The specification defines geometric, optical, mechanical, and measurement parameters with the aim of providing a common reference so that different manufacturers can develop compatible components.

Four optical channels within one fiber

Unlike a conventional single-core fiber, a multicore fiber or MCF (Multicore Fiber) integrates several cores capable of carrying optical signals within the same cladding.

The architecture defined by SDM4 uses four cores arranged in a 2 × 2 square array. The distance between the cores is set at 40 ± 1 micrometers, while the cladding maintains a nominal diameter of 125 micrometers.

This configuration allows for several independent optical paths within the usual external dimensions of a fiber.

This approach is particularly relevant given the growth of infrastructure dedicated to artificial intelligence. The addition of more servers, GPUs, and accelerators is increasing the number of connections required both within data centers and between different facilities on the same campus.

Multicore fiber is presented as one of the technologies capable of increasing the density of these connections without multiplying the number of physical fibers in the same proportion.

Designed for short-range connections

The SDM4 specification establishes optical characteristics geared towards short-range applications operating in the O band, located approximately between 1260 and 1360 nanometers and commonly used in communications centered around 1310 nm.

One of the relevant parameters in a multi-core fiber is crosstalk, that is, the interference that can occur between optical signals circulating through different cores.

The specification sets a crosstalk limit of −40 dB or less at a distance of one kilometer and a wavelength of 1310 nm. The requirement applies to both adjacent and diagonally positioned core pairs.

The document also outlines principles for conducting measurements and includes guidelines on core numbering, rotational alignment, polarization mode dispersion, and marker configurations.

Interoperability between manufacturers

One of the main objectives of a multi-source agreement or MSA (Multi-Source Agreement) is to establish shared technical characteristics that allow compatible products to be developed between different suppliers.

In the case of SDM4 fiber, having common dimensions, optical performance, and measurement procedures provides a reference for manufacturers of fiber, connectors, transceivers, and other optical communications components.

Interoperability becomes especially important as data center architectures increase in scale and need to integrate equipment from different manufacturers.

The release of version 1.0 thus establishes a technical basis on which commercial implementations and subsequent standardization work can be developed.

Towards international standardization of multicore fiber

The specification can also be used as a reference in future international standardization processes.

Among the organizations related to this field are the International Telecommunication Union (ITU-T) and the International Electrotechnical Commission (IEC), while organizations such as the IEEE can address aspects related to transceivers and optical interfaces.

The long-term goal is for the characteristics of this type of fiber to be integrated into standards that facilitate its use in communications infrastructures from different manufacturers.

The publication of a common specification therefore represents a step towards the industrialization of quad-core multicore fiber and its potential use on a larger scale in high-density networks.

More connections for AI infrastructures

The growth of artificial intelligence systems is changing the connectivity needs of data centers. So-called horizontal scaling involves adding more servers, accelerators, and GPUs to distribute workloads across a growing number of nodes.

This model requires a high number of communication links and increases the density of the optical cabling.

Integrating four independent transmission paths within a single fiber can help increase that density while maintaining an outer diameter of 125 micrometers.

Multicore fiber is thus emerging as one of the technologies intended to respond to the increase in optical connections associated with AI data centers, especially in short-range links where available space, cabling density and component interoperability are relevant factors.