Multimode Interference Coupler Research Articles

Experimental verification of phase behaviors of MMI couplers and their application to spectrally flat WDM (de)multiplexers

Phase and amplitude behaviors of several types of 2:2 multimode interference (MMI) couplers are theoretically analyzed and experimentally verified using a fabricated silicon-nanowire waveguide-based interferometer scheme. The phase response of various structural MMI schemes is critical for realizing highly efficient wavelength-division-multiplexing optical (de)multiplexers in terms of spectral flatness, low insertion loss, and minimal crosstalk. We experimentally demonstrate these aspects through the fabrication of a silicon-nanowire-based interference testing scheme. Finally, based on the experimental results, we propose a method for designing spectrally flat optical (de)multiplexers with good fabrication tolerance.

Optimization and multiphysics analysis of high-performance lithium niobate modulators

Thin-film lithium niobate electro-optic modulators (TFLN EOMs) are widely utilized across various fields due to their exceptional performance. In this theoretical study, we optimized a TFLN EOM featuring a multimode interference coupler, cosine-curved waveguides, and a modulating arm straight waveguide structure. By tuning the etching depth, waveguide width, and geometric parameters of the traveling wave electrodes, we achieved a half-wave voltage-length product of 1.6V⋅cm and a 3 dB bandwidth of approximately 140 GHz. Additionally, we performed a multiphysics analysis to investigate the temperature characteristics of the modulator. The results indicate that increased temperature induces thermal-optic effects in the material, leading to changes in the waveguide’s refractive index and the optical beam phase, which results in increased insertion loss. Moreover, the stress generated by thermal expansion is concentrated in the modulator’s waveguide, representing a weak point that is prone to failure.

High‐contrast QPSK pattern recognition device consisting of a 4×$\times$4 MMI coupler

AbstractPattern recognition of optical label is utilized not only in packet switches but also in the field of data security due to its wide range of applications. In particular, pattern recognition using waveguide devices is highly valued because it eliminates the need for encoder processing. However, existing methods suffer from the problem of degraded contrast ratio because half of the input power are emitted from non‐target waveguides. In this article, a method is proposed to improve the contrast ratio employing a 44 multi‐mode interference (MMI) coupler with multiple input signals. The theoretical results reveal that the proposed method improves the contrast ratio from 3.0 dB to 9.5 dB and dB for the three‐input and four‐input cases, respectively, for quadrature phase‐shift keying (QPSK)‐modulated signal. Furthermore, numerical simulation was performed through the three dimension finite‐difference time‐domain (3D‐FDTD) method, and the proposed scheme successfully discriminated each QPSK pattern. The power value at all ports closely matches the theoretical value, and the ratios of 9.5 dB and 30.0 dB are numerically obtained.

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.

N × N MMI performance improvement method by suppressing propagation constant errors

We propose a method to improve the performance of N × N(N⩾3) multimode interference couplers (MMIs) by suppressing propagation constant errors through controlling excited guided modes and the width of MMI, without increasing their fabrication complexity. 5 × 5 MMIs were fabricated on a commercial 220-nm silicon-on-insulator (SOI) platform to verify the method. Measurement results show that insertion losses (ILs) and imbalances (Ims) are reduced from 2.28 dB to 0.16 dB and from 6.94 dB to 0.33 dB, respectively.

Large bandwidth array waveguide grating design for FBG interrogation system

The array waveguide grating (AWG) demodulation method has been widely used in recent years. However, the resolution and total measurement range of AWG-based Fiber Bragg Grating (FBG) interrogation systems are limited by the output characteristics of AWGs. We designed and fabricated a multi-channel SiO2-based AWG as a key component of FBG Interrogation. To increase the dynamic range of demodulation, a multimode interference coupler (MMI) structure is introduced in the middle of the input waveguide and the input slab waveguide. From the simulation results, the 3-dB bandwidth of the AWG is increased from 1.04 nm to 1.86 nm. We test the performance of the interrogation system based on this AWG. The results demonstrate that the system can achieve continuous demodulation in the C-band, with an interrogation accuracy better than 20.22 pm and a wavelength resolution of 1 pm.

