Laser Physics- lecture notes

1. 1
Laser Types
Lecture 6
LASER AND ITS APPLICATIONS
421 Phys
Types of lasers
We will somewhat arbitrarily look at lasers based on whether
the gain medium is a gas, liquid, or solid.
Gas: - Atomic gas laser (He-Ne laser),
- Ionic gas laser ( Ar Ion laser)
- Molecular gas laser (CO2 lasers, Excimer laser)
Liquid: Dye lasers
Solid: Ruby, Nd:YAG laser, Nd:glass, Ti:sapphire laser
Semiconductor: Diode (semiconductor) lasers

2. 2
Atomic gas lasers:
The Helium Neon (He-Ne) Laser
 Helium Neon lasers consist of a discharge
tube inserted between highly reflecting
mirrors.
 The tube contains a mixture of helium and
neon atoms in the approximate ratio
 of He:Ne (5:1).
• By applying a high voltage (a few KV) across the tube, an electrical discharge can
be induced.
• The electrons collide with the atoms and put them in an excited state.
• The light is emitted by the neon atoms, and the purpose of the helium is to assist
the population inversion process.
A typical construction is shown in the Figure.

3. 3
The helium atoms can easily de-excited by collisions with neon atoms in the ground
state according to the following scheme:
It would not be easy to get this population inversion without the helium because collisions
between the neon atoms and the electrons in the tube would tend to excite all the levels of
the neon atoms equally. This is why there is more helium than neon in the tube.
 Optimum performance in the He–Ne laser is found to occur when the product of
tube diameter and total gas pressure is D × P= 3.6 - 4 torr × mm.
Properties of He-Ne laser beam
He–Ne lasers are low power(a few mW for a laser 10–20 cm long).
 The efficiency is quite low, (η ~ 0.02%)
 Excellent beam quality
Narrow laser linewidth → high coherence
 Many applications, e.g. laser interferometers

4. 4
The direct excitation of Neon gas is inefficient, but the direct excitation of
He gas atoms is very efficient.
• An excited state of the He atom has an energy level which is very
similar to the energy of an excited state of the Neon atom
 The excited Helium atoms collide with the Neon atoms, and transfer to
them the energy for excitation.
Thus Helium gas does not participate in the lasing process, but
increases the excitation efficiency so that the lasing efficiency
with it increase by a factor of about 200
The role of the Helium gas in He-Ne laser
Ion gas lasers: Helium cadmium
• The population inversion scheme in HeCd is similar to that in He-Ne except that the
active medium is Cd+ ions.
• The laser transitions occur in the blue and the ultraviolet at 442 nm, 354 nm and 325
nm.
• The UV lines are useful for applications that require short wavelength lasers, such as
high precision printing on photosensitive materials.
• Examples include lithography of electronic circuitry and making master copies of
compact disks.

5. 5
325 nm
Pumping
Ground state
cd+ ion ground state
Energy[eV]
He+ ion ground state
CadmiumHe
Energy
transfer 335.3 nm
441.6
nm
Transitions in He-Cd laser
 The excitation to the upper laser level of the Cadmium atoms in the gas is similar to
the excitation process of the Neon gas in a He-Ne laser: Helium atoms are excited by
collisions with accelerated electrons, and than they pass their energies to Cadmium
atoms by collisions.
 The transitions in Helium-Cadmium laser are between energy levels of singly
ionized Cadmium atoms, and about twelve lines are available.
 These wavelengths are in the shorter wavelength region, violet and Ultra-Violet
(UV). Thus, the main application of the He-Cd laser is in the optics laboratory, for
fabricating holographic gratings.
 The practical problem in Helium-Cadmium laser is to maintain homogeneous
distribution of the metal vapor inside the electrical discharge tube.
 The ions are attracted to the cold windows at the ends of the cavity. In order to
prevent coating of the windows with Cadmium, cold traps are put before the laser
windows.

6. 6
Output wavelengths:
- Blue light 0.4416 [mm]
- Ultra-Violet (UV) light 0.3250 [mm].
Maximum output power:
150 [mW] in the blue line, and 50 [mW] at UV.
Maximum total efficiency:
in the blue line 0.02%, and in the UV 0.01%.
Spectral width:
0.003 [nm] (about 5 [GHz]), and coherence length: about 10 [cm].
Distance between two longitudinal modes:
about 200 [MHz].
Characteristics of He-Cd lasers
Argon ion (Ar+) Lasers
• Argon has 18 electrons with the configuration 1s22s22p63s23p6.
• Argon atoms incorporated into a discharge tube can be ionized by collisions
with the electrons.
• The Ar+ ion has 17 electrons. The excited states of the Ar+ ion are
generated by exciting one of the five 3p electrons to higher levels. The level
scheme is given below.
• The important transitions occur between the 4p and 4s levels of the Ar+ ion.
due to fine structure (spin-orbit coupling) this is actually a doublet.
• The two emission lines are at 488 nm (blue) and 514.5 nm (green).
• Several other visible transitions are also possible, making Ar+ lasers
very good for colorful laser light shows.

