This Letter outlines radio-over-fiber combined with radio-over-free-space optics (RoFSO) and radio frequency free-space transmission, which is of particular relevance for fifth-generation networks. Here, the frequency band of 24-26 GHz is adopted to demonstrate a low-cost, compact, and high-energy-efficient solution based on the direct intensity modulation and direct detection scheme. For our proof-of-concept demonstration, we use 64 quadrature amplitude modulation with a 100 MHz bandwidth. We assess the link performance by exposing the RoFSO section to atmospheric turbulence conditions. Further, we show that the measured minimum error vector magnitude (EVM) is 4.7% and also verify that the proposed system with the free-space-optics link span of 100 m under strong turbulence can deliver an acceptable EVM of <9% with signal-to-noise ratio levels of 22 dB and 10 dB with and without turbulence, respectively.

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Two experimental configurations of a hybrid K-band (25 GHz) microwave photonic link (MPL) are investigated for seamless broadband wireless access networks. Experimental configurations consist of optical fiber, free-space optics (FSO) and radio frequency (RF) wireless channels. We analyze in detail the effects of channel impairments, namely fiber chromatic dispersion, atmospheric turbulence and multipath-induced fading on the transmission performance. In the first configuration, transmission of the 64-quadrature amplitude modulation (QAM) signal with 5, 20 and 50 MHz bandwidths over 5 km standard single-mode fiber (SSMF), 2 m turbulent FSO and 3 m RF wireless channels is investigated. We show that, for QAM with a high bandwidth, the link performance is being affected more by atmospheric turbulence. In the second configuration, the 20 MHz 4/16/64-QAM signals over a 50 km SSMF and 40 m FSO/RF wireless links are successfully transmitted with the measured error vector magnitude (EVM) values of 12, 9 and 7.9%, respectively. It is shown that, for all transmitted microwave vector signals, the bit error rate is lower than the hard-decision forward-error-correction limit of 3.8×10 −3. Moreover, an extended FSO link span of 500 m for 25 GHz hybrid MPL with 16-QAM at 10 Gb/s under the weak and strong turbulence regimes is evaluated via simulation analysis to mimic a practical outdoor system.

We demonstrate halving the record-low loss of interconnection between a nested antiresonant nodeless type hollow-core fiber (NANF) and standard single-mode fiber (SMF). The achieved interconnection loss of 0.15 dB is only 0.07 dB above the theoretically-expected minimum loss. We also optimized the interconnection in terms of unwanted cross-coupling into the higher-order modes of the NANF. We achieved cross-coupling as low as −35 dB into the LP$$_{11}$$ 11 mode (the lowest-loss higher-order mode and thus the most important to eliminate). With the help of simulations, we show that the measured LP$$_{11}$$ 11 mode coupling is most likely limited by the slightly imperfect symmetry of the manufactured NANF. The coupling cross-talk into the highly-lossy LP$$_{02}$$ 02 mode ($$>2000$$ > 2000  dB/km in our fiber) was measured to be below −22 dB. Furthermore, we show experimentally that the anti-reflective coating applied to the interconnect interface reduces the insertion loss by 0.15 dB while simultaneously reducing the back-reflection below −40 dB over a 60 nm bandwidth. Finally, we also demonstrated an alternative mode-field adapter to adapt the mode-field size between SMF and NANF, based on thermally-expanded core fibers. This approach enabled us to achieve an interconnection loss of 0.21 dB and cross-coupling of −35 dB into the LP$$_{11}$$ 11 mode.

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