Fiber Optic Sensors
As optical fiber cables carry the light signals in it, so it is necessary to convert electrical signals into light signals and light signals into an electrical signal. Optical sensors are the devices used to converts light rays into electronic signals.
The physical quantity of light is measured and then converted into a form that is readable by an instrument. Optical sensors are part of a larger system that integrates a source of light, a measuring device, and an optical sensor. This is often linked to an electrical trigger. The trigger responds to a change in the signal within the light sensor. An optical sensor measures the changes from one or several light beams. When any change occurs, the light sensor operates as a photoelectric trigger and therefore either increase or decreases the electrical output. Optical sensors can measures temperature, pressure, Flow, Liquid level, displacement, vibration, rotation, etc.
Digital fiber optic sensors
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Fiber optic sensors working principle
The general structure of an optical fiber sensor system is composed mainly of four components that are the source, fiber optic, transducer, and detector. Optical sources can be Laser, LED, Laser diode, etc., optical fiber type can be single mode or multimode, sensing, or modulator element which convert the measured value to an optical signal. And then optical detector and actuating circuitry are processing electronics, oscilloscope, optical spectrum analyzer, etc. In the FOSs devices, the optical parameters that can be modulated are the amplitude, phase, color (spectral signal), and state of polarization.
Fig.Basic building blocks of an optical fiber sensor system
The optical modulation methods of the sensors can involve the following
Intensity-modulated sensors: The variation of light intensity is proportional to the perturbing environment that can be detected by sensors. The concepts associated with intensity modulation comprise transmission, reflection, and micro bending. For this, a reflective target can be incorporated into the fiber.
Phase-modulated sensors: The phase of the light in sensing fiber is compared to a reference fiber in a device known as an interferometer. This sensor uses a coherent laser light source and two single-mode fibers. The light is divided and injected into the reference and sensing fibers. If the light in sensing fiber is exposed to the perturbing environment, a phase shift appears between them. The phase shift is detected by the interferometer. Phase modulated sensors are more accurate than intensity-modulated sensors.
Intensity Based Fiber Optic Sensors
FOS are often grouped into two basic classes according to the manner in which the optical fiber is used, the sensor can either be an intrinsic one if the modulation takes place directly in the fiber or extrinsic if the modulation is executed by some external transducer.
Intrinsic fiber optic sensor (IFOS): Intensity, polarization, phase, wavelength, or transit time of light can be modulated with (IFOS). The sensors which modulate light intensity generally tend to use multimode fibers, while only mono mode cables are used to modulate other parameters.
An interesting feature of IFOSs is that they can provide distributed sensing over the distances of up to 1 meter. In Intrinsic sensors, the variable of interest (i.e.physical perturbation) must modify the characteristics of optical fiber to modify the properties of light carried by fiber, Intrinsic FOSs directly employs an optical fiber as sensitive material, sensor head, and also as the medium to transport optical signal with information of perturbation environment to be measured.
They operate through the direct modulation of light guided into an optical fiber. These sensors can use interferometric configurations, Long-Period Fiber Grating (LPFG), Fiber Bragg Grating (FBG), or special fibers (doped fibers) designed to be sensitive to specific perturbations. Light intensity is the simplest parameter to manipulate in intrinsic sensors because it requires only a simple source and detector.
Extrinsic fiber optic sensor (EFOS): Optical fiber is simply used to guide light to and from a location at which an optical sensor head is located. The sensor head is located external to the optical fiber and is typically based on miniature optical components, which are designed to modulate the properties of light in response to changes in the environment with respect to physical perturbations.
Therefore, in this arrangement, one fiber transmits optical energy to the sensor head. Then this light is well-modulated and is coupled back via a second fiber, which guides it to the optical detector. Extrinsic fiber optic sensors use normally a multimode cable, to transmit modulated light from a conventional sensor. A major feature of extrinsic sensors, which make it so useful in such a large number of applications, is its ability to reach places that are otherwise inaccessible.
Fiber Optic Sensor Types
Fiber optical sensors are devices that can work in harsh environments where conventional electronic and electrical sensors have difficulties. They can sense various physical properties, like pressure, position, strain, chemical changes, magnetic and electric fields, flow, vibration, light level, radiation, and color.
• Chemical sensor: Chemical sensors are measurement devices that convert a chemical or property of a specific analyte into a measurable signal, whose magnitude is usually proportional to the concentration of the analyte.
• Temperature sensors: A temperature sensor is an electronic device that measures the temperature of its environment and converts the input data into electronic data to record monitor or signal temperature changes.
• Strain sensors: have been designed for application areas in which there is a need to measure the deformation due to external forces acting on already existing, massive components. The device is simply screwed onto the component.
• Biomedical sensors: spectroscopic biomedical sensors, CO2, O2, and PH are often measured simultaneously, flow monitoring by laser dopplerimetry.
• Electrical and magnetic sensors: Magnetic sensor definition is a sensor that is used to notice disturbances as well as changes within a magnetic field such as strength, direction, and flux. There are different types of detection sensors that may work on a number of characteristics like light, pressure, temperature. These sensors are separated into two groups. The first one is used to calculate the total magnetic field, whereas the second one is used to calculate vector components of the field.
• Pressure sensors: is a device for pressure measurement of gases or liquids. The pressure is an expression of the force required to prevent fluid from expanding and is typically stated in terms of force per unit area.
. • Displacement and position sensors: It is one among the primary optoelectronic simple sensors that believe the change in retro reflectance due to a proximal mirror surface, also referred to as liquid level sensors.
Fiber-optic sensors exhibit a number of advantages:
• They contain electrically insulating materials (no electric cables are required), which makes possible their use e.g. in high-voltage environments.
• They will be safely utilized in explosive environments, because there’s no risk of electrical sparks, even within the case of defects.
• They’re resistant to electromagnetic interference (EMI), even to nearby lightning strikes, and don’t themselves electrically disturb other devices.
• Their materials are often chemically passive, i.e., don’t contaminate their surroundings and aren’t subject to corrosion.
• They need a really wide operating temperature range (much wider than is feasible for several electronic devices).
• They need multiplexing capabilities: multiple sensors during a single fiber line are often interrogated with one optical source.