Generally speaking, fiber optic cable can be installed using many of the same techniques as conventional copper cables. For an overview of installation techniques, click on General Guidelines
; for more detailed information on the various processes involved, click on any one of the following: Cable Preparation Component Selection Connector Attachment Connector Finishing Connector Guide Fiber Preparation FiberExpress Brilliance Installation Guide FiberExpress Brilliance Video Splicing Optical Fibers Testing
The following contains information on the placement of fiber optic cables in various indoor and outdoor environments. In general, fiber optic cable can be installed with many of the same techniques used with conventional copper cables. Basic guidelines that can be applied to any type of cable installation are as follows:
Conduct a thorough site survey prior to cable placement.
Develop a cable pulling plan.
Follow proper procedures.
Do not exceed cable minimum bend radius.
Do not exceed cable maximum recommended load.
Document the installation.Conduct a Site SurveyThe purpose of a site survey is
to recognize circumstances or locations in need of special attention. For example, physical hazards such as high temperatures or operating machinery should be noted and the cable route planned accordingly. If the fiber optic cable has metallic components, it should be kept clear of power cables. Additionally, building code regulations, like the National Electric Code (NEC)** must be considered. If there are questions regarding local building codes or regulations, they should be addressed to the authority having jurisdiction, such as the fire marshal or city building inspector.
Develop a Cable Pulling Plan
A cable pulling plan should communicate the considerations noted during the site survey to the installation team. This includes the logistics of cable let-off/pulling equipment, the location of intermediate access points, splice locations and the specific responsibilities of each member of the installation team.
Follow Proper Procedures
Because fibers are sensitive to moisture, the cable end should be covered with an end cap, heavy tape or equivalent at all times. The let-off reel must never be left unattended during a pull because excess or difficult pulls, center-pull or backfeeding techniques may be employed.
Do Not Exceed Cable Minimum Bend Radius
Every Belden® cable has an installation minimum bend radius value. During cable placement it is important that the cable not be bent to a smaller radius. After the cable has been installed, and the pulling tension removed, the cable may be bent to a radius no smaller than the long term application bend radius specification.
The minimum bend radii values still apply if the cable is bent more than 90 degrees. It is permissible for fiber optic cable to be wrapped or coiled as long as the minimum bend radius constraints are not violated.Do Not Exceed Cable Maximum Recommended Load
While fiber optic cables are typically stronger than copper cables, it is still important that the cable maximum pulling tension not be exceeded during any phase of cable installation. In general, most cables designed for outdoor use have a strength rating of at least 600 lbs. Belden fiber optic cables also have a maximum recommended load value for long term application. After cable placement is complete the residual tension on the cable should be less than this value. For vertical installations, it is recommended that the cable be clamped at frequent intervals to prevent the cable weight from exceeding the maximum recommended long term load. The clamping intervals should be sufficient to prevent cable movement as well as to provide weight support.
Leave Extra Cable
A common practice is to leave extra cable at the beginning and at the end of the cable run. Also, extra cable should be placed at strategic points such as junction boxes, splice cases and cable vaults. Extra cable is useful should cable repair or mid-span entry be required.
Document the Installation
Good record keeping is essential. This will help to ensure that the cable plant is installed correctly and that future trouble shooting and upgrading will be simplified. All Belden fiber optic cables have a unique lot number shown on the shipping spool. It is important that this number be recorded. Cable pre- and post- installation test data should be recorded in an orderly and logical fashion.Pulled Installations
In order to effectively pull cable without damaging the fiber, it is necessary to identify the strength material and fiber location within the cable. Then, use the method of attachment that pulls most directly on the strength material—without stressing the fiber.
As a general rule, it is best to install cable prior to connector attachment. After connectors have been attached, it becomes more difficult to protect the fiber from inadvertent stress. If a pull is to be made entirely in one direction, connectors may be pre-installed on one end, leaving the other end for pulling.
If the cable must be installed with connectors attached, every practical means must be taken to protect the connectorized end from damage or stress. Cushioned enclosures should be used to protect connectors during pulling.
The leading end of the cable should be sealed to prevent intrusion of water or other foreign material while pulling.
Bi-directional pulls are possible by laying the cable into large "figure-8"-shaped loops on the ground, from where it can feed from both ends.
