Industrial Laser Design Flashcards
What does LASER Stand for?
Light Amplification by the stimulated emission of radiation
What are the 4 main properties of LASER?
Monochromaticity
Collimation (beam Narrowing)
Coherence (EM waves with fixed relationship - destructive or constructive)
High Energy Density
why is it useful to have lasers that emit at different wavelengths?
Can cut or engrave etc.
How is a laser beam generated?
- Energy is put into active medium to excite atoms.
- Difference Wavelengths are emitted by spontaneous emission in all directions.
- stimulated emission builds up along tube axis to excite other atoms.
- Radiation is amplified and an amount emitted as a laser beam
How is light emitted?
When electrons change energy levels. This is the change in energy/Plancks constant.
What are the types of photon absorption and emission?
On a 2 level energy system, Stimulates absorption, spontaneous emission or stimulated emission.
Spontaneous is far more likely that stimulated emission at thermal equilibrium.
What is stimulated emission?
Incident photon must have correct energy (hv) for electron - monochromaticity.
Emitted photon must have the same wavelength and directionality as incident photon - collimation.
Emitted photo is also in phase with incident photon and so they are constructive - coherence.
Materials are more like to absorb photons than emit. Need to create a population inversions where N2>N1. but in thermal equilibrium N1Β»N2.
Over what distances is a laser collimated?
Infinitely.
What is population inversion?
Population inversion is where it is necessary to excite or pump the material into a non-equilibrium distribution.
Light emission by stimulated emission wouldnβt be possible without this.
What is required for population inversion?
- Optical radiation
- Electrical discharge
- Passage of current
- Electron bombardment
- Release of chemical energy
What is population inversion controlled by?
the Boltzmann distribution:
π2/π1=e[ββπ/ Ξ»ππ] where N2 can never exceed N1.
What is population pumping?
If N1=N2, the rate of stimulated
emission and stimulated absorption
are equal
* Therefore population inversion
cannot be achieved with a simple
two energy level system.
* Most pumping utilise either three or
four level systems
* Decay into radiative state must be
fast (e.g. lifetimes of 10-9 seconds)
* Radiative state should be
metastable β lifetimes of the order
10-3 seconds
What is an optical resonator?
For spontaneous emission to build,
stimulated emission must be sustained.
* Radiation confinement can be obtained
by using a mirror arrangement to
produce an optical resonator
* Mirrors near 100% reflectivity
* Amplitude grows until steady state
reached
* Further growth of wave amplitude within
cavity appears as laser output - high energy density
* Mirror quality (high reflectance, low
absorption, optical flatness, etc.) is very
important.
Where are there losses in the optical oscillator system?
β Transmission at the mirrors
β Absorption and scattering by the mirrors
β Diffraction around the boundary of the mirrors
β Non desired absorption in the laser medium (similar
energy separations)
β Scattering
What is the threshold gain coefficient?
Measure of the amplification of light in the laser medium whilst overcoming the equilibrium between losses and gains.
Final irradience/ Initial Irradience>1<π
1π
2π
2 πβπΎ πΏ
Where R is the mirror reflectance, with a seperation of , k is the gain coefficient and y is the effective volume loss coefficient.
What and why are three line broadening mechanisms required?
As lasers do not produce just one wavelength.
- Collisional (or pressure) broadening β distortions in energy levels
- Natural damping β stems from finite transition times between energy levels
- Doppler broadening β differences in frequencies as atoms travel towards or
away from observer
What are CO2 Lasers?
CO2:N2:He (1:2:8)
* Nitrogen added to increase pumping to upper CO2 level
* Helium added to depopulate CO2 back to E0
* Pumping by electric discharge (either CW or pulsed β DC or RF power switching)
Advantages/disadvantages of CO2 Lasers?
Advantages:
* High average power
* Well established technology
* Suitable for CW or pulsed (mostly CW)
Disadvantages
* Low absorption by metals
* Not transmittable through optic fibres
* Beam size limited by wavelength (generally a few
hundred microns)
* Large devices
Applications: cutting, welding, hardening, cladding
What is an Excimer Laser?
Electric discharge in
a gas mixture of, for example,Kr,F2, Ne and He, at around 4 atm.
* Generates UV photons for the brief period of discharge(usually about 20 ns)
* WavebandΒ± 0.4nm
* Short pulses are very powerful
~35 MW
* Gain is so high that oscillation chamber not always required
Advantages/Disadvantages of Excimer Laser?
Advantages:
* Range of potential wavelengths
* High absorption
* Can break organic bonds of polymers (photochemical)
* Ideal for fine detailed work (e.g. micromachining)
Disadvantages
* Use of corrosive gases β safety and construction
implications
* Sealed system requires periodic refilling due to
* gas degradation
Applications: cleaning, micro drilling, polymer machining
What is the Nd:YAG Laser?
Neodymium doped in YAG (Yttrium
Aluminium Garnet) βproduces Nd3+ ions
* Flash lamp used to provide pump photons, but relatively wide band means it is inefficient (only narrow band provides electron transitions) leading to high temperatures
* Flash lamp lifetimes are short ~1000 h
* Very high powers achievable through multiple coupled fibres
1015 W peak power available from the Vulcan laser at RAL, Didcot
Advantages / Disadvantages of the Nd:YAG Laser?
Advantages:
* Very high peak powers possible
* Good absorption by metals
* Transmittable by fibre optics
* Can operate pulse switched or CW
Disadvantages:
* Not suitable for thicker materials
* Low average power
Applications:
* More commonly: Drilling, marking, cutting
* Less commonly: welding, hardening, cladding
What is a fibre laser?
Fastest growing laser technology
* Doped plastic or glass fibres that
are end- or side-pumped by diode
lasers
* Use of variation of refractive
index:
β For wave guide
β For Bragg grating to generate
oscillations
* Core can be as narrow as 100 ΞΌm
Advantages / Disadvantaged of Fibre Laser?
Advantages:
* Good beam quality
* High power
* Good absorption by metals
* Small, robust devices
* Low maintenance
Disadvantages:
* Beam size limited by wavelength
* Processing speed can be low
Applications: Drilling, marking, cutting, welding,
hardening, cladding
What is a diode laser?
The excited state is that of
electrons in a conduction band
* Electrons jumping from conduction to valance bands emits photon with
energy, hΞ½
* Current and temperature tuneable over
a short range
* Cleaved ends act as mirrors for optical cavity.
* Most prevalent laser technology (low power applications)
Advantages / Disadvantages of 1.2.5 Diode Lasers?
Advantages:
* Relatively cheap
* Easily stackable β kWs from mug sized devices
* Small, robust, reliable
Disadvantages:
* Very high divergence (30 - 40Β°) β Requires
collimating optics
* Suffer from age-related red-shift
* Applications: Often used as pump lasers, plastic
welding, conduction welding, hardening
What is the major problem associated with trying to use an array of diode lasers
to perform micromachining of a material?
Beam overlap and uniformity of power distribution. Diode lasers are very wide angled and should be able to be precisely controlled both of which diodes are not suited for.
What are other uses of the laser systems efficiency?
Not only with the cooling system and extraction units running as well as the laser there can be losses her from the moving and electrical losses.
