How lasers work - a thorough explanation

Summary

In this informative video, Paul from Physics High Lasers explains the fundamental principles behind laser technology. He begins by highlighting the ubiquity of lasers in various industries and applications. The script delves into the atomic structure, the Bohr model, and the concept of discrete energy levels, leading to the explanation of why lasers are monochromatic, coherent, and collimated. Paul discusses the process of stimulated emission and the importance of achieving population inversion to generate a laser beam. The use of mirrors to create a standing wave and the resulting amplification of photons is also covered, culminating in the production of a focused, monochromatic beam of light.

Takeaways

• 🌈 Lasers are widely used in various fields such as industry, medicine, and everyday life like scanning groceries at supermarkets.
• 🔬 The acronym LASER stands for Light Amplification by the Stimulated Emission of Radiation, which is the fundamental principle behind how lasers work.
• 🎨 A laser beam is monochromatic, meaning it produces light of a single color due to the discrete energy levels of electrons in atoms.
• 🌊 The beam is coherent, with all the light waves being in phase, which is a result of the stimulated emission process.
• 📏 The collimated beam of a laser is very focused and travels in a single direction, which is essential for its applications.
• 💡 The Bohr model of the atom helps explain the discrete energy levels and the process of photon absorption and emission.
• 💥 The process of stimulated emission involves an electron absorbing a photon and then releasing two identical photons, leading to amplification.
• 🔝 Population inversion is a key concept where more electrons are in the excited state than the ground state, which is necessary for the laser effect.
• 🔄 The use of a metastable state allows electrons to stay in an excited state longer, increasing the chances of stimulated emission.
• 🔄 Mirrors are used in a laser setup to create a resonance effect, forming a standing wave that amplifies the photon population exponentially.
• 🚀 A partially transparent mirror allows the coherent laser beam to exit the laser cavity while reflecting most of the light back for further amplification.

Q & A

What are some common uses of lasers mentioned in the script?

Lasers are used in various applications such as pointing at objects like wedding rings, in industrial applications, medical devices including eye surgery, and in supermarkets for scanning groceries.

What are the three main characteristics of a laser beam?

The three main characteristics of a laser beam are that it is monochromatic (produces only one color), coherent (waves are in phase with each other), and collimated (a focused beam).

What does the acronym LASER stand for?

LASER stands for Light Amplification by the Stimulated Emission of Radiation.

What is the Bohr model of the atom and why is it relevant to understanding lasers?

The Bohr model of the atom is a simplistic model that depicts electrons existing in discrete orbits or energy levels. It is relevant to understanding lasers because the discrete energy levels determine the specific energy (and thus color) of the photons emitted, contributing to the laser's monochromaticity.

How does the energy difference between electron energy levels relate to the color of the laser light produced?

The energy difference between electron energy levels corresponds to the energy of the photons produced, which determines the color of the laser light. Since the energy levels are discrete, the laser light is monochromatic, meaning it only produces one color.

What is the process of stimulated emission and how does it contribute to the coherence of a laser beam?

Stimulated emission is a process where an electron in an excited state is stimulated by a photon, causing it to drop to a lower energy level and release two photons of the same energy, frequency, and phase. This process contributes to the coherence of a laser beam because it ensures that all photons are in phase with each other.

What is population inversion and why is it necessary for a laser to function?

Population inversion is a condition where there are more electrons in the excited state than in the ground state. It is necessary for a laser to function because it increases the probability that a photon will stimulate the emission of additional photons, leading to a cascading effect that amplifies the light.

What is a metastable state and how does it help in achieving population inversion?

A metastable state is an energy level that an electron can occupy for a relatively longer time compared to other excited states. It helps in achieving population inversion by allowing more electrons to stay in the excited state for a longer duration, increasing the likelihood of stimulated emission when a photon encounters these electrons.

How do mirrors play a role in the production of a laser beam?

Mirrors are used to reflect photons back and forth within the laser medium, creating a standing wave and a resonance effect. This process amplifies the number of photons and ensures that the laser beam is coherent and collimated.

What is the significance of the standing wave in a laser and how does it contribute to the laser's properties?

