What are fiber optic sensors?
The fundamental characteristic of all fiber optic sensors is that they depend on some optical properties, such as intensity, phase, state of polarization and wavelength, to be modulated by measurands. Measurands could be pressure, temperature, electromagnetic field or displacement.
All fiber optic sensors have an optical element that is sensing these property changes. For most sensors, this element is the optical fiber itself or a non-fiber optical element.
Fiber optic sensors with optical fiber as sensor element are called “intrinsic fiber sensor” and sensors with a non-optical fiber sensing element are called “extrinsic fiber sensor”.
1. Intrinsic Fiber Sensors
In the intrinsic fiber sensor, external measurands such as pressure, vibration, temperature interact with optical fiber element and cause fiber bending, fiber distortion and a change in the refractive index of the sensing fiber.
Because of the refractive index change, lights that travel through the fiber are affected accordingly. The changes in light properties, such as light intensity, light wavelength and light phase are then detected. The magnitude of measurands interacting with the fiber can then be determined.
2. Extrinsic Fiber Sensors
Birefringent crystal, intensity mask or thin film absorbers are most often used as sensor elements in extrinsic fiber sensors. Usually they are integrated into the optical path.
When the external force interacts with the sensing element, the light properties are modulated as well. The sensor has light source, optical path and photo detector parts. The magnitude of measurands is detected similar to intrinsic fiber sensors.
The Applications of Fiber Optic Sensors
Wide Area Sensing and Monitoring
Because of optical fiber’s immune to electromagnetic field, fiber sensors have a big potential in these areas. They are widely used in temperature sensing in building, leakage monitoring along oil pipelines and so on.
The above mentioned applications are called wide area sensing or monitoring. The name means that the sensing covers a wide area. In this area, fiber sensors are divide into two categories: distributed sensor and quasi-distributed sensor.
1. Distributed Sensor
Distributed sensors sense measurands continuouly over the entire length of the fiber. The most important criteria is that sensor fibers must be very sensitive to measurands.
A typical example of distributed sensors is a temperature sensor utilizing Raman scattering effect in optical fibers. Another example is OTDR (Optical Time Domain Reflectometer) which senses signal reflection in the whole length of an optical path.
2. Quasi-Distributed Sensor
Quasi-distributed sensors use discrete sensor elements that are carefully arranged in the fiber network. This type if sensor needs to be small size, low cost and high reliability.
High Sensitivity Measurements
Another area for fiber sensors is the high sensitivity measurement applications. This type of sensors typically utilize light interference’s extremely high sensitivity property.
A number of interferometric fiber sensors have been used for measurement of temperature, pressure, vibration and so on. The fiber optic gyroscope is one typical example of this type of applications.
Harsh Environment Measurement
Some extreme environment has no choice but fiber optic sensors. This kind of applications include high temperature, immersion into chemical reagents, radioactive rays factories and so on. The fiber optic sensor’s resistant to this type of harsh environment is extremely important.