Laser Examples Flashcards
Describe some key characteristics of the Helium Neon Laser
active medium 90% He 10% Ne
pumping is electrical discharge
output is 632 nm or 1.15.or 3.39 micrometers
power 1-10mW
cost 100-1000’s
4 level inhomogeneously broadened
how does the population inversion arise HeNe laser
occurs in the neon atoms
neon indirectly excited
electrons collide to excite the He atoms in long lived 21s and 23s states
collisions between helium and neon excite neon into the 2s and 3s states
population inversion established between 3s/3p and 2s/2p states in neon
stimulated emission gives gain
2p and 3p lifetimes are short, they quickly decay to 1s
neon colliding with the walls return to the ground state
why is it sometimes convenient to contain the gain material inside another container
this protects the mirrors from the medium and enables them to be easily adjusted
what does the importance of the wall mean in the helium neon laser
cannot operate at large tube diameters of high gas pressures
what are brewster windows
cut at the brewster angle, they exhibit no reflection for one of the polarisation states, the other state is reflected and suffers increased loss
what happens to the output of lasers with brewster windows
linearly polarised
describe some key characteristics of argon ion lasers
the active medium is ionised argon atoms
pumped by electrical discharge
output is 514 and 488 nm
power 1-10W
cost 10’s of thousands
4 level inhomogeneous
where is the population inversion in the argon ion laser
in 2 levels in the ionised argon
how do argon ion lasers incorporate solenoids
the high currents requires a solenoid to contain the plasma in the centre of the tube
how are the argon ion lasers cooled
using heat exchangers and a water jacket
how does the population inversion arise in the argon ion laser
through electron collision within the discharge, ionisation takes place
collisions excite the ions to the 4p state
population inversion created between the 4p and 4s states
4s state has a short lifetime and decays to ground state giving off UV light
ion recaptures an electron and returns to atomic ground state
how can you select which lasing transition to operate the argon ion laser on
an internal prism
describe the key features of the laser diodes
the active medium is the direct bandgap between the semiconductor
pumping is electrical
output is 670-1300nm
power 1mW-1W
large cost range from 1-20000
4 level
active region covers interface and output beam is rectangular/elliptical
no need for external mirrors due to reflectivity from refractive indices
what happens in the laser diode when a forward bias is applied
active region forms with electrons and holes
recombination occurs and photons are emitted
the small active region and waveguide effect is small so photons diverge away from active region where gain is achievable
we need to feedback some of emitted light for a laser so the diode must have a high threshold current
how do we reduce threshold in a laser diode with forward bias
guide the photons along junction
restrict width of interface to achieve high current density but low current - heterojunctions
how is the width restricted in a heterojunction
use a narrow electrode or increase doping for increased resistance
describe some of the key features of the Nd:YAG laser
active medium is Neodymium impurity in yttrium aluminium garnet
optical pumping
output is 1064nm
power level up to 50W CW
cost 1 to 10 k
4 level homogeneous (Nd in glass inhomogeneous)
