Silicon-nitride photonic integrated waveguide for dual-polarization traveling-wave four-wave-mixing applications

Four-wave mixing (FWM) is a crucial nonlinear optical effect used to achieve various applications such as optical parametric amplification (OPA), phase-sensitive amplification (PSA), wavelength conversion, and phase conjugation. These applications have been well demonstrated on optical fiber platforms. Furthermore, integrated silicon nitride (SiN) platforms have recently garnered attention for such nonlinear applications due to their integration capability and large nonlinear coefficients. The previous studies of signal amplification and wavelength conversion mostly have concentrated on the fundamental transverse-electric (TE₀) mode. However, the evolution of coherent optical communication has necessitated these devices to support dual-polarization (DP) signals. In this paper, we demonstrate the potential of polarization-independent FWM applications using a single SiN waveguide. The polarization dependence can potentially be mitigated through the design of the waveguide dimensions. This design ensures that the intersection of the dispersion parameter (D) between the TE₀ and the fundamental transverse-magnetic (TM₀) modes occurs at a target wavelength, with the pump set at this wavelength. For instance, our design demonstrates that the intersection wavelength can occur at the C-band, with a cross-sectional dimension of 800 × 3000 nm². Numerical analyses show that the OPA gains for both modes can be nearly identical with this waveguide structure at a pump wavelength of 1550 nm. As a proof of concept, we fabricated a SiN spiral waveguide with a 2.8-meter length and investigated its polarization dependency when the device was utilized as a wavelength converter. Additionally, we conducted a high-speed experiment with the device using a 25-Gbaud (50-Gb/s) quadrature-phase-shift-keyed (QPSK) signal and successfully demodulated the QPSK signal from the idler, achieving comparable performance for both modes. This work paves the way for realizing dual-polarization four-wave mixing (DP-FWM) applications with a single integrated waveguide.