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Electrical current monitoring plays an important role in power transmission system, electric instruments, etc. The stability of the monitoring system is highly demanded. In this paper, a new interrogation scheme for a fiber Bragg grating (FBG) current sensor based on a dual-loop optoelectronic oscillator (OEO) with high stability, high precision and simple structure is proposed and experimentally demonstrated. The wavelength shift of the FBG, which is bonded on a magnetostrictive alloy-strip, is converted into the frequency shift of the microwave signal through the OEO system utilizing the wavelength to frequency mapping mechanism. Different from previous single-loop OEO schemes, a length of ∼1 km single mode fiber is used to form the second loop in the OEO-cavity, which can enhance the mode-selection ability and reduce the total gain requirement of the system. In addition, a fiber-ring-laser (FRL) cavity is adopted by integrating the sensing FBG-head as the wavelength-selective component to improve the quality of the optical carrier signal. The measured side-mode suppression ratio (SMSR) is up to 62.59 dB and the free spectral range (FSR) is five times larger than the single-loop structure, resulting in highly-enhanced stability and so the precision. Most importantly, the frequency fluctuation is as low as 0.277 ppm, which is the minimum to the best of our knowledge. Compensating methods for the influence of the temperature and the hysteresis are also discussed. The proposed scheme provides a feasible solution to promoting the development of OEO-based interrogation technology and its commercial applications. [ABSTRACT FROM AUTHOR]

A new temperature fiber ring laser (FRL) sensor based on a cascaded Sagnac loops fiber structure is proposed and experimentally demonstrated. The optical FRL sensor consists of cascaded Sagnac loops by inserting two polarization-maintaining optical fibers (PMF) with slightly different lengths. PMF with length of 56 cm and 75 cm are used in Sagnac loops as a filter and sensing unit in laser cavity. The working principle of the sensor is based on the phase shift (θ) caused by birefringence between two principal polarization modes and enhance the sensitivity by constructing a Vernier-scale. In an appropriate temperature range (25 °C - 31 °C), the detection sensitivity of FRL sensor based on a cascaded Sagnac structure is significantly higher than other FRL sensors. Thanks to the laser sensing system, the sensitivity can be modulated by changing the free spectral range (FSR). The experimental results show that the temperature sensitivity of the cascade Sagnac structure sensor is - 4.031 nm / °C, which is five times higher than that of FRL sensor base on the single Sagnac structure. [ABSTRACT FROM AUTHOR]

A novel microfiber sensor is proposed and demonstrated based on a singlemode-tapered hollow core-singlemode (STHS) fiber structure. Experimentally a STHS with taper waist diameter of $26.5~\mu \text{m}$ has been fabricated and RI sensitivity of 816, 1601.86, and 4775.5 nm/RIU has been achieved with RI ranges from 1.3335 to 1.3395, from 1.369 to 1.378, and from 1.409 to 1.4175 respectively, which agrees very well with simulated RI sensitivity of 885, 1517, and 4540 nm/RIU at RI ranges from 1.3335 to 1.337, from 1.37 to 1.374, and from 1.41 to 1.414. The taper waist diameter has impact on both temperature and strain sensitivity of the sensor structure: (1) the smaller the waist diameter, the higher the temperature sensitivity, and experimentally 26.82 pm/°C has been achieved with a taper waist diameter of $21.4~\mu \text{m}$ ; (2) as waist diameter decrease, strain sensitivity increase and 7.62 pm/ $\mu \varepsilon $ has been achieved with a taper waist diameter of $20.3~\mu \text{m}$. The developed sensor was then functionalized for human chorionic gonadotropin (hCG) detection as an example for biosensing application. Experimentally for hCG concentration of 5 mIU/ml, the sensor has 0.5 nm wavelength shift, equivalent to limit of detection (LOD) of 0.6 mIU/ml by defining 3 times of the wavelength variation (0.06 nm) as measurement limit. The biosensor demonstrated relatively good reproducibility and specificity, which has potential for real medical diagnostics and other applications. [ABSTRACT FROM AUTHOR]

A unique type of refractive index (RI) sensor is proposed using a dual Mach-Zehnder interferometer (MZI) structure based on side-hole fibers (SHFs). The MZI structure contains two single-mode fibers (SMFs), two coreless fiber (CLF) sections and an SHF section, which are spliced together in the order of SMF-CLF-SHF-CLF-SMF. The SMFs and the CLFs enable light lead in/out and beam splitting/combining, respectively. As a special feature of the structure, one hole of SHF is exposed for liquid filling and to form two MZIs as well. Three types of sensors are fabricated, namely S1, S2 and S3. Numerical simulation and experimental studies have been conducted to characterize the sensing performance. The RI sensitivity of the S1 using the ∼550 μm long SHF section reaches 14,000 nm/RIU. When a shorter SHF section is used in S2, the detectable RI range is broadened due to larger FSR. When the closed hole of SHF in S3 is filled with liquid to introduce the Vernier effect, the sensitivity can be further enhanced to over 44,000 nm/RIU (i.e., Refractive Index Unit), which corresponds to the detection limit at the level of 1.0 × 10−5 RIU. This sensor design is original and easy to package, which gives it potential for label-free biochemical analyses. [ABSTRACT FROM AUTHOR]

An adaptive image restoration algorithm based on 2-D bilateral filtering has been proposed to enhance the signal-to-noise ratio (SNR) of the intrusion location for phase-sensitive optical time domain reflectometry (Ф-OTDR) system. By converting the spatial and time information of the Ф-OTDR traces into 2-D image, the proposed 2-D bilateral filtering algorithm can smooth the noise and preserve the useful signal efficiently. To simplify the algorithm, a Lorentz spatial function is adopted to replace the original Gaussian function, which has higher practicability. Furthermore, an adaptive parameter setting method is developed according to the relation between the optimal gray level standard deviation and noise standard deviation, which is much faster and more robust for different types of signals. In the experiment, the SNR of location information has been improved over 14 dB without spatial resolution loss for a signal with original SNR of 6.43 dB in 27.6 km sensing fiber. The proposed method has the potential to precisely extract intrusion location from a harsh environment with strong background noise. [ABSTRACT FROM PUBLISHER]

Besides distinct features on RF/optical signal generation, optoelectronic oscillators (OEOs) have also been rapidly developed as emerging techniques towards sensing, measurement, and detection. In this paper, we start with the conceptual architecture and the analytical model of OEOs. Then, three operation principles behind sensing, measurement, and detection applications are categorized, including the variation on the time delay of loop, the passband reconfiguration of microwave photonic filter in loop, and the oscillation gain from injection locking, which clearly clarify the X-to-frequency mapping (X denotes target parameter or signal) for supporting practical solutions and approaches. Next, a comprehensive review to advances in OEO-based sensing, measurement, and detection applications is presented, including length change and distance measurement, refractive index estimation, load and strain sensing, temperature and acoustic sensing, optical clock recovery, and low-power RF signal detection. As a new application example, a novel approach for in-line position finding is proposed. When a long fiber Bragg grating inserted into OEO is locally heated to slightly broaden its reflection spectrum, the target position heated is mapped into the oscillating frequency shift, according to the first operation principle. A sensitivity of 254.66 kHz/cm is obtained for position finding in the experiment. Afterward, solutions for calibration and stabilization are briefly introduced, which enable us to improve the accuracy and reliability. Finally, features and future prospects on the sensing, measurement, and detection applications are discussed, such as compact and integrated OEOs. [ABSTRACT FROM AUTHOR]