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E 3 ,N 3. Short lived state ( t ~10 -8 s). E 2 ,N 2. Metastable state ( t ~10 -3 s). Optical pumping by intense light source. hf 12. hf 13. hf 12. hf 12. E 1 ,N 1. Ground state. Lecture 7: Lasers and their applications (II). Requirements for Laser action (Y&F 38.6):.
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E3,N3 Short lived state (t~10-8 s) E2 ,N2 Metastable state (t~10-3 s) Optical pumping by intense light source hf12 hf13 hf12 hf12 E1 ,N1 Ground state 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: P1X: Optics, Waves and Lasers Lectures, 2005-06.
Small fraction of light escapes Mirror “Leaky mirror” (partially silvered) ~0.1% transmission • Requirements for laser action: • Pumped light must not matchE2-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). • 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.
N3 Short lived (~10-8 s) E3 DE23=0.48 eV hf13 Metastable (2 ms) N2 E2 l=694.3 nm (red) DE12=1.78 eV Optical Pumping l=550 nm (green) DE13=2.26 eV hf12 hf12 hf12 E1 N1 Ground state 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. P1X: Optics, Waves and Lasers Lectures, 2005-06.
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.
He atom Ne atom 20.61 eV 20.66 eV 5s 20.30 eV 4s 2s 19.78 eV 632.8 nm 4p 18.70 eV Discharge Collisions 3p 18.37 eV 16.70 eV 3s 0 eV • 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: (red) • 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.
f • 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): • Spacing between frequencies: • 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. P1X: Optics, Waves and Lasers Lectures, 2005-06.
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.
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.
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.
