Frequency stabilization of distributed-feedback laser diodes at 1572 nm for lidar measurements of atmospheric carbon dioxide

SBS mitigation by sinusoidal phase modulation of a 1572 nm all-fiber amplifier for lidar CO2 sensing

Lidar C O 2 sensing can be performed by 1572 nm pulsed laser sources. This work presents the development of a fiber amplifier at this wavelength emitting 1 µs FWHM Gaussian pulses at a repetition rate of 7.5 kHz. We obtain the mitigation of stimulated Brillouin scattering (SBS) by shaping the seed laser spectrum into a frequency comb with sinusoidal phase modulation. This frequency comb is compatible with a coherent dual-comb spectroscopy (DCS) method for a targeted lidar C O 2 sensing application. The effect of the harmonics spacing and relative intensity on the SBS threshold is studied. Laser pulses are amplified up to 182 µJ (182 W peak power) from a single-mode erbium (Er) and ytterbium (Yb) co-doped fiber. Those results hold promise for seeding large mode area Er-Yb co-doped fiber power amplifiers.

Double-enhanced multipass cell-based wavelength modulation spectroscopy CH4 sensor for ecological applications

A novel CH₄ sensor based on wavelength modulation spectroscopy with a multipass cell was developed for the soil respiration measurement of CH₄. A home-made double-enhanced Herriot-type multipass cell with an effective absorption length of 73.926 m and a fiber-coupled distributed feedback diode laser emission at 1653.74 nm were used to design the sensor. The double enhancement of the effective optical pathlength of the multipass cell, absorption line locking, laser intensity normalization, and temperature control of the multipass cell were used to improve cell performance and achieve a minimum detection limit of 10 ppbv and a measurement precision of 6.4 ppbv. Finally, the potential of the developed CH₄ sensor for ecological applications was verified by measuring the soil respiration of CH₄ and monitoring of CH₄ in the atmosphere over a long period.

Photon-counting distributed free-space spectroscopy

Spectroscopy is a well-established nonintrusive tool that has played an important role in identifying and quantifying substances, from quantum descriptions to chemical and biomedical diagnostics. Challenges exist in accurate spectrum analysis in free space, which hinders us from understanding the composition of multiple gases and the chemical processes in the atmosphere. A photon-counting distributed free-space spectroscopy is proposed and demonstrated using lidar technique, incorporating a comb-referenced frequency-scanning laser and a superconducting nanowire single-photon detector. It is suitable for remote spectrum analysis with a range resolution over a wide band. As an example, a continuous field experiment is carried out over 72 h to obtain the spectra of carbon dioxide (CO2) and semi-heavy water (HDO, isotopic water vapor) in 6 km, with a range resolution of 60 m and a time resolution of 10 min. Compared to the methods that obtain only column-integrated spectra over kilometer-scale, the range resolution is improved by 2-3 orders of magnitude in this work. The CO2 and HDO concentrations are retrieved from the spectra acquired with uncertainties as low as ±1.2% and ±14.3%, respectively. This method holds much promise for increasing knowledge of atmospheric environment and chemistry researches, especially in terms of the evolution of complex molecular spectra in open areas.

2.05-µm all-fiber laser source designed for CO2 and wind coherent lidar measurement

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.

Multi-frequency differential absorption lidar incorporating a comb-referenced scanning laser for gas spectrum analysis

A multi-frequency differential absorption lidar incorporating a tunable laser and an optical frequency comb is demonstrated for precise spectrum analysis of atmospheric gas. The single frequency tunable laser is stabilized by locking to the optical frequency comb, with a standard deviation of 0.5 MHz. To achieve a high signal-to-noise ratio, a multi-mode superconducting nanowire single-photon detector with an active-area diameter of 50 µm, a quantum efficiency of 31.5%, and dark noise of 100 counts per second is implemented, which enables to avoid the need for high energy lasers. In the experiment, the range-resolved spectrum of atmospheric mixture gases (CO₂ and HDO) in a region of 1572.2 - 1572.45 nm is obtained. Results show different partially overlapped absorption of two gases in different seasons, with a stronger influence of HDO on CO₂ in summer than in winter. The interactions are taken into account by separating the mixture absorption spectrum (one CO₂ line and two HDO lines) with triple-peak Voigt fitting. The retrieved concentrations over 6 km with a range resolution of 120 m and a time resolution of 10 min are compared with in-situ sensors. The uncertainties of the retrieved concentrations are as low as 6.5 µmol/mol (ppm) and 1×10^-3 g/kg for CO₂ and HDO, respectively.

