Characteristic Analysis of 3 × 3 Coupler-Based Nonlinear Optical Loop Mirrors for Spectrum Equalization in Optical Frequency Combs

Research progress on optical frequency combs (OFCs) has provided promising solutions for advancements in various fields. However, the interference from non-ideal components in the temporal waveform of OFC signals often leads to spectral non-flatness, which limits their applicability, especially in ultra-wideband applications. To address this issue, we propose a cavity-free ultra-flat OFC generation scheme, utilizing a nonlinear optical loop mirror (NOLM) based on a 3 × 3 fiber coupler to achieve significant pulse shaping. To the best of our knowledge, this is the first time that a 3 × 3 NOLM has been incorporated into an OFC generation system. In this work, the transmission model for all possible configurations of the 3 × 3 NOLM is derived and analyzed. Under identical input light field conditions, the proposed optimal 3 × 3 NOLM achieves a pulse sidelobe suppression ratio (SSR) improvement of 15.9 dB compared to the optimal 2 × 2 NOLM, resulting in a flatness enhancement of 11.6 dB. When compared to the ideal linear parabolic shape, the proposed optimal 3 × 3 NOLM yields a mean-squared error (MSE) of 0.006, versus 0.3 for the optimal 2 × 2 NOLM-based scheme, following the nonlinear mixing process in OFC generation. The results demonstrate that the proposed scheme offers superior background noise suppression, enhanced central spectral equalization efficiency, reduced pumping power requirements, and improved robustness against modulation parameter variations. This work provides innovative technical support for more efficient OFC generation, which will promote the broader implementation of OFCs across various applications.