Jerry A. Gelbwachs

Atomic resonance filters

The atomic resonance filter (ARF) is an ultranarrowband (Q approximately 10/sup 5/-10/sup 6/), large-acceptance-angle, isotropic optical filter. These features make the device ideally suited for applications in which weak laser signals are detected against a continuum background. The filter properties arise from the physical processes of absorption, emission, and internal energy conversion in atomic vapors. The characteristics of the ARF are described and the underlying physics that governs the operation is discussed. Representative examples of passive, active, and IR filters are presented. A metastable ARF that offers improved solar background-limited performance by filtering signals at Fraunhofer wavelengths is described. >

Water-vapor continuum CO 2 laser absorption spectra between 27°C and −10°C

Water continuum CO2 laser absorption spectra are reported for temperatures between 27 and −10°C. The continuum is found to possess a negative temperature coefficient. The results obtained suggest that the magnitude of this temperature coefficient increases with increasing water pressure and decreasing temperature. The temperature coefficients between 27 and 10°C for air mixtures containing 3.0- and 7.5-Torr water vapor are −2.0 ± 0.4 and −2.9 ± 0.5%/°C, respectively. For mixtures with 3.0-Torr water the 10–0°C temperature coefficient is −7.7 ± 0.2%/°C. The temperature and water pressure dependencies observed for the continuum suggest that while both collisional broadening and water dimer mechanisms contribute to the continuum, the dimer mechanism is more important over this temperature range.

Fluorescence of Atmospheric Aerosols and Lidar Implications

The fluorescence of aerosols in the ambient atmosphere has been monitored in situ using cw argon ion laser excitation in bands of 50 nm and 100 nm over the spectral region of 560-810 nm. The observed broadband aerosol fluorescence may limit lidar (laser radar) determinations of pollutants. The limitation can be overcome by a method in which the aerosol fluorescence excited at two wavelengths is constant while the molecular signals differ. The effectiveness of the technique has been demonstrated by in situ measurements of atmospheric nitrogen dioxide (NO(2)) in the presence of aerosols.

Saturated optical nonresonant emission spectroscopy (SONRES) for atomic detection

We have demonstrated the extremely small detection limits that can be achieved in a high‐pressure environment by the combination of saturated absorption and nonresonance emission spectroscopy. The method of SONRES exploits efficient energy transfer between excited atomic states at high pressures and intense optical fields available from tunable lasers. An estimated 104 atoms/cm3 of sodium were monitored in a flame; the corresponding atomic concentration was 1 part in 1015. A detection limit of ∼10 atoms/cm3 was recorded for sodium in argon. The ultimate detection limit for this method is ∼10−4 atoms.

Iron Boltzmann factor LIDAR: proposed new remote-sensing technique for mesospheric temperature

We describe a new LIDAR technique for middle atmospheric temperature measurement. The proposed LIDAR exploits the Fe layer in the 80-100-km altitude region. Absolute temperatures are inferred by the use of the Maxwell-Boltzmann relationship from the ratio of LIDAR returns from mesospheric Fe atoms excited at 372 and 374 nm, corresponding to the ground-state resonance line and a thermally populated resonance line, respectively. The wavelengths of the new LIDAR are favorable for capturing Rayleigh signals from the middle atmosphere. A simulation indicates that a complete temperature profile from 30 to 100 km can be acquired with the proposed LIDAR by monitoring simultaneously the Rayleigh signals and the Fe fluorescence returns excited by the same transmitter pulse.

Fluorescence determination of atmospheric NO2

The concentrations of nitrogen dioxide (NO2) in the atmosphere have been determined in real time and with a sensitivity of one part per billion. Laser excitation of NO2 was at 4416 and 4880 Å and fluorescence was monitored at 0.7 to 0.8 μm. Results obtained on typical smoggy days in Los Angeles are presented.

Stimulated‐Emission Cross Section of Nd3+ at 1.06 μ in POCl3, YAG, CaWO4, ED‐2 Glass and LG55 Glass

Stimulated‐emission cross sections of Nd3+ at 1.06 μ in POCl3, yttrium aluminum garnet (YAG), calcium tungstate (CaWO4), and two laser glasses (ED‐2 and LG55) were measured by studying the laser threshold gain and inverted population. The technique of end pumping with pulsed xenon ion lasers was used with an identical laser cavity for all the Nd‐doped samples. A direct comparison of the cross sections in the different hosts was obtained.

Carbon dioxide laser absorption spectra and low ppb photoacoustic detection of hydrazine fuels

Absorption cross-section data are reported for the toxic rocket fuels hydrazine, monomethylhydrazine, and unsymmetrical dimethylhydrazine (UDMH), as well as for their selected air oxidation products dimethylamine, trimethylamine, and methanol at up to seventy-eight CO(2) laser wavelengths each. These data are important for the assessment of the capability of CO(2) laser-based spectroscopic techniques for monitoring low levels of hydrazine-fuel vapors in the ambient air. Interference-free detection sensitivities of <30 ppb have been demonstrated for UDMH using a laboratory photoacoustic detection system.

A Fraunhofer-wavelength magnetooptic atomic filter at 422.7 nm

A demonstration of the first magnetooptic atomic filter that overlays a strong solar Fraunhofer line is reported. Compared with alkali magnetooptic filters, this filter enjoys a large reduction in solar interference and a significant decrease in the number of noise passbands. The filter utilizes the strong Ca(4p/sup 1/P/sub 1/-4s/sup 1/S/sub 0/) transition at 422.7 nm. Under the weak magnetic field experimental conditions, a maximum transmission efficiency of 55% and a symmetrical double-peaked transmission spectrum with 1.5 GHz wide passbands were observed. The filters frequency response was measured with a laser intensity modulation technique. No falloff was observed at 176 MHz, the highest frequency available with the apparatus. Calculations indicate that further improvements in filter performance can be achieved by optimizing the magnetic field and the cell temperature. >

Infrared detection by an atomic vapor quantum counter

An efficient technique for the detection of nartowband infrared radiation by up-conversion in an optically saturated atomic vapor is described. The strong transition cross sections provided by the resonance interactions combined with the narrow Doppler spectral response profile contribute to the potential to approach quantum-noise-limited performance. Experimental results confirm the model of the detection process for sodium transitions at 1.48, 2.34, and 3.42 μ. The predicted ultimate noise equivalent power (NEP) of this device is less than 10-17W/Hz1/2in the 1.5-5-\mu spectral region.