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In industries such as coal mining, natural gas transportation and waste-to-energy, methane detection is an essential step. In order to reduce the risk and improve accuracy, laser sensors are used to detect methane. Aiming at the characteristics of the absorption peak of methane gas at 1653.7 nm, the 1653.7 nm distributed feedback laser was obtained from the multi quantum well materials design and ridge-wide pattern design to device packaging by using metal-organic chemical vapor epitaxial deposition, holographic exposure, and nanoimprint lithography. The laser performance achieves a side-mode suppression ratio of 54 dB, a slope efficiency of 0.372 W/A, a threshold current not greater than 12 mA, a saturated optical power greater than 20 mW, and stable optical and electrical properties. Based on this laser, for methane gas with a concentration of 0% to 3%, the loss is stable with the change of gas concentration, and the absorption sensitivity to methane is 0.20237 dB/%. [ABSTRACT FROM AUTHOR]

High-power mid-infrared 2.8- $\mu \text{m}$ ultrafast laser has potential applications in the fields of laser surgery due to its wavelength overlap with the absorption peak of water. In this work, we demonstrate a 2.8- $\mu \text{m}$ picosecond Raman laser in a methane-filled hollow-core anti-resonant fiber (HC-ARF) with average power of 4 W, pulse energy of 40 $\mu \text{J}$ , and beam quality of 1.47. The laser beam quality improvement at mid-infrared Raman conversion in methane-filled HC-ARF is firstly observed. This work heralds a promising approach to generate mid-infrared ultrafast Raman laser with multi-Watt level average power and high beam quality. [ABSTRACT FROM AUTHOR]

Irradiating hohlraum targets with high-power laser pulses is a promising approach to achieve inertial confinement fusion (ICF). Unfortunately, such interaction is accompanied by intense electromagnetic pulses (EMPs). The EMPs can interfere with various physical diagnostic signals and even cause a malfunction of some crucial equipment. In this study, EMPs generated due to kJ-laser coupling with hohlraum targets are recorded using B-dot probes inside the target chamber. The configuration of the hohlraum has a more remarkable effect on EMP amplitudes compared to laser energy. 0.6 mg/cc of He gas filling in the hohlraum and the presence of the capsule inside the hohlraum both dramatically reduce EMPs. Moreover, EMP frequencies corresponding to various hohlraums are basically below 1.75 GHz. Soft X-ray (0.1–4 keV) radiant flux is used to reveal the characteristics of EMPs. The conclusions are expected to enable us to better understand the relationship of EMPs with target configuration and laser energy. [ABSTRACT FROM AUTHOR]

SO2F2 and SOF2 are important characteristic decomposition components of SF6. Accurate detection of these two gases is helpful for fault diagnosis of gas-insulated switchgear (GIS). In this article, the infrared characteristic peaks of SO2F2 and SOF2 were obtained by comparing the simulation and experimental infrared spectrum. Then, cross interference analysis was performed with SF6 characteristic components such as H2S, SO2, CO, and SF6 background gas to determine the appropriate detection bands for SO2F2 and SOF2. Tunable diode laser absorption spectroscopy (TDLAS) experiment platform was built based on quantum cascade laser with center wavelength at 1506.4 and 1340.2 cm−1. The concentration of SO2F2 and SOF2 is detected through this platform. And the concentration information of gas is deduced by detecting the voltage value of second harmonic signal. The experimental results show that, for SO2F2 gas between 1.01 and 19.88 ppm, the $R^{2}$ is 0.989 and the maximum relative error is 2.044%. For SOF2 gas between 1.03 and 9.00 ppm, the ${R}^{{2}}$ is 0.992 and the maximum relative error is 0.728%. SO2 gas may cause weak interference to SOF2 detection, which has some regularities, and the interference can be eliminated by selecting the appropriate SOF2 fitting linear equation. The research results provide a feasible scheme for the quantitative detection of SO2F2 and SOF2 gases. [ABSTRACT FROM AUTHOR]

High spatial resolution two-dimensional (2D) imaging of gas flow is essential for better understanding and characterization of a combustion field. We propose a method for 2D gas measurement with high spatial resolution by combining tunable laser absorption spectroscopy and an imager with an electronic rolling shutter through their common feature of time scanning. The method was demonstrated by oxygen measurement using a 764nm vertical cavity surface emitting laser and a low-cost CMOS camera. 2D imaging of the oxygen concentration distribution were obtained at a frame rate of 15fps with a horizontal spatial resolution of about $28\mu \text{m}$. This method enjoys simple structure, low cost, high spatial resolution and shows potential in the 2D and 3D reconstruction of the combustion field. [ABSTRACT FROM AUTHOR]

In this work, a novel configuration of a photothermal gas sensor is demonstrated. The measured gas sample is delivered to a gas cell placed inside the linear cavity of a 1.55 μm mode-locked fiber laser. The gas refractive index modulation resulting from the excitation by an auxiliary continuous wave laser is efficiently probed by analyzing the phase shift of the self-heterodyne beatnote signal of the mode-locked laser. IQ demodulation combined with Wavelength Modulation Spectroscopy-based signal analysis was employed to optimize and simplify the spectroscopic data acquisition and analysis. A proof-of-concept experiment with detection of carbon dioxide at 2 μm wavelength yielded a noise equivalent absorption of 2.25·10−7 for 1000 s integration time. The proposed sensor configuration is versatile and can be used to probe any gas molecule, provided an appropriate excitation source is used to induce the photothermal effect. [ABSTRACT FROM AUTHOR]

In this letter, an infrared photoinverter with a GeSe 2-D/ $\alpha $ -PbSe heterostructure and a high detectivity ($5.8\times 10 ^{14}$ Jones) and responsivity ($1.26\times 10$ 6 V/W) has been proposed for applications in spectroscopy detectors and quick chemical gas identification. The design concept is based on three criteria: 1) an enhanced heterostructure constructed from a GeSe 2-D nanomaterial with PbSe and an abrupt PN junction; 2) a photoinverting device that can conveniently perform I-V conversion; and 3) a compatible sensing unit that includes a spectral ppb-level trace gas sensing module. Our work, which used a tunable DFB (Distributed Feedback) laser and retained high spectral sensitivity (1550-1750 nm), presents a novel technical method that meets the various requirements for cost-effective trace gas detection in the infrared region. [ABSTRACT FROM AUTHOR]

