This study aims to explore the utilization of optical exceptional points (EPs) in non-Hermitian systems, particularly in the context of photonic crystal fibers (PCFs). EPs induce fascinating physical phenomena and applications, such as unidirectional zero-reflection light transmission and phase transitions in metamaterials. Here, we aim to investigate the implementation of EPs in PCFs to achieve mode conversion and modulate optical interactions between different modes.

To achieve this objective, we introduce a symmetric gain-loss refractive index distribution into the core of PCFs, creating a parity-time (PT) symmetric non-Hermitian system. This approach involves carefully designing and fabricating the PCF structure to ensure the desired refractive index profile. We then analyze the optical properties of the system, including the formation of EPs and their effects on mode coupling and conversion using a beam propagation method.

Our investigation effectively showcases the achievement of optical EPs within the tailored PCF architecture, facilitating the proficient manipulation of mode interactions. Specifically, we successfully realize asymmetric mode conversion between LP₀₁ and LP₁₁ modes spanning a wavelength range of 1.3 to 2.0 μm, boasting an efficiency rate of up to 99%. Furthermore, this structure facilitates the simultaneous conversion from the LP₁₁ mode to the LP₀₁ mode (Fig. 5). Leveraging counterclockwise transmission is instrumental in mitigating mode purity issues stemming from device reflections. Crucially, our proposed scheme demonstrates resilient performance across diverse structural parameters, underscoring its promise for practical applications.

The observed mode conversion and modulation of optical interactions highlight the significance of EPs in non-Hermitian systems, particularly in the context of PCFs. The symmetric gain-loss refractive index distribution plays a crucial role in creating the PT-symmetric system, forming EPs, and enabling effective control over mode coupling. The high efficiency and tolerance to structural variations further enhance the applicability of the proposed scheme in real-world scenarios.

In conclusion, our study presents an innovative approach for mode manipulation in PCFs based on optical exceptional points. By leveraging the unique properties of EPs in non-Hermitian systems, we demonstrate efficient and flexible mode conversion within a wide wavelength range. This research expands the application scope of EPs in photonics. It provides a promising solution for enhancing the functionality of photonic crystal fibers in various optical systems and devices.