Bragg Gratings

Given the great impact of fiber Bragg gratings in DWDM communication networks we present a MatLab based graphical interface to design and model the FBG parameters to compensate the optical fiber link chromatic dispersion. This tool allows the spectral analysis of Apodized and Linearly Chirped FBG's in general as well as the design of the grating device that can compensate for the chromatic dispersion of an specified fiber link. The spectral characteristics of the designed grating are shown as well as an estimation of the deviation from the ideal theoretical compensator. The application is really cost-effective from the computational point of view and finally, it can also be displayed the time domain original pulse to be transmitted, the dispersion broadened pulse and the recompressed one as well as the compression ratio, to check the behavior of the designed system.

We report what we believe to be the first highly symmetric first-order IR femtosecond laser fiber Bragg gratings within the telecommunications C band in free-standing optical fiber, fabricated with a relatively low NA lens and without use of oil immersion techniques. This grating features the smallest dimensions for a pointby-point fiber grating reported so far (to our knowledge). This achievement paves the way to rapid mass manufacturing of highly efficient and stable Bragg gratings using ultrafast lasers in any type of fiber. Mastering this femtosecond grating inscription technique also allowed the fabrication of the first Bragg gratings with direct near-IR femtosecond inscription in photonic crystal fibers, and without the use of techniques that rely on the compensation of the holey structure.

We present a novel device for the characterisation of static magnetic fields through monitoring wavelength shifts of femtosecond inscribed fibre Bragg grating and micromachined slot, coated with Terfenol-D. The device was sensitive to static magnetic fields and can be used as a vectoral sensor for the detection of magnetic fields as low as 0.046 mT with a resolution of ± 0.3mT in transmission and ± 0.7mT in reflection. The use of a femtosecond laser to both inscribe the FBGs and micromachine the slot in a single stage prior to coating the device significantly simplifies the fabrication.

Practical demonstrations of diode laser emission from the broad surface area rather than from the cleaved facet of the wafer are relatively recent. This is so despite the fact that the concepts are many years old. The vertical-cavity approach was demonstrated by Melngailis in 1964, and the grating surface emitting and folded-cavity approach were reported by many authors in the mid to late 1970s. Many, perhaps most, of the concepts discussed in this book were around for many years before they were actively pursued. The advances over the last ten years, mainly in materials, are largely responsible for the capability to implement the ideas presented into working devices. We can look forward to continued progress in materials, processing, and design during the next decade and can expect to see semiconductor laser performance outstripping even our present dreams.
There are now frequent reports on all three principal types of diode laser
and diode laser array surface emitters in the literature. Unlike edge-emitting
semiconductor lasers, the surface emission approach allows the use of mass production techniques throughout the fabrication process. In addition, the surface emission approach allows testing of the completed devices at the wafer level, before dicing and packaging. These same capabilities yielded tremendous reductions in cost and enormous increases in the performance and reliability of transistors and other solid state electronic devices. Surface-emitting approaches also allow the integration of single or numerous lasers to form photonic integrated circuits or high power, monolithic, two-dimensional arrays.
Because of the now extensive literature on surface-emitting diode lasers
and arrays and the proposed and emerging applications of these exciting and
practical new devices in a variety of systems, we feel that this in-depth book
covering the field will be useful to researchers, users, and students interested in the field of lasers, electrooptics, and optical communication. Recent work has been motivated by numerous goals, including low power, integrated sources to replace electrical interconnects with optical interconnects for ultra large-scale integrated circuits; two-dimensional, independently addressable laser arrays for neural networks; steerable output beams for optical computers; high power with large emitting areas for pumping solid-state lasers; and coherent, single frequency, high-power operation with a controlled output beam for space communication and second harmonic generation.
The information in this book is intended to provide the reader with both
knowledge about fundamental concepts and the present state of the art of surface-emitting lasers. There are definitive chapters on vertical-cavity, etched facet-mirror, and grating surface emitters. Additional chapters treat the operation of Bragg grating couplers; edge-emitting diode laser arrays; the theory of phase locking, modes, and beam steering of surface-emitting arrays; external methods of phase locking arrays; coherence and phase control in laser arrays; and thermal considerations in two-dimensional surface-emitting arrays.
We have fortunately been able to enlist some of the leading researchers and
developers of surface-emitting diode lasers to contribute to this book. We wish to thank them for many interesting and productive discussions in connection with the preparation of this work. We also wish· to thank RCA Laboratories (now the David Sarnoff Research Center), Princeton, New Jersey, for providing both of us with talented collaborators, up-to-date equipment, and a pleasant environment in which most of our work described herein was carried out.

