Optical Receiving Apparatus Asynchronous Sampling Clock
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Solution Overview
Problem
In digital coherent reception schemes for optical communication, accurate clock recovery is challenging due to severe waveform distortion, leading to reduced accuracy in digital conversion and poor quality of the recovered clock signal, especially when the clock recovery unit is placed upstream or downstream of the waveform distortion compensator.
Innovation Solution
An optical receiving apparatus that combines signal light with reference light using a 90° phase hybrid circuit, converts the combined optical signals into electrical signals, and generates a sampling clock asynchronous with the signal light based on its symbol rate, allowing for stable digital conversion and demodulation of the received signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the clock recovery unit is placed upstream of the waveform distortion compensator, then the clock can be recovered before distortion affects the signal, but the recovered clock quality deteriorates due to severe waveform distortion
Solution Approach 1:
The patent applies preliminary action by performing waveform distortion compensation before clock recovery. The waveform distortion compensator processes the received signal first to correct distortion, and then the clock recovery unit operates on the compensated signal to generate a high-quality sampling clock. This sequence ensures both stable clock recovery and high clock quality.
2Measurement precision
If the clock recovery unit is placed downstream of the waveform distortion compensator, then the clock signal quality improves, but loop delay increases reducing digital conversion accuracy
Solution Approach 1:
The patent resolves this contradiction by performing waveform distortion compensation as a preliminary action before clock recovery. This allows the clock recovery unit to operate on pre-compensated signals, achieving both high clock signal quality and minimal loop delay, as the compensation happens in advance rather than creating a feedback loop delay.
3Length of stationary object
If severe waveform distortion is present, then signal transmission can occur over long distances, but clock recovery becomes unreliable and digital conversion accuracy decreases
Solution Approach 1:
The patent applies preliminary action by implementing waveform distortion compensation before clock recovery. This pre-processing step corrects the severe waveform distortion that occurs during long-distance transmission, enabling the subsequent clock recovery unit to reliably extract the sampling clock from the compensated signal, thus maintaining clock recovery reliability despite long transmission distances.
4Measurement precision
If a synchronous sampling clock is used, then digital conversion can be performed accurately, but the system cannot handle frequency offsets between local oscillator and signal light
Solution Approach 1:
The patent performs waveform distortion compensation as a preliminary action before digital-to-analog conversion. This pre-compensation handles frequency offsets and waveform distortions, allowing the system to use a simple asynchronous free-running clock for sampling. The preliminary compensation ensures that even with frequency offsets, the distorted waveform is corrected before conversion, maintaining both accuracy and frequency offset tolerance.
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 enables accurate digital conversion and improved reception characteristics by setting the sampling frequency to an integral multiple of the signal symbol rate, reducing errors in demodulation and enhancing the signal-to-noise ratio, even with distorted waveforms, and allows for independent configurations of ADCs and digital signal processing circuits.
Implementation Method 1
a 90° phase hybrid circuit that combines signal light and reference light to two optical signals
Implementation Method 2
an optoelectric converting unit that converts, into electrical signals, two or more optical signals
Data Source
AI summary
An optical receiving apparatus includes a combining unit that combines signal light and reference light; a optoelectric converting unit that converts, into electrical signals, two or more optical signals that enable reconstruction of a complex electric field signal of the signal light obtained by the combining unit; and a sampling clock generating unit that generates a sampling clock that has a frequency preset based on a symbol rate of the signal light and is asynchronous with the signal light. The optical receiving apparatus further includes a digital converting unit that samples at the frequency of the sampling clock signal, an electrical signal obtained by the optoelectric converting unit and converts the electrical signal into a digital signal; and a digital signal processing unit that demodulates a received signal based on a complex digital signal obtained from the digital signal obtained by the digital converting unit.