Lasers and Energy-Assisted manufacturing processes

What does LASER stand for?

Light Amplification by the Stimulated Emission of Radiation

What are the 4 main properties of laser light?

1. Monochromaticity

2. Collimation

3. Coherence

4. High energy density

What is monochromaticity?

Laser light consists of a single wavelength (colour), with a very narrow wavelength range compared to conventional light sources.

What are the three line broadening mechanisms?

1. Collisional (or pressure) broadening

2. Natural dampening

3. Doppler broadening

Collisional broadening

Distortions in energy levels

Natural damping

Stems from finite transition times between energy levels

Doppler broadening

Differences in frequencies as atoms travel towards or away from observer

Collimation

Laser light travels in parallel rays with very low divergence, allowing the beam to spread minimally and maintain its intensity over long distances.

Coherence

Laser light waves maintain a constant phase relationship in space and time

High energy density

Concentration of optical power per unit area, typically expressed as power density (W/cm²) or irradiance

Common Types of Laser for Materials Processing

Gas lasers

Solid state lasers

Diode lasers

Schematic of a typical laser

Generating a laser beam steps

1. Energy is put into active medium to excite atoms/molecules

2. Different wavelength emitted by spontaneous emission in all directions

3. Stimulated emission builds up along tube axis to excite other atoms/molecules

   1. Radiation is amplified and an amount emitted as a laser beam

CO2 gas laser wavelength

10.6 μm

CO2 gas laser applications

Materials processing

CO2 gas laser power

Few W to 40kW CW

Excimer gas laser wavelength

193 nm, 248 nm, 308 nm

Excimer gas laser applications

Micro-machining

Excimer gas laser power

Avg. up to 1kw and 60MW peak

Fibre solid-state laser wavelength

1.07-1.08 μm

Fibre solid-state laser applications

Materials processing

Fibre solid-state laser power

Few W to 50kW

Nd:YAG solid-state laser wavelength

1.064 μm, 532 nm, 366 nm

Nd:YAG solid-state laser applications

Material processing

Nd:YAG solid-state laser power

Avg. up to 2kW, max pulse frequency 50kHz

Diode semiconductor diode lasers wavelength

Visible to IR (< 1μm)

Diode semiconductor diode lasers applications

Pump light source, material processing

Diode semiconductor diode lasers power

Few mW to 10kW

CO₂ lasers include

CO₂:N₂:He (1:2:8)

Why is nitrogen added to CO2 lasers

to increase pumping to upper CO2 level

Why is Helium added to CO₂ lasers

To depopulate CO₂ back to E₀

Advantages of CO₂ lasers

• High average power

• Well established technology

• Suitable for CW or pulsed

Disadvantages of CO₂ lasers

• Low absorption by metals

• Non transmittable through optic fibres

• Beam size limited by wavelength

• Large devices

Applications of of CO₂ lasers

Cutting, welding, hardening, cladding

Excimer laser

Uses an electrical discharge in a gas mixture to produce powerful, short UV laser pulses.
Pulse: ~20 ns

• Pulse: ~20 ns

• Power: ~35 MW

• Wavelength range: ±0.4 nm

• High gain: Resonant cavity may not be required.

Advantages of an excimer laser

• Range of potential wavelengths

• High absorption

• Can break organic bonds of polymers (photochemical)

• Ideal for fine detailed work (e.g. micromachining)

Disadvantages of an excimer laser

• Use of corrosive gases - safety and construction implications

• Sealed system require periodic refilling due to gas degradation

Applications of an excimer laser

Cleaning, micro drilling, polymer machining

Nd:YAG

• Active medium: Neodymium-doped YAG crystal, producing Nd³⁺ ions

• Pumping: Flash lamp provides pump photons

• Efficiency: Low, as only a narrow wavelength band causes transitions

• Drawbacks: High temperatures and short flash-lamp lifetime (~1000 h)

• Power: Very high powers possible using multiple coupled fibres

• Example: Vulcan laser reaches ~10¹⁵ W peak power

Advantages of Nd:YAG Lasers

• Very high peak powers possible

• Good absorption by metals

• Transmittable by fibre optics

• Can operate pulse switched or CW

Disadvantages of Nd:YAG Lasers

• Not suitable for thicker materials

• Low average power

Applications of Nd:YAG Lasers

More commonly: Drilling, marking, cutting

Less commonly: Welding, hardening, cladding

Fibre laser

Active medium: Doped glass or plastic fibre

Pumping: Diode laser, end- or side-pumped

Waveguiding: Refractive index variation confines light

Bragg grating: Creates oscillations within the fibre

Core: Can be as narrow as ~100 μm

Key point: One of the fastest-growing laser technologies

Advantages of fibre lasers

• Good beam quality

• High power

• Good absorption by metals

• Small robust devices

• Low maintenance

Disadvantages of fibre lasers

• Beam size limited by wavelength

• Processing speed can be slow

Applications of fibre lasers

Drilling, marking, cutting, welding, hardening, cladding

Diode laser

Active medium: Electrons in the conduction band

Laser emission: Electrons fall to the valence band, emitting photons

Energy: Photon energy = hνh\nu

Tuning: Current and temperature allow limited wavelength tuning

Optical cavity: Cleaved ends act as mirrors

Key point: Most common laser technology, mainly for low-power applications

Advantages of Diode lasers

• Relatively cheap

• Easily stackable → kWs from mug sized devices

• Small, robust, reliable

Disadvantages of Diode lasers

• Very high divergence (30 - 40°) - requires collimating optics

• Suffer from age related red-shift

Applications of Diode lasers

Often used as pump lasers, plastic welding, conduction welding, hardening

What are the seven main components of an industrial laser processing system?

1. Laser Device / Source (plus power supply and pumping systems).

2. Cooling System (chiller mechanism and pumps).

3. Extraction Unit (removal of gases and ejected workpiece debris).

4. Motion System (actuators for beam or workpiece delivery).

5. Control Unit (central computer controller).

6. Safety & Interlock Systems.

7. Process-Specific Ancillaries (such as shielding gas or heated beds).

IR wavelength range

1500nm - 1mm

Visible light wavelengths range

400nm - 800nm

UV wavelength range

100nm - 400nm

Absorption produces

Heat, Luminescence, Photochemical reactions

Transmission

Amount of radiation will be transmitted by an absorber depending on: material type, wavelength

For opaque materials: Reflectivity =

1 - Absorptivity

For transparent materials: Reflectivity =

1 - (Transmissivity + Absoptivity)

Rayleigh scatter

Scatter from small particles (<< λ). Gives uniform scatter direction but is highly wavelength dependent. Shorter wavelengths are scattered more.