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In this work, magnesium (Mg), copper (Cu), manganese (Mn) and chromium (Cr) in aluminum alloy samples were quantified by femtosecond laser-induced breakdown spectroscopy (fs-LIBS). The different parameters affecting the experimental results, including the laser pulse energy, moving speed of the 2D platform and spectral average number were optimized. The background signal preprocessing methods of median filtering (MF corrected) and Savitzky-Golay filtering (SG corrected) algorithms were used and the effect of the LIBS spectral analysis in the experiment investigated. The calibration curves of Mg, Cu, Mn and Cr elements were established separately and their corresponding detection limits (LODs) were calculated. After background correction, the LODs of Mg, Cu, Mn and Cr elements in MF corrected were 54.52, 11.69, 7.33 and 27.72 ppm, and in SG corrected were 59.15, 17.48, 14.75 and 31.97 ppm. The LODs of these elements in MF corrected and SG corrected have 1.4-5.2 and 1.2-2.5 improvement factors compared to those obtained using the fs-LIBS technique. This work demonstrates that background signal preprocessing methods are very helpful for improving analytical sensitivity and accuracy in quantitative analyses of aluminum alloys.

High-sensitivity fiber-optic interferometric refractive index sensors (FIRSs) have been of interest to researchers due to their potential to fabricate specific physical, chemical, and biological sensors. Fabricating interferometers working near the dispersion turning point (DTP) is an effective approach to improve the sensitivity of FIRSs. However, the group effective refractive index (RI) difference approaching 0 and the ratio of the variation of the effective RI difference to the external RI change being −1 cannot be simultaneously realized in low RI sensing, which restricts the further improvement of sensing sensitivity. Here, dual-path Mach-Zehnder interferometers (MZIs) with unequal geometrical path length are proposed for ultrasensitive RI sensing. The dual-path MZIs contain fiber path and sample path of different geometrical lengths to form the optical path difference. The dual-path MZIs can not only show a turning point where the sensing sensitivity tends to be infinite, which is similar to the previously reported DTP, but also get the constant value −1 for the RI response factor, leading to the result that the dips within an over-500-nm band width around the turning point can achieve high sensitivity reaching 105 nm/RIU level or higher. Ultrahigh sensitivity of −1.26 × 106 nm/RIU has been experimentally demonstrated at the RI around 1.35022. The dual-path MZIs proposed here may enlighten new ideas for developing high-sensitivity FIRSs.

Simultaneous measurement of pressure and temperature in seawater is realized based on polydimethylsiloxane (PDMS) sealed microfiber Mach–Zehnder interferometer (MZI). Benefitting from the high thermal-optic coefficient and large elasticity of PDMS, high sensitivity and robust structure can be obtained. In theoretical modeling, significant enhanced pressure response in fiber is observed clearly, and pressure or temperature sensitivities are calculated theoretically. In experiment, PDMS sealed MZI is fabricated by a two-step sealing method. Using this MZI, sensitivity of 13.31 nm/MPa for pressure sensing and −7.41 nm/°C for temperature sensing are demonstrated, respectively, which are about one order higher than bare fiber MZIs (without PDMS sealing). To verify the accuracy of sensor, several tests under arbitrarily pressure and temperature are performed with average errors of 4.38% and 1.44%, respectively. In addition, effects of encapsulation on avoiding cross-sensitivity with salinity, PDMS thickness on pressure sensitivity, response time, repeatability, time stability, and polarization of the sensor are also evaluated by experiment. Sensors demonstrated here show advantages of low cost, simple fabrication, robust and compact structure, high-pressure resistance, high sensitivity, good repeatability, and long-term stability.

