KEYWORDS: Sensors, Modulation, Signal to noise ratio, Optical testing, Free space optics, Free space optical communications, Direct methods, Fourier transforms, Signal detection, Digital signal processing
Free space optical (FSO) communication need to keep aiming and tracking each other to keep the communication link unimpeded. In this article, quadrant detector (QD) is used to measure beacon angle deviation. But due to the background light and the dark current, the precision of the direct method cannot match the requirement of the angle measurement. To solve this problem, a proposed method based on intensity modulation and Fourier-Transform is adopted to detect the beacon position accurately. We could consider use a cosine signal to modulate the intensity of beacon light at transmitting end. So the QD output photocurrents are also cosine signals with the same frequency. Then, the energy of QD output signal is concentrated at a certain frequency point, so that the signal-to-noise ratio (SNR) at the frequency is enhanced. Finally Fourier-Transform operation can detect the amplitudes of the photocurrents at the certain frequency point which correspond to the energy of the beacon light illuminated to each quadrant respectively. Therefore, we can calculate the beacon spot position accurately. Experiment results show that even when QD output SNR is only -7.66dB, the max absolute error of the proposed method is 0.016mm and is a quarter of the direct method.
The radiation characteristics of the atmospheric boundary observed in limb view is significant in many fields, such as target recognition, infrared warning and weather forecasts and other fields. Different atmospheric conditions, observation geometry and spectral range will affect the radiation characteristics of the limb atmosphere. The specific parameters are different, and the degree of influence is also different. A hemispherical accumulation method (HLA) was proposed to calculate the background radiation of the limb detection. Based on the characteristics of the limb detection, the sensitivity of the mid-and far-infrared two-band fronts detection to atmospheric mode parameters and gas content parameters is studied. Using the HLA model, the sensitivity analysis of the radiation characteristics of the marginal atmosphere was carried out, and the quantitative calculation results are given.
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