Boosting SiN Ring Resonator Linewidth Lower Than 10 fm Through Optical Bistability

We report enhanced nonlinear optics in complementary metal oxide\nsemiconductor compatible photonic platforms through the use of layered two\ndimensional (2D) graphene oxide (GO) films. We integrate GO films with silicon\non insulator nanowires (SOI), high index doped silica glass (Hydex) and silicon\nnitride (SiN) waveguides and ring resonators, to demonstrate an enhanced\noptical nonlinearity including Kerr nonlinearity and four wave mixing (FWM).\nThe GO films are integrated using a large area, transfer free, layer by layer\nmethod while the film placement and size are controlled by photolithography. In\nSOI nanowires we observe a dramatic enhancement in both the Kerr nonlinearity\nand nonlinear figure of merit (FOM) due to the highly nonlinear GO films. Self\nphase modulation (SPM) measurements show significant spectral broadening\nenhancement for SOI nanowires coated with patterned films of GO. The dependence\nof GO Kerr nonlinearity on layer number and pulse energy shows trends of the\nlayered GO films from 2D to quasi bulk like behavior. The nonlinear parameter\nof GO coated SOI nanowires is increased 16 fold, with the nonlinear FOM\nincreasing over 20 times to a FOM greater than 5. We also observe an improved\nFWM efficiency in SiN waveguides integrated with 2D layered GO films. FWM\nmeasurements for samples with different numbers of GO layers and at different\npump powers are performed, achieving up to 7.3 dB conversion efficiency (CE)\nenhancement for a uniformly coated device with 1 layer of GO and 9.1 dB for a\npatterned device with 5 layers of GO. These results reveal the strong potential\nof GO films to improve the nonlinear optics of silicon, Hydex and SiN photonic\ndevices.\n

Optical bistability in a single-bus InGaAs micro-ring resonator array

Silicon-based ring resonators have proven to be essential components for building lab-on-chip systems, thanks to their capability to operate as label-free photonic sensors. Moreover, the possibility of using silicon nitride wafers offer low-loss waveguides across a wide range of wavelengths, from visible to near-infrared. In this work, we report on refractometric sensors based on silicon nitride ring resonators operating in the TE mode around 1310 nm wavelengths. Our preliminary results demonstrate that such a device could be used in the area of biosensing, therefore the aim is to test its sensitivity in terms of displacement of the resonant peak by passing from pure water to ethanol solutions. Subsequently, functionalization procedures will be explored in order to conduct binding experiments.

We demonstrate over 1 dB of photon number difference correlations from two-mode squeezed states having mean photon number above 5, generated with a silicon nitride ring resonator and measured using photon number-resolving transition edge sensors. © 2019 The Author(s)

A detailed analysis of fundamental tradeoffs between ring radius and coupling gap size is presented to draw realistic borders of the possible design space for microring resonators (MRRs). The coupling coefficient for the ring-waveguide structure is estimated based on an integration of the nonuniform gap between the ring and the waveguide. Combined with the supermode analysis of two coupled waveguides, this approach is further expanded into a closed-form equation that describes the coupling strength. This equation permits to evaluate how the distance separating a waveguide from a ring resonator, and the ring radius, affect coupling. The effect of ring radius on the bending loss of the ring is furthermore modeled based on the measurements for silicon MRRs with different radii. These compact models for coupling and loss are subsequently used to derive the main optical properties of MRRs, such as 3-dB optical bandwidth, extinction ratio of resonance, and insertion loss, hence identifying the design space. Our results indicate that the design space for add-drop filters in a wavelength division multiplexed link is currently limited to 5–10 $\mu$ m in radius and gap sizes ranging from 120 to 210 nm. The good agreement between the results from the proposed compact model for coupling and the numerical FDTD and experimental measurements indicate the application of our approach in realizing fast and efficient design space exploration of MRRs in silicon photonic interconnects.

Photonic ring resonators can be considered building blocks of new concept satellite payloads for implementing several functions, such as filtering and sensing. In particular, the use of a high Q-factor ring resonator as sensing element into a Resonant Micro Optic Gyroscope (RMOG), provides a remarkable improvement of the performance with respect to the competitive technologies. To qualify a ring resonator for Space applications, the radiation effects on it in the Space must be carefully evaluated. Here, we investigate the effects of gamma radiation on a high Q InGaAsP/InP ring resonator, for the first time, to our knowledge. The ring resonator under study has a footprint of about 530 mm2 and it is based on a InGaAsP/InP rib waveguide, with a width of 2 µm and a thickness of 0.3 µm, formed on a 0.7 µm thick slab layer on an InP substrate 625 µm thick. For a total dose of about 320 krad Co60 gamma irradiation, a mean variation of about 13% and 4% was measured for Q and extinction ratio (ER), respectively, with respect to the values before irradiation (Q = 1.36 × 106, ER = 6.24 dB). Furthermore, the resonance peak red-shifts with a linear behaviour was observed increasing the total dose of the absorbed radiation, with a maximum resonance detuning of about 810 pm. These non-significant effects of a quite high gamma radiation dose confirm the potential of high-Q InP-based ring resonators into Space systems or subsystems.

