Stable soliton comb from dual ring microresonators Motivation Frequency combs are usually generated using mode locked lasers, featuring narrow comb line spacing (~100MHz). Whereas, microresonator based combs present:  Large comb line spacing (10 to 500 GHz)  Low optical power operation The stability of microresonator based combs depends on the detuning of the laser with respect to the pump resonance. Our work aims at increasing the stability and the spectral purity of the comb lines. Spectroscopy Telecommunication Astronomy Precision measurements Frequency synthesizing LiDAR Frequency comb Applications 1. Helgason, Ó.B., Arteaga-Sierra, F.R., Ye, Z. et al. Dissipative solitons in photonic molecules. Nat. Photonics 15, 305–310 (2021). 2. Xue, Xiaoxiao, Qi, Minghao and Weiner, Andrew M.. "Normal-dispersion microresonator Kerr frequency combs" Nanophotonics, vol. 5, no. 2, 2016, pp. 244-262. References Abstract: We present a packaged SiN dual-ring resonator for frequency comb generation. Using a low frequency noise pump laser and active temperature stabilization we alleviate frequency drifts and detuning fluctuations. The resulting solitons are stable over several hours. Anamika Karunakaran1,2, Angelo Manetta1, Poul Varming1, Óskar B.Helgason3,Patrick Montague1, Minhao Pu2, Victor Torres-Company3, Kresten Yvind2 1 NKT Photonics, Birkerød, Denmark, 2 Technical University of Denmark, Kgs. Lyngby, Denmark, 3Chalmers University of Technology, Gothenburg,Sweden Dual ring microresonator In our experiment, we use dual ring resonators manufactured in Chalmers University of Technology, Sweden [1]. These achieve the localized anomalous dispersion by evanescent coupling between two rings with slightly different free spectral range. The degeneracy point of these rings can be tuned using a heater on the auxiliary ring which enables use of pump lasers at any convenient frequency. Figure : a) Dispersion profile of our microresonator [1] b) Drawing of the dual ring resonator c) Wavelength of mode crossing with respect to heater voltage Microresonators • High Q (~106) with 𝜒3 nonlinearity • Exhibit either anomalous or normal dispersion • Free spectral range ranging from ~10 GHz to 500 GHz In normal-dispersion microresonators, a mode with localized anomalous dispersion can be engineered by coupling different resonator modes.[2] Solitons are then generated by pumping this mode. Normal-dispersion microresonators typically yield combs with higher conversion efficiency than anomalous dispersion microresonators. Comb generation When light is coupled to the main ring, thermal effects redshift its optical resonance. Tuning the laser from the blue side to the red of the main resonance allows to generate our comb. In order to attain a coherent mode-locked state, both the voltage to the auxiliary-ring heater and the laser frequency are finely tuned. Frequency combs are generated via cascaded four wave mixing process exploiting the 𝜒3 properties and high Q value of our microresonator. Figure : Experimental setup for comb generation Figure : Generation of soliton. The laser is first detuned to the blue side of the resonator (Step 1). Fine adjustment of both the laser frequency and heater voltage bring about the appearance of comb lines (Step 2 and 3). Step 4 shows the final comb. Results We generated a soliton comb from a dual ring microresonator. Soliton stability depends on the detuning between the laser and the optical resonance of the main ring. To improve the stability, • Our device was packaged using buttcoupled high NA fibers. • Using low frequency noise NKT single frequency laser, we reduced detuning fluctuations . • By temperature stabilisation of the packaged device, the thermal drift of the resonance was curbed. After adding the TEC, the life span of solitons increased from 20 minutes to up to 5 hours. Figure : (a)The optical spectrum of the generated soliton comb. (b) The corresponding ESA spectrum of the comb lines. Figure (a) The packaged and