3 and 4 Level Systems in Lasers - A Level Physics

This video explains 3 level systems and 4 level systems in lasers for A Level Physics.
In reality a three or four level energy system is used to allow a population inversion, with the atoms excited by pumping them with an external source of energy. In both a three level system and a four level system, we have a metastable energy level where electrons drop down to pretty instantly after being excited. 3 and 4 level systems produce a cascade effect of photons in a laser, allowing them to be more powerful.
I used this as a reference: www.wjec.co.uk/...
Thanks for watching,
Lewis
This video is recommended for anyone studying A Level Physics in the following exam boards:
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[@100millzypie]

I'm a first year Physics student and this was explained far more clearly than in lectures - thank you!

[@philipprije1096]

That was just brilliantly explained! I am from Germany and for me it is usually quite challenging to understand all this technical and science stuff in English, but your explanation of how a laser works, is perfect, so thanks a a lot!

[@PhysicsOnline]

Thank you

[@jagritisharma3136]

very simple and precise explanation... works above A level... as a chemist i needed simple explanation of lasers and this was more than satisfying.

[@Felixkeeg]

Year 4 Chemistry student, very concise explanation, thank you

[@XLRG1]

the best description for laser ever seen so far.... I really appreciate it

[@sv8523]

Thanks for the great explanation!

[@donny3974]

I subscribed to your Year 13 content yesterday on your website and I am loving it! Videos are explained so well and are very informative like this one :D

[@PhysicsOnline]

Donny Thanks

[@vvv2170-v8y]

Amazing explanation !!

[@Roscovanul2]

Thank you. Very good explanation

[@aashwinsharma1859]

What a great explanation....🙏🙏

[@34adnanali26]

Wow wow just like how much good it was really appreciate it love from india bro

[@ombie9439]

Thank you!!

[@PhysicsOnline]

My pleasure

[@pipp33momo]

Am struggling with a couple of points; first .. what is the real advantage of 3 or 4 level lasers given that the higher level is never gonna contain excited elecrons Second .. what would happen if we just pumped white light energy on a two level laser? What does really matter for an atom to behave as a laser medium .. i mean . These mentioned reactions do take place in all atoms .. or just the atoms of certain materials rendering them suitable for a laser medium?

[@bigdx5059]

I like your Feynman diagram symbol on ur profile😂👍👌

[@jonathanmv]

What about the photons when transitioning from E4 to E3 and from E2 to E1? Do we ignore them because we want to produce the red light? Also. It's not clear for me the advantage to add another level. What's the gain using a 4 levels system vs one with 3 if both use pumping? Thank you very much for the videos. They are very clear

[@PhysicsOnline]

Jonathan Morales Vélez the energy released from levels 4 to 3 and from 2 to 1 I’m the 4 level systems are small compared to the photons that go on to cause further stimulated emission of electrons in the meta stable state.

[@PhysicsOnline]

The four level system also allows the level below the meta stable level to empty almost instantly, so there will always be a population inversion.

[@karamany9870]

Thanks from a 1st year EE

[@kavithanaagesh260]

Verryyyyy nice ...

[@azegeaustin5615]

Thank you for the video. I now have a better insight into the topic. Question: do the source of the photon (red) that creates the spontaneous emission different from the pump source?

[@HelenaEngstrom]

why does a 4-level system have a lower threshold than a 3-level system?

[@kamal0885]

Because of low pumping energy as in 4 level system the transition is not direct from metastable to ground state it goes like upper metastable state to lower metastable state and then to ground stand and in 3 level this process is direct from MSS to GS. Thus there will be more electrons in the E3 state of 4 level than in E2 state of 3 level(SAY). Hence low pumping energy is required in 4 level thus the low threshold.

[@jacobb7379]

1:48 how come the ‘non-laser light’ emissions (like here), and the pumping energy, doesn’t interfere with the laser light?

[@abcd-by6rw]

4 level system wasn't explained very well (still good!) But just wondering what is purpose of level N2?

[@joshuagardiner9096]

same

[@kamal0885]

so that the electron does not go directly to the ground state unlike in 3 level system, and because of this low pumping energy is required and thus low threshold in 4 level system.

[@kamal0885]

@@joshuagardiner9096 so that the electron does not go directly to the ground state unlike in 3 level system, and because of this low pumping energy is required and thus low threshold in 4 level system.

[@MiniGorrila]

Question: Population Inversion is import between two energy levels; not the full system? So as long as N3>N2 stimulated emission can happen? It doesn't matter how big N1 and N4 are? Thats why 4 Level lasers are better? Great video btw thanks!

[@kamal0885]

Actually it does matter. Because without N1 you cant not estimate the pumping energy required to achieve population inversion.

[@noorrazaq5915]

ماكو ترجمة للفيديو؟ ؟؟

[@rodgervinsmoke8220]

Why is it hard to get N2 bigger than N1

[@kamal0885]

because N2 state is unstable and every particle tends to be in the stable state.

[@sambhavsingh5088]

GG

[@farahfarisa3164]

Gttd😊

[@BP-qe4qv]

Why does n2 have to be greater than n1 for a population inversion? Also how does 1 photon become 2 become 4 become 8etc? 1 in becomes 2 out so how does that 2 that left become 4? Or is it just u put 2 in and 4 comes out?

[@harrybatty5846]

B P watch the previous videos on lasers, it explains all the above.

[@pamdeepsangha5943]

Great explanation, thank you!