Chirped Laser Dispersion Spectroscopy Research Articles

Stationary and drone-assisted methane plume localization with dispersion spectroscopy

This work presents stationary and mobile retroreflector-based remote sensing techniques for methane leak localization and quantification using chirped laser dispersion spectroscopy equipped with a custom laser transceiver capable of continuous tracking of a flying drone and coupled with inverse atmospheric gas dispersion modeling. The techniques demonstrate the ability to localize leaks as low as 0.13 g CH4·s−1, which are up to 25 times smaller than those typically observed at natural gas facilities, as well as actively track a moving retroreflector mounted on a lightweight (∼250 g) drone to enable spatial plume reconstruction. This system exhibited a 2.3 ppm-m sensitivity over pathlengths of 40–150 m. Source localization to within ±7 m is demonstrated using a modified horizontal radial plume mapping technique with a stationary retroreflector grid. Meanwhile, the mobile system utilizing a drone-mounted retroreflector is able to localize a controlled release within ±1 m of its source location and estimate leak rates using inversion techniques assuming type B Gaussian plume stability class within ±30% error with respect to the actual low flow rate releases.

Mid-IR antiresonant hollow-core fiber based chirped laser dispersion spectroscopy of ethane with parts per trillion sensitivity

Rapid development of a new type of air-core guiding fibers, the so-called Antiresonant Hollow-Core Fibers (ARHCFs), have brought a wide range of new application areas of the optical fiber technology. One of them is the laser spectroscopy of gases, where the interaction path length between the gas molecules and the laser beam is one of the most important factors defining the detection capability of the sensors. Due to the ability of efficient light guiding in an empty core, ARHCFs can be filled with the target gas and form long and low-volume absorption cells in the sensor, delivering the interaction distance required by the user and application. However, successful application of the gas-filled ARHCFs necessitates combining several factors, i.e. low-loss guidance and optimized structure of the ARHCF, proper sensor setup, and an efficient gas sensing technique. In this work, we report for the first time the development of a Chirped Laser Dispersion Spectroscopy (CLaDS) sensor of ethane (C2H6) aided with a 30 m long ARHCF-based absorption cell. Thanks to this unique configuration with an extended laser-gas molecules interaction path, the sensor reached a superb detection capability of ∼15 parts-per-trillion by volume (pptv) for a 25-s integration time with a noise equivalent absorption (NEA) coefficient of 3.5 × 10−10 cm−1. The sensitivity of the sensor beats the performance of the reported up to date detectors utilizing mid-IR hollow-core fibers (HCFs).

Broadband gas phase absorber detection and quantification by chirped laser dispersion spectroscopy at 1.55 µm.

The demonstration and first evaluation of chirped laser dispersion spectroscopy (CLaDS) for quantitative measurements of gas molecules with broad spectral features is reported. The demonstration is conducted on propyne (methyl acetylene) gas, using a widely tunable external cavity near infrared laser, λ ≈ 1.55 µm, whose frequency can be swept at 2.6 MHz/µs. A direct baseband downconversion scheme is implemented to recover molecular dispersion, with a cost-effective 32 GHz radio frequency architecture. Laboratory tests demonstrate in particular the value of laser dispersion spectroscopy for the sensing of turbid media with a large range of variations, owing to a significant immunity of the detection scheme to variations in received optical power. Normalized minimum concentration measurable in the 1.5 ms scan is ∼0.7 ppm.m.√Hz.

Mid-Ir Antiresonant Hollow-Core Fiber Based Chirped Laser Dispersion Spectroscopy of Ethane with Parts Per Trillion Sensitivity

Mid-Ir Antiresonant Hollow-Core Fiber Based Chirped Laser Dispersion Spectroscopy of Ethane with Parts Per Trillion Sensitivity

Fugitive methane detection using open-path stand-off chirped laser dispersion spectroscopy.

We report an open-path chirped laser dispersion spectrometer capable of detecting the atmospheric methane concentration above the background using both specular and diffusive reflective surfaces via two distinct operation modes in a stand-off detection configuration. The system is integrated with simultaneous ranging functionality, which enables average concentration measurements for varying optical pathlengths. The system was first tested for accuracy and characterized to achieve sensitivity of 2.9ppm-m/Hz1/2 and pathlength precision of 0.2m/Hz1/2 with a controlled release of methane outside the laboratory. The instrument was subsequently field-deployed in the proximity of a natural gas compressor station for fugitive methane detection. The instrument successfully detected methane plumes and narrowed down the location of the plume through multi-path measurement. The field measurements were verified by a co-located reference mobile methane sensor.

