Academic literature on the topic 'Multi-Mode interference (MMI) coupler'

In this study, we demonstrate the influence of operating temperature variation and stress-induced effects on a silicon-on-insulator (SOI)-based multi-mode interference coupler (MMI). Here, SiGe is introduced as the cladding layer to analyze its effect on the optical performance of the MMI coupler. SiGe cladding thickness is varied from 5 nm to 40 nm. Characterization of the MMI coupler for ridge waveguides with both rectangular and trapezoidal sidewall slope angle cross-sections is reviewed in terms of power splitting ratio and birefringence. Stress-induced birefringence as a function of operating temperature and cladding thickness for fundamental mode have been calculated. A trapezoidal waveguide with 40 nm of cladding thickness induces more stress and, therefore, affects birefringence more than a rectangular waveguide of any thickness. Simulation results using the finite element method (FEM) confirmed that operating temperature variation, upper cladding thickness, and its stress effect are significant parameters that drastically modify the performance of an MMI coupler.

A semi-analytical method for the S-parameter calculations of an N×M multimode interference coupler (MMI coupler) is presented. The proposed semi-analytical method is based on the mode decomposition and utilizes an effective index method to approximate the channel waveguide using an equivalent slab waveguide whose modes are described by exact analytic expressions. In comparison to the commonly used beam propagation method (BPM) and finite difference time domain method, which require significant time and computational resources, the proposed method accelerates the design process of photonic integrated circuits and basic building blocks such as an MMI coupler. The simulation results obtained using the developed method and the BPM were compared and showed very similar outcomes for different topologies of the MMI coupler. The key advantage of the proposed semi-analytical method over other analytical models is its ability to accurately simulate MMI couplers with an arbitrary position and number of input and output waveguides. In addition, this method can be extended using the theory of local coupled modes by taking into account the reflections from the end face of the MMI box.

The optimal design of a 4x4 multimode interference (MMI) coupler as an optical 90° hybrid based on a weakly-guided optical waveguide was considered. Seven geometrical parameters of a 4x4 MMI coupler were optimized by a real-coded micro-genetic algorithm, and parallelized using a message-passing interface. The beam-propagation method was used to evaluate the fitness of the MMI coupler in the optimization process. The optimized 4x4 MMI coupler showed a common-mode rejection ratio greater than 28.9 dBe and a phase error less than 2.52° across a wavelength range of 1520 to 1580 nm, which satisfied typical system requirements. The optimization process was executed on a Beowulf-style cluster comprising five identical PCs, and its parallel efficiency was 0.78.

We present a comparison of directional couplers and multi-mode interferometers based on the unique properties of high-index contrast ridge waveguides. The two devices are intimately related as the MMI is structurally derived from the DC. For the first time, the continuous evolution from the two-mode coupling characteristic of DC to the multi-mode mixing and interference characteristic of MMI is shown. We show that practical directional couplers with reasonable gap size can also be quite compact and have the same coupling length for both TE and TM polarizations. Consequently, the DC can be just as polarization insensitive as the MMI. These features, however, require careful design control involving a large set of design parameters. On top of that, we also show a novel design of polarization splitter based on both DC and MMI. By comparison, the MMI design is more robust and involves fewer design variables.

We propose a method for generating the electromagnetically induced transparency (EIT) like-transmission by using microring resonator based on cascaded 3 × 3 multimode interference (MMI) structures. Based on the Fano resonance unit created from a 3 × 3 MMI coupler with a feedback waveguide, two schemes of two coupled Fano resonator unit (FRU) are investigated to generate the EIT like transmission. The theoretical and numerical analysis based on the coupled mode theory and transfer matrix is used for the designs. Our proposed structure has advantages of compactness and ease of fabrication. We use silicon waveguide for the design of the whole device so it is compatible with the existing Complementary Metal-Oxide-Semiconductor (CMOS) circuitry foundry. The fabrication tolerance and design parameters are also investigated in this study.

Rate equations are derived for the coupled-cavity laser with a multimode-interference coupler. A strategy and scheme is indicated for iterative self-consistent numerical solution of the steady-state equations. The presence of the linewidth-enhancement parameter is explicitly taken into account. Locking in stable single-mode anti-phase operation is numerically demonstrated and locking ranges are given. Numerical results are given for the output power and the operation frequency as functions of the pump strengths of the individual lasers. The shapes of output-intensity curves agree well with measured curves.

Rate equations are derived for the coupled-cavity laser with a multimode-interference coupler. A strategy and scheme is indicated for iterative self-consistent numerical solution of the steady-state equations. The presence of the linewidth-enhancement parameter is explicitly taken into account. Locking in stable single-mode anti-phase operation is numerically demonstrated and locking ranges are given. Numerical results are given for the output power and the operation frequency as functions of the pump strengths of the individual lasers. The shapes of output-intensity curves agree well with measured curves.

