Multipass Cell Research Articles

Signal-enhanced high-sensitivity atomic magnetometer based on multi-pass cell

Signal-enhanced high-sensitivity atomic magnetometer based on multi-pass cell

Calibration-free infrared absorption spectroscopy using cantilever-enhanced photoacoustic detection of the optical power

We report on sensitive tunable laser absorption spectroscopy using a multipass gas cell and a solid-state photoacoustic optical power detector. Unlike photoacoustic spectroscopy (PAS), this method readily allows a low gas pressure for high spectral selectivity and a free gas flow for continuous measurements. Our photoacoustic optical power detector has a large linear dynamic range and can be used at almost any optical wavelength, including the middle infrared and THz regions that are challenging to cover with traditional optical detectors. Furthermore, our approach allows for compensation of laser power drifts with a single detector. As a proof of concept, we have measured very weak CO2 absorption lines at 9.2 µm wavelength and achieved a normalized noise equivalent absorption (NNEA) of 2.35·10−9 Wcm−1Hz−1/2 with a low-power quantum cascade laser. The absolute value of the gas absorption coefficient is obtained directly from the Beer-Lambert law, making the technique calibration-free.

New experimental measurements of the Collision Induced Absorptions of H2-H2 and H2-He in the 3600-5500 cm−1 spectral range from 120 to 500 K

The Collision-Induced Absorption (CIA) fundamental band of H2 has been studied in the 3600-5500 cm−1 spectral range for temperatures ranging from 120 to 500 K for both a pure H2 gas and a H2-He mixture. We used a simulation chamber called PASSxS (Planetary Atmosphere System Simulation x Spectroscopy) developed at INAF/ISAC which contains a Multi-Pass cell interfaced with a Fourier Spectrometer, aligned to reach an optical path of 3.28 m. The H2-H2 and H2-He binary absorption coefficients (BACs) have been derived for seven temperatures in the chosen range and provided in tabular form, including the unexplored high-temperature range above 300 K. We also calculated the integral of the H2-H2 and H2-He experimental BACs in the reduced 4000-5000 cm−1 spectral range, finding a linear trend with temperature in both cases. The integrals have also been computed with larger uncertainties for the whole band, in the total 3600-5500 cm−1 spectral range including the band's wings, partially affected by the water vapor absorption. The integrals calculated over the whole and reduced spectral ranges are collected in tables. In addition, we performed measurements with a H2-He mix for different mixing ratios to explore possible deviations from the linear combination of the BACs. The experimental BACs have been shown in comparison with Abel and Borysow's ab initio models for a temperature of about 400 K, resulting in a good agreement over almost the whole spectral range, with a maximum deviation around the main peak of the band. Data and models also show a good agreement in the linear trend of the integrated BACs with temperature, apart from the H2-H2 Borysow's BACs, which follow a quadratic trend. Finally, we resolved all the interference dips, which were not taken into account by the existing theoretical models.

Parabolic mirror cavity-enhanced Raman spectroscopy for trace gas detection.

A parabolic mirror cavity-enhanced Raman spectroscopy (PM-CERS) method was reported, utilizing a combined optical path of a parabolic mirror and a multi-pass cell. In this setup, the focal point of the parabolic mirror is aligned with the laser convergence center of the multi-pass cell. This method achieved high excitation intensity at the focal point and broadened the collection range of Raman scattering signals, thus significantly improving both the excitation and collection efficiency of Raman signals. Additionally, the structure of the system was simplified through the integrated design of the collection cavity and sample cell. Nitrogen, oxygen, water vapor, and carbon dioxide could be detected in the air within an exposure time of just 0.5 s, achieving the limits of detection at the ppm level. Compared to the conventional multi-pass cell, the signal intensity and signal-to-noise ratio were each enhanced by approximately 5.6-fold, further demonstrating the excellent potential application in trace gas detection.

Carrier-envelope phase-stabilized ultrashort pulses from a gas-filled multi-pass cell

Few-cycle laser pulses at a high repetition rate with a stable carrier-envelope phase are required for next-generation attosecond time-resolved spectroscopies. One way to generate these pulses is the nonlinear compression of laser pulses via gas-filled hollow-core fibers. Recently, an alternative approach based on multi-pass cells (MPCs) has been shown to be very efficient for post-compression of turn-key, industrial-grade, high average power Yb-doped solid-state laser amplifiers. However, to expand the system for exploring strong-field laser applications, its carrier-envelope phase stability needs to be demonstrated in the compressed pulses. In this Letter, we present the generation of carrier-envelope phase-stabilized 40 fs pulses with 380 μJ energy at 50 kHz by compressing the output of a Yb:KGW amplifier in a gas-filled MPC. Comparable short-term carrier-envelope phase errors of 412 and 435 mrad root mean square were observed from the amplifier and MPC, respectively, indicating that the phase stability of the amplified pulses is well-maintained during pulse compression in the MPC.

