Prof.W.B.Lee Was Invited to Be Honorary Member of Editorial Board of JNUAA

Professor W.B.Lee is the director of Department of Manufacturing Engineering of Hong KongPolytechnic University and a scholar of great attainments in manufacturing science and technology,ma-terials processing,ultra-precision machining.At present he has published over90 papers on the variousjournals and international conferences,2 books and2 keynote papers.Because of his research reputationProfessor W.B.Lee has been invited to be honorary professor of Nanjing University of Aeronautics andAstronautics(NUAA)in1996.On14 Decemeber,this year,Prof.W.B.Lee was one again invited tobe honorary member of Editorial Board of Journal of Nanjing University of Aeronautics and Astronautics(JNUAA).NUAA specially promulgates the formal document for Prof.W.B.Lee and he accepted withpleasure the honorary title.

Advanced modulation formats for underwater visible light communications [Invited]

In this paper, we present a detailed comparison of applying three advanced modulation formats including carrierless amplitude and phase modulation(CAP), orthogonal frequency division multiplexing(OFDM), and discrete Fourier transform spread orthogonal frequency division multiplexing(DFT-S OFDM) in underwater visible light communication(UVLC) systems. Cascaded post-equalization schemes are suggested to compensate the system impairments. For the first time, a two-level post-equalizer is presented to mitigate the nonlinear effect and improve the system performance of UVLC. The first post-equalization is based on a novel recursive least square Volterra. These modulation formats are all experimentally demonstrated with corresponding digital signal processing(DSP) algorithms. The experimental results show that single carrier modulations including CAP and DFT-S OFDM can outperform OFDM. Our experiment results show that up to 3 Gb/s over a 1.2 m underwater visible light transmission can be achieved by using DFT-S OFDM 64 QAM and CAP-64. The measured bit error rate is well under the hard decision-forward error correction(HD-FEC) threshold of 3.8 × 10^(-3).

Silicon photonic platforms for mid-infrared applications [Invited]

Silicon photonic integrated circuits for telecommunication and data centers have been well studied in the past decade, and now most related efforts have been progressing toward commercialization. Scaling up the silicon-oninsulator(SOI)-based device dimensions in order to extend the operation wavelength to the short mid-infrared(MIR) range(2–4 μm) is attracting research interest, owing to the host of potential applications in lab-on-chip sensors, free space communications, and much more. Other material systems and technology platforms, including silicon-on-silicon nitride, germanium-on-silicon, germanium-on-SOI, germanium-on-silicon nitride, sapphireon-silicon, Si Ge alloy-on-silicon, and aluminum nitride-on-insulator are explored as well in order to realize low-loss waveguide devices for different MIR wavelengths. In this paper, we will comprehensively review silicon photonics for MIR applications, with regard to the state-of-the-art achievements from various device demonstrations in different material platforms by various groups. We will then introduce in detail of our institute's research and development efforts on the MIR photonic platforms as one case study. Meanwhile, we will discuss the integration schemes along with remaining challenges in devices(e.g., light source) and integration. A few application-oriented examples will be examined to illustrate the issues needing a critical solution toward the final production path(e.g., gas sensors). Finally, we will provide our assessment of the outlook of potential futureresearch topics and engineering challenges along with opportunities.

Lowering the energy consumption in silicon photonic devices and systems [Invited]

We review current silicon photonic devices and their performance in connection with energy consumption.Four critical issues are identified to lower energy consumption in devices and systems: reducing the influence of the thermo-optic effect, increasing the wall-plug efficiency of lasers on silicon, optimizing energy performance of modulators, and enhancing the sensitivity of photodetectors. Major conclusions are(1) Mach–Zehnder interferometer-based devices can achieve athermal performance without any extra energy consumption while microrings do not have an efficient passive athermal solution;(2) while direct bonded III–V-based Si lasers can meet system power requirement for now, hetero-epitaxial grown III–V quantum dot lasers are competitive and may be a better option for the future;(3) resonant modulators, especially coupling modulators, are promising for low-energy consumption operation even when the power to stabilize their operation is included;(4) benefiting from high sensitivity and low cost, Ge/Si avalanche photodiode is the most promising photodetector and can be used to effectively reduce the optical link power budget. These analyses and solutions will contribute to further lowering energy consumption to meet aggressive energy demands in future systems.

