Design of optical frequency comb based on dual frequency pumped normal dispersion silicon carbide microresonator

Abstract

The scheme of generating optical frequency comb (OFC) mainly includes modelocked laser, electro-optic modulation comb, nonlinear Kerr micro-resonator comb. OFC with frequency spacing in the order of 10~200 GHz can be employed in optical communication, microwave photonics, and other fields. Silicon carbide (SiC) has attracted considerable research interest in integrated nonlinear photonics owing to its high second and third order nonlinear coefficients, low optical loss, without multiphoton absorption loss thanks to the wide bandgap. Single soliton microcomb in anomalous group velocity dispersion regime based on 4H-SiC-on-insulator thin film has been demonstrated with the relative lower pump to comb efficiency, while the OFC in normal dispersion regime based on the SiC microresonator has not been reported. The pump conversion efficiency of OFC in the normal dispersion regime is high, and the pump frequency detuning range for the OFC generation is large, which is conducive to the OFC generation and long-term stable operation. Since there is no modulation instability effect in normal dispersion regime, the key for the OFC generation in normal dispersion regime is that the initial state should be assisted by a multi-frequency laser (or four-wave mixing sideband). The phase-locked dualfrequency laser can be regarded as a pulse pump laser source with wide pulse duration, which can be realized by integrated DFB laser.<br>In this paper, a scheme of generating OFC by pumping the normal dispersion SiC microresonator with phase locked dual-frequency laser is proposed. The flat normal dispersion in 1550nm band is realized through dispersion engineering of the SiC microresonator. The effective mode field area of the TE<sub>0</sub> fundamental mode at 1550nm in the optimized SiC ridge waveguide is about 0.94<i>μm</i><sup>2</sup>, and the nonlinear coefficient is about 3.69<i>W</i><sup>-1</sup><i>m</i><sup>-1</sup>. Meanwhile, dispersion parameters of the microresonator with 100GHz FSR are also obtained. The OFC generation pumped by a phase-locked dualfrequency laser based on normal dispersion SiC microresonator is simulated through using the Lugiato Lefever equation (LLE). The evolution process of the OFC in time and frequency domain related to the pump detuning is studied. The effects of several parameters such as the pump power, microresonator waveguide loss, microresonator dispersion, proportion of the dual-frequency laser and the frequency interval of dualfrequency laser on the performance of the OFC are also investigated. The following conclusions can be obtained through the OFC generation simulation:(1) When the microresonator waveguide loss is larger, the pump detuning range for the OFC generation becomes smaller, and the pulse peak power under the same pulse intensity filling rate decreases. (2) When the input pump power is larger, the pump detuning range for the OFC generation becomes larger, the pulse peak power under the same pulse intensity filling rate increases, and the corresponding spectrum becomes wider. (3) With the increase of absolute dispersion value, the spectrum bandwidth of the generated OFC decreases obviously. (4) The power proportion of dual-frequency laser has little influence on the OFC generation. (5) The frequency spacing of the generated OFC can be tuned through changing the frequency spacing of the two phase locked laser with integral multiple of FSR.<br>The OFC with spectrum bandwidth of about 70nm can be generated in 1500nm- 1600nm through the simulation. The simulation results are beneficial to promoting the research and practical application of high repetition rate broadband optical frequency comb in 1550nm band based on the normal dispersion silicon carbide microresonator.