A multimode waveguide interference (MMI) coupler is combined with a critical linear tapered waveguide on a silicon-on-insulator (SOI) chip. When the TE0 mode is a critical adiabatic mode conversion from a single-mode waveguide to an extreme linear tapered waveguide combined with an MMI, this linear tapered waveguide is achieved to the maximum divergence angle (i.e., the shortest length). The maximum divergence angle is expressed by θ ≤ 2 tan−1[(0.35Wmmi − Ws)/(0.172Lmmi)] under a 1 × 1 MMI combined with this critical linear tapered waveguide. The expression formula is demonstrated by three different widths of a 1 × 1 MMI of 4 μm/8 μm/12 μm combined with the critical linear tapered waveguide. So, the maximum divergence angle is obtained at θ = 16°/14°/8°, with respect to this linear tapered waveguide loss of 0.022 dB/0.172 dB/0.158 dB, and this linear tapper length is reduced by 93.7%/92.9%/87.5% than the divergence angle θ = 1°. The output power of a 1 × 1 MMI combined with a critical linear tapered waveguide is enhanced at least 1.5 times under 0.95 above condition.

The study of designing a compact transverse electric (TE)/transverse magnetic (TM) polarization multimode interference (MMI) combiner based on silicon slot-waveguide technology is proposed for solving the high demands for high-speed ability alongside more energy power and minimizing the environmental impact of power consumption, achieving a balance between high-speed performance and energy efficiency has become an important consideration in an optical communication system. The MMI coupler has a significant difference in light coupling (beat-length) for TM and TE at 1550 nm wavelength. By controlling the light propagation mechanism inside the MMI coupler, a lower order of mode can be obtained which can lead to a shorter device. The polarization combiner was solved using the full-vectorial beam propagation method (FV-BPM), and the main geometrical parameters were analyzed using Matlab codes. Results show that after a short light propagation of 16.15 μm, the device can function as TM or TE combiner polarization with an excellent extinction ratio of 10.94 dB for TE mode and 13.08 dB for TM mode with low insertion losses of 0.76 dB (TE) and 0.56 dB (TM) and the combiner function well over the C-band spectrum. The polarization combiner also has a robust MMI coupler length tolerance of 400 nm. These attributes make it a good candidate for using this proposed device in photonic integrated circuits for improving power ability at the transmitter system.

The key components of a polarization-independent electro-optic (EO) interferometer, including the beam splitter, mode converter, and directional coupler, are designed based on a lithium niobate (LN) platform on an integrated insulator to ensure high extinction ratios. By elaborately designing the geometric structure of the multimode interference (MMI) coupler, beam splitting of equal proportions and directional coupling of higher-order modes are realized. The most prominent characteristic of the proposed interferometer is polarization insensitivity, which is realized by converting TM polarization into TE polarization using a mode converter, providing conditions for the study of light with different polarizations. At 1550 nm, the visibility of the interferometer is 97.59% and 98.16% for TE and TM, respectively, demonstrating the high performance of the proposed LN polarization-independent interferometer.

