WLPI Fiber Optic Sensing Solutions

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  • WLPI fiber optic technology sets standards in precision and flexibility

    • Fiber optic white light polarisation interferometry (WLPI) is a patented fiber optic technology
    • It enables precise measurements to be made in the most challenging of applications
    • It offers maximum flexibility in sensor design such that reliable measurements can be generated even in extremely unfavourable environments

    wlpi fiber optic measurement solutions

    Advantages compared to fiber Bragg technology:

    • Easier installation and handling
    • Easier adjustment to requirements and flexibility
    • Greater stability and better reliability
    • Improved safety
    • Maintenance-free
    • Lightweight, compact design
    • Versatile

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6 unique advantages of fiber optic sensing solutions: 

1. Insensitive to high voltages and electromagnetic interference
2. Long-term stability
3. Intrinsically safe
4. Resistant and robust
5. No drift
6. Maintenance-free after installation "plug & forget"

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Information about the fiber optic WLPI technology

Fiber optic white light polarisation interferometry (WLPI) is a patented fiber optic technology which enables precise measurements to be made in the most challenging of applications. It offers maximum flexibility in sensor design such that reliable measurements can be generated even in extremely unfavourable environments.

Fiber optic measurement systems comprise two main components: The fiber optic sensor and the signal analysis unit. Added to these are a fiber optic cable (FOC) which can fulfil different purposes depending on the technology employed.

A fiber optic sensor is made up of a sealed housing in which the optical sensor element is situated. This sensor element is sensitive to the physical quantity to be recorded. Different fiber optic measurement methods based on one or more of the specific characteristics of light (intensity, phase, polarisation and spectrum) exist. In principle the measurement quantity to be recorded changes one or more of the special characteristics (according to the technique), causing a changed light signal to be reflected back.

Fiber optic sensors can be generally be divided into two classes: Extrinsic and intrinsic sensors. Their differences in terms of design and functioning yield special properties that can have a positive or negative effect depending on the application. For intrinsic sensors the fiber optic cable is an essential part of the measuring mechanism. The optical fiber is the sensor. Fiber optic sensors based on Fiber Bragg technology are popular sensors in this class.

In contrast, extrinsic sensors are characterised by the decoupling of the sensing part and the optical fiber. The optical fiber (FOC) serves solely to transmit the light signal from the sensor unit to the signal conditioning unit . Temperature sensors based on gallium arsenide (GaAs) crystals and the fiber optic sensors based on WLPI technology presented in the following are categorised as extrinsic sensors.

The signal conditioning unit is used to feed the light signal into the fiber, to receive the reflected modified light signal and to process the received signal and convert the results into physical units for the measured quantity. The light source used differs according to measurement technique and technology.

Optical interferometry involving measurement of the phase modulation of light is considered to be the most sensitive fiber optic measurement method. The interferometer is a very precise optical measuring instrument in which at least two light bundles are guided by semi-transparent mirrors along different optical paths, reflected at the ends of the paths by additional mirrors and then recombined.

This results in an interference pattern determined by the difference in optical paths travelled by the individual beams before they are superimposed. A physical quantity can be measured using interferometry provided that changes in the quantity result in changes in the path length in the interferometer. 

The original use of a laser, a source of light with a narrow bandwidth, led to the problem of phase ambiguity because the coherent length of the light source was generally larger than the path length difference in the interferometer. This limited the application possibilities for the fiber optic sensors based on interferometry. The solution to this problem is to use a light source with a short coherence length and an accordingly wide spectrum. 

This type of interferometry is called ‘white light interferometry’ or ‘coherence tomography’. The founders of Opsens are pioneers in the introduction of white light interferometry into fiber optic measurements. In the field of industrial sensors they have developed this technology to the market readiness stage and can now present their latest development in the form of an improved fiber optic measurement technology: white light polarisation interferometry.

Areas of Application


Measuring equipment is needed for monitoring process and environment parameters in diverse industrial applications. Conventional electronic sensors frequently come up against the limits of the technology because interfering effects such as high voltages or EMI limit the operability of the systems. Particularly in these environments the fiber optic products from Opsens can create new possibilities for reliable measurements.

Defence and aerospace

Do you give top priority to the safety and reliability of your measuring equipment even in harsh environments with abrupt changes in weather and temperature such as occur during take-off and landing? Are your product development activities focussed on the core topics of lightweight construction and electromagnetic interference? WLPI technology-based fiber optic sensors offer the advantages you are looking for. Insensitivity to temperature fluctuations and lightning strike immunity to EMI, extremely small dimensions for weight reduction, high reliability and maintenance-free operation are only some of the advantages of our fiber optic sensors. The possible applications are diverse and the solutions numerous and adapted to your requirements. Talk to us.



