Types of Optical Fiber

An optical fiber is a fine fiber encapsulated in a plastic protective sheath, allowing it to bend without breaking. Typically, a transmitter at one end of the fiber uses a light-emitting diode or a laser beam to send light pulses through the fiber, and a receiver at the other end uses a photosensitive element to detect the pulses.

The concept of using optical fiber for communication transmission was first proposed by Charles Kao, former president of the Chinese University of Hong Kong, and George A. Hockham. Kao was awarded the 2009 Nobel Prize in Physics for this contribution.

In daily applications, since the transmission loss of light in optical fiber is much lower than that of electricity in electrical wires, optical fiber is used for long-distance information transmission.

The terms optical fiber and optical cable are often confused. Most optical fibers must be covered by several layers of protective structure before use, and the covered cable is called an optical cable. The protective structure outside the fiber prevents damage from the surrounding environment, such as water, fire, and electric shock. An optical cable consists of the fiber, buffer layer, and jacket. Optical fiber is similar to coaxial cable, except that it has no mesh shielding layer. The center is a glass core through which light propagates.

Due to its low loss, easy splicing, easy system integration, high reliability, relatively simple manufacturing, and low cost, optical fiber quickly replaced the use of traditional cables.

Now, the common types of optical fiber mainly include the following:

1. By material: silica fiber, plastic-clad fiber, multi-component glass fiber, all-plastic fiber, and fluoride fiber

Silica fiber uses silicon dioxide (SiO2) as the main raw material and is generally used for long-distance transmission. Its operating wavelength covers ultraviolet, infrared, and visible light, and its transmission loss is much lower than that of plastic fiber, enabling very long transmission distances. However, silica fiber powder is harmful to the human body and the fiber is prone to breaking.

Plastic-clad fiber is a step-index fiber with a high-purity quartz glass core and a cladding made of plastic such as silicone, which has a slightly lower refractive index than quartz. Compared with silica fiber, it features a larger core diameter and a higher numerical aperture (NA). Therefore, it is easy to couple with LED light sources and has relatively low loss. As a result, it is well suited for local area networks (LANs) and short-distance communication.

All-plastic fiber has the greatest advantage of being made of acrylic, which can be bent at 90 degrees without affecting signal transmission and is harmless to the human body. Therefore, plastic fiber is widely used in scenarios with complex short-distance wiring, such as fiber-to-the-desktop and industrial control. In the medical field, plastic fiber is also used in body function monitoring applications.

2. By mode: single-mode fiber and multimode fiber

Single-mode fiber: This refers to a fiber that can transmit only one propagation mode at the operating wavelength, commonly referred to as single-mode fiber (SMF). Currently, it is the most widely used fiber in cable television and optical communications.

Multimode fiber: A fiber that can support multiple propagation modes at the operating wavelength is called multimode fiber. The core diameter is 50 μm, and since the number of transmission modes can reach several hundred, the transmission bandwidth is mainly dominated by modal dispersion compared with SMF. Historically, it was used for short-distance transmission in cable television and communication systems. Optical cables made of multimode fiber can be used for communication, offering good transmission performance and large information capacity; a single channel can simultaneously accommodate dozens of voice calls and transmit dozens of television programs at the same time.