The standing wave is significant because it represents a resonance condition where the photons are amplified as they travel back and forth between the mirrors. This contributes to the laser's properties by ensuring a high level of coherence and a tightly focused, collimated beam.

How is the wavelength of the laser light determined in a laser setup?

The wavelength of the laser light is determined by the length of the laser tube and the resonance condition set up by the standing wave. By adjusting the length of the tube, the right resonance for a specific wavelength can be achieved, producing a laser beam of that particular color.

Outlines

00:00

🌈 Introduction to Lasers and Their Applications

Paul introduces the ubiquity of lasers in modern society, mentioning their use in industries, medical devices, eye surgery, and everyday life such as supermarket scanners. He then poses questions about the unique properties of lasers, such as their monochromatic nature, coherence, and collimation, and promises to delve into the working principles behind a laser in the video.

05:01

🔬 Understanding the Laser Principle: Atoms and Emission

The script explains the fundamental principle behind lasers, starting with the Bohr model of the atom to illustrate discrete energy levels. It describes how electrons absorb photons that match the energy difference between levels, leading to excitation. The process of spontaneous emission is contrasted with stimulated emission, where an electron in an excited state releases two identical photons upon encountering another photon. This process is key to the amplification and coherence of the laser light.

10:04

🔄 Achieving Coherence and Population Inversion

The paragraph delves into the concept of coherence, explaining that each emitted photon has the same frequency and phase due to stimulated emission. It then addresses the challenge of achieving a cascading effect of photon generation, which requires a high probability of encountering excited atoms. To overcome this, the concept of population inversion is introduced, where more atoms are in an excited state than in the ground state, facilitated by using a material with a metastable energy level that allows electrons to stay in the excited state longer, increasing the likelihood of stimulated emission.

🚀 Enhancing Laser Action with Resonance and Mirrors

This section discusses the practical setup of a laser, including the use of an energy source to stimulate atoms and the introduction of mirrors to create a resonant standing wave of photons. The mirrors reflect photons back and forth, amplifying the photon population exponentially. The standing wave is tuned to a specific frequency by adjusting the length of the laser tube, resulting in a monochromatic beam. The use of a partially transparent mirror allows the laser beam to exit while maintaining its collimation.

Keywords

💡Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of radiation. In the video, Paul explains that lasers are used in various applications such as industry, medical devices, and supermarket scanners, highlighting their ubiquity and versatility. The term 'laser' itself is an acronym for 'Light Amplification by Stimulated Emission of Radiation', which is central to understanding how lasers work.

💡Monochromatic

Monochromatic light refers to light that consists of a single wavelength and therefore a single color. In the context of the video, Paul mentions that lasers are monochromatic because they produce light of only one color. This property is crucial for the precision and applications of lasers, such as in eye surgery where a specific wavelength is needed for a particular procedure.

💡Coherent

Coherence in the context of light refers to the property where the waves are in phase with each other, resulting in a stable and consistent beam. Paul explains that the coherence of a laser beam is due to the fact that all the emitted photons have the same frequency and phase, which is a result of the stimulated emission process.

💡Collimated

A collimated beam is a beam of light that is parallel and does not diverge significantly over a long distance. In the video, Paul describes how lasers produce a collimated beam, which is important for applications that require a focused and precise light source, such as in industrial cutting or alignment systems.

💡Atom Structure

The structure of an atom is the arrangement of its basic components, including the nucleus and the electrons orbiting around it. In the script, Paul uses the Bohr model of the atom to explain how electrons exist in discrete energy levels, which is fundamental to understanding the monochromatic nature of laser light.

💡Electron Energy Levels

Electron energy levels refer to the specific, quantized energy states that electrons can occupy within an atom. Paul explains that the energy difference between these levels is what determines the color (wavelength) of the light emitted by a laser when an electron transitions from a higher energy level to a lower one.

💡Stimulated Emission

Stimulated emission is a process where an incoming photon causes an excited electron to drop to a lower energy level, emitting a photon with the same properties (wavelength, phase, etc.) as the incoming photon. This concept is central to the amplification process in a laser, as it leads to the production of multiple identical photons.

💡Population Inversion

Population inversion is a condition where a majority of the atoms or molecules in a medium are in an excited state rather than the ground state. Paul discusses this concept as a necessary condition for the laser to work efficiently, as it increases the likelihood of stimulated emission events, leading to a cascade of photon production.

