Single-MMI Coherent Receiver Layout Without Waveguide Crossings
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Solution Overview
Problem
Conventional coherent optical communication systems face issues with phase errors and power imbalance due to waveguide crossings and unequal lengths in coherent receiver optical sub-assemblies (C-ROSA), which affect performance and increase design complexity.
Innovation Solution
A photonic integrated circuit (PIC) using a single MMI coupler with non-intersecting optical waveguides and uniform electrical paths is designed to eliminate waveguide crossings and ensure equal optical and electrical path lengths, optimizing performance and reducing complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional C-ROSA designs use multiple optical splitters and combiners with waveguide crossings, then the optical hybrid function is achieved, but phase errors and power imbalance occur due to unequal waveguide lengths
Solution Approach 1:
The patent merges multiple optical components (splitters and combiners) into a single MMI coupler that performs both functions simultaneously. The 4x4 MMI coupler integrates the signal splitting and combining operations that were previously distributed across multiple separate components, eliminating the need for complex waveguide routing between them.
Solution Approach 2:
The patent segments the optical paths within the MMI coupler to create four distinct output ports with controlled phase relationships. By dividing the single coupler into functional regions that correspond to different output ports, the design achieves phase balance without requiring external waveguide adjustments.
2Area of stationary object
If waveguide crossings are used within the optical hybrid, then compact integration is achieved, but signal crosstalk and phase errors are introduced
Solution Approach 1:
The patent converts the potential harm of waveguide crossings into a benefit by using the MMI coupler's inherent multi-mode interference characteristics. Instead of avoiding mode coupling entirely, the design exploits controlled mode interference within the MMI region to achieve the desired signal combining and phase shifting functions without external waveguide crossings.
3Adaptability or versatility
If unequal length waveguides are used to connect optical hybrid outputs to photodetectors, then routing flexibility is improved, but phase errors and non-uniform RF performance result
Solution Approach 1:
The patent creates equipotential optical paths by designing equal length waveguides from each MMI coupler output port to the corresponding photodetector. This ensures that all optical signals experience identical propagation conditions, eliminating phase errors and ensuring uniform RF performance across all detection channels.
4Adaptability or versatility
If multiple optical components are used in the optical hybrid, then functional flexibility is maintained, but manufacturing precision and power balance deteriorate
Solution Approach 1:
The patent makes the single MMI coupler universal by designing it to perform multiple functions simultaneously: signal splitting, signal combining, phase shifting, and power equalization. This multi-functional approach maintains the necessary optical hybrid functionality while eliminating the power balance and precision issues associated with multiple separate components.
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
The solution provides phase balance, power balance, and uniform RF performance, minimizing design and fabrication complexity while maintaining reliable coherent detection.
Implementation Method 1
a single multi-mode interference (MMI) coupler configured to combine the S and LO signals
Implementation Method 2
four photodetectors to convert electrically quadrature signal components from the optical mixing products
Data Source
AI summary
A coherent receiver optical sub-assembly photonic integrated circuit has optical input ports, electrical output ports, photodetectors, and a single multi-mode interference (MMI) coupler. The optical input ports receive a phase modulated optical signal (S) and a continuous wave local oscillator (LO) optical signal. The electrical output ports output electrical signals corresponding to mixing products of the S and LO optical signals. The photodetectors are connected to the electrical output ports through electrical lines. The MMI coupler has a main body, input access waveguides, and output access waveguides. The input access waveguides receive the S and LO optical signals from the input ports and launch light to the main body. The output access waveguides are connected to respective ones of the photodetectors through optical waveguides. The output access waveguides collect multi-mode interference signal products from the main body and deliver the mixing products of the S and LO optic signals.