A Transflective Nano-Wire Grid Polarizer Based Fiber-Optic Sensor

A transflective nano-wire grid polarizer is fabricated on a single mode fiber tip by focused ion beam machining. In contrast to conventional absorptive in-line polarizers, the wire grids reflect TE-mode, while transmitting TM-mode light so that no light power is discarded. A reflection contrast of 13.7 dB and a transmission contrast of 4.9 dB are achieved in the 1,550 nm telecom band using a 200-nm wire grid fiber polarizer. With the help of an optic circulator, the polarization states of both the transmissive and reflective lights in the fiber may be monitored simultaneously. A kind of robust fiber optic sensor is thus proposed that could withstand light power variations. To verify the idea, a fiber pressure sensor with the sensitivity of 0.24 rad/N is demonstrated. The corresponding stress-optic coefficient of the fiber is measured. In addition to pressure sensing, this technology could be applied in detecting any polarization state change induced by magnetic fields, electric currents and so on

Low Profile Stretch Sensor for Soft Wearable Robotics

This paper presents a low profile stretch sensor for integration into soft structures, robots and wearables. The sensor mechanism uses a single piece of highly flexible and light weight optical fibre and is based on the notion that bending an optical fibre modulates the intensity of the light transmitted through the fibre, a technique often referred as macrobending light loss. In this arrangement, the optical fibre originates from sensor’s electronic unit, passes through a stretchable encasing structure in a macrobend pattern, and then loop back to the same unit resulting in a simplified electrical and optical design; the closed optical loop allows for no electronics at one end of the sensor making it safe for human robotics applications, and no optical interference with the external environment eliminating the need for complex conditioning circuitries. Of particular interest of the soft robotics community, the ability of this custom macrobend stretch sensor to flexibly adapt its configuration allows preserving the inherent softness and compliance of the robot which it is installed on. Our experimental results indicate that the optical fibre’s bending radius is the dominant design parameter for sufficiently complex patterns, a finding that can facilitate generalisation of the sensing methods across different scales. The measurement performance of the mechanism and its impact on the stiffness of the encasing structure is benchmarked against a custom calibration and testing system

A Non-linear Model for Predicting Tip Position of a Pliable Robot Arm Segment Using Bending Sensor Data

Using pliable materials for the construction of robot bodies presents new and interesting challenges for the robotics community. Within the EU project entitled STIFFness controllable Flexible & Learnable manipulator for surgical Operations (STIFF-FLOP), a bendable, segmented robot arm has been developed. The exterior of the arm is composed of a soft material (silicone), encasing an internal structure that contains air-chamber actuators and a variety of sensors for monitoring applied force, position and shape of the arm as it bends. Due to the physical characteristics of the arm, a proper model of robot kinematics and dynamics is difficult to infer from the sensor data. Here we propose a non-linear approach to predicting the robot arm posture, by training a feed-forward neural network with a structured series of pressures values applied to the arm's actuators. The model is developed across a set of seven different experiments. Because the STIFF-FLOP arm is intended for use in surgical procedures, traditional methods for position estimation (based on visual information or electromagnetic tracking) will not be possible to implement. Thus the ability to estimate pose based on data from a custom fiber-optic bending sensor and accompanying model is a valuable contribution. Results are presented which demonstrate the utility of our non-linear modelling approach across a range of data collection procedures

Комбінований числово-аналітичний метод параметричного проєктування тера¬герцового підкладкового поляризатора на підставі дротяних сіток

