Light it up
It may be surprising to learn that the idea of using light waves to transmit voice signals is well over a century old. In fact, Alexander Graham Bell’s “photophone” invention used a narrow beam of sunlight focused on a thin mirror that vibrated when hit by human sound waves to transmit voice signals over distances up to 700 feet in 1880! The foundation for modern techniques of transmitting light energy was set in the 1960’s when ruby lasers were first demonstrated and in the 1970’s when workers at Corning Glass Works produced the first optical fibre with signal losses less than 20 dB/km. Since then, tremendous strides have been made in the refinement of semiconductor laser and light emitting diode light sources, as well as the optical fibre cables and components used to support the transmission of light energy.
While optical fibre cabling expertise is commonly thought to fall within the domain of service providers, it can not be overlooked that optical fibre cabling plays an important role in supporting customer-owned telecommunications infrastructures as well. Beyond supporting long-length runs installed between buildings or points in a customer-owned campus environment (commonly referred to as “outside plant cabling”), it’s interesting to note that, on average, 20% of the cabling installed in the enterprise and 40% of the cabling installed in the data centre (particularly between storage devices) is optical fibre cabling. While balanced twisted-pair copper cabling may still be the media of choice due to familiarity, perceived ease-of-termination compared to optical connections, and significantly lower equipment costs, the following benefits are compelling reasons to consider optical fibre cabling in your IT infrastructure:
- Extended distance support beyond the balanced twisted-pair limit of 100 meters
- Smaller media (e.g. two category 6A cables occupy the same space as one 216 fibre cable)
- Lighter media (e.g. 108 category 6A cables weighing 1,000 pounds or one 216-fibre cable weighing 40 pounds can be used to support 108 channels that are 200 feet long)
- Significantly higher port density in the telecommunications closet and line card density in the data centre (up to 1,728 in a 4U housing)
- Smaller pathways required for fiber
- Improved air flow due to less cable damming
- Media robustness; optical fibre cabling can withstand double the pull tension of balanced twisted-pair cabling (50 lbf versus 25 lbf)
- Reduced equipment power consumption and cooling costs
- Centralized optical cabling may be used when deploying centralized equipment in the horizontal to eliminate the need for an optical cross-connect
- Support of passive optical LAN (POL) solutions
- Immune to electromagnetic and radio frequency interference (EMI/RFI)
- Immune to lightning strikes
Signal Transmission over Optical Fibre Cabling:
|Table 1: Summary of Optical Light Sources|
|Light Source Type||Cost||Speed||Transmission
|Source Aperture (approx)|
(Light Emitting Diode)
|850 nm||100 µm|
|VCSEL: “Vertical Cavity Surface Emitting Laser” (Semiconductor laser diode)||Mid||≥ 1 Gb/s||850 nm
|Laser: (Fabrey-Perot edge-emitting semicon- ductor laser diode)||High||≥ 1 Gb/s||1310 nm
Optical communication is the transmission of photon (or light) energy through a low-loss waveguide whose function is to propagate the light signals over long distances. In telecommunications systems, the source of the photon energy may be a light emitting or a semiconductor laser diode, whose function is to produce light energy at a single wavelength. By turning the light source on and off quickly, streams of ones and zeros can be transmitted to form a digital communications channel. LED and laser light sources vary considerably with respect to their cost, transmit speed, and physical properties. Refer to table 1 for an overview of the three light sources used in optical fibre telecommunications systems.
The wavelength of the optical light source describes the frequency of the transmitted light wave (the longer the wavelength, the lower the frequency of the light wave) and has been selected to best match the transmission properties of recognized optical fibre types. A helpful analogy is to think of “wavelength” as the color of the light signal that is being transmitted. As shown in figure 1, the common optical communications wavelengths of 850 nm to 1550 nm fall between the ultraviolet and microwave frequencies in the light spectrum.
Source aperture describes the width of the transmitted light signal pulse. This characteristic is also related to the diameter of the optical fibre (the “waveguide”) that will optimally transmit the light pulses; which helps to explain why there are several types of optical fibre cabling systems available. Larger diameter optical fibre (e.g. 62.5μm and 50μm) is required to adequately support transmission of light sources with larger apertures such as LED’s and VCSEL’s by minimizing signal loss and maximizing transmit distances. Small diameter optical fibre (e.g. 9μm) is required to adequately support transmission of laser light sources.