Iodine-stabilized laser at telecom wavelength using dual-pitch periodically poled lithium niobate waveguide

We demonstrate the third harmonic generation of a 1542-nm laser using a dual-pitch periodically poled lithium niobate waveguide with a conversion efficiency of 66%/W². The generated 514-nm light is used for saturation spectroscopy of molecular iodine and laser frequency stabilization. The achieved laser frequency stability is 1.1×10^-12 at an average time of 1 s, which is approximately one order of magnitude better than the acetylene-stabilized laser at 1542 nm. Uncertainty evaluation and absolute frequency measurement are also performed. The developed frequency-stabilized laser can be used as a reliable frequency reference at the telecom wavelength for various applications including optical frequency combs and precision interferometric measurement.

Error analysis for lidar retrievals of atmospheric species from absorption spectra

We generalize a model for retrieving atmospheric constituents from lidar absorption spectra measured at any laser frequency channels. Random and systematic retrieval errors from measurement noise and model bias, respectively, are analyzed parametrically and numerically to provide deeper insight. By placing four or more channels symmetrically around the absorption peak, retrieval errors from a common laser frequency shift and spectral baseline tilt can be eliminated. By solving for the frequency shift and spectral baseline tilt, atmospheric retrievals degrade only slightly even when such channels are shifted substantially out of symmetry. An etalon can thus be used for the wavelength stabilization.

Continuous-wave operation of a semipolar InGaN distributed-feedback blue laser diode with a first-order indium tin oxide surface grating

A novel approach to realize DFB gratings on GaN based laser diodes is presented and continuous-wave single longitudinal mode operation is achieved. The first order gratings were fabricated on the surface of indium tin oxide (ITO) on top of the laser ridge, which combines the benefits of simplified fabrication, easy scalability to wider ridges, and no regrowth or overgrowth. Under continuous-wave operation, the laser emits with a full FWHM of 5 pm, a SMSR of 29 dB and output power from a single facet as high as 80 mW. To the best of authors' knowledge, this is also the first demonstration of a DFB-LD on semipolar InGaN/GaN system.

Important evidence of constant low CO2 windows and impacts on the non-closure of the greenhouse effect

The CO₂ distribution in the atmosphere remains unclear for the complexity of the long-range vertical transport process and other influencing factors. In this work, regression analysis was used to verify the accuracy of CO₂ concentrations datasets. Geostatistical analyses were used to investigate the spatiotemporal distributions of CO₂ at 7 levels from near the surface to the mid-troposphere (0~5 km). Spatial correlation and time series analyses were used to further determine the diffusion characteristics of the CO₂ concentration based on the horizontal wind (NCEP R2), which is one of the main driving factors. The results showed that the horizontal, not vertical, diffusion of CO₂ becomes increasingly more prominent with the decrease in atmospheric pressure to the mid-troposphere, whereas many regions, such as the Rocky Mountains and Qinghai-Tibet Plateau, have constant low values throughout the year due to the influence of high topography (up to 10.756 ppmv lower than that near the surface). These areas form low CO₂ concentration 'windows' keeping letting thermal infrared energy out into space. This study is the first to question the existing view of the closure of the 'greenhouse effect'. Future research studies should more precisely determine the closure threshold and the uncertainties about the surface fluxes.

Analysis of energy monitoring for a double-pulsed CO2 integrated path differential absorption lidar at 1.57 μm

For double-pulsed 1.57 μm integrated path differential absorption lidar, the transmitted pulse energy measurement is an important factor that can influence the uncertainty of CO₂ concentration measurement. An energy monitoring experiment was performed to determine how to improve the measurement precision of the transmitted pulse energy. Ground glass diffusers were used to reduce the speckle effect during energy monitoring. The roughness and rotational speed of the ground glass diffusers were considered and compared. The normalized energy ratios between on-line and off-line echo pulses and on-line and off-line energy monitoring pulses were analyzed, and the Allan deviation was used to evaluate the energy monitoring results. Averaging 148 shots, the standard deviation of the normalized energy ratio reached 0.0757%, whereas the correlation between the energy ratio of the on-line and off-line energy monitoring pulses and the energy ratio of the on-line and off-line echo pulses was higher than 90%.