An efficient method for equalization of downlink CDMA channels is presented. By describing the observed signal in terms of a state-space model, the method employs the Kalman filter (KF) to achieve an unbiased signal estimate satisfying the linear minimum mean-squared error (LMMSE) criterion. The state-space model is realized at the symbol and chip levels. With the symbollevel model, the KF is used to estimate the transmitted chips that correspond to each symbol interval; whereas at the chip level, the transmitted chips are estimated individually. The symbol-level KF has a built-in tracking capability that takes advantage of the a priori known scrambling sequence, which renders the transmitted signal nonstationary. The chip-level KF reduces the complexity of the symbol-level KF significantly by ignoring the nonstationarity introduced by scrambling. A simple method for further reducing the KF complexity is also presented. The computational complexity of the proposed technique is analyzed and compared with that of several linear approaches based on finite-impulse response (FIR) filtering. Simulations under realistic channel conditions are carried out which indicate that the KF-based approach is superior to FIR equalizers by 1-2 dBs in error-rate performance.

Research thrusts in silicon photonics are developing control operations using higher order waveguide modes for next generation high-bandwidth communication systems. In this context, devices allowing optical processing of multiple waveguide modes can reduce architecture complexity and enable flexible on-chip networks. We propose and demonstrate a hybrid resonator dually resonant at the 1st and 2nd order modes of a silicon waveguide. We observe 8 dB extinction ratio and modal conversion range of 20 nm for the 1st order quasi-TE mode input.

This study proposed the modeling of fiber Bragg grating as a sensor for the optimum reflectivity with minimum discontinuities at the end of center wavelength. Recent advancement in optical sensor technology, make it necessary to generate the optimum results with minimum error. In this analysis, dependence of grating output spectra on various parameters has been studied for minimum bandwidth, side lobes and maximum reflectivity. In previous studies effect of length and other parameters has been studied. In this paper author has included the effect of modulation depth also. Study shows that grating length and modulation depth of refractive index of core are very critical factors which significantly alter the grating characteristics. Here low absorption third window of optical communication is used for the sake of low attenuation. All the simulation has done using GratingMOd software using transfer matrix method.

An implementation of a cellular neural/non-linear network (CNN) for processing black-and-white (B/W) images is presented in which the template terms are 1-bit programmable. Such approach leads to a very compact implementation of the coefficient circuits and fast (digital) programming. In this programming scheme, the more complex templates are split into subtasks that are run successively. The structure allows a direct or algorithmic evaluation of the majority of templates proposed for B/W images. The transient mask is utilized in performing the local logic operations as well as in template operations. The proposed architecture is suitable for high-density implementations. A test structure of a 4 × 4 network has been implemented with a standard digital 0.18-m CMOS process. One cell occupies only 155 m 2 , making possible the implementations of very large networks on a single chip. The algorithms used for the logic function computations and selected template evaluations are described, and the corresponding measurement results are shown.

