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Introduction
With the advent of wavelength-division multiplexing (WDM), fiber-based long-haul communications have experienced significant advancements, leading to dramatic improvements in land-based communication bandwidths and network topologies for terrestrial systems. Emerging communication-related optoelectronic components and photonic integrated circuits have significantly advanced optical transceivers by integrating narrow-linewidth lasers [1], optical waveguide amplifiers [2], frequency microcombs [3, 4–5], and high-speed, high-responsivity photodetectors [6], along with heterogeneous technology [7] to enhance performance and integration while ultra-wideband modulators [8] improve data efficiency. These developments have facilitated the increasing demands for high-speed data across diverse fields such as telecommunications, energy-efficient interconnects, high-definition remote conferencing, virtual environments, healthcare, large-scale computing, and high-energy particle physics experiments [9, 10, 11, 12–13]. However, this growth has led to congestion in the last-mile wireless access networks within the allocated GHz radio frequency (RF) spectrum. This has prompted a shift toward using higher-frequency and optical carriers for wireless communications [14, 15–16]. A range of advantages of optical wireless communication (OWC) includes access to THz-level bandwidth, unrestricted spectrum usage, easy of deployment, smaller and lighter receivers, and enhanced channel security. In 5G and beyond 5G networks, the integration of networked flying platforms—such as unmanned aerial vehicles, drones, and high-altitude balloons—has emerged as a promising approach to expanding network coverage and capacity. These platforms serve as wireless fronthaul/backhaul links in space-air-ground-sea integrated networks, leveraging hybrid millimeter-wave and free-space optical (FSO) links to further enhance connectivity. Moreover, FSO communication is being increasingly used for both terrestrial and satellite links [17, 18, 19–20], enabling building-scale relay networks, backbone links, and three-dimensional geostationary satellite networks for global broadband coverage [21]. Recent trials have demonstrated remarkable Tb/s FSO transmission using multi-laser arrays for wavelength-division multiplexing. These systems incorporate programmable wavelength-selectable switches to equalize intensity across the array and employ electronic control units for double-pass building-to-building links [22, 23]. Significant progress has also been made with coherent detection techniques [24], co-transmitted local oscillator tones in Kramers–Kronig schemes [25], and spatial-mode multiplexing [24, 26], all contributing to further advancements in free-space data transmission.
In parallel, microresonator-based frequency combs have emerged as a transformative chip-scale platform [27, 28, 29, 30, 31, 32, 33–34], offering unique spectral properties that revolutionize a range of applications, including mode-locked oscillators [35], precision frequency metrology [36, 37], astrophysical spectrographs [38, 39],...