Jan Kodet

Time measurement device with four femtosecond stability

We present the experimental results of extremely precise timing in the sense of time-of-arrival measurements in a local time scale. The timing device designed and constructed in our laboratory is based on a new concept using a surface acoustic wave filter as a time interpolator. Construction of the device is briefly described. The experiments described were focused on evaluating the timing precision and stability. Low-jitter test pulses with a repetition frequency of 763 Hz were generated synchronously to the local time base and their times of arrival were measured. The resulting precision of a single measurement was typically 900 fs RMS, and a timing stability TDEV of 4 fs was achieved for time intervals in the range from 300 s to 2 h. To our knowledge this is the best value reported to date for the stability of a timing device. The experimental results are discussed and possible improvements are proposed.

Accuracy of two-way time transfer via a single coaxial cable

In order to find limits of the accuracy of the two-way time transfer (TWTT) via a single coaxial cable, we have carried out a detailed analysis which is presented in this paper. We applied the TWTT concept when a transmission line is driven by pulse current drivers and the times of arrival of the pulses are measured at the ends of the line. In addition to the estimation of the accuracy, the analysis provides several rules for proper design of a TWTT system with optimal performance. Based on this concept, a TWTT system for highly accurate time distribution or comparison has been designed and realized. For distances up to 1?km the accuracy was better than 100?ps without any additional correction or adjustment. After the influence of the non-symmetry of input?output circuits was corrected, the errors were lower than 20?ps for distances up to 2?km. The TWTT system is designated for keeping unified time in a net of event timers distributed in one building or in a relatively small area. The timing units forming the system guarantee the time transfer parallel to the time tagging of external pulses.

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.

Note: Optical trigger device with sub-picosecond timing jitter and stability

We are presenting the design, construction, and overall performance of the optical trigger device. This device generates an electrical signal synchronously to the detected ultra-short optical pulse. The device was designed for application in satellite laser ranging and laser time transfer experiments, time correlated photon counting and similar experiments, where picosecond timing resolution and detection delay stability are required. It consists of the ultrafast optical detector, signal discriminator, output pulse forming circuit, and output driver circuits. It was constructed as a single compact device to optimize their matching and maintain stability. The detector consists of an avalanche photodiode--both silicon and germanium types may be used to cover the wavelength range of 350-1550 nm. The analogue signal of this photodiode is sensed by the ultrafast comparator with 8 GHz bandwidth. The ps clock distribution circuit is used to generate the fast rise/fall time output pulses of pre-set length. The trigger device timing performance is excellent: the random component of the timing jitter is typically 880 fs, the temperature dependence of the detection delay was measured to be 370 fs/K. The systematic error contribution depends on the laser used and its stability. The sub-ps values have been obtained for various laser sources.

Two-way time transfer via optical fiber providing subpicosecond precision and high temperature stability

We are reporting on analysis, design, construction, and key parameters of the device for a two-way time transfer via an optical fiber. The dominant source of errors in the two-way optical time transfer (TWOTT) via relatively short optical fibers is the temperature dependence of internal delays within the terminal units. We have performed an analysis of the influence of the internal delays and their temperature dependence on the TWOTT process considering two different configurations of the terminals with the feedback coupling in optical and electrical domains. The achieved results have been used for the optimal design of a TWOTT system implementing standard small form-factor pluggable optical transceivers. The operational tests of this new TWOTT system confirmed the precision of the time transfer on the subpicosecond level, the time transfer stability characterized by TDEV better than 60 fs for averaging intervals from 100 s to 10 000 s, and the temperature stability better than 100 fs K−1.

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.

Scheduling VLBI observations to satellites with VieVS

Observations of satellites with Very Long Baseline Interferometry (VLBI) radio telescopes provide a variety of new possibilities such as the integration of different geodetic techniques, which is one of the main goals of GGOS, the Global Geodetic Observing System of the IAG. Promising applications can be found, among others, in the field of inter-technique frame ties. With the standard geodetic VLBI scheduling software not being prepared to use satellites as radio sources so far, such observations were complicated due to the need to carefully prepare the required interchange files. The newly developed Satellite Scheduling Module for the Vienna VLBI Software (VieVS) offers a solution to this. It allows the user to prepare VLBI schedule files in a standardized format, providing the possibility to carry out actual satellite observations with standard geodetic antennas, e.g. of the IVS network. First successful observations of GLONASS satellites, based on schedules created with the new VieVS module, took place on the baseline Wettzell-Onsala in January 2014.

Note: electronic circuit for two-way time transfer via a single coaxial cable with picosecond accuracy and precision.

We have designed, constructed, and tested the overall performance of the electronic circuit for the two-way time transfer between two timing devices over modest distances with sub-picosecond precision and a systematic error of a few picoseconds. The concept of the electronic circuit enables to carry out time tagging of pulses of interest in parallel to the comparison of the time scales of these timing devices. The key timing parameters of the circuit are: temperature change of the delay is below 100 fs/K, timing stability time deviation better than 8 fs for averaging time from minutes to hours, sub-picosecond time transfer precision, and a few picoseconds time transfer accuracy.

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.