Kang, Chun Hong; Trichili, Abderrahmen; Alkhazragi, Omar; Zhang, Huafan; Subedi, Ram Chandra; Guo, Yujian; Mitra, Somak; Shen, Chao; Roqan, Iman S.; Ng, Tien Khee; Alouini, Mohamed-Slim; Ooi, Boon S.(Optics Express, The Optical Society, 2019-10-08)[Article]
Underwater wireless optical communication (UWOC) can offer reliable and secure connectivity for enabling future internet-of-underwater-things (IoUT), owing to its unlicensed spectrum and high transmission speed. However, a critical bottleneck lies in the strict requirement of pointing, acquisition, and tracking (PAT), for effective recovery of modulated optical signals at the receiver end. A large-area, high bandwidth, and wide-angle-of-view photoreceiver is therefore crucial for establishing a high-speed yet reliable communication link under non-directional pointing in a turbulent underwater environment. In this work, we demonstrated a large-area, of up to a few tens of cm2, photoreceiver design based on ultraviolet(UV)-to-blue color-converting plastic scintillating fibers, and yet offering high 3-dB bandwidth of up to 86.13 MHz. Tapping on the large modulation bandwidth, we demonstrated a high data rate of 250 Mbps at bit-error ratio (BER) of 2.2 × 10−3 using non-return-to-zero on-off keying (NRZ-OOK) pseudorandom binary sequence (PRBS) 210-1 data stream, a 375-nm laser-based communication link over the 1.15-m water channel. This proof-of-concept demonstration opens the pathway for revolutionizing the photodetection scheme in UWOC, and for non-line-of-sight (NLOS) free-space optical communication.
Shamim, Md. Hosne Mobarok; Alkhazragi, Omar; Ng, Tien Khee; Ooi, Boon S.; Khan, Mohammed Zahed Mustafa(IEEE Access, Institute of Electrical and Electronics Engineers (IEEE), 2019-10-01)[Article]
We implemented a tunable dual-longitudinal-mode spacing InGaN/GaN green (521–528 nm) laser diode by employing a self-injection locking scheme that is based on an external cavity configuration and utilizing either a high-or partial-reflecting mirror. A tunable longitudinal-mode spacing of 0.20 – 5.96 nm was accomplished, corresponding to a calculated frequency difference of 0.22–6.51 THz, as a result. The influence of operating current and temperature on the system performance was also investigated with a measured maximum side-mode-suppression ratio of 30.4 dB and minimum dual-mode peak optical power ratio of 0.03 dB. To shed light on the operation of the dual-wavelength device arising from the tunable longitudinal-mode spacing mechanism, the underlying physics is qualitatively described. To the best of our knowledge, this tunable longitudinal-mode-spacing dual-wavelength device is novel, and has potential applications as an alternative means in millimeter wave and THz generation, thus possibly addressing the terahertz technology gap. The dual-wavelength operation is also attractive for high-resolution imaging and broadband wireless communication.
Mao, Yuan; Ashry, Islam; Alias, Mohd Sharizal; Ng, Tien Khee; Hveding, Frode; Arsalan, Muhammad; Ooi, Boon S.(IEEE Photonics Journal, Institute of Electrical and Electronics Engineers (IEEE), 2019-09-17)[Article]
We experimentally investigated the performance of a distributed acoustic sensor (DAS) designed using a few-mode fiber (FMF), when launching different spatial modes under intentional index perturbation within the fiber. Our demonstration showed that the quasi-single mode (QSM) operated FMF offers higher signal-to-noise ratio (SNR) for the DAS, compared with the case when launching other degenerate higher order modes. Additionally, we compared the behavior of the single-mode fiber (SMF)- and FMF-based DAS when using optical pulses of varying power levels. The FMF enables the realization of a DAS with longer sensing range and higher spatial resolution. The developed FMF-based DAS is further tested via sensing various vibration events produced by piezoelectric transducer (PZT) cylinder, pencil break, and loudspeaker.
Stegenburgs, Edgars; Bertoncini, Andrea; Trichili, Abderrahmen; Alias, Mohd Sharizal; Ng, Tien Khee; Alouini, Mohamed-Slim; Liberale, Carlo; Ooi, Boon S.(IEEE Communications Magazine, Institute of Electrical and Electronics Engineers (IEEE), 2019-08-22)[Article]
We report the use of 3D-printed microscale spiral phase plates to generate orbital angular momentum (OAM) carrying beams. We confirm that the generated beams have high purity, and we have successfully tested them to convey data signals with low bit error rates at the wavelength of 980 nm. This method will open new opportunities for generating OAM beams for many applications in optical communications, including free-space optics, as well as underwater, chip-to-chip, and quantum communications.
