Nonlinear Optical High Order Ring Resonators and Their Applications

Abstract

    Chip-scale integrated photonic all-optical circuits offering the benefit of low power, ultrahigh-speed and wide-band operation have received great research interests due to their potential in all-optical applications. Integrated all-optical components such as switches, wavelength converters, logic gates and analog-to-digital convertors are indispensable components in future all-optical processing systems. Among the nonlinear optical structures, recent interests in the development of nonlinear optical microring resonators are due to its small size while maintaining a very high nonlinear figure of merit. So far most of the nonlinear optics applications and demonstrations are with single ring resonator and they have been well reported and understood, however there are much fewer reports on nonlinear behavior in the high-order ring resonator systems especially experimentally. Since high-order ring resonator system has a wider passband and longer interaction length, it should offer better nonlinear performance than the single ring resonator. Therefore, the aim of this dissertation is to evaluate the nonlinear properties of the high-order ring resonator system and to investigate its performance in all-optical applications.

 
    To achieve our goal, we first obtain a fundamental understanding of the nonlinear behaviors in these systems by the development of nonlinear models to simulate both the static and dynamic behaviors in these systems. These models also allow us to compare the measured results in the later experiments with the model to extract additional valuable information from the measurements. In these models, both spectral and temporal nonlinear coupled mode equations that accounts for both the thermal and Kerr effects are implemented to provide a complete picture of high-order system.

    In the investigation of the static nonlinear effects in high-order ring resonator system, we carried out two experiments. First, a 5^th-order ring resonator filter is used to demonstrate optical instability in these systems by placing a single pump at the edge of the filter passband. We observed that the system is extremely sensitive to the position of the pump with respect to the filter passband as bi- and multi-stable behaviors can be achieved by a slight detuning of the pump position. Unlike in the single ring resonator filter which can only consists of bi-stable behavior. The multi-stability in the high-order system can potentially find applications in A/D conversion. In the second experiment, broadband wavelength conversion in the four-wave mixing with the same 5th order ring resonator filter is demonstrated. Conversion over the entire passband of 3.5 GHz with conversion over 20 THz is obtained all under low input power of about 10 mW input power. Compares with the single ring resonator filter, the high-order filter has a much broader conversion bandwidth and is more suitable for higher communication capacity.

    The dynamic nonlinear behaviors in the high-order ring resonator system is investigated through its implementation in the passive mode-locked circuit. Stable mode-locking is achieved by utilizing an 11^th-order ring resonator filter in the self-locked configuration. Unlike in the single ring resonator case where multiple ring resonator resonances are needed to provide stable mode-locked, only a single passband from the 11^th-order ring resonator filter is needed to provide a stable pulse train with pulse width in the 10’s of ps with repetition rate in the MHz range. When multiple passbands that are separated by one or more free-spectral range (FSRs) are allowed to operate in the self-locked loop, stable mode-locking can also be achieved. However, these pulses are complex pulses that consists of the frequency components from all the passbands involved in the operation. The observation is confirmed from the autocorrelation measurement of the output signal which shows beat signal equal to multiple FSRs of the filter. This burst mode operation can generate signal in the 100s GHz to a few THz range and potentially be a path to the realization of ultrahigh precision frequency clock.

 
    The enhanced nonlinear performance is exhibited with using the high-order resonator from the comparison with that of the single resonator, which provides not only the fundamental rule for the design of nonlinear optical devices with low operation threshold and broadband response, but also a potential reference for coupled system in the fields of plasmonics, condensed matter physics, and even nonlinear quantum optics.