Directional Coupler Research Articles

Compact polarization splitter-rotator based on lithium niobate-on-insulator platform

We propose a compact mode-coupled polarization splitter-rotator (PSR) based on the lithium niobate-on-insulator platform. The PSR is composed of an asymmetric directional coupler, a taper, and an S-bend. Cross-polarization mode coupling is achieved using a ridge waveguide and air cladding to break the symmetry of the waveguide cross section with a length of only 61 μm. Simulation results show that the insertion losses of this device in the 1500–1600 nm range are less than 1.85 and 0.31 dB for TM0 and TE0 modes, respectively, and the extinction ratios are more than 22.5 and 22.7 dB, respectively. In addition, the PSR exhibits a polarization conversion efficiency of 99.97% at 1550 nm. The proposed PSR has great potential for applications in polarization-division multiplexing transmitters and on-chip polarization controllers.

Chaos and regularities in cavity assisted two-channel nonlinear coupler

This paper presents a study of the dynamical behavior in a cavity-assisted two-channel Kerr nonlinear directional coupler. Applying a deterministic periodic laser pump to a two-channel coupled system led to a range of dynamic behaviors, encompassing periodicity, quasiperiodicity, chaoticity and hyperchaoticity. This is confirmed by the exploration of the Lyapunov exponents, Poincaré sections and bifurcation diagrams for resonance and low cavity detuning. For low cavity detuning, chaos arises when the laser pump’s amplitude is large. This is due to higher interactions between photons and the cavity, resulting in more photons inside and higher nonlinearity. On the other hand, the reduced interactions in high cavity detuning displayed regular behavior which is explained qualitatively.

A fixed phase tunable directional coupler based on coupling tuning

The field of photonic integrated circuits has witnessed significant progress in recent years, with a growing demand for devices that offer high-performance reconfigurability. Due to the inability of conventional tunable directional couplers (TDCs) to maintain a fixed phase while tuning the reflectivity, Mach-Zehnder interferometers (MZIs) are employed as the primary building blocks for reflectivity tuning in constructing large-scale circuits. However, MZIs are prone to fabrication errors due to the need for perfect balanced directional couplers to achieve 0-1 reflectivity, which hinders their scalability. In this study, we introduce a design of a TDC based on coupling constant tuning in the thin film Lithium Niobate platform and present an optimized design. Our optimized TDC design enables arbitrary reflectivity tuning while ensuring a consistent phase across a wide range of operating wavelengths. Furthermore, it exhibits fewer bending sections than MZIs and is inherently resilient to fabrication errors in waveguide geometry and coupling length compared to both MZIs and conventional TDCs. Our work contributes to developing high-performance photonic integrated circuits with implications for various fields, including optical communication systems and quantum information processing.

Indigenous Development of MIC-Based C-Band Dual Chain Down Converter Unit for GEOSAT- Spacecraft Checkout System

As part of miniaturization, the development work for the C-band dual-chain down converter, which is extensively used in TM (Telemetry) data acquisition system in GEOSAT spacecraft checkout system has been taken up in MIC (Microwave Integrated Circuit) approach using microstrip line structure. A prototype unit is realized using standard surface mount devices (SMD) and few indigenously developed MIC components namely, the directional couplers, the power dividers etc. The MIC-based down converter module, which forms the heart of the unit is housed in an Aluminium milled box having the dimension of 150 x 150 x 25 mm3. Except internal Local Oscillator (LO), RF Switches and Power supply, all other circuitry are mounted withinthis milled box which is housed in 1U chassis. It meets all the requirements of a conventional down converter. The paper describes the salient features of the unit, its detailed design approach and realization plan and finally the test and evaluation results of the prototype unit developed indigenously.

Quasi-Optical Four-Port Acoustic Filters Based on NEMS Coupled Beam Arrays.

Theoretical models are presented for quasi-optical four-port acoustic devices based on NEMS-coupled beam arrays. Analogies with coupled mode devices in microwaves, ultrasonics, optics, and electron wave optics are first reviewed, together with coupled beam filters. Power transfer between two mechanically coupled, electrostatically driven, coupled beam arrays is then demonstrated using a lumped element model, and the conditions for full power transfer are established. Four-port devices, including directional couplers and coupler filters with complementary transmission ports, are then demonstrated. Predictions are verified for realistic device layouts using the stiffness matrix method.

