Measurement of atmospheric turbulence parameters relevant to adaptive optic system design

Abstract

Experimental measurements to investigate the effects of atmospheric turbulence on two major parameters for laser adaptive optic (AO) system design (turbulence coherence length r 0 and the atmospheric time constant τ 0) were carried out to cover weak, moderate and strong turbulence conditions prevailing in the city of Delhi (India). Diurnal and seasonal variation of atmospheric refractive index structure parameter (C n 2 ) near the ground was measured with a minimum and maximum recorded values of 2 x10 -13 m -2/3 and 10 -11 m -2/3 respectively. Estimated values of r 0 range from 0.42 mm to 4.4 mm (for 0.6328 µm wavelength of He-Ne laser), 0.8 mm to 8.3 mm (for 1.06 µm wavelength of Nd:YAG laser), 1 mm to 10.5 mm (for 1.3 µm wavelength of Chemical Oxy-Iodine laser), 3.7 mm to 38.2 mm (for 3.8 µm wavelength of DF laser) and 12.5 mm to 131 mm for 10.6 µm CO 2 laser wavelength, each for measured C n 2 values of 10 -11 m -2/3 to 2 x10 -13 m -2/3 . Intensity autocorrelation function measurements of laser scintillations indicate that a bandwidth of ~ 0.5 kHz (corresponding to measured τ 0 ~ 2.25 ms)is required for the adaptive optic system to operate effectively in our conditions. Laser beam wander exceeding 100 µrad (both at 0.6328 µm and 1.06 µm wavelengths) at frequencies ranging from 10-20 Hz were observed under strong turbulence conditions. Estimation of number of correction zones for deformable mirror indicate that for the measured turbulence strengths in our atmosphere, the complexity of the AO system at 1.06 µm, 1.3 µm and 3.8 µm wavelengths will be very high as compared to 10.6 µm system where the actuator requirements are much less and within manageable range.

Study of Wavefront Tilt Variance with Various Telescope Apertures in Indoor Convective Turbulence

“…Results are as given in Table 1. The estimated values of Fried parameter are found in good agreement with the actually observed values for near ground turbulence conditions 23,26 . The experimentally measured values of the Hurst exponent (H) show a departure from the classical D -1/3 relation.…”