Highly sensitive methane detection using a mid-infrared interband cascade laser and an anti-resonant hollow-core fiber.

For over a decade hollow-core fibers have been used in optical gas sensors in the role of gas cells. However, very few examples of actual real-life applications of those sensors have been demonstrated so far. In this paper, we present a highly-sensitive hollow-core fiber based methane sensor. Mid-infrared distributed feedback interband cascade laser operating near 3.27 µm is used to detect gas inside anti-resonant hollow-core fiber. R(3) line near 3057.71 cm-1 located in ν3 band of methane is targeted. Compact, lens-free optical setup with an all-silica negative curvature hollow-core fiber as the gas cell is demonstrated. Using wavelength modulation spectroscopy and 7.5-m-long fiber the detection limit as low as 1.54 ppbv (at 20 s) is obtained. The demonstrated system is applied for a week-long continuous monitoring of ambient methane and water vapor in atmospheric air at ground level. Diurnal cycles in methane concentrations are observed, what proves the sensor's usability in environmental monitoring.

Widely tunable continuous-wave fiber laser in the 1.55-1.8††µm wavelength region.

In this paper, we experimentally demonstrate a continuous-wave widely wavelength-tunable fiber laser based on erbium- and bismuth-doped fibers in parallel configuration. A diffraction grating was used for wavelength tuning, and the tuning range of 268 nm (from 1545 to 1813 nm) was obtained using Littrow design. This result is significantly greater than demonstrated previously in the same spectral region with erbium or bismuth doped fiber lasers.

Heterodyne photothermal spectroscopy of methane near 1651 nm inside hollow-core fiber using a bismuth-doped fiber amplifier.

We present laser-based methane detection near 1651 nm inside an antiresonant hollow-core fiber (HCF) using photothermal spectroscopy (PTS). A bismuth-doped fiber amplifier capable of delivering up to more than 160 mW at 1651 nm is used to boost the PTS signal amplitude. The design of the system is described, and the impact of various experimental parameters (such as pump source modulation frequency, modulation amplitude, and optical power) on signal amplitude and signal-to-noise ratio is analyzed. Comparison with similar PTS/HCF-based systems is presented. With 1.3 m long HCF and a fiber amplifier for signal enhancement, this technique is capable of detecting methane at single parts-per-million levels, which makes this robust in-fiber sensing approach promising also for industrial applications such as, e.g., natural gas leak detection.

Characterization of a single-frequency bismuth-doped fiber power amplifier with a continuous wave and modulated seed source at 1687 nm.

In this paper, we report the performance of a bismuth-doped fiber amplifier at 1687 nm. This wavelength region is particularly interesting for laser-based spectroscopy and trace gas detection. The active bismuth-doped fiber is pumped at 1550 nm. With less than 10 mW of the seed power, more than 100 mW is obtained at the amplifier's output. We also investigate the signal at the output when a wavelength-modulated seed source is used, and present wavelength modulation spectroscopy of methane transition near 1687 nm. A significant baseline is observed in the spectra recorded when the fiber amplifier is used. The origin of this unwanted background signal is discussed and methods for its suppression are demonstrated.

Demonstration of mid-infrared gas sensing using an anti-resonant hollow core fiber and a quantum cascade laser.

In this paper we report a mid-infrared sensor based on an anti-resonant hollow core fiber. A quantum cascade laser operating around 4.53 µm is used to target one of the strongest transition of nitrous oxide near 2203.7 cm-1. The system provides 1-second minimum detection limit at single parts-per-billion level using 3.2-m-long fiber with the response time of less than 30 seconds. Presented sensing approach shows a good perspective for compact and sensitive mid-infrared fiber-based spectrometers.

Laser absorption spectroscopy at 2 µm inside revolver-type anti-resonant hollow core fiber.

In this paper, we present a laser-based sensing inside anti-resonant hollow core fiber. A distributed feedback laser diode operating near 2004 nm and a 1.35-m-long silica-based fiber are used to demonstrate carbon dioxide detection with sensitivity down to ~5 ppmv. Gas exchange time as low as 5 seconds is obtained. This performance was achieved in a very simple optical configuration, without any mirrors or lenses in the setup.

Kagome Hollow Core Fiber-Based Mid-Infrared Dispersion Spectroscopy of Methane at Sub-ppm Levels.

