Optical Fiber Communications 101: Key Concepts and Technologies

In 1880, Alexander Graham Bell conducted an experiment where he made a phone call using natural light (sunlight) to convert his voice into light via a “photophone.” This light was transmitted approximately 700 ft. away, converted back to voice for the recipient to hear, and is now believed to be the first instance of wireless transmission of speech. Not surprisingly, this method was initially too difficult to use over longer distances due to the transmission path being space, which caused the light to spread and disperse and was easily further complicated by obstacles like precipitation and physical objects.

It was almost a century later before optical-based communication was put to practical use, thanks in large part to the invention of optical fiber and lasers. A laser’s stable, highly directional beam of light (emitted from tiny semiconductor windows that measure just a few hundred-thousandths of a square millimeter) can carry enormous amounts of information. This combination of this plus optical fiber (a high-performance transmission medium made of glass as thin as a human hair capable of trapping optical signals and transmitting them over long distances without significant attenuation) were game changers and set the stage for optical-based telecommunication innovation.
 

How Optical Fiber Communication Works

The most important elements of optical communication are a transmission medium with extremely low optical attenuation and a highly stable, long-life light source that operates with a small current. With the advent of optical fiber as a transmission medium and semiconductor laser as a light source widespread use of optical communications became practical.

The process of optical communication breaks down into a few simple steps:

• An electrical signal is sent from a terminal (e.g., telephone, computer)
• It converts into an optical signal (digital signals "0" and "1" convert into flashing light) via an electrical-to-optical (E/O) converter
• This transmits along the optical fiber, propagates, and converts back into an electrical signal via the communication partner’s optical-to-electrical (O/E) converter at the receiving terminal

E/O converters use light-emitting elements such as semiconductor lasers, O/E converters use light-receiving elements such as photodiodes, and optical elements such as lenses are used at the input and output of optical fiber. It’s important to note that the size of the light-emitting part of a semiconductor laser is on the order of μm, the light-receiving part of a photodiode is on the order of 100 μm, and the core diameter of an optical fiber is around 10 μm, which necessitates advanced technology to align them accurately.

Optical Fiber Advantages
Compared to conventional metallic cables, optical fiber provides an advantage of low loss (~ 0.2dB/km) and wide bandwidth (several hundred MHz to THz) to enable long-distance, high-capacity communication. Additionally, optical fiber is lightweight and less susceptible to noise (no electromagnetic induction).

Optical Fiber Structure
Optical fiber consists of a cylindrical core that propagates light and a concentric cladding that surrounds it. The cladding’s refractive index is slightly smaller than that of the core, which confines light within the core and propagates by repeated total reflection at the boundary with the cladding. Optical fiber is as thin as a hair (diameter of 125 μm, core of 9 μm – 62.5 μm), with most optical fibers used for communication being made from quartz glass.

Optical Fiber Types
When light travels through an optical fiber, only reflections at a certain angle are reflected repeatedly due to the relationship between the difference in refractive index (between the core and cladding of the optical fiber) and the thickness of the core. This special angle is referred to as propagation mode. When there are many such angles, this is referred to as multimode, and when there is only one, this is called single mode. Among multi-mode optical fibers, there is a graded index (GI) optical fiber that has a gradual change in the refractive index distribution of the core. Fibers commonly used in optical communication are single mode and GI.

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