eFunda: Theory of Fiber Optic Sensors
engineering fundamentals Fiber Optic Sensors: Theory
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Sensor Response

The Fotonic (a popular fiber optic) sensor output function (i.e. light intensity versus distance to target surface) can be divided into three regions: the front slope, the transition, and the back slope. These three regions are shown in the schematic below.

Response of Typical Fotonic Sensors
Light Intensity (Voltage Output) vs. Distance

Interpretation of Sensor Response
Interpreting the Fotonic sensor output function shown above, we can draw the following conclusions:
- When the gap between the probe tip and the target surface is zero, no light can escape from the transmitting fibers to reach the receiving fibers, so the measured intensity of the reflected light is zero.
- As the gap opens up, reflected light can begin to reach the receiving fibers. However, in this region the actual intensity of the reflected light does not change with increasing gap distance, since most of the light is still confined to the surface directly under the probe tip (or more precisely, directly under the transmitting fibers). The measured intensity increases because as the gap opens up, a larger fraction of the reflected light can reach the receiving fibers. In other words, the receiving fibers become more and more "illuminated". As a result, the measured intensity of the reflected light increases almost linearly with gap distance in this front slope region.
- The measured intensity keeps increasing until the gap distance is about the same order as the probe diameter. In this transition region, the receiving fibers are now fully illuminated and the maximum measured reflection is reached.
- As the gap increases past the transition region, the measured intensity drops off following roughly an inverse-square law. This results from the fact that, even though the receiving fibers are fully illuminated, the actual intensity of the reflected light as seen by the probe diminishes.
Effective Sensor Range

In practice, both the front slope and back slope regions of the sensor output curve can be used in displacement measurements. The front slope is more linear and sensitive, while the back slope has a larger effective range.

Finally, re-calibration is generally required, since the reflection index of target surfaces may vary across different locations, work pieces, and materials.

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