Compact Multimode Interference Research Articles

High-performance electro-optic switches with compact multimode interference and Bezier S-bend waveguides

High-speed and energy-efficient electro-optic (EO) switches are crucial for next-generation data communication systems. This paper presents a novel EO switch utilizing a multimode interference (MMI) coupler-based approach on a lithium-niobate (LiNbO3) platform. The switch is designed with Bezier S-bend waveguides, leading to a significant reduction in bending loss to 0.18 dB. This Bezier-bend EO switch shows an excess loss of 0.33 dB and crosstalk of −20.44 dB for the bar port switching, while the cross port switching exhibits an excess loss of 0.64 dB and crosstalk of −13.66 dB. Applying a 3.27 V voltage achieves a balanced splitting ratio of 29:29:29 between the three outputs. The length of this EO switch is 4.09 mm, which requires 4.1 V to create a phase shift, showing a voltage efficiency of 1.68 V.cm. This paper presents a promising pathway for a novel EO switch design and introduces improved signal management for next-generation computing systems.

Near-infrared Y-branch polymer splitters realized with compact MMI structures for efficient power splitting

Optical splitters are promising photonic devices for next-generation photonic integrated circuits, which enable signal distribution and routing between the different components, facilitating complex optical functionalities on a single chip. This research introduces what we believe is a novel numerical technique for enhancing optical network efficiency by incorporating a taper-based step-index (SI) Y-branch multimode interference (MMI) splitter with organic-inorganic hybrid polymer materials. The proposed device comprises a core width of 5 µm for the input and output waveguides to satisfy the single-mode conditions. We designed and optimized the MMI splitter using the beam propagation method (BPM). The splitter demonstrates the power splitting property with an efficiency of 86%. The excess losses for the MMI splitter are 0.52 dB and 0.50 dB for TE and TM modes, respectively, at 1.55 µm. The polarization dependence loss (PDL) and propagation loss (PL) are 0.015 dB and 0.00019 dB/µm, respectively.

Proposal of compact multimode interference optical modulator with high extinction ratio

We proposed a compact 1 × 1 multimode interference (MMI) optical modulator with a high extinction ratio. A triply coupled quantum well structure is designed for the core layer for the enhancement of the quantum-confined Stark effect, and the MMI waveguide with isolation trenches is symmetrically designed. The length of the MMI modulator is reduced by 30%, and the wavelength dependence was reduced compared with that observed in the previously reported MMI modulator. Owing to both the electroabsorption effect and the electric-field induced refractive index change in a multiple-quantum-well core layer, the extinction ratio is expected to be approximately 45 dB at an operating voltage of 3.3 V.

Design of compact polarization-insensitive multimode interference triplexer

A compact polarization-insensitive multimode interference (MMI) triplexer is proposed. The triplexer can process signals among three wavelengths of 1310, 1490 and 1550 nm in the fibre-to-the-home (FTTH) systems. The polarization insensitivity is obtained by adjusting the refractive index of SiN x in the Si/SiN x /Si sandwiched structure. A triangular lattice array air-hole photonic crystal embedded in the MMI waveguide can transmit and reflect downstream and upstream signals, respectively. Furthermore, a cascaded MMI structure is used to separate two downstream signals. The performance parameters of the triplexer are analysed by three-dimensional finite-difference time-domain method. The results show that the size of the triplexer is only 5.5 × 234.5 µm2, the insertion loss and crosstalk are as low as 0.45 dB and −27.2 dB respectively. Besides, the triplexer has a large 3 dB bandwidth up to 105 nm, and it has potential application value in the FTTH systems.