We proposed a six-gas simultaneous detection system for methane (CH4), ethane (C2H6), acetylene (C2H2), ethylene (C2H4), carbon monoxide (CO) and carbon dioxide (CO2) based on wavelength modulation spectroscopy (WMS). Five different near infrared distributed feedback (DFB) lasers combined with a custom $5\times1$ wavelength division multiplexer were used to detect the six gases. The lasers were modulated at five different sinusoidal signals and the detection light intensity signals were demodulated with corresponding sinusoidal signals, which can be named frequency division multiplexing (FDM). Among them, CH4 and C2H6 were measured with one laser based on time division multiplexing (TDM). The principle of four multi-gas detection based on FDM was also derived. To miniaturize the device, a Raspberry Pi was used for data processing based on python, and the detected gas concentrations can be displayed directly on the screen. The six-gas sensing system exhibits detection limits of 247 ppb, 1.28 ppm, 126 ppb, 982 ppb, 2.55 ppm and 4.87 ppm with average time of 1 s for CH4, C2H6, C2H2, C2H4, CO and CO2, respectively. With an average time extended to 100 s, the measurement precision achieves 24 ppb, 135 ppb, 14 ppb, 141 ppb, 495 ppb and 608 ppb, respectively. With a fast response time, compact size and high detection sensitivity, the detection system is promising for dissolved gas analysis (DGA) in transformer oil and other material aging process. [ABSTRACT FROM AUTHOR]

A novel near-infrared light-induced thermoelastic spectroscopy (LITES) based on a multilayer surface-coated quartz tuning fork (QTF) is proposed for gas detection. Thin-film of reduced graphene oxide (rGO) and polydimethylsiloxane (PDMS) is successively coated on the surface of QTF to improve the laser absorption and thermoelastic conversion efficiency, respectively. NH3 is selected as the target analyte for the measurements. The experimental results indicate that introducing a thin layer of PDMS-rGO composite allows a maximum gain factor of 8.64 and 5.53 for the signal amplitude and signal-to-noise ratio (SNR), respectively, compared to a commercial standard QTF. A minimum detectable concentration of 8.98 ppmv was obtained, improved by 5.12 times than that of a bare QTF, and a normalized noise equivalent absorption coefficient (NNEA) of ${1.53}\times {10}^{-{9}}\,\text {cm}^{-{1}} \cdot \text {W} \cdot \text {Hz}^{-\text {1/2}}$ was achieved. The results of Allan deviation analysis show that the detection sensitivity reaches 2.63 ppmv when the averaging time is 555 s. The research work and related results have laid a good foundation for trace gas detection by the cost-effective and easy-to-operate infrared sensor. [ABSTRACT FROM AUTHOR]

Soft-glass fiber lasers have attracted lots of attentions as they are most likely to achieve compact, stable, and highly efficient mid-infrared emission, which has unique applications in many fields. However, due to the immature manufacturing technique of soft glass fibers and the limited types of doped rear-earth, laser power enhancing and wavelength expansion beyond 4 μm are greatly restricted. Gas-filled hollow-core fibers (HCFs) have been proved to be an effective way to generate mid-infrared emission. We previously demonstrated a continuous-wave 4.3 μm fiber source with power of 82 mW in CO2-filled HCFs. Here, the maximum output power is increased to 557 mW, and laser wavelengths densely step-tunable from 4276 to 4428 nm including 46 lines have been demonstrated. The output spectrum and power properties with gas pressure and pump power are investigated in detail. In addition, the influence of buffer gases on laser characteristics is studied experimentally for the first time. This work provides significant guidance for the development of high-power tunable mid-infrared fiber gas lasers. [ABSTRACT FROM AUTHOR]

A scheme for second-harmonic generation (SHG) of a short-laser pulse from a gas jet immersed in a magnetic field has been investigated. A laser pulse with sufficient intensity interacting with gas produces free electrons by tunnel ionization. The electrons accelerated by the laser pulse generate nonlinear currents at the second-harmonic (SH) frequency of the fundamental laser pulse. The ambient magnetic field provides an additional resonance to enhance the SH field amplitude. Based on the plasma current model, we estimate the SH field and provide various scaling laws for optimization and controlling the SHG process. This study might provide a new way to optimize the SH field from the laser interactions with a gas jet immersed in a magnetic field. [ABSTRACT FROM AUTHOR]

Photothermal interferometry (PTI) with hollow core fibers (HCFs) have enabled highly sensitive spectroscopic gas sensors in an all-fiber format. Here we report remarkable improvement in the limit of detection of HCF-PTI, in terms of noise equivalent concentration (NEC), by exploiting the optical-phase-modulation amplifying (OPMA) effect of an HCF resonating cavity. By locking the wavelength of a 1550 nm probe laser to the resonance of a 10-cm-long HCF Fabry-Pérot cavity with a finesse of ∼700, OPMA of more than two orders of magnitude is achieved, which enables ultra-sensitive gas detection with large dynamic range. With 1654 nm, 1532 nm, and 761 nm pump lasers, we demonstrate detection of methane, acetylene, and oxygen with noise-equivalent-concentration of 15 parts-per-trillion (ppt), 2.7 ppt, and 0.56 parts-per-million (ppm), respectively. Further improvement in NEC is possible by use of a higher finesse cavity with a longer length of HCF. Extension of the technique to other gases, other types of phase or dispersion modulation-based sensors, and other optical resonating cavities is straightforward. [ABSTRACT FROM AUTHOR]

We present a high-resolution and large dynamic range π-phase-shifted fiber Bragg grating (π-PSFBG) temperature sensor. A current modulated laser diode with a linearized optical frequency sweep is used to interrogate the sensor. The sensor is comprised of a π-PSFBG sensing head and an HCN gas cell. The gas cell functions as an absolute frequency reference to compensate for the laser frequency drift. The reflection spectrum is sent to a lock-in amplifier to generate the error signal, proportional to the first order derivative of the spectrum. The center frequency difference that carries sensing information between the π-PSFBG and HCN gas cell is calculated by cross-correlation of their error signals. A sinusoidal modulation enables to move the data processing from low-frequency stage to high-frequency stage, by phase-sensitive detection and low-pass filtering, the interferences caused by laser intensity noises and electronic devices’ noises are significantly reduced. In the experiment, a frequency resolution of 3.696 MHz with a 55 GHz mode-hop-free tuning range, is achieved, corresponding to a temperature resolution of 2.6 mK with a dynamic range from 5 to 40 °C. [ABSTRACT FROM AUTHOR]

