Abstract:We investigate simultaneously the temporal and optical and radio-frequency spectral properties of parametric frequency combs generated in silicon-nitride microresonators and observe that the system undergoes a transition to a mode-locked state. We demonstrate the generation of sub-200-fs pulses at a repetition rate of 99 GHz. Our calculations show that pulse generation in this system is consistent with soliton modelocking. Ultimately, such parametric devices offer the potential of producing ultrashort laser pulses from the visible to mid-infrared regime at repetition rates from GHz to THz.

We demonstrate the generation of a supercontinuum spanning more than 1.4 octaves in a silicon nitride waveguide using sub-100-fs pulses at 1 μm generated by either a 53-MHz, diode-pumped ytterbium (Yb) fiber laser or a 1-GHz, Yb:CaAlGdO(4) (Yb:CALGO) laser. Our numerical simulations show that the broadband supercontinuum is fully coherent, and a spectral interference measurement is used to verify that the supercontinuum generated with the Yb:CALGO laser possesses a high degree of coherence over the majority of its spectral bandwidth. This coherent spectrum may be utilized for optical coherence tomography, spectroscopy, and frequency metrology.

We observe strong modal coupling between the TE 00 and TM 00 modes in Si 3 N 4 ring resonators revealed by avoided crossings of the corresponding resonances. Such couplings result in significant shifts of the resonance frequencies over a wide range around the crossing points. This leads to an effective dispersion that is one order of magnitude larger than the intrinsic dispersion and creates broad windows of anomalous dispersion. We also observe the changes to frequency comb spectra generated in Si 3 N 4 microresonators due polarization mode and higher-order mode crossings and suggest approaches to avoid these effects. Alternatively, such polarization mode-crossings can be used as a novel tool for dispersion engineering in microresonators.Optical microresonators are important for a wide range of applications, such as parametric frequency combs [1-10], optomechanics [11,12], and in quantum optics as sources for photon-pairs [13][14][15][16][17][18][19] or squeezed states [20][21]. The microresonator resonances can in principle be precisely calculated using the dispersion of the resonating modes and the resonator length. However, modal coupling between different types of modes can significantly alter the shape and position of their resonances. Mode splitting occurs for strong coupling [22], and coupling between whole families of modes results in avoided crossings [23][24][25][26][27]. This can lead to dramatic localized changes in the effective dispersion near these crossing points, which in general affects any parametric interaction that relies on precise frequency matching of different resonances. In particular it can play an important role in the formation of parametric frequency combs [24][25][26][27][28][29][30][31]. While mode-crossings can be disruptive for comb generation by inhibiting soliton formation [25] and distorting the comb spectrum [27], they can also be beneficial, allowing for comb formation in resonators with normal group-velocity dispersion (GVD) [8,24] or aiding the generation of dark solitons in normal GVD resonators [29]. In the context of frequency comb generation, only modal interactions between different families of spatial modes have been considered thus far. However, in dielectric waveguides, even when the waveguide is 'single mode', there are typically at least two guided fundamental modes, the fundamental quasi transverse electric (TE 00 ) and the fundamental quasi transverse magnetic (TM 00 ) mode, which correspond approximately to the polarization of light in the waveguide.Here, we report on the observation of avoided crossings that result from the strong modal coupling between the TE 00 and TM 00 polarization modes in Si 3 N 4 microring resonators. Similarly, strong polarization mode coupling has been shown to be useful for polarization conversion based on silicon oxinitride technology [31]. Since such a mode interaction can even occur in single-mode waveguides, it is more fundamental than other forms of modal interactions (i.e., between higher-order spatial modes). The physical o...

Silicon nitride (Si₃N₄) waveguides represent a novel photonic platform that is ideally suited for energy efficient and ultrabroadband nonlinear interactions from the visible to the mid-infrared. Chip-based supercontinuum generation in Si₃N₄ offers a path towards a fully-integrated and highly compact comb source for sensing and time-and-frequency metrology applications. We demonstrate the first successful frequency comb offset stabilization that utilizes a Si₃N₄ waveguide for octave-spanning supercontinuum generation and achieve the lowest integrated residual phase noise of any diode-pumped gigahertz laser comb to date. In addition, we perform a direct comparison to a standard silica photonic crystal fiber (PCF) using the same ultrafast solid-state laser oscillator operating at 1 µm. We identify the minimal role of Raman scattering in Si₃N₄ as a key benefit that allows to overcome the fundamental limitations of silica fibers set by Raman-induced self-frequency shift.

We present the first direct carrier-envelope-offset (CEO) frequency detection of a modelocked laser based on supercontinuum generation (SCG) in a CMOS-compatible silicon nitride (Si(3)N(4)) waveguide. With a coherent supercontinuum spanning more than 1.5 octaves from visible to beyond telecommunication wavelengths, we achieve self-referencing of SESAM modelocked diode-pumped Yb:CALGO lasers using standard f-to-2f interferometry. We directly obtain without amplification strong CEO beat signals for both a 100-MHz and 1-GHz pulse repetition rate laser. High signal-to-noise ratios (SNR) of > 25 dB and even > 30 dB have been generated with only 30 pJ and 36 pJ of coupled pulse energy from the megahertz and gigahertz laser respectively. We compare these results to self-referencing using a commercial photonic crystal fiber and find that the required peak power for CEO beat detection with a comparable SNR is lowered by more than an order of magnitude when using a Si(3)N(4) waveguide.