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We use a Fourier transform spectrometer based on a difference frequency generation optical frequency comb to measure high-resolution, low-pressure, room-temperature spectra of methane in the 1250 - 1380 cm$^{-1}$ range. From these spectra, we retrieve line positions and intensities of 678 lines of two isotopologues: 157 lines from the $^{12}$CH${_4}$ ${\nu}$${_4}$ fundamental band, 131 lines from the $^{13}$CH${_4}$ ${\nu}$${_4}$ fundamental band, as well as 390 lines from two $^{12}$CH${_4}$ hot bands, ${\nu}$${_2}$ + ${\nu}$${_4}$ - ${\nu}$${_2}$ and 2${\nu}$${_4}$ - ${\nu}$${_4}$. For another 165 lines from the $^{12}$CH${_4}$ ${\nu}$${_4}$ fundamental band we retrieve line positions only. The uncertainties of the line positions range from 0.19 to 2.3 MHz, and their median value is reduced by a factor of 18 and 59 compared to the previously available data for the $^{12}$CH${_4}$ fundamental and hot bands, respectively, obtained from conventional FTIR absorption measurements. The new line positions are included in the global models of the spectrum of both methane isotopologues, and the fit residuals are reduced by a factor of 8 compared to previous absorption data, and 20 compared to emission data. The experimental line intensities have relative uncertainties in the range of 1.5 - 7.7%, similar to those in the previously available data; 235 new $^{12}$CH${_4}$ line intensities are included in the global model.

We demonstrate an antiresonant hollow-core fiber-based photothermal sensor capable of targeting gases having absorption lines beyond 5 µm. The sensor reached a nitric oxide detection limit of 11 ppbv for 144 s integration time.

In this work we report on Wavelength Modulation Spectroscopy of ethane with sub parts-per-billion detection limit in the mid-IR spectral band aided with a 30-meters long self-fabricated all-silica Antiresonant Hollow-Core Fiber-based absorption cell.

We use comb-based Fourier transform spectroscopy to record high-resolution spectra of 12CH4 and 13CH4 from 1250 to 1380 cm-1. We obtain line positions and intensities of 4 bands with uncertainties of ~450 kHz and ~3%, respectively, which we use to improve a global fit of the effective Hamiltonian.

We report the development of compact, stabilized fiber-based mid-infrared frequency comb sources covering the 7–9 μm and 3.1–3.4 μm spectral ranges. Both sources enable high-precision Fourier-transform spectroscopy of greenhouse gases.

In this work, we present femtosecond laser cutting of microchannels in a nodeless antiresonant hollow-core fiber (ARHCF). Due to its ability to guide light in an air core combined with exceptional light-guiding properties, an ARHCF with a relatively non-complex structure has a high application potential for laser-based gas detection. To improve the gas flow into the fiber core, a series of 250 × 30 µm microchannels were reproducibly fabricated in the outer cladding of the ARHCF directly above the gap between the cladding capillaries using a femtosecond laser. The execution time of a single lateral cut for optimal process parameters was 7 min. It has been experimentally shown that the implementation of 25 microchannels introduces low transmission losses of 0.17 dB (&lt, 0.01 dB per single microchannel). The flexibility of the process in terms of the length of the performed microchannel was experimentally demonstrated, which confirms the usefulness of the proposed method. Furthermore, the performed experiments have indicated that the maximum bending radius for the ARHCF, with the processed 100 µm long microchannel that did not introduce its breaking, is 15 cm.

We propose a unique way to design multipass cells (MPCs), which combines cost-efficient spherical mirrors with the high-density pattern of astigmatic mirrors. Such functionality was accomplished using at least three standard spherical mirrors appropriately tilted, which breaks the parallelism between them. A genetic algorithm (GA) supported the cell configuration optimization. A 16 m and 23.8 m optical path length (OPL) MPC was developed, practically realized, and proved by a time-of-flight (TOF) experiment to demonstrate the principle. Finally, CO2 detection at 2004nm obtaining 0.4 ppmv limit of detection (LOD) using wavelength modulation spectroscopy (WMS) with 10 s averaging was performed.