Compact On-Chip Optical Components Based on Multimode Interference Design Using High-Contrast Grating Hollow-Core Waveguides

Low-loss high-contrast grating (HCG) hollow-core waveguide (HCW) is shown to be a promising candidate for building a variety of compact multimode interference (MMI) based optical components. First, we show that HCG-HCWs demonstrate self-imaging property essential for designing MMI components. Furthermore, strong confinement of light in the waveguide core and low-index guiding leads to compact optical components. Simulation results show extremely compact 2 × 2 coupler with a length of 26 μm (16 λ), footprint (length × width) of 85 μm2 (35 λ2) and a 3-dB splitter with a length of 3.6 μm (2.4 λ), and a footprint of 12 μm 2 (5 λ2). Simulation results agree well with the predictions of MMI theory and the design can be easily extended to realize 1 × N splitters and N × N couplers. As a proof of concept, a 1 × 4 splitter with equal power splitting ratio and low insertion loss is designed. In addition, an ultracompact 2 × 2 optical switch based on HCG-HCW is proposed and simulated. The two waveguide channels are isolated by a single layer of HCG. By a small change of the refractive index of this HCG, light can switch between the channels. By proper optimization of HCG parameters, only higher order modes are excited in the MMI region resulting in an ultracompact optical switch with a switching length of 60 μm, ∼25× smaller than conventional 2 × 2 MMI coupler.

Cylindrical Talbot effect for ultra-compact multimode interference couplers.

Tapered multimode interference (MMI) couplers have been broadly studied for applications in integrated photonic circuits due to their footprints being smaller than conventional MMIs. The operation of an MMI is fundamentally related to the Talbot effect. In this Letter, the Talbot effect is analyzed in cylindrical coordinates, rather than conventional Cartesian coordinates. A cylindrical geometry is found to result in a significant reduction of Talbot distance and, hence, more compact MMIs. Simulation results are compared to theoretical predictions to confirm the accuracy of the method.

CMOS-compatible and fabrication-tolerant MMI-based polarization beam splitter

A compact multimode interference (MMI) coupler based polarization beam splitter (PBS) is proposed and realized, which is fabrication-tolerant and fully compatible with standard silicon photonic foundry processes. The PBS on the 220nm silicon strip waveguide is as small as 4.2μm×132.64μm. Besides, the simulated fabrication tolerances of the width, the length and the etching depth are more than 100nm (80nm), 5μm (5.65μm) and 60nm (120nm) for TE (TM) mode, respectively. In addition, the experimental results demonstrate that the bandwidth is about 26nm with the extinction ratio better than 15dB.

A Library of Ultra-Compact Multimode Interference Optical Couplers on SOI

A library of compact multimode interference (MMI) couplers with MMI region widths ranging from 1.1 μm to 3.2 μm is presented for 220-nm silicon-on-insulator-based photonic circuits. By optimizing the MMI geometrical parameters and considering the Goos-Häanchen shift, insertion losses of lower than 0.1 dB with moderate experimental errors are achieved for 3-dB MMI power splitters at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> band, with the power imbalance around 0.2 dB.

Proposal of Compact Tunable 1×2 Multimode Interference Splitter Based on Multiple Quantum Well

We propose a compact tunable 1×2 multimode interference (MMI) splitter based on an InGaAs/InAlAs multiple quantum well for an active trimmer for Mach–Zehnder modulator with a very high extinction ratio. The light power splitting characteristics of the splitter are theoretically investigated by a beam propagation method and a three dimensional finite-difference time-domain method. It is assumed that the core layer is composed of a multiple five-layer asymmetric coupled quantum well (FACQW) that is expected to exhibit a large electrorefractive index change owing to its unique quantum confined Stark effect. By applying reverse voltages and changing slightly the refractive indices of the localized regions surrounded by trenches for electrical isolation, the power splitting ratio can be tuned over a wide range. The length and width of the proposed MMI with four refractive index modulation regions are 192 and 6 µm, respectively. The length can be reduced to 115 µm if the number of index modulation regions is reduced to two, though the tuning range of the splitting ratio is narrowed. The simulation results show that the proposed compact MMI splitter has a low insertion loss and a low power consumption, and is promising for Mach–Zehnder modulators with a very high extinction ratio.

