Amplitude modulation to phase distortion conversion in photonic bandpass sampling link

Abstract

Recently, for the rapidly improved performance of femtosecond lasers, photonic bandpass sampling based radio frequency (RF) photonic link has shown performance that rivals that of the standard continuous wave (CW) architecture. The front-end photonic bandpass sampling technique overcomes the back-end high sampling rate and multilevel down-conversion requirement, which has found more and more applications in high-performance radars and antennas, aeronautics and astronautics system and compressive sensing, etc. [1-3]. As with the traditional CW-based analog photonic link (APL), the sampled RFs are also distorted by all kinds of nonlinearities arising from the mostly adopted Mach-Zehnder modulator (MZM), due to the nonlinear electronics-optics-electronics conversion in photonic bandpass sampling link. To improve the link's spurious free dynamic range (SFDR) and fidelity, all the generated nonlinear distortions need to be concerned and well suppressed. Specially, both the target signals and derived nonlinear distortions are sampled and frequency-folded within the first Nyquist zone in bandpass sampling link. In our previous work [2], for example, all of the 3rd-order nonlinearities arising from the MZM were well compensated except the significant 2nd-order nonlinear spurs generating from photo-detector (PD). In most situations, distortions caused by PD are not under consideration since the PD shows un-conspicuous nonlinearity in non-saturated linear range. However, high optical incident eventually leads to saturation and causes nonlinear behavior in the PD under pulsed illumination, since the peak optical power is much higher than CW light. It has been demonstrated that the directly detected electrical signal phase changes as a function of the applied optical energy for the un-modulated optical pulse trains due to the PD saturation effect [4-5]. The significant saturation produces the conversion of optical amplitude noise to microwave phase retardation (AM-PM), which resulting in worsening the extracted microwave signal phase noise.