Asymmetric Directional Coupler Research Articles

Deterministic quasi-continuous tuning of phase-change material integrated on a high-volume 300-mm silicon photonics platform

Programmable photonic integrated circuits (PICs) consisting of reconfigurable on-chip optical components have been creating new paradigms in various applications, such as integrated spectroscopy, multi-purpose microwave photonics, and optical information processing. Among many reconfiguration mechanisms, non-volatile chalcogenide phase-change materials (PCMs) exhibit a promising approach to the future very-large-scale programmable PICs, thanks to their zero static power and large optical index modulation, leading to extremely low energy consumption and ultra-compact footprints. However, the scalability of the current PCM-based programmable PICs is still limited since they are not directly off-the-shelf in commercial photonic foundries now. Here, we demonstrate a scalable platform harnessing the mature and reliable 300 mm silicon photonic fab, assisted by an in-house wide-bandgap PCM (Sb2S3) integration process. We show various non-volatile programmable devices, including micro-ring resonators, Mach-Zehnder interferometers and asymmetric directional couplers, with low loss (~0.0044 dB/µm), large phase shift (~0.012 π/µm) and high endurance (>5000 switching events with little performance degradation). Moreover, we showcase this platform’s capability of handling relatively complex structures such as multiple PIN diode heaters in devices, each independently controlling an Sb2S3 segment. By reliably setting the Sb2S3 segments to fully amorphous or crystalline state, we achieved deterministic multilevel operation. An asymmetric directional coupler with two unequal-length Sb2S3 segments showed the capability of four-level switching, beyond cross-and-bar binary states. We further showed unbalanced Mach-Zehnder interferometers with equal-length and unequal-length Sb2S3 segments, exhibiting reversible switching and a maximum of 5 (N+1,N=4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$N+1,N=4$$\\end{document}) and 8 (2N,N=3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${2}^{N},N=3$$\\end{document}) equally spaced operation levels, respectively. This work lays the foundation for future programmable very-large-scale PICs with deterministic programmability.

Compact multichannel reconfigurable mode demultiplexer enabled by phase change material

The reconfigurable mode demultiplexer is a crucial component for flexibly routing modes into different channels in on-chip multimode photonic systems with enhanced information processing capabilities. In this paper, we present a multichannel reconfigurable mode demultiplexer enabled by ultralow-loss phase-changing Sb2Se3. By harnessing the phase-change-mediated mode coupling in asymmetric directional couplers (ADCs), one or more of the higher-order modes including TE1, TE2 and TE3 modes could be selectively dropped from the bus waveguide with low losses. With an optimized ADCs structure, the proposed mode demultiplexer demonstrates insertion loss less than 0.227 dB in the ON (amorphous) state and the extinction ratios large than 23.28 dB over the C-band. By coupling the access waveguides of the higher-order mode in parallel on both sides of the bus waveguide, the device size can be compact with a footprint of ∼ 7 × 75 µm2, and this design approach can be further extended to enable more higher-order mode multiplexing.

Thermo-optic reconfigurable three-mode (de)multiplexer based on an asymmetrical horizontal directional coupler

In this paper, we propose a thermo-optic reconfigurable three-mode (de)multiplexer based on an asymmetrical horizontal three-waveguide directional coupler that includes two identical single-mode waveguides and a three-mode waveguide. Over the whole wavelength range of 1540–1560 nm, and for the TE (TM) polarization, our typical fabricated device with polymer material shows coupling efficiencies as high as 94% (93%) and 93% (92%) for the mode conversions of LP01−LP11a and LP01−LP11b, with the heating powers of 53.57 mW and 71.19 mW, respectively. Our proposed device can be employed in the fields of reconfigurable mode-division multiplexing systems.

Tunable O-band high-tolerance silicon mode multiplexer based on phase-mismatched asymmetric directional couplers

Small silicon-on-insulator (SOI)-based tunable O-band TE0-TE1 and TE0-TE2 mode multiplexers (MUXs) based on phase-mismatched asymmetric directional couplers (ADCs) are theoretically and experimentally demonstrated. We designed ADCs with an intentionally phase-mismatched coupling region to suppress wavelength dependence and improve fabrication error tolerance. The fabricated O-band TE0-TE2 mode MUX can maintain more than −0.71dB transmission by heating. Moreover, we proposed three-section tapered ADCs (3STADCs) divided into three regions. We theoretically show that the TE0-TE1 and TE0-TE2 mode MUXs based on the 3STADC have more than −0.28dB and −0.36dB transmission within ±10nm waveguide width fabrication error, respectively.

