When change of hydrogen(H₂) gas concentration in a certain point is measured, non-contact measurement technology with high temporal and spatial resolution is necessary. In this study, H₂ concentration in the small area of <1cm² under the gas flow was measured by using a Raman lidar. Raman scattering light at the measurement point of 750mm ahead was detected by the Raman lidar. As a result, it was proved that the H₂ concentration of more than 100ppm could be successfully measured.

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 SO₂ and NH₃, typical corrosive gases, as target substance, and verified experimentally the enhancement of Raman signal intensity by resonance Raman effect.

When change over time of hydrogen gas concentration in a certain point is measured, a gas sensor is fixed at the measurement point, or a syringe is utilized to sample the gas, which is analyzed by gas chromatography. However, the gas sensor and the syringe have an influence on gas flow and its measurement. Non-contact measurement technology with high temporal resolution and high spatial resolution is necessary. Therefore, in this study, for the purpose of non-contact measurement of the hydrogen gas concentration in the small area under the gas flow, a specialized experimental device was developed. A laser beam (wavelength 355nm, pulse energy 120µJ, PRF 1kHz) with the beam diameter 1mmϕ at the measurement spot was irradiated to the hydrogen gas. Raman scattering light at the measurement point of 750mm ahead was focused by Fresnel lens, which is arranged at the orthogonal direction against the transmitting laser beam. As a result, it was proved that Raman signal intensity had good linear correlation with hydrogen density of more than 100ppm. The gas concentration in a small area could be quantitatively estimated without interference of the gas flow.