Beyond LED Lighting: Laser based lighting and visible light communications

LED solid-state lighting (SSL) has made tremendous progress over the past decades. Although LED lighting is known for its high energy efficiency, there are several limitations and challenges for the exiting LED lighting technology. Specifically, due to the intrinsic characteristic called ‘efficiency droop’, LEDs become less efficient and convert less of their electrical energy into light under high power operation. Integration of visible light communication capability in LED SSL, or Li-Fi, has been a topic of intense research after the idea was proposed in 2011. To date, a data rate of up to 100Mbps has been demonstrated.

At KAUST, we are developing the next generation of SSL lighting using visible laser diodes. Laser diodes do not suffer efficiency droop at high current densities. This allows for the design of lamps using a single, small footprint, light-emitting chip operating at high current densities. Using a single chip reduces system costs compared with LEDs because the system uses less material per chip, requires fewer chips, and employs simplified optics and a simplified heat- sink. The chip area required for LED technologies will be significantly reduced using laser-based solid-state lighting. This technology will also enable highly controllable beams in term of tunable throw distance, tunable color temperature and rendering index.

In this talk, I will focus on the recent progress of visible diode laser-based lighting technology and high-speed transmitters and receivers for Gbit/s visible light communication (LiFi) and underwater wireless optical communication.

 

Optical breathers

Since the seminal works by A. Hasegawa and co-workers in the 1980s, the modulation instability (MI) phenomenon has been widely studied and used in optical fibers. MI is a phenomenon of common occurrence in various fields of physics such as hydrodynamics, plasma physics and nonlinear optics, and was investigated from the 1960s onwards. This nonlinear process is known as a general precursor of highly localized wave structures through amplification of perturbations. This talk will consider the full nonlinear stage of modulation instability in the case of periodic or localized weak perturbations, by using a unique mathematical formalism based on the nonlinear Schrödinger equation and breather wave solutions. I will review the recent theoretical and experimental advances, more specifically, the distinct experimental configurations for the sake of observation of breather waves in optics. Based on the coherent seeding of the MI process, new wave structures such as rogue breathers, super-regular breathers, breather wave molecules, and some dark-wave counterparts, have been experimentally observed in optical fibers, and in water tanks as well. In conclusion, I will provide an outlook on novel research directions.

 

 

Short-pulsed near- and mid-IR sources based on fibre lasers

Short pulsed fibre lasers with high peak power have found application in various areas based on different wavelengths. Gain-switched laser diode (GSLD) technique offers the simplest and most practical method for generating picosecond (ps) pulses at a controllable repetition rate, and has the capability of being integrated into stable, compact and all-fiberized master-oscillator-power-amplifier (MOPA) system. At 1035 nm wavelength, by using GSLD as seed and Ytterbium doped fibre amplifier, we have realized ~100 ps pulses with average power of ~100 W and peak power of ~200 kW at the output. Such high-power, short-pulsed fibre laser is of great interest for high-throughput material processing. With using the 1μm fibre laser as a pump, frequency conversion through optical parametric generation and amplification based on periodically poled lithium niobate (PPLN) crystal was investigated. Near and mid infrared (IR) pulses with wavelength tunability of 1.4 ~ 3.7 μm and peak power as high as 25 kW are obtained. In the 2 μm wavelength regime, a high power thulium (Tm) fibre MOPA is developed. 35-ps pulses with high peak power of up to ~300 kW at 1.95μm is achieved from a large-mode-area Tm fiber. Mid-IR pulses with wavelength tuning range from 2.5 μm to 8.3 μm are realized from an optical parametric device based on the 1.95μm fiber laser pumped orientation-patterned GaAs crystal. The mid-IR pulses have high peak power (13 kW) and narrow spectral linewidth (< 1.5 cm-1) and therefore can be used for important applications such as chemical sensing and imaging.

