Dense wavelength division multiplexing has been proposed as a means of implementing communications on aircraft. In such applications, power consumption is a critical consideration. The impact of reducing the transmitter power and recovering losses using a shared amplifier has been investigated. By recovering the power loss using a shared amplifier transmitter, power savings can be made. This network has been modeled and savings of 20% are predicted in a realistic aircraft environment.
A method for performing optical beam shaping in the near-field region using diffractive optical elements generated by
Fresnel based Phased Optimised General Error Diffusion algorithm (POGED) was developed and investigated by means
of numerical simulations. POGED was found to deliver significantly higher signal to noise ratio than iterative
Gerchberg-Saxton type algorithm.
Semiconductor-based optical amplifiers (SOAs) offer solutions to a variety of amplification needs covering wavelengths ranging from of 0.6 to 1.6 µm. Gain adjustment, through the bias current, enables automatic power control to be implemented. However, this requires knowledge of the signal strength. The amplified spontaneous emission power, particularly in high gain SOAs, can be significant with respect to the signal strength, and therefore additional components may be required to derive an accurate measure of the signal strength. This increases both the complexity and cost of implementing automatic power control (APC). We report on a method for estimating the signal strength based on measurement of the total output power and the SOA drive current. The method is extendable to other methods of optical amplification, e.g., erbium-doped fiber amplifiers.
The semiconductor optical amplifier (SOA) is a versatile component that can be deployed to meet the expanding applications associated with the introduction of additional functionalities at the optical level in wavelength division multiplexed systems. The future network requires low cost, small footprint, directly controllable amplification throughout the different application layers from long haul through to metro; the intrinsic size and integration capability advantages will ensure that the SOA plays a key role in this evolution. In multi-wavelength gating/amplification applications the gain dynamics, oscillating at timescales comparable to that of the data which is being amplified, introduce issues of pattern dependent waveform distortion (patterning) in single channel, and inter-channel cross-talk in multi-wavelength cases which require management through careful SOA design and understanding of the network application scenarios. In this paper, an optical linear amplifier (OLA) architecture with the unique capability to provide variable gain whilst maintaining linear operation at high output saturation powers will be described. Initial characterisation results for the OLA will be presented.
This paper presents the operation of an optical current transducer (OCT). The OCT operates by measuring changes in the polarization state of light traveling through a magneto- optic sensor head exposed to the magnetic field of a current carrying conductor. The changes in polarization state are directly related to the magnitude of the current flowing within the bus bar but ia also influenced by external environmental parameters such as temperature and mechanical disturbance. Engineering solutions to these influences have been implemented. Thermally induced errors of greater than 25% are shown to be corrected to less than 0.2% over a range of temperatures from -45 degree(s)C to 140 degree(s)C. In addition to this, signal waveform distortion resulting from mechanical vibration is automatically reduced by more than 30 dB over a range of frequencies from 0 Hz to 5 kHz. This paper reports on the design of this OCT and on detailed investigations carried out at the British Short-Circuit Testing Station to establish the operational characteristics of the OCT when subjected to transient current signals often with a significant dc component.
The combination of chemically sensitive, swellable polymer materials with novel optical fiber cable designs to transduce the swelling activity into microbend loss enables a simple yet powerful sensor to be produced. Interrogating such cables with standard optical time domain reflectoctrometry (OTDR) instruments allows particular chemicals of interest to be detected and located along a cable which may extend to several kilometers. We report here on a sensor cable which uses a water swellable material, a hydrogel, to detect positions of water ingress, relative humidity level or pH value. In direct water ingress tests, wet sensor lengths as small as 5 cm in several hundreds of meters have been detected using conventional OTDRs. Following a review of the sensor design, we present the results of an investigation of the mechanical interaction between the hydrogel polymer and the optical fiber within the sensor. The behavior of the sensor is then characterized within environments of different relative humidity levels from 70 percent to 100 percent at temperatures ranging from 0 to 60 degrees C. The sensor was initially designed for applications within civil engineering but can be applied to a much broader range of measurement requirements, for example soil moisture measurement. We will report details on experimental observations on concrete cure within reinforcing tendon ducts and soil humidity measurements within different soil types.
Optical fibre sensors have enormous potential as an important technology in structural monitoring. Their particular advantages include the ability to make distributed measurements, immunity from electromagnetic noise and pick-up and the capability of operating over large areas and/or at interrogation distances of many kilometres. This paper will, first of all, examine the requirements for structural monitoring in both civil engineering and composite structures for aerospace and underground transportation. The benefits and disadvantages of fibre optic techniques will then be explored and the general features of important contributory technologies including fibre Bragg gratings, microbend sensing, interferometric and sub-carrier sensors and non-linear optical techniques, e.g. stimulated Brillouin scatter will be highlighted and their applications and properties described. The paper then examines some specific applications in each of these two principal sectors and concludes by speculating on future prospects for fibre optic sensing and structural assessment.