Long pulse gas lasers utilizing double-discharge circuits are different from the single-short pulse gas lasers in that they do not require all of the desired energy to be deposited into the peaking capacitors prior to ignition of the main gap. Instead, the majority of the energy is held in another capacitor that is then discharged in the form of a longer duration discharge pulse output compared with that of single-short pulse lasers. Extended stability of the discharge is typically achieved by igniting the main gap discharge at higher ignition voltages. Several double-discharge circuit topologies exist in the literature. However, each topology has its own disadvantages primarily centered around the components needed and the resulting complexity. In this article we introduce the application of the new hybrid circuit pulsed power topology as a double-discharge circuit with high ignition voltages. The hybrid circuit pulsed power topology was originally published by the authors of this article in their previous publication titled “A New Hybrid Pulsed Power Circuit Topology for Gas Laser Applications.” The advantage of the hybrid circuit as a double-discharge circuit is that it does not require any special device [e.g., semiconductor opening switch (SOS) diode] and can use commonly available components. [ABSTRACT FROM AUTHOR]

A major decision surrounding the design of pulsed power circuits for gas lasers is the choice between a thyratron and solid-state switches. The thyratron is a mature device technology whose cost is increasing. The ratings of solid-state switches still do not match those of a thyratron. Pulse transformer and pulse compression circuits must be added to overcome the lower ratings of solid-state switches leading to a higher parts count and cost. This article proposes a new solid-state pulse power topology called the hybrid circuit topology to address the parts count and cost issue. The hybrid circuit makes use of a combination of capacitive and inductive energy storage to reduce the time required to transfer the energy from the transformer primary to the secondary side. This ultimately reduces the quantity of magnetic material used in the pulse compression circuit. [ABSTRACT FROM AUTHOR]

One of the key challenges in the design of gas lasers powered by pulsed power circuits pertains to reducing the flow of current through the electrodes during the filamentary discharge phase. This phase occurs after the lasing period (“volume glow”) is terminated due to discharge instabilities and the current flow during this period is referred to as late currents. Late currents contribute to electrode erosion. The ejected electrode material contaminates the gas, and lowering the service life of the laser. Snubber circuits are an economic solution to reducing late currents in pulsed power circuits that do not employ a magnetic switch. In a simple snubber, which is connected directly across the main storage/drive capacitor $C_{0}$ , it was found that the stray series inductances of both $C_{0}$ and the high voltage snubber diodes significantly reduced the magnitude of current flowing through the snubber. A new coaxial capacitor and snubber assembly was proposed to improve the effectiveness of the snubber in reducing the late currents. The design exploited the high degree of mutual coupling between the concentric coaxial cylinders to reduce the total inductance in the snubber loop. Experimental results showed an almost 40% reduction in the total charge that flows through the discharge in the late currents stage. Although the improvement was significant, it is expected that improvements to the construction quality of the prototype could potentially lower the late currents even further. [ABSTRACT FROM AUTHOR]

In nonlinear optics, a major challenge is to maximize the interaction between the light from laser sources and low-density media such as gases. An ultrafast laser beam can be focused to form a highly intense spot over a small concentrated area. The ultrafast laser beam has a short pulse width of less than one picosecond and high peak power in the beam profile (Fig. 1). The beam profile of the laser source describes the energy density and distribution of light, and the beam profile of a laser source is usually affected during the propagation and collimation of the beam. An efficient nonlinear optical sensor also requires high peak power at low energy (low average power) over short duration of laser pulse, a high beam profile and long interaction length. These requirements of a nonlinear optical sensor with low attenuation constant can be achieved in hollow-core photonic crystal fiber (HC-PCF). Gas-filled HC-PCF exhibits optical nonlinearity for an ultrashort temporal and spectral broadening of NIR pulses [1], [2]. The nonlinearity in HC-PCF can be achieved by tuning the gas pressure. Gas-filled HC-PCF nonlinear media are low-cost, replenishable, reconfigurable and exhibit sharp spectral lines [3]. Linear and nonlinear responses are also found in other types of optical fibers. [ABSTRACT FROM AUTHOR]

Continuously high-speed spectroscopic gas detection with a wide spectral span has a long time been a challenge. We propose a method based on an extended optical linear chirp chain (EOLCC) for gas spectrum measurement with a fast acquisition rate and a broad bandwidth. The basic optical linear chirp (OLC) unit moves a fixed frequency when it passes through an optical loop containing a dual-parallel Mach-Zehnder modulator driven by a microwave signal. This method is demonstrated by monitoring a high-speed flow process of acetylene gas under normal pressure, achieving a spectral bandwidth of 88 GHz, a repetition frequency of 0.53 MHz, a 100% duty cycle, and a frequency-chirping linearity error of 5.2 × 10−5. Our proposed method based on the EOLCC is expected to have a repetition frequency on the MHz order, showing great potential in the development of high speed optical gas sensors. [ABSTRACT FROM AUTHOR]

Gas detection based on photoacoustic (PA) spectroscopy has attracted extensive attention due to high sensitivity and portability. Herein, to achieve the simultaneously detection of the transformer oil carbide gases, C2H2 and CO, a gas-detection system based on one cantilever beam sensor and single-pass PA cell was proposed. The infrared light beams were coupled by the coarse wavelength division multiplexer (CWDM) and fiber coupler, the vibration of the cantilever beam was measured by the vibration meter, and the gas detection experiment was carried out. The results showed that cantilever could distinguish multiple kinds of acoustic waves, in the frequency domain, the simultaneous detection of C2H2 and CO can be realized by extracting the fast Fourier transform (FFT) values at 20 and 55 Hz. The system exhibits a good repeatability, and a linear relationship between the pure PA signals and gas concentration was confirmed with the correlation coefficient greater than 0.9. In addition, the pure PA signals also show linear trend with the coupling ratio. The minimum detection limit and resolution of C2H2 are 0.27 and 0.12 per part million (ppm), respectively, when the CWDM is used to couple the light beams, whereas they are 23.42 and 14.94 ppm of CO. Especially, it was found that the PA signals have good superposition whether the modulation frequencies of the external lasers are same or not. This article provides an idea for simultaneous detection of multicomponent in gas mixture. [ABSTRACT FROM AUTHOR]

