Multimode Interference Structure Research Articles

Phase-locked high-brightness interband cascade laser array with multimode interferometer couplers.

We report on the design, fabrication, and characterization of an interband cascade laser (ICL) array incorporating multimode interference (MMI) couplers for phase-locking emitters. The ICL array, emitting at 3-4 µm, was developed to address the challenges of heat dissipation and in-phase operation in mid-infrared lasers. The array features a 7.5-µm-wide ridge design for fundamental transverse mode propagation and employs MMI structures to realize an in-phase operation of multiple emitters. The far-field patterns, characterized by periodic and symmetrical interference fringes, confirm the coherent operation of the array and the efficacy of MMI couplers in achieving phase-locking. The single-ridge side of the array exhibits a single-lobe far-field profile, with higher-order transverse modes effectively suppressed, showcasing a nearly diffraction-limited beam quality (M2 ≈ 1.31) at high output powers (390 mW from the 1 × 4 array). The robust performance and scalable design of the ICL array, validated by experimental results and theoretical simulations, indicate its potential for high-power mid-infrared applications and as an optical phased array.

Polarization-insensitive multimode interference coupler on an x-cut thin-film lithium niobate platform.

Thin-film lithium niobate (TFLN) is a promising integrated photonics platform but currently lacks a polarization-insensitive multimode interference (MMI) coupler, a crucial component for polarization-related optical communication applications such as polarization management, polarization-division multiplexing, and polarization-insensitive modulation systems. This paper presents a novel, to the best of our knowledge, approach by rotating the MMI structure on an anisotropic x-cut TFLN at specific angles to compensate for the difference in the beat length between the two polarizations. A polarization-insensitive 1 × 2 MMI coupler is experimentally achieved with measured transmittances of -2.5 to -4 dB for both output ports and polarization modes in the wavelength range of 1520-1580 nm.

A flat-top arrayed waveguide grating base on cascading ‘A’ shape multimode interference

Today, large bandwidth is required to increase data transmission speeds, this requires an optimized design of the photonic devices. To achieve increasing bandwidth in the wavelength division multiplexing device’s arrayed waveguide grating (AWG). We have cascaded a special ‘A’ shape multimode interference (MMI) structure at the end of the input waveguide. ‘A’ shape structure is better able to get the ideal light field distribution, making the flattening effect more perfect. The ‘A’ shape MMI is designed by the linear spreading equation, the structure of the MMI can be easily and conveniently manufactured in a practical application. Simulation results show that increasing the 1 dB bandwidth to 0.7 nm and the 3 dB bandwidth to 1.6 ± 0.2 nm, the insertion loss is measured at 3.5 ± 0.3 dB, and the channel crosstalk is below −30 dB. It paves the path to achieve increasing bandwidth of AWG.

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.

Silicon photonic delay-based reservoir computing incorporated with a multimode interference structure

Silicon photonic delay-based reservoir computing incorporated with a multimode interference structure

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.

Reconfigurable Generation of PAM-4 Signal Based on Fano Effect for Optical Interconnect Systems

We present a novel low-power architecture for the generation of multilevel pulse amplitude modulation (PAM-4) signal generation. A new structure of a micro-ring resonator based on a 4´4 MMI (Multimode interference) coupler is proposed to control the coupling coefficient and Fano shapes. Based on this structure, a high linearity of the transmission is created, compared with the Mach Zehnder Interferometer (MZI) conventional structure. Here, instead of using directional couplers and MZI as shown in the previous reports, in our method only two 4´4 multimode interference structures on silicon on insulator (SOI) waveguides has been used. The new design is compatible and suitable for the current CMOS fabrication technology. In this work, the special design is as follows: one of two MMI couplers is used to be a micro-ring resonator and two segmented phase shifters are used in the micro-ring resonator to generate 4 levels of the PAM-4 signal. The micro-ring resonator is controlled to work at the over-coupled region, so the Fano effect can be generated. This proposed PAM-4 architecture uses the generated Fano effect, therefore an extreme reduction in power consumption can be achieved. A large fabrication tolerance of ±500 nm and a compact footprint of 10´500 µm2 can be carried-out. The device is simulated and optimally designed using the FDTD (finite difference time domain) and EME (Eigenmode Expansion). This architecture can be useful for optical interconnects and data center network applications.