Numerous optical fibre sensing techniques have been evaluated to monitor composite material structures. Most of these sensor systems aim to measure responses to static loads or vibrational spectra. An alternative approach is to monitor the ultrasonic signature over structure and evaluate changes in this signature in terms of modifications to structural properties. This paper describes the principles and implementation of such a system. A simple line integrating interferometric optical fibre sensor is used to detect the propagation of Lamb waves at typically 250kHz within the composite material. The wavelength of these Lamb waves (typically 2cm) exceeds the dimensions of structural defects of interest to the optical fibre sensor is, in effect, detecting changes in the ultrasonic scattering signature of the test structure and relating these changes to the evolution of faults or damage within the structure. The paper will present preliminary results which explore the basic sensitivity mechanisms within the optical fibre sensor and examine the modifications to the ultrasonic signature with the introduction of predetermined damage and deterioration. The eventual aim is a fully integrated system which will enable “plug in” structural testing. These preliminary results indicate that such systems concepts are a medium term prospect.
This paper describes some initial results of an evaluation technique, applied to composite plates, which effectively measures the ultrasonic transfer function of these plates and interprets these transfer functions in terms of damage thresholds within the material samples. The samples are probed using Lamb waves at frequencies in the region of 250 kHz. The detectors are interferometric optical fiber systems embedded within or mounted on the surface of the sample. The system is sufficiently sensitive to detect damage and/or structural deterioration before its magnitude is sufficient to compromise structural integrity.
The measurement of quasi-static strain field using optical fibers presents a considerable challenge due to the inherent sensitivity of optical fibers to temperature. This paper summarizes recent work we have carried out on two approaches to this problem. Dual mode polarimetric measurements were investigated as a means of implementing distributed temperature measurements and radio frequency subcarrier sensors have been used to perform the same measurement on an integrated basis. These techniques are contrasted and assessed against other technologies such as Bragg gratings and dispersive Fourier transform spectroscopy on the basis of measurement capability, ease of implementation and technological maturity.
Optical fiber sensors for monitoring of structures (OSMOS) is a European collaborative research project which has, over the past three years, embraced a number of technological issues related to the problem of structural monitoring in the civil engineering and aerospace industries. A key technical objective of the program was the measurement of temperature and strain using a single sensor length. A laboratory prototype using the differential sensitivities of polarimeters based on the fundamental, LP<SUB>01</SUB> mode and the first higher order LP<SUB>11</SUB> mode of polarization maintaining fiber demonstrated parameter recovery to within 2 C and 5 (mu) (epsilon) . A receiver enabling quasi-distributed measurements to be made with a linear spatial resolution of 70 cm using white light polarimetry was assembled. White light polarimetry was also used in conjunction with pressure sensitive fiber to detect impact damage on a composite radome structure. Impacts of 5 Joules in magnitude were detected with a spatial resolution of around 1 cm. Microwave radio frequency subcarrier measurement techniques were used to develop the engineering processes necessary to integrate optical sensors into civil engineering structures for simulated applications trials. This enabled issues such as stress transfer, mechanical bonding and sensor protection to be addressed. For the aerospace industry, embedding of optical fiber sensors remains an important issue. Here we developed techniques for embedding connectorized fibers such that the component could be machine finished after curing, an important feature of the manufacturing process.
Over the past twenty years considerable advances have been made in new optical fiber sensor technology able to measure physical parameters as a function of linear position. Such advances are of interest to the civil engineering community who have the problem of implementing measurement systems which can provide data on the structural state of health of large constructions such as bridges. This paper reviews the background to the measurement problem and outlines the relevant optical fiber techniques which have been considered in this context. The potential and the limitations of these measurement approaches is discussed.
The simple observation that the temperature coefficient of most structural materials in which strain may need to be measured differs from that of optical fiber measuring systems indicates immediately that both temperature and strain fields should be mapped with comparable accuracy in order to arrive at a reliable indication of mechanical strain. This paper initially defines the criteria for adequate compatibility between temperature and strain field measuement and then compares the techniques which have emerged in the past five years to address this problem. All techniques require the measurement of two optical parameters which are typically differential delays e.g. in interferometers and grating sensors or combined dellay and dispersion characteristics. The basic features of these measurement techniques will be addressed and comparisons made between the applicability of the various techniques. As an example, our own work measured temperature and strain to within +/- 2 C and 10 (mu) (epsilon) . Other work, e.g. with Bragg gratings achieves +/- a few tens of (mu) (epsilon) and a few degrees centrigrade.
The concept of the smart structure integrates structural engineering, sensing, control systems and actuation to provide a mechanical assembly which is capable of responding to its environment and/or loading conditions. The realization of the smart structure requires integration of skills in a variety of scientific and engineering disciplines ranging from mechanical engineering through materials science into signal processing, data analysis, sensing and actuation. The sensing technology must have a number of key features of which the ability to take distributed measurements of various parameters throughout the structure is paramount. Fiber optics technology therefore promises to have a significant role to play in the evolution of the smart structures concept. This paper analyses this role in detail, presents an assessment of the current state-of-the art in fiber optic technology related to smart structures and presents a scenario for future developments.
This paper will present a brief overview of the current activities in fiber optic distributed, integrating and quasi distributed measurements within the University. The research focuses on applications in structural monitoring and includes a distributed moisture ingress monitor and some simple systems exploiting special cable structures to implement specialized strain and distance measurement for self surveying structures.
