Vertical cavity surface emitting laser based hybrid fiber-free space optic link for passive optical network applications

In this work, a hybrid architecture employing Vertical Cavity Surface Emitting Laser (VCSEL) based Single Mode Fiber (SMF) link followed by Free Space Optic (FSO) transmission, is investigated. The VCSEL is biased below threshold to generate short optical pulses, which is coded in NRZ format. After the 20 km SMF link, a splitter divides the received power into two branches. One portion is coupled to a PIN photodiode for receiver operation, whereas the remaining power is coupled to a lens for free space transmission. The VCSEL-SMF-FSO link is simulated and performance measures are computed and verified with link budget calculation. The maximum FSO link distances under different weather conditions and coupling ratio were found. For BER ≤10−12, under clear weather, the maximum attainable FSO link lengths for 1.25, 2.5, 5 and 10 Gbps data rates are found to be 0.205 km, 0.16 km, 0.14 km and 0.11 km respectively. The same procedure when extended to snowy conditions, yield FSO link length of 155 m, 125 m, 110 m and 90 m for 1.25, 2.5, 5 and 10 Gbps data rates under similar BER performance. The impact of fractional power coupled to FSO link was analyzed for 5 Gbps by varying the splitting ratio from 80:20 to 86:14. The optimum ratio up to which both SMF and the cascaded link system meet the required BER for Gigabit PON application is also determined.