Photon-counter device to synchronize ground-based and orbiting instruments

A laser time-transfer-measuring link is under construction for the European Space Agency for its Atomic Clock Ensemble in Space (ACES) experiment. The device is expected to be delivered to the International Space Station in 2014. The objective of this laser-time-transfer measurement is the synchronization of ground-based clocks and the space station's onboard clock with precision of a few picoseconds (10^-12s) and accuracy of 50ps.¹

The experiment is a spin-off of existing projects of laser ranging to artificial Earth satellites.² The orbital hardware comprises a corner-cube retroreflector (CCR), an optical receiver based on a single-photon avalanche diode (SPAD), and an event-timing device connected to the satellite's local clock or timescale. Its principle is summarized in Figure 1. In operation, ultrashort laser pulses are fired toward the satellite by a ground-based laser-ranging station. The time of the firing is recorded or ‘time tagged’ with respect to the ground station's clock. The pulses are detected by the orbiting SPAD and time-tagged according to the orbiting clock. Simultaneously, the CCR redirects the laser pulse back toward the ground station, providing information about the ground-to-space signal-propagation delay.

Figure 1. Time diagram of laser-time-transfer principle. The time difference between space (S) and ground (G) clock scales, ΔT, is the result of the experiment. τ: Ground-to-space propagation delay.

The signal photon flux at the orbit is of the order of 10¹³ photons/m² in one laser shot. Thus, a multiphoton signal may be obtained, but the optical signal is attenuated to the single-photon level. The photon-counting approach has been selected to reduce the systematic biases as much as possible. The absolute propagation delays associated with the timing chain may be calibrated down to the level of several tens of picoseconds. This is more than one order of magnitude better than with radio-frequency-wavelength measurements.

The requirements put on the absolute calibration of the signal delays within the laser-time-transfer signal chain are very challenging. Among others, the optical-to-electrical detection delay of the photon-counting receiver itself should be calibrated with the uncertainty of 25ps maximum. We have developed a measurement technique with resulting value of 12ps.³

The photon-counting detection module designed in our labs has been used for ground-to-space laser-time-transfer measurements in previous years.^4,5 However, for the ACES mission, the new requirements put on detector performance are significantly higher from the point of view of time resolution, detection delay, stability, and last but not least, the optical-to-electrical detection-delay value itself.

The detector package has a timing resolution better than 25ps rms, the detection delay has a subpicosecond stability for times in the range of 30 seconds to two hours (see Figure 2), and the detection-delay temperature drift is below +1.7ps/K. The total power needed is less than 0.5W (93mA from a single 5.3V power supply). The detector package is space qualified. A radiation dose exceeding 100kRads does not change the detector's dark-count rate.

Figure 2. Timing stability of the entire laser time-transfer chain consisting of the single-photon avalanche diode (SPAD) detector package, laser, timing systems, and cabling. Note subpicosecond stability for averaging time intervals of 30 seconds up to two hours.

Recently, the engineering module shown in Figure 3 has been completed for further operational tests and calibration missions around satellite-ranging stations. The realization of additional, analogous experiments in other missions⁶ is under consideration.

Figure 3. Photon-counting detector for the space mission Atomic Clock Ensemble in Space–European Laser Timing. Included in the housing are input optics and optical filter. Total height is 130mm.

This research has been supported by numerous grants provided by the Grant Agency of the Czech Republic, Ministry of Education of the Czech Republic, and international grant agencies. The recent work has been supported by framework MSM6840770015. The European Space Agency primary contractor is CSRC, Ltd.