Adaptive free-space optical (FSO) communication links have been extensively studied in order to adapt to variable atmospheric channel environments due to factors such as atmospheric turbulence. As a supporting technology, an all-in-one BPSK/QPSK switchable transmission and reception method is proposed and experimentally demonstrated for adaptive modulation format switching in FSO links. The transmission and reception of both modulation formats are realized based on the same IQ modulator and single-photodetector coherent receiver. Simulation and experimental results show that the QPSK signal has a power penalty of about 3–4 dB compared to the BPSK signal with a BER of about 1 × 10⁻³. The basis for format switching is given according to the various atmospheric channel conditions. The proposed method provides a flexible and efficient solution for variable FSO communication environments to improve their performance.

A high-efficiency ultraviolet (UV) optical wireless communication link is proposed and demonstrated between unmanned aerial vehicles (UAVs) employing a novel azimuthally omnidirectional optical antenna based on an annular compound parabolic concentrator (ACPC). The receiving antenna offers a 114° field of view (FOV), covering polar angles from 10° to 124°, while achieving a maximum omnidirectional gain of 21.6 dB at a distance of 9 m. With a data transmission rate of 9.6 kbps, an outdoor transmission of live images between two in-flight UAVs successfully validates the link performance.

A new, to our knowledge, simultaneous bi-directional all-optical multipoint-to-multipoint switching scheme is proposed and experimentally demonstrated for free-space optical (FSO) communication networks. Through tuning the optical switch at the core node, the communication link between each two arbitrary remote nodes can be established. The multipoint transmission performance is tested in a turbulence-tunable atmospheric cell using a 6.25 Gbit/s quadrature phase-shift keying (QPSK) signal. At the bit error rate (BER) of 1×10⁻³, the power penalty is <3.5dB with a 250°C temperature difference of the atmospheric cell. Moreover, this scheme is compatible with all-optical wavelength conversion, which can achieve strict transparency for different modulation formats. Our proposed scheme offers the possibility to provide an efficient, low-cost, and high-speed method for simultaneous bi-directional all-optical multipoint-to-multipoint FSO communications.

A single-source, multipoint free-space optical (FSO) communication network is presented and experimentally demonstrated using modulating retro-reflectors and all-optical switching. By employing modulating retro-reflectors (MRRs), only a single laser source is required at the central node for the whole network, and the remote nodes are free from sources, which leads to effectively reduced size, weight, and power (SWaP). The signal is successfully backhauled to the central node from a remote node based on an MRR, and the signal transfer between two remote nodes is also realized via an all-optical switch device at the central node. The atmospheric channel is experimentally simulated using a rotating phase plate. At a BER of 1×10 ⁻⁶ , the measured backhaul power penalties are 6.5 and 9.4 dB under turbulence conditions generated by the rotating phase plate at rotation speeds of 220 and 180 r/min, respectively. The power penalties increase to 9.4 and 12.0 dB for the transfer link between two remote nodes under the two atmospheric turbulence conditions, respectively.

A receiving improvement method is proposed and experimentally demonstrated based on hybrid spatial-modal diversity for single-input multi-output (SIMO) free-space optical (FSO) communication links. A moderate turbulence with a Rytov variance of 0.4665 is simulated using a rotatable phase plate. For 10 Gb/s quadrature phase shift keying (QPSK) signals, the hybrid spatial-modal diversity is realized by employing a two-aperture array with three-mode photonic lanterns. At the bit error rate (BER) of $1\times 10^{-4}$ , the transmitted power is reduced by about 2.8 dB due to the spatial diversity compared to a single aperture. Meanwhile, a 2.2-dB improvement is achieved using linear polarization modal diversity. The overall improvement reaches 3.9 dB through the hybrid spatial-modal diversity.