Wireless communication systems are based on frequency synthesizers that generate carrier signals, which are used to transmit information. Frequency synthesizers use voltage controlled oscillators (VCO) to produce the required frequencies within a specified period of time. In the process of generating frequency, the VCO and other electronic components such as amplifiers produce some unwanted short-term frequency variations, which cause frequency instability within the frequency of interest known as phase noise (PN). PN has a negative impact on the performance of the overall wireless communication system. A literature study conducted on this research reveals that the existing PN cancellation techniques have some limitations and drawbacks that require further attention. A new PN correction technique based on the combination of least mean square (LMS) adaptive filtering and single-loop single-bit Sigma Delta (SD) modulator is proposed. The new design is also based on the Cascaded Resonator Feedback (CRFB) architecture. The noise transfer function (NTF) of the architecture was formulated in way that made it possible to stabilize the frequency fluctuations within the in-band (frequency of interest) by locating its poles and zeros within the unit circle. The new design was simulated and tested on a commercially available software tool called Agilent © © U Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a Advanced Design System (ADS). Simulation results show that the new technique achieves better results when compared with existing techniques as it achieves a 104 dB signal-to-noise (SNR), which is an improvement of 9 dB when compared with the existing technique accessed from the latest publications. The new design also achieves a clean signal with minimal spurious tones within the inband with a phase noise level of-141 dBc/Hz (lower phase noise level by 28 dBc/Hz) when compared with the existing techniques. © © U Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a © © U Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a

This paper presents a reduced visual impact structural health monitoring system, based on fibre Bragg gratings. This sensing network was developed for the church of Santa Casa da Misericordia of Aveiro and comprises 19 displacement sensors and 5 temperature sensors. All the sensors were custom made, according to the monitoring points' characteristics. The results obtained over the first year of monitoring are presented.

Single mode optical fibres with thin nickel coatings (outer diameter ~350 µm) are successfully embedded into stainless steel 316 components using bespoke laser based additive manufacturing technology. In our approach we manufacture SS 316 components using Selective Laser Melting, incorporating Ushaped grooves with dimensions suitable to hold nickel coated optical fibers. Coated optical fibers containing fiber Bragg gratings for strain monitoring and temperature sensing are placed in the groove. The embedding is completed by melting subsequent powder layers on top of the fibers. Cross sectional microscopy analysis of the fabricated components, together with analysis of the Bragg gratings behaviour during fabrication indicates a strong substance-to-substance bond between coated fibre and added SS 316 material. Temperature and strain cycling of the embedded sensors demonstrates the ability of gratings to survive the embedding process, and act as sensing elements in harsh environments. In-situ strain and temperature measurements from within the component are demonstrated for high dynamic stress levels and elevated temperatures (< 400 ⁰C).

In this paper, we attempt to present a multilayer-structure biosensor. A graphene-based Surface Plasmon Resonance (SPR) biosensor is considered as a structure, which o ers enhancement of sensitivity in comparison with the conventional biosensors. The sensitivity improvement caused by the presence of graphene is discussed through monitoring of the biomolecular interactions and following that the Refractive Index (RI) changes. For this purpose, an RI change, which increases during the course of biomolecular interaction, is considered from 1.462 to 1.52. Our numerical results show that the proposed SPR biosensor with a graphene layer can provide a better sensitivity due to the eld distribution at the binding region. This paper investigates the Sensitivity Enhancement Factor (SEF) according to the S-parameters and the e ects of design parameters such as thicknesses of the gold and graphene layers and the refractive index change during the course of DNA hybridization in this biosensor.

In this paper, we introduce and analytically demonstrate a novel chemical sensor based on a defective one-dimensional photonic crystals (1DP C) in total internal reflection (TIR) geometry, where a gradient refractive index (GRIN) defect layer is located at the end of the structure. This configuration creates an open sensing interface for real-time detection with ultrahigh sensitivity. Using the GRIN lens in the structure not only helps to tune the resonance wavelength to a desired value but also as a consequence of its special refractive index distribution function, a ringshaped reflectance distribution appears on the output plane of the structure that plays a key role in finding the optimized refractive index of the defect layer and achieving the best resonance of the cavity modes. The resonance angle of the cavity modes is highly sensitive to the effective refractive index of the fluid flowing above the sensor. The refractive index variation of ∆nF = 10 −8 RIU that causes ∆θ = 3.2 × 10 −6 degrees shift at the resonance angle can be detected by this sensor.

