Mode-Locked and Repetition-Rate-Tunable Comb Generation Using Dual Coupled Microrings

Abstract

A method incorporating controllable mode interaction is proposed for Kerr frequency comb generation in normal-dispersion microresonators. Repetition-rate-tunable combs and broadband mode-locking transitions are demonstrated by using dual silicon nitride microrings. OCIS codes: (230.4555) Coupled resonators; (190.7110) Ultrafast nonlinear optics Microresonator frequency combs (Kerr combs) are very attractive for their advantages of compactness, ultra-broad bandwidth, and high repetition rate [1]-[6]. Many applications can benefit from this revolutionary technique, such as optical fiber communications [7], photonic radiofrequency filters [8], optical clocks [9], etc. Anomalous dispersion microresonators are generally preferred for comb generation [5]-[6], because it is easy to get modulational instability in the anomalous dispersion regime which is required for frequency combs growing up from just low-level noise. However, since most nonlinear materials currently available have normal dispersion, special efforts are required to get the overall dispersion anomalous by tailoring the microresonator geometry [10]-[11]. Some experiments show comb generation in microresonators constructed of normal dispersion waveguides [12]-[18]. The coupling and interaction between different mode families is recognized as a key role enabling the comb generation [13], [16]-[17]. Mode interaction aided dark soliton excitation and broadband mode-locking transition have been demonstrated in the normal dispersion regime [17]. The mode interaction generally arises from defects of the microresonators which are difficult to control in fabrication. Here we demonstrate a new scheme in which mode interaction can be intentionally introduced and controlled. Repetition-rate-tunable comb generation as well as broadband mode-locking transitions are achieved by using silicon nitride microrings. To the best of our knowledge, this is the first demonstration of a reliable engineering technique for microcomb generation in the normal dispersion regime.Our scheme is based on the well-known coupled-microresonator structure [19]. Figure 1(a) shows the microscopy image of the device which contains two microrings coupled to each other. The cross-section of the waveguide constructing the microrings is 1.3 μm X 600 nm, corresponding to a normal dispersion of ~130 ps 2 /km. The main ring and the auxiliary ring have slightly different radii (main: 60 μm, auxiliary: 58 μm). Figure 1(b) shows Fig. 1. (a) Microscopy image of the dual coupled rings. (b) Full-range transmission of Fig. 2. Repetition rate tunable comb generation by the main ring (blue) and the auxiliary (red) ring respectively. (c) Zoom-in transmission controlling the mode interaction position. ×: pump;of two consecutive resonances of the main ring when the auxiliary ring is thermally tuned. ∇ : mode interaction positon.