Multipass Cavity Research Articles

Simultaneous detection of soot, temperature, and C2H2 in laminar premixed flames using a multi-pass diode laser absorption spectrometer.

We report the development of a multi-pass diode laser absorption spectroscopy system for simultaneous measurements of soot volume fraction (SVF), temperature, and C2H2 concentration using a single diode laser near 1.543 µm. A line-shaped beam spot pattern is chosen for the open-path Herriott multi-pass cavity, enabling sensitive detection at various heights above the burner with an effective optical absorption path length of approximately 1.2 m in a 6 cm diameter flame region. The gas parameters (temperature and C2H2 concentration) and the SVF are determined from the absorption spectra of the target C2H2 line pair and the laser extinction of the soot, which can be extracted from the detected signal, respectively. The performance of the system was confirmed in laminar premixed ethylene and air (C2H4/air) sooting flames produced by a standard bronze plug McKenna burner at four representative equivalence ratios. All the measurement results were compared with the two-dimensional (2D) computational fluid dynamics (CFD) simulations using a skeletal mechanism with the Moss-Brookes model. The good quantitative and qualitative agreement between the TDLAS measurements and 2D CFD simulations confirms the powerful capability of the developed system.

Gas detection using cavity-enhanced Raman spectroscopy with a bidirectional multi-pass cell and polarization beam-splitting optical path

We demonstrate a substantial enhancement of gas Raman scattering using a bidirectional multi-pass cavity CERS system, which incorporates a polarization beam-splitting optical path. The system design allows the laser light to traverse the multi-pass cavity for four specific trips, satisfying the need for quick detection of various gas components. Our gas detection experiments using multi-pass cavities with different times of reflection indicate that the addition of polarization beam-splitting optical path gives 1.5 to 1.68 times enhancement of Raman signal compared with that of the system without polarization beam-splitting. For the detection of CH4, a limit of detection of 1.66 ppm was achieved with our system using a multi-pass cell with 41 times of reflection and an integration time of 30s. Our proposed design, which integrates a bidirectional multi-pass cavity with polarization beam-splitting optical path, gives an economical multicomponent gas detection system and a valuable tool for guiding the design and precise alignment of these cavities. This system shows significant promise for applications in e.g. human breath and environmental monitoring.

Trace Analysis of C4F7N Insulating Gas Mixtures by Spontaneous Raman Spectroscopy and Gas Chromatography.

2,3,3,3-Tetrafluoro-2-(trifluoromethyl) propanenitrile (C4F7N) is being researched as an alternative to sulfur hexafluoride (SF6) for applications in gas-insulated switchgear. We independently assessed the effectiveness of gas chromatography-mass spectrometry (GC-MS) and a novel method of feedback-assisted multipass cavity spontaneous Raman spectroscopy (SRS) for the trace quantification of impurities in C4F7N and its related byproducts. A total of 14 gases were identified with estimated concentrations as low as 20 ppm (ppm) for C3F6 using GC-MS and 7.4 ppm for CH4 using SRS and as high as 500 ppm for CF4 using GC-MS and 1430 ppm for CO using SRS. While GC-MS is highly effective in selectively detecting and quantifying trace contaminants, it necessitates separate detectors for various gases, such as CH4 and H2. SRS succeeded in detecting CF4 and C2F6 at concentrations of 465 and 100 ppm, respectively, and in placing an upper bound of several hundred ppm for the other analytes. Crucially, SRS holds potential for portability-and thus for field applications-in gas-insulated switchgear equipment diagnostics.

Continuous wave cavity ringdown spectroscopy incorporating with an off-axis arrangement, white noise perturbation, and optical re-injection

We present an ultra-sensitive continuous wave cavity ringdown spectroscopy (cw-CRDS) spectrometer to record high resolution spectra of reactive radicals and ions in a pulsed supersonic plasma. The spectrometer employs a home-made external cavity diode laser as the tunable light source, with its wavelength modulated by radio-frequency white noise. The ringdown cavity with a finesse of ∼105 is arranged with an off-axis alignment. The combination of the off-axis cavity and the white-noise perturbed laser yields quasi-continuum laser-cavity coupling without the need of mode matching. The cavity is further incorporated with an extra multi-pass cavity for optical re-injection of light reflected off the master cavity, which significantly increases the throughput power of the high-finesse cavity. A fast switchable semiconductor optical amplifier is used to modulate the cw laser beam to square wave pulses and to initialize timing controlled ringdown events, which are synchronized to the plasma pulses with an accuracy of ∼3 µs. The performance and potential of the cw-CRDS spectrometer are illustrated and discussed, based on the high resolution near-infrared spectroscopic detection of trace 13C13C radicals generated in a pulsed supersonic C2H2/Ar plasma with a pulse duration of ∼50 µs.

