All-optical Fog Sensor for Determining the Fog Visibility Range in Optical Wireless Communication Links Muhammad Ijaz, Zabih Ghassemlooy, Senior Member, IEEE, Hoa Le-Minh, Sujan Rajbhandari, Member, IEEE Optical Communication Research Group, School of computing, Engineering and Information Sciences, Northumbria University, UK Email: z.ghassemlooy@northumbria.ac.uk Abstract: The goal of this research work is to use an all optical based fog sensor to study the atmospheric visibility of fog and its constituents on the optical wireless communication (OWC) links in a controlled laboratory test-bid. The fog sensor measures the transmittance of the Infrared (IR) radiations which is used to determine the link visibility. Experimental results obtained show that using the fog sensor the visibility range from 0.37 – 1 km and above with respect to different fog density can be predicted. 1. Introduction As the number of users using applications requiring a large bandwidth is increasingly growing, the bandwidth limits of current wireless systems in particular radio frequency based technologies (in particular 60 GHz band) are being stretched. Recently, OWC systems with a huge unlicensed bandwidth capability have attracted a great deal of interest from a number of sources including academia, industry, and standardization bodies. This huge bandwidth represents high potentials in terms of capacity and flexibility, thus making OWC technology particularly attractive candidate for multi-gigabit wireless applications (including uncompressed video, audio streaming and multi-gigabit file transferring) offering better quality and user experience, in areas to complement radio frequency based services [1-3]. Despite unique capability of OWC systems such as higher transmission capacity, security, low deployment cost, and immunity to the electromagnetic interference compared to existing communications systems, there are a number of technical challenges that needs addressing. In OWC or also known as free-space optical communications (FSO) for outdoor applications, the optical beam propagation through the atmosphere experiences attenuation and signal power level fluctuation [2 - 4]. Attenuation of optical intensity mainly caused by the absorption, scattering and refraction of optical waves by gas molecules, snow, rain and fog. For link lengths exceeding several hundred meters fluctuations of received optical signal present a severe problem. Fog has the largest impact on FSO links. Under heavy fog conditions the link range is limited to a few hundred meters [5]. The meteorological definition of fog is when the link visibility is less than 1 km [4, 6]. The visibility is the prime parameter for investigating the affects of fog on outdoor FSO systems. The system performance, the link length and the availability of the FSO systems can be optimized by investigating the visibility. One of the major issues in FSO systems operational at different sites is the lack of system design parameters such as the attenuation due to fog which is function of visibility. The visibility data is not available for all the sites. This could be due to the high implementation cost of transmissometer to measure visibility. In this paper we introduced a laboratory optical fog sensor which has been designed to characterise the link visibility for different fog channel conditions. 2. Design of the Fog Sensor The density of the fog, which is function of the visibility, can be measured within a controlled environment, as outlined in Fig. 1. A portable optical fog sensor composed of an IR LED at a wavelength of 880 nm, a photo-detector with a peak response at 880 nm, and an optical band pass filter to block all background radiations is positioned at the centre of the atmospheric chamber, see Figs. 1(a) &(b). The optical source and photo-detector separated apart by 13 cm are heated up to the room temperature using a heating wire mounted around them. This is to ensure that no water droplets will form, thus no further losses due to absorption and refraction of IR light beam. The volume of atmospheric chamber is 5.5×0.3×0.3 m 3 . A modulated laser beam generated at one end is collected at the other end via a photo-detector followed by a trans-impedance amplifier. Characterisation of visibility depends on the fog density within the chamber. A fog generator is used to fill the chamber with a controlled amount of fog from low to thick fog.