Chirped laser dispersion spectroscopy for spectroscopic chemical sensing with simultaneous range detection.

Spectroscopic chemical detection requires knowledge or determination of an optical path for accurate quantification of path-integrated concentration of species. Continuous-wave-laser-based spectroscopic systems operating in an open integrated-path remote sensing configuration are usually not equipped for optical path determination. Here we demonstrate a measurement technique capable of simultaneous spectroscopic chemical quantification and range finding. The range-finding functionality is implemented with chirped laser dispersion spectroscopy. The methodology is potentially useful for remote chemical sensing in a hard-target LIDAR configuration and for automatic calibration of gas cells with unknown or varying lengths.

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.

Molecular dispersion spectroscopy in a CO2-filled all-fiber gas cells based on a hollow-core photonic crystal fiber

CO2 sensing using self-fabricated all-fiber gas cells based on a hollow-core photonic crystal fiber and chirped laser dispersion spectroscopy (CLaDS) is presented. I show that CLaDS enables significant reduction of optical fringes arising from the mutlimode nature of the air-core photonic bandgap (PBG) fiber, hence improving the gas sensing system’s stability and performance. By carefully selecting modulation frequency, it is possible to match the period of interference signal (resulting from coupling between fiber’s guided modes) and, therefore, suppress its contribution to the retrieved signal. CLaDS enables successful use of hollow-core PBG fibers in gas sensing applications.

All-fiber mid-infrared difference frequency generation source and its application to molecular dispersion spectroscopy

A widely tunable, fully monolithic, mid-infrared difference frequency generation source and its application in the dispersion-spectroscopy-based laser trace gas detection of methane and ethane, near 2938 and 2998 cm−1, is presented. Utilizing a fiber pigtailed nonlinear crystal module radically simplified the optical setup, while maintaining a superb conversion efficiency of 20% W−1. Seeded directly from two laser diodes, the source delivered ~0.5 mW of tunable radiation, which was used in a chirped laser dispersion spectroscopy setup, enabling the highly sensitive detection of hydrocarbons.

Optical heterodyne-enhanced chirped laser dispersion spectroscopy

A proof-of-concept heterodyne-enhanced chirped laser dispersion spectroscopy system is presented. In remote sensing systems where low return powers are expected, the addition of an optical local oscillator and subsequent nonlinear processing can provide improved performance in chirped laser dispersion spectroscopy. Details about the system configuration, phase noise cancellation, and experimental verification are discussed.

Fundamental Noise Characteristics of Chirped Laser Dispersion Spectroscopy

Analysis of the fundamental noise characteristics of chirped laser dispersion spectroscopy (CLaDS) is reported. The noise performance of CLaDS under shot noise conditions is investigated and the results are compared to that of direct laser absorption spectroscopy (DLAS). For single spectral point detection, the CLaDS detection limit is a factor of 6 higher than that of DLAS. In the case of full spectral fitting with accurate representation of the noise characteristics and the less complex baseline correction in CLaDS, both techniques show comparable fundamental limit of performance in trace-chemical detection.

Phase sensitive chirped laser dispersion spectroscopy under high absorbance conditions

A whole-fiber methane sensor under high absorbance based on phase sensitive chirped laser dispersion spectroscopy is presented in this paper. The laser source of the sensor is a tunable distributed feedback diode laser with a frequency of 1653.7 nm. A telecom-based electro-optical intensity Mach-Zehnder modulator working in carrier suppression mode is adapted to modulate the single frequency laser beam for generating a dual-sideband spectrum beside the carrier wave. Unlike previous proposed phase sensitive chirped laser dispersion spectroscopy scheme, the beatnote signal generated by the two sidebands is detected experimentally. The refractive index fluctuation around the 23 transition of methane is measured by detecting the phase variation of the dual-sideband beatnote signal through using the heterodyne interferometric method. A lock-in amplifier is employed in the phase demodulation process. By connecting the refractive index (the real part of the complex refraction index) and the absorption coefficient (the imaginary part of the complex refraction index) via Kramers-Kroning relation, the gas concentration information is retrieved from the optical dispersion measurement. Absorption-based wavelength modulation spectroscopy measures the gas concentration encoded in the optical intensity based on Beer-Lambert's law. However, the signal sensitivity of wavelength modulation spectroscopy decreases, and the signal even decreases while the gas concentration is raised in high absorbance condition, which leads to an uncertainty in concentration measurement. Experimental results demonstrate that wavelength modulation spectroscopy has better performance in low absorbance condition. The detection limit is about 38.1 ppmm. However, because the sensitivity decreases in high absorbance conditions, the upper detection limit of wavelength modulation spectroscopy is only 1500 ppmm. The dynamic range is defined through dividing the upper detection limit by the detection limit. Therefore, the wavelength modulation spectroscopy obtains a linear measurement dynamic range of 16 dB. Nevertheless, under the same experimental condition, the phase sensitive chirped laser dispersion spectroscopy has a much larger linear measurement range from 47.3 ppmm to 174825 ppmm with a dynamic range higher than 35 dB. Absorption-based gas measurement technique such as wavelength modulation spectroscopy can achieve a low detection limit by using long optical path at the expense of lower upper limit concentration. Phase sensitive chirped laser dispersion spectroscopy appears to be effective in high absorbance condition, which may be caused by high concentration or long optical path. Furthermore, by combining phase sensitive chirped laser dispersion spectroscopy and long optical path technique such as multi pass cell in sensor design, large linear measurement dynamic range and low detection limit can be obtained at the same time.

