Abstract

Radio over Free Space Optics (RoFSO) is a desirable wireless communication option in 5G and 6G networks, as it combines high bandwidth with flexibility. For this application, we previously proposed a vertical backside-illuminated InP-based InGaAs PIN photodetector (PD) array, with responsivities up to 1.05 A/W, dark current densities of 1 nA and 3 dB bandwidths of 1.8 GHz [1].
In this paper, we report on improved modulation results, largely obtained by the further optimization of the PCB on which the array is mounted. We demonstrate a 3 dB bandwidth of 2.3 GHz for each PD in the array and a generally flatter modulation response. Fiber link experiments were done to mimic free space optical links and to demonstrate high bitrate operation of the PD array. By applying feed forward equalization at the transmitter side, we demonstrate optical transmission in the C-band at 10 Gb/s for NRZ-OOK as well as PAM-4. Low BERs (below 10-12) are obtained for fiber links of 2km and 25 km between the externally modulated transmitter and the PDs. This good performance is obtained for all connected PDs of the array. For the link experiments over 25km of standard single mode fiber, a fiber amplifier is used right after the transmitter to compensate for fiber loss.