1. LASER
LASER stands for Light Amplification by Stimulated Emission
of Radiation. The working principle of a laser is based on induced or
stimulated emission.
2. TYPES OF LASERS
The various types of laser that have been developed so far,
display a very wide range
of physical and operating parameters.
●
according
to the physical state of the active material - solid state (Ruby Laser, Nd:YAG
Laser, Fiber Lasers), liquid Laser (Dye Lasers), gas lasers (He-Ne Laser, CO2 Laser, Argon-Ion
Laser), free-electron lasers.
●
According
to the wavelength of the emitted radiation - infrared lasers, visible lasers,
UV and X-ray lasers.Output powers cover an even larger range of values.
●
lAccording to the Output Mode - Continuous
Wave (CW) Lasers
(these emit a steady, uninterrupted
beam.Their power is measured in Watts. Examples: Laser pointers,
fiber-optic communications, and industrial cutting lasers), Pulsed Lasers
(These concentrate energy into short "bursts." While the average
power might be low, the peak power during the pulse can be trillions of watts. Examples:
Q-switching and Mode-locking used in tattoo removal or high-precision eye
surgery.)
3. ABSORPTION, SPONTANEOUS EMISSION, STIMULATED EMISSION
To
understand these three processes, consider two energy levels of an atom 
(Lower State) and 
(Higher State). These three processes describe how light (photons)
interact with atoms and are the fundamental building blocks of laser physics.
Each involves the movement of electrons between energy levels, typically a
lower energy state (ground state) and a higher energy state (excited state).Here
is the explanation of each process:
Absorption
Absorption occurs when
an atom in a low energy state 
takes
in energy from an incoming photon to move an electron to a higher energy state 
The energy of the incoming photon must exactly match the
difference between the two energy levels & 
. In the result the photon vanishes, and the
atom becomes "excited."
Spontaneous Emission
An atom cannot stay in an excited
state forever; it is naturally unstable. In spontaneous emission, the electron
drops back down to the ground state on its own.
● The Process:- After a short period ( "excited state
lifetime"), the electron falls from 
to
● The Result:- A photon is released.
● Key Characteristic:- The emitted photon
has a random direction and phase. This is the process responsible for most
light we see, such as the glow from a lightbulb or a candle.
Stimulated Emission
This is the process that makes Lasers (Light Amplification by
Stimulated Emission of Radiation) possible. It occurs when an incoming photon
interacts with an atom that is already in an excited state.
● The Process:- Instead of being
absorbed, the incoming photon "nudges" the excited electron to drop
down immediately.
● The Result:- Two photons leave the
atom—the original incoming photon and the newly emitted one.
● Key Characteristic:- The two photons are
identical in every way: they have the same energy, direction, phase, and
polarization. This creates "coherent" light.
Comparison Table
|
Feature |
Absorption |
Spontaneous Emission |
Stimulated Emission |
|
Initial State |
Ground state ( |
Excited state ( |
Excited state ( |
|
Trigger |
External photon |
Natural decay (random) |
External photon |
|
Photons Out |
0 |
1 |
2 (Identical) |
|
Light Type |
N/A |
Incoherent (Random) |
Coherent ( Ordered) |
4. Population Inversion
Population Inversion is the fundamental condition required
for a laser to function.
Under normal thermal equilibrium, according to the Boltzmann
Distribution, more atoms exist in the lower energy state () than in the higher energy state (
). Population inversion is the unnatural state where the
number of atoms in the excited state (
) exceeds the number of atoms in the ground state (
).
Mathematically: >
Why is it Necessary?
In a normal state (
), if a photon enters the medium, it is more likely to be
absorbed by a
ground-state atom than to cause stimulated emission.
To achieve Light Amplification, stimulated emission must
dominate over absorption. This can only happen if there is a "crowd"
of atoms waiting in the excited state, ready to be triggered by an incoming
photon.
5. Metastable State
Achieving population inversion is difficult because atoms
usually stay in an excited state for only about 
seconds. To build up a population, we need a
Metastable State.
●
Excitation: Atoms are pumped from to a very high energy level 
.
●
Rapid Decay: They quickly fall to (the metastable state) via a non-radiative
transition (releasing heat instead of light).
●
Accumulation: Atoms stay in the metastable state () for a much longer time (about 
seconds).
●
Inversion:
Because they "hang out" in longer than it takes to pump new ones up, eventually becomes greater than .
6. Pumping
Since population inversion is not a natural state, energy
must be constantly supplied to the system to maintain it. This is done through
Pumping:
● Optical Pumping: Using light (e.g.,
in Ruby lasers).
● Electric Discharge: Using electric
current (e.g., in He-Ne gas lasers).
● Chemical Reactions: Using energy
from a chemical bond.
