Ivan Prochazka

Active–passive mode-locked Nd:YAG laser with passive negative feedback

Stationary ultrashort light pulses of 10-psec duration and with an energy of 10 μJ per pulse were obtained from an active-passive mode-locked Nd:YAG laser by using a two-photon absorption limiter (GaAs) inserted into the resonator. The stability of energy in the steady-state part of the pulse train, containing approximately 90 pulses, is better than ±1.5%. The dynamics of the pulse-shortening mechanism are described, and it is shown that maximum pulse compression is reached after only approximately 10 round trips.

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.

Ground-based demonstration of the European Laser Timing (ELT) experiment

The development of techniques for the comparison of distant clocks and for the distribution of stable and accurate time scales has important applications in metrology and fundamental physics research. Additionally, the rapid progress of frequency standards in the optical domain is presently demanding additional efforts for improving the performances of existing time and frequency transfer links. Present clock comparison systems in the microwave domain are based on GPS and two-way satellite time and frequency transfer (TWSTFT). European Laser Timing (ELT) is an optical link presently under study in the frame of the ESA mission Atomic Clock Ensemble in Space (ACES). The on-board hardware for ELT consists of a corner cube retro-reflector (CCR), a single-photon avalanche diode (SPAD), and an event timer board connected to the ACES time scale. Light pulses fired toward ACES by a laser ranging station will be detected by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse toward the ground station providing precise ranging information. We have carried out a ground-based feasibility study at the Geodetic Observatory Wettzell. By using ordinary satellites with laser reflectors and providing a second independent detection port and laser pulse timing unit with an independent time scale, it is possible to evaluate many aspects of the proposed time transfer link before the ACES launch.

Photon counting module for laser time transfer via Earth orbiting satellite

For the project of the Laser Time Transfer (LTT) we have developed the photon counting detector package designed to synchronize the ground and space based clocks by laser pulses. The device flying modules were constructed at the Shanghai Observatory, China, and they were tested for operation in a space environment in 2006. Numerous ground indoor tests of time synchronization of the rubidium clock were performed before the satellite launch. The device was launched onboard the Chinese experimental navigation satellite Compass-M1 on 13 April 2007 to a high altitude orbit of 21,500 km. The worlds first space clock synchronization by means of laser pulses was carried out from the satellite laser station Changchun, Jilin, China, 1 August 2007. The results of the space born detector package pre-launch tests along with the first in-flight operation results are presented.

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.

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.

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.