Vertical Directional Coupler Research Articles

Vertical directional coupling based grating emission engineering for optical phased arrays

In this Letter, a novel, to the best of our knowledge, vertical directional coupling waveguide grating (VDCWG) architecture is proposed to increase the length of waveguide grating antennas for large aperture on-chip optical phased arrays (OPAs). In this new architecture, the grating emission strength is engineered by the vertical directional coupler, which provides additional degrees of design freedom. Theoretical analysis and numerical simulation show that the VDCWG can adjust the grating strength in the range of more than two orders of magnitude, corresponding to an effective grating length more than a centimeter. For proof-of-concept, a VDCWG antenna with a length of 1.5 mm is experimentally demonstrated. The grating strength is measured to be 0.17 mm−1, and the far-field divergence angle is 0.061°. A 16-channel OPA is also developed based on the proposed VDCWG, which proves the potential of the new architecture for large aperture OPAs.

Silicon nitride stoichiometry tuning for visible photonic integrated components

In integrated photonics, silicon nitride-based devices operating in the visible range of light may experience auto-fluorescence, an undesired effect that can interfere with the propagating signal. In this article, a reduction in auto-fluorescence has been obtained by studying stoichiometric and silicon-rich silicon nitride, subjected to different post-thermal annealings in different atmospheres. Stoichiometric silicon nitride treated with rapid thermal annealing at 1100 °C in an argon atmosphere reduces the photoluminescence intensity of the material by 95%. Silicon-rich nitride shows a more stable photoluminescence response to different annealings and atmospheres than the stoichiometric. Compared to the stoichiometric material, the emission peaks experienced by the silicon-rich silicon nitride are red shifted between 140 and 190 nm, and the refractive index value is increased by 7% at 633 nm. Also, the interface effects have been studied, showing a remarkable contribution when the annealing is performed in an argon atmosphere, while no contribution from these effects is observed in a nitrogen atmosphere. Finally, taking advantage of the refractive index variation between nitrides, a vertical directional coupler using two asymmetric waveguides, one of each type of silicon nitride, has been designed and simulated, obtaining a coupling length of 9.8 μm with a coupling power of 95.8%, demonstrating the 3D integration capabilities of combining silicon nitride layers of variable composition.

Ultra-Broadband and Low-Modal-Crosstalk Mode Multiplexer Based on Cascaded Vertical Directional Couplers Formed by Adiabatic-Tapered Waveguides Without Mode Conversion

Ultra-Broadband and Low-Modal-Crosstalk Mode Multiplexer Based on Cascaded Vertical Directional Couplers Formed by Adiabatic-Tapered Waveguides Without Mode Conversion

Co-planar arbitrary ratio optical power splitter based on cascaded hybrid-core vertical directional couplers for arbitrary guide modes

We propose a co-planar optical power splitter based on cascaded hybrid-core vertical directional couplers, which serves to split arbitrary ratio power of arbitrary guide modes. We fabricate the optical power splitter using micro-fabrication facilities. Our fabricated chip demonstrates power splitting for the E11, E21 and E12 modes between two few-mode waveguides. The experimental device shows that optical power splitting ranges can be larger than 11.0 dB, 12.1 dB and 11.3 dB for the E11, E21 and E12 modes, respectively, over the C + L band and beyond. The proposed optical power splitter could be used in the mode division-multiplexing systems where power splitting for arbitrary guide modes in few-mode waveguide is required in the ultra-broadband wavelength range for mode manipulating, such as mode power distribution, mode dependent loss equalization and all-optical MIMO signal processes.

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

Three-dimensional mode multiplexer based on adiabatic-tapered waveguides forming vertical directional couplers over C + L band and beyond.

We present a mode multiplexer based on vertical directional couplers that are formed by adiabatic-tapered waveguides. We design and fabricate the device via the micro-fabrication processing to (de)multiplex the E11, E21, and E12 modes from the few-mode bus waveguide. Our experimental device shows a coupling ratio higher than 98.6% and 97.0% for the E21 and E12 modes, respectively, over the C + L band and beyond. The modal cross talk of this device can be lower than -17.1 dB, -18.4 dB, and -15.1 dB caused by the unintended E11, E21, and E12 modes, respectively. This mode multiplexer can work over a broader wavelength range with weak polarization sensitivity, which could be used in the mode-division-multiplexing systems where mode (de)multiplexing is required in the expanded communication wavelength window other than the C-band.