La technologie photonique sur silicium s'appuie sur les procédés matures de fabrication de l'industrie du semi-conducteur pour produire des composants opto-électroniques à échelle industrielle. Les métamatériaux à base de réseaux sub-longueur d'onde permettent de contrôler le confinement du mode et la dispersion, et ont ainsi été implémentés pour démontrer des performances de pointe de composants photoniques intégrés. Les effets de diffraction et de réflexions sont supprimés dans les matériaux sub-longueur d'onde. Leurs dimensions sont petites et sont environ de 100 nm. Jusqu'à présent, la majorité des composants sub-longueur d'onde ont été fabriqués par lithographie électronique. Or, cette technique n'est pas compatible avec une production à large échelle. Aujourd'hui, la lithographie à immersion se déploie dans les fonderies photoniques sur silicium. Elle permet de définir des dimensions aussi petites que 70 nm, avec un modèle de correction d'effets optiques de proximité. Le but principal de cette thèse est d'étudier la faisabilité de l'utilisation de la lithographie à immersion avec la correction d'effets optiques de proximité pour la fabrication de composants photoniques sub-longueur d'onde de pointe. Ces composants ont été développés sur des plaques de 300 mm de diamètre au CEA-Leti. Trois composants ont été étudiés, chacun avec une spécificité technologique : i) un diviseur de puissance avec une seule étape de gravure complète, ii) un réseau de couplage puce-fibre alternant des gravures partielles et complètes, et iii) une matrice d'antennes optiques, couvrant une large surface, avec une étape de gravure partielle. Le diviseur de puissance est constitué d'un coupleur par interférométrie multi-mode (MMI) avec des réseaux sub longueur d'onde pour contrôler la dispersion des modes optiques et ainsi pour obtenirune très large bande passante spectrale, qui a été mesurée expérimentalement à 350 nm, et qui en bon accord avec les simulations. La bande passante d'un MMI conventionnel sans structures sub longueur d'onde n'est que de 100 nm environ. Le réseau de couplage puce-fibre s'appuie sur une géométrie en forme de « L », avec des structures sub-longueur d'onde gravés partiellement et complètement, pour augmenter l'efficacité de couplage. Celle-ci a été mesurée à -1.70 dB (68 %) à une longueur d'onde de 1550 nm et représente la meilleure performance pour une telle structure complexe, utilisant une technologie autre que la lithographie électronique. Néanmoins, la valeur mesurée est inférieure à la valeur simulée de 0.80 dB (83 %). Une des raisons principales de cette performance limitée est la sensibilité de cette structure aux erreurs d'alignement entre les deux étapes de gravure pendant la fabrication. L'antenne optique est constituée de structures sub longueur d'onde partiellement gravées pour obtenir une grande surface d'émission de 48 µm×48 µm, réduisant ainsi la divergence du faisceau. Cette antenne a été implémentée comme antenne unitaire dans une matrice 4×4 à réseau phasé avec un pas de 90 µm×90 µm. A une longueur d'onde de 1550 nm, le faisceau émis par l'antenne unitaire a une divergence à mi-hauteur mesurée de 1.40° et celui émis par la matrice d'antennes a une divergence à mi hauteur de 0.25°. Ces valeurs sont en accord avec les valeurs simulées. Ces résultats servent comme preuve de concept de l'implémentation d'une telle antenne dans une matrice à réseau phasé. En résumé, les résultats de cette thèse illustrent le grand potentiel de la lithographie à immersion avec la correction d'effets optiques de proximité pour la fabrication de composants photoniques sub- longueur d'onde, ouvrant ainsi la voie pour la commercialisation de ces derniers
Silicon photonics technology leverages the mature fabrication processes of the semi-conductor industry for the large volume production of opto-electronic devices. Subwavelength grating (SWG) metamaterials enable advanced engineering of mode confinement and dispersion, that have been used to demonstrate state-of-the-art performance of integrated photonic devices. SWGs generally require minimum feature sizes as small as a 100 nm to suppress reflection and diffraction effects. Hitherto, most reported SWG-based devices have been fabricated using electron-beam lithography. However, this technique is not compatible with large volume fabrication, hampering the commercial adoption of SWG-based photonic devices. Currently, immersion lithography is being deployed in silicon photonic foundries, enabling the patterning of features of 70 nm, when used in conjunction with optical proximity correction (OPC) models. The main goal of this PhD is to study the feasibility of immersion lithography and OPC for the realization of high-performance SWG devices. The SWG devices developed here have been fabricated using the OPC models and 300 mm SOI wafer technology at CEA-Leti. Three devices have been considered as case studies, each with a specific technological challenge: i) a power splitter requiring a single full etch step, ii) a fiber-chip grating coupler interleaving full and shallow etch steps, and iii) an optical antenna array covering a large surface area with a shallow etch step. The power splitter is implemented using a SWG-engineered multi-mode interferometer (MMI) coupler. The SWG is used to control the dispersion of the optical modes to achieve an ultrawide operating spectral bandwidth. This device experimentally showed state-of-the-art bandwidth of 350 nm, in good agreement with simulations. Note that the bandwidth of a conventional MMI without SWG is around 100 nm. The fiber-chip coupler relies on an L-shaped geometry with SWG in full and shallow etch steps to maximize the field radiated towards the fiber. The measured coupling efficiency, of - 1.70 dB (68 %) at a wavelength of 1550 nm, is the highest value reported for an L-shaped coupler fabricated without electron-beam lithography. Still, this value differs from the calculated efficiency of 0.80 dB (83 %), and compares to experimental values achieved with fiber-chip grating couplers without SWG (~ -1.50 dB). One of the main reasons for the limited experimental performance is the strong sensitivity of the structure to errors in the alignment between the full and shallow etch steps. The optical antenna uses shallowly etched SWG teeth to minimize the grating strength, allowing the implementation of a large area emission aperture, of 48 × 48 µm, which is required to minimize the beam divergence. A two-dimensional (2D) optical phased array (OPA) with an antenna pitch of 90 µm × 90 µm, comprising 16 antennas was designed and fabricated. The SWG-based unitary antenna has a measured full width at half maximum divergence of 1.40° at a wavelength of 1550 nm, while the beam emitted from the phased array has a divergence of 0.25°, both in very good agreement with expected values. These results serve as a good proof-of-concept demonstration of this novel antenna architecture. In summary, the results shown in this PhD illustrate the great potential of immersion lithography and OPC for harnessing SWG-engineering, paving the way for their commercial adoption. Devices with full or shallow etch steps exhibited excellent performance close to that predicted by simulations. The fiber-chip grating couplers deviated from expected results, probably due to the tight fabrication tolerances associated with the combination of full and shallow etch steps