In this work, I demonstrate a novel configuration of a photothermal gas sensor. Detection of nitric oxide at a wavelength of 5.26 µm was possible by constructing an absorption cell based on a self-fabricated antiresonant hollow core fiber characterized by low losses at both the pump and probe wavelengths. Proper design of the sensor allowed using the heterodyne interferometry-based signal readout of the refractive index modulation, which yielded a record noise equivalent absorption of 2.81×10−8 cm-1 for 100 s integration time for mid-infrared fiber-based gas sensors. The obtained results clearly demonstrate the full potential of using properly designed antiresonant hollow core fibers in combination with sensitive gas detection methods.

In this work, we present simultaneous and sensitive detection of methane and ethane at ~3.34 µm using a 15-meteres long self-fabricated silica ARHCF and Wavelength Modulation Spectroscopy technique. The ARHCF was filled with a mixture of 10 ppmv and 20 ppmv ethane and methane, respectively via air-tight housings placed at both fiber end-facets. The gas molecules were excited using a self-built continuous wave Difference Frequency Generation source which radiation was coupled into the gas-filled ARHCF. The ARHCF-aided gas sensor reached a minimum detection limit at parts-per-billion by volume level, confirming the suitability of the proposed approach for trace-gas sensing.

Detection of Nitric Oxide at 5.26 µm is performed using photothermal interferometry and a 25 cm-long antiresonant hollow-core fiber as an absorption cell, reaching a minimum detection limit of 11 ppb for 144 seconds averaging. The proposed configuration shows the full potential of combining novel hollow-core fibers with the photothermal detection techniques, which allows separating the pump and the probe part of the sensor.

We present a proof-of-concept of the ARHCF-assisted gas molecules detection setup applying a non-wavelength modulation differential optical absorption spectroscopy (DOAS) technique to reduce an intermodal interferences effect and the compensation of changes in laser beam propagation parameters through ARHCF air-core. Toggled intensities of two air-core propagated laser beams have closely spaced wavelengths, while only one coinciding with targeted absorption line of acetylene at 1532.83 nm (~6524 cm-1). The difference in detected light intensities is used to determine a targeted gas concentration. Self-designed boxcar average unit allowed to 50 000 times/sec signals averaging, providing a minimal detection limit equal to 2.7 ppmv at 10s integration time.

In this work, a method of photothermal spectroscopic signal extraction is presented. The refractive index modulation readout is realized in a purely frequency detection-based approach, by demodulating the beatnotes of a mode-locked fiber laser operating at 1.56 µm. A unique and non-complex self-heterodyne harmonic amplification technique is employed, yielding an increase in the limit of detection by a factor of 22. The sensor’s performance was evaluated by detecting nitric oxide at 5.26 µm, confirming the feasibility of separating the pump and probe sections of the device. The sensor reached a detection limit of 9.6 parts-per-billion by volume for an integration time of 136 s, with only a 20 cm-long laser-gas molecules interaction path length.

Laser-based gas detection techniques are undergoing continuous, rapid development due to the wide area of applications in medicine, environmental monitoring, military applications etc. The development of antiresonant hollow-core fibers (ARHCFs) capable of transmitting with low-loss mid-infrared radiation enabled developing novel, miniaturized and very sensitive gas spectrometers. The unique design of the ARHCFs limits the overlap of the propagating beam with the glass cladding, at the same time ensuring single-transversal-mode operation at the wavelengths suitable for targeting strong transitions of numerous gas molecules. This fact combined with large diameters of the hollow cores (>100 µm) allows for non-complex gas exchange, which ensures low response time of the sensors based on these unique fibers. With proper design, ARHCF-based laser gas detectors can have several-meter-long beam-gas interaction path-lengths, thus reaching minimum detection limits (MDL) matching, or even outperforming sensors based on traditional multipass absorption cells [1] .