Geotechnical engineering and building industry

Does your measurement task require sensors embedded in the soil or permanently positioned in hard-to-reach locations? Does your application involve high temperatures, corrosive liquids or chemicals? Good - because that’s what our fiber optic sensors are meant for. The maintenance-free sensors can be installed and integrated into your application in various ways. Even large cable lengths of up to 3 km are no problem. Do you require a ‘plug and forget’ solution? Talk to us.



Structure health monitoring

Ageing infrastructure, dilapidated structures such as bridges and long-term monitoring of critical components
are becoming critical due to the danger to humans and materials posed by strain and deformation of structural elements. That’s where fiber optic WLPI technology comes in, enabling robust, reliable sensors to be built with no drift over time and no sensitivity to transverse strain. Because they are insensitive to temperature fluctuations the sensors can be welded at the measurement location. For retrofitting, monitoring of initial damage (crack formation) or integration into a new building – we have the right solution for your measurement task. Talk to us.



Energy / renewable energies

In the energy industry the need for monitoring machine conditions or specific environmental conditions is growing. This especially applies to turbines in offshore wind parks as well as final storage facilities e.g. for radioactive waste. Depending on their design, the sensors can be installed via spot welding, complete integration, bonding or embedding in concrete. Do you want to carry out preventive monitoring of your machine or turbine to avoid faults, failure and damage as well as to optimise the functioning?

With our fiber optic sensors based on the WLPI technology you can measure the deformation of the turbine blades and optimise the angle of attack (pitch). Measurements can be carried out without changing the aerodynamics thanks to the integration of the strain sensor into the composite of the turbine blade. Detection of both turbine blade icing and the level of the surrounding water is possible.

Microwaves in chemical and food industries

Microwaves are often used in the chemical and food industries for product research and development: for preparing samples for chemical analysis or carrying out specific heating processes such as drying, cooking, thawing, pasteurising or sterilising of food. For in situ monitoring of processes you need to record quantities such as temperature and pressure precisely and quickly. Our fiber optic sensors are intrinsically safe and immune to microwave radiation as well as electromagnetic fields and hence lend themselves well to direct onsite measurements.

Comparison between WLPI and other fiber optic technologies

Compared with electronic sensors, fiber optic sensors offer a number of significant advantages including insensitivity /immunity to electromagnetic interference and high voltages. They are intrinsically safe, impervious to lightning strikes and capable of being built with extremely small dimensions. White light polarisation interferometry (WLPI) also possesses other advantages over conventional fiber optic technologies such as Fiber Bragg grating sensors.

    • Easier installation / handling
      In contrast to Fiber Bragg technology, the fiber optic cables in WLPI technology only have the task of transmitting the light signal between the sensor and the signal conditioning unit; hence the fiber optic cable length can easily be adjusted. Optical extension cords which can be interconnected with plug-in connections are available for this. Laborious splicing of the FOC is not necessary. Depending upon the application, assembly can be done e.g. via spot welding, bonding or complete integration into a component or a structure.
    • Easier adjustment to requirements / flexibility
      There are several standard sensors available for measuring strain, pressure, displacement and temperature. These sensors are always adapted to the requirements of the given application for ensuring both optimal function and protection of the sensor. The sensor design for a pressure sensor for fuel level monitoring differs greatly from that for a sensor adapted to the harsh conditions and high temperatures encountered at the depth of an oil well, although the functionality and the technology used are identical.
    • Greater stability
      Fluctuations in light intensity have no effect on the WLPI sensors as their function is based on the path length change in the interferometer, not on intensity changes. Thus optical losses, for example due to losses in the plug, cannot affect the movement or bend of the fibers or the performance of the system.The extrinsic character of the WLPI technology is particularly relevant for fiber optic strain measurement due to the insensitivity of WLPI sensors, unlike Fiber Bragg sensors, to transverse strain. Moreover, the temperature compensation required with Fiber Bragg sensors is done away with.

          • Better reliability
            When the system is used continuously with the maximum light intensity the MTBF value is 100.000 hours. In practice the light source is only very rarely operated above 50% of the maximum intensity.
          • Improved safety
            In terms of the generated energy WLPI is safer than laser-based fiber optic measurement techniques. If the light source is working with maximum intensity and the FOC is limited to 1 cm the maximum measurable intensity at the end of the fiber is less than a few microwatts. In comparison, a laser can output a few hundred milliwatts in a fault situation.
          • Maintenance-free
            A broadband light source as used for the WLPI technology does not have to be calibrated. Laser-based methods such as the Fiber Bragg technology require regular maintenance and calibration for compensating the drift and correcting the wavelength and intensity.
          • Lightweight, compact design
            WLPI-based sensors can be built with very small dimensions. Moreover, all components needed for signal analysis can be accommodated on a module approximately the size of a credit card.
          • Versatile
            All supported measurement quantities can be recorded with the same signal conditioning unit.


                  The following pictures show schematic presentations of the sensor design
                  for the corresponding physical values (temperature, pressure, strain, force and displacement):

                  You would like to know more about our WLPI fiber optic sensing solutions? Just call us +49 6195 70060 or send us an email to info@althen.de