We are presenting novel active quenching circuit for Single Photon Avalanche Diodes (SPADs). It was designed and
optimized for satellite laser ranging applications, where the specific requirements are put on the gating performance. The
goal of this work was to be able to detect the photons in short time after gate on with constant detection delay and
sensitivity to minimize the measurement errors on one hand and background photon flux induced false count on the other
hand. The detector sensitivity and especially the detection delay must be stabilized few nanoseconds after the gate
activation. In the new circuit the SPAD can be pulse operated up to 5 volts above its breakdown voltage, the gate is
opened by the incoming external pulse and is closed by the first photon detection. The new circuit was built and tested,
the detector package for the field operation at the satellite laser ranging station was completed. The device performance:
detection sensitivity, detection delay and timing resolution was measured and will be presented.
The Satellite Laser Ranging (SLR) Station Graz is measuring routinely distances to satellites with a 2 kHz laser,
achieving an accuracy of 2-3 mm. Using this available equipment, we developed - and added as a byproduct - a kHz SLR
LIDAR for the Graz station: Photons of each transmitted laser pulse are backscattered from clouds, atmospheric layers,
aircraft vapor trails etc. An additional 10 cm diameter telescope - installed on our main telescope mount - and a Single-
Photon Counting Module (SPCM) detect these photons. Using an ISA-Bus based FPGA card - developed in Graz for the
kHz SLR operation - these detection times are stored with 100 ns resolution (15 m slots in distance). Event times of any
number of laser shots can be accumulated in up to 4096 counters (according to > 60 km distance).
The LIDAR distances are stored together with epoch time and telescope pointing information; any reflection point is
therefore determined with 3D coordinates, with 15 m resolution in distance, and with the angular precision of the laser
First test results to clouds in full daylight conditions - accumulating up to several 100 laser shots per measurement -
yielded high LIDAR data rates (> 100 points per second) and excellent detection of clouds (up to 10 km distance at the
moment). Our ultimate goal is to operate the LIDAR automatically and in parallel with the standard SLR measurements,
during day and night, collecting LIDAR data as a byproduct, and without any additional expenses.
Using our Satellite Laser Ranging (SLR) facility, our experience and our available equipment for Single Photon
detection, we installed a Single Photon Counting Module (SPCM) to measure the photon flux of variable stars, and of
stars with transiting exoplanets; these observations are intended as a complementary application to our standard SLR
activities, to contribute observations to already known - and also to candidate - variable stars and stars with transiting
exoplanets. While it is relatively easy to detect the large - in some cases up to 50% - variations of some stars, it is a
challenge to detect the transiting exoplanets with this method; the decrease in photon flux here is only in the order of a
few percent. In this paper, we present first results.
The paper describes the new achievements in an all solid state photon counting technique with picosecond resolution. The extended dynamical range has been achieved: the dependence of the detection delay on the detected signal strength - the time walk -has been compensated within several orders of optical signal strength. The principal application of the detector is the millimeter resolution satellite laser ranging. The detector is based on silicon avalanche photodiode pulse biased above its break voltage. The external gating and avalanche active quenching electronics is used. The time walk of the avalanche photodiode is of the order of hundreds of picoseconds in the dynamical range of single to one hundred photons input signal strengths. The additional electronics circuit has been developed to compensate for the time walk: the input optical signal strength influences the avalanche current build up time,the maximum build up time difference is 20 psec within the dynamical range 1:1000. This time difference is sensed, stretched by the factor of ten. The stretched time interval is applied, with the negative sign, as a correction to the detector propagation delay. The detector ultimate timing resolution, temporal stability, dynamical range and its dependence on the input laser pulse length have been investigated in detail. The fieldable version of the detector is been used for satellite laser ranging purposes. The timing resolution of the entire detector better than 20 picoseconds r.m.s., the maximum dynamical range > 1000:1 with the item walk bellow +/- psec have ben achieved, the results are presented. The additional applications in spectroscopy, biophysics, rangefinding and fiber optics may be considered.
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.
The application of the streak camera with the circular sweep for satellite laser ranging is described. The Modular Streak Camera system employing the circular sweep option was integrated into the conventional Satellite Laser System. The experimental satellite tracking and ranging has been performed. The first satellite laser echo streak camera records are presented.