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The alternative to glass optic fiber sensors: POF based fiber optic sensors

Fiber optic sensors are especially interesting for applications that may cause problems with EMI (electromagnetic interference) or interactions in chemical or biological processes when using electrical, current-driven sensors. So far, fiber optic sensors made of single-mode glass optical fibers are most well-known. Typically they utilze a wavelength selective readout with Bragg reflectors. Unfortunately, the cost of such a sensor system is too high for many applications.

In the typical fields of application of SHM (Structural Health Monitoring) in

  • Construction industry (strain measurement, vibration measurement),
  • Mechanical engineering (vibration measurement),
  • chemical industry (temperature measurement), and also
  • medical and biotechnology (minimally invasive measurement inside living organisms)

due to cost reasons, fiber optic sensors equipped with Bragg structures can usually be used only in large projects with a corresponding financial budget. In the search for a lower-cost alternative of glass fiber optic sensors, POF offers a number of other advantages in addition to a much simpler system technology compared to glass fiber.

  • POF is not brittle like glass fiber and can be much more stressed in strain sensors.
  • An uncomplicated LED or photodiode coupling is used for light input and output.
  • The installation of the POF sensor system does not require a team of specialists with expensive measurement technology, but can be carried out by any technician after a short training period.

However, the POF has disadvantages, too: its range is limited to about 100m due to the higher light attenuation and there is no simple option to monitor several measuring points with one fiber. But, one device of POF sensor electronics can connect several fibers and thus measure several points without any problems.

Various measurement principles for fiber optic sensors

DieMount already published in 2010 with “Semsors for color measurement via polymer optical fiber (POF)” a contribution to the use of POF-Splitters and LED of different wavelengths in the application of different measurement principles in POF sensor technology. In addition to the classification into duplex and simplex measurement principles, the distinction between intrinsic and extrinsic measurement principles is helpful. The following introduction of the terms duplex and simplex as well as extrinsic and intrinsic for the different measuring principles serves as a rough overview.

Extrinsic sensor

Using fiber optic splitters, the reflected light of a reflecting object can be measured with different wavelengths, by electronically selecting the active LED of interest. The return signal is either led through a second fiber (duplex method) or by the illuminating fiber (simplex method) itself. The simplex method requires special 1×2 splitters with high crosstalk attenuation (low crosstalk splitters) to separate the forward and return signals.

Since in both measurement principles outlined above the optical signal is not modulated by the fiber itself, but by an object located outside the fiber, this sensor type is referred to as extrinsic sensor.

Intrinsic sensor

With the optical fiber POF, intrinsic sensor methods, too, are used in addition to extrinsic sensor methods. Intrinsic means that the change in the optical signal occurs through the fiber itself in response to a physical measurand. The two following sketches show the principle:

fiber optic sensor, intrinsic
fiber optic sensors, intrinsic

The above figures show that in the intrinsic sensor method, the physical variable (e.g. pressure or strain) acts directly on the fiber, changing it and thus causing a measurable change in attenuation.

The possibility to measure in simplex mode is very helpful in practical operation, because the POF has to be connected to the DUT from one side only. For some applications, duplex operation is not possible, e.g. when the fiber is hidden in the object to be monitored from one side.

All the methods shown above, duplex and simplex, intrinsic and extrinsic, use optical power measurement, which can be realized electronically with moderate effort. Neither fast electronics nor optical filters are required. The simplex option is available for many measurement techniques. The loss of optical power budget due to the splitter and simplex design is generally easy compensated by the dynamic range of the opto electronic evaluation unit.

Under applications it is shown how simplex and duplex methods in intrinsic and extrinsic design can be implemented in practice.