Download Week 4 Thursday Notes (Lesson 7)

Solid State Lasers
• Was first type of laser (Ruby 1960)
• Uses a solid matrix or crystal carrier
• eg Glass or Sapphire
• Doped with transition metal or rear earth ions
• eg Chromium (Cr) or Neodynmium (Nd)
• Mirrors at cavity ends
• Typically pumped with light
• Most common a Flash lamp
• Light adsorbed by doped ion, emitted as laser light
• Mostly operates in pulsed mode (newer CW)
Flash Lamp Pumping
• Use low pressure flash tubes (like electronic flash)
• Xenon or Krypton gas at a few torr (mm of mercury pressure)
• Electrodes at each end of tube
• Charge a capacitor bank:
50 - 2000 µF, 1-4 kV
• High Voltage pulse applied to tube
• Ionizes part of gas
• Makes tube conductive
• Capacitor discharges through tube
• Few millisec. pulse
• Inductor slows down discharge
Light Source Geometry
• Earlier spiral lamp: inefficient but easy
• Now use reflectors to even out light distribution
• For CW operation use steady light sources
Tungsten Halogen or Mercury Vapour
• Use air or water cooling on flash lamps
Q Switch Pulsing
• Block a cavity with controllable absorber
• Like an optical switch
• During initial flash pulse switch off
• Recall the Quality Factor of resonce circuit (eg RLC)
Q=
2π energy stored
energy lost per light pass
• During initial pulse Q low
• Allows population inversion to increase without lasing
• no stimulated emission
• Then turn switch on
• Now sudden high stimulated emission
• Dump all energy into sudden pulse
• Get very high power level, but less energy
Q Switch Process During Laser Pulse
• Flash lamp rises to max then declines (~triangle pulse)
• Q switch makes cavity Q switch on after max pumping
• Low Q, so little spontaneous light
• Population inversion rises to saturation
• The Q switch creates cavity: population suddenly declines
due to stimulated emission
• Laser pulse during high Q & above threshold conditions
Energy Loss due to Mirrors & Q
• Q switching can be related to the cavity losses
• Consider two mirrors with reflectance R1 and R2
• Then the rate at which energy is lost is
E
τc
=
(1− R1R2 )E
τr
where τc = photon lifetime
τr = round trip time = 2L/c
E = energy stored in the cavity
• Average number of photon round trips is the lifetime ratio
1
τc
=
τ r (1 − R1R2 )
Q Equations for Optical Cavity
• Rewrite energy equation in terms of photon lifetime τc
• First note the energy lost in the time of one light cycle tf = 1/f
Elost / cycle =
Et f
τc
=
E
fτ c
where f = frequency
• Thus the cavity's Q is
Q=
2πE
Elost / cycle
=
2πE
= 2πfτ c
 E 


f
τ
 c
• Thus for a laser cavity:
Q = 2π fτ c =
2π fτ r
4π fL
4π L
=
=
(1 − R1 R2 ) c(1 − R1 R2 ) λ (1 − R1 R2 )
• Q switch: go form high reflectivity to low reflectivity
on one mirror
• Also Q is related to the bandwidth of the laser
(from resonance cavity circuits).
Q=
f
∆f
• Thus lifetime relates to the bandwidth
∆f =
1
2πτ c
Transition Metal Impurity Ion Energy levels
• Chromium Cr3+ ion
• Atom has energy levels (shells)
(orbit)(shell)(no. electrons)
1s2 2s2 2p6 3s2 3p6
• In ions unfilled orbital electrons interact
• inter-electron coulomb interaction split the energies
(capital letter the L quantum)(spin quantum)
• Ion then interacts with crystal field
splits energy levels more
Rare Earth Impurity Ion Energy levels
• Spin of electrons interacts with orbit
• Splits the inter-electronic levels
Ruby Laser
• First laser built used Ruby rods: Maiman 1960
• Crystal is Aluminium Oxide Al2O3: Sapphire
• 0.05% Cr3+
• 3 level system: absorbs green/blue
• emission at 694 nm
• Pulsed operation
Ruby Laser Design
• Typically uses helix flash lamp
• Mirrors may be plated onto rod
• Seldom used now
Nd: YAG Lasers
• Dope Neodynmium (Nd) into material
• Most common Yttrium Aluminum Garnet - YAG:
Y3Al5O12
• Hard brittle but good heat flow for cooling
• Next common is Yttrium Lithium Fluoride: YLF
YLiF4
• Stores more energy, good thermal characteristics
• Nd in Glass stores less energy but easy to make
Nd: YAG Laser Energy Levels
• 4 level laser
• Optical transitions from Ground to many upper levels
• None radiative to 4F3/2 level
• Typical emission 1.06 microns
Nd: YAG Laser Output
• Note spikes in emission
• Pulse typically microseconds
Nd: YAG Lasers Energy Distribution
• Measure pulse output in total energy, Joules
• Generally trade off high power for low repetition rate
• High power, low rep rate
• Q switch pulse in nanosec range
Typical Nd: Yag layout
Nd: Glass Lasers
• Can make very large laser disks
• meters in diameter
• Large disks use to amplify laser beam
• Used in Laser Fusion projects
• TeraWatt lasers
• Slab type laser: beam bounces through Cavity
Diode pumped Nd: YAG Lasers
• Newest used laser diode to pump Nd: YAG
• Diode laser light carried by fiber optic to YAG cavity
• Means heat losses and power supply separate from laser
Typical Nd: Yag laser parameters
Typical Nd: Yag laser parameters
Alexandrite Lasers
• Alexandrite: Cr3+: BeAl2O4
• Similar to ruby: developed 1973
• 4 level system
• Transition to wide range of bands: 700-820 nm
• Creates a tunable laser
Tunable Alexandrite Laser
• Place prism in cavity at rear
• Wavelength for proper cavity controlled by prisim
Color or F Centre Laser
• Alkali Halids form point defects from X-rays, e-beams
• Clear material becomes coloured
• Defect a cation vacancy: net positive charge
• Electron orbits this: broad absorption band
Color Centre Laser
• Optically pumped, usually by another laser
• Broad band of states so laser tuned
• eg Thallium doped KBr pumped by Nd:Yag
• Emits at 1.4 - 1.6 microns, 20% effeciency