On-chip photonic convolution by phase-change in-memory computing cells with quasi-continuous tuning

Matrix multiplication acceleration by on-chip photonic integrated circuits (PICs) is emerging as one of the attractive and promising solutions, offering outstanding benefits in speed and bandwidth as compared to non-photonic approaches. Incorporating nonvolatile phase-change materials into PICs or devices enables optical storage and computing, surpassing their electrical counterparts. In this paper, we propose a design of on-chip photonic convolution for optical in-memory computing by integrating the phase change chalcogenide of Ge 2 Sb 2 Se 4 Te 1 (GSST) into an asymmetric directional coupler for constructions of an in-memory computing cell, marrying the advantages of both the large bandwidth of Mach-Zehnder interferometers (MZIs) and the small size of micro-ring resonators (MRRs). Through quasi-continuous electro-thermal tuning of the GSST-integrated in-memory computing cells, numerical calculations about the optical and electro-thermal behaviors during GSST phase transition confirm the tunability of the programmable elements stored in the in-memory computing cells within [-1, 1]. For proof-of-concept verification, we apply the proposed optical convolutional kernel to a typical image edge detection application. As evidenced by the evaluation results, the prototype achieves the same accuracy as the convolution kernel implemented on a common digital computer, demonstrating the feasibility of the proposed scheme for on-chip photonic convolution and optical in-memory computing.

Mode-selective power splitters for mode-division multiplexing optical networks

We propose an all-fiber mode-selective power splitter (MSPS) for non-circular-symmetric LP lm (l = 1, 2, …) modes, which is suitable for multicasting and optical performance monitoring in mode-division multiplexing optical fiber networks. The MSPSs are asymmetric two-core few-mode directional couplers composed of a few-mode fiber and a two-mode fiber. We theoretically studied the three conditions required by the MSPSs. By carefully choosing the core-to-core distance and coupling length, the MSPS can achieve arbitrary splitting ratio regardless of the modal field orientation of the input non-circular-symmetric LP mode. By using an asymmetric structure, the MSPS can ensure the power splitting only happens on the target non-circular-symmetric LP mode when the phase matching condition is satisfied. In addition, we designed and numerically simulated LP31 MSPSs with four kinds of splitting ratios, among which the one with 90/10 splitting ratio was fabricated based on tapering and polishing method. The fabricated LP31 MSPS is characterized and the results show that its splitting ratio is much more stable than regular LP31 mode-selective coupler.

Inverse design and demonstration of on-chip silicon high-order mode pass filter

We propose a concept of a high-order mode (HOM) pass filter based on the inverse-designed mode-routing, which enables an ultra-compact footprint and broad bandwidth. To validate the concept, we experimentally demonstrate two types of HOM pass filters using the direct-binary search topology optimization algorithm. In the first HOM pass filter, the mode-routing region is constructed using an inverse-designed adiabatic coupler, while the second filter utilizes a tapered asymmetric directional coupler. The subwavelength units based on the functional regions of both filters have an ultra-compact footprint of 4 µm × 800 nm. The experimental results indicate that the insertion losses of two HOM-pass filters are 3.13 and 1.94 dB, respectively, and their mode cross-talks are −15.8 and −27.36 dB at the center wavelength of 1550 nm. Both HOM pass filters exhibit high performance over a broad bandwidth of 130 nm.

Study of Multi-Channel Mode-Division Multiplexing Based on a Chalcogenide-Lithium Niobate Platform

A multi-channel mode-division multiplexing based on a chalcogenide-lithium niobate platform using chalcogenide films with adjustable refractive index is proposed, with the aim of overcoming issues with narrow bandwidth and large crosstalk in conventional multiplexers. An asymmetric directional coupler, employing chalcogenide-based thin-film modulation, was designed to realize the multiplexing and separation of TE1, TE2, and TE3 modes. Simulations show that the device is capable of obtaining an insertion loss of between 0.03 dB and 0.7 dB and a crosstalk of between −21.66 dB and −28.71 dB at 1550 nm. The crosstalk of the TE1, TE2, and TE3 modes is below −20.1 dB when accessing the waveguide output port in the 1500–1600 nm band. The proposed multiplexer is a promising approach to enhance the transmission capability of thin-film lithium-niobate-integrated optical paths.

