How Lasers Work - A Complete Guide
Summary
TLDRThis script delves into the fascinating world of lasers, explaining their ubiquitous presence in both scientific research and industry. It begins with the history of the laser, from Einstein's concept of stimulated emission in 1917 to the first working laser in 1960. The script highlights lasers' unique properties: their narrow line width for monochromatic light, coherence for synchronized photon behavior, and the ability to focus high intensity light into a small area. It then breaks down the complex quantum mechanics behind laser operation, including stimulated absorption, spontaneous emission, and the crucial stimulated emission process. The explanation of how a laser cavity supports specific frequencies and the role of the gain medium in determining the emitted light's color complete this comprehensive overview.
Takeaways
- 🌟 Lasers are widely used in both scientific research and industry for their unique properties.
- 🔤 The term 'laser' is an acronym for 'Light Amplification by Stimulated Emission of Radiation'.
- 📚 The concept of the laser was introduced by Einstein in 1917, with the first working laser being developed by Theodor Maiman in 1960.
- 🔴 The first laser used synthetic Ruby to excite atoms to higher energy levels, creating a powerful beam of light.
- 🎯 Lasers are characterized by three main properties: narrow line width (monochromaticity), coherence, and the ability to focus high intensity light into a small area.
- 🔬 The purity of a laser's light, known as line width, is much narrower than other light sources, making it ideal for scientific experiments requiring specific energies.
- 🌀 Coherent light from a laser is polarized and in phase, similar to an orchestra playing in sync, allowing for the concentration of light energy over a distance.
- 💡 The high intensity of lasers is useful in applications such as military targeting and medical procedures like laser eye surgery.
- 🤔 The functioning of a laser involves complex quantum mechanics, including stimulated absorption, spontaneous emission, and stimulated emission.
- 🔄 To create a laser, a population inversion is necessary where there are more electrons in the excited state than the ground state, favoring stimulated emission.
- 🔍 A laser cavity with mirrors helps to create standing waves through constructive interference, amplifying the light waves and producing a coherent beam.
- 📏 The allowed frequencies in a laser cavity are determined by the cavity's length and the speed of light, with imperfections in the mirrors broadening these frequencies slightly.
Q & A
What does the acronym LASER stand for?
-LASER stands for Light Amplification by Stimulated Emission of Radiation.
When was the concept of stimulated emission introduced by Einstein?
-The concept of stimulated emission was introduced by Einstein in 1917.
What was the first device based on Einstein's predictions that achieved amplification and generation of electromagnetic waves?
-The first device was the MASER (Microwave Amplification by Stimulated Emission of Radiation), demonstrated by Charles Townes in 1954.
Who developed the first working laser and when was it developed?
-Theodor Maiman developed the first working laser at Hughes Research Lab in 1960.
What are the three unique properties of a laser?
-The three unique properties of a laser are line width (monochromaticity), coherence, and the ability to deliver high-intensity light to a small area.
What is the significance of a laser's narrow line width?
-A narrow line width signifies that the emitted light is close to a single frequency, which is useful for scientific experiments that require analysis with specific energies.
How does coherence in a laser differ from the light emitted by an LED?
-Coherent light from a laser is polarized in the same direction and is in phase, whereas an LED emits incoherent light where the light waves are not synchronized.
What is the process called when an electron in an excited state falls back down to a lower energy state and emits a photon?
-This process is called spontaneous emission.
What is stimulated emission and why is it important for lasers?
-Stimulated emission is when an excited electron is forced to fall back to a lower energy state by a photon, emitting an identical photon in the process. It is important for lasers because it allows for the creation of a beam of identical, coherent photons.
What is a population inversion and why is it necessary for a laser to operate?
-A population inversion is a condition where there are more electrons in the excited state (metastable state) than in the ground state. It is necessary for a laser to operate because it allows for the continuous stimulated emission of photons.
How does a laser cavity contribute to the amplification of light?
-A laser cavity, with a mirror on one side and a partial mirror on the other, allows light waves to reflect back and forth, creating standing waves through constructive interference, which amplifies the light through stimulated emission.