Pumping creates Population Inversion.
An atom undergoes Spontaneous Emission, releasing a seed
photon.
That photon triggers Stimulated Emission in the inverted
population.
The process chain-reacts, resulting in a coherent Laser
Beam.
7. Optically Pumped Laser Systems
Optical pumping is a process in which light is used to raise
(or "pump") electrons from a lower energy level to a higher one in a
laser medium. This creates the population inversion necessary for stimulated
emission.
● Fundamental Principle:- In an optically pumped system, an
external light source (the "pump") provides photons with energy h𝝂. If this energy matches the
difference between two energy levels in the gain medium, electrons are excited
to a higher state.
Three-Level Systems: The laser
transition occurs between an excited state and the ground
state (e.g., Ruby Laser).
Four-Level Systems: The laser
transition occurs between two excited states, which is
generally more efficient because the
lower laser level is naturally depopulated (e.g.,
Nd:YAG Laser).
● Components of the Pumping System:-To maximize efficiency, the system
requires specific components to couple the pump light into the active medium:
● The Pumping Cavity:- Usually an elliptical or circular
reflector. In an elliptical cavity, the pump lamp is placed at one focus and
the laser rod at the other, ensuring maximum light focus on the medium.
● The Optical Source:- Flashlamps: (e.g., Xenon or Krypton)
Used for
pulsed lasers. They provide a broad
spectrum of light.
8. Optical Source
In laser physics, the optical source (often called the
"pump source") provides the energy required to achieve population
inversion within the gain medium. The choice of source depends on the medium's
absorption spectrum and whether the laser needs to operate in pulsed or
continuous-wave (CW) mode.
Flashlamps (Incoherent Sources)
Flashlamps are high-intensity discharge lamps used primarily
for pulsed solid-state lasers.
● Xenon Flashlamps:- Most common for high-energy
pulsed lasers (like Ruby or Nd:YAG). They emit a broad spectrum of light from
ultraviolet to infrared.
● Krypton Flashlamps:- These are often preferred for
continuous-wave (CW) Nd:YAG lasers because their emission lines match the
absorption peaks of Neodymium better than Xenon at lower power densities.
❖ Mechanism: A high-voltage discharge
through the gas creates a plasma that emits radiant energy, which is then
focused onto the laser rod using an elliptical reflector.
Laser Diodes
(Coherent/Semi-Coherent Sources)
Diode pumping has largely replaced lamps in many modern
applications, creating Diode-Pumped Solid-State (DPSS) lasers.
● Spectral Matching:- Unlike
flashlamps, laser diodes can be manufactured to emit at a very specific
wavelength (e.g., 808 nm for Nd:YAG). This matches the absorption band of the
medium perfectly, minimizing heat.
● Efficiency:- Because there is less
"waste" light, the thermal loading on the laser crystal is
significantly reduced.
● Configurations:-
● End-Pumping: The diode beam is
directed along the longitudinal axis of the laser rod. High beam quality but
limited power.
● Side-Pumping: Multiple diode arrays
surround the laser rod. Used for high-power industrial lasers.
Secondary Lasers (Laser Pumping)
In some high-performance systems, one laser is used as the
optical source for another. This is common when the gain medium requires high
spatial coherence or a very specific excitation energy.
● Argon-Ion Lasers:- Frequently used to pump
Ti:Sapphire lasers or dye lasers.
● Frequency-Doubled Nd:YAG (532 nm):- Commonly used to pump tunable dye
lasers or optical parametric oscillators (OPOs).
Solar Pumping
A more specialized optical source where concentrated
sunlight is used to pump a laser medium.
❖ Application:- Primarily researched for
space-based communication or renewable energy applications where external power
is limited but solar flux is high.
Examples of Optically Pumped Lasers
|
Laser Type |
Gain Medium |
Pump Source |
Characteristics |
|
Ruby Laser |
Cr3+ ions in Al2O3
|
Xenon Flashlamp |
Three-level, pulsed, 694.3 nm |
|
Nd:YAG
|
Nd3+ ions in Yttrium Aluminum Garnet
|
Laser Diode or Krypton Lamp
|
Four-level, high efficiency, 1064 nm |
|
Dye Laser |
Organic dyes in liquid solvent |
Flashlamp or another Laser |
Tunable wavelengths across the visible spectrum |
|
Ti:Sapphire |
Ti3+ ions in Sapphire |
Green Argon Laser (514 nm) |
Widely tunable, used for ultrashort pulses |
9. projection geometries
projection geometries" refer to how energy is
distributed into the gain medium (pumping) or how the resulting beam is shaped
and directed onto a target (output).
End-Pumping
(Longitudinal Pumping)
In this setup, the pump light from
the optical source is focused directly into the face (end) of the laser rod,
traveling along the same axis as the resulting laser beam.