Tooth-shaped grating-assisted structure for compact multimode interference coupler

A compact tooth-shaped grating-assisted structure has been proposed for a multimode interference (MMI) coupler and studied theoretically with silica waveguides with silicon oxynitride cores by using a mathematical model based on the sinusoidal mode simple effective index method. The dependence of the access waveguide length and beat length of the grating-assisted MMI (GA-MMI) structure on access waveguide separation (h) is shown. The total length of the GA-MMI coupler is less than that of a grating-assisted two-mode interference (GA-TMI) coupler (h=0) for an access waveguide S bending loss of 0.2 dB. It is also seen that the effect of fabrication tolerance on power imbalance of a GA-MMI coupler is almost close to that of a GA-TMI coupler.

All-Optical Discrete Fourier Transform Based On Multimode Interference Couplers

In this letter, an all-optical discrete Fourier transform (DFT) device based on multimode interference (MMI) couplers is proposed. The DFT device is composed of a discrete sine transform device and a discrete cosine transform device, both of which are based on MMI couplers. The main advantage of this DFT device is that it is composed of two compact MMI couplers, which greatly reduces the fabrication effort on planar lightwave circuits.

Compact High-Performance Multimode Interference Couplers in Silicon-on-Insulator

We demonstrate compact multimode interference (MMI) couplers on silicon-on-insulator that exhibit both high performance and relaxed tolerances. The devices are fabricated with a single shallow etch step on 1.5-mum-thick silicon, and have a footprint of 12.8 mum times 456 mum, including 100-mum-long tapers at input and output. In a Mach-Zehnder configuration, we achieve an extinction ratio larger than 22 dB for transverse-electric polarization and larger than 20 dB for transverse-magnetic polarization. This extinction ratio is maintained in the full 1520- to 1580-nm band and for MMI width variations in a range of 0.45 mu m. We also show appreciable tolerance differences between paired and general excitation MMIs.

Design and fabrication of a compact multimode interference splitter with silicon photonic nanowires

A compact multimode interference (MMI) splitter with silicon photonic nanowires on silicon-on-insulator (SOI) substrate is designed and fabricated. The footprint of the MMI section is only approximately 3×10 (\mu m). The simulation results show that the device may have a low excess loss of 0.04 dB. The transmission loss of the silicon photonics wire is measured to be 0.73±0.3 dB/mm. The compact size and low transmission loss allow the device to be used in ultra-compact photonic integrated circuits. The device exhibits a good light splitting function. In a spectral range of 1549-1560 nm, the excess loss is 1.5 dB and the average imbalance between the two channels is 0.51 dB.

Compact multimode interference couplers with arbitrary power splitting ratio

We show that it is possible to obtain 2 x 2 waveguide couplers with arbitrary power splitting ratios by interconnecting a pair of unequal width waveguides as the phase-tuning section into the middle of two short MMI sections. These couplers have simple geometry and low loss. They offer valuable new possibilities for designing waveguide-based photonic integrated circuits.

A Tapered Structure for Compact Multimode Interference Coupler

A new tapered structure based on general interference has been proposed and realized by using silica waveguides with a silicon oxinitride (SiON) core for the reduction of coupling length of the 2times2 multimode interference (MMI) coupler. The coupling characteristics of the proposed structure are compared with those of conventional MMI structures using a simple mathematical model based on sinusoidal modes. The beat length for the proposed tapered MMI coupler is reduced by 45% of that of a conventional MMI coupler. The effect of power imbalance on fabrication tolerance of the proposed tapered MMI coupler are also studied and compared with other tapered MMI couplers.

Compact multimode interference coupler with tapered waveguide geometry

In this paper, a novel MMI coupler, based on general interference, with tapered waveguide geometry has been proposed for reduction of coupling length. The coupling characteristics and power imbalance of the proposed structure are compared with conventional MMI structures by using a mathematical model based on sinusoidal modes. It is seen that the beat length for tapered MMI coupler with angle of taper ∼1.05° is reduced by ∼24% of that of conventional MMI coupler and the coupling characteristics obtained with the mathematical model, match well with those obtained by more sophisticated BPM computer aided design software. The power imbalance for tapered 3 dB MMI coupler is more sensitive to the wavelength than that for conventional 3 dB MMI coupler and variation of power imbalance with fabrication tolerance for both the MMI coupler is almost same.

