Fiber Optic Vibration Sensor Based on the Tilted Fiber Bragg Grating

A temperature-insensitive fiber optic vibration sensor based on the tilted fiber Bragg grating (TFBG) is presented. The sensing head is formed by insertion of a small section of MMF between a single-mode fiber and the TFBG. The reflection light from this tilted fiber Bragg grating includes two parts: the reflected Bragg mode and the cladding modes. The cladding modes were coupled back into the core mode as a function of the multimode fiber. The power of the cladding modes is sensitive to vibration, so the external vibration measurement can be obtained through the cladding mode average output power. Experiment results show that the root mean square (RMS) of the detection error of the average power was 0.01 μW within the temperature range from 20 to 70°C, so it is proved to be temperature independent; its frequency response has been tested to 1 KHz

Development of a reinforcing fibre light-guide for use as a damage sensor within composite structures

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This study presents the results of an investigation to develop a novel sensor which would give a direct indication of the extent of impact damage in a composite. This was achieved by using glass reinforcing fibres to produce a light-guide, which was embedded within a composite laminate. The laminate was then subjected to impact events or bending stresses of sufficient magnitude to cause damage. The impact energies used in this study varied between 2 and 10 Joules, and the indentation depths varied between 0.125 and 1 mm, allowing damage propagation to be monitored. The fall-off in the transmitted light was used to monitor the level of damage, along with C-scanning and sectioning to provide reference data. The use of reinforcing fibres to produce the sensor meant that the strains required to cause failure in the fibres was realistically close to those of the composite constituents. Changes in the transmission characteristics of the sensor were found to correspond to real failure events occurring during impact.This study is funded by Mr D. Coward at Royal Mail Securities and the EPSRC

Large deployable antenna program. Phase 1: Technology assessment and mission architecture

The program was initiated to investigate the availability of critical large deployable antenna technologies which would enable microwave remote sensing missions from geostationary orbits as required for Mission to Planet Earth. Program goals for the large antenna were: 40-meter diameter, offset-fed paraboloid, and surface precision of 0.1 mm rms. Phase 1 goals were: to review the state-of-the-art for large, precise, wide-scanning radiometers up to 60 GHz; to assess critical technologies necessary for selected concepts; to develop mission architecture for these concepts; and to evaluate generic technologies to support the large deployable reflectors necessary for these missions. Selected results of the study show that deployable reflectors using furlable segments are limited by surface precision goals to 12 meters in diameter, current launch vehicles can place in geostationary only a 20-meter class antenna, and conceptual designs using stiff reflectors are possible with areal densities of 2.4 deg/sq m

Large Deployable Reflector (LDR) system concept and technology definition study. Volume 2: Technology assessment and technology development plan

A study was conducted to define reasonable and representative LDR system concepts for the purpose of defining a technology development program aimed at providing the requisite technological capability necessary to start LDR development by the end of 1991. This volume presents thirteen technology assessments and technology development plans, as well as an overview and summary of the LDR concepts. Twenty-two proposed augmentation projects are described (selected from more than 30 candidates). The five LDR technology areas most in need of supplementary support are: cryogenic cooling; astronaut assembly of the optically precise LDR in space; active segmented primary mirror; dynamic structural control; and primary mirror contamination control. Three broad, time-phased, five-year programs were synthesized from the 22 projects, scheduled, and funding requirements estimated

Ultra-compact all-in-fiber-core Mach–Zehnder interferometer

Optical Mach–Zehnder interferometers (MZIs) are useful components in a variety of optical applications, including optical modulation; signal processing; and physical, chemical, and biological sensing. We introduce here a novel, assembly-free all-in-fiber-core MZI, which is directly written with a femtosecond laser. By introducing a positive refractive index-modified zone in half of the fiber core, the original single-mode fiber section is converted into a few-mode fiber section, where a strong coupling between the two lowest-order guided modes is generated, resulting in a well-defined interference spectrum in transmission. This device promises many significant advantages over existing approaches such as ease of fabrication, stability, small insertion loss, robustness extremely broad operating bandwidth, and precise and controllable cavity lengths. These advantages make this device strikingly attractive with the potential for extensive adoption in fiber communications, signal processing, sensors, and laser wavelength control

Innovative Plastic Optical Fiber Sensors

This thesis describes the development of new types of fiber optic sensors for the measurement of mechanical quantities such as displacement, vibration and acceleration. Also, it describes the realization of specific acquisition systems designed to interrogate the developed sensors. Since optical fibers have been historically associated with high speed telecommunication links because of their very large bandwidth and low attenuation, there is a great interest for their employment in sensor applications. Fiber sensors represent a promising solution in many fields since fibers can be used for the measurement of several quantities, not only mechanical as those investigated in this work, but also chemical with the possibility to detect specific chemical or bio-chemical molecules. Among the physical quantities to be detected, the displacement measurement is required in some applications, especially in structural civil and mechanical fields, where it is possible to evaluate the cracks evolution, providing information about the safety of the structure under monitoring in order to detect eventually risky situations. All the developed sensors are able to measure the displacement along one or two axis, that can be employed also during vibration tests especially at high frequencies, and also acceleration sensors to monitor acceleration at low frequencies. The developed sensors are based on plastic optical fibers instead of the traditional glass fibers, which are traditionally employed in optical communications. This change is related to the aim of realizing sensors maintaining the excellent typical characteristics of the fibers, such as electromagnetic immunity, intrinsically fire safety and flexibility of applications, but with costs comparable to those of commercial electromechanical sensors. Indeed, nowadays, the commercial fiber optic sensors are based on glass fibers because they have very good performance. However, they find limited applications due to the high costs of their complex interrogation systems and also for the procedure required to splice the fibers. On the other hand, plastic optical fibers represent a promising alternative because of their geometrical and optical properties that allow employing low-cost non coherent sources such as LED and also simplifying the procedure for the sensor connection and installation. Therefore, the design of the proposed plastic optical fiber displacement sensors is described with the sensor practical arrangement and the realized prototypes. An acquisition system has been designed and realized to characterize the sensors and the characterization results are also provided. Moreover, the development and the characterization of a plastic optical fiber sensor able to measure the displacement in two directions have been described. The main drawback of the developed sensors are stability issues and for this reason laboratory and in situ-tests have been carried out in order to verify the sensor performance over the time. I The results obtained with the stability tests have highlighted the necessity to develop displacement sensors with increased stability. To this aim, a compensation technique based on two different wavelengths has been developed. The same working principle of the developed displacement sensor has been exploited to realize a fiber vibrometer to be employed during the vibration monitoring for measuring without contact the vibrations of the device under test. The sensor development, a suitable calibration procedure developed to overcome the problem of real targets with a non uniform reflectivity, and the experimental tests have been described. Furthermore, the preliminary results concerning the feasibility study of a plastic optical fiber accelerometer are reporte

