We report a novel, to the best of our knowledge, and simple technique to lock a 642 nm multi-quantum well diode laser to an external linear power buildup cavity by directly feeding the cavity reflected light back to the diode laser for enhancement of gas Raman signals. The dominance of the resonant light field in the locking process is achieved by reducing the reflectivity of the cavity input mirror and thus making the intensity of the directly reflected light weaker than that of the resonant light. Compared with traditional techniques, stable power buildup in the fundamental transverse mode TEM00 is guaranteed without any additional optical elements or complex optical arrangements. An intracavity exciting light of 160 W is generated with a 40 mW diode laser. Using a backward Raman light collection geometry, detection limits at the ppm level are achieved for ambient gases (N2, O2) with an exposure time of 60 s.

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The surface-enhanced Raman scattering (SERS) substrates enable a highly sensitive detection of furfural in the transformer oil. However, detection substrates with long-term stability are still extremely challenging. In this work, we anchored the thiol-containing coupling agents 2, 5-dimercapto-1, 3, 4-thiadiazole (DMTD) and 1, 4-benzenedithiol (BDT) on the surface of bubble copper (B-Cu) and flower-like silver nanoparticles (F-Ag), respectively. The three-dimensional SERS detection substrates with long-term stability by using a combination of chemical reduction and self-assembly methods were constructed. The substrate has a minimum detection limit of 10 −9 M for rhodamine B in oil with an enhancement factor of up to 2.23 � 10 7 . Importantly, the three-crystal B-Cu@F-Ag 1 @Au 5 substrate was used for the detection of furfural in the transformer oil with a detection limit of 2 mg/L and a relative standard deviation value of 2.46%. After 60 days of a simulated operation, the detection signal of furfural in the transformer oil samples at 75°C and still reached the initial value of 77.53%, indicating that the substrate has a good long-term stability. This triple frame structured SERS detection platform shows great potential in tracking furfural in the aging transformer oil mixing systems. | INTRODUCTIONPower transformers are the core equipment in the power transmission systems. During the actual operation of a transformer, its oil-paper insulation system is subjected to electrical, thermal and mechanical stress, and it produces aging characteristics [1][2][3][4]. As a typical aging characteristic, furfural can be used to accurately evaluate the aging degree of the transformer insulation system [5,6]. Chromatographic based spectrophotometry and high-performance liquid chromatography are routine detection methods for furfural in the transformer oil. These methods are expensive, and they are easily affected by the surrounding environmental factors and cannot complete the detection of a complex oil system [7,8]. To achieve a nondestructive and in situ detection of furfural dissolved in oil, Toshihiro et al. applied laser Raman spectroscopy to detect the standard sample of furfural in the transformer oil for the first time, and the minimum detection limit could reach only 104 mg/L [9], which cannot meet the furfural detection accuracy standard (4 mg/L) of the transformer oil after severe aging.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

In this article, with an anti-resonant hollow core fiber (ARHCF), fiber-enhanced Raman spectroscopy (FERS) for trace-gas sensing in a high-concentration gas background is demonstrated for the first time. The performance of the apparatus is verified by detecting trace-gas in the high concentration SF6 and gaseous impurities in the high concentration C2H6. With a 1.5 W laser source and 60 s exposure time, the limit of detection (LOD) of gases at tens of ppm levels is achieved, including carbonyl sulfide (COS), carbon tetrafluoride (CF4), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), acetylene (C2H2), ethylene (C2H4), propyne (C3H4), propylene (C3H6), and propane (C3H8). Quantification of multi-gas with great accuracy exceeding 94% is also realized. It shows that the FERS can demonstrate the ability of multi-gas sensing with high selectivity, sensitivity, and accuracy.

C 2 H 2 and H 2 , as important chemical and energy raw materials, can be produced effectively and environmentally friendly by the partial oxidation (POX) of CH 4 . Simultaneous analysis of intermediate gas compositions in the multiprocess (cracking, recovery, degassing, etc) of POX can regulate product generation and improve production efficiency. To overcome the disadvantage of common gas chromatography, we propose a fluorescence noise eliminating fiberenhanced Raman spectroscopy (FNEFERS) technique for simultaneous and multiprocess analysis of the POX process, in which the fluorescence noise eliminating (FNE) method can effectively eliminate the horizontal and vertical spatial noise to ensure ppm level limits of detection (LOD). The vibration modes of gas compositions related to each POX process such as cracked gas, synthesis gas, and product acetylene are analyzed. Meanwhile, the composition of three-process intermediate sample gases from Sinopec Chongqing SVW Chemical Co., Ltd is quantitatively and qualitatively analyzed simultaneously, along with the ppm level LODs (