Multi-frequency differential absorption LIDAR system for remote sensing of CO2 and H2O near 1.6 µm

The specifications and performance of a ground-based differential absorption LIDAR (light detection and ranging) system (DIAL) using an optical parametric oscillator (OPO) are presented. The OPO is injection-seeded with the output of a confocal filter cavity at frequencies generated by an electro-optic phase modulator (EOM) from a fixed-frequency external cavity diode laser (ECDL). The number of seed frequencies, frequency spacings, and duration is controlled with an arbitrary waveform generator (AWG) driving the EOM. Range resolved data are acquired using both photon current and photon counts from a hybrid detection system. The DIAL measurements are performed using a repeating sequence of 10 frequencies spanning a range of 37.5 GHz near 1602.2 nm to sequentially sample CO₂ and H₂O at 10 Hz. Dry air mixing ratios of CO₂ and H₂O with a resolution of 250 m and an averaging time of 10 min resulted in uncertainties as low as 6 µmol/mol (ppm) and 0.44 g/kg, respectively. Simultaneous measurements using an integrated path differential absorption (IPDA) LIDAR system and in situ point sensor calibrated to WMO (World Meteorological Organization) gas standards are conducted over two 10 hr nighttime periods to support traceability of the DIAL results. The column averaged DIAL mixing ratios agree with the IPDA LIDAR results to within the measured uncertainties for much of two measurement periods. Some of the discrepancies with the in situ point sensor results are revealed through trends observed in the gradients of the range resolved DIAL data.

Characterization of Frequency-Doubled 1.5- m Lasers for High-Performance Rb Clocks

We report on the characterization of two fiber-coupled 1.5- diode lasers, frequency-doubled and stabilized to Rubidium (Rb) atomic resonances at 780 nm. Such laser systems are of interest in view of their implementation in Rb vapor-cell atomic clocks, as an alternative to lasers emitting directly at 780 nm. The spectral properties and the instabilities of the frequency-doubled lasers are evaluated against a state-of-the-art compact Rb-stabilized laser system based on a distributed-feedback laser diode emitting at 780 nm. All three lasers are frequency stabilized using essentially identical Doppler-free spectroscopy schemes. The long-term optical power fluctuations at 780 nm are measured, simultaneously with the frequency instability measurements done by three beat notes established between the three lasers. One of the frequency-doubled laser systems shows at 780 nm excellent spectral properties. Its relative intensity noise <10^-12 Hz^-1 is one order of magnitude lower than the reference 780-nm laser, and the frequency noise <10⁶ Hz²/Hz is limited by the laser current source. Its optical frequency instability is at s, limited by the reference laser, and better than at all timescales up to one day. We also evaluate the impact of the laser spectral properties and instabilities on the Rb atomic clock performance, in particular taking into account the light-shift effect. Optical power instabilities on long-term timescales, largely originating from the frequency-doubling stage, are identified as a limitation in view of high-performance Rb atomic clocks.

Analysis of a random modulation single photon counting differential absorption lidar system for space-borne atmospheric CO2 sensing

The ability to observe the Earth's carbon cycles from space provides scientists an important tool to analyze climate change. Current proposed systems are mainly based on pulsed integrated path differential absorption lidar, in which two high energy pulses at different wavelengths interrogate the atmosphere sequentially for its transmission properties and are back-scattered by the ground. In this work an alternative approach based on random modulation single photon counting is proposed and analyzed; this system can take advantage of a less power demanding semiconductor laser in intensity modulated continuous wave operation, benefiting from a better efficiency, reliability and radiation hardness. Our approach is validated via numerical simulations considering current technological readiness, demonstrating its potential to obtain a 1.5 ppm retrieval precision for 50 km averaging with 2.5 W average power in a space-borne scenario. A major limiting factor is the ambient shot noise, if ultra-narrow band filtering technology could be applied, 0.5 ppm retrieval precision would be attainable.

