Development and application of an airborne differential absorption lidar for the simultaneous measurement of ozone and water vapor profiles in the tropopause region.

A new, combined, lidar system has been developed that is able to simultaneously measure profiles of ozone and water vapor onboard aircraft. The concurrent measurement of these complementary trace species in the upper troposphere and lower stratosphere allows inferring exchange processes in the tropopause region. Whereas an advanced H2O differential absorption lidar at 935 nm has successfully been developed and extensively tested at DLR in the past, we describe here an amendment of this lidar by the addition of an ultraviolet (UV) channel to measure ozone. The transmitter of the ozone differential absorption lidar (DIAL) is based on a near-IR optical parametric oscillator that is frequency-converted into the UV spectral range by intracavity sum frequency mixing. Hereby, a continuous UV tuning range of ∼297-317 nm has been achieved. The average output power in this range is higher than 1 W corresponding to more than 10 mJ per pulse at a repetition rate of 100 Hz. The ozone DIAL system has been carefully characterized both on the ground and in flight. The first simultaneously measured two-dimensional cross-sections of ozone and water vapor in the upper troposphere and lower stratosphere have been recorded during the Wave-driven Isentropic Exchange (WISE) field campaign in 2017 demonstrating the high potential of this system for studying exchange processes in this region of the atmosphere.

Energy calibration of integrated path differential absorption lidars.

The stringent requirements for energy reference measurement represent a challenging task for integrated path differential absorption lidars to measure greenhouse gas columns from satellite or aircraft. The coherence of the lidar transmitter gives rise to speckle effects that have to be considered for accurate monitoring of the energy ratio of outgoing on- and off-line pulses. Detailed investigations have been performed on various measurement concepts potentially suited for deployment within future satellite missions.

Upconversion detector for range-resolved DIAL measurement of atmospheric CH4.

We demonstrate a robust, compact, portable and efficient upconversion detector (UCD) for a differential absorption lidar (DIAL) system designed for range-resolved methane (CH4) atmospheric sensing. The UCD is built on an intracavity pump system that mixes a 1064 nm pump laser with the lidar backscatter signal at 1646 nm in a 25-mm long periodically poled lithium niobate crystal. The upconverted signal at 646 nm is detected by a photomultiplier tube (PMT). The UCD with a noise equivalent power around 127 fW/Hz1/2 outperforms a conventional InGaAs based avalanche photodetector when both are used for DIAL measurements. Using the UCD, CH4 DIAL measurements have been performed yielding differential absorption optical depths with relative errors of less than 11% at ranges between 3 km and 9 km.

CHARM-F-a new airborne integrated-path differential-absorption lidar for carbon dioxide and methane observations: measurement performance and quantification of strong point source emissions.

The integrated-path differential-absorption lidar CHARM-F (CO2 and CH4 Remote Monitoring-Flugzeug) was developed for the simultaneous measurement of the greenhouse gases CO2 and CH4 onboard the German research aircraft HALO (High Altitude and Long Range Research Aircraft). The purpose is to derive the weighted, column-averaged dry-air mixing ratios of the two gases with high precision and accuracy between aircraft and ground or cloud tops. This paper presents the first measurements, performed in the spring of 2015, and shows performance analyses as well as the methodology for the quantification of strong point sources applied on example cases. A measurement precision of below 0.5% for 20 km averages was found. However, individual measurements still show deviations of the absolute mixing ratios compared to corresponding data from in situ profiles. The detailed analysis of the methane point source emission rate yields plausible results (26±3 m3/min or 9.2±1.15 kt CH4 yr-1), which is in good agreement with reported numbers. In terms of CO2, a power plant emission could be identified and analyzed.

Upconversion-based lidar measurements of atmospheric CO2.

For the first time an upconversion based detection scheme is demonstrated for lidar measurements of atmospheric CO2-concentrations, with a hard target at a range of 3 km and atmospheric backscatter from a range of ~450 m. The pulsed signals at 1572 nm are upconverted to 635 nm, and detected by a photomultiplier tube, to test how the upconversion technology performs in a long range detection system. The upconversion approach is compared to an existing direct detection scheme using a near-IR detector with respect to signal-to-noise ratio and quantum efficiency. It is for the first time analyzed how the field-of-view of a receiver system, for long range detection, depends critically on the parameters for the nonlinear up-conversion process, and how to optimize these parameters in future systems.

