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)
what sort of laser can be used to pump Nd:YAG lasers
high power diode lasers
what does end pumping allow for
development of compact lasers
what does side pumping allow
power scalability
Describe some of the key features of the Ti:Sapphire laser
Titanium ion embedded in a matrix of Sapphire at about 1% doping
pumped optically
output 700-1000nm
power 1W
4 level system can be CW or pulsed
what are some of the applications of the Ti Sapphire laser
multiphoton imaging, high resolution spectroscopy and laser cooling
why are Ti: Sapphire lasers called vibronic
due to a mixture of vibrational and electronic transitions
what setup will result in a tunable single frequency output from the Ti:Sapphire laser
a bowtie cavity
what are Erbium doped amplifiers important for
telecoms, especially the 1.55 micrometer band
what is the most efficient pumping for erbium doped fibre amplifiers
close to the lasing wavelength around 980nm
ions excited then decay non radiatively before lasing at 1550nm
what is in band pumping
a longer wavelength of pumping is used to put ions in the top of the maifold, then they rapidly decay non radiatively to the bottom of the manifold
compare the upper state lifetimes for erbium doped fibre amplifiers to the higher lying levels
the upper state lifetime is around 10ms, all higher levels have at most microseconds because of fast photon decay
what does the bandwidth for gain depend on for erbium:glass
the pumping rate
what happens at long wavelengths in erbium doped fibre amplifiers
we have positive gain for low pumping levels (4 level characteristic) and lasing occurs at the top of the manifold which has a low population and rapidly relaxes to the bottom
what happens at shorter wavelengths in erbium doped fibre amplifiers
very high pumping rate is required for positive gain (3 level characteristic)
lasing occurs to the bottom of the manifold which is relatively populated
at what pumping ration (N2/N1) can sufficient net gain around 1550 nm be achieved in fibres a few metres long
0.8
how is a flat gain region achieved in EDFAs for telecoms applications
insert gain flattening filters after the amplifier sections of fibre which lower the high gain regions to lower values
what assumptions are made in describing the small signal gain for a fibre laser by the 4-level system
the gain cross section over the range of interest is half its peak value
the mode area for pump and laser radiation is assumed equal to the core area
V is chosen to be a maximum of 2.4 indicating single mode operation
what is a wavelength division multiplexer
a device which allows two wavelengths to be mixed (the input signal and the pump light source)
describe how a EDFA might be fabricated using a modified chemical vapour deposition technique
volatile erbium compound is introduced with GeCl4, SlCl4 and oxygen
erbium is deposited as a component of the soot
dopant concentration and profile can be carefully controlled via building up layers of specified composition
OR a solution containing the dopant is flown through the tube with the normal soot layer and then flushed out, applying heat leaves traces of the erbium in the soot voids
more heat compacts the coating to tube wall and collapses the tube to a solid preform
what problem had to be overcome to achieve high power fibre lasers and what was the solution
high powers needed to pump a fibre laser need to come from divergent laser diode arrays which could not be focussed onto a small diameter fibre end by end pumping
solution - cladding pumping
where are high power optical fibre lasers applied
industrial cutting and welding (lower wavelengths result in better absorption in metals)
What does the design of a cladding pumped laser look like
two cladding layers with the intermediate layer having n somewhere between the core and the outer cladding
with a reasonable thickness of around 100mm, the intermediate layer will accept lots of the pump power from a divergent laser diode array
all light confined by outer cladding will intercept at some point the core
a powerful oscillator can be formed if reflectors are added to the ends of the fibre
how are claddings designed for cladding pumping
because some linearly polarised modes have null intensity on the axis and the mode profile in the cladding is the superposition of the modes a core placed on the axis will not recieve optimal pumping
need to break the symmetry
What are the two main outcomes of a fbre laser
a high power output and takes a poor quality laser diode mode and turns it into a high quality spatial mode
what is the most important material for cladding pump laser systems
ytterbium
the 915nm band is suited to semiconductor laser pumping
and no higher energy levels until UV means internal loss due to absorption of excited states is minimised