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The increasing demand for life extension of both military and civil aircrafts led to significant advances in repair technology of cracked metallic structures. Consequently, bonded composite repairs of metallic structures became a rapidly growing technology in the field of aerospace. Given the specific characteristics of a bonded composite repair and the differences in the materials used in each case (metal, composite & adhesive), the applicability of a number of Non Destructive Testing methods to trace crack propagation under a composite patch repair of a cracked metallic structure is examined, following mechanical testing in fatigue. In this paper, the main NDT methods examined include the eddy current method applied over a bonded composite repair (i.e. without removing the repair) as well as Bragg grating optical sensors embedded into composite patches. The capability and the reliability of the eddy-current method to detect cracks under a composite obstacle of significant thickness were checked for several patch thickness. The eddy-current method was found to be fully capable of tracing the crack propagation under the composite patch, requiring only proper calibration of the generator. Small differences in the crack lengths between the patched and the unpatched side of the specimen which were examined were explained by their nonsymmetric configuration, which induced different stress intensity factors at the patched and the unpatched sides, as finite element analysis has clearly shown. On the other hand, Bragg grating sensors were proven capable of tracing crack propagation with high accuracy, through interpretation of the differences caused in the strain field over the crack, after comparison with finite element analysis results.

In this paper, dual antenna receiver architectures are studied including RAKE, chip-level linear equalizer, and their combination. The arithmetic complexity of single and dual antenna receiver methods is analyzed. Cost of such receivers when implemented with customized hardware or software on application-specific instruction set processors (ASIP) is estimated. The study shows that feasible dual antenna detection can be obtained with less than 70% additional costs. More flexible implementation supporting several standards can be obtained with software but it requires higher power consumption due to additional memory.

In this paper, we report, to our knowledge, the first extended study of the inscription of Bragg gratings in surface-core fibers and their application in refractive index and directional curvature sensing. The research ranges from fiber fabrication and grating inscription in untapered and tapered fibers to the performance of simulations and sensing measurements. Maximum sensitivities of 40 nm/RIU and 202.7 pm/ m À1 were attained in refractive index and curvature measurements respectively. The obtained results compares well to other fiber Bragg grating based devices. Ease of fabrication, robustness and versatility makes surface-core fibers an interesting platform when exploring fiber sensing devices.

ABSTRACT form only given. Complex coupling (CC) in distributed feedback (DFB) lasers can lead to high-quality modal properties such as high monomode yield, stable narrow linewidth, high side-mode suppression ratio (SMSR) and strong immunity to feedback without the use of AR coatings. We fabricate second order DFB lasers with strong vertical emission to create the complex coupling by a simple and reliable method. The vertical radiation also presents the potential advantage of high power, high brightness and low diffraction angles. In this paper, we use a 2-dimensional model of the laser cavity to describe the relations between the facet phases and the vertical radiation profile of as-cleaved second order DFB lasers. We demonstrate a method that uses the radiation profile to derive the left and right facet phases. Previously, Morthier et al. (1995) proposed a method based on numerical fits of subthreshold spectra to determine the coupling coefficients and the facet phases. However, this method requires high-resolution spectra and precise relative intensity measurements of the modal peaks below threshold. It is preferential to measure the facet phases under actual lasing conditions. Our new method requires only one measurement of the surface near-field emission profile.

Abstract: InGaAs quantum-well complex-coupled DFB lasers with gain coupling induced by vertical emission are demonstrated. Continuous-wave operation of these lasers at room temperature provides single-mode emission at a wavelength around 980 nm. A spectral linewidth as narrow as 15 MHz and a side-mode suppression ratio larger than 40 dB are obtained. The use of this gain coupling mechanism permits to reduce the influence of the regrown interface quality on the laser performance. OCIS codes:(140.3490) Lasers, ...