💡Metastable State

A metastable state is an energy level that an electron can occupy for a relatively long time before decaying to a lower level. In the video, Paul describes how using a material with a metastable state can help achieve population inversion, as electrons spend more time in this state, increasing the chance for stimulated emission to occur.

💡Resonance

Resonance in the context of lasers refers to the condition where the length of the laser cavity is set to support the oscillation of photons at a particular frequency, creating a standing wave. Paul explains that by setting up the correct resonance, the laser can produce a highly monochromatic and directional beam of light.

💡Mirrors

In a laser, mirrors are used to reflect the light back and forth within the laser cavity, creating the resonance necessary for a laser beam. Paul mentions that the mirrors are crucial for the amplification process, as they help to establish the standing wave and ensure that the light is emitted in a collimated beam.

Highlights

Lasers are now ubiquitous and can be found in various applications such as industry, medical devices, and everyday use like supermarket scanners.

A laser is characterized by being monochromatic, coherent, and collimated, which are key to its functionality and applications.

Laser stands for 'Light Amplification by the Stimulated Emission of Radiation', which explains the process of how a laser operates.

The Bohr model of the atom is used to understand the discrete energy levels that are fundamental to the laser's monochromaticity.

Electrons in atoms can absorb a photon's energy and jump to a higher energy level, then release the same energy as they return to the ground state, explaining the monochromatic nature of laser light.

The principle of stimulated emission is key to the amplification process in lasers, where one photon can lead to the emission of multiple identical photons.

Coherence in lasers is achieved when emitted photons are in phase with each other, maintaining the same wavelength and frequency.

A quantum phenomenon ensures that photons produced through stimulated emission are always in phase, contributing to the laser's coherence.

Population inversion is necessary for the laser effect, where more electrons are in the excited state than the ground state, facilitating the cascading photon generation.

The use of a metastable state in laser materials allows electrons to remain in the excited state longer, increasing the probability of stimulated emission.

The introduction of a third energy level in laser materials helps achieve population inversion by providing a longer-lived excited state.

Mirrors are used in laser setups to create a standing wave and resonance, which amplifies the photon generation process.

The length of the laser tube and the use of mirrors determine the resonance and thus the specific wavelength of the laser light.

A partially transparent mirror is used to allow the laser beam to exit the tube while maintaining the resonance for photon amplification.

Lasers produce a tightly focused and collimated beam due to the standing wave setup and the use of mirrors.

The explanation of how a laser works includes the use of light as a pump source, the stimulated emission process, and the resulting monochromatic, coherent, and collimated beam.

Transcripts

hi

i'm paul from physics high lasers now

these are

quite ubiquitous that is you can find

them fairly cheaply these days

and you can use them to do all matter of

things such as pointed things such as

at my wedding ring lasers are quite

common in industry and medical devices

they're used for example in eye surgery

they're used in industrial applications

and of course every time you go to the

supermarket and get your groceries

scanned

they use lasers but what makes a laser a

laser let's turn the light off and

examine the laser

by using this deodorant spray

so what you notice is three things first

of all it's monochromatic that is it

only produces one color

secondly we have a beam that is coherent

that is the waves that are coming out

are all in phase with one another and

thirdly we have what we

call a collimated beam that is a very

nice focused beam

but why is it monochromatic why is it

coherent and why is it collimated

and what does laser actually stand for

well today i'm going to discuss

the working principles behind a laser so

stay tuned

[Music]

