Speed Collection! The most comprehensive introduction to optical fiber cables ever!

Latest update time：2018-08-30

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Optical fiber

Optical fiber, the full name is optical fiber, and its English name is OPTIC FIBER.

It is a fiber made of glass or plastic that acts as a light transmission tool.

The main purpose of optical fiber is communication. Currently, the optical fiber used for communication is basically quartz-based optical fiber, whose main component is high-purity quartz glass, namely silicon dioxide (SiO2).

Fiber optic communication system uses optical fiber to transmit light waves that carry information to achieve the purpose of communication.

▎The development history of optical fiber communication

In 1880, Alexander Graham Bell invented the "optical telephone".

In 1887, British scientist Charles Vernon Boys pulled out the first optical fiber in his laboratory.

In 1938, Owens Illinois Glass Company in the United States and Nitto Bosei Corporation in Japan began producing long glass fibers.

The concept of cladding was proposed by optical physicist Brian O'Brian in 1951.

In 1956, a student at the University of Michigan made the first glass-clad optical fiber by using a tube of glass with a low refractive index melted onto a rod of glass with a higher refractive index.

In 1960, Theodore Maiman demonstrated the first laser. This ignited people's interest in optical communications. Lasers seemed to be a promising way of communication that could solve the transmission bandwidth problem, and many laboratories began experiments.

In 1966, British-Chinese scholar Charles Kao pointed out the possibility and technical approach of using optical fibers to transmit information, laying the foundation for modern optical communication - fiber-optic communication.

In 1970, Corning, an American company, successfully developed quartz optical fiber with a loss of 20dB/km.

In 1973, Bell Laboratories in the United States achieved even greater success, reducing optical fiber loss to 2.5dB/km.

In 1976, Nippon Telegraph and Telephone (NTT) reduced optical fiber loss to 0.47 dB/km (wavelength 1.2μm).

▎Characteristics of fiber optic communication

Huge communication capacity

Theoretically, one optical fiber can transmit 10 billion voice channels simultaneously. Currently, experiments on transmitting 500,000 voice channels simultaneously have been successful, which is thousands or even hundreds of thousands times higher than traditional coaxial cables, microwaves, etc.

Long relay distance

Optical fiber has an extremely low attenuation coefficient. When equipped with appropriate optical transmission and receiving equipment, optical amplifiers, forward error correction and RZ coding modulation technology, its relay distance can reach thousands of kilometers. Traditional cables can only transmit 1.5km and microwaves 50km, which is simply incomparable.

Good confidentiality performance
Adaptable

It has the advantages of not being afraid of interference from strong external electromagnetic fields and corrosion resistance.

Small size and light weight
The raw materials are abundant and cheap

▎Optical fiber structure

The typical structure of optical fiber is a multi-layer coaxial cylinder , which is mainly composed of core , cladding and coating .

Fiber Core

Located in the center of the optical fiber, it is made of high-purity silica with a very small amount of dopants. The refractive index of the core is slightly higher than that of the cladding, and the loss is lower than that of the cladding. The light energy is mainly transmitted in the core.

layers

Located around the core, the cladding is also made of high-purity silica containing a very small amount of dopants. The cladding provides a reflective surface and optical isolation for light transmission, and plays a certain mechanical protection role.

Coating

The outermost layer of the optical fiber, composed of acrylic, silicone rubber and nylon. The coating protects the optical fiber from moisture erosion and mechanical abrasion.

How Fiber Optics Work

Total reflection principle

If a light beam is directed from a denser medium to a less dense medium, the angle of refraction is greater than the angle of incidence, as shown in the figure.

If θ0 is continuously increased, the refraction angle θ1 can reach 90°. At this time, θ1 is called the critical angle.

When light travels from a denser medium to a less dense medium and the incident angle is greater than the critical angle, total internal reflection occurs.

Optical fiber uses this total reflection to transmit light signals.

Fiber Dispersion

Causes of Fiber Dispersion

In optical fiber, optical signals are composed of many different components. Since the propagation speeds of the various frequency components or mode components of the signal are different, after being transmitted through the optical fiber for a certain distance, time delay differences occur between different components, causing distortion of the transmission signal waveform and pulse broadening. This phenomenon is called fiber dispersion.

Effects of Fiber Dispersion

The existence of fiber dispersion causes the transmitted signal pulse to be distorted and broadened, thus generating inter-symbol interference. In order to ensure the communication quality, the inter-symbol interval must be increased, that is, the signal transmission rate must be reduced, which limits the communication capacity and transmission distance of the optical fiber system.

