Laser imaging

6 Laser imaging

Lasers

Light and the Laser

White light

• Made up of varying wavelengths

• Ranges from ultraviolet in the short wave, through visible light, to infrared in the long wavelengths

• Very short coherence lengths, due to the number of different wavelengths and the anti-phase components cancelling one another out

Fig. 6.1 White light – a cacophony in vision.

Laser light

• Only one wavelength

Fig. 6.2 Laser light – coherent light of only one wavelength.

The First LaserPulsed laser
In 1960, Theodore Maiman, working at the Hughes Aircraft Electronic Research Laboratory, built the first laser

• It was basically a photographer’s electronic flashgun wound around a crystal of synthetic ruby (aluminium oxide mixed with a small amount of chromium)

• The synthetic ruby was in the form of a tube with a mirror at each end of the tube

• One mirror was a partial reflector

• The distance between the mirrors was precisely defined

• It was tuned to make the light produced bounce back and forth in a regular and reinforcing pattern to create standing waves of light in the rod which had a pulse of about three thousandths of a second

Fig. 6.3 Maiman’s original synthetic ruby laser.

The Operation of a Laser

The Basic Physics	• If a photon is fired at an atom, the atom can absorb the photon and thus be raised to an excited state • The only way that the atom can return to stability is by releasing this absorbed energy in the form of a photon of light (spontaneous emission) • But if a photon is fired at an already excited atom, the atom will release TWO light photons and then return to its stable state (stimulated emission) (Fig. 6.4 )
Inside a Laser	• The atoms are stimulated by an external power source to produce stimulated emission • When a stage is reached, whereby half the atoms are in an excited state, population inversion occurs (Fig. 6.5 ) • If more photons are then fired, stimulated emission takes place. Stimulated emission occurs because it is more likely that the photons will strike an already excited atom rather than a stable atom • If the above events occur in a tube, the tube can be ‘tuned’ to encourage the light produced to oscillate in a regular pattern • The light then breaks out as an extremely coherent, parallel beam of light which contains a considerable amount of energy – laser light
Continuous Wave Gas Laser	• Uses helium and neon as the lasing material • The light is very coherent and uniform and can be switched off and on like an electric light • They are used in laser imagers