Understanding Fiber Opitc Cable, Single Mode, Multi Mode

Basic structure of optical fiber
The general structure of optical fibers includes three parts: Fiber core, Cladding, and Jacket.



Both the core and cladding are made of glass of different refractive indices. The innermost part is the core, and the second layer on the top of the core is the cladding of an optical fiber. They have different refractive indices, and the refractive index of the core is always greater than the index of the cladding. This index difference causes total internal reflection to occur at the index boundary along the length of the fiber. A light ray is injected into the fiber optic cable, total internal reflection takes place so that it can transmit down the fiber and does not escape through the sidewalls.

The jacket protects the fiber from damage and moisture. It is made up of flexible and abrasion resistant varieties of plastic to increase the flexibility of the fiber. As we knew, the core and cladding are made of glass and cannot be bent and fragile. Usually, the jacket has another layer beneath it called buffer. The buffer and the jacket together protect the cable from environmental hazards.

Some of the optical fiber also has an outer jacket for further protection. The color of this outer layer of fiber is standardized so you can tell what type of fiber you have.

Normally, there are two types of optical fiber: single mode and multi mode. Single-mode fiber is a single glass fiber strand used to transmit a single mode or ray of light. Single mode fiber with a relatively narrow diameter, through which all signals travel straight down the middle without bouncing off the edges. Single-mode fiber features only one transmission mode. But Multimode fiber can carry multiple light rays (modes) at the same time because of its bigger diameter.

Transmission characteristics of optical fiber
Fiber has two main transmission characteristics: Signal loss and fiber dispersion. Signal loss expresses the loss as the attenuation per dB of length, which is measured in decibels(dB). The loss of optical fiber directly affects the transmission distance of optical fiber communication system or the distance between relay stations. Fiber dispersion means that the signal transmitted by the fiber is carried by different frequencies and modes, which will lead to different transmission speeds, resulting in signal distortion.

Fiber dispersion includes Material dispersion, Waveguide dispersion and Modal dispersion. The material dispersion and waveguide dispersion are usually caused by different signal frequencies. And the modal dispersion is caused by the signal’s multimode. Signals that are not in a single mode can cause modal dispersion. Single-mode fiber is so small in diameter that rays of light can reflect internally through one layer only. So there is only material dispersion and waveguide dispersion, and there is no modal dispersion. However, multimode fibers have modal dispersion. The dispersion of the fiber not only affects the transmission capacity of the fiber, but also limits the relay distance of the fiber-optic communication system. Compared with multimode fiber, single-mode fiber has higher bandwidth and can carry signals for longer distances.

Single mode optical fiber
Single mode fiber optic cable has a small diametral core that allows only one mode of light to propagate. Single Mode Fiber with a relatively narrow diameter of 8.5 to 9.5μm, through which only one mode will propagate typically 1310 or 1550nm.

Multi mode optical fiber
Multi mode fiber is an optical fiber that allows multiple guided modes to be transmitted. Multi-mode fiber has a larger diameter core, typically 50 or 62.5μm. This larger core allows multiple modes of light to propagate. The standard wavelengths of the multimode are 850 nm and 1300 nm. Multimode fiber is available in four classifications: OM1 (62.5/125 µm), OM2, OM3, OM4 (50/125 µm). There’s also a new multimode fiber standard known as WBMMF (wideband multimode fiber) which uses the wavelengths between 850nm and 953nm.

Both single mode fiber and multimode fiber have a cladding diameter of 125μm.



Single mode fiber or multi mode fiber?
Signal transmission distance
Single-mode fiber features a core diameter of nominally 9 µm, whereas makes the light reflection more compact, allowing only one mode of light to travel, thus making the optical signal travel farther. For single-mode transmission, modal dispersion is not a factor. So single mode fiber can get you 10km, 40km, and even farther without affecting the signal. It would be ideal for long distances that require more bandwidth such as telecommunications companies, cable television providers, and colleges and universities.

Multimode fiber has a large diameter core, which can transmit a variety of modes of light. In multimode, since the fiber core size is large, the signal “disperses” quickly, and the inter-modal dispersion is larger. Because the light travels different paths, the longer the cable is, the more distortion is caused by the different paths arriving at the receiving end at slightly different times. This is why single mode cable works better for longer distances. Multimode optical fiber is commonly used short distances, audio/video applications, and Local Area Networks (LANs). And OM3/OM4/OM5 multimode fibers can support high rate data transmission.


Bandwidth capacity
Bandwidth, which is defined as the information-carrying capacity. The main factors affecting the fiber transmission bandwidth are various dispersion, and the modal dispersion is the most important factor. The dispersion of the single mode fiber is small, so that the light can be transmitted over a long distance in a wide frequency band. As for the multi mode fiber, it can carry less in bandwidth and capacity than single mode fiber because of some complex problems such as interference and interference. The latest generation of multimode fiber bandwidth, OM5, is set at 28000MHz/km, while single mode fiber bandwidth is larger.

Cost
Why would we use multimode fiber if single mode has higher bandwidth and can carry this information over longer distances? Cost is the key. 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. However, optical transceiver is very expensive, then the cost of using single-mode fibers is higher than the cost of multimode fiber optic cables. This fact drives most data centers to use multimode for cost savings.

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