Types of Optical Fibers
Optical fibers are classified into two main types: single-mode (SM) and multi-mode (MM). This classification is based on the number of paths that light can follow within the fiber's core. Single-mode fibers allow light to propagate along a single path, whereas multi-mode fibers allow for multiple paths of two or more paths. The term 'mode' here refers to the individual light paths or rays within the fiber core.
The overall outer diameter of the optical fiber's cross-section is 125 um, and the core diameter varies depending on the mode as follows.
1. Single-Mode Fiber
It has a very small core diameter of 8 ~ 9 µm, allowing light to propagate in only a single path. It is designed to transmit single mode at wavelengths above 1260 nm, specifically at 1310 nm and 1550 nm. For reference, shorter wavelengths like 850 nm will have multiple modes in SM optical fibers. It provides low loss and high bandwidth for long-distance data transmission, making it suitable for communications over distances of up to several hundred kilometres. Additionally, there is minimal signal distortion due to the absence of interference between modes. Its applications include communication networks, data centers, and high-speed internet connections.
2. Multimode Fiber
Multimode fiber has a wider core, typically 50 to 62.5 µm, which allows light to travel along multiple paths within the fiber. These multiple paths, or modes, can cause the light to spread out and distort, a phenomenon known as modal dispersion. Because of this, multimode fiber is best suited for shorter distances, usually within a few meters to 2 km. It's commonly used in local area networks (LANs), data centers, and connecting devices within a building.
Next, there is the PM (Polarization Maintaining) fiber, which is widely used in optical devices sensitive to polarization states. PM fiber has a special structure that allows it to maintain the polarization state while transmitting light within the fiber. I will cover PM fiber in detail in the following section to better understand its properties.
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