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  • 1
    Publication Date: 2011-08-17
    Description: The effects of optical feedback on the performance of Pb(1-x)Sn(x)Se semiconductor lasers are reported. Experimental results are presented to demonstrate that uncontrolled optical feedback can occur from optics external to the laser cavity and that these external optical elements can result in the formation of a composite cavity. This composite cavity can sustain a number of narrow closely spaced emission modes with the spectral characteristics of these modes dependent upon the parameters of the cavity. With weak feedback, the spectral width of the amplitude envelope of the new lasing modes resembles that of the original laser mode with no optical feedback. With strongly coupled feedback the envelope of the external cavity modes substantially broadens the apparent linewidth of the laser. It is shown that these effects can be eliminated, and, moreover, they offer the potential for obtaining spectrally narrow single-mode emission from Pb-salt lasers through the use of carefully designed external cavities.
    Keywords: LASERS AND MASERS
    Type: Applied Optics; 18; Dec. 1
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  • 2
    Publication Date: 2011-08-17
    Description: An infrared heterodyne radiometer has been used to study the seasonal variation of the vertical distribution of atmospheric ammonia. The ammonia profiles are inferred from high-resolution atmospheric solar transmittance measurements around the ammonia absorption feature at 927.32323 kaysers. The transmittance data are obtained from six IF channels with spectral resolutions ranging from .0067 to .033 kaysers. Results from this study show a marked decrease in the tropospheric ammonia level with a decrease at ground level from approximately 10 ppb in March 1979 to 1.3 ppb in August 1979.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters; 7; May 1980
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  • 3
    Publication Date: 2011-08-18
    Description: Ultrahigh resolution (0.007 per cm) atmospheric solar absorption spectra that have been obtained from a tunable infrared heterodyne radiometer are discussed. The radiometer was developed for ground based observations in the 8-12 micron region, and tunability is achieved through the use of Pb-salt semiconductor laser local oscillators. Spectra have been obtained in a piecewise fashion from 9.1 to 11.1 microns using laser emission modes that exhibit characteristics suitable for local-oscillator operation. Spectra showing absorption features of HNO3, O3, CO2, and H2O are presented along with comparisons of experimental and synthetic spectra calculated using a line-by-line atmospheric transmission model.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: Optical Engineering; 21; Mar
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  • 4
    Publication Date: 2016-06-07
    Description: The noise characteristics of the TDL were studied for frequencies less than 20 kHz. For heterodyne applications, the high frequency ( 1 MHz) characteristics are also important. Therefore, the high frequency noise characteristics of the TDL were studied as a part of a full TDL characterization program which has been implemented for the improvement of the TDL as a local oscillator in the LHS system. It was observed that all the devices showed similar high frequency noise characteristics even though they were all constructed using different techniques. These common high frequency noise characteristics are reported.
    Keywords: LASERS AND MASERS
    Type: NASA. Langley Res. Center Heterodyne Systems and Technol., Pt. 1; p 129-142
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  • 5
    Publication Date: 2016-06-07
    Description: A tunable diode laser heterodyne radiometer was developed for ground based measurements of atmospheric solar absorption spectra in the 9 to 12 micron spectral range. The performance and operating characteristics of this tunable infrared heterodyne radiometer (TIHR) is discussed along with recently measured heterodyne solar absorption spectra in the 10 to 11 micron spectral region.