● The Process: The optical source beam is aligned
with the longitudinal axis of the crystal. Because the pump beam and the laser
beam overlap almost perfectly, the energy conversion is extremely efficient.
● Beam Quality: Since the energy is concentrated in
the center of the rod, it produces a very clean, high-quality beam.
● The Limitation: Because all the energy is focused
on a small spot at the end of the rod, it creates significant thermal stress.
If you pump too much power, the crystal can crack or suffer from "thermal
lensing." This limits the total power output
Side-Pumping (Transverse Pumping)
In this configuration, multiple diode arrays are arranged
around the sides of the laser rod, perpendicular to the laser beam's path.
● The Process:- The pump light enters from the
circumference of the rod. This allows developers to pack many diode bars along
the length of the rod, flooding the entire volume of the crystal with energy.
● Power Capability:- This is the preferred method for
high-power industrial lasers. Because the heat is distributed across the entire
surface area of the rod rather than just the tips, the system can handle much
higher energy levels without destroying the crystal.
● The Trade-off:- The overlap between the pump light
and the laser beam is less precise than in end-pumping. This results in
slightly lower beam quality, though it is more than sufficient for heavy-duty
tasks like melting or cutting steel.
Slab Geometry (The
"Zig-Zag" Path)
To overcome the power limits of a round "rod,"
engineers use a flat, rectangular Slab of laser crystal.
● The Geometry:- The laser beam does not travel in a
straight line. Instead, it reflects off the top and bottom surfaces in a
zig-zag pattern using Total Internal Reflection (TIR).
● Thermal Advantage:- Because the slab is thin and has a
large surface area, it can be cooled much more effectively than a rod. The
zig-zag path also "averages out" thermal distortions, leading to a
high-power beam that stays very straight and focused.
Thin-Disk Geometry
This is a modern geometry designed for extreme power with
almost no thermal distortion.
● The Geometry:- The laser medium is a very thin
disk (often only 100–200 micrometers thick) mounted on a high-performance heat
sink.
● The Path:- The pump light is reflected back
and forth through the disk multiple times using a parabolic mirror.
● Result:- Because the disk is so thin, heat
is removed instantly from the back. This allows for multi-kilowatt power levels
with the beam quality of a small lab laser
10. power supply
The power supply (often called a Laser Power Supply Unit or
LSU) is the component responsible for converting electrical energy from a wall
outlet or battery into the specific form required to "pump" the laser
medium.
Because different lasers require
vastly different types of energy—ranging from high-voltage sparks to steady
low-voltage currents—power supplies are highly specialized. There are some
types of Laser Power Supplies
Diode Laser Drivers (Low Voltage, High
Precision)
Used for semiconductor lasers and DPSS (Diode-Pumping
Solid-State) lasers.
● Mechanism: These provide a very
stable, constant DC current.
● Critical Requirement: Laser diodes
are extremely sensitive to "spikes." A tiny surge in voltage can
destroy the diode instantly. Therefore, these supplies include
"slow-start" circuits and heavy surge protection.
● Cooling: Often integrated with TEC
(Thermoelectric Cooler) controllers to keep the diode at a constant
temperature.
Gas Laser Supplies (High Voltage)
Used for Helium-Neon (HeNe) or CO2 lasers.
● Mechanism: These require a massive
"ignition voltage" (often 10,000V to 20,000V) to ionize the gas and
start the plasma discharge.
● Operation: Once the gas is glowing,
the voltage drops to a lower "sustaining voltage" (around 2,000V to
5,000V) to keep the laser running.
Flashlamp / Arc Lamp Drivers
(High Energy Storage)
Used for older solid-state lasers (like Ruby or Nd:YAG
lasers) that aren't diode-pumped.
● Mechanism: These use large capacitor
banks to store energy.
● The Pulse: The power supply charges
the capacitors and then releases all that energy in a microsecond-long burst
into a xenon flashlamp, creating an intense white light that pumps the laser
rod.
Pulsed vs. Continuous Power
● CW (Continuous Wave): The power
supply provides a steady stream of energy. It must be designed for high heat
dissipation.
● Pulsed (Q-Switched): The power
supply "pumps" energy into a storage medium (like a capacitor or the
laser crystal itself) and then releases it all at once. This requires a supply
that can The Quality Factor handle high peak loads followed by recovery
periods.
11. Quality Factor (Q factor)
The quality factor (Q factor) is a dimensionless parameter
that describes how well a laser resonator (the cavity between the mirrors)
stores energy versus how much energy it loses per cycle. We define the Q factor
of the cavity by the following equation:
Q = ω0
Here ω0 is the oscillation frequency of the mode.
