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This work designs a 60 GHz rhombic patch array antenna with high gain, low sidelobe level (SLL), and reduced array area. This antenna consists of eight linear serial arrays with varying number of patches. A slot-coupled center-fed structure of substrate-integrated waveguide (SIW) is designed to simplify the complex feed network required for direct excitation of planar arrays and reduce insertion loss. The rhombic configuration utilizes the quantity ratio of patches for equivalent taper excitation to reduce SLL, rather than the use of tapered width as in traditional designs, which can achieve high gain and avoid cumbersome resizing of patches and connecting lines. An array factor analysis method is proposed to predict the SLL of the antenna. To form a planar array and further reduce the H-plane SLL, a −25-dB Dolph-Chebyshev SIW feed network is constructed. The measured gain of the rhombic array antenna is 18.2 dBi, and the E-plane and H-plane SLLs are less than −20 dB and −25 dB, respectively. This work also simulates a reference uniformly excited high-gain 8 $\times $ 10 array antenna. Compared to this reference antenna, the proposed rhombic array antenna has similar simulated gain but much lower SLL and 23% smaller array area.

A full-duplex self-recovery optical fibre transport system is proposed on the basis of a novel passive single-line bidirectional optical add/drop multiplexer (SBOADM). This system aims to achieve an access network with low complexity and network protection capability. Polarisation division multiplexing technique, optical double-frequency application and wavelength reuse method are also employed in the transport system to improve wavelength utilisation efficiency and achieve colourless optical network unit. When the network comprises a hybrid tree-ring topology, the downstream signals can be bidirectionally transmitted and the upstream signals can continuously be sent back to the central office in the reverse pathways due to the remarkable routing function of the SBOADM. Thus, no complicated optical multiplexer/de-multiplexer components or massive optical switches are required in the transport system. If a fibre link failure occurs in the ring topology, then the blocked network connections can be recovered by switching only a single optical switch preinstalled in the remote node. Simulation results show that the proposed architecture can recover the network function effectively and provide identical transmission performance to overcome the impact of a breakpoint in the network. The proposed transport system presents remarkable flexibility and convenience in expandability and breakpoint self-recovery.

Given the rising demand for free-space with underwater environmental monitoring, calamity precaution, and developed industry applications, a free-space optical (FSO)-underwater wireless optical communication (UWOC) convergence is constructed to be an attractive architecture for affording a high transmission capacity. A 256-Gb/s four-channel space-division-multiplexing (SDM)-based four-level pulse-amplitude modulation (PAM4) FSO-UWOC convergent system that exploits two-stage injection-locked red-light vertical-cavity surface-emitting laser transmitters is, thus, offered and practically demonstrated. With a four-channel SDM scheme and PAM4 modulation, the total transmission capacity is drastically enhanced to 256 Gb/s. To the best of our understanding, this architecture is the leading one to build a high-speed 256 Gb/s FSO-UWOC convergent system with competent performances. It directs an approach to expedite extensive applications in the integration of high-speed free-space with underwater links.

With the increasing demand for free-space/underwater environmental monitoring, disaster precaution, and manufacturing industry applications, free-space optical (FSO)-plastic optical fiber (POF)-underwater wireless optical communication (UWOC) convergence is designed to be a promising framework for providing long-haul free-space with underwater links. An 82-m 9 Gb/s four-level pulse amplitude modulation (PAM4) FSO-POF-UWOC convergent system employing a 405-nm blue-light injection-locked laser diode (LD) is thereby offered and practically demonstrated. Results reveal that a 1.8-GHz 405-nm blue-light injection-locked LD can be effectively applied for a 9-Gb/s PAM4 signal transmission over 50-m FSO link, 30-m graded-index (GI)-POF transportation, and 2 m clear ocean underwater channel. To the authors' understanding, this study is the first to practically build an 82-m 9 Gb/s PAM4 FSO-POF-UWOC convergent system that effectively constructs a long-haul optical wireless-wired-wireless link using doublet lenses, GI-POF, and optical beam reducer. The performances of the proposed convergent systems are analyzed by bit error rate and non-return-to-zero eye diagram in real-time over an 82-m transport. This framework is the leading one to establish a long-haul FSO-POF-UWOC convergent system with qualified transmission performances. It guides a promising way to facilitate wide applications in the convergence of FSO, POF, and UWOC.