Compact and low-crosstalk multimode waveguide crossing utilizing subwavelength holey metamaterial waveguides

To further increase the transmission capacity of on-chip optical communication systems, hybrid division multiplexing technology has emerged as a crucial alternative solution, in which multimode waveguide crossings are highly desired. In this paper, we propose and experimentally demonstrate a compact multimode (i.e., three different modes) waveguide crossing that employs subwavelength holey metamaterial waveguides (SHMWs). The used SHMW, formed by inserting subwavelength periodic holes into a multimode interference (MMI) coupler, deservedly exhibits synergetic advantages of the two kinds of structures, enabling an attractive three-mode (e.g., TE0, TM0, and TM1) waveguide crossing with flexible design, small size, and good performance. Simulation results show that the realized device has a low insertion loss (< 0.74 dB), low reflection loss (<−13.1 dB), and low crosstalk (<−31.6 dB) at a central wavelength of 1550 nm for all the modes with a compact footprint of 27.4 µm × 27.4 µm. The experimental results prove that insertion losses are as low as 0.72 dB, 0.27 dB, and 0.90 dB for TE0, TM0, and TM1 mode, respectively, with the corresponding crosstalk below −38 dB at 1550 nm. The proposed device can be widely applied in photonic integrated circuits to construct photonic systems with the abilities of mode control and multiplexing.

A triplexer chip with cascaded butterfly multimode interference couplers by femtosecond laser microprocessing

A triplexer chip utilizing three cascaded butterfly multimode interference (MMI) couplers to output three optical signals from their respective ports is proposed. The chip consists of a borosilicate glass substrate, a JA photoresist core layer, and an RZJ photoresist cladding. The beam propagation method (BPM) is used to design and optimize the chip. The femtosecond laser processing is investigated, and the chip is processed with a laser power of 1.6 mW and an ablation speed of 4 mm/s. Tests show that the chip meets the design specifications with a low insertion loss of 4.34 dB, an extinction ratio of more than 15 dB, a crosstalk of less than −15 dB, and a 3 dB bandwidth up to 76.8 nm.

300-nm-thick, ultralow-loss silicon nitride photonic integrated circuits by 8-in. foundry production

Silicon nitride (Si3N4) photonic integrated circuits are rapidly developing in recent decades. The low loss of Si3N4 attracts significant attention and facilitates a wide range of applications in integrated photonics. In this work, we demonstrate the foundry fabrication of a 300-nm-thick 8-in. wafer-scale Si3N4 platform, with a microresonator intrinsic quality factor of up to 15×106, corresponding to an ultralow loss of 2.2 dB/m. Leveraging this platform, we develop a mature process design kit, achieving a single-mode waveguide propagation loss of less than 5 dB/m, an edge coupler loss of 1.3 dB, and an insertion loss of 0.07 dB for multimode interference couplers. Utilizing the processed Si3N4 chip, we realize a hybrid integrated tunable external cavity laser with a tuning range from 1534 to 1602 nm, a record-high side-mode suppression ratio of up to 76 dB, an optical power of 26 mW, and an intrinsic linewidth of down to 314 Hz. Our work lays a solid foundation for the further development of applications, including nonlinear optics, quantum optics, optical communications, and ranging.

Low-loss silica waveguide 1×8 thermo-optic switch based on large-scale multimode interference couplers

Optical switches play key role in signal crossing and interconnection. Low-loss and compact optical switches are highly demanded in on-chip optical routing. A silica waveguide 1 × 8 thermo-optic switch based on cascaded 1 × 8 multimode interference (MMI) and 8 × 8 MMI couplers is experimentally demonstrated. Beam propagation method is adopted in the theoretical design and optimization. Standard CMOS technology fabrication is used in switch preparation. Air trenches are introduced to enhance the thermal modulation. At wavelength 1550 nm, the measured insertion loss and crosstalk of this 1 × 8 switch is less than 3.69 dB and −16.29 dB, respectively. And the extinction ratio is larger than 16.68 dB for all routing states. The rise time and fall time are 1.0 ms and 1.24 ms respectively. Compared with the 8-channel switch based on cascaded stages structure, the size reduces by 30 %. With future bandwidth improvement, this demonstrated switch has good potentials in the application of all-optical network routing.

Polarization-Insensitive Lithium Niobate-on-Insulator Interferometer.

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.

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.

Ultracompact 3D Splitter for Single‐Core to Multi‐Core Optical Fiber Connections Fabricated Through Direct Laser Writing in Polymer

AbstractThe deployment and advancement of high‐bandwidth communication networks, quantum information systems, and sensing platforms relying on multi‐core optical fibers (MCFs) are challenged by the scarcity of cost‐effective, compact, and efficient optical interfacing components. This study introduces an unprecedented 3D‐printed 1 × 4 splitter for MCFs fabricated with 2‐photon polymerization‐based direct laser writing. The pivotal element is a triangular cross‐section 3D multimode interference (MMI) coupler, supplemented with S‐bends and adiabatic tapers to facilitate the splitting of a signal from a single core of a single‐mode fiber into four cores of a multi‐core fiber. All components are initially designed and assessed to minimize loss and polarization dependence across the C‐ and L‐bands using optical simulation. Subsequently, a proof‐of‐concept model of the splitter, compactly integrated within the fiber volume, featuring a remarkably short length of 180 µm and insertion loss of ≈−3 dB, is fabricated. The manufacturing speed, minimal loss, component compactness, and flexibility of the approach, collectively present promising avenues for pioneering developments in MCF‐coupling components.