Investigations on the beam pointing stability of a pulsed optical parametric oscillator.

Although the beam pointing stability of optical parametric oscillators and amplifiers is important for various applications few results on this parameter have been published. Here, we investigate the beam pointing stability of an injection-seeded, nanosecond optical parametric oscillator, compare it to its pump laser, and measure correlations between them. Although correlation between both quantities are found, the beam pointing stability of the OPO is significantly better that the one of its pump. Furthermore, the concept of the Allan variance is applied to analyze the temporal components of the pointing stability.

Influence of molecular scattering models on aerosol optical properties measured by high spectral resolution lidar.

The influence of molecular scattering models on aerosol optical properties measured by high spectral resolution lidar (HSRL) is experimentally investigated and theoretically evaluated. The measurements analyzed in this study were made during three field campaigns by the German Aerospace Center airborne HSRL. The influence of the respective theoretical model on spaceborne HSRL retrievals is also estimated. Generally, the influence on aerosol extinction coefficient can be neglected for both airborne and spaceborne HSRLs. However, the influence on aerosol backscatter coefficient depends on aerosol concentration and is larger than 3% (6%) at ground level for airborne (spaceborne) HSRLs, which is considerable for the spaceborne HSRL, especially when the aerosol concentration is low. A comparison of the HSRL measurements and coordinated ground-based sunphotometer measurements shows that the influence of the model is observable and comparable to the measurement error of the lidar system.

Fast-switching system for injection seeding of a high-power Ti:sapphire laser.

A high frequency switching and tunable seed laser system has been designed and constructed for injection seeding of a high-power pulsed Ti:sapphire laser. The whole laser system operates as the transmitter of a scanning, ground-based, water-vapor differential absorption lidar (DIAL). The output of two seed lasers can be tuned in the wavelength range of 815-840 nm up to the power of 20 mW and switched between the online and offline wavelengths of the DIAL at frequencies of 0-1 kHz. The frequency stability of online and offline seed lasers is better than +/-20 MHz rms and the mode-hop-free tuning range is greater than 40 GHz with external cavity diode lasers. The advantage of this system for efficient injection seeding of the Ti:sapphire cavity is that it is modular, robust, fully fiber-coupled, and polarization maintaining.

Compact, passively Q-switched, all-solid-state master oscillator-power amplifier-optical parametric oscillator (MOPA-OPO) system pumped by a fiber-coupled diode laser generating high-brightness, tunable, ultraviolet radiation.

We report on a compact, tunable ultraviolet laser system that consists of an optical parametric oscillator (OPO) and a longitudinally diode-pumped Nd:YAG master oscillator-power amplifier (MOPA). The pump energy for the whole laser system is supplied via a single delivery fiber. Nanosecond pulses are produced by an oscillator that is passively Q-switched by a Cr(4+):YAG crystal. The OPO is pumped by the second harmonic of the Nd:YAG MOPA. Continuously tunable radiation is generated by an intracavity sum-frequency mixing process within the OPO in the range of 245-260 nm with high beam quality. Maximum pulse energies of 1.2 mJ were achieved, which correspond to an optical efficiency of 3.75%, relating to the pulse energy of the MOPA at 1064 nm.

Airborne high spectral resolution lidar for measuring aerosol extinction and backscatter coefficients.

An airborne high spectral resolution lidar (HSRL) based on an iodine absorption filter and a high-power frequency-doubled Nd:YAG laser has been developed to measure backscatter and extinction coefficients of aerosols and clouds. The instrument was operated aboard the Falcon 20 research aircraft of the German Aerospace Center (DLR) during the Saharan Mineral Dust Experiment in May-June 2006 to measure optical properties of Saharan dust. A detailed description of the lidar system, the analysis of its data products, and measurements of backscatter and extinction coefficients of Saharan dust are presented. The system errors are discussed and airborne HSRL results are compared to ground-based Raman lidar and sunphotometer measurements.