The features of the terahertz substrate-based wire grid polarizer designing are considered in the work. For this purpose, a literature review of different type polarizer constructions comparative characteristics was presented and the advantage of wire grid constructions was described. An overview of the materials used for the manufacture of suitable polarizers for different radiation wavelengths is given. Based on this, a combined numerical-analytical method of terahertz polarizer parametric design was presented in the paper. The method consists of effective anti-reflective characteristics analytical calculation of the polarizer unit cell structure and their further refinement and expansion by numerical finite element simulation of the electromagnetic wave propagation problem within this structure in the frequency domain with specific boundary conditions. This brings the scientific novelty. The anti-reflective characteristics calculations are based on the analytical effective medium theory which enables designing polarizer as a thin dielectric film with minimum reflectance. The finite element model considers a polarizer unit cell as a waveguide with specific periodic and port conditions. The combination of both approaches brings the possibility to design a polarizer grid and substrate sizes which provide minimum reflectance. An example of terahertz substrate-based polarizer parameters and characteristics calculation by the developed combined method was given. The obtained results are consistent within 1 % error with the assumed theoretical calculations that were obtained by other scientific group. This indicates the adequacy and practical value of the proposed method for the design of polarizers with anti-reflective, high-transmittance, low-loss and robust characteristics. In addition, by using a numerical finite element component, the proposed method is quite simple to extend by changing the geometry of the unit cell to study such promising objects as multilayer polarizers or polarizers based on more complex meta- or composite materials.Розглянуто комбінований числово-аналітичний метод параметричного проєктування підкладкового поляризатора терагерцового електромагнітного випромінювання, побудованого на підставі дротяних сіток. Здійснено літературний огляд порівняльних характеристик можливих конструкцій поляризаторів різних типів та описано перевагу конструкцій на підставі дротяних сіток. Наведено огляд матеріалів, що використовують для виготовлення відповідних поляризаторів для різної довжини хвилі випромінювання. Проаналізовано методи, що застосовують для розрахунку характеристик і параметрів поляризаторів на підставі дротяних сіток. Розроблений числово-аналітичний метод параметричного проєктування базується на аналітичному розрахунку ефективних антивідбивних характеристик конструкції елементарної комірки поляризатора. У межах методу розроблено скінченно-елементну модель поляризатора, реалізовану за допомогою програмного забезпечення COMSOL Multiphysics, що дає змогу уточнити характеристики конструкції елементарної комірки і можливості для їх розширення за допомогою числового моделювання методом скінченних елементів задачі поширення електромагнітних хвиль у цій конструкції в частотній області зі специфічними граничними умовами, що сукупно дає наукову новизну. Отримані результати моделювання добре узгоджуються з відомими експериментальними результатами та тісно корелюють з результатами моделювання, що отримали інші наукові групи, що свідчить про адекватність та практичну цінність методу для завдань проєктування поляризаторів терагерцового випромінювання з антивідбивними, високопропускними, низьковтратними та надійними характеристиками. Окрім цього, завдяки використанню числової скінченно-елементної складової, запропонований метод досить просто розширити способом зміни геометрії елементарної комірки для дослідження таких перспективних об'єктів, як багатошарові поляризатори чи поляризатори на підставі складніших мета- чи композиційних матеріалів

Macrobend optical sensing for pose measurement in soft robot arms

This paper introduces a pose-sensing system for soft robot arms integrating a set of macrobend stretch sensors. The macrobend sensory design in this study consists of optical fibres and is based on the notion that bending an optical fibre modulates the intensity of the light transmitted through the fibre. This sensing method is capable of measuring bending, elongation and compression in soft continuum robots and is also applicable to wearable sensing technologies, e.g. pose sensing in the wrist joint of a human hand. In our arrangement, applied to a cylindrical soft robot arm, the optical fibres for macrobend sensing originate from the base, extend to the tip of the arm, and then loop back to the base. The connectors that link the fibres to the necessary opto-electronics are all placed at the base of the arm, resulting in a simplified overall design. The ability of this custom macrobend stretch sensor to flexibly adapt its configuration allows preserving the inherent softness and compliance of the robot which it is installed on. The macrobend sensing system is immune to electrical noise and magnetic fields, is safe (because no electricity is needed at the sensing site), and is suitable for modular implementation in multi-link soft continuum robotic arms. The measurable light outputs of the proposed stretch sensor vary due to bend-induced light attenuation (macrobend loss), which is a function of the fibre bend radius as well as the number of repeated turns. The experimental study conducted as part of this research revealed that the chosen bend radius has a far greater impact on the measured light intensity values than the number of turns (if greater than five). Taking into account that the bend radius is the only significantly influencing design parameter, the macrobend stretch sensors were developed to create a practical solution to the pose sensing in soft continuum robot arms. Henceforward, the proposed sensing design was benchmarked against an electromagnetic tracking system (NDI Aurora) for validation