For ease of cable installation, the area of the cable divided by the area of the duct or conduit should be less than 53% per a single cable. Permissible area to be occupied for 2 cables is 31%, for 3 or more cables it is 40%.
Direct Attachment: Strength member is tied directly to the pulling fixture. The cable end must be sealed to prevent intrusion of moisture while pulling.
With direct attachment, cable strength material is tied directly to the pulling fixture. Conventional cable tools may be used. Loose fiberglass threads are not suitable for direct attachment because they may break if knotted. Fiberglass epoxy rods are too rigid to tie, but may be secured to the pulling fixture by using tight clamping plates or screws.
Indirect Attachment: Pulling forces are distributed over the outer cable structure.
With indirect attachment, pulling forces are distributed over the outer portion of the cable structure. If cable strength materials are located directly beneath the jacket, this method will produce the least amount of stress on the fiber.
A popular type of pulling fixture for indirect attachment is the "Chinese Basket" or "Kellems Grip".* The Kellems Grip is usually reliable for cables of 1/4" diameter or more. Large pulling forces are possible with a Kellems Grip if the grip’s diameter and length are properly matched to cable characteristics.
A Kellem Grip should spread pulling forces over a 1-1/2 to 3-foot length of cable. For small cables, pre-stretching and taping the Kellems Grip to the cable helps to assure even pulling.
Many lubricants are available for lowering friction forces. These include greases, waxes, clay slurries and water-based gels. Fiber optic jacket materials are compatible with most of these. For new conduit, lubrication of the conduit before pulling is suggested—particularly if there are several bends.
Air Plenums, Trays, Raceways
Installation procedures for open placement of fiber optic cables are the same as for electrical cables. Care should be taken to avoid sudden, excessive force so as not to violate tensile load and radius limits. Sharp bending and scraping at entrances and covers should be avoided.
For indoor applications, NEC**-rated OFNR (riser) and OFNP (plenum) should be used to satisfy building code regulations. It is always recommended to check local authorities prior to cable installation.
Belden outdoor cables may be buried directly in the ground. Environmental hazards include freezing water, crushing forces from rocky soil, ground disruption from construction, and rodents. Burying the cable 36 to 48 inches deep may help prevent most of these hazards.
Direct plow-in installation requires a cable capable of withstanding uneven pulling forces. Loose tube cables are best suited for these types of installations.
Double jacketing, gel filling, metal sheathing and armoring are used as water barriers.
Use of double jacketed armored cables can sometimes be avoided by burying polyethylene pipe to form a simple conduit. The pipe makes a smooth passageway and may be curved to allow easy access at manholes and other pull points. Cables may be subsequently replaced without digging.
All Belden Loose tube cables are compatible with helical lashing.
Aerial lashing of a fiber optic cable.
It is frequently required to store cables prior to installation. Temperature ranges for cable storage are listed in the corresponding catalog pages. It is recommended that cable ends be sealed to prevent intrusion of moisture.
Polyethylene pipe can be used as a simple conduit. This allows use of less expensive cables in direct burial applications.
Harvey Hubbell Trademark.
** Trademark of National Fire Protection Association, Quincy, MA.
The following is a general description of cable preparation and termination procedures.
The procedure for stripping fiber optic cables is very similar to electronic cables. However, care should be taken not to cut into the layer of aramid directly beneath the jacket. This would either reduce the pull strength of the cable, or weaken the connection. For this reason, if a blade must be used, a cut which does not completely penetrate the jacket can be made. This will weaken it sufficiently and allow the jacket to be peeled.
Most Belden cables utilize a ripcord capable of tearing the outer sheath.Cutting and Trimming Aramid
Aramid can be easily cut with sharp scissors if the threads are confined in movement so that cutting pressure can be applied. Ceramic scissors may also be used.Steel and Fiberglass Epoxy Rod Members
Temperature stabilized cables of both loose and tight buffer constructions often have steel or fiberglass epoxy rods. Use of heavy-duty cutters is recommended for these hard materials.
Buffer Tube Trimming
Buffer tubes are made of plastic materials with various characteristics of hardness and flexibility. Belden buffer tubes are both flexible and strong, but may be trimmed easily. The simplest way is to score one side of the buffer tube firmly with a razor blade, then bend the tube sharply away from the score mark. The broken-off piece is then pulled straight off, leaving the fiber intact.