What is the role of the gain medium in a laser?
-The gain medium is the material in a laser that provides the energy levels necessary for stimulated emission. Different materials will emit photons of different energies, determining the laser's output frequency.
How do the allowed frequencies in a laser cavity relate to the laser's output frequency?
-The allowed frequencies in a laser cavity are the frequencies that can resonate within the cavity and produce standing waves. The laser's output frequency must be one of these allowed frequencies that also falls within the gain medium's emission range.
What factors cause the broadening of the frequency range emitted by a laser?
-Factors such as the Doppler effect, Stark effect, and other quantum mechanical behaviors cause the broadening of the frequency range emitted by a laser, resulting in a gain curve rather than a single frequency.
Outlines
🌌 Introduction to Lasers and Their History
This paragraph introduces the ubiquity of lasers in both scientific research and industry. It explains the acronym LASER, which stands for 'Light Amplification by Stimulated Emission of Radiation', and provides a brief historical context starting with Einstein's introduction of stimulated emission in 1917. The first working laser was developed by Theodor Maiman in 1960 using a synthetic ruby. The paragraph also outlines the collaborative nature of the laser's development and hints at the unique properties of lasers that make them so useful.
🔍 The Unique Properties and Utility of Lasers
This section delves into the three main properties that make lasers so versatile: line width, coherence, and power. Line width refers to the purity of the laser light, which is much narrower than any other light source, making it monochromatic. Coherence describes the laser light's uniform polarization and phase, akin to an orchestra playing in sync. Lastly, the power of lasers allows for the concentration of high-intensity light in a small area, which is beneficial for various applications, including military and medical uses such as laser eye surgery.
🔬 Quantum Mechanics Behind Laser Operation
This paragraph explains the complex workings of a laser, grounded in quantum mechanics. It discusses the fundamental processes of stimulated absorption, spontaneous emission, and stimulated emission. Stimulated absorption occurs when a photon boosts an electron to a higher energy state. Spontaneous emission is the natural decay of an excited electron back to a lower state, releasing a photon. Stimulated emission is pivotal for lasers, where an incoming photon triggers an excited electron to emit an identical photon, leading to a coherent light beam. The paragraph also introduces the concept of metastable states, which are essential for creating the population inversion necessary for laser action.
🏗️ Constructing a Laser: The Laser Cavity and Resonance
This section describes the construction of a laser cavity, which is crucial for amplifying light waves through constructive interference. The cavity typically consists of a fully reflective mirror and a partially reflective mirror, allowing some light to escape as the laser beam. The paragraph explains how light waves are amplified through stimulated emission within the cavity, leading to the formation of standing waves due to resonance. These standing waves are the result of multiple light waves interfering constructively, and the conditions for their formation are governed by an equation relating the cavity's length, the mode number, and the speed of light.
📊 Laser Cavity Frequencies and Gain Medium
The final paragraph discusses the allowed frequencies within a laser cavity and how they are determined by the cavity's dimensions and the speed of light. It explains that not all frequencies can resonate within the cavity, and the specific frequencies are influenced by the cavity's imperfections. The paragraph also introduces the concept of the gain medium, which is the material used in a laser to produce light through stimulated emission. Different materials are used for different laser frequencies, and the emitted frequencies are influenced by various quantum mechanical effects, resulting in a gain curve that represents the range of frequencies the laser can emit.
🌈 Conclusion: The Variety of Laser Materials
Concluding the script, this paragraph highlights the wide variety of laser materials available, including gases, solids, and even liquids, which enable the creation of lasers emitting a broad spectrum of frequencies. The paragraph wraps up by encouraging viewers to engage with the content and supporting the channel, reinforcing the educational value of the video.
Mindmap
Keywords
💡Laser
💡Stimulated Emission
💡Line Width
💡Coherence
💡Power
💡Quantum Mechanics
💡Metastable State
💡Population Inversion
💡Cavity
💡Gain Medium
💡Doppler Effect
Highlights
Lasers are ubiquitous in both scientific research and industry, used for various applications from hair removal to popping balloons.
