Types and Classifications of Optical Fibers Explained

Optical fibers come in many types. Depending on their applications, the required functions and performance vary. The classification of optical fibers mainly depends on working wavelength, refractive index distribution, transmission modes, raw materials, and manufacturing methods. For optical fibers used in communication, their design and manufacturing principles are generally the same, such as:
① low attenuation;
② certain bandwidth and low dispersion;
③ easy connection;
④ easy fiber drawing;
⑤ high reliability;
⑥ relatively simple manufacturing;
⑦ low cost.

1. Classification by Working Wavelength

Ultraviolet optical fibers
Visible-light optical fibers
Near-infrared optical fibers
Infrared optical fibers

2. Classification by Refractive Index Distribution

Step-index (SI) fibers
Graded-index (also called gradient-index, GI) fibers
Others (such as triangular, W-type, depressed-cladding types)

3. Classification by Transmission Modes

Single-mode fibers (including polarization-maintaining and non-polarization-maintaining fibers)
Multimode fibers

4. Classification by Raw Materials

Silica optical fibers
Multi-component glass optical fibers
Plastic optical fibers
Composite optical fibers (such as plastic-clad fibers, liquid-core fibers)
Infrared-material fibers

5. Classification by Manufacturing Methods

Preform manufacturing: VAD, CVD, PCVD, MCVD, OCVD
Fiber drawing: rod-in-tube method and crucible method

Silica Optical Fibers

Silica optical fibers are made primarily of silicon dioxide (SiO₂), and by adjusting different doping concentrations, the refractive index distribution of the core and cladding can be controlled. Silica-based fibers have the characteristics of low loss and wide bandwidth. Compared with optical fibers made of other materials, silica fibers also offer full-spectrum transmission from ultraviolet to near-infrared (190–2500 nm).

Multi-Component Glass Optical Fibers

Multi-component glass fibers, as composite fibers, are produced by mixing oxides such as sodium oxide (Na₂O), boron oxide (B₂O₃), potassium oxide (K₂O), etc., with SiO₂. Their characteristics include a lower softening point than quartz glass and a large refractive-index difference between the core and the cladding.

Plastic-Clad Optical Fibers

Plastic-clad fibers use high-purity quartz glass as the core and plastics such as silicone or resin—with slightly lower refractive indices than quartz—as the cladding, forming a step-index fiber. Compared with silica fibers, they have larger core diameters and higher numerical aperture (NA). Therefore, they easily match LED light sources and offer relatively low transmission loss.

Plastic Optical Fibers

Plastic optical fibers have both the core and cladding made entirely of plastic (polymer). Early products were mainly used for decoration, illumination, and short-distance optical communication. Common raw materials include PMMA, polystyrene (PS), and polycarbonate (PC). Their attenuation is limited by the inherent C–H bonding structure of the polymers and typically reaches several tens of dB per kilometer.

Single-Mode Optical Fibers

Single-mode optical fibers refer to fibers that can transmit only one propagation mode at the operating wavelength, commonly abbreviated as SMF (Single Mode Fiber). SMFs have no multimode dispersion and have wider transmission bandwidth compared with multimode fibers.

Multimode Optical Fibers

Optical fibers capable of transmitting multiple modes are called multimode fibers (MMF: Multi Mode Fiber). Based on refractive index distribution, MMFs are divided into graded-index (GI) type and step-index (SI) type. The refractive index of GI fibers is highest at the center of the core and gradually decreases toward the cladding. In SI fibers, different optical paths result in timing differences during reflection and forward transmission, causing distortion of the output signal and narrowing of the transmission bandwidth.