A hybrid Internet/cable television (CATV)/5G fiber-free-space optical (FSO) integrated system with a triple-wavelength polarization multiplexing scenario is attainably demonstrated for the first time. In a triple-wavelength polarization multiplexing scenario with vestigial sideband (VSB) four-level pulse amplitude modulation (PAM4)/CATV/16-quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) modulation, the channel capacity is considerably multiplied and the overall performance is significantly improved. With a tunable optical band-pass filter, polarization-tracking free de-multiplexing is attained by removing other wavelengths with different polarizations, and the optical VSB PAM4/CATV/16-QAM-OFDM signal is created by partially reducing the optical signal's linewidth. Compared with established polarization multiplexing fiber-FSO integrations with complex polarization de-multiplexing mechanism, it demonstrates a notable one with the benefits of simplicity and flexibility. Sufficiently low bit error rate, acceptable PAM4 eye diagrams, clear constellation map, and high composite second-order/composite triple beat values are attained through a 40 km single-mode fiber transport with 200 m free-space link. This demonstrated triple-wavelength polarization multiplexing fiber-FSO integrated system is promising because it not only incorporates fiber backhaul with optical wireless feeder, but it also simplifies system's structure and enhance system's overall performance.

For the first time up to our knowledge, a centralized-light-source two-way eight-level pulse amplitude modulation (PAM8)/four-level pulse amplitude modulation (PAM4) free-space optical (FSO) communication with parallel optical injection-locked vertical-cavity surface-emitting laser (VCSEL) transmitter is practically demonstrated. With the assistance of parallel optical injection locking, injection locking in the polarization sideband causes a simultaneous generation of the free-running orthogonal polarization sideband to form a centralized-light-source scheme. With the centralized light source, additional light source or wavelength reuse component, such as Fabry-Perot laser diode or reflective semiconductor optical amplifier, is not needed at the receiver side. It is very attractive because it enables the receiver side to share the light source remotely located at the transmitter side. Based on the centralized-light-source scheme and parallel optical injection-locking operation, the injection-locked parallel polarization sideband is used for downlink transmission, and the induced free-running orthogonal polarization sideband is transmitted and used as the optical carrier for uplink transmission. Over a 200-m free-space link, good bit-error-rate (BER) performance, the qualified PAM8, and PAM4 eye diagrams are obtained for two-way FSO communications. This proposed centralized-light-source two-way FSO communication provides a practical choice for two-way high transmission capacities and considerably develops the scenario characterized by parallel optical injection locking.

This paper aims to propose and conduct a practical demonstration of a flexible two-way phase modulation (PM)-based fiber-free-space optical (FSO) convergence system employing one optical carrier transmission scheme and vertical cavity surface emitting laser (VCSEL)-based tunable optical band-pass filter (TOBPF). One optical carrier is delivered to efficaciously reduce the dispersion effect caused by a 40-km single-mode fiber (SMF) transportation and the distortion caused by beating among multiple optical sidebands. Using a VCSEL-based TOBPF at the receiving site, the selected optical signal will be acquired flexibly from one of the multiple injected optical signals. This is the first time that an optical carrier transmission scheme and a VCSEL-based TOBPF are used in a two-way PM-based fiber-FSO convergence system. The downstream light is flexibly phase-remodulated with microwave/millimeter-wave data signal for uplink delivery. In-depth observation shows that bit error rate performs brilliantly via a 40-km SMF transportation with a 100-m FSO link. Such a flexible two-way PM-based fiber-FSO convergence system would be very attractive for the integration of fiber backbone and indoor networks to provide data signal flexibly.

