Two-layer laser anti-reflection coatings

Two-layer anti-reflection coatings on crown glass have been made for helium-neon lasers, using arsenic trisulphide and magnesium fluoride as the high and low refractive index layers. The thicknesses of both films were varied and the reflectivity spectra measured. The most efficient coating had a reflectivity of 0·005±0·002% at λ632·8 nm. The coatings are durable and stable. Good agreement was obtained between the measured and calculated reflectivities; the efficiency of a coating is limited by non-uniformities in the film thicknesses. The effect on the reflectivity of departures in film thicknesses from their optimum values was calculated. The optimum thicknesses of the films were calculated for substrates of different refractive indices and for wavelengths from 0·58 to 1·0 μm.