Fiber-optic Time Transfer Research Articles

High-precision fiber-optic time transfer with an unlimited compensation range.

We present a fiber-optic time transfer system with high transfer stability and an unlimited compensation range of the delay variation. We first stably transmit a frequency signal from a voltage-controlled oscillator to the remote site. The time signal is then embedded in the frequency signal by simply selecting its one cycle per second with a tunable gate signal. Therefore, the proposed time transfer system inherits both the stability and the unlimited adjustment range of the frequency transfer yet with no need for demodulation. The time deviation of 1.93 ps is achieved at 1000-s averaging. This simple and demodulation-free time transfer system is applicable for scalable distributed applications that require high-precision time synchronization and wide-range delay compensation.

Kalman Filter Based Pseudo-Code Ranging and Carrier Phase Measurement for Fiber Optical Time Transfer Method

In the optical fiber time transfer system, fiber optic time transfer is limited by the pulse signal time delay measurement precision, and cannot benefit from a higher precision of the carrier phase information. Its transfer precision compared to the frequency transfer exists in a larger gap transfer. This paper proposes a time delay measurement method based on carrier phase and pseudo-code ranging for optical time transfer. The time signal is modulated with pseudo-random code and carrier at the transmitter, and the time delay is measured at the receiver by the methods of pseudo-code ranging and carrier phase measurement. The time transfer is achieved by eliminating the transmission link delay through a two-way method. The first-order difference value of the carrier phase measurement and the pseudo-code ranging measurement are used as the observation quantities, and they are fused through a Kalman filtering method to finally obtain the high-precision time difference measurement. We validate the theory on the common-clock experimental platform over a 50 km fiber link, The time transfer stabilities of the systems are 5.2254×10−14/s and 2.146×10−16/104 s (modified Allan deviation), 3.0169×10−14/s and 1.2392×10−12/104 s (time deviation). The standard deviation of the time transfer system after fusion can reach 2.4255 ps.

Out-of-Band Fibre-Optic Time and Frequency Transfer Using Asymmetric and Symmetric Opto-Electronic Repeaters.

Signal repeaters for fibre-optic communication can be realized with back-to-back connected transceivers. This configuration can provide high gain (≈30 dB) at low cost, and the needed semiconductor lasers and modulators can be realized for practically any relevant wavelength. Unfortunately, for time and frequency transfer the uncorrelated wavelength drifts in the transceiver lasers can compromise transfer stability, and device replacement due to failure may result in large time offsets that have to be measured via global navigation satellite services. This work demonstrates that good results can nevertheless be obtained with standard telecom DWDM tranceivers over long time periods (years). More importantly, a simple wavelength-symmetric repeater is proposed that can be used to cancel the detrimental effects of wavelength drifts and which lessens the need for link re-calibrations after transceiver replacements. In a proof-of-concept test setup, timing drift due to wavelength drift of a repeater laser is reduced by approximately two orders of magnitude.

Secure Two-Way Fiber-Optic Time Transfer Against Sub-ns Asymmetric Delay Attack With Clock Model-Based Detection and Mitigation Scheme

Secure Two-Way Fiber-Optic Time Transfer Against Sub-ns Asymmetric Delay Attack With Clock Model-Based Detection and Mitigation Scheme

A High-Precision Transfer of Time and RF Frequency via the Fiber-Optic Link Based on Secure Encryption

This paper presents a high-precision transfer system of time and RF frequency via the fiber optic link based on secure encryption. On the basis of the two-way time transfer of optical fiber, a security strategy composed of an SM2 encryption algorithm is introduced, which can resist the security risk of time information being tampered with. The experimental results show that the developed picosecond-precision fiber-optic time transfer equipment can ensure high stability while realizing the encryption function. Time synchronization stability in terms of time deviation (TDEV) of 1 PPS can reach around 10.7 ps at 1 s and 7.1 ps at 10 s averaging time. The stability of the 10 MHz frequency can reach around 4.7 × 10−12 at 1 s and 1.1 × 10−12 at 10 s averaging time. There is no significant difference in time transfer accuracy, compared with unencrypted conditions. Furthermore, this paper realizes a ring time transfer network via a 150 km fiber-optic link with three nodes using three devices. The TDEV of 1PPS can reach around 20.8 ps at 1s averaging time. This paper provides a reference to establish a high-precision, safe, and stable time synchronization fiber network in the future.

