## 2 Scheme of Multiple-Access Time and Frequency Transfer  
One of the most commonly used methods in multiple-access time and frequency transfer is that the master site and each slave site use an independent continuous wave (CW) laser as the light source [34; 37]. Assuming the frequency of the radio-frequency (RF) signal in the master site is _2f_, the signal is then loaded onto the optical carrier by an amplitude modulation electro-optic modulator (EOM) and the modulated carrier signal is transmitted directly from the master site to each slave site through the link after beam splitting. The user can recover the transmitted _2f_ frequency signal from the optical carrier by the photodetectors. At the same time, each slave site uses an RF signal with a frequency of \(f\), loads the signal onto the optical carrier, and sends the optical signal to the master site and back to the original slave site in the same link through wavelength multiplexing. At the slave site, the two signals with frequencies of _2f_ and \(f\) can be mixed, and the error signal containing phase drift information can be extracted and fed back to the frequency source of the slave site, thereby achieving the stabilization of the phase of the RF source. Although this method can realize multiple-access time and frequency transfer on a single optical fiber, and therefore has the application capability of a practical system, such methods also have certain limitations. For example, firstly, each slave site needs to use two photodetectors, which greatly increases the cost and complexity of the system when applied to a large range of multiple sites; then, the system uses a large number of wavelength division multiplexers, and the optical signal of the slave site needs to go back and forth twice on the same optical channel, which not only increases the complexity of the system but also reduces the intensity of the optical signal and loses the signal-to-noise ratio (SNR). Therefore, the traditional method of using CW laser and wavelength multiplexing to achieve multiple-access time and frequency transfer cannot be applied to the future large-scale multiple-site scenario for high reliability, low cost, and low complexity. In view of the limitations of the above traditional techniques, we propose a multiple-access time and frequency transfer technique that combines a femtosecond OFC with CW lasers. The schematic of the technique is shown in Figure 1.  
In our scheme, the light source of master site A is a femtosecond OFC, assuming that its repetition frequency is \(F_{\text{rep}}\) and the initial phase is \(\varphi_{\text{A}}\). The pulsed laser generated by the OFC passes through an optical beam splitter (BS) and outputs one part to the photodetector PD1 to generate an electrical signal with \(F_{\text{rep}}\). We use a phase-locking technique to lock the repetition frequency signal to an external reference signal, thus achieving the stability of the femtosecond OFC [38]. At the same time, the other part from the optical beam splitter is passed through a circulator and an optical coupler to output multiple optical signals. Each optical signal can be disseminated to each slave site through an optical link (optical fiber or free space). To facilitate the explanation of the scheme, Figure 1 shows the situation where the multiple-access time and frequency transfer is realized between master site A and two slave sites, B and C. In the actual system, more slave sites can be designed. At slave sites B and C, each has a CW laser as its light source. The frequencies of RF signals transmitted by the slave sites have been carefully designed while corresponding to the repetition frequency and harmonic frequencies of the OFC in the master site. As shown in the figure, the frequency of site B is the repetition frequency \(F_{\text{rep}}\), and the initial phase is \(\varphi_{\text{B}}\). The frequency of site C is the second harmonic of the repetition frequency \(2F_{\text{rep}}\), and the initial phase is \(\varphi_{\text{C}}\).