Terahertz PAM4 Direct Modulation Technique Based on Dual-Channel OOK Combination

To enhance spectral efficiency and augment the aggregate data throughput in terahertz frequency direct modulators, this paper introduces technique utilizing Pulse Amplitude Modulation 4-level (PAM4) facilitated by a dual-channel combination approach, while preserving an all-digital modulation framework. The proposed methodology synergistically integrates high-balance broadband waveguide couplers, a broadband 180-degree phase delay line, and high-speed On-Off Keying (OOK) direct modulators. Through EM-simulation, our research successfully demonstrates the implementation of a PAM4 direct modulator, driven by a fully digital modulation signal, operating at 138 GHz. This system exhibits a on-off ratio of over 16 dB and maintains an amplitude inconsistency to mean ratio exceeding 13 dB between adjacent modulation states.