A high-speed (400 Gb/s) and long-reach (180 m) four-level pulse amplitude modulation (PAM4) optical wireless communication system employing Mach-Zehnder modulator (MZM)-optoelectronic oscillator (OEO) broadband light source (BLS) and doublet lenses is proposed. Analytic results show that the data rate is markedly improved by PAM4 modulation and MZM-OEO BLS with multiple wavelengths, and the free-space transmission distance is considerably increased by a couple of doublet lenses. With the support of PAM4 modulation, MZM-OEO BLS, and doublet lenses, good bit error rate performance and clear PAM4/nonreturn-to-zero eye diagram are achieved at a 400 Gb/s/180 m operation. The proposed system is presented to be an eminent one not only as a result of its evolution in free-space optical (FSO) links, but also on account of its high-speed and long-reach characteristics in FSO links.

A high-speed 84 Gb/s vestigial sideband (VSB)-eight-level pulse amplitude modulation (PAM8) fiber-invisible laser light communication (IVLLC) integration based on vertical-cavity surface-emitting laser (VCSEL) transmitter with injection locking scheme is proposed. To the authors' knowledge, this study is the first to realize a high-speed 84 Gb/s VSB-PAM8 VCSEL transmitter-based fiber-IVLLC integration. It is effectual to adopt an optical VSB-PAM8 signal in fiber-IVLLC integrations to suppress the linewidth of an optical signal so as to reduce the fiber dispersion. An injection locking scheme and a linear equalizer possess enhancements in frequency response, and thereby provide good transmission performances in a high-speed VSB-PAM8 fiber-IVLLC integration. Impressive bit error rate performance and clear eye diagrams are acquired in real time at a 25-km single-mode fiber operation with a 10-m free-space link. Such proposed 84 Gb/s VSB-PAM8 VCSEL transmitter-based fiber-IVLLC integration is a notable option for providing high transmission rates at optical fiber and optical wireless communications.

A 25-Gbps underwater wireless optical communication (UWOC) system with a two-stage injection-locked 680-nm red-light vertical-cavity surface-emitting laser (VCSEL) transmitter to enhance the frequency response and a laser beam expander to expand the collimated beam diameter over a 5-m highly turbid harbor water link is proposed and practically demonstrated. In highly turbid harbor water link, the overall attenuation coefficient at 680 nm is smaller than that at 520 and 450 nm, thereby a 680-nm red-light VCSEL transmitter is adopted in this proposed 5 m/25 Gbps UWOC system rather than a 520-nm green-light laser diode (LD) transmitter or a 450-nm blue-light LD transmitter. A satisfactory bit error rate performance (3 × 10-9) and a clear eye diagram are acquired in real time. This proposed UWOC system with a two-stage injection-locked 680-nm VCSEL transmitter and a laser beam expander brings important improvements in the scenario characterized by high turbidity.

A wavelength-division-multiplexing (WDM) free-space optical (FSO) communication system of a high-speed hybrid signal is proposed and demonstrated in this study. This study is the first to transmit a high-speed hybrid signal mixed with four wavelengths that are modulated with different signals, namely 10, 25, 28, and 32 Gb/s, using a single beam. Favourable bit error rate (BER) and clear eye maps are achieved by adopting parameters after investigating the bandwidth of tuneable optical bandpass filter, channel spacing, and number of mixed light channel effects on system performance. The WDM-FSO communication system of a high-speed hybrid signal has a simple configuration, low cost, and low BER.

A 103.12-Gb/s wavelength-division-multiplexing (WDM) four-level pulse amplitude modulation (PAM4) transmission based on 850-nm and 880-nm vertical-cavity surface-emitting lasers (VCSELs) with light injection and optoelectronic feedback techniques is proposed and experimentally demonstrated. Results show that two such 7.5-GHz VCSELs with light injection and optoelectronic feedback techniques are potent for 103.12-Gb/s WDM PAM4 transmissions. To the authors' knowledge, it is the first one to successfully adopt two VCSEL transmitters with light injection and optoelectronic feedback techniques in a WDM PAM4 transmission. A total transmission rate of 103.12 Gb/s (51.56 Gb/s/λ × 2 λs) is achieved in the proposed WDM PAM4 transmissions. The link performances of the proposed WDM PAM4 transmissions have been evaluated in real time. Good real-time bit error rate performance and three independent clear eye diagrams are obtained at a 180-m OM4 multimode fiber operation. Such a proposed 103.12-Gb/s WDM PAM4 VCSEL-based transmission has great potential for providing efficient bandwidth in short-reach optical communications.

