# Finesse of an Fabry-Pérot interferometer

during an undergrad experiment we have to estimate the finesse of an Fabry-Pérot interferometer.

The reflectance $R$ is given so the theoretical value can be calculated by $\mathcal{F} = \frac{\pi\sqrt{R}}{1-R}$. For an estimation of the practical value we can use the definition $\mathcal{F} = \frac{\Delta\lambda}{\delta\lambda}$, where $\delta\lambda$ is the full-width half-maximum of an transmission band and $\Delta\lambda$ is the free spectral range (FSR).

In our experiment we had an expandet, monochromatic source in an magnetic field, so due to the Zeeman effect we had in fact an splitted line. How can we get the FSR? Or is there another way to calculate the finesse?

• Which is your question? Or to be more precise: what are you able to adjust in your experiment? Do you know the wavelength separation of your "split line", and can you change the wavelength peak of your FP interferometer? – Carl Witthoft Dec 29 '13 at 13:58
• We know the wavelength separation, and we are only able this one (separated) wavelength. The FP interferometer is also fixed. We know the wavelength resolution of the etalon. – Linosch Artox Dec 29 '13 at 14:48