The Selection of Optical Fiber
1.Structure, types of Optical fiber and Characteristics of Optical fiber telecommunication
1) structure
Optical fiber compose of a thin glass fiber or plastic fiber of 0.1 mm diameter. The special structure of optical fiber guarantee that the light travel longitudinally along the length of the fiber. Optical fiber is the main media for data transmission. Presently, the working wavelength ranges from 800nm~1800nm.
Optical fibers is a thin coaxial concentric cylinder made of quartz glass. Optical fibers consist of core, cladding and coating, as shown in below picture:
The core locates in the center of the optical fiber, it’s the transmission channel for light. Multi mode optical fiber is 50 or 62.5 μm in diameter, single mode optical fiber is usually 8 ~10 μm in diameter.
Cladding wraps the core, and the cladding helps seal the light in the core. The Refractive index of the cladding is smaller than that of the core to accomplish total reflection of light in the core. The cladding diameter is usually 122~128 μm.
Cladding is wrapped with coating, which helps protect the outer surface of the optical fiber and increase the tensile performance. The core and cladding is usually is usually coated twice to form the final optical fiber. It’s first time coated with silicone resin or polyurethane and coated again with nylon, polyethylene or polypropylene based on different application scenarios. The second coating can be loose tube, tight buffer or ribbon.
In loose tube structure, coated fibers are place in a relatively large tube, optical fiber have free room inside the loose tube, the tube is the second layer of coating. Loose tube structure has good mechanical performance, water resistance, and is easy to make optical cable without extra attenuation. A loose tube can usually carry 2~12 fibers, however. In some special designs,for example, in micro fiber optic cable for mini sized ducts, a loose tube can carry maximum 24 fibers. The structure has a strict requirement for manufacturing equipment. In tight buffer optical fiber structure, the coated optical fiber is tightly wrapped a second layer of buffer coating. This structure is simple in structure and easy to use, but it’s poor in crush resistance and sensitive to temperature in attenuation. It’s usually use for indoor cable and optical patch cords. Ribbon fiber structure usually incorporates some parallel fibers side by side, all of them seal with polyester into a ribbon shape. The ribbon structure is similar to tight buffer structure. It’s is place in loose tubes during cable production, so that the cable can achieve advantages of both loose tube structure and tight buffer structure. Typically a ribbon can incorporate 4~12 fibers. The outer polyester coating has weak tensile force, which make it easy to split the ribbon so that the fibers can be used individually. Ribbon structure features large fiber capacity, easy application. It’s is very suitable for densely populated areas, and ribbon fiber optic cable is the main cable for ftth.
2) Types of optical fiber
There are many ways for categorizing optical fibers. Optical fibers can be divided by raw material, manufacturing techniques, optical transmission mode, refractive index profile of fiber cross section, and working wavelength. An optical fiber can have different names according to different categorizing methods. In telecommunication lines, optical fibers can be divided with the following methods.
A. Categorizing according to raw material, optical fibers can be divided into glass fiber and plastic fiber.
Glass fiber’s core and cladding usually incorporates quartz. This optical fiber features low attenuation and medium dispersion. Most optical fibers use quartz as raw material.
Plastic optical fiber is a new optical fiber, the core and cladding of which are made of plastic. It has larger attenuation than quartz optical fiber. Plastic optical fiber has large core diameter, which is easy for installation, and large Numerical aperture, which is easy for joint connection and the production cost for plastic optical fiber is much lower so it’s usually used in short distance transmission.
B. Categorizing by the refractive index on the cross section of optical fibers, they can be divided into Step-Index Fiber (SIF) and Graded-Index Fiber (GIF).
The refractive index of SIF’s core and cladding features stepped changed at the interface between core and cladding; The refractive index of GIF’s core become smaller gradually as diameter goes bigger and eventually equals to that of its cladding.
C. Categorizing by the transmission mode, optical fibers can be divided into multi mode optical fiber (MMF) and single mode optical fiber (SMF).
MMF is an optical fiber that allows multiple transmission modes at certain wavelength. Multi-mode optical fiber can be further divided into Step-Index Fiber also called multi-mode uniform fiber and Graded-Index Fiber multi-mode non-uniform fiber. Multi-mode fiber’s core diameter is usually 50μm and 62.5 μm and cladding diameter is usually 125 μm. The large core diameter and multiple transmission mode cause higher attenuation and smaller bandwidth, and is not suitable for long distance transmission. So it’s usually used for short distance transmission.
