Fiber-optic Frequency Transfer Research Articles

Analysis and Reduction of Nonlinear Effects in Optical Fiber Frequency Transfer

Nonlinear effects in optical fiber frequency transfer have a significant impact on the precision of frequency transfer. We investigate the main nonlinear effects, including the Brillouin scattering and the Raman scattering, in optical fiber frequency transfer through theoretical and simulation calculations in detail. The calculation results show that the threshold powers of the Brillouin scattering and the Raman scattering decrease with the increase in the fiber length; however, the fiber length has little to no impact on the threshold powers when the fiber length is greater than 10 km. The threshold powers, including the Brillouin scattering and the Raman scattering, increase as the attenuation coefficient increases. Conversely, when it comes to the gain coefficients, the outcomes exhibit a reverse trend. When the linewidth Δvlaser of the laser source is from 1 Hz to 1 MHz, the linewidth Δvlaser does not affect the threshold powers of the Brillouin scattering. This study seeks to offer design guidance aimed at mitigating nonlinear effects in optical fiber frequency transfer. The calculated results hold considerable potential in guiding various applications reliant on Brillouin and Raman scattering properties, such as laser technology and optical fiber sensing.

Stable frequency transfer for clock synchronization for telecom networks

A stable and reliable clock synchronization system that transfers reference clocks to nodes is essential for telecom networks. Mainly, the clock synchronization of telecom networks is carried out through satellite links. However, for the evolving ubiquitous wireless communications, especially in some harsh environments, such as underground and trenches, the satellite-based clock synchronization scheme bears inevitable obstacles, which may cause strategic risks for the telecom networks, since no control and manipulation can be imposed on the satellite systems. Therefore, to ensure the robust operation of telecom networks with performance guarantees, some auxiliary means are desirable to assist clock synchronization that is solely based on satellite links. In this article, we report a phase-stabilized frequency signal transmission on branching optical fiber for clock synchronization for telecom networks. Particularly, the phase fluctuation due to optical carriers separation link and fiber link is compensated by a feedback network, which employs a high-precision voltage-controlled oscillator in the phase-locked loop to drive the acousto-optic frequency shifter for fast phase correction. Furtherly, the factors that cause performance limitations of the frequency distribution system are analyzed. Eventually, the experiment results obtained show that a stabilized fiber-optic frequency transfer scheme can be used as a reliable method for clock synchronization with high accuracy.

Research on the impact of temperature variation on stability of optical fiber frequency transfer system

Temperature variation is a key factor affecting the stability of fiber optic frequency transfer systems. In a long-distance optical fiber frequency transfer system, a dispersion compensation fiber (DCF) placed in the machine room and a non-buried single-mode fiber (SMF) are collectively referred to as a bare optical fiber. Due to the impact of the external environment, the temperature of the bare optical fiber varies drastically, which seriously deteriorates the long-term stability (10,000 s stability) of the frequency transfer system. To investigate the relationship between the temperature variation of the bare optical fiber and the frequency transfer stability, we use VPItransmissionMaker optical simulation software to simulate and study the fiber frequency transfer system of the bare optical fiber under the impact of different temperatures. The simulation found that the system stability is 1.4 × 10 − 16 / 104 s in a 3000 km link when the temperature variation of DCF is 5°C, whereas the impact on the system stability is on the order of 10 − 16 / 104 s when the length of the non-buried SMF is more than 300 km and the temperature variation is more than 10°C. To verify the correctness of the simulation, the stability of the actual 80 km fiber optic frequency transfer system was compared with that of the simulation, and the stability of the simulation was found to be 4.2 × 10 − 17 / 104 s, whereas the stability of the experimentally measured system was 7.9 × 10 − 17 / 104 s, and the simulation results were basically consistent with the experimental results.

Determination of the orthometric height difference based on optical fiber frequency transfer technique

The development of remote frequency transfer techniques, especially the appearance of optical clocks with unprecedented stability, has prompted geoscientists to study their applications in geodesy. Using remote frequency transfer technique, by frequency comparison of two optical clocks at two points P and Q connected by optical fibers, one can measure the signal's frequency shift between them, and the geopotential difference between them can be determined based on the gravity frequency shift equation. Given the orthometric height of P, the orthometric height of Q can be determined. Since the present stability of the optical clock has achieved 1 × 10−18 or better and comparing the frequency transfer via optical fiber provides stability at 10−19 level, the optical clock network enables determining the orthometric height at centimeter-level. This study provides a formulation to determine the height difference at one-centimeter level between two points on the ground based on the optical fiber frequency transfer technique.

