Adaptive optical beam tracking and alignment system with a wide field-of-view for optical wireless communication

Optical wireless communication (OWC) can provide the last mile high data rate and broadband access to indoor users. Infrared-based communication (IRC) can also achieve high data rate OWC transmission using a directional and collimated line-of-sight (LOS) optical beam. However, a collimated optical beam with beam steering ability is required to provide the LOS communication channel to different mobile users. Different optical beam steering approaches have their limitations. Although the mechanical-based approach using fast steering mirror (FSM) usually has a large footprint and limited optical steering speed; it can provide a large field-of-view (FOV) and support a wide wavelength window with polarization, optical mode, and modulation format independence operations. These are crucial for the OWC systems as wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) are employed to boost the transmission capacity. In this work, we propose and reveal a proof-of-concept experimental demonstration of an adaptive fast steering mirror (FSM)-based optical beam tracking and alignment system. To enhance the FOV and the stability of the OWC transmission, high-precision adaptive optical tracking via a two-FSM system with a light spot approaching method is proposed and demonstrated. The operation mechanism of optical alignment using two FSMs to establish the reference optical axis path is discussed in detail. The first FSM is utilized to correct the beam displacement issue from the reference optical axis, while the second FSM is utilized to correct the incident beam angle issue from the reference optical axis. Experimental result shows that the FOV can be enhanced from 0.05° (i.e., using only a single fiber collimator) to 51.85° using the proposed FSM-based system. A high-speed and stable orthogonal frequency division multiplexing (OFDM) OWC transmission at 89.3 Gbit/s, 87.03 Gbit/s, and 84.47 Gbit/s after free space propagation distances of 3 m, 10 m, and 20 m, respectively, can be achieved, fulfilling the hard-decision forward-error-correction (HD-FEC) requirement (i.e., bit-error-rate, BER = 3.8 × 10^-3).