If W(t) represents the energy in the mode at time t, then from Eq. (7.24) we
obtain
Q = ω0
= - 
whose
solution is
W(t) = W(0) 
Thus if 
represents the cavity lifetime, the time in
which the energy in the mode decreases by a factor 
then,
= 
12. Pulsed Operation of Lasers
In many applications of lasers, one wishes to have a pulsed
laser source. In principle it is possible to generate pulses of light from a
continuously operating laser, but it would be even more efficient if the laser
itself could be made to emit pulses of light. In this case the energy contained
in the population inversion would be much more efficiently utilized. There are
two standard techniques for the pulsed operation of a laser; these are
Q-switching and mode locking. Q-switching is used to generate pulses of high
energy but nominal pulse widths in the nanosecond regime. On the other hand
mode locking produces ultrashort pulses with smaller energy content. We shall
see that using mode locking it is possible to produce laser pulses in the
femtosecond regime.
13. Q-Switching
Imagine a laser cavity within which we have placed a shutter
which can be opened and closed at will. Let us assume that the shutter is
closed (does not transmit) and we start to pump the amplifying medium. Since
the shutter is closed, there is no feedback from the mirror and the laser beam
does not build up. Since the pump is taking the atoms from the ground state and
disposing them into the excited state and there is no stimulated emission the
population inversion keeps on building up. This value could be much higher than
the threshold inversion required for the same laser in the absence of the
shutter. When the inversion is built to a reasonably high value, if we now open
the shutter, then the spontaneous emission is now able to reflect from the
mirror and pass back and forth through the amplifying medium. Since the
population inversion has been built up to a large value, the gain
provided by the medium in one round trip will be much more
than the loss in one round trip and as such the power of the laser beam would
grow very quickly with every passage. The growing laser beam consumes the
population inversion, which then decreases rapidly resulting in the decrease of
power of the laser beam. Thus when the shutter is suddenly opened, a huge light
pulse gets generated and this technique is referred to as Q-switching. High
losses imply low Q while low losses imply high Q. Thus when the shutter is kept
closed and suddenly opened, the Q of the cavity is suddenly increased from a
very small value to a large value and hence the name Q-switching. For
generating another pulse the medium would again need to be pumped while the
shutter is kept closed and the process repeated again. Figure shows
schematically the time variation of the cavity loss, cavity Q, population
inversion, and the output power. As shown in the figure an intense pulse is
generated with the peak intensity appearing when the population inversion in
The cavity is equal to the threshold value. Figure shows a Q-switched pulse
emitted from a neodymium–YAG laser. Using this phenomenon it is possible to
generate extremely high power pulses for use in various applications such as
cutting, drilling, or in nuclear fusion experiments. Pulse width of Q-switched
laser can be approximated by
 |
Pulse width (Δt) ≈ 
Where
- cavity decay time
- threshold population inversion
- initial population inversion
before switching
14. TECHNIQUES FOR Q-SWITCHING
The techniques for Q-switching are generally classified into
two categories:
● Active (controlled by external
electronics) and
● Passive (self-triggered by the light
intensity).
Electro-Optic Q-Switching
(Active)
This is the most precise method,
utilizing the Pockels Effect. A Pockels cell (a crystal like LiNbO3)
and a polarizer are placed inside the cavity. When a specific voltage (the
quarter-wave voltage) is applied, the crystal induces a phase shift that
changes the polarization of the light. The polarizer then rejects this light,
preventing feedback (Low Q). When the voltage is rapidly switched off, the
polarization remains unchanged, the light passes through the polarizer, and the
"giant pulse" is released (High Q). The switching speed of this
method is Extremely fast (< 1 ns).
Acousto-Optic Q-Switching
(Active)
Used extensively in high-repetition-rate lasers (like
Nd:YAG). A piezoelectric transducer is attached to a transparent medium
(usually quartz). When an RF signal is applied, it creates ultrasonic waves
that produce periodic variations in the refractive index, acting like a
diffraction grating. This grating deflects the beam away from the cavity axis
(Low Q). Switching the RF signal off stops the diffraction, allowing the beam
to oscillate normally (High Q).
This method has high reliability and is capable of switching
at rates of several hundred kHz.
Passive Q-Switching (Saturable
Absorber)
This technique does not require external drivers or power
supplies, making it compact and cost-effective.A Saturable Absorber (a dye or a
doped crystal like Cr4+ :YAG) is placed in the cavity. Initially,
the absorber has high opacity, preventing lasing.
As the pump source increases the
population inversion, the intra-cavity photon density rises. Once the light
intensity reaches the "saturation intensity" of the absorber, the
ground-state population of the absorber is depleted, making it suddenly
transparent (the "bleaching" effect). The pulse is released, and once
the intensity drops, the absorber "recovers" its opacity.
