Digital Optical Spectral Shaping for Out-of-Band Noise Suppression
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Solution Overview
Problem
Conventional optical communications systems face limitations in suppressing out-of-band noise, which interferes with adjacent channels and reduces spectral capacity, while attempting to maintain accurate clock and carrier recovery, often requiring a compromise that sacrifices spectral capacity.
Innovation Solution
A digital filter processes input data signals using a compensation function and shaping function to generate multi-bit sample streams that modulate an optical carrier, addressing impairments and modifying the baseband spectrum to mitigate out-of-band noise and ensure accurate clock and carrier recovery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a Low-Pass Filter is used to suppress harmonic bands and reduce out-of-band noise, then out-of-band noise is reduced, but frequency components of the baseband signal near ±Fs/2 are suppressed, degrading clock and carrier recovery
Solution Approach 1:
The patent applies digital filtering and spectral shaping to the baseband signal before modulation, pre-compensating for potential out-of-band noise issues. By shaping the spectrum in advance and applying pre-distortion compensation, the system reduces out-of-band noise while preserving the necessary frequency components for clock and carrier recovery, avoiding the need for aggressive analog filtering that would degrade recovery accuracy.
Solution Approach 2:
The patent replaces the conventional analog Low-Pass Filter with a digital filtering approach implemented through digital signal processing. Instead of using a physical filter with finite roll-off that suppresses both out-of-band noise and useful frequency components, the system uses digital spectral shaping that can precisely control the frequency response, maintaining sharp attenuation of out-of-band noise while preserving the baseband signal components near ±Fs/2 needed for recovery circuits.
2Object-generated harmful factors
If guard-bands are inserted between adjacent WDM channels to prevent out-of-band noise interference, then adjacent channel interference is reduced, but spectral capacity is reduced
Solution Approach 1:
The patent applies spectral shaping and pre-distortion compensation to the modulated signal before transmission, pre-compensating for potential out-of-band emissions. This preliminary processing ensures that the transmitted signal has minimal out-of-band noise, allowing WDM channels to be placed closer together without requiring large guard-bands, thereby increasing spectral capacity while preventing adjacent channel interference.
Solution Approach 2:
The patent changes the spectral characteristics of the transmitted signal through digital spectral shaping, modifying the frequency distribution to concentrate energy within the allocated channel bandwidth and suppress out-of-band emissions. By adjusting the spectral parameters through digital filtering and shaping, the system achieves better spectral confinement without requiring guard-bands, thus maximizing spectral capacity while preventing interference with adjacent channels.
Data Source
AI summary
A method and system for generating an optical channel signal for transmission through an optical fiber link of an optical communications system. A digital filter processes an input data signal using a compensation function and a shaping function to generate a pair of multi-bit sample streams representing a target optical E-field envelope of the optical channel signal. A modulator modulates an optical carrier light using the pair of multi-bit sample streams to generate the optical channel signal. The compensation function is designed to at least partially compensate impairments of the optical fiber link. The predetermined shaping function is designed to modify a baseband spectrum of the target optical E-field envelope.