7. 7
514.5 nm 488 nm
Pumping
Ground state of Ar
atom
Ground state of Ar ion
Radiative decay
laser transition
Energy[eV]
Transition In Ar+ Laser
First: The electrons in the tube collide with argon atoms and ionize them according to
the scheme: Ar (ground state) + lots of energetic electrons
Ar+ (ground state) + (lots + 1) less energetic electrons .
The Ar+ ground state has a long lifetime and some of the Ar+ ions are able to collide with
more electrons before recombining with slow electrons.
this puts them into the excited states according to:
Ar+ (ground state) + high energy electrons Ar+ (excited state) + lower energy
electrons
Since there are six 4p levels as compared to only two 4s levels, the statistics of the
collision process leaves three times as many electrons in the 4p level than in the 4s level.
Hence we have population inversion. Moreover, cascade transitions from higher excited
states also facilitates the population inversion mechanism. The lifetime of the 4p level is
10 ns, which compares to the 1 ns lifetime of the 4s level. Hence we satisfy tupper > tlower
and lasing is possible.
Population inversion is achieved in a two-step process

8. 8
 The diagram below shows a typical arrangement used in an Ar+ laser.
 Argon lasers tend to be much bigger than helium-Neons.
 The tube length might be 1–2 m, and the tube might be running at 50 A with a voltage
of 250 V. Hence water-cooling is usually necessary.
 Output powers up to several tens of Watts are possible.
 The tube is enclosed in a magnet to constrain the Ar+ ions and protect them from
deflections by stray fields.
Metal segmented
structure Solenoid
(magnetic coil)
prism
output
mirror
Anode
Water Jacket
Gas return line
Beryllium
oxide tube
Cathode
The windows at the ends of the
tube are cut at Brewster’s angle
(which satisfies tanθ = n) to
reduce refection losses (there is
no reflected beam for vertically
polarized light at this angle.)
Since there are several laser
transitions with similar
wavelengths, it is necessary to
use a prism to select the emission
line that is to be used.
In addition to laser light shows, argon lasers are used for pumping
tunable lasers such as dye lasers and Ti:sapphire lasers. There are
also some medical applications such as laser surgery, and scientific
applications include fluorescence excitation and Raman
spectroscopy- Microscopy- forensic medicine-ophthalmic surgery
Applications of Ar laser

9. 9
Excimer lasers
“Excimer”: excited dimer (E.g., He2)
“Exciplex”: excited complex (dissimilar atoms) (E.g., ArF)
Excimers and exciplexes are molecules characterised by a dissociative
ground state, but by a bound potential for an excited electronic state:
Since the lower state is very short-lived, a population inversion can also be
achieved relatively easily.
Excimer lasers are pulsed, high power lasers
 The name Excimer comes from the combination of the two words:
exited dimer, which means that the molecule is composed of two
atoms, and exists only in an excited state
E.g., ArF (193 nm), KrF (248 nm), XeF (351 nm), KrCl (222 nm), XeCl (308
nm), XeBr (282 nm)
 An electric discharge is used to pump the laser.
 Note that the Excimer laser can be
changed by exchanging the gas
mixture (along with the HR and OC).
 Although the ground energy level
is short-lived, in some cases the
lower level potential may be very
slightly bound, allowing some
tuneability of the laser.
5
4
3
2
1
0
248nm
Energy[eV]
Lasertransitions
Kr* + F
Kr + F
r0

10. 10
The composition of the gas mixture inside the tube of the Excimer laser is:
 Very little halogen (0.1-0.2%).
 Little noble gas (Argon, Krypton or Xenon). about 90% Neon or Helium.
 The halogen atoms can come from halogen molecules such as: F2, Cl2, Br2, or from
other molecules which contain halogens such as: HCl, NF3.
 The advantage of using a compound and not a pure halogen, is the strong
chemical activity of the halogen molecule (especially Fluorine).
 Excimer lasers emit in the Ultra-Violet (UV) spectrum.
 The radiation is emitted only in short pulses.
 The length of each pulse is between pico-seconds to micro-seconds (10-12-10-6s).
 The gas pressure inside the laser tube is high: 1-5 [At].
 The efficiency of commercial Excimer lasers is up to a few percent.
Operating of the Excimer Laser
Properties of Excimer Lasers
- Active media : excimers, e.g. ArF, KrK, XeCl,…
- Pumping mechanism : electron impact in a gas discharge, ion-ion
recombination, harpooning reactions
- Low efficiency (with respect to the partial pressures of the initial reactants, i.e.
typically in the regime of 1 bar); compensation of low efficiency by large partial
pressures, large media length (typically 1 m), and large pump rate (i.e. in high-
voltage gas discharge, typically in the regime of 20 kV) efficiency (optical
output/electric input) of approx. 1 %
- Possibility of large output powers and repetition rates (e.g. excimers provide very
high-energy pulses in the UV and near-VUV regime), e.g. several 100 mJ per
laser pulse with repetition rates of several 100 Hz
- Small lifetime excited state (approx. 10 ns) large pump rate required.
- laser wavelengths from 108 nm (NeF) to 397 nm (XeF)
- Every pulse of Excimer laser radiation contains a large number of photons, since
it has a very high peak power.
Properties of Excimer laser :

