Thermal nonlinearity based optical pulse generation in microrings

Abstract

The next generation of optical communication will depend heavily on all optical signal processing. Because digital clock pulses are needed for driving functional gates and synchronizing optical analog to digital (A/D) converters and other optical components, great effort has been placed on generating optical pulses. Approaches include non-linear polarization rotation, photonic crystal based methods, and cross gain modulation of semiconductor optical amplifiers. Although these methods share the advantage of high optical bandwidth, they each suffer from complicated fabrication and poor scalability. In this work, we propose using optical self-heating in silicon nitride microring resonators and thermal nonlinearity for creating all optical signal pulses. They key factor in our design is the passive framework we have chosen which result in scalability of our design. The resonances in the reflection spectrum of a plain microring are caused by coherent optical backscattering from the natural sidewall roughness created during dry etching. Light absorbed through the ring resonator will be converted to heat resulting in a temperature rise inside the ring. This temperature increase will alter the material refractive index which in turn will result in a red shifting of the resonance wavelength. Shifting the resonance wavelength changes the optical energy stored in the resonator as well as the absorption of the ring. Hence, the thermal and optical dynamics of the ring are coupled through the self-heating and thermo-optic effect [1]. Considering these dynamics, if we tune to a wavelength that is slightly on the blue side of the resonance and we increase the input signal power, the resonance wavelength will move away from our set wavelength and consequently the reflected power will decrease. Conversely, reducing the input power creates an increase in reflected power. Thus, by altering the input power, it is possible to switch between two states of the ring resonator and more importantly to realize an inverter function. By feeding back the reflection signal to the input of the ring resonator (Fig. 1(a)), we were able to generate all optical pulses (Fig. 1(b)) with a controllable frequency.