This paper reports on the basic design and preliminary evaluation of an entirely novel cable configuration which enables the detection of water, pH or similar variables as a function of position along the length of an optical fibre. This sensing capability is realised through a combination of Optical Time Domain Reflectometry (OTDR) and a microbend transducer activated by chemically sensitive water swellable polymers (hydrogels). Experiments with a water sensor prototype have demonstrated the detection of wetted sections of less than 25 cm length in cables longer than 100 m and indicate that interrogation of sensors several kilometres long is possible. The present experiments have demonstrated the principal of measurement through the development of a distributed water detector. However the technique can be used to monitor various chemical parameters such as pH or ionic concentration by selecting the appropriate gel as the responsive medium.
Many attempts have been made to use optical fibres to measure strain or temperature. However, since fibres are sensitive to both strain and temperature, practical strain measurements require that the effects of temperature are simultaneously measured or compensated for. This necessitates the measurement of two parameters which respond differently to changes in strain and temperature, a condition which may be met using fibres operating in the dual mode regime.
We report on preliminary experimental trials aimed at assessing the suitability of a distributed water monitor as a means of determining the presence of grout in reinforced tendon ducts of civil engineering structures. This sensing capability is realized through a combination of Optical Time Domain Reflectometry (OTDR) and chemically sensitive water swellable polymers (hydrogels). This form of sensor cable can detect water as a function of linear position along its length with a spatial resolution of a few centimeters<SUP>1,2</SUP>. The experiments carried out here indicate that this approach has considerable potential as a means of providing quality assurance during the grouting procedure.
At present, there is an acute need for techniques in monitoring civil engineering structures, and optical fiber sensors are acknowledged to be amongst the best candidates. For more than ten years, interferometric optical fiber sensors have been widely investigated and now provide a rich extended basis for measuring strains experienced by structural elements. However, because of their periodic response, those sensors need extending measuring techniques to fulfill civil engineering requirements. Amongst different methods, Thomson-CSF and the University of Strathclyde have recently employed a microwave subcarrier system . A specific sensor dedication to the arena of large civil engineering structures has been designed and tested.
This paper describes the framework and objectives of the OSMOS (Optical fiber Sensing system for MOnitoring of Structures) project. OSMOS is a CEC funded BRITE project with the aim of demonstrating the industrial feasibility of manufacturing optical fiber smart structures for Civil Aeronautics and Civil Engineering. The sensor concepts and associated technological issues to be addressed are presented in connection with the specific applications investigated in the framework of this project.
Determination of the state of cure of epoxy resin based systems is of considerable interest to manufacturers of large carbon fiber reinforced plastic and glass reinforced plastic structures. Optical methods designed to indicate the cure state have been developed using a loss mechanism which is a function of the refractive index of the curing system. Such techniques are however subject to corruption from losses arising from other influences and consequently are limited in their measurement resolution. In this paper, two techniques which are able to provide a high degree of accuracy of measurement of refractive index are investigated as a means of performing cure measurements. The methods investigated involve the interaction of the evanescent field of a side polished optical fiber with an overlay waveguide or a surface plasmon. Coupling between the fiber and the overlay waveguide (or plasmon) is strongly influenced by the refractive index of the bulk superstrate above the overlay (in this case the curing resin system). Both sensing schemes are self referencing and are not influenced by loss.
A collaborative European Programme N degree(s) RI 1B 0173-C(CD) under the auspices of BRITE (Basic Research in Industrial Technologies for Europe), jointly sponsored by the Commission of the European Communities and European Industry, was launched in 1988 to explore and develop an optical sensor network embedded in composite for measuring the strain and temperature distributions. Its objectives and first results were presented at the `Fiber Optic Smart Structures and Skins II' conference (Sept. 1989, Boston). This paper will describe the work and the main results which have been obtained since then. Three main areas have been covered which have concerned the implementation of a coherence based parallel quasi- distributed sensing system, the simultaneous measurement of temperature and strain and the mechanical properties of composite material with embedded sensor. All results have shown the high interest of such an optical sensing network for structure monitoring.
A technique based upon the differential sensitivities of dual mode and polarimetric sensing schemes is shown to be capable of resolving simultaneously temperature and strain variations to within 20 micro-epsilon and 1 K over a strain and temperature excursion of 2 micro-epsilon and 45 K. The technique is evaluated experimentally over an 80 cm sensing length of unembedded optical fiber and in an 8 ply unidirectional carbon/epoxide laminate subject to temperature and strain cycling. A comparative analysis of the performance of the embedded and the unembedded fiber sensors is presented.
The paper proposes polarimetric and two-mode differential interferometric schemes incorporated in an elliptical-core fiber to resolve strain and temperature simultaneously. Initial results indicate accuracies of +/- 10 microns/m and +/- 5 C for strain and temperature measurements, respectively. A technique, based on the evaluation of the condition number of a matrix, is shown to be useful in evaluating comparative merits of multiparameter sensing schemes. Experimental results for four fibers are presented, and cross-sensitivity issues are discussed.