How to Design Fiber Optic Transmission Systems

The first step in any fiber optic transmission systems design requires making careful decisions based on operating parameters that apply for each component of a fiber optic transmission systems. The main questions, given in the table below, involve data rates and bit error rates in digital systems, bandwidth, linearity, and signal-to-noise ratios in analog systems, and in all systems, transmission distances. These questions of how far, how good, and how fast define the basic application constraints.

System-Design-Considerations

All of these considerations are inter-related, and transmission distance is the predominant consideration. Transmission distance affects the strength of the fiber optic transmitter output, which dictates the type of light source used. It impacts fiber type, as single-mode fiber is better suited to long distance transmission. Fiber Optic Transmitter and fiber type dictate fiber optic receiver type and sensitivity. Transmission distance also dictates the modulation scheme as some are better for longer distances than others. While designing a system can be complex, several techniques simplify this process. One such technique is used to determine the link’s optical loss budget, which evaluates the transmitter output power, the operating wavelength, fiber attenuation, fiber bandwidth, and receiver optical sensitivity. This process is described at length in the article, Optical Link Loss Budget. Another technique determines the link’s rise time budget, which describes the transmission device’s ability to turn on and off fast enough. A sensitivity analysis determines the minimum optical power that must be received in order to achieve the required system performance. The receiver sensitivity can be affected by source intensity noise, inherent to the light source being used, fiber noise, inherent to the optical fiber, receiver noise, inherent in the detector used, time jitter, intersymbol interference, and bit error rate. Environmental considerations must be made. Temperature affects the performance of LEDs and lasers as well as the optical fiber itself. Building installations will generally require safety testing for fire safety, EMI radiation, or other parameter specific to the application environment. Certain environments present more hazards for fiber optic systems than others, which may impact the type of cable that can be specified. A good fiber optic system design must consider these factors. The cost of a fiber optic transmission system can also be a critical consideration. Component considerations such as light emitter type, emitter wavelength, connector type, fiber type, and detector type will have an impact on both the cost and performance of a system. Common sense goes a long way in designing the most cost-effective system to meet an application’s requirements. A properly engineered system is one that meets the required performance limits and margins with little extra. Excess performance capability often means the system costs too much for the specific application. Fortunately, there’s no need to design a system on your own. Once you’ve determined your need for fiber and the basic system requirements, a sales engineer or applications engineer can step you through the technical details.

Some common questions you’ll be expected to answer include:

1. What is the fiber loss for your system? This is not the same as optical loss; it refers to the bandwidth distance product which describes how much optical attenuation occurs over a certain length of fiber. If the system is previously installed and is being upgraded, this information is probably readily available. If the installation is new, knowing the transmission distance (i.e. the distance between the transmitter and the receiver) can help an applications engineer calculate the fiber loss. The fiber loss will determine transmitter optical output requirements and/or the inclusion of regenerators in the fiber path.

2. What type of signals do you wish to transmit? This includes fiber optic video signals, fiber optic audio signals, fiber optic data signals, and also indicates whether or not the signal will be digital or analog.

3. What type of fiber will be used? As Table 1 notes, the choices are multimode or single-mode. Transmission distance, signal type and application will predetermine the best fiber type. Typically long distance, high speed, or multichannel transmission require single-mode fiber, while short distance, low speed, and single channel transmission will allow the use of less expensive multimode fiber.

4. What optical connectors will be used? As with fiber type, different systems will have different requirements. Connectors may be specified to reduce backreflection, increase ease of installation, meet dense packaging requirements, or interface with connectors in an existing system.

5. What quality is expected at the receive end? This usually refers to video quality, and while it may seem obvious to answer, “the best quality,” the helpful answers include: surveillance quality, high quality, broadcast quality, studio quality, etc. The required video quality can impact fiber type and required electronics.

6. What configuration will the system require? This generally refers to the topology of the system, which may be point-to-point, ring, or fanout. In broadcast networks, configurations also include add/drop/repeat topologies. A fiber optic system checklist is available in Adobe Acrobat PDF format by clicking here. This list provides a means to specify these and many other details of a fiber optic system design.

Analog Fiber Optic CATV System Design and Introduction

Analog AM fiber optic systems have begun to replace coax cable for local distribution within a CATV network, while digital systems are being used for headend or hub site elimination and for transmitting various data services. In the past, these analog and digital transmissions systems are operated separately from each other over separate optical fibers. However, as these CATV systems grow and expand, the current trend in CATV system design incorporates wavelength-division multiplexing to combine both the analog and digital signals for transmission using the same fiber. This allows system expansion by increasing the number of signals transmitted on fiber currently installed. As these systems grow, the forward path transmission ceases to be the only required path. Today’s CATV system may also require a return path network to handle data from the Internet via cable modems. This article will focus on both two fiber and single fiber two-signal WDM CATV system design. For additional information on WDM systems using more than two signals, see articles on CWDM and DWDM. Unidirectional CATV Transmission (Forward Path) Before 1980, most CATV systems were coax based, but by the early 1980′s the CATV industry began using direct modulated 1310 nm VSB/AM links for distribution super trunks. Figure 1 illustrates a typical system architecture including a super trunk. By transporting a high quality replica of the headend signals, this system reduced the number of cascaded amplifiers required.

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Figure 1 – Typical Super Trunk CATV Architecture

By the early 1990′s, CATV providers began using multichannel digital systems to transport large numbers of uncompressed, broadcast-quality, digitized video channels between the headends. Still operating in the 1310 nm wavelength window, in this configuration, a previous separate headend is replaced by very high quality signals that are transported by a multichannel digital system from a “master” headend. Figure 2 illustrates this configuration. The advent of high performance externally modulated 1550 nm VSB/AM transmitters and erbium-doped fiber amplifiers (EDFAs) changed the architecture of CATV system design once again. These 1550 nm links are used to carry signals between headend sites over long distances, using the EDFA as an in-line amplifier.

