Optical Phase Conjugation for Nonlinear Impairment Mitigation
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Solution Overview
Problem
Long-haul optical transmission systems face limitations due to optical noise accumulation and nonlinear impairment, which previous solutions attempted to address with complex and costly phase sensitive amplifiers requiring high-quality carrier regeneration and phase lock loops.
Innovation Solution
An optical phase conjugate copier generates a phase conjugate duplicate of the original optical signal, which is then processed in a digital signal processor to cancel out nonlinear phase shifts, simplifying the system and reducing costs by eliminating the need for additional hardware.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If signal power is increased to overcome optical noise accumulation, then signal-to-noise ratio is improved, but non-linear impairment increases and harms signal quality
Solution Approach 1:
The patent creates a phase-conjugate copy (idler) of the original optical signal. This copy travels through a separate fiber and experiences similar non-linear phase shifts. At the receiver, the two signals are combined and processed to cancel out the non-linear impairments while maintaining the signal-to-noise ratio benefits of higher power transmission.
Solution Approach 2:
The patent converts the harmful non-linear phase shifts into a beneficial effect by having two signals experience identical distortions that can then be cancelled through digital signal processing. The non-linear impairment that would normally degrade the signal is instead used to create correlated distortion patterns that enable cancellation.
2Reliability
If phase sensitive amplifiers are used to increase OSNR and mitigate nonlinear phase noise, then signal quality is improved, but system complexity and cost increase due to required carrier regeneration and phase lock loops
Solution Approach 1:
Instead of using complex phase sensitive amplifiers with carrier regeneration and phase lock loops, the patent uses a simpler phase-conjugate copying approach. The optical phase conjugate copier generates an idler signal that is a phase-conjugate copy of the original signal, eliminating the need for complex feedback control mechanisms.
Solution Approach 2:
The patent replaces the mechanical/optical feedback control systems (phase lock loops, carrier regeneration circuits) with a digital signal processing approach. The phase conjugation and signal combination are performed in the digital domain, simplifying the overall system architecture.
3Reliability
If twice of phase conjugation operations are performed in optical domain using phase sensitive amplifiers, then nonlinear phase noise is mitigated, but system cost is largely increased due to requiring two sets of optical phase conjugation setups
Solution Approach 1:
The patent generates a single phase-conjugate copy (idler) of the original signal at the transmitter. This single copying operation is sufficient to enable non-linear phase noise mitigation through digital signal processing at the receiver, eliminating the need for multiple optical phase conjugation setups.
Solution Approach 2:
The patent moves the second phase conjugation operation from the optical domain to the digital domain. Instead of requiring a second optical phase conjugate copier, the digital signal processor performs the conjugation and combination operations electronically, significantly reducing system complexity and cost.
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 enhances the signal-to-noise ratio and increases system stability while maintaining nonlinear tolerance, simplifying the system setup and reducing costs compared to previous methods.
Implementation Method 1
The optical phase conjugate copier with digital domain processing employs: 1) four-wave mixing (FWM) in highly nonlinear fiber (HNLF) to generate a phase conjugate copy of the original signal
Implementation Method 2
four-wave mixing (FWM) in highly nonlinear fiber (HNLF) to generate a phase conjugate copy of the original signal
Data Source
AI summary
A long haul transmission system uses a digital signal processor DSP instead of an additional optical phase conjugate copier because the optical phase conjugate copier requires high quality optical carrier regeneration to recover the pump and optical PLL to maintain phase matching between signal and pump. Therefore, the use of DSP to process the signal and idler at receiver end greatly simplifies the system setup, increases the system stability and decreases the system cost.