Mechanical Q-Switching
The oldest and simplest form of Q-switching. One of the
resonator mirrors is rotated at high speeds (up to 30,000 RPM). The cavity only
has a high Q-factor during the very brief window when the rotating mirror is
perfectly parallel to the stationary mirror. It is difficult to synchronize
with external events, and mechanical wear is a significant factor.
15. MODE-LOCKING
Mode-locking is a technique in optics used to produce
extremely short pulses of light, typically in the femtosecond (10-15
s) range. While a standard laser might produce a continuous beam or relatively
long pulses, a mode-locked laser concentrates all its energy into a tiny,
high-intensity "packet" of light that bounces back and forth within
the laser cavity.
Q-switching produces very high energy pulses but the pulse
durations are typically in the nanosecond regime. In order to produce
ultrashort pulses of durations in picoseconds or shorter, the technique most
commonly used is mode locking. In order to understand mode locking let us first
consider the formation of beats when two closely lying sound waves interfere
with each other. In this case we hear beats due to the fact that the two sound
waves (each of constant intensity) being of slightly different frequency will
get into and out of phase periodically (Fig. 15.1). When they are in phase then
the two waves add constructively to produce a larger intensity. When they are
out of phase, then they will destructively interfere to produce no sound. Hence
in such a case we hear a waxing and waning of sound waves and call them as
beats.
Figure - 15.1
Mode locking is very similar to beating except that instead
of just two waves now we are dealing with a large number of closely lying
frequencies of light. Thus we expect beating between the waves; of course this
beating will be in terms of intensity of light rather than intensity of sound.
In order to understand mode locking, we first consider a laser oscillating in
many frequencies simultaneously. Usually these waves at different frequencies
are not correlated and oscillate almost independently of each other, i.e.,
there is no fixed-phase relationship between the different frequencies. In this
case the output consists of a sum of these waves with no correlation among
them. When this happens the output is almost the sum of the intensities of each
individual mode and we get an output beam having random fluctuations in
intensity. In Figure - 15.2 We have plotted the output intensity variation with
time obtained as a sum of eight different equally spaced frequencies but with
random phases. It can be seen that the output intensity varies randomly with
time resembling noise.
Figure - 15.2
Now, if we can lock the phases of each of the oscillating
modes, for example bring them all in phase at any time and maintain this phase
relationship, then just like in the case of beats, once in a while the waves
will have their crests and troughs coinciding to give a very large output and
at other times the crests and troughs will not be overlapping and thus giving a
much smaller intensity.
In such a case the output from the laser would be a
repetitive series of pulses of light and such a pulse train is called
mode-locked pulse train and this phenomenon is called mode locking. 
Figure -15.3
Figure - 15.4 shows the output intensity variation with time
corresponding to the same set of frequencies as used to plot Fig. 15.2, but now
the different waves have the same initial phase. In this case the output
intensity consists of a periodic series of pulses with intensity levels much
higher than obtained with random phases. The peak intensity in this case is
higher than the average intensity in the earlier case by the number of modes
beating with each other. Also the pulse width is inversely proportional to the
number of frequencies.
Figure - 15.4
Types of Mode-Locking
There are two primary ways to achieve this synchronization:
1.
Active
Mode-Locking
- Passive
Mode-Locking
Active Mode-Locking
An external signal is used to
modulate the light inside the cavity. This is often done with an Acousto-Optic
Modulator (AOM), which acts like a fast shutter that only opens when the pulse
is expected to pass through, forcing the light to organize into a pulse.
Figure 15.5
Passive Mode-Locking
This
does not require external electronics. Instead, a Saturable Absorber is placed
in the cavity. This material absorbs low-intensity light but becomes
transparent (saturates) when high-intensity light hits it. This naturally
favors the formation of a single intense pulse over a continuous weak beam.
Because these pulses are so short, they are used in:
● Ultrafast Spectroscopy: To "freeze" and study
chemical reactions as they happen.
● Precision Machining: For surgery (like LASIK) or
micro-drilling where heat must not spread to surrounding tissue.
● Optical Communications: Sending high-speed data packets
over fiber optics.
● Frequency Combs: Used in optical clocks for
ultra-precise timekeeping.
16. Ruby Lasers
The first laser to be operated successfully was the ruby
laser which was fabricated by Maiman in 1960. Ruby, which is the lasing medium,
consists of a matrix of aluminum oxide in which some of the aluminum ions are
replaced by chromium ions. It is the energy levels of the chromium ions which
take part in the lasing action. Typical concentrations of chromium ions are
~0.05% by weight. The energy level A diagram of the chromium ion is shown in
Fig. 16.1. As is evident from figure this a three-level laser. The pumping of
the chromium ions is performed with the help of flash lamp (e.g., a xenon or
krypton flashlamp) and the chromium ions in the ground state absorb radiation
around wavelengths of 5500 Å and 4000 Å and are excited to the levels marked E1
and E2.
