MMI Coupler Research Articles

Electro-optic MMI coupler as wavelength demultiplexer for advanced SDM wireless-WDM optical signal converter

Electro-optic MMI coupler as wavelength demultiplexer for advanced SDM wireless-WDM optical signal converter

High-performance thin-film lithium niobate optical 90° hybrid with a novel CMRR assessment method

Ultra-Compact 90° Hybrid DesignThe optical 90° hybrid uses a 4 × 4 MMI coupler with an ultra-compact footprint of ~9 μm × 107 μm, which is 3 times shorter than previous work. Novel CMRR Calculation MethodA new method to assess the CMRR using a single unequal Mach-Zehnder interferometer (MZI), reducing fabrication complexity and measurement errors. Excellent PerformanceThe device features low excess loss (0.55 to 1.4 dB), low phase errors (< ±4.5°), and high CMRR (< −17.8 dBe) over a wide wavelength range (1530 to 1605 nm).

Integrated Photonic Passive Building Blocks on Silicon-On-Insulator Platform

Integrated photonics on Silicon-On-Insulator (SOI) substrates is a well developed research field that has already significantly impacted various fields, such as quantum computing, micro sensing devices, biosensing, and high-rate communications. Although quite complex circuits can be made with such technology, everything is based on a few ’building blocks’ which are then combined to form more complex circuits. This review article provides a detailed examination of the state of the art of integrated photonic building blocks focusing on passive elements, covering fundamental principles and design methodologies. Key components discussed include waveguides, fiber-to-chip couplers, edges and gratings, phase shifters, splitters and switches (including y-branch, MMI, and directional couplers), as well as subwavelength grating structures and ring resonators. Additionally, this review addresses challenges and future prospects in advancing integrated photonic circuits on SOI platforms, focusing on scalability, power efficiency, and fabrication issues. The objective of this review is to equip researchers and engineers in the field with a comprehensive understanding of the current landscape and future trajectories of integrated photonic components on SOI substrates with a 220 nm thick device layer of intrinsic silicon.

Mode-insensitive and mode-selective optical switch based on asymmetric Y-junctions and MMI couplers

Driven by the large volume demands of data in transmission systems, the number of spatial modes supported by mode-division multiplexing (MDM) systems is being increased to take full advantage of the parallelism of the signals in different spatial modes. As a key element for photonic integrated circuits, the multimode waveguide optical switch (MWOS) is playing an important role for data exchange and signal switching. However, the function of the traditional MWOS is simple, which could only implement the mode-insensitive or mode-selective switching function; it is also difficult to scale to accommodate more spatial modes because of the limitation of the device structure. Therefore, it is still challenging to realize a multifunctional and scalable MWOS that could support multiple modes with low power consumption and high flexibility. Here, we propose and experimentally demonstrate a multifunctional MWOS based on asymmetric Y-junctions and multimode interference (MMI) couplers fabricated on a polymer waveguide platform. Both mode-insensitive and mode-selective switching functions can be achieved via selectively heating different electrode heaters. The fabricated device with the total length of ∼0.8 cm shows an insertion loss of less than 12.1 dB, and an extinction ratio of larger than 8.4 dB with a power consumption of ∼32 mW for both mode-insensitive and mode-selective switching functions, at 1550 nm wavelength. The proposed MWOS can also be scaled to accommodate more spatial modes flexibly and easily, which can serve as an important building block for MDM systems.

A Semi-Analytical Method for the S-Parameter Calculations of an N × M Multimode Interference 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.

Compact and broadband dual-polarization waveguide crossing utilizing subwavelength-hole-assisted MMI couplers.

In this Letter, an ultracompact silicon-based waveguide crossing for dual polarizations is proposed and experimentally demonstrated using subwavelength-hole-assisted multimode interference couplers. Thanks to the flexible and easy dispersion engineering in the introduced subwavelength-hole-assisted multimode interference couplers, the reduced and equal beat lengths for dual polarizations are accessible via careful parametric optimization, consequently enabling a substantially reduced device size. Experimental results indicate that the proposed crossing (13.6 × 13.6 µm2 in size) features a low insertion loss of 1.03 dB (0.76 dB) and low crosstalk of -32.5 dB (-37.8 dB) at a central wavelength of 1550 nm for TE (TM) mode, with a broad bandwidth of ∼80 nm for crosstalk of <-18 dB.

