How Many Types of Fiber Optic Sensors Are There? What Are the Differences Between Different Types?

Fiber optic sensors can be divided into point sensors and distributed sensors according to their working principles. In point sensor types, the sensor operates at a single point, typically placed at the end of an optical fiber. An example of this is a fiber Bragg grating distributed along the fiber length. It is used to measure temperature or strain. This single-point fiber sensing method uses phase changes to activate the sensor. In distributed sensor types, the sensor operates over a distribution of points along the fiber. In this approach, the sensor functions along a long chain of sensors or an array of optical fibers.

External and Intrinsic Sensors:

Fiber optic sensors can be classified into two types based on where the beam changes occur: external fiber sensors and intrinsic fiber sensors. An external fiber sensor is one where the beam leaves the cable or feed and changes before reaching the fiber detector. If the beam does not leave the cable but changes within the cable itself, it is called an intrinsic fiber sensor. Intensity-based fiber pressure sensors used to measure pressure between two plates are examples of intrinsic fiber sensors.

Comparison of Different Types of Fiber Optic Sensors:

Based on the sensing method and the placement of the fiber emitter and receiver, there are three types of fiber optic sensors: through-beam, reflective, and retro-reflective.

In a through-beam sensor, the emitter and receiver are placed facing each other to form a straight optical path. When any object enters the path, the light intensity changes and the object can be detected.

In a reflective sensor, the emitter and receiver are placed in parallel. Light emitted by the emitter is reflected by an object and measured by the receiver. When a red LED is used as the light source, this type of sensor has difficulty distinguishing red from white because they reflect similar amounts of light.

In a retro-reflective sensor, the emitter and receiver are housed together and a reflector made of special reflective material is used. The emitted beam is reflected by the reflector and received by the receiver. If anything interrupts the beam path, detection occurs based on differences in beam intensity and other parameters at the receiver.