IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.1 Abstract-We report on a photonic system for generation of high quality Continuous Wave (CW) sub-THz signals. The system consists on a Gain-Switching-based Optical Frequency Comb Generator (GS-OFCG), a two-optical-modes selection mechanism and a n-i-pn-i-p superlattice photomixer. As mode selection mechanism, both selective tunable optical filtering using FabryPerot Tunable Filters (FPTF) and Optical Injection Locking (OIL) are evaluated. The performance of the reported system surpasses in orders of magnitude the performance of any commercially available optical mm-wave and sub-THz generation system in a great number of parameters. It matches and even overcomes those of the best commercially available electronic THz generation systems. The performance parameters featured by our system are: linewidth <10 Hz at 120 GHz, complete frequency range coverage (60-140 GHz) with a resolution in the order of 0.1 Hz at 120 GHz (10 -12 of generated frequency), and high long term frequency stability (5 Hz deviation over one hour). Most of these values are limited by the measurement instrumentation accuracy and resolution, thus the actual values of the system could be better than the reported ones. The frequency can be extended straightforwardly up to 1 THz extending the OFCG frequency span. This system is compact, robust, reliable, and offers a very high performance, especially suited for sub-THz photonic Local Oscillators and high resolution spectroscopy.

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A report is presented on the photonic synthesis of ultra-narrow line-width continuous-wave (CW) sub-THz signals using a gain-switching (GS) based optical frequency comb generator (OFCG), selective optical filtering and a n-i-pn-i-p superlattice photomixer. This setup provides continuous tunability with a tuning resolution in the range of 0.1 Hz at 120 GHz and full width at half maximum of the generated signals below the limits of the measurement setup (< 10 Hz). The advantages of this system make it a very good candidate for applications requiring extremely low phase noise and continuous tunability, such as high resolution spectroscopy in the sub-THz and THz range

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