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Homework Help: Science: Physics: Lasers
by Emily McPherson
LASER stands for Light Amplification by the Stimulated Emission of
Radiation. In 1917 Albert Einstein calculated the conditions necessary for
stimulated emission; but, it was only much later, in 1960, that the first
visible LASER was demonstrated by T. Maiman. However, a MASER, a similar
device that emitted energy in the microwave region was the first to be
developed.
Basic Concepts
Normally when an electron gains energy there is a random time delay
before it spontaneously emits the energy as light. Sometimes this emission
is said to come from a 'forbidden transition' and the time delay is much
longer, though still random. This is called Spontaneous emission.
If during this waiting period the electron is hit by a photon of light the
electron will instantly emit its energy. This is called Stimulated emission.
An unusual feature of this is that the photon emitted will be traveling in
the same direction as the original photon.
For this process to be sustained the rate of stimulated emission needs to be
greater than that of spontaneous emission. This state is called a Population
Inversion, where there are more excited electrons than unexcited electrons.
However, with just two energy state involved this will never be achieved.
Three or four energy levels are generally used.
When a photon is absorbed by an electron and becomes excited there is a
rapid transition into another long-lived energy state. Thus, this long-lived
state soon becomes more populated than the unexcited state, which ensures a
sustained process of stimulated emission.
Types of Lasers
Doped Insulator Lasers
Two very common doped insulator lasers are made from rods of Ruby and
Garnet, which contain some metal impurities. Silica glass can also be made,
purposely doped with impurities, and used either as a rod or a more recently
as an optical fiber.
The red color of Ruby comes from a small amount of Chromium
contaminating the mineral corundum (Aluminum Oxide, with impurities
giving any other colors designated as Sapphires). The Chromium impurity is
also the basis for its use as a laser.
Yttrium Aluminum Garnet (YAG) that is doped with a metal called
Neodymium.
The lasers need some energy source to replenish the energy lost through
spontaneous emission (a process called pumping). This is achieved through
absorption of light, for instance from a Xenon flashtube or a
semiconductor(Diode) laser.
Gas Lasers
Some of the most common type of lasers are gas lasers, for instance,
Helium Neon, Carbon Dioxide and Argon ion lasers. It is based on a gas
discharge tube, such as is found in Neon lights or Mercury fluorescent
lights. High energy electrons bombard the gas, leading to excited electrons.
These types of lamps can be modified to transform the spontaneous light
emission in stimulated light emission.
Semiconductor Lasers
These are based on semiconductor device called a PN-junction. This is
where two types of semiconductor are put together. On one side there is a
deficit of electrons (the Positive side) and on the other side there is an
excess of electrons (the Negative side). These sides are usually created by
'doping', adding atoms to the semiconductor which have one more or one less
electron.
If a voltage is applied across the junction (called Forward-biased) some of
the excess electrons cross to the other side and recombine with the
electron-deficient atoms. However, if a voltage is applied across the PN
junction in the opposite direction (called Reverse-biased) there is a large
resistance to the flow of electrons. This is the basis of a diode, where the
electrons (and thus electric current) practically only flows in one
direction. However, if the voltage is large enough then the electrons will
be able to
flow. The energy needed for the electrons to cross the junction can come
from many sources. For instance in a Photodiode the absorption of light of
sufficient energy can produce a small current. This is also the basis of a
Solar Cell, designed to deliver the maximum power in the device.
The recombining of the atom and the electron that occurs releases the
energy that the free electron possessed. This energy can be in the form of
light and PN junctions can be designed so that photons with visible energies
are emitted. This is the basis of a Light Emitting Diode (LED). So far, the
easiest way to create colors other than the common red diode laser is to
start off with a red or infrared laser and double or triple the frequency.
Unfortunately this is not a very efficient process and produces low energy
devices.
As mentioned previously if an atom is stimulated by a photon prior to the
spontaneous emission of a photon then lazing occurrs. So, now we have a
light source all we need is sufficient energy input and an optical cavity to
cause a constant population inversion.
Mirrors are normally used to control the direction of the photon beam
ensuring efficient lazing. Mirrors could be used, but the high refractive
index of the material means that a large proportion of the light is
reflected at an air interface.
A simple PN junction needs a large number of electrons to be injected to
sustain the lazing, due to optical and electron losses. These types of diode
lasers, called homojunctions, therefore have a high 'threshold current'. The
losses increase greatly with temperature, as the electrons are given more
thermal energy.
In order to reduce the losses the junction is sandwiched between a material
of different optical and electrical properties. These lasers are called
heterojunctions. Another method that is used to contain the light output is
to make the junction only in a thin stripe on the semiconductor (called a
stripe laser).
Homework Help: Science: Physics
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