What’s difference between single-mode optical fiber and multi-mode optical fiber?

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An optical fiber is a flexible, transparent fiber made of extruded glass or plastic, slightly thicker than a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than wire cables. Optical fibers typically include a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by the phenomenon of total internal reflection which causes the fiber to act as a waveguide.

In general, there are two kinds of optical fiber: fibers that support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those that support a single mode are called single-mode fibers (SMF). But what’s difference between them? Reading this text will help you get the answer.

What’s single-mode optical fiber?

In fiber-optic communication, a single-mode optical fiber (SMF) is an optical fiber designed to carry light only directly down the fibre – the transverse mode. For single-mode optical fiber, no matter it operates at 100 Mbit/s or 1 Gbit/s date rates , the transmission distance can reach to at least 5 km. Typically, it is used for long-distance signal transmission.

What’s multi-mode optical fiber?

Multi-mode optical fiber (MMF) is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus. Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km (100BASE-FX), 1 Gbit/s up to 1000m, and 10 Gbit/s up to 550 m. There are two kinds of multi-mode indexes: step index and graded index.

Types of Fiber Optic
Multimode :
(1) SR:(short range):
SR stands for Short range,these trancievers support links length of 300m over multimode fiber
and use 850nm lasers.
2:LRM:
LRM means long reach Multimode ,these trancievers support distance up to 220m over multimode
fiber and use 1310nm lasers.

Single Mode:
LR:
LR stands for long reach, these trancievers support distance up to 10 km over single mode fiber
ans use 1310nm laser.
ER:
ER means extended reach, the data rate of these trancievers support distance up to 40 km over single mode fiber and use 1550nm lasers.
ZR:
ZR also stands for extended reach which can transmit 10 g data rate and 80 km distance over single mode fiber and use 1550nm laser.

nowaday 120 km is available.
                     
What’s difference between SMF and MMF?

• Core diameter

The main difference between multi-mode and single-mode fiber is that the former has much larger core diameter, typically has a core diameter of 50 or 62.5 µm and a cladding diameter of 125 µm. While a typical single-mode fiber has a core diameter between 8 and 10 µm and a cladding diameter of 125 µm.

• Optical source
  Both lasers and LEDs are used as light sources. Laser light sources are significantly more expensive than LED light sources however they produce a light that can be precisely controlled and which has a high power. Because the LED light sources produce a more dispersed light source (many modes of light) these light sources are used with multi-mode
  cable. While a laser source is used (which produces close to a single mode of light) with single-mode cable.

• Bandwidth
  Since multi-mode fiber has a larger core-size than single-mode fiber, it supports more than one propagation mode. Besides, like multi-mode fibers, single-mode fibers do exhibit modal dispersion resulting from multiple spatial modes, but the modal dispersion of single-mode fiber is less than multi-mode fiber. For these reasons, single-mode fibers can have a higher bandwidth than multi-mode fibers.

• Jacket color
  Jacket color is sometimes used to distinguish multi-mode cables from single-mode ones. The standard TIA-598C recommends, for non-military applications, the use of a yellow jacket for single-mode fiber, and orange or aqua for multi-mode fiber, depending on type. Some vendors use violet to distinguish higher performance OM4 communications fiber from other types.

• Modal dispersion
  The LED light sources sometimes used with multi-mode fiber produce a range of wavelengths and these each propagate at different speeds. This will lead to much modal dispersion, which is a limit to the useful length for multi-mode fiber optic cable. In contrast, the lasers used to drive single-mode fibers produce coherent light of a single wavelength. Hence its modal dispersion is much less than multi-mode fiber. Due to the modal dispersion, multi-mode fiber has higher pulse spreading rates than single mode fiber, limiting multi-mode fiber’s information transmission capacity.

• Price
  For multi-mode fiber can support multiple light mode, the price of it is higher than single-mode fiber. But in terms of the equipment, because single-mode fiber normally uses solid-state laser diodes, therefore, the equipment for single-mode fiber is more expensive than equipment for multi-mode fiber. And for this reason , the cost of using multi-mode fiber is much less than using single-mode fiber instead.

What kind of optical fiber should I choose?
This is based on transmission distance to be covered as well as the overall budget allowed. If the distance is less than a couple of miles, multi-mode fiber will work well and transmission system costs (transmitter and receiver) will be in the $500 to $800 range. If the distance to be covered is more than 3-5 miles, single-mode fiber is the choice. Transmission systems designed for use with this fiber will typically cost more than $1000 due to the increased cost of the laser diode.