Chirped laser dispersion spectroscopy with parametric downconversion for open-path gas sensing

Chirped laser dispersion spectroscopy (CLaDS) is a sensitive and robust method for gas detection. In CLaDS, a multitone laser beam is sent through the sample and focused onto the detector where RF beat note is generated. Spectroscopic information is subsequently retrieved through frequency demodulation of this beat note. When CLaDS is used for open-path sensing (i.e., when the pressure-broadened transitions are measured), demodulation at high frequency is required (1 GHz and beyond). We address this issue with a parametric downconversion of the beat note signal. This approach moves CLaDS toward compact, cheaper, and more robust implementations and can be used in both near- and mid-infrared regions. A setup operating at 1650 nm is presented and characterized. Ambient methane detection with an open-path multipass cell is 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.

Remote Sensing of Atmospheric Methane with Simultaneous Ranging Using Chirped Laser Dispersion Spectroscopy

We present a new sensing technology that allows for simultaneous sensing and ranging using chirped laser dispersion spectroscopy (CLaDS). In conjunction with previous works demonstrating the effectiveness of CLaDS for remote sensing, this new configuration yields spectroscopic and ranging information from a single measurement, and is implemented for continuous, multi-path detection of atmospheric methane. 1. INTRODUCTION In remote sensing of atmospheric trace gases, knowledge of the path length is crucial to accurate extraction of the species concentration. However, in most case, this optical path length must be known beforehand or measured separately. Currently, the standard technique for atmospheric ranging and sensing is lidar (light radar), where pulsed light probes the target environment. The timing of the returned backscatter is used to extract the path length, and the received intensity is analyzed to determine the path-integrated concentration. We present an alternative, continuous wave (CW), approach based on the fundamentals of chirped laser dispersion spectroscopy (CLaDS). A proof-of-concept study has shown that both species concentration and path length can be extracted from a single, CW measurement. Development of a system that combines this CW sensing and ranging feature, with a CLaDS system that monitors atmospheric methane will be presented.

Fiber dispersion measurement using chirped laser dispersion spectroscopy technique.

Utilizing the fundamentals of chirped laser dispersion spectroscopy, an alternative method of fiber dispersion measurement is presented. The dispersion of the device under test is probed through the interaction of copropagating electromagnetic waves and subsequent heterodyne detection and frequency demodulation. Measurement of the dispersion parameter, D, is possible with this direct measurement scheme.

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.

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.

Analysis and demonstration of atmospheric methane monitoring by mid-infrared open-path chirped laser dispersion spectroscopy.

Atmospheric methane concentration levels were detected using a custom built laser dispersion spectrometer in a long open-path beam configuration. The instrument is driven by a chirped distributed feedback mid-infrared quantum cascade laser centered at ~1283.46 cm-1 and covers intense rotational-vibrational transitions from the fundamental ν4 band of methane. A full forward model simulating molecular absorption and dispersion profiles, as well as instrumental noise, is demonstrated. The instrument's analytical model is validated and used for quantitative instrumental optimization. The temporal evolution of atmospheric methane mixing ratios is retrieved using a fitting algorithm based on the model. Full error propagation analysis on precision gives a normalized sensitivity of ~3 ppm.m.Hz-0.5 for atmospheric methane.

Open-path sensor for atmospheric methane based on chirped laser dispersion spectroscopy

The design and characterization of a near-IR Chirped Laser Dispersion Spectroscopy (CLaDS) sensor for atmospheric methane detection are reported. The near-IR CLaDS system exhibits the benefits of the prior mid-IR CLaDS systems implemented for open-path sensing while taking advantage of the robust fiber-optic components available in the near-IR. System noise, long-term stability, and comparison with existing technology for methane detection are presented.