We present a study on the resolution limits and resolution factors of terahertz (THz) ptychography. Simulations of a binary amplitude object show that ptychography shares the same intrinsic resolution factors with digital holography, i.e. it is diffraction-limited. Reconstructions of amplitude and phase objects obtained from holographic and ptychographic experiments are comparable. A lateral resolution of around one wavelength λ is achieved on an amplitude object, while a depth resolution of around λ/5 is reported on a weakly diffracting phase object. THz ptychography is expected to complement THz holography for imaging biological samples and THz transparent specimens.
The foundation pillars of successful technical products are performance, cost, and reliability. The development of reliable components and the operation of highly available systems is a comprehensive engineering task combining probability theory, materials science, and experience. Components have to be as reliable as necessary in order to build systems that are dependable and cost efficient during the whole life cycle. Reliability engineering is an ongoing process starting at the conceptual phase of a product design and continuing throughout all phases of a product life cycle. Theprimary objective is to identify and eliminate potential reliability problems as early as possible. While it may never be too late to improve the reliability of a product, corrections are orders of magnitude less expensive in the early design phase rather than once the product is manufactured and in service. This paper comprises an introduction to basic reliability engineering terms, reliability analysis methods such as reliability block diagrams, failure mode and effects analysis, Markov processes, the concept of redundancy, failure rate prediction models and the physics of failure approach, qualification and accelerated reliability testing. Examples of electronic and optical components, as well as complex opto-electronic systems and networks are given for illustration.
The paper gives an overview of reliability, availability, and maintainability of fiber optical sensors, three key factors on which standards and validation should be based and which are required for successful industrialization. The examples given are based on two long term applications with fiber optical Bragg gratings - the surveillance of two bridges (civil engineering). However, similar reflections are required for any type of application and any optical fiber sensors. Recommendations are given to improve the confidence and acceptance of possible users in fiber optical sensing systems. It is shown that with proper installation lifetimes of 50 years are possible.
This paper discusses direct focused ion beam (FIB) writing of microstructured photonic elements and sensors like apertures on tips of optical fibers, microlenses, or photonic crystals. Emphasis is mainly on the fabrication side.
Silicone and acrylic elastomers have received increased attention as dielectric electroactive polymer (EAP) materials for actuator technology. The goal of this work was to develop and characterize a new class of silicones (DC3481) and to compare it with acrylic elastomers. The influence of various types of hardeners, hardener concentration, prestrain and high dielectric organic fillers was studied by mechanical, electrical and electromechanical experiments. Furthermore the temperature dependence and the viscoelastic properties were investigated. The results show that by changing type and concentration of hardener, the Young's modulus can be varied. In order to increase the dielectric constant, the silicone was blended with organic materials. Compared to acrylic elastomers, this new class of silicone elastomers has the advantage of a constant stiffness over a wide range of temperature and a lower viscosity that results in a higher response speed of the actuator.
Fiber optic sensors are potentially very well suited for condition monitoring of environment, materials, structures, and facilities. However, there is a long way from a laboratory prototype to a reliable industrial sensor system. Based on the examples of two fiber Bragg grating systems, both used for long term monitoring of strain and temperature on bridges, general sensor system reliability will be discussed. In addition, specific reliability considerations and lifetime tests, especially for optical fibers and Bragg gratings, coatings, and adhesives will be presented.
For the extension of existing optical fiber links to higher data transfer rates in the multi-gigabit range and the expected higher power density due to DWDM one must consider possible limitations of the deployed cables. Many investigations have identified polarization mode dispersion PMD as a crucial parameter especially for cables exposed to environmental stresses. Three aerial optical fiber cable links were characterized by measuring PMD, optical time domain reflectometry OTDR, polarization-OTDR, and bit error rate BER. Measurement results over several days are correlated to temperature data from weather stations along the cable lines.
We report on application tests of novel sensor elements for long term surveillance of tunnels. The sensors are made of glass fiber reinforced polymers (GFRP) with embedded optical fiber Bragg gratings. The tests were made in a tunnel near Sargans in Switzerland and we will present strain and temperature data of more than one year of operation of the sensor elements. Two sensor types were tested. First, GFRP rockbolts with a diameter of 22 mm were produced. They have a load-bearing function as anchors for tunnel or mine roofs and in addition measure distributed strain fields and temperature with embedded optical fiber Bragg grating arrays. Rockbolts are key elements during construction and operation of tunnels. Data about strain inside the rockbolts can support decision about precautions to be taken and reveal information about the long term movement of the rock. Second, thin and flexible GFRP wires of 3 mm in diameter were found to be robust and versatile sensors not only for tunnel surveillance but for many civil engineering applications where they can be attached or embedded (e.g., in concrete). The fabrication of both sensor types and solutions for the connection of the embedded fiber sensors to a fiber cable will be presented. Moreover, laboratory and tunnel data of functionality and long term stability tests will be discussed and compared.