As a result of the variance in source aperture and optical fiber size, there are two ways that light can propagate through optical fiber. Since 62.5μm and 50μm optical fibre diameters are relatively large compared to the wavelength of the transmitted light signal (i.e. 850 nm to 1550 nm), there are many paths or “modes” that light energy may take when it propagates through the optical fiber. This type of transmission is referred to as multimode. Since the 9μm optical fibre diameter is similar to the wavelength of the transmitted light signal, only the one wavelength associated with transmission propagates through the optical fiber. This type of transmission is referred to as singlemode.
Careful examination of multimode signal propagation quickly raises a concern about how the design of the optical fibre itself may adversely impact signal propagation. The earliest optical fibre design, referred to as step index, was constructed with a uniform index of refraction. This meant that all energized paths of light, whether propagating at the core or at the edge of the optical fiber, traveled at the same speed. The undesired result is that, over some distance, the energized modes in a step index optical fibre will support different path lengthsand the output pulse will subsequently have a lower amplitude and wider spread (longer duration) than the input pulse due to the faster and slower light paths. Modal dispersion describes the degree to which the output pulse has spread compared to the input pulse and effectively limits the bit rate or bandwidth of the step index optical fibre to 20 – 30 million signal cycles per second transmitted over the distance of one kilometer (20-30 MHz·km). To compensate for this phenomenon, graded index optical fibre is constructed so that the index of refraction gradually changes from a maximum at the centre (“slowing” the light signal) to a minimum (“speeding up” the light signal) near the edge of the optical fiber. This increases the bandwidth of graded-index fibre to greater than 1 billion signal cycles per second transmitted over the distance of one kilometer (1 GHz·km). Virtually all multimode fibers manufactured today are graded index. Since singlemode optical fibre supports only one wavelength of light, modal dispersion is not a concern for this media. See figure 2 for examples of multimode and singlemode light propagation.
An additional improvement to multimode optical fibre design involves optimizing the media to specifically support the VCSEL light source. Because the source aperture of an LED light source exceeds the diameter of the largest optical fiber suitable for telecommunications (62.5μm), all modes of a multimode fibre are energized and the pulse output is fairly easy to control with graded index optical fiber. However, since the source aperture of a VCSEL light source is much less than the diameter of the smallest optical fibre suitable for telecommunications (50μm), only a portion of the available transmission paths in a multimode fibre are energized. Second generation “laser-optimized” graded-index optical fibre is even more tightly specified to ensure that the pulse output of a VCSEL source exhibits well-controlled and limited modal dispersion.
In consideration of next generation applications that will employ more complex transmission schemes, such as transmitting more than one wavelength over a single fibre (e.g. wavelength division multiplexing), emphasis is placed on ensuring that optical fibers have a smooth attenuation profile over the range of possible transmission wavelengths. Of particular concern is attenuation increase in the 1360 – 1480 nm (the “E Band” or “water peak”) range due to hydroxyl (also specified as OH¯) ions that are absorbed into singlemode fibers during the manufacturing process. Low water peak (LWP) singlemode fibers have undergone an additional manufacturing step to reverse the water absorption and have a nearly smooth attenuation profile. Zero water peak (ZWP) singlemode fibers undergo a more complex process which eliminates all losses in the water peak range and further lowers attenuation loss across the entire spectrum.
