Due to the extremely complex working conditions, various health and safety hazards are present in underground coal mines, which cause economic losses and heavy casualties. Among these hazards, methane gas explosion and coal combustion are recognized as the two major hazards to miners. Traditional electronic sensors in mine safety monitoring systems have problems such as low precision, a large amount of maintenance, and monitoring dead zones. In the past decade, gas sensors based on tunable diode laser absorption spectroscopy (TDLAS) have been extensively studied and tailored for use in the coal mine industry because of their advantages of high sensitivity, high stability, fast response, intrinsic safety, and remote monitoring. This invited paper introduces the recent progress and typical applications of TDLAS-based methane sensors, carbon monoxide sensors, and multi-gas monitoring systems in coal mine gas monitoring, fire prevention, and early warning in intelligent coal mines.

With the development of energy efficiency technologies such as 5G communication and electric vehicles, Si-based GaN microelectronics has entered a stage of rapid industrialization. As a new generation of microwave and millimeter wave devices, High Electron Mobility Transistors (HEMTs) show great advantages in frequency, gain, and noise performance. With the continuous advancement of material growth technology, the epitaxial growth of semiconductor heterojunction can accurately control doping level, material thickness, and alloy composition. Consequently, HEMTs have been greatly improved from material structure to device structure. Device performance has also been significantly improved. In this paper, we briefly describe MOCVD growth technology and research progress of GaN HEMT epitaxial films, examine and compare the “state of the art” of enhanced HEMT devices, analyze the reliability and CMOS compatibility of GaN devices, and look to the future directions of possible development.

Owing to interference fringes in the multireflective gas cell, the detection sensitivity of a system in tunable diode laser absorption spectroscopy (TDLAS) will decrease significantly. In this work, a combined scheme of complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) and wavelet threshold denoising (WTD) is proposed. Theoretical simulations were performed to validate the effects of the proposed algorithm, which was also verified via a CO₂ detection experiment. After CEEMDAN-WTD processing, the noisy intrinsic mode function (IMF), pure IMF, and residual components of the detection signal were identified and reconstructed successfully. Based on analysis of the simulations, CEEMDAN-WTD algorithm improved the signal-to-noise ratio by 1.87 times and decreased the root mean-squared error by 37.6% than the moving average algorithm. For the CO₂ detection system, R² = 0.999 was determined by the calibration experiment. Additionally, based on Allan variance analysis and a long-time experiment, the limit of detection was estimated to be 3.08 ppm for an average time of 148 s and measurement accuracy of 0.65%, respectively. The obtained results sufficiently validate that the CEEMDAN-WTD algorithm can effectively suppress interference fringe noise in TDLAS.

Semiconductor lasers have developed rapidly with the steady growth of the global laser market. The use of semiconductor laser diodes is currently considered to be the most advanced option for achieving the optimal combination of efficiency, energy consumption, and cost parameters of high-power solid-state and fiber lasers. In this work, an approach for optical mode engineering in planar waveguides is investigated. The approach referred to as Coupled Large Optical Cavity (CLOC) is based on the resonant optical coupling between waveguides and allows the selection of high-order modes. The state-of-art of the CLOC operation is reviewed and discussed. We apply the CLOC concept in our waveguide design strategy. The results in both numerical simulation and experiment show that the CLOC approach can be considered a simple and cost-efficient solution for improving diode laser performance.

Recent experiments have shown that more than 85% of photo-generated carriers can escape from multiple quantum wells (MQWs) sandwiched between p-type and n-type layers (PIN). In this work, we quantitatively analyze the relationship between the energy of carriers and the height of potential barriers to be crossed, based on the GaAs/InGaAs quantum well structure system, combined with the Heisenberg uncertainty principle. It was found that that the energy obtained by electrons from photons is just enough for them to escape, and it was found that the energy obtained by the hole is just enough for it to escape due to the extra energy calculated, based on the uncertainty principle. This extra energy is considered to come from photo-generated thermal energy. The differential reflection spectrum of the structure is then measured by pump–probe technology to verify the assumption. The experiment shows that the photo-generated carrier has a longer lifetime in its short circuit (SC) state, and thus it possesses a lower structure temperature than that in open circuit (OC). This can only explain a thermal energy reduction caused by the continuous carrier escape in SC state, indicating an extra thermal energy transferred to the escaping carriers. This study is of great significance to the design of new optoelectronic devices and can improve the theory of photo-generated carrier transports.

According to the classical theory, multi-quantum wells (MQWs) have quantum confinement effect on photo-generated carriers, which limits its application in light-to-electric devices. However, relevant experiments showed that a large proportion of photo-generated carriers can escape from MQWs sandwiched in the p-type layer and n-type layer (PIN structure), but not in the NIN structure. In order to study this phenomenon carefully, we applied positive and negative bias to an NIN structure respectively to simulate the PIN structure. By analyzing the photoluminescence (PL) spectra, we observed a weak escape behavior of photon-generated carriers among QWs in NIN structure. The experiment results indicate that strong electric field could drive carriers to escape from QWs rather than relaxation and recombination, while in NIN structure the inhomogeneous distribution of the electric field intensity reduces the carrier transport efficiency. The further study will give new ideas to design and produce photoelectric devices.