SMF is an optical fiber that allows only one transmission mode at certain wavelength. It allows transmission at the basic mode and there is no time spacing between different modes, so the fiber has much larger bandwidth than multi-mode fiber. The refractive index of single mode optical fiber features stepped change. SMF’s mode field diameter is usually 8~10 μm, the cladding diameter is usually 125 μm. The bandwidth of single mode optical fiber is about dozens of GHz, it’s two orders of magnitude higher than the grade index multi-mode fiber.
D. Folllowing ITU-T standards, optical fibers can be divided into G.651, G652, G.653, G.654, G.655, G656, G.657. To keep the advantage and security of its standards, the International Telecommunication Union, often modifies and add new content to its released standards according to optical fiber’s technical development.
A) G.651 optical fiber. This fiber works at both wavelengths of 1310nm and 1550nm, and has lowest dispersion at 1310nm and lowest attenuation at 1550nm. G651 optical fiber can be further divided into A, B, C and D types. And their Core diameter, Cladding diameter, and Numerical aperture are respectively:
G.651 type A: 50 μm /125 μm /0.2
G.651 type B: 62.5 μm /125 μm /0.275
G.651 type C: 85 μm / 125 μm / 0.275
G.651 type D: 100 μm / 140 μm / 0.316
G.651 optical fiber is mainly applied in area network, and among them type A and type B is more commonly used.
B) G.652 optical fiber. This fiber is called standard single mode fiber, also called non dispersion shifted optical fiber. The fiber works at wavelength of 1310nm and 1550nm. Its non-zero dispersion wavelength is 1310nm. It has the lowest attenuation at wavelength of 1550nm yet the dispersion is high at wavelength of 1550nm. So this fiber has the best transmission performance at 1310nm. G.652 optical went commercial from year 1983. Today, over 70 million kilometers of the fiber is installed, which makes it the most widely used fiber in the world. G.652 optical fiber can be further divided into 4 types.
G.652 type A optical fiber is mainly used in the SDH transmission system as per ITU-T G.957 standards and the max STM-16 single-channel SDH transmission system with optical amplification as per ITU-T G.691.
G.652 type B optical fiber is mainly used in the SDH transmission system as per ITU-T G.957 standards and the max STM-64 single-channel SDH transmission system with optical amplification as per ITU-T G.691.
G.652 type C optical fiber is also called low water peak optical fiber. The attenuation changes little at the wavelength of 1310~1360nm, so the fiber has much more reusable wavelengths than G.652B fiber, and is suitable for city area network.
G.652 type D optical fiber is similar to G652C in optical performance, however G.652D works at a wider range of wavelength than G652C does.
G.652 optical fiber has its shortcomings. Its dispersion at 1550nm is too high, and the dispersion coefficient reaches 18 ps/(nm?km), which is not suitable for long distance transmission and the application of Dense WDM System.
C) G.653 optical fiber. This fiber is also called dispersion shifted fiber. It has the lowest attenuation and no dispersion at 1550nm and is suitable for long distance transmission system, mainly applied in the submarine transmission system that has single channel transmission stretching thousands of kilometers and the backbone system of land based long distance transmission. G.653 optical fiber is not suitable for WDM System because Nonlinear effects of four-wave mixing occurs when using EDFA to transmit WDM signals.
D) G.654 optical fiber. This optical fiber has the extremely low attenuation at 1550nm (0.18 dB/km). The fiber has low bend loss. But the production process is complex, and cost is high so it’s less applied. It’s mainly used for submarine transmission system that features the long distance transmission and no active equipment.
E) G.655 optical fiber. This fiber is also called non-zero dispersion shifted single mode fiber. It’s designed and made for optical amplification Dense WDM System. The dispersion of G.655 optical fiber at 1550nm is 0.1~6.0ps/(nm?km), which better balances the non-linear effect of four-wave mixing, and is suitable for high speed, large capacity, dense WDM long distance transmission. G.655 optical fiber can be further divide into A, B and C types.
G.655 type A mainly supports the application of ITU-T G.691, G.692, G.693, G.959.1. For the DWDM system of G.692, total input power is limited according to channel wavelength and dispersion characteristics, typically the minimum spacing between channels is no less than 200GHz.
G.655 type B mainly supports the application of ITU-T G.691, G.692, G.693 and G.959.1. In the DWDM system of G.692, according to the channel wavelength and required dispersion characteristics, the optical fiber allows higher total input power that G.655A does, and typically the minimum spacing between channels is no less than 100GHz.
