Shinichi Ueno

Development of 1.6 μm continuous-wave modulation hard-target differential absorption lidar system for CO 2 sensing

We have demonstrated the 1.6 mum cw modulation hard-target differential absorption lidar system for CO(2) sensing. In this system, ON and OFF wavelength laser lights are intensity modulated with cw signals. Received lights of the two wavelengths from the hard target are discriminated by modulation frequencies in the electrical signal domain. The optical circuit is fiber based, and this makes the system compact and reliable. It is shown that a stable CO(2) concentration measurement corresponding to a fluctuation of 4 ppm (rms) (ppm is parts per million) has been achieved in 32 s measurement intervals and the 1 km path.

Feasibility study on 1.6 μm continuous-wave modulation laser absorption spectrometer system for measurement of global CO 2 concentration from a satellite

A feasibility study is carried out on a 1.6 μm continuous-wave modulation laser absorption spectrometer system for measurement of global CO(2)concentration from a satellite. The studies are performed for wavelength selection and both systematic and random error analyses. The systematic error in the differential absorption optical depth (DAOD) is mainly caused by the temperature estimation error, surface pressure estimation error, altitude estimation error, and ON wavelength instability. The systematic errors caused by unwanted backscattering from background aerosols and dust aerosols can be reduced to less than 0.26% by using a modulation frequency of around 200 kHz, when backscatter coefficients of these unwanted backscattering have a simple profile on altitude. The influence of backscattering from cirrus clouds is much larger than that of dust aerosols. The transmission power required to reduce the random error in the DAOD to 0.26% is determined by the signal-to-noise ratio and the carrier-to-noise ratio calculations. For a satellite altitude of 400 km and receiving aperture diameter of 1 m, the required transmission power is approximately 18 W and 70 W when albedo is 0.31 and 0.08, respectively; the total measurement time in this case is 4 s, which corresponds to a horizontal resolution of 28 km.

Performance improvement and analysis of a 1.6 μm continuous-wave modulation laser absorption spectrometer system for CO 2 sensing

In a previous study, we developed a 1.6 μm continuous-wave (cw) modulation laser absorption spectrometer system for CO(2) sensing and demonstrated the measurement of small fluctuations in CO(2) corresponding to a precision of 4 parts per million (ppm) with a measurement interval of 32 s. In this paper, we present the process to achieve this highly specific measurement by introducing important points, which have not been shown in the previous study. Following the results of preliminary experiments, we added a function for speckle averaging on the optical antenna unit. We additionally came up with some ideas to avoid the influences of etalon effects and polarization dependence in optical components. Because of the new functions, we realized a calibration precision of 0.006 dB (rms), which corresponds to a CO(2) concentration precision of less than 1 ppm for a 2 km path. We also analyzed the CO(2) sensing performance after the improvements described above. The measured short time fluctuation of the differential absorption optical depth was reasonably close to that calculated using the carrier-to-noise ratio of the received signal.

All-solid-state high-power conduction-cooled Nd:YLF rod laser

A high-average-power conduction-cooled diode-pumped Nd:YLF rod laser has been developed. A new conduction-cooled side-pumping scheme with a solid prismatic pump-light confinement cavity was employed. A transparent, high-thermal-conductivity MgF>(2) prism was used as a highly efficient pump cavity as well as a low-thermal-resistance heat spreader. The pumping efficiency and thermal resistance of the cavity were 85% and 0.20 degrees C degrees W, respectively. When this scheme was combined with heat pipes for heat removal, a maximum average output power of 72 W was demonstrated, with an optical slope efficiency as high as 49%.

Laser absorption spectrometer using frequency chirped intensity modulation at 1.57 μm wavelength for CO 2 measurement

We have demonstrated the laser-absorption spectrometer system using frequency chirped intensity modulation at 1.57 μm wavelength for measurement of CO(2) concentration. Using this technique, backscattered laser radiation from different ranges can be discriminated in the frequency domain of the electrical signal. We have reported the discrimination of two signals from the targets with different ranges. It is shown that stable measurements with short time fluctuation corresponding to 4 ppm (rms) were obtained with 32 s measurement intervals. Furthermore, there is qualitative good agreement on, at least, the diurnal changes between the results of the laser absorption spectrometer system and the in-situCO(2) sensor.