A noise immune temperature measurement method is proposed by using tunable diode laser absorption spectroscopy (TDLAS). Noise immunity is achieved through spectrum shifting by a Mach–Zehnder interferometer. The driving current of a distributed feedback (DFB) laser is tuned for linear wavenumber scanning and followed by a saw-tooth climbing laser signal. The laser signal travels through the interferometer, and a high-frequency carrier signal of megahertz level modulates the saw-tooth signal away from background noises. When the output laser of the interferometer passes through the target gas, the absorption spectrum is imposed on the intensity profile. In this way, the absorption spectrum is shifted to a high-frequency spectral band from a low-frequency band where both the absorption spectrum and the background noises appear. A part of the output laser of the interferometer is split to provide a reference for baseline extraction and hence absorption spectrum extraction. For the dual-color temperature extraction method, laser absorption spectra of the target gas centered at two different wavenumbers are achieved. Numerical simulations and actual experiments at different temperatures, e.g., using a heating device and Bunsen burner verified the noise immunity of the proposed method. Performance comparisons with the direct absorption spectroscopy (DAS) showed that the proposed method generated more precise temperature values in a noisy background. [ABSTRACT FROM AUTHOR]

Stimulated Raman scattering (SRS) enabled by the emerging gas-filled low-loss anti-resonant hollow-core fiber (ARHCF) technology opens up a competitive way towards the development of novel lasers in the molecular fingerprint region. In this article, the characteristics of noise and long-term stability of near- and mid-infrared (near-IR and mid-IR) gas-filled fiber Raman lasers have been investigated for the first time. The results reveal that an increase in Raman pulse energy is associated with a decrease in noise, and that the relative pulse peak intensity noise (RIN) is always lower than the relative pulse energy noise (REN). We also demonstrate that long-term drift of the pulse energy and peak power are directly linked with the high amount of heat release during the Raman Stokes generation. The demonstrated noise and long-term stability performance provide necessary references for potential spectroscopic applications as well as further improvements of the emerging IR gas-filled ARHCF Raman laser technology. [ABSTRACT FROM AUTHOR]

Physical sensors form the fundamental building blocks of a multitude of advanced applications that detect and monitor the surroundings and communicate the acquired physical data. The everlasting need for more compliant, low-cost, and energy-efficient sensor solutions has led to considerable interest in enhancing their features and operation limits even further. While graphene has emerged as a promising candidate material, due to its outstanding electrical and mechanical properties, it is still not available in large volumes for practical applications. Meanwhile, Laser-Induced Graphene has opened new perspectives for a versatile, durable, printed physical sensing platform capable of detecting various physical parameters across a range of conditions and subjects. In this review, LIG physical sensors were categorized into four broad types based on their transduction mechanism: mechanical, thermal, magnetic, and electromagnetic. We summaries various design strategies established for preparing reliable physical sensors without the involvement of chemical treatments, synthesis, and multi-step fabrication processes. The review considers the effects of laser choice, lasing environment, and parameters on graphene properties. We also discuss a broad spectrum of applications of LIG physical sensors in fields ranging from healthcare, tactile sensing, environmental monitoring, energy harvesting, and soft robotics to desalination and THz modulation. [ABSTRACT FROM AUTHOR]

In this paper, we propose a scheme to eliminate the laser power losses that occur in photoacoustic spectroscopy (PAS) multi-point gas sensing that uses a reference gas channel, and we then design a new two-optical-path PAS gas cell that is constructed using both sensing and reference gas cells. The second harmonic PAS signals are extracted from the sensing and reference gas cells using a lock-in amplifier, and the ratio of these signals is used to calculate the gas concentration in an approach that can effectively eliminate the influence of laser power losses on the system’s detection performance. Acetylene (C2H2) gas is detected at 1532.83 nm in multi-point PAS experiments and the effectiveness of the proposed loss elimination scheme is verified. Using three new two-optical-path PAS gas detection devices, the linear responses of the proposed sensor to the acetylene concentration are determined, with yielded R-square values of 0.99834, 0.99788, and 0.99603 being obtained. [ABSTRACT FROM AUTHOR]

In this article, we report the synthesis techniques of ZnO nanoparticles (NPs) in open air atmosphere by pulse laser ablation (PLA) method using solid ZnO polycrystalline palette followed by fabrication of formaldehyde (HCHO) sensor. The crystalline phase and morphology of the obtained ZnO NPs were studied by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The XRD patterns showed that the NPs were polycrystalline structure with good crystallinity. The FESEM images revealed that the NPs were spherical in shape and loosely agglomerated. The average diameter of the NPs was in the range of 30–43 nm. Moreover, the ZnO NPs-based sensor exhibited excellent formaldehyde sensing performance at a temperature of 350 °C. The sensor exhibited a gas response of about 5.2 toward 300 ppm formaldehyde with 25 s response and 12 s recovery time. Furthermore, the ZnO NPs-based sensor exhibited excellent reliability and reproducibility to formaldehyde. On the other hand, the sensor showed high sensitivity of about 0.2% ppm −1 to formaldehyde with a 50 ppm lower detection limit. In addition, the sensor showed an excellent linear relationship (R2 = 0.7948) between the response and the concentration of formaldehyde in the range of 50–400 ppm. This work demonstrates that PLA in open air is a rapid and cost-effective method for synthesizing metal oxide NPs for gas-sensing applications without the need for wet chemical routes. [ABSTRACT FROM AUTHOR]

The results of experimental studies of the formation of narrow-band high-energy radiation in a discharge single-module KrF laser are presented. The conditions for the combustion of a diffuse discharge in a KrF laser are revealed, which provide a maximum specific energy of radiation pulses of 9.5 J/l, with an internal energy efficiency of ~4%. The duration of radiation generation for several half periods of the pump current is realized. The operating conditions of the optical system generator-amplifier allowing narrow-band (2 pm) high-energy radiation in the spectral range of 247.5–249.6 nm are determined. [ABSTRACT FROM AUTHOR]