Silicon and hybrid silicon photonic devices for intra-datacenter applications: state of the art and perspectives [Invited]

We review the state of the art and our perspectives on silicon and hybrid silicon photonic devices for optical interconnects in datacenters. After a brief discussion of the key requirements for intra-datacenter optical interconnects, we propose a wavelength-division-multiplexing(WDM)-based optical interconnect for intra-datacenter applications. Following our proposed interconnects configuration, the bulk of the review emphasizes recent developments concerning on-chip hybrid silicon microlasers and WDM transmitters, and silicon photonic switch fabrics for intra-datacenters. For hybrid silicon microlasers and WDM transmitters, we outline the remaining challenges and key issues toward realizing low power consumption, direct modulation, and integration of multiwavelength microlaser arrays. For silicon photonic switch fabrics, we review various topologies and configurations of high-port-count N-by-N switch fabrics using Mach–Zehnder interferometers and microring resonators as switch elements, and discuss their prospects toward practical implementations with active reconfiguration.For the microring-based switch fabrics, we review recent developments of active stabilization schemes at the subsystem level. Last, we outline several large challenges and problems for silicon and hybrid silicon photonics to meet for intra-datacenter applications and propose potential solutions.

Few-layer MoS2 saturable absorbers for short-pulse laser technology: current status and future perspectives [Invited]

Few-layer molybdenum disulfide（MoS2） is emerging as a promising quasi-two-dimensional material for photonics and optoelectronics, further extending the library of suitable layered nanomaterials with exceptional optical properties for use in saturable absorber devices that enable short-pulse generation in laser systems. In this work, we catalog and review the nonlinear optical properties of few-layer MoS2, summarize recent progress in processing and integration into saturable absorber devices, and comment on the current status and future perspectives of MoS2-based pulsed lasers.

2D noncarbon materials-based nonlinear optical devices for ultrafast photonics [Invited]

Ultrafast lasers play an important role in a variety of applications ranging from optical communications to medical diagnostics and industrial materials processing. Graphene and other two-dimensional（2D） noncarbon materials, including topological insulators（TIs）, transition metal dichalcogenides（TMDCs）, phosphorene, bismuthene, and antimonene, have witnessed a very fast development of both fundamental and practical aspects in ultrafast photonics since 2009. Their unique nonlinear optical properties enable them to be used as excellent saturable absorbers（SAs） that have fast responses and broadband operation, and can be easily integrated into lasers. Here, we catalog and review recent progress in the exploitation of these 2D noncarbon materials in this emerging field. The fabrication techniques, nonlinear optical properties, and device integration strategies of 2D noncarbon materials are first introduced with a comprehensive view. Then, various mode-locked/Q-switched lasers（e.g., fiber, solid-state, disk, and waveguide lasers） based on 2D noncarbon materials are reviewed. In addition, versatile soliton pulses generated from the mode-locked fiber lasers based on 2D noncarbon materials are also summarized. Finally, future challenges and perspectives of 2D materials-based lasers are addressed.

Silicon and silicon nitride photonic circuits for spectroscopic sensing on-a-chip [Invited]

There is a rapidly growing demand to use silicon and silicon nitride（Si3N4） integrated photonics for sensing applications, ranging from refractive index to spectroscopic sensing. By making use of advanced CMOS technology,complex miniaturized circuits can be easily realized on a large scale and at a low cost covering visible to mid-IR wavelengths. In this paper we present our recent work on the development of silicon and Si3N4-based photonic integrated circuits for various spectroscopic sensing applications. We report our findings on waveguide-based absorption, and Raman and surface enhanced Raman spectroscopy. Finally we report on-chip spectrometers and on-chip broadband light sources covering very near-IR to mid-IR wavelengths to realize fully integrated spectroscopic systems on a chip.

