Micro-combs: A novel generation of optical sources

The quest towards the integration of ultra-fast, high-precision optical clocks is reflected in the large number of high-impact papers on the topic published in the last few years. This interest has been catalysed by the impact that high-precision optical frequency combs (OFCs) have had on metrology and spectroscopy in the last decade (Holzwarth et al., 2000; Jones et al., 2000; Udem et al., 2002; Cundiff and Ye, 2003; Hänsch, 2005). OFCs are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally-spaced spectral lines that represent precise marks in frequency. Their importance was recognised worldwide with the 2005 Nobel Prize being awarded to T.W. Hänsch and J. Hall for their breakthrough in OFC science (Hänsch, 2005). They demonstrated that a coherent OFC source with a large spectrum - covering at least one octave - can be stabilised with a self-referenced approach, where the frequency and the phase do not vary and are completely determined by the source physical parameters. These fully stabilised OFCs solved the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Very recent advancements in the fabrication technology of optical micro-cavities (Vahala, 2003) are contributing to the development of OFC sources. These efforts may open up the way to realise ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates, in the form of compact and integrated devices. Indeed, the fabrication of high-quality factor (high-Q) micro-resonators, capable of dramatically amplifying the optical field, can be considered a photonics breakthrough that has boosted not only the scientific investigation of OFC sources (Spillane et al., 2002; Del’Haye et al., 2007; Del’Haye et al., 2008; Ferdous et al., 2011; Levy et al., 2010; Razzari et al., 2010; Herr et al., 2012) but also of optical sensors and compact light modulators (Vahala, 2003; Xu et al., 2005). In this framework, the demonstration of planar high-Q resonators, compatible with silicon technology (Ferdous et al., 2011; Levy et al., 2010; Razzari et al., 2010; Herr et al., 2012; Xu et al., 2005) , has opened up a unique opportunity for these devices to provide entirely new capabilities for photonic-integrated technologies. Indeed, it is well acknowledged by the electronics industry that future generations of computer processing chips will inevitably require an extremely high density of copper-based interconnections, significantly increasing the chip power dissipation to beyond practical levels (Asghari and Krishnamoorthy, 2011; Caulfield and Dolev, 2010; Chen et al., 2011); hence, conventional approaches to chip design must undergo radical changes. On-chip optical networks, or optical interconnects, can offer high speed and low energy per-transferred-bit, and micro-resonators are widely seen as a key component to interface the electronic world with photonics. Many information technology industries have recently focussed on the development of integrated ring resonators to be employed for electrically-controlled light modulators (Xu et al., 2005; Asghari and Krishnamoorthy, 2011; Caulfield and Dolev, 2010; Chen et al., 2011) , greatly advancing the maturity of micro-resonator technology as a whole. Recently (Razzari et al., 2010; Levy et al., 2010; Herr et al., 2012), the demonstration of OFC sources in micro-resonators fabricated in electronic (i.e. in complementary metal oxide semiconductor (CMOS)) compatible platforms has given micro-cavities an additional appeal, with the possibility of exploiting them as light sources in microchips. This coincidence of events is creating fierce competition in developing highly efficient OFC generators based on micro-cavities which can radically change the nature of information transport and processing. Even in telecommunications, perhaps a more conventional environment for optical technologies, novel time-division multiplexed optical systems will require extremely stable optical clocks at ultra-high pulse repetition-rates towards the THz scale. Furthermore, arbitrary pulse generators based on OFC (Jiang et al., 2007; Cundiff and Weiner, 2010) are seen as one of the most promising solutions for this next generation of high-capacity optical coherent communication systems. This review will summarise the recent exciting achievements in the field of micro-combs, namely optical frequency combs based on high-Q micro-resonators, with a perspective on both the potential of this technology, as well as the open questions and challenges that remain.