Design of a compact multimode interference coupler based on deeply-etched SiO2 ridge waveguides

A compact multimode interference (MMI) coupler is realized by using deeply-etched SiO2 ridge waveguides, which have a strong confinement in the lateral direction and improve the quality of self-images in the MMI section. A 1×4 MMI coupler is designed as a numerical example and demonstrates good performances such as a low insertion (0.0275dB) and small non-uniformity (0.0507dB). The total device size of the present MMI coupler is reduced to about 1/16 of that based on SiO2 buried waveguides.

Improved QS-MMI' 1.31/1.55&#x03BC;m 파장분리기의 최적화 설계 및 제작

다중모드간섭 기반의 <TEX>$1.31/1.55{\mu}m$</TEX> 파장분리기의 크기 및 성능개선을 위해 'Improved Quasi-State' 불완전 단일상 개념을 설계에 도입하였다. 코어와 클래딩의 굴절률 차이가 작은 경우의 모드간 위상오차를 역이용 하도록 설계하여 'Quasi-State'의 출력 파워와 소멸비를 월등히 개선하였다. 다중모드간섭기의 폭이 <TEX>$14.4{\mu}m$</TEX>, 입력도파로의 수평 이동이 <TEX>$5.3{\mu}m$</TEX>가 되도록 설계한 구조를 유효굴절률법과 MPA를 사용하여 분석한 결과 최대 소멸비는 양 파장대역 모두 -25dB 이하로 나타났다. 설계된 파장분리기는 일반적인 다중 모드간섭기의 길이의 1/5정도에 불과한 <TEX>$2620{\mu}m$</TEX>의 간섭길이를 가진다. 소프트 리소그래피 공정을 통해 설계된 파장분리기를 제작하였으며, <TEX>$1.31{\mu}m$</TEX>와 <TEX>$1.5{\mu}m$</TEX>의 성공적인 파장분리를 확인하였다. We designed and fabricated a compact multi-mode interference (MMI) wavelength demultiplexer using the concept of 'Improved Quasi-State' modes. The output power and extinction ratio were improved by utilizing modal phase error which is specially occurred in low-index contrast. For a designed demultiplexer, the mode propagation analysis with effective index approximation shows significant improvement of extinction ratio to -25 dB for both <TEX>$1.31{\mu}m\;and\;1.51{\mu}m$</TEX> wavelength region and the split-length was reduced about 1/5 of other MMI devices. The fabricated device shows successful characteristics for both 1.31 and <TEX>$1.55{\mu}m$</TEX> wavelengths. These results demonstrate the potential of low-index materials system and the embossing process for photonic integrated circuits.

Design of a compact three-dimensional multimode interference phased array structures (3-D MMI PHASAR) for DWDM applications

In this paper, we present a three-dimensional multimode interference phased array structures (3-D MMI PHASAR) (de)multiplexer as a multilevel extension to the planar two-dimensional (2-D) MMI PHASAR. This device is compatible with the planar integrated optics technology. This opens the route for new high-density multilevel optical circuits. In this paper, we discuss 3-D integrated optics and give a detailed description of this new device. The demultiplexing of 65 dense wavelength division multiplexing (DWDM) channels with a channel spacing of 0.2 nm using the proposed structure is considered. The comparison with a 2-D planar PHASAR shows a net improvement in the structure compactness and performance.

Multimode interference couplers with tunable power splitting ratios

New, compact multimode interference couplers with tunable power splitting ratios have been realized. Experiments show large tuning ranges. Such couplers are needed to optimize ON-OFF ratios in interferometric devices and may find applications as extremely compact switches.