etalon

noun

1. An optical device containing parallel mirrors, used as a narrow band filter, often in laser design.

   “It will be shown that the simple theory outlined in the preceding paragraph, which is applicable to single-plate etalons, can be used to predict the main features of the spectral curve of a multi element etalon.”

   “The laser output will be composed of longitudinal modes that fall inside the transmission band of the etalon, hence a narrowing of the laser line width is observed.”

   “The next step of this experiment is to compare the etalon m₁m₂ (shortest length) with the etalon m₃m₄ (the next one). For this two etalons are arranged side by side as shown in Fig. 7.13a.”