Laser stands for Light Amplification by Stimulated Emission of Radiation, a term coined by Einstein in 1917.
The first laser was developed in 1960 by Theodor Maiman at Hughes Research Lab, using synthetic ruby as the lasing medium.
Lasers are valuable due to their narrow line width, meaning they emit light at a very specific frequency, which makes them monochromatic.
Coherence is a key property of lasers, meaning the emitted light is all polarized in the same direction and in phase.
Lasers can deliver high-intensity light to a small area, which is crucial for applications like laser eye surgery and military uses.
The process of laser operation involves stimulated absorption, spontaneous emission, and stimulated emission.
Stimulated absorption occurs when a photon excites an electron to a higher energy state.
Spontaneous emission happens when an excited electron falls back to a lower energy state, releasing a photon.
Stimulated emission is critical for lasers, where an incoming photon causes an excited electron to emit a second, identical photon.
Lasers require a population inversion, where more electrons are in an excited state than in the ground state.
Laser cavities amplify light by constructive interference, using mirrors to reflect light back and forth.
The gain medium in a laser determines the wavelength of the emitted light, with different materials producing different colors.
Standing waves in the laser cavity produce resonance, amplifying the light to create the laser beam.
Modern lasers can use various materials, including gases, solids, and liquids, to produce a wide range of wavelengths.
Transcripts
everyone has seen them and have probably
teased many cats with them maybe some of
you have had unwanted hair removed or
maybe you have built one and popped some
balloons with it bottom line lasers are
ubiquitous not only in scientific
research but also in Industry just how
do these little devices manage to put
out that nice powerful cated beam of
light all this and more coming up as
some may or may not know laser is
actually an acronym it stands for light
amplification by stimulated emission of
radiation however nowadays it is so
common that people don't bother to
capitalize it and simply write laser a
very brief history of the laser starts
in 1917 when Einstein introduced the
concept of stimulated emission which
will be explained shortly then in 195
before the first Mesa was demonstrated
by Charles towns the M standing for
microwave the ammonia Mesa was the first
device based on Einstein's predictions
and obtained the first amplification and
generation of electromagnetic waves with
a wavelength of about 1 cm which is in
the microwave range this is recognized
as the precursor to the laser it wasn't
until 1960 when Theodor mayam developed
the first working laser at Hughes
research lab mayon's early laser used a
powerful energy source to excite atoms
in a synthetic Ruby to higher energy
levels the development of the laser was
a collaborative effort by scientists and
Engineers who were leaders in Optics and
photonics okay so why are lasers useful
why are they ubiquitous the answer can
be broken down to three unique
properties the laser holds the first
being line width the purity of a laser
referred to as the line width can can be
quite narrow more so than any other
light source in layman's terms this is a
measure of what frequencies are
contained in the emitted light the
narrower the line withd the closer the
emitted light is to a single frequency
single color if you will thus a laser is
said to be monochromatic in reality it
does output a small range of
frequencies the smaller this range the
better the line width and quality of the
laser in contrast an incandescent bulb
has a very large line width and emits
the broad spectrum which is why the
emitted light is white white light is a
superposition of all the colors in the
visible
spectrum having a narrow line width is
useful because many scientific
experiments want to analyze stuff with
certain energies different wavelengths
of light corresponds to different
energies hence having a source with one
energy is helpful the second is
coherence the light emitted by a laser
is coherent light this means it is all
polarized in the same direction as well
as being in Phase the laser is said to
Output highly coherent monochromatic
light and led on the other hand is also
monochromatic one color but it emits
incoherent light an analogy with
synchronization and Harmony can be made
imagine an orchestra playing if the
orchestra is in sync and everyone is
playing the parts correctly it will be
pleasing to the ear the laser if some
players are playing out of sync but
still playing the parts correctly it
won't sound as good the D coherence is
important because all the photons add
their energies together and we can then
focus them on a small spot over some
distance lastly power lasers make it
possible to deliver High intense light
to a small area of course militaries are
particularly interested in this aspect
of the laser as well as medical
applications laser ey surgery for
example now let's take a look at how a