A stripping tool which barely cuts through the tube is also satisfactory. If it is intended to cut through both the buffer tube and the fiber, use diagonal cutters and cut through cleanly.Breakout Element Trimming
Breakout subunit element jackets are most easily removed by a stripping tool which cuts circumferencially. The jacket may then be pulled straight off, exposing the aramid.
Component SelectionSource Emitters
Optical emitters couple light into a fiber according to NA and core size. Using a light source not matched to a particular fiber's NA and core size will cause less than optimum light coupling for the system.
LEDs are relatively inexpensive, reliable and easy-to-use because their electronic circuitry is less complex than that required for a laser.
Semiconductor lasers and LEDs are both direct transducers from electrical to optical radiation. LEDs couple less power into the fiber because they emit the optical radiation over a broader angle area. The laser is a much more complicated structure due to the requirement for a small dual-face cavity. Also its output is temperature dependent and the lifetime is less than the LED.
Several different LED packaging styles are commercially available:
The LED or laser diode can be packaged so that the fiber cable plugs directly into the device package. An alternative is fastening the fiber directly to the chip and leaving the opposite end available for a connector.
Matched transmitter and receiver units, plus a wide variety of other fiber optic components ranging from discrete elements like LEDs, laser diodes, and detectors to complete rack-mounted modules are all readily available.
Lightwave receivers use photodetectors, where the photons of light generate photoelectrons. A minimum average number of photons in each pulse is necessary to achieve a given error probability (21 photons for 10-9 error probability). Considerable amplification is necessary. For an avalanche-photodiode (APD) initial amplification is internal. For positive-intrinsic-negative detectors (PIN) this amplification is by external electronic amplifiers.
Optical Fiber Loss
We've already considered core size and numerical aperture as measures of fibers ability to accept the optical power. Now let's consider what happens to the optical signal once it's launched.
In coaxial cable, the higher the frequency, the more signal strength decreases with distance and this is referred to as attenuation. Fiber frequency is constant until it reaches its bandwidth limit. Thus optical loss is proportional to distance.
This attenuation within the fiber is caused by absorption and scattering of lightwaves due to chemical impurities and molecular structure. These fiber properties absorb or scatter the optical radiation so that it escapes the core and is lost.
Attenuation within a fiber is specified by the manufacturer at certain wavelengths: For example 3 dB/km at 850 nanometers. This is done because fiber loss varies with wavelength.
These wavelengths are measured in nanometers (nm)--billionths of a meter-- which represent the distance between two cycles of the same wave. Wavelength is a descriptive property of electromagnetic radiation. Light and infrared radiation are a portion of the total electromagnetic spectrum.
Microwaves, radar, television and radio operate in the longest wavelength areas. In between the ultraviolet and the microwave spectrums, we have fiber optic wavelengths, which are in the infrared spectrum.
Fibers are therefore optimized for operation at certain wavelengths. For example, less than 1 dB/km loss is attainable in 50/125 µm multimode fiber operating at 1300 nm, and less than 3 dB/km (50% loss) is attainable for the same fiber operating at 850 nm. The 50/125 nomenclature indicates both the outside diameter of the core (50 microns) and the cladding (125 microns).
The favorable transmission regions within the optical spectrum for a fiber are referred to as "windows". The 800 to 900 nanometer region is the first window, 1100 to 1300 nanometers is the second window, and the third window occurs at about 1550 nanometers. In these spectral windows fibers have very low attenuation. The lowest losses occur in the infrared region around 1300 nm and again around 1550 nm.
Great improvements have been made in all fiber types so that premium fibers exhibit losses of less than 0.5 dB/km at wavelengths of 1300 and 1550 nm. However, source emitters and detectors for these regions are currently more expensive.
If the fiber is to perform well, the source chosen should provide optical radiation at the specified wavelength, and the detector should be sensitive to the same wavelength.
In coaxial and other metallic cables, very high frequency signals tend to be attenuated rapidly with distance. As a result, amplifiers and equalizers are required at periodic intervals to build up signals to usable levels.
However, each time an analog amplifier is added, noise is introduced to the metallic system and the overall system signal-to-noise ratio degrades.
