Josef Blazej

Time-walk-compensated SPAD: multiple-photon versus single-photon operation

The SPAD has proven already its capability of timing single- photon events with picosecond accuracy; it does that also for multi-photon events, but introduces here a time walk effect: with received energies of 1000 photons and more, the measured epoch time is shifted 200 ps or more towards earlier times; although the specific SPAD type used shows the lowest time walk effect of all measured silicon avalanche diodes, this effect still might introduce range errors of up to 30 mm, when measuring distances to satellites. It has been shown that this time walk effect is connected with a very small change of the avalanche rise time; this effect has been successfully used to develop an electronic circuit which measures this rise time difference, and uses it to compensate automatically almost all of the time walk effect. Some prototypes have been built and tested successfully in the satellite laser ranging station Graz; improved versions of the circuit are operated or tested now successfully in other SLR stations. It has been shown that the time walk effect can be reduced to more or less zero, for a dynamical range from single photon up to more than 1000 photons. For best time walk compensation, the circuit is adjusted for a specific laser pulse length; it has been shown however, that this adjustment also gives good time walk compensation for other laser pulse lengths.

Measurement of the optical to electrical detection delay in the detector for ground-to-space laser time transfer

We present a new type of measurement and the first results of determination of the optical to electrical delay of a photon counting detector. This type of measurement has not been reported for photon counting. The absolute value of the time interval between the time of arrival of the signal photon onto the detector input aperture and the time when the electrical output signal exceeds the pre-defined level must be determined. The optical to electrical delay value is required for ground-to-space laser time transfer with picosecond accuracy. The laser time transfer link is under construction for the European Space Agency for its application in the experiment Atomic Clock Ensemble in Space. We have developed the measurement technique and have measured the detection delay of the solid state photon counter. The experiment is described along with the first results.

Note: Solid state photon counters with sub-picosecond timing stability

We are reporting on a design, construction, and performance of photon counting detector packages based on solid state sensors. These photon counting devices have been optimized for extremely high stability of their detection delay. The detectors have been designed for applications in fundamental metrology and optical time transfer. The single photon avalanche diode structure manufactured on silicon using the K14 technology is used as a sensor. The active area of the sensor is circular with a diameter of 100 or 200 μm. The sensor is operated in an active quenching and gating mode. The photon detection efficiency exceeds 40% in a wavelength range spanning from 500 to 800 nm. The timing resolution is better than 20 ps rms. Its detection delay is stable within ±600 fs over several days of operation, in a sense of time deviation the detection delay stability of 150 fs has been achieved. The temperature change of the detection delay is as low as 280 fs∕K. This timing performance is preserved even under extremely high background photon fluxes exceeding 100 Mc/s. The detectors have been qualified for operation in space missions.

Photon number resolving in geiger mode avalanche photodiode photon counters

Abstract The paper reports on research and development in the field of avalanche photodiodes operated as photon counters in a Geiger mode. A technique has been developed and tested that permits estimation of the photon number involved in a detection process. It can be applied in a time correlated photon counting experiment simultaneously with original required time interval estimation. A time walk compensation circuit provides uniform electrical pulses, and the time interval between them is related to the number of photons detected. Employing a picosecond event timing device, the photon number can be estimated within the dynamic range 1–1000 photons with resolution better than a factor of three.

Note: Space qualified photon counting detector for laser time transfer with picosecond precision and stability

The laser time transfer link is under construction for the European Space Agency in the frame of Atomic Clock Ensemble in Space. We have developed and tested the flying unit of the photon counting detector optimized for this space mission. The results are summarized in this Note. An extreme challenge was to build a detector package, which is rugged, small and which provides long term detection delay stability on picosecond level. The device passed successfully all the tests required for space missions on the low Earth orbits. The detector is extremely rugged and compact. Its long term detection delay stability is excellent, it is better than ±1 ps/day, in a sense of time deviation it is better than 0.5 ps for averaging times of 2000 s to several hours. The device is capable to operate in a temperature range of -55 °C up to +60 °C, the change of the detection delay with temperature is +0.5 ps/K. The device is ready for integration into the space structure now.

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.

Photon counting altimeter and lidar for air and spaceborne applications

We are presenting the concept and preliminary design of modular multipurpose device for space segment: single photon counting laser altimeter, atmospheric lidar, laser transponder and one way laser ranging receiver. For all the mentioned purposes, the same compact configuration of the device is appropriate. Overall estimated device weight should not exceed 5 kg with the power consumption below 10 W. The device will consists of three main parts, namely, receiver, transmitter and control and processing unit. As a transmitter a commercial solid state laser at 532 nm wavelength with 10 mW power will be used. The transmitter optics will have a diameter at most of 50 mm. The laser pulse width will be of hundreds of picoseconds order. For the laser altimeter and atmospheric lidar application, the repetition rate of 10 kHz is planned in order to obtain sufficient number of data for a distance value computing. The receiver device will be composed of active quenched Single Photon Avalanche Diode module, tiny optics, and narrow-band optical filter. The core part of the control and processing unit including high precision timing unit is implemented using single FPGA chip. The preliminary device concept includes considerations on energy balance, and statistical algorithms to meet all the mentioned purposes. Recently, the bread board version of the device is under construction in our labs. The concept, construction, and timing results will be presented.

Development and construction of the photon counting receiver for the European laser time transfer space mission

We are presenting the work progress and recent results in the development and construction of the photon counting receiver, which is prepared for the European Laser Timing experiment in space. It is an optical link prepared in the frame of the ESA mission Atomic Clock Ensemble in Space. The ultra short laser pulses will be used to synchronize the time scales ground to space with picosecond precision. To minimize the timing biases the photon counting concept of the space born receiver was selected. The requirements put on the photon counting receiver are quite challenging in terms of the long term detection delay stability, wide operation temperature range, extremely high background photon flux and others. Recently, the bread board version of the detector has been constructed and is under extensive test in our labs. The concept and construction will be presented along with the achieved device parameters.

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.

Development of the European Laser Timing instrumentation for the ACES time transfer using laser pulses

We are presenting the work progress and recent results in the field of the European Laser Timing instrumentation for the ACES time transfer using laser pulses. European Laser Timing (ELT) is an optical link presently under preparation in the frame of the ESA mission “Atomic Clock Ensemble in Space” (ACES). The on-board hardware consists of a comer cube retro-reflector (CCR), an optical receiver based on a single-photon avalanche diode (SPAD) and an event timer board connected to the ACES time scale. Short laser pulses fired towards ACES by a laser ranging station will be time tagged with respect to the ground time scale. They will also be detected in space by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse towards the ground station providing precise ranging information. This procedure provides ground-to-space and ground-to-ground time transfer with a precision and accuracy outperforming the radiofrequency techniques. Extensive work has been done on the photon counting detector. Three different versions of the photon counter for space application have been developed and tested. The main areas of interest were: minimal power consumption requirements of the optical detector package, long term stability of detection delay and a broad operational temperature range. The detector shall be capable to operate within a temperature range of - 50° to + 50° C while keeping the detection delay stability on the picosecond level for temperature variations of 6.5 K peak-to-peak. The detection delay shall be characterized and controlled - e.g. delay between the event of photon absorption and the appearance of the electrical pulse on the detector output. A configuration of the optical receiver able to maintain uniform sensitivity over a wide field of view of 120 degrees was developed. The parameters of the optical receiver obtained in the ground tests meet safely all the requirements for the ELT mission.