Yasukuni Shibata

Development of a 1.6 μ m differential absorption lidar with a quasi-phase-matching optical parametric oscillator and photon-counting detector for the vertical CO 2 profile

We have developed a 1.6 microm carbon dioxide (CO(2)) differential absorption lidar utilizing a quasi-phase-matching optical parametric oscillator (OPO) and a photon-counting detector. The operating wavelengths were chosen based on their low interference from water vapor and low temperature sensitivity. The online wavelength was in the (30012<--0001) band of CO(2), which was insensitive to atmospheric temperature. The established OPO laser achieved a 10 mJ, 200 Hz repetition rate at the online and offline wavelengths. Our observations confirmed the statistical error of 2% with 5 h of accumulation for the CO(2) density profile less than 5.2 km. Also, the statistical error of 1% at an altitude of 2 km was demonstrated. The results of the vertical CO(2) concentrations acquired using a 1.6 microm wavelength are presented.

Stimulated Raman Scattering Laser Oscillation around 1.6 µm Carbon Dioxide Absorption Line

A solid-state stimulated Raman scattering (SRS) laser oscillation around the 1.6 µm carbon dioxide absorption lines is demonstrated. The stokes output of the SRS radiation at 1.57 µm is generated from the frequency conversion of the 1.35 µm laser radiation of Nd3+:KGd(WO4)2 (Nd:KGW) in the cavity. The maximum output energy was 13.8 mJ with a repetition rate of 10 Hz, in response to the incident laser pumped from the laser diode to the Nd:KGW. To our knowledge, this result of a 1.57 µm intracavity SRS oscillation at CO2 absorption lines around 1.6 µm is gained for the first time.

System Evaluation of an Incoherent Wind Doppler LIDAR Using an Iodine Filter

This study is carried out system error evaluation of an incoherent wind Doppler LIDAR system that uses an iodine filter. Sharp slopes of an iodine filter provide better wind-measuring sensitivity than a Fabry–Perot etalon. The magnitude and sign of the Doppler shift are obtained from the ratio of the intensities of two signals arising from two slopes of an iodine absorption line. Systematic errors of wind measurement are caused mainly by the fluctuations of laser frequency and iodine absorption linewidth. The standard deviation of the zero Doppler shift is experimentally evaluated to be 1.8 MHz, which corresponds to the uncertainty in line-of-sight wind velocity of 0.5 m/s. This result shows a good agreement with the result of an experiment of the tuning stability of laser frequency with respect to the absorption line slope. Moreover, comparison of wind profiles is made between the Doppler LIDAR and concurrent radiosonde measurements. The measured horizontal wind speeds are from 8 to 21 m/s in the altitude range from 8 to 25 km, indicating a reasonable agreement between the two instruments. Experimentally, the total optical efficiency of this Doppler LIDAR system is estimated to be 4.0%, which is close to the value expected from the evaluation of each component of the instrument.

Measurement of Pressure-Induced Broadening and Shift Coefficients of Carbon Dioxide Absorption Lines around 1.6 µm for using Differential Absorption Lidar

A precise measurement of the vertical profiles of carbon dioxide is required for reducing the uncertainty in the carbon budget. In order to achieve measurements of the vertical CO2 distribution with an uncertainty better than approximately 4 ppm, a precise knowledge of the pressure-dependent broadening and shift coefficients of CO2 absorption lines is indispensable. In this paper, we report the measurement of air pressure-induced shift coefficients for eight absorption lines at around 1.57 µm. On average, the pressure shift coefficients are -0.30 MHz/Torr for pure CO2 and -0.24 MHz/Torr under an air-mixed condition.

Direct detection 1.6μm DIAL for measurements of CO2 concentration profiles in the troposphere

Inverse techniques using atmospheric transport models are developed to estimate the CO2 sources and sinks based on the observed data. In comparison with the ground-based monitoring network, CO2 measurements for vertical profiles in the troposphere have been due to the limited observations by using campaign-style aircrafts and the commercial airlines with limited spatial and temporal coverage. The differential absorption lidar (DIAL) with the range resolution is expected to bring several advantages over passive measurements, for example, daytime coverage and neglecting influences of aerosol and cirrus layers. We have succeeded to develop the 1.6 μm DIAL technique using direct detection method for measurement of CO2 concentration profiles in the atmosphere. This paper describes the advanced CO2 1.6 μm DIAL technique consisting of the optical parametric generator (OPG) transmitter (10mJ/pulse) that excited by the LD pumped Nd:YAG laser with high repetition rate (500Hz) and the receiving optics that included the large telescope with 60cm diameter and the photomultiplier tube with high quantum efficiency (~8%) operating at the photon counting mode and the narrowband interference filter (0.5nm bandwidth) for daytime observations. The CO2 concentration profiles from ground to an altitude of 12km are conducted to measure with better than 1% standard deviation using 500m bins by this CO2 DIAL.

