Technology
The Fuel of POF : Consumer and Automotive
Holger Herrmann, Pete Doyon, Schleuniger AG
      POF stands for plastic or polymer optical fiber. These fibers are used to transmit light for data transmission or lighting applications, respectively. The major areas of growth will be in the consumer and automotive areas, and to a less extent in the interconnect, building and industrial control application markets.

      Why POF is being used and what applications are fueling the demand and what quality issues have to be considered in processing ?

      According to a study published by Information Gatekeepers, the worldwide market for POF cable was approximately US$ 500m in 2002, and is expected to grow to US$2 billion by 2006. The standard POF cable has a core size of 0.98 mm, and is about 8-times larger in diameter compared to standard glass optical fiber (GOF). The large size makes it easier to work with POF compared to GOF, since the tolerances are not so tight.

      POF has several advantages over other transmission mediums such as GOF or copper. The inherent characteristics of plastics make it more flexible and more resistant to damage than single-mode GOF, meanwhile, it weighs less and takes up less volume than copper. Having much higher bandwidth capabilities than copper, both POF and GOF don't cause any electromagnetic interference (EMI) or radio frequency interference (RFI) , none of them are affected by either of them. In general, all optical fiber transmissions are extremely difficult to tap, making both POF and GOF a good choice where data security is important. The higher the frequency of the transmission, the more economical it becomes to use POF compared to any copper-based solution, including twisted pair and coax. POF is also very resistant to vibration. While the large core makes it easier to align the light path inside of connectors, light emitters and detectors, POF is also selected over GOF because it is easier to terminate, polish and connect resulting in lower installation costs.

      Of course, POF has some disadvantages. Compared to GOF it has a high attenuation (loss), which means it can only be used for short distances typically up to 50m. It is also not well suited for high temperature applications such as on engine-mounted sensors.

      Due to its characteristics and capabilities, POF is an ideal medium for building networks over short distances. Although these networks can be installed in homes, offices, houses and any kind of vehicle, currently they are being used primarily in cars. As automobiles become increasingly complex, the weight and volume of copper based networks creates a problem for automobile designs. A single POF can replace many individual copper wires, resulting in major weight and space savings. Many newly engineered vehicles now also have a navigation system, hands-free mobile phone and a TV/DVD rear-seat entertainment system. The communication between all these devices must be done without them interfering with one another and other vehicle networks. While POF works very well in this application, it is also a promising solution for many of the safety systems and probably upcoming x-by-wire technology. Although most automotive use has been for data transmission, POF can also be used for illumination purposes such as dome lights, door lights and instrument panels. One of the advantages is that a centralized light source can be used to replace dozens of individual bulbs, making the system more service-friendly.

      Automobile manufacturers in Europe have been installing POF in automobiles for a few years. The reason they are further ahead of the U.S. or Japanese automakers is that they have adopted a common standard (known as MOST media orientated system transport), paving the way for POF use in cars. For example, the new BMW 7 series has more than 100meter of POF in it. The U.S. and Japanese car manufacturers are looking at using a different standard known as IDB-1394, which however has not been finalized yet. High volume use of POF in U.S. and Japanese vehicles is expected to occur within 2006.

      The same reasons that make POF a good choice for the automotive industry make it a good choice for aerospace applications. POF is currently being used in flight entertainment systems and is being looked at for a variety of other applications, such as in lighting for the instrument control panels. POF is also frequently used as a means to connect between sensors and control systems. The reason is that POF offers good electrical isolation and electromagnetic immunity. POF sensors are currently measuring a variety of conditions, such as pres-sure, temperature or chemical characteristics. One application where it has a major advantage is in hazardous locations such as in fuel tanks or other explosive environments.

      POF is also used in the sign and illumination industries for many different purposes, including decorative, architectural, traffic signals and medical applications. Some types of POF are designed to emit light along their entire length while others are engineered to project light out of their ends. The variety of POF types and related components is growing rapidly and it is being used in many new areas. Applications include the use of POF embedded in highway-telematics regulate traffic, underwater lights for swimming pools, museum lighting for delicate objects and the lighting of footpaths or building facades.

Stringent requirements for processing, quality
      When it comes to processing POF cables, keeping the dB-loss at a minimum is crucial. The most important factors in this context are the quality of the end surface and the connection process. In order to achieve optimum results especially for data transmission applications POF cables are sometimes not only cut and stripped, but go through a surface treatment process before being connected.

      After cutting and stripping, the end surface should already be smooth and show no cracks. The final dB-loss, however, can be further reduced by means of a subsequently applied grinding process. Tests have shown that the differences in dB-loss between a cable whose surface has been treated and one that has simply been cut to length can amount to 1dB or more.
      Once the end surface has been treated, the POF cable needs to be connected. There are different ways to assemble a ferrule onto a POF cable including crimping, gluing and laser welding. While gluing has remained a complicated process that requires long cycle times and crimping doesn't provide the desired results, laser welding is currently considered the state-of-the-art connection technology. POF ends are precisely positioned in the ferrules, so that the required position is achieved during the welding process. While the typical tolerance range for MOST ferrules is 0.01 to 0.1mm, a recessed fiber position is usually needed in order to avoid scratching during handling or mating. By controlling the most important process parameters (time and temperature), laser welding provides excellent quality and short cycle times.

      The ever-increasing importance of quality control both during and after manufacture has also been addressed by most POF processing equipment manufacturers. In order to keep cycle times low, usually only the insertion loss is measured in an automated production line. The process sounds simple: a sender emits a defined amount of light into a POF cable, and a receiver measures how much of that light gets to the other end. The complexity lies in the calibration of the sender/receiver and the precise positioning of the POF ends.

      Apart from insertion-loss testing, other quality-assurance measures include visual control of the surface with a microscope, measuring of the distance from the POF-cable end (inside of the ferrule) to the end of the ferrule, or applying destructive tests, such as pull-force experiments. These quality assurance tests are typically done with semi-automatic test equipment.

      Briefly: POF has some advantages over other transmission mediums, making it an excellent choice for use in a wide variety of applications. As standards evolve and mature, the production volume will increase, and the need for increased automation will follow. Knowledge of POF technology and its use will be fundamental.
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