Fast and on-line link optimization for the long-distance two-way fiber-optic time and frequency transfer.

The performance of long-distance two-way fiber-optic time and frequency (T/F) transfer is directly affected by link parameters. In this paper, a fast on-line scheme to maximize the signal-to-noise ratio at the T/F transfer link ends (denoted by SNRW and SNRE) is proposed by optimizing the gains of bidirectional Erbium-doped fiber amplifiers (Bi-EDFAs). The dedicatedly defined objective optimization functions, ln(1/SNRW + 1/SNRE) for microwave and time transfer, and ln(1/SNRW) for optical frequency transfer, are proven to be a convex function and enable the computation time to be reduced significantly by employing the fast and mature convex optimization algorithm. The linear equations about the objective optimization function and the fiber parameters required by optimization are constructed, and then the fiber parameters are proposed to be on-line obtained accordingly by measuring the concerned SNR at the link end under different EDFA gain configurations and solving the linear equations. The transmission matrixes for the proposed optimization scheme are further derived, which can simplify the implementation in the long-distance multiple EDFAs links. The simulation shows that the proposed algorithm can obtain the optimal parameters consistent with the reported experimental results, and can reduce the computation time by more than four orders compared with the scanning optimization method for a link with more than 6 Bi-EDFAs.

High precision time transfer based on laser wavelength tracking

In a high-precision optical fiber time transfer system, in order to solve the scientific problem of time transfer dispersion deviation caused by the inconsistency of the two-way laser wavelengths, a high-precision time transfer method based on laser wavelength tracking is proposed in this paper. In the two-way time comparison, the laser emitted by the local site is used as a reference, and other lasers in the system track the reference, and the difference between the two-way laser wavelengths is small enough and stable for a long time, thereby greatly reducing the time transfer deviation caused by the inconsistency of the two-way laser wavelengths. In order to verify the performance of laser wavelength tracking, a double-layer temperature controlled externally modulated laser is designed, an experimental device for automatic laser wavelength tracking is designed, and laser wavelength tracking experimental modules are developed. The results show that the standard deviation of wavelength jitter is 55 fm, and the long-term relative stability of laser wavelength tracking is better than 5 fm@1×10<sup>4</sup> s, which ensures that the two laser wavelengths can remain relatively stable for a long time. In the case of long-distance fiber time transfer, by optimizing the setting value of the wavelength difference between each laser and the reference light on the link, the dispersion deviation of time transfer can be further reduced. In order to verify the feasibility of the high-precision optical fiber time transfer method based on laser wavelength tracking, a long-distance multi-station optical fiber time transfer experimental setup is built in our laboratory. The experiment is carried out in the laboratory by using 0.005, 250, 500, 750 km optical fiber links. The experimental results obtained on 750 km link show that the time transfer deviation is better than 5 ps, the stability is 4.7 ps@1 s, 0.4 ps@4×10<sup>4</sup> s, and the uncertainty is 8.4 ps. In the engineering application of optical fiber time transfer, by optimizing the working sequence of the system, the holding time length of the remote site clock can be reduced, and the accuracy of optical fiber time transfer can be further improved, which lays a foundation for realizing high-precision long-haul optical fiber time transfer.

