Wavelength Domain Research Articles

Reconfigurable all-fiber mode exchange enabled by mechanically induced LPFG for short-reach MDM networks

Data exchange is an attractive network function for enhancing efficiency and flexibility and it has been widely investigated in the wavelength domain. Recently, mode division multiplexing (MDM) based on few-mode fiber (FMF) has been proposed to increase network capacity by transmitting signals over parallel spatial modes. In this paper, we propose a reconfigurable all-fiber mode exchange (ME) scheme based on mechanically induced long period fiber grating (LPFG) for short-reach MDM networks. The reconfigurable functionality is realized by changing the loaded pressure on LPFG. We experimentally demonstrate flexible data exchange between two linear polarization (LP) modes with on–off-keying (OOK) modulation and direct detection. Error-free performance is achieved for wavelength division multiplexing (WDM) signals in each LP mode.

Metameric Indoor Localization Schemes Using Visible Lights

In indoor environments, visible light communications paradigm is emerging as a viable promising solution complementary to well-known radio frequency technology. At the same time, the information about user's location is useful for accessing the medium via space-division multiplexing, handling over or providing access to location-based contents. In this paper, we present two localization mechanisms based on the wavelength domain by assuming that each anchor point uses a spectrally dedicated signature for the user to readily identify it. The first approach, i.e., wavelength-based localization, assumes a simultaneous transmission of three different pulse streams emitted by the red–green–blue (RGB) light emitting diodes (LEDs). The second method, i.e., color-based localization, considers the subsequent transmission of RGB pulses. Localization is then computed through traditional received signal strength and time difference of arrival approaches. Moreover, we resort to the properties of metamerism so that the red, green, and blue components used by LEDs provide the white light sensation to the human eye. The performances of the two proposed schemes are close to theoretical bounds. Even in the worst cases, the estimation error variance is in the order of $10^{-4}$ m $^2$ . Finally, the signaling request for estimating user position is less than others in the literature and is independent from the number of anchor points.

Eruptive Solar Prominence at 37 GHz

On 27 June 2012, an eruptive solar prominence was observed in the extreme ultraviolet (EUV) and radio wavebands. At the Aalto University Metsahovi Radio Observatory (MRO) it was observed at 37 GHz. It was the first time that the MRO followed a radio prominence with dense sampling in the millimetre wavelengths. This prompted us to study the connection of the 37 GHz event with other wavelength domains. At 37 GHz, the prominence was tracked to a height of around $1.6~\mathrm{R}_{\odot}$ , at which the loop structure collapsed. The average velocity of the radio prominence was $55 \pm 6~\mbox{km}\,\mbox{s}^{-1}$ . The brightness temperature of the prominence varied between $800 \pm 100$ K and $3200 \pm 100$ K. We compared our data with the Solar Dynamic Observatory (SDO)/Atmospheric Imaging Assembly (AIA) instrument’s 304 A EUV data, and found that the prominence behaves very similarly in both wavelengths. The EUV data also reveal flaring activity nearby the prominence. We present a scenario in which this flare works as a trigger that causes the prominence to move from a stable stage to an acceleration stage.

Experimental demonstration of directly modulated buried heterostructure DFB semiconductor laser based on reconstruction equivalent chirp technique

A directly modulated buried heterostructure distributed feedback (DFB) semiconductor laser based on the reconstruction equivalent chirp (REC) technique in the 1.3 μm wavelength domain is experimentally demonstrated. The packaged laser shows a good single longitudinal mode (SLM) property, with the side mode suppression ratio (SMSR) higher than 40 dB when the injection current is higher than 20 mA, as the threshold current is 10 mA. The direct modulation performance of the laser is also delightful, with the modulation bandwidth up to 10GHz at a low injection current (40mA), and the measured spurious free dynamic range (SFDR) up to 86 dB·Hz2/3.

