Objective Cavity optomechanical system enables a hybrid system where optical resonant mode and mechanical oscillation mode are coupled together. Benefiting from the compatibility of the manufacturing process with the semiconductor industry, a strong optomechanical interaction between the optical cavity and mechanical resonator can be achieved in nanoscale silicon photonics devices. The nanoscale optomechanical system can considerably improve the interaction between optics and mechanics mediated by the optical force, enabling the development of ultra-stable and ultra-narrow linewidth mechanical oscillation signals. Cavity optomechanics research on classical mechanics and quantum systems has been widely utilized to provide an unprecedented platform for high-precision sensing, cavity optomechanical devices and circuits, and high-efficiency microwave-optical conversion devices. We propose a nonlinear cavity optomechanical coupling scheme by introducing a nonlinear mechanical oscillator integrated with a linear mechanical oscillator to achieve the high-performance optomechanical frequency comb. This novel nonlinear optomechanical system can provide a solution for the high-performance cavity optomechanical frequency comb and hence effectively promote the development and application of photonic microwave source technology. Methods A nonlinear mechanical oscillator integrated with a linear mechanical oscillator is introduced into an optomechanical system, and the optomechanical frequency comb is experimentally observed with a series of equally spaced and discrete frequencies. In this optomechanical system, a zipper cavity consists of two photonic crystal nanobeam (PCN) cavities. One PCN is doubly clamped to work as a fixed PCN for laser pumping, and the other movable PCN is integrated with a 2-degree-of-freedom (2-DOF) vibration system to transform the optical force acting on two mechanical oscillator. In the 2-DOF oscillator system, a curved nonlinear mechanical oscillator is integrated with a linear mechanical oscillator. Based on the degenerated multimode mixing method, the mechanism of the optomechanical frequency comb is theoretically analyzed. These nonlinear mechanics are coupled to the optomechanical interaction, and the response of the light field can be eventually detected to observe the optomechanical frequency comb. Results and Discussions The optical performance of the zipper cavity is investigated for the following detection of mechanical spectra, and the optical spectrum is divided to identify the laser sweeping range for the observation of optomechanical frequency comb. In this coupled PCN cavity with a high Q(o), the even cavity mode has a high optomechanical coupling strength to detect the in-plane oscillation of the 2-DOF oscillator system. The characterization of the nonlinear optomechanical device, a power spectral density (PSD) map, is conducted to observe the mechanical oscillations by the wavelength swept through the resonance of TE1,e and TE2,e modes. When the laser is swept into the optomechanical frequency comb zone at the TE2,e resonance mode, two sidebands of the mechanical resonant frequency are generated with some symmetrical side lobes with a frequency-spacing of 81.5 kHz, which behaves very similar to an optical frequency comb. At the TE1,e resonance mode, cavity energy inside the fundamental optical mode is higher than that of TE2,e mode, so the nonlinear optomechanical system is driven by a higher optical force. The optomechanical frequency comb at the TE1,e resonance is observed with more excited side lobes with a frequency-spacing of 81.3 kHz due to the stronger optomechanical coupling between the 2-DOF vibrator and optical cavity induced by higher cavity energy. As a result, the measured optomechanical frequency comb in the TE1,e cavity mode is clearer than that of TE2,e cavity mode when the pump power is 25 mu W, which indicates that the generation of the optomechanical frequency comb is highly dependent on the energy in the intracavity. Conclusions In the paper, the optomechanical frequency comb is experimentally observed in a nonlinear optomechanical system. In this optomechanical system, a nonlinear oscillator is integrated with a linear oscillator to form a 2-DOF oscillator. In analogue to the generation of optomechanical frequency comb, a numerical model is theoretically proposed to analyze the nonlinear optomechanical coupling by introducing a nonlinear term. When the nonlinear force is acted on the 2-DOF oscillator, the degenerate four-wave mixing of mechanical modes can contribute to the generation of an optomechanical frequency comb. In the experiments, a demonstration of the optomechanical frequency comb is conducted using the TE1,e and TE2,e modes. From the analysis of the optomechanical frequency comb, it can be concluded that first, the optomechanical frequency comb always arises at the red-detuned sideband, and it is highly dependent on the optical power coupled into the cavity. Second, a remarkable optomechanical frequency comb is excited in the fundamental cavity mode much more visible than that of the second cavity mode. In future practical applications, the optomechanical frequency comb can be potentially used as a microwave photonic source, which can provide a reference clock signal in the frequency range of MHz.
