Tetsuo Aoki

Coherent 2 μm differential absorption and wind lidar with conductively cooled laser and two-axis scanning device

A coherent 2 microm differential absorption and wind lidar (Co2DiaWiL) was developed to measure CO(2) concentration and line-of-sight wind speed. We conductively cooled a pumping laser head to -80 degrees C and diode arrays to approximately 20 degrees C. A Q-switched laser outputs an energy of 80 mJ (pulse width 150 ns (FWHM), pulse repetition frequency up to 30 Hz). CO(2) measurements made over a column range (487-1986 m) for 5 min accumulation time pairs achieved 0.7% precision. Line-of-sight wind speeds for ranges up to approximately 20 km and returns from a mountainside located 24 km away from the Co2DiaWiL were obtained.

Earth-satellite-Earth laser long-path absorption experiment using the Retroreflector in Space (RIS) on the Advanced Earth Observing Satellite (ADEOS)

This paper reports the results of the laser long-path absorption experiments carried out with the Retroreflector in Space (RIS) on the Advanced Earth Observing Satellite (ADEOS). The RIS is a 0.5 m diameter single-element hollow retroreflector with a unique optical design which uses a curved mirror surface to correct velocity aberrations caused by the satellite movement. In the RIS experiments a laser beam was transmitted from a ground station, reflected by the RIS, and received back at the ground station. The absorption of the intervening atmosphere was measured in the round-trip optical path. After the launch of the ADEOS in August 1996, the optical characteristics of the RIS were tested, and it was confirmed that the RIS worked well in orbit. The spectroscopic measurement was carried out with the single-longitudinal-mode TEA lasers by means of the method utilizing the Doppler shift of the reflected beam caused by the movement of the satellite. The spectrum of ozone was successfully measured in the region, and the measurement of the column contents of ozone was validated with the simultaneous heterodyne spectrometer measurement. In June 1997, however, the experiment with the RIS was discontinued due to the malfunction of the ADEOS solar paddle.

Laser transmitter/receiver system for earth-satellite-earth long-path absorption measurements of atmospheric trace species using the retroreflector in space

A laser transmitter/receiver system using a retroreflector in space (RIS) to experiment with earth-satellite-earth laser long-path absorption measurements of atmospheric trace species is described. The system employs two single-longitudinal-mode pulsed CO2 lasers to measure the atmospheric spectrum and a second-harmonic Nd:YAG laser for active satellite tracking using the image of the RIS reflection. Details of the laser system and the initial experiment are reported.

Conductive-cooled 2micron laser for CO2 and wind observations

We have developed 2micron Q-switched lasers with conductive-cooled side pumped laser head. Q-switched output of 100mJ at 20Hz has been achieved in a Tm,Ho:YLF laser oscillator with a composite rod. The experiments showed that very good thermal conduction is held between rod and heat sink. This laser head may be used for laser amplifier of 500mJ output. Another type of Tm,Ho:YLF laser head has been developed for the oscillator of 50-100mJ output at 20-40Hz. These 2micron lasers are applied to coherent lidar systems, which will be used to measure atmospheric CO2 and wind profiling.

Development of coherent 2-μm differential absorption and wind lidar with laser frequency offset locking technique

We developed a coherent 2-μm differential absorption and wind lidar to measure CO2 concentration and line-of-sight wind speed. The wavelength of on-line laser was set at the R30 absorption line center of CO2 and the atmospheric transmission for the on-line backscattered signal caused by CO2 is large. Measurable range of CO2 measurement was limited. A laser frequency offset locking system was installed into the laser system to improved measurable range of CO2 measurement. Two single-frequency continuous wave lasers are used for the laser frequency offset locking. One laser (center, λCenter) of the two continuous lasers is directly locked to the R30 absorption line center of CO2 and the other (on-line, λOn) is frequency-shifted to λCenter laser. Although long-range CO2 measurement depends on the laser frequency offset, the installation of the laser frequency offset realized vertical CO2 measurement in a range of up to the upper troposphere.

