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

Nd-YAG Construction

ME 677: Laser Material Processing Instructor: Ramesh Singh

• 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

Diode Pumped Solid State Lasers

• Tiny fraction of the power is absorbed by Nd3+
• Waste heat causes distortion
• Diode lasers have high wall plug efficiency and

good coupling with Nd3+

ME 677: Laser Material Processing Instructor: Ramesh Singh

good coupling with Nd

Diode Lasers

• Similar to LED
• Difference in Fermi energy in conduction and

valence band at p-n junction

• Photons can be emitted

ME 677: Laser Material Processing Instructor: Ramesh Singh

• Photons can be emitted
• Stacked configuration

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

ME 677: Laser Material Processing Instructor: Ramesh Singh

without lasing

• Pumped by diode lasers

Advantages

• High beam quality
• High wall plug efficiency
• Portability

Long life

ME 677: Laser Material Processing Instructor: Ramesh Singh

• Long life

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/cm2
• Optics

– Fused silica,

• Gas 4-5 MPa

ME 677: Laser Material Processing Instructor: Ramesh Singh

• Gas 4-5 MPa

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 Laser Nd:YAG CO2 Disc Wall Plug Efficiency 30% ~ 5% ~10% 15% Output Powers to 50kW to 6kW to 20Kw to 4kW BPP (4/5 kW) < 2.5 25 6 8 Life 100,000 10,000 N.A. 10,000

ME 677: Laser Material Processing Instructor: Ramesh Singh

Life 100,000 10,000 N.A. 10,000 Cooling Air/water water water water Floor Space (4/5 kW) < 1 sq. m 6 sq. m 3 sq. m 4 sq. m Operating Cost $21.31 $38.33 $24.27 $35.43 Maintenance Not Required Often Require d Often

Market Share

ME 677: Laser Material Processing Instructor: Ramesh Singh

Laser-Summary

• Types of Lasers
• Optical cavity Design
• Cooling

Comparative study

ME 677: Laser Material Processing Instructor: Ramesh Singh

• Comparative study

Laser Optics-I

ME 677: Laser Material Processing Instructor: Ramesh Singh

Laser Optics-I

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 is Planck’s constant

p h = λ

ME 677: Laser Material Processing Instructor: Ramesh Singh

– 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.

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/m3) which absorb light – No. of photons absorbed is equal to the photons in the opaque area, is the opaque area in m2 of each particle

ME 677: Laser Material Processing Instructor: Ramesh Singh

– is the opaque area in m2 of each particle – Fraction of photons absorbed = Opaque area/Total Area

• r, σ N A dz/A =dI(z)/I(z)

– I(z) @z=0 = I0

Derivation of Beer Lambert’s Law

σ σ

σNz z z z

e I z I Nz I z I Ndz I z dI

−

= − = − =

• )

( ) / ) ( ln( ) (

A

dz

I(z)

y x

ME 677: Laser Material Processing Instructor: Ramesh Singh

intensity and wavelength medium

• f

function a is β

βz

e I z I

−

= ) (

A x

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

ME 677: Laser Material Processing Instructor: Ramesh Singh

• 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)

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

ME 677: Laser Material Processing Instructor: Ramesh Singh

• Vapor has little capability of photon absorption but plasma

again has the capability due to presence of free electrons

Absorption

• The electron density in plasma is given by

Saha Eqn. where Ni = ionisation density; No = density

ME 677: Laser Material Processing Instructor: Ramesh Singh

where Ni = ionisation density; No = density

• f atoms;

Vt = ionization potential, eV; T = absolute temperature, K. This indicates that temperatures of the

• rder of 10,000-30,000°C are required for

significant absorption

Sequence of Absorption

• Energy Intensity

104 W/mm^2 103 W/mm^2

ME 677: Laser Material Processing Instructor: Ramesh Singh

105 W/mm^2

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

ME 677: Laser Material Processing Instructor: Ramesh Singh

– Some of the key effects are presented in the next slides