A new-generation approach to PAM-4 Based on the Fano resonance effect using a multimode interference structure

The goal of this research is to come up with a new optical structure for the generation of multilevel pulse amplitude modulation (PAM-4) signalling, which is used for optical interconnects and data center networks. The Fano effect is made with a structure made up of a 4x4 MMI coupler, feedback waveguides, and phase shifters. For the production of PAM-4, we employ the Fano effect. The new proposed approach can reduce power consumption and extend the linear region of the device transfer function compared with conventional approaches based on Mach Zehnder Interferometers (MZI) and ring resonators. We show that an extreme reduction of power consumption of 3 to 30 times, compared with the conventional structure based on MZI and ring resonators, is achieved. In addition, the proposed structure based on silicon wave guides has the advantages of extreme high bandwidth, large fabrication tolerance, and a compact footprint. The proposed structure can be applied to generate the PAM-4 based on the Fano effect with low power consumption, and it is suitable for complex systems with a lot of transceivers. The device is numerically simulated and designed using the Finite Difference Method (FDM) and Finite Difference Time Difference (FDTD). The PAM-4 structure based on silicon waveguides can be compatible with CMOS’s existing technologies.

LPCVD Silicon Nitride Photonic Integrated Components Designed at 532 Nm Wavelength for a Novel Retinal Projection Concept and Visible Domain Applications

This paper describes the design, fabrication, and experimental characterization of several photonic integrated components specifically made for a novel retinal projection concept for augmented reality applications. The retinal projection concept is based on the combination of integrated optics and holography to generate a self-focusing image on the retina. The photonic integrated circuit used for the retinal projector is made of several passive photonic components fabricated with stoichiometric Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> : single-mode strip waveguides, variable radius bent waveguides, MMI couplers (Multi-Mode Interference), diffraction grating couplers and waveguide crossings. The photonic components are designed to work in the visible spectrum at λ = 532 nm. They are compact and generally show low losses, which are the two main requirements to easily insert the photonic integrated circuit on wearable glasses and enhance the image quality of the retinal projector. Our photonic components can also find applications in a variety of other fields related to the visible spectrum, such as biophotonics, optical phased arrays, visible light communication and more.

On chip optical neural networks based on MMI microring resonators for image classification

We propose a new on-chip optical neural network (OONN) based on multimode interference-microring resonators (MMI-RRs). The suggested structure eliminates the need for wavelength division multiplexers (WDM) to create an optical neuron on a single chip. New microring resonator structure based on 4×4 MMI coupler with a size of 24µm × 2900 µm is used for the basic elements of the computation matrix, as a result a higher bandwidth and free spectral range (FSR) can be achieved. The Si3N4 platform along with the graphene sheet is designed to modulate the signals and weights of the neural networks at a very high speed. The Si3N4 can provide wide range of operating wavelengths and can work directly with the wavelengths of color images. The structure's benefits include rapid computing speed, little loss, and the ability to handle both positive and negative values. The OONN has been applied to the MNIST dataset with a speed faster than 2.8 to 14x times compared with the conventional GPU methods.

Parabolic MMI Coupler for 2 × 2 Silicon Optical Switch With Robustly High Extinction Ratio for Four Paths

We propose and experimentally demonstrate a Mach-Zehnder silicon thermo-optic switch based on a parabolic multi-mode interference coupler. By judiciously designing the parabolic structure, the 2×2 MMI can exhibit extremely small imbalance of output power around 0.1 dB over the C band with good fabrication tolerance according to simulation. As such, for all four possible switching paths, high average extinction ratios over 35 dB can be achieved from 1520 nm to 1580 nm in experiment, with maximum values concurrently over 40 dB. The proposed parabolic MMI occupies a small area of merely 2.4 μm × 9.5 μm.