ABSTRACT High quality second-order complex coupled distributed feedback lasers have been fabricated. The gain coupling is caused by the vertical radiation. The InGaAs/GaAs/AlGaAs as-cleaved lasers are 500 μm long and operate CW at room temperature at 980 nm. The average threshold current density is 300 A/cm2. The sidemode suppression ratios are as high as 47 dB

RESUMEN: Se estudia el comportamiento de la anchura del modo fundamental en una Fibra de Cristal Fotónico (FCF) guiada por índice cuando se aplica un estiramiento en un rango de valores comprendido entre 0.5 y 2.5%ε. Se identifican las regiones de frecuencia normalizada (Λ/λ) y de la fracción de llenado de aire d/Λ en los que se observa la máxima influencia del estiramiento en la sensibilidad. Finalmente, se analiza y discute su evolución.ABSTRACT: The fundamental mode width behaviour is studied for an index guided Photonic Crystal Fibre (PCF) when strain is applied in a range of values between 0.5 and 2.5%ε. The regions of normalized frequency (Λ/λ) and of the filling air fraction d/Λ, in which maximum strain influence is observed, are identified. Finally, its evolution is analyzed and argued

In this paper we present a 4-element Vivaldi antenna array and beamsteering receiver IC for fifth-generation mobile network (5G) New Radio (NR). The implemented receiver utilizes a delay-based local-oscillator (LO) phase-shift technique for accurate beamsteering, and it exhibits 1 to 2.4 degree phase tuning capability for 2-5 GHz bandwidth accordingly. On-chip delay measurement is performed with pilot signal generation and delay estimation capable of 2 ps accuracy. The IC is fabricated on 28 nm CMOS technology, it occupies an area of 1.4 × 1.4 mm 2 including bonding pads and consumes 22.8 mW at 2 GHz for single receiver path operation. The receiver demonstrates wideband over-the-air reception with the prototype antennas.

All-plastic fiber-based pressure sensor We present a feasibility study and a prototype of an all-plastic fiber-based pressure sensor. The sensor is based on long period gratings inscribed for the first time to the best of our knowledge by a CO2 laser in polymethyl methacrylate (PMMA) microstructured fibers and coupled to a pod-like transducer that converts pressure to strain. The sensor prototype was characterized for pressures up to 150 mbars, and various parameters related to its construction were also characterized in order to enhance sensitivity. We consider this sensor in the context of future applications in endoscopic pressure sensors.

A review of refractive index measurement based on different types of optical fiber sensor configurations and techniques is presented. It addresses the main developments in the area, with particular focus on results obtained at INESC Porto, Portugal. The optical fiber sensing structures studied include those based on Bragg and long period gratings, on micro-interferometers, on plasmonic effects in fibers and on multimode interference in a large spectrum of standard and microstructured optical fibers.

The increasing demand for life extension of both military and civil aircrafts led to significant advances in repair technology of cracked metallic structures. Consequently, bonded composite repairs of metallic structures became a rapidly growing technology in the field of aerospace. Given the specific characteristics of a bonded composite repair and the differences in the materials used in each case (metal, composite & adhesive), the applicability of a number of Non Destructive Testing methods to trace crack propagation under a composite patch repair of a cracked metallic structure is examined, following mechanical testing in fatigue. In this paper, the main NDT methods examined include the eddy current method applied over a bonded composite repair (i.e. without removing the repair) as well as Bragg grating optical sensors embedded into composite patches. The capability and the reliability of the eddy-current method to detect cracks under a composite obstacle of significant thicknes...

Plasmonic sensors are one of the most powerful optical biosensors, which can be used for the detection of small numbers of molecules at ultra-low concentrations. However, the detection of dilute analytes of low molecular weight (<500 Da) is still a challenging task using currently available plasmonic biosensors. Here, we demonstrate the detection of both lower and heavier molecular weight biomolecules close to single molecule level using a novel plasmonic biosensor platform based on hyperbolic metamaterials. We show experimentally the extraordinary spectral and angular sensitivities of the biosensor platform, from visible to near infrared (NIR) wavelengths, by exciting the bulk plasmon polaritons associated with hyperbolic metamaterials.

We propose and demonstrate an anti-jamming system to defend against wideband jamming attack. Free space optical communication is deployed to provide a reference for jamming cancellation. The mixed signal is processed and separated with photonic signal processing method to achieve large bandwidth. As an analog signal processing method, the cancellation system introduces zero latency. The radio frequency signals are modulated on optical carriers to achieve wideband and unanimous frequency response. With wideband and zero latency, the system meets the key requirements of high speed and real-time communications in transportation systems.

