Veijo Lyöri

Fiber optic sensors for traffic monitoring applications

A novel fiber optic sensors for traffic monitoring applications are presented. They are useful for monitoring the traffic entering and leaving guarded areas, counting traffic on public roads as well as in determining lane occupancy at traffic lights. When installing sensor arrays, the sensors may also be used to provide additional information: measure the speed, wheel base of a vehicle and also to determine the number of axles and vehicle type. The sensors are based on optical fiber or cable, installed inside the asphalt layer of the road, to measure compression or vibrations near the road surface. Two sensor principles were used. One based on the microbending effect and another that utilizes the speckle phenomenon. In both cases the whole length of the fiber acts as a sensor. The microbending sensor requires a special fiber and special set-up, whereas in the speckle sensor a standard cable may be used. Both sensor types were tested in field applications where the harsh environment, especially the heat, sets great demands on installation. In these experiments, speed and vehicle type measurements were carried out with good results. In the paper, we will discuss the advantages and disadvantages of both sensor types and present some field test results. We will also show the benefits of these particular fiber optic sensors over traditional sensors.

Indicating cure and stress in composite containers using optical fibers

Long optical fibers embedded inside a composite material enable on-line monitoring of the manufacturing process and can also be used to assess damage by monitoring strain and cracks in the structure. We report on measurements of the microbending loss in long single-mode and multimode telecommunication fibers that were successfully installed inside the walls of composite containers during the normal manufacturing process. The work was largely empirical, and theoretical aspects were not investigated thoroughly. The measurements were carried out using the optical through power technique and a commercial optical time-domain reflectometer that gives not only the attenuation but also spatial information. We found that the coating material and its thickness have an effect on loading sensitivity and on the applicability of the method for cure monitoring. Although we discovered that repeatability issues are a problem, the measurement data indicate that the end-of-curing process of the composite can be determined by optical sensing fibers and damaging external loading of the structure can be detected.

Pulsed time-of-flight radar for fiber-optic strain sensing

This article describes a fiber-optic interrogation device based on the pulsed time-of-flight technique. The apparatus is capable of measuring time delays between wideband reflectors, such as connectors, along a fiber path with a precision of about 280 fs (rms value) and a spatial resolution of about 3 ns (0.30 m) in a measurement time of 25 ms. Potential application areas include measuring integral strain and its derivatives such as cracks, deflections, and displacements, particularly in large civil engineering and composite structures. The operation and basic blocks of the measurement system are presented in detail together with measurement results obtained in laboratory and field conditions. It is shown that by using a fiber loop sensor with a reference fiber, it is possible to achieve a strain precision below 1 microstrain and a measurement frequency of 4 Hz. System performance proved adequate for the study of both static and dynamic phenomena in a bridge deck.

Mirror fabrication on optical fibres using maskless excimer laser-assisted methods

This work deals with two novel methods for the fabrication of mirrors on single mode optical fibres (SiO doped with GeO 22 in the core). In both procedures, XeCl excimer laser pulses were utilised for either the activation—for consecutive chemical metallisation—of the core (additive process) or the ablative patterning of the preliminary chemically deposited metal film (subtractive method). In situ reflectance measurements (at 1350 nm) allowed us to tailor the metal growth and reach the proper mirror thickness. Both methods were found to be suited to create reflective copper coatings with maximum relative reflectivity of ;9 dB. Optical microscopy has been involved in the characterisation of Cu mirrors. 2002 Elsevier Science B.V. All rights reserved.

A High Resolution Reflectometer for Measuring Dynamic Strain in a Single Mode Optical Fibre

A high resolution, pulsed time-of-flight (TOF) reflectometer for measuring length and dynamic strain in a single mode optical fibre is reported. The object of the paper is to discuss the measurement principle and to present the results of some practical experiments, including a pull test on an OPGW cable and an elongation test on a standard telecommunication fibre with a dual-acrylate coating. Applications in the field of optical fibre-based strain sensing are also briefly reviewed.

