New research trends on high-precision time transfer technology

High-precision time transfer plays an important role in the areas of fundamental research and applications. Accompanying w ith the remarkable improvements in the ability of generating and measuring high-accuracy time-frequency signal,seeking for new time-transfer techniques betw een distant clocks w ith much further improved accuracy attracts attentions w orld-w idely. The time-transfer technique based on optical pulses has the highest precision presently,and the further improvement in the accuracy is heavily dependent on the time-domain properties of the pulse as w ell as the sensitivity of the applied measurement on the exchanged pulse. The application of optical frequency comb in time transfer for a precision up to femtosecond level are currently the focus of much interest,and has recently achieved many breakthroughs. Further investigations show that,utilizing quantum techniques,i.e. quantum measurement technique and quantum optical pulse source,can lead to a new limit on the measured timing information. Furthermore,it can be immune from atmospheric parameters,such as pressure,temperature,humidity and so on.Such quantum improvements on time-transfer have a bright prospect in the future applications requiring extremely high-accuracy timing and ranging. The potential achievements w ill form a technical basis for the future realization of sub-femtosecond time transfer system.

Coherent optical frequency transfer via 972-km fiber link

We demonstrate coherent optical frequency dissemination over a distance of 972 km by cascading two spans where the phase noise is passively compensated for.Instead of employing a phase discriminator and a phase locking loop in the conventional active phase control scheme,the passive phase noise cancellation is realized by feeding double-trip beat-note frequency to the driver of the acoustic optical modulator at the local site.This passive scheme exhibits fine robustness and reliability,making it suitable for long-distance and noisy fiber links.An optical regeneration station is used in the link for signal amplification and cascaded transmission.The phase noise cancellation and transfer instability of the 972-km link is investigated,and transfer instability of 1.1×10^(-19)at 10^(4)s is achieved.This work provides a promising method for realizing optical frequency distribution over thousands of kilometers by using fiber links.

Coherent Transfer of Optical Frequency over 112 km with Instability at the 10^(-20) Level

We demonstrate optical-carrier transfer over a 112-km single-span urban fiber link, By actively compensating the phase noise induced along the fiber link, a noise suppression of 55 dB at 1 Hz is obtained. A fractional frequency instability of 2.5× 10^-16 at i s is achieved, and reaching 7.5× 10^-20 at 10000 s. The system is stable and able to run for a long time. This work will contribute to optical frequency distribution and remote comparison among atomic clocks.

An all-optical phase detector by amplitude modulation of the local field in a Rydberg atom-based mixer

Recently,a Rydberg atom-based mixer was developed to measure the phase of a radio frequency(RF)field.The phase of the signal RF(SIG RF)field is down-converted directly to the phase of a beat signal created by the presence of a local RF(LO RF)field.In this study,we propose that the Rydberg atom-based mixer can be converted to an all-optical phase detector by amplitude modulation(AM)of the LO RF field;that is,the phase of the SIG RF field is related to both the amplitude and phase of the beat signal.When the AM frequency of the LO RF field is the same as the frequency of the beat signal,the beat signal will further interfere with the AM of the LO RF field inside the atom,and then the amplitude of the beat signal is related to the phase of the SIG RF field.The amplitude of the beat signal and the phase of the SIG RF field show a linear relationship within the range of 0 toπ/2 when the phase of the AM is set with a differenceπ/4 from the phase of the LO RF field.The minimum phase resolution can be as small as 0.6°by optimizing the experimental conditions according to a simple theoretical model.This study will expand and contribute to the development of RF measurement devices based on Rydberg atoms.

Cascaded optical frequency transfer over 500-km fiber link using regenerative amplifier

We demonstrate a 300-km+200-km cascaded coherent phase transfer via fiber link.The transfer is divided into a 300-km span and a 200-km span with independent phase locking loops,aiming to extend the phase control bandwidth of the whole link.The phase noise and transfer instability of the cascaded transmission are investigated and compared with those in the case of a single-span 500-km transfer.We achieve the transfer instabilities of 1.8×10^-14 at 1 s,8.9×10^-20 at 104 s for the 300-km+200-km cascaded transmission,and 2.7×10^-14 at 1 s for the 500-km single-span transfer.

Direct Digital Frequency Control Based on the Phase Step Change Characteristic between Signals

We present a new digital phase lock technology to achieve the frequency control and transformation through high precision multi-cycle group synchronization between signals without the frequency transformation circuit. In the case of digital sampling, the passing zero point of the phase of the controlled signal has the phase step characteristic, the phase step of the passing zero point is monotonic continuous with high resolution in the phase lock process, and using the border effect of digital fuzzy area, the gate can synchronize with the two signals, the quantization error is reduced. This technique is quite different from the existing methods of frequency transformation and frequency synthesis, the phase change characteristic between the periodic signals with different nominal is used. The phase change has the periodic phenomenon, and it has the high resolution step value. With the application of the physical law, the noise is reduced because of simplifying frequency transformation circuits, and the phase is locked with high precision. The regular phase change between frequency signals is only used for frequency measurement, and the change has evident randomness, but this randomness is greatly reduced in frequency control, and the certainty of the process result is clear. The experiment shows that the short term frequency stability can reach 10-12/s orders of magnitude.

Optical frequency transfer link with remote site compensation

In this paper,we present a remote time-base-free technique for a coherent optical frequency transfer system via fiber.At the remote site,the thermal noise of the optical components is corrected along with the link phase noise caused by environmental effects.In this system,a 1×2 acousto-optic modulator(AOM)is applied at the remote site,with the first light being used to eliminate the noise of the remote time base and interface with remote users while the zeroth light is used to establish an active noise canceling loop.With this technique,a 10 MHz commercial oscillator,used as a time base at the remote site,does not contribute to the noise of the transferred signal.An experimental system is constructed using a 150 km fiber spool to validate the proposed technique.After compensation,the overlapping Allan deviation of the transfer link is 7.42×10^(-15)at 1 s integration time and scales down to 1.07×10^(-18)at 10,000 s integration time.The uncertainty of the transmitted optical frequency is on the order of a few 10-19.This significantly reduces the time-base requirements and costs for multi-user applications without compromising transfer accuracy.Meanwhile,these results show great potential for transferring ultra-stable optical frequency signals to remote sites,especially for point-to-multi-users.