Arbitrary-ratio 1 × 2 optical power splitter based on thin-film lithium niobate.

Optical power splitters (OPSs) have been widely used in photonic integrated circuits, but an OPS with a large fabrication tolerance and free choice of power splitting ratio (PSR) is still highly desired for thin-film lithium niobate (TFLN) platform. Here, we propose and experimentally demonstrate several 1 × 2 OPSs with PSRs from 50:50 to 5:95 using TFLN platform. The proposed devices are built by multimode interference structure to achieve a broad bandwidth and large fabrication tolerance. Various PSRs can be obtained by adjusting the geometry structure of the multimode interference region. All of our fabricated devices feature an insertion loss lower than 0.3 dB at the wavelength of 1550 nm, and a PSR variation less than 3% in the range of 1520 nm to 1590 nm.

Optical fiber pH sensor based on a multimode interference device with polymer overlay.

An optical fiber pH sensor based on a multimode interference structure is presented. The sensitive element is a piece of no-core fiber (NCF) coated with a mixture of polyallylamine hydrochloride and polyacrylic acid by a modified layer-by-layer (LbL) self-assembly method. It is experimentally shown that by reducing the diameter of the NCF by chemical etching, the sensitivity is increased from -0.31n m/p H to -2n m/p H. The sensor exhibits a high linear response of 0.997 over a pH range from 5 to 11.3 with a rapid response time lower than 1s.

Mode Conversion Between Beams Carrying Orbital Angular Momentum With Opposite Topological Charges Using Two-Dimensional Multimode Interference Waveguides

Recently, the implementation of rectangular waveguides for generation and manipulation of orbital angular momentum (OAM) modes, has been developed to exploit the outstanding features of these modes in more complex integrated devices. Multimode interference (MMI) structures have been widely used in both one and two dimensions as the basic elements in many integrated optical devices like optical beam splitters, mode converters, couplers, wavelength-division (de)multiplexers, and switches. According to the various applications of OAM modes in quantum and classical communications, the study of their propagation properties in MMI waveguides is useful in order to facilitate the way into higher security and capacity systems. This paper presents the results of numerical investigation of a proposed integrated optical OAM mode converter which is performed based on the mode decomposition properties of propagating OAM modes in 2D MMI waveguides, as well as the fact that any order of OAM mode can be represented as superposition of an odd mode and a quarter-wave shifted even mode. The proposed device, which consists of two MMI waveguides connected by phase shifters and linear waveguides, provides mode conversion between beams carrying OAM with odd opposite topological charges. The design procedure is performed for OAM modes with topological charge values of l = ±1 and l = ±3 using beam propagation method. The proposed device is passive with reciprocal behavior and has the output mode purity of 94% and 82% for beams carrying l = ±1 and l = ±3, respectively.

Vector dark–bright pulses from a ytterbium doped fiber laser mode-locked by nonlinear multimode interference

We report dark–bright pulse pair generation from the ytterbium doped fiber laser mode-locked by cascaded nonlinear multimode interference (MMI) effect under an all-normal dispersion configuration. The MMI is implemented in a step index single mode–multimode–single mode fiber structure. The combination of two MMI structures, acting like a saturable absorber, has been used for stable mode-locking operation in the resonator. By adjusting the polarization controller attached to one of the MMI structure, stable dark–bright pulses are observed at fundamental, second and third harmonic repetition rates. The optical spectrum of the dark–bright pulses is doubly peaked at 1036.6 and 1040.6 nm with a spectral width ∼0.7 nm. The width of the dark pulses varied from 72 to 50 ns and that of the bright pulses from 30 to 14 ns when the pump power is increased from 102 to 170 mW at the fundamental repetition rate mode-locking. The darkness of the dark and intensity of bright pulses increase with the pump power. Dark and bright pulses in the pair are resolved by placing a polarizing component confirming their vector nature.

