1 / 20 The Nyquist soliton Kerr comb Xiaoxiao Xue 1 , Bofan Yang, Mian Wang, Shangyuan Li, Xiaoping Zheng 2 , and Bingkun Zhou Department of Electronic Engineering, Beijing National Research Center for Information Science and Technology, Tsinghua University, Beijing 100084, China 1 xuexx@tsinghua.edu.cn; 2 xpzheng@tsinghua.edu.cn Abstract Dissipative Kerr cavity solitons (DKSs) are localized particle-like wave packets that have attracted people’s great interests in the past decades. Besides being an excellent candidate for studying nonlinear physics, DKSs can also enable the generation of broadband frequency combs which have revolutionized a wide range of applications. The formation of DKSs are generally explained by a double balance mechanism. The group velocity dispersion is balanced by the Kerr effect; and the cavity loss is compensated by the parametric gain. Here, we show that DKSs can emerge through the interplay between dispersive loss and Kerr gain, without the participation of group velocity dispersion. By incorporating rectangular gate spectral filtering in a zero-dispersion coherently driven Kerr cavity, we demonstrate the generation of Nyquist- pulse-like solitons with unprecedented ultra-flat spectra in the frequency domain. The discovery of pure dissipation enabled solitons reveals new insights into the cavity soliton dynamics, and provides a useful tool for spectral tailoring of Kerr frequency combs. Coherently driven dissipative temporal cavity solitons (DKSs), which were first demonstrated in a fiber ring cavity [1], are now being investigated intensely for their great potential in broadband optical frequency comb generation [2], [3]. With integrated miniature microcavities, soliton microcombs with ultra-compact volume and low power consumption have been demonstrated [4], [5], which may revolutionize a wide range of applications from high- precision metrology to large-capacity communications [6]-[14]. The formation of DKSs relies on an elegant balance of group velocity dispersion and Kerr effect as well as dissipative loss and parametric gain [2]. Depending on the sign of dispersion, DKSs with a rich diversity have been observed. In the anomalous dispersion region, the DKSs show similar temporal and spectral features to the classical solitons in optical fibers [1], [15]. In the normal dispersion region, rather different DKSs such as dark pulses and flat-top platicons may exist [16], [17]. To tailor the comb spectrum, most efforts to date have been focused on global dispersion engineering and mode crossing management [18]-[20]. Nevertheless, it has been found that the frequency dependence of cavity loss may also affect the comb generation. Spectral filtering induced modulational instability [21], [22] and pulse mode-locking [23], [24] akin to that in normal-dispersion mode-locked lasers have been reported. Despite the intense studies, the physics of DKSs is still far from being thoroughly explored and fully understood. For applications such as spectroscopy [6], wavelength-division multiplexed communication [7], lidar ranging [8], [9], photonic neutral computing [11], [12], microwave photonic signal processing [13], [14], etc., the frequency comb should ideally have a rectangular spectrum with uniform lines in a designed bandwidth. When the comb line phases are identical, the optical time-domain waveform can be described by a sinc function which is