Classification of Fiber Dispersion

According to the cause of dispersion, optical fiber dispersion can be divided into mode dispersion, material dispersion, waveguide dispersion and polarization dispersion.

▎Electromagnetic spectrum of optical fiber

Fiber loss

Optical fiber loss refers to the reduction in optical power due to absorption, scattering, etc. after the optical signal is transmitted through the optical fiber.

Classification of optical fiber loss

Schematic diagram of the attenuation of ordinary single-mode optical fiber varying with wavelength

▎Classification of optical fiber

Step-index fiber

In the core and cladding regions, the refractive index distribution is uniform, n1 and n2 respectively. At the boundary between the core and cladding, the refractive index changes stepwise (n2<n1).

Graded Index Fiber

The refractive index at the center of the optical fiber is the largest (n1), but it gradually decreases with the increase of the radial direction of the cross-section. At the boundary between the core and the cladding, it drops to the refractive index n2 of the cladding region.

Multimode fiber (MMF)

It can transmit multiple modes of light. However, its inter-modal dispersion is large, which limits the frequency of transmitting digital signals, and it becomes more serious with the increase of distance.

Single-mode fiber (SMF)

Only one mode of light can be transmitted, so its inter-modal dispersion is very small, making it suitable for long-distance communication.

Comparison between multimode fiber and single-mode fiber

Applications of multimode fiber and single mode fiber

Fiber Optic Interface

There are several types of fiber optic interfaces:

FC round with thread (mostly used in patch panels)
ST snap-on round
SC card-type square type (mostly used on routers and switches)
The LC connector is similar in shape to the SC connector, but smaller than the SC connector.
MT-RJ square, dual-fiber transceiver at one end
MPO/MTP type
BFOC type
DIN Type
FDDI type
MU type

What do common expressions such as "FC/PC", "SC/PC", and "SC/APC" mean?

The part before “/” indicates the connector model of the pigtail. FC and SC are as mentioned above and are omitted here.

The part after “/” indicates the cross-section process of the optical fiber connector, that is, the grinding method.

"PC: Physical Contact":

The cross section of the connector is flat, which is actually a micro-spherical grinding and polishing, and is most widely used in the equipment of telecom operators.

"APC":

It is at an angle of 8 degrees and is micro-spherical ground and polished. It is a model that is widely used in radio and television and early CATV. The fiber pigtail head uses an angled end face to improve the quality of TV signals. The main reason is that TV signals are analog light modulated. When the coupling surface of the connector is vertical, the reflected light returns along the original path.

"UPC":

The attenuation is smaller than that of "PC" and is generally used for equipment with special requirements. Some foreign manufacturers use FC/UPC for internal fiber jumpers in ODF racks, mainly to improve the indicators of the ODF equipment itself.

Fiber Optic Module

Optical module, whose full name is optical transceiver, is an important device in fiber-optic communication system.

Generally, network devices include the following types:

SFP (Small Form-factor Pluggable transceiver):

Small package pluggable transceiver (LC interface), SFP supports rates of 100M, 155M, 622M, 1000M, 1250M, and 2500M.

GBIC (GigaBit Interface Converter):

Gigabit Ethernet Interface Converter (SC Interface)

XFP (10-Gigabit small Form-factor Pluggable transceiver):

10 Gigabit Ethernet interface small form factor pluggable transceiver (LC interface)

XENPAK (10 Gigabit EtherNet Transceiver PAcKage):

10 Gigabit Ethernet interface transceiver package (SC interface)

▎Fiber fusion

Fusion splicing is a wiring technology that uses the heat energy generated by discharge between electrode rods to melt the optical fibers into one piece. It is divided into the following two categories:

Fiber core alignment method

This is a fusion splicing method that observes the core of the optical fiber under a microscope, uses image processing to position it, aligns the central axis of the core, and then performs discharge. Positioning is performed from two directions using a fusion splicer equipped with a two-way observation camera.

Fiber fusion splicer

Fixed V-groove centering method

This is a fusion splicing method that uses high-precision V-grooves to arrange optical fibers and uses the core alignment effect generated by the surface tension when melting the optical fibers to align the outer diameter. Recently, due to the development of manufacturing technology, the dimensional accuracy of the optical fiber core position has been improved, so low-loss wiring can be achieved.

This method is mainly used for multi-core one-time wiring.

