In this paper, we demonstrate a wavelength division multiplexing (WDM)-based system for simultaneously delivering ultra-stable optical frequency reference, 10 GHz microwave frequency reference, and a one pulse per second (1 PPS) time signal via a 50 km fiber network. For each signal, a unique noise cancellation technique is used to maintain their precision. After being compensated, the transfer frequency instability in terms of the overlapping Allan deviation (OADEV) for the optical frequency achieves 2 × 10${}^{-17}$/s and scales down to 2 × 10${}^{-20}$/10,000 s, which for the 10 GHz microwave reference, approaches 4 × 10${}^{-15}$/s and decreases to 1.4 × 10${}^{-17}$/10,000 s, and the time uncertainty of the 1 PPS time signal along the system is 2.08 ps. In this scheme, specific channels of WDM are, respectively, occupied for different signals to avoid the possible crosstalk interference effect between the transmitted reference signals. To estimate the performance of the above scheme, which is also demonstrated in this 50 km link independent of these signals, the results are similar to that in the case of simultaneous delivery. This work shows that the WDM-based system is a promising method for building a nationwide time and frequency fiber transfer system with a communication optical network.

A polarization-insensitive recirculating delayed self-heterodyne method (PI-RDSHM) is proposed and demonstrated for the precise measurement of sub-kilohertz laser linewidths. By a unique combination of Faraday rotator mirrors (FRMs) in an interferometer, the polarization-induced fading is effectively reduced without any active polarization control. This passive polarization-insensitive operation is theoretically analyzed and experimentally verified. Benefited from the recirculating mechanism, a series of stable beat spectra with different delay times can be measured simultaneously without changing the length of delay fiber. Based on Voigt profile fitting of high-order beat spectra, the average Lorentzian linewidth of the laser is obtained. The PI-RDSHM has advantages of polarization insensitivity, high resolution, and less statistical error, providing an effective tool for accurate measurement of sub-kilohertz laser linewidth.

We demonstrate a coherent phase transfer via a 224 km cascaded fiber link comprising two 112 km links stabilized by two phase-locking loops, respectively. The optical signal is regenerated employing heterodyne optical phase locking (HOPL) after the first 112 km transfer. With a gain of more than 50 dB, the HOPL is capable of tracking the frequency of the incoming carrier with a fluctuation of 0.48 mHz and preserving the instability of the incoming laser to $6\times {10}^{-20}$ at 1000 s. The phase noise cancellation of each span is investigated, and the out-loop transfer instability of the 224 km link reaches $7.7\times {10}^{-19}$ at 10,000 s. The relation between the transfer instability of each span and that of the whole link is also deduced in the paper, in agreement with experimental results of the 224 km link.

We report on the realization of a multiple-branch optical frequency transfer method, which can be adopted to deliver an optical reference to multiple independent remote sites. In comparison with the previous techniques, the proposed technique mitigates the frequency allocation constraints of the driving frequencies for the acousto-optic modulators (AOMs) at the remote sites by adding a short loop fiber link at the local site. The end-to-end optical path phase noise detection and compensation for each remote site is simultaneously performed at the remote site, which significantly simplifies the configuration at the local site, and can improve the signal-to-noise ratio (SNR) and reduce the cycle slip rate. We also conduct theoretical analysis for the effect of frequency aliasing from the perspective of different refractive index in triple-pass optical frequency transfer, analyze the effect of the SNR and cycle slips on the fractional frequency instability, and present a comprehensive equation for calculating noise contributions from end-to-end optical frequency transfer over the multiple-branch fiber network, demonstrating that the implementation of the fiber noise cancellation (FNC) for the each branch consisting of two fiber topologies with the single FNC setup is practical. This technique paves the way to integrate the optical frequency networks into the existing fiber communication networks with the higher compatibility and reliability.

We for the first time present an all-passive cascaded optical frequency transfer technique over a fiber link, in which the phase noise of the each fiber link introduced by environmental perturbation and the laser repeater station are simultaneously compensated by using passive phase noise cancellation. The laser repeater station consisting of a cavity-stabilized laser is employed as a regenerative amplifier with a 45 dB optical gain. We demonstrate a cascaded optical frequency transfer with two 100 km spooled fiber links and one laser repeater station, illustrating an instability of $4.8\times 10^{-15}$ at the integration of 1 s and $4.9\times 10^{-19}$ at 10,000 s improved by a factor of 2 compared to the single-span 200 km fiber link. Dividing the whole fiber link into sub-sections could significantly improve the bandwidth of the phase noise compensation. Additionally, all-passive phase noise cancellation has the advantage of avoiding the servo bumps in the transferred light relative to the conventional active technique. The proposed technique provides a promising solution for high-performance and robust long-distance optical frequency transfer.