Towards quantitative atmospheric water vapor profiling with differential absorption lidar

Differential Absorption Lidar (DIAL) is a powerful laser-based technique for trace gas profiling of the atmosphere. However, this technique is still under active development requiring precise and accurate wavelength stabilization, as well as accurate spectroscopic parameters of the specific resonance line and the effective absorption cross-section of the system. In this paper we describe a novel master laser system that extends our previous work for robust stabilization to virtually any number of multiple side-line laser wavelengths for the future probing to greater altitudes. In this paper, we also highlight the significance of laser spectral purity on DIAL accuracy, and illustrate a simple re-arrangement of a system for measuring effective absorption cross-section. We present a calibration technique where the laser light is guided to an absorption cell with 33 m path length, and a quantitative number density measurement is then used to obtain the effective absorption cross-section. The same absorption cell is then used for on-line laser stabilization, while microwave beat-frequencies are used to stabilize any number of off-line lasers. We present preliminary results using ∼300 nJ, 1 μs pulses at 3 kHz, with the seed laser operating as a nanojoule transmitter at 822.922 nm, and a receiver consisting of a photomultiplier tube (PMT) coupled to a 356 mm mirror.

Compact rubidium-stabilized multi-frequency reference source in the 1.55-μm region

Combining light modulation and frequency conversion techniques, a compact and simple frequency-stabilized optical frequency comb spanning over 45 nm in the 1.56-μm wavelength region is demonstrated. It benefits from the high-frequency stability achievable from rubidium atomic transitions at 780 nm probed in a saturation absorption scheme, which is transferred to the 1.56-μm spectral region via a second-harmonic generation process. The optical frequency comb is generated by an electro-optic modulator enclosed in a Fabry-Perot cavity that is injected by the fundamental frequency stabilized laser. Frequency stability better than 2 kHz has been demonstrated on time scales between 1000 s and 2 days both at 1560 nm, twice the rubidium wavelength, and for a comb line at 1557 nm.

Method for wavelength stabilization of pulsed difference frequency laser at 1572 nm for CO(2) detection lidar

High-accuracy on-line wavelength stabilization is required for differential absorption lidar (DIAL), which is ideal for precisely measuring atmospheric CO(2) concentration. Using a difference-frequency laser, we developed a ground-based 1.57-μm pulsed DIAL for performing atmospheric CO(2) measurements. Owing to the system complexity, lacking phase, and intensity instability, the stabilization method was divided into two parts-wavelength calibration and locking-based on saturated absorption. After obtaining the on-line laser position, accuracy verification using statistical theory and locking stabilization using a one-dimensional template matching method, namely least-squares matching (LSM), were adopted to achieve wavelength locking. The resulting system is capable of generating a stable wavelength.

Impact of broadened laser line-shape on retrievals of atmospheric species from lidar sounding absorption spectra

We examine the impact of broadened laser line-shape on retrievals of atmospheric species from lidar-sounding absorption spectra. The laser is assumed to be deterministically modulated into a stable, nearly top-hat frequency comb to suppress the stimulated Brillouin scattering, allowing over 10-fold pulse energy increase without adding measurement noise. Our model remains accurate by incorporating the laser line-shape factor into the effective optical depth. Retrieval errors arising from measurement noise and model bias are analyzed parametrically and numerically to provide deeper insight. The stable laser line-shape broadening minimally degrades the column-averaged retrieval, but can significantly degrade the multiple-layer retrievals.

Error reduction in retrievals of atmospheric species from symmetrically measured lidar sounding absorption spectra

We report new methods for retrieving atmospheric constituents from symmetrically-measured lidar-sounding absorption spectra. The forward model accounts for laser line-center frequency noise and broadened line-shape, and is essentially linearized by linking estimated optical-depths to the mixing ratios. Errors from the spectral distortion and laser frequency drift are substantially reduced by averaging optical-depths at each pair of symmetric wavelength channels. Retrieval errors from measurement noise and model bias are analyzed parametrically and numerically for multiple atmospheric layers, to provide deeper insight. Errors from surface height and reflectance variations are reduced to tolerable levels by "averaging before log" with pulse-by-pulse ranging knowledge incorporated.

Laser frequency offset locking via tripod-type electromagnetically induced transparency

We have demonstrated laser frequency offset locking via the Rb87 tripod-type double-dark resonances electromagnetically induced transparency (EIT) system. The influence of coupling fields' power and detuning on the tripod-type EIT profile is studied in detail. In a wide coupling field's detuning range, the narrower EIT dip has an ultranarrow linewidth of ∼590  kHz, which is about one order narrower than the natural linewidth of Rb87. Without the additional frequency stabilization of the coupling lasers, we achieve the relative frequency fluctuation of 60 kHz in a long time of ∼2000  s, which is narrower than the short-time linewidth of each individual laser.