This work is dedicated to a method for preionization of a laser medium (that is the most popular ones-CO2- and CO-lasers) using X-ray radiation created by a nanosecond sliding discharge (NSD). An experimental study has been carried out on the influence of the initial pressure and the composition of the gaseous medium on the intensity of X-ray bremsstrahlung. In order to estimate this intensity, a special sensor is used; it can measure free electron density that, in our, case is created during photoionization by X-ray radiation from an NSD. It is shown that in case an X-ray emitter is filled with argon, the photoelectron density in the vicinity of a cathode is about 15 times higher than in case the X-ray emitter is filled with helium. Moreover, when the gas volume of the X-ray emitter is filled with argon, the energy spectrum of the “runaway” electrons shifts to the region of lower values of their energy. [ABSTRACT FROM AUTHOR]

The near-infrared (NIR) polarized photodetector has wide range of applications including the object identification. Wavelength of 1550 nm is the least loss band for transmission communication, and the study of photodetectors working at 1550 nm demonstrate special scientific and applied significances. GaSb has attracted tremendous attention in the field of photoelectric detection due to its suitbale band gap, high mobility, sensitivity, and good compatibility with modern microelectronics technology. In this work, a highly polarization-sensitive GaSb nanowire-based photodetectors under short-wave near-infrared photoelectric detection is achieved. The device has a good optical detection ability in the near-infrared band, and showed the responsivity up to 77.3 A/W with the external quantum efficiency of 6.18 × 103 % (illumination intensity of 1.59 mW/mm2) under 1550 nm. Furthermore, obvious photocurrent anisotropy are investigated under the near-infrared band from 808 nm to 1550 nm. The largest dichroic ratio reaches 3.3 at 1550 nm. These results indicate that GaSb nanowire based photodetectors are not only promising candidates for NIR detection but also have promising potentials in polarization sensitive applications. [ABSTRACT FROM AUTHOR]

We demonstrated MIR-pump NIR-probe photothermal spectroscopy with the first harmonic (PTS- $1{f}$) detection of formaldehyde, one of the most common volatile organic compounds (VOCs), in a silica hollow-core negative curvature fiber (HC-NCF). The photothermal gas sensor adopts a mid-infrared interband cascade pump laser at $3.6~\mu \text{m}$ and a near-infrared fiber probe laser at $1.56~\mu \text{m}$. At the optimal modulation frequency (8 kHz) and modulation index (1.8) of the pump laser, we obtained a normalized noise equivalent absorption (NNEA) coefficient of $4\times 10^{-9}$ cm−1WHz $^{-1/2}$. The use of HC-NCF with an inner diameter of $65~\mu \text{m}$ enables the sensitive photothermal detection even for a very low pump power of micro-watts. The background-free PTS- $1{f}$ detection was observed to enhance the sensitivity by a factor of 2.4 compared to the second harmonic ($2{f}$) detection. A theoretical model was established in this work to interpret the experimental results. [ABSTRACT FROM AUTHOR]

We present an all-optical photoacoustic (PA) multigas analyzer by using a digital fiber-optic acoustic detector, which consists of a spectrum demodulation based fiber Fabry–Perot (F–P) pressure sensor and a digital virtual lock-in amplifier. The concentration of multigas is quantified by using an intensity-modulated infrared thermal source and two wavelength-modulated laser diodes. Light beams from the three sources are coupled to a single nonresonant PA cell. The generated PA pressure is detected by the fiber F-P pressure sensor, which is demodulated by a near-infrared spectrometer. A lock-in white-light interferometer is developed for detecting first-harmonic ($1f$) and second-harmonic ($2f$) signals. The noise equivalent detection limits with a 100-s averaging time are 37, 9, 6, 17, 4, and 60 ppb for CH4, C2H2, C2H4, C2H6, CO, and CO2, respectively. [ABSTRACT FROM AUTHOR]

The HERMES III accelerator is an 18–20-MeV linear induction accelerator constructed at Sandia National Laboratories in the late 1980s and which continues operation to this day. As part of recent modernization efforts, the laser triggering system on the accelerator has been replaced with a newly designed solid-state system. This system consists of ten Nd:YAG lasers, each having a nominal output energy of 40–45 mJ at a wavelength of 266 nm. The beam from each laser is split such that it triggers two of the Rimfire gas switches on the accelerator. Compared to the previous laser triggering system, this arrangement makes it possible to more readily tailor the final output pulse shape, and the overall reliability of this key accelerator system is now improved. The design of the new laser triggering system is presented here, along with details pertaining to the energy budgeting, optical beam paths, and electrical triggering of the lasers. The initial operational data from the HERMES III accelerator using this new triggering system are also presented. [ABSTRACT FROM AUTHOR]

We demonstrate that tunable diode laser spectroscopy (TDLS) can be used as a reliable, convenient and versatile investigative technique for non-invasive, high-sensitivity and time-resolved studies of the expression of various traits by bacterial strains. We use a 2004 nm vertical cavity surface emitting laser and a 4320 nm mid-infrared quantum cascade laser to show that the bioluminescence and simultaneous CO2 emission from Photobacterium leiognathi are correlated. The bioluminescence peaks when the growth rate is maximum. This supports the hypothesis that bioluminescence is a cell density-dependent phenomenon. We also show that TDLS can be used for ultra-rapid antimicrobial susceptibility testing without any need of sample preparation. The mole fraction of CO2 emitted during the growth of E. coli was recorded for different standardized doses of ampicillin to show the drastic suppression of growth as the antibiotic dose was increased to the recommended minimum inhibitory drug concentration. Growth suppression was detectable within 2 h, which is much shorter than the time required by conventional methods. We conclude that TDLS is a potent technique for validation and refinement of mathematical models of microbial growth that are necessary to better understand the dynamics of the expression of various traits at specific stages of growth. It is also important for rapid antimicrobial susceptibility testing. [ABSTRACT FROM AUTHOR]

