Lifang Xue

Temperature-insensitive fiber Bragg grating liquid-level sensor based on bending cantilever beam

A temperature insensitive fiber Bragg grating (FBG) liquid level sensor based on bending cantilever beam (BCB) is proposed and demonstrated. The BCB induces axial strain gradient along the sensing FBG, resulting in a Bragg bandwidth modulation. The broadening of FBG spectrum bandwidth and the reflection optical power change with the liquid level and they are insensitive to spatially uniform temperature variations. For a liquid level variation of 500 mm and a temperature range from 0/spl deg/C to 80/spl deg/C, the measured liquid level fluctuates less than 2% without any temperature compensation. By optical power detection via a pin photodiode, the liquid-level sensor avoids complex demodulation process and potentially costs low.

Temperature-insensitive fiber Bragg grating force sensor via a bandwidth modulation and optical-power detection technique

A technique for temperature-insensitive force measurement using a single fiber Bragg grating (FBG) based on strain-gradient-induced reflection spectrum-bandwidth modulation and optical-power detection is demonstrated. A specially designed bending cantilever beam (BCB) is used to induce axial-strain gradient along the sensing FBG, resulting in a Bragg bandwidth modulation. The broadening of the FBG spectrum bandwidth and reflection optical power linearly change with the applied force, and both of them are insensitive to spatially uniform temperature variations. For a temperature range from 20 degC to 80 degC, a linear response of force measurement up to 20 N with fluctuation less than 0.8% full-scale is achieved without any temperature compensation. The demodulation process is simplified by optical-power detection via a p-i-n photodiode, and the sensing system is potentially cost-effective

Temperature-insensitive fiber Bragg grating dynamic pressure sensing system

Temperature-insensitive dynamic pressure measurement using a single fiber Bragg grating (FBG) based on reflection spectrum bandwidth modulation and optical power detection is proposed. A specifically designed double-hole cantilever beam is used to provide a pressure-induced axial strain gradient along the sensing FBG and is also used to modulate the reflection bandwidth of the grating. The bandwidth modulation is immune to spatially uniform temperature effects, and the pressure can be unambiguously determined by measuring the reflected optical power, avoiding the complex wavelength interrogation system. The system acquisition time is up to 85 Hz for dynamic pressure measurement, and the thermal fluctuation is kept less than 1.2% full-scale for a temperature range of -10 degrees C to 80 degrees C.

Simultaneous measurement of stress and temperature with a fiber Bragg grating based on a loop thin-wall section beam.

We designed and demonstrated what we believe to be a novel sensor for simultaneous measurement of stress and temperature. A fiber Bragg grating is flatly adhered to the surface of a loop thin-wall section beam. The theoretical analyses and the experimental results show that both the central wavelength shift and the chirped bandwidth of the grating reflection spectrum have a linear relationship with the stress and the temperature, respectively, and the slopes of them are different. Therefore, the temperature and stress can be discriminated by interrogating the chirped fiber grating. Moreover, we also investigated the strain of the loop thin-wall section beam, and the results show that the strain is cosine proportional to the double positional angle.

Modal cutoff properties in germanium-doped photonic crystal fiber

The germanium-doped photonic crystal fiber (PCF) has some characteristics that differentiate it from pure-silica PCF for a germanium element being doped in the core, such as the intensified nonlinearity, the enhanced photosensitivity, and so on. To pave the way for the application of the Ge-doped PCF successfully, it is necessary to study its properties. We investigated the modal cutoff properties of Ge-doped PCF quantitatively by using the beam propagation method. The numerical results show that the effective refractive indices and the normalized frequency V of Ge-doped PCF not only depend on the normalized pitch delta/lambda but also depend on the normalized hole size d/delta, the modal cutoff boundary for the single mode-multimode of the Ge-doped PCF shift to the low d/delta side in contrast to the pure-silica PCF.

Force sensing with temperature self-compensated based on a loop thin-wall section beam

A novel method about force sensing with temperature self-compensated based on a loop thin-wall section beam is proposed and demonstrated. Two identical fiber Bragg gratings are rigidly affixed on the outer surface of the beam with the positional angle difference between the two gratings of 90/spl deg/. The wavelength space between the two reflected peaks of the gratings is proportional to the force and insensitive to temperature when the direction of the force and one of the two gratings are located along a diameter; therefore, the force sensing with temperature self-compensated can be achieved.

Enhanced nonlinearity in a simultaneously tapered and Yb 3+ -doped photonic crystal fiber

The nonlinearity of photonic crystal fibers (PCFs) can be enhanced by simutaneously tapering the fiber and doping rare-earth ions (such as Yb3+) or chalcogenide in the core. The properties of the Yb3+-doped and tapered PCF is numerically investigated by the beam propagation method. The results show that the difference of the optical field distribution between the doped and the pure silica PCF is not huge, and the mode field area of the doped PCF is only a little smaller than that of pure silica PCF. However, the nonlinearity of the doped PCF is enhanced dramatically by doping Yb ions in the core. Furthermore, the optimal boundary profile of the transverse structural parameters for maximal nonlinearity coefficient is acquired, and the boundary normalized pitch decreases exponentially with the boundary normalized hole size increasing. Finally, the longitudinal criterion for tapering is proposed, namely, that the normalized distance of the taper slope should be longer than 30 μm. All these results will provide references for enhancing the nonlinearity of PCFs.

Conditions for higher-order resonant modes to be excited in a photonic-crystal fiber Bragg grating

The modal cutoff properties in a germanium-doped solid-core photonic-crystal fiber Bragg grating (PCFBG) are investigated with the beam propagation method. The results show that the normalized frequency V of a PCFBG depends not only on the normalized pitch Lambda/lambda but also on the normalized hole size d/Lambda. In addition, the single-mode-multimode boundary profile of the PCFBG shifts to the low normalized hole-size side in contrast to that of the pure-silica solid-core photonic-crystal fiber (pure-solid PCF). Furthermore, besides the phase-matching condition and the electric field overlapping with the grating region, the inequality VPCF>pi also should be fulfilled for the higher-order resonant modes to be excited in the PCFBG.

A novel method for independent tuning of the center wavelength and the bandwidth of fiber Bragg grating

A novel method for independent tuning of the center wavelength and the bandwidth of a fiber Bragg grating (FBG) was proposed and demonstrated. An FBG was adhered to the outer surface of a loop thin-wall section beam. The center wavelength and the bandwidth of the FBG can be tuned independently through changing the force while the fiber grating locates at different special positions. Quasi-chirp-free wavelength tuning with 6.706 nm and bandwidth tuning with 5.368 nm under quasi-shift-free center wavelength were obtained experimentally. The experimental results are in good agreement with the theoretical analyses.

A digital liquid level sensor system based on parallel fiber sensor heads

The design and development of a digital liquid level sensor system based on parallel fiber sensor heads are presented. A novel parallel fiber sensor heads based on side-polished common single-mode optical fiber is designed and applied. When the sensor head contacts with the liquid, we observe a sharp decrease in the output intensity. The digital liquid level sensor system based on 16 sensor points is operated by using an optical source with limited power. The measurement resolution and range of the liquid level sensor can be selected by adjusting the distance between the sensor heads. The advantages of this sensor system include low cost, high sensitivity, simple structure, excellent real-time character, high stability, and temperature-insensitiveness in normal temperature and pressure.