[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We describe the characteristics of the planar-waveguide external cavity diode laser (PW-ECL). To the best of our knowledge, it is the first butterfly-packaged 1064 nm semiconductor laser that is stable enough to be locked to an external frequency reference. We evaluated its performance from the viewpoint of precision experiments. Using a hyperfine absorption line of iodine, we suppressed its frequency noise by a factor of up to 10<sup>4</sup> at 10 mHz. The PW-ECL's compactness and low cost make it a candidate to replace traditional Nd:YAG nonplanar ring oscillators and fiber lasers in applications that require a single longitudinal mode.
[Show abstract][Hide abstract] ABSTRACT: We report on our development effort for a trace-gas-sensing lidar transmitter to be used in future Earth-orbiting satellites. Our lidar transmitter is based on an optical parametric oscillator (OPO), whose output wavelength is switched at a rate of 5 kHz across the target line. The OPO cavity length and the seed laser wavelengths are stabilized to molecular and atomic references. We demonstrated the concept of the OPO-based lidar transmitter at 1,651 nm, achieving ̃300-μJ output energy and <300-MHz linewidth, which are anticipated to be required for a future methane lidar spaceborne mission.
Applied Physics B 01/2014; 116(4). DOI:10.1007/s00340-014-5783-4 · 1.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The last century has seen enormous progress in our understanding of the Universe. We know the life cycles of stars, the structure of galaxies, the remnants of the big bang, and have a general understanding of how the Universe evolved. We have come remarkably far using electromagnetic radiation as our tool for observing the Universe. However, gravity is the engine behind many of the processes in the Universe, and much of its action is dark. Opening a gravitational window on the Universe will let us go further than any alternative. Gravity has its own messenger: Gravitational waves, ripples in the fabric of spacetime. They travel essentially undisturbed and let us peer deep into the formation of the first seed black holes, exploring redshifts as large as z ~ 20, prior to the epoch of cosmic re-ionisation. Exquisite and unprecedented measurements of black hole masses and spins will make it possible to trace the history of black holes across all stages of galaxy evolution, and at the same time constrain any deviation from the Kerr metric of General Relativity. eLISA will be the first ever mission to study the entire Universe with gravitational waves. eLISA is an all-sky monitor and will offer a wide view of a dynamic cosmos using gravitational waves as new and unique messengers to unveil The Gravitational Universe. It provides the closest ever view of the early processes at TeV energies, has guaranteed sources in the form of verification binaries in the Milky Way, and can probe the entire Universe, from its smallest scales around singularities and black holes, all the way to cosmological dimensions.
[Show abstract][Hide abstract] ABSTRACT: This paper reports on an all-sky search for periodic gravitational waves from sources such as deformed isolated rapidly spinning neutron stars. The analysis uses 840 hours of data from 66 days of the fifth LIGO science run (S5). The data were searched for quasimonochromatic waves with frequencies f in the range from 50 to 1500 Hz, with a linear frequency drift f˙ (measured at the solar system barycenter) in the range -f/τ<f˙<0.1f/τ, for a minimum spin-down age τ of 1000 years for signals below 400 Hz and 8000 years above 400 Hz. The main computational work of the search was distributed over approximately 100 000 computers volunteered by the general public. This large computing power allowed the use of a relatively long coherent integration time of 30 hours while searching a large parameter space. This search extends Einstein@Home’s previous search in LIGO S4 data to about 3 times better sensitivity. No statistically significant signals were found. In the 125-225 Hz band, more than 90% of sources with dimensionless gravitational-wave strain tensor amplitude greater than 3×10^-24 would have been detected.
