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Several years ago, researchers reported the use of a photonic sensor to monitor the level of liquid in a container. This non-explosive sensor is based on the use of the fiberoptic shockwave sensor to detect position
What Are Photonic Crystals?
Photonic crystal fibers are a variation of fiber optics which present greater advantages than conventional optical fibers.
Optical fibers were a revolutionary advancement in the field of sensing, combining great sensitivity with precise guidance of systems based on light signals. Their adaptability to almost any geometry, low cost and efficiency led to their widespread use in sensor technology. This resulted in the development of robust, flexible, small but sensitive sensors which could be used for remote monitoring and in multiplex configurations. They are also fit for use in electromagnetic fields, high noise levels, electrical voltages and radioactivity, as well as in hot, corrosive or explosive media.
The chief drawback lay in the inherent limitations of silica-based fibers, which required the cladding and core to possess similar chemical, optical and thermal characteristics.
This was addressed through intensive research; however, the invention of photonic crystal fibers (PCFs) in 1996 has helped overcome these restrictions.
PCFs have a fiber along which air holes are arranged in periodic pattern and a hollow core. The waveguide properties of these fibers arise from the air hole pattern. The fact that PCFs are made of just one material and can be in several geometric shapes allows greater design adaptability.
The Advantages of Photonic Sensors
Why are photonic sensors being used in fluid level measurement? For one thing, their use precludes the need for electrical or optoelectronic components for signal conversion, near or inside the tank holding the fluid.
PCFs can have cores and cladding holes of varying sizes and locations, which allows features such as the spectrum of transmission, mode shape, nonlinear nature and birefringence to be fine-tuned. This means that the values attained go much beyond those possible with conventional optical fibers. Light propagation through air or the inserted fluids is also a possibility that can be exploited for sensing applications.
Photonic Sensors for Fluid Monitoring
The use of photonic sensors for various types of sensors is now well established and widely used for sensing of physical phenomena such as pressure, temperature, gas, strain and torsion, as well as for chemical sensing including various gases and biological molecules. However, the use of this technology for fluid sensing is still in the infant stage.
Detection of liquid chemicals is among the most difficult of industrial production tasks. It is also an important facet of production, as chemicals in liquid form are used in most industrial processes and require smart sensing to optimize safety.
Vapor sensing has been explored in terms of sensing various liquids. Another method uses absorption as the basis of a chemical sensing application, utilizing a hollow-core PCF. However, PCFs with liquid-filled cores permit high interaction between the sample and light, and are a promising new reliable sensing technology for fluids.
Some sensors use microstructured core PCFs filled with various chemicals, resulting in better field interactions for liquid sensing. However, using hollow-core fibers filled with liquid allows excellent adjustments to be made to the thermal and electrical properties of the fiber. A third possibility describes the use of infiltrated hollow-core fibers. Hollow-core PCFs filled with liquid have also demonstrated their capability to detect liquid mixtures.
Some recent innovations include the use of a lever with optical interferometry to produce a photonic liquid level sensor. The PCF is sandwiched between two optical single-mode fibers, while a rotatable lever linked to a fixed point measures the level using buoyancy. This type of sensor is simple to make, inexpensive and very sensitive.
Sources and Further Reading
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