On June 12, 2023, the Center, in collaboration with Prof. John E. Bowers' group at the University of California, Santa Barbara, has made progress in the study of microcavity dark pulses, reporting technological breakthroughs such as sub-milliwatt pumping thresholds, simple robust excitation, and broadband scanning of mode-locked microcavity optical combs, which was published in the journal Advanced Photonics under the title "Submilliwatt, widely tunable coherent microcomb generation with feedback-free operation".
Screenshots of the paper
Optical frequency combs are pulsed lasers mixed from a series of equally spaced frequency components, which serve as scales in the time/frequency domain, and have now revolutionized modern science and technology in many fields such as optical communications, spectral detection, optical ranging, and optical frequency clocks.
However, the traditional optical comb is bulky and complex, and is still only functioning in the "ivory tower"; on the other hand, based on the on-chip micro-cavity structure of the "key" optical comb, due to the incompatibility between electronic control part and the material of the optical part, it is also difficult to realize the mass production based on CMOS process. In addition, conventional on-chip optical frequency combs have a very limited spectral tuning range, which greatly affects their applicability in application scenarios such as LIDAR and spectral measurements.
Robust excitation and wide tuning properties of microcavity optical combs
Faced with the above problems, the research team launched a study on the simple excitation technique and broadband scanning technique for microcavity optical combs. Unlike most studies, the team focused on the mode-locked state of dark pulses in normally dispersive microcavities, which naturally has the energy efficiency advantage of high conversion efficiency. Using this mode-locked state, the cavity optical power does not undergo large abrupt changes during the evolution from the non-mode-locked state to the mode-locked state, so the microcavity resonance peaks do not undergo large drifts due to intracavity thermal effects, which avoids the use of techniques such as fast frequency sweeping. In addition, upon reaching the mode-locked light comb state, negative feedback on key support parameters such as detuning amount is created in the cavity due to thermal effects in the cavity. At a pump power input of 150 mW, the "dark pulse" optical frequency comb will be generated in the range of 97.5 GHz, which exceeds the entire free spectral range. This not only means that the comb has an ultra-wide frequency tuning range, but also a breakthrough in the operation of the comb from the closed-loop frequency stabilization control system - just ordinary on-chip laser is enough to make the "dark pulse" comb output stably. This is an important step towards the integration and commercialization of optical frequency combs. Utilizing the strong thermo-optic effect of AlGaAs material, the mode-locked optical comb can be generated and stabilized for a long period of time without external feedback, and the stable operation of the microcavity optical comb for up to 7 hours has been tested continuously on the experiment. In addition to this, the "dark pulse" optical comb has an excellent frequency chirp capability. By modulating the pump light, the modulation frequency is shifted to all spectral lines of the comb, generating frequency shifts of up to more than 10 GHz, which corresponds to ranging accuracies on the order of centimeters. Chirp is a phenomenon in which the frequency of a signal varies with time, and it is also affected by nonlinear effects such as stimulated Raman scattering and higher-order dispersion. However, due to the low pump power required for the "dark pulse" compared to a conventional optical frequency comb, the chirp frequency shift due to the above nonlinear effects is negligible.
The co-first authors of the paper are Hao-Wen Shu, assistant professor of the Center; Chang Lin, assistant professor of the School of Electronics; Cheng-Hao Lao, a doctoral student of the School of Physics; and Bitao Shen, a doctoral student of the Center. Prof. Xingjun Wang and Prof. John E. Bowers are the co-corresponding authors of the paper. Academician Shaohua Yu of Pengcheng Laboratory participated in this work and provided important guidance. Key collaborators also include Dr. Wei-Qiang Xie (now Associate Professor at Shanghai Jiaotong University) at the University of California, Santa Barbara, Center PhD students Xuguang Zhang, Ming Jin, Yuan-Sheng Tao, Zi-Han Tao, and Hua-Chong Khong, post-doctoral fellow RuiXuan Chen, and Assistant Professor QiFan Yang at the School of Physics. This work was done by the State Key Laboratory of Regional Optical Fiber Communication Network and Novel Optical Communication System, School of Electronics, Peking University as the first unit.
Link to the original paper:
https://www.researching.cn/articles/OJa164fb956bc0d621
