1. Subject: Engineering Physics (PHY-1) Common For All Branches
Unit: 2.1 LASER
Syllabus: Spontaneous and stimulated emissions, Laser action,
characteristics of laser beam-concepts of coherence, He-Ne and
semiconductor lasers (simple ideas), applications.
Prepared By: www.kukworld.in
Spontaneous and Stimulated Emission
Spontaneous emission: Spontaneous emission is when an electron in a higher energy
level drops down to a lower energy level and a photon is emitted with an energy equal
to the energy difference between the two levels. There is no interference in this
process from outside factors. Usually spontaneous emission happens very quickly after
an electron gets into an excited state. In other words, the lifetime of the excited state is
very short (the electron only stays in the high energy level for a very short time).
However, there are some excited states where an electron can remain in the higher
energy level for a longer time than usual before dropping down to a lower level. These
excited states are called metastable states.
Stimulated emission: As the picture above shows, stimulated emission happens when
a photon with an energy equal to the energy difference between two levels interacts
with an electron in the higher level. This stimulates the electron to emit an identical
photon and drop down to the lower energy level. This process results in two photons at
the end.
How a laser works
A laser works by a process called stimulated emission - as you can tell from what `laser'
stands for! You can imagine that stimulated emission can lead to more and more identical
photons being released in the following way: Imagine we have an electron in an excited
metastable state and it drops down to the ground state by emitting a photon. If this photon
then travels through the material and meets another electron in the metastable excited state
this will cause the electron to drop down to the lower energy level and another photon to be
emitted. Now there are two photons of the same energy. If these photons then both move
through the material and each interacts with another electron in a metastable state, this will
2. result in them each causing an additional photon to be released, i.e. from 2 photons we then
get 4, and so on! This is how laser light is produced.
Figure 1: Spontaneous emission is a two step process, as shown here. First, energy from
an external source is applied to an atom in the laser medium, raising its energy to an
excited (metastable) state. After some time, it will decay back down to its ground state and
emit the excess energy in the form of a photon. This is the first stage in the formation of a
laser beam.
Figure 2: Stimulated emission is also a two step process, as shown here. First, a laser
photon encounters an atom that has been raised to an excited state, just like in the case of
spontaneous emission. The photon then causes the atom to decay to its ground state and
emit another photon identical to the incoming photon. This is the second step in the creation
of a laser beam. It happens many, many times as the laser photons pass through the optical
cavity until the laser beam builds up to full strength.
3. This can only happen if there are many electrons in a metastable state. If most of the
electrons are in the ground state, then they will just absorb the photons and no extra
photons will be emitted. However, if more electrons are in the excited metastable state than
in the ground state, then the process of stimulated emission will be able to continue. Usually
in atoms, most of the electrons are in the lower energy levels and only a few are in excited
states. When most of the electrons are in the excited metastable state and only a few are in
the ground state, this is called population inversion(the populations in the excited and
ground states are swapped around) and this is when stimulated emission can occur. To
start off the process, the electrons first have to be excited up into the metastable state. This
is done using an external energy source.
Population inversion
Population inversion is when more atoms are in an excited state than in their
ground state. It is a necessary condition to sustain a laser beam, so that there are
enough excited atoms that can be stimulated to emit more photons.
4. Therefore, materials used to make laser light must must have metastable states which can
allow population inversion to occur when an external energy source is applied.
What is Difference between Spontaneous Emission and Stimulated Emission:
1. Spontaneous emission does not depend on the higher and lower energy level state. Stimulated
emission is the difference between the higher and lower energy level state.
2. In spontaneous emission the energy transfer is less. In stimulated emission energy transfer is
twice that of spontaneous emission.
3. Spontaneous emission is caused by laser. Stimulated emission is caused by LED.
4. The light emitted in spontaneous emission is monochromatic and non-polarized. The light
emitted in stimulated emission is monochromatic and polarized.