Highly sensitive temperature sensor based upon a multimode interference structure filled with an ethanol-glycerol solution

A temperature sensor based upon a single mode - multimode-single mode – multimode - single mode (SMSMS) fiber structure is experimentally demonstrated. The SMSMS structure is covered by ethanol-glycerol solution, and changing the ratio of ethanol to glycerol changes the temperature sensitivity. A theoretical calculation was developed to characterize the influence of the ethanol-glycerol content upon the sensor performance,. Multimode interference (MMI) occurs between single mode fibers (SMF) and multimode fibers (MMF), and temperature measurements were achieved by monitoring the movement of the interference profile in the transmission spectrum. The results show that covering the SMSMS structure with an ethanol-glycerol solution improves the temperature sensitivity of the sensor. The temperature sensitivity was is 0.700 nm/°C from 30 °C to 80 °C.

Three-Dimensional Polymer Variable Optical Attenuator Based on Vertical Multimode Interference with Graphene Heater.

Low-power-consumption optical devices are crucial for large-scale photonic integrated circuits (PICs). In this paper, a three-dimensional (3D) polymer variable optical attenuator (VOA) is proposed. For monolithic integration of silica and polymer-based planar lightwave circuits (PLCs), the vertical VOA is inserted between silica-based waveguides. Optical and thermal analyses are performed through the beam propagation method (BPM) and finite-element method (FEM), respectively. A compact size of 3092 μm × 4 μm × 7 μm is achieved with a vertical multimode interference (MMI) structure. The proposed VOA shows an insertion loss (IL) of 0.58 dB and an extinction ratio (ER) of 21.18 dB. Replacing the graphene heater with an aluminum (Al) electrode, the power consumption is decreased from 29.90 mW to 21.25 mW. The rise and fall time are improved to 353.85 μs and 192.87 μs, respectively. The compact and high-performance VOA shows great potential for a variety of applications, including optical communications, integrated optics, and optical interconnections.

Research on Mid-Infrared External Cavity Quantum Cascade Lasers and Applications

In this paper, we review the progress of the development and application of external cavity quantum cascade lasers (ECQCLs). We concentrated on ECQCLs based on the wide tunable range for multi-component detection and applications. ECQCLs in the mid-infrared band have a series of unique spectral properties, which can be widely used in spectroscopy, gas detection, protein detection, medical diagnosis, free space optical communication, and so on, especially wide tuning range, the tuning range up to hundreds of wavenumbers; therefore, ECQCLs show great applications potential in many fields. In this paper, the main external cavity structures of ECQCLs are reviewed and compared, such as the Littrow structure, the Littman structure, and some new structures. Some new structures include the intra-cavity out-coupling structure, multimode interference (MMI) structure, and acousto-optic modulator (AOM) control structure. At the same time, the application research of ECQCLs in gas detection, protein detection, and industry detection are introduced in detail. The results show that the use of diffraction gratings as optical feedback elements can not only achieve wide tuning, but it also has low cost, which is beneficial to reduce the complexity of the laser structure. Therefore, the use of diffraction gratings as optical feedback elements is still the mainstream direction of ECQCLs, and ECQCLs offer a further new option for multi-component detection.

Ultra-Broadband and Low-Loss Silicon-Based Power Splitter Based on Subwavelength Grating-Assisted Multimode Interference Structure

High-performance and compact power splitters are fundamental components in on-chip photonic integrated circuits (PICs). We propose a silicon-based power splitter based on a subwavelength grating (SWG)-assisted multimode interference (MMI) structure. To shorten the device size and enhance the device performance, an inverse-tapered SWG is embedded in the central region of the MMI and two rows of uniform SWG are embedded on both sides, together with two right-angled cutting structures on the input side. According to the results, the MMI length was obviously reduced to 3.2 μm (5.2 μm for conventional MMI structure under the same waveguide width), while the insertion loss (IL) and reflection loss were 0.08 dB and <−35 dB, respectively. Moreover, the allowable working bandwidth could be extended to 560 nm by keeping IL <0.6 dB, covering the whole optical communication band. On the basis of these features, we believe that such a power splitter is very promising for building on-chip large-scale PICs where power splitting is indispensable.

