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Guidelines for Selecting Fiber Optic Cables and Connectors

As a signal transmission technology, optical fiber utilizes the transmission medium. The transmission medium in fiber optic technology is fiber optic cable. Typically, a fiber optic cable network consists of several fiber optic cables. These fibers are integrated into the network using specific fiber optic connectors. Since cables and connectors are fundamental elements of fiber optic networks, it is important to select the correct type of fiber optic cable and connector for a specific application. However, the selection of these two elements is a complex process as the range of types, properties and specifications varies. As technology has advanced, fiber optic cables and connectors have evolved to have several beneficial parameters, and as a result, the choice has become a bit more complicated.

Fiber Optic Cable and Connector Definitions

What is Fiber Optic Cable?

Fiber or fibre optic technology is an effective cabled-based communication system. It is reliable, versatile, and widely used in many applications and industries.
Optical fibre cabling is used to transfer information via pulses of light, which pass along one or more transparent plastic or glass pipes. In some cases, this can be more than several hundred pipes. Each of these strands is little wider than an average hair and is normally surrounded by a further layer of cladding which is also in plastic or glass but constructed at a different density to the main inner strand.
 
A sheath made up of several layers of insulated casing is also wrapped around the cladded fibres. This usually comprises of a protective wrapper, known as a buffer tube, followed by a final outer jacket designed to protect the entire multi-stranded cable.

Selection Parameters for Fiber Optic Cables

The following is the list of selected parameters of fiber optic cables for specific applications.
Buffered Fiber Cable: The buffered fiber optics cable construction is the encapsulation of multiple optical fibers inside a buffer tube or a jacket. The buffer tube bounds all fiber strands together and also offer protection against external forces.
There are two specific types of buffered fiber cables, which are listed below.
 
  • Loose Buffer Construction: The loose buffer construction has multiple optical fibers inside the buffer tube but each fiber strand is jacketed with a water-blocked polymer tube. This construction keeps the fiber strands distanced from each other, therefore, the level of attenuation is negligible here. Also, having polymer protection, the central strength of the fiber ultimately increases. Due to the same reason, these types of optical cables are highly immune to external forces like external pull, moisture, noise, etc. Having a loose gap between the fiber strands, it is called loose pack constructions. This type is suited for outdoor device-to-device connectivity applications.
  • Tight Buffer Construction: In these cables, the optical strands are covered with buffer material, which is placed directly on the fiber in the form of winding. The tight winding of buffer material makes a protective layer on the fibers. Further, the buffered fibers are jacketed with the buffer tube. This construction makes the compact, lightweight and flexible. These fiber optic cables are suited for conduits and enclosures.
Simplex Cable Construction: The simplex fiber optic cable construction features a single tight-buffered optical strand inside a cable jacket. In this type, the fiber optic strand becomes a single channel that enables a direct device to device data transmission. Due to single-channel transmission, these cables offer uniformity and consistency in the transmission which further makes the interconnection of two devices more convenient.
Multichannel Cable Construction: Multichannel cable construction features multiple tight or loose buffered cables jacketed together. Each buffered optical strand makes a channel. Therefore, this type of cable construction comprises multiple channels. The buffered optical strands are supported by a central strength member. This type of cable construction is suitable for indoor as well as outdoor applications. However, outdoor fiber optic cables feature a water-blocked polymer between the strands.
All these types of constructions are available in two construction standards or configurations. The following cable configurations are available in fiber optic cables.
  • Zip-cord Configuration: This configuration features two or more bundles of buffered optical strands inside the protective jacket. However, each bundle is covered in its own jacket and then multiple bundles are held together by an outer protective jacket. This configuration can have bundles simplex or duplex optical strands under the Siamese version of fiber optic cable design and manufacturing.
  • Loose Tube Configuration: This is Telco standard fiber optic configuration. It often features a non-buffered type of optical strands surrounded by water repellent gel. This water-block gel acts as buffer material. This construction can be single or multi-channel. This configuration is often utilized for telecommunication purposes, however, it claims not-suited for video transmission over Ethernet.
Above mentioned fiber optic cable construction and configuration criteria are further tested for compatibility with application requirements and operational environments during optical fiber cable selection. For specific application and environment, one or more types of cable constructions and configurations can be suited, however, the best fit is advised to be selected.
The following are industrial environment-based constructions available in fiber optic cables.
  • Water-blocked cable construction- Marine or outdoor fiber optic applications.
  • Armored cable construction- Construction and aerospace fiber optic applications.
  • Aerial cable construction- Aeronautical and telecommunication fiber optic applications.
  • Tactical cable construction- Military and defense fiber-optic applications.
 
Along with the mentioned selection factors, the fiber optic cable has to be compatible with the fiber optic connectors. There is a vast range of fiber optic connectors available, therefore, the selection itself is a complex procedure. Similar to fiber optic cables, the selection of fiber optics connectors takes consideration of several section parameters.
 

What is optical fiber connector?