now laser stands for light amplification

by the stimulated emission of radiation

now we're going to explore that but that

means but we also want to address why it

is monochromatic coherent and of course

collimated

so to start off with we need to have a

look at the structure of the atom which

is really about the substance that

actually

generates the photons that we want to

have for the laser now we have here the

bohr model the atom and i have a video

that you can have a look at where i

explore the ball model now it is a

simplistic model

and needless to say that the atom is far

more complex

where the liquid lips and probability

clouds around the nucleus but in this

case this model will suffice to help us

understand what's going on because the

electrons

do exist in what we call discrete orbits

they don't radiate energy in these

orbits and so

they have various energy levels and so

my electron can exist in one energy

level

it can exist in the other energy level

and there could be more

further up each case it is a step

up in terms of its energy but they are

discrete

energy levels or quantum energy levels

that means that the

energy between one energy level and the

other one is a very discrete

amount and this is where we're going to

have an

excitation going on here so i will have

a photon coming

in and if and only if that photons

energy is exactly equal to the

difference between these two energy

levels

that photon is absorbed by my electron

and as a result my electron jumps at

energy level

and so has a higher energy state and

then what happens

almost immediately is that an electron

jumps back to the

lower energy level what we refer to as

the ground state and so it

drops down and therefore releases energy

but guess what it releases the energy

with exactly the same photon of energy

being released

so that is under the principle of what

we refer to as

e is equal to h f where

f is the frequency of my photon h is

planck's constant and e of course is the

energy

now that explains to us why it is

monochromatic as we'll see as we go on

all the photons we're going to be

talking about are exactly the amount of

energy that we are dealing here with

their energy levels

and so therefore we will only produce

photons of a very specific amount of

energy and hence

it's monochromatic let's move on and

let's look

now a bit closer because we've talked

about emission and in this case we've

got spontaneous emission photon coming

in electron getting excited

dropping back down and then

spontaneously emitting the same

photon energy but we want a stimulated

and so what we could have is a situation

like so

we have my electron here and it of

course

is stimulated by my photon

comes in because my electron that is

absorbing that energy

jumps an energy level now what if now

for example

while this electron is in this

stimulated stage

what happens if it encounters another

photon

and in this case it absorbs that energy

but before

it drops down or whilst it drops down

into

its energy level it now releases two

photons

and so what we get here is an increase

of photons and so what we have here is

an amplification

i started with one photon and end up

with two photons

so in that sense that explains the

implication process though there's more

to it as you'll see

but the thing is is that the second

photon is exactly the same

so in other words it is the same

wavelength the same phase

same everything so we say it's coherent

because they are

in phase with each other now why they

are actually going to be exactly the

same is actually a quantum phenomena

that i'm not going to delve into now

and that could be something that you can

look into further but needless to say

we have now a duplication of our photon

and so that explains why it's coherent

because they are always going to be in

phase every photon we're going to be

producing

will have exactly the same frequency and

phase as a result

so i can have a photon going in and a

single photon going out which is

spontaneous emission i can have a photon

going in

and if the electron is already in the

stimulated stage i can have two come out

and so now i've got stimulated

emission so what happens if now if those

two photons

encounter other stimulated electrons

well we start off with two of course

then we got four

we have eight then we have 16 and of

course

that continues on and so what we get

this is cascade effect of all these

photons being generated

as long as they encounter stimulated

electrons but

here is the problem the time it takes

for the photon to start in its excited

state to back to ground state

is very very quick so what is the

probability

that we have multiple atoms in the

excited state well

really really small so small in fact

that really this is not going to produce

our cascading

effect and there's a problem right there

at least in the simplistic model so we

need to find a way of

increasing the population of our excited

state and so what we get now is what we

call population

inversion let me explain what that means

so in this case my population is in its

ground state now i've got here six

representative

electrons in their ground state and so

the number

of electrons in the excited state

is going to be definitely different to

the number of electrons

in the ground state so clearly our n2 is

less than n1

if we want to have x a

continuing cascading effect we need that

to be reversed we need more

in the n2 state than the in one state

and so

what we want is what we refer to as a

population inversion because the

population is

inverted we've got more in the excited

state and less

in the ground state so how do we get

them up there remember as i said to you

when they're actually pushed up there

they very quickly jump back so even if

we actually have a few stimulator before

we have anyone

encountering another electron very

quickly you'll find

they will be jumping back down to their

grain state and as a result the number

n ends up being mostly in the ground

state so how do we solve that problem

well the solve the problem

