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 have been investigating the influence of atmospheric turbulence on the propagation of a picosecond laser pulse. The figure of merit of presented results is the time of propagation, its absolute delay and jitter. Phase wavefront deformation or beam profile changes were not studied. The correlation of the atmospheric turbulence with the propagation delay fluctuation was measured. The research was motivated by the needs of highly precise laser ranging of ground, air, and space objects; and highly precise and accurate time transfer ground-to-space and ground-to-ground by means of picosecond optical laser pulse. Firstly for comparison, lets briefly summarize the effects of a turbulent atmosphere to continuous laser beam. The total effect of atmospheric turbulences on a continuous laser beam propagation is a highly complex subject. Atmospheric turbulences can be defined as random spatial variations in the refraction index of the atmosphere resulting in a distortion of the spatial phase fronts of the propagating signal. Spatial phase front distortion induces the variable path of light energy and thus all effects described later on. Variations of the refraction index are caused by the turbulent motion of the atmosphere due to the variations in temperature and gradients in the water vapour. Following (Degnan, 1993), the optically turbulent atmosphere produces three effects on low power laser beams: 1) beam wander, 2) beam spread and 3) scintillations. Severe optical turbulence can result in a total beam breakup. Beam wander refers to the random translation of the spatial centroid of the beam and is generally caused by the larger turbulent eddies through which the beam passes. In astronomical community it is usually referred as seeing. Beam spread is a short term growth in the effective divergence of the beam produced by smaller eddies in the beam path. The two effects are often discussed together in terms of a “long term” and “short term” beam spread. The “long term” beam spread includes the effects of beam wander, whereas the “short term” beam spread does not. For more details, see (Degnan, 1993). Maximum turbulence occurs at mid-day in the desert (low moisture) under clear weather conditions. For the usual laser wavelength of 532 nm one can expect 2.4-4.6 cm for the coherence length at zenith angles of 0° and 70° respectively. At the tripled Nd:YAG wavelength (355 nm) the corresponding values are 3.1 and 1.6 cm (Degnan, 1993). Turbulence induced beam spreading will only have a significant impact on beam divergence (and hence signal level) if the coherence length is on the order of, or smaller than, the original effective beam waist radius. Since a typical 150 μrad beam implies an effective waist radius of 2.26 mm, the effect of beam spread on signal level for such systems is relatively small, i.e. a few percent.

We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of the superhump period is found to be composed of three distinct stages: an early evolutionary stage with a longer superhump period, a middle stage with systematically varying periods, and a final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods of less than 0.08 d show positive period derivatives. We present observational characteristics of these stages and give greatly improved statistics. Contrary to an earlier claim, we found no clear evidence for a variation of period derivatives among different superoutbursts of the same object. We present an interpretation that the lengthening of the superhump period is a result of the outward propagation of an eccentricity wave, which is limited by the radius near the tidal truncation. We interpret that late-stage superhumps are rejuvenated excitation of a 3:1 resonance when superhumps in the outer disk are effectively quenched. The general behavior of the period variation, particularly in systems with short orbital periods, appears to follow a scenario proposed in Kato, Maehara, and Monard (2008, PASJ, 60, L23). We also present an observational summary of WZ Sge-type dwarf novae. Many of them have shown long-enduring superhumps during a post-superoutburst stage having longer periods than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently with the mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives, and are excellent candidates for those systems around or after the period minimum of evolution of cataclysmic variables.

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.

The paper reports the results of research and development of a single photon avalanche detector (SPAD) for use in the harsh and hostile conditions of outer space. The photon counting detector was developed for space projects related to the synchronization of timescales via a space clock using optical pulses. The detector is based on a SPA D manufactured on silicon using the K 14 process, and operated in an active quenching mode. Its operation over an extreme temperature range and under high optical overload has been tested together with its sensitivity to radiation in space. The technology demonstrator of the detectors for the China Laser Time Transfer mission was developed and tested. The mission launch is expected in the year 2008.