Design and analysis of a compact and broadband polarization beam splitter on silicon nitride on silicon-on-insulator multilayer platform using a partially overlapped vertical asymmetrical directional coupler

Design and analysis of a compact and broadband polarization beam splitter on silicon nitride on silicon-on-insulator multilayer platform using a partially overlapped vertical asymmetrical directional coupler

Fiber-connectorized ultrafast-laser-inscribed K-band integrated optics beam combiner for the CHARA telescope array.

A fiber-connectorized K-band integrated-optics two-telescope beam combiner was developed for long-baseline interferometry at the CHARA telescope array utilizing the ultrafast laser inscription (ULI) technique. Single-mode waveguide insertion losses were measured to be ∼1.1d B over the 2-2.3µm window. The development of asymmetric directional couplers enabled the construction of a beam combiner that includes a 50:50 coupler for interferometric combination and two ∼75:25 couplers for photometric calibration. The visibility of the bare beam combiner was measured at 87% and then at 82% after fiber-connectorization by optimizing the input polarization. These results indicate that ULI technique can fabricate efficient fiber-connectorized K-band beam combiners for astronomical purposes.

Design and experimental demonstration of a silicon five-mode (de)multiplexer based on multi-phase matching condition.

A compact 5-mode (de)multiplexer [(De)MUX] is proposed and experimentally demonstrated based on the principle of multi-phase matching. The proposed device comprises a cascaded asymmetric directional coupler (ADC) based on 3-mode phase-matching, a polarization beam combiner, and a taper waveguide connecting them. The multiple modes in the access waveguides are matched to different modes in the same bus waveguide, which eliminates the need for additional taper structures and results in a total coupling length of only 18.9 µm. Experimental results exhibit that the insertion losses of the five modes are below 3.4 dB, and the mode crosstalks are below -15 dB at the central wavelength. The 3-dB bandwidths of TM0, TM1, TE0, TE1, and TE2 modes are greater than 100 nm, 46 nm, 100 nm, 28 nm, and 37 nm, respectively. The proposed device can serve as a key functional component in highly integrated on-chip mode-division multiplexing systems.

Flexible Data Rate Allocation Using Non-Orthogonal Multiple Access (NOMA) in a Mode Division Multiplexing (MDM) Optical Power Splitter for System-on-Chip Networks.

We put forward and demonstrate a silicon photonics (SiPh)-based mode division multiplexed (MDM) optical power splitter that supports transverse-electric (TE) single-mode, dual-mode, and triple-mode (i.e., TE0, TE1, and TE2). An optical power splitter is needed for optical signal distribution and routing in optical interconnects. However, a traditional optical splitter only divides the power of the input optical signal. This means the same data information is received at all the output ports of the optical splitter. The powers at different output ports may change depending on the splitting ratio of the optical splitter. The main contributions of our proposed optical splitter are: (i) Different data information is received at different output ports of the optical splitter via the utilization of NOMA. By adjusting the power ratios of different channels in the digital domain (i.e., via software control) at the Tx, different channel data information can be received at different output ports of the splitter. It can increase the flexibility of optical signal distribution and routing. (ii) Besides, the proposed optical splitter can support the fundamental TE0 mode and the higher modes TE1, TE2, etc. Supporting mode-division multiplexing and multi-mode operation are important for future optical interconnects since the number of port counts is limited by the chip size. This can significantly increase the capacity besides wavelength division multiplexing (WDM) and spatial division multiplexing (SDM). The integrated SiPh MDM optical power splitter consists of a mode up-conversion section implemented by asymmetric directional couplers (ADCs) and a Y-branch structure for MDM power distribution. Here, we also propose and discuss the use of the Genetic algorithm (GA) for the MDM optical power splitter parameter optimization. Finally, to provide adjustable data rates at different output ports after the MDM optical power splitter, non-orthogonal multiple access-orthogonal frequency division multiplexing (NOMA-OFDM) is also employed. Experimental results validate that, in three modes (TE0, TE1, and TE2), user-1 and user-2 achieve data rates of (user-1: greater than 22 Gbit/s; user-2: greater than 12 Gbit/s) and (user-1: greater than 12 Gbit/s; user-2: 24 Gbit/s), respectively, at power-ratio (PR) = 2.0 or 3.0. Each channel meets the hard-decision forward-error-correction (HD-FEC, i.e., BER = 3.8 × 10-3) threshold. The proposed method allows flexible data rate allocation for multiple users for optical interconnects and system-on-chip networks.