Incoherent Doppler lidar using two wavelengths for wind measurement

A new incoherent Doppler lidar technique for atmospheric wind measurement is presented. This direct detection Doppler lidar is realized by the transmitting laser with two wavelengths and the receiving system with only one bandpass or absorption filter. We fabricated actually the Doppler lidar system using the frequency doubled Nd:YAG laser which was alternately tuned to the both slopes of one absorption line of the molecular iodine with an AO wavelength shifter, and performed simultaneous measurements of the eastward and northward wind components from 8 to 25km altitude with 500m vertical resolution. This Doppler lidar system is very useful to measure the wind profiles between lower troposphere and stratosphere by one effort.

A direct detection 1.6μm DIAL with three wavelengths for high accuracy measurements of vertical CO2 concentration and temperature profiles

The accurate vertical CO2 profiles in the troposphere are highly desirable in the inverse techniques to improve quantification and understanding of the global budget of CO2 and also global climate changes. Moreover, wind information is an important parameter for transport simulations and inverse estimation of surface CO2 flux. A differential absorption lidar (DIAL) is an attractive method for obtaining vertical CO2 profiles and we have developed an 1.6μm DIAL system to perform simultaneous measurements of CO2 concentration, atmospheric temperature profile and wind profile. The absorption cross sections of gas and air density depends on atmospheric temperature and pressure. Then precise temperature and pressure profiles are necessary for accurate CO2 mixing ratio measurement by DIAL. Laser beams of three wavelengths around a CO2 absorption line are transmitted alternately to the atmosphere for simultaneous measurements of CO2 concentration and temperature. The receiving optics include the near-infrared photomultiplier tube and a fiber Bragg grating (FBG) filter to detect a Doppler shift.

Low-Altitude Wind Measurement Using Incoherent Doppler Lidar by Utilizing Both Absorption Slopes of the Iodine Absorption Spectrum

In this study, we perform wind observation in the lower troposphere using the incoherent Doppler wind lidar that we previously developed for the upper troposphere and stratosphere. This Doppler lidar uses both slopes of the iodine absorption spectrum, and the Doppler shift is obtained from the ratio of the two transmission signals. Because this ratio is influenced by Mie contamination, a precise Mie measurement improves the quality of wind observation in the Mie contamination region. If the iodine absorption spectrum completely absorbs the Mie signal, wind measurement will be independent of Mie contamination. The effect of Mie contamination on wind measurement by Mie-Rayleigh backscattering is quantified and the need for the absorption of the Mie signal is assessed. Moreover, we compare the wind profiles measured in the Mie contamination region using the Doppler wind lidar and a wind profiler. The average wind difference between the lidar and the wind profiler is 1.39 m/s. The Mie contamination was completely excluded by the iodine absorption spectrum and lidar comparison experiment demonstrated the high feasibility and good capability of the lidar for measuring wind in the Mie contamination range.

Development of airborne DIAL for water vapor measurement

For the future spaceborne water vapor DIAL system, an airborne differential absorption lidar (DIAL) system in the near infrared has been developed. An injection seeded triple pulse Ti:Sapphire laser is used for the transmitter and an LD pumped conductive-cooled Nd:YLF laser is employed for the pump laser of the Ti:sapphire laser. The entire system including the receiving telescope, receiving optics, an APD detector and signal processing system are developed. The system was onboard the Beechcraft B200 aircraft and flight tests of the system were conducted at three nights of November 1999.

Multipurpose lidar system for observations of equatorial atmosphere

We have constructed the lidar facility for survey of atmospheric structure over troposphere, stratosphere, mesosphere and low thermosphere over Kototabang (100.3E, 0.2S), Indonesia in the equatorial region. The lidar system consists of the Mie and Raman lidars for tropospheric aerosol, water vapor and cirrus cloud measurements, the Rayleigh lidar for stratospheric and mesospheric temperature measurements and the Resonance lidar for metallic species such as Na, Fe, Ca ion measurements and temperature measurements in the mesopause region. The laser system included in this lidar facility consists of three pulsed Nd:YAG lasers, a pulsed Ti:Sapphire laser seeded by a ring Ti:Sapphire laser and a dye laser. And, the receiving system consists of a Schmidt-Cassegrain telescope with 20cm diameter, a Schmidt-Cassegrain telescope with 35cm diameter and five Newtonian telescopes with 45cm diameter. The most parts of this lidar system are remotely controlled via the Internet from Tokyo Metropolitan University (TMU) in Japan.