# Research on Tbps and Kilometer-Range Transmission of Terahertz Signals
## 2 Terahertz Signal
### Photonic Approaches  
Photonic approaches fundamentally employ the dual-laser optical heterodyne beating technique for high-speed signal generation [58; 59; 60; 61]. This technology leverages an opto-electro-optical seamless bridging architecture for signal generation and transmission: Two independent continuous-wave (CW) optical waves--one serving as the LO and the other as the modulated optical carrier--are generated by lasers. They are combined in an optical coupler for heterodyne beating, producing a THz signal whose frequency equals the difference between the two optical wavelengths. A high-speed photodetector then performs the optical-to-electrical (O/E) conversion, and the resulting THz signal is radiated by an antenna. At the receiver, a reverse conversion process reconstructs the optical carrier from the THz signal, which is finally received by a coherent receiver. This Fiber-Wireless (FiWi) integrated access system effectively combines the bandwidth and long-distance transmission advantages of optical fiber communications with the mobility and seamless coverage benefits of wireless communications. By utilizing the relatively simple and cost-effective photonically assisted THz technology, it overcomes the bandwidth bottleneck inherent in electronic devices to generate ultra-high-speed wireless THz signals.  
Prior to reference [62], real-time fiber-THz-fiber link transmissions exceeding 100 Gbit/s at frequencies above 350 GHz remained unreported. This gap stemmed from the immense challenges associated with achieving high-speed, stable, end-to-end real-time transmission at such high frequencies, including the generation, modulation, transmission loss, and real-time signal processing of high-frequency signals. Addressing this challenge, through  
Figure 4: Pure-electronic terahertz real-time transmission system; Tx: transmission; Rx: reception.  
the experimental setup in Figure 5, we successfully constructed and demonstrated, for the first time, a photonically assisted THz wireless transmission system operating in the 330-500 GHz band achieving an unprecedented Aggregated Interface User Rate (AIUR) of 1.0488 Tbps, alongside seamless integration of 10 km of fiber and a 10 m wireless link.  
The core innovation lies in fully leveraging the unique advantages of photonics: The system employs free-running tunable ECLs as high-purity, low-phase-noise optical carriers (ECL-1 @ 193.47 THz, ECL-2 @ 193.565 THz). A high-speed optical I/Q modulator (Fujitsu FTM7961 EX/301, 3 dB bandwidth > 35 GHz) efficiently modulates the 46 Gbaud PS-64 QAM baseband signal directly in the optical domain. PDM enables parallel transmission of dual-polarization signals within a single fiber. After 10 km of Standard Single-Mode Fiber (SSMF) transmission, a third ECL (ECL-3 @ 193.1 THz) serves as the LO at the receiver for optical heterodyne beating with the incoming signal. The critical O/E conversion is performed by a high-performance UTC-PD (NTT IOD-PMAN-13001). This device features an ultra-wide response bandwidth (300-2500 GHz typical), high saturation power (up to 15 dBm optical input), and good responsivity (0.15 A/W @ 1550 nm), enabling efficient, linear conversion of the heterodyne-generated difference frequency signal into powerful THz radiation. This photonically assisted THz generation method overcomes the bandwidth constraints faced by purely electronic approaches (>300 GHz), offering superior frequency flexibility, tunability, and potential bandwidth.  
Finally, employing a meticulously designed 2 \(\times\) 2 MIMO wireless link and an innovative real-time signal processing algorithm based on a Vector Quantized Variational Autoencoder (VQ-VAE)--specifically addressing the limitations of conventional equalizers for high-order PS-QAM signals--we successfully achieved a landmark AIUR of 1.0488 Tbps with a net rate of 828 Gbps. This groundbreaking result powerfully demonstrates the core value and immense potential of photonic technology in constructing future 6G networks demanding Tbps-level extreme connectivity.