## 1 Introduction  
Due to the great demands for wireless communication bandwidth for applications such as the Internet of Things (IoT), online gaming and conferencing, cloud-based storage and processing, etc., the radio-frequency (RF) spectrum has been exhausted. Utilizing the optical frequency spectrum for future wireless communication, which is known as optical wireless communication (OWC), could be a promising solution [1; 2; 3; 4; 5]. Visible light communication (VLC) [6; 7; 8; 9; 10; 11; 12; 13; 14; 15] is one implementation of OWC using the visible light spectrum. VLC has been developing rapidly in the past decades to provide both communication and illumination simultaneously since it can integrate with the existing light-emitting diode (LED) illumination infrastructure. Furthermore, it can offer the advantages of license-free and electromagnetic interference (EMI)-free wireless transmission. As the optical signal does not interfere with the RF signal, VLC can be used to augment RF wireless communication to provide extra communication capacity without degrading the performance of both signals. In future 6G wireless systems, VLC is also considered one of the promising candidates [16; 17]. In addition, VLC could also provide many value-added functions of lighting, including underwater optical wireless communication (UWOC) [18; 19; 20; 21], visible light positioning (VLP) [22; 23; 24; 25], and optical camera communication (OCC) [26; 27; 28; 29]. VLC transmission can be mainly divided into two categories: non-line-of-sight (NLOS) diffused transmission and direct line-of-sight (LOS) transmission. They have their pros and cons. Although diffused NLOS transmission does not require critical alignment between the transmitter (Tx) and receiver (Rx), the received optical power is low. Furthermore, in NLOS transmission, pulse spreading and multipath-induced inter-symbol interference (ISI) issues will significantly affect the VLC transmission performance. On the other hand, although the LOS VLC transmission requires additional mechanisms to ensure alignment between the Tx and Rx, higher performance can be obtained. In order to achieve high-performance LOS VLC transmission, optical alignment between the optical Tx and Rx is very critical to ensuring the VLC signal can be well received by the Rx. There are mainly two ways to improve LOS transmission. (i) One way is to perform precise optical alignment between the Tx and Rx based on optical beam steering. The optical beam steering is to ensure a narrow optical beam reaches the Rx from the Tx. Optical beam-steering schemes based on mechanical [30], tunable laser [31], diffractive [32], spatial light modulator (SLM) [33], as well as active optical phased array (OPA) [34; 35] approaches have been proposed and demonstrated. (ii) Another way to improve the LOS transmission is by increasing the Rx field-of-view (FOV); hence, the optical alignment becomes less critical. The paragraphs below are about different ways to increase the FOV. Many creative optical antennas have been demonstrated using special optical materials as well as special Rx to enhance the FOV of VLC systems. In 2016, [PERSON] et al. proposed and demonstrated a tight array of polystyrene fibers doped with an organic dye (Saint-Gobain BCF-92), forming a rectangular detector with an increased detection area [36]. They used optical waveguides doped with wavelength-shifting dyes. The incident modulated optical signal was absorbed by the dye molecules independently of the incidence angle of the optical signal and subsequently re-emitted at a different wavelength. In 2018, [PERSON] et al. proposed and demonstrated a free-space optical communication (FSOC) system for industrial vehicles using two types of optical fibers (i.e., light-diffusing fiber and wavelength-shifting fiber) providing both downlink (DL) and uplink (UL) transmissions [37]. In 2019, [PERSON] et al. reported a large-area scintillating fiber-based Rx using ultraviolet (UV)-to-blue color conversion for underwater wireless optical communication [38].