Multimode interference coupler based on general interference for integrated optical and quantum optic devices: a review

Integrated optics play important role in the development for optical network devices, optical processing, instrumentation and quantum information device. Here, multimode interference (MMI) coupler is one of basic component preferred for these integrated optic devices in which MMI coupler based on general interference (GIMMI) has been focused for large scale integrated optic devices because of compact size in comparison to MMI coupler based restricted interference (RIMMI). In this paper, we have reviewed different GIMMI structures reported previously. Different waveguide materials are mentioned and compared for fabrication of GIMMI device. The coupling behaviors and modal analysis of GIMMI couplers and their different tapered structures are estimated by using simple model based on sinusoidal modes showing reduced coupling lengths in down tapered structures in comparison to other structures. The excess loss, polarization dependence loss and crosstalk versus fabrication tolerances are obtained to compare their performances revealing almost same losses and fabrication tolerance. The paper reports the use of the GIMMI coupler in wavelength multiplexing, power splitting, switching, optical processing. Finally, Quantum optic application of GIMMI coupler is shown indicating high quantum fidelity of 50:50 coupling ratio.

Design of a 1 × 3 Power Splitter Based on Multimode Interference in a Parabolic-Type Slot-Waveguide Structure

Multimode interference (MMI) couplers based on silicon slot-waveguide structures have received widespread attention in recent years. The key issues that need to be addressed are the size and loss of such devices. This study introduces a 1 × 3 silicon-based slot-waveguide multimode interference power splitter. The device uses a gallium-nitride slot-waveguide structure to reduce the length of the coupling region and decrease additional losses. To reduce the width of the coupling region, the multimode interference coupling area is designed with a parabolic-shaped structure. The introduction of a tapered structure between the input/output waveguides and the coupling region improves additional losses and non-uniformity. Furthermore, we conducted an analysis of the fabrication tolerances of the coupling region. In this paper, we use mode solution to simulate the design of the device in the 1550 nm optical wavelength range. The eigenmode expansion method is used to simulate and optimize the parameters of the device. The device is simulated using the eigenmode expansion solver. The simulation results show that the total length of the coupling region for the device is only 4 μm. The normalized transmission of the device is 0.992, and its excess loss and imbalance are 0.036 dB and 0.003 dB, respectively. The proposed power splitter can be applied to integrated optical circuit design, optical sensing, and optical power measurement.

Silicon‐photonic four‐mode triple‐band multiplexing device for hybrid wavelength/mode division multiplexing networks

SummaryWhile wavelength division multiplexing (WDM) technology combines several wavelengths onto a single waveguide, the technology of mode division multiplexing (MDM) allows many orthogonal modes of the same wavelength to operate simultaneously without interchannel crosstalk. Thus, the hybrid WDM and MDM network in which the two above‐mentioned techniques cooperate could give a several‐fold increase in the overall network capacity. Constructing this network requires hybrid wavelength‐and‐mode multiplexers, especially ones with high integration and complementary metal‐oxide‐semiconductor (CMOS) compatibility. In this paper, we propose a design of a four‐mode triple‐band multiplexer that is capable of multiplexing up to 12 separate optical signal flows by utilizing four eigenmodes (TE0, TE1, TE2, and TE3) and three‐wavelength windows, which center at 1310, 1490, and 1550 nm. The device is on silicon‐on‐insulator (SOI) platform, consisting of four butterfly‐shaped multimode interference (MMI) couplers, four directional couplers, and a cascaded asymmetric Y‐junction coupler. Via numerical simulations, the proposed design is verified to be able to operate effectively on the three aforementioned bandwidth slots with an optical conversion efficiency of over 93% in all functions. Moreover, it exhibits insertion loss less than 1.5 dB and crosstalk smaller than −16 dB within 25 nm bandwidth at each wavelength window. These results can affirm the success of wavelength–mode combination, which leads to a massive improve in the channel capacity on the same optical multiplexing system for optical telecommunications and photonics on‐chip interconnections.

Polarization-insensitive multimode interference coupler based on a subwavelength grating structure.

