## 2 Limit of Optical Phase Compensation  
The most efficient way to achieve RF transfer over fiber link with a continuous wave (CW) laser is to use an electro-optic modulator or a current modulation technique to load the microwave onto the laser. After the laser light is modulated, the light is sent to the fiber link. On the other sites of the fiber, the light is extracted, and a RF signal is recovered by a high-speed diode. This process achieves a simple point-to-point fiber-based frequency transfer. However, in practice, due to the influence of environmental factors on the fiber link, the optical signal will introduce a timing fluctuation when it propagates through the optical fiber. To suppress the timing fluctuation in the fiber link, a phase compensation technique should be proposed. In general, the optical phase compensator is widely used to suppress the timing fluctuation in a frequency transfer. Figure 1 shows the schematic of a fiber-based RF transfer with a fiber stretcher-based phase compensator. On the transmitter site shown in Figure 1, there is a light source provided by a CW diode laser. The light source produces a 1550 nm laser light beam (COFIBER, NLL-1550). The light passes through an electro-optic modulator (EOM), and the intensity of the light is modulated by a RF signal from a frequency source (RS, SMA100B). The modulated light then passes through a circulator and a fiber stretcher, where the stretcher introduces a phase delay to the light. The phase-delayed light is sent into a fiber-optic  
Figure 1: Schematic of typical fiber-based frequency transfer with optical compensation. CW: continuous wave, EOM: electro-optical modulator, BPF: band-pass filter, HV: high voltage, PD: photodiode, PI: proportional-integral controller, FFT: fast Fourier transform. link, on the receiver site, the light is received by another circulator and a coupler, and the transmitted light is split into two parts. The high-power part is sent back to the fiber link by the circulator, while the remaining light is converted by a high-speed InGaAs photodiode (PD2) into an RF signal that can be used by the users. At the same time, the returning light reaches the transmitter site through the same optical fiber link. On the transmitter, the returning light passes through the same fiber stretcher again, and this time, another same phase delay is introduced. The light passing through the fiber stretcher is split out in the circulator, and converted to another RF signal by a high-speed InGaAs photodiode (PD1). With a phase detector or a mixer, the phase of the RF signal from PD1 is compared with the reference signal, so as to obtain an error voltage signal. Finally, the error signal is fed back to the fiber stretcher through a proportional-integral (PI) controller (AS, NB1005), that is, to achieve the goal of compensating for time fluctuations. It should be noted that when two different directions of light pass through the same optical path, the vibration and temperature drift from the optical fiber link will introduce the same timing fluctuation to the transmission link in both directions. With the phase compensation scheme shown in Figure 1, the timing fluctuation suppression can be successfully achieved in the frequency transfer over a short-distance optical fiber link. However, for a long-distance link, there is a potential problem that the amplitude of the timing fluctuation may exceed the adjustable range of the fiber stretcher. This is because that a normal fast PZT-based stretcher can only provide a maximum millimeter-scale tunable range. For example, the maximum delay range of a commercial fiber stretcher (General Photonics, FST-001-B) is about 3 mm (10 ps in time domain), and the timing fluctuation introduced in a long fiber may exceed this range. The limit of the range inevitably causes the timing fluctuation in fiber link to fail to be compensated, thus seriously affecting the performance of the frequency transfer.