Cost-effective demonstration of fiber-optic frequency transfer.

In this paper, we demonstrate a cost-effective solution for fiber-optic frequency transfer. By employing the commercially available small form pluggable (SFP) transceivers and other components for telecom, the scheme is compatible with the existing telecom networks. An experiment testbed based on the common round trip frequency transfer is carried out to investigate the corresponding performance in detail. Compared with the conventional analog electro-optic interconversion scheme, the optical modulation and demodulation employing SFP transceiver has an inferior performance in terms of additive phase noise. However, the scheme enables a much larger operation range of receiving optical power for an optimal system performance, which indicates the certain capability of adapting to different fiber transmission links. On the basis of the established testbed, fiber-optic frequency transfer with a frequency of 1 GHz is demonstrated over a 50 km dispersion-compensated fiber link. Furthermore, the additive phase noise of -84dBc/Hz @1 Hz and -130dBc/Hz @10 kHz is achieved. Simultaneously, the Allan deviation (ADEV) of 9.6×10-14/s and 8.4×10-17/104s is reached, respectively. The scheme can provide a relatively feasible solution to implement fiber-optic frequency transfer over the existing telecom network infrastructure.

Formulation of Determining the Gravity Potential Difference Using Ultra-High Precise Clocks via Optical Fiber Frequency Transfer Technique

Based on gravity frequency shift effect predicted by general relativity theory, this study discusses an approach for determining the gravity potential (geopotential) difference between arbitrary two points P and Q by remote comparison of two precise optical clocks via optical fiber frequency transfer. After synchronization, by measuring the signal’s frequency shift based upon the comparison of bidirectional frequency signals from P and Q oscillators connected with two optical atomic clocks via remote optical fiber frequency transfer technique, the geopotential difference between the two points could be determined, and its accuracy depends on the stabilities of the optical clocks and the frequency transfer comparison technique. Due to the fact that the present stability of optical clocks achieves 1.6×10−18 and the present frequency transfer comparison via optical fiber provides stabilities as high as 10−19 level, this approach is prospective to determine geopotential difference with an equivalent accuracy of 1.5 cm. In addition, since points P and Q are quite arbitrary, this approach may provide an alternative way to determine the geopotential over a continent, and prospective potential to unify a regional height datum system.

Fiber-optic radio frequency transfer based on passive phase noise compensation with frequency dividing and filtering.

In this Letter, a passive phase noise compensation-based fiber-optic frequency transfer scheme with frequency dividing and filtering is proposed. The probe signal is divided at the remote site and effectively discriminated from the backscattering noises by electrical bandpass filtering at the local site. By combining the bidirectional transmission with the same wavelength on a single fiber, the scheme can reach the maximum bidirectional propagation symmetry and effectively suppress the effect of backscattering at the same time. The proposed scheme is experimentally demonstrated and compared with the scheme with backscattering. The relative frequency stabilities of 3.9×10(-14)/s and 1.2×10(-16)/10(4) s are reached for 40 km transfer, which obviously outperform the scheme with backscattering, especially in terms of short-term stability, and are closed to the floor of the back-to-back experimental system.

Fiber-Optic Joint Time and Frequency Transfer With Active Stabilization of the Propagation Delay

In this paper, we describe the extension of our fiber-optic frequency transfer system to the time transfer capability. In contrast to standard two-way transfer schemes which offer only comparisons of two distant clocks, our system displays distribution functionality, reproducing the time and frequency signals of the reference clock in the remote location. By using active compensation of the fiber delay fluctuations, we obtained a time deviation of 0.3 ps (for time transfer) and an Allan deviation of 1.2 × 10-17 (for frequency transfer) at 105-s averaging. The experiments presented were carried out using a 60-km-long fiber loop, forming a part of the real urban network around Krakow.