Performance Analysis of SiGe-Cladded Silicon MMI Coupler in Presence of Stress

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.

Low-loss, broadband MMI coupler based on thin film lithium niobate platform

A low-loss broadband power splitter based on X-cut lithium niobate is proposed by utilizing the multimode interference coupler with a shallow-etched slot, wedge-shaped structure and SiO2 cladding. The multimode waveguide with a shallow-etch slot could reduce the sensitivity of multi-mode region on the wavelength and reach to high-quality imaging requirements. Results show that there is an excess loss around 0.01 dB at the operation wavelength of 1550 nm, a broad operation bandwidth of ∼ 1314 nm and large tolerance in the shallow-etching. Additionally, our proposed device has been compared with the published and conventional multimode interference coupler for presenting good low-loss and broad bandwidth.

Design of a Power Splitter Based on a 3D MMI Coupler at the Fibre-Tip

Planar MMI couplers based on inorganic material platforms have played an essential role in photonic integrated circuits development. Advances in organic polymer fabrication techniques enable the design of components beyond a single plane, thus facilitating vertical integration for a wide range of components, including the MMI coupler. This paper presents the design of two 3D IP-dip polymer-based MMI power splitters operating in the near-infrared part of the spectrum at a wavelength of 1550 nm. The resulting output power ratio, modal field distributions, spectral characteristics, and the effects of input fibre misalignment are investigated using the beam propagation method. The fabrication method used to realise the designed splitters was direct laser writing. The function of the splitters was then verified by a highly resolved near-field scanning optical microscope.

Low-Loss Calibration-Free 2 × 2 Mach-Zehnder Switches With Varied-Width Multimode-Interference Couplers

Large-scale photonic switches are essential components for telecommunications and interconnects, incorporating many cascaded 2 × 2 optical switches. To keep the excess loss of the entire switch low, it is crucial to develop ultralow-loss 2 × 2 optical switches by introducing new coupler designs. In this paper, a low-loss calibration-free 2 × 2 Mach-Zehnder switch (MZS) designed with varied-width multimode-interference (MMI) couplers is proposed and fabricated. In this design, the MMI coupler has a varied-width MMI region and two multimode output waveguides. The MMI coupler optimized by the particle swarm optimization (PSO) algorithm has an ultralow excess loss < 0.053 dB and a suppressed higher-order mode excitation < −26 dB across the C-band (1530–1565 nm). Such a varied-width MMI coupler with two multimode output waveguides can be used compatibly for the MZS with widened phase shifters, so that the random phase imbalance between the two MZS arms can be suppressed effectively, which thus is promising for calibration-free operation. For the present MZS fabricated with standard 180-nm silicon photonic foundry processes, the excess loss is ∼0.5 dB across the C-band and the extinction ratio is ∼25 dB without calibration.

Improved Half-Wave Coupled V-Cavity Laser Using Aggregating-Image Multimode Interference Coupler

We theoretically analyze and experimentally demonstrate improved half-wave couplers for widely tunable V-cavity lasers. An overlapping-image 6×6 MMI coupler and an aggregating-image 16×16 MMI coupler are investigated, which give optimal coupling phase and coefficient for achieving high single-mode selectivity in V-cavity lasers while producing a low loss. A single-electrode controlled tuning of 38 channels is achieved with channel spacings of 100 GHz and 150 GHz. By combining two single-electrode tuning segments at different temperatures, a wavelength tuning range of 52 channels from 1515 nm to 1585 nm is obtained, and the side mode suppression ratios (SMSRs) reach 45 dB.

