Etalon Free Spectral Range
Fabry-Pérot etalon: FSR = λ²/(2nd cos θ). Transmission follows the Airy function.
FSR at λ: λ²/(2nd cos θ)
120.1 nm
order m = 12.90 at λ
FSR in Frequency c/(2nd cos θ)
14.99 THz
exact for a non-dispersive gap
Reflecting Finesse ℱ = π√R/(1 − R)
14.05
coefficient F = 4R/(1 − R)² = 80.00
Fringe FWHM
1.069 THz
exact 4 asin(1/√F); FSR/ℱ = 1.067 THz
Nearest Transmission Peak
1538.462 nm
order 13
Peak FWHM (wavelength)
8.441 nm
resolving power λ/δλ = 182.3
Contrast T_max/T_min
81.00
1 + F (lossless mirrors)
Gap Angle θ
0.00°
sin θ = sin θ₀/n
Three orders around the design wavelength, sampled densely at each peak so high-finesse fringes are not aliased. Lossless mirrors without reflection phase; absorption, flatness and beam divergence lower the real finesse.
Model and references
Exact. The equations are exact within the stated assumptions, or are a standard's tabulated values. The results are as good as the inputs. About the tiers
Ideal etalon: lossless identical mirrors without reflection phase, non-dispersive gap, plane-wave illumination; absorption, flatness and beam divergence lower the real finesse.
- Born M., Wolf E. (1999). Principles of Optics, 7th ed., §7.6.1. Cambridge University Press. doi:10.1017/CBO9781139644181
- Hecht E. (2017). Optics, 5th ed., §9.6.1. Pearson.