Enhancement of the laser-induced excitation probability of the hyperfine ground state of muonic hydrogen by a multipass cavity setup

Enhancement of the laser-induced excitation probability of the hyperfine ground state of muonic hydrogen by a multipass cavity setup

Fresnel Lens Array-Based Phase Mask Location Method for Adjustable Multi-Pass Cavity

The modulation accuracy of Multi-Plane Light Conversion (MPLC) mainly depends on the positioning accuracy of the phase mask on the Spatial Light Modulator (SLM). To improve positioning accuracy, the impact of phase mask shift on modulation accuracy is investigated, and a position method is proposed. In order to investigate the influence of phase mask offset on the input light conversion effect, a convolution transmission model for the adjustable multi-pass cavity is established. Then, the positioning process for the phase masks is analyzed and simulated, and a method of positioning the phase masks is presented. This method reduces the positioning time and increases the positioning accuracy to 8 μm. Finally, experiments are performed to verify the feasibility of the method. Experimental results show that the similarity of the adjustable multi-pass cavity positioned by this method can reach 93.44%.

Sensitive and Stable Atomic Vector Magnetometer to Detect Weak Fields Using Two Orthogonal Multipass Cavities

This paper presents a compact low-temperature atomic vector magnetometer for weak-field measurements, using an atomic cell containing two orthogonal multipass cavities. At the working temperature of $75{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, the magnetic field sensitivities at all three axes are better than $45\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.2em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$ at 10 Hz limited by photon noise, and $85\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.2em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$ at 0.1 Hz. This sensor also shows measurement stabilities better than 1.5 pT at three axes for an integration time of ${10}^{4}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$, even with the laser frequency unlocked. The sensor response to a rotation is demonstrated, which is also developed to measure the effective gyromagnetic ratio of atoms in this sensor when the bias field is nulled. This magnetometer makes an important step towards long-term stable measurements and calibrations of ultralow fields.

Cavity-Enhanced Raman Spectroscopy for Detection of Trace Gaseous Impurities in Hydrogen for Fuel Cells

Gaseous impurities contained in hydrogen (H2) profoundly affect the performance of hydrogen proton-exchange membrane fuel cells. We demonstrate the utility of cavity-enhanced Raman spectroscopy as a unique approach for detection of gaseous impurities. A dense-pattern multipass cavity which is composed of four spherical mirrors placed in a Z-shaped configuration is used to enhance the Raman signal by extending the laser-gas interaction length. A total of 85 spots are identified on the 2-inch-diameter front (or rear) mirror, which indicates that 510 beams exist in the cavity. Detection limits of the impurity gases, including oxygen (O2), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ammonia (NH3), and hydrogen sulfide (H2S), reach sub-ppm- and ppb-levels at a total pressure of 0.1 and 2.5 MPa, respectively. This satisfies the detection requirements according to the maximum allowable concentration for these gases. Our cavity-enhanced Raman spectroscopy (CERS) apparatus can simultaneously measure multiple gases with high sensitivity and selectivity with no sample destruction. It has excellent application prospects in gaseous impurity analysis for the quality assessment of gaseous energy.

Multipass optical cavity for low density plasma polarimetry: numerical analysis and detection scheme

Measurements of internal magnetic fields are of primary importance in laboratory plasma physics; the most common diagnostic method is Faraday rotation measurement by means of polarimetry. Faraday rotation measurements are integrated along a line of sight: the overall rotation is proportional to the plasma density, magnetic field and probing wavelength, as well as to the integration segment length and hence to the plasma dimensions.As a results, measurements of small magnetic fields in small, low density plasmas becomes non-trivial; on the other hand, this is typically the situation of small, self organized laboratory experiments.Being the probing wavelength practically limited to the THz region by diffraction phenomena, the use of a multipass scheme is actually the only way to improve the diagnostic sensitivity. This work discuss an application study of a multipass cavity to the PROTO-SPHERA experiment, but the results can be easily generalized to others of similar characteristics.In particular, different layouts are analyzed (straight, folded, annular cavity) discussing the overall stability region, the impact of changes in plasma refraction and the overall performances in terms of photon lifetime. The performance analysis features a discussion on continuous vs pulsed regime as well as on some basic detection schemes (direct, closed loop feedback).

Ambient Hydrocarbon Detection with an Ultra-Low-Loss Cavity Raman Analyzer.

