Normal‐dispersion microcombs enabled by controllable mode interactions

Abstract

We demonstrate a scheme incorporating dualcoupled microresonators through which mode interactions are intentionally introduced and controlled for Kerr frequency comb (microcomb) generation in the normal-dispersion region. Microcomb generation, repetition rate selection, and mode locking are achieved with coupled silicon nitride microrings controlled via an on-chip microheater. The proposed scheme shows for the first time a reliable design strategy for normal-dispersion microcombs and may make it possible to generate microcombs in an extended wavelength range (e.g. in the visible) where normal material dispersion is likely to dominate.

Advanced photonic filters based on cascaded Sagnac loop reflector resonators in silicon-on-insulator nanowires

“…[7][8][9] Moreover, the resulting group delay response and mode interaction are useful for enhancing light-material interaction and dispersion engineering in nonlinear optics. [10][11][12][13][14] Photonic resonators can be classified into two categories-travelling-wave (TW) resonators, exemplified by ring resonators, and standing-wave (SW) resonators represented by photonic crystal cavities, distributed feedback cavities, and Fabry-Pérot (FP) cavities. 3 The majority of work on mode splitting in photonic resonators has been based on TW resonators [15][16][17][18][19] although some recent work has investigated device structures consisting of both TW and SW resonators.…”

Normal-dispersion microresonator Kerr frequency combs

“…Modulational instability may thus occur on the upper branch of the bistability curve, which is generally impossible for normal dispersion microresonators in the case of no mode coupling (recall the discussion in the last section). This mode coupling induced modulational instability has been found to play a key role that enables comb generation in normal dispersion microresonators in experiments [40,[45][46][47][48]. Figure 7A shows one example of a Si 3 N 4 microring resonator [45].…”

“…A dual-coupled microresonator scheme was proposed in Ref. [47] to introduce programmable mode coupling for reliable comb generation in the normal dispersion regime. Similar structures have been demonstrated previously for high-efficiency on-chip four-wave mixing [85].…”

“…Broadband phaselocked Kerr combs have been demonstrated in the second stage via subcomb offset frequency matching [33], ultrashort pulse generation [35], bright soliton formation [36], and self-injection locking [37]. Kerr comb generation in normal dispersion microresonators has also been demonstrated [31,[40][41][42][43][44][45][46][47][48], but has been less explored. Although achieving modulational instability for comb initiation in the general normal dispersion regime is more difficult, comb generation using normal dispersion microresonators also offers potential advantages.…”

On Frequency Combs in Monolithic Resonators

“…It is also possible to use a set of coupled resonators to control the GVD using the mode interaction principle. In this way Kerr comb generation, repetition rate selection, and mode locking are achieved with coupled SiN microrings controlled via an on-chip microheater [234]. The frequency combs generated in this way have relatively narrow optical spectra but are characterized with high stability and high relative coherence of the frequency harmonics.…”