CMOS monolithic photodetector with a built-in 2-dimensional light direction sensor for laser diode based underwater wireless optical communications.

Underwater wireless optical communications (UWOC) are considered an emerging high-speed wireless network for underwater applications and compete with underwater radio frequency (RF) communications and underwater acoustic communications (UAC). Even though the utilization of laser diodes (LDs) enhances the -3dB modulation bandwidth extraordinarily from a few tens of MHz to GHz, LDs have the features of high collimation and narrow spectrum. Without the point-to-point optical alignment, the performance of the LD-based UWOC system drops exponentially because the received optical power determines the signal-to-noise ratio (SNR) of the UWOC system. To achieve a high-performance and reliable UWOC link based on LDs requires focusing optics and an alignment system. In this paper, we demonstrated a CMOS monolithic photodetector with a built-in 2-dimensional light direction sensor for the UWOC link by using a 450 nm LD and none-return-to-zero on-off keying (NRZ-OOK) modulation method. Employing this innovative technique, the field of view (FOV) was enlarged to 120°, and data rates up to 110 Mb/s at a bit error rate (BER) of 2.3×10-10 were obtained. The establishment of a proposed UWOC physical link showed enhanced communication performance for more practical and robust wireless communication applications.

Variable self-powered light detection CMOS chip with real-time adaptive tracking digital output based on a novel on-chip sensor.

This paper provides a solution for a self-powered light direction detection with digitized output. Light direction sensors, energy harvesting photodiodes, real-time adaptive tracking digital output unit and other necessary circuits are integrated on a single chip based on a standard 0.18 µm CMOS process. Light direction sensors proposed have an accuracy of 1.8 degree over a 120 degree range. In order to improve the accuracy, a compensation circuit is presented for photodiodes' forward currents. The actual measurement precision of output is approximately 7 ENOB. Besides that, an adaptive under voltage protection circuit is designed for variable supply power which may undulate with temperature and process.

Active tracking system for visible light communication using a GaN-based micro-LED and NRZ-OOK.

Visible light communication (VLC) holds the promise of a high-speed wireless network for indoor applications and competes with 5G radio frequency (RF) system. Although the breakthrough of gallium nitride (GaN) based micro-light-emitting-diodes (micro-LEDs) increases the -3dB modulation bandwidth exceptionally from tens of MHz to hundreds of MHz, the light collected onto a fast photo receiver drops dramatically, which determines the signal to noise ratio (SNR) of VLC. To fully implement the practical high data-rate VLC link enabled by a GaN-based micro-LED, it requires focusing optics and a tracking system. In this paper, we demonstrate an active on-chip tracking system for VLC using a GaN-based micro-LED and none-return-to-zero on-off keying (NRZ-OOK). Using this novel technique, the field of view (FOV) was enlarged to 120° and data rates up to 600 Mbps at a bit error rate (BER) of 2.1×10-4 were achieved without manual focusing. This paper demonstrates the establishment of a VLC physical link that shows enhanced communication quality by orders of magnitude, making it optimized for practical communication applications.