Hyperspectral and Lidar Intensity Data Fusion: A Framework for the Rigorous Correction of Illumination, Anisotropic Effects, and Cross Calibration

The fusion of hyperspectral imaging (HSI) sensor and airborne lidar scanner (ALS) data provides promising potential for applications in environmental sciences. Standard fusion approaches use reflectance information from the HSI and distance measurements from the ALS to increase data dimensionality and geometric accuracy. However, the potential for data fusion based on the respective intensity information of the complementary active and passive sensor systems is high and not yet fully exploited. Here, an approach for the rigorous illumination correction of HSI data, based on the radiometric cross-calibrated return intensity information of ALS data, is presented. The cross calibration utilizes a ray tracing-based fusion of both sensor measurements by intersecting their particular beam shapes. The developed method is capable of compensating for the drawbacks of passive HSI systems, such as cast and cloud shadowing effects, illumination changes over time, across track illumination, and partly anisotropy effects. During processing, spatial and temporal differences in illumination patterns are detected and corrected over the entire HSI wavelength domain. The improvement in the classification accuracy of urban and vegetation surfaces demonstrates the benefit and potential of the proposed HSI illumination correction. The presented approach is the first step toward the rigorous in-flight fusion of passive and active system characteristics, enabling new capabilities for a variety of applications.

Experimental performance of semiconductor optical amplifiers and praseodymium-doped fiber amplifiers in 1310-nm dense wavelength division multiplexing system

One of the key optical transmission components is optical amplifiers. Studies on the amplification properties of the 1310-nm optical amplifiers are presented. The evaluated optical amplifiers are semiconductor optical amplifier (SOA) and praseodymium-doped fiber amplifier (PDFA). The study is aimed at the dynamic operation in single- and multiwavelength domains with high rate signals. The maximum obtained gain was 25.0 dB for SOA and 20.9 dB for PDFA. For the SOAs, the minimum achieved value of the receiver sensitivity was −11.5 dBm for a single channel and −11.5 dBm for a dense wavelength division multiplexing case while for PDFA those values were −11.0 dBm and −10.9, respectively. The main advantage of the PDFA in comparison to the measured SOAs is its higher saturation power. The SOAs proved to be viable candidates for high-speed amplification in the 1310-nm wavelength domain.

Interrogation of a spectral profile division multiplexed FBG sensor network using a modified particle swarm optimization method

This paper applies the concept of spectral profile division multiplexing to track each Bragg wavelength shift in a serially multiplexed fiber Bragg grating (FBG) network. Each sensor in the network is uniquely characterized by its own reflected spectrum shape, thus spectral overlapping is allowed in the wavelength domain. In contrast to the previous literature, spectral distortion caused by multiple reflections and spectral shadowing between FBG sensors, that occur in serial topology sensor networks, are considered in the identification algorithm. To detect the Bragg wavelength shift of each FBG, a nonlinear optimization function based on the output spectrum is constructed and a modified dynamic multi-swarm particle swarm optimizer is employed. The multiplexing approach is experimentally demonstrated on data from multiplexed sensor networks with up to four sensors. The wavelength prediction results show that the method can efficiently interrogate the multiplexed network in these overlapped situations. Specifically, the maximum error in a fully overlapped situation in the specific four sensor network demonstrated here was only 110 pm. A more general analysis of the prediction error and guidelines to optimize the sensor network are the subject of future work.

An Efficient Spectral Amplitude Coding (SAC) Technique for Optical CDMA System using Wavelength Division Multiplexing (WDM) Concepts

This article introduces an improved method for Optical Code Division Multiple Access system (OCDMA). In this scheme, a hybrid technique is used in which Wavelength Division Multiplexing (WDM) is merged with Spectral Amplitude Coding (SAC) to efficiently diminish Multiple Access Interference (MAI) and alleviate the impact of Phase Induced Intensity Noise (PIIN) appearing in photo-detecting process. The proposed technique SAC-OCDMA/WDM MP (SW-MP) is implemented by using Matrix Partitioning (MP) code family, which is constructed via merging mathematics sequence and algebraic approaches. The key notion is to create the code patterns in SAC domain, then diagonally replicate it in the wavelength domain as blocks which preserves the same code patterns of a given code weight. The SW-MP code family preserves convenient code length property which gives flexibility in transmitter-receiver design. It is reported that the proposed scheme has potential to remove MAI proficiently and improve the system performance significantly.

