Coherent Optical Receiver Clock Synchronization via Digital Feedback
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Solution Overview
Problem
Existing coherent optical receivers face challenges in synchronizing the clock with optical phase-modulated signals and compensating for fiber dispersion and atmospheric fluctuations, particularly when operating at high transmission speeds and handling both intensity and coherent modulation schemes without using optical PLL.
Innovation Solution
A coherent optical receiver design that includes a 90-degree optical hybrid circuit, I and Q channel photo detectors, a clock extraction circuit, sampling circuits, and a digital signal processing section to synchronize the clock with the demodulated signal, allowing for real-time compensation of fiber dispersion and atmospheric phase fluctuations, and enabling operation with both intensity and coherent modulation schemes without requiring optical PLL.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If optical PLL is used to synchronize local oscillation light with signal light, then frequency synchronization is achieved, but device complexity increases and adaptability to different modulation schemes is reduced
Solution Approach 1:
The patent extracts the clock signal directly from the received optical signal through optical-to-electrical conversion and digital processing, removing the need for complex optical PLL circuits. The clock extraction circuit obtains synchronization information directly from the signal itself, simplifying the overall system architecture while maintaining reliable frequency synchronization.
Solution Approach 2:
The receiver is designed to handle multiple modulation schemes (intensity modulation and coherent modulation) through a unified architecture. By using digital signal processing to extract clock and data signals, the system achieves universal adaptability without requiring scheme-specific hardware configurations, thereby reducing device complexity while improving versatility.
2Measurement precision
If digital processing is used to compensate fiber dispersion and atmospheric fluctuation, then signal quality is improved, but processing speed is limited by converter operation speeds
Solution Approach 1:
The patent performs clock extraction and signal synchronization before the main digital processing operations. By pre-synchronizing the sampling clock with the received signal and pre-compensating for major distortions in the optical domain or through preliminary digital filtering, the system reduces the computational burden on high-speed converters, enabling both high signal quality and processing speed.
3Measurement precision
If sampling frequency is increased to match high transmission speeds, then data accuracy is improved, but analog-to-digital converter requirements become more stringent and costly
Solution Approach 1:
The patent replaces high-speed analog sampling with a combination of optical-domain signal conditioning and lower-speed digital sampling. By performing initial signal processing, clock recovery, and synchronization in the optical or baseband domain, the system reduces the required sampling frequency of the ADC, thereby simplifying converter requirements while maintaining data accuracy through digital signal processing techniques.
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 design allows for increased transmission speed, reduced digital processing load, and real-time compensation of signal distortions, enabling efficient handling of various modulation schemes and maintaining high-speed data processing in sync with the phase-modulated signal.
Implementation Method 1
The 90-degree optical hybrid circuit mixes the input signal light and the local oscillation light having the same frequency as the input signal light so as to directly obtain low-frequency electric signals IPD1 and IPD2
Implementation Method 2
I-channel and Q-channel photo detectors which respectively receive the quadrature components I and Q output from the 90-degree optical hybrid circuit
Data Source
AI summary
A coherent optical receiver includes a 90-degree optical hybrid circuit to which a received signal light is input, I-channel and Q-channel photo detectors to which the outputs of the hybrid circuit are input, a clock extraction circuit which reproduces a clock whose speed is the same as a demodulated signal obtained by demodulating the received signal light and which is synchronized therewith, I-channel and Q-channel sampling circuits which sample the signal outputs from the I-channel and Q-channel photo detectors by use of the clock, and a digital signal processing section which digitally processes the sampled signals, converts them to a digital signal, and outputs the digital signal. The digital signal processing section feeds a phase offset signal detected there back to the clock extraction circuit to thereby control the phase of the clock, and compensates dispersion of light within a fiber and phase fluctuation during free-space propagation.