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ABSTRACT: The SO2 molecule is of considerable interest in the context of atmospheric pollution, and in many laser monitoring techniques the ultraviolet absorption band at 300 nm is used to determine SO2 concentrations in the atmosphere. Recent laboratory experiments with a resolution of 2 × 10-3 nm showed that variations could occur in absorption cross-section measurements made with different laser bandwidths due to unresolved fine structure. We have investigated absorption spectra with a line width of 3 × 10-6 nm, using a frequency-doubled continuous-wave dye laser, and have confirmed the existence of fine structure in the absorption even when collisionally broadened with an atmosphere of nitrogen. These measurements provide a data base from which valid absorption cross sections may be calculated for all monitoring laser bandwidths. We estimate the pressure broadening coefficient for nitrogen in this wavelength region as 83 +/- 38 kHz Pa-1 (11 +/- 5 MHz torr-1). The temperature dependence of the absorption cross-section was also investigated.
Optics Communications - OPT COMMUN. 01/1980; 33:287-291.
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ABSTRACT: A servocontrolled 1-m plane-parallel Fabry-Perot interferometer has been developed at NPL for the precise intercomparison of laser wavelengths. This instrument has been used to measure the wavelength ratio of a 679-nm radiation and that from a 633-nm iodine-stabilized He-Ne laser, achieving an accuracy of 2.9 parts in 10(11). The 679-nm light was derived from a stabilized CO(2) laser radiation by upconversion, and the wavelength of this 9.3-microm laser radiation can be calculated from the visible wavelength result. Frequency measurements on the same CO(2) laser radiation have already been made in this laboratory, so that the experiment reported here leads to a precise value for the speed of light in vacuum and to the value of 473, 612, 380.5 +/- 0.3 MHz for the absolute frequency of the visible radiation from a He-Ne laser stabilized to component d of (127)I(2).
Applied Optics 04/1978; 17(7):1048-54. · 1.41 Impact Factor
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ABSTRACT: Early determinations of the speed of light, based on astronomical observations or terrestrial time-of-flight experiments, were largely superseded in the 1940's by measurements of the frequency and wavelength of microwave radiation. The results were limited by the uncertainty in the wavelength measurements, and it was apparent that greater accuracy could be achieved by using radiation of shorter wavelength. It was, however, not until the development of lasers, and the nonlinear optical techniques made possible by their high output intensities, that frequency measurements could be extended towards the visible region of the spectrum.Stimulated by a demand for a more precise knowledge of the speed of light for application in space research, geodesy, and metrology, a new series of determinations has recently taken place. The measurements were made upon the radiations from a number of different stabilized-laser systems operating in the near infrared and visible spectral regions. Several different interferometric techniques were developed for the wavelength measurements. The results have led to a new recommended value for the speed of light, 299 792 4581.2 m s–1, and to the possibility of re-defining the unit of length.
Optical and Quantum Electronics 12/1975; 8(1):1-14. · 0.82 Impact Factor
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ABSTRACT: WAVELENGTH measurements, which are part of an accurate determination of the speed of light1, have been made on the radiation from a carbon dioxide laser, stabilised to the R(12) transition at 9.3 µm by saturated fluorescence in an external CO2 cell, which has a measured frequency2 reproducible to better than one part in 109. The problems of a direct intercomparison of infrared and visible wavelengths were avoided by mixing the CO2 radiation with light from a 10-mW He–Ne laser at 0.63 µm to give a difference frequency sideband at a wavelength of 0.68 µm. This mixing process, known as up-conversion, was performed in a cooled, single crystal of proustite3,4. The wavelength of the CO2 radiation was then determined using the relationship λ9.3 = λ0.63/(1—R), in which it is assumed that c is the same for the two visible radiations. The wavelength measurements were thus of the ratio R = λ0.63/λ0.68 and the value of λ0.63 with respect to the primary length standard.
09/1974; 251(5470):46-47.
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ABSTRACT: WE report here the completion of a determination of the speed of light
at the National Physical Laboratory1. The value was obtained
from the product of the measured frequency and the wavelength,
determined through up-conversion, of the radiation from a CO2
laser stabilised to the R(12) transition of CO2 at 9.3
µm.
Nature 08/1974; 251:46. · 36.28 Impact Factor
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ABSTRACT: 1] Research into the role of volatile organic compounds (VOCs) in the chemistry of the lower atmosphere has led to requirements for stable and accurate standard gas mixtures to underpin long‐term monitoring of global background and polluted urban concentrations. We describe a novel method for the preparation of standard mixtures containing thirty compounds in matrices of nitrogen and whole air. The estimated uncertainty in the amount fractions of the compounds in these mixtures is typically 2% (relative) and typical stability is better than 0.2% per annum. We also report the results of a comparison exercise between eight laboratories using these mixtures, which indicate that typical uncertainties achievable by laboratories making measurements of these compounds are better than demonstrated previously. (2010), Standard mixtures of ambient volatile organic compounds in synthetic and whole air with stable reference values, J. Geophys. Res., 115, D14302, doi:10.1029/2009JD012933.
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ABSTRACT: Measurement of the speed of light is reviewed for two broad classes of measuring techniques: time-of-flight techniques where a light pulse is timed over a measured distance and techniques where the wavelength and frequency of an oscillator are measured. A brief historical review of the more significant early measurements and the limitations of the techniques is presented. It was not until the development of lasers and the nonlinear optical techniques made possible by their high output intensities that frequency measurements could be extended to the visible region of the spectrum. Experimental results are compared and discussed, which have resulted in the new value for the speed of light recommended by CCDM and endorsed by other international organizations.
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ABSTRACT: Accurate, high-resolution measurements have been made of the SO2 absorption spectrum in the ultraviolet from 297.0 to 301.0 nm, using a frequency-doubled dye laser of bandwidth 2 × 10-3 nm. These results are applicable to SO2 pollution monitoring in the atmosphere and fine structure is observed in the spectrum under atmospheric conditions at ambient temperatures, giving results significantly different from previous publications. Measurements were also made at 100°C in order to simulate more realistically the conditions occurring within power plant plumes. The latter measurements result in significantly differing absorption coefficients. Measurements of the NO2 spectrum were also made to determine the degree of spatial interference when both gases are present together in the atmosphere.
Optics Communications.
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ABSTRACT: Laboratory measurements of the absorption coefficients of NO2 at wavelengths suitable for the operation of differential absorption lidar (DIAL) systems, at around 446 nm and 489 nm are reported. Such measurements are necessary for the valid determination of absolute levels of concentration of NO2 in the atmosphere and have been made in order to assess anomalies in existing data. A theoretical assessment of some of the optional systems currently available for the application of DIAL to the measurement of atmospheric concentrations of SO2 is described.
Optics & Laser Technology.
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ABSTRACT: Measurements have been made of the energy generated in the wavelength region 3.3 to 4.0 μm, by difference-frequency mixing between a tunable dye laser and a Nd: YAG laser in lithium niobate. The highest mixing efficiencies have been observed with the beams aligned in a non-collinear geometry. It is demonstrated that this non-collinear geometry corresponds to the tangential phase-matching condition which increases the acceptance angle of a second-order nonlinear interaction over that achieved in a non-critically phase-matched geometry.
Optics Communications.
