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

A Fiber Optical Sensor For Non-Contact Vibration Measurements

This paper describes an intensity based optical sensor for the evaluation of accelerations from non-contact displacement measurements. Plastic optical fibers are used to collect the reflected light from several points on the vibrating surface, allowing the reconstruction of the vibration distribution. Two compensation techniques to reduce systematic effects due to the target reflectivity are also described and compared: one is based on the spectral analysis of the received optical signal and the other takes advantage of a reference displacement sensor. Experimental results in real conditions during vibration tests have demonstrated the capability to measure sub-micrometer vibration amplitudes up to about 40 kH

Assessment of a Dual-Wavelength Compensation Technique for Displacement Sensors Using Plastic Optical Fibers

The paper analyzes the performance of a dual-wavelength technique devised to compensate power fluctuations in intensity-modulated plastic optical fiber sensors, which were specifically conceived for the measurement of displacements in industrial and civil applications. These sensors retrieve the displacement from the variation of the attenuation along the light path and use two signals at different wavelengths to compensate for the effects of parasitic quantities, such as temperature and strains along the fiber. The theoretical behavior of the compensation technique is presented, and the results of experiments carried out with different combinations of signal wavelengths and plastic fibers are reported. The experimental setup has proved that, by proper choice of the compensation signal wavelength, it is possible to monitor displacements in the range (0 to 10) mm, even for low received power and under severe perturbation conditions, thus significantly improving the long-term stability of the sensor

Two-Dimensional Displacement Sensor Based on Plastic Optical Fibers

An inexpensive fiber-based displacement sensor for two-dimensional crack monitoring is proposed and analyzed. The device is packaged as conventional crack monitoring gages based on sliding plates and exploits the dependence of the transmitted power between facing optical fibers with the displacement. The use of multi-core polymeric fibers with high numerical aperture allows a compact form factor and simplifies the sensor assembly. The position detection algorithm has been optimized through simulations; then experimental tests have shown a good agreement with simulations and have proved that even with simplified layout and artisanal realization the sensor can measure displacements in a square area of 3 mm by 3 mm with an uncertainty better than 0.05 m

Static Characterization of Curvature Sensors Based on Plastic Optical Fibers

Sensors able to measure curvature changes are emerging as an effective alternative to the more common strain gauges for structural health monitoring applications. Particularly interesting is the all-optical fiber implementation for its unique properties and the possibility of being embedded. This paper, after a brief description of curvature sensors using plastic optical fibers, focuses on their characterization in applications where high sensitivity is required, and compares their performance with commercial strain sensors based on fiber Bragg gratings. The choice of plastic optical fibers allows the realization of simple, compact and cheap sensors. A characterization setup to test different sensor typologies is proposed and the main uncertainty contributions are investigated

Plastic optical fiber sensor for displacement monitoring with dual-wavelength compensation of power fluctuations

The paper describes the use of a dual-wavelength technique for compensating power fluctuations in intensity-based plastic optical fiber sensors for displacement measurements. The sensor consists of a fiber that collects the reflected light from a target and retrieves its distance from the signal attenuation. Besides a measurement signal at 450 nm that travels back and forth the displacement region and is attenuated according to the target position, a compensation signal at 650 nm propagates along the fiber and is back-reflected at the fiber tip by a dichroic filter. The compensation signal undergoes power fluctuations due to environmental stress of the fiber and is therefore used as a reference to correct the measurement signal. A demonstrator, which includes a sensor, the acquisition circuitry and signal processing software, was built and tested in laboratory and proved that the measurement signal is capable of monitoring displacements in the range (0 ÷ 10) mm even at low power, while the reference signal is not influenced by the position of the reflecting target and can therefore be used as reliable referenc

Design and characterization of curvature sensors based on plastic optical fibers for structural monitoring2013 IEEE International Instrumentation and Measurement Technology Conference (I2MTC)

Sensors able to measure curvature changes are emerging as an effective alternative to the more common strain gauges in structural health monitoring. Particularly interesting is the optical fiber implementation of such sensors and this paper discusses the design and characterization of a simple, cheap, and compact curvature sensor based on plastic optical fibers. A characterization setup is also proposed in order to test the sensor performances with dynamic deformation

Optical fiber sensors for long- and short-term crack monitoring

This paper describes a fiber displacement sensor and analyzes its performance in evaluating the evolution of cracks for structural health monitoring applications, both over short and long time periods. The sensor is based on plastic optical fibers, a choice that allows having all the advantages typical of optical fibers, but with costs comparable to the more common electro-mechanical systems. Two sensor topologies have been realized and some experimental results about laboratory and in field tests are reported to assess the effectiveness of the proposed solutio