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The optical qualities of supersonic jets of four different gases were investigated. The ratios (optical wavelength)/(jet thickness) were ~0.8 × 10−4. Near-field schlieren photographs and interferograms and far-field intensity distributions of a circular beam traversing the jets are presented. The jets tested could be arranged in the following order of improving optical quality: Ar, N2, He, 62% He/38% Ar. Higher optical quality could be qualitatively correlated with lower values of |n(jet) − n(ambient air)| and [n(jet) − 1], where n is the refractive index. The far-field Strehl ratios for circular beams passing just below the nozzle exit ranged from ~0.65 for Ar for which the refractive-index functions were 3.4 × 10−4 and 6.0 × 10−4, respectively, to as high as 0.92 for the He–Ar mixture where the jet refractive index matched that of the ambient air and [n(jet) − 1] was 2.7 × 10−4. Measured far-field Strehl ratios are compared with values calculated from measurements of near-field phase errors. Methods of improving the optical quality of the jets are discussed.
© 1982 Optical Society of America
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