Objective Free-space laser time-frequency transmission technology has the advantages of high capacity, extensive coverage, long transmission distance, and high confidentiality. Its accuracy is expected to reach the standard quantum limit, making it an important technical development direction for space high-precision time-frequency transmission in the future. The precision of laser range directly affects the accuracy of laser time-frequency transmission, which is determined by the accuracy of measuring the pulse round-trip time interval. Compared with traditional time interval counting method, the measurement accuracy of event timing method has reached ps level, and it has become an essential measurement technology in high-repetition-rate laser ranging technology. In this research, an index evaluation model for the measurement performance of the event timer was established in order to provide a reference for the assessment of event timer in free-space laser time-frequency transmission with greater precision and longer distance.
Methods The performance assessment model for event timers presented in this work mixes deterministic and random elements. Frequency stability (time-domain variance) and power-law spectral noise are random parameters, and frequency accuracy and frequency drift rate are deterministic ones. Time series from accurate frequency sources can be measured to obtain a number of indicators that describe the effectiveness of the event timer, such as those based on the minimum double-dimensional frequency accuracy and frequency drift rates, as well as time-dome differences that describe the stability of frequencies, such as Allan variance, Modified Allan variance, Time variance and Hadamard variance. The power law spectrum model contains five random noises, including random walk noise, frequency-modulated white noise, frequency-modulated flicker noise, phase-modulated white noise and phase-modulated flicker noise, which can be separated from the event timer measurement data. The measurement method was created using the performance assessment model (Fig.2), which contrasts the performance of two common, high-precision event timers (A033 and GT668) that are on the same accuracy level.
Results and Discussions The high-precision event timers A033 and GT668 are being evaluated for their measurement performance using the specified event timer performance assessment model. The dispersion degree of the GT668 was lower than that of the A033 (Tab.2). The frequency measurement accuracy of the event timer A033 was superior to \text7 \times \text1\text0^ - 12 (Fig.6); The event timer GT668 was superior to \text3\text.1 \times \text1\text0^ - 12. Compared to frequency drift rate, the frequency drift rate indicators of A033 was \text2\text.096 \times \text1\text0^ - 15, and frequency drift rate of the GT668 was - 1\text.071 \times \text1\text0^ - 15. The short-term stability Allan standard deviation (1 d) increased from \text7 \times \text1\text0^ - 12 to \text4 \times \text1\text0^ - 12 (Tab.4). There is no discernible difference between FM scintillation noise and FM white noise, and GT668 is more steady in the trend of the random walk noise curve (Fig.8). The performance of high-precision event clocks may be assessed and studied using the performance evaluation model, and the variations between various event timers on the same performance index can be observed.
Conclusions The performance of the picosecond event timer was assessed from the perspectives of deterministic factors and random factors, according to the designed performance evaluation method, and the key performance indicators such as frequency accuracy, frequency drift rate, dispersion degree, frequency stability (time domain variance), and power law spectrum noise were obtained. The experiments validated the event timer assessment method by measuring the event timing of the fixed latency of the passive hydrogen maser VCH-1008 (VREMYA-CH) using the event timers A033 and GT668. The experiment demonstrated that the performance of a high-precision event timer was able to be assessed and analyzed by the evaluation model presented in this paper, ascertain its accuracy and dependability, and provide an analytical foundation for its applications.
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