Highly sensitive CH4-TDLAS sensor based on 3D-printed multi-pass cell

In this paper, a highly sensitive methane (CH4) sensor based on tunable diode laser absorption spectroscopy (TDLAS) with 3D-printed multi-pass cell (MPC) is reported. A MPC design model based on ray tracing mathematical method with the law of reflection in vector form was established. The designed MPC has an optical path length (OPL) of 37.8 m and a ratio of optical path length to volume (RLV) of 13.9 cm−2 to enhance the optical absorption. The 3D-printed gas chamber equipped with the MPC and fiber-coupled input greatly simplifies optical alignment. A minimum detection limit (MDL) of the constructed CH4-TDLAS sensor based on fiber-coupled 3D-printed MPC with dense spot pattern was determined to be 30.93 ppb. It can be further improved to 11.79 ppb when the averaging time of the system was 300 s according to the Allan deviation analysis.

Field deployable laser sensor for calibration-free and real-time measurement of NOx from stationary sources

Nitrogen oxides (NOx) are common pollutants emitted from stationary sources, such as industrial boilers and power plants, with significant adverse impacts on the environment and biosphere. The abatement of NOx from anthropic industrial processes requires reliable and sensitive monitoring systems. To address this challenge, we report a sensitive, robust, and compact quantum cascade laser (QCL)-based sensor to measure NO and NO2 emissions from industrial exhausts. This sensor uses two quantum cascade lasers, which take advantage of the intense absorption spectra of NO at 1929 cm−1 and NO2 at 1599 cm−1. A high-temperature multipass cell is designed to withstand harsh environments, which increases the light path length. The NOx concentrations are directly determined from the wavelength modulation spectroscopic model without the need for calibration. We conducted several laboratory experiments to evaluate the sensor's performance, with a focus on accuracy, sensitivity and selectivity. In the end, we showcased the field deployment of our sensor by continuously measuring NOx concentrations in a 2.8-MW gas-fired industrial boiler.

Multipass Faraday rotators and isolators

Faraday isolators are usually limited to Faraday materials with strong Verdet constants. We present a method to reach the 45° polarization rotation angle needed for optical isolators with materials exhibiting a weak Faraday effect. The Faraday effect is enhanced by passing the incident radiation multiple times through the Faraday medium while the rotation angle accumulates after each pass. Materials having excellent thermo-optical properties in the ultraviolet and mid-infrared range become available for optical isolators. Herriott-type multipass cells offer a simple and compact way to realize the desired propagation length in usual optical materials of standard sizes. A proof-of-principle experiment was carried out, demonstrating polarization rotation of a 532 nm laser beam by an angle of 45° in anti-reflection-coated fused silica surrounded by a standard neodymium ring magnet.

A hybrid learning approach to better classify exhaled breath's infrared spectra: A noninvasive optical diagnosis for socially significant diseases.

Early diagnosis is crucial for effective treatment of socially significant diseases, such as type 1 diabetes mellitus (T1DM), pneumonia, and asthma. This study employs a diagnostic method based on infrared laser spectroscopy of human exhaled breath. The experimental setup comprises a quantum cascade laser, which emits in a pulsed mode with a peak power of up to 150 mW in the spectral range of 5.3-12.8 μm (780-1890 cm-1), and a Herriott multipass gas cell with a specific optical path length of 76 m. Using this setup, spectra of exhaled breath in the mid-infrared range were obtained from 165 volunteers, including healthy individuals, patients with T1DM, asthma, and pneumonia. The study proposes a hybrid approach for classifying these spectra, utilizing a variational autoencoder for dimensionality reduction and a support vector machine method for classification. The results demonstrate that the proposed hybrid approach outperforms other machine learning method combinations.

Spatial mode cleaning and efficient nonlinear pulse compression to sub-50 fs in a gas-filled multipass cell.

We demonstrate a gas-filled multipass cell (MPC) that cleaned the spatial mode of a spatial-filter-free 250 W, 100 kHz, 445 fs driven source based on an Innoslab amplifier and compressed the pulse duration to 41 fs simultaneously. The multipass cell acted as a spatial filter and benefited from its discrete waveguide nature, in which the input beam quality factor M2 was improved from 1.53 to a near-diffraction-limited value of 1.21 at 96% transmission.