5. Spontaneous emission is coherent. Stimulated emission is non-coherent.
Spontaneous and Stimulated Radiation
Laser Action
Interaction of electromagnetic radiation with matter produces absorption and spontaneous
emission. Absorption and spontaneous emission are natural processes. For the generation of
laser, stimulated emission is essential. Stimulated emission has to be induced or stimulated and is
generated under special conditions as stated by Einstein in his famous paper of 1917. i.e. ?when
the population inversion exists between upper and lower levels among atomic systems, it is
possible to realize amplified stimulated emission and the stimulated emission has the same
frequency and phase as the incident radiation?. Einstein combined Plank? law with Boltzmann?s
statistics in formulating the concept of stimulated emission. In electronic, atomic, molecular or
ionic systems the upper energy levels are less populated than the lower energy levels under
equilibrium conditions. Pumping mechanism excites say, atoms to a higher energy level by
absorption.
The atom stays at the higher level for a certain duration and decays to the lower stable ground
level spontaneously, emitting a photon, with a wavelength decided by the difference between the
upper and the lower energy levels. This is referred to as natural or spontaneous emission and the
photon is called spontaneous photon. The spontaneous emission or fluorescence has no preferred
direction and the photons emitted have no phase relations with each other, thus generating an
5. incoherent light output (Fig.4). But it is not necessary that the atom is always de-excited to
ground state. It can go to an intermediate state, called metastable state with a radiation less
transition, where it stays for a much longer period than the upper level and comes down to lower
level or to the ground state. Since period of stay of atoms in the metastable state is large, it is
possible to have a much larger number of atoms in metastable level in comparison to the lower
level so that the population of metastable state and the lower or ground state is reversed. i.e. there
are more atoms in the upper metastable level than the lower level. This condition is referred to as
population inversion. Once this is achieved, laser action is initiated in the following fashion. The
atom in the metastable state comes down to the ground state emitting a photon. This photon can
stimulate an atom in the metastable state to release its photon in phase with it. The photon thus
released is called stimulated photon. It moves in the same direction as the initiating photon, has
the same wavelength and polarization and is in phase with it, thus producing amplification. Since
there are a large number of initiating photons, it forms an initiating electromagnetic radiation
field. An avalanche of stimulated photons is generated, as the photons traveling along the length
of the active medium stimulates a number of excited atoms in the metastable state to release their
photons. This is referred to as the stimulated emission. These photons are fully reflected by the
rear reflector (100% reflective) and the number and consequently the intensity of stimulated
photons increases as they traverse through the active medium, thus increasing the intensity of
radiation field of stimulated emission. At the output coupler, a part of these photons are reflected
and the rest is transmitted as the laser output. This action is repeated and the reflected photons
after striking the rear mirror, reach the output coupler in the return path. The intensity of the laser
output increases as the pumping continues. When the input pumping energy reduces, the
available initiating and subsequently the stimulated photons decrease considerably and the gain
of the system is not able to overcome the losses, thus laser output ceases. Since the stimulation
process was started by the initiating photons, the emitted photons can combine coherently, as all
of them are in phase with each other, unlike in the case of spontaneous emission and coherent
laser light is emitted (Fig.5). Though the laser action will continue as long as the energy is given
to the active medium, it may be stated that pulsed laser is obtained if the population inversion is
available in a transient fashion and continuous wave (CW) laser is possible if the population
inversion is maintained in a steady-state basis. If the input energy is given by say a flash lamp,
the output will be a pulsed output and the laser is called a pulsed laser. If equilibrium can be
achieved between the number of photons emitted and the number of atoms in the metastable
level by pumping with a continuous arc lamp instead of a flash lamp, then it is possible to
achieve a continuous laser output, which is called continuous wave laser.
6. We may conclude that, laser action is preceded by three processes, namely, absorption,
spontaneous emission and stimulated emission - absorption of energy to populate upper levels,
spontaneous emission to produce the initial photons for stimulation and finally, stimulated
emission for generation of coherent output or laser.
Characteristics of Laser Beam
Laser light has three unique characteristics, that make it different than "ordinary" light. It is:
Monochromatic
Directional
Coherent
Monochromatic means that it consists of one single color or wavelength. Even through some
lasers can generate more than one wavelength, the light is extremely pure and consists of a very
narrow spectral range.
Directional means that the beam is well collimated (very parallel) and travels over
long distances with very little spread.
Coherent means that all the individual waves of light are moving precisely together
through time and space, i.e. they are in phase.