The paper reports on detailed tests of active area uniformity of three different single photon avalanche detectors (SPADs). Relative sensitivity, timing jitter, and relative detection delay have been measured for a series of points across the detection area; the resulting spatial profiles have been plotted and analysed. Unique features have been found in the case of silicon-based K14 SPADs: their active area is extremely uniform, especially with respect to the detection delay, in contrast to the remaining two SPADs, which exhibit significant fluctuations of the delay. The perfect uniformity of the K14 SPADs enables the detector to maintain its high temporal resolution of 40 ps FWHM even for large detection areas of 200 mu m diameter.

Avalanche photodiodes specifically designed for single photon counting semiconductor avalanche structures have been developed on the basis of various materials: Si, Ge, GaP, GaAs and InGaAs 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 avalanche multiplication 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 the results of research and development of the Single Photon Avalanche Detector (SPAD) for application in a Laser Time Transfer (LTT) space mission. For the joint project with the Shanghai Observatory, Academy of Sciences of China, we have developed the detector package dedicated for the project of synchronizing the hydrogen maser-based time scales by laser pulses. The technology demonstrator of a dual detector has been built and tested in our labs. The main parameters are: detection efficiency 10% at 532 nm, timing resolution 80 psec, dark count rate 8 kHz, non gated operation. The detector's active area is 25 um in diameter. The total mass, including bias stabilizing circuit, is 2 grams, and the total power consumption is below 0.5 Watt per detecting channel. The detector can be operated in a wide range of temperatures ranging from -30o C to +60o C without any additional temperature control. Figure 1: The technology demonstrator of the dual photon counting detectors. The detection chips (protective caps installed for handling) are on the left. The ruggedness of the detector is superb. Optical power of 2 mW has been focused onto a sensitive area while the detector has been biased for 8 hours. No detectable degradation has been experienced. The overload tolerance negates the need for any mechanical Sun protection shutter in space. The recovery time from optical overload to full functionality is less than 0.1 second. The detector package has been successfully integrated into the LTT timing electronics and the pre-flight test was performed in China during the period July-September 2006.

The Laser Induced Fluorescence is a well-known and established analytical technique. We are reporting on the research and development of the remote sensing technique for water pollution of the organic pollutants dissolved in the water or flowing on the water level. The recent development in diode pumped compact micro lasers, solid state photon counters and fast timing electronics opens quite new application possibilities. We are relying on the diode pumped Nd:YAG microlaser, passively Q-switched and frequency doubled. It provides pulse energy exceeding 100 nJ in 600 ps long pulses at 532 nm with the repetition rate of 10 kHz within a compact, small and low power package. The fluorescence signal is detected by the customized silicon photon counting detector. The compact time-to-digital converter with 20 ps timing resolution and a personal computer interface has been constructed for the device. The small receiving optics apertures together with advanced time gating of the detected signal permits to operate in an outdoor environment in daylight background conditions with acceptable signal to noise ratio. The first measurement results. the capabilities of the technique in the environmental monitoring along with the device construction will be presented.

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 report the first direct measurements, to our knowledge, of optical signal path delay fluctuations caused by optical turbulence in the atmosphere. The experiments were based on satellite laser ranging. Our initial motivation was to identify all the random error contributors in satellite laser ranging. We measured and identified the random path fluctuations caused by the atmosphere in the range of units of picoseconds. An appropriate fluctuation model was developed.

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.

This article deals with the problem how to transform an ontology between different formalisms. The presented methodology requires defining meta-models of the source and the target formalisms and defining a mapping between them. This allows us then to convert any ontology encoded in the source formalism to the target one. It is further shown that an extension of the SumatraTT system can be used to implement such transformations easily. The research presented in the paper has been motivated by the needs of the CIPHER research project (Communities of Interest Promoting Heritage of Europian Regions).

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.