Mid-infrared comb-based Fourier transform spectroscopy allows high-resolution measurements of entire molecular absorption bands. However, most previous implementations were limited to wavelengths

One of the most unique features of the hollow-core fibers (HCF) is the possibility to fill the fiber core with desirable gas, which can be useful, especially in laser absorption spectroscopy (LAS) of gases. Sensitivity of LAS-based gas sensors can be easily and efficiently enhanced by increasing the gas molecules-laser light interaction path within the sensor’s setup. In commonly used multi-pass cells-based sensors, the increase in optical path length comes at the cost of the increased sensor’s complexity, leading to the reduced detection capability. HCFs, that are long, flexible, and devoid of additional optics, allows for constructing gas absorption cells that give the possibility to design less-complex sensor's. One of the main problems in developing HCFs-based sensors is the filling time of the fiber core with the target gas. The core diameter of an Anti-Resonant Hollow Core Fiber (ARHCF) is typically on the order of tens of micrometers, leading to a low volumetric flow rate. It was reported in [1] that the time required to fill a 1 meter-long ARHCF with a core diameter of ~84 μm with the target gas using relatively low overpressure (800 Torr) is about 20 seconds. Additionally, the nodeless type of ARHCF is made of separated capillaries, forming its cladding ( Fig. 1a ). The velocity of gas flow in the gaps between the cladding capillaries is far lower than velocity in the core. We believe that this has a significant impact on the time of pressure-driven gas-filling of such fibers, hence it is necessary to perform simulations on more complex geometric models than for the gap-less ARHCFs, where the core can be approximated by a simple 2D channel.

One of the main challenges of laser-based gas sensingis the development of sensors delivering simultaneously high sensitivity, selectivity, fast-response time and non-complex design. Mostly, the detection capability of such sensors depends on the interaction path length between the laser light and the measured gas. Hence, long optical paths are highly desired for e.g. low-concentration gas sensing. Our proposal is to use Antiresonant Hollow-Core Fibers (ARHCFs), which filled with the target gas mixture form absorption cells with potentially any length, delivering low-volume, long and versatile optical paths within the sensor configuration. Currently, the ARHCF core is filled with the target gas via specially designed bulk-optics-based cells placed at the fiber’s ends. This solution provides relatively fast fiber core filling time, however being only efficient while an overpressure is used to force the gas flow through the core, not the diffusion. Therefore, searching for alternative ways of fiber filling with the target gas is necessary. We propose a method of processing the fiber structure using a femtosecond laser allowing for non-invasive accessing the fiber core for more efficient and faster gas diffusion into it through the fabricated microchannels. The fiber structure modification was optimized in a way that does not introduce any unwanted damage of the fiber e.g. cracks on the glass parts or cladding capillaries. The performed experiments have indicated that the laser-processing of the ARHCF structure introduces negligible transmission loss regardless of the number of fabricated microchannels and their length (0.2dB loss for 25 microchannels), confirming the proposed method suitability.

In this work, we present a borosilicate Antiresonant Hollow-Core Fiber-based gas sensor enabling detection of nitric oxide with a parts-per-billion by volume level sensitivity targeting its transition at 5.26 µm using Wavelength Modulation Spectroscopy technique.

We demonstrate fabrication of microchannels in the outer cladding of a nodeless antiresonant hollow-core fiber (ARHCF) using developed femtosecond laser-based micromachining process.

We present a novel, miniaturized configuration of a sub-ppm levels nitric oxide gas sensor. The gas sample concentration induces a photothermal effect inside a monolithic diode- pumped solid-state laser resonator in the Nd:YVO4/YVO4/Air-Gap structure.