A fibre-optic time-of-flight laser radar for measuring integral strain in a composite structure

A measurement system based on a sophisticated time-of-flight technique and a single mode optical fibre with internal reflectors has been developed for measuring integral strain inside a composite structure. The strain and spatial resolutions are 0.1 mm and 5 m respectively and the measurement time 1 s. The measurement system is widely tested with different types of composite structures.

Monitoring of railway embankment settlement with fiber-optic pulsed time-of-flight radar

This paper deals with a fiber-optic pulsed time-of-flight (PTOF) laser radar used for monitoring the settlement of a railway embankment. The operating principle is based on evaluating the changes in the lengths of the fiber-optic cables embedded in the embankment by measuring the time separation of the optical pulses reflected from both ends of the sensor fiber. The advantage of this method is that it integrates the elongation of the whole sensor, and many sensor fibers can be connected in series. In a field test, seven polyurethane-coated optical cables were installed in a railway embankment and used as 20-m long sensors. The optical timing pulses were created using specially polished optical connectors. The measured precision was 0.28 ps, which corresponds 1.8 μstrain elongation using a 20 m long sensor fiber, using an averaged value of 10,000 pulses for a single measurement value. The averaged elongation value of all sensors was used for cancelling out the effect of temperature variation on the elongation value of each individual sensor. The functionality of the method was tested by digging away a 7.5 m long and approximately 18 mm high section of sand below one sensor. It was measured as a +3 mm change in the length of the sensor fiber, which matched well with the theoretically calculated elongation value, 2.9 mm. The sensor type proved to be strong but flexible enough for this type of use.

Fiber optic time-of-flight radar with a submeter spatial resolution for the measurement of integral strain

This paper describes a fibre-optic interrogation device based on a pulsed time-of-flight (TOF) technique for the measurement of integral strain. The precision of the measurement system is 100 μm (1 ps) and it has a spatial resolution of less than 0.50 m (5 ns), achieved by the use of ultra-short probe pulses of about 500 ps, a GHz band receiver channel and a custom-made time-to-digital converter (TDC) implemented in a standard CMOS process. The TDC can simultaneously measure the distance to 9 reflectors (e.g., Bragg gratings) in the fibre core using the same optical pulse. Combined with a common receiver channel and an ultra-fast timing discriminator, this capability makes the system fast and stable, thus enabling both long-term and dynamic measurements. Potential application areas of the system include measurement of integral strain and its derivatives, especially in large civil engineering structures and composite materials. Pull tests with bare optical fibres have demonstrated that the obtained results are in good agreement with those of a reference sensor.

Determination of far-field strain in a composite structure using time-of-flight and Fabry-Perot optical fiber sensors

Measurement system based on a time-of-flight technique and Fabry-Perot optical fiber sensors have been used to determine the strain state in the composite structure. In theory the optical fiber causes a local disturbation to the structure such that measured strain slightly differs from the farfield strain. However, the sensors used in this study were insensitive to the disturbance.

Experiences from Embedded Optical Fiber Based Cure and Stress Sensing in Composite Structures

We report here on experiencies with optical fiber embedding and cure monitoring in a composite container manufacturing process, and with external stress measurements. Microbending loss in long single-mode and multi-mode telecommunication fibers was investigated. The measurements were carried out using the optical through power technique and a commercial optical time domain reflectometer. Simple miniature size electronics, applicable even for embedding was developed for example as an alarm indicator. We found that the coating material and its thickness have an effect on loading sensitivity and on the applicability of the method for cure monitoring. Though we discovered that repeatability issues are a problem, the measurement data indicate that the end of the curing process of the composite can be determined by optical sensing fibers. Further, sensing fibers in different wall layers can be used to follow the progress of the curing inside the wall at different depths. The response of damaging external loading on the structure can also be detected.