Optimization of time and frequency fiber-optic links exploiting bi-directional amplifiers, based on real-time performance measurement

Long-distance fiber-optic time and frequency transfer links exploit bi-directional optical amplifiers whose gains need to be optimized to limit the noise propagating along with the desirable signals. Unlike the usual optimization of such links that are based on simulations or on a trial and error approach, we propose here a technique where the link is initially set-up with a pre-set algorithm, using optical powers measured in the amplifiers installed along the link. After that the final optimization is performed based on jitters measured in the end terminals of the link with a dedicated hardware. All the required parameters (i.e. the optical powers and jitters) are measured in a real-time in a live link. Various methods of assessment of the link performance, using the jitter measurements performed either in one or both end terminals, as well as various optimization criteria (like minimax, square root of sum of RMS jitters or analysis of a single-end jitter measurements), are investigated. The developed procedures were tested experimentally in the laboratory and also during a field trial in a real fiber telecom infrastructure. Obtained results (55% jitter reduction, 7 dB SNR improvement, comparing to the link state after its initial set-up) suggest that the proposed solutions allow optimizing effectively the link performance and may be used to supplement or replace simulation methods, particularly on initial start-up or when reconfiguring the link (e.g. due to a link route modification or after a fiber break).

High-precision fiber-optic two-way time transfer network with time–frequency transform measurement

Time transfer over optical fiber is finding numerous applications and attracting extensive research. In this paper, we present a high-precision fiber-optic two-way time transfer network based on the time–frequency domain transform (TFDT) measurement. The time signals of different sites are embedded into the chirp pulse signals, the relative time interval of which is converted to the mutual beat frequency and then measured in frequency domain. Accordingly enhancing the transformation factor can improve the measurement accuracy to enable the two-way fiber-optic time transfer with picosecond-level resolution. The accuracy of the TFDT measurement is experimentally tested under different pulse widths and signal-to-noise ratios. Furthermore, we implement a three-node time transfer network over a 5.9 km fiber loop link by the picosecond-precision two-way time transfer. The time stabilities of both remote sites can reach around 0.36 ps at 10000 s averaging time.

Picoseconds-Accurate Fiber-Optic Time Transfer With Relative Stabilization of Lasers Wavelengths

In this paper we focus on analyzing the accuracy of time transfer in bidirectional fiber optic links. It has been pointed out that one of the main uncertainty contributions in such links is related to insufficient stabilization of the lasers’ wavelengths and to the asymmetry of propagation caused by the fiber's chromatic dispersion. To reduce these contributions, we propose a scheme where the difference (offset) of the wavelengths of forward and backward lasers is stabilized and where optical interleavers are used as diplexers. This allows implementing a λ-swapping technique to assess dispersion-related link asymmetry during the link calibration. For the investigation of these approaches and the measurement of the associated stability we utilize a frequency-synchronized offset laser module (FSOLM) to realize an offset of 25 GHz. In this paper, we show that with the proposed λ-swapping technique the uncertainty associated with the propagation asymmetry due to fiber chromatic dispersion can be reduced to values below 2.5 ps for links up to 1000 km long, making it insignificant compared to other uncertainty contributions. A laboratory proof-of-concept experiment, in which distances up to 540 km were tested, showed good agreement between the measured and the anticipated propagation delay of investigated links.

Bidirectional optical amplifier for time transfer using bidirectional WDM transmission

In this letter, we propose a bidirectional optical amplification scheme for fiber-optic time transfer based on bidirectional wavelength-division multiplexing (WDM) transmission. The proposed scheme employs single unidirectional erbiumdoped fiber amplifier (Uni-EDFA) for commercial optical networks to implement bidirectional optical amplification. Since including isolators, the effect of backscattering due to the accumulated amplification can be efficiently suppressed by the proposed amplifier. The proposed scheme is validated over different length fiber links in laboratory. Experimental results show that the proposed amplifier can support fiber-optic time transfer over several thousands of kilometers with an additional bidirectional delay difference fluctuation at picosecond magnitude.

Time transfer through the optical supervisory channel in wavelength division multiplexing systems.