Using a compact fiber-based difference frequency generation comb and a Fourier transform spectrometer we record spectra of the N2O ν1 band at 1285 cm-1 in the Doppler limit. Fitting Gaussian line shapes to the individual absorption lines yields center frequencies with

We use a Fourier transform spectrometer based on a compact mid-infrared difference frequency generation comb source to perform broadband high-resolution measurements of nitrous oxide, $^{14}$N$_2$$^{16}$O, and retrieve line center frequencies of the $\nu$$_1$ fundamental band and the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band. The spectrum spans 90 cm$^{-1}$ around 1285 cm$^{-1}$ with a sample point spacing of 3 ${\times}$ 10$^{-4}$ cm$^{-1}$ (9 MHz). We report line positions of 72 lines in the $\nu$$_1$ fundamental band between P(37) and R(38), and 112 lines in the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band (split into two components with e/f rotationless parity) between P(34) and R(33), with uncertainties in the range of 90-600 kHz. We derive upper state constants of both bands from a fit of the effective ro-vibrational Hamiltonian to the line center positions. For the fundamental band, we observe excellent agreement in the retrieved line positions and upper state constants with those reported in a recent study by AlSaif et al. using a comb-referenced quantum cascade laser [J Quant Spectrosc Radiat Transf, 2018;211:172-178]. We determine the origin of the hot band with precision one order of magnitude better than previous work based on FTIR measurements by Toth [http://mark4sun.jpl.nasa.gov/n2o.html], which is the source of the HITRAN2016 data for these bands.

Detection of Nitric Oxide at 5.26 µm is performed using photothermal interferometry and a 25 cm-long antiresonant hollow-core fiber as an absorption cell, reaching a minimum detection limit of 11 ppb for 144 seconds averaging.

We use offset-frequency-free difference frequency generation comb sources and a Fourier transform spectrometer with comb-mode-width limited resolution to measure and analyze spectra of molecular species of atmospheric relevance: CH3I and CH2Br2 around 3000 cm-1, and 14N 162O around 1280 cm-1.

In this work, we present the first demonstration of a gas sensor utilizing the Wavelength Modulation Spectroscopy technique and a mid-IR guiding Antiresonant Hollow-Core Fiber with a record length of 30 m. The self-fabricated ARHCF was used as an air-tight, low-volume absorption cell delivering an extended laser-gas molecules interaction path within the sensor’s setup, which enabled it to efficiently detect ethane at 2996.88 cm−1. Benefiting from the unique guidance properties of the ARHCF, the sensor reached a minimum detection limit of 670 parts-per-trillion by volume for 14 s integration time, which is at a level comparable to the bulk optics-based systems utilizing more complex detection techniques or multipass cells with tens of meters long optical path lengths. The obtained results confirm that ARHCF-based gas sensors can successfully compete with or even outperform bulk optics-based mid-IR gas sensor configurations. The obtained results show the potential for the development of novel, low-volume, compact, and reliable types of gas detectors allowing precise analysis of various gaseous substances at their trace concentration level.

In this work, we present for the first time a laser-based dual gas sensor utilizing a silica-based Antiresonant Hollow-Core Fiber (ARHCF) operating in the Near- and Mid-Infrared spectral region. A 1-m-long fiber with an 84-&micro, m diameter air-core was implemented as a low-volume absorption cell in a sensor configuration utilizing the simple and well-known Wavelength Modulation Spectroscopy (WMS) method. The fiber was filled with a mixture of methane (CH4) and carbon dioxide (CO2), and a simultaneous detection of both gases was demonstrated targeting their transitions at 3.334 &micro, m and 1.574 &micro, m, respectively. Due to excellent guidance properties of the fiber and low background noise, the proposed sensor reached a detection limit down to 24 parts-per-billion by volume for CH4 and 144 parts-per-million by volume for CO2. The obtained results confirm the suitability of ARHCF for efficient use in gas sensing applications for over a broad spectral range. Thanks to the demonstrated low loss, such fibers with lengths of over one meter can be used for increasing the laser-gas molecules interaction path, substituting bulk optics-based multipass cells, while delivering required flexibility, compactness, reliability and enhancement in the sensor&rsquo, s sensitivity.