The detection of ambient outdoor trace hydrocarbons was investigated with a multipass Raman analyzer. It relies on a multimode blue laser diode receiving optical feedback from a retroreflecting multipass optical cavity, effectively creating an external cavity diode laser within which spontaneous Raman scattering enhancement occurs. When implemented with ultra-low-loss mirrors, a more than 20-fold increase in signal-to-background ratio was obtained, enabling proximity detection of trace motor vehicle exhaust gases such as H2, CO, NO, CH4, C2H2, C2H4, and C2H6. In a 10-min-long measurement at double atmospheric pressure, the limits of detection obtained were near or below 100 ppb for most analytes.

Highly sensitive multi-pass cavity enhanced Raman spectroscopy with novel polarization filtering for quantitative measurement of SF6 decomposed components in gas-insulated power equipment

The decomposed components analysis (DCA) is highly valued for potential faults diagnosis in SF6 gas-insulated power equipment (GIE). The current method and technology for effective online monitoring of these components are still lacking. Here, we present a novel multi-pass cavity enhanced Raman spectroscopy (MPC-CERS) as a promising analytical tool to detect SF6 decomposed products. A rear reflector is applied that provides a gain factor of 2.24 for Raman signal intensity and a gain factor of 1.7 for signal-to-noise ratio (SNR) compared to the traditional MPC approach, and a polarizer is installed to suppress the fluorescence background noise by a factor of 0.72. The limits of detection (LODs) at extremely low-ppm levels are achieved for SF6, SO2F2, COS, CF4, SO2, CO2, and CO, and the quantification performance for multi-gases is investigated with a maximum accuracy error < 7.3% as well as a long-term instability of 0.28%. These results pave a promising way for MPC-CERS methodology to be applied in fault diagnosis of SF6-GIE in the future.

Influence of disk aberrations on high-power thin-disk laser cavities.

We present a systematic study on the influence of thin-disk aberrations on the performance of thin-disk laser oscillators. To evaluate these effects, we have developed a spatially resolved numerical model supporting arbitrary phase profiles on the intracavity components that estimates the intracavity beam shape and the output power of thin-disk laser oscillators. By combining this model with the experimentally determined phase profile of the thin-disk (measured with interferometry), we can predict the operation mode of high-power thin-disk lasers, including mode degradation, higher-order mode coupling, and stability zone shrinking, all of which are in good agreement with experiment. Our results show that one of the main mechanisms limiting the performance is the small deviation of the disk's phase profile from perfect radial symmetry. This result is an important step to scaling modelocked thin-disk oscillators to the kW-level and will be important in the design of future active multi-pass cavity arrangements.

Signal-enhanced Raman spectroscopy with a multi-pass cavity for quantitative measurements of formaldehyde, major species and temperature in a one-dimensional laminar DME-air flame

While Raman spectroscopy provides opportunities for simultaneous detection and quantitative analysis of multiple species, it also suffers from certain limitations, such as low signal strength, which often makes it unsuitable for the study of minor species. The present work employs a high-repetition-rate and high-power laser and re-introduces an amplification technique to improve the detection limit, providing an up to 45-time signal enhancement. The amplification is achieved by employing a multi-pass cavity, and different modes were tested and optimized for the measurements. This multi-pass setup has enabled the detection of formaldehyde (CH2O), which is present in stoichiometric dimethyl ether (DME)-air flames on a sub-percent level, and, therefore, known to be challenging to study with Raman spectroscopy. Furthermore, Raman cross-sections of CH2O and DME, which have not been reported in the literature to the authors’ knowledge, are obtained ab initio using Raman scattering activities in the literature. The cross-sections are 6.63 and 22.43, respectively, for CH2O and DME, normalized by the Raman cross-section of nitrogen. The CH2O detection limit in flame with was estimated to 40 ppm with the present experimental settings. Mole fractions of CH2O, nitrogen (N2), water (H2O), carbon dioxide (CO2), oxygen (O2), DME (CH3OCH3), carbon monoxide (CO), and hydrogen (H2) measured in the flame show good agreement when compared with modeling results obtained using two chemical kinetic mechanisms. Flame temperatures were evaluated from the N2 Raman signals, and the steep temperature gradient is successfully resolved experimentally with the multi-pass setup and well predicted by the models.

Dense-pattern multi-pass cavity based on spherical mirrors in a Z-shaped configuration for Raman gas sensing.

We report a dense-pattern multi-pass cavity (MPC) based on four spherical mirrors placed in a Z-shaped cavity configuration for improving the Raman signals from gases. The folding structure of the cavity causes dense patterns of spots, and at least 420 beams are reflected in the cavity. Raman spectra of ambient air, methane, and ethylene are recorded to demonstrate the performance of our apparatus. At atmospheric pressure, ppm-level detection limits of the gases are achieved with 10 s of exposure time. The Raman signal intensities of the gases show excellent linearity with the gases' partial pressures, which means that high-accuracy detection is also feasible.

