Polarization Interleaving for Nonlinear Interference Reduction
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Solution Overview
Problem
Conventional optical systems experience nonlinear interference between orthogonal polarization-multiplexed signals due to nonlinear optical effects in fibers, leading to deteriorated signal quality, which cannot be effectively equalized by linear adaptive equalizers.
Innovation Solution
An optical transfer system is designed with a pulse-signal generation unit producing pulses with a width of Ts/2, a data modulation unit modulating single-polarization signals, and a polarization interleaving unit creating a delay difference of Ts/2 between polarization components, ensuring orthogonal polarization components are alternately present on the time axis with no simultaneous presence, thereby reducing nonlinear interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If orthogonal polarization components are multiplexed simultaneously on the time axis, then the signal transfer capacity is increased, but nonlinear interference occurs between polarization components due to nonlinear optical effects in the fiber
Solution Approach 1:
The patent applies periodic action by using return-to-zero modulation format where each polarization component is transmitted in periodic time slots with regular intervals. The signal is modulated such that it returns to zero between symbol periods, creating a periodic transmission pattern that prevents continuous overlap between orthogonal polarization components, thereby reducing nonlinear interference while maintaining high transfer capacity
Solution Approach 2:
The patent extracts the harmful simultaneous presence of orthogonal polarization components by separating them in the time domain. By using polarization de-multiplexing followed by time-division separation, the system extracts and eliminates the nonlinear interaction that occurs when both polarization components are present simultaneously, allowing each to be processed independently
2Loss of information
If polarization multiplexing is used to double the number of transfer bits per symbol, then spectral efficiency is improved, but linear adaptive equalizers cannot effectively equalize nonlinear interference
Solution Approach 1:
The patent replaces the conventional linear adaptive equalizer with a nonlinear equalizer that incorporates nonlinear compensation algorithms. This substitution enables the system to effectively equalize and compensate for nonlinear interference effects that linear equalizers cannot handle, thereby maintaining high signal quality while utilizing polarization multiplexing for improved spectral efficiency
3Productivity
If high-density wavelength multiplexing is realized, then the optical transfer capacity is increased, but optical signal-to-noise power limitations become more severe
Solution Approach 1:
The patent employs parameter changes by utilizing advanced modulation formats such as QPSK and 16-QAM that encode multiple bits per symbol. This changes the information density parameter, allowing high transfer capacity to be achieved without proportionally increasing optical power, thereby maintaining acceptable optical signal-to-noise ratios even in high-density wavelength multiplexing scenarios
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 configuration significantly reduces nonlinear interference and improves signal transfer quality by eliminating delay differences and adapting to other distortions in the digital signal processing unit.
Implementation Method 1
an optical interference unit that causes an interference between light generated by the local oscillation light source and an optical signal received from the optical transmission unit
Implementation Method 2
a photoelectric conversion unit that converts an output from the optical interference unit to an electric signal; linear photoelectric conversion using synchronous detection
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
In an optical transfer system according to the present invention, an optical transmission unit 100 generates an optical signal in which respective polarization components are alternately present on a time axis, a time period during which the respective polarization components are simultaneously present on the time axis is substantially zero, and a symbol repetition cycle of optical signals of the respective polarization components becomes Ts, an optical reception unit 300 causes an interference between local oscillation light and a received optical signal and converts an interfered optical signal to an electric signal, and a received electric-signal processing unit 400 performs analog-digital conversion of an electric signal, elimination of a delay difference of Ts/2 between the respective polarized signal components, and adaptive equalization of a distortion other than the delay difference.