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.

Real-time monitoring of CH4 and N2O emissions from livestock using mid-infrared external cavity quantum cascade laser absorption spectroscopy

The largest share of greenhouse gas (GHG) emissions related to livestock originates from methane (CH4) and nitrous oxide (N2O) which have a far higher influence on global warming, it is therefore necessary to accurately monitor CH4 and N2O emissions to provide theoretical and practical basis for further estimating and regulating GHG emissions from livestock and improving livestock production performance. For the purpose of sensing CH4 and N2O emissions during livestock living process in real time, an optical sensor based on continuous-wave (CW) external cavity quantum cascade laser (EC-QCL) operating at room temperature was developed. CH4 and N2O absorption lines, located around 8 μm, of the ν4 and ν1 fundamental vibrational bands, respectively, were chosen for direct absorption spectroscopy, which allows for sensitive, selective and simultaneous measurement of CH4 and N2O concentrations. Use of a Herriot multi-pass cell with an effective path-length of 100 m, 1σ (SNR = 1) limits of detection of 26.8 ppbv, 20.3 ppbv and 0.01 % for CH4, N2O and H2O vapor were achieved, respectively. Field measurement of CH4 and N2O emissions from horses has been carried out in a stable over two weeks at the Vernaelde farm in Couderkerque Branche city, France. Concentrations of CH4 and N2O up to 10 times and 1.5 times higher than their levels in the local ambient air (∼ 2.12 ppmv and ∼ 427 ppbv) were observed, respectively.

Highly sensitive CH4, C2H2 and CO simultaneous measurement LITES sensor based on multi-pass cell with overlapped spots pattern and QTFs with low resonant frequency.

This paper presents a simultaneous measurement light-induced thermoelectric spectroscopy (LITES) sensor with high sensitivity for detecting methane (CH4), carbon monoxide (CO) and acetylene (C2H2). It employs a multi-pass cell (MPC) with an overlapped spots pattern and low resonant frequency circle-head quartz tuning forks (QTFs) for the first time. The fiber-coupled MPC with an optical length (OPL) of 40 m was combined with a thin-film filter (TFF) to improve the laser absorption and enable light spots multiplexing on mirrors. Three self-designed circle-head QTFs with low resonant frequencies of less than 10 kHz and a quality factor of ∼ 11500 were adopted to improve the detection ability. The LITES sensor detected multi-gas signals based on their unique absorption spectrum, enabling real-simultaneous measurement of CH4, CO, and C2H2. After optimization, the minimum detection limits (MDLs) of 0.5 ppm, 126.9 ppm and 0.4 ppm for these gases were obtained, respectively. With integration times of 300 s for CH4 and 200 s each for CO and C2H2, the MDLs could be further reduced to 0.09 ppm, 57.1 ppm, and 0.07 ppm, correspondingly. The paper concludes with a discussion of potential strategies for further improving the performance of such LITES sensors.

Symmetric optical multipass matrix systems and the general rapid design methodology

We proposed an original type of multipass cell named symmetric optical multipass matrix system (SMMS), in which the same matrix patterns of various sizes can be formed on both sides. According to its special symmetric configurations, the SMMS design problem is modeled as a variant of the classical traveling salesman problem, which can be rapidly solved by evolutionary optimization algorithms. Two sets of 3-mirror SMMSs are designed, analyzed and built, which show superior characteristics of high stability, desirable beam quality and adjustable optical path lengths. Additionally, they can support simultaneous detection of multiple species with multi-laser channels. The proposed method is further extended to design a 4-mirror SMMS, which verifies the universality and robustness of the design methodology. The experimental observations are in consistent with the theoretical calculations. The newly proposed SMMSs have a broad application prospect in trace gas measurement.

High-sensitivity trace gas detection based on differential Helmholtz photoacoustic cell with dense spot pattern

A high-sensitivity photoacoustic spectroscopy (PAS) sensor based on differential Helmholtz photoacoustic cell (DHPAC) with dense spot pattern is reported in this paper for the first time. A multi-pass cell based on two concave mirrors was designed to achieve a dense spot pattern, which realized 212 times excitation of incident laser. A finite element analysis was utilized to simulate the sound field distribution and frequency response of the designed DHPAC. An erbium-doped fiber amplifier (EDFA) was employed to amplify the output optical power of the laser to achieve strong excitation. In order to assess the designed sensor's performance, an acetylene (C2H2) detection system was established using a near infrared diode laser with a central wavelength 1530.3 nm. According to experimental results, the differential characteristics of DHPAC was verified. Compared to the sensor without dense spot pattern, the photoacoustic signal with dense spot pattern had a 44.73 times improvement. The minimum detection limit (MDL) of the designed C2H2-PAS sensor can be improved to 5 ppb when the average time of the sensor system is 200 s.