The many variables associated with optical fibre transmission, including the capabilities of the light source, modal dispersion, chromatic dispersion (a second order effect characterizing slight shifts in the transmit light spectrum), bandwidth, and losses in the transmission line contribute to the bit rate and distance capabilities of various optical fiber media. In general, lasers transmitting over singlemode fiber support the highest bandwidth and longest distance while LED’s transmitting over large diameter (62.5μm) multimode fibre support the lowest bandwidth and shortest distances.
|Table 2: Optical Fibre Types and Standards References|
|Description||TIA and ISO Standard Reference|
|OM1||62.5μm Multimode||TIA 492AAAA-B
IEC 60793-2-10 A1a.1 type
|OM2||50μm Multimode||TIA 492AAAB-A
IEC 60793-2-10 A1a.2 type
|OM3||850nm laser-opti- mized 50μm Multimode||TIA 492AAAC-B
IEC 60793-2-10 Ed 4.0 A1b type
|OM4||850nm laser-opti- mized high bandwidth
IEC 60793-2-10 Ed 4.0 A1a.3
IEC 60793-2-50 Ed 4.0 B1.1 type
(see also ITU-T G.652 a/b)
|OS2||Singlemode for outdoor loose-tube constructions||TIA 492CAAB
IEC 60793-2-50 Ed 4.0 B1.3 type
(see also ITU-T G.652 c/d)
Unlike balanced twisted-pair media, optical fibre cablingcan be considered an application dependent media. This means that considerations such as distance, application, and equipment cost plays a significant role in the media selection process.
TIA and ISO (through reference to IEC and ITU-T specifications) recognize six grades of multimode and singlemode optical fibre as shown in table 2. Physical dimensions related to the optical fibre (e.g. diameter, non-circularity, and mechanical requirements) and optical specifications (e.g. attenuation and bandwidth) are specified. It is important to keep in mind that these specifications are for the “raw” optical fibre before it is subjected to the cabling process. TIA and ISO use these optical fibre requirements to then specify requirements for OM1, OM2, OM3, OM4, OS1, and OS2 optical fibre cables and cabling.
While media selection may seem onerous, comparing the throughput and distance needs in your target environment against a performance chart such as shown in table 3 is a good way to initiate the selection process. Although this and other similar tables may lead to the conclusion that singlemode fiber is the optimum media under all scenarios, there are trade-offs to consider related to the cost of optoelectronics and application implementation. In particular,
- Singlemode optoelectronics rely on much more powerful and precise light sources and can cost 2 – 4 times more than multimode optoelectronics
- Multimode media is typically easier to terminate and install in the field
- It is always more cost effective to transmit at 850nm for multimode applications and at 1310 nm for singlemode applications
- Optoelectronics that use multiple transmit lasers (e.g. 10GBASE-LX4 uses four separate laser sources per fiber) or other multiplexing techniques cost significantly more than optoelectronics that transmit over one wavelength
A good rule of thumb is to consider multimode fibre to be the most cost-effectivechoice for applications up to 550 meters in length.
|Table 3: Supportable Application Distances by Fibre Type (meters)|
Optical Fibre Cabling Configurations:
Optical fibre cabling is typically deployed in pairs (one fibre is used to transmit and one fibre is used to receive). Due to its extended distance support of applications compared to balanced twisted-pair cabling, optical fibre cabling is the perfect media for use in customer-owned outside plant (OSP), backbone cabling, and centralized cabling applications.
Customer-owned OSP cabling is deployed between buildings in a campus environment and includes the terminating connecting hardware at or within the structures. Interestingly, customer owned OSP cabling is typically intended to have a useful life in excess of thirty (30) years, so great care should be taken to specify robust cabling media. Requirements pertaining to customer- owned outside plant cabling and pathways can be found in ANSI/TIA-758-A and BS EN 50174-3 .
Backbone cabling is deployed between entrance facilities, access provider spaces, service provider spaces, common equipment rooms, common telecommunications rooms, equipment rooms, telecommunications rooms, and telecommunications enclosures within a commercial building. Backbone cabling must be configured in a star topology and may contain one (main) or two (main and intermediate) levels of cross connects. Backbone cabling requirements are specified in ANSI/TIA-568-C.0 , ANSI/TIA-568-C.1 , and ISO/IEC 11801 2nd, Edition.
Centralized optical fibre cabling may be deployed as an alternative to the optical cross-connect to support centralized electronics deployment in single tenant buildings. Centralized optical fibre cabling supports direct connections from the work area to the centralized cross-connect via a pull-through cable and the use of an interconnect or splice in the telecommunications room or enclosure. Note that the maximum allowed distance of the pull-through cable between the work area and the centralized cross-connect is 90 m (295 ft). Centralized cabling requirements are specified in ANSI/TIA-568-C.0 and ISO/IEC 11801 2nd Edition. A typical schematic for centralized optical fibre cabling using an interconnection is show in figure 3.
