Lasers and their Applications

1
Lasers and their Applications
Objectives
i) to understand what is meant by coherent and
incoherent light sources ii) to understand the
terms spontaneous and stimulated emission
and population inversion and to be able to
describe the requirements for laser action iii)
to understand the operation of simple 3 and 4
level lasers iv) to be able to describe the
main properties of some commonly used lasers, and
some of their many applications such as in
precision measurement, medical physics and
optical communications.
2
Lecture 6 Lasers and their applications (I)
Coherent and incoherent light sources (YF 35.1)

• Coherence
• A monochromatic source of light (with the same
  frequency and wavelength) and with a constant
  phase between two points on the wave front at a
  given time is said to be coherent.
• Two conditions for coherence
• Temporal longitudinal space coherence is a
  measure of how well points A and B remain in
  phase (eg. A continuous sine wave is perfectly
  coherent).

3

• Spatial the lateral displacement coherence is a
  measure of the phase relationship between two
  points A and B equidistant from the source (e.g
  a point source has perfect spatial coherence)

4

• Light sources bursts of waves
• Thermal filament is heated and radiated energy
  can give light (eg. Electric light bulb).
• Characteristics
• Wide range of wavelengths and frequencies.
• (ie. wide bandwidth)
• Produces short wave-packets of different l and
  phase --gt short coherence time
• Normally incoherent light, white in colour (a
  mixture of wavelengths).

5

• Gas discharge gas excited by electric discharge
  with electrons jumping to outer levels and then
  de-exciting to give off photons.

De-excitation
Excitation
Photon emitted with frequency f h Plancks
constant6.63x10-34 J s

• Light observed has different colours
• e.g sodium (orange), neon (red), spectral lines
  of helium and mercury, each corresponding to
  transitions from different energy levels
• More monochromatic means more coherence and
  longer wavepackets emitted
• Why do some materials glow under ultraviolet
  light? Three level system absorbs UV, emits
  visible

6

• Example The first three energy levels of a
  hydrogen atom are
• E1-13.6 eV, E2-3.4 eV, E3 -1.5 eV. The Balmer
  series of spectral lines are the lines emitted
  when electrons jump from levels n3,4,5, down
  to level n2. What is the wavelength of light
  emitted when an electron jumps from n3 to n2?
  And from n2 to n1?

Unit of energy electronvolt (eV) Energy required
to accelerate one electron by a potential
difference of 1V 1 eV 1.6x10-19 Cx1V 1.6x10-19
J h Plancks constant6.63x10-34 J s c 3x108
ms-1
(red)
For E2-E110.2eV and wavelength is l121.9 nm
(ultraviolet).
7

• Lasers very monochromatic, highly coherent
  light with long wavepackets
• LASER Light Amplification by Stimulated Emission
  of Radiation
• Characteristics
• Radiation coherent in space (across wavefront
  and in time (along wavefront)
• Very monochromatic
• Very bright e.g. CO2 laser can have power gt100
  kW in continuous light pulsed laser 2.5x1013 W
  over 10-12 s.
• High power density (intensity) gt1017 W/cm2
• Can have very short pulses (10 fs 10-15 s)
• Collimated beam ---gt very directional
• LASER action produced by absorption and emission
  processes, especially stimulated emission

8
Spontaneous and stimulated emission, population
inversion (YF 38.6)

• Atom populations absorption, emission and
  stimulated emission
• Einstein (1917) was the first to fully explain
  the absorption and emission processes in atoms
  when light radiation interacts with them.
• Consider a gas of atoms with 2 possible energy
  states E1 and E2 with populations of N1 and N2
  atoms/cm3.
• If gas in thermal equilibrium, N1 and N2 depend
  on temperature T
• with k Boltzmanns constant 1.38x10-23 JK-1
• Example If E2-E1 2 eV 3.2x10-19 J, with
  temperature of filament T3000 K, then
• Many more atoms in state 1 than 2 (N2ltltN1)

9

• Possible transitions from one state to another
• a) Absorption (resonant absorption of photons)
• An atom with an electron in the lower of the two
  states can absorb radiation of a certain
  frequency f and jump to the higher state.

Note it can only absorb radiation of that
frequency.
b) Spontaneous emission of photons If an
electron of an atom is in a higher state and
there is a lower available state the the electron
can spontaneously jump to the lower state
emitting a photon of energy
10
c) Stimulated emission of photons This effect
was predicted by Einstein. He realised that there
exists another type of emission that can be
stimulated by radiation of frequency f, causing a
jump from a higher level to a lower level only if
Two photons are emitted from the stimulated
transition and they have the same energy,
direction, phase and polarization, the two
photons are coherent.
Summary
11

• Light amplification
• The two photons emitted by stimulated emission
  can further stimulate emission from state 2 to
  state 1 so a large number of photons of the same
  wavelength can be emitted by this mechanism
• If N1gtN2 there are more electrons in the lower
  state so there is more absorption than emission
  (light attenuation).
• If N2gtN1 there is more emission than
  absorption, causing a chain reaction (light
  amplification).
• Population inversion
• Since the normal population condition is that
  N1ltN2, one needs to achieve a non-equilibrium
  situation where there are more states in the
  higher level than the lower level (population
  inversion).
• Population inversion can only be achieved if
  there are more than two states available and if
  we can pump electrons to higher energy levels
  (e.g. optical pumping, electric discharge,
  injection of electrons).