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Figure 2 – Hybrid Analog/Digital CATV Architecture

The high performance 1550 nm systems vary slightly in that a few additional optical components are required. Illustrated in Figure 3, this system also incorporates optical splitters in addition to the EDFA. In this configuration, the transmitter is assumed to have dual outputs, a common feature for these new transmitters. The first optical output of the 1550 nm transmitter feeds a secondary headend 1310 nm transmitter. The second optical output goes into a 1 x 2 optical splitter. The first output feeds directly into a 1550 nm receiver for distribution from the main headend to a 1310 nm transmitter. The second output of the optical splitter feeds an EDFA. The signal is amplified optically and forwarded to the optical receiver which supplies a third headend located many miles away in the system.

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Hybrid 1310 nm & 1550 nm VSB/AM CATV Architecture

The first three architectures use no WDM components and represent completely analog architectures. As CATV systems grow, the need to expand each fiber’s transmission capacity grows with it. Wavelength-division multiplexing allows both analog and digital signals to co-exist on a single fiber. Figure 4 illustrates a unidirectional WDM AM CATV/Digital transport system.

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Figure 4 – Unidirectional Analog/Digital CATV Transport using WDM

In the configuration shown in Figure 4, the signal from the 1310 nm CATV AM transmitter and the 1550 digital transmitter are wavelength-division multiplexed onto one fiber. At the receive, the signals are demultiplexed and output to the correct receivers. In order to maintain system quality, the WDM must be a high isolation type that prevents interference between the 1310 nm analog signal and the 1550 nm digital signal. A bidirectional configuration of this analog/digital CATV transport system is illustrated in Figure 5.

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Figure 5 – Bidirectional Analog/Digital CATV Transport Using WDM

Fiber Optic Security Control and Data Acquisition (SCADA) Networks

SCADA (Supervisory Control And Data Acquisition) networks, originally developed in the 1960s, are used for industrial measurement, monitoring, and control systems, especially by electricity and natural gas utilities, water and sewage utilities, railroads, telecommunications, and other critical infrastructure organizations. They enable remote monitoring and control of an amazing variety of industrial devices, such as water and gas pumps, track switches, and traffic signals. A SCADA system gathers information, such as where a leak on a pipeline has occurred, transfers the information back to a central site, alerting the home station that the leak has occurred, carrying out necessary analysis and control, such as determining if the leak is critical, and displaying the information in a logical and organized fashion. SCADA systems range from relatively simple networks that monitor environmental conditions of a given location to incredibly complex systems that monitor all the activity in a power plant or a municipal water system. for example.

SCADA Network Components
A SCADA network consists of one of more Master Terminal Units (MTUs) which the operators utilize to monitor and control a large number of Remote Terminal Units (RTUs). The MTU is often a general purpose computing platform, like a PC, running SCADA management software. The RTUs are generally small dedicated devices which are hardened for outdoor use and industrial environments. Fiber optic data transceivers are ideal in SCADA networks because they offer EMI immunity. When transceivers are used for the master and remote terminal units, a fault tolerant self-healing ring network is easy to configure. Figure 1 illustrates a self-healing ring network topology.

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Figure 1— Self-healing Ring Network

Reliable operations of SCADA systems depends on proper configuration, cyber security measures, and other factors.

Multi-format Analog and Digital Video/Audio Broadcast Transmission Systems

Broadcasters and cable providers, faced with the impending FCC mandated deadline to digitize all over-the-air transmission, need a multi-format analog and digital transmission solution. As the term implies, these multi-format systems allow the broadcast and cable industries to either transmit digitized analog video/audio channels, purely digital video/audio channels, or both formats at the same time, allowing them to maintain their current quality of service (QoS) while they make the transition to the all-digital broadcast requirements outlined in the Telecommunications Act of 1996. In addition to meeting HDTV transmission capabilities, the systems usually satisfy the requirements of additional audio SAP (Secondary Audio Programming) channels, also mandated by the FCC in accordance with the Americans with Disabilities Act of 1990. The flexibility of these systems allows broadcasters and cable service providers the convenience of a smooth transition from an all-analog transmission scheme to all-digital. Figure 1 illustrates a system that can transport analog and digital video signals simultaneously.

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Figure 1 — Simultaneous transmission of Digital and Analog Video Signals

System Requirements
A transitional transport platform should offer a variety of features to accommodate today’s digitized analog transmission as well as tomorrow’s digital transmission. First, the system should offer broadcast-quality RS-250C short-haul transmission that includes enough audio channels to meet stereo audio as well as SAP and surround sound transmission. Second, the system should offer broadcast-quality, all-digital transmission that complies with the SMPTE 259M DTV television transmission standard. This standard defines the transmission equipment required to transmit digital video signals. Third, the system should comply with other standards, such as ” title=”smpte_310m”>SMPTE 310M and DVB-ASI, to provide maximum system flexibility. Ideally, these transport options should be interchangeable within the system, allowing broadcasters and cable providers to upgrade their transmission system without replacing 100% of their currently installed hardware. Table 1 outlines a few basic performance parameters of these various signals.

elements-of-analog-digital-transport-system

Desirable System Features
Broadcasters and cable operators require additional system features to create a fail-safe network as well as a network that is flexible and easily upgradeable. Modular technology is beginning to dominate fiber optic system design, allowing unprecedented system flexibility and reliability. By isolating the power supply and various A/V modules from the optical transport chassis, a broadcaster or cable operator could specify any combination of digitized RS-250C, SMPTE 259M, SMPTE 310M, or DVB-ASI in a single chassis. Hot-swappable power supplies allow end users to quickly replace a defective power supply without removing power from the system or replacing the entire video transport unit, reducing cost and system down-time. Another desirable feature for future-proof broadcast networks involves the ability to multiplex multiple video and audio signals into a single optical stream. This allows the capacity of the optical fiber to increase without installing additional optical fibers. CWDM and DWDM, operating in the 1400-1600 nm wavelength window allows a number of signals at different wavelengths to be combined. Additionally, incorporating erbium-doped fiber amplifiers, which operate in the 1550 nm window, could greatly extend the transmission range of the broadcast or cable operator, allowing them to reach a wider audience. While many solutions exist for both analog and digital video transmission, broadcasters and cable operators would most benefit from a transport platform that supports both, providing a migration path to implement FCC mandated DTV transmission while still allowing current analog equipment to be utilized. This system would offer a number of network configurations, V/A channel counts, and transmission distance options. See Multi-format Analog and Digital V/A Network Configurations for details on various analog and digital video/audio transport platforms and applications.

