Phase-shifted fiber Bragg gratings (πFBGs)

Phase-shifted FBGsNowadays, the special type of FBGs whose reflection spectrum has a notch arise from a π-phase discontinuity in the center of the grating (called π-phase-shifted FBGs) attracts ground interests among researchers. Because of their highly sensitive ultrasonic detection, πFBG may provide a solution to the sensitivity problems of the FBG. By introducing a π- phase shift into a refractive index modulation of the fiber Bragg grating during its fabrication, the spectral transmission has a narrow bandpass resonance appearing within the middle of the reflection lobe of the FBG. Such an element allows reaching a very narrow transmission band of few picometers.

Fabrication of π-phase-shifted FBGs is achieved by splitting the standard FBG into two identical sub-FBGs with a half-period phase difference between them. The two sub FBGs create an interfere with each other and generate an ultra-narrow transmission window at the center of the FBG spectrum.

Due to the phase discontinuity, a πFBG can be conceptually described as a Fabry–Perot cavity formed by two FBG mirrors. When the two FBGs are highly reflective, the quality factor of the Fabry–Perot cavity is increased, leading to an extremely narrow spectral notch for highly sensitive ultrasonic detection.

Using special structures, even multiple transmission bands are possible.

The primary method used for the fabrication of π-Phase-shifted fiber Bragg grating is based on the UV laser and phase mask method. The occurrence of two peaks/dips is attributed to the refractive index modulations along with the fiber core, with the periodicity of the π-phase mask that has been observed previously in images of gratings that cause destructive interference in a reflected wave at the Bragg condition owing to the phase difference between the grating phases. Thus, the standard phase mask technique produced an alternative type of pi-phase-shifted grating at twice the design Bragg wavelength.

The phase-shifted gratings have found application in distributed feedback lasers, wavelength division multiplexing, athermal setup, or temperature stabilization, as well as to a tuning setup. Also, the π-phase-shifted FBG can be used in highly accurate wavelength references, ASE filtering, spectroscopy, and optical CDMA.

 

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.