This document provides information about lasers, specifically discussing spontaneous emission, stimulated emission, how lasers work, population inversion, and characteristics of laser beams. It then describes the Helium-Neon laser in detail, including how it is pumped through electron collisions, its gain medium of Helium and Neon gases, and the optical resonator that allows stimulated emission to produce coherent laser light. Key points are that lasers require population inversion to produce stimulated emission of coherent, monochromatic, and directional laser light.
Laser is very important technological device these days.There is a use of laser in almost every field of science and technology. It also gives it's application in medicines also.
This presentation shows how it works and what is the mechanism behind this laser phenomenon. Here it is explained from atom theory to application.
Very good explanation with photographs.
Laser is very important technological device these days.There is a use of laser in almost every field of science and technology. It also gives it's application in medicines also.
This presentation shows how it works and what is the mechanism behind this laser phenomenon. Here it is explained from atom theory to application.
Very good explanation with photographs.
A detailed presentation on fraunhofer diffraction and also an introduction to the concept of diffraction.There is also a brief discussion on fresnel diffraction and the difference between former and the latter.
Introduction to semiconductor lasers, and its working. construction of semiconductor laser, Ga As laser, and construction, achievement of population inversion, pumping.
Laser, Pumping schemes, types of lasers and applicationsPraveen Vaidya
The document gives good insite into the different pumping schemes, different types of lasers and Applications like Holographys, laser cutting and Laser Beam Welding.
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation
A detailed presentation on fraunhofer diffraction and also an introduction to the concept of diffraction.There is also a brief discussion on fresnel diffraction and the difference between former and the latter.
Introduction to semiconductor lasers, and its working. construction of semiconductor laser, Ga As laser, and construction, achievement of population inversion, pumping.
Laser, Pumping schemes, types of lasers and applicationsPraveen Vaidya
The document gives good insite into the different pumping schemes, different types of lasers and Applications like Holographys, laser cutting and Laser Beam Welding.
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation
This article discusses the basics of Laser operations. A lucid idea about the types of emission i.e. stimulated, spontaneous, Einstein's Coefficient, Pumping, Lasing, Population Inversion will be sought. Types of Laser and applications of Laser are also discussed in this article.
What is Fluorescence Electrons in an atom or a m.pdfapnashop1
What is Fluorescence? Electrons in an atom or a molecule can absorb the energy in
the electromagnetic radiation and thereby excite to an upper energy state. This upper energy state
is unstable; therefore, electron likes to come back to the ground state. When coming back, it
emits the absorbed wavelength. In this relaxation process, they emit excess energy as photons.
This relaxation process is known as fluorescence. Fluorescence takes place much more rapidly.
Generally, it completes in about 10-5 s or less time from the time of excitation. In atomic
fluorescence, gaseous atoms fluoresce when they are exposed to radiation with a wavelength that
exactly matches one of the absorption lines of the element. For example, gaseous sodium atoms
absorb and excite by absorbing 589 nm radiations. Relaxation takes place after this by
reemission of fluorescent radiation of the identical wavelength. Because of this, we can use
fluorescence to identify different elements. When excitation and reemission wavelengths are the
same, the resulting emission is called resonance fluorescence. Other than fluorescence, there are
other mechanisms by which an excited atom or molecule can give up its excess energy and relax
to its ground state. Nonradiative relaxation and fluorescence emissions are two such important
mechanisms. Because of many mechanisms, the lifetime of an excited state is brief. The relative
number of molecules that fluoresce is small because fluorescence requires structural features that
slow the rate of the nonradiative relaxation and enhance the rate of fluorescence. In most
molecules, these features are not there; therefore, they undergo nonradiative relaxation, and
fluorescence does not occur. Molecular fluorescence bands are made up of a large number of
closely spaced lines; therefore, usually it is hard to resolve. What is Phosphorescence? When
molecules absorb light and go to the excited state they have two options. They can either release
energy and come back to the ground state immediately or undergo other non-radiative processes.
