The Spatial And Temporal Correlation Of Sunlight

Usually, the coherence we refer to is the first-order coherence, such as Young’s double slit experiment. For a long time, people’s understanding of coherence is limited to the first-order coherence. Until 1956, Hanbury Brown and Twiss introduced the second-order coherence of the light field in the measurement of the diameter of the stellar angular. He first proved that there exists the correlation between the intensities of thermal light field. Since then, the re-search of the coherence of light field deep into the second-order coherence. Based on the second-order coherence of the optical field, on the base of the second-order correlation function, people performed the "ghost" image and the "ghost" interference experiment using entangled two-photon source, pseudo thermal light source and true thermal light source. But we can find that the sources people used in experiments are all laboratory light sources at that time.In this article we use sunlight—the most common true thermal source in the nature to perform the temporal and spatial HBT experiments. in the experiments, we obtained the secondary light source with the spectrum of 0.01 nm by utilizing a Faraday anomalous dispersion optical filter(FADOF), because the sunlight has continuous wide spectrum and short coherent time. Different from the light source indoors, the sun is always running around the earth. Thus the sunlight is collected by a Meade LXD650 astronomical telescope, which can automatically track the sun. The detection system is made of two single-photon detectors and a time correlation single photon counter module. The experimental results of temporal and spatial correlation functions of sunlight is shown that( 2)g(0) =1.05, which means that the visibility of the temporal and spatial HBT of sunlight is about 2.6%. Though it is much less than the theoretical value of 1/3, and even less than the visibility of true thermal light HBT experiment, it proves that the sunlight has the second-order temporal and spatial intensity correlation just as the common thermal light source. Therefore, it is possible that sunlight is used to perform the correlation imaging and interference. The main reason of low visibility is that the coherent time of the light source is less than the time jitter of detection system, so the measured value of the normalized correlation function (2)g is much less than its true value. We deduce the relation between the measured and true values of(2)g, according to the semi-classical theory of photodetection, and obtain the true values of(2)g(0) =1.81, It can be inferred that this method can be used in ghost imaging and interference experiments with thermal light such as sunlight, etc, when the time jitter of detection system is much larger than or almost equal to the coherent time of light source.