A 56 Gb/s four-level pulse amplitude modulation (PAM4) light-based WiFi (LiFi) transmission based on a 680-nm/5.4-GHz vertical-cavity surface-emitting laser (VCSEL) with a two-stage injection-locked technique is proposed and demonstrated. Experimentally results show that a 5.4-GHz VCSEL with a two-stage injection-locked technique is effective for 56 Gb/s PAM4 LiFi transmissions. To the authors' knowledge, it is the first one to adopt a 680-nm VCSEL transmitter with two-stage injection-locked technique in a 56 Gb/s PAM4 LiFi transmission. A pair of doublet lenses is employed in the proposed PAM4 VCSEL-based LiFi transmissions to enhance the free-space link. The link performances of the proposed PAM4 LiFi transmissions have been analyzed in real time. Good bit error rate performance and three independent clear eye diagrams are obtained over a 20-m free-space link. Such a proposed 56 Gb/s PAM4 VCSEL-based LiFi transmission with two-stage injection-locked technique has the potential to play a significant role in future wireless infrastructure for providing high transmission rate and long free-space transmission distance effectively.

Fiber-wireless and fiber-invisible laser light communication (IVLLC) convergences that adopt the Mach-Zehnder modulator-optoelectronic oscillator (MZM-OEO)-based broadband light source for microwave (MW)/millimeter-wave (MMW)/baseband (BB) signal transmission are proposed and demonstrated. For downlink transmission, light is optically promoted from 15- and 25-GHz radio-frequency (RF) signals to 10-Gb/s/30-GHz MW, 15-Gb/s/50-GHz MMW, 20-Gb/s/60-GHz MMW, and 25-Gb/s/100-GHz MMW data signals based on fiber-wireless and fiber-IVLLC integrations. Bit error rate (BER) performs efficiently in the 40-km single-mode fiber (SMF) and the 10-m RF/25-m optical wireless transport scenarios. For uplink transmission, downstream light is successfully intensity-modulated with a 25-Gb/s BB data stream based on fiber-IVLLC integration. BER performs efficiently in the 40-km SMF and the 100-m optical wireless transport scenario. Such a hybrid MW/MMW/BB lightwave transmission system is an attractive alternative. It is shown to be a prominent one to present advancements in integrating fiber backbone and radio-frequency (RF)/optical wireless feeder networks.

A 50-m/40-Gb/s 680-nm vertical-cavity surface-emitting laser (VCSEL)-based free-space optical (FSO) communication employing three-stage injection-locked technique and the afocal scheme is proposed and experimentally demonstrated. The three-stage injection-locked technique, which can significantly increase the resonant frequency of the VCSEL, is expected to provide higher transmission rate in FSO communication. The afocal scheme, which can reduce the beam size of laser beam, is expected to provide longer free-space link in FSO communication. As far as we know, it is the first time that a three-stage injection-locked 680-nm VCSEL transmitter and afocal scheme that will successfully build up a 50-m/40-Gb/s FSO communication has been employed. Such a 50-m/40-Gb/s FSO communication provides the advantages of optical wireless links for longer transmission distance and higher transmission rate, which is thoroughly useful for high-speed light-based WiFi (LiFi) applications.

This paper proposes and presents the experimental demonstration of 320-Gb/s free-space optical (FSO) communication based on dense-wavelength-division-multiplexing (DWDM) technology and afocal scheme. To the best of our knowledge, this is the first one that adopts DWDM technology and afocal scheme to successfully demonstrate 50-m/320-Gb/s FSO communication. Results show that the free-space transmission distance is greatly increased by the afocal scheme and that the free-space transmission rate is significantly enhanced by the DWDM technology. DWDM FSO communication over a 50-m free-space link with a total transmission rate of 320 Gb/s (40 Gb/s/λ × 8λ = 320 Gb/s) is achieved. With the aid of a low-noise amplifier and clock/data recovery at the receiving site, good bit-error-rate (BER) performance and clear eye diagram are obtained at a 50-m/320-Gb/s operation. Such 50-m/320-Gb/s DWDM FSO communication provides the advantages of optical wireless communications for long transmission distance and high transmission rate, which is thoroughly useful for long-haul and high-speed light-based WiFi (LiFi) applications.