We propose and experimentally demonstrate spatial filtering by photonic crystals in a Bragg configuration. Compared to the Laue configuration, where spatial filtering was already demonstrated before, the Bragg configuration is more technologically challenging, as the longitudinal periods of such structures must be shorter than the operational wavelength. The proposed configuration is designed and analyzed by FDTD simulations and the multilayer structure is fabricated by physical vapour deposition on the microstructured substrate. The measurements of the angle/wavelength transmission of the fabricated structure show the signatures of the angular filtering.

We present a high-temperature-resistant distributed Bragg reflector fiber laser photowritten in Er/Yb codoped phosphosilicate fiber that is capable of long-term operation at 500 o C. Highly saturated Bragg gratings are directly inscribed into the Er/Yb fiber without hydrogen loading by using a 193 nm excimer laser and phase mask method. After annealing at elevated temperature, the remained gratings are strong enough for laser oscillation. The laser operates in robust single mode with output power more than 1 dBm and signal-to-noise ratio better than 70 dB over the entire temperature range from room temperature to 500 o C.

There is a growing interest for esophageal measurements which can provide important and reliable data when diagnosing the motor function of the sphincters and the esophageal body. Biocompatibility, sensing resolution and the comfort of the patient are key parameters for manometric sensing systems. A new sensing approach which could fulfill all these needs is presented in this paper consisting of an embedded polymer fiber sensor, based on multiplexed fiber Bragg gratings. A response to a radial pressure almost 6 times that of a comparable silica fiber based sensor is obtained.

We study the phase and amplitude stability of RF signals, generated at different frequencies by heterodyning pairs of modes from the optical spectrum of an active mode-locked laser, and we focus on the dependence of noise on the RF frequency. A specific theoretical model is derived, and the amplitude and timing jitter behaviour of the RF signals are analyzed and experimentally validated. The timing jitter reveals to be constant at any RF generated frequency, making the considered RF generation method suitable for the realization of flexible and highly stable micro-and millimeter-wave coherent radar transceivers. The performance of the considered RF generation architecture are compared with a state-of-the-art RF synthesizer, proving that the optically generated RF signals meet the high stability requirements of the new generation of coherent radar systems, even at extremely high frequencies (EHF).

In this paper, we report, to our knowledge, the first extended study of the inscription of Bragg gratings in surface-core fibers and their application in refractive index and directional curvature sensing. The research ranges from fiber fabrication and grating inscription in untapered and tapered fibers to the performance of simulations and sensing measurements. Maximum sensitivities of 40 nm/RIU and 202.7 pm/ m À1 were attained in refractive index and curvature measurements respectively. The obtained results compares well to other fiber Bragg grating based devices. Ease of fabrication, robustness and versatility makes surface-core fibers an interesting platform when exploring fiber sensing devices.

Pilot symbol assisted modulation and a minimum mean-square error (MMSE) channel predictor are used to employ feedback MMSE power control over a frequency nonselective slow Rayleigh fading channel. Feedback is assumed to be noiseless and delayless. First, the performance of the pilot symbol system using MMSE power control is derived in the case of the frame size of two. Lag error is noticed to cause severe performance degradation, even when the channel is very slowly fading. In order to decrease the lag error and to get a good system performance, the number of estimator coefficients is found to become quite large. Operating strategy of MMSE power control is studied by Monte Carlo simulations and compared to various strategies presented in the literature. Spectral efficiency of the pilot symbol system is increased by transmitting more than one data symbol in a given frame. Finally, the performance of the pilot symbol system using MMSE power control is derived in the case of an optimal frame size.

We present an approach for the efficient design of polarization insensitive polymeric optical waveguide devices considering stress-induced effects. In this approach, the stresses induced in the waveguide during the fabrication process are estimated first using a more realistic model in the finite element analysis. Then we determine the perturbations in the material refractive indices caused by the stress-optic effect. It is observed that the stresses cause non-uniform optical anisotropy in the waveguide materials, which is then incorporated in the modal analysis considering a multilayer structure of waveguide. The approach is exploited in the design of a Bragg grating on strip waveguide. Excellent agreement between calculated and published experimental results confirms the feasibility of our approach in the accurate design of polarization insensitive polymer waveguide devices.

