The heart of the innovation lies in the cladding. Photonic crystal fibers, first proposed by Philip Russell and co-workers in the 1990s, replace the doped glass of conventional fiber with a periodic lattice of air holes running along the entire length. Because air has a refractive index close to 1.0, while silica sits near 1.45, the air-hole lattice lowers the effective refractive index of the cladding far more aggressively than doping ever could. In the new design, nine solid silica cores—one at the center and eight arranged symmetrically in an octagonal ring—stand out as the highest-index regions in the structure, each trapping its own light. The surrounding triangular lattice of air holes, with a hole diameter of 2 micrometers and a pitch of 2.7 micrometers, provides the optical barrier that keeps the channels isolated.

Geometry is everything in this architecture. Each core has a diameter of 50 micrometers, and the inter-core distances of 250 and 194 micrometers were deliberately chosen so that every core sits at least seven core radii from its neighbors, a threshold beyond which coupling between cores becomes practically negligible. The researchers confirmed this with full-vectorial finite element method simulations, solving the Helmholtz equation for the transverse permittivity profile and extracting the coupling coefficient between adjacent cores from the overlap of even and antisymmetric supermodes. The result was a coupling coefficient of roughly 3 × 10⁻⁷ per meter, which translates into the −90.5 decibel crosstalk figure. For comparison, conventional weakly coupled multicore fibers typically manage only −60 to −80 decibels.

The air-hole cladding also delivers a high numerical aperture of 0.3, corresponding to a critical acceptance angle of 11.8 degrees. That matters for practical systems: a wide acceptance cone makes it far easier to couple light efficiently from inexpensive sources such as vertical-cavity surface-emitting lasers and light-emitting diodes, which is precisely the regime relevant for short- and medium-reach links. Conventional multicore fibers, with numerical apertures around 0.1 to 0.2, demand more precise and more expensive launch optics. The trade-off is a relatively large outer cladding diameter of 720 micrometers, which imposes larger bending radii than standard telecom fiber, but the authors argue this is acceptable for fixed links, laboratory systems, and spatial-division multiplexing platforms where the benefits of high channel density and simplified integration outweigh the handling constraints.

To predict how signals actually behave over distance, the team turned to the power flow equation, a computationally efficient framework first formulated by Detlef Gloge in 1972 that describes how optical power redistributes among the guided modes of a multimode fiber. Mode coupling in real fibers arises from unavoidable imperfections—microbending, diameter fluctuations, density inhomogeneities—that scatter power between modes as light propagates. Using an explicit finite difference method with a coupling coefficient of 6.4 × 10⁻⁶ radians squared per meter, the researchers simulated two Gaussian launch conditions with angular widths of 1 degree and 5 degrees, tracking the output power distribution along all nine cores at distances from 10 meters up to nearly 2 kilometers.

The simulations reveal a clear and quantitatively rich picture of how the fiber reaches equilibrium. In short fibers, power concentrates in low-order modes, but as light travels, coupling progressively shifts energy toward higher-order modes until the distribution settles into the equilibrium mode distribution. For the narrower 1-degree launch, this occurs at a coupling length of about 705 meters; for the wider 5-degree launch, at about 624 meters. A fully steady-state distribution arrives even later, at roughly 1850 meters and 1700 meters respectively. Wider launch beams therefore shorten the characteristic lengths, because the light starts out spread across more modes and has less distance to travel before mixing completes.

Bandwidth tells the complementary story. At 10 meters, the narrow launch delivers a −3 dB optical bandwidth of 54 gigahertz, versus just 4 gigahertz for the wide launch, because a tightly collimated beam excites fewer high-order modes and suffers less modal dispersion. By 100 meters the figures fall to 0.97 gigahertz and 0.31 gigahertz, and by 500 meters they converge to tens of megahertz. Beyond the steady-state length, the bandwidth becomes entirely independent of how the light was launched, settling at about 12 megahertz. The practical conclusion is that this nine-channel fiber is best suited to short- and medium-haul transmission up to roughly 500 meters, where narrow launch beams can dramatically boost per-channel bandwidth, rather than to long-haul submarine or terrestrial backbone routes.

What elevates the design beyond a communication curiosity is its sensing potential. Because the nine channels are so well isolated, each core can be selectively functionalized to interact with a different analyte, and local changes in refractive index modulate that channel’s effective propagation constant without contaminating its neighbors. The architecture naturally supports differential sensing, in which unfunctionalized cores serve as reference paths that compensate for ambient temperature and mechanical fluctuations in real time. The authors envision parallel, real-time detection of multiple chemical and biological species for environmental surveillance, biomedical diagnostics, and chemical process monitoring—applications where conventional fibers offer only single-parameter or spatially limited sensing.

Fabrication, the team argues, is within reach of existing technology. The fiber can be made by the standard stack-and-draw procedure, in which silica capillaries are stacked into the triangular lattice while solid rods occupy the nine core positions, with multi-stage drawing and active pressure control preventing hole collapse. Alternatively, high-precision CNC ultrasonic drilling or laser-assisted preform machining can fix the core positions before the capillaries are inserted. Standard manufacturing tolerances of ±1 percent on hole diameter and ±0.5 percent on pitch introduce only negligible shifts in cladding index and numerical aperture, and the analysis shows that even with a ±1 micrometer core-position deviation, the inter-core separation stays well above the critical coupling threshold, keeping crosstalk isolation above 88 decibels.

The work arrives amid a broader surge of interest in multicore and photonic crystal fiber architectures, from polymer-fiber multiplexers for short-reach routing to eight-channel C-band demultiplexers built on multicore photonic crystal fibers. By combining an octagonal nine-core layout, an air-silica microstructured cladding, and a rigorous power-flow analysis of bandwidth and mode coupling, the Serbian and Chinese team has sketched a single component that could serve simultaneously as a compact spatial-division multiplexing platform and a nine-channel parallel sensor. As data demand keeps climbing and sensing systems grow ever more parallel, fibers that keep their channels whisper-quiet to one another may prove to be exactly the kind of quiet revolution optical networks need.

Subject of Research: Design and bandwidth analysis of a nine-core multimode step-index silica photonic crystal fiber for space-division multiplexed communication and multi-channel optical sensing.

Article Title: Design and analysis of multimode step-index multicore silica photonic crystal fibers for communication and sensing applications

Article References: Simović, A., Drljača, B., Savović, M., Deng, X., & Savović, S. (2026). Design and analysis of multimode step-index multicore silica photonic crystal fibers for communication and sensing applications. Results in Optics, 25, Article 101155. https://doi.org/10.1016/j.rio.2026.101155

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101155

Keywords: photonic crystal fiber, multicore fiber, space-division multiplexing, inter-core crosstalk, optical bandwidth, mode coupling, silica fiber, optical sensing, power flow equation, fiber optics, refractive index, telecommunications