Analytical Investigation of crosstalk suppression using optical bandpass filters for non-orthogonal Nyquist WDM signals with heterogeneous bandwidths
Year: 2026
Type: article
Abstract: We investigated the effectiveness of filtering 16-QAM two-channel non-orthogonal frequency division multiplexed Nyquist-WDM signals with heterogeneous bandwidths by using an optical bandpass filter. A wideband and a narrowband signal are overlapped in the frequency domain to enhance the spectral efficiency. To extract one of two channels, cascaded optical filters having transmissivity with super-Gaussian shapes with orders of 1 and 5, or a single stage optical filter with super-Gaussian shapes in the order of 5, are utilized, and the optimized filter bandwidth was verified for minimizing the error vector magnitude (EVM) of the transmitted signals. In the case of the combination of 70- and 30-Gbaud bandwidth signals overlapped, and the channel spacing down to 58 GHz can be allowed to keep EVM < 15% with the scaling factor, indicating the retained Nyquist-shaping bandwidth, of 0.7 and the 2 nd filter bandwidth coefficient, normalized by the standard bandwidth corresponding to the signal baud rate, of 0.65. In addition, the bandwidth of the Nyquist WDM signal is also changed in the range from 4- to 20-Gbaud for a narrow-bandwidth signal under the condition of a 20-Gbaud wide-bandwidth signal, and the amount of frequency overlap and the ratio of the baud rate of the narrow-bandwidth signal to that of the wide-bandwidth signal (Baud rate ratio) are compared. We have verified that the bandwidth restriction of the signals using Nyquist shaping is effective for keeping EVM under the narrow frequency spacing between two channels. And the single stage filter is more effective than cascaded super-Gaussian filters for both narrow- and wide-bandwidth signals over a wide range of the Baud rate ratio in the case of Nyquist WDM signal filtering, and this is different from the conventional WDM signal filtering with the cascaded filter. Finally, spectral efficiency (SE) of the non-orthogonal WDM signals was evaluated, and while Nyquist-WDM has higher EVM than WDM, the reduced occupied bandwidth enables higher SE, achieving 2.63–3.23 bps/Hz compared with 2.2–2.6 bps/Hz for non-orthogonal WDM across the evaluated baud-rate ratios.
Source: Optics Continuum
Authors: Onon Bayarsaikhan, Hiroyuki Uenohara
Language: English
Topic: Optical Network Technologies
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,372
OpenAlex ID: W7168292638