Generation schemes of optical frequency combs mainly include mode-locked laser, electro-optic modulation comb, nonlinear supercontinuum-based comb, and nonlinear Kerr microresonator comb. Compared with other generation methods of optical frequency combs, the Kerr microresonator comb is considered a new type of coherent light source that features unique and promising advantages of lower power consumption and whole system integratability.

The Kerr microresonator pumped in the anomalous group velocity dispersion (GVD) regime leads to the dissipative Kerr soliton comb. The dissipative soliton states are sometimes inaccessible due to the intracavity thermal dynamics and therefore require special tricks to align the pump laser and the resonances in soliton formation. These approaches need benchtop laser sources and complex control protocols, which are not suitable for integrated photonic systems. Furthermore, due to the small temporal overlap between the driving continuum wave laser and the ultrashort pulse, the pump-to-comb conversion efficiency is rather low. Meanwhile, Kerr comb pumped in the normal GVD regime has the benefits including relatively easy access to high pump-to-comb conversion efficiency, large pump frequency detuning range for comb generation, and lower power falloffs within the spectral region of interest which are more ideal for optical communications. Since there is no modulation instability (MI) in the normal GVD regime, the most prevalent method to generate a normal GVD comb is to modify the microresonator dispersion via mode splitting. Common mode splitting mechanisms contain mode coupling to different polarization modes, spatial modes, injection locking, and auxiliary resonator modes. However, the above-mentioned methods are quite complicated. Another method to generate a normal GVD comb can be achieved by pump direct modulation or electro-optic pulse generator based on electro-optic intensity and phase modulators at the resonator free spectral range (FSR), but the electro-optic pulse generator is quite bulky. The phase-locked dual-frequency laser can be regarded as a pulse pump laser source with a wide pulse duration, which can be realized by an integrated DFB laser.

Silicon nitride is widely applied to nonlinear optics. It has an ultra-broad transparency window, low intrinsic loss, and a refractive index that allows for moderate optical field confinement in waveguides. However, fabricating thick films with high yield is challenging owing to the large tensile stress in as-deposited stoichiometric silicon nitride films, which can result in the formation of cracks crossing the photonic devices. An alternative way to overcome the high tensile stress is varying the composition of the material itself. In particular, silicon-riched silicon nitride can dramatically reduce the film stress. Silicon-riched silicon nitride waveguides also have a higher nonlinear Kerr coefficient and refractive index than stoichiometric silicon nitride, but the normal GVD comb based on the silicon-riched silicon nitride has not been reported. Thus, we propose a generation scheme of optical frequency combs by adopting a phase-locked dual-frequency laser-pumped normal dispersion silicon-riched silicon nitride microresonator. The proposed optical frequency comb has potential applications in astronomy, optical communication, and microwave photonics.

Firstly, the flat normal dispersion in the 1550 nm band is realized via dispersion engineering of the silicon-riched silicon nitride microresonator by the finite element method mode solver. The effective mode field area of the TE₀ fundamental mode at 1550 nm in the optimized silicon-riched silicon nitride waveguide is about 1.005 μm², and the nonlinear coefficient is about 4.587 W^-1·m^-1. Meanwhile, the dispersion parameters of the microresonator with 100 GHz free spectral range (FSR) are also optimized. Then, the optical frequency comb generation pumped by a phase-locked dual-frequency laser based on the normal dispersion silicon-riched silicon nitride microresonator is simulated by employing the Lugiato Lefever equation (LLE). The evolution process of the optical frequency comb in time and frequency domains related to the laser pump detuning is studied. Additionally, the effects of several parameters on the performance of the optical frequency comb are also investigated.

The silicon-riched silicon nitride waveguide structure with optimized normal dispersion and nonlinear coefficient is obtained by dispersion engineering (Fig. 1). The dispersion parameters such as resonant mode frequency spacing D₁/(2π), second-order dispersion D₂/(2π), third-order dispersion D₃/(2π), dispersion parameter D_int/(2π) of a microresonator with bending radius of 206.5 μm are also obtained (Fig. 2). The schematic diagram of the optical frequency comb generated via phase-locked dual-frequency laser pumped normal dispersion silicon-riched silicon nitride microresonator is shown (Fig. 3). The optical frequency comb generation is simulated by the LLE. The time-frequency evolution process of the optical frequency comb in time and frequency domains related to the pump detuning is studied (Fig. 4). The optical frequency comb in the normal GVD regime can be generated within a relatively large pump detuning range. The laser pump detuning is intrinsically linked to the intensity filling rate of a pulse state. When the pump detuning increases, the pulse becomes narrow with a broad corresponding spectrum. The effects of several parameters such as the pump power, the power proportion of the dual-frequency laser, microresonator waveguide loss, microresonator dispersion, and the frequency interval of dual-frequency laser on the performance of the optical frequency comb are also investigated. The following conclusions can be obtained by the simulation. Firstly, under the higher laser pump power, the pump detuning range for the optical frequency comb generation becomes larger and the pulse peak power under the same pulse intensity filling rate increases, with a wider corresponding spectrum (Fig. 5). Secondly, the power proportion of dual-frequency laser has little influence on optical frequency comb generation (Fig. 6). Thirdly, when the microresonator waveguide loss is larger, the pump detuning range for optical frequency comb generation becomes smaller, and the pulse peak power under the same pulse intensity filling rate decreases (Fig. 7). Fourthly, with the increasing absolute dispersion value, the spectrum bandwidth of the optical frequency comb under the same pulse intensity filling rate reduces obviously (Fig. 8). Finally, the frequency spacing of the optical frequency comb can be tuned via changing the frequency spacing of the phase-locked dual-frequency laser with integral multiple of FSR (Fig. 9).

We propose a generation scheme of optical frequency combs by adopting a phase-locked dual-frequency laser-pumped normal dispersion silicon-riched silicon nitride microresonator. By optimizing the structure of silicon-riched silicon nitride microresonator and dispersion engineering, an optical frequency comb with bandwidth from 1520 nm to 1580 nm is realized via the simulation. The time-frequency evolution process of optical frequency comb generation is analyzed. The simulation results show that a dual-frequency pumped optical frequency comb in the normal GVD regime can be generated within a relatively large pump detuning range, which will benefit long-term comb stabilization and real applications. Additionally, the effects on the performance of optical frequency combs such as the pump power, the power proportion of the dual-frequency laser, microresonator waveguide loss, microresonator dispersion, and the frequency interval of dual-frequency lasers are also studied. Our study shows that silicon-riched silicon nitride waveguides have potential benefits for the 1550 nm broadband optical frequency comb based on normal dispersion nonlinear optical microresonator.