In this paper, we demonstrate the laser-based gas sensing of methane near 3.3 µm inside hollow-core photonic crystal fibers. We exploit a novel anti-resonant Kagome-type hollow-core fiber with a large core diameter (more than 100 µm) which results in gas filling times of less than 10 s for 1.3-m-long fibers. Using a difference frequency generation source and chirped laser dispersion spectroscopy technique, methane sensing with sub-parts-per-million by volume detection limit is performed. The detection of ambient methane is also demonstrated. The presented results indicate the feasibility of using a hollow-core fiber for increasing the path-length and improving the sensitivity of the mid-infrared gas sensors.

Hollow core fiber-assisted absorption spectroscopy of methane at 3.4 µm.

A laser-based spectrometer exploiting a novel Kagome-type hollow core photonic crystal fiber, which serves as a gas cell is demonstrated. Low attenuation of this silica-based fiber in the 3.4 µm wavelength region enables accessing strong, fundamental transitions of methane, which was used as a target analyte in the presented experiment. With an all-fiber differential frequency generation source combined with wavelength modulation spectroscopy technique detection limit at single parts-per-million by volume level was obtained. These results show potential for developing compact and sensitive Kagome-fiber-based mid-infrared laser spectrometers.

Heterodyne interferometric signal retrieval in photoacoustic spectroscopy.

A new heterodyne interferometric method for optical signal detection in photoacoustic or photothermal spectroscopy is demonstrated and characterized. It relies on using one laser beam for the photoacoustic excitation of the gas sample that creates refractive index changes along the beam path, while another laser beam is used to measure these changes. A heterodyne-based detection of path-length changes is presented that does not require the interferometer to be balanced or stabilized, which significantly simplifies the optical design. We discuss advantages of this new approach to photoacoustic signal detection and the new sensing arrangements that it enables. An open-path photoacoustic spectroscopy of carbon dioxide at 2003 nm and a novel sensing configuration that enables three-dimensional spatial gas distribution measurement are experimentally demonstrated.

Laser-Based Monitoring of CH4, CO2, NH3, and H2S in Animal Farming-System Characterization and Initial Demonstration.

In this paper, we present a system for sequential detection of multiple gases using laser-based wavelength modulation spectroscopy (WMS) method combined with a Herriot-type multi-pass cell. Concentration of hydrogen sulfide (H2S), methane (CH4), carbon dioxide (CO2), and ammonia (NH3) are retrieved using three distributed feedback laser diodes operating at 1574.5 nm (H2S and CO2), 1651 nm (CH4), and 1531 nm (NH3). Careful adjustment of system parameters allows for H2S sensing at single parts-per-million by volume (ppmv) level with strongly reduced interference from adjacent CO2 transitions even at atmospheric pressure. System characterization in laboratory conditions is presented and the results from initial tests in real-world application are demonstrated.

Chirped laser dispersion spectroscopy for laser-based hydrogen sulfide detection in open-path conditions.

In this paper the design and characterization of a near-IR Chirped Laser Dispersion Spectroscopy (CLaDS)-based setup for hydrogen sulfide (H2S) detection is reported. This system can be implemented for open-path sensing also in standoff configuration. Target transition selection, system noise and detection limit are discussed and characterized. Furthermore, the cross-interference with other molecules is analyzed. CLaDS-based detection is shown to be highly immune to background carbon dioxide changes, which is a critical issue in accurate open-path sensing of hydrogen sulfide.

Chirped lasers dispersion spectroscopy implemented with an electro-optical intensity modulator--signal strength and shapes under different experimental conditions.

Signals measured with Chirped Laser Dispersion Spectroscopy (CLaDS) setup implemented with an intensity modulator are analyzed. We investigate the signal amplitude dependence on the modulator bias voltage and the signal generator output power. Potential strategies for signal retrieval are discussed. We demonstrate that choosing a bias voltage, an RF generator output power and a demodulation frequency is critical for CLaDS and strongly affects its performance.

Field Test of a Remote Multi-Path CLaDS Methane Sensor.

Existing technologies for quantifying methane emissions are often limited to single point sensors, making large area environmental observations challenging. We demonstrate the operation of a remote, multi-path system using Chirped Laser Dispersion Spectroscopy (CLaDS) for quantification of atmospheric methane concentrations over extended areas, a technology that shows potential for monitoring emissions from wetlands.

High frequency modulation capabilities and quasi single-sideband emission from a quantum cascade laser.

