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Optics Express

Flat-top optical resonance in a single-ring resonator based on manipulation of fast- and slow-light effects

Menghao Huang, Simin Li, Min Xue, Lei Zhao, Shilong Pan

Published Aug 24, 2018• 33 citations
Publisher

Abstract

Optical single-ring resonance inherently generates Lorentzian-shape magnitude and group-delay responses, leading to critical performance limitation in photonics, cavity quantum electrodynamics, cavity optomechanics, and atomic and optical physics. Here, we propose a new type of microresonator that stimulates flat-top resonance in a single-ring cavity. By manipulating the fast and slow light effects in the microresonator, the flat-top group delay can be tuned with an ignorable magnitude variation. In addition, the bandpass response can be switched to a notch, which can enable function-reconfigurable photonic integrated circuits (PICs) without a physical change in the architecture. Our demonstration provides the possibility of developing microresonator-based PICs with unprecedented high flexibility and capacity.

DOI
10.1364/oe.26.023215

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Optical single-ring resonance inherently generates Lorentzian-shape magnitude and group-delay responses, leading to critical performance limitation in photonics, cavity quantum electrodynamics, cavity optomechanics, and atomic and optical physics. Here, we propose a new type of microresonator that stimulates flat-top resonance in a single-ring cavity. By manipulating the fast and slow light effects in the microresonator, the flat-top group delay can be tuned with an ignorable magnitude variation. In addition, the bandpass response can be switched to a notch, which can enable function-reconfigurable photonic integrated circuits (PICs) without a physical change in the architecture. Our demonstration provides the possibility of developing microresonator-based PICs with unprecedented high flexibility and capacity.
We utilize the contra-propagating cavity modes that arise from the evanescent coupling of both the resonators to the bus waveguide in a twin coupled traveling-wave microresonators (MRs) system to generate flat-band slow light (SL). The contra-propagating cavity modes will generate multipeaks in the resonance spectra. Flat-band SL can be generated if such multipeaks become undistinguishable and merge into one single broadened peak that is maximally flat when the inter-resonator coupling strength is optimized relative to the resonators-to-bus-waveguide coupling strengths. The bandwidth and the group delay can be tuned by adjusting the coupling strengths. It is shown that the delay-bandwidth products of the output light at the through (reflection) port are 3- to 12-fold (6- to 24-fold) higher than that of conventional MR-based SL systems. Fabrication tolerance and cavity losses analyses have also revealed that the proposed scheme is rather robust to the fabrication errors and limitations of current state-of-the-art semiconductor processing technology.
We comprehensively simulate and experimentally demonstrate a novel approach to generate tunable electromagnetically induced transparency (EIT) in a fully integrated silicon photonics circuit. It can also generate tunable fast and slow light. The circuit is a single ring resonator with two integrated tunable reflectors inside, which form an embedded Fabry-Perot (FP) cavity inside the ring cavity. The mode of the FP cavity can be controlled by tuning the reflections using integrated thermo-optic tuners. Under correct tuning conditions, the interaction of the FP mode and the ring resonance mode will generate a Fano resonance and an EIT response. The extinction ratio and bandwidth of the EIT can be tuned by controlling the reflectors. Measured group delay proves that both fast light and slow light can be generated under different tuning conditions. A maximum group delay of 1100 ps is observed because of EIT. Pulse advance around 1200 ps is also demonstrated.
Broadband squeezed light functioning as a Markovian reservoir can exponentially enhance light-matter interactions, benefiting quantum technologies. However, conventional single-cavity sources face a trade-off between squeezing depth and spectral bandwidth. We propose a scalable scheme for generating broadband squeezed vacuum using a cascade of parametric microring resonators coupled to a common bus waveguide. By analyzing the output, we identify the specific conditions that yield a broad, flat-topped squeezing spectrum, even under realistic intracavity pump attenuation. We demonstrate that this architecture is robust against fabrication imperfections, including inhomogeneous resonator frequencies and component failures. We show that the flat-topped spectrum converges to the Markovian limit significantly faster than a single-cavity Lorentzian profile. An array of as few as N=5N=5N=5 coupled resonators with an intrinsic loss ratio of κI/κ=0.1κ_I/κ= 0.1κI​/κ=0.1 reduces the required bandwidth to a quarter of that needed by a single cavity to achieve same squeezing. This rapid convergence relaxes the low-QQQ and high-gain constraints of single broadband cavities, distributing the squeezing process across moderately pumped resonators to provide a practical route for engineering squeezed reservoirs on mature integrated photonic platforms.
As a potential enabling technology, slow light has captured much research interest over the past few years for achieving a continuously tunable optical delay line [1]. In principle, slow light is generated by tailoring an enhanced group-index resonance within a given nonlinear medium. This effective refractive index change is experienced by the data stream passing through and thus induces a controllable group delay onto the signal. Themain feature of a slow-light-based optical delay line is the capability for fine-grain temporal manipulation of optical pulses. This fine tunable delay element is envisioned to be useful for various high-bandwidth signal processing functions and applications.
Dynamically tuning Q factor in optical resonators is now more feasible in information processing applications, such as light storage and wavelength conversion. This paper proposed and demonstrated a new dynamically tuning Q approach that is realized in coupled two rings structure. One resonator uses an add-drop configuration, coupled to another single micro-ring. The heating causes an increase in the refractive index, which in turn causes resonant wavelength redshift. This shift is a switch to control the coupling between the two rings, to tune the linewidth of light confined in the ring, equivalent to tune the quality factor. And we adjust the detuning and coupling state between the two rings to control the light coupled into the resonator. The Q factor decreases from 56,525 to 16,450 in the transmission spectrum with the different heater power. In this way, we successfully realized a modulation of the Q from high to low states in the structure. Besides, the original coupling spacing between the two rings also has an influence on the Q factor. The larger the coupling spacing, the higher the Q factor. In addition, we also study the changes of phase and delay time in the tunable process. The results show that the change of fast light (high Q) and slow light (low Q) can be realized at the same time, which would enable applications for on-chip adjustable time delay, fast/slow light and light storage.
Manipulating the speed of light has never been more exciting since electromagnetic induced transparency and its classical analogs led to slow light. Here, we report the manipulation of light group velocity in a terahertz metamaterial without needing a dark resonator, but utilizing instead two concentric split-ring bright resonators (meta-atoms) exhibiting a bright Fano resonance in close vicinity of a bright Lorentzian resonance to create a narrowband transmittance. Unlike earlier reports, the bright Fano resonance does not stem from an asymmetry of meta-atoms or an interaction between them. Additionally, we develop a method to determine the metamaterial "effective thickness", which quantifies the influence of the substrate on the metamaterial response and has remained challenging to estimate so far. By doing so, very good agreement between simulated and measured group delays and velocities is accomplished. The proposed structure and method will be useful in designing optical buffers, delay lines, and ultra-sensitive sensors.