In this thesis, feasibility of Fiber Bragg Grating sensors for Structural Health Monitoring applications of composite structures is studied. The method and essentials for the manufacturing process of composite beams with embedded Fiber Bragg Grating sensors are presented. Composite beams instrumented with surface bonded and embedded Fiber Bragg Grating sensors are tested under static torsional load. In addition, Finite Element Analyses of composite beams under torsional load is conducted using the commercial finite element code ABAQUS®. Subsequently, the results obtained from the tests and from the Finite Element Analyses are compared to observe the consistency between the torque and shear strain results. Furthermore, as a case study, static torsion test of a composite beam equipped with both a strain gage and a Fiber Bragg Grating sensor is performed for the purpose of identifying the agreement between the shear strain results.  M.S. - Master of Scienc

With the rapid proliferation of wireless services, the frequency spectrum has become increasingly crowded, and the interferences and composite signals will be ubiquitous in the wireless receiver. For deeply dissecting and detecting the expected signals, the research community has to investigate the smart signal processing technology to resisting the influence of detrimental signals. For this purpose, blind source separation has been shown to be a promising method for achieving simultaneous spectrum utilization and wireless adaptive interference cancellation. The attractive features and appealing advantages of blind source separation make it an attractive theory for source extraction or recovery, which plays a crucial role in helping realize intelligent signal processing for wireless communication. It can recover the unobserved sources only from the wireless received mixed signals based on the features of the source signal exempted from channel estimation and synchronization manipulation. Wireless communication systems can benefit the high spectrum efficiency, strong anti-interference, and adaptive signal processing through the blind separation mechanism. So far, numerous researchers have made tremendous efforts to investigate this field for enhancing spectrum efficiency, anti-interference ability, and signal detection performance through employing the philosophy of blind separation. These meaningful and appealing research works motivate us to make a comprehensive survey with regard to this area. In this paper, the fundamental principle of the blind separation mechanism, involving independent component analysis, sparse component analysis, non-negative matrix factorization, and bounded component analysis will be reviewed briefly, and then, the critical technologies applied in various wireless communication systems will be overviewed, such as in direct-sequence code division multiplexing access, frequency hopping, orthogonal frequency-division multiplexing, multiple input multiple output, wireless sensor networks, cognitive radio networks, radio frequency identification devices, and communication security. In addition, the important research challenges and meaningful research directions pertaining to the area of blind separation applied in wireless communications systems are also discussed. INDEX TERMS Adaptive signal processing, blind source separation, wireless communication systems, independent component analysis, sparse component analysis, bounded component analysis, artificial intelligence, machine learning. I. INTRODUCTION With the advent of artificial intelligence (AI) era, the blind source separation as a significant machine learning theory will be a powerful tools and hot research topic with substantial potential application areas, such as seismic monitoring, stock prediction, text document analysis, audio signal

Drying shrinkage and creep are two of the most important time-dependent properties of concrete, and the health monitoring of any large-scale concrete structure is practically the monitoring of the combined effects of these two physical properties. We present a fibre-optic sensing technique using fibre Bragg grating (FBG) sensors for the experimental investigation of drying shrinkage and creep of structural grade (40 MPa) concrete. It offers many advantages over the conventional electrical and mechanical methods for both structural health monitoring and standard tests of the mechanical properties. The FBG sensors are directly embedded into concrete specimens and the data are obtained by an optical measurement system. Standard mechanical method, as specified by the Australian Standards, is also used for comparison. Good agreements between the two methods are achieved for both drying shrinkage and creep. The long-term behaviours are predicted from the experimental data, and the accurac...

We present here a method to create spectrally addressable phase masks by encoding phase profiles into volume Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying the Bragg condition of the hologram. Moreover, this approach enables the capability to encode and multiplex several phase masks into a single holographic element without cross-talk while maintaining a high diffraction efficiency. As examples, we demonstrate fiber mode conversion with near-theoretical conversion efficiency as well as simultaneous mode conversion and beam combining at wavelengths far from the original hologram recording wavelength. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.