laser works the workings of a laser are
quite complex as it requires an
understanding of quantum mechanics there
are some commonalities behind every
laser the first part can be broken down
to three key pieces stimulated
absorption spontaneous emission and
stimulated emission which is what the SE
part of laser stands for let's take a
look at the first concept stimulated
absorption we will need a nucleus that
is made up of protons and neutrons that
has an overall positive charge and an
electron that has a negative charge
hey there little guy most textbooks show
electrons existing in discrete energy
states of a material but actually
electrons exist in probability density
clouds around the nucleus as they have
wave likee Behavior and the orbitals
represent the average distance one is
likely to find it let's use this average
distance to define the orbital and
ignore the probability distribution for
Simplicity mostly always electrons are
found in the lowest energy state or
ground state everything in nature wants
to be in a low energy State as it is
easier for it to exist at this level in
other words it minimizes energy think of
a ball on a hill and how easy it is for
it to roll down it wants to roll down
because the energy state is lower closer
to the Earth's core than further away in
this case potential
energy however it is possible to excite
electrons by some kind of external means
just like we can exert a force on the
ball that has rolled down and push it
back up light can be this push to excite
electrons if a photon of Light which is
one unit of light comes across an
electron in a low energy state it can
sacrifice itself and push the electron
to a higher energy State the photon is
annihilated but the energy of it is now
part of the excited electron it should
be noted that each material has
different levels of energy in other
words if the ground state is one unit
and the next energy level is 5 units
then the photon of light must have
exactly four units of energy to excise
the electron to that energy level
anything lower will not suffice and
anything higher would not as well as
there is nowhere for that extra energy
to go unless a higher energy State
exists if the incident photon is very
high in energy the electron would be
ionized to continue our analogy it would
be like trying to push the ball up the
hill with not enough Force the ball
would just roll back down too much force
and it would roll down the other side go
to another Plateau or be launched into
space an exact amount of energy is
required to elevate it to a particular
energy State again this process is
called stimulated absorption as we are
stimulating the electron and it absorbs
the photon's energy the next mechanism
we will look at is spontaneous emission
we now have an excited electron what
happens now well again this higher
energy level is quite unstable and after
a very very short time about 100 nond of
being there the electron will eventually
fall for some perspective light travels
about 29 m in 100 NS when it falls back
down it will release a photon with
energy equal to the difference in energy
levels the higher the fall the higher
the energy of the photon will be should
the energy value of the photon that is
released be in the visible range we
would perceive it as color you may be
thinking if the electron reaches the
higher energy level through the
previously mentioned stimulated
absorption mechanism why exactly does it
fall back down well referring back to
the ball example imagine the ball on a
hill but now with the top having zero
friction and a sharp point the ball can
remain there only if it is perfectly
balanced but any tiny little force in
either direction will cause it to start
rolling the electron in this higher
energy state is in a similar situation
the forces that push it are small
perturbations in vacuum energy this is a
quantum mechanical effect space or
vacuum is not as empty as we think
things are popping into and out of
existence constantly it is these vacuum
events that perturb the electron this is
also responsible for why things are
ferromagnetic that's a different story
though again this process is called
spontaneous emission as the process that
the electron falls back down to the
lower energy state is more or less
spontaneous the last Quantum process we
will talk about and the most important
for lasers is stimulated emission this
occurs when a photon interacts with an
electron that is already excited this
Photon can act as a type of pertubation
and force the electron to fall back down
to a lower energy State and emit a
photon we then will have two photons
photons actually like to be together so
if one comes near a situation where
another one could be present such as the
the electron falling back to a lower
energy State the situation usually will
play out the important part is that the
emitted Photon will be identical to the
one that stimulated it meaning same
frequency phase and polarization they
will be coherent with each other so if
we could somehow Avalanche this process
we would have a laser after all that is
basically what a laser is a zip tillan
identical coherent photons being emitted
