## 1 Introduction  
Laser-based high-precision time and frequency transfer is a method that uses a laser modulation technique to transmit extremely accurate time and frequency signals between sites [1; 2; 3; 4]. The key idea is to use the high stability and coherence of lasers to achieve accurate transfer and comparison of time and frequency. In the past decade, high-precision laser time and frequency transfer has rapidly developed and now plays an important role in many application fields such as clock standards [5], satellite navigation systems [6], communication networks [7; 8], basic scientific research [9], etc. Many studies have reported on the use of lasers in optical fiber and free-space links to achieve high-precision time and frequency transfer experiments, with transmission distances ranging from kilometers to hundreds of kilometers, and accuracy reaching picoseconds or femtoseconds [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. With the rapid development of time and frequency transfer techniques for synchronization networks, the demand for the transfer between multiple sites in synchronization networks is also rapidly increasing. Although the existing time and frequency transfer technique can achieve long distances and high precision, most schemes are only applicable to point-to-point applications and are difficult to apply directly to the fields of multiple sites. At present, there are a few experimental reports on multiple-access optical fiber and the free-space time-frequency transfer technique, but these schemes are not very effective, and there are also certain limitations. For example, the independent lasers used in the sites go through the optical link two or three times, which will result in the loss of at least half of the optical power and also reduce at least half of the signal-to-noise ratio of the optical signal. The use of a wavelength division multiplexer (WDM) reduces the efficiency of the optical channel. This is because, with as many slave sites as there are, they need to occupy many wavelength division multiplexing channels; the slave site uses multiple photodetectors, which increases the complexity of the system and the cost. Therefore,it is of great significance to study practical methods to overcome the certain limitations above, which can be further applied to multiple-access time and frequency transfer in synchronization networks [34; 35; 36; 37]. In this paper, we propose a multiple-access time and frequency transfer in optical fiber and free-space links based on an optical frequency comb (OFC), which can realize high-precision time and frequency transfer between a master site and multiple slave sites. The master site light source uses an OFC, and each slave site adopts an independent frequency-modulated continuous wave (CW) laser. By using the master-slave bidirectional phase comparison and the multi-channel time-phase difference measurement, high-precision time and frequency transfers between multiple sites are realized. With this transfer scheme, we have built a multiple-access time and frequency transfer experiment in both fiber and free space, which includes a master site and two slave sites, and the transfer distance between the master and each slave site is about 4 km (3.9 km for optical fiber and 100 m for free-space link).