Fiber Optic Network Topologies for ITS and Other Systems

All networks involve the same basic principle: information can be sent to, shared with, passed on, or bypassed within a number of computer stations (nodes) and a master computer (server). Network applications include LANs, MANs, WANs, SANs, intrabuilding and interbuilding communications, broadcast distribution, intelligent transportation systems (ITS), telecommunications, supervisory control and data acquisition (SCADA) networks, etc

In addition to fiber optic tranmsmission for its oft-cited advantages (i.e., bandwidth, durability, ease of installation, immunity to EMI/RFI and harsh environmental conditions, long-term economies, etc.), optical fiber better accommodates today’s increasingly complex network. architectures than copper alternatives. Figure 1 illustrates the interconnection between these types of networks.

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Figure 1 — Interconnections Between Different Network Types

Networks can be configured in a number of topologies. These include a bus, with or without a backbone, a star network, a ring network, which can be redundant and/or self-healing, or some combination of these. Each topology has its strengths and weaknesses, and some network types work better for one application while another application would use a different network type. Local, metropolitan, or wide area networks generally use a combination, or “mesh” topology.

Bus Network
A bus network topology, also called a fiber optic daisy-chain topology has each computer directly connected on a main communication line. One end has a controller, and the other end has a terminator. Any computer that wants to talk to the main computer must wait its turn for access to the transmission line. In a straight network topology, only one computer can communicate at a time. When a computer uses the network, the information is sent to the controller, which then sends the information down the line of computers until it reaches the terminating computer. Each computer in the line receives the same information. Figure 2 illustrates a bus network topology. A bus network with a backbone operates in the same fashion, but each computer has an individual connection to the network. A bus network with a backbone offers greater reliability than a simple bus topology. In a simple bus, if one computer in the network goes down, the network is broken. A backbone adds reliability in that the loss of one computer does not disrupt the entire network. Figure 3 illustrates this topology with a backbone.

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Figure 2 — Bus Network Topology

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Figure 3 — Bus Network with Backbone

Star Network
Star networks incorporate multiport star couplers in to achieve the topology. Once again, a main controlling computer or computer server interconnects with all the other computers in the network. As with the bus topology with a backbone, the failure of one computer node does not cause a failure in the network. Figure 4 illustrates a star network topology. Both the bus and the star network topologies use a central computer that controls the system inputs and outputs. Also called a server, this computer has external connections, to the Internet for example, as well as connections to the computer nodes in the network.

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Figure 4 – Star Network Topology

Ring Networks
Ring networks operate like bus networks with the exception of a terminating computer. In this configuration, the computers in the ring link to a main communication cable. The network receives information via a “token” containing information requested by one or more computers on the network. The token passes around the ring until the requesting computer(s) have received the data. The token uses a packet of information that serves as an address for the computer that requested the information. The computer then “empties” the token, which continues to travel the ring until another computer requests information to be put into the token. Figure 5 illustrates this topology. An advanced version of the ring network uses two communication cables sending information in both directions. Known as a counter-rotating ring, this creates a fault tolerant network that will redirect transmission in the other direction, should a node on the network detect a disruption. This network uses fiber optic transceiver, one controlling unit in set in “master” mode along with several nodes that have been set as “remote” units. The first remote data transceiver receives the transmission from the master unit and retransmits it to the next remote unit as well as transmitting it back to the master unit. An interruption in the signal line on the first ring is bypassed via the second ring, allowing the network to maintain integrity. Figure 6 illustrates this configuration as it might be used in a ITS installation.

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Figure 5 – Token Ring Network Topology

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Figure 6 — Self-healing Ring Topology

See also: “Overview of Telecommunications Networks” for additional information on fiber optic networks.

Multi-channel Video, Audio, Data Fiber Transmission Systems

Fiber optic transmission is now the dominant medium for terrestrial transmission of digital signals, and digital fiber optic transmission systems are well established for transporting high quality video, audio, and data signals. Systems must make efficient use of optical fiber by transporting multiple channels of video and audio on a single fiber. A digital system working within a digital domain should be capable of expanding, inserting, routing, and switching signals within a network in such a way that video and audio performance is not affected. Of growing importance is the ability of these networks to accept a variety of signal formats and to interface with public television communication networks. Signal formats for transmission of video might include video encoding at various levels of digitizing accuracy, compressed video, advanced or high definition video, as well as digital high speed data. Understanding aspects of multiplexing, modulation schemes, and digital systems are important to implementing a multichannel transmission system. All video over fiber / audio over fiber / data over fiber transmission systems share a number of elements in common that form the basic system building blocks for any v/a/d system. These include: transmitters, receivers, signal regenerators, repeaters, coders, decoders, switches, modulators, amplifiers, A/D and D/A converters, splitters, combiners, signal fanouts, which allow signals to be added and dropped from a network or utilize smaller system components for the signal distribution, A/B switching for redundant circuit protection, network control data interfaces, and synchronizing clock interfaces. Figure 1 illustrates a digital transmission system with a two-level time-division multiplexing (TDM) hierarchy.

