Hideo Tai

A portable remote methane sensor using a tunable diode laser

A portable remote methane sensor using a 1.65 µm InGaAsP distributed-feedback laser is developed. It is designed as a man-portable long-path absorption lidar using a topographical target with a range of up to about 10 m. An operator can search for gas leaks easily by scanning the laser light. High sensitivity is accomplished by means of second-harmonic detection using frequency-modulation spectroscopy. The experimental detection limit (signal-to-noise ratio = 1) with a diffusive target of magnesium oxide (6 m range, normal incidence) is 450 ppb m with a time constant of 100 ms. Measurements of the reflectance of real targets show that the sensor can distinguish small gas leaks (typically 10 cm3 min -1) within a range of several metres.

Remote detection of methane with a 1.66-μm diode laser

High-sensitivity real-time remote detection of methane in air with a 1.66-microm distributed-feedback diode laser operating at room temperature is demonstrated by laboratory simulations. The laser current was modulated at a high frequency of ~5 MHz, and the laser-center frequency was locked onto a methane-absorption line. The laser light directed toward the probed region was received after one-way transmission or further reflection from a topographic target. The methane absorption was detected by the second-harmonic component in the optical-power variation. The minimum-detectable concentration-path-length product in the transmission scheme was 0.3 part in 10(6) m for a signal averaging time of 1.3 s. In the reflection scheme, the amount of methane could be measured from the ratio of the fundamental and second-harmonic signal intensities independently of the received power.

Long-distance simultaneous detection of methane and acetylene by using diode lasers coupled with optical fibers

Remote simultaneous detection of methane and acetylene by using two single-mode diode lasers emitting at 1.66 and 1.53 mu m, respectively, coupled with 4-km optical fibers has been demonstrated by laboratory simulations. The lasers were modulated at different frequencies to measure in real time the absorption of the two chemical species without interference. The gases could be detected quantitatively by the ratio of the phase-sensitive-detected fundamental and second-harmonic signals. When the absorption pathlength was 10 cm, the minimum detectable concentrations of methane and acetylene in air were 5 and 3 p.p.m., respectively, for a signal averaging time of 1 s.<<ETX>>

Real-time monitoring of environmental methane and other gases with semiconductor lasers: a review

Abstract We have developed a gas-monitoring system using distributed-feedback semiconductor lasers in the 1.3–1.7 μm wavelength region. The principle, performance and applications of this system are reviewed in this paper, together with recent improvements achieved in a sturdy portable model. By wavelength modulation of the single-mode semiconductor lasers, in combination with a second-harmonic detection technique, a minimum detectable absorption of as low as 3 × 10 −6 is attained, which corresponds to a concentration-pathlength product of 70 ppb m for methane. This high sensitivity and the portability of the system have made various real-time sensing measurements of methane and other molecules possible, such as the precise monitoring of atmospheric gases, field measurements of methane emission rates, itinerant detection of leaks from pipelines and isotope-ratio analyses. We also investigate theoretically the effects of the temperature and pressure of a gas sample on the signal intensity in second-harmonic detection, so that we can precisely determine the gas concentration from a measurement.

Tunable solid-state UV lidar system for NO monitoring

A differential absorption lidar (DIAL) system, based on a tunable solid-state Ti:sapphire laser, was developed for nitric oxide monitoring in the UV region. The UV light was generated through second-harmonic generation (SHG) of sum- frequency mixing between a Ti:sapphire laser, pumped by the SH of a Nd:YAG laser, and the fundamental of a second Nd:YAG laser. The output pulse energy of 4 mJ at 226.8 nm with 2 micrometers linewidth was achieved. As a demonstration of DIAL capability, the spatial distribution of nitric oxide emission from a diesel engine was investigated. The detection error achieved was +/- 0.05 ppm up to a distance of 180 m, with the spatial resolution of 20 m.

Tunable, UV Solid-State Lidar for Measurement of Nitric Oxide Distribution

A solid-state differential absorption lidar (DIAL) system was developed for nitric oxide monitoring. The ultraviolet (UV) light was generated through second-harmonic generation, subsequent to the sum-frequency mixing of the pulses from a Ti:sapphire laser, pumped by a frequency-doubled Nd:YAG laser, and from the fundamental of another Nd:YAG laser. The output pulse energy achieved in this way was 4 mJ at 226.8 nm with 2 pm linewidth. This DIAL system made it possible to measure the spatial distribution of nitric oxide emitted from a diesel engine installed at a distance of 130 m from the lidar for demonstration of stack plume, with the detection accuracy of 0.065 ppm with a range resolution of 10 m. We also measured the background nitric oxide of the order of 10 ppb in the urban atmosphere.

Analysis of atmospheric NO x distribution in an urban area by solid state DIAL technique

Atmospheric NOx distribution in an urban area of Japan in winter season were measured by a tunable solid-state lidar system based on a Ti:sapphire laser and a Nd:YAG laser. We show the possibility of long-time air pollution monitoring. We also discuss the reason of the change of NOx concentration by weather conditions.

NO Monitoring by a Tunable Solid-State UV

A differential absorption lidar (DIAL) system, based on a tunable solid-state Ti:sapphire laser, was developed for nitric oxide monitoring in the UV region. The UV light was generated through second-harmonic generation (SHG) of sum-frequency mixing (SFM) between a Ti:sapphire laser, pumped by the SH of a Nd:YAG laser, and the fundamental of a second Nd:YAG laser. As a demonstration of DIAL capability, the spatial distribution of NO emission fromadiesel engine was investigated.

Atmospheric NOx distribution monitoring in urban areas using a tunable solid state lidar

Atmospheric NO and NO2 distribution in urban area near the capital city of Japan were measured by an all solid-state differential absorption lidar (DIAL). These were measured over 15 hours in December 1996. According to the wind speed, the concentration show temporal variation in a NO2 case. NOx analyzer data shows a similar behavior to the result measured by the lidar.

Estimation of various atmospheric constituent measurements by using solid state tunable lasers

Detectivities of differential absorption lidar (DIAL) using solid state lasers have been evaluated. Harmonic-generation and mixing of a Ti:Al/sub 2/O/sub 3/ laser and a Nd:YAG laser was considered as a light source, because of its wide-range wavelength tunability from ultraviolet to infrared. Twelve gases were selected for the evaluations because of their importance for the global environment, atmospheric pollution, and industrial gas leakage. As a result, detection limits of H/sub 2/O, CH/sub 4/, and NO/sub 2/ were 4.2 km, 4.1 km, and 1.8 km for average atmospheric concentrations, assuming that S/N=10 is the limit.<<ETX>>