An optical fiber connector joins optical fibers, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so light can pass. Better connectors lose very little light due to reflection or misalignment of the fibers. In all, about 100 different types of fiber optic connectors have been introduced to the market.

Selection Parameters for Fiber Optic Cables

Consideration of the following parameters while a selection of fiber optics connectors is advised by industrial experts.
PC: PC stands for physical contact. There are two types in this polishing style, namely, flat physical contact and the rounded type. This polishing can offer a slight air gap or close fit contact. The reflection loss in this type of polishing is lesser than (-40 dB).
UPC: UPC stands for ultra-physical contact. This polishing style gives lesser than (-50 dB) reflection loss.
APC: APC stands for angled physical contact. In this style of polishing, the contact surface is angled at 8 degrees which reduces back reflection to the value lower than (-60 dB). Therefore, APC gives the optimum efficient contact interface to the fiber optic connectors.
 
Anaerobic Adhesive Termination: In this type of termination, the connector ferrule has a quick setting adhesive. This type is suitable for indoor fiber optic applications as the adhesives may not sustain water, high temperature, or rough handling. This type of termination can be adopted for the on-off type of connections.
Epoxy-Polish Termination: In this type of termination, the ferrule is glued with epoxy and then the end is polished with a special film. The connectors with this termination type are suitable for applications where low loss is expected.
Crimp Termination: Instead of glue or epoxy, this type of termination uses a crimp to attach the ferrule and fiber. The connectors with this termination type are suited for quick and firm termination but losses may be more in this case.
Pre-Polished Termination: The pre-polished termination features factory installation. It involves in-built short stub fiber inserted and glued into the ferrule, therefore, the operator only has to insert the termination fiber like splicing. This is a quick and easy way of termination, however, these connectors are expensive. If cleaved during installation, these connectors can give nearly zero losses.
Connector Contact Polishing Style: The connector contact polishing style determines the type of contact enabled between the connector and cable. The polishing style impacts the back reflection, which is a measure of light reflected from the contact end of the connector. The higher the back reflection, the higher is the reflection losses in the fiber-optic network. There are three main styles of connector polishing available as listed below.
Termination Types: Since fiber optic connectors terminate the fiber optic cables, they are available in various termination types. One must consider termination types during the selection of connectors as it determines the connective force between the ferrule and the cable.
 
Type of Fiber/Fiber Mode: The fiber optics connectors are associated with standard types of fiber optic cables and modes of transmission. The connector must complement the type of fiber cable being connected. The mode of transmission impacts the speed of transmission, input and output attributes, etc. Therefore, the connectors should be selected under fiber optic networking standards like 10GBase-LX4, 10GBase-LR, 10GBase-ER, 10GBase-LRM, 10GBase-SW, 10GBase-LW, 10GBase-EW, etc. These fiber optic cable network standards define the speed, mode, and distance of transmission. The fiber optic connector must complement the type of fiber optic transmission standard adopted by the user.
Fiber Count: Different types of fiber optic connectors are suitable for a specific number of fiber-count in the optical fiber cable. For simplex cables that feature only one optical strand, FC, SC, ST, LU, etc type of connectors are suitable. For duplex cables with two optical strands, MU, MJRT, etc types of connectors are preferred. Further, for multi-fiber cables, which may have from 4 to 24 number of optical strands advanced connectors like MT connectors are suitable.
Ferrule Material and Dimensions: The ferrules are long cylindrical bored slots on the end of connectors. The fiber optics cable is inserted through the ferrule itself. Therefore, ferrule has a slightly larger diameter than the cable outer diameter. Generally, 1.25 mm and 2.5 mm ferrule diameters are used in the industry. Depending on the application, the material of ferrule may also impact the operation. Ferrules are made of metals, ceramic or plastic materials. The operator must test the durability and functionality of the ferrule for specific application during selection.
Coupling/ Attachment Mechanism: The connectors are designed for various types of coupling mechanisms. The coupling mechanism is basically a mechanical linkage to attach the connector with the cable. There are three common coupling mechanisms available in fiber optic connector designs, namely, push-pull latch coupling, screw coupling, and bayonet coupling. A push-pull latch offers a quick connection but reliability is limited in it. If integrated with an alignment pin, it can be highly reliable. The screw coupling mechanism offers easy but time-consuming installation and moderate durability if not tightened harshly. The bayonet is a twist and snaps type of coupling mechanism. This mechanism is quick, easy, and reliable, therefore, connectors with this type of coupling mechanism can be used for a permanent or on-off type of fiber-connector linkage.
Connector Body: The connector body is the outer structure or housing that supports the ferrule and coupling mechanism. Its quality directly impacts the life-cycle of the connector. Thus, based on your application, metallic or plastic made connector body can be chosen.
Losses: The fiber optic connectors are subjected to signal losses to some extent. The losses can potentially impact the output of a fiber optic transmission network, therefore connectors with minimum loss estimation must be selected. There are two types of losses that must be considered while selecting the fiber optic connectors, namely, insertion loss and return/reflection loss.
  • Insertion Loss: The insertion loss takes place at the connecting point of the cable and connector. This can happen due to several reasons like end gaps, coaxial attachment, core mismatch, axial run-out, etc.
  • Return/Reflection Loss: Return loss occurs due to back reflection. Generally, the signals reflect back and return to the source, and it is known as return loss. Whereas, if the signal reflects back over the connector material it is known as back reflection. Both conditions can cause delays in data packet delivery and sometimes, permanent loss of signal.
Mode of Transmission: The fiber optic transmission is often performed in one of the two modes of transmission, single-mode and multi-mode. Based on the applications, the mode of transmission is selected, eventually a suitable cable us selected. The highlighted features of single-mode and multi-mode fibers that make these cables suitable for specific applications are listed below.
  • Single-mode fiber optic cables: The beneficial features of single-mode fiber optic cables are as described below. The single-mode fiber optic cables enable optical signal transmission via a single path. Therefore, from transmitting source to receiver, the light waves of different frequencies transmit on the same path. This type of cable offers a minimum of attenuation. Therefore, loss of signal is not a concern in this mode of transmission. The source of light in these cables is a laser, therefore, the light intensity does not diminish. Single-mode fiber optic cables are suitable for long-distance transmission.
  • Multi-mode fiber optic cables: The beneficial features of multi-mode optical fiber are as follows. Multi-mode fiber optic cables are suitable for short-distance transmissions. These cables are affordable, therefore make a cost-effective choice for fiber-optic networks. The LED bulbs are sources of light in multimode optical fiber, therefore, it remains a sustainable fiber optic solution.
Core Diameter: There is a varying range of core diameters available in the fiber optic cables. The core diameter and core material define the refraction phenomenon that means they define the mode of transmission. Therefore, the core diameter is an essential selection criterion for fiber optic cables. The single-mode cables are available in core diameter sizes 8µm to 10µm and multi-mode cables are available in 50µm to 62.5µm.
Distance of Transmission: The distance of transmission is one of the important considerations while choosing between two types of fiber optic cables. The single-mode optical fiber cables are suitable for nearly 40km distance of transmission. Whereas, the multi-mode optical fiber cables are suitable for 550 meters to 2km of transmission distance.
Bandwidth: For high-end fiber optic applications, extremely high bandwidth may be required. Single-mode fibers offer theoretically unlimited bandwidth. In case of limited bandwidth requirements, multi-mode fiber cables can be chosen which offer 28000MHz*km.
 
