Design of an RF Synthesizer using an Optical Frequency Comb and Active De-multiplexer

An optical frequency comb (OFC), sometimes referred to as a multi-wavelength source, can be considered to be a ruler of equidistant spectral lines [1]. The precise spacing or the free spectral range (FSR) between the spectral lines is what qualifies the OFC for a plethora of applications. Another of its key distinguishing features is the strong phase correlation exhibited by the spectral components across the entire bandwidth of the OFC. In the past decade, OFCs have been extensively studied, becoming an attractive solution in diverse applications such as spectroscopy [2], ranging [3], atomic clocks [4], steganography [5], spectrally efficient optical communications networks [6], millimetre and terahertz generation [7], [8] and many more.Over the last decade or two, several techniques have been proposed to generate OFCs. The choice of the suitable generation method depends mainly on the application that the OFC will be used for. There is no single technique that can be labelled as the ‘optimum’ one. Depending on the required applications, the desired comb characteristics would be defined and then the most suitable generation method be chosen. In addition, many OFC uses require the separation of the individual comb line e.g., to modulate each one with data. Hence, there has been a lot of interest in devising efficient methods of filtering of comb tones. In particular, the active demultiplexer has drawn significant attention, as its incorporation introduces an additional layer of flexibility, whilst also overcoming some of the shortcomings of the OFC.In this study, the authors will demonstrate the realization of an ultra-pure RF synthesizer designed using a wavelength tunable OFC and active de-multiplexer [9]. A very attractive feature of the proposed architecture is the potential of reducing the footprint via photonic integration. A proof-of-concept PIC designed and fabricated by Pilot Photonics, comprising of a comb source, demultiplexers and combiners, will be characterised and presented.