Figure - 16.1 Figure- 16.2
 |
 |
The chromium ions excited to these levels relax rapidly through a
non-radiative transition to the level marked M which is the upper laser level.
The level M is a metastable level with a lifetime of ∼ 3 ms. Laser emission occurs between level M
and the ground state G at an output wavelength of λ0 = 6943 Å. The flashlamp
operation of the laser leads to a pulsed output of the laser. As soon as the
flashlamp stops operating the population of the upper level is depleted very
rapidly and lasing action stops till the arrival of the next flash. Even during
the short period of a few tens of microseconds in which the laser is
oscillating, the output is a highly irregular function of time with the
intensity having random amplitude fluctuations of varying duration as This is
called laser spiking, the formation of which can be understood as follows: when
the pump is turned on, the intensity of light at the laser transition is small
and hence the pump builds up the inversion rapidly. Although under steady-state
conditions the inversion cannot exceed the threshold inversion, on a transient
basis it can go beyond the threshold value due to the absence of sufficient
laser radiation in the cavity which causes stimulated emission. Thus the
inversion goes beyond threshold when the radiation density in the cavity builds
up rapidly. Since the inversion is greater than threshold, the radiation
density goes beyond the steady-state value which in turn depletes the upper
level population and reduces the inversion below threshold. This leads to an
interruption of laser oscillation till the pump can again create an inversion
beyond threshold. This cycle repeats itself to produce the characteristic
spiking in lasers. Figure 16.2 shows a typical setup of a flashlamp pumped
pulsed ruby laser. The helical flashlamp is surrounded by a cylindrical
reflector to direct the pump light onto the ruby rod efficiently. The ruby rod
length is typically 2–20 cm with diameters of 0.1–2 cm. typical input
electrical energies
required are in the range of 10–20 kJ. In addition to the
helical flashlamp pumping scheme shown in Fig. 16.2, one may use other pumping
schemes such as that pump lamp and the laser rod are placed along the foci of
an elliptical cylindrical reflector. It is well known that the elliptical
reflector focuses the light emerging from one focus into the other focus of the
ellipse, thus leading to an efficient focusing of pump light on the laser rod.
In spite of the fact that the ruby laser is a three-level laser, it still is
one of the important practical lasers. The absorption bands of ruby are very
well matched with the emission spectra of practically available flashlamps so
that an efficient use of the pump can be made. It also has a favorable
combination of a long lifetime and a narrow linewidth. The ruby laser is also
attractive from an application point of view since its output lies in the
visible region where photographic emulsions and photodetectors are much more
sensitive than they are in the infrared region. Ruby lasers find applications
in pulsed holography, in laser ranging, etc.
17. Neodymium-Based Lasers
The Nd:YAG laser (YAG stands for yttrium aluminum garnet
which is Y3Al5O12) and the Nd:glass laser are
two very important solid-state laser systems in which the energy levels of the
neodymium ion take part in laser emission. They both correspond to a four-level
laser. Using neodymium ions in a YAG or glass host has specific advantages and
applications.
(a) Since glass has an amorphous structure the
fluorescent linewidth of emission is very large leading to a high value of the
laser threshold. On the other hand YAG is a crystalline material and the
corresponding linewidth is much smaller which implies much over thresholds for
laser oscillation.
(b) The fact that the linewidth in the case of the
glass host is much larger than in the case of the YAG host can be made use of
in the production of ultrashort pulses using mode locking since as discussed in
Section 7.7.3, the pulsewidth obtainable by mode locking is the inverse of the
oscillating linewidth.
(c) The larger linewidth in glass leads
to a smaller amplification coefficient and thus the capability of storing a
larger amount of energy before the occurrence of saturation. This is especially
important in obtaining very high-energy pulses using Q-switching.
(d) Other advantages of the glass host
are the excellent optical quality and excellent uniformity of doping that can
be obtained and also the range of glasses with different properties that can be
used for solving specific design problems.
(e) As compared to YAG, glass has a much
lower thermal conductivity which may lead to induced birefringence and optical
distortion.
From the above discussion we can see that for continuous or
very high pulse repetition rate operation the Nd:YAG laser will be preferred
over Nd:glass. On the other hand for high energy-pulsed operation, Nd:glass
lasers may be preferred. In the following we discuss some specific
characteristics of Nd:YAG and Nd:glass laser systems.
18. Nd:YAG Laser
The Nd:YAG laser is a four-level laser and the energy level
diagram of the Neodymium ion is shown in Fig. 18.1. The laser emission occurs
at λ0 ≈ 1.06μm.