Speed of Transmission: While selecting fiber optic cable, it is important to match up the speed of transmission. Single-mode fiber often offers 10Gbps to 40Gbps transmission speed whereas multi-mode fiber offers 100Mbps to 10Gbps speed of transmission.
Attenuation: There is a reduction in the power of a light signal during transmission, which is known as attenuation. Attenuation leads to loss of data. Therefore a fiber optic cable with the least possible attenuation. In single-mode fibers, the attenuation range is 0.4 dB/km to 1dB/km. However, multi-mode fiber cable may give higher attenuation beyond 2km distance. Therefore, transmission distance to attenuation comparison must be done during the selection of fiber optic cables.
Cable Construction: The optical fiber cable construction or the construction defines its functional reliability and sustainability for several applications. The utility of fiber optic cable for indoor, outdoor, commercial, domestic, industrial applications can be decided based on the construction itself. Therefore, the fiber cable construction becomes an important selection criterion. The following are three common fiber optic cables based on the construction.
 
Lower is the loss value, higher is the performance efficiency of the connector.
Along with the above-mentioned selection factors, the operator has to consider color codes of the connectors which are stated under TIA 568 standards. Consideration of color codes makes the selection of fiber optic connectors convenient for indoor, commercial, and industrial applications.
Despite the availability of more than 100 types of fiber optic connectors in the market, however, a few specific types are preferred by the industry experts. Experts’ choice types of connectors are listed below in a tabular format along with the general specifications, from which the operator can find a suitable one for specific applications.
Types of Fiber Optic Connectors and their Specifications
Sr. No. Type of Connector Coupling Mechanism Fiber Count End Polishing Style Applications
1 Ferrule Connector (FC) Screw Coupling 1 PC/UPC/APC LAN connections
2 ST Bayonet 1 PC/UPC Telecommunication
3 Standard Connector (SC) Bayonet 1 PC/UPC/APC CATV and Surveillance Equipment
4 LC Bayonet 1 PC/UPC/APC Ethernet multimedia transmission
5 MU Push-Pull Latch 1 PC/UPC/APC Tactical fiber optic networks
6 MT-RJ Bayonet 2 N/A Asynchronous transmission mode
7 MT Push-Pull Latch 4 to 24 N/A Device to Device interconnection under large network, signal distribution networks.
 
 

Conclusion

Fiber optic networking is a widely used but complex technology that relies on cables and connectors to build and expand networks. The performance efficiency of fiber optic networks depends on the types of cables and connectors used, their quality, specifications, and mutual compatibility.

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