Lake health assessment (LHA), a powerful tool for lake ecological protection, provides the foundation for sustainable water environment management. However, existing methods have not yet considered the effects of fuzziness and randomness on LHA. In addition, most of the current studies on LHA focus on the plain areas, lack of quantitative studies in mountain areas, such as the Qinghai-Tibet Plateau. The Pythagorean fuzzy cloud (PFC) integration algorithm drawing on the advantages of Pythagorean fuzzy sets (PFS) and cloud model was proposed. A novel hybrid decision-making framework combining PFC integration algorithm and TOPSIS model was developed to determine the lake health levels with fuzziness and randomness. An indicator system incorporating ecosystem integrity (physical habitat, water quantity and quality, aquatic life) and non-ecological performance (social services) was established. To comprehensively investigate the lake health level in the Qinghai-Tibet Plateau, the Nam Co was selected as study area. Our results confirm that the developed framework in this study can overcome the shortcomings of existing methods and provide a more effective approach for LHA with fuzziness and randomness. In Nam Co, the non-ecological performance was significantly better than the ecosystem integrity. Health levels exhibited a remarkable spatial variation influenced by tourism and grazing, with decreasing health status from the northwestern to southeastern Nam Co. Approximately 85% of the sampling sites were at excellent or healthy levels, 15% were subhealthy, and no sampling sites were unhealthy and sick. Our results highlight that tourism has affected health levels at Nam Co, and effective measures are needed to minimize the impact in ecological fragile areas.

An all-fiber hybrid structure Mach-Zehnder interferometer (MZI) based on silica fiber and fluorinated polyimide microfiber (FPMF) for temperature or salinity sensing in seawater is proposed. Theoretically, transmission characteristic and sensing sensitivity of the MZI are derived for this kind of hybrid structure, and typical interference spectrum is observed around optical communication band. Experimentally, by tracking the shift of interference peaks, temperature and salinity sensing are demonstrated with sensitivities of about 140 pm/°C and 64 pm/‰, respectively, which are much higher than those of existing fiber Bragg gratings and microfiber knot resonators. In addition, dependences of sensitivity on sensor parameters, such as fiber diameter and fiber lengths, are also investigated. Unlike the other microfiber based sensors that higher sensitivity usually comes from the thinner fiber, sensors demonstrated here show that sensitivities can be tuned by changing the path-length difference of the two arms, which greatly decreases the difficulty in component fabrications. Temperature or salinity sensor demonstrated here show advantages of simple fabrication, low cost, tunable sensitivity and dynamic range, relatively high sensitivity and compatible with standard fiber optics system, which may have potential applications in developing low cost and compact size optical sensors in ocean or other liquid surroundings.

Rivers provide the basis for sustainable socio-economic development, but increasingly intense human activities poses a threat to river health. To comprehensively investigate the river health in the Qinghai-Tibet Plateau, China, the Lhasa River is selected as study area, and a comprehensive indicator system incorporating ecosystem integrity (physical habitat, water quantity and quality, aquatic life) and non-ecological performance (social services) is constructed. The random impacts of sampling methods and environmental factors on the accuracy of monitoring data are effectively reduced by the wavelet noise reduction. Combined weights of river health indicators are calculated using the minimum deviation principle. A novel multidimensional similarity cloud model (MSCM) is then developed for assessing river health with fuzzy monitoring data. Results show that the upper reaches of Lhasa River is healthy in terms of ecosystem integrity and non-ecological performance. And the health level exhibits significant spatial variability influenced by urban construction and water conservancy projects, with decreasing health level from upper to lower reaches. 52% of the sampling sites are at excellent or healthy levels, 40% are subhealthy, and 8% are unhealthy in the Lhasa River. Accordingly, recommendations and measures are proposed to improve river health level and explore sustainable water-management policies in the Qinghai-Tibet Plateau, China.

An ultrasensitive Panda-microfiber (PMF)-based multimode interferometer for salinity measurement is proposed and demonstrated. By inserting a section of PMF supporting a few modes into a conventional optical fiber system, an inline interference between the high-order modes of PMF, i.e., ${\rm{TE}}_{{\rm{01}}}$ and ${\rm{TM}}_{{\rm{01}}}$ , is realized. The sensitivity is as high as 2938.16 pm/‰ and its resolution is up to 0.0014‰ (0.0001 S/m), which is two magnitudes higher than other fiber salinity sensors. With the features of compactness, reconfigurability, stability, and compatibility with other fiberized components, PMF has great potential in tunable filtering, multiwavelength lasing, etc.

