Solid State Lasers • Was first type of laser (Ruby 1960) • Uses a solid matrix or crystal carrier • eg Glass or Sapphire • Doped with ~1%-0.001% transition metal or rear earth ions • eg Chromium (Cr) or Neodynmium (Nd) • Mirrors at cavity ends (either on the rod or separate) • Typically pumped with light • Most common a Flash lamp • Newer ones pumped by laser diodes (more efficient) • 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 • Most solid states use Q switching to increase pulse power • Block a cavity with controllable absorber or switch • Acts like an optical switch • During initial pumping flash pulse switch off • Recall the Quality Factor of resonance circuits (eg RLC) 2π energy stored Q = 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 (1− R R )E = 1 2 τ c τ r 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 τ 1 c = 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 Et f E Elost / cycle = = τ c fτ c where f = frequency • Thus the cavity's Q is 2πE 2πE Q = = = 2πfτ c Elost / cycle ⎛ E ⎞ ⎜ ⎟ ⎝ fτ c ⎠ • Thus for a laser cavity: 2π fτ r 4π fL 4π L Q = 2π fτ c = = = ()1 − R1R2 c()1 − R1R2 λ()1 − R1R2 • 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). f Q = ∆f • Thus lifetime relates to the bandwidth 1 ∆f = 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 4 • None radiative to F3/2 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 • Diodes very efficient and λ tuned to max absorption of YAG • Result: increase YAG efficiency for <5% to >50% • Diode laser light can be carried by fiber optic to YAG cavity • Means heat losses and power supply separate from laser Frequency Doubling & Higher Harmonics • Nd:Yag is often run as frequency doubled or higher laser • Generates visible or UV light that way • Works due to non-linear optical effects in materials • Called Second Harmonic Generation or frequency doubling • Certain crystals have non-linear relation between E field polarization & applied E fields • At high laser power E field from light causes effect • Polarization P of the light becomes 2 3 P = ε 0 (χ1E + χ2 E + χ3E + K) • Where χ1 is the linear polarization, χ2 second order polarization • Thus when a sine wave photon is applied then E = E0 sin()ωt 2 2 3 3 P = ε 0 (χ1E0 sin()ωt + χ2 E0 sin (ωt) + χ3E0 sin (ωt) + K) ⎛ 1 2 ⎞ P = ε 0 ⎜ χ1E0 sin()ωt + χ2 E0 []1− cos()2ωt + K⎟ ⎝ 2 ⎠ • Thus get both fundamental and 2nd harmonic light out Frequency Doubling • Direct high power laser light at 2nd harmonic or higher crystal • Done outside of laser cavity • Generates visible or UV light that way • For Nd:Yag get λ=1064 nm & λ2=1064/2=532 nm • Filter out fundamental and get a 2nd harmonic laser out in green • Get ~70% efficiency of conversion for green • 3rd harmonic 354 nm in UV much lower ~30-40% • 4th harmonic use two doubling crystals 266 nm ~15% efficient • 5th harmonic use 2nd & 3rd type crystals get 213 nm at ~6% • Typical crystals KTP, Lithium Niobate • Crystals have finite lifetime ~ few years depending on usage Typical Nd: Yag laser parameters Typical Nd: Yag laser parameters Alexandrite Lasers 3+ • Alexandrite: Cr : 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 .