We have discussed at length the importance of light in our tutorials thus far, but we have yet to see where this light comes from… until now!
Light is generally created by the rapid change of state of an electron from a condition of high energy to one of low energy – this energy is expelled in the form of light. In fibre optics, there are two ways light is created: lasers and LEDs. The two are similar in many regards – indeed, both are made from semiconductors – yet they are fundamentally different; specifically concerning the type of light they emit and their respective applications.
So, how are lasers and LEDs different?
The term laser is actually an abbreviation for “Light Amplification by Stimulation of Emitted Radiation”. This basically means that an optical source is stimulated and light is then emitted in a specific way. This is a controlled process, called stimulated emission. In this particular case, it is possible to control the wavelength (colour), phase, and direction of the resulting light when creating the laser. All of the light waves which result from a laser are aligned with one another; this is known as coherent light, and it is the reason why a laser is simply a tiny dot. In general, lasers can produce relatively high power compared to LEDs and are much more expensive than their counterparts. Finally, a laser’s focal point is narrow enough to pass into a single-mode fibre, and can also be used with multimode fibres by extension. This being said, lasers are almost always paired with single-mode fibres.
Conversely, LED stands for “Light Emitting Diode”. This simply means that energy that passes through the diode is emitted as light. While it is technically true that both lasers and LEDs are made up of “diodes”, LEDs are much simpler than lasers. Many parameters that can be controlled in a laser cannot be controlled in an LED. For this reason, LEDs produce a much more erratic light, called incoherent light. It is very diffuse in contrast with the very precise, refined dot of a laser. Because the light is so spread out, LEDs can only be used with multimode fibres; even with a lens focusing the light, it cannot be concentrated into the tiny opening of a single-mode fibre. LEDs redeem themselves by being much less expensive than lasers.
In summary, lasers and LEDs are the two types of light sources used in fibre optics. Lasers provide precise, aligned, powerful light used mainly with single-mode fibres. The less expensive LED light source provides a scattered light which is only compatible with multimode fibres.
Thanks for tuning in! Be sure to check out our next chapter, which will focus on optical detectors.
June 15, 2018