Coherent Optical Receiver MxN Coupler Direct-Detection Interference
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Solution Overview
Problem
Coherent optical receivers face interference from direct-detection WDM terms, which degrades performance, especially in colorless operation with multiple WDM channels, and existing solutions like optical bandpass filters or increased LO power are not sufficient for massive WDM environments.
Innovation Solution
A coherent optical receiver apparatus and method using an M×N coupler, detectors, AC couplers, subtractors, and a digital signal processor to filter and process mixed signals, eliminating direct-detection terms and recovering optical signals by generating in-phase and quadrature signals, with optional ADCs for digitization and multipliers for scaling based on feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If optical bandpass filter is used to eliminate WDM channels, then direct-detection interference is reduced, but colorless receiver operation capability is lost
Solution Approach 1:
The patent extracts and separately processes the directly detected signal term from the total detector output by utilizing the specific mathematical relationship between the detector outputs and the signal components. This allows the direct-detection interference to be identified and removed through signal processing rather than physical filtering, thereby maintaining colorless operation capability while eliminating interference.
Solution Approach 2:
The patent introduces digital signal processing as an intermediary between the physical detection stage and the final signal recovery. Through mathematical operations on the detector outputs, the direct-detection terms are eliminated as an intermediate step, allowing the desired beat terms to be recovered without physical channel separation.
2Power
If LO power is increased to improve LO to signal power ratio, then coherent detection performance is improved, but OSNR penalty increases in massive WDM environments
Solution Approach 1:
The patent employs feedback through mathematical processing of multiple detector outputs to estimate and eliminate the direct-detection signal terms. This feedback mechanism allows the system to maintain optimal performance without requiring excessive LO power, as the interference cancellation is achieved through intelligent signal processing rather than brute-force power increases.
3Object-affected harmful factors
If balanced receivers are used to suppress direct-detection terms, then interference is reduced according to CMRR, but high balance precision is required for mixer and detector pairs
Solution Approach 1:
The patent replaces the mechanical/optical balancing approach with digital signal processing. Instead of requiring precise physical balancing of mixers and detectors, the system uses mathematical operations on the detector outputs to eliminate direct-detection terms, thereby reducing hardware precision requirements while achieving the same interference suppression goal.
4Adaptability or versatility
If M×N coupler with M,N>2 is used for colorless reception, then multiple WDM channels can be received simultaneously, but device complexity increases
Solution Approach 1:
The patent makes the M×N coupler configuration universal by demonstrating that the same mathematical processing approach works for any M and N values greater than 2. This allows a single receiver design to handle different numbers of WDM channels flexibly, achieving versatility without proportionally increasing complexity, as the signal processing algorithm remains consistent regardless of the specific coupler size.
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
Effectively reduces interference from direct-detection WDM terms, improving the Optical Signal-to-Noise Ratio (OSNR) and enabling effective colorless operation with reduced LO power requirements.
Implementation Method 1
an M×N coupler for receiving an optical signal into a first of M input ports of the M×N coupler and for receiving a local oscillator signal into a second of the M input ports
Implementation Method 2
N detectors, each detector for detecting a mixed signal from a respective output of the M×N coupler to generate one of N respective detected signals
Implementation Method 3
N Alternating Current (AC) couplers, each AC coupler for filtering a respective one of the detected signals to correspondingly generate one of N filtered signals
Data Source
AI summary
One coherent optical receiver includes a 3×3 coupler for receiving a signal and a local oscillator into a first and a third input port respectively, and three detectors for detecting a respective output of the coupler to generate corresponding first, second and third detected signals. A detected signal is filtered by an Alternating Current (AC) coupler to generate a respective first, second or third filtered signal. An adder adds the first, the second and the third filtered signals to determine a directly detected signal term. A first subtractor subtracts the directly detected signal term from the first filtered signal to determine an in-phase signal. A second subtractor subtracts the directly detected signal term from the third filtered signal to determine a quadrature signal. A digital signal processor processes the in-phase signal and the quadrature signal to recover the optical signal.