Low-loss optical resonators fabricated in chip-scale waveguides are of great interest to a wide range of technologies [1]-[4]. Notably, low-loss silicon nitride ring resonators have been demonstrated to function as optical gyroscopes [5]. Virtually all applications of optical resonators benefit from the minimization of propagation loss. Previous analyses of ultralow loss silicon nitride ring resonators have attributed a majority of propagation loss to absorption and sidewall scattering, but here we present new resonators which yield a record-low loss of 0.123 dB/m at an optical wavelength of 1563 nm, and near-infrared images of these resonators indicate that loss is heavily impacted by the presence of randomly positioned scattering elements incorporated into the structure during fabrication.

We analyze theoretically the nonlinear phenomenon of optical bistability inside a ring resonator formed with a silicon-waveguide nanowire and derive an exact parametric relation connecting the output intensity to the input intensity. Our input-output relation accounts for linear losses, the Kerr nonlinearity, two-photon absorption, free-carrier-induced absorption and dispersion, and thermo-optic effects within the resonator. Based on our study, we generalize the standard definition of effective length to allow for all possible losses within a silicon ring resonator. We also present a simplified version of the bistable phenomenon valid for resonators operating in a regime in which losses resulting from two-photon absorption are relatively small. Our analytical results provide clear insight into the physics behind optical bistability and may be useful for designing silicon-based optical memories.

We introduce and experimentally demonstrate a miniaturized integrated spectrometer operating over a broad bandwidth in the short-wavelength infrared (SWIR) spectrum that combines an add-drop ring resonator narrow band filter with a distributed Bragg reflector (DBR) based broadband filter realized in a silicon photonic platform. The contra-directional coupling DBR filter in this design consists of a pair of waveguide sidewall gratings that act as a broadband filter (i.e., 3.9 nm). The re-directed beam is then fed into the ring resonator which functions as a narrowband filter (i.e., 0.121 nm). In this scheme the free spectral range (FSR) limitation of the ring resonator is overcome by using the DBR as a filter to isolate a single ring resonance line. The overall design of the spectrometer is further simplified by simultaneously tuning both components through the thermo-optic effect. Moreover, several ring-grating spectrometer cells with different central wavelengths can be stacked in cascade in order to cover a broader spectrum bandwidth. This can be done by centering each unit cell on a different center wavelength such that the maximum range of one-unit cell corresponds to the minimum range of the next unit cell. This configuration enables high spectral resolution over a large spectral bandwidth and high extinction ratio (ER), making it suitable for a wide variety of applications.

We propose a simple iterative method for calculating the dynamic behavior of ring resonators with fast and slow (cumulative) optical nonlinearities when an optical pulse with an arbitrary-shaped envelope is incident into them. In the case of a slow nonlinearity, the nonlinear phase shift and nonlinear absorption are temporally-integrated over the incident pulse. In this paper, we consider two types of single-ring resonators made out of As2Se3 chalcogenide glass with high nonlinearity and investigate the dynamic properties (especially the effect of the cumulative nonlinearity on optical bistability) using known nonlinear material parameters. It is found that the cumulative nonlinearity suppresses overshoot and ringing after switching, decreases the width of the hysteresis loop between the input and output powers, and shifts its center corresponding to the operating point. The obtained results are useful in developing chalcogenide-based bistable optical devices and the proposed approach is applicable to modeling of a variety of nonlinear optical devices.

Chapter 3 - Fiber Interferometers

We demonstrate a hybrid device consisting of a thin film lithium niobate membrane transfer-printed onto a silicon nitride ring resonator. We measure quality factors in the 105 range at telecom wavelengths.

We experimentally demonstrate a novel RF interference mitigation technique using a network of low-loss silicon nitride ring resonators. The rings are used for complex (phase and amplitude) line-by-line shaping of higher-order sidebands from an electro-optic modulator to discriminate large and small RF signal input, thereby achieving strong (30 dB) mitigation of a large signal and virtually no mitigation for small signals.

We investigate frequency comb generation in silicon nitride ring resonators by using a pump subject to a weak amplitude modulation. We show that a partial locking is obtained when the external modulation frequency differs from the resonator free-spectral-range by up to hundreds of MHz.

We propose and theoretically study a tunable frequency matching method for four-wave-mixing Bragg-scattering frequency conversion in microring resonators. A tunable coupling between the clockwise and counterclockwise propagating modes in the resonators was designed to introduce adjustable mode splitting, thus compensating for the frequency mismatching under different wavelengths. Using a silicon nitride ring resonator as an example, we showed that the tuning bandwidth approaches 35 number of FSRs. Numerical simulations further revealed that the phase-matching strategy is valid under different wavelength combinations and is robust to variations in waveguide geometry and fabrication. These results suggest promising applications in high-efficiency frequency conversion, integrated nonlinear photonics, and quantum optics.