Nd-YAG Construction Nd-YAG laser can be passed through barium borate - PowerPoint PPT Presentation

Nd-YAG Construction • Nd-YAG laser can be passed through barium borate (BBO) or lithium niobate (LBO) crystals can yield 530 nm • This process is called frequency doubling ME 677: Laser Material Processing Instructor: Ramesh Singh

Diode Pumped Solid State Lasers • Tiny fraction of the power is absorbed by Nd 3+ • Waste heat causes distortion • Diode lasers have high wall plug efficiency and good coupling with Nd 3+ good coupling with Nd ME 677: Laser Material Processing Instructor: Ramesh Singh

Diode Lasers • Similar to LED • Difference in Fermi energy in conduction and valence band at p-n junction • Photons can be emitted • Photons can be emitted • Stacked configuration ME 677: Laser Material Processing Instructor: Ramesh Singh

Diode Pumped Fiber Lasers • A laser in which the active gain medium is an optical fiber doped with rare-earth elements – Erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium • Doped fiber amplifiers provide light amplification without lasing without lasing • Pumped by diode lasers ME 677: Laser Material Processing Instructor: Ramesh Singh

Advantages • High beam quality • High wall plug efficiency • Portability • Long life Long life ME 677: Laser Material Processing Instructor: Ramesh Singh

Wavelengths of Solid State Lasers ME 677: Laser Material Processing Instructor: Ramesh Singh

Functioning of Excimer Laser • Excitation by 35-50 kv pulse • Current density up to 1 kA/cm 2 • Optics – Fused silica, • Gas 4-5 MPa • Gas 4-5 MPa ME 677: Laser Material Processing Instructor: Ramesh Singh

Comparison Between Lasers -Power ME 677: Laser Material Processing Instructor: Ramesh Singh

Efficiency-1 ME 677: Laser Material Processing Instructor: Ramesh Singh

Capital Cost • 1987 data ME 677: Laser Material Processing Instructor: Ramesh Singh

Operating Cost • 1987 data ME 677: Laser Material Processing Instructor: Ramesh Singh

Comparison with Fiber Properties of various lasers (courtesy IPG Photonics) Properties Fiber Nd:YAG CO 2 Disc Laser Wall Plug 30% ~ 5% ~10% 15% Efficiency Output Powers to 50kW to 6kW to to 20Kw 4kW BPP (4/5 kW) < 2.5 25 6 8 Life Life 100,000 100,000 10,000 10,000 N.A. N.A. 10,000 10,000 Cooling Air/water water water water Floor Space (4/5 < 1 sq. m 6 sq. m 3 sq. m 4 sq. kW) m Operating Cost $21.31 $38.33 $24.27 $35.43 Maintenance Not Often Require Often Required d ME 677: Laser Material Processing Instructor: Ramesh Singh

Market Share ME 677: Laser Material Processing Instructor: Ramesh Singh

Laser-Summary • Types of Lasers • Optical cavity Design • Cooling • Comparative study Comparative study ME 677: Laser Material Processing Instructor: Ramesh Singh

Laser Optics-I Laser Optics-I ME 677: Laser Material Processing Instructor: Ramesh Singh 1

Outline • Electromagnetic Radiation • Laser-Matter Interaction • Nonlinear Optics ME 677: Laser Material Processing Instructor: Ramesh Singh 2

Nature of Electromagnetic Radiation • Wave-particle duality – de Broglie hypothesis relates wavelength (λ), and momentum (p): h λ = p – h is Planck’s constant – h is Planck’s constant – momentum of a photon is given by p = E/c wavelength by λ = c/ � where c is the speed of light in vacuum. ME 677: Laser Material Processing Instructor: Ramesh Singh

Photon Properties of Different Lasers ME 677: Laser Material Processing Instructor: Ramesh Singh

Interaction of Electromagnetic Radiation with Matter • When electromagnetic radiation strikes a surface – Relection/Absorption/Transmission – Absorption is governed by Beer Lambert’s Law ME 677: Laser Material Processing Instructor: Ramesh Singh

Beer Lambert’s Law • z as an axis parallel to the motion; A and dz are the area and thickness, respectively – dz is sufficiently small that one particle in the slab cannot obscure another particle – N is the concentration of opaque particles in the slab (particles/m 3 ) which absorb light – No. of photons absorbed is equal to the photons in the opaque area, – � is the opaque area in m 2 of each particle � is the opaque area in m 2 of each particle – Fraction of photons absorbed = Opaque area/Total Area or, σ N A dz/A =dI(z)/I(z) – I(z) @z=0 = I 0 ME 677: Laser Material Processing Instructor: Ramesh Singh

Derivation of Beer Lambert’s Law z z = � ( ) dI z � dz − σ Ndz I z 0 0 y = − σ ln( ( ) / ) I z I Nz I(z) 0 − σ Nz ( ) = I z I e A x x 0 0 A − β z ( ) = I z I e 0 β is a function of medium wavelength and intensity ME 677: Laser Material Processing Instructor: Ramesh Singh

Laser-Matter Interaction • If the frequency does not correspond to natural frequency absorption does not occur • Forced vibration induced by electric field, E is small and incapable of vibrating an atomic nucleus The energy transmission occurs due to • photons interacting with free or bound electrons • The process of photons being absorbed by The process of photons being absorbed by electrons is called inverse Bremsstrahlung effect • This could result in either re-radiation in all directions or the electron is bound by lattice phonons (bonding energy in solid or liquid structure) ME 677: Laser Material Processing Instructor: Ramesh Singh

Laser-Matter Interaction • The phonons will cause the structure to vibrate • The vibration is transmitted by diffusion type process q=-kAdT/dx • If sufficient energy is absorbed the vibration intensifies and molecular bonds could be broken which can lead to melting • Vapor has little capability of photon absorption but plasma • Vapor has little capability of photon absorption but plasma again has the capability due to presence of free electrons ME 677: Laser Material Processing Instructor: Ramesh Singh

Absorption • The electron density in plasma is given by Saha Eqn. where Ni = ionisation density; N o = density where Ni = ionisation density; N o = density of atoms; V t = ionization potential, eV; T = absolute temperature, K. This indicates that temperatures of the order of 10,000-30,000°C are required for significant absorption ME 677: Laser Material Processing Instructor: Ramesh Singh

Sequence of Absorption • Energy Intensity 10 3 W/mm^2 10 4 W/mm^2 10 5 W/mm^2 ME 677: Laser Material Processing Instructor: Ramesh Singh

Nonlinear Optics • First observed in 1961 Peter Franken at the University of Michigan – Ruby laser passing through a quartz crystal gave rise to UV radiation – Some of the key effects are presented in the next – Some of the key effects are presented in the next slides ME 677: Laser Material Processing Instructor: Ramesh Singh