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Figure 1 — Digital System with a Two-level TDM Hierarchy

At the transmit end, a digital coder converts the incoming analog video signal to PCM digital data. (See Digital Modulation for details.) The coder also contains a time-division multiplexer, called a high level mux, which creates a digital subchannel that is time-division multiplexed with the PCM video data. This high level mux/video coder outputs a digital data stream and a clock signal, used for synchronicity. These signals then enter the optical transmitter for digital line coding, light source modulation, and an interface to the optical transmission fiber. This high level mux receives its input from a second TDM, called the low level mux, which controls a data bus that can interface with a variety of input signals such as digitized audio and digital data signals. At the receive end, an optical receiver converts the signal from optical to electrical and provides line decoding plus clock recovery. The PCM data and recovered clock are then sent to a digital video decoder that converts the digital video back to an analog signal. The video decoder also contains a high level demux, which separates the subchannel and sends it to a low level demux. At the low level demux, the signals are demultiplexed and put back on the data bus where they are decoded to system output.

Practical Digital Fiber Optic Multichannel System Considerations
A desirable feature in multichannel digital networks is the capacity to add video channels, or “contribution circuits,” that are digitally transmitted from a remote site to a network node. The contribution circuit must be bidirectional for transmission from the network node back to the remote site; a system clock must be at the remote site as well. In a TDM structure all inputs are time-division multiplexed in a fully synchronous digital system without interaction or crosstalk allowing signals to be added or dropped without affecting the rest of the system. Diplexed, reverse direction data channels provide a cost effective and convenient method to enable synchronous multiplexing of remote unidirectional signals into a digital network. This also provides a low-cost method for adding bidirectional, low speed data channels to single direction, high speed video links. Synchronous transmission is not possible without a bidirectional contribution circuit. To achieve this, one needs a duplex transmission link operating on one fiber at the same optical wavelength in both directions. This is achieved by diplexing the forward and reverse optical signals using optical couplers; however, this causes signal interference which degrades signal-to-noise ratio. Optical couplers can minimize this degradation, as can shaping the power spectrums of the forward stream and reverse data streams. Provided the frequency is synchronous with the signal clock, the digital ports for a synchronous TDM should have the ability to interface with a variety of input signal formats such as digital coding and/or framing patterns. This allows signals to be multiplexed and transported over the same optical channel. These universal ports can be achieved by scrambling or coding the incoming data on each TDM port. The system should only require that the incoming data be frequency synchronous but not necessarily phase coherent with the rest of the system. This facilitates the use of video codecs of different sample accuracies or coding formats, transporting digitized non-video signals, and adding signals such as digitized SDTV/HDTV or digitally compressed MPEG-2 signals all in the same system. Practical CATV or RF-based networks should interface to existing CATV signal formats and co-exist as easily as possible with the RF portions of the signal distribution plant. This requires that the digital video coders and decoders in the system be capable of accepting variations to the “standard” video signal. Because the final signal format for CATV networks is VSB/AM, a valuable feature for any digital transport system would include the ability to output a VSB/AM signal, rather than baseband video, to the RF portions of the distribution plant.

System Examples
The simplest multichannel fiber optic video transmission system is the point-to-point link. The example in Figure 2 illustrates an eight channel digital video system.

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Figure 2 — Point-to-point Video Transport System

It is useful to think of a partition in the system between its signal codecs and the optical transmitter and receiver. In this example, the system uses an eight channel optical link. It is important to decide how these digital channels are utilized. Figure 2 shows all eight inputs and outputs on the transmission terminals in use, leaving no room for future expansion. More system capacity could be achieved with additional optical terminals or optical terminals that offer a higher channel count. Many multichannel video transport systems require linking one transmit site to several receive sites in a point-to-multipoint transmission configuration. Figure 3 illustrates an example of this type of system. In this figure, there are three end-of-the-line receive sites and one intermediate receive site where a digital drop/add function has been incorporated. At the intermediate hub, all channels from the headend are dropped for local use and are retransmitted to the next site. Two channels are also added to the intermediate hub for transmission to the next hub.

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Figure 3 — Point-to-multipoint Configuration

As video over fiber transmission systems evolve into multi-service communications networks, with stringent up-time requirements, system redundancy becomes a must to provide remote monitoring of the system. A redundant path with automatic switching, added to the system, allows full monitoring and control at each site, with communication back to the headend. The redundant elements required include digital signal fanouts as well as digital A/B switches. Failure on the primary fiber will cause the system to automatically switch to the secondary fiber keeping network performance unaffected. Figure 4 illustrates this enhanced system feature.

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Figure 4 — Digital Video Transport System with System Redundancy

Digital systems can be used to build fiber optic video transport networks from a set of basic common components that can provide configurations that range from simple point-to-point links to complex point-to-multipoint systems with enhancements for system redundancy. As the system grows, this building block approach allows for system expansion using existing parts. Signal performance remains uniform while any number of channels and services can be transported independent of the distance of the link or the number of links. This offers a level of “future-proofing” that allows the installed system to keep up with the customer’s needs.

Description of basic fiber optic Transmission Systems

Fiber optic transmission Systems uses the same basic elements as copper-based transmission systems: A fiber optic transmitter, a fiber optic receiver, and a medium by which the signal is passed from one to the other, in this case, optical fiber. Figure 1 illustrates these elements. The fiber optic transmitter uses an electrical interface to encode the use information through AM, FM or digital modulation. A laser diode or an LED do the encoding to allow an optical output of 850 nm,1310 nm, or 1550 nm (typically).

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Figure 1 – Elements of a Fiber Optic Link

The optical fiber connects the fiber optic transmitter and the fiber optic receiver. This fiber may be either single-mode or multi-mode. The fiber consists of three main regions, as illustrated in Figure 2. The core, the center of the fiber, actually carries the light. The cladding surround the core in a glass with a different refractive index than the core, allowing the light to be confined in the fiber core. A coating or buffer, typically plastic, provides strength and protection to the optical fiber. The receiver uses either a PIN photodiode or an APD to receive the optical signal and convert it back into an electrical signal. A data demodulator converts the data back into its original electrical form. These elements comprise the simplest link, but other elements may also appear in a fiber optic transmission system. For example, the addition of WDM components allows two separate signals to be joined into a composite signal for transmission, and then can be separated into their original signals at the receive end. Other wavelength-division multiplexing techniques allow up to eight signals (CWDM) or more (DWDM) to be combined onto a single fiber. These are discussed in separate articles as linked in this paragraph. Long distance fiber optic transmission leads to further system complexity. Many long-haul fiber optic transmission systems require signal regenerators, signal repeaters, or optical amplifiers such as EDFAs in order to maintain signal quality. System drop/repeat/add requirements, such as those in multichannel broadcast networks, further add to the fiber optic system, incorporating add-drop multiplexers, couplers/splitters, signal fanouts, dispersion management equipment, remote monitoring interfaces, and error-correction components. See the linked articles for additional information on these components.

