Theoretical investigation of the capture effect in intensity-modulation direct-detection microwave photonic links

Abstract

We introduce the capture effect concept in microwave photonic links (MWPLs) for the first time to our knowledge. The capture effect or the small-signal suppression is the change in the amplitude ratio of the two signals between input and output of the intensity-modulation direct-detection (IMDD) MWPLs. An analytical explanation of the performance of external IMDD MWPLs due to the effects of nonlinearity combined with sum of several input sinusoidal signals is given. We have investigated the suppression of a weaker signal in these links. General analytic expression for the small-signal suppression is derived using a nonlinear analytical approach. We show that the small-signal suppression is quite dependent on the input back-off, the power ratio of input signals, and on the number of input sinusoidal signals. The theoretical maximum possible signal suppression was found to be 6 dB. This analytical asymptotic value is verified by numerical results. We show the influence of the capture effect of the nonlinear MWPL on the optoelectronic oscillator operation that is verified by experimental data in the literature that has already been published.

Low-Drift Optoelectronic Oscillator Based on a Phase Modulator in a Sagnac Loop

“…It is clear from (8) that the output voltage of the photodetector consists of the fundamental frequency and its harmonic components located in different regions of the spectrum, the so-called zero zone (at low frequencies), first or fundamental zone (in the vicinity of the input frequency range), and higher order zones (in the vicinity of the harmonics of the fundamental) [63].…”

“…As it is clear from (28) and (29), this model of phase noise is reminiscent of the well-known Leeson's model [63]. Hence, f L will be the Lesson frequency and the differences are that: 1) here the equivalent quality factor (Q eq ) contains the effect of the optical fiber and the IF BPF simultaneously and 2) the noise due to the optical part (high-Q cavity) is also considered in the model.…”

Graphical approach to designing high‐gain low‐noise MWPLs

“…3). The constant-gain circles (or constant G S circles) that should be plotted on the Smith chart in the Γ S plane can be expressed as [17] Γ S − C s = r s (12) where C s and r s are the centre and the radius of the constant-gain circles (constant G S circles) in the Γ S plane, respectively,…”

“…Electrical performance of MWPLs and their parameters such as gain, small-and large-signal noise figure (NF), spurious-free dynamic range, composite second-order/composite triple beat, signal-to-noise ratio, and capture effect operating under small-and large-signal modulation have already been extensively studied [8][9][10][11][12][13]. In [14], the effect of mismatch and loss of the input lossy impedance matching network on the NF of MWPLs operating under large RF signal modulation was investigated.…”