Laser-based gas sensors utilizing various light-gas interaction phenomena have proved their capacity for detecting different gases. However, achieving reasonable sensitivity, especially in the mid-infrared, is crucial. Improving sensor detectivity usually requires incorporating multipass cells, which increase the light-gas interaction path length at a cost of reduced stability. An unconventional solution comes with the aid of hollow-core fibers. In such a fiber, light is guided inside an air-core which, when filled with the analyte gas can serve as a low-volume and robust absorption cell. Here we report on the use of a borosilicate antiresonant hollow-core fiber for laser-based gas sensing. Due to its unique structure and guidance, this fiber provides low-loss, single-mode transmission > 5 µ m . The feasibility of using the fiber as a gas cell was verified by detecting nitrous oxide at 5.26 µm with a minimum detection limit of 20 ppbv.

We present the results of fabrication of microchannels in the antiresonant hollow-core fiber using ultra-short laser pulses for application in laser-based gas sensing.

An unique photothermal gas sensor configuration employing a simple, mode-locked fiber laser is presented. The concentration of the measured gas sample was encoded in the deviation of pulse repetition frequency, which can be easily demodulated.

We report a compact, actively stabilized fiber-based mid-infrared frequency comb source covering the 6.5 – 9 μm wavelength range with up to 5 mW of average output power at 125 MHz repetition frequency.

Here we present a novel, miniaturized and sub-ppm level gas sensor configuration. To estimate the gas sample concentration the photothermal effect inside a monolithic, diode-pumped solid-state laser (Nd:YVO4/YVO4/Air-Gap) resonator is induced.

We have fabricated an Antiresonant Hollow-Core Fiber enabling efficient guidance in the Near- and Mid-IR and demonstrated its application to simultaneous detection of methane and carbon dioxide using Wavelength Modulation Spectroscopy.

A compact and robust all-fiber difference frequency generation-based source of broadband mid-infrared radiation is presented. The source emits tunable radiation in the range between 6.5 µm and 9 µm with an average output power up to 5 mW at 125 MHz repetition frequency. The all-in-fiber construction of the source along with active stabilization techniques results in long-term repetition rate stability of 3 Hz per 10 h and a standard deviation of the output power better than 0.8% per 1 h. The applicability of the presented source to laser spectroscopy is demonstrated by measuring the absorption spectrum of nitrous oxide (N2O) around 7.8 µm. The robustness and good long- and short-term stability of the source opens up for applications outside the laboratory.

Laser-based gas sensors are undergoing constant and rapid development due to their usability in numerous applications focused on e.g. monitoring of greenhouse gases concentration, safety in harsh environments or medicine. Amongst many types of such sensors, gas detectors employing the photoacoustic or photothermal effect are characterized by an exceptional capability of delivering high selectivity combined with superb signal-to-noise ratio (SNR). In this work, we demonstrate a novel, compact and sensitive nitric oxide (NO) sensor targeting its strong transition near 5.26 μm by combining the photothermal interferometry (PTI) technique and a gas absorption cell based on a self-fabricated antiresonant hollow-core fiber (ARHCF). Having only a 25 cm long interaction length, the sensor reached a minimum detection limit (MDL) of 11 ppbv (parts per billion by volume) for an integration time of 144 s. Proper design of the ARHCF utilized as the gas cell enabled combining a mid-infrared (mid-IR) pump laser to induce the refractive index modulation and a Fabry-Perot interferometer-based readout of the photothermal effect with a near-infrared probe laser. The presented results document the first experimental attempt to detect gas molecules having transitions beyond 4.5 μm in an ARHCF-aided photothermal detection scheme.

Widely-tunable, fully-monolithic, mid-infrared (mid-IR) deference frequency generation source (DFG) is presented. By using a custom designed fiber-pigtailed periodically poled lithium niobate (PPLN) crystal module the idler beam was generated with an efficiency of 21%/W, yielding 2.6 mW of optical output power. The proposed all-fiber configuration radically simplified the optical frequency conversion setup, making it robust and easily configurable. The usefulness of the constructed source was verified by performing simultaneous wavelength modulation spectroscopy (WMS) laser trace gas detection of methane, near 2999 cm−1, and ethane, near 2997 cm−1, via two independently generated, tunable idler beams.