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Figure 2 – Cross-section of an Optical Fiber

Types of Fiber Optic Connectors

Fiber Optic Connectors
We’ll start our section on termination by considering connectors. Since fiber optic technology was introduced in the late 70s, numerous connector styles have been developed. Each new design was meant to offer better performance (less light loss and back reflection), easier and/or termination and lower cost. Of course, the marketplace determines which connectors are ultimately successful.

Fiber Optic Connectors and Splice Loss Mechanisms
Connector and splice loss is caused by a number of factors. Loss is minimized when the two fiber cores are identical and perfectly aligned, the connectors or splices are properly finished and no dirt is present. Only the light that is coupled into the receiving fiber’s core will propagate, so all the rest of the light becomes the connector or splice loss.
End gaps cause two problems, insertion loss and return loss. The emerging cone of light from the connector will spill over the core of the receiving fiber and be lost. In addition, the air gap between the fibers causes a reflection when the light encounters the change n refractive index from the glass fiber to the air in the gap. This reflection (called fresnel reflection) amounts to about 5% in typical flat polished connectors, and means that no connector with an air gap can have less than 0.3 dB loss. This reflection is also referred to as back reflection or optical return loss, which can be a problem in laser based systems. Connectors use a number of polishing techniques to insure physical contact of the fiber ends to minimize back reflection. On mechanical splices, it is possible to reduce back reflection by using non-perpendicular cleaves, which cause back reflections to be absorbed in the cladding of the fiber.
The end finish of the fiber must be properly polished to minimize loss. A rough surface will scatter light and dirt can scatter and absorb light. Since the optical fiber is so small, typical airborne dirt can be a major source of loss. Whenever connectors are not terminated, they should be covered to protect the end of the ferrule from dirt. One should never touch the end of the ferrule, since the oils on one’s skin causes the fiber to attract dirt. Before connection and testing, it is advisable to clean connectors with lint-free wipes moistened with isopropyl alcohol.
Two sources of loss are directional; numerical aperture (NA) and core diameter. Differences in these two will create connections that have different losses depending on the direction of light propagation. Light from a fiber with a larger NA will be more sensitive to angularity and end gap, so transmission from a fiber of larger NA to one of smaller NA will be higher loss than the reverse. Likewise, light from a larger fiber will have high loss coupled to a fiber of smaller diameter, while one can couple a small diameter fiber to a large diameter fiber with minimal loss, since it is much less sensitive to end gap or lateral offset.
These fiber mismatches occur for two reasons. The occasional need to interconnect two dissimilar fibers and production variances in fibers of the same nominal dimensions. With two multimode fibers in usage today and two others which have been used occasionally in the past and several types of singlemode fiber in use, it is possible to sometimes have to connect dissimilar fibers or use systems designed for one fiber on another. Some system manufacturers provide guidelines on using various fibers, some don’t. If you connect a smaller fiber to a larger one, the coupling losses will be minimal, often only the fresnel loss (about 0.3 dB). But connecting larger fibers to smaller ones results in substantial losses, not only due to the smaller cores size, but also the smaller NA of most small core fibers.

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Guide to Fiber Optic Connectors
Check out the “spotters guide” below and you will see the most common fiber optic connectors. (All the photos are to the same scale, so you can get an idea of the relative size of these connectors.)

ST fiber optic Connector
ST (an AT&T Trademark) is the most popular connector for multimode networks, like most buildings and campuses. It has a bayonet mount and a long cylindrical ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. And because they are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference.

fiber optic cable
FC/PC has been one of the most popular singlemode connectors for many years. It screws on firmly, but make sure you have the key aligned in the slot properly before tightening. It’s being replaced by SCs and LCs.

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SC is a snap-in connector that is widely used in singlemode systems for it’s excellent performance. It’s a snap-in connector that latches with a simple push-pull motion. It is also available in a duplex configuration.

Besides the SC Duplex, you may occasionally see the FDDI and ESCON* duplex connectors which mate to their specific networks. They are generally used to connect to the equipment from a wall outlet, but the rest of the network will have ST or SC connectors. *ESCON is an IBM trademark

Below are some of the new Small Form Factor (SFF) connectors:

LC fiber optic Connector
LC is a new connector that uses a 1.25 mm ferrule, half the size of the ST. Otherwise, it’s a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for singlemode.

MT-RJ fiber optic Connector
MT-RJ is a duplex connector with both fibers in a single polymer ferrule. It uses pins for alignment and has male and female versions. Multimode only, field terminated only by prepolished/splice method.

Opti-Jack fiber optic Connector
Opti-Jack is a neat, rugged duplex connector cleverly designed aournd two ST-type ferrules in a package the size of a RJ-45. It has male and female (plug and jack) versions.

Volition fiber optic Connector
Volition is a slick, inexpensive duplex connector that uses no ferrule at all. It aligns fibers in a V-groove like a splice. Plug and jack versions, but field terminate jacks only.

LX-5 fiber optic Connector
E2000/LX-5 is like a LC but with a shutter over the end of the fiber.

MU fiber optic Connector
MU looks a miniature SC with a 1.25 mm ferrule. It’s more popular in Japan.

MT fiber optic Connector
MT is a 12 fiber connector for ribbon cable. It’s main use is for preterminated cable assemblies.