Nonlinear optical absorption of few-layer molybdenum diselenide （MoSe2） for passively mode-locked soliton fiber laser [Invited]

In this paper,both nonlinear saturable absorption and two-photon absorption（TPA） of few-layer molybdenum diselenide（MoSe2） were observed at 1.56 μm wavelength and further applied to mode-locked ultrafast fiber laser for the first time to our knowledge.Few-layer MoSe2 nanosheets were prepared by liquid-phase exfoliation method and characterized by x ray diffractometer,Raman spectroscopy,and atomic force microscopy.The obtained fewlayer MoSe2 dispersion is further composited with a polymer material for convenient fabrication of MoSe2 thin films.Then,we investigated the nonlinear optical（NLO） absorption property of the few-layer MoSe2 film using a balanced twin-detector measurement technique.Both the saturable absorption and TPA effects of the few-layer MoSe2 film were found by increasing the input optical intensity.The saturable absorption shows a modulation depth of 0.63% and a low nonsaturable loss of 3.5%,corresponding to the relative modulation depth of 18%.The TPA effect occurred when the input optical intensity exceeds 260 MW∕cm2.Furthermore,we experimentally exploit the saturable absorption of few-layer MoSe2 film to mode lock an all-fiber erbium-doped fiber laser.Stable soliton mode locking at 1558 nm center wavelength is achieved with pulse duration of 1.45 ps.It was also observed that the TPA process suppresses the mode-locking operation in the case of higher optical intensity.Our results indicate that layered MoSe2,as another two-dimensional nanomaterial,can provide excellent NLO properties（e.g.,saturable absorption and TPA） for potential applications in ultrashort pulse generation and optical limiting.

Mode-locking of fiber lasers using novel two-dimensional nanomaterials: graphene and topological insulators [Invited]

The paper summarizes the recent achievements in the area of ultrafast fiber lasers mode-locked with so-called lowdimensional nanomaterials: graphene, topological insulators（Bi2Te3, Bi2Se3, Sb2Te3）, and transition metal sulfide semiconductors, like molybdenum disulfide（MoS2）. The most important experimental achievements are described and compared. Additionally, new original results on ultrashort pulse generation at 1.94 μm wavelength using graphene are presented. The designed Tm-doped fiber laser utilizes multilayer graphene as a saturable absorber and generates 654 fs pulses at 1940 nm wavelength, which are currently the shortest pulses generated from a Tm-doped fiber laser with a graphene-based saturable absorber.

Optical Waveguide Switches Employing Total-Internal-Reflection (TIR) Effect (Invited Paper)

In this paper, our recent research work on the total-internal-reflection optical switch is presented. The thermo-optic effect of polymeric materials and the photon-induced carrier effect of GaAlAs/GaAs are used in our devices.

Visible light positioning: moving from 2D planes to 3D spaces [Invited]

The global navigation satellite system(GNSS) is a well-established outdoor positioning system with industry-wide impact due to the multifaceted applications of navigation, tracking, and automation. At large, however, is the indoor equivalent. One hierarchy of solutions, visible light positioning(VLP) with its promise of centimeter-scale accuracy and widespread coverage indoors, has emerged as a viable, easy to configure, and inexpensive candidate. We investigate how the state-of-the-art VLP systems fare against two hard barriers in indoor positioning: the need for high accuracy and the need to position in the threedimensions(3D). We find that although most schemes claim centimeter-level accuracy for some proposed space or plane, those accuracies do not translate into a realistic 3D space due to diminishing field-of-view in 3D and assumptions made on the operating space. We do find two favorable solutions in ray–surface positioning and gain differentials. Both schemes show good positioning errors, low-cost potential, and single-luminaire positioning functionality.