    Keywords: LASERS AND MASERS
    Type: Heterodyne Systems and Technol., Pt. 1; p 209-220
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  • 6
    Publication Date: 2019-07-18
    Description: We report the first in-situ measurements of hydrogen cyanide (HCN) and acetonitrile (CH3CN) from the Pacific troposphere (0-12 km) obtained during the NASA/Trace-P mission (Feb.-April, 2001). Mean HCN and CH3CN mixing ratios of 243 (+/-118) ppt and 149 (+/-56) ppt respectively, were measured. The in-situ observations correspond to a total HCN column of 4.4-4.9 x 10(exp 15) molec. cm(exp -2) and a CH3CN column of 2.8-3.0 x 10(exp 15) molec. cm(exp -2). This HCN column is in good agreement with available spectroscopic observations. The atmospheric concentrations of HCN and CH3CN were greatly influenced by outflow of pollution from Asia. There is a linear relationship between the mixing ratios of HCN and CH3CN, and in turn these are well correlated with tracers of biomass combustion (e.g. CH3Cl, CO). Relative enhancements with respect to known tracers of biomass combustion within selected plumes in the free troposphere, and pollution episodes in the boundary layer allow an estimation of a global biomass burning source of 0.8+/-0.4 Tg (N)/y for HCN and 0.4+/-0.1 Tg (N)/y for CH3CN. In comparison, emissions from automobiles and industry are quite small (〈0.05 Tg (N)/y). The vertical structure of HCN and CH3CN indicated reduced mixing ratios in the MBL (Marine Boundary Layer). Using, a simple box model, the observed gradients across the top of the MBL are used to derive an oceanic flux of 6.7 x 10(exp -15) g (N) cm(exp -2)/s for HCN and 4.8 x 10(exp -15) g (N) cm(exp -2)/s for CH3CN. An air-sea exchange model is used to conclude that this flux can be maintained if the oceans are under-saturated in HCN and CH3CN by 23% and 17%, respectively. It is inferred that oceanic loss is a dominant sink for these nitrites, and they deposit some 1.3 Tg (N) of nitrogen annually to the oceans. Assuming reaction with OH radicals and loss to the oceans as the major removal processes, a mean atmospheric residence time of 4.7 months for HCN and 5.1 months for CH3CN is calculated. A global budget analysis shows that the sources and sinks of HCN and CH3CN are roughly in balance. There are indications that biogenic sources may also be present. Mechanisms involved in nitrate formation during combustion and removal in the oceans are poorly understood.
    Keywords: Geophysics
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  • 7
    Publication Date: 2019-07-20
    Description: Ammonia concentrations reached minimal levels (approximately 0.1 ppb) in early winter, followed by a sudden later winter increase. A direct relationship between ambient ammonia levels and air temperature was inferred from the data (linear correlation coefficient r=0.53). Ammonia concentrations were determined to be directly related to the absolute humidity of the air (r=0.72); a weaker relationship between ammonia concentrations and relative humidity was discovered (r=0.37). The data also indicated that ammonia levels were generally higher within continental air masses than those of maritime origin. Soil parameters such as pH and moisture content were found to have a major bearing on the release of gaseous ammonia from soils in the region.
    Keywords: METEOROLOGY AND CLIMATOLOGY
    Type: NASA-CR-164188
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  • 8
    Publication Date: 2019-07-13
    Description: A Beer's Law experiment was performed with a tunable diode laser to find the N2 pressure broadening characteristics of a single 03 absorption line at 1129.426 cm for N2 pressures from 10 to 100 torr (O3 pressure = 3.16 torr). SO2 line positions were used for wavelength calibration. Line shapes were interatively fitted to a Lorentz function. Results were delta (HWHM in MHz) = 47.44 (+ or - 5.34) MHz + 1.730 (+ or - 0.088) MHz/torr *p(torr) with sigma = 0.9897. This intercept compares well with the Doppler O3 - O3 broadened (at 3.16 torr) width of 44.52 Hz. This result in a HWHM line width of 0.44 cm atm at 760 torr and 285 K. The line strengths integrated over delta nu = 0.55 cm were found to be N2 pressure dependent.
    Keywords: OPTICS
    Type: NASA-CR-168999 , NAS 1.26:168999 , PTR-81-10
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  • 9
    Publication Date: 2019-07-13
    Description: A tunable infrared heterodyne radiometer (TIHR) which uses a diode laser as the local oscillator is described as well as methods for the evaluation of the excess noise characteristics of the tunable diode laser. Preliminary atmospheric absorption data taken with the TIHR are presented and show the capability of the TIHR for making the highest resolution atmospheric measurements to date.
    Keywords: LASERS AND MASERS
    Type: NASA-CR-162827 , PTR-80-3
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  • 10
    Publication Date: 2019-07-13
    Description: Heterodyne systems would be much more versatile if a broadly tunable laser, such as a semiconductor diode laser (TDL), could be used as the local oscillator (LO). Previous studies have shown that while a TDL can be used as an LO, the TDL lack sufficient power to cause the signal-to-noise ratio to be shot noise limited. The heterodyne system with a HgCdTe photodiode as the LO was characterized and the beat frequency response of the heterodyne systems was mapped out.
    Keywords: LASERS AND MASERS
    Type: NASA-CR-155784 , PGSTR-AP77-63
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