Silicon-based compact eight-channel wavelength and mode division (de)multiplexer for on-chip optical interconnects.

A compact wavelength and mode division (de)multiplexer is proposed for multiplexing a total of eight guided TE modes of a 220nm thick silicon-on-insulator waveguide with input channels at two wavelengths of 1.55 and 2µm for wavelength division multiplexing. The (de)multiplexer is composed of four sequentially arranged sections with bus waveguides of increasing widths. The first section uses an asymmetric directional coupler to couple one TE mode at 1.55µm, while each of the next three sections consists of two collocated directional couplers to simultaneously couple two TE modes of the bus waveguide, one at each wavelength of 1.55 and 2µm. Three linear adiabatic tapers are designed to connect the consecutive bus waveguides. The fundamental TE mode of the bus waveguide at 1.55 or 2µm is coupled by using another adiabatic taper from a single-mode input waveguide. The simulation results show that over a broad bandwidth of >100n m the insertion loss and crosstalk for both wavelength bands is <1.15d B and <-27d B, respectively. In addition, a compact device footprint with a total coupling length of ∼61µm is achieved due to the use of collocated directional couplers in three sections.

A silicon micro-ring resonator with unprecedented large free spectral range via double injection

A silicon-on-insulator submicron multimode micro-ring resonator symmetrically coupled to two bus waveguides via bent asymmetrical directional coupler to achieve unprecedented-large FSR is proposed based on double-injection configuration. An analytical model based on the transfer matrix method is developed to analyze the proposed structure and the physical mechanism for the FSR enlargement is thoroughly analyzed. In addition, the coupling angle is cleverly selected to improve the spectral shape and the extinction ratio at the resonant wavelengths. Finite difference time domain simulations show that ultra large FSRs of 180 nm and 206 nm can be obtained.

Design and analysis of all-optical half and full adder/subtractor using nonlinear asymmetric directional couplers

The utilization of nonlinear asymmetric directional couplers has emerged as a viable alternative to conventional systems for the execution of arithmetic operations using optical signals. The present study explores the utilization of a coupler-based framework for the development of an all-optical half- and full-adder/subtractor circuit designed with the basic logic gates (AND, OR, XOR) and a soliton pulse as input signals. Nonlinear asymmetric directional couplers have been analysed theoretically and numerically, including the detailed mathematics of coupled wave theory. The switching characteristics of XPM effect based All-optical directional couplers have been examined for appropriate values of the phase shift (∆φ). Appropriate values of ∆φ, extinction ratio (dB), and corresponding controlled pump signal levels and other parameters are investigated for an efficient generation of fundamental logic gates. Further, the detailed analysis of layout generation and design aspects of an all-optical half- and full-adder/subtractor circuit along with the basic logic gates circuits has been carried out. Simulation results are validated with the standard result. The research findings demonstrate that soliton pulses might potentially be useful for optical computing applications when used as the input signal to nonlinear asymmetrical directional couplers.

Mode division (de)multiplexer based on transverse mode conversion in photonic crystal using asymmetric directional couplers.