A novel compact directional coupler with high directivity based on CRLH transmission lines

Designing and fabricating a directional coupler with composite right/left-hand (CRLH) transmission lines with a coupling level of 15 dB is presented in this paper. Compared to conventional couplers, this asymmetric structure has achieved this coupling level compared to the simple transmission line. This structure is comprised of interdigital capacitors and short connection stubs. The distance between two coupled lines is equal to 0.4 mm, and the stubs are spirally designed to compress the structure. The subs’ length is approximately equal to λ/4, and the impedance of the ports is equal to 50 Ω. After designing and simulating, the coupler was fabricated and measured. The directional coupler at the frequency of 1.9 GHz has an isolation level of 44 dB and a directivity of 67.1 dB. Overall, construction and simulation results are in good agreement.

Ultracompact polarization beam splitter based on hybrid plasmonic waveguide

A compact polarization beam splitter (PBS) based on a three-waveguide plasmonic directional coupler was proposed and modeled. The directional coupler comprised two common silicon-on-insulator (SOI) ridge waveguides, with a metal-capped plasmonic SOI waveguide sandwiched between them. The geometrical structure of the three-waveguide directional coupled waveguide was optimized, realizing the phase matching of transverse magnetic (TM) modes but not of transverse electric (TE) modes. The structure and performance of the PBS was optimized using a finite-difference time-domain method. Simulation results showed that the birefringence comparison of TE and TM modes in a hybrid plasmonic waveguide was enhanced using a dielectric loading waveguide to obtain an ultra-compact structure; the final optimized coupling length of the device was 6.1 µm. At the center wavelength of 1.55 µm, the polarization extinction ratio of the TE and TM modes was 34.57 dB and 23.24 dB, and the insertion loss was 0.01 dB and 0.13 dB, respectively.

Selecting robust silicon photonic designs after Bayesian optimization without extra simulations

Optimization methods are frequently exploited in the design of silicon photonic devices. In this paper, we demonstrate that pushing the objective function to its minimum during optimization often results in devices that gradually become more sensitive to perturbations of design variables. The dominant strategy of selecting the design with the smallest objective function can lead to fabrication failure or yield loss due to manufacturing process variations. To address this issue, we propose an intuitive selection criterion that can identify designs not only possessing small objective functions but that are also robust to variations. Our simulation results on the Y-splitter, direction coupler, and bent waveguide designs demonstrate that the proposed method can achieve 2x higher coverage of robust designs with almost negligible run time, compared to the two baseline methods.

Silicon nitride directional coupler-based polarization beam splitter utilizing shallow ridge waveguides for improved fabrication tolerance

This study presents the design and fabrication of a silicon nitride (Si3N4) asymmetrical directional coupler (DC)-based polarization beam splitter (PBS) utilizing a shallow ridge waveguide structure. The use of Si3N4 with a moderate refractive index contrast and the design of a shallow ridge waveguide enables the DC structure to have a wider coupling gap of approximately 600 nm, which can be readily achieved with standard I-line lithography. The simulation results indicate that the polarization splitting is highly efficient with minimal insertion loss. This is corroborated by the experimental measurements, which show consistent broadband performance. The fabricated polarization beam splitter (PBS) exhibits a high polarization extinction ratio (PER) of over 20 dB for both transverse electric (TE) and transverse magnetic (TM) modes across a broad bandwidth of 120 nm (1500-1620 nm). This work demonstrates the potential of shallow ridge Si3N4 waveguides to enhance the fabrication tolerance and performance of integrated photonic devices.

A photonic integrated chip for distinguishing the optical wave packets based on a neural-network

In this work, we use an optical-neural network to recognize a series of optical triangular and square wave packets. This chip is designed based on the reservoir computing, consisting of the directional couplers, phase shifters, Mach-Zehnder modulators, spiral waveguide and grating couplers. The non-linear activation function is obtained by the directional couplers. The optimization of the optical reservoir computing system is performed by tuning the voltage on the phase shifters to obtain the largest extinction ratio of the output signals for different input wave packets. The photonic integrated chip is fabricated on silicon-on-insulator and is characterized at an operation frequency of 3 GHz. The results demonstrate that the photonic integrated chip can efficiently distinguish between optical triangular and square wave packets.

