Free spectral range

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Free spectral range (FSR) is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer or diffractive optical element.^[1]

The FSR is not always represented by ${\displaystyle \Delta \nu }$ or ${\displaystyle \Delta \lambda }$, but instead is sometimes represented by just the letters FSR. The reason is that these different terms often refer to the bandwidth or linewidth of an emitted source respectively.