Quasi-waveguide amplifiers based on bulk laser gain media in Herriott-type multipass cells.

We present here what we believe to be a new geometry for laser amplifiers based on bulk gain media. The overlapped seed and pump beams are repetitively refocused into the gain medium with a Herriott-type multipass cell. Similar to a waveguide, this configuration allows for a confined propagation inside the gain medium over much longer lengths than in ordinary single pass bulk amplifiers. Inside the gain medium, the foci appear at separate locations. A proof-of-principle demonstration with Ti:sapphire indicates that this could lead to higher amplification due to a distribution of the thermal load.

UV 30 fs laser pulse generation using a multi-pass cell.

Ultrashort ultraviolet (UV) pulses are pivotal for resolving ultrafast electron dynamics. However, their efficient generation is strongly impeded by material dispersion and two-photon absorption, in particular, if pulse durations around a few tens of femtoseconds or below are targeted. Here, we present a new (to our knowledge) approach to ultrashort UV pulse generation: using the fourth-harmonic generation output of a commercial ytterbium laser system delivering 220 fs UV pulses, we implement a multi-pass cell (MPC) providing 5.6 µJ pulses at 256 nm, compressed to 30.5 fs. Our results set a short-wavelength record for MPC post-compression while offering attractive options to navigate the trade-off between upconversion efficiency and acceptance bandwidth for UV pulse production.

Increasing Optical Path Lengths in Micro-Fluidic Devices Using a Multi-Pass Cell.

This study presents a novel absorption cell with a circular geometry that can be integrated into microfluidic devices for optical spectroscopy applications. The absorption cell is made of PDMS/SU8 and offers an optical path length that is 8.5 times its diameter, resulting in a significant increase in the sensitivity of the measurements. Overall, this design provides a reliable and efficient solution for optical spectroscopy in microfluidic systems, enabling the precise detection and analysis of small quantities of analytes.

Radiant fluence from ray tracing in optical multipass systems

Optical multipass cells are used in photochemical reactors and laser excitation of weak transitions. In these applications, estimation of the radiation dose in a volume of interest allows us to assess the performance and optimize the design of the cell. We adopt radiant fluence as the figure of merit and employ the radiative transfer equation to derive analytical expressions for average radiant fluence in a given volume of interest. These expressions involve quantities that are either obtained with Monte Carlo ray tracing methods or approximated on the grounds of geometry arguments. Furthermore, we compute the spatial distribution of fluence by applying either of the two methods to individual volume elements in a rectangular voxel grid. Ray tracing is performed with Zemax OpticsStudio 18.9.

NH3 line broadening coefficients and intensities measurement and impurities determination in emerging applications: CCUS, Biomethane and H2

A mid-infrared quantum cascade laser (Mid-IR QCL) coupled with a Single Pass Cell and a Multi Pass Cell, was utilized to measure ammonia (NH3) absorption spectroscopic parameters and determine NH3 impurities toward three emerging applications. We for the first time measured the pressure broadening coefficients perturbed by Air, O2, N2, He, CO2, CH4, and H2 and the line intensities of six NH3 transition lines near 1084.6 cm−1. The measured NH3-He, NH3-Air, and NH3-CO2 broadening coefficients align with HITRAN database, while NH3-H2 coefficients exhibit a maximum discrepancy of 46 %. Deviations between the measured line intensities and HITRAN database are minimal. Nevertheless, the uncertainties of line intensities have been significantly reduced from 20 % in HITRAN to below 3 %. The newly measured line parameters are utilized to address NH3 impurity requirements outlined in CCUS (ISO 27913:2016), Biomethane (EN 16723:2016), and H2 (ISO 14687:2019) standards. Based on the concept of optical gas standard (OGS), the NH3 impurity detection requirements in all three standards have been fulfilled with an uncertainty of 1.35 %. The precision of the NH3-OGS is 800 part per trillion (ppt) with an integration time of 100 s. The repeatability of the NH3-OGS is 130 ppt for a continuous measurement time of 48 min. Notably, the NH3-OGS effectively addresses the highly nonlinear adsorption–desorption dynamics, underscoring the potential of OGS as a calibration-free and SI-traceable metrological gas analysis instrument.