This PDF file contains the front matter associated with SPIE Proceedings Volume 8872, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

The design of space-flight hardware is typically required to be at a Technology Readiness Level (TRL) of 6 before the build of the actual flight hardware can begin. At the early design phase the "relevant environment" for TRL-6 is frequently not well defined. For the ICESat-2 laser relevant environment was defined as the qualification levels in GEVS (General Environmental Verification Standard, GSFC-STD-7000). Our approach to dealing with the high-frequency content of the 14.1 grms random vibration levels in GEVS was a flexure mounted canister design that filtered the highfrequency content. In our talk we will discuss the program and system level implications of this design approach.

A laser diode module (LDM) space certification and qualification program was developed for NASA’s Ice, Cloud and Land Elevation Satellite-2, ICESat-2 mission. The ICESat-2 laser transmitter is a high performance diode-pumped solid state laser that requires high reliability, high efficiency and high brightness fiber coupled LDMs, capable of supporting a 27,000 hour mission life. The test centric LDM space certification and qualification programs consisted of several key phases including a technology plausibility study, laser diode and LDM pedigree reviews, environmental acceptance and qualification testing, and extensive life testing. The intent of the plausibility study was to analytically and experimentally demonstrate that a commercial-off-the-shelf (COTS) LDM design was capable of being space-certified. A pedigree review of the laser diode population was conducted to reject out-of-family laser diodes from the population. The laser diode pedigree review was a statistical analysis of several laser diode performance metrics (efficiency, operating current, etc.) with outliers being rejected. All LDMs underwent environmental acceptance testing including vibration, thermal cycling and an extended burn-in. The primary purpose of the acceptance testing was to highlight internal workmanship issues. The pedigree review of the acceptance tested LDMs was conducted to reject out-of-family LDMs in statistical analysis of several performance metrics (operating current, coupling efficiency, etc.). A sub-set of the flight-certified LDMs will be exposed to environmental qualification testing and will subsequently be tested to failure to determine the LDM capability. Multiple LDMs are being life tested under flight-like conditions and show no signs of degradation with run times of 22,000 hours, which is over 80% of the mission life. Details of the LDMs space certification and qualification programs are presented.

Compact, reliable and conductively-cooled solid state 2-micron laser technology is a critical component of the 3-D Winds mission envisioned in the NRC Decadal survey. In order to mature the 2-micron laser technology to a Technical Readiness Level of 5 (TRL-5), we are developing a conductively-cooled single-frequency 2-micron laser meeting the performance requirements for this wind LIDAR mission and able to operate in vacuum. Conductive cooling is accomplished via heat pipes attached to a reconfigurable condenser plate. The ruggedized mechanical design is based upon design concepts used and validated for the NASA Ice, Cloud and Land Elevation Satellite-2 (ICESat-2) Mission. Achieving TRL-5 has particular challenges for this technology due to its unique requirements. The cold operating temperature mandated by the laser crystal (Ho:Tm:LuLF) and the long resonator required to generate <100 ns pulsewidths needed to maximize the LIDAR resolution, make stabilizing the laser more difficult than many other lasers. The completion and demonstration of this laser provides a platform for further maturation of solid state 2 micron laser technology to the TRL-6 required for space-based deployment.

National Institute of Information and Communications Technology (NICT) has made efforts in order to develop a 2-μm coherent lidar for measuring CO₂ concentration and line-of-sight (LOS) wind speed. Experimental horizontal CO₂ measurements were made to examine the detection sensitivity of the 2-μm coherent lidar in April and May, 2008, and October, 2009. Experimental vertical CO₂ measurements were made for the Greenhouse gas Observing SATellite (GOSAT) data products validation in February 2010 and in January and February 2011. Bias and random error in the LOS wind speed measurements were also investigated in order to evaluate of the 2-μm coherent lidar for wind measurements in 2010. In the paper, we present an overview of our 2-μm coherent lidar developed at the NICT and also of the experimental results.