Alkhazragi, Omar; Sun, Xiaobin; Zuba, Viktor; Amhoud, El Mehdi; Oubei, Hassan M.; Ng, Tien Khee; Jones, Burton; Alouini, Mohamed-Slim; Ooi, Boon S.(IEEE Photonics Journal, Institute of Electrical and Electronics Engineers (IEEE), 2019-08-22)[Article]
To allow for reliable wireless optical links in realistic underwater environments, we study the dependence of turbulence-induced fading on the wavelength using a laser-based white-light interrogator in emulated realistic conditions. We experimentally show that the scintillation index decreases significantly with the increase of wavelength. The results are verified for longer distances using a Monte Carlo simulation. We numerically and experimentally demonstrate that the use of longer wavelengths lowers the bit error ratio by as much as three orders of magnitude. We conclude that using green light is more reliable in turbulent channels than blue. The correlation between different wavelengths under turbulence is studied, which was made possible by the use of the laser-based white-light interrogator.
Zhou, Lyu; Song, Haomin; Liang, Jian Wei; Singer, Matthew; Zhou, Ming; Stegenburgs, Edgars; Zhang, Nan; Xu, Chen; Ng, Tien Khee; Yu, Zongfu; Ooi, Boon S.; Gan, Qiaoqiang(Nature Sustainability, Springer Science and Business Media LLC, 2019-08-09)[Article]
Radiative cooling is a passive cooling strategy with zero consumption of electricity that can be used to radiate heat from buildings to reduce air-conditioning requirements. Although this technology can work well during optimal atmospheric conditions at night, it is essential to achieve efficient cooling during the daytime when peak cooling demand actually occurs. Here we report an inexpensive planar polydimethylsiloxane (PDMS)/metal thermal emitter thin film structure, which was fabricated using a fast solution coating process that is scalable for large-area manufacturing. By performing tests under different environmental conditions, temperature reductions of 9.5 °C and 11.0 °C were demonstrated in the laboratory and an outside environment, respectively, with an average cooling power of ~120 W m– 2 for the thin film thermal emitter. In addition, a spectral-selective structure was designed and implemented to suppress the solar input and control the divergence of the thermal emission beam. This enhanced the directionality of the thermal emissions, so the emitter’s cooling performance was less dependent on the surrounding environment. Outside experiments were performed in Buffalo, New York, realizing continuous all-day cooling of ~2–9 °C on a typical clear sunny day at Northern United States latitudes. This practical strategy that cools without electricity input could have a significant impact on global energy consumption.
Alwadai, Norah M.; Ajia, Idris A.; Janjua, Bilal; Flemban, Tahani H.; Mitra, Somak; Wehbe, Nimer; Wei, Nini; Lopatin, Sergei; Ooi, Boon S.; Roqan, Iman S.(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2019-07-25)[Article]
One-dimensional (1D) structures-based UV-light-emitting diode (LED) has immense potential for next-generation applications. However, several issues related to such devices must be resolved first, such as expensive material and growth methods, complicated fabrication process, efficiency droop, and unavoidable metal contamination due to metal catalyst that reduces device efficiency. To overcome these obstacles, we have developed a novel growth method for obtaining a high-quality hexagonal, well-defined, and vertical 1D Gd-doped n-ZnO nanotube (NT) array deposited on p-GaN films and other substrates by pulsed laser deposition. By adopting this approach, the desired high optical and structural quality is achieved without utilizing metal catalyst. Transmission electron microscopy measurements confirm that gadolinium dopants in the target form a transparent in situ interface layer to assist in vertical NT formation. Microphotoluminescence (PL) measurements of the NTs reveal an intense ZnO band edge emission without a defect band, indicating high quality. Carrier dynamic analysis via time-resolved PL confirms that the emission of n-ZnO NTs/p-GaN LED structure is dominated significantly by the radiative recombination process without efficiency droop when high carrier density is injected optically. We developed an electrically pumped UV Gd-doped ZnO NTs/GaN LED as a proof of concept, demonstrating its high internal quantum efficiency (>65%). The demonstrated performance of this cost-effective UV LED suggests its potential application in large-scale device production.
Sun, Xiaobin; Kong, Meiwei; Shen, Chao; Kang, Chun Hong; Ng, Tien Khee; Ooi, Boon S.(Optics Express, The Optical Society, 2019-06-27)[Article]
We experimentally demonstrated high-speed diffuse line-of-sight optical wireless communication across a wavy water-air-interface. The testbed channel was evaluated, in terms of data rate, coverage and robustness to the dynamic wave movement, based on the performance of different modulation schemes, including non-return-to-zero on-off keying (NRZ-OOK) and quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM). Under the emulated calm water condition, 8-QAM-OFDM offers a data rate of 111.4 Mbit/s at the aligned position, while only 55 Mbit/s is achieved using NRZ-OOK. On the other hand, effective communication can still be maintained at a high data rate of 11 Mbit/s when the photodetector is off aligned laterally by 5 cm based on NRZ-OOK modulation, leading to a coverage of ~79 cm2. By utilizing OFDM modulation scheme, a data rate of 30 Mbit/s can be achieved up to 2.5-cm misalignment, leading to a coverage of ~20 cm2. Furthermore, in the presence of strong waves (15-mm wave height, causing a scintillation index of 0.667), 4-QAM-OFDM modulation showed a better resilience to channel instability than NRZ-OOK modulation. Our studies pave the way for the eventual realization of communication across a challenging water-air interface without the need for an interface relay, which is much sought-after for implementing a robust and large-coverage underwater-to-terrestrial internet-of-things.
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