Download P1X

Lecture 7: Lasers and their applications (II)
Requirements for Laser action (Y&F 38.6):
 Theoretical laser:
• We want a three level system in which the intermediate state is
metastable (ie. the time for transition is much longer than the time
for transition to other states).
• If we can optically pump a three level system such that electrons
are stimulated from the bottom E1 level to the top level E3, and
these decay to a metastable intermediate state E2 then we obtain
population inversion:
E ,N Short lived state (t~10-8 s)
3
3
E2 ,N2 Metastable state (t~10-3 s)
Optical pumping by
intense light source
hf12
hf13
hf12
hf12
E1 ,N1 Ground state
P1X: Optics, Waves and Lasers Lectures, 2005-06.
1
 Requirements for laser action:
• Pumped light must not match E2-E1 = hf12, since it would deplete
the N2 population by stimulated emission.
• Need intense pumping action to achieve population inversion
N2>N1, with state N2 being metastable.
• Use partially silvered mirrors to re-circulate the initial laser light to
boost stimulated emission (create an optical cavity).
Small fraction of
light escapes
Mirror
“Leaky mirror” (partially
silvered) ~0.1% transmission
 Methods of pumping:
• Optical pumping (eg. flash light).
• Gas discharge (eg. gas lasers).
• Chemical excitation (eg. dye lasers).
• Electron excitation (eg. semiconductor lasers).
P1X: Optics, Waves and Lasers Lectures, 2005-06.
2
3 and 4 level lasers:
 The ruby laser:
• First laser constructed in 1960 by T.H. Maiman.
• Ruby: Al2O3 crystal (sapphire) containing
0.05% Cr3+.
• Example of a 3 level pulsed laser.
• Pumping is pulsed with a Xe flash lamp (~10-3 s)
• Ruby laser only emits short pulses (~10-6 s)
since metastable state depletes quickly and is
not replenished.
• Inefficient process: needs large input power
since excitation is from ground state.
N3 Short lived (~10-8 s)
E3
DE23=0.48 eV
hf13
Optical Pumping
l=550 nm (green)
DE13=2.26 eV
E2
hf12
E1
hf12
hf12
Metastable (2 ms)
N2
l=694.3 nm (red)
DE12=1.78 eV
N1 Ground state
P1X: Optics, Waves and Lasers Lectures, 2005-06.
3
 The helium-neon laser (Y&F 38.6):
• First constructed in 1961.
• Mixture of He and Ne (~10-3 atm) sealed in a glass enclosure with two
electrodes to create ionisation by continuous gas discharge.
• Laser action not from ground level so the population in the lower state
is small. It is easy to obtain N2>N1 so it can operate continuously.
• He-Ne laser output small (few mW) and gain low (~1.05 per metre)
due to losses in mirrors, walls, etc.
P1X: Optics, Waves and Lasers Lectures, 2005-06.
4
 The helium-neon laser (cont):
• Example of a 4 level continuous laser.
• Collisions between the He atoms and Ne atoms cause excitations. The
2s state of He is metastable, so collisions of He atoms in this high state
cause excitations in the Ne atoms. These decay to intermediate states:
He atom
20.61 eV
Ne atom
2s
Discharge
20.66 eV 5s
19.78 eV
4p
Collisions
16.70 eV
20.30 eV 4s
632.8 nm
18.70 eV
3p
18.37 eV
(red)
3s
0 eV
hc 6.63  1034  3.0  108
DE  20.66  18.70  1.96eV  l 

 632.8nm
19
DE
1.6  10  1.96
• Other transitions: 543 nm (2.29 eV, green), 1.15 mm (1.08 eV, infrared)
and 3.39 mm (0.36 eV, infrared).
P1X: Optics, Waves and Lasers Lectures, 2005-06.
5
 Modes of vibration of lasing cavity:
• Resonant system at optical frequencies.
• Light has to be in phase after round trip (d=length of optical cavity):
2d  ml 
m
 f c
c  fl 
2d
• Spacing between frequencies:
c
Df 
2d
• Could have atomic transitions with a width of ~1000 MHz: many
longitudinal modes are transmitted.
 Example: Line width of He-Ne laser is
1.5 x 109 Hz. The mirror separation is 0.3
m. Calculate Df and estimate the number of
longitudinal modes that resonate in the
system.
Df
f
9
c
3  108
1
.
5

10
Df 

 5  108 Hz  Num modes 
3
8
2d 2  0.3
5  10
P1X: Optics, Waves and Lasers Lectures, 2005-06.
6
Applications of lasers (Y&F 38.6):
 Helium-neon lasers:
• Characteristics: l=633 nm and a few mW in power.
• High coherence of light is useful for measuring length accurately (by
interferometry or surveying by triangulation).
• Accurate machining.
• Bar-code readers.
 Argon ion laser:
• Characteristics: blue (l=488 nm) and green (l=514 nm) with 30 W
power.
• Used in eye surgery to weld detached retinas.
• Interferometry in gravitational wave detectors.
 CO2 laser:
• Characteristics: kW of power at 10.6 mm (infra-red).
• Used for metal cutting and drilling precision holes, shapes.
P1X: Optics, Waves and Lasers Lectures, 2005-06.
7
 Nd-YAG (neodynium doped ytrium-aluminium-garnet) laser:
• Characteristics: pulsed laser at 1.06 mm (infra-red).
• Used in medicine to clear blocked arteries and to destroy tissue
• (e.g. tumors).
• Nd-glass laser used in induced fusion experiments.
 Semi-conductor laser (e.g. gallium arsenide):
• Characteristics: mainly at 800 nm but also at 680 nm.
• Used for optical communications through optic fibres.
• CD players.
• Supermarket checkouts.
• Pump for Nd-YAG laser.
• Laser pointers.
P1X: Optics, Waves and Lasers Lectures, 2005-06.
8
 Holography:
• Make 3D images by recording amplitude and phase information from
object.
• How do you make a hologram? Make interference pattern between
laser light reflected from object and reference beam. Coherence of
laser light is vitally important for holograms.
• How do you reconstruct the image of the hologram?
Reconstruct hologram by passing laser light of same
wavelength through interference pattern.
P1X: Optics, Waves and Lasers Lectures, 2005-06.
9