The modeling of a temperature optical fiber sensor is proposed and experimentally demonstrated in this work. The suggested structure to obtain the sensing temperature characteristics is by the use of a mechanically induced Long Period Fiber Grating (LPFG) on a tapered single mode optical fiber. A biconical fiber optic taper is made by applying heat using an oxygen-propane flame burner while stretching the single mode fiber (SMF) whose coating has been removed. The resulting geometry of the device is important to analyze the coupling between the core mode to the cladding modes, and this will determine whether the optical taper is adiabatic or non-adiabatic. On the other hand, the mechanical LPFG is made up of two plates, one grooved and other flat, the grooved plate was done on an acrylic slab with the help of a computerized numerical control machine (CNC). In addition to the experimental work, the supporting theory is also included.
The modal characteristics of tapered single mode optical fibers and its strain sensing characteristics by using mechanically induced long period fiber gratings are presented in this work. Both Long Period Fiber Gratings (LPFG) and fiber tapers are fiber devices that couple light from the core fiber into the fiber cladding modes. The mechanical LPFG is made up of two plates, one flat and the other grooved. For this experiment the grooved plate was done on an acrylic slab with the help of a computer numerical control machine. The manufacturing of the tapered fiber is accomplished by applying heat using an oxygen-propane flame burner and stretching the fiber, which protective coating has been removed. Then, a polymer-tube-package is added in order to make the sensor sufficiently stiff for the tests. The mechanical induced LPFG is accomplished by putting the tapered fiber in between the two plates, so the taper acquires the form of the grooved plate slots. Using a laser beam the transmission spectrum showed a large peak transmission attenuation of around -20 dB. The resultant attenuation peak wavelength in the transmission spectrum shifts with changes in tension showing a strain sensitivity of 2pm/μɛ. This reveals an improvement on the sensitivity for structure monitoring applications compared with the use of a standard optical fiber. In addition to the experimental work, the supporting theory and numerical simulation analysis are also included.
Lately, there has been a huge demand for smart structures. In particular the interest has growth in those structures able to detect deterioration conditions and possible failure. Failure prevention requires an appropriate monitoring and maintenance system. Currently, there are available several types of sensors capable of detecting problems in structures, among them, sensors based on optical fibers have been proposed as they represent a non-invasive technique. Some optical fiber sensors are based on Bragg gratings. A grating is a periodical index perturbation of the fiber core which is most commonly achieved through UV radiation. Another technique used to fabricate the gratings, which has not been studied extensively, is electric arc. Therefore, in this work we propose the use of this technique to fabricate fiber optical sensors based on Long Period Fiber Gratings (LPFG). Manufacturing LPFG through electric arc has the advantage of being quite flexible, inexpensive, present very high temperature stability and can be applied to any type of optical fiber. LPFG with a period of 500 microns and 20 mm of length were fabricated through electric arc on standard monomode fibers with the help of a fusion machine and its spectrum was observed by an Optical Spectrum Analyzer (OSA). This type of LPFG is tunable by changing the fabrication parameters of the electric arc which in turns will vary its sensitivity to measure strain on structures when it is used as a sensor. Also, in this paper a theoretical and analytical examination of arc induced LPFG is presented. Mathematical analysis and simulation of the sensor based on LPFG were carried out using the software MATLAB.
This work presents preliminary results on wavelength sensitivity due to mechanically induced long period fiber grating (LPFG) on both standard single-mode and Er-doped fibers. The work presents and compares results for both types of fibers under different torsion conditions. In order to apply the torsion one of the fiber ends is fixed while torsion is applied on the other end. A LPFG whose period is 503μm is used to press on the fiber after the torsion, this will allow for micro curvatures to be formed on the fiber, which will in turn generate a periodical index perturbation on it. Here, it was noted that the rejection band shifts to shorter wavelengths for Er-doped fibers. It was detected that for torsion of 6 turns applied to 10cm doped fiber the wavelength peaks can shift up to 25nm, which is longer than similar results reported on standard fibers. Therefore, by using Er-doped fibers this technique will give more sensitive and accurate results on the real conditions of the structure under study. These results can be employed for sensing applications, especially for small to medium size structures, being these structures mechanical, civil or aeronautical. Theoretical calculations and simulations are employed for experimental results validation.
Fiber optic sensors are a mature choice for highly sensitive applications. Most modern pressure sensors are based on the
piezoelectric effect (pressure causes a material to conduct electricity at a certain rate, leading to a specific level of charge
flow associated with a specific level of pressure). In this paper, we describe theoretical calculations which predict
encouraging experimental results on pressure sensing with optical fibers. These results may be used in applications for
distributed sensors in structural health monitoring (SHM). The sensing fiber is capable of propagating 3 modes with a
straight fiber length of 30cm at a lambda of 1550nm. In our experiments, a perpendicular force of F=200gr cause a core
compression of nearly 2um, according to Poisson’s elastic coefficient for silica, which in turn provoked the loss of half
the number of modes indicating a 50% sensitivity as shown in our results included here. The proposed set-up intends to
measure force vs propagating modes in a standard single mode fiber. A full set of results will be included in our
The temperature sensitivity of a tapered Yb-doped fiber laser is numerically investigated. The laser rate equations are modified to analyze the output characteristics of the tapered fiber laser in the continuous wave regime under different temperature conditions. Numerical analysis shows that for different pump schemes, high sensitivity can be achieved when the pump power is reduced to close to the threshold value. Our results are reproducible and contribute new information to the development and optimization of tapered Yb-doped fiber lasers and temperature fiber laser sensors.