We present an optical fiber time transfer scheme through the optical supervisory channel (OSC) in wavelength division multiplexing (WDM) systems. A sub-band of the standard OSC band is used to transmit time signals by only adding sub-OSC filters in commercial WDM systems. The transmission of the existing communication data service is not affected. The wavelengths of bidirectional time transmission in the same sub-band can be very close, or even the same, to maximize the bidirectional propagation symmetry and improve the precision of time transfer. Combined time, optical supervisory signal, and communication data transmission is experimentally demonstrated along a single 100km optical fiber in a WDM system in laboratory.

Optical Amplification for BTDM-SFSW-Based Time Transfer

Optical amplification, which is necessary for extending the distance of fiber-optic time transfer, has significant impacts on the time transfer performance. In this paper, we investigate and optimize the optical amplification for the time transfer based on bidirectional time division multiplexing transmission over a single fiber with the same wavelength (BTDM-SFSW). Three optical amplification schemes, namely single fiber bidirectional amplifier (SFBA), single fiber bidirectional amplifier with magneto-optical switch (SFBA-MOS), and single-fiber bidirectional-transmission unidirectional optical amplifier (SFBT-UOA) are considered. For each optical amplification scheme over different fiber links, the timing jitter and uncertainty of time transfer are theoretically and experimentally analyzed. A simple timing jitter optimization method for the BTDM-SFSW links is proposed. Further evaluations are performed from the point of view of system complexity, operation cost, and compatibility with telecom optical networks. Results show that SFBA is more suitable over shorter fiber links for its lower complexity and cost, while SFBA-MOS or SFBT-UOA should be employed for longer fiber links to achieve the accepted time transfer performance.

Large-dynamic-range time pre-compensation scheme for fiber optic time transfer

In this paper we experimentally demonstrate a transmission delay compensation scheme for precise fiber-optic time transfer. The scheme is based on a clock counter and an electronic variable delay line, which theoretically can provide unlimited compensation range. We perform successive tests in three optical fiber links of different lengths in which both continuous drifts and abrupt hop of the transmission delay are effectively compensated. The total transmission delay variation induced in the experiments is much larger than most of the reported cases. This large-dynamic compensation scheme is quite suitable for time transfer links whose transmission delay varies a lot.

Uncertainty analysis of BTDM-SFSW based fiber-optic time transfer

In this paper, the uncertainty of time transfer based on a bidirectional time division multiplexing transmission over a single fiber with the same wavelength (BTDM-SFSW) is investigated via theoretical models and experimental measurements. According to the principle and system architecture, the uncertainty evaluation schemes for BTDM-SFSW based time transfer are presented and the uncertainty sources are identified accordingly. In order to avoid the effect of the temperature-dependent fiber delay, the measured time intervals at two sites are regarded as a whole to obtain an overall uncertainty of time interval measurements. For the uncertainty of time transfer modem calibration, aside from the type A uncertainty obtained under the applied calibration scheme, the system reproducibility against practical operation and the contribution of optical power-dependent receiving delays are also included. A mathematical model considering fiber dispersion, polarization mode dispersion (PMD) and the Sagnac effect is established to evaluate the uncertainty from the fiber link. The characteristics of the uncertainty sources in a long-distance fiber-optic time transfer testbed are then explored in detail. The combined expanded uncertainties with a coverage factor of 2 are calculated and experimentally validated over various non-calibrated fiber extensions.

