Fiber optic sensing is becoming pervasive across multiple industries including medical, aerospace, automotive, civil, manufacturing and energy. Within the market, there are dozens of types of technology, each with their own capabilities. Navigating the fiber optic sensing landscape can be challenging due to the variety of technologies. For more detailed information about different types of fiber optic sensors and their applications, download the paper at the end of this post.
Fiber optic sensors are capable of measuring a wide variety of parameters including strain, temperature, internal and applied loads, deflection, liquid level and more. Fiber optic sensors are low cost, lightweight, robust and immune to electromagnetic interference. While at the same time, they are highly sensitive. Most fiber optic sensors fall into two broad categories: Intrinsic and extrinsic.
Extrinsic vs. Intrinsic
At a very basic level, extrinsic sensors use the fiber to guide the light to a sensing region where the optical signal leaves the cable and is modulated in another medium. There are many different variations of extrinsic sensors and in some cases, the optical fiber is used only to transmit data from the sensor to the acquisition unit. One benefit of this method is the ability to reach places that would otherwise be inaccessible. For example, by using a fiber to transmit radiation into a radiation pyrometer located outside an engine, extrinsic methods can measure temperature inside of an aircraft jet engine.
Extrinsic Fiber Optic Sensor Types and Methods
- Optical Coherence Tomography—most commonly used to create images of the retina
- Encoders—can measure linear and angular position.
- Laser Doppler velocimetry—the Doppler shift in a laser can be used to take flow measurements by determining the velocity of a substance.
- Pyrometers—a remote sensing thermometer whereby an optical system focuses the thermal radiation of an object onto a detector.
- Spectroscopy—used in laboratories to identify the chemical composition of sample.
In the case of intrinsic sensors, the light remains within the waveguide so that it measures the effects of the optical signal as it moves down the fiber. Intrinsic fiber optic sensors can use the natural backscattering of light, fiber bragg gratings (FBGs) or other elements to obtain measurements. Intrinsic optical sensors can measure temperature, strain, load, and other parameters by observing the resulting changes in the intensity, phase, polarization, wavelength or transit time of light in the fiber. Sensors that vary the strength of light within the fiber are the simplest to measure, as only one source and detector are required. Intrinsic fiber optic sensors can provide spatially continuous sensing along the entire length of a fiber.
Within the division of intrinsic sensors, users typically deploy scattering or FBG techniques, each with their own advantages. While scattering techniques offer fully distributed data points along a fiber, FBG techniques can have a handful of sensing points or can be fully distributed. By placing FBGs throughout the fiber, engineers can analyze the changes in the way the light reflects and interpret this information to provide accurate measurements. Scattering techniques depend on naturally occurring random imperfections in the fiber optic cable to attain readings. Since FBGs are fabricated to be well defined sensors they have a much higher signal to noise ratio than scattering techniques.
Intrinsic Fiber Optic Sensor Methods
- Scattering based sensors, such as:
- Raman— utilizes Raman anti-stokes signals to obtain temperature measurements.
- Rayleigh—utilizes the Rayleigh backscatter as a function of length in an optical fiber. Strain and temperature cause spectral shifts in Rayleigh backscattering which can be identified by an interrogator.
- Brillouin—sensitive to both strain and temperature, has the longest measurement distance of the scattering technologies.
- Fiber bragg grating based sensors, such as:
- Point FBG Based Sensors – use fiber bragg gratings to obtain a signal at 10-20 points along an optical fiber to measure strain, temperature, pressure and more.
- Spatially continuous FBG based sensors – continuously written fiber bragg gratings obtain data along the entire length of the optical fiber and are capable of measuring strain fields, temperature distributions, internal and applied loads, liquid level, 3D shape and more. Sensuron provides this type of fiber optic sensing technology.
Fiber Optic Sensing vs. Legacy Technology
Point sensors have been the primary form of sensing technology essentially since sensors were invented. These technologies have been good enough for decades and will remain part of the landscape for years to come. The issue with technologies such as strain gauges, thermocouples, and other transducers is not a question of accuracy or reliability, rather, they do not provide enough information about the devices they are measuring to test and monitor the innovations of today. In other words, point sensors can miss events that occur between critical points, leaving blank spaces in the picture painted by the data. Some types of fiber optic sensors can provide a profile between critical points, which allows engineers to obtain precise measurements of full strain fields, temperature distributions, and other parameters.
The abilities and use cases of fiber optic sensing technology are expansive. While each method has its own unique capabilities, choosing the right method will always depend on the application. Sophisticated sensing systems enable engineers to collect and analyze material and structural data to ensure precise measurement and optimal performance every time. Sensuron’s fiber optic sensing platforms are designed to help engineers in the aerospace, automotive, civil and energy industries better validate their structural and thermal models, perform structural analysis, conduct nondestructive damage evaluation, and employ structural health monitoring.