By William Graham
Modern day machines have been known to contain many surprises for the electrician and datacom person. One surprise can be to find that instead of a piece of copper wire carrying the data a piece of plastic optical fiber is used instead.
History of Plastic fiber
Many of us remember plastic optical fiber in cars as far back as the 60’s. Then it was used for illumination purposes. Perhaps a piece of plastic fiber ran from the dash lights to the ash tray to provide light, or perhaps the vehicle utilized plastic fiber from the headlights to the dash to indicate that the headlights were on. As time went on, Plastic Fiber improved in quality to the point where it became useful to carry data as pulses of light.
Today, Plastic Optical Fiber is becoming common for two applications:
For Data Communication systems, Plastic Fiber is the solution that fits between copper pairs and glass fiber. Many people feel that plastic fiber is the quickest way to bring fiber to the desk or throughout the home.
For Illumination, Plastic optical fiber has come into its own and is now a part of the illumination requirements for many casinos and buildings.
First let’s look at the data communications aspect of plastic Fiber.
The plastic optical fiber system, or any fiber optic system, in it’s most basic form, consists of a digital electrical signal which is changed to a pulsating (digital) light signal. This signal is then sent through the optical fiber to the receiver as light, and turned back into an electrical digital signal. This system consists of four basic components:
We don’t count connectors on a copper system usually because loss should be negligible, but with optical fiber, and especially plastic fiber, every pair of connectors or splice represents a loss that must be calculated.
As with any new product, if we are to want to use it, and learn the new technology, we must see some advantages. It has to make our life easier, bring us business or in some way have positive advantages. Plastic Fiber has just that.
Some of these advantages are:
Wide Bandwidth: is the increased capacity to carry information and at the speed of light. Of course, this is because the wavelength of light is 500nm (billionths of a metre).
Electromagnety Immunity: Unlike copper cables, fiber optic cables do not pick up or radiate (EMI) electromagnetic radiation. Because of this, signals do not get distorted and the fibers offer a high standard of error-free transmission. We can run fiber optic cable close to high voltage cables, large motors or switching equipment. Many factories use fiber optics because of it’s immunity to EMI. Motors switching on and off cause interference that can throw a computerized piece of equipment out of sequence.The big car manufacturers and car part plants are our best example at this time. Most of their robots have been built with plastic fiber optics.
Security: Eavesdropping, with a copper cable, is easy, but with fiber, a tap will introduce an error, and a detectable loss.
Light Weight: Plastic fiber is a fraction of the weight of copper with a weight ratio of about 3:1. This weight is important in aircraft and ships.
Small Size: A fiber optic cable is smaller than its copper counterpart. A plastic fiber might be as little as 960 microns in diameter, about the diameter of a nail. A fiber optic cable will often replace several copper pair because of the greater signal carrying capacity.
Safety: Optical fiber is a dielectric and does not carry electricity. There is no spark, shock or fire hazard, it cannot cause explosions. It is intrinsically safe. Fiber optics works well in hazardous locations.
Cost: Because of the improvements in the fiber Optic cable and accessories it is now considered by most people to be the most cost effective system to install in a new installation, or for a major system upgrade. The only speed limitation on optical fiber is the electronics. As we start coming to the end of this decade we see an overwhelmingly large number of new products to provide cheaper and easier installation of fiber optic systems both in the home and in industry. These products include quick and simple connectors for the plastic fiber.
Maintenance: Optical fiber systems are considered the most reliable for data communications. Some companies claim that maintenance and down time is less than 20% of what is required for copper systems.
Ease of Use: Many copper installers learn fiber and eventually find that it is easier to install without problems and if there are faults, they are easier to identify and correct. Plastic Fiber is installed and connectorized with simple hand tool.
Plastic optical fiber for data communications will have a plastic core and a plastic cladding.
Compared to other fibers, these have high losses and lower bandwidth, or information carrying capacity. Plastic fiber seems to be the accepted medium for the in-between range of category 5 copper and glass fiber. Plastic is now accepted for ATM use of up to 50 meters.
Typical plastic applications are increasing and include:
High Voltage Switchgear
Plastic fiber is rapidly becoming more popular for industrial use. The industry appears waiting with baited breath for the next generation of low loss plastic fibers.
Installation and Termination
Plastic Fiber can be installed in raceway of any type. There is less concern for mechanical damage. The plastic in a LAN system will be duplex (Zipcord), and will consist of a jumper plugging into the data wall plug on one end and the computer at the other end.
Splicing and Connecting Plastic Fiber
Plastic fiber can be spliced by simply cutting the ends square with a cutter or hot knife and pushing each end into a small brass sleeve with barbs until the ends touch. It is important to cut the ends as square as possible and clean them well before joining them.
The barbs hold the fiber from pulling out. A sleeve can cover this joint. And I like to put a bit of epoxy under the sleeve to keep it from pulling apart and also to seal it from water and dirt.
Simplex and Duplex 1 mm (960um) plastic fiber cable is the most common size and consists of one or two fibers. It is small being about 1.2mm for simplex and about 2.5mm for duplex.