in contrast if two electrons undergo
spontaneous emission the emitted photons
will unlikely be traveling in the same
direction nor be in Phase but in order
for electrons in the excited energy
level to be able to undergo stimulated
emission and not spontaneous emission
enough time has to be available the
lifetime of an electron in the excited
level is just too short however some
materials have so-called meta stable
States these are excited states with
slightly lower energy than the excited
States States these states allow the
electron to remain there for much longer
lifetimes milliseconds instead of Nan
seconds enough time that a passing
Photon can cause it to undergo
stimulated emission of course an initial
spontaneous emission from the metastable
state to the ground state must occur in
order to have the initial Photon that
can stimulate other excited electrons in
the metastable states to sum up if a
ground state electron is hit with a
photon it will absorb it and move from
the ground state to the excited state
the photon must have the energy equal to
the difference between these levels this
electron will then transition to the
metast stable state if one exists this
transition does not emit a photon and is
said to be a radiationless transition
the energy difference is dissipated in
other ways heat or phons now this
electron if a photon stimulates it will
emit a photon with equal energy phase
and Direction these are the ones that
make up the laser beam it should be
apparent that the photon which pumps the
electron from the ground state to the
excited state has a different energy
than the photons that are being lazed
this is because the energy difference
between the ground state and the excited
state is different than the difference
between the meta stable State and the
ground state the pumping photons are
always higher in energy than the photons
being
lazed we obviously want lots of
electrons in this meta stable State more
so than the ground state in order for
them to be in a situation where
stimulated emission can occur something
known as creating a population inversion
is required if we only had a two levels
we would reach a point of saturation
where 50% of the electrons are excited
and 50% are not the excited electrons
simply spontaneously emit to fast
essentially our medium becomes
transparent to photons by introducing
the metas stable State we force the
pumping photons to excite the ground
state electrons that then transition to
the metastable state so the photons that
are emitted by the transition from the
metastable state to the ground state are
primarily used to stimulate other
electrons in the metastable state enough
time exists for this to happen yes some
of these photons will excite ground
state electrons directly into the
metastable state but the pumping photons
should take care of the majority and
create a situation where there are more
excited electrons in the metast stable
State than ground state electrons a
population
inversion by the way the above is
describing a three-level laser four
level lasers exist and are more
efficient again we want to create an
avalanche effect where the spontaneously
emitted Photon that was created when an
electron transitioned from the
metastable state to the ground state get
Amplified through the means of
stimulated emission
we don't want just a single puny Photon
we want lots all working together it is
not practical to create a laser that is
extremely long so the solution is to put
the laser medium in a cavity let's take
a closer look at how a cavity will
influence the light waves and how
exactly this will create the
amplification we desire since light is a
wave it will be subject to constructive
and destructive
interference we want constructive
interference in our cavity to take place
in order to have a high intensity beam a
laser cavity has a mirror on one side
and a partial mirror on the other it is
partial because we want some of the beam
to escape that's the beam we see now
when light waves are created through
spontaneous emission they will initially
travel in random directions but the ones
traveling perpendicular to the mirrors
will reflect back and forth let's take a
look at one of these light waves it is
first emitted via spontaneous emission
and quickly becomes large in amplitude
through stimulated emission it travels
towards the mirror and is reflected back
because we continue to stimulate atoms
in the left and right directions we get
two waves in the cavity again one moving
to the left and one moving to the right
waves will add their amplitudes when
interfering with each other in this case
we will get a standing wave meaning
instead of a wave noticeably moving to
the left or right the combin wave will
appear to be going up and down rest sure
this is just an illusion this is the
effect of two waves hitting each other
head on and their left and right
components cancel out but their up and
down components add together so when the
wave looks flat this is a moment when
the two waves are destructively
interfering with each other and at the
maximum they are in a constructive
