Single Mode vs Multimode Fiber
Optical fiber is a flexible transparent fiber made of extruded glass or plastic, slightly thicker than human hair. Optical fiber is the most commonly used means for transmitting light at both ends, and it is widely used in optical fiber communications. Optical fiber has longer transmission distance and higher bandwidth than wired cable. An optical fiber is usually composed of a transparent core with a low refractive index and a transparent cladding material. As an optical waveguide, the optical fiber causes total reflection of light in the core.
Generally speaking, there are two kinds of fibers: the fiber that supports multiple propagation paths or transverse modes is called multimode fiber (MMF), and the fiber that supports a single mode is called single mode fiber (SMF). But what is the difference between them?
What is single-mode fiber?
In optical fiber communication, single-mode fiber (SMF) is an optical fiber that directly transmits optical signals in transverse mode. Single-mode fiber runs at a data rate of 100M/s or 1 G/s, and the transmission distance can reach at least 5 kilometers. Normally, single-mode fiber is used for long-distance signal transmission.
What is multimode fiber?
Multimode fiber (MMF) is mainly used for short-distance fiber communication, such as in buildings or campuses. The typical transmission speed is 100M/s, the transmission distance can reach 2km (100BASE-FX), 1 G/s can reach 1000m, and 10 G/s can reach 550m. There are two types of refractive index: graded refractive index and step refractive index.
What is the difference between single-mode and multimode fiber?
1. Core diameter
The main difference between multi-mode and single-mode fibers is that the former has a larger diameter, usually 50 or 62.5 µm core diameter, while typical single-mode fibers have core diameters of 8 and 10 µm. The layer diameters are all 125 µm.
2. Light source
Usually lasers and LEDs are used as light sources. The laser light source is obviously more expensive than the LED light source because the light it produces can be precisely controlled and has a high power. While the light generated by LED light sources is more scattered (light in many modes), these light sources are mostly used in multi-mode fiber jumpers. At the same time, a laser light source (which produces a near-single mode of light) is usually used for single-mode fiber patch cords.
Since multi-mode fiber has a larger core size than single-mode fiber, it supports multiple transmission modes. In addition, like multi-mode fiber, single-mode fiber also exhibits modal dispersion caused by multiple spatial modes, but the modal dispersion of single-mode fiber is smaller than that of multi-mode fiber. For these reasons, single-mode fiber has a higher bandwidth than multi-mode fiber.
4. Sheath color
The color of the sheath is sometimes used to distinguish between multimode fiber patch cords and multimode fiber patch cords. According to the definition of TIA-598C standard, for non-military use, single-mode fiber uses a yellow outer sheath, and multi-mode fiber uses an orange or aqua-green outer sheath. According to different types, some manufacturers use purple to distinguish high-performance OM4 fiber from other types of fiber.
5. Modal dispersion
LED light sources are sometimes used in multimode fibers to create a series of wavelengths that propagate at different speeds. This will cause multi-mode dispersion, which limits the effective transmission distance of multi-mode fiber jumpers. In contrast, a laser used to drive a single-mode fiber produces a single wavelength of light. Therefore, its modal dispersion is much smaller than that of multimode fiber. Due to modal dispersion, multi-mode fiber has a higher pulse spread rate than single-mode fiber, which limits the information transmission capacity of multi-mode fiber.
Multimode fiber can support multiple optical modes, and its price is higher than single-mode fiber. But in terms of equipment, since single-mode fiber usually uses solid-state laser diodes, single-mode fiber equipment is more expensive than multi-mode fiber equipment. Therefore, the cost of using multi-mode fiber is much less than the cost of using single-mode fiber.