Fiber optics are used to transmit light energy over long distances. Optical fibers are thin, transparent strands of optical quality glass or plastic that can be as thin as a strand of hair. This article explains the differences between glass and plastic fiber optics and how to choose the right fiber optic technology for your application.
Plastic Fiber Optics
Plastic fiber optics are usually single strands of fiber optic material (typically 0.01 to 0.06 inch or 0.25 mm to 1.5 mmn diameter). Most plastic fiber optic assemblies are terminated on the sensing end with a probe and/or a threaded mounting tip. The control (sensor) end of most plastic fiber optic assemblies is left unterminated, so that it may be easily cut by the user to the proper length.
Advantages of Plastic Fiber Optics
Plastic fibers have some advantages over glass fibers:
-Plastic fibers are less expensive than glass fibers, whether they are standard versions, special versions, and especially the Vantage Line of high-quality inexpensive fiber optics packaged for OEM applications.
-Plastic fibers allow less signal attenuation than glass fibers. Attenuation is caused by inherent material properties of the fiber optic, such as absorption, scattering, and impurities.
-Plastic fibers are more flexible, and also survive well under repeated flexing. Pre-coiled plastic fiber optics are available for sensing applications on reciprocating mechanisms. Because of their size, plastic fibers can be routed into extremely tight areas.
-Unlike glass fibers, plastic fibers can be cut to length in the field.
For both plastic and glass fibers, It is relatively easy, fast, and inexpensive to create a fiber optic assembly to fit a specific space or sensing environment. These are called special fiber optic assemblies.
Plastic absorbs certain bands of light wavelengths, including the light from most infrared LEDs. Therefore, plastic fiber optics require a visible light source for effective sensing. Plastic fibers are less tolerant of temperature extremes, and are sensitive to many chemicals and solvents. Sheathing materials are used to shield plastic fiber optic assemblies in harsh environments. For many plastic fibers, the bend radius of the cable affects the transmission of light.
Glass Fiber Optics
Glass fiber optics are made up of a bundle of very small (usually about 0.002 inch or 50 micron diameter) glass fiber strands. A typical glass fiber optic assembly consists of a bundle of several hundred glass fibers protected by a sheathing material, usually a flexible armored cable. The cable terminates in an end tip that is partially filled with rigid clear epoxy. The sensing face is optically polished so that the end of each fiber is perfectly flat.
The outer sheath of a glass fiber optic assembly is usually made from stainless steel, sometimes referred to as "square lock" flexible conduit, but may be PVC or some other type of flexible plastic tubing. Even when a non-armored outer covering is used, a protective steel coil is usually included beneath the sheath to protect the fiber bundle.
Advantages of Glass Fiber Optics
-It is relatively easy, fast, and inexpensive to create a glass fiber optic assembly to fit a specific space or sensing environment. These are called special fiber optic assemblies.
-Most glass fiber optic assemblies are very rugged and perform reliably in extreme temperatures.
-Glass fiber optic assemblies can transmit both visible and infrared light, where plastic fiber optics can only transmit visible light. Use a glass fiber optic assembly with a 2.2 mm "T5" termination with infrared versions of the DF-G fiber optic amplifiers.
-Glass fibers can often be used in corrosive and wet environments.
-Some versions of glass fibers are built to go into vacuum environments.
The most common problem experienced with glass fibers is breakage of the individual strands resulting from sharp bending or continued flexing, as occurs on reciprocating mechanisms.
This chart summarizes the differences between plastic and glass fiber optics and how to choose between them based on common application considerations.
Plastic fibers are an all around excellent, cost-effective choice for many fiber optic sensing applications.
Glass fibers are a good choice for challenging environments with high temperatures, corrosive chemicals, and vacuum chamber applications.
For more information about fiber optic sensing, contact an engineer to discuss your application requirements.