4

I was wondering what does the term cavity finesse mean, physically? I understand that it is defined by— $$F = \frac{ \text{Free Spectral Range (FSR)}}{\text{Linewidth}},$$ where FSR is the frequency separation of the cavity modes given by the expression $\frac{\pi c}{nd}$, $d$ being the separation of the mirrors.

However, I have read in some places (e.g. https://www.rp-photonics.com/finesse.html) that finesse is the measure of the cavity losses and is independent of resonator separation. From what I understand, the linewidth of the cavity is entirely dependent upon the reflectance of the mirrors and is independent of $d$. This is puzzling me.

Buzz
  • 17,816
HoOh the legendary chicken
  • 53

1 Answers1

3

that finesse is the measure of the cavity losses and is independent of resonator separation

I guess I would agree with that definition, but would add the word relative before "measure".

The FSR is dictated by the resonator geometry. A shorter resonator will have a larger FSR, i.e. the peaks are spaced farther away.

The linewidth (peak width) is controlled by "how good" the reflection of the mirrors is, which determines "how good" the interference is.

However, "good" is relative.
For example, the following plot shows two normalised Lorentzians with the same linewidth, but with a factor of $10$ in FSR. The blue curve looks "better" doesn't it? enter image description here

Finess is a relative measure to how good a cavity is, combining the information of the resonator (FSR) and the materials' quality (linewidth). Physically, you can say: how many times does the linewidth fit in my FSR? That would give you a sense of resolution.

Community
  • 1
SuperCiocia
  • 25,602