The recommended design approach for the 3D Tropospheric Winds mission is a hybrid Doppler lidar which combines the best elements of both a coherent aerosol Doppler lidar operating at 2 μm and a direct detection molecular Doppler lidar operating at 0.355 μm. In support of the mission, we built a novel, compact, light-weighted multi-field of view transceiver where multiple telescopes are used to cover the required four fields of view. A small mechanism sequentially selects both the “transmit” and “receive” fields of view. The four fields are combined to stimulate both the 0.355 μm receiver and the 2 μm receiver. This version is scaled (0.2 m diameter aperture) from the space-based version but still demonstrates the feasibility of the hybrid approach. The primary mirrors were conventionally light-weighted and coated with dielectric, high reflectivity coatings with high laser damage thresholds at both 2 μm and 0.355 μm. The mechanical structure and mounts were fabricated from composites to achieve dimensional stability while significantly reducing the mass. In the laboratory, we demonstrated the system level functionality at 0.355 μm and at 2 μm, raising the Technology Readiness Level (TRL) from 2 to 4.

We report on the implementation and testing of a new wavelet-based motion estimation algorithm to estimate horizontal vector wind fields in real-time from horizontally-scanning elastic backscatter lidar data, and new experimental results from field work conducted in Chico, California, during the summer of 2013. We also highlight some limitations of a traditional cross-correlation method and compare the results of the wavelet-based method with those from the cross-correlation method and wind measurements from a Doppler lidar.

Atmospheric carbon dioxide (CO₂) is an important greenhouse gas that significantly contributes to the carbon cycle and global radiation budget on Earth. Active remote sensing of CO₂ is important to address several limitations that contend with passive sensors. A 2-micron double-pulsed, Integrated Path Differential Absorption (IPDA) lidar instrument for ground and airborne atmospheric CO₂ concentration measurements via direct detection method is being developed at NASA Langley Research Center. This active remote sensing instrument will provide an alternate approach of measuring atmospheric CO₂ concentrations with significant advantages. A high energy pulsed approach provides high-precision measurement capability by having high signal-to-noise ratio level and unambiguously eliminates the contamination from aerosols and clouds that can bias the IPDA measurement. Commercial, on the shelf, components are implemented for the detection system. Instrument integration will be presented in this paper as well as a background for CO₂ measurement at NASA Langley research Center.

Methane is a potent greenhouse gas and on a per molecule basis has a warming influence 72 times that of carbon dioxide over a 20 year horizon. Therefore, it is important to look at near term radiative effects due to methane to develop mitigation strategies to counteract global warming trends via ground and airborne based measurements systems. These systems require the development of a time-resolved DIAL capability using a narrow-line laser source allowing observation of atmospheric methane on local, regional and global scales. In this work, a demonstrated and efficient nonlinear conversion scheme meeting the performance requirements of a deployable methane DIAL system is presented. By combining a single frequency 1064 nm pump source and a seeded KTP OPO more than 5 mJ of 1.6 μm pulse energy is generated with conversion efficiencies in excess of 20%. Even without active cavity control instrument limited linewidths (50 pm) were achieved with an estimated spectral purity of ~95%. Tunable operation over 400 pm (limited by the tuning range of the seed laser) was also demonstrated. This source demonstrated the critical needs for a methane DIAL system motivating additional development of the technology.

Improvements to the original NCAR/NSF Raman-shifted Eye-safe Aerosol Lidar (REAL) made between 2008 and 2013 are described. They are aimed mainly at optimizing and stabilizing the performance of the system for long-term, unattended, network-controlled, remote monitoring of the horizontal vector wind field and boundary layer height, and observing atmospheric boundary layer phenomena such as fine-scale waves and density current fronts. In addition, we have improved the polarization purity of the transmitted laser radiation and studied in the laboratory the effect of the beam-steering unit mirrors on the transmitted polarization as part of a longer-term effort to make absolute polarization measurements of aerosols and clouds.

I survey the theoretical foundations of the slowly-but-surely emerging field of multiple scattering lidar, which has already found applications in atmospheric and cryospheric optics that I also discuss. In multiple scattering lidar, returned pulses are stretched far beyond recognition, and there is no longer a one-to-one connection between range and return-trip timing. Moreover, one can exploit the radial profile of the diffuse radiance field excited by the laser source that, by its very nature, is highly concentrated in space and collimated in direction. One needs, however, a new class of lidar equations to explore this new phenomenology. A very useful set is derived from radiative diffusion theory, which is found at the opposite asymptotic limit of radiative transfer theory than the conventional (single-scattering) limit used to derive the standard lidar equation. In particular, one can use it to show that, even if the simple time-of-flight-to-range connection is irretrievably lost, multiply-scattered lidar light can be used to restore a unique profiling capability with coarser resolution but much deeper penetration into a wide variety of optical thick media in nature. Several new applications are proposed, including a laser bathymetry technique that should work for highly turbid coastal waters.