 

 

Advanced optical pulse coding for Brillouin distributed fibre sensing

During the past 30 years, distributed optical fibre sensing based on Brillouin optical time-domain analysis (BOTDA) has become a mature technology to spatially monitor environmental variables, such as temperature and strain, over long optical fibres. Nowadays a standard BOTDA can reach a sensing distance of 50 km, with a spatial resolution of 1 m and an acquisition time below 1 minute, for a typical temperature accuracy of 1 K. It has been demonstrated that the signal-to-noise ratio (SNR) that scales the overall performance of BOTDA, is determined by both the pump and probe powers that are limited by nonlinear effects presented in optical fibres. Therefore, any approach that can improve the SNR while avoiding the onset of nonlinear effects, leads to a better performing BOTDA. Among various advanced techniques in the last decade, optical pulse coding has been widely considered as an efficient solution, which offers a coding gain (i.e., SNR improvement) being half of the square-root of the code length, with similar measurement time and moderate hardware overhead compared to the single-pulse BOTDA. In this talk, the fundamentals of optical pulse coding techniques, such as Golay codes and Simplex codes, along with the design rules for the optimization, will be presented. Furthermore, the presentation will introduce a novel advanced implementation of optical pulse coding technique, which pushes the overall performance of BOTDA beyond state-of-the-art.

 

 

Modulation instability in higher-order nonlinear Schrödinger equations

The nonlinear Schrödinger equation is used widely for modelling wave propagation in nonlinear dispersive media. It is the simplest member of a family of integrable equations, and has a variety of analytic solutions in the form of solitons, breathers and rogue waves. In particular, the family of breather solutions describe the phenomenon called modulational instability, which has been studied extensively in optics, hydrodynamics, plasmas and biology.

While the accuracy provided by the nonlinear Schrödinger equation is sufficient for describing the main features of these phenomena, the inclusion of higher-order dispersion and nonlinear terms become important when the properties of the solution extend beyond the approximations used in deriving the equation. In this work, we present, through analytical approach, the dynamics of modulational instability in an extended nonlinear Schrödinger equation, where the higher-order terms are introduced in such a way that the integrability of the equation is preserved.

 

 

 

Planar Optical Sensors for Aviation

Optical sensors are attractive for the aviation sector as they offer lightweight and small footprint solutions for monitoring. Furthermore, they can also operate in environmental extremes such as high temperatures, pressures and in proximity to flammable material, which can be restrictive for electronic alternatives. This talk reviews some the latest Optoavionic research being conducted at the University of Southampton that seeks to achieve lighter and more fuel efficient aircraft through enhanced monitoring capability. The research discussed shall include developments of new sensor platforms that enable multidimensional strain mapping of advanced composites (e.g. carbon fibre reinforced polymer); removal of parasitic mass aboard aircraft through enhanced quality monitoring of fuels; condition monitoring of systems through real-time interrogation of lubricants and use of digital learning and development of concepts such as digital twin (virtual clones) that could be used to make aircraft significantly lighter and therefore more fuel efficient. The core optical technology described is based upon planar silica and silica fibre. Performance of the sensors in harsh environments and in application shall be reviewed and considerations discussed for future deployment for such advances in an industry that is notably conservative.

 

 

 

Photon-atom interface

Nitrogen-vacancy (NV) center emerges as an important system exhibiting promising properties for applications in quantum technologies, including quantum information processing, quantum metrology as well as single photon sources. In our work, the dynamics of nitrogen color centers in diamond are investigated on a single photon level. The system is pumped with quantum light of a well defined number state and fluorescence photons are observed.

