Conectronica Revista Maganize industrial de electronica

The optical fiber sensors constitute a very old field of investigation. In fact, great part of the investigation in photonics has concentrated in the areas of instrumentation and sensors, and manifolds developments of the optoelectronic industry arose indeed for applications of sensors. The reduction in the losses of the fiber, as well as the improvements of quality, sensitivity and dynamic margin of the fiber sensors have turned, them into ideal substitutes of the used traditional sensors in measures of rotation, acceleration, electric fields and magnetic, temperature, pressure, acoustic vibration, chemical position, pressure, humidity, substances, etc.

Their main advantages are in which is measurement techniques which they require of a minimum space, usually they are noninvasive and they are controlled of remote form, they work in hostile surroundings, and the devices are of low weight and are immune to the electromagnetic interferences. In this article we commented some basic principles of its operation and coverall we presented/displayed commercial product examples and applications. Half shared but not as much In the optical fiber sensors the information of the parameter to measure comes determined by a change in the phase, the polarization, the frequency or the intensity of the optical signal (as well as any combination of them). The sensors that detect changes in the intensity of the signal are very simple. However, those that work with the phase, the polarization or the frequency modulation are more complex, because the signal must be processed previously since the photoreceptor only detects optical power. In these cases interferometric structures of the Mach type are used, Michelson, Fabry-Perot or Sagnac, that provide a high sensitivy as well. More recently, networks of Bragg like sensors for spectral measures or discrimination of wavelengths are also being used. Basically, the fiber sensors can be classified in two categories. Those in which the optical fiber is only used like means to guide the light from the emitter to the sensorial element, and from this one last one to the photodetector (called extrinsic). Or those in which the own fiber is used like sensorial element and of reference (called intrinsic). The different propose configurations to date are innumerable. Next we will comment some of most representative.

Between the interferometric structures, most typical it is the Mach interferometer. Its application as sensorial one imagines in figure 1. A coherent optical source generates a signal that is divided in two ways by means of a directional coupler of 3 dB. One of the ways is the one of reference, whereas in the other a transducer exists that turns certain physical parameter of measurement into a change of phase of the optical signal. Later, both signals are again combined in an exit coupler, obtaining themselves two signals (sum and difference) that will be received by individual photodetectors. Of this way, the changes of phase of the optical signal of one of the branches of the interferometer can be moderate from the optical powers in both detectors. The equations come given by:

being L1 and L2 the lengths of the arms of the interferometer, k the constant of propagation and DF the phase angle produced in the transducer. On the other hand, with regard to the intensity sensors, also they exist multitude of possibilities for his implementation. Between the known techniques more we can enumerate: interruption of the light by displacement of a lamina (figure 2), relative displacement of two fibers, modulation of the losses of the nucleus or the cover by curvature or evanescent mode coupling with another fiber, etc. Like example, in figure 2 are the foundations of the technique based on a lamina that prevents the connection of light between two fibers. A sensible mechanism to certain physical parameter (p.ej pressure or vibration) in this way moves to the lamina in major or minor degree, modulating the optical intensity that is detected when coming out of the system.

Products and applications

As one has already commented previously, the fiber sensors find application in multiple fields. Next we presented/displayed some examples of it, along with the companies commercialize that them. The high prices of the energy and the necessity to restitute the reserves, are stimulating to the oil companies to invest in heavy petroleum deposits. Heavy and viscous petroleums present/display challenges in the analysis of flowed and obstacles for their recovery, that are being surpassed with the new technology and the modifications of the methods developed for conventional petroleums.

The method of gravitational drainage attended by steam (steam-assisted gravity drainage, SAGD) works for extra-heavy petroleums. A pair of parallel horizontal wells is perforated, being placed a well about 5 to 7 ms over the other. The steam injected in the well superior warms up heavy petroleum, reducing its viscosity. The gravity causes that mobilized petroleum flows in descendent sense towards the horizontal producer inferior. The initial communication settles down between the injector and the producer by means of injection of steam, cyclical steam or injection of reliable. The recovery factor considered for this method oscillates between 50 and 70%. However, the stratification of the formation can affect recovery SAGD significantly. Method SAGD is used in many fields of Canada, including the fields Christina Lake and MacKay River. In figure 3 a schematic diagram of method SAGD for heavy petroleum extraction can be seen. Evidently, the control of the pressure and the temperature has a great importance in this process, reason why it is required of sensors for his measurement. The Canadian company Inc. Opsens, leader in the manufacture of fiber sensors, has developed products for this concrete application. For example, the sensor of pressure and temperature OPP-W, based on a Fabry-Perot interferometer. Their specifications are in table I. In combination with the fiber cable WFC (rigid cable of 8 fibers for hostile surroundings) and the measuring equipment WellSens (interferometry of polarization of white light), the processes of heavy petroleum extraction can be monitored of effective form (figure 3), as well as any other type of similar application. Image The pressure sensors also find application in the field of the medicine. An example of it is the fiber sensors of company FISO Technologies Inc. (figure 4).

Their applications go from measures of pressure of flowed of the human body in hospitable interventions critics to animal tests in surroundings of high level of electromagnetic interferences. Models FOP-MIV and FOP-M260 are obtained from technologies of micromanufacture of silicon, reason why they are smaller, more precise, more trustworthy and easier to use than the traditional catheters. Additionally, sensor FOP-F125, with a diameter of only 125 microns, is the smallest sensor available commercially for these applications. One is placed directly in the end of the fiber, without needing using no type of adhesive, reason why it allows his integration in minimumly invasive medical and surgical devices. Finally, another example of company that makes fiber sensors is the one of Swiss SMARTEC. In this case it has a line of sensors based on networks of Bragg (FBG) to multiple applications. Among them we can find accelerometers with ranks of measurement of 15g or sensors of deformation for the monitoring of structures (figure 5).

The deformation sensors consist of transducers that transform static or dynamic variations of distance into changes in the wavelength reflected in a FBG submissive tension. The sensor is made up of active and passive parts. The active element contains the fiber that measures the deformation between two ends, obtaining itself a displacement in the wavelength of the FBG based on the distance variation. On the other hand, the passive element, that is insensible to the deformations, is used to connect the sensor with the reading unit. Optionally, an additional FBG for the measurement of temperature and its corresponding compensation can be installed. More companies exist many than they count on fiber sensors in his product catalogue. The applications of the same are innumerable, but like it has seen, his advantages with respect to the traditional sensors it is what it is facilitating that every time are gotten up more in diverse sectors of engineering, the medicine or the society generally.

Author: Francisco Ramos Pascual. Doctor Ingeniero de Telecomunicación. Head lecture of the Polytechnical University of Valencia. Email: framos@upvnet.upv.es This electronic mail is protected against robots of Spam, needs to have activated Javascript to be able to see it