The distortions of the inverted lidar signals can be caused by (i) the constant offset that remains in the backscatter signal after removing the background component, (ii) the multiplicative distortion component, which level is related with the lidar signal intensity, and (iii) the signal noise in the wide wavelength spectra; the latter includes lowfrequency components, which do not obey common random-noise statistics. These distortions, even minor, may yield significant distortions in the retrieved outputs obtained by the inversion of the lidar signal. Implicit and explicit premises and assumptions required for any solution of the lidar equation are additional sources of the uncertainty in the inversion results. There is no reliable way for checking whether used assumptions are valid, therefore, the lidar signal inversion can yield significantly biased results. As a result, instead some statistically mean profile of the atmospheric parameter of interest with the corresponding probability function, one obtains some qualitative estimate of the profile with unknown uncertainty, which depends on the validity of used assumptions. It means that lidar profiling is not a measurement but a result of some simulation based on past observations.

DRDC Valcartier has developed a unique underwater lidar for the measurement of different sea water and ice properties. The lidar head is designed for underwater operation and consists of four telescopes that are connected to the detection and emission unit via five 42 m fused silica optical fibers. Three telescopes are used for data collection, while the fourth is used for laser emission. The laser source and the detection unit are located on a surface vessel. The laser beam is injected into a 100 μm diameter optical fiber. The collimation of the laser beam is done in the lidar head via a lens with 25 mm diameter and 45 mm focal length; the laser beam is linearly polarized using a polarization beamsplitter. A 50 mm receiving telescope co-aligned with the laser beam is used for linear depolarization measurements. A second 50 mm telescope is used to collect off-axis scattered light while a third 50 mm telescope is used to collect inelastic scattered radiation (Raman and induced fluorescence signal). The laser source and detection units are mounted on a small optical table for easy access/modification. Various laser sources and lidar detection techniques (Q-switched pulses or frequency modulated) could be easily implemented. The lidar head can be deployed underwater or mounted on an airborne platform. In this work, the lidar system will be described in detail and preliminary results obtained with a Q-Switch, 532 nm, 1 ns pulse laser source will be presented and compared with the anticipated performance for different water bodies.

Recent advances in the INO broadband SWIR/MWIR spectroscopic lidar will be presented. The system is designed for the detection of gaseous pollutants via active infrared differential optical absorption spectroscopy (DOAS). Two distinctive features are a sub-nanosecond PPMgO:LN OPG capable of generating broadband (10 to <100 nm FWHM) and tunable (1.5 to 3.8 μm) SWIR/MWIR light, and an in-house gated MCT-APD focal plane array used in the output plane of a grating spectrograph. The operation consists in closely gating the returns from back-scattering off topographic features, and is thus, for now, a path integrated measurement. All wavelengths are emitted and received simultaneously, for low concentration measurements and DOAS fitting methods are then applied. The OPG approach enables the generation of moderate FWHM continua with high spectral energy density and tunable to absorption features of many molecules. Recent measurements demonstrating a minimum sensitivity of 10 ppm-m for methane around 3.3 μm with ∼ 2 mW average power in less than 10 seconds will be described. Results of enhancements to the laser source using small or large bandwidth seeds constructed from telecom off-the-shelf components indicate that the OPG output spectral energy density can have controllable spectral widths and shapes. It also has a slightly more stable spectral shape from pulse to pulse than without the seed (25 % enhancement). Most importantly, the stabilized output spectra will allow more sensitive measurements.

Standard backscatter lidars encounter problems when solving the two unknowns (aerosol backscatter coefficient and extinction coefficient) from the only one recorded lidar equation. With the help of the high-spectral-resolution filter, high spectral resolution lidars (HSRLs) can provide unambiguous retrieval without critical assumptions. Spectral discrimination between scattering from molecules and aerosols or cloud particles is the basis of the HSRL technique, and several lidar approaches have been developed to obtain this discrimination. Iodine cell filter, which is a kind of atomic/molecular absorption filter, is robust, stable, and can achieve very good separation of aerosol Mie scattering from atmosphere molecular Cabannes scattering. However, absorption filters are lossy and gaseous absorption lines do not exist at many convenient laser wavelengths. Fabry-Perot interferometers are simple and can be tuned to any wavelength, but are limited by acceptance angle. Field-widened Michelson interferometer (FWMI) is considered to have the ability to overcome the deficiencies of the aforementioned filters as it can perform well at relatively large off-axis angles, is nearly lossless, and can be built to any wavelength. In this paper, the development process of an FWMI that is introduced to be the spectroscopic filter for a polarized near-infrared HSRL instrument will be present. The retrieval process of the aerosol optical properties, the design requirements with special focus on the selection of the free spectral range (FSR) of the FWMI, as well as the design result will be described in detail.

