Multi-core Few-mode Fibre

Multi-core Few-mode Fiber (MC-FMF) - a fibre optic communication transmission medium.

1.What is Multi-core Few-mode Fibre?

Multi-core Few-mode Fiber (MC-FMF) is a special type of optical fibre, which combines the characteristics of Few-mode Fiber (FMF) and Multi-core Fiber (MCF), and can achieve both Mode Division Multiplexing (MDM) and Core Space Division Multiplexing (CSDM) in the same optical fibre. It is expected to become the core transmission medium in the new generation of optical fibre communication system, which is one of the main research directions for the development of future optical communication technology.

2.Why do we need multi-core and less-mode optical fibres?

With the rapid development of the Internet, cloud computing, big data and other technologies, the global data traffic shows explosive growth. Although the traditional single-mode optical fibre is very mature, its capacity is gradually approaching its physical limit. In order to meet the increasing communication demands, new solutions need to be explored to increase the capacity of optical fibre communication systems.

Among various new transmission technologies that break through the theoretical limit of single-mode fibre, multi-core, less-mode technology is expected to fundamentally avoid the capacity bottleneck of single-core fibre, which is one of the main evolutionary directions for future optical fibre communications, and is expected to be the first one to find a landing point in the scenario of short-distance interconnection of ultra-large-capacity data centres in the future.

Concepts related to multi-core and less-mode optical fibre

1. Core

The core is the core component of an optical fibre, which is a glass core with a high refractive index, responsible for the transmission of optical signals. In addition, the middle of the fibre is a low refractive index silica glass cladding, and the outermost is a resin coating for reinforcement.

2. Mode

In the field of optical fibre communication, "mode" is used to describe the propagation of optical signals within the core, i.e. the mode is the propagation path of light.

The generation and transmission of optical fibre modes is based on the total reflection property of optical fibres, where light entering the fibre is guided by the difference in refractive indices between the core and cladding, and is reflected repeatedly in the fibre. If the core diameter is small enough, the light can only propagate along the central axis (fundamental mode), and if the core diameter is large, the fibre can propagate along different paths (multimode), each mode having a different optical range and optical range difference.

Thus, in a single-mode fibre, light travels along one path; in a multimode fibre, light travels in multiple paths.

Advantages and disadvantages of various fibre types

Single-mode optical fibre, multi-mode optical fibre, less-mode optical fibre, multi-core optical fibre, multi-core less-mode optical fibre are introduced here.

Single-mode Fibre (SMF)

An optical fibre that transmits only a single fundamental mode at a given operating wavelength, with a smaller core diameter, usually between 8-10 microns.

Compared with multi-mode fibre, it has smaller dispersion, lower attenuation and stronger anti-interference ability, and is widely used in large-capacity long-distance transmission, such as backbone, metro and data centre interconnection.

Multi-mode Fibre (MMF)

An optical fibre that allows optical signals to be transmitted in multiple modes, with a large core diameter, usually between 50-100 microns.

Compared with single-mode fibre, the dispersion is more obvious, the transmission distance is shorter (generally not more than 2 km), and the signal transmission speed is slower (generally at the level of Gbps), which is mainly used for short-distance data transmission, such as office LANs, data centres and so on.

Few-mode Fibre (FMF)

An optical fibre with a core area large enough to transmit parallel data streams using several separate spatial modes.

Compared with multi-mode fibre, the degree of mode crosstalk and dispersion of FMF is significantly reduced, and each of its modes can be used as an effective signal channel for signal transmission, which greatly improves the capacity of the system, and solves the bandwidth crisis of single-mode fibre in the future.

The disadvantage is that with the increase in the number of transmission modes, the multiplexing and demultiplexing of transmission modes will become very complex, but also due to its large mode field diameter, resulting in transmission distance is limited.

Multi-core Fibre (MCF)

An optical fibre that contains multiple independent cores within a single fibre cladding. Each core is single-mode and can transmit multiple independent optical signals at the same time, making it suitable for building high-capacity, high-speed transmission networks.

Compared to single-mode optical fibres, the disadvantage is that the signals between cores may interfere with each other due to the close spacing of the cores.

In addition, due to the increase in the number of cores, the cladding thickness of the optical fibre may be relatively reduced, which leads to an increase in the limiting loss and bending loss of the optical fibre, further affecting the signal transmission distance and quality. For example, how to reduce crosstalk between different fibre cores, how to improve the bending performance of optical fibres, etc.