Multiplexing ability of optical sensors

The ability to multiplex several optical sensors on one single fiber provides some advantages for the sensing application.

Different measurement tasks like temperature, strain, pressure and more can be integrated on the same optical fiber. The fiber can also be embedded into complex structures.

The following table gives an overview about the optical sensor concepts and their ability to be multiplexed

Sensor conceptMultiplexing ability
FBG – Cantilever SensorYes
FBG with pivot coupling of the seismic massYes
FBG tube sensorYes
FBG based Fibersensing GS6500 – COTSYes
FBG based waveguide sensorYes
Microbend TransducerNo
Fiber gap sensor with pivotsYes (limited)

 

The last sensor concept is marked with „Yes (limited)“. We mean here by „limited“ that the multiplexing of this sensor will enhance a high loss of light per sensor.

To clarify this effect, an intensity based measurement concept is shown simplified on the following picture.

fiber coupler

The input fiber of the sensors is illuminated with a broadband light source. If all sensors are in static conditions, the light propagates through all sensors and the reflected peaks of each sensor have certain intensity.

If acceleration is applied to the first sensor, the surfaces of the input fiber and output fiber are displaced to each other. The light which illuminates FBG1b and the further gratings is attenuated. The difference between the amplitude of FBG1a and FBG1b can be interpreted as an acceleration signal.

The second sensor is illuminated with the attenuated light of the first sensor. Due to this, the sensitivity of the second sensor depends on the deflection of the first sensor. The signal quality (SNR) reduces itself with the growing number of sensors. In worst case the first sensor is outside the measurement range and no light is transmitted to its output fiber. This would make all following sensors unusable for this time.

Due to the dependency of the sensor sensitivity to the number of sensors and the worst case problem, all sensors which use intensity based measurement concepts are not able to achieve the requirements in terms of multiplex ability.

The acquisition of optical sensors is very expensive. Beside the application in radiated environments, optical sensors are mostly used for measurement application where several measurement points must be acquired or the sensors should be embedded into a mechanical structure. Focusing on this argument, the multiplexing ability is the most important trade criterion and has to be supported by the sensor concept.