Free-space Terahertz Spectrum Analysis with an Optoelectronic Hybrid System

Motivated by the recent advances in 5G and 6G technologies, we developed an optoelectronic hybrid system capable of high-resolution spectrum analysis in the terahertz range. The concept involves the phase-locking of three continuous-wave (CW) distributed-feedback diode lasers by mixing the laser light with the radiation of a high-power, electronic narrow-band emitter. To synchronize the laser beat signals with the reference source, two heterodyne optical phase-locked loops consisting of a photoconductive antenna and a PID controller are implemented in the setup. This approach eliminates the need for frequency extenders currently required in state-of-the art spectrum analyzers, resulting in a simpler and more cost-effective architecture. Using our hybrid system, we are able to generate laser difference frequencies ranging from 10 MHz to over 1 THz, with precision in the range of several Hz. Moreover, the frequency can be easily extended to higher frequencies by incorporating suitable electronic emitters or by cascading even more lasers with our concept. To demonstrate the system's resolution and stability, we measure various spurious harmonics of electronic emitters, by implementing the laser frequency synchronization setup in a CW free-space terahertz spectrum analyzer.