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Glass Fiber vs. Plastic Fiber in fiber optic applications

The question concerning glass versus plastic optical fiber is fundamental, but it comes up more often than you’d think, so it’s worth a few words here, just for the record.

We’ll not get into the structure of the actual fiber itself, as it follows the same basic principles of differing refractive indexes between the core and the cladding that leads to total internal reflection and light propagation. Instead, we’ll look directly at the materials and performance.

Typical end emitting plastic optical fibers (POFs) are made from polymethyl methacrylate (PMMA), commonly known as acrylic or polystyrene. They function well for illumination, sensing, and many home, transportation, and industrial applications where weight and vibration may be factors. They’re also easier to work with in that they can be sliced and reconnected in the field (for data transmission applications). They are available in several fixed diameters, (.25mm – 3mm) and 2 numerical apertures, and exhibit good transmission efficiency between 400 and 700nanometers. Even the 3mm diameter will have a reasonable bend radius. They have the same chemical resistance as acrylic.

Glass and synthetically fused silica (quartz) fibers are more stable under temperature extremes, with good resistance in excess of 400° C versus 70° C for PMMA.

Glass fibers can be made in a wide variety of diameters and different numerical apertures, with good transmission efficiency from 200-2200 nanometers. Glass fibers stop bending (they will deflect, but will not bend to form a circle for practical applications) when the diameters increase beyond .4mm. Silica fibers will exhibit better bending capability (mostly because of the buffer)… but their bend radius is also limited… more to do with NA and power loss through tight bends, than actual physical bending property.

Chemical resistance is very good (for glass)… and OK for silica (due to a typical polyimide buffer requirement).

The decision to use glass or plastic fibers, as in so many cases, defaults to application requirement(s). While there are a multitude of applications where either type can be used, some clear limitations (and preferences) make a fiber choice straight forward.

Heat is a major differentiating factor. While there are protective measures that can be put in place to make POF more stable, temperature intolerance will remain a fundamental limitation; any part of the light guide exposed to temperatures over 70C will destroy the light guide prematurely.

Vibration favors plastic; it’s more capable of withstanding a dynamic environment than glass.

Because larger fiber diameters are still flexible, plastic fibers should be considered when the light guide diameter matches a plastic fiber diameter; a single plastic fiber will transmit more power than a bundle of glass fibers… the only losses in a single fiber are Fresnel and wavelength attenuation over length.

Material cost favors plastic, and it could cost is less to build a single plastic fiber light guide. However, because plastic fiber is manufactured and collected on a spool, multi-fiber plastic light guides could cost more than glass… (the labor content to form the bundle is higher than the same light guide made from glass) but never more than silica fiber, which is by far the most expensive of the three types.

If you need a wider, or more narrow, Numerical Aperture, plastic is limited to .48 or .63. Flint glass range is .25 – 1. Silica range is typically .22 .37 and .5.

If the coupled power is high, Plastic will solarize much faster than glass. Silica has even better resistance. (None of these fibers will withstand nuclear radiation).

As always, if you have any questions on your next design and you’re not sure which way to go, plastic or glass, contact us.

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