Since the energy difference between the lower laser level
and the ground level is
∼ 0.26 eV, the ratio of its population to that
of the ground state at room temperature
(T= 300 K) is 
. Thus the lower laser level is almost
unpopulated and hence inversion is easy to achieve. The main pump bands for
excitation of the neodymium ions are in the 0.81 and 0.75 μm wavelength regions
and pumping is done using arc lamps (e.g., the Krypton arc lamp). Typical
neodymium ion concentrations used are ∼1.38 × 1020 
. The spontaneous lifetime corresponding to the
laser transition is 550 μs and the emission line corresponds to homogeneous
broadening and has a width v ∼ 1.2 × 1011 Hz which corresponds to λ ∼ 4.5Å. 
We know that the Nd:YAG laser has a much lower threshold of
oscillation than a ruby laser.
With the availability of high-power compact and efficient
semiconductor lasers, efficient pumping of Nd ions to upper laser level can be
accomplished using laser diodes. This leads to very compact diode pumped
Nd-based lasers. Diode laser pumping is simpler than lamp pumping and also
produces much less heat in the laser medium leading to increased overall
efficiency. Since the laser diode output is narrow band unlike a normal lamp,
the output at 808 nm can be efficiently used for pumping. Typical output powers
of 150 W are commercially available. In fact an intracavity second-harmonic
generator can efficiently convert the laser wavelength to 532 nm (the second
harmonic of 1064 nm of Nd:YAG) leading to very efficient
green lasers. Nd:YAG lasers find many applications in range
finders, illuminators with Q-
switched operation giving about 10–50 pulses per
second with output energies in
the range of 100 mJ per pulse, and pulse width ∼ 10 ns. They also find applications
in resistor trimming, scribing, micromachining operations as
well as welding, hole
drilling, etc.
19. Nd:Glass Laser
The Nd:glass laser is again a four-level laser system with a laser
emission around 1.06 μm. Typical neodymium ion concentrations are ∼2.8 × 1020 cm–3 and various silicate and phosphate glasses
are used as the host material. Since glass has an amorphous structure different
neodymium ions situated at different sites have slightly different
surroundings. This leads to an inhomogeneous broadening and the resultant
linewidth is v ∼ 7.5 ×
1012 Hz which corresponds to λ ∼ 260 Å. This width is much larger than in Nd:YAG lasers and
consequently the threshold pump powers are also much higher. The spontaneous
lifetime of the laser transition is ∼ 300 μs. Nd-doped fiber lasers are also efficient. Chapter 2 discusses fiber lasers and primarily
erbium-doped fiber lasers. But similar analysis can also be carried out for
Nd-doped fiber lasers.
Nd:glass lasers are more suitable for high energy-pulsed
operation such as in laer fusion where the requirement is of subnanosecond
pulses with an energy con- tent of several kilojoules (i.e., peak powers of
several tens of terawatts). Other
applications are in welding or drilling operations requiring high pulse
energies
20. AMPLIFIERS FOR LASERS AND
THEIR CHARACTERISTICS
AOptical amplifiers increase the
power of a laser beam through stimulated emission without converting the
optical signal into an electrical one. These are broadly classified into Laser
Amplifiers (using a gain medium) and Nonlinear Amplifiers.
Laser Amplifiers (Based on
Stimulated Emission)
These amplifiers use a gain medium—such as a crystal, glass,
or semiconductor—that is pumped to achieve population inversion.
● Fiber Amplifiers:- Utilize rare-earth-doped fibers
(e.g., Erbium (EDFA) or Ytterbium (YDFA)).
➢ Pros- High gain, excellent beam
quality, and effective heat dissipation due to high surface-to-volume ratio.
➢ Common Use- Long-distance fiber
optic communication (EDFAs at 1550 nm).
● Solid-State
Amplifiers: Use bulk crystals or glass doped with ions like Nd³⁺ or
Ti³⁺.
➢ Pros- High peak power scaling; ideal
for high-energy applications.
➢ Common Use- Industrial laser
marking, welding, and scientific research.
● Semiconductor Optical Amplifiers
(SOA):
Based on laser diode structures with anti-reflective coatings to prevent
oscillation.
➢ Pros- Compact and electrically
pumped, allowing for easy integration into photonic circuits.
➢ Common Use- Optical switching and
pre-amplifiers in data centres.
Amplification Architectures
The configuration of the amplifier
significantly impacts its gain and efficiency.
|
Configuration |
Description |
Best Use Case |
|
One-Pass |
Beam passes through the gain medium once. |
High-power inputs with high gain media (e.g., Nd:YAG). |
|
Multipass |
Uses mirrors to guide the beam through the medium multiple
times at different angles. |
Amplifying ultrashort pulses (<50 fs) while minimizing
nonlinear distortions. |
|
Regenerative |
Pulse is trapped in an optical resonator using a Pockels
cell and makes many round trips. |
Achieving extremely high gain (up to 10⁶) from very weak
seed pulses. |
Nonlinear Amplifiers
These do not rely on energy storage in the medium; gain is
provided only while the pump light is present.
