Paolo Pintus

Silicon ring isolators with bonded nonreciprocal magneto-optic garnets

A ring isolator is demonstrated for the first time by directly bonding a cerium-substituted yttrium iron garnet (Ce:YIG) onto a silicon ring resonator using oxygen plasma enhanced bonding. The silicon waveguide is 600 nm wide and 295 nm thick with 500-nm-thick Ce:YIG on the top to have reasonable nonreciprocal effect and low optical loss. With a radial magnetic field applied to the ring isolator, it exhibits 9-dB isolation at resonance in the 1550 nm wavelength regime.

Characterization of Insertion Loss and Back Reflection in Passive Hybrid Silicon Tapers

The optical properties of two hybrid silicon taper designs are investigated. These tapers convert the optical mode from a silicon waveguide to a hybrid silicon III/V waveguide. A passive chip was fabricated with an epitaxial layer similar to those used in hybrid silicon lasers. To separate optical scattering and mode mismatch from quantum-well absorption, the active layer in this paper was designed to be at 1410 nm, to allow measurements at 1550 nm. Using cutback structures, the taper loss and the taper reflection are quantified. Taper losses between 0.2 and 0.6 dB per taper and reflections below -41 dB are measured.

Integrated TE and TM optical circulators on ultra-low-loss silicon nitride platform

In this paper, we present two four-port optical circulators for TE and TM modes, respectively. Exploiting the recent technological development concerning Ce:YIG pulse laser deposition on silicon nitride platform, we design two integrated circulators, which can be used to implement several functions in integrated optics, such as de-interleavers, input/output amplifier isolators and output laser isolators. The proposed devices combine the benefit of low loss silicon nitride waveguides with the non-reciprocal properties of magneto-optical materials. The ring cross-section has been optimized in order to maximize the non-reciprocal phase shift and finally the scattering coefficients have been computed using the transfer matrix method. The material stability and refractive index regularity of silicon nitride, the small micro-ring footprint, and the high wavelength selectivity make these devices particularly attractive.

Analysis and Design of Microring-Based Switching Elements in a Silicon Photonic Integrated Transponder Aggregator

In this paper, we present and investigate a new architecture of a silicon photonic transponder aggregator as a new interconnect subsystem enabling the implementation of colorless, directionless, and contentionless ROADMs. Such subsystem is based on a microring resonator switching fabric integrated in a silicon photonics platform to achieve high functional integration together with reduction of cost, footprint, and power consumption. In the proposed device, microring resonators perform simultaneous add and drop of wavelength channels which suffer from two detrimental effects: residual dropped signal crosstalk and residual added signal crosstalk, respectively. Considering three microring-based switching elements, the transfer matrix method has been used to compute the add/drop transfer functions of the switches as a function of their geometrical parameters. The two crosstalk effects have been evaluated jointly with other important transmission parameters, such as bandwidth, insertion losses, side lobe suppression, adjacent channel rejection, extinction ratio, and group dispersion. In addition, device sensitivity with respect to the ring-waveguide coupling coefficients has been calculated. Finally, the performance of the different switches has been assessed to demonstrate that, by a proper design, the proposed transponder aggregator can support 100 Gb/s DP-QPSK modulated signal transmission.

BER evaluation of a low-crosstalk silicon integrated multi-microring network-on-chip

The operation of an integrated silicon-photonics multi-microring network-on-chip (NoC) is experimentally demonstrated in terms of transmission spectra and bit error rates at 10 Gb/s. The integrated NoC consists of 8 thermally tuned microrings coupled to a central ring. The switching functionalities are tested with concurrent transmissions at both the same and different wavelengths. Experimental results validate the analytical model based on the transfer matrix method. BER measurements show performance up to 10(-9) at 10 Gb/s with limited crosstalk and penalty (below 0.5 dB) induced by an interfering transmission.

Silicon-based all-optical multi microring network-on-chip

An optical multi microring network-on-chip (MMR NoC) is proposed and evaluated through numerical simulations. The network architecture consists of a central resonating microring with local microrings connected to the input/output ports. A mathematical model based on the transfer matrix method is used to assess the MMR NoC performance and to analyze the fabrication tolerances. Results show that the proposed architecture exhibits a limited coherent crosstalk with a bandwidth suitable for 10  Gb/s signals, and it is robust to coupling ratio variations and ring radii fabrication inaccuracies.

Accurate vectorial finite element mode solver for magneto-optic and anisotropic waveguides.

In this work, a dielectric waveguide mode solver is presented considering a general nonreciprocal permittivity tensor. The proposed method allows us to investigate important cases of practical interest in the field of integrated optics, such as magneto-optical isolators and anisotropic waveguides. Unlike the earlier developed mode solver, our approach allows for the precise computation of both forward and backward propagating modes in the nonreciprocal case, ensuring high accuracy and computational efficiency. As a result, the nonreciprocal loss/phase shift can be directly computed, avoiding the use of the perturbation method. To compute the electromagnetic modes, the Rayleigh-Ritz functional is derived for the non-self adjoint case, it is discretized using the node-based finite element method and the penalty function is added to remove the spurious solutions. The resulting quadratic eigenvalue problem is linearized and solved in terms of the propagation constant for a given frequency (i.e., γ-formulation). The main benefits of this formulation are that it avoids the time-consuming iterations and preserves the matrix sparsity. Finally, the method is used to study two examples of integrated optical isolators based on nonreciprocal phase shift and nonreciprocal loss effect, respectively. The developed method is then compared with the perturbation approach and its simplified formulation based on semivectorial approximation.

Design of Magneto-Optical Ring Isolator on SOI Based on the Finite-Element Method

In this letter, we present the design of an integrated optical isolator realized by bonding a silicon micro-ring resonator with a Ce:YIG garnet. The nonreciprocal phase shift effect induced by applying a radial magnetic field has been studied using the finite-element method; numerical results clearly point out how to optimize the thickness of the silicon ring and Ce:YIG garnet in order to maximize the nonreciprocal effect between the forward and backward TM modes.

Electrically Driven and Thermally Tunable Integrated Optical Isolators for Silicon Photonics

Optical isolators are required to block undesired reflections in many photonic integrated circuits (PICs), but the performance of on-chip isolators using the magneto-optic effect has been limited due to high loss of such materials. Moreover, they require precise positioning of a permanent magnet close to the chip, increasing footprint and impeding packaging. In this paper, we propose an optical isolator on the silicon-on-insulator platform with record performance and without the use of any external permanent magnets. A metallic microstrip above the bonded silicon microring (MR) is used to generate the magnetic field required for the nonreciprocal behavior. Simultaneously, the microstrip can be used to provide 0.6 nm of thermal tuning while preserving over 20 dB of isolation. We measure 32 dB of isolation near 1555 nm with only 2.3 dB excess loss in a 35 μm radius MR. The tunability, compactness, and lack of permanent magnets suggest this device is a major step towards integration in PICs.

Modeling of strain-induced Pockels effect in Silicon.

We propose a theoretical model to describe the strain-induced linear electro-optic (Pockels) effect in centro-symmetric crystals. The general formulation is presented and the specific case of the strained silicon is investigated in detail because of its attractive properties for integrated optics. The outcome of this analysis is a linear relation between the second order susceptibility tensor and the strain gradient tensor, depending generically on fifteen coefficients. The proposed model greatly simplifies the description of the electro-optic effect in strained silicon waveguides, providing a powerful and effective tool for design and optimization of optical devices.