Optical Cable

Optical cable: Use appropriate materials and cable structures to contain and protect communication optical fibers, protecting them from mechanical and environmental influences and damage, and making them suitable for use in different occasions.

▎Optical cable structure

Optical cable is a structure made of one or more optical fibers or optical fiber bundles that conform to chemical, mechanical and environmental properties. Regardless of the structure of the optical cable, it is basically composed of three parts: cable core, reinforcement element and sheath.

Cable core

The cable core structure should meet the following basic requirements:

① Keep the optical fiber in the best position and state in the cable to ensure stable optical fiber transmission performance. When the optical cable is subjected to certain external forces such as pulling and lateral pressure, the optical fiber should not be affected by external forces.

② The reinforcement elements in the cable core should be able to withstand the allowable tensile force.

③ The cross section of the cable core should be as small as possible to reduce costs. The cable core contains optical fiber, casing or skeleton and strengthening elements. The cable core also needs to be filled with grease, which has reliable moisture-proof performance to prevent moisture from spreading in the cable core.

Sheath

The sheath of the optical cable is to protect the optical fiber core of the cable to avoid damage from external mechanical forces and the environment, so that the optical fiber can adapt to various laying occasions. Therefore, the sheath is required to be pressure-resistant, moisture-proof, have good temperature characteristics, be light in weight, resistant to chemical corrosion and flame-retardant.

The sheath of the optical cable can be divided into an inner sheath and an outer sheath. The inner sheath is generally made of polyethylene or polyvinyl chloride, and the outer sheath can be determined according to the laying conditions, such as an LAP outer sheath composed of aluminum tape and polyethylene plus steel wire armor.

Reinforcement elements

The reinforcement element is mainly used to bear the external force applied during laying and installation. The configuration of the optical cable reinforcement element is generally divided into "central reinforcement element" and "peripheral reinforcement element".

Generally, the reinforcing elements of stranded and skeleton optical cables are located in the center of the cable core, which are called "central reinforcing elements" (reinforced core); the reinforcing elements of central tube optical cables are moved from the cable core to the sheath, which are called "peripheral reinforcing elements".

The reinforcement components generally include metal steel wire and non-metal fiberglass reinforced plastic (FRP). Non-metallic optical cables using non-metallic reinforcement components can effectively resist lightning strikes.

▎Typical structure of optical cable

Commonly used optical cable structures include stranded type, skeleton type, central bundle tube type and ribbon type.

Structure diagram of stranded optical cable

Skeleton optical cable Structure diagram

Central bundle tube optical cable Structure diagram

Ribbon optical cable Structure diagram

▎Classification of optical cables

Outdoor Optical Cable

Mainly used for direct burial, pipeline and overhead construction of trunk lines and metropolitan area networks.

Ribbon Cable

It is mainly used for the construction of metropolitan backbone networks with large core count and high density.

Figure 8 optical cable

The optical cable integrates the cable core and the steel wire suspension wire into an "8"-shaped PE sheath to form a self-supporting structure. During the laying process, there is no need to set up suspension wires and hooks, which has high construction efficiency and low construction costs. It can be easily laid overhead between poles, between poles and buildings, and between buildings.

Indoor optical cable

Mainly used for LAN construction in buildings and vertical wiring in buildings

▎Optical cable model

According to the relevant recommendations of ITU-T, the current optical cable model consists of two parts: the optical cable type code and the optical fiber specification code, separated by a short horizontal line.

The type code of optical cable consists of five parts: classification, reinforcement components, derived characteristics, sheath and outer layer.

Optical cable classification codes and their meanings

GY: Indoor (field) optical cable for communication

GM: Mobile optical cable for communication

GJ: Indoor optical cable for communication

GS: Optical fiber cables for communication equipment

GH: Submarine optical cable for communication

GT: Special optical cable for communication

Codes and meanings of reinforcement components

No symbol: Metal reinforcement

F: Non-metallic reinforcement

Derived feature codes and their meanings

The structural characteristics of optical cables should be able to indicate the main types of cable cores and the derived structures of optical cables. When there are several structural characteristics of optical cables that need to be indicated, they can be indicated by combined codes, which are arranged in order from top to bottom according to the following corresponding codes.

D: Fiber ribbon structure

No symbol: Fiber loose tube coating structure

J: Optical fiber tight-buffered coating structure

Unsigned: Layered structure

G: Skeleton slot structure

X: Central beam tube structure

T: Ointment filling structure

Z: Self-supporting structure

B: Flat shape

Z: Flame retardant