Low-voltage-tunable electromechanical photonic directional coupler in silicon nitride for telecom wavelengths.

Silicon nitride is a low-loss photonic integrated circuit (PIC) platform. However, silicon nitride also shows small nonlinear optical properties and is dielectric, which makes the implementation of programmability challenging. Typically, the thermo-optic effect is used for this, but modulators based on this effect are often slow and cross talk-limited. Here, we present a different approach to programmability in silicon nitride photonics. Micro-electromechanical elements are added to a photonic directional coupler, forming two H-shaped structures. The coupling can be changed by applying a voltage to electrodes placed onto the H-structure, which are then attracted by an electrostatic force. These suspended directional couplers show an insertion loss of 0.67 dB and demonstrate switching with 1.1±0.1 µs rise times, representing a valuable addition to the thermal photonic modulators in silicon nitride technology that offer higher modulation speeds while keeping a comparable insertion loss.

Mode-division multiplexing for visible photonic integrated circuits.

Visible wavelength photonic integrated circuits (PICs) are critical for a wide range of applications including quantum photonics, high-resolution imaging, optogenetics, and portable displays. These applications require functions such as wavefront structuring and dense optical routing on-chip to serve as compact optical interfaces for qubits and cells. The transverse spatial modes of a waveguide can provide the basis for these functions. However, the excitation of these modes in visible PICs has been limited due to fabrication challenges at shorter wavelengths. Here we demonstrate mode-division multiplexing of three higher-order waveguide modes at visible wavelengths (473 nm) with low crosstalk for the first time, to our knowledge. We use adiabatic linearly tapered asymmetric directional couplers that have high theoretical bandwidths of greater than 100 nm and fabrication tolerance to width variations of greater than 45 nm for future integration into large-scale visible PICs with operation across the red, blue, and green spectrum.

Mode Conversion Trimming in Asymmetric Directional Couplers Enabled by Silicon Ion Implantation.

An on-chip asymmetric directional coupler (DC) can convert fundamental modes to higher-order modes and is one of the core components of mode-division multiplexing (MDM) technology. In this study, we propose that waveguides of the asymmetric DC can be trimmed by silicon ion implantation to tune the effective refractive index and facilitate mode conversion into higher-order modes. Through this method of tuning, transmission changes of up to 18 dB have been realized with one ion implantation step. In addition, adjusting the position of the ion implantation on the waveguide can provide a further degree of control over the transmission into the resulting mode. The results of this work present a promising new route for the development of high-efficiency, low-loss mode converters for integrated photonic platforms, and aim to facilitate the application of MDM technology in emerging photonic neuromorphic computing.

Low-Loss Silicon Directional Coupler With Arbitrary Coupling Ratios for Broadband Wavelength Operation Based on Bent Waveguides

Low-Loss Silicon Directional Coupler With Arbitrary Coupling Ratios for Broadband Wavelength Operation Based on Bent Waveguides

Ultra‐Wideband Terahertz Integrated Polarization Multiplexer

AbstractPolarization‐division multiplexing is crucial for increasing channel capacity and spectral efficiency in communications. A key component is the polarization multiplexer, which combines or separates signals with orthogonal polarizations. Existing planar multiplexers lack an ultrawide band for terahertz communications. While microwave‐inspired orthomode transducers (OMTs) provide broadband operation, they suffer from high ohmic loss and bulkiness at terahertz frequencies. Optical integrated polarization beam splitters (PBSs) offer low loss and good integrability but have narrow bandwidths. This work presents a substrateless all‐silicon polarization multiplexer based on tapered directional couplers and air‐silicon effective mediums, integrated monolithically on a compact footprint. The device demonstrates a 37.8% fractional bandwidth, an average insertion loss of 1 dB, and a polarization extinction ratio above 20 dB over 225–330 GHz. This superior performance is enabled by the anisotropy of the effective medium claddings, affecting the two orthogonal guided modes differently. The multiplexer enables simultaneous two‐channel terahertz fiber communications with polarization diversity. It supports aggregated data rates up to 155 and 190 Gbit with bit error rates below hard‐ and soft‐decision forward‐error‐correction limits, respectively. This advancement holds promise for 6G terahertz integrated systems with enhanced channel capacities.

Ultra-Broadband Mode (De)Multiplexer on Thin-Film Lithium Niobate Platform Adopting Phase Control Theory.

