Control of Intercore Crosstalk Accumulation over the C+L band with Spectral Inversion Technique

Introduction

The continuous growth in optical communication capacity demand has driven the development of various multiplexing technologies, including Wavelength Division Multiplexing (WDM) and Space Division Multiplexing (SDM). Although these technologies have evolved independently, their combination significantly enhances transmission capacity. In uncoupled Multicore Fibers (MCFs) used for SDM, Intercore Crosstalk (IXT) is a primary factor affecting signal quality. This paper explores the use of spectrum inversion technology to mitigate the wavelength dependency of IXT [1].

Fundamentals of Spectrum Inversion

The fundamental concept of spectrum inversion involves dividing the point-to-point MCF transmission line into multiple segments. In this system, signals transmitted along one core are affected by IXT from adjacent cores, where the IXT accumulates approximately linearly along the wavelength axis.

Figure 1: Basic concept of the proposed spectrum inversion method, showing transmission line configuration and IXT spectrum modification.

WDM signals undergo spectrum inversion at the optical repeater section of the transmission line. During this process, long-wavelength signals, which typically experience higher IXT, are converted to shorter wavelengths with lower IXT, and vice versa. This conversion effectively reduces the wavelength dependency of cumulative IXT. The optimal placement of the spectrum inversion repeater is at the midpoint of a point-to-point system, as asymmetric segment lengths lead to an imbalanced cumulative IXT spectrum and increase the peak-to-peak IXT variation across the entire bandwidth.

Figure 2: Comparison of 4-span transmission line configurations, showing (a) the SI method and (b) the BPSI method, along with the IXT spectrum for each span.

Advanced Technique: Bandwidth Partitioned Spectrum Inversion (BPSI)

Traditional spectrum inversion methods still retain a residual slope because the wavelength dependence of IXT is approximately linear in dB/nm, and its accumulation in the linear scale cannot achieve complete flattening. To overcome this limitation, the Bandwidth Partitioned Spectrum Inversion (BPSI) technique was developed. In BPSI, the signal bandwidth is divided into multiple sub-bands, each undergoing spectrum inversion at strategically selected repeater sections.

BPSI applies sub-band and full-band inversions at specific points along the transmission line. This sophisticated method mitigates residual slope more effectively than traditional spectrum inversion. The effectiveness of BPSI is primarily attributed to its ability to manage IXT accumulation at multiple scales, simultaneously addressing both local and global wavelength dependencies.

Figure 3: Comparison of cumulative IXT spectra at the end of each span for eight different full-band and sub-band inversion repeater placement patterns.

Optimization of IXT Flattening Design

Optimizing the placement of spectrum inversion repeaters requires careful consideration of transmission line characteristics. Analysis shows that the most effective configuration is to divide the transmission line into four equal-length segments, regardless of the number of spans within each segment. This layout ensures optimal IXT suppression across the entire wavelength range.

Figure 4: (a) BPSI method extended to an arbitrary number of spans and (b) optimal span allocation for N equal-length spans under different conditions.

Experimental Validation

Extensive experimental validation was conducted using lithium niobate-based periodically poled (PPLN) optical phase conjugation. The results demonstrated significant improvements in system performance. The wavelength dependency of IXT in the C+L band was reduced from 8 dB to less than 1 dB, while the IXT slope was reduced from 0.11 dB/nm to 0.02 dB/nm. These improvements translated into substantial signal quality enhancements, with OSNR loss reduction in the L-band of up to 1.5 dB.

Figure 5: (a) Cumulative IXT spectrum after 4 spans, (b) OSNR loss and GMI for different wavelength channels with and without SI or BPSI, and (c) constellation diagrams for channels ch9 and ch16.

Conclusion

Spectrum inversion and BPSI technology have made significant advancements in managing wavelength-dependent IXT in MCF transmission. These methods effectively reduce IXT variations across the C+L band, particularly at longer wavelengths, leading to more efficient utilization of transmission capacity and improved signal quality.

Reference

[1] J. Oh et al., "Metasurfaces for Free-Space Coupling to Multicore Fibers," Journal of Lightwave Technology, vol. 42, no. 7, pp. 2385-2396, April 1, 2024, doi: 10.1109/JLT.2023.3335334.