Design and simulation of 4-mode (de)multiplexers implemented in conventional and subwavelength grating Si/SiO2 platforms

This work presents two 4-mode, broadband, low loss, compact, and fabrication tolerant (de)multiplexers comprised of a 1 × 4 mode converter-splitter, multimode interferometer MMI, and phase shifter devices. The first device is designed using conventional waveguides. The second one is built utilizing subwavelength grating technology introduced in the multimode section of the MMI coupler. To the best of our knowledge, this is the first demonstration of a 4 × 4 (de)multiplexer based on a SWG-MMI coupler. Both devices are built in a silicon-on-insulator (SOI) platform. In the operation of the designed devices, four input modes TE0, TE1, TE2, and TE3 are split and converted into a fundamental TE0 mode at the output end. 80 nm (1510–1590 nm) operating bandwidth, losses > −0.5 dB, and crosstalk < −15 dB are achieved from the conventional version of the (de)multiplexer, while 140 nm (1480–1620 nm), losses > −0.5 dB, and crosstalk < −15 dB, are achieved from the subwavelength grating assisted device. The overall size of the designed devices is 4.5 × 173 µm2 and 4.5 × 123 µm2, for the conventional and subwavelength grating devices, respectively. Subwavelength grating technology offers flexibility in refractive index engineering for extra bandwidth (75% additional bandwidth) and compactness (30% size reduction) compared to the conventional counterpart.

Optical mode conversion based on silicon-on-insulator material [formula omitted]-junction coupler and multimode interferometer

In mode-division multiplexing (MDM) networks, converting modes back and forth between a higher-order mode and a lower-order mode plays a vital role in improving network capacity and flexibility. This paper presents a spatial optical mode conversion supporting three modes based on two Ψ-junction couplers and two multi-mode interference couplers (MMIs). The proposed device can arbitrarily convert a TEi mode to a TEj mode (where i, j = 0, 1, 2) by setting three phase-shifters at 0 or 180 degrees, which are introduced at the Ψ-junction and MMI couplers. Through numerical simulations using the spatial three-dimensions beam propagation method (3D-BPM), the mode converter shows a high conversion efficiency with the insertion loss smaller than 0.2 dB and the crosstalk below −20 dB at the center wavelength of 1.55 μm. The device works effectively in C band, partially in S and L bands within a wideband up to 80 nm. We believe that the proposed device would be a potential platform for applications in MDM networks and photonic integrated circuit systems.

Polymer Mode Selecting Switch Based on Cascaded MMI Couplers

A polymer waveguide mode selecting switch (MSS) composed of two cascaded MMI couplers is demonstrated. The MSS can switch different data channels over E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> modes. Polymer waveguide dimensions are optimized with beam propagation method. The simulated results indicate that the worst insertion loss is 5.5 dB at 1550 nm and the switching powers for E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10 </sub> mode between two different channels are 4.5 and 12.3 mW, respectively. The extinction ratio and crosstalk are less than 19.7 and -19.6 dB, respectively. The switching performances of fabricated MSS prove the functionality of proposed design, which promises its potentials in mode-division-multiplexing applications.

A compact and polarization-insensitive silicon waveguide crossing based on subwavelength grating MMI couplers.

In this work, we proposed and experimentally demonstrated a compact and low polarization-dependent silicon waveguide crossing based on subwavelength grating multimode interference couplers. The subwavelength grating structure decreases the effective refractive index difference and shrinks the device footprint. Our designed device is fabricated on the 220-nm SOI platform and performs well. The measured crossing is characterized with low insertion loss (< 1 dB), low polarization-dependence loss (< 0.6 dB), and low crosstalk (< -35 dB) for both TE and TM polarizations with a compact footprint of 12.5 μm × 12.5 μm.

InP Monolithically Integrated Transmitters Based on High Speed Directly Modulated DFB Lasers

In this paper we summarize results concerning high-speed integrated multi-wavelength transmitters fabricated using a generic InP integration platform. The photonic integrated circuits (PICs) include tunable directly modulated DFB lasers emitting in the C-band, monitoring photodetectors, MMI couplers and output spot size converters. The directly modulated lasers, designed in order to fit within a 100 GHz grid, show a maximum 3-dB small signal bandwidth of 21 GHz and a tunability range of about 4 nm. In particular, we report an 8-channel transmitter showing operation up to 30 Gb/s per channel (for 200+ Gb/s applications) and a 4-channel transmitter capable of operating up to 34 Gb/s per channel (for 100+ Gb/s applications). Error free operation (bit error rates <10−9) is achieved for all the channels in all the cases.