Construction of Fully Integrated and Energy Self-Sufficient NO2 Gas Sensors Utilizing Zinc-Air Batteries.

Exploration of novel self-powered gas sensors free of external energy supply restrictions, such as light illumination and mechanical vibration, for flexible and wearable applications is in urgent need. Herein, this work constructs a flexible and self-powered NO2 gas sensor based on zinc-air batteries (ZABs) with the cathode of the ZABs also acting as the gas-sensitive layer. Furthermore, the SiO2 coating film, serving as a hydrophobic layer, increases the three-phase interfaces for the NO2 reduction reaction. The constructed sensors exhibit a high sensing response (0.3 V @ 5 ppm), an ultralow detection limit (61 ppb), a fast sensing process (129 and 103 s), and excellent selectivity. Moreover, the sensors also possess a wide working temperature range and a low working temperature tolerance (0.34 V at -15 °C). Simulations indicate that the hydrophobic surface at the cathode-hydrogel interface will accommodate more NO2 gas molecules at the reaction sites and prevent the influence of inner water evaporation and direct dissolution of NO2 in the electrolyte, which is beneficial to the enhanced gas sensing abilities. Finally, the self-powered sensing device is incorporated into a smart sensing system for practical applications. This work will pave a new insight into the construction of integrated and energy self-sufficient smart gas sensing systems.

Concentration Retrieval in a Calibration-Free Wavelength Modulation Spectroscopy System Using Particle Swarm Optimization Algorithm.

This paper develops a concentration retrieval technique based on the particle swarm optimization (PSO) algorithm, which is used for a calibration-free wavelength modulation spectroscopy system. As compared with the commonly used technique based on the Levenberg-Marquardt (LM) algorithm, the PSO-based method is less dependent on the pre-characterization of the laser tuning parameters. We analyzed the key parameters affecting the performance of the PSO-based technique and determined their optimal parameter values through testing. Furthermore, we conducted a comparative analysis of the efficacy of two techniques in detecting C2H2 concentration. The results showed that the PSO-based concentration retrieval technique is about 63 times faster than the LM-based one in achieving the same accuracy. Within 5 s, the PSO-based technique can produce findings that are generally consistent with the values anticipated.

Absorption path extension tunable diode laser absorption spectroscopy system with a dual fiber loop configuration.

Increasing absorbance by lengthening the absorption path is a direct and effective approach to improve the signal-to-noise ratio of infrared gas absorption spectroscopy. However, once the absorption path is extended by designing and optimizing the gas cell structure to a certain extent, a bottleneck will appear due to the difficulties in the optical alignment and the interference effect. A modified tunable diode laser absorption spectroscopy system with a dual fiber loop configuration is proposed that can extend the effective absorption path length of the original multipass cell several times. The relevant theoretical model has been established and its effectiveness has been verified through experiments.

A Robust Distributed Multipoint Fiber Optic Gas Sensor System Based on AGC Amplifier Structure.

A harsh environment-oriented distributed multipoint fiber optic gas sensor system realized by automatic gain control (AGC) technology is proposed. To improve the photoelectric signal reliability, the electronic variable gain can be modified in real time by an AGC closed-loop feedback structure to compensate for optical transmission loss which is caused by the fiber bend loss or other reasons. The deviation of the system based on AGC structure is below 4.02% when photoelectric signal decays due to fiber bending loss for bending radius of 5 mm, which is 20 times lower than the ordinary differential system. In addition, the AGC circuit with the same electric parameters can keep the baseline intensity of signals in different channels of the distributed multipoint sensor system at the same level. This avoids repetitive calibrations and streamlines the installation process.

A Time Difference Method for Measurement of Phase Shift between Distributed Feedback Laser Diode (DFB-LD) Output Wavelength and Intensity.

A time difference method to conveniently measure the phase shift between output wavelength and intensity of distributed feedback laser diodes (DFB-LDs) was proposed. This approach takes advantage of asymmetric absorption positions at the same wavelength during wavelength increase and decrease tuning processes in the intensity-time curve by current modulation. For its practical implementation, a measurement example of phase shift was demonstrated by measuring a time difference between the first time and the second time attendances of the same gas absorption line in the intensity-time curve during one sine or triangle modulation circle. The phase shifts at modulation frequencies ranging from 50 Hz to 50 kHz were measured with a resolution of 0.001π. As the modulation frequency increased the shift value increased with a slowed growth rate.

Demodulation algorithm used in single-beam system immune to light power drift.

