Underwater wireless optical communication (UWOC) has gained increasing research interest worldwide from both academic and industrial communities, because of its high bandwidth, compact antennas, low latency, cost-effectiveness, and low power consumption. In the underwater world, the wireless optical links can be complementary to or even more competitive than its acoustic counterpart. Nevertheless, the hostile underwater environment sets up natural obstacles to most information carriers, including the lightwave, even at the right wavelength. Much attention has been recently paid to this interesting and challenging area, leading to impressive progresses. Hence, we launched this focus issue to discuss recent advances and progress in UWOC, and aimed to further stimulate future advancements in this emerging field.

实验研究了弱谐振腔法布里-珀罗激光器在受到光注入扰动时的非线性动力学特性．通过测定“－3模”、“0模”、“13模”三个纵模输出的时间序列, 功率谱和光谱分布, 对其非线性动力学状态进行了判定．研究结果表明: 引入光注入后, 通过改变注入强度及频率失谐, 这三个纵模均可呈现出四波混频、单周期态、准周期态、混沌态以及稳定的光注入锁定态等非线性动力学特性, 其动力学演化路径为经准周期分岔进入混沌态．此外, 为实现对动力学状态的精确控制, 绘制了这三个纵模在光注入强度0 μW到450 μW和频率失谐－16 GHz到16 GHz构成的参量空间的动力学状态分布图．结果显示: “0模”与“13模”在正、负频率失谐均可观察到混沌区域, 而“－3模”在负频率失谐没有出现混沌区域; “0模”存在两个分离的稳定的光注入锁定态区, 而在“－3模”和“13模”仅观测到一个稳定的光注入锁定态区.

A few-mode (FM) vertical cavity surface emitting laser (VCSEL) chip with heavily zinc-diffused contact layer and oxide-confined cross-section is demonstrated for carrying pre-leveled 16-quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) data in OM4 multi-mode fiber (MMF) over 100 m for intra-data-center applications. The FM VCSEL chip, which has an oxide-confined emission aperture of 5 μm, demonstrates high external quantum efficiency, provides an optical power of 2.2 mW at 38 times threshold condition, and exhibits 3 dB direct-modulation bandwidth beyond 22 GHz at a cost of slight heat accumulation. At a DC bias point of 5 mA (22.6Ith) the FM VCSEL chip, with sufficiently normalized modulation output, supports Baud and data rates of 25 and 100 Gb/s, respectively, with forward error correction (FEC) certifying receiving quality after back-to-back transmission. After passing through 100 m OM4 MMF with a receiving power penalty of 4 dB, the FM VCSEL chip demonstrates FEC-certified transmission of the pre-leveled 16-QAM OFDM data at 92 Gb/s.

Wavelength tunable photoluminescence (PL) of Si-rich silicon nitride (SRSN) film with buried Si nanocrystals (Si-ncs) grown by plasma enhanced chemical vapor deposition (PECVD) under SiH4 and NH3 environment is investigated. Intense broadband visible emissions tunable from blue to red can be obtained from the as-deposited SiNx thin films with increasing NH3 flow rate from 150 to 250 sccm and detuning the SiH4/NH3 flow ratio during deposition. To date, the normalized PL wavelength of SiNx films after annealing could be detuned over the range of 385~675 nm by decreasing the NH3 flow rate, corresponding to an enlargement on Si-nc size from 1.5~2 to 4~5 nm. The PL linewidth is decreased with increasing ammonia flow rate due to the improved uniformity of Si-ncs under high NH3 flow rate condition. In addition, the PL intensity is monotonically increasing with the blue shift of PL wavelength due to the increasing density of small-size Si-ncs. The ITO/SiNx/p-Si/Al diode reveals highly resistive property with the turn-on voltage and power-voltage slope of only 20 V and 0.18 nW/V, respectively. The turn-on voltage can further reduce from 20 to 3.8 V by improving the carrier injection efficiency with p-type Si nano-rods.

The effect of annealing condition on sputtered indium tin oxide (ITO) films on quartz with the thickness of 200 nm is characterized to show enhanced optical transparency and optimized electrical contact resistivity. The as-deposited grown ITO film exhibits only 65% and 80% transmittance at 532 and 632.8 nm, respectively. After annealing at 475 oC for 15 min, the ITO film is refined to show improved transmittance at shorter wavelength region. The transmittances of 88.1% at 532 nm and 90.4% at 632.8 nm can be obtained. The 325-nm transmittance of the post-annealed ITO film is greatly increased from 12.7% to 41.9%. Optimized electrical property can be obtained when annealing below 450 oC, leading to a minimum sheet resistance of 26 \Omega/square. Such an ITO film with enhanced ultraviolet (UV) transmittance has become an alternative candidate for applications in current UV photonic devices. The morphology and conductance of the as-deposited and annealed ITO films are determined by using an atomic force microscopy (AFM), showing a great change on the uniformity distribution with finite improvement on the surface conductance of the ITO film after annealing.

The interfacial Si nano-pyramid-enhanced electroluminescence (EL) of an ITO/SiOx/p-Si/Al metal-oxide-semiconductor (MOS) diode with turn-on voltage of 50 V, threshold current of 1.23 mA/cm2, output power of 16 nW, and lifetime of 10 h is reported.

Enhanced near-infrared (NIR) electroluminescence (EL) of a metal-oxide-semiconductor light emitting device (MOSLED) made on CO2 laser-annealed SiOx film is demonstrated. An EL power of near 50 nW from CO2 laser rapid-thermal-annealing (RTA) MOSLED under a biased voltage of 85 V and a current density of 2.3 mA/cm2 is preliminarily reported.