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As we know, lasers have longitudinal modes, the seperation between neighbouring modes (measured in frequency) is:

[tex]\Delta \nu = \frac{c}{2L}[/tex]

and by using the fact that:

[tex]\frac{\Delta \nu}{\nu} = \frac{\Delta \lambda}{\lambda}[/tex]

we obtain the seperation between neightbouring modes as (in wavelength):

[tex]\Delta \lambda = \frac{\lambda^2}{2L}[/tex]

Now, the question I have is that I have read that there is something called the Free Spectral Range (FSR) which is defined as the seperation between longitudinal modes (and has the same formula as [tex]\Delta \lambda[/tex]...however for various reasons I think that the FSR is a term than can only be used to describe Fabry-Perot Interferometers (which, as I understand just simulate a laser cavity and will only transmit modes that obey the "integer number of half wavelengths")...

Erm, yes the question I have, what I want to know is can I use the term FSR in place of longitudinal modes when talking about laser cavities and is the [tex]\Delta \lambda[/tex] equation above correct for laser cavities (and thus for FPI's too?)

Thanks!

James

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# Laser Modes/Free Spectral Range

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