With optical communications, all of the light energy is at approximately the same frequency or wavelength. As a result, the attenuation of a specific wavelength is dependent only on distance. The chart below shows a comparison of attenuation differences between coaxial and fiber optic cable. The requirement for repeaters is, therefore, minimized and the need for equalizers is eliminated in fiber systems.
Connector loss is a function of the physical alignment of one fiber core to another fiber core.
Scratches and dirt can also contaminate connector surfaces and severely reduce system performance, but most often the connector loss is due to misalignment or end separation.
Belden fiber optic cables are available on a custom-order basis with connectors pre-attached. These cable assemblies are ready for direct connection to their mating components and feature 100% optical attenuation testing.Fiber Optic Connectors
Fiber optic connectors position fiber ends to receive or transmit light. The surfaces must be smooth and perpendicular to the fiber axis for greatest efficiency in accepting light rays. Rough, slanted or dirty end surfaces block and scatter light.
Because glue-and-polish connectors are widely used, the following discussion of cable termination procedures will be based on using this type of connector.Termination Procedure
The first step in the termination procedure is to slide the heat-shrink tubing and connector retaining assembly over the cable jacket. Then, cable materials may be stripped to the appropriate lengths-- as specified by the connector manufacturer. The cable is now ready for termination.
Most glue-and-polish systems use two-part epoxy to fasten fibers into ferrules. Setting time can range from 5 minutes to 24 hours. Five-minute epoxies are useful for fast installation in non-critical environments. Longer setting epoxies are better suited for applications where fiber strain and temperature extremes are major concerns.
Heat setting epoxies are useful for situations where many connectors are to be attached. Since these epoxies set only when heat is applied, the need for mixing separate batches is often eliminated.
Before ferrules are filled with epoxy, a trial insertion of fiber should be made. Coating residue on the fiber, or a clogged ferrule may prevent insertion. detecting such problems at this stage will help save time and ferrules.
Apart from polishing, the most critical part of connector installation is cementing the fiber securely in the ferrule. If the fiber is not evenly glued on all sides, it may crack below the end surface. See connector manufacturer instructions for recommendation of quantity of adhesive to be used.
To provide strain relief, the connector retaining assembly is crimped over the aramid strength material. This type of strain relief can typically withstand pulling tension of up to 50 Ibs.
The assembly can now be set aside to cure.
After the epoxy has been thoroughly cured, the long piece of fiber may be removed by a simple cleave (see section on cleaving) and break from the point where it protrudes from the ferrule.
The grinding/polishing fixture allows the ferrule tip to protrude beyond a flat reference surface, which provides a physical limit for grinding and polishing perpendicular to the fiber axis.
Wet papers are used for grinding and polishing. The coarse grinding process may use from 50 to 600 grit size. Also, dry papers are often used.
The initial grinding step removes excess fiber and epoxy from the ferrule tip. It also establishes a rough dimension for tip length.
A smooth level surface is needed for this operation. Very light pressure should be used for initial grinding until a flat surface develops on the end face. Best results can be obtained by gently moving the fixture in a small figure-8 pattern (about 1" loops) against the grinding surface. Grinding motion should be smooth and light. DO NOT PRESS HEAVILY.
After completing this step, the fixture and connector must be thoroughly rinsed in clean water to remove all coarse grit.
This stage removes most of the coarse scratches from the grinding process. Wet papers, typically of 3-13 micron grit, are used. Use the same figure-8 motion with slightly more pressure - still not heavy. Upon completion, the ferrule end should appear satin. A fine scratch pattern will be seen under magnification. Again, the fixture and connector must be rinsed thoroughly.
The final step utilizes papers as fine as 0.3 micron. This is also a wet procedure. Apply the same firm, but not heavy pressure as in the prior polishing step.
Final polishing requires approximately 45 seconds to complete. A high-gloss surface should develop on plastic and stainless steel ferrules.
Ceramic-tipped connectors afford more precision in fiber placement than similar metal types, but require modified polishing methods. Almost no ceramic is ground away because of the material's hardness, and it is possible to grind the fiber below the ceramic with a concave end surface, thereby reducing optical efficiency.
Manufacturers' methods minimize epoxy bead formation around the protruding fiber and use polishing devices which produce convex end surface on the fiber end. Follow manufacturer's directions.