Optical fibre cabling may also be used in the horizontal cabling infrastructure, although there are no provisions allowing extended distances in the TIA and ISO Standards.
Horizontal cabling is deployed between the work area and the telecommunications room or enclosure. Horizontal cabling includes the connector and cords at the work area and the optical fibre patch panel. A full cross-connect or interconnect may be deployed along with an optional multi-user telecommunications outlet assembly (MUTOAs) or consolidation point (CP) for a total of four connectors in the channel. The maximum horizontal cable length shall be 90 m (295 ft) and the total length of work area cords, patch cords or jumpers, and equipment cords shall be 10 m (32 ft) for both optical fibre and balanced twisted-pair cabling channels. Horizontal cabling requirements are specified in ANSI/TIA-568-C.0, ANSI/TIA-568-C.1, and ISO/IEC 11801, 2nd Edition.
Optical Fibre Cable:
The optical fibre that enables light transmission is actually an assembly of 3 subcomponents: the core, the cladding, and the coating. The core is made of glass (or, more accurately, silica) and is the medium through which the light propagates. The core may have an overall diameter of 9 μm for singlemode or 50μm or 62.5μm for multimode transmission. Surrounding the glass isa second layer of glass with a vastly different index of refraction that focuses and contains the light by reflecting it back into the core. This second layer is called the cladding and, regardless of the glass core construction, has an overall diameter of 125μm. Combining the core and cladding diameters is the source of optical fibre descriptors, such as 50/125μm or 62.5/125μm, that are applied to optical fibers commonly used for telecommunications applications. The purpose of the outermost layer, called the coating, is to add strength and build up the outer diameter to a manageable 250μm diameter (about 3 times the diameter of a human hair). The coating is not glass, but rather a protective polymer, such as urethane acrylate, that may be optionally colored for identification purposes. See figure 4 for a cross-sectional view of an optical fiber.
Cabling optical fibers makes them easier to handle, facilitates connector termination, provides protection, and increases strength and durability. The cablemanufacturing process differs depending upon whether the optical fibers are intended for use in indoor, outdoor, or indoor/outdoor environments.
Indoor optical fibre cables are suitable for inside (including riser and plenum) building applications. To facilitate connector terminations, a 900μm plastic buffer is applied over the optical fibre core, cladding, and coating subassembly to create a tight buffered fiber. Up to 12 tight buffered fibers are then encircled with aramid yarns for strength and then enclosed by an overall flame-retardant thermoplastic jacket to form a finished optical fibre cable. For indoor cables with higher than 12-fibre counts, groups of jacketed optical fiber cables (typically 6- or 12-fibre count) are bundled together with a central strength member (for support and to maintain cable geometry) and are enclosed by an overall flame-retardant thermoplastic jacket. Supported fibre counts are typically between 2 and 144.
Outdoor (also known as outside plant or OSP) optical fiber cables are used outside of the building and are suitable for lashed aerial, duct, and underground conduit applications. To protect the optical fibre core from water and freezing, up to 12 250μm optical fibre cores are enclosed in a loose buffer tube that is filled with water-blocking gel. For up to 12-fibre applications, the gel-filled loose tube is encircled with water-blocking tapes and aramid yarns and enclosed within an overall ultraviolet and water resistant black polyolefin jacket. For outdoor cables with higher than 12-fiber counts, groups of loose buffer tubes (typically 6- or 12-fiber count) are bundled together with a central strength member and water-blocking tapes and aramid yarns and then enclosed within an overall ultraviolent and water resistant black polyolefin jacket. Corrugated aluminum, interlocking steel armor, or dual jackets may be applied for additional protection against crushing and rodent-damage. Supported fiber counts are typically between 12 and 144.
