Lasers
The term laser is an acronym for ‘light amplification by the stimulated emission of radiation’.
When atoms are excited there are normally fewer excited atoms than atoms in lower energy states.
The diagrams below show the relative numbers of excited and unexcited atoms normally and when there
is population inversion.
In a laser there must be
• a population inversion; i.e. there must be more excited atoms than unexcited atoms,
• an excited state that is metastable; i.e. one in which the atoms remain for a longer time than is usual.
How this is achieved depends on the type of laser.
Helium-neon laser
This consists of a mixture of 15% helium gas and 85% neon.
Some helium atoms are excited into the metastable state E1 by electrical discharge through the gas.
Helium atoms collide with neon atoms, exciting them to the metastable state E 2’. The helium provides this
energy by dropping into the ground state (providing 20.61 eV) and by losing kinetic energy (0.05 eV).
There is population inversion between this and the E2’ level.
When some of the excited atoms fall into the E1’ level the photons collide with other excited atoms and
stimulate them to fall. The emitted photons cause further atoms to emit photons. These reflect back and
forth from mirrors in the laser tube as shown above right. The result is an intense light in which all the
photons have the same frequency and phase and move in the same direction.
Schematic representation of laser operation
Energy is pumped in to maintain the number of atoms in the excited metastable state.
Photons bounce back and forth, increasing in number by stimulating emission of further photons.
Some (about 1 %) of the photons pass through the partial reflecting mirror. Energy is continually pumped
in and the laser light can be emitted continuously.
32.1 Principles of production
1 Production Principle
E1 and E2 are 2 different energy levels in an atom. Laser light can be produced as a result of receiving
stimulating photons in an excited atom. When laser light is produced
(1) the frequency of the stimulating photon
is the same as the frequency of the stimulated
photon.
(2) the phase of the stimulating photon is the same as the stimulated photon.
The Ruby Laser
The quartz flash tube is used to stimulate the electrons in the ruby rod to a higher energy level so that
laser light is produced when electrons at a higher energy level fall to a lower energy level.
The ends of the ruby rod are plated with silver to enable alternating photon reflection between the 2 ends
until high intensity laser light is produced and emerges from the end that is thinly silver-plated.
32.2 Characteristics
2 Characteristics
For ordinary light, intensity,
For lasers, intensity,
where
n = number of waves
a = amplitude of each wave
Lasers are
1 coherent and in-phase
2 monochromatic
3 of high intensity
4 do not diverge for long distances
32.3 Uses
3 Uses
1 Measuring long distances
2 Cutting metal plates in industry
3 Surgery (eye, brain, treatment of tumours)
4 Reading prices from bar-codes on goods sold in supermarkets
5 Reading information from compact discs in computers
6 Transmission of information through fibre-optic cables
7 Creation of 3 dimensional pictures in holograms
8 Printing through the use of laser printers
Use of lasers in CDs
Within a CD, there is a metal layer with a spiral track of tiny steps (bumps) on it. These and the spaces
between represent the Os and 1 s of the digitized signal. Light from a laser is focused onto the track and
reflected. But where there is a step, the reflected light is cancelled because of interference effects (see
C3). The result is a series of light pulses which the detector converts into electrical signals for processing.
Other uses of lasers
The laser beam is a very narrow beam of intense energy and as a result it has many and varied uses.
The following is a list of some of the uses of lasers:
In medicine
• to destroy tissue in a localized area
• to break up kidney stones
• to repair broken tissue (e.g detached retina)
• to restore sight impaired by a cateract
• to remove decay in teeth (white teeth reflect the laser energy but darker decayed areas do not)
In industry
• to drill fine holes in hard material
• to produce very accurate surveys
• to produce holograms
In communications
• to produce the light beam in fibre optic transmission
Laser safety
Because of their intense localized energy great care has to be take to avoid injury when using lasers.
Even with low-energy beams the eyes are particularly vulnerable, It is necessary to avoid inadvertently
looking at a beam reflected from a mirror or other good reflector. The use of special spectacles is
advisable.