Takao Kobayasi

Laser-Raman radar —Laser-Raman scattering methods for remote detection and analysis of atmospheric pollution

This paper reviews the recent progress of the laser radar method, providing single-ended and range-resolved means for remote detection and analysis of various molecular species in the polluted as well as ordinary atmosphere, based on the existence of their Raman backscattering; simply called the laser-Raman radar method. A brief description is given of the operational principle including the basic concept and fundamental characteristics of this scheme, along with critical discussions of their advantages. The differential backscattering cross section and the spectral distribution of Raman scattering of molecular species present in the atmosphere are discussed and summarized from the practical view-point for providing molecular density evaluation and interference-free detection. After presenting experimental approaches of the laser-Raman scheme to show the feasibility of in-field monitoring and analysis of chemical species in various air conditions, the estimation of molecular concentrations is performed from observed Raman spectra. Analytical discussions on the minimum detectable concentration and the range detectivity are also presented to indicate the present capability and limitation of this laser-Raman technique. The potentiality for further progress in resonance Raman scattering for range-resolved measurement of dispersed pollutants in ambient air is pointed out.

SPECTROSCOPIC DETECTION OF SO2 AND CO2 MOLECULES IN POLLUTED ATMOSPHERE BY LASER‐RAMAN RADAR TECHNIQUE

Raman backscatter of a Q‐switched ruby laser beam from SO2 and CO2 molecules were selectively detected in the polluted atmosphere by means of a laser‐Raman radar system. The spectrum of these Raman components was compared quantitatively with that for the ordinary and sulphur dioxide atmospheres, along with the estimation of relative concentrations of these molecules.Raman backscatter of a Q‐switched ruby laser beam from SO2 and CO2 molecules were selectively detected in the polluted atmosphere by means of a laser‐Raman radar system. The spectrum of these Raman components was compared quantitatively with that for the ordinary and sulphur dioxide atmospheres, along with the estimation of relative concentrations of these molecules.

Remote monitoring of NO 2 molecules by differential absorption using optical fiber link

The feasibility of using an optical fiber system for differential visible absorption measurements of NO/sub 2/ using an argon-ion laser is examined. This technique can also be applied to air pollution monitoring of other molecules in the new infrared region. (AIP)

Infra-red heterodyne laser radar for remote sensing of air pollutants by range-resolved differential absorption

An infra-red heterodyne laser radar scheme incorporating range-resolved differential absorption using elastically backscattered, pulsed signal from distributed particles in the atmosphere is proposed and analysed. The basic requirements of the system arrangement and operating parameters involved in this scheme are discussed in detail. This method is shown to offer a remote and spatially-resolved measurement of various molecuar pollutants dispersed in the air for ranges up to several km and detection sensitivities to less than 0.1 ppm with laser pulses of 100 W average power. This power is 103 to 106 times smaller than that required with the conventional direct detection technique in the infra-red. The measuring accuracy and its limiting processes inherent to the infra-red range-resolved differential absorption method are also analysed.

Detection of sulphur dioxide in stack plume by laser Raman radar

Laser Raman radar offers a promising approach to the remote sensing of atmospheric pollutants in stack plume. An analysis revealed that the combination of the second harmonic of a Nd:YAG laser, a synchronous single photoelectron counting (SSPC) scheme, and narrow passband interference filter employed for light source, signal processor, and separation of Raman-scattered light respectively gave the most sensitive system for stack effluent monitoring. Using a laser Raman radar constructed on the basis of the analysis, the concentration of SO2 in the plume emitted from a 150 m high stack of a power plant was measured at a distance of 228 m.

A two-wavelength oscillation CO2 laser for differential absorption measurements of atmospheric trace molecules

A two-wavelength oscillation C02 laser was constructed for the differential absorption measurement of low concentrations of atmospheric trace molecules and pollution molecules. A diffraction grating was alternately angle-modulated for the two-branch oscillation using a single C02 laser. The output power of the two wavelengths was balanced to be equal. With these characteristics, the phase-sensitive detection technique can be applied to give highly sensitive detection of the received power difference, which is proportional to the absorption in the optical path of the transmitted beams. A differential absorption spectrometer was used to demonstrate the usefulness of this two-wavelength laser oscillation method incorporating the output power balancing technique. The concentration of ethylene molecules was detected in a calibration cell and was also measured in real time in the exhaust of a combustion engine. From these results it has been shown that this differential absorption scheme offers a minimum detectable absorption of approximately 0.8%.

Two-wavelength and power-balanced oscillation of a CO2 laser for application to differential absorption measurements

A new technique of alternate two-wavelength oscillation of a CO2 laser is discussed for application to various differential absorption spectroscopic measurements. Power-balanced, two-adjacent branch oscillation using a single CO2 laser was achieved by modulating the angle of a mirror inside the laser cavity and adjusting automatically the cavity length. The two-wavelength modulation frequency was extended up to about 1.2 kHz. Line tuning and power modulation characteristics were studied. The laser was used in long-path differential absorption measurements of ethylene air pollution molecules to demonstrate the capability of this power-balanced, two-wavelength oscillation method. The minimum detectable absorption was nearly 1×10−3 in a short-path cell experiment and 3×10−3 in a long-path experiment.

Infrared laser radar technique using heterodyne detection for range-resolved sensing of air pollutants

Abstract An infrared, pulsed heterodyne laser radar using differential absorption technique with tunable lasers is proposed as a practical means of measuring dispersed air pollutants with range resolution. A system analysis shows that this scheme offers an adequate sensitivity for ranges up to several km.