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.

Lasers

Monochromatic?

Monochromatic light

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

Coherent light?

Lasers - How do they work?

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.

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.

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.

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)

Metastable states

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

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.

Helium-Neon laser

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.

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.

Helium-Neon laser

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

Uses of lasers

Destroying tumours

Reattaching damaged retinas

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

Unblocking arteries and heart valves

Cutting and sealing nerves in neurosurgery

Correcting myopia in cornea operations

Welding

Cutting and drilling metals

Compact discs

Measuring distances

Reading barcodes

Optical fibre communication

Laser guided “smart” weapons

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