Unit 4 Lasers Flashcards
What is a LASER, difference btw ordinary and laser
light amplification stimulated emission radiation
ORDINARY
not very intense
Moderatly Monochromatic
non directional
High divergence
Moderately coherant
LASER
very intense
Highly Mono Chromatic
Directional
Low Divergence
Highly Coherent
(phi diff neither equal not constant)
HOW is light produced
interaction of atoms with EM radiation
1) Absorption
2) Spontaneous Emission
3) Stimulated emission(Einstien)
atom-> discrete energy levels
Spontaneous: makes atom go to higher energy/absorption once it happens 2 things can happen: if u dont interfere it drops and E is released in terms of photons
hv + E1=E2
2) when photon projected at atom it forces atom to jump down(photon not absorbed by atom) and hence it also releases hv, thus
hv+E2=E1+2hv(Stimulated emission)
properties of laser
1) Monochromaticity (spectral line broadening)
Light from a laser typically comes from an atomic transition with a single precise wavelength. So the laser light expected to have single spectral colour
laser light is highly
monochromatic but not truly monochromatic!
2) Coherance
In the stimulated emission the emitted photons are “in phase”
phase difference btw 2 points remain constant in time
temporal coherance
If the electric fields of a laser are sampled at different times and
if the samples exhibit a well defined phase correlation, then the
laser is said to be temporally coherent
Spatial coherence:
* When EM fields at different spatial regions have a phase correlation (either zero or a constant phase difference), the beam is said to be
spatially coherent
phase difference at any location remains constant for constant intervals of time
- Divergence (directionality): very low divergence
- Intensity
* The high intensity of a Laser arises out of the properties of
Monochromaticity, Coherence and Low divergence
A lot of energy is concentrated in a small cross sectional area
High power IR lasers are used to cut metals
Helium Neon laser
1) Material He and Ne 10:1
2) pumping -> electrical (using e-) DC source
3) Resonant Cavity: glass plates(Brerusters window)=> to polarize light
3s →2p⇒632.8 nm,
3s → 3p ⇒ 3391.2nm,
2s → 2p⇒1152.3nm
Absorption levels in the He atoms and lasing levels in the Ne atomic transitions
pumping starts from He
Energetic electrons in the discharge excite electrons in the He atoms to 23
S and 21S states which are meta-stable
Excited He atoms transfer their energy to Ne atoms by collisions and excites the Ne atoms to 2s and 3s levels (Energy levels of these Ne states are close
to the He excited states -This process is referred to resonant energy transfer)
This create population inversion
Ne atoms de-excites to the ground states through the intermediate states 3p and 2p
Transition from 3s to the 2p states gives the
characteristic red laser (wavelength of 632.8 nm
3s to 3p and 2s to 2p lines give rise to wavelengths
in the Infra red of 3.39 μm and 1.152 μm
WORKING
apply 1000V
e get energised in their journey from cathode to anode
Energetic e bombard He then exciting it e+He=e+He
Energetic He bombard Ne thus exciting it He+Ne=He+Ne
- low power laser
ISA if Ne produced laser why He?
helps in population inversion, uses lesser energy
Why do u need methane
will absorb IR via rotational /vibrational
CH4 absorbs 99% IR
Uses four-level pumping scheme
Lasing operates in continuous wave mode
Einstien coefficients (probability measures)
ISA! what are these coefficients for
=> for atomic probability
=> each energy level, atom depends on 2 energy level
A –> A21
B –> B12, B21 are EINSTIENS COEFFICIENTS
Rate of absorption
Ra ∝ N1
Rab ∝ E(v)
Rab=B12N1E(v)
B12 is coefficient of absorption
Rate of Spontaneous emission
R spe ∝ N2
R spe = A21*N2
A21 = coeff for spontaneous
Rate of Stimulated emission
Rste ∝ N2
Rste ∝ E(v)
Rste = B21N2E(v)
B21 is coefficient of stimulated emission
condition for lasers
Under thermal equilibrium, N 1»_space; N2 Which leads to R Ab > R StEm
RA»»R StE
N1»»N2
TO PRODUCE LASER:
Rsfe»_space;> Ra => N2»»N1
ISA =>
(population inversion) =>
getting more atoms on the higher energy level than in lower energy level
Stimulated Emission is crucial for laser emission
* Only Stimulated Emission can ensure same wavelength and phase (Coherence)
Stimulated Emission has to dominate over spontaneous
Emission and absorption during emission
*For this to happen, we must have Population Inversion
(N2>N1)
*To achieve population inversion we need an external energy
source called energy pump
*Thermal equilibrium is disrupted
two level system ISA!!
intially N1>N2
need N2>N1
has E1 (lower), E2(higher level)
by pumping we can move e- from E1 -> E2, excited level has short time, thus it will be de-excited to lower level
thus population inversion cannot be achieved
problem with 2 system: doesnt have a meta level state
meta level state: an excited energy level with higher life time
ISA !!!!
can 2 level system get population inversion
=> NO, at best can get N1=N2
Three level system
has three energy levels E1(Lower Lasting), (sharp level)
E2(Upper Lasting, meta stable),
E3 (broad level)
From pumping mechanism we can raise the atoms from E1 -> E3 excited level, lifetime is very short, this they quickly transfer to E2, thus atoms can be retained for a long time, thus population inversion is present
Thus stimulated emission of radiation takes place btw meta stable state and ground level leading to LASER light
POPULATION inversion takes a lot of energy/pumping power required
as population inversion is created btw excited level and boundary level
Requirments of Laser
Material -> atoms/molecules/ions/colloids/semi-conductor
Law required=> energy levels that support population inversion
Pumping Source: to excite atoms to a higher energy levels
uses electrical/optical currently
Resonant Cavity:
has 2 mirrors on both sides
makes sure laser bream travels in only one direction
also used for light amplification
Photons travelling in directions not perpendicular to the mirrors are not amplified
ISA! why are the mirrors there:
to increase the optical length
Consists of two mirrors of various
geometries and coatings creating standing waves
Resonant cavity must obey the condition that the gain should be greater than or less than to threshold gain!
