Polymer optical fiber

Polymer optical fiber (POF), also called plastic optical fiber, guides light in a polymer core instead of silica glass. The standard type is a step-index fiber with a polymethyl methacrylate (PMMA) core of 980 µm diameter, refractive index about 1.49, inside a thin fluorinated polymer cladding that brings the outer diameter to 1 mm. Its numerical aperture is about 0.5, and its attenuation is lowest in the visible: roughly 0.15 dB/m near 650 nm, with roughly half that near 520 and 570 nm. The large core and wide acceptance angle make the fiber tolerant of misalignment and simple to terminate, at the cost of loss and bandwidth far worse than glass fiber.

Geometry and guidance

An NA of 0.5 corresponds to an acceptance half-angle of 30° in air. With a core index of 1.49, the cladding index that produces this NA is

$$n_2 = \sqrt{1.49^2 - 0.5^2} = 1.404.$$

The fiber is highly multimode. With a core radius of 490 µm at 650 nm, the V-number is about 2370 and the number of guided modes, roughly $V^2/2$, is about 2.8 × 10⁶, so ray optics describes propagation well.

Attenuation

Loss in PMMA is set mainly by overtones of the carbon-hydrogen vibrational absorption, which rise steeply into the red and near infrared, plus scattering. The windows between absorption bands, near 520, 570 and 650 nm, are where POF links operate; typical specifications are about 0.15–0.2 dB/m at 650 nm, and a 50 m link therefore loses about 7.5 dB at 0.15 dB/m. Standard single-mode glass fiber loses about 0.2 dB/km at 1550 nm, or 0.01 dB over the same 50 m (see fiber attenuation). Tight bends add bend loss as in any multimode fiber, so installation guidelines specify a minimum bend radius.

Modal dispersion and bandwidth

In a step-index multimode fiber, rays at the edge of the acceptance cone travel a longer zigzag path than axial rays. The spread in arrival time, described under modal dispersion, is approximately

$$\Delta t \approx \frac{L\,\text{NA}^2}{2\,n\,c}.$$

For a 50 m link with NA = 0.5 and $n = 1.49$:

$$\Delta t \approx \frac{50 \times 0.25}{2 \times 1.49 \times c} = 14\ \text{ns}.$$

Taking the bandwidth as about $0.44/\Delta t$ gives 31 MHz, a bandwidth-length product near 1.6 GHz·m (1.6 MHz·km). This full-launch figure is pessimistic: sources that underfill the NA, mode-dependent loss and mode coupling in real fiber raise the measured bandwidth. Standards such as IEEE 802.3bv (1000BASE-RH) reach 1 Gb/s over 50 m of step-index POF by combining a red source with multilevel modulation and equalization.

Graded-index and perfluorinated POF

A graded-index profile reduces modal dispersion in POF as it does in glass multimode fiber. Graded-index fibers made from perfluorinated polymers such as CYTOP replace carbon-hydrogen bonds with carbon-fluorine bonds, whose vibrational overtones lie at longer wavelengths, so absorption in the 850–1300 nm region is much lower; losses of a few tens of dB/km are typical. With cores of about 50–120 µm, these fibers support multi-Gb/s rates over 100 m or more and can use the same 850 nm VCSEL sources as glass multimode links.

Uses

• Automotive networks. The MOST (Media Oriented Systems Transport) bus used 1 mm POF with 650 nm LEDs for in-vehicle infotainment, at up to 150 Mb/s in MOST150.
• Home and industrial links. POF carries Ethernet within buildings and provides electrically isolated, noise-immune links in factory automation and power electronics, where distances are typically under 100 m.
• Lighting and sensing. Large-core POF bundles deliver illumination, and POF sensors exploit the material's elasticity and low cost for strain, bend and liquid-level measurement.

The standard source is a red light-emitting diode near 650 nm, matched to the loss window and inexpensive; resonant-cavity LEDs and red laser diodes are used at higher rates.

Handling and limits

POF is terminated by cutting with a hot or sharp blade and, when lower loss is needed, polishing the end face on fine abrasive film, often directly in a crimped connector; no cleaver or fusion splicer is required. The PMMA core softens at elevated temperature, so standard fiber is typically rated to about 85 °C; high-temperature grades with other core polymers extend this for engine-compartment use. PMMA also absorbs water, which slowly raises loss in humid environments.

Common questions

What is the difference between plastic and glass optical fiber?

Glass optical fiber has loss of fractions of a dB per km and, in single-mode form, very high bandwidth over long distances. Standard POF has loss near 0.15 dB/m and bandwidth limited to tens to hundreds of MHz over 50 m, but its 1 mm core, flexibility and simple termination suit short, low-cost links.

What wavelength does plastic optical fiber use?

PMMA POF is used in its visible loss windows, most often at 650 nm, sometimes at 520 or 570 nm. Perfluorinated graded-index POF is used at 850 to 1300 nm.

How far can plastic optical fiber transmit?

Standard step-index POF is used up to roughly 50–100 m, limited by attenuation and modal dispersion. Perfluorinated graded-index POF reaches a few hundred metres at Gb/s rates.

References: Saleh & Teich, Fundamentals of Photonics 3rd ed. (Wiley, 2019); O. Ziemann, J. Krauser, P. E. Zamzow, W. Daum, POF Handbook: Optical Short Range Transmission Systems 2nd ed. (Springer, 2008); Y. Koike, Fundamentals of Plastic Optical Fibers (Wiley-VCH, 2015).