11. 11
The Common Excimer lasers
Commercial Excimer lasers can emit (UV) radiation up to an average power of 100 Watts.
• Since the emitted wavelengths are very short, each individual photon carries a large amount
of energy, which is enough to break the bond between molecules in the material that absorbed
the radiation. Thus, the Excimer laser is the perfect cutting tool for almost every material
Special Applications:
• Photolithography - Material processing at a very high accuracy (up to parts of microns !).
• Cutting biological tissue without affecting the surrounding.
• Correcting vision disorders - Cutting very delicate layers from the outer surface of the
cornea, thus reshaping it, to avoid the necessity for glasses.
• Marking on products - Since the short wavelength radiation from the Excimer laser is
absorbed by every material, it is possible with a single laser to mark on all kinds of materials,
such as plastics, glass, metal, etc.
• The price of an Excimer laser is relatively high (tens of thousands of dollars), but it is used a
lot because of its unique properties.
• Pump laser for dye laser systems
Applications of the Excimer Laser:

12. 12
The carbon dioxide laser CO2
CO2 has 3 normal modes
of vibration:
 The symmetric stretch
(ν1) at 1354 cm-1
 The bending vibration
(ν2) at 673 cm-1
 The asymmetric stretch
(ν3) at 2396 cm-1.
The CO2 laser is a high power infrared laser of high efficiency that may be pulsed or CW.
It lases between vibrational levels of CO2.
Since the lower state is very short-lived, a population inversion can also be
achieved relatively easily.
m
m
1000
0200
0001
Energy transfer by resonance & collision
Pumping
TransitionsinCO2laser

13. 13
 Lasing action in a CO2 molecule was first demonstrated by C. Patel in 1964.
 He transmitted an electric discharge pulse through pure CO2 gas in a laser tube,
and got a small laser output.
 CO2 is the gas in which the lasing process occurs, but other gas additives
to the laser tube improve the total efficiency of the laser.
 The standard CO2 laser includes in the active medium a mixture of CO2 with
N2 and He.
 The optimal proportion of these 3 gases in the mixture depends on the laser
system and the excitation mechanism.
 In general, for a continuous wave laser the proportions are:
CO2:N2:He ( 1:1:8)
 Pumping is achieved by electric discharge – some CO2 molecules are
directly excited, together with efficient transfer from excited N2 to CO2.
 Transitions between vibrational levels also involve rotational transitions,
giving rise to a relatively large number of closely spaced emission lines
(the laser can be tuned between these transitions).
 Very high laser (up to kW) powers can be achieved.
 CO2 is a linear molecule, and the three atoms are situated on a straight
line with the Carbon atom in the middle.

14. 14
Various designs of CO2 lasers
1. Longitudinal flow Output
Mirror
Power
Supply
laser tube
(ceramic)
Al sheet
Gas InGas out
Mirror
2. Sealed off- Laser2CO
He, Ne, CO2 gas
mixture
Mirror Mirror
Rf power
supply
Laser tube
Laser beam
3. Waveguide 2CO lasers
Rf PowerSupply
Insulator
(W
aveguide reflectors)
Output mirror
Rear mirror
Metallicelectrodes
Waveguide bore
region
Laser
Beam
4. Transverse-flow 2CO lasers
Power
supply
Gas flow
Electrode
Output
Mirror
Rear Mirror
Laser Beam
Electrode

15. 15
5. Gas dynamic Laser2CO
 100kWCan produce powers in the range of
Diffuser
Combustion
chamber
Nozzles
Laser beam
 High output power. Commercial CO2 Lasers produce more than 10,000
watts continuously.
 Output spectrum is in the Infra-Red (IR) spectrum: 9-11 [mm].
 Very high efficiency (up to 30%).
 Can operate both continuously or pulsed.
 Average output power is 75 [W/m] for slow flow of gas, and up to few
hundreds [W/m] for fast gas flow.
 Very simple to operate, and the gasses are non-toxic.
Properties of CO2 Laser