Experimental study on overtopping performance of a circular ramp wave energy converter

Circular ramp overtopping wave energy converter (CROWN) is an offshore overtopping wave energy converter that consists of a reservoir surrounded by a circular ramp and an outflow duct installed with a low water head turbine. Physical model experiments were conducted in a wave tank as a preliminary study at Ocean University of China. The wave overtopping behaviors demonstrated that guide vanes can effectively lead the wave water motion on the ramp for overtopping into the reservoir. An analysis of the overtopping discharge shows that the resonant zone is located in the smaller relative wave length domain. Within the scope of shape parameters used in this study, guide vanes and milder ramp slopes were found to significantly enhance the overtopping discharge. The orientation of the guide vane angle also had a large influence on the overtopping flow rates for smaller relative wave heights and lengths. The experimental results are presented with the aim of designing and optimizing the shape parameters of CROWN for a future prototype device.

Multi-parameter measurements using optical fibre long period gratings for indoor air quality monitoring

An array of three long period gratings (LPGs) fabricated in a single optical fibre and multiplexed in the wavelength domain was used to measure simultaneously temperature, relative humidity (RH) and volatile organic compounds (VOCs), which are key indoor air quality (IAQ) indicators. Each LPG sensor was designed with optimised response to a particular measurand. The first, with no surface modification, was used to measure temperature. The second, modified by a mesoporous coating of silica nanoparticles (SiO2 NPs), was used to measure RH and the third, modified with a coating of SiO2 NPs infused with a functional material, p-sulphanatocalix[8]arene (CA[8]), was employed to monitor VOC concentration. The LPGs were fabricated with periods such that they operated at or near the phase matching turning point. The sensors were calibrated in the laboratory and the simultaneous measurement of the key indoor air quality parameters was undertaken in laboratory and office environments. It was demonstrated successfully that the data produced by the LPG sensor array under real conditions was in a good agreement with that produced by commercially available sensors. The average differences between values obtained by the optical fibre sensor and standard temperature and RH sensors were better than 0.5°C and 5% respectively. Further, the potential application of fibre optic sensors for VOC detection at high concentrations has been demonstrated.

Gravitational lens system SDSS J1339+1310: microlensing factory and time delay

We spectroscopically re-observed the gravitational lens system SDSS J1339+1310 using OSIRIS on the GTC. We also monitored the r-band variability of the two quasar images (A and B) with the LT over 143 epochs in the period 2009–2016. These new data in both the wavelength and time domains have confirmed that the system is an unusual microlensing factory. The C iv emission line is remarkably microlensed, since the microlensing magnification of B relative to that for A, μBA, reaches a value of 1.4 (~0.4 mag) for its core. Moreover, the B image shows a red wing enhancement of C iv flux (relative to A), and μBA = 2 (0.75 mag) for the C iv broad-line emission. Regarding the nuclear continuum, we find a chromatic behaviour of μBA, which roughly varies from ~5 (1.75 mag) at 7000 Å to ~6 (1.95 mag) at 4000 Å. We also detect significant microlensing variability in the r band, and this includes a number of microlensing events on timescales of 50–100 d. Fortunately, the presence of an intrinsic 0.7 mag dip in the light curves of A and B, permitted us to measure the time delay between both quasar images. This delay is ΔtAB = 47+5-6 d (1σ confidence interval; A is leading), in good agreement with predictions of lens models.

Nd:LYAG and Nd:LGAG Mixed Crystals: Potential Candidates for 1.83 μm Laser Source

We report here, for the first time, diode-pumped continuous-wave laser operation of Nd:LuYAG (LYAG) and Nd:LuGdAG (LGAG) mixed crystals around 1.83 μ m. Cooling the laser samples down to about 6 °C, output powers of about 0.56 and 0.46 W with slope efficiencies of about 5.5% and 5.6% can be achieved, respectively, for the two crystals. These ${\rm{Nd}}^{3+ }{- \rm{doped}}$ garnet mixed crystals, like single crystal Nd:YAG, are believed to be the potential for laser generation at the important 1.83- μ m near-infrared wavelength domain.

First On-Sky Fringes with an Up-Conversion Interferometer Tested on a Telescope Array.

