Fiber optic inclinometer demonstrates the versatility of FBG sensors

FBG inclinometerFiber optic inclinometer is considered to be one of the most important optical fiber devices used in numerous fields of application. For example, in geophysical engineering, the inclinometer based on the fiber Bragg grating technology (FBG) allows monitoring the angle of inclination at regular intervals along the borehole to profile the land displacement.

Current fiber optic inclinometers utilizing FBG sensors demonstrate a high level of sensitivity and angle of measurement resolution of 0.006°. It should be noted that traditional inclinometers have disadvantages when it comes to their involuntary twist during the process of installation. And optical fiber sensors were widely thought to overcome these limitations due to the absence of moving parts.

Nevertheless, the combination of traditional inclinometers with FBG technology enables us to resolve the challenge and make the fiber optic inclinometers’ operation independent of twisting, therefore, the precision remains constant. Moreover, despite numerous fiber optic techniques that measure temperature, strain, pressure, and other parameters, new fiber optic inclinometer based on a fiber’s taper and a long-period fiber Bragg grating have been recently developed.

The principle of the fiber optic device is based on the use of conventional fiber Mach-Zehnder interferometer that allows dividing the incoming light between two optical fibers and recombining it at a second junction. To be precise, in a Mach-Zehnder system, a long-period fiber Bragg grating dissipates light at its resonant wavelength from the optical fiber core into the cladding, while a second-long period FBG takes light from the cladding back into the fiber core.

The novel fiber optic inclinometer is a simpler device that includes only a single long-period fiber Bragg grating. Moreover, it uses the taper that enables to increase the core field so that part of its light is coupled into the cladding. The fiber optic inclinometer offers the following advantages in its operation:

  • the  immunity  to electromagnetic  interference;
  • the  high sensitivity;  
  • the  compact size;  
  • the multiplexing and remote interrogation abilities.

This optical fiber device based on the interferometer was tested in the measurement of angular displacement. During the experiment, the optical fiber on either side of the taper was sheathed in capillary tubes and then twisted accurately at the taper. Thus, such a bend influences the fringe visibility, but not the location of the fringes.

Optromix is a FBG sensor manufacturer that provides top of the line FBG sensing systems suitable for monitoring of the constructions’ inclination. If you have any questions or would like to buy a FBG product, please contact us at info@optromix.com

Inclination measurement system: incline FBG sensors

Fiber Bragg Grating sensors one of the most requested fiber optic technologies have superior sensitivity and frequency specifications, making them well suited for many spheres of applications. The FBG inclinometers can be used to identify internal damage at a very early stage.  The FBG inclinometers are devices used to monitor subsurface movements through sensors designed to measure inclination with respect to vertical. When installing the FBG inclinometer casing, it is important to select the appropriate diameter. The large-diameter casing is better suited to shear zones, multiple shear zones, and slope failures. Moderate- to small-diameter casing can be used for short-term installations or slopes where smaller displacements distributed along the borehole are anticipated. Correct installation of the casing is important; and deep holes, particularly the influence of helical deformation must be considered. A conventional FBG inclinometer system consists of a plastic casing that is installed in a nearly vertical position in the ground, with a servo-accelerometer or electro-level sensor inserted into the casing to measure the local tilt of the casing in response to ground movement. The sensor element is lowered and raised, guided by grooves in the inner surface of the casing, with the tilt of the casing being recorded at fixed spatial intervals.

Incline FBG sensors have been widely used to monitor ground movements in various applications, for example, landslides, tunnels, and foundations, etc., where they provide vital ground movement information including magnitude, rate, and location. The produced information can be used for checking design assumptions and provide early warning of problems.

Another type of FBG sensors that can monitor inclination is a tiltmeter. Tiltmeters are devices used to monitor the change in the inclination of a ground surface point. The device consists of a gravity sensing transducer capable of measuring changes in inclination as small as one arc second. They are used to monitor slope movements where the landslide failure mode is expected to contain a rotational component. The advantages of using tiltmeters are their lightweight, simple operation, and relatively low cost. They may be combined with an incline FBG sensor and extensometers in what has been termed as integrated pit slope monitoring systems.