Single-mode operated multimode AlGaAs-on-insulator microring resonators for Kerr comb generation

. We present a tapered coupler design to achieve single-mode operation in multimode microring resonators and experimentally demonstrate Kerr comb generation in a high-Q, single-mode operated AlGaAs-on-insulator multimode microring resonator.


Introduction
Microresonator-based Kerr optical frequency comb (microcomb) has gained significant attention due to its potential for revolutionizing metrology, spectroscopy astronomy, and optical communications.Microcombs can be generated via optical parametric oscillation in high-Q nonlinear microresonators.A wide range of integrated nonlinear material platforms has already been investigated for efficient Kerr comb generation [1].Among them, AlGaAs-on-insulator (AlGaAsOI) stands out as a promising candidate for achieving efficient nonlinear processes due to its ultra-high device effective nonlinearity [2].High-quality-factor (Q) microresonators are indispensable for nonlinear photonics, as they can significantly enhance the nonlinear efficiency, enabling not only Kerr comb generation [2] but also the optical signal processing [3], second harmonic generation [4], and photon pair generation [5].
To achieve high-Q microresonators, multimode waveguides are commonly used to minimize scattering loss by reducing mode overlap with the sidewalls.However, the interaction between the fundamental and higher-order modes may result in excessive avoided mode crossings (AMXs), distorting the comb spectrum [6].Moreover, exciting the higher-order modes in the microresonator increases the coupler loss, leading to the degradation of the Q [7].Therefore, single-mode operation of multimode microresonators is desired.
Several methods have been proposed to prevent the excitation of high-order modes in multimode resonators.For microring resonators, tapered resonator waveguide designs can eliminate excited higher-order modes but at the expense of compromising the Q of the fundamental mode [8], [9].An adiabatic coupler design can be used to prevent the excitation of the higher-order mode, but this requires changing the circular shape of the resonator, which may degrade the Q and alter the resonator dispersion [10].A pulley coupler design is also an effective method for selectively exciting the fundamental mode in a ring resonator and has been successfully demonstrated in low-confinement waveguides [7].However, the phase matching condition of the coupler can be significantly altered if the waveguide width is deviated from the design due to fabrication imperfection.This may result in insufficient bus-to-resonator coupling, especially for high-index contrast material platforms such as AlGaAsOI.For racetrack resonators, mode mismatch between the straight and curved sections has to be carefully addressed to avoid high-order mode excitationin addition to the coupler design [11]- [14].
Here, we present a tapered coupler design that enables the single-mode operation of a multimode microring resonator and ensures high tolerance against geometric deviations.We validate our design in high-Q AlGaAsOI microring resonators with two different multimode waveguide dimensions and demonstrate Kerr comb generation.Fig. 1(a) shows a microring resonator defined by the resonator radius (RR), resonator waveguide width (WR), the coupling gap (G), and the coupling angle (θ).Although a standard pulley coupler can selectively excite the fundamental mode [7], precise phase-matching is required to achieve the desired coupling.We propose a tapered coupler design where the bus waveguide width is linearly tapered from W1 to W2 with an average width of W0.The taper always ensures a bus waveguide dimension phase-matched to the resonator waveguide so that the busto-ring coupling strength increases with an increasing coupling angle [15].Fig. 1(b) shows the effective index of the fundamental TE mode for bus waveguides with different widths.Here we employ a circular resonator with a multimode dimension (360 nm×780 nm, RR=25 µm) that supports three TE mode families, as shown in the insets of Fig. 1(b).We design the W0 to be the phase-matched waveguide width (585 nm), while the bus waveguide is tapered from 535 nm to 635 nm.
To verify the single-mode operation of the proposed design, we fabricate and compare 780-nm-wide AlGaAsOI resonators with a standard point coupler and a bus-tapered coupler.The AlGaAsOI wafers were made by wafer bonding and substrate removal process [13].The devices were patterned using electron-beam lithography and dry etching [16].The measured transmission spectrum of the multimode resonators with a point coupler design (bus waveguide width = 470 nm, bus-to-ring gap = 250 nm) is shown in Fig. 1(c), where one can find three mode families.In contrast, only one mode family (TE00) is visible in Fig. 1(d) owing to the proposed tapered coupler employed in the multimode resonator.
To increase the Q of AlGaAsOI microring resonators, we also fabricated microring resonators with a 1-µm-wide resonator waveguide design, which supports more modes.The bus waveguide is tapered from 790 nm to 730 nm with a coupling angle 90°.As shown in Fig. 2(a), the measured transmission spectrum only contains one mode family, indicating that the higher-order modes have been wellsuppressed.The extracted group velocity dispersion is -373 pm/nm/km, and the intrinsic Q is 7.6×10 5 , as shown in Fig. 2(b) and 2(c), respectively.
We generate a Kerr frequency comb in the normal dispersion microresonator by pumping the resonator with a continuous-wave tunable laser at about 1580 nm.An AMX induces a local anomalous dispersion which initiates the parametric oscillation required for comb generation.Fig. 2(d) shows the generated Kerr comb spectrum covering the whole telecom C-and L-band with a 250-GHz repetition rate.

Conclusion
We have proposed a simple and generic bus-to-resonator coupling scheme for achieving single-mode operation in multimode microresonators.Using a coupler with a tapered bus waveguide, we have demonstrated high-Q multimode microring resonators with clean single-mode family transmission response, simplifying the Kerr comb generation at the user end., 07004 (2023)

Fig. 1 .
Fig. 1.(a) Schematic of the tapered coupler.(b) The effective index of the TE00 mode (black line) for 360-nm-thick waveguides with different bus widths.The effective index of the TE00 and TE10 modes for the resonator waveguide (WR=780 nm, RR=25 µm) are indicated by blue and purple lines, respectively.The insets are the profiles of the guided modes in the resonator waveguide.(c, d) Transmission spectrum of the multimode microring resonators using a (c) point coupler and (d) tapered coupler, respectively.