Investigating the wastewater treatment efficiency (WTE) is an important component in fostering sustainable water management. This study proposes an environmental production technology framework based on eutrophication levels and heavy metal concentrations caused by wastewater discharge. The slack based measure (SBM) model with undesirable outputs and logarithmic mean Divisia index (LMDI) decomposition model are developed to quantitatively investigate the spatiotemporal distribution and driving effects in urban wastewater treatment under the context of China's River Chief system. Results showed that the distribution curve of WTE changes from double peaks to a single peak, and WTE value appears a decreasing (2001–2010)-increasing (2011–2020) trend. Provinces with high WTE are mainly distributed in eastern China, with low WTE are in western China. Overall, wastewater treatment efficiency has been generally improved through the implementation of China’s River Chief system. And provinces with low WTE are shrinking, with high WTE are extending from southeastern to northwestern China. To achieve a simultaneous improvement in wastewater treatment efficiency, it is necessary to improve the WTE in corresponding provinces that belong to barely effective development and inefficient development. Meanwhile, provinces in superiorly effective development and slightly effective development should be encouraged to maintain their high-level WTE.

This paper investigates the transmission spectral characteristics of microfiber MZ interferometer with a knot resonator (MZIKR). The MZIKR exhibits a combined effect of resonance and interference in its transmission spectra. By adjusting the coupling coefficients to convert the relative strength of resonance and interference, the transmission spectra show various shapes, mainly reflected in the direction of fringes. The obtained upward fringe exhibits an extinction ratio of 17 dB and a FWHM of 0.3 nm. The tunable transmission spectrum demonstrated here may have great potential for narrow-band filtering, and the coexisting resonance and interference effect also allows the MZIKR to perform dual-parameters sensing.

Dual-points temperature measurement in seawater is demonstrated based on a seawater temperature sensor assembled by the highly-birefringent elliptical-core fiber (HBECF). The theoretical and experimental transmission spectra of the sensor are studied respectively, which shows good agreement with each other. And a matrix equation is established by analyzing the experiment results, by which the temperature changes can be obtained by measuring the wavelength shifts of spectra. The minimum temperature change that can be detected is 0.016 °C and the measurement error is about 0.1%. The advantages of simple structure, high accuracy, high sensitivity, fast response and anti-electromagnetic interference make the sensor meet the demands of seawater temperature measurement.

It is significant to develop new methods for temperature and salinity measurement in seawater since they are two important parameters in physical oceanography application. In this paper, a new approach for simultaneous measurement of temperature and salinity in seawater based on microfiber Mach-Zehnder (MZ) interferometer with a knot resonator is proposed. The typical transmission spectra show the combined effect of MZ interference and knot resonance. When the temperature and salinity change, both the interference dip and the resonance dip shift linearly. Based on the linear relation, the measurement matrix is established, and the temperature and salinity of seawater sample can be obtained simultaneously by collecting the transmission spectra. The hybrid structure demonstrated here shows advantages of high sensitivity and good accuracy, which may have great potential for temperature and salinity measurement in ocean and other liquid surroundings.

Combined with non-adiabatic tapering and mode field mismatch between two different fibers, a splicing point tapered fiber Mach-Zehnder interferometer is proposed. Theoretically, two sensing units are carefully designed for dual-parameter sensing, and mode exciting and evolution in fibers are analyzed and shown dynamically. In experiment, transmission spectrum is obtained with two sets of clear interference as designed. After a simple capsulation, simultaneous measurement of seawater temperature and salinity are realized with sensitivities of −994.83pm/°C and 290.47pm/‰, respectively. Additionally, short response time of 33ms and good repeatability are also demonstrated. And effects of encapsulation on avoiding strain and press are verified experimentally. The MZI demonstrated here shows advantages of low cost, easy fabrication, simple construction, compact and robust structure, and dual-parameter sensing with high sensitivity and fast response.

