High-Performance Terahertz Bandpass Filter Design Considering the Mode Integrity

This study introduces an innovative design approach for high-performance terahertz (THz) bandpass (BP) filters, addressing mode coupling challenges in the THz band to achieve mode integrity of high-Q_u (unloaded quality factor) resonant modes and consequently enhance the overall filter quality factor. Through comprehensive analysis of mode-coupling effects and development of a specialized parameter extraction technique, the proposed method enables effective mode separation and efficient filter optimization. Furthermore, a novel differential screw has been implemented to achieve precise frequency adjustments, compensating for manufacturing errors that would otherwise deteriorate filter performance. The effectiveness of this methodology is demonstrated through a THz filter with a 1% fractional bandwidth and two transmission zeros (TZs). The filter was fabricated using CNC milling techniques. The prototype achieves exceptional measured performance: insertion loss better than 0.5 dB and a Q-factor exceeding 2100—representing the highest value reported so far for filters in this class.