Stephen E. Maxwell

Multiheterodyne spectroscopy with optical frequency combs generated from a continuous-wave laser

Dual-drive Mach-Zehnder modulators were utilized to produce power-leveled optical frequency combs (OFCs) from a continuous-wave laser. The resulting OFCs contained up to 50 unique frequency components and spanned more than 200 GHz. Simple changes to the modulation frequency allowed for agile control of the comb spacing. These OFCs were then utilized for broadband, multiheterodyne measurements of CO2 using both a multipass cell and an optical cavity. This technique allows for robust measurements of trace gas species and alleviates much of the cost and complexity associated with the use of femtosecond OFCs produced with mode-locked pulsed lasers.

Quantum phase transitions and continuous observation of spinor dynamics in an antiferromagnetic condensate.

Condensates of spin-1 sodium display rich spin dynamics due to the antiferromagnetic nature of the interactions in this system. We use Faraday rotation spectroscopy to make a continuous and minimally destructive measurement of the dynamics over multiple spin oscillations on a single evolving condensate. This method provides a sharp signature to locate a magnetically tuned separatrix in phase space which depends on the net magnetization. We also observe a phase transition from a two- to a three-component condensate at a low but finite temperature using a Stern-Gerlach imaging technique. This transition should be preserved as a zero-temperature quantum phase transition.

Number Fluctuations and Energy Dissipation in Sodium Spinor Condensates

We characterize fluctuations in atom number and spin populations in F=1 sodium spinor condensates. We find that the fluctuations enable a quantitative measure of energy dissipation in the condensate. The time evolution of the population fluctuations shows a maximum. We interpret this as evidence of a dissipation-driven separatrix crossing in phase space. For a given initial state, the critical time to the separatrix crossing is found to depend exponentially on the magnetic field and linearly on condensate density. This crossing is confirmed by tracking the energy of the spinor condensate as well as by Faraday rotation spectroscopy. We also introduce a phenomenological model that describes the observed dissipation with a single coefficient.

Hyperspectral image projection of a pig kidney for the evaluation of imagers used for oximetry

Hyperspectral image projection applied to optical medical imaging can provide a means to evaluate imager performance. This allows repeated viewing of unique surgical scenes without the need for costly experiments on patients. Additionally, the generated scene can be well characterized and used repeatedly as a standard for many different imagers at different times and locations. This paper describes the use of a hyperspectral image of a pig kidney. The scene of the kidney is projected with the full spectral content allowing the oxygenation status of the tissue to be observed and evaluated spatially.

Construction of a high power OPO laser system for differential absorption LIDAR

Our goal is to develop and characterize optical measurement technology to enable accurate quantification of greenhouse-gas emissions from distributed sources and sinks. We are constructing a differential absorption LIDAR (DIAL) system that will be sensitive to the three primary greenhouse gases, carbon dioxide, methane, and nitrous oxide. Our system uses a high energy optical parametric oscillator (OPO) operating from 1585 nm to 1646 nm. Here we describe this OPO system and initial characterization of its output. The OPO uses a Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) design. The commercially available RISTRA cavity is machined from a solid block of aluminum. The compact single piece cavity design requires no mirror adjustments and image rotation provides efficient light conversion efficiency and excellent beam quality. The injection seeded OPO has demonstrated total output energy of 50 mJ/pulse when pumped with 220 mJ/pulse of 1064 nm radiation. The pump laser has a repetition rate variable from 1 Hz to 100 Hz and a temporal pulse width of 4.2 ns. In the current configuration the seed laser is locked to a mode of the cavity.

Validation of spectral radiance assignments to integrating sphere radiance standards for the Advanced Baseline Imager

The Advanced Baseline Imager (ABI) is the next-generation imaging sensor for the National Oceanic and Atmospheric Administration’s (NOAA’s) operational meteorological satellites in geostationary orbit. One pathway for traceability to reference standards of the visible and near-infrared radiometric response for ABI is to a 1.65 m diameter integrating sphere source standard of spectral radiance. This source illuminates the full entrance pupil via the ABI Earth-view port, thus determining the absolute spectral radiance responsivity in the visible and shortwave infrared. The spectral radiance values of the large sphere are assigned by Exelis using a double monochromator and a 15.24 cm diameter integrating sphere source standard that is calibrated by NIST. As part of the ABI program, Exelis was required by NASA to have the spectral radiance values assigned by Exelis to the large sphere be validated by NIST. Here we report the results of that activity, which took place in April, 2013. During the week of April 8, Exelis calibrated the 1.65 m diameter sphere at all 24 levels that correspond to the ABI calibration protocol. During the week of April 15, the NIST validation exercise for five selected levels took place. NIST deployed a portable spectral radiance source, a filter radiometer restricted to the visible and near-infrared, and two spectroradiometers that covered from 350 nm to 2500 nm. The NIST sphere source served as the validation standard. The comparison results, which are reported at the ABI bands, agreed to within the combined uncertainties. We describe the methodology, results, and uncertainty estimates related to this effort.