Explain the terms monochromatic and coherent.

1
Lesson 3

• Explain the terms monochromatic and coherent.
• Identify laser light as a source of coherent
  light.
• Outline the mechanism for the production of laser
  light (students should be familiar with the term
  population inversion).
• Outline an application of the use of a laser
  medical applications, communications, bar-code
  scanners, laser disks, surveying, welding and
  machining metals, drilling tiny holes in metals,
  production of CDs, reading and writing CDs, DVDs,
  etc.

2
Lasers
3
Monochromatic?
4
Monochromatic light

• Mono chromatic light has a very narrow range of
  frequencies (i.e. it is only made of light of one
  colour)

5
Coherent light?
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
Lasers - How do they work?
10
Spontaneous emission

• An atom in an excited (high energy level) state
  will eventually fall back to a less excited
  state, emitting a photon of light. This is called
  spontaneous emission.

11
Spontaneous emission

• Many atoms producing the same spontaneous
  emission at random may produce monochromatic
  light, but the phases of all the waves will be
  different, they will travelling in different
  directions, and their polarisation will be
  different.

12
Stimulated emision

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

13
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)
14
Metastable states

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

15
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.

16
Helium-Neon laser
17
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 heliums kinetic
  energy and the helium atom is now back in its
  ground state.

18
Helium-Neon laser

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

19
Helium-Neon laser

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

20
Uses of lasers
21
Destroying tumours
22
Reattaching damaged retinas

• The laser damages part of the tissue so that the
  resulting scar welds the retina back into place.

23
Unblocking arteries and heart valves
24
Cutting and sealing nerves in neurosurgery
25
Correcting myopia in cornea operations
26
Welding
27
Cutting and drilling metals
28
Compact discs
29
Measuring distances
30
Reading barcodes
31
Optical fibre communication
32
Laser guided smart weapons
33
Questions

• Page 605 questions 4, 6, 7, 8, 9 and 10.