Radio frequency arbitrary waveform generators (AWGs) are an important category of signal sources that can generate arbitrary user-defined waveforms. They have attracted significant attention in many applications ranging from wireless communications, to radar, measurement systems, and others [1–3]. Most approaches to RF AWGs are based on electronic technologies and while these are very mature with advanced instrumentation available commercially, they are bulky, very expensive, and subject to limitations in speed and linearity due largely to the required use of digital-to-analogue converters [4].
Photonic approaches [5–7], on the other hand, offer very high bandwidths and low phase noise that are difficult to obtain purely electronically [4]. There are a wide variety of approaches to realizing photonic RF arbitrary waveform generation [4, 8–15], such as spatial-to-temporal mapping [8, 9], wavelength-to-time mapping [4, 10], and Fourier synthesis [11] based on line-by-line control of optical frequency combs [12, 13]. However, while offering many advantages, these approaches face challenges of one form or another. For the spatial-to-time mapping and wavelength-to-time mapping AWGs, the synthesized waveforms are generally single-shot pulses, which are not suitable for RF applications that require continuous waveforms. For the Fourier synthesis approach, the synthesized signal bandwidth is subject to the resolution of the line-by-line spectral shaping, thus is unable to reach low-frequency RF bands.
Integrated optical Kerr frequency comb sources, or ‘micro-combs’ [16–22], have come into focus as a fundamentally new and powerful tool due to their ability to provide highly coherent multiple wavelength sources for RF applications [23–30]. They can greatly increase the capacity of communications systems and allow the processing of RF spectra for a wide range of advanced signal processing functions [31–35]. Micro-combs have the potential to provide a much larger number of wavelengths, an ultra-large Nyquist bandwidth compared to mode-locked lasers, as well as to offer a greatly reduced footprint and degree of complexity.
RF transversal filtering is a powerful approach to RF signal processing that is particularly well suited to multiwavelength optical sources [24]. For RF transversal filter functions, the number of wavelengths dictates the available number of channels to provide the RF time delays. Thus, with micro-combs, the performance of RF signal processing functions and other systems such as beamforming devices can be greatly enhanced in terms of the quality factor and angular resolution. In particular, the combination of a low FSR (for microcombs) of 50GHz or less, together with the use of soliton crystals, has proven extremely successful for a wide range of RF photonic applications [25–33]. Based on these advantages, a wide range of RF applications have been demonstrated, such as optical true time delays [25], transversal filters [26–28], signal processors [29–31], channelizers [34, 35], and phase-encoded signal generators [33]. Previously, we reported a photonic RF phase-encoded signal generator that achieved a phase encoding rate ranging from 1.98 to 5.95 Gb/s in a compact footprint [22]. In that approach, an RF single-cycle pulse was multicast onto a spectrally flattened micro-comb, with the progressively delayed replicas assembled arbitrarily in time according to the designed binary phase codes.
In this letter, we propose and demonstrate a user-defined RF arbitrary waveform generator based on a soliton crystal micro-comb source. Eighty-one wavelength channels are used, compared with 60 wavelengths for our previous work [33], which significantly enhances the speed and flexibility of our signal generation system. We present the architecture of the arbitrary waveform generator and then the results of experiments generating user-defined waveforms. These include a tunable square waveform with a duty ratio ranging from 10–90%, sawtooth waveforms with tunable slope ratios from 0.2 to 1, and symmetric concave quadratic chirp waveforms with an instantaneous frequency reaching down to the sub GHz range. Our successful results stem from the soliton crystal’s extremely robust and stable operation and generation, as well as its much higher intrinsic efficiency, all of which are enabled by an integrated CMOS compatible platform. The high performance as well as the good agreement between theory and experiment confirms our approach as being an effective way to implement user-defined RF waveform generation with reduced footprint, lower complexity, and potentially lower cost.