Optical glass fibers are well known for their telecommunications functions. However, fiber optics can be applied to a wide variety of sensor applications. By taking advantage of the properties of glass and the geometry of the fiber, measurements of temperature, pressure, humidity, and other quantities can be collected.
What are Fiber Optic Sensors?
Fiber optic sensors are instruments that use an optical glass fiber as the sensing element. For sensor applications, multi-mode fibers with large core diameters (>10 micrometers) are used.
Fiber optic sensors convert the behavior of light passing through the sensor fiber to an engineering unit such as temperature or pressure. The conversion is based on a variety of factors, depending on the type of measurement being made.
Types of Fiber Optic Sensors
The most commonly used fiber optic sensor types include:
- Strain Sensing - Strain is one of the most common functions for fiber optic sensors, because it can be directly measured. Mechanical strain in the fiber changes the geometric properties of the fiber, which change the refraction of the light passing through it. These changes can be correlated to the applied strain. Some strain sensors use fiber Bragg gratings as measurement points. Mechanical strain changes the critical wavelength of the grating, which can be easily measured.
- Temperature Sensing - Temperature sensors are essentially identical to strain sensors, except the strain in the fiber is caused by thermal expansion or contraction of the fiber itself. The strain measurement can be correlated with changes in temperature.
- Pressure Sensing - Fiber-optic pressure sensors are constructed from a fiber that is inserted inside a rigid tube, usually made of steel. A diaphram at the end of the tube couples the tube to the fiber, and acts as the sensing point. External pressures cause the fiber to expand or contract
- Humidity Sensing - Like the other sensor types, fiber optic humidity sensors do not measure relative humidity directly. These types of sensors have fiber Bragg gratings inscribed into the fiber core, and the outer surface of the fiber at the grating location is coated with a hygroscopic coating, such as polyimide. The coating absorbs or releases water vapor based on the relative humidity of the sensing environment. This causes expansion or contraction of the coating, which induces a strain on the fiber. The strain results in a change in the critical wavelength of the grating, and this strain can be correlated to humidity levels. Individual sensors require calibration due to potential variations in coating thicknesses.
For fibers that use Bragg gratings as part of the sensor, multiple gratings can be inscribed at different points along one fiber. Each grating will be inscribed for a specific critical wavelength, so the resulting signal will contain a pulse for each sensor that can can be separated out.
Applications for Fiber Optic Sensors
Fiber optic sensors are attractive options for a variety of applications due to their thin cross section and their ability to have extremely long cable lengths. Some applications include:
- Biomedical monitoring (small size, non-reactive materials)
- Petroleum drilling sites (long lead lengths, high resistance to temperature)
- Aircraft engines (high resistance to temperature)
Fiber optic sensors are also used in locations where localized power is not available, because the sensor does not require electrical power to operate. Fiber optic sensors provide a variety of benefits to engineers with unique test and measurement requirements.
 |
