Sodium Doublet on Fabry-Perot

The image below shows the appearance of the interference fringes of the sodium doublet produced by Fabry-Perot optics. This is the appearance at one position. As the micrometer is adjusted to increase the mirror spacing, the appearance changes as the fringes merge together.

If the micrometer is advanced until the fringes look the same again, then you have advanced one of the fringe sets one full order with respect to the other. This allows you to calculate the wavelength difference. The button below displays a movie of the progression from the appearance at left through a cycle and back to that appearance. Load movie

Fabry-Perot Calculation

A high-resolution interferometer, the Fabry-Perot has a resolvance of

=	for r = = reflectance d = mm = etalon separation m = = order of interference.

which means that the least separation of two spectral lines is given by

= = nm

for λ = = nm

The interferometer can also be characterized by its free spectral range, the change in wavelength necessary to shift the fringe system by one fringe:

Free spectral range:

= = nm

Fabry-Perot Resolution

A high-resolution interferometer, the Fabry-Perot Interferometer has a resolvance of

which means that the least separation of two spectral lines is given by

Resolvance examples

Calculation

This separation means that the two wavelengths satisfy the Rayleigh criterion. The interferometer can also be characterized by its free spectral range, the change in wavelength necessary to shift the fringe system by one fringe: