Optical frequency comb in silicon nitride microresonator(Invited)
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摘要: 具有高品质因子(Q 值)的光学谐振腔能够长时间将光束缚在较小的模式体积内,极大地增强了光与物质的相互作用,成为集成光学器件中具有重大潜力的重要组成部分。聚焦于目前广泛应用于集成非线性光学领域的氮化硅材料平台,为了解决大尺寸氮化硅微环腔由拼接误差、表面粗糙等因素导致的散射损耗较大的问题,进行了一系列的工艺改进以提高大尺寸氮化硅微环腔的品质因子。结果表明:通过薄膜再沉积工艺可以有效降低氮化硅波导的散射损耗,半径为560 μm的大尺寸氮化硅微环腔的本征Q值得到了平均26% 的提升。得益于提高的微腔Q 值,在氮化硅微环腔中实现了重复频率40 GHz 的光学频率梳。Abstract: Optical resonators with high quality (Q) factor can restrict light in a small mode volume for a long time, greatly enhancing the interaction between light and matter, and becoming an important component with great potential in integrated optical devices. Focusing on the silicon nitride material platform, which is currently widely used in the field of integrated nonlinear optics, in order to solve the problem of large scattering loss in the large size on-chip silicon nitride microring resonator caused by the stitching error, the surface roughness and other factors, a series of fabrication process improvements were made to improve the quality factor of the large size silicon nitride microring resonator. The results show that the scattering loss of the silicon nitride waveguide can be effectively reduced by thin film redeposition process, and the intrinsic Q of the large size silicon nitride microring resonator with a radius of 560 μm is increased by 26% on average. Thanks to the improved Q of the large size microring resonator, the frequency comb with the repetition rate of 40 GHz is realized in the on-chip silicon nitride microring resonator.
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图 2 (a) 再次沉积薄膜前的氮化硅微环腔示意图,耦合波导上方已沉积二氧化硅保护层;(b) 再次沉积薄膜后的氮化硅微环腔的侧面视图
Figure 2. (a) Schematic diagram of the silicon nitride microring resonator before redeposition of film, silica protective layer has been deposited above the coupling waveguide; (b) Side view of the silicon nitride microring resonator with the redeposited film
图 3 (a) 氮化硅微环腔在沉积薄膜前(蓝色直方图)与后(橙色直方图)的本征Q值的统计直方图,包括了在1550~1625 nm之间所有TM基模的本征Q值;(b)~(c)沉积前后,本征Q值最高的光学模式的透射功率谱,通过洛伦兹拟合的Q值分别为2.84×106和3.44×106,相应的本征Q值分别为2.91×106和4.15×106
Figure 3. (a) Statistical histograms of intrinsic Q of silicon nitriding microring resonator before (blue histogram) and after (orange histogram) deposition of thin films, which includes intrinsic Q of all fundamental TM modes between 1550 nm and 1625 nm; (b)-(c) The transmission power spectrum of the optical mode with the highest intrinsic Q before and after the redeposition. The loaded Q by Lorentz fitting is 2.84×106 and 3.44×106, respectively, and the corresponding intrinsic Q is 2.91×106 and 4.15×106, respectively
图 4 (a)氮化硅微环腔的TM模式的透射谱;(b)随着泵浦光从腔模的蓝失谐扫到红失谐,腔内光功率和透射光功率的变化,能观测到明显的孤子台阶;(c)产生的光学频率梳的频谱,包络抖动由避免模式交叉导致;(d)光学频率梳频谱与相应的集成色散曲线,色散曲线上的色散突变与频率梳频谱上的包络抖动相对应
Figure 4. (a) Transmission spectrum of the TM mode in the silicon nitride microring resonator; (b) The change of the intracavity power and the transmitted power with the pump light sweeping from the blue detuning to the red detuning, where distinct soliton steps can be observed; (c) The generated spectral spectrum of the soliton microcomb, the envelope perturbation is caused by avoided mode crossings; (d) The spectral spectrum of the soliton microcomb and the corresponding integrated dispersion curve, the dispersion abrupt change on the dispersion curve corresponds to the envelope perturbation on the soliton microcomb spectrum
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