Core and cladding

Every index-guiding dielectric waveguide has a core, the region of higher refractive index $n_1$ in which most of the light travels, and a cladding of lower index $n_2$ around it. Light in the core that meets the boundary at a glancing enough angle is reflected back by total internal reflection, so it stays guided. In telecom fiber both are fused silica: the core is raised in index by germanium doping, or the cladding is lowered by fluorine, and the two indices differ by only a few tenths of a percent. Standard single-mode fiber has a core about 8–9 µm across in a 125 µm cladding. On a silicon photonic chip the core is a silicon strip of index 3.48 and the cladding is silica of index 1.44.

Index difference and numerical aperture

The size of the index step is quoted as the relative index difference

$$\Delta = \frac{n_1^2 - n_2^2}{2n_1^2} \approx \frac{n_1 - n_2}{n_1},$$

where the approximation holds when the step is small, as in fiber; the index contrast entry compares platforms. The step sets the numerical aperture of a step-index fiber,

$$\text{NA} = \sqrt{n_1^2 - n_2^2} \approx n_1\sqrt{2\Delta}.$$

With $n_1 = 1.450$ and $\Delta = 0.36\%$, the cladding index is 1.4448 and NA = 0.123, an acceptance half-angle of 7.1° in air. Inside the core, the critical angle at the boundary is 85.1° from the normal, so only rays traveling within 4.9° of the fiber axis are guided. Datasheets of standard fiber quote NA 0.14, measured from the far field at the 1% power level, which overstates the NA of the equivalent step; the single-mode-fiber entry explains the difference.

Silicon in silica has $\Delta = 41\%$, more than a hundred times larger. The light is held so tightly that waveguides of 500 × 220 nm guide a single mode and bend around radii of a few micrometers, where fiber needs centimeters.

Standard fiber geometries

Fiber sizes are written core/cladding in micrometers:

The 50/125 and 62.5/125 multimode fibers are graded-index designs whose core index falls smoothly toward the cladding, which equalizes the travel times of the modes; for them the NA refers to the peak core index.

Coating and buffer

The glass cladding is protected by a polymer coating, usually a dual acrylate layer that brings the outside diameter to 250 µm (200 µm in reduced-diameter fiber). The coating is mechanical: it preserves the strength of the glass surface and cushions it against microbending. Patch cords add a 900 µm tight buffer and a jacket.

Cladding modes

Light launched outside the core, or scattered out of it at a splice, can be guided by the cladding itself, with the coating or air as its outer boundary. These cladding modes carry power that is not in the core mode and can distort measurements of insertion loss and of coupled power. Standard acrylate coatings have a higher index than silica, so most cladding light leaks into the coating within a few meters of fiber. Where it must be removed deliberately, a cladding mode stripper, a section of bare fiber in contact with a high-index gel or polymer, draws it out; in high-power fiber lasers the stripper also removes unabsorbed pump light and must be cooled.

A double-clad fiber makes use of cladding guidance. Its inner cladding, typically 125–600 µm across, is surrounded by a low-index polymer or fluorine-doped glass and guides multimode pump light, which is absorbed gradually by the doped core it crosses.

Claddings on chips

In integrated optics the cladding is the material below and above the core: the buried oxide of a silicon-on-insulator wafer, typically 2–3 µm thick, below, and a deposited oxide, air or a polymer above. The buried oxide must be thick enough that the evanescent tail of the mode does not reach the silicon substrate, which would leak light.

Common questions

What is the difference between core and cladding?

The core has the higher refractive index and carries the light; the cladding has a slightly lower index and confines the light to the core by total internal reflection.

Why does the cladding need to be 125 µm thick?

Optically a few tens of micrometers would hold the evanescent field. The 125 µm diameter is a mechanical and handling standard that keeps the core away from the coating, sets the bend stiffness and lets all connectors share one ferrule bore.

Does light travel in the cladding?

Part of the guided mode does: in standard single-mode fiber about 19% of the power travels in the cladding near the core at 1310 nm and 27% at 1550 nm. Cladding modes, by contrast, are separate modes guided by the whole cladding.

References: B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019); A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman and Hall, 1983); G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010).