Masers and Lasers

1. Chapter 8: Lasers and Masers

2. Unique Properties of Lasers
• Monochromaticity - light with a single
frequency (one colour).

3. • Coherence – all rays of the beam are in phase.

4. • Directionality

7. Three fundamental phenomena
• Absorption
• Spontaneous emission
• Stimulated emission

8. Absorption
• An atom initially at level one (ground level)
receives an external stimulus (energy).
• Energy raises it to level two (an excited state)

9. Spontaneous emission
• Atom in an excited state (high energy level)
falls back to a less excited state, emitting a
photon of light.
• This is called spontaneous or radiative
emission.

10. • Spontaneous emission is random
• Atoms involved in spontaneous emission may
produce monochromatic light…
• … but the phases of the waves will be different
(non-coherent) and …
• … they will travelling in different directions.

11. Stimulated emission
• Emission stimulated by another photon.
• The emitted photon is in the same direction
and in phase with the incident photon.

12. Population inversion
• Stimulated emission is the basis of the laser,
but you need many more atoms in the excited
state than the ground state. This is called
population inversion. It is achieved by
pumping.
Most atoms in ground state
Most atoms in excited state
(population inversion)

13. Metastable states
To achieve population inversion we must have
metastable states. These are excited states
where electrons stay for unusually long times.

14. Metastable states
Normally an electron in an excited state will
make the transition to a lower state in a time
of 10-7s. In contrast an electron may stay in a
metastable state for 10-3s.

15. • Laser equipment has three basic components:
– a resonant optical cavity
– a gain medium (active medium) and
– a pump source to excite the particles in the gain
medium.

16. • Active medium placed between two mirrors.
• Light bounces up and down between the
mirrors resonantly.
• Laser cavity made up of two mirrors, one of
which is semi-transparent to release the laser
that is generated in the cavity.

17. • Stimulated emission process takes place in the
gain medium.
• Gain medium amplifies light of any direction.
• However, only the light that bounces up and
down between the resonator mirrors is
amplified many times and therefore reaches a
high intensity.

18. • Monochromatic because the photon energy
has to match a particular energy transition.
• Since the amplification process maintains the
phase and direction of the light, the laser
output is directional and coherent.

19. • The active particles in the laser gain medium
need to be in a state of inversion for the laser
to operate.
• This is done through a process known as
pumping.
• The beam gains an extra photon whenever an
atom returns to the ground state

20. • The beam (laser) is reflected back and forth in
the cavity until it has enough intensity to be
sent out of the cavity.
• After that, the flash bulb can again excite
enough atoms for the creation of a new pulse.

21. • Laser is released continuously in a Continuous
Wave (CW) laser.
• An obstacle is placed in the laser cavity to stop
the laser discharge.
• The obstacle is removed when the flash bulb
has excited a large number of atoms it
releasing one gigantic pulse.

22. Helium-Neon laser

23. Helium-Neon laser
• The helium is used to create population
inversion in the neon. An electric field is used
to excite the Helium to a metastable state
20.61 eV above its ground level. When this
metastable helium atom collides with a neon
atom it excites the neon atom to a 20.66 eV
metastable state. The extra 0.05 eV comes
from the helium’s kinetic energy and the
helium atom is now back in its ground state.

24. Helium-Neon laser
The neon atoms transition to a
18.7 eV state, giving out photons
of λ = 632.8 nm. These photons
are reflected by mirrors at the
ends of the laser tube causing
stimulated emission in other
metastable neon atoms.

25. Helium-Neon laser
One of the mirrors is semi
transparent, allowing the
monochromatic, coherent,
directional and very intense
laser beam to emerge.

26. Uses of lasers

27. Destroying tumours

28. Reattaching damaged retinas
The laser
damages part
of the tissue
so that the
resulting scar
“welds” the
retina back
into place.

29. Unblocking arteries and heart valves

30. Cutting and sealing nerves in
neurosurgery

31. Correcting myopia in cornea
operations

32. Welding

33. Cutting and drilling metals

34. Compact discs

35. Measuring distances

36. Reading barcodes

37. Optical fibre communication

38. Laser guided “smart” weapons