A bidirectional fiber-wireless and fiber-invisible laser light communication (IVLLC) convergence system that adopts a dual-polarization modulation scheme and a Mach-Zehnder modulator (MZM)-optoelectronic oscillator (OEO)-based broadband light source (BLS) for hybrid cable television (CATV)/microwave (MW)/millimeter-wave (MMW)/baseband signal transmission is proposed and experimentally demonstrated. The MZM employed in the MZM-OEO is operated at the minimum transmission point, which results in the format of optical carrier suppression for the y-axis component of the light. Using a dual-polarization modulation scheme, the optical carrier modulated with CATV signal (x-polarization) and the optical sidebands modulated with MW and MMW data signals ( y-polarization) are separated and polarized orthogonally automatically. Through an in-depth observation, good carrier-to-noise ratio, composite second-order, composite triple-beat, and bit error rate performances are obtained over a 40-km single-mode fiber and a 10-m RF/100-m optical wireless transport. Such a bidirectional fiber-wireless and fiber-IVLLC convergence system is a notable option; it would be attractive for providing broadband heterogeneous services such as CATV, Internet, and big data services.

An 8 m/9.6 Gbps underwater wireless optical communication (UWOC) system based on a two-stage injection-locked 405 nm blue light laser diode (LD) transmitter with a 16-quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) modulating signal is proposed and experimentally demonstrated. To the best of our knowledge, it is the first one that employs a two-stage injection-locked technique to improve the transmission performance of UWOC systems. With the assistance of an equalizer at the receiver site, acceptable bit error rate (BER) performance and a constellation map are obtained over an 8-m underwater link. Such a two-stage injection-locked 405 nm blue light LD transmitter-based UWOC system is shown to be a promising alternative, presenting its feasibility for long-range and high-speed underwater links.

A progressive bidirectional cable television (CATV)/wireless-over-fiber lightwave transmission system based on one broadband light source (BLS), one arrayed waveguide grating, two intensity modulators, and one phase modulator is proposed and experimentally demonstrated. A continuous amplified spontaneous emission BLS is split into eight channels in the C-band with 50-GHz spacing. The multiple equal-space optical spectra exhibit a high optical signal-to-noise ratio (≥ 55 dB) and good flatness (≤ 5 dB). A high-quality comb multicarrier provides a single wavelength and twin millimeter-wave bands for baseband CATV and wireless transport systems. Several measured values (carrier-to-noise ratio ≥ 64 dB, composite second order ≥ 62 dB, composite triple beat ≥ 63 dB, and bit error rate $\leq 10^{-9} $) can be obtained from distortion-reduced and noise-free performance. Moreover, downstream and upstream high-bit-rate wireless transmissions exhibit an interference-eliminating property. Thus, the proposed system does not only offer a future for progressive fiber backbone and CATV/wireless hybrid networks, but also provides a full-duplex interference-free for high-data-rate mobile communication and in-home CATV access networks as well.

An innovative free-space optical (FSO) link using laser light propagation to achieve transmission rate of 40 Gb/s at a wavelength of 1550 nm is proposed and experimentally demonstrated. Over a 20-m free-space link, brilliant bit-error-rate performance and clear eye diagram are obtained in the proposed 1550-nm distributed feedback (DFB) laser diode (LD)-based FSO links. As far as we know, it is the first time that a 1550-nm externally modulated laser transmitter cascaded with an erbium-doped fiber amplifier and a pair of fiber collimators to successfully set up a 20-m/40-Gb/s FSO link has been employed. Compared with the 680-nm vertical-cavity surface-emitting laser-based FSO link, this proposed 1550-nm DFB LD-based FSO link is attractive not only because it has longer free-space transmission distance but because it supplies higher bandwidth operation as well. Such a 1550-nm DFB LD-based FSO link provides the benefits of optical wireless communications for longer transmission distance and higher transmission rate, which is thoroughly helpful for optical wireless network applications.