interference Point here are a few
examples of some standing waves in a
cavity that are resonating resonance is
just a fancy word for having these waves
being in a state where standing waves
are being produced a mode being just
what n you have Nal 1 is a mode Nal 2 is
another one n equal 3 Etc is there an
equation that will tell us what modes
can exist in the cavity sure there is
the left part is the frequency that
exists in the cavity n is the mode which
is always an integer V is the velocity
of the wave and L is the distance
between the two sides of the cavity the
Velocity in our equation is the speed of
light C which is 300,000
km/s the L is just the distance between
the mirrors light traveling from the
left of the cavity will now interfere
with light traveling from the right so
again we have these possible modes where
the light can produce standing waves and
be in resonance not all frequencies are
able to exist in a cavity but a lot are
also let's be clear that the standing
waves produces are a collection of
trillions and trillions of light waves
all working together they are produced
by stimulated emission and the cavity
allows them to keep amplifying each
other they are coherent with each other
recall this was one of the big reasons
why we care about lasers if we didn't
have this Synergy between light waves we
would just have an ugly LED I bet you
can't make your cat go crazy with a red
LED well maybe but you get my point
question what frequencies are allowed to
exist in a red Las a point of cavity
answer a cheap red laser pointer has a
cavity length of about 1 mm and the
speed of light is c 300 million
m/s plugging in these values to our
equation we would get a difference
between allowed frequencies of about
10050
GHz Now red light has a frequency of
about 400.0 5 terz which corresponds to
an N value of
2,667 recall n must be an integer so if
400.0 5 terz is an allowed frequency
then the next one would be when n equal
2,668 which is a frequency of 400.2
terz we can plot all allowed frequencies
as we know 150 GHz will separate them
the plot will look like this here we
have an equal to
2,667 and the corresponding frequency of
400.0 five terz here is
2,668
2,669 and so on these are the
frequencies that are allowed to resonate
in this laser cavity so if you wanted
your laser to have a frequency of 400.1
terz you would first have to change the
cavity length for this to be allowed as
it is not possible in this red Laser's
cavity about 2,600 frequencies in the
visible spectrum would be able to
resonate in this red lasers cavity
now there is slightly more to the story
about these allowed frequency lines we
have assumed the mirrors are perfect
which is practically impossible the
imperfectness of the mirrors and other
slight variations add a thickness to the
frequency lines the actual allowed
frequencies in a laser cavity looks like
this again this is due to
imperfections the last piece of the
puzzle is to mention the gain medium
itself gain medium is just the material
we are using for our laser different
materials will have different energy
levels hence photons of different energy
will be released during stimulated
emission for example different materials
will need to be used to create a blue
laser than that of a red laser since the
energy levels in a material are discrete
one would think that exactly one
frequency would be emitted out of a
laser but only if this is also a
frequency allowed in our laser cavity we
can superimpose these ideas on this
graph
we assume here that indeed the
stimulated emitted photon is a frequency
that is allowed in the cavity however
there is much more to the story The
frequencies being emitted out of the
laser actually takes a shape like this
this was briefly mentioned at the
beginning of this video when discussing
L width what is going on here are
complicated events such as the Doppler
effect Stark effect and other quantum
mechanical Behavior the takeaway is that
the gain medium does output a small
range of frequencies and has this gain
curve it is still extremely narrow and
said to be monochromatic it's not but
it's close enough to sum up certain
frequencies are allowed to exist in a
laser cavity there is some relaxation to
these frequencies as the mirrors and
such are not perfect the laser game
medium emits photons in a certain
frequency as well but again there is
some broadness to this as certain
effects influence this we can
superimpose these two frequency Plus
spots and get the following the
frequencies under the game curve that
have enough intensity to overcome other
cavity losses are the ones the laser
emits there are plenty of laser active
medium these days any frequency you wish
to lace is pretty much possible here is
a picture of different laser material
and the frequency they output some are
in the gas State Some solid and it is
even possible to use a liquid as a
Lessing
material this concludes this episode on
the laser if you enjoyed the content and
learned something please consider doing
all that stuff every other video asks
you to do you know what I am talking
about
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