 

 

 

Plasmonics with graphene flakes: a quantum-mechanical approach

Plasmonics offers opportunities to spectrally sensitively tailor electromagnetic field distributions, in particular to spatially confine the field and enhance it locally by orders of magnitude. Noble-metal-based plasmonic devices that are usually considered for this purpose, however, suffer from poor quality factors, extremely short lifetimes, and an inability to dynamically tune their properties. On the contrary, graphene is a versatile, broadband, adjustable, and tunable plasmonic material. Its ability to focus electromagnetic fields into nanometric regions of space is well beyond that of noble metals. Graphene is characterized by heavily suppressed absorption losses for photon energies below a threshold corresponding to the Fermi energy. A suitable level of doping, via electric gating, or chemical means can also shift the Fermi energy on-demand. Combined with the plasmonic lifetimes reaching hundreds of optical cycles, this renders graphene-based plasmonic nanostructures perfect candidates to implement tunable nanoscale cavity QED systems.

In our work, we develop and extend existing analytical frameworks and numerical tools to account for dynamics and optical properties of hybrid systems made of graphene nanoflakes coupled to adatoms in the presence of an external electromagnetic field. An adatom is an atomic system positioned near the flake. Contrary to previous works in this context, where its coupling to the graphene flake was only realized optically, we allow electron exchange with selected carbon sites at the flake. Our method combines the tight-binding model of finite flakes to characterize eigenstates and eigenenergies of the system with the master equation approach to trace the time evolution of charge density under external illumination, similarly to as it has been proposed in Ref. 2. Here, we additionally exploit the Anderson impurity model and integrate it in the temporal picture to account for the adatom's influence on the dynamics of the graphene flake, as well as the back-action from the latter. Study of the method reveals connections between different basic models of quantum physics. On the application side, we show that the mutual impact of the graphene flake and the adatom can in specific conditions be profound.

 

 

 

Short absorption length step-index large mode area fiber

Ultra-fast fiber lasers have made impressive advancement in the last decade and progressively replaced solid-state lasers as they offer compact and low ownership cost for industry applications requiring high average power and high energy output. The progress has hinged on advances of large mode area (LMA) fibers that push power scaling limitations such as nonlinear scatterings and phase shift. It is known that large mode area and short fiber length are key parameters to suppress the nonlinearities. In particular, the short fiber length requirement becomes more important as the mode area scaling is coupled to beam quality. In many cases, the large mode area is fulfilled by lowering core numerical aperture to generate reasonable beam quality. This implies low ytterbium concentration, thus low pump absorption, which counteracts the short length requirement. This is particularly true in a step-index fiber. This talk will present a development route for realizing a very short absorption length step-index LMA fiber. High power outputs demonstrated with the fiber will be presented.

 

 

 

Dirac Plasmon-Assisted Asymmetric Hot Carrier Generation for Room-Temperature Infrared Detection

The talk will outline a novel strategy for uncooled, tunable, multispectral infrared detection. Due to the low photon energy, detection of infrared photons is challenging at room temperature. Thermoelectric effect offers an alternative mechanism bypassing material bandgap restriction. Infrared detection by the photo-thermoelectric effect critically depends on the generation of a temperature gradient (ΔT) for the efficient collection of the generated hot-carriers; however, in theory, the magnitude of ΔT is limited by the Seebeck coefficient of the material. In this article, we demonstrate, for the first time, an asymmetric plasmon-induced hot-carrier Seebeck photodetection scheme at room temperature that exhibits a remarkable responsivity of 2900 V/W, detectivity (D*) of 1.1x10⁹ Jones along with an ultrafast response of ~ 100 ns in the technologically relevant 8 – 12 µm band, the performance of which compares favorably even with present cryogenically cooled detection schemes. This is achieved by engineering the asymmetric electronic environment of the generated hot carriers on chemical vapor deposition (CVD) grown large area nanopatterned monolayer graphene, which leads to a record ΔT of 4.7 K across the device terminals thereby enhancing the photo-thermoelectric voltage beyond the theoretical limit for graphene.