Hazardous Gas Detection by Cavity-Enhanced Raman Spectroscopy for Environmental Safety Monitoring.

We demonstrate the practicability of cavity-enhanced Raman spectroscopy (CERS) with a folded multipass cavity as a unique tool for the detection of hazardous gases in the atmosphere. A four-mirror Z-sharped multipass cavity results in a greatly extended laser-gas interaction length to improve the Raman signal intensity of gases. For Raman intensity maximization, the optimal number of intracavity beams of a single reflection cycle is calculated and then the cavity parameters are designed. A total of 360 intracavity beams are realized, which are circulated four times in the cavity based on the polarization. ppb-Level Raman gas sensing at atmospheric pressure for several typical explosive gases and toxic gases in ambient air, including hydrogen (H2), methane (CH4), carbon monoxide (CO), hydrogen sulfide (H2S), and chlorine (Cl2), is achieved at 300 s exposure time. Our CERS apparatus, which can detect multiple gases simultaneously with ultrahigh sensitivity and high selectivity, is powerful for detecting hazardous gases in the atmosphere, and it has excellent potential for environmental safety monitoring.

Spontaneous Raman scattering at trace gas concentrations with a pressurized external multipass cavity

A setup is described with which the spontaneous Raman scattering rate from pressurized gas samples is greatly enhanced inside a near-concentric optical cavity. The pump source is a high power multimode semiconductor laser diode that receives optical feedback from the cavity by way of a volume Bragg grating, resulting in a linewidth of 80 pm and circulating power close to 100 W. Under pressurization (up to 8 bar) a limit of detection below 100 parts-per-billion was obtained for gaseous hydrogen, measured in ambient air and in breath. Furthermore, isotopic trace detection is demonstrated by headspace measurements of semiheavy water.

Isotopic trace analysis of water vapor with multipass cavity Raman scattering.

Cavity-enhanced spontaneous Raman scattering was investigated as a means of simple and inexpensive isotopic water analysis. A multimode blue laser diode equipped with a feedback-generating multipass cavity provided a 100-fold Raman enhancement at a pump linewidth of 3.5 cm-1. Samples containing trace amounts of 1H2H16O were probed at deuterium-hydrogen concentration ratios ranging from 157 parts-per-million (local seawater) down to 8 parts-per-million (deuterium depleted water). All measurements were performed in argon or dried air at atmospheric pressure at 1H2H16O concentrations nearing 100 parts per billion with an uncooled camera at exposure times as short as a few minutes.

Modified ray transfer matrix method for accurate non-sequential ray tracing between arbitrary reflective mirrors.

The ray transfer matrix method is widely used for analyzing beam-transmission properties and designing multi-pass systems. In this paper, a modified ray transfer matrix method is proposed for tracing an accurate non-sequential ray in three-dimensional space based on a vector reflection theory. The modified ray transfer matrix method can be utilized to analyze a ray transmitting between arbitrary surfaces, which is not confined to rotational symmetrical structures. There is no need to project rays onto two perpendicular planes, nor to introduce a paraxial approximation in our calculation. Since the length and angle of every ray can be calculated accurately, almost no deviation is accumulated in multi-pass system. The modified ray transfer matrix method can be applied in optical design, especially in the design of multi-pass cavities and multi-pass cells.

A dual-gas sensor for simultaneous detection of methane and acetylene based on time-sharing scanning assisted wavelength modulation spectroscopy.

Methane (CH4) and acetylene (C2H2) are important bioscience and chemical gases. The real-time monitoring and analysis of them have important research value in industrial process control. The time-sharing scanning assisted wavelength modulation spectroscopy (WMS) technique is developed for real-time and simultaneous detection of CH4 and C2H2. This system involves two near-infrared distributed feedback (DFB) lasers and a compact multipass cavity with an effective optical path of 52.2m. The selected strong absorption lines of methane and acetylene are located at 6046.96cm-1 and 6531.7cm-1, respectively. The experiment environment is conducted at room temperature 23°C and pressure 760Torr. The sensor performance, including the minimum detection limit (MDL) and the stability, was improved by eliminating the influence of light intensity fluctuation using the WMS-2f/SAW technique. Allan deviation analysis indicates that a MDL of 0.1ppm for CH4 and 0.2ppm for C2H2 are achieved with 1-s integration time. And the instrument response time is about 44s through the continuous analysis of standard gases. This sensitive, simple, reliable, and lowcost dual-gas sensor is very suitable for applications in the field environment, chemical process, and many other gas-phase analysis areas.

基于多程腔技术的长纳秒脉冲532 nm Nd∶YAG调Q绿光激光器

基于多程腔技术的长纳秒脉冲532 nm Nd∶YAG调Q绿光激光器