Datacenter optics:requirements,technologies,and trends(Invited Paper)

We review over a decade of technology evolution and advancement of intra-datacenter optical interconnect, mainly driven by the explosive bandwidth growth of web and cloud-based services. Emerging trends and tech-nology options to scale interface bandwidth beyond 400 Gb/s will also be discussed.

Photoconductive devices for terahertz pulsed spectroscopy:a review[Invited]

Photoconductive switches were the key components that allowed the generation and detection of coherent broadband electromagnetic pulses at terahertz frequencies, opening the possibility for performing spectroscopy and,therefore, measuring complex dielectric properties of materials in this band, which was mostly unexplored. In this paper, we present a brief introduction to the operation principles of these devices. Subsequently, we present a review of the current state-of-the-art in this field and discuss the challenges to be faced in future development of these devices.

Lensless Vanderlugt optical correlator using two phase-only spatial light modulators(Invited Paper)

A lensless Vanderlugt optical correlator using two phase-only spatial light modulators （SLMs） is proposed. The SLMs are used for displaying input and filter patterns respectively. The SLMs are also used as programmable lenses in order to realize the lensless construction. This lensless system is simple and its alignment adjustment is easy. The performance of the SLMs as programmable lenses is also described.

Integrated optical delay lines:a review and perspective[Invited]

Optical delay lines（ODLs） are one of the key enabling components in photonic integrated circuits and systems.They are widely used in time-division multiplexing, optical signal synchronization and buffering, microwave signal processing, beam forming and steering, etc. The development of integrated photonics pushes forward the miniaturization of ODLs, offering improved performances in terms of stability, tuning speed, and power consumption. The integrated ODLs can be implemented using various structures, such as single or coupled resonators, gratings, photonic crystals, multi-path switchable structures, and recirculating loop structures.The delay tuning in ODLs is enabled by either changing the group refractive index of the waveguide or changing the length of the optical path. This paper reviews the recent development of integrated ODLs with a focus on their abundant applications and flexible implementations. The challenges and potentials of each type of ODLs are pointed out.

Quantum dot lasers and integrated optoelectronics on silicon platform Invited Paper

Chip-scale integration of optoelectronic devices such as lasers, waveguides, and modulators on silicon is prevailing as a promising approach to realize future ultrahigh speed optical interconnects. We review recent progress of the direct epitaxy and fabrication of quantum dot （QD） lasers and integrated guided-wave devices on silicon. This approach involves the development of molecular beam epitaxial growth of self- organized QD lasers directly on silicon substrates and their monolithic integration with amorphous silicon waveguides and quantum well electroabsorption modulators. Additionally, we report a preliminary study of long-wavelength （〉 1.3 μm） QD lasers grown on silicon and integrated crystalline silicon waveguides using membrane transfer technology.

Advances in quantum dots for classical and non-classical light sources Invited Paper

Recent advances in quantum dots （QDs） for classical and non-classical light sources are presented. We have established metal organic chemical vapor deposition （MOCVD） technology for InAs-based QD lasers at 1.3 μm and achieved ultralow threshold in QD lasers with photonic crystal （PhC） nanocavity. In addition, single photon emitters at 1.55 μm, GaN-based single photon sources operating at 200 K, and high-Q PhC nanocavity have been demonstrated.

Nonlinear terahertz metamaterial perfect absorbers using GaAs[Invited]

We investigate the nonlinear response of terahertz(THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz time-domain spectroscopy. The resonance frequency redshifts 20 GHz and the absorbance is reduced by 30% as the incident peak field is increased from 30 to 300 kV/cm. The nonlinear response arises from THz field driven interband transitions and intervalley scattering in the GaAs. To eliminate the Fresnel losses from the GaAs substrate, we design and fabricate a flexible metamaterial saturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.