Both intensity- (IM) and frequency-modulation (FM) behavior of a directly modulated quantum cascade laser (QCL) are measured from 300 Hz to 1.7 GHz. Quantitative measurements of tuning coefficients has been performed and the transition from thermal- to electronic-tuning is clearly observed. A very specific FM behavior of QCLs has been identified which allows for optical quasi single sideband (SSB) modulation through current injection and has not been observed in directly modulated semiconductor lasers before. This predestines QCLs in applications where SSB is required, such as telecommunication or high speed spectroscopy. The experimental procedure and theoretical modeling for data extraction is discussed.

Chirped laser dispersion spectroscopy with differential frequency generation source.

A feasibility study of open-path methane detection at 3.4 µm using chirped laser dispersion spectroscopy (CLaDS) based on nonlinear differential frequency generation (DFG) laser source is performed. Application of a DFG source based on telecom laser sources and modulators allows mid-infrared CLaDS system to be optimized for measurements of gases at atmospheric conditions for which modulation in the GHz range is required. Excellent agreement between observed CLaDS signals and spectroscopic models has been observed.

Cryogen-free heterodyne-enhanced mid-infrared Faraday rotation spectrometer.

A new detection method for Faraday rotation spectra of paramagnetic molecular species is presented. Near shot-noise limited performance in the mid-infrared is demonstrated using a heterodyne enhanced Faraday rotation spectroscopy (H-FRS) system without any cryogenic cooling. Theoretical analysis is performed to estimate the ultimate sensitivity to polarization rotation for both heterodyne and conventional FRS. Sensing of nitric oxide (NO) has been performed with an H-FRS system based on thermoelectrically cooled 5.24 µm quantum cascade laser (QCL) and a mercury-cadmium-telluride photodetector. The QCL relative intensity noise that dominates at low frequencies is largely avoided by performing the heterodyne detection in radio frequency range. H-FRS exhibits a total noise level of only 3.7 times the fundamental shot noise. The achieved sensitivity to polarization rotation of 1.8 × 10(-8) rad/Hz(1/2) is only 5.6 times higher than the ultimate theoretical sensitivity limit estimated for this system. The path- and bandwidth-normalized NO detection limit of 3.1 ppbv-m/Hz(1/2) was achieved using the R(17/2) transition of NO at 1906.73 cm(-1).

Chirped lasers dispersion spectroscopy implemented with single- and dual-sideband electro-optical modulators.

We report new approaches for signal generation in Chirped Laser Dispersion Spectroscopy (CLaDS). Two optical arrangements based on electro-optical modulators significantly reduce CLaDS system complexity and enable optimum performance when applied to detection of GHz-wide molecular transitions. Proof-of-principle experiments in the near-infrared spectral range are presented and potential strategies for application in the mid-infrared are discussed.

Measuring optically thick molecular samples using chirped laser dispersion spectroscopy.

In this Letter, a dispersion-based gas sensing method applied to detection of optically thick samples is presented. We show that chirped laser dispersion spectroscopy (CLaDS) technique provides perfectly linear signal response over a wide range of target analyte concentrations. Using the most convenient chirp-modulated CLaDS detection scheme, it enables spectroscopic measurements in a line-locked mode from the minimum detection limit up to >99% peak molecular absorption.

Signal-to-noise ratio in chirped laser dispersion spectroscopy.

Quantitative studies and experimental validation of noise sources occurring in chirped laser dispersion spectroscopy (CLaDS) are reported. Their impact on the signal-to-noise ratio (SNR) achievable with the CLaDS sensing method is analyzed through a noise model supported by experimental results. In particular the model shows that the SNR is optimal for a given value of the laser chirp rate. The experimental studies are conducted with a quantum cascade laser operating at 5.2 µm for the detection of nitric oxide. Optical fringing has been found to be a significant non-random source of noise and an effective reduction method that can improve the SNR is also discussed.

Chirped laser dispersion spectroscopy for remote open-path trace-gas sensing.

In this paper we present a prototype instrument for remote open-path detection of nitrous oxide. The sensor is based on a 4.53 µm quantum cascade laser and uses the chirped laser dispersion spectroscopy (CLaDS) technique for molecular concentration measurements. To the best of our knowledge this is the first demonstration of open-path laser-based trace-gas detection using a molecular dispersion measurement. The prototype sensor achieves a detection limit down to the single-ppbv level and exhibits excellent stability and robustness. The instrument characterization, field deployment performance, and the advantages of applying dispersion sensing to sensitive trace-gas detection in a remote open-path configuration are presented.