The ST/SC/FC/FDDI/ESON connectors have the same ferrule size – 2.5 mm or about 0.1 inch – so they can be mixed and matched to each other using hybrid mating adapters. This makes it convenient to test, since you can have a set of multimode reference test cables with ST connectors and adapt to all these connectors. Likewise, the LC, MU and E2000/LX-5 use the same ferrule but cross-mating adapters are not easy to find.

Connector Types
The ST is still the most popular multimode connector because it is cheap and easy to install. The SC connector was specified as a standard by the old EIA/TIA 568A specification, but its higher cost and difficulty of installation (until recently) has limited its popularity. However, newer SCs are much better in both cost and installation ease, so it has been growing in use. The duplex FDDI, ESCON and SC connectors are used for patchcords to equipment and can be mated to ST or SC connectors at wall outlets. Singlemode networks use FC or SC connectors in about the same proportion as ST and SC in multimode installations. There are some D4s out there too. EIA/TIA 568 B allows any fiber optic connector as long as it has a FOCIS (Fiber Optic Connector Intermateability Standard) document behind it. This opened the way to the use of several new connectors, which we call the “Small Form Factor” (SFF) connectors, including AT&T LC, the MT-RJ, the Panduit “Opti-Jack,” 3M’s Volition, the E2000/LX-5 and MU. The LC has been particularly successful in the US.

Connector Ferrule Shapes & Polishes
Fiber optic connectors can have several different ferrule shapes or finishes, usually referred to as polishes. early connectors, because they did not have keyed ferrules and could rotate in mating adapters, always had an air gap between the connectors to prevent them rotating and grinding scratches into the ends of the fibers.

fiber optic Connectors

Beginning with the ST and FC which had keyed ferrules, the connectors were designed to contact tightly, what we now call physical contact (PC) connectors. Reducing the air gap reduced the loss and back reflection (very important to laser-based singlemode systems ), since light has a loss of about 5% (~0.25 dB) at each air gap and light is reflected back up the fiber. While air gap connectors usually had losses of 0.5 dB or more and return loss of 20 dB, PC connectors had typical losses of 0.3 dB and a return loss of 30 to 40 dB.

Soon thereafter, it was determined that making the connector ferrules convex would produce an even better connection. The convex ferrule guaranteed the fiber cores were in contact. Losses were under 0.3dB and return loss 40 dB or better. The final solution for singlemode systems extremely sensitive to reflections, like CATV or high bitrate telco links, was to angle the end of the ferrule 8 degrees to create what we call an APC or angled PC connector. Then any reflected light is at an angle that is absorbed in the cladding of the fiber.

Loss Budget Standard of fiber optic cables

The EAI/TIA 568 standard for premises cabling is used by most manufacturers and users of premises cabling systems in the US. Internationally, IEC/ISO 11801 is very similar for fiber optics, although there are differences in various countries. TIA-568 has been under continual revision since its inception. The current version is “568 B.3″ covering fiber optics. It includes some major changes from earlier versions for fiber optics. TIA 568 “C” is expected to be published sometime in 2008, but fiber optic cabling changes are not substantial. These include:
1. Adds 50/125 micron fiber (OM2 or OM3) as an alternative fiber type and specifies performance.
2. Allows alternate connectors to the SC, esp. small form factor connectors.
3. Adds performance standards for all connectors.
4. Includes bend radius specifications for cables.
5. Specifies requirements for connecting hardware.

Fiber Optic Cable Performance Standards
568 B3 adds 50/125 fiber as an acceptable type and specifies the performance of cabled fiber as follows:

Fiber Type
Wavelength (nm)
Max Attenuation Coefficient (dB/km)
Bandwidth (MHz-km with overfilled launch)
50/125 (OM2, OM3)
850
3.5
500 (OM2), 2000 (OM3)
1300
1.5
500
62.5/125 (OM1)
850
3.5
160
1300
1.5
500
Singlemode (OS1, OS2)
(Premises)
1310
1.0
NA
1550
1.0
NA
Singlemode (OS1, OS2)
(Outside Plant)
1310
0.5
NA
1550
0.5
NA

Note that these specs are quite conservative, compared to what is routinely available in the marketplace, especially now that more fiber manufacturers are offering high bandwidth 62.5/125 fiber and 50/125 fiber with even higher bandwidth than OM3 (currently being called OM3+ by most manufacturers, but an OM4 spec with 850 nm BW of approximately 3500 MHz-km.) The spec notes also that the cable manufacturer can use the fiber manufacturer’s data on bandwidth, so they do not have to test it.

Hybrid Cables
The standard notes that hybrid cables are permitted, with markings per ANSI/EIA/TIA-598-A. ( Hybrid cables contain both multimode and singlemode fibers.)

Premises Cables
Horizontal cables with 2-4 fibers require a 25 mm bend radius after installation or 50 mm while being pulled with a tension of 50 pounds (222 N).
Other premises cables require a bend radius of 10 times the cable outside diameter unloaded and 15 times the OD when under the maximum rated pulling tension for that cable.

Outside Plant Cables
The standard calls for water-blocked cables with a minimum pulling tension of 600 pounds (2670 N).
Minimum bend radius is 20 times the cable diameter under max rated pulling tension and 10 times unloaded.

Drop Cables
The standard adds a definition for “drop cables,” low fiber count cables used to connect high fiber count cables to smaller fiber count cables or patchcords in a single location. These cables must withstand 300 pounds pulling tension (1335 N).

Connectors and Connecting Hardware
Any connector design is permitted as long as it has a FOCIS document (Fiber Optic Connector Intermateability Standard). All with FOCIS documents are acceptable.

Color Codes: Multimode connectors are beige for 62.5/125 fiber, black for 50/125 fiber, singlemode are blue, angle-polished singlemode are green, and outlets are also color coded accordingly. Cable color codes are the same as TIA-598.

Duplex connectors are keyed and are always crossover – that is Position A of one connector connects to Position B on the other end! Patchcords have this feature too, to permit correct connection of transmitters and receivers!

Outlet boxes must have provision for termination of at least 2 fibers.

Patch panels and outlets must provide unique identification for the connecting cabling.

