eFunda: Introduction to Fiber Optic Sensors
engineering fundamentals Fiber Optic Sensors: Introduction
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Overview

A popular fiber optic sensor, the Fotonic sensor is a displacement sensor containing two groups of fiber optics, one set connected to a light source and termed the transmitting fibers, and the other set connected to a photo detector (photodiode) and known as the receiving fibers. These two groups of fibers are bundled into a common probe.

Referring to the schematic below, the light generated from the source is channeled through the transmitting fibers to the probe tip. The light then travels to the target surface and part of it is reflected back to the probe. A portion of the reflected light is caught by the receiving fibers and transmitted to the photo detector where its intensity is measured. The intensity of the reflected light is a function of distance (gap) between the probe tip and the target surface.

Typical Fotonic Sensors
Common Specifications
Common specifications for commercially available fotonic sensors are listed below:

  Diameter: ~2.75 mm (0.109 in)
  Pattern: Random distribution (most frequently used), concentric with transmitting fibers inside, concentric with transmitting fibers outside, and hemispheric. See schematic below.
  *Sensitivity: 63 mV/µm (1.6 mV/µin)
  *Linearity: ±1% from 25 to 100 µm (0.001 to 0.004 in)
  Frequency Response Flat (±3dB) from DC to 50 kHz
  Output: 0 to 10 V DC
   *- Evaluated at the front slope region.

Commonly-Used Fiber Patterns
(Cross section of probe)

Pros and Cons

  • Pros:
  - Non-contacting measurement.
  - No moving parts, less likely to break.
  • Cons:
  - Re-calibration is generally required often, since the reflection index of target surfaces may vary.
  - Works well on highly reflective surfaces, less effective on duller surfaces.
  - May be affected by surrounding lighting conditions.
  - The characteristic length of the target surface's roughness should be at least an order smaller than the spacing of the transmitting and receiving fibers. In other words, a fotonic sensor can measure the roughness of the target surface up to the order of the spacing of the transmitting and receiving fibers.

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