● Optical Parametric Amplifiers (OPA): Based on $\chi^{(2)}$ nonlinearity
in crystals; wavelength is tunable via phase-matching.
● Raman Amplifiers: Exploit Stimulated Raman
Scattering (SRS) in optical fibers.
21. SEMICONDUCTING LASER
Semiconductor lasers or laser diodes play an important part
in our everyday lives by providing
cheap and compact-size lasers. They consist of complex
multi-layer structures requiring nanometer scale accuracy and an elaborate
design. Their theoretical description is important not only from a fundamental
point of view, but also in order to generate new and improved designs. It is
common to all systems that the laser is an inverted carrier density system. The
carrier inversion results in an electromagnetic polarization which drives an
electric field In
In most cases, the electric field is confined in a
resonator, the properties of which are also important factors for laser performance.
In semiconductor laser theory, the optical gain is produced
in a semiconductor material. The choice of material depends on the desired
wavelength and properties such as modulation speed. It may be a bulk
semiconductor, but more often a quantum hetero structure. Pumping may be
electrically or optically (disk laser). All these structures can be described
in a common framework and in differing levels of complexity and accuracy. Light
is generated in a semiconductor laser by radiative recombination of electrons
and holes. In order to generate more light by stimulated emission than is lost
by absorption, the system has to be inverted, see the article on lasers. A laser is, thus, always a
high carrier density system that entails many-body interactions. These cannot
be taken into account exactly because of the high number of particles involved.
Correlation effects
Taking the collision terms into
account explicitly requires a large numerical effort, but can be done with
state-of-the-art computers. Technically speaking, the collision terms in the
semiconductor Bloch equations are included in the second-Born approximation.
This microscopic model has the advantage
of having predictive character, i.e., it yields the correct line width for any temperature or excitation
density. In the other models, the relaxation time has to be extracted from the experiment, but
depends on the actual parameters meaning the experiment has to be redone for
any temperature and excitation intensity.
Intrinsic Semi Conducting Laser
A semiconductor is a material whose conductivity lies
between those of conductor and insulator.
Semiconductors are of two types:
a) Intrinsic semiconductors or pure
semiconductors
b) Extrinsic semiconductors or doped
semiconductors. Extrinsic semiconductors are further classified into two types
depending upon the type of majority carriers:
i) n-type semiconductors where
electrons are majority carriers.
ii) p- type semiconductors where
holes are majority carriers.
When a p-type semiconductor and a n-
type semiconductor are joined by special techniques, there will be flow of
electrons from n side to p side and flow of holes from p side to n side. After
some time, an electric field will be created which will oppose this flow and
flow stops. Thus, there will be formation of depletion regions. This region is
called so because it is depleted from charge carriers.
22.
Colour Centre Lasers
A Colour Centre Laser (or Color Center Laser) is a type of
tunable solid-state laser that uses crystal defects, specifically F-centers, as the
active gain medium. These lasers are primarily used for high-resolution
spectroscopy in the near-infrared region.
The Active Medium: F-Centers
The term "F-center" comes from the German Farbzentrum,
meaning "colour centre".
● Structure:- An F-center is a point defect where an anionic vacancy in an
ionic crystal (typically an alkali halide like NaCl or KCl) is occupied by one
or more unpaired electrons.
● Optical Effect:- These trapped electrons absorb light
in the visible spectrum, causing normally transparent crystals to become
coloured.
● Laser-Active Types:- Not all defects are suitable for
lasing.
Operational Principle
Colour
centre lasers operate similarly to dye lasers but use a solid crystal instead
of a liquid solution.
Pumping:
They require optical pumping, usually provided by another laser
such as an Argon-ion, Krypton-ion, or Nd:YAG laser.
● Four-Level System: They typically function as a four-level laser
system.
The electron is excited to a higher state, relaxes to a "shifted"
excited state via phonon interactions, undergoes stimulated emission, and then
relaxes back to the ground state.
● Broad Tunability: Because of strong electron-phonon
coupling, the emission bands are very broad, allowing the laser to be smoothly
tuned over a wide range of wavelengths.
● can produce ultrashort pulses in
mode-locked operation.
Applications
● Spectroscopy:- Their primary use is in molecular
spectroscopy
and high-resolution gas sensing.
● Fiber Optics:- Testing of optical components in
the 1.3μm$ to 1.6μm telecommunications window.
● Metrology:- Used in precision measurement and
as components in frequency chains.
-: Ghanshyam Ji Yadav Sir