碩士
國立中山大學
光電工程學系研究所
106
The rapid advancement in information science and technology, such as big data and deep learning opens even higher demand for data processing and transmission, which is beyond the capability of currently mature technology based on electronic circuits. Although integrated optics and silicon photonics serves one of the solution to such a demand, the electronic response in data modulation and processing still set the limit to the speed. Femtosecond response time are easily achieved in nonlinear optical materials, for example the Kerr effect can be used for all optical processing in ultrafast regime. Even though silicon itself is one of the good nonlinear material, its two photon absorption (TPA) become an obstacle for implementing efficient nonlinear optical processing device. Ta2O5, on the other hand, is a wide band gap material providing high transmission and low loss in communication bandwidth. In the meanwhile, it has large Kerr nonlinear coefficient that is comparable to silicon but does not suffer from TPA. In this dissertation, we implement and demonstrate non-linear all-optical optical switcher using nonlinear multiple interference effect in Ta2O5 waveguide of sub-wavelength thickness. We design the device based on multiple-mode nonlinear Schrödinger equations and split-step Fourier method for simulation. We designed and fabricated multi-mode waveguides of width 8um, height 0.1um, length 0.4557cm(31L), and length 0.7497cm(51L) and characterize them by a femtosecond oscillator emitting 100fs mode locked pulses at 80MHz repetition frequency and the wavelength of 1064 nm. It is observed that the transmission through the nonlinear MMI waveguide drops as the peak intensity of the incident laser increases. The transition in the power depending transmission is consistent with the simulation when the modal losses are justified unequally.

Large on-chip bandwidths required for high performance electronic chips will render optical components essential parts of future on-chip interconnects. Silicon photonics enables highly integrated photonic integrated circuit (PIC) using CMOS compatible process. In order to maximize the bandwidth density and design flexibility of PICs, vertical integration of electronic layers and photonics layers is strongly preferred. Comparing deposited silicon, single crystalline silicon offers low material absorption loss and high carrier mobility, which are ideal for multi-layer silicon PIC. Three different methods to build multi-layer silicon PICs based on single crystalline silicon are demonstrated in this dissertation, including double-bonded silicon-on-insulator (SOI) wafers, transfer printed silicon nanomembranes, and adhesively bonded silicon nanomembranes. 1-to-12 waveguide fanouts using multimode interference (MMI) couplers were designed, fabricated and characterized on both double-bonded SOI and transfer printed silicon nanomembrane, and the results show comparable performance to similar devices fabricated on SOI. However, both of these two methods have their limitations in optical interconnects applications. Large and defect-free silicon nanomembrane fabricated using adhesive bonding is identified as a promising solution to build multi-layer silicon PICs. A double-layer structure constituted of vertically integrated silicon nanomembranes was demonstrated. Subwavelength length based fiber-to-chip grating couplers were used to couple light into this new platform. Three basic building blocks of silicon photonics were designed, fabricated and characterized, including 1) inter-layer grating coupler based on subwavelength nanostructure, which has efficiency of 6.0 dB and 3 dB bandwidth of 41 nm, for light coupling between layers, 2) 1-to-32 H-tree optical distribution, which has excess loss of 2.2 dB, output uniformity of 0.72 dB and 3 dB bandwidth of 880 GHz, 3) waveguide crossing utilizing index-engineered MMI coupler, which has crossing loss of 0.019 dB, cross talk lower than -40 dB and wide transmission spectrum covering C-band and L-band. The demonstrated integration method and silicon photonic devices can be integrated into the CMOS back-end process for clock distribution and global signaling.
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A novel high power laser configuration based on phase-locked array resonators (fig. 1) is presented in this paper [1.2]. This configuration provides an efficient way of extracting high optical power from the lower power elements. Using quantum-well intermixing (QWI), we have monolithically integrated the four amplifiers/lasers with the passive multi-mode interference (MMI) coupler and output waveguide.

We have reported a low loss Si wire arrayed waveguide grating (AWG) using parabolic rib waveguide taper [1] to realize 100GHz-class channel spacing. We proposed recently a more simple structure using multi-mode interference coupler with minimum terrace area [2] for loss reduction. In this report we demonstrate an experimental results showing lower loss obtained by foundry dependent improved fabrication process for the latter device.

We present an optical 90° hybrid based on cascaded deformed multi-mode interference couplers. The fabricated hybrid demonstrated the common-mode rejection-ratio >25dB, excess-loss ~1dB, and phase deviation less than ±5° within the wavelength range over 45nm.