A theory about scales in atmospheric optical turbulence vortex from the point view of spatial correlation function is described. Then an experiment is carried out to prove this theory by the fiber optical turbulence sensor array near the ground. Results show that the outer scale has a mean value of 0.62m and varies from 0.34m to 0.95m by doing a nonlinear fitting on spatial correlation functions. With this method, the value of the outer scale can be given directly without any hypothesis when the optical turbulence is well-developed. A question about how the trend of the spatial correlation function show when the displacement approaches the outer scale is solved. This research can be regarded as a progress about understanding the characters of spatial correlation function in optical turbulence.
In this article, a high quality fiber optical turbulence sensing array contains several sensors is proposed to obtain time series of air refractive index fluctuations. A fixed sensor is supposed to be set as the origin and others to be arranged in Cartesian coordinates respectively. Under the spatial configurations above, two-point correlation algorithm is used to give two order structure parameters and multipoint correlation algorithm for more structure information about scalar turbulence. For each direction, two-point spatial correlation coefficients varying with distance are provided. Meanwhile spatial power spectrum and outer-scales according to the data are introduced. Multipoint correlations can give more structure information such as the interactions between scales and the spatial structure of relevant fluctuations. In the one-dimension circumstance for instance the x-axis, spatial correlation coefficient tends to take oscillation. After a short-time averaging, it tends to decrease with the increase of spatial displacement, and then tends to zero after outer scale. Further study show that within the limit of outer scale, diurnal variation of the spatial correlation coefficient and intensity reveal a higher similarity, the relevancy is about 60% and keeps stable; once the distance goes across the outer scale, they are uncorrelated. In short, utilizing the fiber optical turbulence sensing array is a new method for measuring spatial correlation of optical turbulence. It can overcome some problems from single-point measurement, especially when using Taylor’s frozen-turbulence hypothesis. Some structural information of optical turbulence not only makes the theory of scalar field more abundant, but also in favor of some problems about optical propagation.
An integrated fiber-optic turbulence sensor based on non-balanced fiber-optic Mach-Zehnder interferometer with a small
air gap as light path difference has been designed for detecting air refractive index fluctuation. For avoiding sensing
signal fading and perturbations from circumstance during signal transmission, the phase generated carrier is used. The
turbulence induced air refractive index fluctuations are demodulated by the algorithm of correlation. Background noise
of the sensor is below10−17 . By comparing with the refractive index structure constant measured by fine-wire resistance
thermometer, results show good agreement in both their magnitude and tendency. For its outstanding property of
corrosion protection, the sensor is especially suitable for maritime atmospheric optical turbulence research, which is
verified by one month sea beach investigation. Some results of the maritime optical turbulence intensity are reported in
the end.
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