655 type C is similar to G.655B, but has higher standard in polarized mode dispersion. The fiber allows over 400km transmission of STM-64 system and supports the application of G.959.1 STM-256.
G.655 optical fiber features low dispersion at 1550nm, and is suitable for over 10Gb/s high speed transmission system that combines EDFA and DWDM system.
F) G656 optical fiber. This fiber is “broadband optical transmission non-zero dispersion optical fiber”, which means it has active dispersion at wavelength of 1460~1625nm. G.656 optical fiber’s wavelength at working wavelengths should be larger than required value, it has good effective area, and its dispersion slope is almost zero. So, G.656 optical fiber can largely lower the cost for dispersion compensation, helps further develop the quartz’s potential in bandwidth. G.656 optical fiber guarantees 400km transmission distance for the system of 100GHz channel spacing and 40Gb/s. G.656 optical fiber is suitable for the expansion of wavelengths used for DWM transmission in backbone or city area network.
G) G.657 optical fiber. G.657 optical fiber is also called “access network use bend-insensitive single mode optical fiber”, G.657A optical fiber has a minimum bend radius of 15 mm. G.657B optical fiber has a minimum bend radius of 7.5mm. G.657 optical fiber is often used in access network. This optical fiber has been widely applied because of high cost.
Above is ITU’s category of optical fibers in different times and for different application scenarios. Among them the most commonly used are G.651 multimode optical fiber, G652 single mode optical fiber, and G.655 single mode optical fiber. ITU also mentions a dispersion compensating fiber. It’s a passive parts. In optical transmission, its negative dispersion is used to compensate for the positive dispersion that occurs during optical transmission. What’s worth mentioning is the most widely used optical fiber is still G652 optical fiber.
3) Characteristics of optical fiber transmission
To sum up, optical fiber telecommunication has below advantage:
A. Wider bandwidth, reaching 30 THz.
B. Low attenuation. The most commonly used optical fiber is quartz optical fiber. High purity quartz glass is used to make optical fiber, so the attenuation is really low. The relay distance is longer, thus reducing the use of repeater and lowering the construction cost.
C. Free from electromagnetic influence. Optical fiber cable is used to transmit light, which is free from electromagnetic influence, and won’t radiate itself.
D. Small diameter, light weight. Optical fiber is about 0.1mm in diameter, optical fiber cables are smaller in diameter and weight, and thus optical cables of larger number of fibers can be easily made to increase space utilization.
E. Abundant raw material. The main raw material for optical fiber is quartz, which is easily obtained.
Optical fibers has certain shortcomings, for example, it’s fragile and it is not easy to do fiber joint, cut, split and adapt. Installer must be well trained to do these work.
2.The attenuation of optical fiber and bandwidth
Parameters of optical fibers can be described in 3 aspects, geometrical parameters, optical parameter, and transmission performance parameters. Here we will introduce some typical parameters of optical fiber.
The most important is attenuation and bandwidth. Attenuation refers to the optical power loss during transmission, and bandwidth refers to the scattering of optical power during transmission. Bandwidth is measured in MHz?km, it’s the product of maximum modulation frequency pulse multiplied by optical fiber length.
1) Attenuation of optical fiber
Optical signal goes weaker as distance becomes longer, and this is the attenuation of optical fiber. Usually the optical loss that occurred in a unit length of optical fiber is defined as attenuation. It’s calculated with below formula:
α=10 lgPi/P0
Note: α refers to optical attenuation coefficient, dB/km;
Pi refers to input optical power, W;
P0 refers to output optical power, W
Suppose the attenuation coefficient of a single mode optical fiber is 0.18 dB/km. After amplification, a optical signal’s input power is enlarged into +5 dBm, the receiving sensitivity of the receiving end is -28 dBm. So the gain of the amplification is 33 dB. Divide the gain by attenuation coefficient and you get transmission distance 33/0.18=183 km. Considering other factors like the aging of transmission line, the fiber can achieve a transmission distance of 120 km throughout its service life.
There are many causes for attenuation, main causes includes absorb attenuation, scattering attenuation, and bend attenuation. And the absorb attenuation is the major cause of attenuation.
Absorb attenuation consists of intrinsic absorption, impurities absorption and atomic defect absorption. The material of optical fiber and impurities have strong capability of absorbing optical fibers. And this is an important cause for optical signal attenuation.
