White Light Polarization Interferometry (WLPI) technology offers a great degree of flexibility in the design of various types of fiber optic sensors.
How does WLPI work?
WLPl technology stands for white Light polarization interferometry and is a technique used in fiber optic sensors. These can be used for measuring various parameters and are extremely suitable for use in heavy duty applications or for use in hazardous environments. These sensors offer a number of advantages over the standard fiber technology.
The white light polarization interferometry (WLPI) technology offers a great degree of flexibility in the design of various types of fiber optic transducers. Therefore numerous measurement and sensing applications can benefit from this advantageous feature combined with its outstanding performances. The WLPI technology is aimed to respond to the most demanding and hazardous applications! Therefore perfectly suited for civil, offshore and oil & gas industries.
Fiber optic sensors are made up of two main parts: the fiber optic transducer (also called the fiber optic gauge or the fiber optic probe) and the signal conditioner (also called the readout or the interrogation unit).
The fiber optic transducer is made of a customized body which contains an optical device that is sensitive to the physical magnitude to be measured, the measurand.
For non-distributed sensors, the sensitive part of the transducer is usually mounted at the tip of an optical fiber that connects to the signal conditioner unit. The latter is used for injecting light into the optical fiber, receiving the modified light signal returned by the transducer as well as for processing the modified light signal and converting the results into physical units of the measurand.
This figure shows the schematic design for each transducer of the specified measurand. For all of these types of transducers, a change in the magnitude of the applied measurand results into a change of the path length difference´s of the transducer sensing interferometer.
Therefore the path length difference can be thought as the output of the transducer although we know that the physical or real output is the light signal that carries the information about s.
There are different methods for fiber optic sensing which are based on the specific properties of the light radiation (intensity, phase, polarization, and spectrum) to be modulated by the measurand. Among them, optical interferometry, which concerns the phase modulation of the light radiation, is recognized as the most sensitive method for fiber optic sensing. Indeed, the interferometer is known as a very accurate optical measurement tool for measuring a physical quantity by means of the measurand-induced changes of the interferometer path length difference.
However, when using a narrowband light source (such as a laser source), the coherence length of the source is generally greater than the path length difference of the interferometer and therefore the measurement suffers from a 2? phase ambiguity, due to the periodic nature of the interferogram fringes.
This problem may severely restrict the measuring applications and this is why it has prohibited many interferometric fiber optic sensors to meet acceptance within the measurement industry. The phase ambiguity problem is avoided by using a light source with short coherence length that is a light source with a broadband spectrum.
In this case, the fringes of the interferogram are narrowly localized into a path length difference region so the variation of the path length difference can be determined without the 2? ambiguity by locating the fringe peak or the envelope peak of the interferogram. This type of interferometry is known as white-light or low-coherence interferometry.
Fiber Optic Sensor technology enables precise measurements to be made in the most challenging applications.