A demodulation algorithm based on the head-tail technique is proposed for single-beam water vapor detection under rough environmental conditions, which is immune to fluctuations of light power. In the head-tail technique, collected data are processed by adding the head and tail data together and gradually approaching the center. The majority of photocurrent attenuation caused by optical loss can be effectively compensated by combining an optical intensity normalization coefficient in the method. The experiment indicates that, when the light power attenuates 4%, the deviation in a single-beam system is 1.29%, which is obviously superior to a dual-beam subtraction system whose deviation is 8.45%. The connection and advantages compared to a previous single-beam detection system have been discussed. The whole arrangement is simply designed without a beam splitter, of which the reliability and validity are fully verified by the experimental results.

Measurement and analysis of water vapor inside optical components for optical fiber H2O sensing system.

Water vapor existing inside internal end-face gaps of optical components of an optical fiber H2O sensing system makes it possible to influence the measurement accuracy and stability. The influence principle has been briefly analyzed based on the structure of three main optical components: a distributed feedback laser diode (DFB-LD), a collimator, and a photoelectric diode (PD). With application of a differential technique, the influence of water vapor inside the DFB-LD can be removed. With reasonable recombination of the collimator and the PD in a dual-beam detection system, the influence of water vapor inside the collimator and the PD's end-face gaps has been suppressed from more than 1.57×10(-3) to as low as -2.175×10(-5) in absorbance. After H2O isolation processing water vapor inside the end-face gaps of the DFB-LD, the collimator, and the PD can be utilized as a reference to design a simple but feasible H2O sensor. As a result, good linearity with an R2 of 0.9964 has been realized in a concentration range of 39-2110 ppm during an application test, and a long-term test of the designed H2O sensor against the S8000 with a difference of 10 ppm has been achieved.

Reliability analysis and comparison of demodulation methods for dual-beam wavelength-modulation spectroscopy water vapor detection.

Subtraction, division, and balanced ratiometric detection (BRD) are three extensively used demodulation methods for dual-beam wavelength-modulation trace gas detection. However, reliability comparisons among these methods under changing environmental conditions were rarely researched. In this paper, the influences of ambient temperature and bend loss of fibers are analyzed in detail, and the reliabilities of the subtraction, division, and BRD methods are quantitatively compared for the first time to our knowledge. When the ambient temperature is increased by 1°C, the deviation of the division method is only 0.29%, which obviously outperforms the subtraction method (2.90%) and the BRD method (0.55%). Furthermore, a concept, "power fluctuation rejection ratio," is introduced to compare the suppression effects of the subtraction, division, and BRD methods on the laser light source power fluctuation. The study results demonstrate that the division method provides better reliability when the ambient temperature or bending loss is varied. The validity and reliability are fully verified by the fact that the experimental results give good agreement with the theoretical simulation.

High-sensitive measurement of water vapor: shot-noise level performance via a noise canceller.

Taking advantages of distributed feedback laser diode a technique is described to achieve high-sensitive measurement for water vapor concentration. This technique, with a modified balanced ratio metric detection system, has improved the accuracy of measured absorption spectrum by two main aspects. Improvement by matching equivalent conductivity of signal or reference photo detector (PD) is presented, and with the additional matched resistance suppression for the power variation in the signal-beam has been improved from 53 to 88 dB. The importance of integrating amplifier bandwidth design from the circuit to the measured absorption spectrum has been demonstrated in our experiment. For a scan rate of 32 Hz with an optimal corresponding bandwidth of 15.9 kHz, the absorption spectrum is well described by Voigt profile, with a difference of 1% at an atmosphere pressure of 1 atm and a room temperature of 296 K. With the application of averaging and filtering, absorption sensitivity of 1.093×10(-6) for water vapor at 1368.597 nm has been demonstrated, and the corresponding concentration is 71.8 ppb in just a 10 cm path length.

Acquisition of phase-shift fiber grating spectra with 23.5 femtometer spectral resolution using DFB-LD.

A novel method based on distributed-feedback laser diode (DFB-LD) continuous wavelength-scanning for acquiring precise spectra of phase-shift fiber gratings is presented. Compared to the traditional method, the spectral resolution retrieved by this method is only limited by the optical line-width of the light source, which can reach up to the order of femtometer and is much higher than that of high-resolution optical spectrum analyzer (generally on the order of picometer). In addition, a Signal-to-Noise Ratio (SNR) advantage can be provided owing to a much higher spectral density of DFB-LD than amplified spontaneous emission (ASE) source. Precise spectra of three phase-shift fiber grating samples have been obtained at a resolution of 23.5 femtometer.