Replication of microwave signals is widely applied to microwave detection and frequency measurement [1]. Due to the high speed and large bandwidth, photonics-assisted replication of microwave signals, based on passive fiber coupler arrays [2] and fiber recirculating loops [3], are proposed to generate multi-frequency radar signals [4] and achieve high-speed analog-to-digital converter [5]. The method based on passive couplers makes it difficult to maintain the same pulse interval and pulse amplitude in the replicated pulse train [6]. On the other hand, an optical fiber replication loop has a tunable pulse delay and multiple replication times. It is more convenient to achieve decades of replication using a fiber recirculating loop. However, in fiber recirculating loops, nonlinear effects cannot be ignored since long fiber is used. For example, self-phase modulation (SPM) and cross-phase modulation (XPM) broaden the spectra of optical pulses [7,8]. Four-wave mixing (FWM) may transfer energy from one channel to adjacent channels [9]. Stimulated Brillouin scattering (SBS) leads to backscattered Stokes light. In previous work, digital signal processing (DSP) [10] has been successfully adopted to compensate for nonlinear effects,

However, in microwave photonics, an analog signal is transmitted rather than a digital signal. It is necessary to find out the physical principle of nonlinear optics to mitigate the effect. By precise control of optical gain, decades of replicated microwave signals have been achieved using a fiber recirculating loop. However, periodical amplitude fluctuations are still observed in the replicated microwave signals, which may lead to distortion of the optical pulse waveform. For instance, the fluctuation of the amplitude of the replicated signal in the fiber recirculating loop causes the fluctuation of the signal-to-noise ratio (SNR) and increases the bit error rate [11]. Fluctuated signal power along the system also significantly degrades waveform distortion compensation [12].

In 2019, an experiment based on a fiber recirculating loop was implemented to examine the space-time evolution of a modulationally unstable plane wave [13]. It is shown that periodic amplitude fluctuations may be attributed to the impacts of modulation instability (MI). MI is a steady-state modulation caused by the interaction of nonlinear and dispersion effects. It has been used in frequency comb generation [14], high-intensity optical rogue waves [15], and fiber interferometer switches [16]. To analyze the impacts of MI in a fiber loop, Adrien E. Kraych [17] studied the space-time dynamics of a modulationally unstable plane wave perturbed by a localized peak in a fiber system. In our previous work [18], the numerical simulations show that MI can amplify the noise component in the gain range and lead to the periodic broadening and compression of the bandwidth, which affects the amplitude fluctuations of replicated signals and reduces the SNR. According to previous research [19], the periodic fluctuation of replicated signal power may lead to the fluctuation of SNR along the fiber, which affects the SNR of the replicated RF signals. Moreover, high input power can shorten the fluctuation period, which deteriorates the stability, and makes it difficult to accurately acquire signals.

In this work, we analyze the amplitude variations in photonic-assisted microwave signal replication, which are caused by the nonlinear effects in fiber-recirculating loops. Numeric simulations are implemented. It is shown that periodical amplitude fluctuations can be caused by MI. A demonstrated experiment is carried out, which meets well with the simulations. The amplitude fluctuation and SNR evolution of the replicated microwave signal under different input optical powers are measured experimentally, and the period of the amplitude fluctuations is shortened with the increase of optical power. This work theoretically explains the amplitude variations in the photonics-assisted replication of microwave signals, which demonstrate the impacts of nonlinear effects on the photonic-assisted microwave signal replication.