A Fabry-Perot formed by a plate with two parallel faces or two mirrors held apart, transmitting only frequencies whose round trip fits whole wavelengths. Tilted inside a laser cavity it forces single-longitudinal-mode operation; outside it serves as a filter or wavelength reference. Unwanted etalons from any pair of parallel surfaces, such as windows or flat filters, add fringes to spectra and power, which is why such parts are wedged.
That ripple in the transmission is an etalon from the cuvette windows, tilt it a few degrees and it goes away.
Also heard as
- Fabry-Perot etalon
- FP
Related terms
Free spectral range
The frequency spacing between neighboring resonances of a laser cavity, etalon or interferometer.
Finesse
The sharpness of an optical cavity's resonances: free spectral range divided by the resonance linewidth.
Longitudinal mode
One of the discrete frequencies at which a standing wave fits a whole number of half-wavelengths into the resonator.
Coherence length
The path difference over which a light source keeps a fixed phase relationship and can still form interference fringes.