This document provides a summary of different laser types, including He-Ne lasers, injection lasers, Nd-YAG lasers, and dye lasers. It discusses the construction, working principles, and energy diagrams of each laser. The He-Ne laser was the first continuous laser and uses a mixture of helium and neon gases as its active medium. Injection lasers use a semiconductor PN junction as the active medium and achieve population inversion through carrier injection. Nd-YAG lasers use a neodymium-doped yttrium aluminum garnet crystal as the active medium and operate as a four-level laser system. Dye lasers can be widely tuned by changing the organic dye solution

1. He-Ne laser, Injection laser
Nd-YAG laser & Dye laser
PRESENTATION ON
By
ILLYAS M K
213118010

2. contents
1. He-Ne LASER
2. INJECTION LASER
3. Nd-YAG LASER
4. DYE LASER
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3. He-Ne LASER
He-Ne stands for Helium-Neon, Was the first continuous laser.
In this laser active medium consists of mixture of helium and neon gases which do not interact, so
one type of atomic gas lasers
He-Ne gases are taken in 10:1 ratio
Electric discharge is used as pumping source
It is a four level laser
Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum.
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4. CONSTRUCTION
The setup consists of a discharge tube of length 80 cm and bore diameter of 1.0 cm.
Mixture of helium and neon gases maintained at low pressure (an average 50 Pa per cm of cavity
length ) in a glass envelope.
consists of a glass envelop with a narrow capillary tube through the center. The capillary tube is
designed to direct the electrical discharge through its small bore to produce very high current
densities in the gas.
The plasma tube has a large cylindrical metallic cathode and a smaller metallic anode. The current is
directed from cathode to anode.
The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts
through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for
CW operation.
The optical cavity of the laser typically consists of a plane, high reflecting mirror at one end of the
laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end.
He-Ne lasers are normally small, with an optical output powers ranging from 1 mW to 100 mW.
Output power of these lasers depends on the length of the discharge tube and pressure of the gas
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6. WORKING
In the He-Ne laser the light is produced by atomic transitions within the Neon atom.
The Helium does not directly produce laser light but it acts as a buffer gas, the purpose of which is
to assist/help the atoms of the other gas to produce lasing in as manner.
When energy from the pumping source is applied (voltage applied) in He-Ne gas mixture then some
of the energy is observed by the Helium atoms.ie, the helium atoms achieve an excited state.
Now when the Helium atoms move within the laser tube, they collide with the Neon atoms. At each
collision some of the energy within the helium atom is transferred to the Neon atom and so raising
it to an excited meta-stable state.
When a sufficient number of Neon atoms reach to this state then population inversion occurs and
hence the lasing can take place.
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7. ENERGY DIAGRAM
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8. On collision with fast moving electrons He atoms get excited to F2
and F3 energy levels, where
lifetime of helium atoms is more. So maximum possibility of energy transfer between He and Ne
atoms through atomic collisions.
Thus Ne atoms get excited to E4 and E6, continuous excitation attains population inversion.
Transitions from E6 to E5 and E4 to E3 corresponds to infrared region
Transition from E6 to E3 corresponds to visible region, Red colour in visible spectrum.
Ne atoms present in E3 level are de-excited to E2 level by spontaneous emission and from there on
colliding with walls of the tube gets de-excited to ground level E1
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9. INJECTION LASER
 An injection laser, also known as an laser diode or diode laser
 It is a specially fabricated PN junction device, which emits coherent light when forward biased
 The basic mechanism responsible for light emission from a semiconductor is the recombination of electrons and holes at a p-n
junction when a current is passed through a diode
 Conversion of electrical energy to light energy
 The material which often used in Laser diode is the gallium Arsenide (GaAs) in (homojunction)
 In order to design a laser diode, the p-n junction must be heavily doped (degenerately doped).
 Population inversion is required for producing stimulated emission. A P-N diode consist of electrons and holes distributed in
respective energy bands. Therefore laser action in this type of diode involves energy bands rather than discrete levels
 Unlike in other lasers population inversion is not obtained by exciting electrons in spatially isolated atoms but injecting into the
conduction band from the external circuit, thus the name injection laser
 Thus attaining a condition of large concentration of electrons in the conduction band and a large concentration of holes in the
valence band.
 Therefore the conduction band plays the role of excited level while the valence band plays the role of ground level.17 April 2019 9

10. CONSTRUCTION
 Consist of a heavily doped PN junction, starting with a heavily
doped N type GaAs material and a heavily zinc doped layer
constitutes the heavily doped P region formed on its top.
 By doping, the Fermi level of the n-side will lie in the
conduction band whereas the Fermi level in the p-region will lie
in the valance band.
 The top and bottom surfaces are provided with metal contacts
(metallized) to pass current through the diode
 The front and rear faces are polished parallel to each other and
perpendicular to the plane of the junction
 The remaining two sides of the diode are roughened to
eliminate lasing action in that direction
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11. WORKING
 When PN junction is forward biased, electron and holes are injected into the junction region (active region) in high
concentrations. at low forward current the electron hole recombination causes spontaneous emission of photons and
the junction acts as an LED.
 On increasing the forward current the intensity of the light increases linearly and when it reaches the threshold value,
the carrier concentration in the junction region will rise to a very high value as a result junction region contains a large
concentration of electrons within the conduction band and simultaneously a large number of holes within valence band
(holes represent absence of electron)
 Thus the upper energy level in the narrow region are having a high electron population while the lower energy levels in
the same region are vacant (holes), the condition of population inversion attained in active region or depletion region or
inversion region
 Recombination acts of electron and hole pairs lead to spontaneous emission of photons which propagating in the
junction plane stimulate the conduction electron to jump into vacant states of valence band
 This stimulated electron hole recombination produces coherent radiation, GaAs laser emits at a wavelength of 9000Å
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13. Nd-YAG LASER
Neodymium-doped Yttrium Aluminum Garnet ( Y3Al5O12), Nd: YAG laser is a solid state laser in which
Nd: YAG is used as a laser medium.
Nd: YAG laser is a four-level laser system, which means that the four energy levels are involved in
laser action. These lasers operate in both pulsed and continuous mode.
Nd: YAG laser generates laser light commonly in the near-infrared region of the spectrum at 1064
nanometers (nm). It also emits laser light at several different wavelengths including 1440 nm, 1320
nm, 1120 nm, and 940 nm
 In Nd: YAG laser, light energy sources such as flashtube are used as energy source to supply energy
to the active medium.
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14. CONSTRUCTION
Energy source
The energy source or pump source supplies energy to the active medium to achieve population
inversion. In Nd: YAG laser, light energy sources such as flashtube or laser diodes are used as energy
source to supply energy to the active medium.
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15. Active medium
The active medium or laser medium of the Nd:YAG laser is made up of a synthetic crystalline material
(Yttrium Aluminum Garnet (YAG)) doped with a chemical element (neodymium (Nd)). The lower energy
state electrons of the neodymium ions are excited to the higher energy state to provide lasing action in
the active medium.
Nd3+ ions act as active medium or active centres. YAG is just the host.
Optical resonator
The Nd:YAG crystal is placed between two mirrors. These two mirrors are optically coated or silvered.
Each mirror is silvered or coated differently. One mirror is fully silvered whereas, another mirror is partially
silvered. The mirror, which is fully silvered, will completely reflect the light and is known as fully reflecting
mirror.
On the other hand, the mirror which is partially silvered will reflect most part of the light but allows a
small portion of light through it to produce the laser beam. This mirror is known as a partially reflecting
mirror
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16. WORKING
Nd: YAG laser is a four-level laser system, which means that the four energy levels are involved in laser
action.
In Nd:YAG laser, the lower energy state electrons in the neodymium ions are excited to the higher energy
state to achieve population inversion.
Consider a Nd:YAG crystal active medium consisting of four energy levels E1, E2, E3, and E4.
The energy levels will be E1 < E2 <E3 <E4. The energy level E1 is known as ground state, E2 is the next
higher energy state or excited state, E3 is the metastable state and E4 is the excited state.
When flashtube or laser diode supplies light energy to the active medium (Nd:YAG crystal), the lower
energy state (E1) electrons in the neodymium ions gets excited to the higher energy state E4.
The lifetime of higher energy state E4 is very small (230 microseconds) so the electrons in the energy
state E4 do not stay for long period. After a short period, the electrons will fall into the next lower energy
state or metastable state E3 by releasing non-radiation energy (releasing energy without emitting
photons).
The lifetime of metastable state E3 is high compared to E4. This results in an increase in the number of
electrons in the metastable E3 and hence population inversion is achieved.
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17. After some period, the electrons in the metastable state E3 will fall into the next lower energy state
E2 by releasing photons or light. The emission of photons in this manner is called spontaneous
emission.
The lifetime of energy state E2 is very small just like the energy state E4. Therefore, after a short
period, the electrons in the energy state E2 will fall back to the ground state E1 by releasing radiation
less energy.
When photon emitted due to spontaneous emission is interacted with the other metastable state
electron, it stimulates that electron and makes it fall into the lower energy state by releasing the
photon. As a result, two photons are released. The emission of photons in this manner is called
stimulated emission of radiation.
Spontaneous emission is a natural process but stimulated emission is not a natural process. To
achieve stimulated emission, we need to supply external photons or light to the active medium.
The Nd:YAG active medium generates photons or light due to spontaneous emission. The light or
photons generated in the active medium will bounce back and forth between the two mirrors. This
stimulates other electrons to fall into the lower energy state by releasing photons or light. Likewise,
millions of electrons are stimulated to emit photons.
The light generated within the active medium is reflected many times between the mirrors before it
escapes through the partially reflecting mirror
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18. ENERGY DIAGRAM
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1064nm

19. DYE LASER
• A dye laser is a laser which uses an organic dye as the lasing medium, usually a liquid solution.
• A dye is a coloured substance which imparts its colour to the material it is being applied
• Active medium for the dye laser is the organic dye
• Some of the organic dyes are Rhodamine 6G, Fluorescein, coumarin
• Flash lamps and several types of lasers can be used to optically pump dye lasers
• The main advantage of this type of laser is its tunability, which means lasing wavelength for a dye
may be varied over wide range,
• so it is also called as tunable lasers
• Tuning over 500 angstrom has been obtained.
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20. CONSTRUCTION
Dye laser consisted of a 1cm long quartz glass tube filled with solutions of organic dyes
Organic dyes are dissolved in solvents like water, ethyl alcohol, methanol.
Active medium = organic dye + water, benzene, ethanol
The dye solution typically has a concentration in the range of 10-2 to 10-4 M
Energy for optical pumping is provided by flash lamp
Tuning can be obtained by replacing one of the mirrors of the resonant cavity with a diffraction
grating.
By rotating the diffraction grating wavelength of the laser output can be altered.
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22. WORKING & ENERGY DIAGRAM
The main difference is the use of dye as active medium which is dissolved in suitable solvent.
Absorption from S0 to S1, S2
Rapid collisional relaxation to ground vibrational level of S1
Fluorescence to various vibrational states of S0 (basis for laser
emission)
ISC (inter system crossing) to state T1
pumping at 1 or 2; laser emission at 5
If flash lamp is used for optical pumping output is pulsed laser
If other laser sources are used for optical pumping output is
continuous wave laser
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24. THANK YOU
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