WDM Link Coherent Detection Using Optical Frequency Comb Sources
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Solution Overview
Problem
Conventional WDM systems require a high number of costly and complex optical phase locked loops (OPLLs), electrical phase locked loops (EPLLs), and feed-forward carrier recovery systems for coherent detection, leading to increased system complexity, size, and power consumption.
Innovation Solution
Implementing a mode-locked optical frequency comb source at both the transmitter and receiver, with approximately constant frequency and phase offsets between components to reduce the number of required PLLs and carrier recovery systems, leveraging the mode-locked nature to achieve coherent detection and demodulation with a reduced number of phase locked loops or feed-forward carrier recovery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional WDM systems use separate laser sources and full PLL systems for each channel, then coherent detection performance is maintained, but system complexity and cost increase significantly
Solution Approach 1:
The patent merges multiple separate laser sources into a single optical frequency comb source that generates multiple frequency components simultaneously. The comb source produces N frequency components with constant frequency and phase relationships, replacing N separate lasers and their associated PLL systems. This merging reduces device complexity while maintaining coherent detection performance through the inherent stability of the comb structure.
Solution Approach 2:
The optical frequency comb source serves multiple functions: it acts as the light source for multiple WDM channels simultaneously, provides frequency references for coherent detection across all channels, and establishes phase relationships without requiring individual PLLs for each channel. This multi-functionality reduces the overall system complexity while maintaining detection performance.
2Device complexity
If multiple separate PLL systems are used for each WDM channel, then frequency and phase locking is achieved, but system size and power consumption increase
Solution Approach 1:
The patent combines multiple individual PLL systems into a single shared PLL system that processes beat signals from multiple channels simultaneously. The receiver mixes incoming optical signals with local comb components, generates beat signals, and uses one PLL to recover the intermediate frequency reference that serves all channels. This merging reduces the number of PLL systems from N to 1, thereby reducing power consumption and system size.
Solution Approach 2:
Instead of implementing separate PLL circuits for each channel, the system generates a single intermediate frequency reference through one PLL and then distributes copies of this reference to all channel demodulation processes. This copying approach maintains the frequency and phase locking functionality for all channels while using only one physical PLL system, reducing power consumption and complexity.
3Device complexity
If an optical frequency comb source is used for multiple WDM channels, then the number of laser sources is reduced, but maintaining constant phase relationships between components becomes challenging
Solution Approach 1:
The patent employs feedback mechanisms to maintain constant phase relationships in the optical frequency comb. The receiver mixes incoming signals with local comb components and generates beat signals that are processed by a PLL system. The PLL outputs a corrected intermediate frequency reference that feeds back to adjust the comb source, ensuring that frequency and phase relationships remain constant despite environmental variations. This feedback loop stabilizes the comb structure and maintains coherent detection performance.
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 significantly reduces system complexity, leading to more compact, cheaper, and lower power consumption systems while maintaining high optical power efficiency and data signal strength.
Implementation Method 1
The comb is generated via parametric light generation (e.g. four wave mixing). This results in a mode locked comb with the mode locking provided by the nature of the parametric generation processes.
Implementation Method 2
Mixing refers here to combining (summing) the incoming light with at least a portion of the locally generated light, thus generating an interference signal. This effectively reamplifies the optical signal, making it easier to overcome the electronic noise floor of the receiver.
Data Source
AI summary
An optical data link has a transmitter and a receiver with coherent detection at the receiver and more than one optical carrier frequency. The optical carrier frequencies are generated by a frequency comb source in both the transmitter and the receiver. The frequency comb sources generate frequency combs that have frequency components and a free spectral range. The optical carrier frequencies transport more than one optical channel. Either at least one frequency component or the free spectral range of the optical comb generated at the receiver is locked to the comb generated at the transmitter by an optical phase locked loop, or an electrical phase locked loop or a feed-forward carrier recovery generates an intermediate frequency carrier reference that is routed to more than one channel to demodulate the data.