Performance evaluation of a 1.6-µm methane DIAL system from ground, aircraft and UAV platforms

Methane is an efficient absorber of infrared radiation and a potent greenhouse gas with a warming potential 72 times greater than carbon dioxide on a per molecule basis. Development of methane active remote sensing capability using the differential absorption lidar (DIAL) technique enables scientific assessments of the gas emission and impacts on the climate. A performance evaluation of a pulsed DIAL system for monitoring atmospheric methane is presented. This system leverages a robust injection-seeded pulsed Nd:YAG pumped Optical Parametric Oscillator (OPO) laser technology operating in the 1.645 µm spectral band. The system also leverages an efficient low noise, commercially available, InGaAs avalanche photo-detector (APD). Lidar signals and error budget are analyzed for system operation on ground in the range-resolved DIAL mode and from airborne platforms in the integrated path DIAL (IPDA) mode. Results indicate system capability of measuring methane concentration profiles with <1.0% total error up to 4.5 km range with 5 minute averaging from ground. For airborne IPDA, the total error in the column dry mixing ratio is less than 0.3% with 0.1 sec average using ground returns. This system has a unique capability of combining signals from the atmospheric scattering from layers above the surface with ground return signals, which provides methane column measurement between the atmospheric scattering layer and the ground directly. In such case 0.5% and 1.2% total errors are achieved with 10 sec average from airborne platforms at 8 km and 15.24 km altitudes, respectively. Due to the pulsed nature of the transmitter, the system is relatively insensitive to aerosol and cloud interferences. Such DIAL system would be ideal for investigating high latitude methane releases over polar ice sheets, permafrost regions, wetlands, and over ocean during day and night. This system would have commercial potential for fossil fuel leaks detection and industrial monitoring applications.

Micropulse differential absorption lidar for identification of carbon sequestration site leakage

A scanning differential absorption lidar (DIAL) instrument for identification of carbon dioxide leaks at carbon sequestration sites has been developed and initial data has been collected at Montana State University. The laser transmitter uses two tunable discrete mode laser diodes operating in the continuous-wave mode with one locked to the online absorption wavelength and the other operating at the offline wavelength. Two in-line fiber optic switches are used to switch between online and offline operation. After the fiber optic switch, an acousto-optic modulator is used to generate a pulse train used to injection seed an erbium-doped fiber amplifier to produce eye-safe laser pulses with maximum pulse energies of 66 μJ, a pulse repetition frequency of 15 kHz, and an operating wavelength of 1.571 μm. The DIAL receiver uses a 28 cm diameter Schmidt-Cassegrain telescope to collect that backscattered light, which is then monitored using a photomultiplier tube module operating in the photon counting mode. The DIAL has measured carbon dioxide profiles from 1 to 2.5 km with 60 min temporal averaging. Comparisons of DIAL measurements with a Licor LI-820 gas analyzer point sensor have been made.

Error reduction methods for integrated-path differential-absorption lidar measurements

We report new modeling and error reduction methods for differential-absorption optical-depth (DAOD) measurements of atmospheric constituents using direct-detection integrated-path differential-absorption lidars. Errors from laser frequency noise are quantified in terms of the line center fluctuation and spectral line shape of the laser pulses, revealing relationships verified experimentally. A significant DAOD bias is removed by introducing a correction factor. Errors from surface height and reflectance variations can be reduced to tolerable levels by incorporating altimetry knowledge and "log after averaging", or by pointing the laser and receiver to a fixed surface spot during each wavelength cycle to shorten the time of "averaging before log".

Precision and fast wavelength tuning of a dynamically phase-locked widely-tunable laser

We report a precision and fast wavelength tuning technique demonstrated for a digital-supermode distributed Bragg reflector laser. The laser was dynamically offset-locked to a frequency-stabilized master laser using an optical phase-locked loop, enabling precision fast tuning to and from any frequencies within a ~40-GHz tuning range. The offset frequency noise was suppressed to the statically offset-locked level in less than ~40 μs upon each frequency switch, allowing the laser to retain the absolute frequency stability of the master laser. This technique satisfies stringent requirements for gas sensing lidars and enables other applications that require such well-controlled precision fast tuning.