We report on the development of a measuring system for dynamic long-term monitoring of highway pavement; i.e. the relative vertical displacements within the pavement layers. Beside the knowledge of weight and frequency of vehicles it is more and more of interest to know the vertical deformations within the pavement layers due to the traffic loads. Our displacement measurement is based on the magnetostrictive principle. Several positions along one measuring axis corresponding to the different layers of the pavement are monitored. The development of the measuring system, the calibration, the embeddment in a test pavement and first test results will be presented.
In civil engineering it is of interest to monitor long-term performance of structures made of new lightweight materials like glass or carbon fiber reinforced polymers (GFRP/CFRP). In contrast to surface applied optical fiber sensors, embedded sensors are expected to be better protected against rough handling and harsh environmental conditions. We report on two recently done fiber optical sensor applications in civil engineering. Both include structurally embedded fiber Bragg grating (BG) arrays but have different demands with respect to their operation. For the first application fiber BGs were embedded in GFRP rockbolts of 3 - 5 m in length either of 3, 8, or 22 mm diameter. The sensor equipped rockbolts are made for distributed measurements of boulder motion during tunnel construction and operation and should withstand strain up to 1.6%. Rockbolt sensors were field tested in a tunnel near Sargans in Switzerland. For a second application fiber BGs were embedded in CFRP wires of 5 mm diameter used for the pre- stressing cables of a 56 m long bridge near Lucerne in Switzerland. The permanent load on the cable corresponds to 0.8% strain. Due to the embedded sensors, strain decay inside the cable anchoring heads could be measured for the first time during loading and operation of the cables. For both applications mechanical and thermal loading tests were performed to assess the function of these new elements. Also, temperature and strain sensitivity were calibrated. Reliability studies with respect to stress transfer, fiber mechanical failure, and wavelength shift caused by thermal BG decay as well as monitoring results of both applications are presented.
Rockbolt anchors for tunnel or mine roofs are key elements during construction and operation. We report on the fabrication of glass fiber reinforced polymer (GFRP) rockbolts with embedded fiber optical Bragg grating sensors and their first field application in a test tunnel. Optical fibers and in-fiber Bragg grating sensors were embedded in GFRP rockbolts during a continuously ongoing pultrusion process on an industrial production machine. Depending on their outer diameter the rods equipped with fiber sensors serve as measuring rockbolts or as extensometric sensors for the motion of boulders in the tunnel roof. The adhesion and force transfer of different fiber coatings were tested by push-out experiments. By temperature and strain cycle tests the performance of the rockbolt sensors was evaluated. We will present these results and the measurements made during a first installation of fiber optical rockbolt sensors in a tunnel.
Based on the example application of Emmenbridge, a newly built steel-concrete-composite bridge in Switzerland with 47 m long built-in carbon fiber reinforced polymer (CFRP) prestressing cables, we will present and analyze the process chain leading to a reliable surveillance of modern civil engineering structures with embedded fiber optical Bragg gratings. This consists first in the embedding of optical fibers and in-fiber Bragg gratings in long CFRP wires in an industrial environment, including fiber optical monitoring of the curing process. Then, various qualifying tests were done: annealing experiments for determining optical lifetime of the Bragg gratings used, dynamic and static tensile tests for estimating their mechanical lifetime under operation, push-out experiments to check adhesion of fiber/coating/matrix interfaces, and performance tests to determine strain and temperature sensitivity of the embedded Bragg gratings. Finally, the prestressing cables were equipped with the CFRP sensor wires and built into the bridge.
The hydro-electric power dam of Luzzone in Switzerland has been heightened by 17 m to 225 m. Distributed fiber optical sensors, based on wavelength multiplexed Bragg gratings, have been embedded in the concrete to monitor temperature and strain evolution. The sensors were designed to make them suitable for embedding and operation in this harsh environment. Data taken during curing and the first year afterwards are presented.