Round trip gain
Let R1 and R2 be the reflective coefficients of the mirrors
* L, the distance between the mirrors
In a round trip even if we achieve a gain compared to the loss over billions of such trips the total gain would be significant
4 level energy level system
E1
E2(lower level, excited level)
E3(meta level state, Upper level )
E4
Raising atoms from E1 to E4, atoms then make fast transitions to E3, btw E3 and E2 laser is produced, from E2 atoms go back to E1 and the cycle repeats
Since both E2, and E3 are excited levels, pumping power required for population inversion is lower
CO2 Laser 4 level
Material - CO2, N2,He
pumping- electrical
Resonant Cavity: A specialized optical resonator
Symmetric stretching (n00)
2. Asymmetric stretching (00n)
3. Bending mode(0n0)
are quantized energies
- Symmetric stretching:
Bond lengths always equal
2 Asymmetric stretching:
Bond lengths unequal - Bending mode
WORKING
- switch on high voltage
electrons get energised - energetic electrons bombard N2,exciting it
-excited N2 bombards CO2 thus exciting Asymmetric mode
CO2(001)=>CO2(100)+radn
λ=10.6 μm
CO2(001)=>CO2(020)+radn
λ=9.4 μm
CO2(100)=>CO2(020)+radn
CO2(010) + He => CO2(000)
He* collides wiht walls thus losing energy
ISA: if Co2 produces laser why is N needed
N is meta stable thus has population inversion
ISA why is Co2 more efficient than atomic lasers,
efficiency of Co2 more efficient than He
Extensively used for welding, cutting ,drilling
Uses four-level pumping scheme - continuous wave mode
Uses four-level pumping scheme - continuous wave mode
* The active centers are CO2 molecules
* Helium (He) helps in the depopulation of lower levels, also as He has
thermal conductivity to keep temperature of CO2
low (to avoid
population in the lower level by thermal excitation).
Ruby Laser
Material Al2O3(passive)+Cr 3+
pumping: optical
Resonant Cavity:
Silver coatings of the polished ends of the rod form the mirror arrangement
The ends cut and polished at Brewster’s angle also
eliminates the need of mirrors.
wrapped around with a tub of Xe, around a rod doped with Cr
active component produces laser, passive is support system
Ruby => pulsed laser system
➢This corresponds to photons with 550 nm
(green) and 410nm (blue) wavelengths as the
absorption wavelengths
One of the intermediate energy states of Cr3+
ions is a Meta Stable
The transition from the upper absorption
energy levels to the intermediate state is
through non radiative transitions.
This leads to an increase in the population of
the intermediate state while the population of
the ground state is depleted rapidly.
Non radiative transitions
Thus, For a short duration of time the population of the intermediate state > the population of the ground state - POPULATION INVERSION
TRANSISTIONS FROM THE INTERMEDIATE STATE TO THE GROUND STATE CAN
LEAD TO PHOTONS OF THE STIMULATED TYPE leading to a LASER with a
wavelength of 694.3 nm.
Typical ruby laser pulse widths are of the order of a millisecond.
semi conductor laser
gives off light => when it is forward biased
semiconductors with a direct band gap
DIRECT BAND:ISA!!
GaAs(similar to Si) -> LED
➢ Conduction band e- can recombine directly with a hole in the valence band
=> recombination => spontaneous, stimulated;
hole doesn’t move only e- moves
INDIRECT band gap
releases heat
=>λ=885 nm (IR)
=> a barrier btw n, p
materiail
Active medium: Heavily doped direct band gap semiconductors
* Energy Pump: Electrical
* Laser Cavity: Two opposite sides of diode laser are cleaved for
emission of laser
electron meeting hole -> recombination=>
light(1.4 ev), Heat
- low cost, simple, compact and portable
- small in size
- operate at low powers
- Have high efficiency of the order of 40%
- Mass produced
LASER HOMOJUNCTION AND HETROJUNCTION
Homo junction
n, p type are heavily doped
-> Heat produced =>high
a barrier btw n, p
HETRO JUNCTION
material => AlGaAs, GaAs
(Al=> Refractive index lower compared to GaAs) (As=> Eg increases (2ev))
=> Charge confinement:
GaAs active layer =>
lower band gap than AlGaAs layers
GaAs => recombination occurs here
=>High efficiency by
Charge confinement
=> energy pit=> more recombination
ISA=> what produces laser/name of mechanism => double Hetro Junction
Essential features of laser
1) mono chromatic,
degree of mono chromacity = Δv/v
Δv= spread of f, v=central f
-> line widh => Δv
CAUSES OF BROADNING:
1) Natural broadening of energy gap
>Doppler shift ->gas lasers >Collisional broadening:
interruption in radn, when atom collides a atom releasing radn
2) Coherence:
ISA numerical = Zc = c/Δv
Zc-coherence length
3) gain curve=>frequency comb-> series of discrete equally spaced f lines
ISA =>MADE OF locking’s(same phase wavelenghts) all have phases lined up
Holograph vs normal pic
PICTURE
2D pic, lacks depth info
▪ Captures the intensity of light reflected from an
object on a film
▪ no info of the phase of the EMF field
HOLOGRAPH:
3D images =>
capturing phase and intensity
ISA=> DIFF btw real and virtual image, (are laterally inverted)
interference of the reference beam
with light reflected from the object => hologram
recorded interference pattern => 3D image