Mode division multiplexing technology has the potential to increase the channel capacity of a single wavelength carrier. Attaining cost-effective high-bandwidth-density devices with small footprints is a concern, and photonic crystal based devices are promising for ultra-small on-chip communications. This paper presents a 2D photonic crystal based mode division (de)multiplexer on a silicon on insulator platform. The device comprises two coupling regions of asymmetric directional couplers that perform mode conversion operations between the fundamental mode and higher-order modes. Each coupling section is dedicated to converting a specific mode. Mode conversion is achieved by designing a multimode waveguide to satisfy the phase-matching condition of the desired mode with the single mode waveguide. Two linear adiabatic tapers are introduced for the smooth transition of modes between waveguide sections. The device is designed and simulated for three-channel modes at 1550nm using the finite-difference time-domain technique. The obtained insertion loss and cross talk are <0.41d B and <-20.14d B, respectively. The overall size of the proposed mode division (de)multiplexer is 328.5µm 2. A fabrication tolerance study for the proposed device is performed by varying the rod radius and position in the device structure's taper and bus waveguide regions. The proposed 2D photonic crystal based mode division (de)multiplexer has the potential to be used in large-capacity optical communication systems.

Silicon-based subwavelength grating polarization beam splitter with asymmetric directional coupler

In this paper, a novel polarization beam splitter (PBS) is proposed. The silicon-based subwavelength grating (SWG) is successfully applied to the asymmetric directional coupled PBS, and the research shows that its performance has improved in many aspects. By carefully optimizing structural parameters, the transverse electric (TE) polarization is in a reflected state in the SWG, which effectively weakens the coupling strength, while the transverse magnetic (TM) polarization is almost unaffected. Simulation results show that the bandwidths of TE polarization and TM polarization are 200 nm (1450–1650 nm) and 141 nm (1450–1650 nm) in the extinction ratio of >20 dB, respectively, while the insertion loss is <0.5 dB and <1.3 dB, respectively. The PBS is very robust to the process production. With the manufacturing error exceeding 20 nm, the performance is only reduced by 0.6 dB, effectively solving the problem of complex preparation process.

Ultra-compact, broadband polarization beam splitter based on x-cut lithium-niobate-on-insulator platform using hybrid plasmonic waveguide

We propose an ultra-compact, high-performance polarization beam splitter (PBS) on the x-cut lithium-niobate-on-insulator (LNOI) platform. The device based on asymmetrical directional coupler (ADC) consists of a single-mode waveguide (SMW) and a hybrid plasmonic waveguide (HPW). Whole length of the device is only 47 μm, which is the shortest PBS on the x-cut LNOI platform due to polarization-dependent characteristic of plasmonics. When choosing proper width of coupling waveguides (WGs), the phase-matching condition can be satisfied in TE polarization while phase-mismatching exists in TM polarization at the same time. Simulation result shows that bandwidth of the device can achieve at least 200 nm with −10 dB extinction for TE polarization and at least 200 nm with less than −20 dB extinction for TM polarization, which successfully covers entire C-band and L-band. Additionally, the device exhibits large fabrication tolerance and easily fabricated characteristic.

Compact hybrid five-mode multiplexer based on asymmetric directional couplers with constant bus waveguide width.

We experimentally demonstrate a hybrid mode division multiplexer (MDM) based on asymmetric directional couplers (ADCs) without transition tapers in between. The proposed MDM can couple five fundamental modes from access waveguides into the bus waveguide as the hybrid modes (TE0, TE1, TE2, TM0, and TM1). To eliminate the transition tapers between cascaded ADCs as well as to enable arbitrary add-drop to the bus waveguide, we maintain the bus waveguide width to be the same, while a partially etched subwavelength grating is introduced to reduce the effective refractive index of the bus waveguide. The experimental results demonstrate a working bandwidth of up to 140 nm.

Compact and broadband polarization beam splitter on lithium-niobate-on-insulator using a hybrid plasmonic waveguide

A compact and broadband polarization beam splitter (PBS) is proposed based on an asymmetric directional coupler consisting of a hybrid plasmonic waveguide and a ridge waveguide on lithium-niobate-on-insulator. Due to the surface plasma polariton (SPP) effect, the phase-matching condition is satisfied for transverse electric polarization, whereas the transverse magnetic polarization has a significant phase mismatch by choosing reasonable widths of both waveguides. A short ( ∼ 28 μm long) PBS is designed while the gap width is chosen to be 200 nm to make it easy to fabricate. Numerical simulations show that the designed PBS has a broad bandwidth (>130 nm) for an extinction ratio of >15 dB and a large fabrication tolerance for the variation of the waveguide width (over ± 50 nm).