Filterless Coherent Receiver Beat Noise Reduction
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Solution Overview
Problem
Filterless coherent optical receivers face significant performance penalties due to signal-signal interference, which increases with the number of WDM channels, and existing solutions like balanced detection or 3×3 couplers increase component costs and system complexity.
Innovation Solution
A system that detects signal-signal beat noise terms simultaneously across the entire WDM signal band and uses digital signal processing to cancel interference, employing a standard coherent receiver frontend and an additional frontend for signal-signal beat-noise detection, with polarization beam splitting and digital subtraction to reduce interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If balanced detection is used to suppress signal-signal interference, then interference suppression is improved, but component cost and system complexity increase
Solution Approach 1:
The patent segments the interference suppression function into two parts: (1) analog suppression through optical filtering that separates signal-signal beating terms from signal-LO beating terms in the frequency domain, and (2) digital suppression through DSP that further cancels residual interference. This segmentation allows each part to handle a portion of the suppression task, achieving good interference rejection without requiring complex balanced detection hardware.
Solution Approach 2:
The patent introduces an optical filter as an intermediary component that selectively passes the signal-LO beating terms while blocking the signal-signal beating terms. This intermediary element simplifies the detection architecture by performing the heavy lifting of interference suppression in the optical domain, reducing the burden on subsequent electronic components and simplifying the overall system.
2Object-affected harmful factors
If balanced detection is used to suppress signal-signal interference, then interference suppression is improved, but component cost increases
Solution Approach 1:
The patent replaces expensive balanced photodiode assemblies with cheaper direct-detection photodiodes combined with optical filtering. The optical filter, while adding a component, is a relatively low-cost element compared to precision-matched balanced photodiodes. This substitution achieves similar interference suppression performance with lower component cost and easier manufacturing.
3Object-affected harmful factors
If 3×3 couplers are used instead of 90 degree hybrid, then noise and interference are minimized, but implementation cost and complexity increase
Solution Approach 1:
Instead of using complex 3×3 couplers to achieve interference suppression, the patent inverts the approach by using simple optical filters combined with direct detection. The filtering is performed in the frequency domain before detection, rather than trying to achieve suppression through complex interferometric arrangements. This inversion of the detection architecture simplifies implementation while maintaining effectiveness.
4Object-affected harmful factors
If 3×3 couplers are used instead of 90 degree hybrid, then noise and interference are minimized, but implementation cost increases
Solution Approach 1:
The patent replaces expensive 3×3 coupler assemblies with cheaper combinations of standard photodiodes and optical filters. The optical filter provides the necessary frequency selectivity at a fraction of the cost of 3×3 couplers, making the system more economically viable for commercial deployment while achieving similar noise and interference minimization.
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 effectively suppresses signal-signal interference, reducing performance penalties and component costs by using fewer photodiodes and ADCs, while maintaining system performance across increasing WDM channels with minimal increase in DSP complexity.
Implementation Method 1
The front-end of a coherent receiver is an optical hybrid combining the signal with the LO. After square-law photodetection, the output photocurrent consists of a desired signal-LO beating term plus undesired interferences arising from signal-signal and LO-LO beating.
Implementation Method 2
After square-law photodetection, the output photocurrent consists of a desired signal-LO beating term plus undesired interferences
Implementation Method 3
separating the SSI signal into separate SSI X and Y polarization with a polarization beam splitter (PBS)
Data Source
AI summary
Systems and methods are disclosed for a filter-less coherent receiving system with a filter-less coherent receiver frontend; a signal-signal beat-noise detector coupled to the filter-less coherent receiver frontend; and a real-time processor coupled to the filter-less coherent receiver frontend and the signal-signal beat-noise detector to reject signal-signal interference.