Aerosol observations with ceilometers have been made worldwide recently. To use ceilometer data to retrieve aerosol profiles, raw signals should be accurately converted to the attenuated backscattering coefficient. Hence, the calibration coefficient for the system constant has to be determined correctly. We conducted a ceilometer–lidar comparative experiment to evaluate the Lufft CHM15k Nimbus product. The attenuated backscattering coefficient using CHM15k was smaller by a factor of 1.48 compared to that of lidar. The calibration coefficient should be periodically corrected using the ceilometer signal itself since lidar data are generally unavailable in the field observations. We recalibrated the product using both Rayleigh fitting and cloud attenuation methods. The correction factor, determined from the recalibration, was 15% (9%) smaller when using the Rayleigh fitting (cloud attenuation) method than the factor determined from lidar. Uncertainties from backscattering ratios at the reference height and the lidar ratio can cause systematic errors in the correction factor determined from the Rayleigh fitting method. Uncertainties due to the multiple scattering factor contribute to systematic errors for the cloud attenuation method. We propose a calibration method using depolarization ratios for future polarization-sensitive ceilometers, which can estimate the calibration coefficient without multiple scattering factors.
A vertical pin photodiode with a thick intrinsic layer is integrated in a 0.5-μm BiCMOS process. The reverse bias of the photodiode can be increased far above the circuit supply voltage, enabling a high-drift velocity. Therefore, a highly efficient and very fast photodiode is achieved. Rise/fall times down to 94 ps/141 ps at a bias of 17 V were measured for a wavelength of 660 nm. The bandwidth was increased from 1.1 GHz at 3 V to 2.9 GHz at 17 V due to the drift enhancement. A quantum efficiency of 85% with a 660-nm light was verified. The technological measures to avoid negative effects on an NPN transistor due to the Kirk effect caused by the low-doped I-layer epitaxy are described. With a high-energy collector implant, the NPN transit frequency is held above 20 GHz. CMOS devices are unaffected. This photodiode is suitable for a wide variety of high-sensitivity optical sensor applications, for optical communications, for fiber-in-the-home applications, and for optical interconnects.
The mobile demonstrator for biological aerosol standoff detection has been designed and built to test and develop
reliable optical methods for the identification of biological aerosols in a 10 km range. Disciplines such as agriculture,
defense and security are increasingly concerned with distinguishing certain classes of biological particles from remote
distances. The instrument combines backscatter channels for 3 laser wavelengths, 2 nitrogen Raman channels,
depolarization and fluorescence channels with 2 ultraviolet excitation wavelengths. Aerosol size distribution, particle
shape and refractive index as well as fluorescence excitability by different laser wavelengths and spectral fluorescence
information are the distinguishing variables for the identification of unknown biological aerosols.
The propagation of ultrashort, ultra-intense laser pulses gives rise to strongly nonlinear processes. In particular, filamentation is observed, yielding an ionized, conducting plasma channel where white-light supercontinuum due to self-phase modulation occurs. This supercontinuum, extending from the UV to the IR, is a suitable "white laser" source for atmospheric remote sensing, and especially Lidar (Light Detection and Ranging). Recent significant results in this regard are presented, as well as lightning control using ultrashort laser pulses. The application of ultrashort-pulse lidar to aerosol monitoring is also discussed.
High-power femtosecond laser pulses can lead to strong nonlinear interactions during the propagation through a medium. In air the well known self-guiding effect produces long intense and moderately ionized filaments, in which a broad white-light continuum from the near UV to the mid IR is generated. The forward directed white-light can be used to do range resolved broadband absorption measurements, which opens the way to a real multi-component lidar for the simultaneous detection of several trace gases. On the other hand, enhanced nonlinear scattering and characteristic emission from the filament region, as well as from the interaction of intense pulses with aerosols, can be observed. This opens perspectives towards a novel kind of analysis of atmospheric constituents, based upon nonlinear optics. Additionally, the conductivity of the filaments can be used for lightning control. Here we present the basic concepts of the femtosecond lidar, laboratory experiments and recent results of atmospheric measurements.