To understand fiber, single mode fiber, multimode fiber.

March 11, 2022

Author: Amy Doelly

A bare fiber is generally divided into three layers: core, cladding and coating.

The fiber core and cladding are composed of glass with different refractive indices, the center is the high-refractive-index glass core (germanium-doped silica), and the middle is the low-refractive-index silica glass cladding (pure silica). Light enters the fiber at a specific angle of incidence, and full emission occurs between the fiber and the cladding (because the refractive index of the cladding is slightly lower than the core), so that it can propagate in the fiber.

The main function of the coating layer is to protect the optical fiber from external damage while increasing the flexibility of the optical fiber. As mentioned earlier, the core and cladding are made of glass, which cannot be bent or brittle, and the use of the coating layer protects and prolongs the life of the fiber.

A layer of outer sheath is added to the non-bare fiber. In addition to protecting, the outer sheath of different colors can also be used to distinguish various fibers.

Optical fibers are divided into single-mode fibers (Single Mode Fiber) and multi-mode fibers (Multi Mode Fiber) according to the transmission mode. Light enters the fiber at a specific angle of incidence, and full emission occurs between the fiber and the cladding. When the diameter is small, only one direction of light is allowed to pass through, which is a single-mode fiber; when the diameter of the fiber is large, light can be allowed to pass. It enters and propagates at multiple angles of incidence, and it is called a multimode fiber at this time.

Transmission characteristics of optical fibers

Optical fibers have two main transmission properties: loss and dispersion. fiber loss refers to the attenuation per unit length of the fiber, in dB/km. The level of optical fiber loss directly affects the transmission distance of the optical fiber communication system or the distance between the repeater stations. Fiber dispersion refers to the distortion of the signal due to the fact that the signal transmitted by the fiber is carried by different frequency components and different mode components, and the transmission speeds of different frequency components and different mode components are different.

Fiber dispersion is divided into material dispersion, waveguide dispersion and modal dispersion. The first two kinds of dispersion are caused by the fact that the signal is not of a single frequency, and the latter kind of dispersion is caused by the fact that the signal is not of a single mode. A signal that is not a single mode causes modal dispersion. A single-mode fiber only transmits a single fundamental mode, so there is only material dispersion and waveguide dispersion, and no modal dispersion. Multimode fibers, on the other hand, have intermodal dispersion. The dispersion of the optical fiber not only affects the transmission capacity of the optical fiber, but also limits the relay distance of the optical fiber communication system.

single mode fiber

Single-mode fiber (Single Mode Fiber), light enters the fiber at a specific angle of incidence, and full emission occurs between the fiber and the cladding. When the diameter is small, only light in one direction is allowed to pass through, which is a single-mode fiber; single mode fiber; Mode fibers have a thin central glass core, typically 8.5 or 9.5?m in diameter, and operate at wavelengths of 1310 and 1550 nm.

Multimode fiber

Multimode fiber is a fiber that allows multiple guided modes to transmit. The core diameter of a multimode fiber is generally 50?m/62.5?m. Due to the large core diameter of a multimode fiber, light of different modes can be transmitted on one fiber. The standard wavelengths for multimode are 850nm and 1300nm, respectively. There is also a new multimode fiber standard called WBMMF (Wideband Multimode Fiber), which uses wavelengths between 850nm and 953nm.

Single-mode fiber and multi-mode fiber, both with a cladding diameter of 125?m.

Single mode fiber or multimode fiber?

Transmission distance

The smaller diameter of a single-mode fiber makes the reflection tighter, allowing only one mode of light to travel, allowing the light signal to travel farther. As the light passes through the core, the number of light reflections is reduced, reducing attenuation and generating further signal propagation. Because it has no intermodal dispersion or very small intermodal dispersion, single-mode fiber can transmit 40 kilometers or more without affecting the signal. Therefore, single-mode fiber is generally used for long-distance data transmission, and is widely used in telecommunications companies, cable TV providers and colleges and universities, etc.

Multimode fibers have larger diameter cores that can propagate multiple modes of light. In multimode transmission, due to the larger core size, the dispersion between modes is larger, that is, the optical signal "spreads" faster. Signal quality degrades over long distances, so multimode fiber is often used for short distances, audio/video applications, and local area networks (LANs), and OM3/OM4/OM5 multimode fiber can support high-speed data transmission.

bandwidth, capacity

Bandwidth is defined as the ability to carry information. The main factor affecting the width of the optical fiber transmission bandwidth is various dispersion, and the modal dispersion is the most important. The dispersion of the single-mode fiber is small, so it can transmit light over a long distance with a wide frequency band. Because multi-mode fiber will cause complex problems such as interference and interference, it is inferior to single-mode fiber in terms of bandwidth and capacity. The latest generation of multimode fiber bandwidth OM5 is set to 28000MHz/km, while the single mode fiber bandwidth is much larger.

cost

If single-mode fiber has higher bandwidth and can travel farther, why do you need multi-mode fiber? Cost may be the crux of the matter. Since the core diameter of the single-mode fiber is too small, it is difficult to control the beam transmission, so a laser is required as the light source body. Since the optical transceiver is very expensive, the cost of using single-mode fiber will be higher than the cost of multi-mode fiber optic cable. This fact motivates most data centers to use multimode fiber to save costs.

Which mode of fiber to choose depends more on the required application environment. Yiyuantong can provide various types of fiber patch cords. HYC is a national high-tech enterprise focusing on the research and development, manufacturing, sales and service of passive basic components for optical communication. The company's main products are: optical fiber connectors (high-density optical connectors for data centers), wavelength division multiplexers, optical splitters and other three core optical passive basic components, which are widely used in fiber-to-the-home, 4G/5G mobile Communication, Internet data center, defense communication and other fields.