Machine Design

Fiber-Optic Connections

Three major components in any fiber-optic system are the transmitter, receiver, and fiber cable.

Transmitters are often referred to as launchers and are typically either injection laser diodes (ILDs) or light-emitting diodes (LEDs). The total power of any launcher never completely couples into a fiber. Therefore, the power output specification of a source alone is of little help in determining available power. As an example, an LED may have a total output of 1,000∝W, but may only launch 50 ∝W into a fiber. This constitutes a 26-dB loss. To minimize losses, devices specifically designed as launchers contain either pigtails or special ferruled connectors. A pigtail is a short length of cable permanently fixed to the launcher. A ferruled connector consists of a special ring or nut that draws the cable up to the LED. Manufacturers using either of these methods usually list the package output power.

Receivers typically employ PIN (positive-intrinsic-negative) or avalanche photodiodes (APDs) as detectors. PINs have better short and long wavelength response, while APDs have higher sensitivity, having 10 times the gain.

Cable optical qualities determine overall system performance. This is because manufacturers optimize cables for certain wavelengths and transmission modes. Standard cable lengths for OEMs are one and two kilometers, though some suppliers provide longer lengths. Cable originally developed for long-distance telecommunications, still used in LANs today, is designated 50/125. The material has a 50-∝m-diameter core and a 125-∝m-diameter cladding. It is used for its low cost and attenuation and high bandwidth.

Many manufacturers recommend cables larger than 50/125. Cables with core diameters of 100 to 200 ∝m are widely used. The large diameters are easier to splice and interconnect and carry more light. A plastic-coated glass fiber with a 200-∝m core can transmit seven times more light than one with a 100-∝m core and 65 times more than a 50-∝m core.

Larger fibers also have higher collection factors, making them more efficient. Collection factors refer to the amount of incident light captured by the cable. Higher light efficiency allows the use of LEDs rather than laser launchers.

Most connectors use an epoxy/polish termination. After the fiber is stripped to the proper dimension, it is attached with epoxy to the connector. Strengthening members are then crimped to the connector body. Finally, the fiber-connector end is polished to a fine finish.

Exceptions to the epoxy/polish system are reusable mechanical splices and epoxyless connectors. Epoxyless connectors are generally comparable to standard epoxy/polish connectors. They bring the applied cost of fiber-optic terminations in line with that of copper conductors.

Connectors must be functionally compatible with similar ones, and interchangeable among the various manufacturers. For example, large telecommunications companies like AT&T in the U.S. and NTT in Japan design their system connectors to be both mechanically and optically compatible. In other cases, specifications describe the type of connector to be used, such as MIL-C-83522 for SMA-style connectors in military applications.

Each connection to a fiber incurs some light loss. Since losses add, the number of connections influences the maximum length of the system. Typical losses vary from as low as 0.2 dB for splices to about 4 dB for inexpensive connectors.

A variety of factors cause connector losses. Typical ones include variations in fibers, such as differences in core diameters or numerical apertures, and core concentricity.

Many connectors maintain a small gap between the fibers. In some cases this gap is designed to prevent damage to the fiber end finish. But the gap causes losses from Fresnel reflection and from the spreading of light from the fiber end. To reduce these losses, some manufacturers fill the gap with a material having a refractive index close to that of the fibers.

Misalignment of the center axis of the two fibers also causes attenuation. A 10% offset from the core diameter results in a loss of about 0.5 dB. This means that for a 50-∝m core, the connector must hold each fiber to within 5 ∝m. Most connector manufacturers attempt to limit lateral offset to 5% of the core diameter.

Angular misalignment also causes losses. For a given misalignment angle, attenuation increases as the numerical aperture of the fiber decreases.

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