Unsteady flow induced by hydropower stations exerts a significant impact on the water level in multi-approach channels, which directly threatens the safe passage of ships. In this study, a one-dimensional and a two-dimensional hydrodynamic model are adopted to simulate the water level fluctuations at the entrance of multi-approach channels and the lower lock head of a ship lift with consideration of initial water surface elevation, base flow, flow amplitude, regulation time, and locations of hydropower stations, unfavorable conditions are successfully identified, and the fluctuations at the approach channel entrance and the lower lock head of a ship lift under single-peak and double-peak regulating mode are analyzed considering the flow regulating of the Gezhouba Hydropower Station (GHS), thus, the water level oscillation process in the multi-approach channels is presented. Results show that the largest wave amplitude in the multi-approach channels manifests under unfavorable conditions including lower initial water surface elevation, smaller base flow, larger flow variation, and shorter regulation time, and water level fluctuation in the multi-approach channel is primarily induced by flow amplitude and net flow between the Three Gorges Hydropower Station (TGHS) and the GHS, with consideration of the counter-regulation process of the GHS. This research contributes to providing a reference for a similar large-scale cascade hydropower station regarding regulation and control of navigation conditions.

The nitrate concentration is extremely low in seawater, which needs highly sensitive sensors to measure. In this paper, a new method to improve the sensitivity of nitrate concentration measurement in seawater based on dispersion turning point (DTP) theory is demonstrated using a microfiber mode interferometric sensor. The high sensitivity is achieved by the interference of HE11 mode and HE12 mode. Different parameters that influence DTP have been calculated theoretically, including positions of dips and diameters for obtaining DTP, the relationship between DTP and group effective refractive index difference, and the effects of refractive indexes of solutions on the DTP. Calculated results show that the sensitivity of dip will be ultra-high near the DTP and the ultra-high sensitivity region is very narrow in a certain band or a certain diameter. In the experiment, six sensors with different diameters are fabricated to realize the concentration of sodium nitrate sensing in seawater. By optimizing the parameters of sensor, the maximum sensitivity and detection limit of sodium nitrate in seawater can reach to be 5.98 pm/ppm (refractive index sensitivity is 50396.09 nm/RIU) and 0.17 ppm, respectively. The sensor demonstrated here shows advantages of easy to construct, low cost, high precision and high sensitivity, which provides a new optical detection method for in-situ detection of marine environment or low concentration substances sensing in other liquids.

In this paper, an ultrahigh sensitivity sensing in aqueous solution for microfiber modal interferometer (MMI) is realized. The group refractive index (RI) difference between HE11 and HE12 mode will come down to 0 at a certain wavelength, resulting in ultrahigh sensitivity near this wavelength. MMI with different diameters have their individual ultrasensitive wavelength band, which indicates the broad range of optional probing wavelength and more liberal diameter condition on fiber fabrication. In the experimentation, infrared absorption band in water around 1360-1600nm is easy to keep away by adjusting the microfiber diameter. As a result, an ultrahigh sensitivity of 14.95 pm/ppm is realized for sodium nitrate at 1320nm, whose equivalent sensitivity is about 1.26 × 105-nm/RIU for RI, which is much higher than most of the existing naked sensors with magnitude of 102-104nm/RIU.

A new method for seawater temperature sensing based on a high-birefringence elliptic fiber (HBEF) Sagnac loop is proposed in this letter due to the linear relationship between the change of temperature and the shift of interference peak. The transmission spectra of HBEF Sagnac loop at different seawater temperatures are studied theoretically and experimentally, which show good agreement with each other. The results show that sensing sensitivity increases with the increasing HBEF length and the decreasing wavelength, which is just opposite to the dependence of free spectrum range on the fiber length and the probing wavelength. Thus, there may exist an optimal fiber length and probing wavelength for the highest sensitivity and the largest detection range. By optimizing the parameters of probing system, the sensitivity of −472 pm/°C is obtained for temperature sensing, and the detection range from 10 °C to 30 °C is kept with the wavelength of 1310 nm (communication window) and the fiber length of 98.9 cm. Seawater temperature sensors demonstrated here show the advantages of small size, low cost, easy fabrication, high sensitivity, and easy integration with fiber systems.

