During the last two decades, free-space optical links got considerable importance due to their benefits of higher data rates, license free-spectrum, easy and rapid deployment and mobility. Free-space optical links use carrier frequency in the range of 20 THz to 375 THz (in near infrared (IR) region and visible band in wavelengths) to establish a communication link for terrestrial communication, inter-satellite links, deep space links, ground-tosatellite and satellite-to-ground links. Free-space optical links are also useful for different military applications, disaster recovery and last mile access. However, despite of having all these advantages the performance of freespace optical links depends upon the atmospheric conditions and parameters of system design. Geometrical losses of free-space optical links are directly related to parameters of system design or internal parameters. In this paper we analyzed different parameters of system design to minimize the geometrical losses. We presented the analysis of internal design parameters like divergence angle, diameter of receiver aperture, diameter of transmitter aperture, link distance and suggested the suitable parameters of system design.

Free-space optical links use modulated beam of light to transmit high amount of data from transmitter to receiver to get line-of-sight communication link. Free-space optical is cost effective solution to provide higher data rate to end-users. Free-space optical links are also considered as a better alternative to RF links due to their advantages of low power consumption and higher data rate of the range Gbps, and are highly secure to electromagnetic interference. Despite of having all these advantages free-space optical links are highly affected by the severe weather conditions like fog, rain, snow, smoke and dust or aerosol particles suspended in air. Fog is one of a major challenge for free-space optical to achieve carrier class availability and causes high amount of attenuation. In this paper we presented the performance analysis of free-space optical links by estimating signal-to-noise ratio, bit error rate, under fog conditions during winter season in Lahore, Pakistan.