemented inside the transmitter, and also performs a digital pre-distortion in order to compensate for the nonlinearity of the modulator. The digital pre-distortion method mainly consists in converting a target signal into a pre-distorted signal by subtracting a signal proportional to the distortion signal from the target signal. The non-orthogonality of the modulator is mitigated by an appropriate bias-related linear transform that is selected as to obtain the best quality performance using a scan method or as to obtain a maximum value of cross-correlation using a pilot tone method.