Scattering attenuation includes linear scattering, non-linear scattering, structural incomplete scattering.This attenuation is usually caused due to micro change of material density and the non-uniformity of SiO2, GeO2, PeO5, which cause the non-uniformity in the scattering of refractive index and allows light to be scatter outside the fiber, or it’s caused due to the such defects as bubbles in the core and cladding that leads to the scattering of lights. The size of structural defects is usually way larger than the wavelength of optical signal, and cause extra attenuation to all wavelengths. But this attenuation is little compared to absorption attenuation.
Bend attenuation has two forms, one is called macro bend attenuation that occurs because the bend radius is larger than the optical fiber diameter, and the other is called micro bend attenuation that occurs because tiny twist of optical fiber.
2) Dispersion and bandwidth of optical fiber
Dispersion refers to the phenomena where light pulse lasts longer at the output end after travelling through optical fibers. Dispersion measures the increased pulse lasting time on every unit of spectrum width and every unit of fiber length. Dispersion is the main cause for narrowing of bandwidth, which further limits the data transmission capacity. Categorizing by the causes, dispersion can be divided into 4 types, mode dispersion, material dispersion, waveguide dispersion and polarized dispersion.
A. Mode dispersion. This type of dispersion occurs to multimode optical fibers. Due to the difference in transmission route, light signal of different modes reach output end at different time and cause the light pulse to last longer. This type of dispersion is call mode dispersion.
B. Material dispersion. This type of dispersion occurs because material’s refractive index varies with regard to different wavelength, which can further increases the lasting time of output pulse.
C. Waveguide dispersion. Due to small refractive index gap between core and cladding, when full reflection happens, some lights went into cladding. The light intensity is related to light’s wavelength. And this means lights of different wavelengths travels through different lengths of routes. So the light pulse emitted by the light source lasts longer at the receiving ends because the light pulse consists of lights of different wavelengths which travel through different lengths of route to the receiving end. The longer the wavelength, the more light intensity that goes into cladding and the longer the transmission route. And this is called waveguide dispersion.
D. Polarized dispersion. It’s also call polarized mode dispersion. It has two causes.
Internal factor. Core non-concentricity error, uneven stress distribution, crush force, bending and torsion on optical fibers during production process causes birefringence. In transmission of lights in single mode fibers, transmission group speed difference occurs between two orthogonal polarized linear modes, and this cause polarized mode dispersion.
External factor. Outer birefringence stands for unstable polarization of lights in single mode fibers caused by outer factors. There are many outer factors, so the formula for outer birefringence can’t be united as one. Outer factors can cause birefringence of optical fibers in that they cause new anisotropy on optical fiber. For example, in the stranding process or installation process, optical fibers may suffer from bending, torsion, shaking, and crushing, the randomness of these forces could cause birefringence inside optical fibers. Also strong electromagnetic field and temperature variation may also have an influence on it.
Both material dispersion, and waveguide dispersion are related to wavelength, so they are also called wavelength dispersion. Mode dispersion occurs inside multimode fibers only. The rest types of dispersion can happen in single mode optical fibers. The dispersion follows below sequence: Mode dispersion > material dispersion > waveguide dispersion > polarized dispersion. It’s clear that multimode fiber’s bandwidth is determined by mode dispersion while single mode fiber has no mode dispersion and thus has very wide bandwidth.
Optical fiber’s transmission bandwidth limits the signal transmission speed. Generally, when the receiving end just failed to receive signals, the product of optical signal speed and optical fiber length is the bandwidth of the length of the optical fiber. The fiber length is usually a unit length, so in writing, we usually ignore it. That’s why we see bandwidth measured in MHz, GHz, etc.
3.The selection of optical fibers
The selection of optical fibers must be done with great caution. The selection should be done based on the service requirement on transmission speed, transmission distance, quality transmission system etc.. The selection of optical fiber must also take into consideration the future need of bandwidth, and cost performance. The planner must fully understand the all kinds of optical fiber, then select the right fiber based on their transmission performance, system single channel rate, with WDM or DWDM, etc..
1) Multi mode optical fiber
Multimode optical fibers have low price, large core diameter, and easy to do joint compared to single mode fiber, yet they have high attenuation coefficient, large dispersion, and small bandwidth. They are suitable for only short distance transmission.
2) Single mode optical fiber
Single mode optical fiber, compare to multi mode optical fibre, has lower attenuation coefficient, high bandwidth, and is suitable for long distance transmission.