Ultra-long-distance distribution of low-phase-noise two-tone lightwave for THz seeding

We investigated the generation of high-quality two-tone optical THz reference (300 GHz) to be used for advanced THz transmission and demonstrated long-distance distribution over fiber links while maintaining the signal quality. We observed that the two-tone generation method considerably affected the dependence of transmission quality on the distribution distance. Noise accumulation in the high frequency range was suppressed by using narrowband fiber Bragg grating (FBG) filters regardless of the tone generation method, and an excellent reference signal quality, expressed by an integrated phase error value of 0.034 degrees, was obtained for a distribution distance of 12000 km and a time interval of 1/32 ns. This result also indicates the possibility of distributing high-quality references to many users. Numerical simulations using the measured results show that THz wireless transmission with 32 GBaud and 32-64 QAM modulation would be possible without using dedicated digital signal processing for phase and frequency offset estimation. However, when using an optical comb for tone generation, the THz signal quality would degrade strongly with the tone distribution distance, possibly due to the accumulation of noise within the FBG bandwidth. When two frequency-locked lasers are used instead of an optical comb, the THz signal quality only weakly depends on the tone distribution distance.