Optical Signal Pre-Distortion for ROADM Spectrum Equalization
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Solution Overview
Problem
The cascading of optical filters in optical transmission systems for super-channels leads to distortion of the OFDM signal spectrum, resulting in variable error rates and degraded reception performance due to non-rectangular spectral transfer functions of current ROADM multiplexers.
Innovation Solution
A method of pre-distorting optical signals with frequency multiplexed subcarriers by applying corrective factors to the electrical subcarriers based on measured parameters such as signal-to-noise ratio or power, to equalize the spectrum and compensate for the impact of optical components like ROADM multiplexers, thereby maintaining signal integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If cascaded ROADM multiplexers are used for optical routing, then transparency and reconfigurability are improved, but spectral distortion and signal degradation occur due to non-rectangular transfer functions
Solution Approach 1:
The patent applies preliminary digital pre-distortion to the electrical signal before optical conversion. Correction factors are calculated based on the known spectral response of the ROADM multiplexers and applied to pre-compensate the signal, counteracting the anticipated spectral distortion before it occurs in the optical domain.
Solution Approach 2:
The patent modifies the electrical signal parameters (amplitude and phase) of individual subcarriers based on calculated correction factors. These parameter adjustments compensate for the non-rectangular spectral transfer function of the ROADM multiplexers, ensuring uniform power distribution across all optical subcarriers after transmission.
2Measurement precision
If narrow spectral transfer function filters are used for sub-band multiplexing, then spectral selectivity is improved, but cascading multiple filters causes significant spectrum distortion
Solution Approach 1:
The patent calculates correction factors based on the cumulative spectral response of cascaded narrow filters and applies pre-distortion to the electrical signal before optical conversion. This preliminary compensation counteracts the expected spectrum shape degradation from cascading multiple selective filters.
Solution Approach 2:
The patent introduces digital signal processing as an intermediary between the electrical signal and optical conversion. The correction factors act as a mediator that compensates for the cumulative effect of cascaded narrow filters, maintaining spectral integrity without requiring modification of the filters themselves.
3Reliability
If optical pre-equalization is implemented, then chromatic dispersion tolerance is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces complex optical pre-equalization hardware with digital signal processing in the electrical domain. By applying correction factors to electrical subcarriers before optical conversion, the system achieves dispersion compensation without requiring additional optical components or complex optical processing hardware.
Data Source
Figure 1A~1B
Figure 2
Figure 3~4
AI summary
The invention relates to a method for pre-distorting an optical signal (SOPT), intended to be transmitted over an optical transmission line (30) and comprising a plurality of frequency-multiplexed optical subcarriers, this optical signal being obtained from the optical conversion of an electrical signal (SE) comprising a plurality of frequency-multiplexed subcarriers. This method includes, in particular, obtaining (E2) a plurality of correction factors (αA,i), corresponding to the electrical subcarriers, as a function of a plurality of values, determined for the optical subcarriers, of a first parameter dependent on the power of said subcarriers, and applying (E3) the correction factors to the amplitudes of the corresponding electrical subcarriers, so as to equalize, at the output of said optical transmission line (30), the values of the first parameter for the subcarriers of the optical signal.