A 20-km/60-Gb/s two-way passive optical network (PON) based on directly modulated two-stage injection-locked 1.55-μm vertical-cavity surface-emitting laser (VCSEL) transmitters and negative dispersion fibers (NDFs) is proposed and demonstrated. A one-stage injection-locked technique has been confirmed as an effective approach for increasing the frequency response of a VCSEL. A two-stage injection-locked technique, which can further increase the frequency response of a VCSEL, is thereby expected to have excellent transmission performance in a two-way PON. To be the first to employ directly modulated two-stage injection-locked 1.55-μm VCSEL transmitters in a two-way PON, brilliant bit-error-rate performance and a clear eye diagram are obtained over a 20-km NDF transport. A directly modulated VCSEL has a positive chirp, whereas an NDF has a negative dispersion property in the transmission fiber; this negative dispersion property compensates the laser positive chirp and results in systems with better transmission performance. Such an innovative two-way PON provides the advantage of a communication link for high data rates, which is an attractive feature that is useful for broadband network applications.

This paper proposes and validates a bidirectional hybrid lightwave transport system based on fiber-invisible laser light communication (IVLLC) and fiber-visible laser light communication (VLLC) convergences with light injection and optoelectronic feedback techniques. This paper is also the first to employ light injection and optoelectronic feedback techniques in a bidirectional lightwave transport system based on fiber-IVLLC and fiber-VLLC integration. Light is successfully modulated directly with cable television (CATV), 16-quadrature amplitude modulation (QAM), and 16-QAM-orthogonal frequency-division multiplexing (OFDM) signals. Good carrier-to-noise ratio, composite second order, composite triple beat, and bit error rate (BER) are obtained for CATV, 16-QAM, and 16-QAM-OFDM signal transmissions over a combined 40-km single-mode fiber, a 1.43-km photonic crystal fiber, and 6-m free-space transmission. The proposed bidirectional hybrid lightwave transport system exhibits significant potential in providing broadband integrated services, such as CATV, Internet, and telecommunication, via optical fiber and free-space indoor networks.

A hybrid orthogonal frequency-division multiplexing (OFDM) and quadruple-frequency radio-over-fiber (RoF) transport system is developed based on colorless optical polarization beam splitters and an optical polarization-arrangement technique. No optical filter is required to eliminate any specific optical wavelength from the hybrid transport system; the employed optical wavelength and central carrier frequency of the transmitted radio-frequency signal can be dynamically adjusted without alerting the transmission performance of the proposed hybrid transport system. To prove the concept of the quadruple-frequency technique, a 40-GHz millimeter-wave (MMW) signal quadrupled from a 10-GHz signal is experimentally demonstrated in an optical domain. Its advancement is then employed to experimentally accomplish a hybrid OFDM and quadruple-frequency RoF signal transmission over a 25-km span of single-mode fiber. The advancement of the hybrid transport system is evaluated and demonstrated by error-free transmissions and clear eye and constellation diagrams. This proposed system is a great candidate for greatly reducing the cost of simultaneously transmitting OFDM and MMW signals over optical access networks and makes it more practical to be employed.

A hybrid wavelength-division-multiplexing (WDM) lightwave transport system for millimeter-wave (MMW)/microwave (MW)/baseband (BB) signals that are transmission based on fiber-wireless and fiber-visible laser light communication (VLLC) convergences is proposed and demonstrated. A broadband light source with an optical signal-to-noise ratio enhancement scheme in a hybrid lightwave transport system is employed. Light is optically promoted from a 5-Gbps/15-GHz RF data signal to 5-Gbps/60-GHz MMW and 5-Gbps/30-GHz MW data signals in fiber-wireless convergence. Light is also optically demoted from a 5-Gbps/15-GHz RF data signal to a 5-Gbps data stream in fiber-VLLC convergence. Over a 40-km single-mode fiber and a 4-m RF wireless/a 10-m free-space VLLC transmission, bit error rate performs impressively for 60-GHz MMW, 30-GHz MW, and 5-Gbps BB signal transmission. Such a hybrid WDM lightwave transport system could have practical applications for fiber-wireless and fiber-VLLC convergences to provide broadband integrated services.