A multimode interference (MMI) coupler is one of the basic components for photonic integrated circuits. However, MMI couplers realized by conventional waveguides are polarization sensitive, which is undesired for many applications, such as optical switches and communications. In this Letter, we propose a polarization-insensitive MMI coupler on a 220-nm silicon-on-insulator platform by constructing different effective interference lengths for TE and TM modes assisted with subwavelength grating structures. The designed MMI coupler shows an excess loss of <0.24(0.43) dB and a power imbalance of <0.6(0.5) dB for the TE(TM) mode over the wavelength range of 1.5-1.6 µm in theory. Experimentally, the fabricated MMI exhibits low excess loss <0.64(0.53) dB and power imbalance <1(0.85) dB for the TE(TM) mode over a wavelength range of 1.55-1.61 µm.

Compact, Scalable, Fast‐Response Multimode 2 × 2 Optical Switch Based on Inverse Design

AbstractMode‐division multiplexing (MDM) introduces a new dimension to on‐chip optical interconnection, where the multimode optical switch is one of the core components. However, researchers are still struggling to reduce the chip size and response time, despite great efforts being made. Here, a compact, scalable, fast‐response multimode 2 × 2 switch supporting four modes is demonstrated based on the specific inverse design. The device consists of two multimode 2 × 2 CB couplers (including mode converters, multimode interference couplers, and crossings) and a thermo‐optic (TO) phase shifter. It achieves extinction ratios exceeding 15 dB for all modes at the central wavelength of 1.55 µm. All the fundamental components are designed by an improved general shape optimization method, realizing a compact size and a larger manufacturing tolerance (±20 nm). Besides, a hydrogen‐doped indium oxide microheater is introduced as a TO phase shifter to achieve a fast response (≈3.5 µs in average) for all modes. This solution provides an effective avenue toward large‐scale multimode optical switch matrices for the future MDM systems.

Damascene Process Development for Low-Loss Photonics Devices with Applications in Frequency Comb

Silicon nitride (SiN) is emerging as a material of choice for photonic integrated circuits (PICs) due to its ultralow optical losses, absence of two-photon absorption in telecommunication bands, strong Kerr nonlinearity and high-power handling capability. These properties make SiN particularly well-suited for applications such as delay lines, chip-scale frequency combs and narrow-linewidth lasers, especially when implemented with thick SiN waveguides, which is achieved through low-pressure chemical vapor deposition (LPCVD). However, a significant challenge arises when the LPCVD SiN film thickness exceeds 300 nm on an 8-inch wafer, as this can result in cracking due to high stress. In this work, we successfully develop a damascene process to fabricate 800 nm-thick SiN photonics devices on an 8-inch wafer in a pilot line, overcoming cracking challenges. The resulting 2 × 2 multimode interference (MMI) coupler exhibits low excess loss (−0.1 dB) and imbalance (0.06 dB) at the wavelength of 1310 nm. Furthermore, the dispersion-engineered SiN micro-ring resonator exhibits a quality (Q) factor exceeding 1 × 106, enabling the generation of optical frequency combs. Our demonstration of photonics devices utilizing the photonics damascene process sets the stage for high-volume manufacturing and widespread deployment.

Monolithically integrated multimode interference coupler-based master oscillator power amplifier with dual-wavelength emission around 830 nm

A monolithically integrated dual-wavelength multimode interference coupler-based master oscillator power amplifier is presented. It consists of two shallowly etched, laterally separated ridge waveguide laser cavities as master oscillators with individual distributed Bragg reflector gratings as cavity mirrors. A deeply etched coupling section containing S-bend shaped waveguides and a multimode interference coupler is used to couple the laser emission of the master oscillators into a shallowly etched single waveguide serving as power amplifier. Changing the etch depth for the coupling section enables a compact device layout. In addition, increased radiation angles of modes not coupled into the power amplifier help to suppress beam steering, otherwise indicated by laterally separated far-field intensity distributions. The device provides 0.5 W of dual-wavelength emission around 830 nm in individual and common operation. As designed, both emission wavelengths are separated by 0.5 nm with spectral widths below 20 pm, limited by the spectral resolution of the spectrometer. Both peak wavelengths remain within spectral windows of 50 pm within the available power range. This enables full flexibility selecting operating points for applications such as shifted excitation Raman difference spectroscopy and the generation of THz emission by photomixing. The emission wavelengths can additionally be non-continuously tuned by applying a heater current to resistors implemented next to the distributed Bragg reflector gratings. As an example, selected spectral distances of 0.5 nm, 1.0 nm, 1.5 nm, and 2.0 nm are demonstrated. Near field widths of 5 μm and far field angles of 17° result in beam propagation ratios of 1.4 (1/e2) in all operation modes and enable easy beam shaping or optical single-mode fiber coupling.