A four-off-beam acoustic micro-resonator (AmR) structure is proposed to improve the photoacoustic signal intensity in quartz-enhanced photoacoustic spectroscopy (QEPAS) for trace gas sensing. Two vertical-array-based double-off-beam AmR structures are arranged on each side of the quartz tuning fork (QTF) horizontally to construct a four-off-beam AmR structure using a rectangular prism. This QEPAS scheme based on the four-off-beam AmR is evaluated in the water-vapor detection of air at normal atmospheric pressure and room temperature (27 °C). Comparing with the bare QTF scheme, the new QEPAS configuration enhances the photoacoustic spectroscopy signal by a factor of 25.84 times, achieving a minimum detection limit of 278 ppbv with a normalized noise equivalent absorption coefficient of 1.27 × 10−9 W/Hz1/2 in water vapour detection at a wavelength of 1368.597 nm. [ABSTRACT FROM AUTHOR]

Supercontinuum (SC) lasers provide an excellent opportunity for absorption spectroscopy by virtue of their high brightness and large bandwidth. Here we present the detection of multiple industrial toxic gasses from 1480 nm to 1700 nm, with a simple custom SC laser source. Using readily available optical components, we demonstrate an all-fiber system for the detection of ammonia (NH3) and methane (CH4) in a 2.4 m long hollow-core photonic bandgap fiber with 20- $\mu \text{m}$ core diameter. The responsivity, selectivity, and performance of the system for continuous detection are demonstrated. [ABSTRACT FROM AUTHOR]

A compact, sensitive and stable hollow waveguide (HWG) mid-infrared gas sensor, based on gas absorption lines using wavelength modulation spectroscopy with a second harmonic (WMS-2ƒ) detection scheme, was developed for simultaneous measurements of ambient CO2 and water vapor. Optimization of the laser modulation parameters and pressure parameter in the HWG are performed to improve the strength of the WMS-2f signal and hence the detection limit, where 14.5-time for CO2 and 8.5-time for water vapor improvement in system detection limit is achieved compared to those working at 1atm. The stability of the sensor has been improved significantly by optimizing environmental disturbances, incoupling alignment of the HWG and laser scanning frequency. An Allan variance analysis shows detection limit of the developed sensor of ∼3 ppmv for CO2 and 0.018% for water vapor, which correspond to an absorbance of 2.4 × 10−5 and 2.7 × 10−5, with a stability time of 160 s, respectively. Ambient CO2 and water vapor measurement have been performed in two days in winter and spring separately. The measurement precision is further improved by applying a Kalman adaptive filter. The HWG gas sensor demonstrates the ability in environmental monitoring and the potential to be used in other areas, such as industrial production and biomedical diagnosis. [ABSTRACT FROM AUTHOR]

A steady-state kinetic model of argon–helium high-pressure radio frequency dielectric barrier discharge is developed using a simplified reaction rate package appropriate for pressures ranging from 200 to 500 torr. Electron production and loss rates are analyzed as a function of pressure and Ar–He mixture, highlighting the importance of the Ar2* dimer for higher pressures and Ar rich mixtures. As pressure increases, the primary ionization mechanism shifts between ionization of ground state Ar to dimer ionization, which affects the steady-state reduced electric field due to the electron energy required for the two dominant ionization mechanisms. The resulting reduced electric field profile controls the metastable Ar(1s5) production, which, combined with the Ar(1s5) loss rates, reveals the metastable density as a function of pressure and Ar-fraction. This simplified 0-D steady-state model shows close agreement with previous time-dependent simulations using a robust reaction rate package. [ABSTRACT FROM AUTHOR]

The use of lasers to initiate the closure of high-power gas switches was initially explored soon after the development of suitable high-power pulsed lasers in the 1960s. The low jitter, excellent triggering range, and galvanic isolation provided by laser triggering have been extensively exploited in the triggering of megavolt switches in large pulsed power machines. The fact that the lasers and optical systems used in these machines are very large, complex, and expensive and have led to the perception that these are unavoidable characteristics of laser triggered gas switches, leading to a reluctance among the directed energy and compact pulsed power community to consider their use in compact pulsed power systems. The Advanced Pulsed Energy, Ionization, and Discharge Center (APERIODIC) Research Group at The University of New Mexico (UNM), Albuquerque, NM, USA, has been investigating novel triggering technologies for compact pulsed power. One of the technologies under investigation is a system we have named the microintegrated laser switch (MILS). This system holds the potential to overcome the limitations of traditional laser triggering of gas switches in compact applications. As part of the preliminary work on developing the MILS concept, UNM has collaborated with MegaWatt Lasers, Hilton Head Island, SC, USA, to conduct laser triggering experiments using a commercial off-the-shelf (COTS) erbium-doped glass passively Q-switched microlaser. This 28 mm $\times 9$ mm laser produces 4-ns pulses at 1535 nm with an average energy per pulse of $244~\mu \text{J}$. One of the principal advantages of this laser is that, due to the wavelength and energy, it is a class 1 laser and can be operated without the stringent controls necessary for the lasers typically used for laser triggering. [ABSTRACT FROM AUTHOR]

The Applied Pulsed Energy, Ionization, and Discharge Center (APERIODIC) research group at The University of New Mexico, Albuquerque, NM, USA, has been investigating the use of low-energy diode-pumped solid-state (DPSS) passively q-switched lasers operating at a wavelength of 1535 nm to trigger gas switches. This wavelength was chosen due to the improved eye safety and ready supply of commercial microlasers at this wavelength. Laser triggering of switches in a pulsed power system often presents challenges in beam transport from the laser to the switch optics. In many cases, it is most convenient if the beam transport can be accomplished directly through the insulating medium of the device. It is also possible that this laser wavelength could be used to trigger an oil-insulated spark gap switch. Thus, the absorption of the laser wavelength in insulating oils is of interest. In this work, the absorption coefficient of the 1535-nm laser pulses has been measured for various common insulating oils. The results indicate that beam transport through oil at this wavelength will likely be impractical due to relatively high absorption. However, the absorption is low enough that the use of this wavelength to trigger an oil-insulated gap could be possible. [ABSTRACT FROM AUTHOR]