Coherence:
Definition: a fixed phase relationship between the electric field values at different locations or at
different times
Coherence is one of the most important concepts in optics and is strongly related to the ability of
light to exhibit interference effects. A light field is called coherent when there is a fixed phase
relationship between the electric field values at different locations or at different times.Partial
coherence means that there is some (although not perfect) correlation between phase values. There
are various ways of quantifying the degree of coherence, as described below.
It is also common to call certain processes or techniques coherent or incoherent. In that case,
“coherent” essentially means phase-sensitive. For example, the general method of coherent beam
7. combining relies on the mutual coherence of beams, whereas spectral (incoherent) beam
combining does not
− Spatial Versus Temporal Coherence
There are two very different aspects of coherence:
Spatial coherence means a strong correlation (fixed phase relationship) between the electric fields at
different locations across the beam profile. For example, within a cross-section of a beam from a laser with
diffraction-limited beam quality, the electric fields at different positions oscillate in a totally correlated way,
even if the temporal structure is complicated by a superposition of different frequency components. Spatial
coherence is the essential prerequisite of the strong directionality of laser beams.
Temporal coherence means a strong correlation between the electric fields at one location but different
times. For example, the output of a single-frequency laser can exhibit a very high temporal coherence, as
the electric field temporally evolves in a highly predictable fashion: it exhibits a clean sinusoidal oscillation
over extended periods of time.
He-Ne Laser
How the Helium-Neon Laser Works?
10. There are three principal elements of a laser, which are (1) an energy pump, (2) an
optical gain medium, and (3) an optical resonator. These three elements are described
in detail below for the case of the HeNe laser .
(1) Energy pump.
A 1400 V high voltage, DC power supply maintains a glow discharge or plasma in a
glass tube containing an optimal mixture (typically 5:1 to 7:1) of helium and neon gas,
as shown in Fig. 1 and indicated in the diagram ofFig. 2. The discharge current is
limited to about 5 mA by a 91 k ballast resistor. Energetic electrons accelerating
from the cathode to the anode collide with He and Ne atoms in the laser tube,
producing a large number of neutral He and Ne atoms in excited states. He and Ne
atoms in excited states can deexcite and return to their ground states by spontaneously
emitting light. This light makes up the bright pink-red glow of the plasma that is seen
even in the absence of laser action.
The process of producing He and Ne in specific excited states is known as pumping
and in the HeNe laser this pumping process occurs through electron-atom collisions in
a discharge. In other types of lasers, pumping is achieved by light from a bright
flashlamp or by chemical reactions. Common to all lasers is the need for some process
to prepare an ensemble of atoms, ions or molecules in appropriate excited states so
that a desired type of light emission can occur.
(2) Optical gain medium.
To achieve laser action it is necessary to have a large number of atoms in excited
states and to establish what is termed a population inversion. To understand the
significance of a population inversion to HeNe laser action, it is useful to consider the
processes leading to excitation of He and Ne atoms in the discharge, using the
simplified diagram of atomic He and Ne energy levels given in Fig. 3. A description
of the rather complex HeNe excitation process can be given in terms of the following
four steps.
(a) An energetic electron collisionally excites a He atom to the state labeled
21
S0 in Fig. 3. A He atom in this excited state is often written He*(21
S0), where the
asterisk means that the He atom is in an excited state.
(b) The excited He*(21
S0) atom collides with an unexcited Ne atom and the atoms
exchange internal energy, with an unexcited He atom and excited Ne atom, written
Ne*(3S2), resulting. This energy exchange process occurs with high probability only
because of the accidental near equality of the two excitation energies of the two levels
in these atoms.
11. (c) The 3S2 level of Ne is an example of a metastable atomic state, meaning that it is
only after a relatively long period of time - on atomic time scales - that the Ne*(3S2)
atom deexcites to the 2P4 level by emitting a photon of wavelength 6328 Å. It is this
emission of 6328 Å light by Ne atoms that, in the presence of a suitable optical
configuration, leads to lasing action.
(d) The excited Ne*(2P4) atom rapidly deexcites to its ground state by emitting
additional photons or by collisions with the plasma tube walls. Because of the extreme
quickness of the deexcitation process, at any moment in the HeNe plasma, there are
more Ne atoms in the 3S2 state than there are in the 2P4 state, and a population
inversion is said to be established between these two levels.