After polishing is completed, the fiber end should be inspected with a 30x to 200x microscope. This will show if the fiber is well centered and free from chips. The fiber end should be viewed under oblique light which will most clearly show scratches. If a major scratch appears across the center of the core area additional polishing may remove it. Scratches outside the core area are generally not considered critical.
When polishing is successfully completed, remove the fixture and position the strain relief in place behind the ferrule cap. When not in use, always protect connectors with dust caps.
ST Compatible - Small size connector with keyed bayonet coupling for simple ramp latching or disconnect; dry connection. Available in multimode and single mode versions. Fully compatible with existing ST hardware. For data processing telecommunications and local area networks, premise installations, instrumentation and other distribution applications. Low insertion and return loss.
SMA - Small size connector with SMA coupling nut; dry connection. For use with multimode cables in data communications applications such as local area and data processing networks, premise installations and instrumentation. Low insertion loss. Fully compatible with all existing SMA hardware.
Biconic - Small size connector with screw thread, cap and spring loaded latching mechanism. Low insertion and return loss. Compatible with all biconic hardware
ESCON* - Compatible with IBM Escon hardware. Available in single-mode and multimode versions.
FDDI - Duplex fiber optic connector system with ceramic ferrule, fully compatible with ANSI FDDI PMD document. For data communications applications, including FDDI backbone, front-end or back-end networks and IEEE 802.4 token bus. Dry connection, with positive latching mechanism. Low insertion loss.
FC- One piece connector design for easy termination. Compatible with NTT-FC and NTT-D3 hardware. Dry connection with screw type strength member retention. Available in multimode and single mode versions. Applicable for telecommunications and data communications networks, premise installations and instrumentation. Low insertion and return loss.
D4 - Compatible with NTT-D4 hardware. Ferrule alignment key for consistent re-mating. Rugged construction for long life and durability. Low insertion and return loss.
SC - Square design for high packing density. Push-pull operation simplifies connections. Available in single mode and multimode versions. Low insertion and return loss.
* IBM trademark
NOTE: Information on connectors is presented for selection assistance only. Belden does not assume any liability or responsibility for the accuracy of descriptive or performance data herein. Product and performance specifications should be verified with the connector manufacturer.
Fiber PreparationFiber Stripping
Optical fibers must be stripped of buffer coatings to allow a close fit within precision connectors. (Note: always wear safety glasses or goggles when working directly with fibers.)
Buffer coatings are usually removed mechanically with sharp blades or calibrated stripping tools. In any type of mechanical stripping, the key is to avoid nicking the fiber.
(Note: Dispose of broken pieces of fiber by placing them on a piece of tape. Glass fibers are difficult to see and may not be felt until through the skin. Eyes should not be rubbed while working with fibers.)
Splicing Optical Fibers
Preparation of fibers for splicing is very similar to the process described under connectorization. After jacket materials, strength members and buffer tubes have been cut to the appropriate lengths, the fiber buffer coatings must be removed.Cleaving
After the buffer coatings have been removed, fibers must be cleaved in preparation for splicing. Cleaving is a method of breaking a fiber in such a way as to create a smooth, square end on the fiber.Principles of Cleaving
Glass is typically strong until a flaw occurs and creates a region of high stress under pressure. The first step in the cleaving process is to create a slight flaw or "scribe" in the outer surface of the fiber.
Optical fibers can be scribed with a sharp blade of hard material such as a diamond, ruby, sapphire or tungsten carbide. The scribe is made by lightly touching the cleaned fiber, at a right angle, on the desired cleave point with a scribing tool. Only the lightest pressure is required to produce a scribe if the blade is sharp. NOTE: DO NOT USE A SAWING MOTION. A crude or slanted scribe will produce shattered or scalloped end surfaces.
After the scribe is made, a straight pull will produce the cleanest break. If bending accompanies pulling, a square break is less likely, especially with large fibers. Dispose of broken fiber pieces by placing them on a piece of tape. ALWAYS WEAR SAFETY GLASSES WHEN WORKING WITH OPTICAL FIBERS.
The level of quality required for a given cleave depends on the application. For fusion splicing, mechanical splicing and some connectors systems, cleaves must be nearly perfect. Some connector and splicing systems use cleaving to produce the final end surface on the fiber (no subsequent grinding or polishing). However, for quick continuity checks with a flashlight, less than perfect cleaves may be acceptable.
A 30x to 50x hand microscope is useful for quick checks of cleave quality.
