Introduction to Fiber-Optic Communication and the Key Properties of Optical Fiber

Introduction

We are in the era of big data, where people, things and objects all connect through the network. Demand for data transmission keeps growing, and so does the bandwidth it requires. Today we introduce fiber-optic communication.

Fiber-Optic Communication

Traditional communication uses electricity to carry signals, with wires as the transmission medium. But as everyone knows, conductors always have resistance; even highly conductive materials are limited in how far they can carry a signal. Fiber-optic communication emerged to solve this. It converts the electrical signal into an optical signal, sends it through optical fiber as the medium, and converts it back into an electrical signal for decoding at the receiver.

Structure of an Optical Fiber

An optical fiber consists of three main parts, from the inside out:

1. Core: carries the light. Marked 1 in the figure below.

2. Cladding: has a slightly lower refractive index than the core, confining light within the core. Marked 2 in the figure below.

3. Coating (jacket): protects the fiber. Marked 3 in the figure below.

The figure below illustrates the structure of an optical fiber.

Image source: https://en.wikipedia.org/wiki/Core_(optical_fiber)

Whenever light crosses an interface, part of it is refracted and part is reflected. When light travels from an optically denser medium to a less dense one, that is, from a higher refractive index to a lower one, total internal reflection occurs once the angle of incidence exceeds a certain critical angle, meaning the refracted portion drops to zero. This is why the cladding's refractive index must be slightly lower than the core's: it keeps the transmitted signal fully confined within the fiber core.

Fiber Modes

By mode, optical fiber is divided into single-mode and multimode fiber. So what is a mode? Simply put, it is a way light propagates; if there is only one way for light to propagate, the fiber is single-mode. The main difference between single-mode and multimode fiber is the core diameter: multimode fiber typically has a core of about 50 µm, while single-mode fiber is about 9 µm. As described above, light energy reaches the other end through total internal reflection between the core and cladding. In multimode fiber, you can think of different incidence angles as different modes, i.e., different propagation paths. As the core shrinks from 50 µm toward 9 µm, fewer angles satisfy the total internal reflection condition, until at 9 µm only one angle remains. That fiber is called single-mode fiber.

Numerical Aperture (NA)

As mentioned, fiber-optic communication converts electrical signals into optical signals and transmits them through fiber. A key parameter for coupling the optical signal into the fiber is the numerical aperture (NA). Simply put, NA is the fiber's acceptance angle: if light enters the fiber within the NA condition, it propagates through total internal reflection; otherwise, it does not.

Dispersion

Dispersion is the main factor limiting fiber bandwidth. The greater the dispersion, the more light spreads out as it travels through the fiber, and the more it spreads, the less far and fast it can go. Dispersion falls into three main categories:

1. Modal dispersion

This occurs in multimode fiber. Different modes propagate differently, meaning at different angles, so after long-distance transmission the different modes of the same light pulse reach the receiver at different times, creating a time spread within a single signal. Modal dispersion depends on how many modes the fiber supports; the closer to single-mode propagation, the less dispersion. The figure below illustrates different modes propagating in a fiber. Higher-order modes have smaller reflection angles, meaning a longer path and longer time from the source on the left to the right end. The longer the distance, the larger the difference between high- and low-order modes, which is modal dispersion.

Image source: https://community.fs.com/blog/types-of-optical-fiber-dispersion-and-compensation-strategies.html

2. Material dispersion

There is no truly single-wavelength light in the world, and a material's refractive index varies with wavelength. Different refractive indices produce different propagation behavior; simply put, different wavelengths have different total internal reflection angles in the fiber. The figure below illustrates material dispersion: red, green and blue light have different refractive indices in the fiber and therefore different reflection angles. After traveling some distance, the different colors arrive at the right end at different times, causing dispersion.

Image source: https://community.fs.com/blog/types-of-optical-fiber-dispersion-and-compensation-strategies.html

3. Waveguide dispersion

Propagation in single-mode fiber is more properly described with wave optics. Solving it with wave-optics mathematics shows that part of the light wave actually travels in the cladding, and the refractive index difference between cladding and core creates a time difference. This is called waveguide dispersion.

In short, as light travels through fiber, dispersion creates a time spread in the signal, which delays it and can cause the receiver to misread it.

Attenuation

Dispersion causes timing spread in signal transmission, while attenuation weakens the signal. Signal attenuation falls into two main categories:

Fiber absorption: every material absorbs some light, even a transparent fiber. Over long distances, even a tiny absorption coefficient can cause severe loss.

The figure below shows fiber loss at different wavelengths in dB/km. It shows that however low the fiber loss, it becomes significant over long transmission distances.

Image source: http://www.fowiki.com/b/understand-fiber-attenuation/

Finally, let's wrap up with the fiber spectrum:

• 850 nm: loss is higher here, but today's very mature GaAs VCSELs emit at 850 nm and offer a low-cost advantage, so applications at this wavelength usually use multimode fiber.

• 1310 nm: also called the O-band. Dispersion is nearly zero here and loss is fairly low, so single-mode fiber is usually used for transmission.

• 1550 nm: also called the C-band. It has slight dispersion but the lowest loss, so this wavelength is typically paired with single-mode fiber for the longest-reach transmission.