Physical Review D 01/2013; 80(4):042003. DOI:10.1103/PhysRevD.80.042003 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report airborne measurements of the column abundance of atmospheric methane made over an altitude range of 3-11 km using a direct detection integrated-path differential-absorption lidar with a pulsed laser emitting at 1651 nm. The laser transmitter was a tunable, seeded optical parametric amplifier pumped by a Nd:YAG laser, and the receiver used a photomultiplier detector and photon-counting electronics. The results follow the expected changes with aircraft altitude, and the measured line shapes and optical depths show good agreement with theoretical calculations.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate the airborne measurement of atmospheric methane using a pulsed lidar at 1650 nm using an integrated path differential absorption scheme. Our seeded nanosecond-pulsed optical parametric amplifier (OPA)-based instrument works up to the highest altitudes flown (> 10 km). Our airborne measurements are in good agreement with the expected atmospheric absorption. The difference between the measured absorption expressed in terms of the differential optical depth (DOD) and the predicted DOD calculated using Global Modeling and Assimilation Office (GMAO) data is 0.027 +/- 0.038 (standard deviation), with the DOD ranging from 0.5 to 2. For large sections of the flight, the measured absorption profile agrees within 2% rms of the predicted absorption profile. Overall, the retrieved atmospheric methane concentration lies within 11% of the true concentration, the latter estimated from our onboard insitu Picarro cavity ring down spectrometer.
Proceedings of SPIE - The International Society for Optical Engineering 10/2012; DOI:10.1117/12.929990 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the development effort of a nanosecond-pulsed optical
parametric amplifier (OPA) for remote trace gas measurements for Mars
and Earth. The OPA output has ˜500 MHz linewidth and is widely
tunable at both near-infrared and mid-infrared wavelengths, with an
optical-optical conversion efficiency of up to ˜39%. Using this
laser source, we demonstrated open-path measurements of CH4
(3291 and 1652 nm), CO2 (1573 nm), O (1652 nm), and CO (4764
nm) on the ground. The simplicity, tunability, and power scalability of
the OPA make it a strong candidate for general planetary lidar
instruments, which will offer important information on the origins of
the planet's geology, atmosphere, and potential for biology.
[Show abstract][Hide abstract] ABSTRACT: We have developed a linearly polarized ytterbium-doped fiber ring laser with a single longitudinal mode output at 1064 nm. A fiber-coupled intracavity phase modulator ensured mode-hop free operation and allowed fast frequency tuning. The fiber laser was locked with high stability to an iodine-stabilized laser, showing a frequency noise suppression of a factor ∼ 105 at 1 mHz.
[Show abstract][Hide abstract] ABSTRACT: 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".
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Optical fiber and semiconductor laser technologies have evolved dramatically over the last decade due to the increased demands from optical communications. We are developing a laser (master oscillator) and optical amplifier based on those technologies for interferometric space missions, including the gravitational-wave missions NGO/SGO (formerly LISA) and the climate monitoring mission GRACE Follow-On, by fully utilizing the matured wave-guided optics technologies. In space, where simpler and more reliable system is preferred, the wave-guided components are advantageous over bulk, crystal-based, free-space laser, such as NPRO (Non-planar Ring Oscillator) and bulk-crystal amplifier.
Journal of Physics Conference Series 06/2012; 363(1). DOI:10.1088/1742-6596/363/1/012054
[Show abstract][Hide abstract] ABSTRACT: NASA is currently developing several Earth science laser missions that
were recommended by the US National Research Council (NRC) Earth Science
Decadal Report. The Ice Cloud and Land Elevation Satellite-2 (ICESat-2)
will carry the Advanced Topographic Laser Altimeter System (ATLAS) is
scheduled for launch in 2016. The Active Sensing of CO2 Emissions over
Nights, Days, and Seasons (ASCENDS) mission and will measure column
atmospheric CO2 concentrations from space globally. The Gravity Recovery
And Climate Experiment (GRACE) Follow-On (GRACEFO) and GRACE-2 missions
measure the spatially resolved seasonal variability in the Earth's
gravitational field. The objective of the Lidar Surface Topography
(LIST) mission is to globally map the topography of the Earth's solid
surface with 5 m spatial resolution and 10 cm vertical precision, as
well as the height of overlying covers of vegetation, water, snow, and
ice. This paper gives an overview of the laser transmitter and receiver
approaches and technologies for several future missions that are being
investigated by the NASA Goddard Space Flight Center.
Proceedings of SPIE - The International Society for Optical Engineering 05/2012; 8381:23-. DOI:10.1117/12.920783 · 0.20 Impact Factor