Optical Fiber Sensing (1)
The technology to use optical fibers as sensors has been in development for more than 30 years. Here, measurement technology using optical fiber sensors is called optical fiber sensing and has the following advantages providing a means to solve some problems of electrical sensors.
Optical Fiber Sensing Advantages
- No power supply to sensor
- Remote detection
- Unaffected by electromagnetic noise
- Lightning resistant
Optical fiber sensing has a wide application range in various fields, including structural health monitoring, such as bridges and buildings, wind turbines and power transmission lines, as well as the security of important facilities, such as airports, by measuring physical quantities including strain and displacement. It also supports monitoring of power transmission and maintenance of industrial facilities by measuring physical quantities, such as electrical current and vibration.
The following sections describe the various types of optical fiber sensing, their features, and required light sources.
Optical fiber sensing can be broadly classified into two types: point type, and distributed type.
- Point-type sensors are specially processed on optical fiber lines to function as sensors. A typical example is the Fiber Bragg Grating sensor.
- The distributed type uses technology making the entire optical fiber function as a sensor. It includes OTDR, which measures the presence and location of optical fiber breaks and losses, as well as R-OTDR and B-OTDR, which read information about backscattered light generated when light passes through an optical fiber.
This first issue of Optical Fiber Sensing discusses point-type optical fiber sensing.
Each point-type sensing method is described below.
1. FBG Sensor
The FBG sensor is a type of optical filter with a periodic grating at one part of an optical fiber. Figure 1 shows the principle. When light with a broad wavelength spread, such as from a wavelength swept light source or SLD, is injected at one end of the fiber, only light with a specific wavelength (called the filter wavelength) is reflected by the FBG sensor part, and light at other wavelengths passes through to the other end.
Figure 1: Principle of FBG sensor
Here, physical expansion, contraction, or temperature change in the FBG sensor causes a corresponding change in the filter wavelength, and the physical quantity can be determined by measuring the wavelength of this reflected light. Multi-point measurement is possible by connecting multiple FBG sensors with different filter wavelengths in series. FBG sensor measurement methods include 'SLD and spectrometer' type as well as more accurate 'wavelength swept light source and photodetector' type.
Figure 2: Example of FBG sensor measurement – Monitoring airport security
2. BOF Sensor
As shown in Fig. 3, the BOF sensor has a dielectric multilayer film attached to the tip of the optical fiber and the blue stripes in the figure are the sensor part. Applying heat to the sensor part increases the refractive index and changes the reflectance spectrum to the longer-wavelength side. Applying pressure to the sensor part makes the film physically thinner and changes the reflectance spectrum to the shorter-wavelength side. The physical quantity is obtained from this change in wavelength, Δλ.
At multi-point measurement, multiple points are measured simultaneously by pulsing the light source and setting a different distance between each sensor for identification.
Figure 3: Principle of BOF sensor
Sensing with BOF sensors is ideal for measurement in harsh environments, such as high-humidity locations. It uses relatively broad-spectrum devices, such as SLDs and LEDs, as light sources, but SLDs with high light intensity are best for fiber focusing and multipoint measurement.
Figure 4: Example of BOF sensor measurement – Measuring generator eccentricity, vibration, and surface blur
3. Hetero-core Fiber Optic Sensor
The hetero-core optical fiber sensor forms a narrow core-diameter part (hetero-core part) as shown in Fig. 5. Bending at this part increases light intensity loss, allowing determination of physical quantity. Conventional optical fiber sensing requires temperature compensation when measuring static physical quantities, but this method is unaffected by temperature variation and the ability to perform sensing without compensation is a major advantage.
Figure 5: Principle of hetero-core fiber optic sensor
This sensor performs various types of sensing, such as displacement, strain, water level, and acceleration, using the curvature of the hetero-core. Since this method measures physical quantities as light intensity differences, it can be used with any device with stable light output that can be injected into the optical fiber, such as LDs, LEDs, SLDs, and DFB-LDs, making it a very versatile method.
Figure 6: Example of hetero-core optical fiber sensor measurement – Monitoring dam water level
4. Current Sensor
Although there are various methods for measuring electrical current, the advantages of current sensors using optical fiber sensing are freedom from electromagnetic induction noise effects, as well as compact size and light weight, because the optical fiber winding itself is an insulator, reducing the weight and size of the required electrical insulation. Additionally, the optical fiber winding diameter is easily adapted to larger-diameter electrical conductors.
As shown in Fig. 7, the optical fiber current sensor is wound around the electrical conductor to form the fiber sensing section with a mirror at one end of the section to reflect back the propagated light. The incident light is first polarized linearly at polarizer 1 before passing via an optical coupler and through the current-sensor fiber sensing section to be reflected by a mirror at the end of the section back through the optical coupler and then through polarizer 2. The angle of polarizer 2 is adjusted so the transmittance is maximum when no current flows through the electrical conductor. When electric current passes through the conductor, the linear polarization deflection angle α changes in proportion to the magnetic field H and the amount of light transmitted through polarizer 2 decreases, facilitating measurement of current from the amount of emitted light.
Figure 7: Principle of current sensor
Current sensors use LDs, SLDs, ASE, and gas lasers as light sources but SLDs are ideal due to their relatively high output and low interference noise.
Figure 8: Example of current sensor measurements – Monitoring power plants/sub-stations and high-power electrical structures
5. Faraday Effect Sensors
This last section explains an example of optical fiber sensing by adding a polarizer, magnetic material called a Faraday rotator, and a magnet to the end of an optical fiber. In theory, this system supports light in free space, but in practice, optical fiber is used to measure remotely from the device to the sensing point, forming an example of optical fiber sensing.
Figure 9 shows the principle of the Faraday sensor. The rotation angle of light transmitted through the Faraday rotator changes according to the strength of the magnetic field to which the Faraday element is subjected, and this is converted to light intensity on passage through a birefringent element called a polarizer.
Figure 9: Principle of Faraday sensor
Depending on the structural arrangement of the magnet position, this sensor can be applied to rain gauges, disaster prevention sensors, such as levee scour and rock-fall detection, or detection sensors for opening and closing sluice gates.
If a sensor part is prepared, a sensing system can be configured using an OTDR to measure the optical fiber transmission loss and disconnection points.
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Figure 10: Example of Faraday sensor measurement – Monitoring opening and closing of sluice gates
There are various other methods in addition to these introduced point-type optical fiber sensing methods, such as Fiber-Optic Gyroscope (FOG) attitude-control sensors using the Sagnac effect as the detection principle, and electric field sensors using the Pockels effect as the detection principle.
Anritsu has a full range of light sources for optical fiber sensing. For product information on our SLDs, DFB-LDs and Wavelength Swept Light Sources, visit our Optical Sensing for Industry web page.
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