The influence of Earth atmospheric turbulence on the propagation of a picosecond laser pulse has been investigated
from point of view detection with high temporal resolution. The results have been interpreted for optical time scale
synchronization link allowing picosecond precision and accuracy in ground-to-space time transfer on a single photon
signal levels. The details in laser beam position changes, phase wave-front deformation or beam profile changes were
not studied like in adaptive optics as the goal of time transfer link is not the imaging but time tagging. The figure of
merit of presented results is the time of propagation, its absolute delay and jitter. The correlation of the atmospheric
turbulence with the propagation delay fluctuation was measured. The physical reason of the fluctuation of propagation
time of laser pulse on picosecond level is the same, but the entirely different approach in comparison to adaptive optics
was used to describe the effect.
We are presenting the results of the studies related to propagation of ultrashort optical pulse through the turbulent
atmosphere. The correlation of the atmospheric turbulence with the propagation delay fluctuation was measured. The
entirely different approach in comparison to adaptive optics was developed to describe the effect. The experiments
described enabled us for the first time to determine the L0 parameter on the basis of direct measurement. The recent
achievements in the field of pulsed lasers, fast optical detectors and timing systems enable us to resolve the effects of
propagation differences monitoring on the level of units of picosecond propagation time. Three independent types of
path configurations have been studied: horizontal path, slant path at elevation 10 - 80 degrees to a flying target and slant
path from ground to space. Additionally, new techniques of optical receivers signal processing give a way to distinguish
the atmospheric fluctuations contribution from the energy dependent detection delay effects.
Single photon avalanche diodes (SPADs) based on various semiconductors have been developed at the Czech Technical
University in Prague during the last 20 years. Much attention has been also paid to development of high-speed active
quenching circuits for these detectors. Recently, we have performed a series of experiments to characterize our silicon-based
photon counters and their capability of operation in a gated mode with the gate duration of single nanoseconds and
the detector sensitivity rise time of hundreds of picoseconds. This performance has been achieved by optimizing the
active quenching circuit and its components. The fast gating is needed in cases, when the photons of interest are
generated short time after a strong optical signal, which cannot be suppressed in optical domain. The time dependence of
detection sensitivity, detection delay and timing resolution within the nanosecond gates has been measured.
Atmospheric turbulence induces random delay fluctuations to any optical signal transmitted through the air. These fluctuations can influence for example the measurement precision of laser rangefinders. We have found an appropriate theoretical model based on geometrical optics that allows us to predict the amplitude of the random delay fluctuations for different observing conditions. We have successfully proved the applicability of this model by a series of experiments, directly determining the amplitude of the turbulence-induced pulse delay fluctuations by analysis of a high precision laser ranging data. Moreover, we have also shown that a standard theoretical approach based on diffractive propagation of light through inhomogeneous media and implemented using the GLAD software is not suitable for modeling of the optical signal delay fluctuations caused by the atmosphere. These models based on diffractive propagation predict the turbulence-induced optical path length fluctuations of the order of micrometers, whereas the fluctuations predicted by the geometrical optics model (in agreement with our experimental data) are generally larger by two orders of magnitude, i.e. in the submillimeter range. The reason of this discrepancy is a subject to discussion.
We are reporting on research and development of a water pollution remote sensing technique, based on laser induced fluorescence of organic pollutants floating on the water level or dissolved in water. We are relying on a diode pumped Nd:YAG microlaser, providing 1 microjoule 600 ps long pulses at 532 nanometers wavelength with repetition rate of 10 kHz within a compact, small and low power package. The fluorescence signal is detected by a silicon photon counting detector. A compact time to digital converter with 20 picoseconds timing resolution and a personal computer interface has been constructed for the device. The small receiving optics apertures together with advanced time filtering of the detected signal permits to operate in an outdoor environment in a daylight background conditions with acceptable signal to noise ratio. The indoor tests of the device indicate its capability of detection of sub-micrometer oil films on the water level even at high noon. The capability of detection of dissolved organic substances has been demonstrated as well.
Solid state single photon detectors are getting more and more attention in various areas of applied physics: optical sensors, communication, quantum key distribution, optical ranging and Lidar, time resolved spectroscopy, opaque media imaging and ballistic photon identification. Avalanche photodiodes specifically designed for single photon counting semiconductor avalanche structures have been developed on the basis of various materials: Si, Ge, GaP, GaAsP and InGaAs/InGaAsP at the Czech Technical University in Prague during the last 20 years. They have been tailored for numerous applications. Recently, there is a strong demand for the photon counting detector in a form of an array; even small arrays 10x1 or 3x3 are of great importance for users. Although the photon counting array can be manufactured, there exists a serious limitation for its performance: the optical cross-talk between individual detecting cells. This cross-talk is caused by the optical emission of the avalanche photon counting structure which accompanies the photon detection process. We have studied in detail the optical emission of the avalanche photon counting structure in the silicon shallow junction type photodiode. The timing properties, radiation pattern and spectral distribution of the emitted light have been measured for various detection structures and their different operating conditions. The ultimate limit for the cross-talk has been determined and the methods for its limitation have been proposed.
We are presenting preliminary results of the development of the Technology Demonstrator of the photon counting laser altimeter for planetary studies. This device is expected to be a universal instrument applicable in various space missions. The device should provide altimetry and surface radiometry in the range of 400 to 1400 km with one meter range resolution under rough conditions - Sun illumination, space radiation, etc. The Technology Demonstrator is the modular test equipment dedicated to test individual critical components, concepts and technologies: the laser source, the photon counting detector and its electronics. The concept and techniques to be investigated are: the energy budget of the altimeter, range resolution, the signal to noise ratio under various background light conditions, photon counting data acquisition, signal mining and processing techniques.
We are presenting experimental data on atmospheric fluctuations measurements and their influence on laser ranging precision. Three independent path configurations have been studied: 4.3-kilometer horizontal path, slant path at elevation 10-80 degrees and slant path from ground to space. The laser ranging has been performed using the satellite laser ranging system in Graz, Austria. The system precision is 6 picoseconds (single shot RMS) and the measurement repetition rate is 2 kHz. That enables us to monitor fast fluctuations with period of the order of milliseconds. The atmospheric seeing conditions have been measured simultaneously. We have identified and measured contribution of the atmospheric fluctuations to the ranging precision and time spectrum of these fluctuations for the first time.
We have estimated the contribution of atmospheric turbulence effects to the satellite laser ranging precision. This work was motivated by the observed discrepancy between the precision of laser ranging to short baseline ground targets and space born targets. The contribution of the atmosphere is expected to be the limiting factor to the satellite laser ranging precision on millimeter level. Two different atmospheric optical models were investigated. The geometry approach showed that at some situations the turbulence-induced random ranging error could reach the millimeter level, as observed in laser ranging experiment. This effect significantly decreases with the station’s altitude above sea level and satellite altitude above horizon. The results depend on the value of the atmospheric outer scale parameter; its value is only approximate due to hardly predictable nature of the turbulence strength height profile. A novel experiment with high repetition rate satellite laser ranging is introduced, which should prove the turbulence contribution to the satellite laser ranging precision.