is by introducing a material that you

can

actually have a third level or a level

three

material and so what we want to do is

stimulate

our electron beyond to the level that we

want and how do we do that well

we have here our second we're a second

and now a third level so imagine i fire

a

white light photon now you're gonna say

hold on a white light photon doesn't

exist

you are correct actually what we have is

let's say

light that has basically white so it has

multiple

photon wavelengths in there and

hopefully

one of those photons of course will

excite it up to

this level right here and so now what we

have

is our super excited electron but the

materials chosen here that these two

energy levels

or this energy levels is very very

unstable

and so what happens is as i pump the

white light in we have our

electrons jumping up into this third

level here

but very very quickly it jumps down into

this second level and so what we now get

is an electron that is in the second

level

that will stay there a little longer now

this

state here is referred to as the

metastate

and the beauty about the metastate is

that the time that the electron exists

in the metastate is actually a little

longer

up to a thousand times longer than let's

say

in the normal situation so what that

means is

you're going to now get a situation

where you're going to have a lot more

electrons sitting in this

metastate for a certain period of time

which means if i now have my photon

coming in

and it's experiencing an electron in

that meta state

very quickly what we're going to get is

that electron jumping down

and we there will produce two photons

it's more probabilistic for us to

produce

more photons that way so in essence what

we get

is this so here is multiple atoms and

what we start to see

in these electrons and see in these

atoms we start to see

pairs of photons coming off as they come

out

and then of course they're interacting

with other atoms as well

and then what you're going to get of

course is an increasing or a cascading

number

of photons being generated in your

material

but you can see a problem is that

they're all going in different

directions we're not going to

get let's say a strong focused beam how

do we do

that well the first thing we need to do

is with our atom as i said to you

is we need to apply some sort of energy

source now it can be a light source but

it can also be an

electrical source as well so this is

what the stimulation aspect so in this

case we're using light

and so there's our light aspect we've

talked already about the fact that it's

amplified and we've already talked about

that it's by stimulated

emission so we've actually covered most

of the terms already of the term

laser but what we want to do is increase

the effect

so how do we do that now the first thing

we do is we add mirrors

what does that do well we have of course

photons going

all different directions but any photon

that is going

in let's say that direction is going to

reflect back and go back in that

direction and then of course when it

gets to the other side it's going to

reflect back

in that direction and so forth it's

going to go backwards and forwards and

every single time you start to see

a stimulated emission you're getting

more and more photons as it goes

backwards and forwards and backwards and

forwards

in essence if we then look at the light

in terms of its wave phenomena what we

end up

setting up is a standing wave it's

actually what we call a resonance and so

what we get

here is a huge amplification

as we get to generate a standing wave

of photons basically going backwards and

forwards

every single time increasing

exponentially

as they encounter electrons now at this

stage we've got it in

a tube with two mirrors but the beauty

here of course is is that along this

path

it's very narrow anything that goes in

the other direction will be bouncing off

the mirror and goes

out to the side but we're going to

definitely get an increasing effect

along the line here perpendicular to our

mirrors

now being a standing wave is that that

standing wave

is determined by the wave formula which

is f

is equal to nv over 2 l

where n is basically equal to

the different harmonics in this case

i've got a harmonic of

two the reality is is that the harmonic

we

generate here is in the thousands the

frequency of course is the frequency of

the photon

and v of course is the speed of light

and the length is the length of the tube

so in other words if you set the length

of the tube right you'll create

the right resonance for the wavelength

that you're interested in

and in this case for example a red

wavelength let's say 632.8 nanometers

which is the wavelength for a helium

neon laser so now that we have set up a

standing wave we need to now somehow let

the light out well i need to change the

transparency of my mirror

now by changing the transparency of a

mirror usually only about about one

percent so in other words it's now 99

reflective i'm going to get my some of

my light going out

and i have my laser beam because the

light now is only

what is going perpendicular to my mirror

along that line that central line

it now explains why we get a really

tight beam

and it is collimated and in some lasers

what they might also do is put a small

lens

to really adjust for any imperfections

that may exist

so in summary let's quickly review

what do we use to pump the energy into

our tube

we used light what did we end up getting

we ended up getting more photons so we

had amplification

how did we do that well we had to have

emission but it had to occur with

electrons that were already stimulated

so we had stimulated emission and as a

result we get a nice

monochromatic coherent collimated beam

which we can call

radiation well i hope that has helped

you understand how a laser works please

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comment down below

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via patreon my name is paul from physics

high

take care and bye for now