The fiber optic microwave frequency transmission technology is based on the loopback method to detect and suppress fiber optic noise. Under the adoption of existing communication fiber links, the frequency transmission stability can reach 10^-18/d-10¹⁹/d, which meets the remote transmission and comparison of current microwave atomic reference frequency standards, with broad application prospects. At present, most of the solutions are point-to-point master-slave station transmission structures, and in many application scenarios, it is necessary to deliver a reference frequency source to multiple users (multiple access), and even add a midway download point in the transmission fiber link, such as square kilometer array (SKA), radio telescope array, and very-long-baseline interferometry (VLBI). The high-precision ground-based time service project undertaken by the National Time Service Center, Chinese Academy of Sciences includes the construction of a thousand-kilometer level fiber optic microwave frequency transmission network, which employs the unique electrical phase compensation transmission scheme of the center. As a supplement to the backbone transmission network, the local frequency distribution network needed for the future is constructed, and a set of multi-access transmission and mid-download equipment schemes compatible with the existing system is designed to enhance the regional service capability of the high-precision ground-based timing system. As a result, the standard frequency signal in the backbone network can reach the client within 100 km of the urban node to complete the local frequency signal distribution task of the city node.

The system diagram of multi-access fiber microwave frequency transmission technology is shown in Fig. 1, where TX is the common transmitting end and RX1-RXN are multiple receiving ends, or there are multiple addresses. At the transmitter TX, the reference signal is modulated onto the laser carrier by a Mach-Zehnder modulator (MZM) and an external modulation. Then, by adopting a fiber splitter, the signal light is divided into N channels and transmitted to N addresses respectively. Firstly, a principle analysis of microwave frequency transmission technology of multi-access fiber is carried out to show the feasibility of the experimental scheme. Then the experimental verification is carried out in the laboratory: the 2 km fiber spool and 50 km fiber spool are connected by the 2×2 fiber coupler, and the beam splitting ratio of the fiber coupler is 9∶1. After the 50 km fiber spool, the fiber dispersion compensation fiber of the corresponding length is fused to reduce the dispersion effect in the fiber link.

By utilizing the above experimental design and device link, Symmetricom 5125A is adopted to measure the residual phase noise at the receiving end of the multi-access fiber optic microwave transmission technology. The results are shown in Fig. 6. The phase noise spectrum of the 7.5 GHz signal at the receiving end is greatly optimized via phase noise elimination of the scheme. Meanwhile, the stability of multi-access fiber microwave transmission technology is measured by phase comparison method. The voltage signal is tested by digital multimeter 3458A according to the test block diagram in Figs. 4 and 5. The results are shown in Fig. 7. The frequency stability of the remote 7.5 GHz signal is 3.5×10^-14/s and 1.2×10^-17/10⁵ s. The stability of mid-download is 4.1×10^-14/s and 6.5×10^-17/10⁵ s. In the process of multi-access fiber microwave frequency transmission technology, the unidirectional erbium-doped fiber amplifier (EDFA) is employed to amplify the optical power, which will increase the asymmetry in the fiber link and affect the propagation stability of the system. Due to the nonlinear effect of devices, nonlinear effects inevitably occur during signal processing, which affects the phase detection accuracy. To avoid the influence of nonlinearity, we adopt 1.875 GHz and 3 GHz as the propagation frequencies. Signals at 1.875 GHz and 3 GHz do not have harmonic overlap and cannot generate intermodulation components of similar frequencies.

We propose an fiber optic microwave frequency transmission system with multi-access frequency transmission and midway download, which adopts different optical wavelengths to transmit microwave signals and thus avoid the effects of parasitic reflection and backscattering of signal light. The scheme has two main features. Firstly, the link noise compensation is implemented in the receiving end by electrical phase compensation method, and the fiber optic link is compensated for due to temperature noise disturbance caused by external environmental changes such as stress. Secondly, the dispersion compensation fiber in the transmission system is also placed at the receiving end to compensate for the dispersion impact in the fiber link. The stability of remote frequency transmission is 3.5×10^-14/s and 1.2×10^-17/10⁵ s, and that of mid-download is 4.1×10^-14/s and 6.5×10^-17/10⁵ s. The index of multi-access fiber optic microwave frequency transmission system can meet the requirement of microwave atomic frequency signal long-distance transmission in various applications, with a wide application prospect.