Photonic Integrated Chip on TriPleX Platform for Realizing Optical Haar Transform and Compression in the Visible Spectrum

The design and characterization of a new photonic chip on TriPleX <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sup> integrated platform technology for investigating all-optical Haar transform (HT) based compression in the visible spectrum is proposed. A building block (BB) based on a multimode interference (MMI) structure is designed for green light wavelength (532 nm), enabling the optical HT operation in the circuits of the optical chip. The designed TriPleX chip demonstrates two types of MMI based optical circuits, i.e., restricted interference (RI) and general interference (GI), allowing the implementation of the optical HT and compression on the input image pixels. An optical image compression rate of 75% is achieved for a linear 8-pixels array of an image by the designed circuit in the chip. An experimental coupling ratio (CR) of ~98:2 is obtained by the designed circuit in the chip with identical optical inputs, proving high capability of the proposed design for the realization of optical HT and image compression.

Realization of visible light integrated circuits for all-optical haar transform

Photonic integrated circuits have been designed, simulated, and tested on TriPleX™ platform technology for implementing optical Haar Transform (HT) in the visible spectrum. Optical circuits contain the new building block (BB), designed using the multimode interference (MMI) structure, performing optical HT on a pair of the optical signals. The Outputs of the BB demonstrate the sum and subtraction of the input signals according to the HT operation. 1st-order and 2nd-order optical HT achieved for the green light input signals. Simulation and experimental results have successfully validated the designed photonic circuits’ capability in implementing optical HT.

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.

Tunable characteristic of phase-locked quantum cascade laser arrays* *Project supported by the National Basic Research Program of China (Grant Nos. 2018YFA0209103 and 2018YFB2200504), the National Natural Science Foundation of China (Grant Nos. 61991430, 61774146, 61790583, 61674144, 61774150, and 61805168), the Beijing Municipal Science & Technology Commission, China (Grant No. Z201100004020006), and the Key Projects of the Chinese Academy of Sciences

A multimode interference (MMI) structure is designed to simplify the fabrication of quantum cascade laser (QCL) phase-locked arrays. The MMI geometry is optimized with a sufficient output channel distance to accommodate conventional photolithography and wet etching process by which power amplifier array is fabricated without using the complicated two-step etching-regrowth or dry etching technique. The far-field pattern with periodically modulated peaks reveals that the beams from the arrays are phase-locked. Furthermore, the frequency tuning performance of the MMI-based phase-locked arrays is studied using the Littrow-configuration external cavity structure. A wavelength tuning range of more than 60 cm−1 is demonstrated, which will eventually realize the high power, frequency tunable, large-scale phase-locked arrays, and their application in spectroscopy.

Carbon Nanotubes/Graphene Oxide Composite Applications in Fiber RH Sensor

We propose a high-sensitivity optical fiber relative humidity (RH) sensor based on multiwalled carbon nanotubes (MWCNTs)/graphene oxide (GO) composite, which functions as the humidity-sensitive film. The optical component of this demonstrated sensor is a single-mode fiber-solid core fiber-single-mode fiber-based multimode interference structure. For the composite sensitive film, GO is used as a dispersant for two types of MWCNTs with different lengths. Subsequently, GO-dispersed MWCNT hybrid films are formed on fiber surfaces by the dip-coating method. Herein, the hydroxylated MWCNTs can function as supporting structures to avoid the formation of layer-by-layer stacked GO, and this destacking humidity-sensitive film is beneficial to enhance water molecules' absorptive capacity. To clarify the principle, the interaction between composite humidity-sensitive film and water molecules is investigated. The experimental results reveal that the interference fringe shows a redshift with the increase in RH. When RH changing from 40% to 90%, the sensor probe with long-range MWCNT-based film has higher sensitivity than that with short-range MWCNT-based film, which demonstrates the effect of the supporting action of MWCNTs. Besides, a larger specific surface area and pore size of long-range GO-MWCNTs benefit sensitivity enhancement. In addition, this proposed sensor has good repeatability, stability, and a large linear measurement range (40% RH-90% RH).