Indoor/outdoor optical fibre cables offer the ultraviolet and water resistance benefits of outdoor optical fibre cables combined with a fire retardant jacket that allows the cable to be deployed inside the building entrance facility beyond the maximum 15.2 m (50 ft) distance that is specified for OSP cables. (Note that there is no length limitation in countries outside of the United States that do not specify riser or plenum rated cabling.) The advantage of using indoor/outdoor optical fibre cables in this scenario is that the number of transition splices and hardware connections is reduced. Indoor/outdoor optical fibre cables are similar in construction to outdoor optical fibre cables except that the 250μm optical fibre cores may be either tight buffered or enclosed within loose buffer tubes. Loose tube indoor/outdoor optical fibre cables have a smaller overall diameter than tight buffered indoor/outdoor optical fiber cables, however tight buffered indoor/outdoor cables are typically more convenient to terminate because they do not contain water-blocking gel or require the use of breakout kits.
|Table 4: Optical Fibre Cable Transmission Performance|
|Wavelength (nm)||Maximum Attenuation (db/km)||Minimum Over- filled Modal Bandwidth (MHz•km)||Minimum Ef- fective Modal Bandwidth (MHz•km)|
|Inside Plant (OS1 and OS21)||1310
|Indoor-Out- door (OS1 and OS21)||1310
|Outside Plant (OS1 and OS21)||1310
Figure 5 shows examples of 72-fibre optical cables featuring tight buffered indoor and loose tube outdoor constructions. Optical fibre cable is characterized by its maximum attenuation, minimum overfilled modal bandwidth (LED light source – multimode only), and minimum effective modal bandwidth (VCSEL light source – multimode only) per kilometer at two transmission wavelengths as shown in table 4.
Optical Fibre Interconnections: Unlike the plug and jack combination that compr ises a mated balanced twisted-pair connection, an interconnection is used to mate two tightbuffered optical fibers. An optical fiber interconnection typically consists of two plugs (connectors) that are aligned in a nose-to-nose orientation and held in place with an adapter (also called a coupler or bulkhead). The performance of the optical fiber interconnection is highly reliant upon the connector’s internal ferrule and the adapter’s alignment sleeve. These components work in tandem to retain and properly align the optical fibers in the plug-adapter-plug configuration. The internal connector ferrule is fabricated using a high-precision manufacturing process to ensure that the optical fiber is properly seated and its position is tightly controlled. The high-tolerances of the alignment sleeve ensure that the optical fibers held in place by the ferrule are aligned as perfectly as possible. Although more expensive, ceramic alignment sleeves maintain slightly tighter tolerances than metal or plastic alignment sleeves, are not as susceptible to performance variations due to temperature fluctuations, and may be specified for extremely low loss applications. All Siemon adapters come standard with ceramic alignment sleeves. See figure 6 for an example of an optical fibre plug-adapter-plug configuration.
Accurate plug-adapter-plug alignment minimizes light energy lost at the optical fibre interconnection and maintaining precision tolerances becomes especially cr itical as the optical fiber diameter decreases. For example, if two 62.5μm optical fibers are off-centre by 4 μm in opposite directions, then 13% of the light energy escapes or is lost at the interconnection point. This same misalignment in a 9μm singlemode fibre would result in almost a total loss of light energy! The critical nature of the core alignment is the reason why different optical fiber types, including 62.5μm and 50μm multimodefiber, should never be mixed in the same link or channel.
Optical fibre breakout kits are used to facilitate termination of loose-tube optical fibers used in indoor/outdoor and outdoor applications. Once the water-blocking gel is thoroughly removed from the optical fibers, the breakout kit allows furcation tubes (typically 1.2mm to 3.0mm in diameter) to be installed over the 250μm optical fibers; increasing the diameter and forming a short “jacket” so that the optical fibers may be terminated to the desired optical fibre connector. Selection of the correct furcation tube ensures compatibility with all optical fibre connectors.
There are many choices for the optical fibre connector.
