Spectrally Efficient Photonic Terahertz Transmitter for Kramers-Kronig Reception Without Guard-Band Based on Optical Injection Locking

Kramers-Kronig (KK) receivers are recognized as a cost-effective alternative to the coherent receivers in photonics terahertz (THz) communications for high-speed and complex signal transmission. Essential to KK reception is the inclusion of an additional reference carrier to satisfy the single-sideband minimum phase condition. This, however, necessitates a frequency guard band between the modulated signal and the reference carrier to accommodate the frequency drift inherent in lasers, consequently diminishing spectral efficiency. In this work, we propose a spectrally efficient photonic THz transmitter employing optical injection locking (OIL) for KK reception, which allows synchronization of the optical beat frequencies, eliminating the frequency drift of the output THz signals. This approach ensures close frequency alignment between the transmitted THz signal and the reference carrier, thereby negating the necessity for guard bands. Finally, THz wireless transmission and KK reception of up to 120 Gbps PS-64QAM signals are experimentally demonstrated in D-band, achieving a bit error rate (BER) below the 25% SD-FEC threshold.