Cleaving tools are available in inexpensive hand models or more sophisticated mechanized tools.Splicing Methods
There are two basic types of splices: Fusion and Mechanical.Fusion Splicing
Fusion splices are made by positioning cleaned, cleaved fiber ends between two electrodes and applying an electric arc to fuse the ends together. A perfusion arc is applied to the fiber while the ends are still separated to vaporize volatile materials which could cause bubbles.
Final precise alignment is done by moving fiber ends together until there is slight pressure between end surfaces.
An ideal fusion cycle is short and uses a ramped or gradually increasing arc current. A short, ramped cycle is considered least likely to produce excessive thermal stress in fibers. Cold temperatures require increased time and arc current.
Experienced operators consistently produce fusion splices with losses less than 0.2 dB per splice and averaging 0.3 dB on multimode fibers. Sophisticated fusion splicing systems for single-mode fibers produce typical splice losses of 0.05 to 0.1 dB.Mechanical Splicing
Mechanical splicing systems position fiber ends closely in retaining and aligning assemblies. Focusing and collimating lenses may be used to control and concentrate light that would otherwise escape. Index matching gels, fluids and adhesives are used to form a continuous optical path between fibers and reduce reflection losses.In-Line Connector/Connector Splicing
Connector-to-connector splicing may be used in situations where there is an abundance of optical power. Connectorized cable assemblies are joined through an alignment bushing which fits snugly over the tip of each connector.
Insertion losses for connector-to-connector splices can be as high as 1.0 to 1.5 dB. If these losses are considered excessive, an alternative method should be used.
TestingThe Flashlight Test
A simple continuity test for short-to-medium length fiber optic links is to shine a flashlight into a cleaved or connectorized link and observe if light comes out of the other end. On short lengths, it may be necessary to cleave only the end where the flashlight injects light into the fiber.
This simple check can be made on cable lengths of up to a mile and more. If cable ends are outdoors, sunlight may be used. NOTE: on longer lengths, the light observed at the opposite end may appear red in color. This is normal and is caused by the filtering of light within the fiber.
CAUTION: NEVER LOOK DIRECTLY INTO A FIBER CONNECTED TO LIGHT LAUNCHING EQUIPMENT. THIS CAN CAUSE PERMANENT EYE DAMAGE.Optical Power Measurements
When an optical cable has been installed, all splices made and connectors attached, it must be determined if the system is capable of delivering the required power. The simplest test requires a light source of the same type, wavelength and approximate power as that of the equipment to be used. The system equipment itself is often a satisfactory source.
The first step is to obtain an approximate measure of system launch power. A short test cable with the same fiber and connector style as the installed cable can be used for this procedure. One end of the short cable is connected to the light-launching equipment. The other end is connected to an optical power meter.
After the initial reading is taken on the short length of test cable, a second similar reading is taken with the installed cable in place. The difference between the two readings indicates the additional power losses due to fiber length and differences in optical qualities of connectors. Because approximate fiber losses are known, losses greater than 1.0 to 1.5dB above fiber losses might indicate an inferior connection – requiring either re-polishing or replacement.Optical Power Meters
Power meters often read directly in power units, such as dBm and dBµ. By using connector adapters and light sources of the same wavelength as the installed equipment, an accurate measure of link losses with connectors and splices may be obtained.The Optical Time Domain Reflectometer (OTDR)
OTDRs are typically used to measure distance and attenuation over the entire fiber link. They are also used to identify specific points along the link where losses occur, such as splices.
An OTDR is an optical radar which measures time of travel and the return strength of a short pulse of light launched into an optical fiber. Small reflections occur throughout the fiber, becoming weaker as power levels drop with distance. At major breaks, large reflections occur and appear as strong peaks on an oscilloscope.
Testing of short and medium distance fiber optic systems seldom requires an OTDR. In smaller systems, optical power meter tests are faster and more useful.
Many instrument rental companies are now offering OTDR's as well as other fiber optic splicing and test equipment.Magnifying Glasses and Microscopes
Because the naked eye cannot detect scratches or defects in optical fibers, use of magnification equipment is required. For most routine inspections, and ordinary battery-powered illuminated microscope of 30x to 100x can produce satisfactory results.
Some microscopes are available with special adapters specifically designed for use with fiber optic connectors.