Mode (de)multiplexers (MDMs) serve as critical foundational elements within systems for facilitating high-capacity communication, relying on mode conversions achieved through directional coupler (DC) structures. However, DC structures are challenged by dispersion issues for broadband mode coupling, particularly for high-order modes. In this work, based on the principles of phase control theory, we have devised an approach to mitigate the dispersion challenges, focusing on a thin-film lithium niobate-on-onsulator (LNOI) platform. This solution involves integrating a customized inverse-dispersion section into the device architecture, offsetting minor phase shifts encountered during the mode coupling process. By employing this approach, we have achieved broadband mode conversion from TE0 to TE1 and TE0 to TE2 within a 300 nm wavelength range, and the maximum deviations were maintained below -0.68 dB and -0.78 dB, respectively. Furthermore, the device exhibited remarkably low crosstalk, reaching down to -26 dB.

Branch Waveguide Couplers with a frequency of 510 GHz for Terahertz Transmit/Receive Isolation Applications.

To address the requirement of functioning as a transmit/receive isolation device in terahertz transceiver systems, in this paper, we present two high-isolation multi-branch waveguide directional couplers operating at a center frequency of 510 GHz. One is a high-performance five-branch directional coupler, and the other is a new type of three-branch waveguide coupler with lower processing difficulty. Both couplers were fabricated using low-cost CNC milling technologies. The performance of these couplers was verified through measurement results, demonstrating high isolation at the center frequency.

Topological polarization beam splitter based on directional coupling between asymmetric valley photonic crystal waveguides.

Compared with symmetric directional couplers (DCs), asymmetric DCs constructed by two or over two different parallel waveguides offer a more flexible structure and allow for easier expansion of mode channels. In this Letter, we propose a kind of asymmetric topological DC based on two different valley photonic crystal waveguides (VPCWs). According to the coupled-mode theory, phase matching induces the complete coupling for the guide modes of two different VPCWs, whereas significant phase mismatching indicates no coupling occurs. Furthermore, the asymmetric topological DCs exhibit backscattering immunity and anti-disturbance robustness owing to the topological edge states (TESs), which greatly improve the performance of asymmetric DCs. We further design a new, to the best of our knowledge, kind of topological polarization beam splitter (TPBS) at the communication wavelength of 1550 nm by ensuring that the transverse electric mode satisfies the phase matching condition, while the transverse magnetic mode is phase-mismatched. The simulated results demonstrate that the proposed TPBS exhibits highly effective polarization separation and is robust against defects. This design holds significant potential for applications in optical communication systems.

Broadband and low-cross talk mode multiplexer based on subwavelength-assisted phase matching.

Integrated mode multiplexers are key components in photonic networks and are crucial to achieving higher data density. Asymmetric directional couplers represent one of the most common multiplexing architectures, owing to their low losses and scalability, but further bandwidth enhancements are sought after. In this Letter, we demonstrate a multiplexer/demultiplexer link based on asymmetric directional couplers assisted by subwavelength grating metamaterials. Through subwavelength-assisted phase matching, low losses and cross talk are achieved on a small form factor over a broad bandwidth. Experimental measurements show insertion losses under 0.8 dB and cross talk under -19.8 dB in a 170 nm bandwidth (1500 nm to 1670 nm) for a full multiplexer/demultiplexer link.

Inverse-designed compact silicon waveguide reflector for on-chip resonators

Waveguide reflectors are crucial components in integrated optics and on-chip resonators, with their efficiency and dimensions significantly impacting system performance and integration. This paper presents an compact silicon on-chip waveguide reflector, designed using photonics inverse design, featuring a footprint of only 1.64μm×1.78μm and a simple, smooth structure. The simulation shows a 1 dB bandwidth over 400 nm and a reflectivity above 97.6% in the 1520–1570 nm wavelength range. Experimental measurements, conducted through Fabry-Pérot resonator spectrum analysis, confirm the excellent performance of the waveguide reflector, attaining a reflectivity over 92.8%. Additionally, the study presents two more waveguide reflectors tailored for the 1.31μm and 2μm wavelength, emphasizing the scalability of the design. The compact size and broadband reflectivity characteristics of the waveguide reflector provide increased flexibility in designing cavity lengths and directional couplers, making it a promising choice for applications in resonators and highly integrated photonics systems.