Fibre optic hydrogen sensing for long term use in explosive environments

Hydrogen is an explosive and flammable gas with a lower explosive limit of just 4% volume in air. It is important to monitor the concentration of hydrogen in a potentially hazardous environment where hydrogen may be released as a by-product in a reaction or used as a principal gas/liquid. A fibre optic based hydrogen sensor offers an intrinsically safe method of monitoring hydrogen concentration. Previous research studies have demonstrated a variety of fibre optic based techniques for hydrogen detection. However the long-term stability of the hydrogen sensor and interrogation system has not yet been assessed and is the focus of this study. In the case of sensor heads being permanently installed in-situ, they cannot be removed for regular replacement, making long-term stability and reliability of results an important feature of the hydrogen sensor. This thesis describes the investigation and characterisation of palladium coated fibre optic sensor heads using two designs of self-referenced refractometer systems with the aim of finding a system that is stable in the long term (~6 months). Palladium was the chosen sensing material owing to its selective affinity for absorbing hydrogen. Upon hydrogen absorption, palladium forms a palladium- hydride compound that has a lower refractive index and lower reflectivity than pure palladium. The refractometers measured the changes in the reflectivity to enable calculation of the concentration of hydrogen present. A low detection limit of 10ppm H2 in air was demonstrated, with a response time of 40s for 1000ppm H2 in air. A further aspect to sensor stability was investigated in the form of sensor heads that had a larger area for palladium coverage. Hydrogen induced cracking in palladium is a common failure mechanism. A hypothesis is presented that a larger sensor area can reduce the probability of catastrophic failure resulting from cracks, which may improve the predictability of the sensor’s performance. Two sensor head designs have been proposed – fibre with a ball lens at the tip and fibre with a GRIN lens at the tip, both of which potentially offer a larger area than the core of a singlemode optical fibre. The limit of detection and response times of the sensor heads were characterised in hydrogen. For long term stability assessment of the sensor head and the interrogation unit, the system was left running for a period of 1 to 4 weeks and the noise and drift in the system was quantified using an Allan deviation plot

Nanowire Alignment: Techniques, Quantification, and Applications in Large-Area Devices

Nanowire alignment is essential for their integration into large-area devices, as well as to obtain certain functionality such as the ability to polarize light and increase surface-enhanced Raman scattering. Various nanowire alignment methods have been developed, however, major drawbacks have limited their application such as complex processing, high cost, limited compatible nanowire materials, and limited scalability. In addition, the methods used to quantify the quality of nanowire alignment are lacking in accuracy, speed, and applicability to all kinds of nanowires. In this thesis, two simple and large-area alignment methods are studied that are applicable for nanowires synthesized by any method and compatible with large-area electronic device fabrication processes. The first method is accomplished by depositing nanowires on polyvinyl alcohol films followed by film stretching, which achieves high-quality alignment (with an order parameter S=0.93). Nanowire breakage, which is commonly encountered in similar techniques, is minimized and the average length of nanowires after alignment is nearly the same (~99.3%) as before alignment. The second alignment method is accomplished directly during rod-coating deposition of the nanowires, without the need of any additional step. Two image processing methods based on edge-detection and skeletonication are presented to recognize nanowires from microscopy images. Then an order parameter and an orientational distribution function are used for alignment quantification. Compared with previously reported studies, these methods are fast and automated, reliable without bias, generally applicable, easy to implement, and computationally efficient. The alignment methods described above are applied in two applications. Firstly, the electrical and optical anisotropy of slightly aligned silver nanowire films, which can be used as transparent electrodes, are investigated. Their transparency to polarized light is increased by 7.3 percentage points compared to typical randomly oriented silver nanowire films, which may benefit end uses such as liquid crystal displays and the touch sensors on top of them. Secondly, a crossed film structure consisting of semiconductor nanowires aligned in one direction and metal nanowires orthogonally aligned is designed. The metal nanowires are intended to act as interconnects to substantially reduce semiconductor nanowire-nanowire junction resistances while avoiding lithographically-defined metal pads, the latter which can have poor mechanically flexibility and involve fabrication processes not desired for large-area electronics. Such a device structure can be developed further for use in large-area flexible devices such as light, strain and chemical sensors and energy generators

Design of compact optical devices based on periodic meta-structures

Ankara : The Department of Electrical and Electronics Engineering and the Graduate School of Engineering and Science of Bilkent Univ., 2013.Thesis (Master's) -- Bilkent University, 2013.Includes bibliographical references leaves 115-133.Manipulation of the flow of light is demanded for several applications such as communication, data storage, sensor, photovoltaic cells, microscopy, lasers and light emitting diodes for the purpose of designing compact, high-throughput and high efficiency optical devices. Nevertheless, the control of the propagation of the light becomes much harder in devices with smaller geometries mostly because of diffractions, loss mechanisms and fabrication difficulties. Furthermore, materials that are already available in the nature do not provide unprecedented optical properties for nanoscale optical applications. Due to this fact that fabrication of artificial materials is needed for utilizing novel and intriguing optical devices. For this purpose, some relatively new research fields have emerged like photonic crystals, metamaterials and high contrast gratings. We propose several designs based on aforementioned meta-structures to achieve compact and practically realizable optical devices. We presented compact optical demultiplexer, diode-like device and electro-optic modulator designs that are based on photonic crystals. We also proposed two circular polarizer designs based on metamaterials and high contrast gratings. Further, we investigated unidirectional transmission and polarization manipulation properties in chiral metamaterials. For most of the proposed designs, we also experimentally verified the numerical and theoretical findings. In conclusion, we can claim that the utilization of artifically structured materials give opportunity to realize the control of light much more easily in nanoscale designs.Akosman, Ahmet EminM.S