Characterization of fog by liquid water content for use in free space optics

Free space optical (FSO) communication became a topic of much interest in recent years. The main reason of that is that it is one of the not too many possibilities making wireless transmission of Gbit/sec and other wideband signals a reality. In this paper in an introductory section some of the main points relevant to this technique are briefly listed, i.e. propagation impairments (including fog effect), practical advantages and application possibilities. The bulk of the paper deals with an equipment elaborated in Budapest University of Technology and Economics (BME) measuring one of the parameters characterizing fog i.e. liquid water content (LWC). A method of calibration is given. Results of measurement taken during 6 fog events of total length of about 70 hours show a very high correlation between LWC and fog attenuation along the FSO link. Also the dependence of measured attenuation from measured LWC has similar characteristics as published relationship of attenuation vs. LWC. Thus LWC as measured with this instrument is a good estimator of fog effect on FSO propagation.