Bubbly flow widely exists in many industrial applications of energy, metallurgy, and chemistry, etc. Due to the clusters and overlap of dense bubbles at a high void fraction, it is nearly impossible to obtain the information of flow structures and characteristics in the spatial field with traditional measurement methods. In this paper, a novel laser scanning based 3-D reconstruction method for dense bubbly plume is developed. The measurement area is scanned by a laser sheet through a rotating hexagonal optical prism, and a high-speed camera captures the sequentially sliced images in the flow field, which is parallel to the scanning direction. Meanwhile, a scanning mathematic model is established, and its linearization is analyzed in detail. An image preprocessing method is developed to extract the features of the bubbly plume. To be specific, a method involves adaptive wavelet threshold denoising is developed to remove the noise. Moreover, methods regarding sliced image-matching and interpolation based on Log-polar transformation are presented to improve the spatial resolution effectively, and a set of image evaluation standards are designed to investigate the interpolation efficiency and accuracy. The experimental results conclude that the reported 3-D reconstruction method for dense bubbly flow based on laser scanning is valid with high precision, which explores a new way for the visualization of the 3-D structures and measurement of the volumetric flow field and the complex flow characteristics. [ABSTRACT FROM AUTHOR]

A self-consistent two-dimensional model of the discharge KrF laser on Ne/Kr/F2 gas mixtures is described. The model predictions are compared with the experimental results for a KrF laser with an excitation pulse duration of ~ 30 ns. An agreement within 10% for the discharge current and laser energy is obtained. Evolution of the discharge width and of the radiation intensity distribution between the electrodes over the time of a radiation pulse is demonstrated. In addition, the effect of the electrode radii on laser energy is analyzed. [ABSTRACT FROM AUTHOR]

In this paper, a stable and reliable carbon monoxide (CO) monitoring system with high sensitivity (at sub-ppm level) was designed and demonstrated with particular reference to use in the mining industry, tailoring the design specifically for forecasting spontaneous combustion, a major hazard to miners. An appropriate strong CO absorption line was used to minimize the interferences expected from gases present in ambient air, with several preferred CO absorption lines selected and investigated, therefore choosing a distributed feedback (DFB) laser operating at a wavelength of 2330.18 nm as the excitation source. Through a detailed investigation, a minimum detection limit of ~0.2 ppm and a measurement precision of <50 ppb were achieved with a 1 s averaging time. Further in tests, a long-term continuous monitoring evaluation was carried out, demonstrated the excellent stability and reliability of the developed CO monitor. The results obtained have validated the potential of this design of a CO monitoring system for practical monitoring applications underground to enhance safety in the mining industry. [ABSTRACT FROM AUTHOR]

Electron–ion recombination effect on electron acceleration by a high-intensity laser pulse propagating through a tunnel ionizing gas is investigated in order to observe the actual electron energy gain during acceleration. An intense short-pulse laser with a Gaussian radial profile propagates through a vacuum followed by gas. The point at which the peak of the pulse interacts with the electron is the initial point of the gas region. Tunnel ionization causes defocusing of the laser pulse due to high-density plasma formation on the propagation axis. The electron experiences an additional acceleration during the trailing part of the pulse and, thus, gains net energy. In the presence of electron–ion recombination, the laser pulse focuses more, and hence, the net energy gain is affected significantly for specific parameters region. A model that self-consistently evolves the laser electron acceleration as it ionizes a neutral gas is presented. The model incorporates the electron–ion recombination effects for multiple ionization stage and for tempospatial variations in the neutral gas density appropriate for studying gas-jet system. The electron energy gain during acceleration is calculated in He gas, where the conditions are appropriate for recombination. It is found that for a given laser intensity, there is always an optimal spot size and focal position with respect to the gas jet, which minimizes the refraction and maximizes the acceleration length for higher energy gain of the electrons. The inclusion of electron–ion recombination is more realistic if the pulse duration is longer in a laser–gas-jet experiment. [ABSTRACT FROM AUTHOR]

We demonstrate tunable diode laser spectroscopy (TDLS)-based absolute measurements of the mole fraction of carbon monoxide (CO) and carbon dioxide (CO2) in the plume of an aerosol-based fire extinguishing agent. The mole fraction values of CO and CO2 in the plume for two different compositions of the aerosol-forming compound are measured to study the effect of chemical composition on gas emission during controlled discharge of aerosol. The aerosol plume from the first composition contained 336 ± 39 ppm CO and 1389 ± 99 ppm CO2 on an average while that from the second composition contained 691 ± 56 ppm CO and 1971 ± 118 ppm CO2. These measurements show that the CO content in the plume can be reduced by more than 50% if the chemical composition of the aerosol-forming compound is properly chosen. The technique can be readily extended to measure other species (such as nitrogen oxides) that are often present in fire-extinguishing aerosol plumes. Two mid-infrared continuous-wave distributed-feedback quantum cascade lasers are used to interrogate the rotational-vibrational transitions of CO at 4559.217 nm and CO2 at 4319.30 nm. The mole fractions of CO and CO2 are extracted by a digital signal processor from least-squares fit of a simulated Voigt lineshape to the gas absorption line obtained using direct absorption spectroscopy. [ABSTRACT FROM AUTHOR]

Infrared spectral range between 1.7 and 2.5 μm is of particular interest for biomedical sensing applications due to the presence of first overtone C–H stretch and a combination of stretch and bending vibrations of C–H, N–H, and O–H bonds. These vibrations are molecule specific and can be used to selectively sense important biomolecules such as glucose, lactate, urea, ammonia, serum albumin, etc. In this paper, we review recent developments of swept-wavelength lasers based on GaSb type-I gain-chip technology, their key performance parameters for spectroscopy applications and provide experimental data on spectroscopic sensing of the key biomolecules both in synthetic solutions as well as whole blood. [ABSTRACT FROM AUTHOR]

In this work, we report on real-time gas sensing using a swept fiber laser source (SFLS) based on a micro-electro-mechanical-system (MEMS) tunable filter, a semiconductor optical amplifier (SOA) gain medium and a fiber ring. The MEMS filter is implemented using deeply-etched silicon-on-insulator (SOI) silicon/air Bragg mirrors attached to a micro-actuator enabling a relatively high-speed tunability and compatibility with single-mode fibers. A theoretical analysis is presented relating the required laser output power and spectral linewidth to the gas detection limits taking into account the signal-to-noise ratio (SNR). The minimum detectable concentrations of nitrous oxide, carbon dioxide, carbon monoxide and acetylene gases were determined. The dependence of the gas spectrum in terms of its absorption line strength, spectral width and spacing on the laser source was emphasized in the analysis, revealing the limits of distinguishing between different gases. The fiber-laser source is then characterized under static and dynamic conditions. An experimental parametric study was carried out varying the fiber cavity length from 20 m to 2 km, the sweep rate up to 2 kHz and the pumping current up to 6 times the threshold current. Finally, the SFLS is exploited to measure the transmission spectrum of carbon dioxide gas – as an example – at various pressures. The effects of the pumping current, the sweeping speed, and the sweeping direction on the measured spectrum were studied in order to determine the optimal conditions. The measured spectrum is compared with the theoretical one taking into consideration the FWHM of the laser source. The comparison shows a good match. [ABSTRACT FROM AUTHOR]

