Laser diode

Lasers in the form of laser diodes are in widespread use today. These laser diodes are used in a large number of products today, the most common of which is probably in CD and DVD drives for computers and television and audio systems. Laser diodes are also used in other applications including copying machines, printers, and many more common applications. Apart from this there are many more specialist applications where laser didoes are used including optical fibre communications, medical surgery, some areas of IC manufacture and a host of other uses.

The reason for the use of lasers is their high directionality, the fact that they are monochromatic, they are a coherent light source and they have a very high power density. Apart from this they possess a high switching speed and this enables them to be used for optical communications where a wide bandwidth is a necessity to enable the required data rates to be achieved.

Background
The name laser comes from the words Light Amplification by Stimulated Emission of Radiation. Lasers operate because of a phenomenon called stimulated emission that was first postulated by Albert Einstein before 1920. Although a number of media including gases liquids and amorphous solids can be used for lasers the first ones were realised in the 1960s using rubies. A helium-neon gas laser followed this in 1961 but it was not until 1970 that semiconductor laser diodes were made to run at room temperature by Hayashi. This represented the final step in research work that had been undertaken by a number of people and organisations over the years. It had required an in depth study of the properties of gallium Arsenide, the material that is used as the basis for many laser diodes and much work on the properties of the diode structures.

Construction
There may appear to be many similarities between a light emitting diode and a laser diode, the two are fundamentally different from an operational point of view. The laser diode is consists of heavily doped n+ and p+ regions. For manufacture it is normal to start with an n+ substrate and then the top layer can be grown onto this. The doping can be included in a variety of ways, either by diffusion, ion implantation or even deposited during the epitaxy process. A variety of materials can be used for laser diodes, although the most common starting substrates are Gallium Arsenide (GaAs) and Indium Phosphate (InP). These are known as type III-V compounds because of their places in the chemical periodic table of elements. Whatever material is used, it must be possible to heavily dope it as either a p type or n type semiconductor. This rules out most of the type II-VI materials, leaving the group III-V materials as the ideal option.

Apart from the basic semiconductor requirements, there are a number of optical requirements that are needed to enable the laser diode to operate. It needs an optical resonator. This must occur in the plane of the required light output. To achieve this the two walls of the laser diode that form the resonator must be almost perfectly smooth, forming a mirror surface from which the light can be reflected internally. One of the walls is made slightly less reflecting to enable the light to come out from the laser diode. Another requirement is that the two mirror surfaces must be perfectly perpendicular to the junction, otherwise the laser action does not occur satisfactorily. The two other surfaces perpendicular to the one of the required light output are roughened slightly to ensure that the laser action does not occur in this plane as well. In this way a resonant optical cavity is created. Although it is many wavelengths long it still acts as a resonant cavity.

Operation
There are three main processes in semiconductors that are associated with light. The first is absorption. The second is spontaneous emission, and the third is stimulated emission.

Absorption occurs when light enters a semiconductor and its energy is transferred to the semiconductor to generate additional free electrons and holes. This effect is widely used and enables devices like to photo-detectors and solar cells to operate.

The second effect known as spontaneous emission occurs in LEDs. The light produced in this manner is what is termed incoherent. In other words the frequency and phase are random, although the light is situated in a given part of the spectrum.

Stimulated emission is different. A light photon entering the semiconductor lattice will strike an electron and release energy in the form of another light photon. The way in which this occurs releases this new photon of identical wavelength and phase. In this way the light that is generated is said to be coherent.

The key to the process occurs at the junction of the highly doped p and n type regions. In a normal p-n junction current flows across the p-n junction. This action can occur because the holes from the p-type region and the electrons from the n-type region combine. With an electromagnetic wave (in this instance light) in passing through the laser diode junction diode junction it is found that the photo-emission process occurs. Here the photons release further photons of light occurs when they strike electrons during the recombination of holes and electrons occurs.

Naturally there is some absorption of the light, resulting in the generation of holes and electrons but there is an overall gain in level.

The structure of the laser diode creates an optical cavity in which the light photons have multiple reflections. When the photons are generated only a small number are able to leave the cavity. In this way when one photon strikes an electron and enables another photon to be generated the process repeats itself and the photon density or light level starts to build up. It is in the design of better optical cavities that much of the current work on lasers is being undertaken. Ensuring the light is properly reflected is the key to the operation of the device.

Summary
The laser diode is now well established, and used in a wide variety of applications. Although not nearly as cheap as many other forms of diode, laser diodes are still produced in vast quantities and at a relatively low cost, as demonstrated by the fact that laser diodes are even used in the light pencils used for illustrating overhead projector slide presentations. At the other end of the market, laser diodes for use in optical communications systems have been shown with data rates in excess of 20 Gbits per second. With performance levels in this region, they are being used increasingly in many communications applications.