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.
