05-07-2013, 12:39 PM
Introduction to Fiber-Optic Sensors
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Fiber-Optic Sensing Basics
The telecommunication industry has significantly changed due to recent advances in fiber optic technology [1]. It was possible to carry gigabytes of information at the speed of light, and there were improvements and cost reductions in optoelectronic components. The emergence of fiber-optic sensors (FOS) began when designers tried to combine the product outgrowths of fiber optic telecommunication with optoelectronic devices.
Besides, the loss in fibers was greatly reduced and the sensitivity of detection of losses increased making it possible to sense changes in phase, intensity and wavelength from outside perturbations on the fiber and this marked the birth of fiber optic sensing. The mechanisms that fibers are made to be immune against in telecommunication systems are now made to be the ones that fibers should be sensitive to for sensing applications.
Compared to their electronics counter parts, FOS have a number of advantages which make them more suitable for use in many sectors. These include immunity to electromagnetic interference, insulation against electric current, robustness and more resistance to harsh environments, high sensitivity. There are also other important features like chemical passivity, wide operating temperature range, multiplexing capabilities to form sensing networks, light weight and easy integration into a variety of structures.
Spatial Resolution
It is the measure of the smallest separation of distance between two points over which the sensor
can identify any sensible change in spatial variation of the meausurand (strain, temperature) to be
detected.
For a distributed fiber optic sensor, it is often defined either as the minimum distance over which
the system is able to indicate the value of the measurand within the specified uncertainty
(measuring spatial resolution) or as the minimum distance that generates results which are within
10% of the measurand transition amplitude (detection spatial resolution).
Fiber Bragg Grating Based Sensors
Fiber Bragg Gratings (FBGs) are simple, intrinsic sensing elements which can be photo inscribed into a silica fiber and have all the advantages of a silica fiber [5]. They have an inherent self-referencing capability and are easily multiplexed along a single line. They are widely used in distributed embedded sensing in smart structures, as the sensing elements in grating based chemical sensors, pressure sensors and accelerometers.
In the following sections, an introduction to FBG manufacturing is presented which is then followed by the operating principle and a few basic implementation schemes for fiber optic sensors based on FBGs.
FBG Manufacturing Techniques
There have been a number of techniques for the manufacturing of FBGs, all of which are based on the basic principle of photo-induced refractive index changes. That is, the index variation is “written” or “inscribed” systematically into the core of a special type of optical fiber using an intense ultra-violate (UV) source. Mostly, a photosensitive germanium-doped silica fiber, so that the refractive index of the core changes with exposure to UV light, is used in the manufacturing of FBGs.
Normally A higher amount of germanium concentration is needed to produce strong gratings but standard fiber that is pre- soaked in hydrogen to enhance its photo-sensitivity can be used. Another method of manufacturing is using a photomask which possesses the intended grating features. By placing the photomask between the UV light source and the photosensitive fiber, the grating structure is determined by the transmitted intensity of light striking the fiber. Chirped FBG are produced using this technique.
FBG Interrogation Methods
Fig 2.2 shows one of the most commonly used and basic detection schemes for FBG-based point
sensors. It consists in a broadband optical source followed by a coupler that is used to carry the
optical signal to the FBG and leads the light reflected back from the DBG to the detector. The
scheme shows both the reflective and transmission detection options for detection and hence
there is also another detector at the end of the span. The detector in each case is used to monitor
the shift in the wavelength which is caused by a change in the environmental parameter.
The most commonly used method of interrogating one or more FBGs is based on passive
broadband illumination of the FBG with light of wavelength range covering that of the specific
FBG and analyzing the reflected narrowband signal or the notch in the transmitted signal.
Usually the wavelength shift measurement is not very straight forward and hence a very general
principle is to convert the wavelength shift to some easily measurable parameter such as
amplitude, phase or frequency.
Matched Filter Fiber Grating
In this method, whose basic scheme is shown in Fig.2.4, light from a broadband source is input
to the sensor grating via a fiber coupler [8]. The light reflected from the sensing FBG then
propagates to the receiving grating, which is mounted on a piezoelectric stretcher. Both the
receiving and sensor gratings are fabricated so that they both have the same central reflecting
wavelength. When the sensor grating is exposed to varying temperature or strain, its center
wavelength varies in proportion to the measurand and hence in practice the two center
wavelengths will not be identical.
If a Piezoelectric transducer (PZT), is used to derive the change in central wavelength of the
receiving FBG, at one point in the sweep the reflection wavelengths of both FBGs will match. In
this condition a strong signal will be reflected from the receiving grating and detected by the
photodiode. Then, the instantaneous wavelength value of the sensor grating could be determined
from knowledge of the PZT driving voltage and the wavelength of the receiving grating.