A sort of analytical method of fast diagnosis of chromophoric dissolved matter (CDOM) in water is discussed. The total luminescence spectra (TLS) of CDOM in several types of water samples with laser-induced fluorescence (LIF) measurements using a 405 nm wavelength excitation source were measured in the laboratory, and the spectra of CDOM were pointed out and obtained with spectral fluorescence signature (SFS) technique. The spectrum of water Raman scattering and fluorescence of CDOM were separated from TLS with fitting Gaussian of the least squares method, and the curve of fluorescence peak intensity of CDOM against corresponding concentration of CDOM is showed. High correlation (R² = 0.93) was observed between concentration of CDOM and fluorescence normalized to water Raman scattering. The results have presented the capability of the LIF technique as an integrated tool for research and observations.

High spectral resolution lidars (HSRLs) are becoming more and more important in profiling atmospheric aerosols as accurate measurement can be achieved by employing the high-spectral-resolution spectroscopic filter. A field-widened Michelson interferometer (FWMI) is specially designed to be the spectroscopic filter for a near-infrared HSRL. The FWMI is superior to other commonly used filters, such as F-P interferometric filters and atomic/molecular absorption filters for its large angular acceptance, high photon efficiency and wavelength flexibility. It consists of a cubic beam splitter and two orthogonal arms. In this paper, the designing procedure of determining the materials and dimensions of the arms is described in detail. The result designed to the spectral line of 1064nm is presented and is analyzed to have good working performance. A tolerance evaluation model is also established to assist the design process.

High spectral resolution lidars (HSRLs) have shown great advantages for the measurement of backscatter and extinction coefficients of aerosols and clouds due to its spectral discrimination process, which brings about more straightforward and accurate retrieval without additional assumptions in contrast to standard backscatter lidars. We have developed a tilted, field-widened Michelson interferometer (FWMI) to obtain this spectral discrimination. The interferometer is composed of a cubic beam splitter, a solid arm and an air-solid blending arm, and will be employed as the spectroscopic filter of an HSRL to block the aerosol signals but transmit the molecular backscattered photons optimally. In this paper, a comprehensive radiometric model is developed to evaluate the spectral discrimination performance of the FWMI, especially under varies of practical imperfections, such as fabrication errors and utility defections. The principle of the modeling for tilted FWMI as the spectroscopic filter of the HSRL has been presented, through which, the effects on transmittance characteristics of the FWMI from these practical imperfections are analyzed in detail. The model can be used to evaluate the machinery tolerance budgets for new FWMI designs and decide optimal state of usage for FWMI appliance.

The concept and experimental results of photon counting detector package based on a single photon avalanche diode with active gating and quenching circuit are presented. The single shot timing resolution of time-correlated photon counting experiment is about 30 ps, however demonstrated long-time stability allows averaging for both indoor and satellite laser ranging or similar photon counting experiments. The picosecond laser is employed as signal source to demonstrate detector capability. The long term delay stability sub-picosecond range and small temperature drifts of all elements of measuring chain are crucial to obtain useful data for final satellite laser experiments.

A number of ICESat-2 system requirements drove the technology evolution and the system architecture for the laser transmitter Fibertek has developed for the mission.. These requirements include the laser wall plug efficiency, laser reliability, high PRF (10kHz), short-pulse (<1.5ns), relatively narrow spectral line-width, and wave length tunability. In response to these requirements Fibertek developed a frequency-doubled, master oscillator/power amplifier (MOPA) laser that incorporates direct pumped diode pumped Nd:YVO₄ as the gain media, Another guiding force in the system design has been extensive hardware life testing that Fibertek has completed. This ongoing hardware testing and development evolved the system from the original baseline brass board design to the more robust flight laser system. The final design meets or exceeds all NASA requirements and is scalable to support future mission requirements.