Recent progress in tunable optical sources has led to the development of remote and imaging detection of hazard leak gases into the atmosphere. In this paper, we describe basic principles and requirements of the laser sensing systems of the leak gases for safety and environmental surveillance applications.

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In recent years, a requirement has emerged for cost-effective techniques for the detection of leaks of methane gas, the principle ingredient of natural gas. To realize such techniques, laser absorption spectroscopy at the absorption line of methane has been intensively researched. The author has developed two types of equipment for methane gas leak detection using a compact and low-cost near-infrared diode laser (InGaAsP DFB laser,λ=1.65372μm). One system can survey the leaks in underground gas pipes while driving along the roadabove the pipes. The other can provide remote detection and pinpointing of the leaks in exposed gas pipes. To accomplish highly sensitive detection, despite weak absorption in near-infrared range, these two equipments employ second-harmonic detection of wavelength modulation spectroscopy.

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A portable sensor has been developed that is capable of producing real-time video images of fugitive methane emissions. Active imaging is performed using a pulsed optical parametric generator, OPG, that illuminates the scene at wavelengths corresponding to an absorption feature of methane near 3.27 um. The imager is capable of displaying single-wavelength images as well as differential-wavelength images. The transmitted radiation has a fan-shaped beam profile, and the radiation backscattered from surfaces is imaged onto a single row of detectors on a focal plane array. By sweeping the field-of-view, a two-dimensional image is formed in which a methane plume appears as an area of attenuation. Two pulses offset in wavelength can be transmitted to produce a differential image that suppresses background features and enhances gas plume visibility. The ima-ger can be operated with a battery power source and carried by a single operator. Tests were conducted at leak-detection training facilities in Japan, and results are presented here.

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Detection of leaks in a busy urban environment is an important problem. Walking surveys do not easily inspect street locations. Extractive and fixed path optical vehicle mounted system do not survey the entire street width. The Japan Gas Association in support of the Japan National R & D Project Against Pipeline Leaks undertook the development of a differential absorption LIDAR (DIAL) system for sensitive detection of methane in natural gas leaks. This paper presents the concept of this laser system, receiver, processing and display and its implementation. The compact laser system, electronics and all support hardware were mounted inside a Toyota RAV-4 vehicle and delivered to JGA in early 2004. The DIAL system was designed to image the street surface directly in front of the vehicle. A 4-m wide by 4-m deep footprint is imaged with 0.5 m by 0.5 m resolution starting approximately 2.5 meters in front of the RAV4. This DIAL system determines methane leak concentration by comparing the measured reflectance from two sets of laser pulses separated slightly in wavelength. The ratio of the absorbed to non-absorbed light determines the concentration. This system was designed to detect leaks 10 ppmV/m and greater. Our system was tested with various calibrated methane-filled pillows representing severaleak equivalences

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Detection of atmospheric methane is based on a huge amount of fundamental data provided by high-resolution spectroscopy in the infrared region. Doppler-limited and sub-Doppler resolution spectroscopy of methane have been discussed in particular for the 3.3 and 1.67μm band transitions. To attain sub-Doppler resolution in the 1.67μm band transitions, we employed a Fabry Perot cavity as an absorption ce11. This enabled us to observe nonlinear saturation spectra with a spectral resolution of about 1 MHz even using the weak optical field from a laser diode, because the optical field strength in the cavity is greatly enhanced at the antinodes of the optical standing wave. We precisely measured small Stark shifts of seven tetrahedral components of the vibration rotation transitions, sixty-six difference frequencies between the components, and the absolute frequency of two transitions. To enhance the sensitivity, noise-immune cavity-enhanced optical- heterodyne molecular spectroscopy (NICE-OHMS) was also applied.

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We demonstrate the generation of femtosecond pulses with targeted time-dependent polarization. In order to shape desired pulses, we feed back analyzed results from a homemade polarization characterization system. The characterization of momentary polarization is based on the Fourier transform analyses of dual-channel spectral interferometry. The genetic algorithm (GA) is employed to find optimum modulation parameters for the pulse shaper. We successfully shape femtosecond pulses whose ellipticity increases at a constant rate. With the target rate of 3.44×10^-3 fs^-1, we achieve pulses with the rate of 3.24×10^-3 fs^-1 at the 150 th generation of GA. The relative error between the shaped pulse and the target pulse is less than 6% over the main part of the pulse. This shows that our shaping technique is capable of shaping femtosecond pulses with desired timedependent polarization. Time-dependent polarization pulses have many potential applications as discussed in the text.

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A bi-directional fiber ring laser with a SOA (semiconductor optical amplifier) has been developed in which a frequency difference between the counter-propagating oscillations is controlled by an introduced birefringent medium. With the material which causes the retardation change by a physical phenomenon to be measured, the laser in this study is applicable to a novel fiber sensor of which the sensing signal is obtained in a frequency domain and its application for temperature sensing has been demonstrated.

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