The Astronomical Light Optical Hybrid Analysis project investigates the combined use of a telescope array interferometer and nonlinear optics to propose a new generation of instruments dedicated to high-resolution imaging for infrared astronomy. The nonlinear process of optical frequency conversion transfers the astronomical light to a shorter wavelength domain. Here, we report on the first fringes obtained on the sky with the prototype operated at 1.55 μm in the astronomical H band and implemented on the Center for High Angular Resolution Astronomy telescope array. This seminal result allows us to foresee a future extension to the challenging midinfrared spectral domain.

Red Light Activation of Ru(II) Polypyridyl Prodrugs via Triplet-Triplet Annihilation Upconversion: Feasibility in Air and through Meat.

Triplet-triplet annihilation upconversion (TTA-UC) is a promising photophysical tool to shift the activation wavelength of photopharmacological compounds to the red or near-infrared wavelength domain, in which light penetrates human tissue optimally. However, TTA-UC is sensitive to dioxygen, which quenches the triplet states needed for upconversion. Here, we demonstrate not only that the sensitivity of TTA-UC liposomes to dioxygen can be circumvented by adding antioxidants, but also that this strategy is compatible with the activation of ruthenium-based chemotherapeutic compounds. First, red-to-blue upconverting liposomes were functionalized with a blue-light sensitive, membrane-anchored ruthenium polypyridyl complex, and put in solution in presence of a cocktail of antioxidants composed of ascorbic acid and glutathione. Upon red light irradiation with a medical grade 630 nm PDT laser, enough blue light was produced by TTA-UC liposomes under air to efficiently trigger full activation of the Ru-based prodrug. Then, the blue light generated by TTA-UC liposomes under red light irradiation (630 nm, 0.57 W/cm2) through different thicknesses of pork or chicken meat was measured, showing that TTA-UC still occurred even beyond 10 mm of biological tissue. Overall, the rate of activation of the ruthenium compound in TTA-UC liposomes using either blue or red light (1.6 W/cm2) through 7 mm of pork fillet were found comparable, but the blue light caused significant tissue damage, whereas red light did not. Finally, full activation of the ruthenium prodrug in TTA-UC liposomes was obtained under red light irradiation through 7 mm of pork fillet, thereby underlining the in vivo applicability of the activation-by-upconversion strategy.

Low-Loss Passive Si$_{3}$N $_{4}$ Serial-to-WDM Interface for Energy-Efficient Optical Interconnects

The increasing number of computational servers in data centers is imposing tighter constraints on the networking infrastructure. Scalable power efficient optical interconnect network becomes necessary to leverage the bandwidth capacity of current electronic switches or opto-electronic components. Hence, novel optical interconnect technology can enhance the network capacity by harnessing the feasibility of simultaneous processing of optical signal in the wavelength and time domains. In this paper, we present a four channel optical passive wavelength-striped mapping (PWSM) device, which passively time compresses/expands serial packets through optical wavelength multiplexing/demultiplexing. The PWSM device, which has a 1 $\times$ 4 channel optical wavelength demultiplexer with integrated optical delay lines, is designed in a low-loss Si $_{3}$ N $_{4}$ (propagation loss $\sim$ 3.1 dB/m) waveguide platform. The PWSM device multiplexes/demultiplexes four WDM channels and offsets in time the adjacent channels to optically serialize/de-serialize data packets. In this demonstration, a 64 ns long data packet is formed at the output of the device by combining four 16 ns data segments of the packet in time domain. Incremental optical insertion loss between adjacent channels is $\sim$ 9.7 dB due to the integrated passive optical delay waveguides. The data rate of the four segmented packets and the combined packet is 25 Gb/sec. We have measured a bit error rate performance below 1 $\times 10^{-9}$ for the 64 ns serial data packet regenerated by the PWSM device for a received optical power of 6.7dBm.

Wavelength-scale light concentrator made by direct 3D laser writing of polymer metamaterials.

We report on the realization of functional infrared light concentrators based on a thick layer of air-polymer metamaterial with controlled pore size gradients. The design features an optimum gradient index profile leading to light focusing in the Fresnel zone of the structures for two selected operating wavelength domains near 5.6 and 10.4 μm. The metamaterial which consists in a thick polymer containing air holes with diameters ranging from λ/20 to λ/8 is made using a 3D lithography technique based on the two-photon polymerization of a homemade photopolymer. Infrared imaging of the structures reveals a tight focusing for both structures with a maximum local intensity increase by a factor of 2.5 for a concentrator volume of 1.5 λ3, slightly limited by the residual absorption of the selected polymer. Such porous and flat metamaterial structures offer interesting perspectives to increase infrared detector performance at the pixel level for imaging or sensing applications.