The spark channel plasma generated in the atmospheric nitrogen trigatron is studied with the laser Mach–Zehnder (M–Z) interferometer. The development of the spark channel plasma is described by analyzing the temporal and spatial evolution of the plasma electron density from the interferograms. The regions of the shock wave and the plasma are distinguished by the phase shift. Different spatial distributions of the anodic and cathodic plasma channels are obtained. The anodic spark channel shows good axial symmetry with constant radial expansion velocity. On the periphery of the main spark channel, a secondary spark channel occurs simultaneously in the cathodic region. After the collision of the two channels, the stagnation forms in the overlapping region and the secondary channel merges with the main one gradually. The leftward expansion velocity of the secondary channel is higher than the rightward velocity of the main channel, but both of them are lower than that near anode. The speculation on the production of the secondary spark channel is given and its influence on the development of the spark discharge in the gap is discussed in this work. [ABSTRACT FROM AUTHOR]

In this paper, we propose and demonstrate a gas pressure fiber probe with high sensitivity magnified by Vernier effect. The probe is composed of two cascaded Fabry–Perot interferometers based on a SMF–SOHST–OFC structure (SMF: single-mode fiber; SOHST: side-opened hollow silica tube; OFC: optical fiber column). The high-frequency CO2 laser drilling method for hollow silica tube can effectively maintain the transient balance of the air pressure inside and outside the cavity without destroying the reflective ends of the optical fibers. Experimental results show that the prepared fiber probe with the SOHST length of 375.2 μm and column length of 247.3 μm has high gas pressure sensitivity of 80.3 pm/kPa by demodulating Vernier envelope, and it has relatively low temperature cross-sensitivity of –1.33 kPa/°C. This sensor is highly sensitive and of compact size, which not only can be applied in gas pressure sensing but also has the potential for application in microfluidic detection. [ABSTRACT FROM AUTHOR]

The theoretical analysis of large-area diodes shows that the electron flow in the diode is subject to the transit-time instability (TTI). The TTI can modulate the electron beam energy at the anode and add a transverse beam temperature. Both of these effects can have negative consequences on many diode applications that require a high-quality mono-energetic electron beam. A distributed transmission line model of the diode is developed which allows techniques from microwave engineering to be applied to the unstable diode. The theory suggests that a resistively loaded, periodic structure of slots in the cathode can stabilize the TTI. The results from particle-in-cell (PIC) simulations of a large-area electron-beam diode in 2-D are presented that support this conclusion. [ABSTRACT FROM AUTHOR]

This paper consists of two parts. In the first part, the emission was generated in air, and the waveform recording the spectral line at 337 nm (= prominent N2 laser line) was noted. The pulse forming network (PFN) Marx generator having a large ($50~\Omega$) internal impedance was used. The pulses of the nitrogen emission at 337.1 nm of long ($< 3~\mu \text{s}$) duration and of short (≈10 ns) duration were obtained. The photodiode registered both the spontaneous emission and the stimulated (laser) emission in the 337 ± 5 nm spectral range. The data show that the laser emission predominates in the pulses of long duration. In the second part of this paper, it is suggested that the ultraviolet (UV) radiations emitted by the sun, together with the electric fields present at high altitudes, are capable of exciting the N2 molecules to the upper excited $\text{C}^{3}\Pi _{u} $ level to yield the laser emission of long duration. On the basis of the laser energy content dissipated into the plasma, the size of the transient luminous events (TLEs) known as the elves and the sprites was evaluated. The dynamics of initially small size, highly compressed, and very hot plasma formed inside the sprite was presented. It is proposed that the TLEs named “crepuscular twilight in the sky of New York” observed on December 28, 2018; “mysterious bright flash” occurred in Russia on January 9, 2018; and maybe the Tunguska event happened in Siberia on the morning of June 30, 1908 should be included in the list of the TLEs. [ABSTRACT FROM AUTHOR]

A compact pulse power system has been developed and successfully tested for soft X-ray lasing at 46.9 nm in argon plasma generated by fast capillary discharge scheme. The system has been designed with the aim to generate high current with a high rate of rise in the gas-filled ceramic (Al2O3) capillary (radius 1.4 mm and length 150 mm). The electrical parameters for the compact system were fixed using circuit simulation and the setup design was optimized using electrostatic field simulation software. A 10-stage 100–400 kV, 6-nF Marx generator is used to pulse charge the 3.2 nF, 35-nH water filled coaxial capacitor. The improvement in the output laser energy has been observed due to faster rise time of discharge current. The highest current observed was 70 kA, achieving a rate of rise of current $\sim 2\times 10^{12}$ A/s in 97 mm long, 2.8 mm inner diameter capillary. We strongly believe that the performance of the system is suitable for study of discharge plasmas from gasses other than the argon for exploring recombination-based X-ray lasing at wavelengths shorter than 46.9 nm. [ABSTRACT FROM AUTHOR]

Laser direct joining of metal and plastic plays a significant role in manufacturing of multiple material structures. To improve the strength of the joint, a controllable laser joining system was developed for polyethylene terephthalate plastic and titanium based on the detected multiple signal sources. The main contribution of this paper lies in sensing 1) the intensity of thermal radiation to control process heat by adjusting the laser power; and 2) the reflection of the helium–neon laser to control the movement of the specimens. The response speeds of thermal radiation intensity when increasing or decreasing the laser powers were measured to guide the adjustment of laser power. The intensities of reflective light before and after the joining process were measured to guide the movement control. Then, the integrated control process was set up. Width, tensile shear strength, and fracture position of the joint produced by the controllable joining system were compared with those produced by the conventional constant power method, which validated the advantage of this newly developed system. [ABSTRACT FROM AUTHOR]