Optical solitons are waveforms that preserve their shape while propagating, relying on a balance of dispersion and nonlinearity [1,2]. Soliton-based data transmission schemes were investigated in the 1980s, promising to overcome the limitations imposed by dispersion of optical fibers. These approaches, however, were eventually abandoned in favor of wavelength-division multiplexing (WDM) schemes that are easier to implement and offer improved scalability to higher data rates. Here, we show that solitons may experience a comeback in optical communications, this time not as a competitor, but as a key element of massively parallel WDM. Instead of encoding data on the soliton itself, we exploit continuously circulating dissipative Kerr solitons (DKS) in a microresonator [3,4]. DKS are generated in an integrated silicon nitride microresonator [5] by four-photon interactions mediated by Kerr nonlinearity, leading to low-noise, spectrally smooth and broadband optical frequency combs [6]. In our experiments, we use two interleaved soliton Kerr combs to trans-mit a data stream of more than 50 Tbit/s on a total of 179 individual optical carriers that span the entire telecommunication C and L bands. Equally important, we demonstrate coherent detection of a WDM data stream by using a pair of microresonator Kerr soliton combs one as a multi-wavelength light source at the transmitter, and another one as a corresponding local oscillator (LO) at the receiver. This approach exploits the scalability advantages of microresonator soliton comb sources for massively parallel optical communications both at the transmitter and receiver side. Taken together, the results prove the significant potential of these sources to replace arrays of continuous-wave lasers in high-speed communications. In combination with advanced spatial multiplexing schemes [7,8] and highly integrated silicon photonic circuits [9], DKS combs may bring chip-scale petabit/s transceivers into reach.The first observation of solitons in optical fibers [2] in 1980 was immediately followed by major research efforts to harness such waveforms for long-haul communications [1]. In these schemes, data was encoded on soliton pulses by simple amplitude modulation using on-off-keying (OOK). However, even though the viability of the approach was experimentally demonstrated by transmission over one million kilometres [10], the vision of soliton-based communications was ultimately hindered by difficulties in achieving shape-preserving propagation in real transmission systems [1] and by the fact that nonlinear interactions intrinsically prevent dense packing of soliton pulses in either the time or frequency domain. Moreover, with the advent of wavelength-division multiplexing (WDM), line rates in long-haul communication systems could be increased by rather simple parallel transmission of data streams with lower symbol rates, which are less dispersion sensitive. Consequently, soliton-based communication schemes have moved out of focus over the last two decades. More recently, frequ...

Gallium phosphide (GaP) is an indirect bandgap semiconductor used widely in solid-state lighting. Despite numerous intriguing optical properties-including large χ (2) and χ (3) coefficients, a high refractive index (> 3), and transparency from visible to long-infrared wavelengths (0.55 − 11 µm)its application as an integrated photonics material has been little studied. Here we introduce GaPon-insulator as a platform for nonlinear photonics, exploiting a direct wafer bonding approach to realize integrated waveguides with 1.2 dB/cm loss in the telecommunications C-band (on par with Si-on-insulator). High quality (Q > 10 5 ), grating-coupled ring resonators are fabricated and studied. Employing a modulation transfer approach, we obtain a direct experimental estimate of the nonlinear index of GaP at telecommunication wavelengths: n2 = 1.2(5) × 10 −17 m 2 /W. We also observe Kerr frequency comb generation in resonators with engineered dispersion. Parametric threshold powers as low as 3 mW are realized, followed by broadband (> 100 nm) frequency combs with sub-THz spacing, frequency-doubled combs and, in a separate device, efficient Raman lasing. These results signal the emergence of GaP-on-insulator as a novel platform for integrated nonlinear photonics.

One of the essential prerequisites for detection of Earth-like extra-solar planets or direct measurements of the cosmological expansion is the accurate and precise wavelength calibration of astronomical spectrometers. It has already been realized that the large number of exactly known optical frequencies provided by laser frequency combs (astrocombs) can significantly surpass conventionally used hollow-cathode lamps as calibration light sources. A remaining challenge, however, is generation of frequency combs with lines resolvable by astronomical spectrometers. Here we demonstrate an astrocomb generated via soliton formation in an on-chip microphotonic resonator (microresonator) with a resolvable line spacing of 23.7 GHz. This comb is providing wavelength calibration on the 10 cm/s radial velocity level on the GIANO-B high-resolution near-infrared spectrometer. As such, microresonator frequency combs have the potential of providing broadband wavelength calibration for the next-generation of astronomical instruments in planet-hunting and cosmological research.The existence of life on other planets and the evolution of our Universe are questions that extend far beyond a purely astronomical context into other domains of science and society. Observational contributions relevant to both questions can be made by measuring minute wavelength shifts of spectral features in astronomical objects. For instance, an Earth-like planet, too faint for a direct observation, can reveal its presence by periodically modifying the radial velocity of its host star and hence Doppler-shifting characteristic features in the stellar spectrum 1,2 . Similarly, it has been suggested that the changing expansion rate of the Universe could be directly measured by observing the cosmological redshift in distant quasars 3,4 . The major challenge for such measurements is the requirement of a precisely and accurately calibrated astronomical spectrometer capable of detecting frequency shifts equivalent to radial velocities of the order of 10 cm/s or smaller. Conventional approaches of spectrometer calibration typically rely on the emission lines of hollow-cathode gas lamps that are used as calibration markers. However, the limited stability over time, the sparsity and different intensities of emission lines as well as the sensitivity to line blending impose limitations that are incompatible with the observational requirements. Over the last decade it has been realized that laser frequency combs (LFCs) 5-11 provide new means of wavelength calibration with unprecedented accuracy and precision 12-15 . Such LFCs are typically derived from mode-locked lasers and consist of large sets of laser lines whose optical frequencies ν n are equidistantly spaced: nu n = n * f rep + f 0 (n is an integer number). The two parameters f rep and f 0 are radio-frequencies (RF) accessible by conventional electronics and refer to the pulse repetition rate and carrier-envelope offset frequency of the mode-locked laser.Via self-referencing and stabilization schemes, f rep and ...

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Dissipative Kerr solitons in optical microresonators provide a unifying framework for nonlinear optical physics with photonic-integrated technologies and have recently been employed in a wide range of applications from coherent communications to astrophysical spectrometer calibration. Dissipative Kerr solitons can form a rich variety of stable states, ranging from breathers to multiplesoliton formations, among which, the recently discovered soliton crystals stand out. They represent temporally-ordered ensembles of soliton pulses, which can be regularly arranged by a modulation of the continuous-wave intracavity driving field. To date, however, the dynamics of soliton crystals remains mainly unexplored. Moreover, the vast majority of the reported crystals contained defects -missing or shifted pulses, breaking the symmetry of these states, and no procedure to avoid such defects was suggested. Here we explore the dynamical properties of soliton crystals and discover that often-neglected chaotic operating regimes of the driven optical microresonator are the key to their understanding. In contrast to prior work, we prove the viability of deterministic generation of perfect soliton crystal states, which correspond to a stable, defect-free lattice of optical pulses inside the cavity. We discover the existence of a critical pump power, below which the stochastic process of soliton excitation suddenly becomes deterministic enabling faultless, device-independent access to perfect soliton crystals. Furthermore, we demonstrate the switching of soliton crystal states and prove that it is also tightly linked to the pump power and is only possible in the regime of transient chaos. Finally, we report a number of other dynamical phenomena experimentally observed in soliton crystals including the formation of breathers, transitions between soliton crystals, their melting, and recrystallization. arXiv:1903.07122v2 [physics.optics]