The microfiber knot-type ring resonator is widely used in sensing, and the resonant mode is an essential concern. In this paper, resonant mode characteristics of microfiber knot-type ring resonator are studied theoretically. Results show that resonant mode split occurs when the deviation angle is large enough under the condition that the input is linearly or elliptically polarized light. Moreover, in the case that the input is partially polarized light, the resonant peak positions of split modes are identical with those in the case of inputting linearly or elliptically polarized light under the same deviation angle. The split mode characteristic is further studied by salinity sensing simulation and experiment. The split modes have the same free spectral range, and their resonant peak shifts have the same response to salinity changing. The experimental split mode characteristic is in good agreement with theoretical simulating, which demonstrates the resonant mode split theory of the microfiber knot-type ring resonator. The analysis on resonant mode presented here may offer useful references for sensing applications.

Ocean internal-wave phenomena occur with the variation in seawater vertical temperature, and most internal-wave detections are dependent on the measurement of seawater vertical temperature. A seawater temperature sensor based on a microfiber knot resonator (MKR) is designed theoretically and demonstrated experimentally in this paper. Especially, the dependences of sensing sensitivity on fiber diameter and probing wavelength are studied. Calculated results show that sensing sensitivity increases with the increasing microfiber diameter with the range of 2.30–3.91 μm and increases with the increasing probing wavelength, which reach good agreement with results obtained by experiments. By choosing the appropriate parameters, the maximum sensitivity measured can reach to be 22.81 pm/°C. The seawater temperature sensor demonstrated here shows advantages of small size, high sensitivity, easy fabrication, and easy integration with fiber systems, which may offer a new optical method to detect temperature of seawater or ocean internal-wave phenomenon and offer valuable reference for assembling micro sensors used for other parameters related to seawater, such as salinity, refractive index, concentration of NO3− and so on.

The salinity and temperature of seawater are important parameters in oceanography. Based on the directional coupler assembled by polyimide-coated micro/nanofibers, optical sensors with high sensitivity for simultaneous salinity and temperature sensing in seawater are proposed. Dependences of sensitivities on wavelength, salinity, and temperature are investigated theoretically, with which performances of such sensor under general sea conditions can be evaluated. Results show that salinity and temperature sensitivities can reach levels of nm/‰ and nm/°C, which are much higher than those of fiber Bragg gratings, knot resonators, and photonic crystal fibers. Other considerations for system design such as the length of the coupling area, the diameter difference between two fibers, and the thickness of polyimide coatings are also discussed. Sensors proposed here suggest a simple approach to realize high-sensitivity micro/nanofiber optical sensing of salinity and temperature in seawater simultaneously and may find applications in developing miniature sensors used in seawater.

An all-fiber Mach-Zehnder interferometer (MZI) for two quasi-continuous points' temperature sensing in seawater is proposed. Based on the beam propagation theory, transmission spectrum is designed to present two sets of clear and independent interferences. Following this design, MZI is fabricated and two points' temperature sensing in seawater are demonstrated with sensitivities of 42.69pm/°C and 39.17pm/°C, respectively. By further optimization, sensitivity of 80.91pm/°C can be obtained, which is 3-10 times higher than fiber Bragg gratings and microfiber resonator, and higher than almost all similar MZI based temperature sensors. In addition, factors affecting sensitivities are also discussed and verified in experiment. The two points' temperature sensing demonstrated here show advantages of simple and compact construction, robust structure, easy fabrication, high sensitivity, immunity to salinity and tunable distance of 1-20 centimeters between two points, which may provide references for macroscopic oceanic research and other sensing applications based on MZIs.