Traditional optical fibre connectors are represented by the SC and ST connector styles. These two types of optical fiber connectors were recognized when optical fibre cabling was described in the first published TIA and ISO/IEC telecommunications cabling Standards. The ST connector features a round metal coupling ring that twists and latches onto the adapter and is only available as a simplex assembly (two assemblies are required per link or channel). SC connectors feature a quick push-pull latching mechanism and have an advantage in that they may be used in conjunction with a duplexing clip that more easily supports the interconnection of the 2 optical fibers in a link or channel. SC optical fibre connectors are generally recommended over ST optical fibre connectors for use in new installations due to their duplexing capability. Both ST and SC connectors may be field-terminated using an epoxy polish or mechanical splice method. In addition, the SC connector may be quickly and reliably field-terminated using Siemon’s proven XLR8® mechanical splice technology.
Small form factor (SFF) refers to a family of optical fiber interfaces that support double the connector density of traditional optical fibre connectors. The most common SFF interface is the LC connector; with the MT-RJ having some limited legacy market presence. Both interfaces feature duplex configurations and a small pluggable form with external plug latch that is approximately the same size as the 8-position modular plug used for copper connections. The LC connector may by field-terminated using an epoxy polish method or mechanical splice method such as Siemon’s XLR8 technology. The MT-RJ connector is field-terminated using a traditional no-epoxy/no-polish mechanical splice termination method. The main difference between the MT-RJ and LC optical connector is related to the performance of the internal ferrule. The internal ferrule of the LC connector maintains sufficiently tight tolerances to fully support both singlemode and multimode applications, while the MT-RJ connector is recommendedfor use in legacy applications only. Siemon does not recommend field termination of MT-RJ connectors for singlemode applications.
Array optical fibre connectors are the latest recognized style of optical fibre interfaces and are intended to support extremely high density environments (e.g. those supporting 40GBASE-SR4 and 100GBASE SR10), as well as emerging technologies such as 100GBASE-SR4 and 100GBASE LR4 that will require more than 2 optical fibers per link or channel. There are typically 12 or 24 fibers in an array connector, although one array connector may support as many as 144 fibers. A multi-fibre push on (MPO) style interface is the most basic array interface. MTP® optical fiber connectors are intermateable with MPO connectors including those used in active equipment; however they are engineered to deliver improved mechanical and optical performance and are recommended for deployment in new installations. MPO/MTP connectors cannot be field terminated. Array or “plug & play” modules are self-contained and typically support the interconnection of two 12-fiber MPO/MTP interfaces with 24 LC connections or one 12-fiber MPO/MTP interface with 12 SC or LC connections.
Optical fibre connector performance is specified for the parameters of insertion loss (0.75 dB maximum) and return loss (20 dB minimum for multimode, 26 dB minimum for singlemode, and 55 dB minimum for singlemode used to support broadband analog video (e.g. CATV) applications). Examples of common optical fibre connectors are shown in figure 7.
Optical Fibre Cabling Deployment:
The most common optical fibre cabling deployment approach is to field terminate the optical fibre connectors to the optical fibre cable using the appropriate epoxy polish or no-epoxy/no polish mechanical termination method. However, the MPO/MTP plug and play modules and MPO/MTP array connectors are not supported by field termination and there are other considerations, such as installer expertise and the IT construction/upgrade schedule, which may favor the use of factory-terminated pigtails or trunking assemblies over field termination methods. The pros and cons of each of these methods are described below.
Field termination supports the lowest raw material cost for SC, ST, LC, and MT-RJ optical fibre cabling systems. However, the time needed for field-termination is the longest of the three deployment options and installer skill level requirements are higher, which may increase the project installation cost. No-epoxy/no-polish termination methods require less installation skill than the epoxy polish method, however the connectors used in conjunction with mechanical termination methods are more expensive and the performance (especially using the no-epoxy/no polish method) may be lower and more variable.
Optical fibre pigtails feature a factory pre-terminated and tested SC, ST, LC or MT-RJ optical fibre connector and a 1 meter stub of 62.5/125μm multimode, 50/125μm multimode, or singlemode optical fiber. The stub end of the pigtail is then fusion spliced to the optical fiber. Fusion splicing provides a consistent, nearly loss-free termination and can be fast with proper technicians and equipment. The main benefits to this approach are the assurance of low-loss performance at the interconnection and the elimination of the need for end-face inspections and possible connector re terminations.