Lukas Kral

Optical signal path delay fluctuations caused by atmospheric turbulence

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.

Single photon counting module for space applications

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 SPAD manufactured on silicon using the K14 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.

Satellite laser ranging precision ultimate limit

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.

Picosecond laser pulse propagation delay fluctuation through atmosphere

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.

Photon counting timing uniformity–unique feature of the silicon avalanche photodiodes K14

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 µm diameter.

Silicon photon counting detector optical cross-talk effect

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 10×1 or 3×3 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.

Atmospheric-fluctuation-induced laser ranging jitter

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.

Picosecond laser pulse distortion in a turbulent atmosphere

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.

Random fluctuations of optical signal path delay in the atmosphere

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.

Optical cross-talk effect in a semiconductor photon counting detector array

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.