Connector Mating Loss:

Fiber Type
Wavelength (nm)
Loss (dB)
Optical Return Loss
(dB)
Multimode
850
0.75
20
1300
0.75
20
Singlemode
1310
0.75
26 (CATV:55)
1550
0.75
26 (CATV:55)

 Remember these connector losses are maximum values. The loss is high to allow prepolished/splice connectors which have higher loss than adhesive/polish connectors. Users may specify lower loss for installations if agreed upon by all parties involved. 

Notes:
Loss is tested per FOTP-171, single cable reference.
Maximum loss spec holds over temperature (0-60C), humidity (95% @ 40C), impact, pull strength of coupling (7.4 lb./33N), durability (500 matings), cable retention (11 lbs./50 N straight, 4 lbs./19N at 90)flex and twist.

Splices
Fusion or mechanical splices shall not have a loss of more than 0.3 dB. Multimode splices must have a return loss of better than 20 dB. Singlemode splices must be better than 26 dB ORL for general applications, 55 dB ORL for CATV broadband analog video.

What is Fiber Optical Amplifiers?

With the demand for longer transmission lengths, optical amplifiers have become an essential component in long-haul fiber optic systems. Semiconductor optical amplifiers (SOAs), erbium doped fiber amplifiers (EDFAs), and Raman optical amplifiers lessen the effects of dispersion and attenuation allowing improved performance of long-haul optical systems.

Semiconductor Optical Amplifiers
Semiconductor optical amplifiers (SOAs) are essentially laser diodes, without end mirrors, which have fiber attached to both ends. They amplify any optical signal that comes from either fiber and transmit an amplified version of the signal out of the second fiber. SOAs are typically constructed in a small package, and they work for 1310 nm and 1550 nm systems. In addition, they transmit bidirectionally, making the reduced size of the device an advantage over regenerators of EDFAs. However, the drawbacks to SOAs include high-coupling loss, polarization dependence, and a higher noise figure. Figure 1 illustrates the basics of a Semiconductor optical amplifier.

soa
Figure 1 – Semiconductor Optical Amplifier

Modern optical networks utilize SOAs in the follow ways: Power Boosters: Many tunable laser designs output low optical power levels and must be immediately followed by an optical amplifier. ( A power booster can use either an SOA or EDFA.) In-Line Amplifier: Allows signals to be amplified within the signal path. Wavelength Conversion: Involves changing the wavelength of an optical signal. Receiver Preamplifier: SOAs can be placed in front of detectors to enhance sensitivity.

EDFAs
The explosion of dense wavelength-division multiplexing (DWDM) applications make these optical amplifiers an essential fiber optic system building block. EDFAs allow information to be transmitted over longer distances without the need for conventional repeaters. The fiber is doped with erbium, a rare earth element, that has the appropriate energy levels in their atomic structures for amplifying light. EDFAs are designed to amplify light at 1550 nm. The device utilizes a 980 nm or 1480nm pump laser to inject energy into the doped fiber. When a weak signal at 1310 nm or 1550 nm enters the fiber, the light stimulates the rare earth atoms to release their stored energy as additional 1550 nm or 1310 nm light. This process continues as the signal passes down the fiber, growing stronger and stronger as it goes. Figure 2 shows a fully featured, dual pump EDFA that includes all of the common components of a modern EDFA.

dual-pump-EDFA
Figure 2 – Block Diagram of an EDFA

The input coupler, Coupler #1, allows the microcontroller to monitor the input light via detector #1. The input isolator, isolator #1 is almost always present. WDM #1 is always present, and provides a means of injecting the 980 nm pump wavelength into the length of erbium-doped fiber. WDM #1 also allows the optical input signal to be coupled into the erbium-doped fiber with minimal optical loss. The erbium-doped optical fiber is usually tens of meters long. The 980 nm energy pumps the erbium atom into a slowly decaying, excited state. When energy in the 1550 nm band travels through the fiber it causes stimulated emission of radiation, much like in a laser, allowing the 1550 nm signal to gain strength. The erbium fiber has relatively high optical loss, so its length is optimized to provide maximum power output in the desired 1550 nm band. WDM #2 is present only in dual pumped EDFAs. It couples additional 980 nm energy from Pump Laser #2 into the other end of the erbium-doped fiber, increasing gain and output power. Isolator #3 is almost always present. Coupler #2 is optional and may have only one of the two ports shown or may be omitted altogether. The tap that goes to Detector #3 is used to monitor the optical output power. The tap that goes to Detector #2 is used to monitor reflections back into the EDFA. This feature can be used to detect if the connector on the optical output has been disconnected. This increases the backreflected signal, and the microcontrolled can set to disable the pump lasers in this event, providing a measure of safety for technicians working with EDFAs. Figure 3 shows a two-stage EDFA with mid-stage access. In this case, two single-stage EDFAs are packaged together. The output of the first stage EDFA and the input of the second stage EDFA are brought out the user. Mid-stage access is important in high performance fiber optic systems. To reduce the overall dispersion of the system, dispersion compensating fiber (DCF) can be used periodically. However, problems can arise from using the DCF, mostly the insertion loss reaching 10 dB. Placing the DCF at the mid-stage access point of the two-stage EDFA reduces detrimental effects on the system, and allows the users noticeable gain.