If the excited molecule undergoes a non radiative process, it emits some energy and come to a
triplet state where the energy is somewhat lesser than the energy of the exited state, but it is
higher than the ground state energy. Molecules can stay a bit longer in this less energy triplet
state. This state is known as the metastable state. Then metastable state (triplet state) can slowly
decay by emitting photons, and come back to the ground state (singlet state). When this happens
it is known as phosphorescence. What is the difference between Fluorescence and
Phosphorescence? • When light is supplied to a sample of molecules, we immediately see the
fluorescence. Fluorescence stops as soon as we take away the light source. But phosphorescence
tends to stay little longer even after the irradiating light source is removed. • Fluorescence takes
place when excited energy is released, and the molecule comes back to the gro.
Laser Action
The combination of spontaneous emission first, and then stimulated emission, causes the laser to "lase," which means it generates a coherent beam of light at a single frequency.
Fluorimetry, principle, Concept of singlet,doublet,and triplet electronic sta...Vandana Devesh Sharma
Content-Principle
concept of singlet, doublet and triplet electronic stages,
Internal and external conversions,
Factors affecting fluorescence,
quenching,
Instrumentation and
applications
Types of luminescence including
bioluminescence,
chemiluminescence,
Fluorescence, and
phosphorescence
These various forms of luminescence differ in their method of emitting light.
Bioluminescence
Chemiluminescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation
In fluorescence, absorption and emission light takes place in very short time (10-12 or 10-9 seconds)
Fluorescence starts immediately after the absorption of light and stops as soon as the incident light is cut off
Eg -The fluorescent clothes, shoes
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation
In fluorescence, absorption and emission light takes place in very short time (10-12 or 10-9 seconds)
Fluorescence starts immediately after the absorption of light and stops as soon as the incident light is cut off
Eg -The fluorescent clothes, shoes
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation
In fluorescence, absorption and emission light takes place in very short time (10-12 or 10-9 seconds)
Fluorescence starts immediately after the absorption of light and stops as soon as the incident light is cut off
Eg -The fluorescent clothes, shoes
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation
In fluorescence, absorption and emission light takes place in very short time (10-12 or 10-9 seconds)
Fluorescence starts immediately after the absorption of light and stops as soon as the incident light is cut off
Eg -The fluorescent clothes, shoes
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation
In fluorescence, absorption and emission light takes place in very short time (10-12 or 10-9 seconds) Fluorimetry
An analytical technique for identifying and characterizing minute amounts of substance by excitation of the substance with a beam of ultraviolet/Visible light and detection and measurement of the characteristic wavelength of fluorescent light emitted.Excited – State Processes in molecules
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight.
In molecular spectroscopy, a Jablonski diagram is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity.
Instructions for Submissions thorugh G- Classroom.pptxJheel Barad
This presentation provides a briefing on how to upload submissions and documents in Google Classroom. It was prepared as part of an orientation for new Sainik School in-service teacher trainees. As a training officer, my goal is to ensure that you are comfortable and proficient with this essential tool for managing assignments and fostering student engagement.
Palestine last event orientationfvgnh .pptxRaedMohamed3
An EFL lesson about the current events in Palestine. It is intended to be for intermediate students who wish to increase their listening skills through a short lesson in power point.
A Strategic Approach: GenAI in EducationPeter Windle
Artificial Intelligence (AI) technologies such as Generative AI, Image Generators and Large Language Models have had a dramatic impact on teaching, learning and assessment over the past 18 months. The most immediate threat AI posed was to Academic Integrity with Higher Education Institutes (HEIs) focusing their efforts on combating the use of GenAI in assessment. Guidelines were developed for staff and students, policies put in place too. Innovative educators have forged paths in the use of Generative AI for teaching, learning and assessments leading to pockets of transformation springing up across HEIs, often with little or no top-down guidance, support or direction.
