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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 [1–5]. 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 [5]. 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 [6] 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 [7–13] but also of optical sensors and compact light modulators [6,14].
Fig. 1. Micro-combs have been generated by a large class of resonator devices, such as whispering-gallery mode-based bulk toroids, spheres, monolithic toroids and integrated ring-resonators.
In this framework, the demonstration of planar high-Q resonators, compatible with silicon technology [10–14], 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 [15–17]; hence, conventional approaches to chip design must undergo radical changes. On-chip optical networks, or optical interconnects, can offer high speed and low energy pertransferred-bit, and micro-resonators are widely seen as a key component to interface the electronic world with photonics.
Fig. 2. Examples of silica-based micro-resonators: (a) cleaned and cleaved optical fibre (b) silica micro-sphere; scanning electron micro-graphs of (c) silica micro-disc resonators after under-etching the silicon substrate (d) silica micro-toroid cavity after thermal re-flow.
Many information technology industries have recently focused on the development of integrated ring resonators to be employed for electrically-controlled light modulators [14–17], greatly advancing the maturity of micro-resonator technology as a whole. Recently [11–13], 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 scenario 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 [18,19] 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 microresonators, with a perspective on both the potential of this technology, as well as the open questions and challenges that remain.
Fig. 6. Silicon nitride micro-resonators, as fabricated by Purdue University, with a free spectral range (FSR) of (a) 231 GHz (b) 75 GHz (c) 37 GHz (d) 25 GHz. The spatial scale reported in panel (a) is the same for all the pictures. Integrated ring-resonators are usually characterised by small footprints, leading to short travelling times and high FSRs of the circular micro-cavity in the order of hundreds of GHz. To decrease such FSRs, different geometries can be used.
[1] R. Holzwarth, T. Udem, T. Hänsch, J. Knight, W. Wadsworth, P. Russell, Optical frequency synthesizer for precision spectroscopy, Phys. Rev. Lett. 85(2000) 2264–2267.[2] D.J. Jones, S.A. Diddams, J.K. Ranka, Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis, Science 288 (2000) 635–639.[3] T. Udem, R. Holzwarth, T.W. Hänsch, Optical frequency metrology, Nature 416 (2002) 233–237.[4] S.T. Cundiff, J. Ye, Colloquium: Femtosecond optical frequency combs, Rev. Modern Phys. 75 (2003) 325–342.[5] T.W. Hänsch, Passion for Precision, Nobel Lecture, 2005.[6] K.J. Vahala, Optical microcavities, Nature 424 (2003) 839–846.[7] S. Spillane, T. Kippenberg, K. Vahala, Ultralow-threshold Raman laser using a spherical dielectric microcavity, Nature 415 (2002) 621–623.[8] P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, T.J. Kippenberg, Optical frequency comb generation from a monolithic microresonator,Nature 450 (2007) 1214–1217.[9] P. Del’Haye, O. Arcizet, A. Schliesser, R. Holzwarth, T.J. Kippenberg, Full stabilization of a microresonator-based optical frequency bomb, Phys. Rev.Lett. 101 (2008) 053903.[10] F. Ferdous, H. Miao, D.E. Leaird, K. Srinivasan, J. Wang, L. Chen, L.T. Varghese, A.M. Weiner, Spectral line-by-line pulse shaping of on-chip microresonator frequency combs, Nature Photon. 5 (2011) 770–776.[11] J. Levy, A. Gondarenko, M. Foster, A. Turner-Foster, A. Gaeta, M. Lipson, CMOS-compatible multiple-wavelength oscillator for on-chip optical interconnects, Nature Photon. 4 (2010) 37–40.[12] L. Razzari, D. Duchesne, M. Ferrera, R. Morandotti, S. Chu, B. Little, D. Moss, CMOS-compatible integrated optical hyper-parametric oscillator, Nature Photon. 4 (2010) 41–45.[13] T. Herr, K. Hartinger, J. Riemensberger, Universal formation dynamics and noise of Kerr-frequency combs in microresonators, Nature Photon. 6 (2012) 480–487.[14] Q. Xu, B. Schmidt, S. Pradhan, M. Lipson, Micrometre-scale silicon electro-optic modulator, Nature 435 (2005) 325–327.[15] M. Asghari, A. Krishnamoorthy, Silicon photonics: Energy-efficient communication, Nature Photon. 5 (2011) 268–270.[16] H.J. Caulfield, S. Dolev, Why future supercomputing requires optics, Nature Photon. 4 (2010) 261–263.[17] X. Chen, C. Li, H.K. Tsang, Device engineering for silicon photonics, NPG Asia Mater. 3 (2011) 34–40.[18] Z. Jiang, C.-B. Huang, D.E. Leaird, A.M. Weiner, Optical arbitrary waveform processing of more than 100 spectral comb lines, Nature Photon. 1 (2007)463–467.[19] S.T. Cundiff, A.M. Weiner, Optical arbitrary waveform generation, Nature Photon. 4 (2010) 760–766.
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