This experiment is divided into two parts; the first part is to illustrate the interference fringes from an etalon and the second part is to measure an etalon’s finesse. 

 

Part I. Etalon Interference Fringes A diffused HeNe laser beam is incident on an etalon followed by a focusing lens. On the focal plane, one observes and characterized an interference pattern with concentric rings. The refractive index and thickness of the etalon are correlated through the experimental data.  

 

Part II. Etalon Finesse Measurement Etalon finesse is an important index for spectral selectivity. In this experiment, a collimated HeNe laser beam is incident on a spinning etalon. From the oscilloscope trace of the transmitted laser intensity, an etalon’s free spectral range and finesse are deduced.