Common sizes of plastic fiber used for communication would include:
1.00mm is the more common size used for data communications with smaller sizes more popular for the sensor market.
The minimum bend radius for the 1.00mm plastic fiber optic cable would generally be about 40 to 50 times the outer diameter of the jacketed fiber. The exact figure will depend on the type of cable and will be specified by the manufacturer.
Where we are using the plastic fiber, will determine our connectorizing method. If we are connecting a few feet of fiber to a sensor, sometimes we will cut the fiber square and put it into the sensor receptacle. In other cases of sensor and machine applications, a plug-in type of connector is used. In this case we remove about 1 inch (3cm) of the outer jacket, push the fiber through the connector until the bare end comes out the tip and the jacket is secured in the brass holding sleeve. A drop of epoxy on the bare fiber will ensure it stays put and does not piston. Next we cut the end of the plastic fiber square with the tip. If epoxy is used that requires being heat cured, my personal experience has been not to exceed 70 degrees Celsius.
This is usually an adequate finish but sometimes tradespersons tend to be fussy and can improve the connection by polishing the end with polishing film and water. A 15um film would be good for a start then a 5um and finish off with a 1um. Cleaning well between each polishing will ensure a great low-loss finish. All polishing should be done with water on the film and a soft surface such as a rubber pad or a soft book. Optionally, we might have left a tiny bit of fiber sticking out the tip and touched it on a hot oven to give it a smooth finish. This connector is available in a duplex model and simplex (single) and duplex couplers are also available. It is probably the most common connector for process sensor applications.
For a computer application we would look for a very low loss finish. For this we need better alignment of the axis of the fibers cores which would mean a better quality connector such as an “ST” type connector. This connector is a spring-loaded, push-in and turn bayonet type connector. We would polish this connector in about the same manner except that we would give it a fine finish with .3 polishing film. We could expect a loss of about 1 dB per mated pair with this connector. If you do cure the epoxy in a heat oven, do not cure at a temperature more than 75 degrees Celsius.
1998 saw the approval of plastic optical fiber (POF) for high-speed ATM (Asynchronous Transfer Mode) networks, the ATM Forum approved standard number AF-PHY-POF 155-0079.000. This long awaited approval should signal the start of commercial manufacture of POF, which will be allowed in ATM data transmission for 155 Mbps at distances up to 50 meters. POF allows for higher speed, greater security and more reliability.
Plastic Optic Fiber For Illumination
Imagine a light in the bottom of a gasoline fuel tank. Why not? If it is a piece of optical fiber, it is intrinsically safe, it will not cause a spark, explosion or even an over temperature condition.
As we enter the start of the last year of the century, I am communicating with at least two companies that are trying to develop plastic optical fiber lighting systems for mining lighting applications where intrinsic safety is a necessity.
Some of the uses of optical fiber for illumination include:
Pools and water fountains
Illuminating glass blocks
Isle, track and step lighting
Explosion proof lighting – Mining
Nightclub and theatre lighting
Signage – replacing Neon
Roof outline lighting
These illuminating fibers are used in two ways:
This application is excellent for many of the uses mentioned above that would require a beam of light emitting from the end to light an object etc.
This is a totally different situation from end light. Scattering is a good thing in this application. As light passes along the length of the fiber, it scatters light producing the side glow effect. Generally fibers for side illumination are illuminated from both ends. Rayleigh scattering causes some color shift along the length of the fiber. The illuminator has some effect on the nature of the color shifting. Tungsten Halogen will produce very little blue light so the fibers will appear slightly greenish. Metal Halide produces a much more even distribution of light and the same fiber will appear whiter in color.
The recommended length for even illumination is usually about 100 feet (32 meters) Longer lengths are possible with an illuminator with an illuminator at both ends.
End and side lit fibers come in two ways:
As solid fibers in three sizes: 1/4″, 3/8″ and 1/2″ and usually in lengths of 80 feet or more.
As bundles of smaller fibers in a transparent plastic tube.
There are arguments in favor of both ways.
This should not be less than 8 times the fiber diameter. If it is there will be excessive loss and hot spots (bright) at the bends.
Fibers are usually made of Polymer and are pliable and easy to shape and install. They will become stiffer in time and show an improvement in the optical properties after a period of several days of continuous illumination.
We will find these decorative fibers in many of our new buildings, casinos, swimming pools etc. The electrical contractor is finding decorative plastic optical fiber just another way to increase the bottom line.
Life Span of Plastic Fiber:
Plastic fiber is new but has survived for five years with no noticeable deterioration of the optical quality.
The end of the fiber should never be exposed to water.
All plastics will degrade if left exposed to sunlight or other UV sources. Fiber in this condition is enclosed in a UV stabilized material, such as a UV protected polycarbon track or similar product with no less than 92% UV resistance.
Plastic fiber will work best in service temperatures of 80 degrees C. and maximum temperatures should not exceed 200 degrees C. A “Cool Clamp” system is used in conjunction with illuminator cooling fans to maintain this temperature.
Fiber shows no loss of optical properties at temperatures as low as -40 degrees C.