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.
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