Optical fiber

An optical fiber is a cylindrical waveguide: a core of glass with refractive index $n_1$ surrounded by a cladding of slightly lower index $n_2$, so that light launched within a limited range of angles is trapped in the core by total internal reflection and carried over long distances. Standard telecom fiber is fused silica with a germanium-doped core about 8–9 µm across, a cladding 125 µm in diameter, and a polymer coating usually 250 µm (increasingly 200 µm) in diameter. The index difference is small, a few tenths of a percent, and the loss at 1550 nm is about 0.2 dB/km, so 1% of the launched power remains after 100 km of fiber.

Structure and guidance

The core and cladding are drawn together from a single preform heated to its softening point, which is why the interface is optically smooth and the two glasses are concentric to within a fraction of a micrometre. The coating carries no light; it protects the glass surface from the scratches and moisture that would otherwise weaken it. How strongly the fiber guides light is described by its numerical aperture:

$$\mathrm{NA} = \sqrt{n_1^2 - n_2^2} \approx n_1\sqrt{2\Delta},$$

where $\Delta = (n_1 - n_2)/n_1$ is the fractional index difference. For $n_1 = 1.4504$ and $\Delta = 0.36\%$, NA = 0.123, and light is accepted from air within a half-angle of 7.1°. The number of guided modes is set by the V-number,

$$V = \frac{2\pi a}{\lambda}\,\mathrm{NA},$$

with $a$ the core radius. For $a$ = 4.1 µm and NA = 0.123, $V$ = 2.04 at 1550 nm, below the single-mode limit of 2.405; the theoretical cutoff wavelength for this core is about 1.32 µm. In cabled lengths the second mode is stripped by bends, and standard fiber is specified with a cable cutoff of at most 1260 nm, so it is also single mode at 1310 nm.

Main families

The commercial grades (OM1 to OM5 multimode and the ITU-T single-mode families) are compared in Types of optical fiber.

• Single-mode and multimode. A small core carries one mode and gives the highest bandwidth over distance; multimode cores of 50 or 62.5 µm carry hundreds of modes and are easier to couple to. The trade-offs are covered in single-mode vs multimode.
• Index profile. A uniform core with a sharp boundary is a step-index fiber; a parabolic profile that equalizes mode delays is a graded-index fiber.
• Dispersion-engineered fibers. Shifting or flattening the zero of chromatic dispersion gives dispersion-shifted and non-zero dispersion-shifted designs.
• Specialty fibers. Polarization-maintaining fiber holds a linear polarization; photonic crystal fiber and hollow-core fiber guide with air-hole microstructures; few-mode and multi-core fibers carry parallel channels; rare-earth-doped and double-clad fiber form the gain medium of fiber lasers and amplifiers.
• Polymer fiber. Plastic optical fiber with a core of about 1 mm couples easily to LEDs but loses on the order of 0.15 dB/m in the visible, which confines it to links of tens of metres.

Loss and signal speed

Attenuation in silica has a minimum near 1550 nm, set by Rayleigh scattering on the short-wavelength side and infrared absorption on the long side, with typical values of about 0.2 dB/km at 1550 nm and about 0.33 dB/km at 1310 nm; the mechanisms, measurement methods and historical numbers are in fiber attenuation. Bending adds loss, covered under bend loss and microbend loss. Signals travel at $c/n_g$ with a group index near 1.468, about 4.9 µs per kilometre.

Connecting and terminating fiber

Fiber is joined permanently by fusion splicing, which needs a flat end prepared by fiber cleaving, and demountably by connectors whose ferrule ends are finished by fiber polishing. The common body styles are listed under fiber connector types. Power is divided among many fibers by a fiber optic splitter, the central component of passive optical networks.

Where it is used

Telecommunications and data centres account for most fiber by length, including submarine cables that span oceans with optical amplifiers spaced roughly 50–100 km apart. Fiber is also the sensing element in fiber optic sensors, gyroscopes and distributed acoustic sensing, the gain medium of high-power fiber lasers, the delivery path for laser surgery and machining, and the illumination and imaging channel in endoscopes.

Common questions

What is an optical fiber made of?

Almost all communications fiber is high-purity fused silica (SiO₂) made by vapour deposition. The core is raised in index by germanium dioxide doping, or the cladding is lowered by fluorine doping in pure-silica-core designs. A dual-layer acrylate coating surrounds the glass. Plastic optical fiber uses PMMA or fluorinated polymers, and mid-infrared fibers use fluoride or chalcogenide glasses.

How does an optical fiber carry light around bends?

The ray picture says light meets the core-cladding boundary at angles beyond the critical angle, about 85° from the normal for the fiber above, and is fully reflected each time. A gentle bend keeps those angles above critical. A tight bend tilts some rays below it, and in the wave picture the outer part of the mode would have to travel faster than light in the cladding, so it radiates; that is the origin of bend loss.

How far can light travel in a fiber without amplification?

At 0.2 dB/km, 50 km of fiber passes 10% of the launched power and 100 km passes 1%. Unamplified links typically reach 40–80 km depending on the transmitter power and receiver sensitivity; longer spans use optical amplifiers such as the erbium-doped fiber amplifier.

References: Saleh & Teich, Fundamentals of Photonics 3rd ed. 2019, Ch. 10; G. P. Agrawal, Fiber-Optic Communication Systems 4th ed. 2010; A. W. Snyder & J. D. Love, Optical Waveguide Theory 1983.