Fiber-optic curvature sensor based on cladding-mode bragg grating excited by fiber multimode interferometer

A sensing structure consisting of a short multimode fiber (MMF) spliced in just upstream of a uniform fiber Bragg grating (FBG) is proposed and experimentally demonstrated for temperature-independent curvature measurement. The short MMF section generates cladding modes in the fiber that contains the FBG. Several of these modes get reflected back by the FBG at shorter wavelengths and re-enter the launch fiber after passing through the MMF section. The net recoupling efficiency between the incident forward core mode and reflected cladding modes is $-$20 dB when the fiber is straight, but decays when a curvature applied on the sensing structure. A maximum sensitivity of 0.74 dB/m -1 is obtained in a large measurement range up to ∼ 15.5 m -1, with no orientation dependence of the sensitivity. The reflected coupled power variation is measured to be ±0.15 dB over the temperature range from 0 to 70 °C, allowing for temperature-independent curvature measurements

Polymer, Metal, and Ceramic Microtubes by Strain-driven Self-rolling

A thin polymer bilayer film was transformed into micro- and nano-tubes using strain driven self-rolling phenomena of polystyrene (PS)/poly (4-vinyl pyridine) (P4VP) film. Polymer bilayer was produced by consecutive deposition of PS and P4VP, from toluene and chloroform solutions, respectively, by dip-coating technique. The object formation proceeds from a opening in the film made by photolithography or by mechanical scratching followed by immersion of patterned sample in dodecylbenzene sulfonic acid (DBSA) solution. DBSA forms supramolecular complexes with pyridine rings of P4VP and increases the specific volume of the polymer. Since the solution is neutral to PS layer, bilayer film develops strain due to unequal swelling of polymers in solution of DBSA and hence the film bends and scrolls in order to minimize its free energy and form tubes. The length of the tubes and the direction of rolling are determined by mechanical patterning of the film. UV-photolithography is used to fabricate patterns of polymer bilayer in order to create tube in a precise manner. The kinetics of the tube formation was studied with respect to acidity of the solution and UV dose. Rate of rolling increased with the acidity of the solution. Tube diameter and rate of rolling decreased with the increase of the UV exposure time. Films with 2-dimensional gradients of layer thicknesses were prepared to study a broad range of parameters in a single experiment. Furthermore, polymer micro-toroids and triangles were also fabricated using self-rolling approach of PS/P4VP layer. Moreover, the kinetics of toroid formation is also studied in the present work. The equilibrium dimensions of toroid are determined by the balance of the bending and the stretching energies of the film. The width of the rolled-up bilayer is larger for the films with higher values of the bending modulus and smaller values of the effective stretching modulus. Moreover, self-rolling phenomena of polymer layer was also explored as a template to fabricate metal, ceramic and metal/ceramic hybrid tube. In order to fabricate metallic and V bimetallic tube, the cross-linked polymer film is capped by metallic layer. After rolling, polymer template is removed by pyrolysis resulting in pure metal microtubes. The fabrication of silica and silica/gold hybrid tubes of high aspect ratio is also demonstrated. Polydimethylsiloxane (PDMS) is used as a precursor of the silica and it is converted into silica by pyrolysis at high temperature. Entire polymer moiety is also removed at this temperature. In order to fabricate hybrid tube of silica with gold, a thin gold layer is deposited on the polymer layer by physical vapour deposition. Self-rolling of polymer bilayers is a very convenient approach for interfacing the interior of microtubes with external electrical circuits and it can be used in particular for creating devices as micro-bubble generators exploiting electrolytic decomposition of fluids. A demonstration of microbubble generation inside the polymer tube is shown in this work. Possibility to functionalize the hidden walls of the tubes is one of the major advantages of the self-rolling approach. One can modify the surface of the film prior to rolling by magnetron sputtering of metal and upon rolling, tube and toroids with metallized inner surface could be obtained. The tube and toroids with metallic inner surface are promising for the future research as IR-frequency range resonators. Polymer and metallic microtubes fabricated by self-rolling approach may find applications in such fields as IR-waveguiding, microfluidics, enzyme bi-reaction, chemical and biochemical sensing. The silica and silica/gold hybrid tubes have potential use in optoelectronic devices and in catalytic applications

Index to NASA Tech Briefs, January - June 1966

Index to NASA technological innovations for January-June 196