 

 

 

Multi-context holographic memory exploiting a wavelength-dependent optimization technique

This paper presents a proposal of a new wavelength-dependent optimization technique for two-dimensional holographic memories in order to improve the recording density of the holographic memories. When using a holographic memory as an electrical memory, a photodiode array is frequently used for converting the diffraction light from the holographic memory to electrical signals. However, when lasers with a variety of wavelengths are used to read multi-context, although the diffraction limitation of light of each laser is different from each other, the resolution of a photodiode array must be designed based on the diffraction limitation of light of the longest wavelength laser. However, the proposed technique has achieved the best recording density on a two-dimensional holographic memory for every wavelength lasers by adjusting the size of each recording region of the holographic memory for each context to satisfy the resolution of a photodiode array.

 

Hexagonal Boron Nitride as a Flexible Random Laser

Random lasing has been a platform for fundamental understanding of coherent phenomena in disordered media. Simple fabrication and design (as mirrors and other optical components are not needed to get lasing) and reduced dimensionality (sub-micron scale) makes it appealing for sensing and biomarking applications. Remarkable progress in studying different materials, geometries, and external pumping dependences of laser properties and efficiency of a random laser have been made to date. In particular, I will talk about random lasing properties from samples containing Aluminium Nitride nanocolumns prepared on hexagonal Boron Nitride (hBN) and without hBN with rhodamine 6G as the gain medium. Observation of stable emission and differences on threshold behaviour will be discussed.

 

Optical Intensity Sensor using Power-Imbalance of Non-Interfering Signals in Mach-Zehnder Structure

A new approach for optical intensity-based sensing is achieved by exploiting the optical power imbalance between the arms of a Mach-Zehnder structure. This approach demonstrates immunity to any variation in the input optical power, while its interrogation is simple and performed through RF spectrum analyzer.

 

Absorption Enhancement of Tunable Terahertz Hybrid Graphene-Metal Antenna with Stacked Graphene Configuration

In this article, gate tunable graphene-metal hybrid nanoantenna is proposed, with detailed numerical simulations are carried out to evaluate the absorption of multi-layer graphene nanoantenna at terahertz frequencies, specifically at far-infrared. The investigated graphene nanoantenna is composed of multilayer graphene stack formation, which is placed above the gold hexagon radiator to efficiently couple activated graphene plasmons. The efficiently coupled plasmons between graphene layers and gold hexagon increase the absorption of the antenna. Furthermore, the relationship between the absorption and multilayer graphene is analysed by changing the Fermi energy of the graphene sheet, thus providing tunability of broadband absorption at a wide range of frequencies. The investigated nanoantenna has a resonant frequency at 30.5 THz with a bandwidth of 0.6 THz, while the tri-layer graphene nanoantenna results in better absorption almost reaching 100% with a bandwidth of 2.5 THz. The simulation of the graphene-metal antenna is performed in CST studio by using an FDTD solver. The designed antenna has various utilizations in the field of photonics i.e. terahertz imaging, sensing, and spectroscopy applications.

 

Photoluminescence and Raman Studies on GaAs1-xBix Grown on GaAs

Photoluminescence (PL) and Raman studies were carried out to investigate the effect of growth rate on GaAs1-xBix alloys. The samples were grown between 0.09 and 0.5 µm/hour. At room temperature, the lowest PL peak energy (1.07 eV) was obtained by sample grown at 0.23 µm/hour. Samples grown at lower and higher than 0.23 µm/hour showed PL peak energy redshift and blueshift, respectively. Raman data show peaks at 162, 228, 270 and 295 cm-1 which correspond to the GaAs like phonons. The GaBi like phonons were also observed at 183 and 213 cm-1 but their intensity are significantly weaker compared to GaAs like phonons. PL and Raman data show that Bi incorporation into GaAs lattice is dependent on the growth rate and in this study, the highest Bi concentration was obtained for sample grown at 0.23 µm/hour.

