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# Single frequency lasers and single mode lasers

#### Gardevar

##### New member
Hello everyone, I have a question that I don't understand in the laser operation: how many transverse modes does the single frequency laser have and how many longitudinal modes does multimode laser have? Well, when i started the search for this question there were no straight answer, but as i get it: single frequency laser means also single mode if actually I can say it, so, single frequency laser has 1 trasverse mode and 1 longitudinal mode, hasn't it? And also about multimode: it has N longitudinal modes due to the number of transverse mode, isn't it? I also tried to draw the frequency spectra of single frequency laser as I understood the material.

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#### kecked

##### Active member
That is a very complicated question. You understand what a mode is? It’s the result of standing waves and Phase. You can and do have modes in single wavelengths. I being careful or I’ll need to write a lot. Now onto wavelength. You know the standing wave is the result of the cavity correct? This determines the fundamental of oscillation BUT. There are integer overtones possible too. How these interact in phase and amplitude can lead to modes. Over simplified but basic idea. Now you say single wavelength. Well no laser is generally one wavelength. Because of this and that avoiding pages here there is a freq spread about a mean. Very carefully setup lasers with gratings can narrow this to a few mhz maybe even better. Diodes dpss gas dye. Are all different in this respect. Well I think I gave you a summary but it’s full of holes so think of it as a very over wide response Like an led vs a laser.

I didn’t address transverse and long cuz it’s a long answer. There is also circular and several other types of phase. Unless you are referring to the electric vs the magnetic component of electromagnetic radiation in which case generally both exist and are orthogonal to each other. Again being careful because in certain rare situations you can have them separate buts it very rare and in weird situations like exchange of photons in a nucleous and even then not usual.

#### Gardevar

##### New member
That is a very complicated question. You understand what a mode is? It’s the result of standing waves and Phase. You can and do have modes in single wavelengths. I being careful or I’ll need to write a lot. Now onto wavelength. You know the standing wave is the result of the cavity correct? This determines the fundamental of oscillation BUT. There are integer overtones possible too. How these interact in phase and amplitude can lead to modes. Over simplified but basic idea. Now you say single wavelength. Well no laser is generally one wavelength. Because of this and that avoiding pages here there is a freq spread about a mean. Very carefully setup lasers with gratings can narrow this to a few mhz maybe even better. Diodes dpss gas dye. Are all different in this respect. Well I think I gave you a summary but it’s full of holes so think of it as a very over wide response Like an led vs a laser.

I didn’t address transverse and long cuz it’s a long answer. There is also circular and several other types of phase. Unless you are referring to the electric vs the magnetic component of electromagnetic radiation in which case generally both exist and are orthogonal to each other. Again being careful because in certain rare situations you can have them separate buts it very rare and in weird situations like exchange of photons in a nucleous and even then not usual.
Thank you for your answer, i get the main idea that longitudinal modes correspond to frequency, and transverse modes correspond to intensity distribution or am i wrong? I undestand that longitudinal modes have a contidion that there are integer number of half-waves and than we get frequency distribution. And transverse modes: according to the diffraction losses we get the specific kind of beam in the output.
But then I read the information that modes are characterised by 3 numbers n,m,l and n correspond to changes of amplitude of electric field strength in the longitudinal direction and m,l /=/ in the transverse direction. And transverse mode is a mode with known m,l numbers and same thing about longitudinal . And here comes an interesting question: can they exist separately or not? And n,0,0 in the picture corresponds to longitudinal and trasverse modes or only longitudinal?

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#### kecked

##### Active member
that's the long reply and no long and tran are not freq and amplitude. I'll answer better later. It's dinner.

watch this and you’ll learn what you need to know to start asking the right questions.

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#### Gardevar

##### New member
To conceptualize "longitudinal modes", these three web pages give a relatively clear simplified verbal general overview explanations w. graphics, if that helps:
Thank you, i have already looked through these pages, but the main thing that i don't understand by now is the frequency spectrum of modes, as i already said in the other reply, i wanna know the connection between the modes, so when we have n,0,0 it's TEM 00, isn't it? But in terms of the frequency spectrum it's also a longitudinal mode to n+1,0,0?

#### Gardevar

##### New member
that's the long reply and no long and tran are not freq and amplitude. I'll answer better later. It's dinner.

watch this and you’ll learn what you need to know to start asking the right questions.
Thank you, i'll try to watch it as soon as possible.