This Gasta posits a strategic approach to integrating AI into HEIs to prepare staff, students and the curriculum for an evolving world and workplace. We will highlight the advantages of working with these technologies beyond the realm of teaching, learning and assessment by considering prompt engineering skills, industry impact, curriculum changes, and the need for staff upskilling. In contrast, not engaging strategically with Generative AI poses risks, including falling behind peers, missed opportunities and failing to ensure our graduates remain employable. The rapid evolution of AI technologies necessitates a proactive and strategic approach if we are to remain relevant.
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
The empire's roots lie in the city of Rome, founded, according to legend, by Romulus in 753 BCE. Over centuries, Rome evolved from a small settlement to a formidable republic, characterized by a complex political system with elected officials and checks on power. However, internal strife, class conflicts, and military ambitions paved the way for the end of the Republic. Julius Caesar’s dictatorship and subsequent assassination in 44 BCE created a power vacuum, leading to a civil war. Octavian, later Augustus, emerged victorious, heralding the Roman Empire’s birth.
Under Augustus, the empire experienced the Pax Romana, a 200-year period of relative peace and stability. Augustus reformed the military, established efficient administrative systems, and initiated grand construction projects. The empire's borders expanded, encompassing territories from Britain to Egypt and from Spain to the Euphrates. Roman legions, renowned for their discipline and engineering prowess, secured and maintained these vast territories, building roads, fortifications, and cities that facilitated control and integration.
The Roman Empire’s society was hierarchical, with a rigid class system. At the top were the patricians, wealthy elites who held significant political power. Below them were the plebeians, free citizens with limited political influence, and the vast numbers of slaves who formed the backbone of the economy. The family unit was central, governed by the paterfamilias, the male head who held absolute authority.
Culturally, the Romans were eclectic, absorbing and adapting elements from the civilizations they encountered, particularly the Greeks. Roman art, literature, and philosophy reflected this synthesis, creating a rich cultural tapestry. Latin, the Roman language, became the lingua franca of the Western world, influencing numerous modern languages.
Roman architecture and engineering achievements were monumental. They perfected the arch, vault, and dome, constructing enduring structures like the Colosseum, Pantheon, and aqueducts. These engineering marvels not only showcased Roman ingenuity but also served practical purposes, from public entertainment to water supply.
Honest Reviews of Tim Han LMA Course Program.pptxtimhan337
Personal development courses are widely available today, with each one promising life-changing outcomes. Tim Han’s Life Mastery Achievers (LMA) Course has drawn a lot of interest. In addition to offering my frank assessment of Success Insider’s LMA Course, this piece examines the course’s effects via a variety of Tim Han LMA course reviews and Success Insider comments.
The French Revolution, which began in 1789, was a period of radical social and political upheaval in France. It marked the decline of absolute monarchies, the rise of secular and democratic republics, and the eventual rise of Napoleon Bonaparte. This revolutionary period is crucial in understanding the transition from feudalism to modernity in Europe.
For more information, visit-www.vavaclasses.com
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
Letter from the Congress of the United States regarding Anti-Semitism sent June 3rd to MIT President Sally Kornbluth, MIT Corp Chair, Mark Gorenberg
Dear Dr. Kornbluth and Mr. Gorenberg,
The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
This Congress will not stand idly by and allow an environment hostile to Jewish students to persist. The House believes that your institution is in violation of Title VI of the Civil Rights Act, and the inability or
unwillingness to rectify this violation through action requires accountability.
Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
students that opportunity and have been hijacked to become venues for the promotion of terrorism, antisemitic harassment and intimidation, unlawful encampments, and in some cases, assaults and riots.
The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
Welcome to TechSoup New Member Orientation and Q&A (May 2024).pdfTechSoup
In this webinar you will learn how your organization can access TechSoup's wide variety of product discount and donation programs. From hardware to software, we'll give you a tour of the tools available to help your nonprofit with productivity, collaboration, financial management, donor tracking, security, and more.
Welcome to TechSoup New Member Orientation and Q&A (May 2024).pdf
Laser notes pdf
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