We are developing the remote measurement system for detecting harmful substances such as nitrogen dioxide and chemical materials using the resonance Raman scattering effect. Although the most remote measurement systems were designed to use the light from ultraviolet to visible wavelengths region, the characteristic Raman spectra of complex materials were observed in deep-ultraviolet (DUV) wavelengths region. The development of the standoff detection system by measuring the DUV light contributes to identify the various materials remotely. In this study, the design results of the Raman scattering light receiving equipment (light receiving system) consisting of telescope, relay optical system and spectrometer were considered. The optical design of the light receiving system was performed based on the estimation model of the signal-to-noise ratio (SNR) and the ray tracing method. The important parameters (the attenuation of the laser pulse and Raman light in the atmosphere, the amplification factor of the scattering cross section in DUV wavelength region etc.) are included in the model, and SNR of the Raman spectrum can be evaluated by changing each parameter. The estimation results suggest that SNR depends strongly on the reflectance of mirror and the amplification factor of the scattering cross section in DUV wavelength region. The durability test of the mirror has been performed to evaluate the use in outdoor. The results show that the mirrors with high reflectance in DUV wavelength region have enough tolerance to the temperature change and the scratch.
Laser Emission of a blue ECDL at a single transverse mode (TEM<sub>00</sub>) was confirmed and Raman spectra of O<sub>2</sub> and N<sub>2</sub> in the atmosphere were acquired using the intra-cavity beam of the ECDL. The center wavelength of the diode laser is 416 nm in the blue region of the visible spectrum. A Fabry-Perot resonator composed of two high reflectivity mirrors was used as an external cavity and a fraction of the intra-cavity beam was feedbacked to the anti-reflection coated facet of the diode laser. The optical feedback makes it possible that the diode laser oscillates stably at a specific frequency in the gain band of the laser diode chip. This configuration includes only four optics and has potential to be utilized as a compact Raman gas sensor that can be mounted on drones or the other machines. The detection limits in 1 second integration time were estimated to be lower than 100 ppm in the case of N<sub>2</sub> gas and 200 ppm in O<sub>2</sub>, respectively.
In this study, a hydrogen leak simulation apparatus was made, and then hydrogen diffusion behavior in the ground and in the atmosphere was measured, in order to clarify how to detect leaked hydrogen or respond after the leakage. The hydrogen leak simulation apparatus was composed of an underground simulation tank of 7 m in diameter and 1.35 m in depth, and an atmospheric simulation tank of 8 m wide x 8 m depth x 3 m height. The pipe was set in the underground simulation tank at the burial depth 1.2 m. A pinhole of 1 mm in diameter was made on the buried pipe, and the diffusion behavior of hydrogen gas released from the pinhole was measured. For the diffusion behavior in the ground, the concentration distribution was measured by 40 sensors buried in decomposed granite soil and crushed stone. For the diffusion behavior in the atmosphere, the concentration distribution was measured by Raman imaging. The hydrogen gas passing through the asphalt and diffusing into the atmosphere was irradiated with the third harmonic generation from the Nd: YAG laser, and the Raman scattering light was visualized by a high sensitivity camera. The characteristics of the hydrogen gas diffusion behavior were found. In addition, the hydrogen diffusion behavior was reproduced by simulation analysis, and compared with the experimental results. As a result, it is confirmed that the simulation of the diffusion behavior in the ground and in the atmosphere is valid even under the condition with pavement.
For Lidar technology that can identify the location of the target substances and measure spatial distribution, the establishment of that technology is required so that it can comprehensively provide remote measuring hazardous substances that cause harm to human bodies, such as toxic substances and combustible substances. Hazardous substances exist in a very wide range of forms, for example, chemical species, physical conditions, and organisms or inorganisms. In addition, substances developed for the purpose of attacking the human body, represented by nerve agents, exhibit their effects by a small amount. Therefore, in order to realize remote sensing of hazardous substances, it is necessary to apply an excellent measurement principle that can respond to the diversity and the detection of trace components of these objects. The Raman effect is a useful phenomenon that enables identification of many individual substances, but the extremely weak response has led to significant limitations in applicable fields. In this study, we conducted basic experiments for the realization of remote sensing technology of hazardous substances based on the resonance Raman effect. The resonance Raman effect is a phenomenon in which the intensity of Raman scattering light is greatly enhanced by excitation with light of a wavelength corresponding to the electronic transition energy of the target substance. The presence of electronic transition energy of substances can be confirmed by observing the ultraviolet absorption spectra. Many hazardous substances exhibit ultraviolet absorption in the deep ultraviolet wavelength region of 300 nm or less. Therefore, in this study, we constructed a resonance Raman spectrum measuring device capable of wavelength sweeping in the deep ultraviolet wavelength range, selected SO<sub>2</sub> and NH<sub>3</sub>, typical corrosive gases, as target substance, and verified experimentally the enhancement of Raman signal intensity by resonance Raman effect.
As a fundamental study for improving the detection accuracy of Raman spectroscopy under noisy conditions, this paper proposes a novel spectrum decomposition method, where the observed spectrum from an unknown substance is decomposed into some known spectra. Raman spectroscopy can be used for a remote sensing method, where a laser is irradiated to the target and then the Raman scattering light is analyzed to detect the target constituents. The spectrum decomposition is the method to analyze the observed spectrum, that is the Raman scattering light, with some known spectra, which are previously developed as a database. The purpose of the decomposition is to find a linear combination of the known spectra so that the linear combination appropriately represents the observed spectrum. The coefficients of the linear combination show the density of molecules contained in the target. The coefficients can be found with multiple linear regression method. However, the coefficients can contain large errors under low signal-noise-ratio conditions. The proposed method tries to overcome the noise problem by using three techniques. The first technique is to employ the nonnegative least squares method, which is the least squares method with non-negative constraints for the coefficients. The second technique is to select the wavelengths of the observed and known spectra for the spectrum decomposition. The third technique is to select the wavelength of the laser irradiated to the target. This paper conducts numerical experiments to show the effectiveness of the proposed method.
Hydrogen is expected to become an energy source in the next generation. Although hydrogen gas is a combustible gas
with a large explosion concentration range, leakage is presently monitored by contact type gas sensors. The technology
for locating a leak and remote sensing of gas concentration distribution is required in case of hydrogen gas leaks. In this
study, remote sensing technology of hydrogen gas concentration distribution using a Raman lidar was developed. The
lidar system consisted of a pulsed Nd:YAG laser of wavelength 354.7 nm and a Galilean telescope of aperture 170 mm.
The system could detect hydrogen gas by vibrational Raman scattering. In this method, hydrogen gas concentration
could be measured based on the ratio of the Raman scattering signals from hydrogen gas and from atmospheric nitrogen,
which were simultaneously measured. In this manner, the geometrical form factor of the biaxial lidar and the
instrumental function were canceled. Hydrogen gas concentration of 0.6-100% could be measured at a distance 13m
using this system.