 

Gallium Antimonide Thermophotovoltaic: Simulation and Electrical Characterization

Gallium Antimonide (GaSb) Thermophotovoltaic (TPV) cell is a well-known device for waste-heat harvesting technology. To date, the conversion efficiency of GaSb TPV cell remains low due to the presence of electrical losses. An experimental characterization under AM1.5 G standard testing condition was performed to obtain the electrical characteristic and performance of GaSb TPV cell. Consecutively, a GaSb TPV cell model is developed using Silvaco TCAD simulation software. An error of less than 0.5% was demonstrated for current density and open circuit voltage between the experimental measurement and simulation model. The finding in this study demonstrates the importance of minimizing these losses for developing high-performance GaSb TPV cell.

 

Gas sensor using tapered optical fiber coated with nanostructures via chemical bath deposition

Typical gas sensors are based on electrical signal such as conductometric, surface acoustic wave (SAW) or quartz crystal microbalance (QCM). The electrical sensors are popular because of their low cost and highly sensitive towards gases. However, it is easily affected by electromagnetic interference (EMI) and thus compromise the signal response. Alternatively, a growing interest in optical sensors using optical is due to their small size, light weight, resistant to EMI and resilient in high temperature environment. Nevertheless, cylindrical dimension of the optical fiber possesses coating challenges in producing uniform active layer for any sensing applications.  Herein, optical hydrogen (H₂) sensor using tapered fiber coated with metal oxide nanostructures via chemical bath deposition (CBD) will be presented. The simple and low-cost deposition technique produces highly homogeneous nanostructures with controlled thickness around the cylindrical tapered fiber which is difficult to be achieved via other physical methods. The selected metal oxides are zinc oxide (ZnO) and manganese dioxide (MnO₂). The synthesized nanomaterials showed high purity and crystallinity with unique shapes. The nanostructures were characterized by FESEM, XRD and EDX to confirm the material properties. It was discovered that the sensors are sensitive towards different concentrations of H₂ in synthetic air at specific operating temperatures. By varying the material deposition time, different thickness of sensing layer can be obtained. Further investigation with sensor samples of as-prepared and annealed were also carried out to study its sensing performance towards H₂. The absorbance response of annealed metal oxide nanostructures coated on the tapered fibers show  higher optical response as compared to as-prepared sensor upon 1% H₂ exposure in the synthetic air when measured in the visible to near infra-red optical wavelength.

 

Optical fiber taper biosensor for dengue detection

Controlling the Dengue epidemic relies heavily on early and accurate diagnosis, giving rise to various commercial methods with their own advantages and disadvantages. Most conventional methods detect non-primary analytes that appear some time after infection and the results are either qualitative or can only be attained through complex procedures and equipment. Fiber optic sensors have been recognized as a major player in compact diagnostic applications and its capability as a bio-sensing transducer can open up the paths towards better diagnostic tools. This talk reviews past works done by the UPM Wireless and Photonic Networks Research Centre (WiPNET) on dengue virus (DENV) detection using optical fiber. This includes the tapered fiber transducer and the chosen anti-DENV II E protein antibodies probe. The use of organic and inorganic nano-materials (Graphene Oxide and PAMAM) as sensing layer for enhancement of the virus detection will also be discussed.

 

Optical modulation using two dimensional materials

In the advancement of photonics engineering, the venture of new optoelectronics materials that able to cater different optical properties is a highlighted research direction. The emerging of 2D materials such as graphene, black phosphorus and transition metal dichalcogenides have provided great potential in the evolution of photonics technologies. Owing to the tailorability of 2D materials, the optical properties of 2D materials, which including energy band-gap, third-order nonlinearity, nonlinear absorption, and the thermo-optics coefficient can be a tailor for different types of optical applications. Since the past decade, the explorations of 2D materials in photonics applications have to extend to all-optical modulators, all-optical switch, all-optical wavelength converter that covered visible and near-infrared wavelength range. Herein, we reported three different methods to incorporate 2D materials into all-optical system as optical modulator, which are 2D material film, 2D material coated side-polished fiber, 2D material coated tapered fiber. From above methods, we have successfully modulate the time, amplitude, phase and polarization state of signal light in all-optical system.