When a population inversion is established between the 3S2 and 2P4 levels of the Ne
atoms in the discharge, the discharge can act as an optical gain or amplification
medium for light of wavelength 6328 Å. This is because a photon incident on the gas
discharge will have a greater probability of being replicated in a 3S2-->2P4 stimulated
emission process (discussed below) than of being destroyed in the complementary
2P4-->3S2 absorption process.
(3) Optical resonator or cavity.
As mentioned in 2(c) above, Ne atoms in the 3S2 metastable state decay
spontaneously to the 2P4 level after a relatively long period of time under normal
circumstances; however, a novel circumstance arises if, as shown inFig. 2, a HeNe
discharge is placed between two highly reflecting mirrors that form an optical cavity
or resonator along the axis of the discharge. When a resonator structure is in place,
photons from the Ne* 3S2-->2P4transition that are emitted along the axis of the cavity
can be reflected hundreds of times between the two highly reflecting end mirrors of
the cavity. These reflecting photons can interact with other excited Ne*(3S2) atoms
and cause them to emit 6328 Å light in a process known as stimulated emission. The
new photon produced in stimulated emission has the same wavelength and
polarization, and is emitted in the same direction, as the stimulating photon. It is
sometimes useful for purposes of analogy to think of the stimulated emission process
as a "cloning" process for photons, as depicted in Fig. 4. The stimulated emission
process should be contrasted with spontaneous emission processes that, because they
are not caused by any preceding event, produce photons that are emitted isotropically,
with random polarization, and over a broader range of wavelengths.
As stimulated emission processes occur along the axis of the resonator a situation
develops in which essentially all 3S2-->2P4 Ne* decays contribute deexcitation
photons to the photon stream reflecting between the two mirrors. This photon
multiplication (light amplification) process produces a very large number of photons
12. of the same wavelength and polarization that travel back and forth between the two
cavity mirrors. To extract a light beam from the resonator, it is only necessary to have
one of the two resonator mirrors, usually called the output coupler, have a reflectivity
of only 99% so that 1% of the photons incident on it travel out of the resonator to
produce an external laser beam. The other mirror, called the high reflector, should be
as reflective as possible. The small diameter, narrow bandwidth, and strong
polarization of the HeNe laser beam are determined by the properties of the resonator
mirrors and other optical components that lie along the axis of the optical resonator.
Semiconductor Laser
Definition: lasers based on semiconductor gain media
Semiconductor lasers are lasers based on semiconductor gain media, where optical gain is
usually achieved by stimulated emission at an interband transition under conditions of a high
carrier density in the conduction band.
The physical origin of gain in a semiconductor (for the usual case of an interband transition) is
illustrated in Figure 1. Without pumping, most of the electrons are in the valence band. A pump
beam with a photon energy slightly above the bandgap energy can excite electrons into a higher
state in the conduction band, from where they quickly decay to states near the bottom of the
conduction band. At the same time, the holes generated in the valence band move to the top of
the valence band. Electrons in the conduction band can then recombine with these holes, emitting
photons with an energy near the bandgap energy. This process can also be stimulated by
incoming photons with suitable energy. A quantitative description can be based on the Fermi–
Dirac distributions for electrons in both bands.
Most semiconductor lasers are laser diodes, which are pumped with an electrical current in a
region where an n-doped and a p-doped semiconductor material meet. However, there are
also optically pumped semiconductor lasers, where carriers are generated by absorbed pump
light, and quantum cascade lasers, where intraband transitions are utilized.
Figure 1: Physical origin of gain in a semiconductor.
Common materials for semiconductor lasers (and for other optoelectronic devices) are
13. GaAs (gallium arsenide)
AlGaAs (aluminum gallium arsenide)
GaP (gallium phosphide)
InGaP (indium gallium phosphide)
GaN (gallium nitride)
InGaAs (indium gallium arsenide)
GaInNAs (indium gallium arsenide nitride)
InP (indium phosphide)
GaInP (gallium indium phosphide)
These are all direct bandgap semiconductors; indirect bandgap semiconductors such as silicon do
not exhibit strong and efficient light emission. As the photon energy of a laser diode is close to
the bandgap energy, compositions with different bandgap energies allow for different emission
wavelengths. For the ternary and quaternary semiconductor compounds, the bandgap energy can
be continuously varied in some substantial range. In AlGaAs = AlxGa1−xAs, for example, an
increased aluminum content (increased x) causes an increase in the bandgap energy.
