Research On Silicon-Based Parametric Wavelength Conversion And On-Chip Photonic Temporal Differentiator

Optical device is crucial for the high-speed and intelligent optical communication networks. All-optical signal processing and photonic integration are the technology trends of optical devices. All-optical signal processing is an effective approach to overcome the inherent speed limitations of conventional electronics, while photonic integration can decrease the device dimensions and power consumption, lower the cost, and improve the reliability. Promoted by the mature fabrication process of integrated circuits, silicon is emerging to be the most prevalent platform for photonic integration.The parametric wavelength converter and the photonic temporal differentiator are two important kinds of the all-optical devices. Based on the silicon-on-insulator (SOI) platform, this work focuses on the advanced technologies for these devices. To improve the wavelength conversion efficiency, the quasi-phase-matching (QPM) technology is researched, and novel schemes are proposed. Besides, the theoretical model and the implementation for the on-chip microring-based fractional differentiator are ameliorated. The main contents and innovations of this work are as follows:1. Study on the silicon-based parametric wavelength conversion(1) A novel QPM scheme is proposed in silicon waveguide with the microring phase shifter, which diminishes the effect of phase mismatch on the conversion efficiency, and provides more freedom to design the waveguide geometry and choose the pump wavelength. Over-coupled all-pass microring resonators are used to obtain the additional phase shift.By suppressing the parametric attenuation process, the conversion efficiency is increased observably. Besides, the influences of key factors on the improvement of conversion efficiency are discussed. It is revealed that enhancing the coupling strength, properly adjusting the pump power, and increasing the waveguide length are beneficial, while the nonlinear loss is detrimental. Furthermore, the microring assisted QPM can also be used to flatten the spectra of conversion efficiency. Simulation results reveal that, by appropriately selecting the parameters, flattened spectrum is obtained in the wavelength range of 10 nm.(2) A novel QPM scheme is proposed in a vertically etched silicon grating, which can realize efficient and broadband wavelength convention without dispersion engineering. For the designated signal wavelength, the QPM is achieved by properly tailoring the period and duty cycle of the gratings. Besides, by alternating the phase-mismatch between two values with opposite signs, the conversion efficiency is significantly improved. Furthermore, larger waveguide length is beneficial to obtain greater conversion efficiency. In this work, as the conversion bandwidth of the proposed wavelength converter is insensitive to the propagation length, the tradeoff between the conversion bandwidth and the peak conversion efficiency is alleviated. Simulation results reveal that, for a continuous-wave (CW) pump at 1550 nm, a conversion bandwidth of 331 nm and a peak efficiency of-12.8 dB can be realized in a 1.5-cm-long grating with serious phase-mismatch.2. Study on the on-chip photonic temporal differentiator(1) A novel theoretical model for the microring-based photonic fractional differentiator is proposed. In this model, the time-reverse character is considered, where the under-coupled and the over-coupled resonators are modeled as time-reversed ideal differentiators. Besides, the time delay characteristic is analyzed, which is varied according to the differentiation order. In addition, by considering the bandwidth of the pulse, the effect of input pulse width on the differentiation order is explained. Furthermore, as smaller fitting error can be obtained, it is demonstrated that the response of the microring resonator is more suitable for realizing the differentiation order n≤1.(2) The influences of key factors on the output deviation are analyzed for the microring-based fractional differentiator. When the carrier and the resonance are mismatched, the deviation will increase in the notch part of the output pulse. Besides, the variation of input pulse width may cause a change in the differentiation order, where the phase response of the microring resonator plays an important role. In addition, by reducing the input pulse width, although less deviation can be obtained for small differentiation orders, the deviation for order n>1 will be increased. Furthermore, the finite slope on the resonance in the phase response will lead to an amplitude deviation of the side peak in the output waveform. If the slope is further decreased, the position of the side peak can also be changed.(3) A novel scheme of tunable fractional-order differentiator is proposed based on the inverse Raman scattering (IRS) in a silicon microring resonator. By controlling the power of the pump light-wave, the intracavity loss is adjusted, and the coupling state of the microring resonator can be changed. Thus, the differentiation order can be tuned continuously. Besides, the field enhancement effect improves the pump intensity and the IRS process in the resonator, so the input pump power can be reduced. Furthermore, the narrow attenuation bandwidth of IRS process in silicon provides the potential to achieve different differentiation orders simultaneously at different resonant wavelengths, which has offered great flexibility. Simulation results reveal that, by increasing the pump power continuously from 60 mW to 95 mW, the Gaussian pulse with a full width at half-maximum (FWHM) of 50 ps can be differentiated with the order from 1.6 to 0.3, and the output deviation from an ideal fractional-order differentiator is maintained less than 5%.