High-precision long-haul fiber-optic time transfer between multi stations

To achieve high-precision fiber-optic time transfer, the method of two-way transmission is usually used. Therefore in this paper we propose to develop a high-precision long-haul fiber-optic time transfer between multi stations by simultaneously transferring the 1 pluse per second signal, time code signal and 10 MHz frequency signal over single fiber with the same wavelength, and adopting the time division multi address (TDMA) as well as the purification and regeneration method at individual station. In this proposal, the equipment at each remote station has its own address, and the equipment at the local station can establish the periodic two-way time transfer with any remote station by using the TDMA method, therefore each remote station is synchronized with the local station. To avoid the superimposed effect of optical noises during propagation in fiber, the optical-electro-optical relay amplifiers are utilized. In the meantime the propagation delay of the fiber link is compensated for at each remote station. With the self-developed engineering prototypes, the experimental verifications are subsequently conducted both in laboratory and real field. In the laboratory, the experimental setup is built by cascading 11 rolls of 50 km-long fiber coils, and locating three monitoring devices at different fiber distances of 50, 300, and 550 km from the local station. The stabilities of the time transfer at these three points are achieved to be 16.7, 16.8, and 18.4 ps in standard deviation, and the time deviations are 1.78, 2.09, and 2.92 ps at an averaging time of 1000 s respectively. In the real field test, a field fiber link of 871.6 km in length is utilized, along which 11 self-developed time-frequency transceivers are set at the cascaded fiber-optic stations. Since only the 11th remote station is co-located at the local station, the performance and the time transfer between the 11th remote station and the local station are measured accurately. The time transfer is experimentally demonstrated with the time standard deviation of 29.8 ps and the time deviations of 3.85 ps/1000 s. The timing uncertainty on the field fiber link is also checked and gives a value of 25.4 ps. To further improve the long-term stability of time transfer, the more accurate thermal control of the lasers used in the system should be adopted to reduce the optical wavelength drift. By compressing the bandwidth of the phase locked loop module in each remote device, the short-term stability of time synchronization can also be better. This proposal can also be extended to the fiber networks with star-shaped and chain-shaped connections. Therefore time synchronization in even larger areas and more stations can be realized.

High-Precision Ultralong Distance Time Transfer Using Single-Fiber Bidirectional-Transmission Unidirectional Optical Amplifiers

This paper demonstrates a fiber-optic time transfer over a 6000 km noncalibrated fiber link using proposed single-fiber bidirectional-transmission unidirectional optical amplifiers (SFBT-UOAs). The proposed SFBT-UOA employs a 2 × 2 optical switch to enable the time-division-multiplexed time signals for both directions to pass through the same UOA in different time slots. The bidirectional delay symmetry is guaranteed to the maximum extent by allowing the time signals to be transmitted back and forth using the same wavelength over a single fiber. Meanwhile, the noises suffered in the systems employing single fiber bidirectional optical amplifiers (SF-BOAs), such as double Rayleigh backscattering, are effectively suppressed. Consequently, high-precision fiber-optic time transfer can be implemented over ultralong distance by adopting the proposed SFBT-UOAs. Moreover, the SFBT-UOA can support the conventional network services via wavelength division multiplexings (WDMs) and, therefore, offers the potential to perform high-precision time transfer over long-haul commercial fiber links. Over 6000 km, the measured time deviations of fiber-optic time transfer in the laboratory are less than 190 ps/s and 61 ps/105 s, respectively. The calculated combined uncertainty is not beyond 70 ps, which agrees well with the experimental verification over the noncalibrated fiber link.

链路时延波动对光纤时间传递稳定性的影响

链路时延波动对光纤时间传递稳定性的影响

温度对双向时分复用光纤时间传递精度的影响

温度对双向时分复用光纤时间传递精度的影响

Stable fiber-optic time transfer by active radio frequency phase locking.

In this Letter we demonstrate a fiber link capable of stable time signal transfer utilizing our active long-distance radio frequency (RF) stabilization technology. Taking advantage of the chromatic dispersion in optical fiber, our scheme compensates dynamically the link delay variation by tuning the optical carrier wavelength to phase lock a round-trip RF reference. Since the time signal and the RF reference are carried by the same optical carrier, a highly stable time transfer is achieved at the same time. Experimentally, we demonstrate a stability of the time signal transfer over 50-km fiber with a time deviation of 40 ps at 1-s average and 2.3 ps at 1000-s average. The performance of the RF reference delivery is also tested, with an Allan deviation of 2×10(-15) at 1000-s average. According to our proposal, a simultaneous stable time and frequency transfer is expected.