Development of 2-micron airborne coherent Doppler lidar at NICT

We have studied a 2-micron airborne coherent Doppler lidar to observe wind profile downward from flying object. We investigated the algorithms required to extract the Doppler-shifted frequency compensating for a speed of the flying object. The airborne experiments were conducted to demonstrate the feasibility of the airborne coherent Doppler lidar from a flying object in 2002, 2004 and 2006. We extracted the Doppler-shifted frequency corresponding to aircraft speed with developed algorithms and obtained wind profiles through airborne experiment. To examine wind profiles measured by the airborne coherent Doppler lidar, we compared those profiles with profiles measured by a GPS-dropsonde and a windprofiler. Although the volume measured by the airborne coherent Doppler lidar system differed spatially and temporally from those by other instruments, the wind profiles observed by the airborne coherent Doppler lidar agreed well with those observed by other instruments.

Development of coherent Doppler lidar for wind profiling

Space-borne Doppler lidar is expected to make wind profile observations on a global scale with an accuracy of 1 to 2 m/s. It may solve the problem of the shortage of the accuracy and distribution in the current wind data. We have studied an eye-safe coherent Doppler lidar (CDL) model that could be deployed on the exposed facilities of Japanese Experiment Module (JEM) and that would meet the science requirements. We have good prospects of 500mJ output at 10Hz in a conduction cooling sub-scale laser, which could be a small model of space-borne laser for JEM/CDL. We are making studies on improving the system’s efficiency, reducing its weight, and establishing the fundamental technologies involved. Research on another possibility, e.g. a free flyer, for a demonstration mission besides of JEM/CDL is also valuable to be considered. Development of algorithm for application of coherent lidar system is also in progress through air-borne experiments and ground-based observations.

Development of coherent Doppler lidar at CRL

Global wind profiling with a space-borne Doppler lidar is expected to bring big progress in the studies on global climate change and Numerical Weather Prediction. A feasibility study has been done for an eye-safe 2micron coherent Doppler lidar aiming at demonstration of the technology onboard the Japanese Experiment module of the International Space Station. We are now developing an airborne coherent Doppler lidar system to measure wind profile under a jet plane for simulation of the Doppler lidar measurement in space. This system is also operated in the ground to develop algorithm of the wind measurements and the results of the wind profiles are compared with those derived from other instruments.

Plan for the experiment with the retroreflector in space (RIS) on ADEOS

Experiments on the earth-satellite-earth laser long-path absorption measurements of atmospheric trace species will be carried out with the retroreflector in space (RIS) for the Advanced Earth Observing Satellite (ADEOS). The RIS is a single-element hollow retroreflector with an effective diameter of 0.5 m, which was designed for spectroscopic measurement in the infrared region. The ground system for the experiment employs two single-longitudinal-mode pulsed CO2 lasers. High-resolution atmospheric absorption spectra are measured by using the Doppler shift of the return beam caused by the satellite movement. Vertical profiles of O3 and CH4, and column contents of CFC12, HNO3, CO, N2O, etc. will be obtained from the measured spectra.

New automated lidar system and multimedia virtual laboratory

We are developing a new remote-operational lidar system, with multimedia technology. This system reduces the necessity to go to the remote place. The system is located in Rikubetsu (43.3N, 143.5E), in north portion of Japan, which is a famous place for a fair weather all the year round. We use SHG of Nd:YAG laser and receive P, S components of Mie back scatter with photon counter. All necessary controling of instruments such as the roof of the housing, tilt mirror, laser, photon counters, oscilloscope, and so on, are done by personal computers. Operator uses a web browser from a remote site to operate the lidar system, with monitoring the weather and the total state of the system. There is another workstation which runs a newly developed desktop video conference system. With these system, several persons can simultaneously observe and watch the data, discuss with each other. We can now observe the aerosols more frequently than ever. This will let us to get precise information about the aerosols in north regions of Japan.