two-stage-EDFA
Figure 3 – Two-stage EDFA with Mid-stage Access

The optical input first passes through optical Isolator #1. Next the light passes through WDM #1, which provides a means of injecting the 980 nm pump wavelength into the first length of erbium-doped fiber. WDM #1 also allows the optical input signal to be coupled into the erbium-doped fiber with minimal optical loss. The erbium-doped optical fiber is usually tens of meters long. Like the fully feature, dual pumped EDFA, the 980 nm energy pumps the erbium atoms into an excited state that decays slowly. When light in the 1550 nm band travels through the erbium-doped fiber it causes stimulated emission of radiation. As the optical signal gains strength, output of the erbium-doped fiber then goes into the optical isolator #2, the output of which is available to the user. Typically, a dispersion compensating device will be connected at the mid-stage access point. The light then travels through isolator #3 and WDM #2, which couples additional 980 nm energy from a second pump laser into the other end of a second length of erbium-doped fiber, increasing gain and output power. Finally, the light travels through isolator #4. Photons amplify the signal avoiding almost all active components, a benefit of EDFAs. Since the output power of an EDFA can be large, any given system design can require fewer amplifiers. Yet another benefit of EDFAs is the data rate independence means that system upgrades only require changing the launch/receive terminals. The most basic EDFA design amplifies light over a narrow, 12 nm, band. Adding gain equalization filters can increase the band to more than 25 nm. Other exotic doped fibers increase the amplification band to 40 nm. Because EDFAs greatly enhance system performance, they find use in long-haul, high data rate fiber optic communication systems and CATV delivery systems. Long-haul systems need amplifiers because of the lengths of fiber used. CATV applications often need to split a signal to several fibers, and EDFAs boost the signal before and after the fiber splits. There are four major applications that generally require optical fiber amplifiers: power amplifier/booster, in-line amplifier, preamplifier or loss compensation for optical networks. Below are detailed description of each application. Power Amplifier/Booster Figure 4 illustrates the first three application for optical amplifiers. Power amplifiers (also referred to as booster amplifiers) are placed directly after the optical transmitter. This application requires the EDFA to take a large signal input and provide the maximum output level. Small signal response is not as important because the direct transmitter output is usually -10 dBm or higher. The noise added by the amplifier at this point is also not as critical because the incoming signal has a large signal-to-noise ratio (SNR).

EDFA-APPS
Figure 4 – Three Applications for an EDFA

In-Line Amplifiers In-line amplifiers or in-line repeaters, modify a small input signal and boost it for retransmission down the fiber. Controlling the small signal performance and noise added by the EDFA reduces the risk of limiting a system’s length due to the noise produced by the amplifying components. Preamplifiers Past receiver sensitivity of -30 dBm at 622 Mb/s was acceptable; however, presently, the demands require sensitivity of -40 dBm or -45 dBm. This performance can be achieved by placing an optical amplifier prior to the receiver. Boosting the signal at this point presents a much larger signal into the receiver, thus easing the demands of the receiver design. This application requires careful attention to the noise added by the EDFA; the noise added by the amplifier must be minimal to maximize the received SNR. Compensating for Loss in Optical Networks Inserting an EDFA before an 8 x 1 optical splitter increases the power to almost +19 dBm allowing each of the eight output legs to provide +9 dBm, making the output almost equal to the original transmitter power. The optical splitter alone has a nominal optical insertion loss of 10 dB. The transmitter has an optical output of +10 dBm, meaning that the optical splitter outputs without an EDFA would be 0 dBm. This output power would be acceptable for most digital applications; however, in analog CATV applications this is the minimal acceptable received power. Therefore, inserting the EDFA before the optical splitter greatly increases the output power.

edfa-POWERAMP
Figure 5 – Loss Compensation in Optical Networks

Wideband EDFAs Optical communication systems carrying 100 or more optical wavelengths require and increase in the bandwidth of the optical amplifier to nearly 80 nm. Normally employing a hybrid optical amplifier, consisting of two separate optical amplifiers, allows for separate amplification, one for the lower 40 nm band and the second for the upper 40 nm band. Figure 6 exemplifies the optical gain spectrum of a hybrid optical amplifier. The solid lines illustrate the response of two individual amplifier sections. The dotted line, which has been increased by 1 dB for clarity, shows the response of the combined hybrid amplifier.

wideband-edfa
Figure 6 – Optical Gain Spectrum of a Hybrid Optical Amplifier

Raman Optical Amplifiers
Raman optical amplifiers differ in principle from EDFAs or conventional lasers in that they utilize stimulated Raman scattering (SRS) to create optical gain. Initially, SRS was considered too detrimental to high channel count DWDM systems. Figure 7 shows the typical transmit spectrum of a six channel DWDM system in the 1550 nm window. Notice that all six wavelengths have approximately the same amplitude.

srs-transmitted-spectrum
Figure 7 – DWDM Transmit Spectrum with Six Wavelengths

By applying SRS the wavelengths, it is obvious that the noise background has increased, making the amplitudes of the six wavelengths different. The lower wavelengths have a smaller amplitude than the upper wavelengths. The SRS effectively robbed energy from the lower wavelength and fed that energy to the upper wavelength.

srs-received-spectrum
Figure 8 – Received Spectrum After SRS is on a Long Fiber

A Raman fiber optic amplifier is little more that a high-power pump laser, and a WDM or directional coupler. The optical amplification occurs in the transmission fiber itself, distributed along the transmission path. Optical signals are amplified up to 10 dB in the network optical fiber. The Raman optical amplifiers have a wide gain bandwidth (up to 10 nm). They can use any installed transmission optical fiber. Consequently, they reduce the effective span loss to improve noise performance by boosting the optical signal in transit. They can be combined with EDFAs to expand optical gain flattened bandwidth. Figure 9 shows the topology of a typical Raman optical amplifier. The pump laser and circulator comprise the two key elements of the Raman optical amplifier. The pump laser, in this case, has a wavelength of 1535 nm. The circulator provides a convenient means of injecting light backwards in to the transmission path with minimal optical loss.

raman-amp-config
Figure 9 – Typical Raman Amplifier Configuration

Figure 10 illustrates the optical spectrum of a forward-pumped Raman optical amplifier. The pump laser is injected at the transmit end rather than the receive end as shown in Figure 9. The pump laser has a wavelength of 1535 nm; the amplitude is much larger than the data signals.

raman-transmitted-signal
Figure 10 – Example of Raman Amplifier — Transmitted Spectrum

As before, applying SRS makes the amplitude of the six data signals much stronger. The energy from the 1535 nm pump laser is redistributed to the six data signals.

raman-received-signal
Figure 11 – Example of Raman Amplifier — Received Spectrum