Path-averaged atmospheric CO2 measurement using a 1.57 μm active remote sensor compared with multi-positioned in situ sensors

The Green-house gas Observation SATellite (GOSAT) was launched to determine the continental CO2 inventories. Its sensor is based on a passive remote sensing technique developed to achieve less than 1% relative accuracy for atmospheric CO2 measurements. Meanwhile, a laser remote sensor with the differential absorption spectrometry has been developed for a candidate of a future space-based mission to observe the atmospheric CO2 or other trace gases. A prototype of the newly developed active remote sensor has been performed to demonstrate a properly validated performance for ground-based and airborne systems. This study shows the results of the in-house and field measurements. The in-house measurement demonstrated the linearity with the correlation coefficient of over 0.99 between the instrumental response and the known CO2 density in the cell. The diurnal variation obtained from our system is consistent (correlation coefficient of 0.95) with that of multi-positioned in situ sensors, indicates the spatial responsibility of the atmospheric CO2 obtained from our remote sensor with two ~3-km observation paths.

A compact thermal infrared imaging radiometer with high spatial resolution and wide swath for a small satellite using a large format uncooled infrared focal plane array

In this paper, we present a feasibility study for the potential of a high spatial resolution and wide swath thermal infrared (TIR) imaging radiometer for a small satellite using a large format uncooled infrared focal plane array (IR-FPA). The preliminary TIR imaging radiometer designs were performed. One is a panchromatic (mono-band) imaging radiometer (8-12μm) with a large format 2000 x 1000 pixels uncooled IR-FPA with a pixel pitch of 15 μm. The other is a multiband imaging radiometer (8.8μm, 10.8μm, 11.4μm). This radiometer is employed separate optics and detectors for each wave band. It is based on the use of a 640 x 480 pixels uncooled IR-FPA with a pixel pitch of 25 μm. The thermal time constant of an uncooled IR-FPA is approximately 10-16ms, and introduces a constraint to the satellite operation to achieve better signal-to-noise ratio, MTF and linearity performances. The study addressed both on-ground time-delayintegration binning and staring imaging solutions, although a staring imaging was preferred after trade-off. The staring imaging requires that the line of sight of the TIR imaging radiometer gazes at a target area during the acquisition time of the image, which can be obtained by rotating the satellite or a steering mirror around the pitch axis. The single band radiometer has been designed to yield a 30m ground sample distance over a 30km swath width from a satellite altitude of 500km. The radiometric performance, enhanced with staring imaging, is expected to yield a NETD less than 0.5K for a 300K ground scene. The multi-band radiometer has three spectral bands with spatial resolution of 50m and swath width of 24km. The radiometric performance is expected to yield a NETD less than 0.85K. We also showed some preliminary simulation results on volcano, desert/urban scenes, and wildfire.

Performance analysis on 1.6 micron CW modulation laser absorption spectrometer for CO2 sensing

For the application to the global CO2 monitoring from the space-borne active sensor have been studied. We have developed the Laser Absorption Sensor (LAS) system for ground-based CO2 monitoring using the wavelength of 1.6 micron. Furthermore, we have also reported about measurement result with short time fluctuation corresponding to the concentration of 4 ppm (rms) in 32 s intervals and 1 km path. In this paper, we discuss how to achieve this performance.

Development of 1.6 micron CW modulation ground-based DIAL system for CO2 monitoring

We have demonstrated the 1.6 micron CW modulation hard-target DIfferential Absorption Lidar (DIAL) system for CO2 sensing. In this system, ON and OFF wavelength laser lights are intensity modulated with CW modulation signal. Received lights of the two wavelengths from the hard-target are discriminated by modulation frequencies in electrical signal domain. Since the optical circuit is fiber-based, the system is compact, flexible, and reliable. It is shown that stable CO2 concentration measurement corresponding to 4 ppm(rms) can be realized in the measurement time of 32s. This measurement stability is better than those obtained by the conventional CO2 sensing DIAL systems in the same measurement time. And the diurnal change of the measured results is in good agreement with the ones measured by an in-situ CO2 meter.

Performance improvement on fiber-based CW modulation laser absorption spectrometer system for CO 2 sensing

For the application to the global CO2 sensing, DIfferential Absorption Lidar or Laser Absorption Spectrometer (LAS) systems using the conventional coherent and incoherent methods have been studied1–4. But the conclusion about the best system configuration has not been determined yet, because of the severe requirements for the satellite-borne sensor in addition to the precise measurements in real-time. As the results of our consideration to overcome this issue, we invented the compact fiber-based Continuous Wave (CW) modulation LAS system, which enables simultaneous transmission of two (ON and OFF) wavelength and discrimination of wavelengths in the electrical signal domain. And we have developed the ground-based model and demonstrated high precision measurement corresponding to 4 ppm with short time interval of 32 s5. This high precision measurement was not easy to achieve and we have found out some important points to improve the performance and added some functions, which have not been shown in our previous literature. In this paper, we show the functions to realize this high precision measurement.