KEYWORDS: Temperature metrology, Phase measurement, Fringe analysis, Thermal effects, Fluctuations and noise, Quantitative analysis, Head, Cameras, Laser stabilization, Body temperature
We report on the application of ESPI to measure deformations induced by thermal load on lightweight honeycomb panels for space applications. The panel was mounted isostatically onto a vibration isolated table. A housing for temperature stabilization was constructed enclosing the panel, heating elements, fans and the ESPI-head made of Invar. Emphasis is put on the quantitative analysis of the deformation of this large object (0.8 X 0.8 m2) viewed from a relatively short distance of 1.1 m and illuminated sequentially from three non-orthogonal directions. Influences of laser stability, rigid body displacements, temperature inhomogeneities as well as possible deformations of the measurement head are discussed in order to derive the measurement uncertainty and to estimate corrections. Beside the sensitivity vector analysis it is important to take into account the optical light path changes due to temperature changes. Out of plane deformation fields of the panel are presented.
There is strong interest to develop fiber-optical sensing systems for long term surveillance and structural monitoring. Although many detection schemes have been proposed, industrial acceptance of optical fibers as validated replacement of other sensors is limited. Low cost manufacturability, reliability, and long term stability are very important for usability in concrete and composite material structures. Lifetime for major structures in civil engineering of 50 - 100 years are very demanding on the sensors and require accurate aging models and test data to demonstrate their reliability and durability. Acceleration factors of several orders of magnitude can be achieved under reasonable testing conditions depending on temperature, mechanical stress, humidity, chemical environment and activation energy of the damaging process. We report on accelerated aging tests and failure mechanisms of optical fibers and Bragg gratings at elevated temperature, humidity and mechanical stress. Aging behavior is discussed and results from field measurements of large civil structures are presented.
We report on applications of surveillance and test systems for civil engineering structures. The system key elements are optical-fiber Bragg grating sensors and conventional resistance strain gauges. A recently built stay cable bridge with a world novelty of two carbon-fiber-reinforced-polymer cables was equipped with both types of sensors. The sensor system on the bridge is now operational for ten months and the bridge is open for traffic for 4 months. Results of the bridge surveillance are presented. To monitor a large concrete structure, the electrical power dam of Luzzone in the Swiss Alps, a prototype sensor rod was designed. First measurements with a sensor rod embedded in a concrete test prism are discussed. Several redundant measurements are made to compensate for temperature drift and to monitor the reliability of the measurement chain.
Surveillance of facilities and critical structures by monitoring mechanical strength and integrity is necessary for safety in use. Fiber-optic sensors still have poor industrial acceptance due to their lack of demonstrated reliability and long-term stability. Reliability testing for objects with expected life times of 10 to 100 years has to rely on accelerated aging procedures. We report on a series of aging tests of optical fibers and Bragg gratings at elevated temperature, humidity and mechanical stress performed in regard to field applications. Tensile and climate tests were performed with optical fibers embedded in glass fiber reinforced polymers (GFRP) and surface attached to carbon fiber reinforced polymers (CFRP). For surface attached fiber sensors relative humidity was found to be a critical parameter with strong influence on life time. A cable-stayed bridge (Storchenbrucke in Winterthur, CH, still under construction) where for the first time two steel cables are replaced by CFRP cables was equipped with optical-fiber Bragg gratings and standard resistance strain sensors.
A specially designed optical time domain reflectometer was used to measure the speed of propagation of rapidly running induced cracks along high density polyethylene pipes. Optical fibers were wound in a helical form around each pipe sample. Upon arrival, the propagating crack succesively broke and shortened the fibers; thereby the travel time of a reflected light pulse was reduced. The time dependent lengths of the fibers indicated the position of the edge of the crack. With this method crack speeds in the range of 100 m/s to 200 m/s were measured. The results agree well with those obtained by a conventional method. The fiber optical measurement even allowed to determine the form of the crack path.
We report on civil engineering applications of wavelength multiplexed optical-fiber Bragg grating arrays produced directly on the draw tower for testing and surveying advanced structures and material like carbon fiber reinforced concrete elements and prestressing tendons. We equipped a 6 m X 0.9 m X 0.5 m concrete cantilever beam reinforced with carbon fiber lamellas with fiber Bragg grating sensors. Static and dynamic strain levels up to 1500 micrometers /m were measured with a Michelson interferometer used as Fourier spectrometer with resolutions of about 10 micrometers /m for all sensors. Comparative measurements with electrical resistance strain gauges were in good agreement with the fiber optic results. We used the fiber sensors in two different arrangements: some Bragg grating array elements measured the local strain while others were configured in an extensometric way to measure moderate strain over 0.1-1 m.
Intrinsic fiber optic Fabry-Perots (FP) adhered to the surface of a cantilever beam are used as strain sensors.
Low coherence demodulation with a scanning Michelson interferometer (MI) and interfringe resolution
techniques allow absolute interrupt-immune measurements of static longitudinal strains with a resolution of a
few microstrains. Low reflecting FPs arranged either in parallel or in series can be multiplexed by giving them a
difference in their lengths.
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