Photonic Oscillators for THz Signal Generation

Abstract

The photonic oscillator concept is a rather new technique for providing low-phase noise continuous-wave signals in the THz regime. Compared to other electrical and optical generation techniques, photonic oscillators exhibit a number of unique features such as ultra-wideband tuneability, compactness and ability to operate over a broad temperature range making it an interesting device for several THz applications. According to a recent study initiated by the European Space Agency (ESA), photonic oscillators utilizing advanced photodetectors are considered as one of the most promising candidates for the generation of THz signals [1]. This chapter reviews the state-of-the-art in photonic oscillators for THz generation and compares this new technique with other existing electrical and optical approaches. Since the development of terahertz photonic oscillators is strongly related to the invention of ultra high-frequency photodetectors we will also cover recent achievements in high-frequency photodetectors in this chapter. Explicitly, we will discuss the high-frequency performance of distributed travelling-wave photodetectors which exhibit a great potential for efficient local oscillator (LO) generation at THz frequencies. This fact is experimentally proven by demonstrating compact photonic oscillators employing advanced travelling-wave photodetectors which are indeed capable of providing sufficient LO power at THz frequencies (e.g. to pump a superconductor-insulator-superconductor (SIS) mixer at around 500 GHz). A key and unique feature of a photonic oscillator compared to other THz sources is its ability to allow for tuning the LO frequency over a wide frequency range. As an example, compact photonic oscillator modules exhibiting an amazingly large tuning range of almost 1 THz will be presented.