Hi-Bi FBG Research Articles

Highly birefringent side-hole fiber Bragg grating for high-temperature pressure sensing.

We demonstrate a novel, to the best of our knowledge, high-temperature pressure sensor based on a highly birefringent fiber Bragg grating (Hi-Bi FBG) fabricated in a dual side-hole fiber (DSHF). The Hi-Bi FBG is generated by a femtosecond laser directly written sawtooth structure in the DSHF cladding along the fiber core through the slow axis (i.e., the direction perpendicular to the dual-hole axis). The sawtooth structure serves as an in-fiber stressor and also generates Bragg resonance due to its periodicity. The DSHF was etched by hydrofluoric acid to increase its pressure sensitivity, and the diameter of two air holes was enlarged from 38.2 to 49.6 µm. A Hi-Bi FBG with a birefringence of up to 1.8 × 10-3 was successfully created in the etched DSHF. Two distinct reflection peaks could be observed by using a commercial FBG interrogator. Moreover, pressure measurement from 0 to 3 MPa at a high temperature of 700°C was conducted by monitoring the birefringence-induced peak splits and achieved a high-pressure sensitivity of -21.2 pm/MPa. The discrimination of the temperature and pressure could be realized by simultaneously measuring the Bragg wavelength shifts and peak splits. Furthermore, a wavelength-division-multiplexed (WDM) Hi-Bi FBG array was also constructed in the DSHF and was used for quasi-distributed high-pressure sensing up to 3 MPa. As such, the proposed femtosecond laser-inscribed Hi-Bi FBG is a promising tool for high-temperature pressure sensing in harsh environments, such as aerospace vehicles, nuclear reactors, and petrochemical industries.

Simultaneous Measurement of Strain and Temperature by a Sawtooth Stressor-Assisted Highly Birefringent Fiber Bragg Grating

We propose and demonstrate a highly birefringent fiber Bragg grating (Hi-Bi FBG) fabricated for the simultaneous measurement of strain and temperature. A birefringence of up to 2.96 × 10−4 was created in the FBG by inscribing a sawtooth stressor near the fiber core with femtosecond laser irradiation. This provides a method for measuring the strain and temperature simultaneously, using one Hi-Bi FBG with two parameters (i.e., the Bragg wavelength $\lambda _{B}$ and the birefringence-induced wavelength difference $\lambda _{D}$ ). Such a sawtooth stressor-assisted Hi-Bi FBG exhibits strain sensitivities of 1.24 pm/μϵ in $\lambda _{B}$ and 2.14 × 10−2 pm/ μϵ in $\lambda _{D}$ , and temperature sensitivities of 9.52 pm/°C in $\lambda _{B}$ and 0.13 pm/°C in $\lambda _{D}$ . Moreover, the proposed sawtooth stressor can significantly increase the sensitivities of the birefringence to the applied strain and temperature. The strain and temperature sensitivities in $\lambda _{D}$ of the proposed sawtooth stressor-assisted Hi-Bi FBG are of several orders of magnitude, higher than those (i.e., 6.07 × 10−5 pm/μϵ and 9.42 × 10−3 pm/°C in $\lambda _{D}$ ) of a straight stressor-assisted Hi-Bi FBG. Consequently, the proposed sawtooth stressor-assisted Hi-Bi FBG sensors are promising in many areas, such as smart structures and intelligent robotics.

Tunable dual-wavelength actively Q-switched Er/Yb double-clad fiber laser

We demonstrate experimentally a dual-wavelength tunable actively Q-switched fiber laser using 3 m of Er3+/Yb3+ co-doped fiber as the gain medium. For wavelength tuning we used a tunable Hi-Bi FBG having two reflection wavelengths separated by 0.4 nm. The laser emits a dual-wavelength signal that is tunable in a range of 11.8 nm. Laser operation can be switched between single and double wavelength emission. The laser operates at repetition rates from 30 to 110 kHz with pulse durations of 280 ns and pulse energies near 0.5 μJ.

Multi-component strain development in superconducting magnet coils monitored usingfibre Bragg grating sensors fabricated in highly linearly birefringent fibre

The commissioning of superconducting magnet coils was monitored using embedded opticalfibre Bragg grating sensors (FBG) fabricated in highly linearly birefringent (HiBi) fibre.The HiBi FBG sensors monitored the internal strain developed in the coils during theenergization of the coils. The development of multiple components of strain in the coilswhen the magnet was energized and quenched was monitored, revealing phenomena that ithad not been previously possible to measure using other sensor technologies.

Novel in-line fiber polarization beam splitter using high-birefringence fiber Bragg grating

This study proposes a novel fiber-type polarization beam splitter (PBS) based on a high-birefringence fiber Bragg grating (Hi-Bi FBG). The reflective and transmitted modes of the Hi-Bi FBG are used to separate the p- and s-waves of an optical signal. The experimental results show that a 36dB extinction ratio can be obtained in the reflective mode. However, in the transmitted mode, the extinction ratio is just 4.78dB since the reflectivity of the Hi-Bi FBG is only 34.75%. It is shown analytically that the extinction ratio in the transmitted mode can be improved to 20dB by specifying a Hi-Bi FBG with a reflectivity of 99%. Finally, it is shown that the Bragg wavelength of the in-line fiber PBS can be tuned electrically to comply with the working wavelength.

Radial-basis function network for the approximation of FBG sensor spectra with distorted peaks

The implementation of a radial-basis function network to approximate spectra of the signal reflected by a fibre Bragg grating sensor is reported. This algorithm helps the interpretation of the data acquired with equipment as an optical spectrum analyser. Results using a double-peaked spectrum from a uniform fibre Bragg grating sensor show that the common practice of fitting the spectrum with different interpolation methods and finding its peak, or directly finding the maximum intensity position of the raw spectrum, would cause a larger error when compared to searching for the peak of an approximated spectrum using the proposed neural network. An example is demonstrated through two experiments measuring the volumetric shrinkage of polymeric resin using a uniform FBG and a HiBi FBG embedded in the material. The obtained accuracy is higher than that obtained with the simple non-processed peak detection.