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This publisher's note contains a correction to Opt. Lett.49, 969 (2024)10.1364/OL.510598.
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We present the development of a multi-spectral, integrated-path differential absorption (IPDA) lidar based on a dual-comb spectrometer for greenhouse gas monitoring. The system uses the lidar returns from topographic targets and does not require retroreflectors. The two frequency combs are generated by electro-optic modulation of a single continuous-wave laser diode. One of the combs is pulsed, amplified, and transmitted into the atmosphere, while the other acts as a local oscillator for coherent detection. We discuss the physical principles of the measurement, outline a performance model including speckle effects, and detail the fiber-based lidar architecture and signal processing. A maximum likelihood algorithm is used to estimate simultaneously the gas concentration and the central frequency of the comb, allowing the system to work without frequency locking. H2O (at 1544 nm) and CO2 (at 1572 nm) concentrations are monitored with a precision of 3% and 5%, respectively, using a non-cooperative target at 700 m. In addition, the measured water vapor concentrations are in excellent agreement with in-situ measurements obtained from nearby weather stations. To our knowledge, this is the first complete experimental demonstration and performance assessment of greenhouse gas monitoring with a dual-comb spectrometer using lidar echoes from topographic targets.
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We present a hybrid fiber/bulk laser source designed for CO2 and wind monitoring using differential absorption LIDAR (DIAL) and coherent detection at 2.05â µm. This source features a master oscillator power amplifier (MOPA) architecture made of four fiber stages and one single-pass, end-pumped, bulk amplifier. This Letter focuses on the single-pass bulk amplifier performance and on the hybrid architecture benefits for DIAL and coherent detection. The bulk material is a holmium-doped YLF crystal that provides high efficiency amplification at 2.05â µm. This laser offers an energy breakthrough as compared to the classical stimulated Brillouin scattering (SBS) limit encountered in a fiber laser without compromising robustness, thanks to very few free-space optical elements and a small optical path. It delivers pulse energy and repetition frequency of 9.0 or 1.2â mJ/20â kHz with 200â ns quasi Fourier-transform limited pulses.
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We present a theoretical, numerical and experimental assessment of the impact of speckle on a dual electro-optic frequency comb (EOFC) based system for integrated path differential absorption (IPDA) measurements. The principle of gas concentration measurements in a dual EOFC configuration in the absence of speckle is first briefly reviewed and experimentally illustrated using a C2H2 gas cell. A numerical simulation of the system performance in the presence of speckle is then outlined. The speckle-related error in the concentration estimate is found to be an increasing function of the product between the roughness of the backscattering surface and the EOFC line-spacing. As this product increases, the speckle-induced power fluctuations in the comb lines are no longer correlated to each other. To confirm this, concentration measurements are conducted using backscattered light from two different surfaces. Experiment results are in very good agreement with numerical simulations. Though detrimental for IPDA measurements, it is finally shown that decorrelation of speckle noise can be advantageously exploited for surface characterization in a dual EOFC configuration.
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This work reports on an all-fiber pulsed laser source for simultaneous remote sensing of CO2 concentration and wind velocity in the 2.05 µm region. The source is based on a polarization-maintaining master oscillator power amplifier (MOPA) architecture. Two narrow-linewidth master oscillators for ON-line/OFF-line CO2 differential absorption lidar operation alternately seed a four-stage amplifier chain at a fast switching rate up to 20 kHz. The MOPA architecture delivers laser pulses of 120 µJ energy, 200 ns duration (600 W peak power) at 20 kHz pulse repetition rate (2.4 W average power). The output linewidth is lower than 5 MHz, close to the pulse Fourier transform limit, and the beam quality factor is M2=1.12. The source also provides a pre-amplified 20 mW local oscillator with a relative intensity noise of -160dB/Hz that ensures optimal performance for future coherent detection.
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We have designed and built a wavelength-tunable optical source for standoff detection of gaseous chemicals by differential absorption spectrometry in the long-wave infrared. It is based on a nanosecond 2 µm single-frequency optical parametric oscillator, whose idler wave is amplified in large aperture Rb:PPKTP crystals. The signal and idler waves are mixed in ZnGeP2 crystals to produce single-frequency tunable radiation in the 7.5-10.5 µm range. The source was integrated into a direct detection lidar to measure sarin and sulfur mustard inside a closed chamber, in an integrated path configuration with a noncooperative target.
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We discuss and evaluate the expected performance of a tunable multi-wavelength integrated-path differential absorption lidar operating in the long-wave infrared between 7.5 and 11 µm, for standoff measurement of chemical agents. Interference issues with natural gas compounds throughout the entire 7.5-11 µm band are first discussed. Then, the study focuses on four interest species, three warfare agents, and a simulant. A performance model is derived and exploited to assess the expectable measurement precision of the lidar for these four species in the integrated-path mode within a 2 min alert time and seventeen emitted wavelengths. Measurement precisions better than the targeted sensitivity levels look reachable at the kilometer range with laser power below 100 mW. Performance optimization strategies are discussed, either by adjusting the pulse energy/pulse repetition rate for a given laser power and lidar range or by reducing the wavelength sequence in an optimal way. Finally the system's receiving operating characteristic curves are derived to describe the expected detection performance in terms of probability of false alarm rate and probability of detection.
