All-optical RZ-QPSK generation through cross-phase modulation-based aggregation of RZ-BPSK and NRZ-OOK in an octagonal highly nonlinear dispersion-flattened PCF

In this paper, what we believe to be a novel all-optical modulation format synthesizer capable of directly generating a 20 Gbps return-to-zero quadrature phase-shift keying (RZ-QPSK) signal through cross-phase modulation (XPM) between independent 10 Gbps RZ-BPSK (probe) and 10 Gbps NRZ-OOK (control) data streams is proposed and numerically validated. The nonlinear interaction is realized in a specially engineered octagonal highly nonlinear dispersion-flattened photonic crystal fiber (HNL-DF PCF), whose geometry is adapted from previously reported designs optimized for strong mode confinement and ultraflattened dispersion. The fiber exhibits a nonlinear coefficient γ  = 27 W −1 km −1 , group velocity dispersion β 2  ≈ 0 ± 0.5 ps/nm/km, and effective area A eff  ≈ 3.6  μ m 2 , enabling efficient phase transfer with minimal walk-off and confinement loss. The XPM-induced phase shift on the probe adds the information from the control signal’s intensity onto RZ-BPSK, yielding a spectrally efficient RZ-QPSK output after filtering. Numerical simulations confirm the successful synthesis of high quality RZ-QPSK signal, as demonstrated by clean constellation clusters, narrow spectral sidebands, and stable amplitude-phase statistics. System-level analysis reveals an OSNR penalty near about 3.1 dB, maintaining a log(BER) FEC threshold, along with robust transmission performance of less than 100 km of standard single-mode fiber without using an in-line EDFA and dispersion compensation. The proposed architecture eliminates the need for delay-line interferometers, dual-branch processing, or electronic processing, providing a compact, power-efficient, and fully optical solution for advanced modulation format generation in next-generation photonic networks.