Volterra Series Based Linearity Analysis of a Phase-Modulated Microwave Photonic Link

An analytical technique based on Volterra series for systematically computing distortion in analog microwave photonic (MWP) links is presented in this paper. In the analysis, we express both modulator and photodetector responses in the MWP link as Taylor series expansions, which are then nested to give rise to Volterra series for systems with memory. Closed-form expressions of the link photocurrent and optical power based on the nth-order Volterra kernels are derived from the functional Taylor series of the MWP link. These expressions are applicable in both direct detection and phase-modulated conditions hitherto referred to as memoryless case and with memory, respectively. The synthesized Volterra kernels are primarily used to determine key figures of merit such as nth-order harmonic distortion, third-order intermodulation distortion (IMDn), and third-order intercept point (OIPn) for an arbitrary MWP link. We apply the technique to a photonic link having a ring-assisted Mach-Zehnder interferometer (RAMZI) filter and compare its OIP3 performance with that of an MZI discriminator. Within a bandwidth of 20 GHz, it is shown that RAMZI has 10-dB better OIP3 performance than MZI. We also illustrate that OIP3 and IMD measurements done using three-tone tests would result in more accurate predictions of MWP link parameters than standard two-tone tests.