While the most common semiconductor lasers are operating in the near-infrared spectral region,
some others generate red light (e.g. in GaInP-based laser pointers) or blue or violet light (with
gallium nitrides). For mid-infrared emission, there are e.g. lead selenide (PbSe) lasers (lead salt
lasers) and quantum cascade lasers.
Apart from the above-mentioned inorganic semiconductors, organic semiconductor compounds
might also be used for semiconductor lasers. The corresponding technology is by far not mature,
but its development is pursued because of the attractive prospect of finding a way for cheap mass
production of such lasers. So far, only optically pumped organic semiconductor lasers have been
demonstrated, whereas for various reasons it is difficult to achieve a high efficiency with
electrical pumping.
Types of Semiconductor Lasers:
There is a great variety of different semiconductor lasers, spanning wide parameter
regions and many different application areas:
Small edge-emitting laser diodes generate a few milliwatts (or up to 0.5 W) of output power in
a beam with high beam quality. They are used e.g. in laser pointers, in CD players, and for optical
fiber communications.
External cavity diode lasers contain a laser diode as the gain medium of a longer laser cavity. They
are often wavelength-tunable and exhibit a small emission linewidth.
Both monolithic and external-cavity low-power levels can also be mode-locked for ultrashort
pulse generation.
Broad area laser diodes generate up to a few watts of output power, but with significantly
poorer beam quality.
High-power diode bars contain an array of broad-area emitters, generating tens of watts with
poor beam quality.
14. High-power stacked diode bars contain stacks of diode bars for the generation of extremely high
powers of hundreds or thousands of watts.
Surface-emitting lasers (VCSELs) emit the laser radiation in a direction perpendicular to the wafer,
delivering a few milliwatts with high beam quality.
Optically pumped surface-emitting external-cavity semiconductor lasers (VECSELs) are capable of
generating multi-watt output powers with excellent beam quality, even in mode-locked operation.
Quantum cascade lasers operate on intraband transitions (rather than interband transitions) and
usually emit in the mid-infrared region, sometimes in the terahertz region. They are used e.g. for
trace gas analysis.
Typical Characteristics and Applications:
Some typical aspects of semiconductor lasers are:
Electrical pumping with moderate voltages and high efficiency is possible
particularly for high-power diode lasers, and allows their use e.g. as pump sources
for highly efficient solid-state lasers (→ diode-pumped lasers).
A wide range of wavelengths are accessible with different devices, covering much
of the visible, near-infrared and mid-infrared spectral region. Some devices also
allow for wavelength tuning.
Small laser diodes allow fast switching and modulation of the optical power,
allowing their use e.g. in transmitters of fiber-optic links.
Typical Characteristics and Applications:
Some typical aspects of semiconductor lasers are:
Electrical pumping with moderate voltages and high efficiency is possible
particularly for high-power diode lasers, and allows their use e.g. as pump sources
for highly efficient solid-state lasers (→ diode-pumped lasers).
A wide range of wavelengths are accessible with different devices, covering much
of the visible, near-infrared and mid-infrared spectral region. Some devices also
allow for wavelength tuning.
Small laser diodes allow fast switching and modulation of the optical power,
allowing their use e.g. in transmitters of fiber-optic links.
Uses/Applications of LASER :
Although the first working laser was only produced in 1958, lasers are now found in many
household items. For example, lasers are well-known through their use as cheap laser pointers.
However, lasers can be very dangerous to the human eye since a large amount of energy is
focused into a very narrow beam. NEVER POINT A LASER POINTER INTO SOMEBODY'S
EYES - IT CAN BLIND THEM FOREVER.
Other uses include:
Semiconductor lasers which are small, efficient and cheap to make are used in CD
players.
15. He-Ne Lasers are used in most grocery shops to read in the price of items using
their barcodes. This makes the cashiers' job much quicker and easier.
High energy lasers are used in medicine as a cutting and welding tool. Eye surgery
in particular make use of the precision of lasers to reattach the retinas of patients'
eyes. The heat from cutting lasers also helps to stop the bleeding of a wound by
burning the edges (called cauterising).
Applications:
laser printers
laser communication and fibre optics
optical storage
using lasers as precision measurement tools
