Bistable 2x2 add/drop filter based on polarization rotator switch using helix-free ferroelectric liquid crystal with twisted alignment

Efficient, high-speed, and high-bandwidth communication is critical for enabling the future of a data-driven society. The rapid growth of mobile devices, cloud computing, and AI-driven demands from data centers has created an unprecedented need for increased transmission capacity and bandwidth in optical networks. To meet these demands, wavelength division multiplexing (WDM) has become the most efficient and widely adopted solution. Ferroelectric Liquid Crystals (FLCs) are significantly faster, has no FFE compared to nematic LC and present a promising alternative for next-generation optical switches. FLCs offer various electro-optical modes, including bistable mode, Electrically Suppressed Helix mode, Deformed Helix mode, and Kerr effect in vertically aligned Deformed Helix FLC. In this work, we provide an overview of these modes and demonstrate a 2×2 bistable add/drop filter based on polarization rotation using a helix-free FLC with a twisted alignment structure. The design achieves an extinction ratio of up to 25 dB. Additionally, we show a method to achieve true bistability in helix-free FLCs, maintaining stability for over 24 hours. The long-term bistability stems from the twisted alignment structure, where the twist angle is optimized to match the molecular free energy minimum under specific boundary conditions. Photoalignment technology is used to fine-tune the anchoring energy and twist angle, resulting in a fast, bistable device that is independent of temperature, applied frequency, and cell gap. The filter operates at a threshold voltage as low as 0.5 V/μm with a response time of 32 μs at 1.5 V/μm, making it a robust solution for optical switches.