Performance evaluation of high-capacity FSO communication utilizing non-coherent (EAM and MZM modulators) and coherent VCSEL-QAM OFDM detections

The integration of optical devices with wireless communication system has possibilities for enhancing telecommunications networks. Free space optics (FSO) technology is recognized for its potential to transmit high-capacity wireless data over space. The suggested work contributes FSO designs using a vertical cavity surface emitting laser (VCSEL). Three configurations, are suggested; direct detection system utilizing Mach–Zehnder intensity modulator (MZM) and electro- absorption modulator (EAM), and a quadrature-amplitude electrical modulator (QAM) with coherent detection orthogonal frequency-division multiplexing (OFDM) system. The proposed systems are assessed under different tolerance weather conditions such as, clear weather, moderate fog, dense fog and snow, with corresponding attenuation factors of 0.23 dB km −1 15.5 dB km −1 , 25 dB km −1 and 45 dB km −1 . based on the standard performance metrics; Q-factor, BER, eye diagram, and OSNR, both MZM and EAM, allow data transmission of (10 Gbps) over the range of 3.4 km, 2.2 km and 1.4 km under the moderate fog, dense fog, and Snow. However, acceptable Q-factor of around 6 and significant opening eye diagram is achieved, the MZM system outperforms the EAM over the same transmission range and attenuation. Under the attenuation of 25 dB km −1 , OSNR of the MZM and EAM is decreased from 47 dB, and 44 dB to around 10 dB and 9 dB, by increasing the range from 0.4 km to 2.2 km. Furthermore, within the range power fluctuation, the total received optical power decreases linearly from −10 dBm to slightly around −56 dBm. The OFDM coherent detection has been suggested to enhance data rate to 20 Gbps and distance range. Using the 4-QAM-OFDM system, the proposed FSO setup enables data transmission over distances of up to 2.4 km at 20 Gbps, and an attenuation of 45 dB km −1 achieved a Q-factor of 9. In contrast, the initial configuration (EAM and MZM) supports only a transmission distance of 1.2 km, at a rate of 10 Gbps, under the same attenuation conditions. The Q-factor measurements and constellation diagram demonstrate that the FSO-OFSM-based VCSEL model operates well at a bias modulating current of 5 mA.