Resonant Cavity 4-λ Integrated 4×4 PD-Array for High Optical Alignment Robustness WDM-FSO Communications

We present a newly developed resonant cavity 4-λ integrated two-dimensional 4 × 4 photodetector array (2D-PDA) device and its application for a wavelength-division-multiplexing (WDM) free-space optical (FSO) communication. The 2D-PDA enables direct optical coupling to the FSO beam, simplifying the optical alignment process needed for 10 μm single-mode fiber coupling and reducing the need for beam-tracking function. In the WDM-FSO communication demonstration, a four-wavelength beam in free space was successfully received by 4-different cavity length PD pixels in 4 × 4 PDA with crosstalk between each channel. This paper discusses about the fabrication and characteristic of the 2D-PDA device, in addition to the 25 Gbps/λ FSO communication demonstration using the 4 × 4 2D-PDA. Considering the crosstalk, the bit error rate is discussed in the 2-WDM and 3-WDM FSO communication demonstrations at 25 Gbps/λ.