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We report on the performances of a coherent DIAL/Doppler fiber lidar called VEGA, allowing for simultaneous measurements of methane and wind atmospheric profiles. It features a 10µJ, 200 ns, 20 kHz fiber pulsed laser emitter at 1645 nm, and it has been designed to monitor industrial methane leaks and fugitive emissions in the environment. The system performance has been assessed for range-resolved (RR) and integrated-path (IP) methane measurements in natural background conditions (i.e. ambient methane level). For RR measurements, the measured Allan deviation at τ=10 s is in the range of 3-20 ppm, depending of the aerosol load, at a distance of 150 m, with 30 m range resolution, and a beam focused around 150-200 m. For IP measurements, using a natural target at 2.2 km of distance, the Allan deviation at τ=10 s is in the range of 100-200 ppb. In both cases, deviation curves decrease as τ-1/2, up to 1000 seconds for the longest averaging time. Finally, the lidar ability to monitor an industrial methane leak is demonstrated during a field test.
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A multi-channel Raman lidar has been developed, allowing for the first time simultaneous and high-resolution profiling of hydrogen gas and water vapor. The lidar measures vibrational Raman scattering in the UV (355 nm) domain. It works in a high-bandwidth photon counting regime using fast SiPM detectors and takes into account the spectral overlap between hydrogen and water vapor Raman spectra. Measurement of concentration profiles of H2 and H2O are demonstrated along a 5-meter-long open gas cell with 1-meter resolution at 85 meters. The instrument precision is investigated by numerical simulation to anticipate the potential performance at longer range. This lidar could find applications in the French project Cigéo for monitoring radioactive waste disposal cells.
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Atmospheric gravity waves and turbulence generate small-scale fluctuations of wind, pressure, density, and temperature in the atmosphere. These fluctuations represent a real hazard for commercial aircraft and are known by the generic name of clear-air turbulence (CAT). Numerical weather prediction models do not resolve CAT and therefore provide only a probability of occurrence. A ground-based Rayleigh lidar was designed and implemented to remotely detect and characterize the atmospheric variability induced by turbulence in vertical scales between 40 m and a few hundred meters. Field measurements were performed at Observatoire de Haute-Provence (OHP, France) on 8 December 2008 and 23 June 2009. The estimate of the mean squared amplitude of bidimensional fluctuations of lidar signal showed excess compared to the estimated contribution of the instrumental noise. This excess can be attributed to atmospheric turbulence with a 95% confidence level. During the first night, data from collocated stratosphere-troposphere (ST) radar were available. Altitudes of the turbulent layers detected by the lidar were roughly consistent with those of layers with enhanced radar echo. The derived values of turbulence parameters Cn2 or CT2 were in the range of those published in the literature using ST radar data. However, the detection was at the limit of the instrumental noise and additional measurement campaigns are highly desirable to confirm these initial results. This is to our knowledge the first successful attempt to detect CAT in the free troposphere using an incoherent Rayleigh lidar system. The built lidar device may serve as a test bed for the definition of embarked CAT detection lidar systems aboard airliners.
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In this paper, we propose signal-processing tools adapted to supercontinuum absorption spectroscopy, in order to predict the precision of gas species concentration estimation. These tools are based on Cramer-Rao bounds computations. A baseline-insensitive concentration estimation algorithm is proposed. These calculations are validated by statistical tests on simulated supercontinuum signals as well as experimental data using a near-infrared supercontinuum laser and a grating spectrometer.
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We address an original statistical method for unsupervised identification and concentration estimation of spectrally interfering gas components of unknown nature and number. We show that such spectral unmixing can be efficiently achieved using information criteria derived from the Minimum Description Length (MDL) principle, outperforming standard information criteria such as AICc or BIC. In the context of spectroscopic applications, we also show that the most efficient MDL technique implemented shows good robustness to experimental artifacts.
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A new concept of spectrum analyzer is proposed for short-range lidar measurements in airborne applications. It implements a combination of two fringe-imaging Michelson interferometers to analyze the Rayleigh-Mie spectrum backscattered by molecules and particles at 355 nm. The objective is to perform simultaneous measurements of four variables: the air speed, the air temperature and density, and the particle scattering ratio. The Cramer-Rao bounds are calculated to evaluate the best expectable measurement accuracies. The performance optimization shows that a Michelson interferometer with a path difference of 3 cm is optimal for air speed measurements in clear air. To optimize density, temperature, and scattering ratio measurements, the second interferometer should be set to a path difference of 10 cm at least; 20 cm would be better to be less sensitive to the actual Rayleigh-Brillouin line shape.