Atomic transition and cavity axial-mode frequencies

[DivGradCurl]

Hello folks,

I would like to know whether or not the cavity axial-mode frequencies have any influence on the frequency output of the laser beam. I suppose that the atomic transition from excited state to ground state is the only occurrence responsible for the wavelength, whereas the axial-mode frequencies are only important to understand the round-trip dynamics inside the cavity. Please help me clarify these ideas. Thank you.

 

[DivGradCurl]

I've been looking into this question and it seems that the cavity axial (i.e. longitudinal) modes are the frequencies that you can amplify using the lasing medium. The frequency range that is actually amplified falls within the atomic gain profile, and one can design the cavity in order to specify such range. The mode (i.e. resonant frequency) spacing comes into play in order to achieve this result. So, here comes another naive question along the same lines: if you want to design, for example, an (ArF)* excimer laser at 193 nm, you probably must have both the excited complex transition at 193 nm AND the longitudinal resonance at the same wavelength (possibly a narrowband of resonant frequencies that you can filter out later), right? Thanks.

 

[Claude Bile]

The actual frequencies you will get out of the laser will be the axial modes (also called longitudinal modes) that overlap with the gain spectrum of the laser transition.

As far as designing a laser with a specific wavelength is concerned, the simplest solution would be to add greater wavelength selectivity by adding a second resonance condition. A Fabry-Perot Etalon such as a microscope slide is perfect for this. You can then tune the laser simply by tilting the etalon.

Claude.