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Reference.org
Free spectral range
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<p>Free spectral range (FSR) is the spacing in optical <a href="/facts/Frequency/QUSbPaW8">frequency</a> or <a href="/facts/Wavelength/kcJU0ymz">wavelength</a> between two successive reflected or transmitted optical intensity maxima or minima of an <a href="/facts/Interferometer/X3E1SsQT">interferometer</a> or <a href="/facts/Diffractive_optical_element/Mk80xXHT">diffractive optical element</a>. </p><p>The FSR is not always represented by Δ ν {\displaystyle \Delta \nu } or Δ λ {\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. </p>