LASER
It’s the abbreviation of Light Amplification by Stimulated Emission of Radiation
Creating of laser light:
1.) Energy level system of atoms and molecules
atomic energy levels, electron transitions, electrons gain energy (drawing-away from the
atomic nucleus), electrons lose energy (if they approach the atomic nucleus)
ground- (they occupy the orbits with the least energies possible), excited-state electrons
(they are on orbits that have more energies than orbits that they could occupy with the least
energies)
electrons have quantized (defined) energy levels
molecular energy levels (figure 1)
figure 1
absorption: when the ground state electron absorbs a photon with a given energy, then it
can get to a higher state, but only if between the excited state and the ground state the
energy difference is exactly the photon’s energy (this is Bohr’s energy resonance condition)
ΔE = E2-E1 = h*f
The photon gets absorbed, it ceases to exist → gives it’s energy to the atom, which gets
excited. These two processes are coinstantaneous!
2.) Fundamental radiation processes:
spontaneous emission: The excited state electron gets back to ground state after a certain
time, while it gives away it’s extra energy emitting a photon with defined energy. This is
spontaneous (direction, time) without outside impact.
figure 2
stimulated emission: in 1917 Einstein predicted that emission could have a version which is
not spontaneous, but it happens on outside impact. This is called stimulated (induced)
emission. It happens when a photon -with energy that satisfies the Bohr condition for
excited state electrons- get by an excited-state electron.
At this time the electron gets back to the ground state while emitting a photon with the
same energy. The outside photon and the emitted photon both have the same energy thus
frequency, so the number of photons got doubled. The emitted photon also has the same
moving direction, phase, and polarization as the outside electron. From this Einstein
realized with this method it would be possible to create collimated light with high intensity.
So in this case from one photon we get two so we amplify light. Laser operation’s most
important condition is that in the interaction the number of photons should increase, so
light amplification should happen.
Σ:AMPLIFICATION! 1 → 2 photons
The cause is an outside photon! same direction, time, phase, energy and wavelength
The created photon:
-has the same frequency as the original
-travels in the same direction as the original
-they have the same polarization
-they have the same phase
Light amplification’s next step is a geometric optical solution: how to direct the light more
than one time through the amplifier material (the gain medium) so we can use the
stimulated emission in every case.
From the point of the gain medium we can speak about the following lasers:
-solid-state (crystal+metal contamination)
-gas (CO2), He-Ne
-dye: organic dye solution
-semiconductor lasers
figure 3
Processes between an atom’s two energy states: a)spontaneous emission b) absorption c) stimulated
emission, E1,2-energy levels, photon (hv). Source: Kecik J.,2006). Source:
http://www.szrfk.hu/rtk/kulonszamok/2005_cikkek/nanai_laszlo.pdf
3.) Population inversion: to get high intensity light we need numerous excited
electrons, so we need to create lot of them in a given material. We can reach this
with energy-investment, so we need to invest energy into the material from the
outside, that is called laser pumping. If the number of electrons in the ground state is
N1, in the excited state is N2 than basically we have N1>>N2. On the other hand after
pumping we have N2>N1. This is called population inversion, which is depicted in the
following figure.
figure 4
if the number of excited state atoms is higher than ground-sate atoms we call this inverse
population, or population inversion-than in this case induced emission has a higher
possibility than absorption
We need at least a three energy level laser (we can’t make laser in a two energy level
system) and from the higher levels at least one must have a long lifetime (laser energy level)
so the possibility of spontaneous emission will be little.
Pumping can be achieved by
- thermal excitation (warming),
- optical excitation (with flash light) or by
- electric discharge
Laser is amplified light, that we create in an optical resonator. An optical resonator contains
the gain material between two collateral mirrors, it ensures the positive feedback, and the
proper frequency for the resonance.
4.) Optical resonator: two collateral plane or concave mirrors. It reflects back part of the
departing light to the gain material. positive feedback, self-excitation, resonance
figure 5
Resonance condition in the laser: 2L = mλ (the double of the resonator length should be
equal to one of the wavelength’s integral multiples, where L is the distance between the
mirrors, λ is the wavelength in the given material, m: integer number) The resonator’s
natural vibrations are standing waves (standing wave: it’s the superposition of two waves
which have the same frequency, amplitude but travel in the opposite directions).
We make a direction more important by using mirrors. The photons that aren’t perpendicular to the
mirrors escape from the resonator cavity. The photons that bounce back and forth between the two
mirrors induce the emission of more and more photons. (figure 5). If we pump continuously (the
occupation of the higher energy level) then more and more energy will be concentrated in the
resonator cavity in the form of coherent photons. We can draw off from this energy by using a
partially transparent mirror, so a given proportion of the created photons can leave the resonator
cavity continuously. This leaving collimated, monochromatic, coherent radiation called laser
radiation.
Laser light begins with a photon emitted spontaneously in axial direction.
This gets multiplied up in the optical cavity by the induced emission. The photons travelling
in the wrong direction get scattered out of the laser beam.
Σ For laser oscillation we need:
1-gain medium
2-intensive electron excitation (pumping)
3-positive feedback
4-optical resonator
5.) Attributes of the laser light:
- monochromatic: narrow bandwidth (the relative frequency bandwidth of a laser is Δf / f
~10-10)
- The gained light is coherent, it is able to create interference even in the case of big pathdifferences.
Time coherency: the photons emitted in different times have the same frequency
Spatial coherency: phase identity through the laser beam’s cross-section .
-The laser beam is a narrow beam with very little divergence, so it’s approximately
collimated.
- Laser energy is concentrated in a little space, in pulsed mode this happens during a very
little time-interval, so laser light’s power density (E/At) can be much higher than ordinary
light sources.
-polarized
-possibility of ultrashort-pulses (10-15 s)
Σ What is needed for lasers?
•Pumping
•Population inversion
•Stimulated emission
•Optical resonance
•Mirrors with high reflectivity
6.) Comparison
Ordinary light sources
vs
LASER
wide wavelength bandwidth
monochromatic (narrow spectral
bandwidth)
divergent beam
collimated beam
not coherent
coherent
power density: for example: welding:~103 W/m2
non polarized
~1015 W/m2
polarized
7.) Laser types: lasers can be categorized by many aspects. For example considering the gain
medium we can differentiate semiconductor, gas, solid-state and dye lasers. Considering
the operation method we can speak about two big types: continuous and pulse lasers.
Considering the energy we differentiate the laser types into four big classes.
8.) Application of lasers:
-moulding, drilling, spot-welding,
-surgical operation, retinal laser surgery,
-gene surgery,
-barcode reading,
-CD-player laser playback head,
- lengthiness and velocity measurement using interference,
-direction setout
- light source used to create holography (Gábor Dénes hologramme=whole picture)