Visualization of the process of metal heating and melting in the anode zone in an arc discharge with a tungsten electrode

Pictures of an electric arc burning in argon, obtained by means of a digital camera, within the different domains of wavelengths of the visible spectrum are presented. Maps of the thermal fields of the heating spot are plotted, and the plasma temperature in the anode arc zone is calculated. The application potential of the digital image technology in the visible wavelength domain for analysis of the processes directly in the anode arc zone and for estimation of the arc column parameters is shown.

HERschelObservations of Edge-on Spirals (HEROES)

We investigate the dust energy balance for the edge-on galaxy IC 2531, one of the seven galaxies in the HEROES sample. We perform a state-of-the-art radiative transfer modelling based, for the first time, on a set of optical and near-infrared galaxy images. We show that taking into account near-infrared imaging in the modelling significantly improves the constraints on the retrieved parameters of the dust content. We confirm the result from previous studies that including a young stellar population in the modelling is important for explaining the observed stellar energy distribution. However, the discrepancy between the observed and modelled thermal emission at far-infrared wavelengths, the so-called dust energy balance problem, is still present: the model underestimates the observed fluxes by a factor of about two. We compare two different dust models, and find that dust parameters and thus the spectral energy distribution in the infrared domain are sensitive to the adopted dust model. In general, the THEMIS model reproduces the observed emission in the infrared wavelength domain better than the popular Zubko et al. BARE-GR-S model. Our study of IC 2531 is a pilot case for detailed and uniform radiative transfer modelling of the entire HEROES sample, which will shed more light on the strength and origins of the dust energy balance problem.

X-ray diffraction gratings: Precise control of ultra-low blaze angle via anisotropic wet etching

Diffraction gratings are used from micron to nanometer wavelengths as dispersing elements in optical instruments. At shorter wavelengths, crystals can be used as diffracting elements, but due to the 3D nature of the interaction with light are wavelength selective rather than wavelength dispersing. There is an urgent need to extend grating technology into the x-ray domain of wavelengths from 1 to 0.1 nm, but this requires the use of gratings that have a faceted surface in which the facet angles are very small, typically less than 1°. Small facet angles are also required in the extreme ultra-violet and soft x-ray energy ranges in free electron laser applications, in order to reduce power density below a critical damage threshold. In this work, we demonstrate a technique based on anisotropic etching of silicon designed to produce very small angle facets with a high degree of perfection.

Towards Petabit/s All-Optical Flat Data Center Networks Based on WDM Optical Cross-Connect Switches with Flow Control

Scaling the capacity while maintaining low latency and power consumption is a challenge for hierarchical data center networks (DCNs) based on electrical switches. In this work we present a novel all-optical flat DCN architecture OPSquare that potentially addresses the scaling issues by employing parallel intra-/inter-cluster switching networks, distributed fast WDM optical cross-connect (OXC) switches, and a novel top-of-rack (ToR) switch architecture. The fast (nanoseconds) WDM OXC switches allow flexible switching capability in both wavelength and time domains and statistical multiplexing. The OPSquare DCN performance targeting Petabit/s capacity has been thoroughly assessed. First the packet loss, latency, throughput, and scalability are numerically investigated under realistic data center traffic model. Results indicate that when scaling the DCN size up to 1024 ToR switches, a packet loss ratio below 10−6 and a server end-to-end latency lower than 2 µs can be guaranteed at load of 0.3 with limited 20 kB buffer. Then, the experimental evaluation of the DCN by employing 4 × 4 OXC prototypes shows multi-path dynamic switching with flow control operation. The case deploying 32 × 32 and 64 × 64 OXC switches connecting 1024 and 4096 ToRs are emulated and limited performance degradation has been observed. The potential of switching higher-order modulation and waveband signals further proves the suitability of OPSquare architecture for Petabit/s and low-latency DCN by using optical switches with moderate radix.