A laser diode is a semiconductor device that generates coherent light of high intensity when the electric current is applied to it. The word LASER is actually an acronym for Light Amplification by Stimulated Emission of Radiation. It converts electrical energy into a light beam. It’s like a light-emitting diode (LED) with a PN junction but unlike LEDs which emits spontaneous light, it emits coherent and monochromatic one. It’s a very important device as we will see later in its application.
It’s made of oppositely doped semiconductors. The two parts are sandwiched to form a junction (which is depleted of charge carriers). Other diodes may have an intrinsic region between p and n-type. The active part is the junction or intrinsic region. The device’s outer surface is highly polished to form a mirror surface except for one side where the beam is emitted. There may be other optical parts for directing the light in the required direction. The semiconductor used are alloys of germanium, arsenide, indium that are doped with aluminum for n and p types.
When direct current is applied to the diode in forward biased manner an electric field is created. The holes in p-type are attracted to the n-type while at the same time electrons at n-type are attracted to p-type. Laser diode works in three key processes as highlighted below.
- Absorption– as electrons are drifted to the p-type side by the electric field, they have to cross the intrinsic region or junction. The free electrons strike other electrons around an atom, they absorb energy and jumps to high energy levels. Where the electrons leave a hole is created creates (a space that needs to be filled). The electrons are highly unstable in higher energy levels compared to low ones. It’s key to understand electrons surround the nucleus of an atom occupying discrete energy levels.
- Spontaneous emission– unstable electrons in high energy levels after some time will return to the stable low energy level to fill holes (vacancy devoid of the electron). To fall to the lower level, it loses energy in form of a photon of light. This is the working principle of an LED. The process of electron filling a hole is referred to as recombination.
- Stimulated emission– unlike in the spontaneous case where electrons freely and naturally fall to a lower energy level, this is a triggered or stimulated process. A light photon is applied to strike the unstable electron at the high energy level. This causes it to lose energy in form of light photon and fall into a lower energy level. The stimulating photon and the stimulated one are inherent or in phase. The two photons of light have the specific same amount of energy and wavelength hence monochromatic. To increase the emission, some of the emitted light is reflected back by polished surfaces and they stimulate more and more until its intensity becomes high. The amount of intensity is measured by the amount of stimulating light that is measured by a photodiode. To control the intensity the current is adjusted accordingly.
Key properties of the emitted light are worth noting.
- Coherence- light waves are in phase
- Monochromatic- light has a single wavelength and frequency. It can be reflected by optical lenses as a single color in the desired direction.
- Brightness- the beam is bright owing to high intensity. The light beam is concentrated to exit at only one opening.
- Directionality- it’s highly directional. The light is directed by a waveguide and only a small opening for emission is allowed. The polished surfaces reflect the light to one exit only. The Beam has high power and when focused can travel far.
The gadgets that work by laser diode technology are numerous and they cut across many fields. Some are enumerated below;
- Medical – laser beam is used to kill cancerous cells and remove unwanted tissues.
- Fiber-optic communication– data transmission
- Printing and scanning machines
- Instrumentation– contactless ranger finder
- CD and DVD readers and recorders
- Laser points – used in PowerPoint presentation and remote-control modules.
- Barcode reader– used in sales to determine product details and security systems
- Industry– for machines precise cut use
A laser diode is a small device with low power consumption and high efficiency yet a powerful one. Technology has really made life easier for humanity. Your remote-control module utilizes the laser for you to switch TV channels at the comfort of your couch. Semiconductor technology is a solution to many world problems. A word of caution; never ever look at a laser beam directly, its high intensity can damage your eyes. What more can you do with a laser beam? I would love to hear from you.