Low Power Consuming Mode Switch Based on Hybrid-Core Vertical Directional Couplers with Graphene Electrode-Embedded Polymer Waveguides

We propose a mode switch based on hybrid-core vertical directional couplers with an embedded graphene electrode to realize the switching function with low power consumption. We designed the device with Norland Optical Adhesive (NOA) material as the guide wave cores and epoxy polymer material as cladding to achieve a thermo-optic switching for the E11, E21 and E12 modes, where monolayer graphene served as electrode heaters. The device, with a length of 21 mm, had extinction ratios (ERs) of 20.5 dB, 10.4 dB and 15.7 dB for the E21, E12 and E11 modes, respectively, over the C-band. The power consumptions of three electric heaters were reduced to only 3.19 mW, 3.09 mW and 2.97 mW, respectively, and the response times were less than 495 µs, 486 µs and 498 µs. Additionally, we applied such a device into a mode division multiplexing (MDM) transmission system to achieve an application of gain equalization of few-mode amplification among guided modes. The differential modal gain (DMG) could be optimized from 5.39 dB to 0.92 dB over the C-band, together with the characteristic of polarization insensitivity. The proposed mode switch can be further developed to switch or manipulate the attenuation of the arbitrary guided mode arising in the few-mode waveguide.

Tunable Mode-Dependent Loss Equalizer Based on Hybrid-Core Vertical Directional Couplers

Tunable Mode-Dependent Loss Equalizer Based on Hybrid-Core Vertical Directional Couplers

Tunable delay line based on high-performance three-dimensional switches using graphene heaters

We propose a novel, to the best of our knowledge, tunable delay line based on high-performance three-dimensional (3-D) switches driven by graphene heaters. The 3-D switches, which are constructed with a vertical directional coupler (VDC), provide continuous tuning functionality of the proposed delay line by tuning the coupling ratio of the VDC. The use of the graphene electrode by placing it in direct contact with the upper waveguide core, combined with the air slots formed on both sides of the waveguides, causes the switch to have a quite low power consumption of 3.6 mW. Our designed device, which has a footprint of 20 m m × 6.8 m m , is capable of providing a continuous tunable delay from 0 to 150 ps with minimum operating bandwidth of > --> 3.34 G H z , from 0 to 100 ps with minimum operating bandwidth of > --> 5 G H z , and from 0 to 50 ps with minimum operating bandwidth of > --> 10 G H z . The cross talk for different delay cases is less than − 21.6 d B over the whole C-band and less than − 45.2 d B at 1550 nm.

State-of-the-Art and Perspectives on Silicon Waveguide Crossings: A Review.

In the past few decades, silicon photonics has witnessed a ramp-up of investment in both research and industry. As a basic building block, silicon waveguide crossing is inevitable for dense silicon photonic integrated circuits and efficient crossing designs will greatly improve the performance of photonic devices with multiple crossings. In this paper, we focus on the state-of-the-art and perspectives on silicon waveguide crossings. It reviews several classical structures in silicon waveguide crossing design, such as shaped taper, multimode interference, subwavelength grating, holey subwavelength grating and vertical directional coupler by forward or inverse design method. In addition, we introduce some emerging research directions in crossing design including polarization-division-multiplexing and mode-division-multiplexing technologies.

Photon-number-resolving segmented detectors based on single-photon avalanche-photodiodes.

We investigate the feasibility and performance of photon-number-resolved photodetection employing single-photon avalanche photodiodes (SPADs) with low dark counts. While the main idea, to split n photons into m detection modes with a vanishing probability of more than one photon per mode, is not new, we investigate here a important variant of this situation where SPADs are side-coupled to the same waveguide rather than terminally coupled to a propagation tree. This prevents the nonideal SPAD quantum efficiency from contributing to photon loss. We propose a concrete SPAD segmented waveguide detector based on a vertical directional coupler design, and characterize its performance by evaluating the purities of Positive-Operator-Valued Measures (POVMs) in terms of number of SPADs, photon loss, dark counts, and electrical cross-talk.

Power divider based on vertical elliptical directional couplers with increased isolation and bandwidth

In this study, a novel high-isolation wideband divider based on multi-layer elliptical defected ground structure coupler is presented along with its analytical theory and design procedure. The proposed divider could be conveniently applied for high- power applications with less loss for the absence of isolation resistors. The simulation results show that in 3.1–10.6 GHz, the return loss S11 and S22 are −4 dB within 4–9 GHz, and the amplitude and phase difference between port 2 and port 3 is <1 dB and 2°, respectively.

High-Order-Mode-Pass Mode (De)Multiplexer With a Hybrid-Core Vertical Directional Coupler

We propose a high-order-mode-pass mode (de)multiplexer based on the structure of a vertical waveguide directional coupler formed with a few-mode core and a single-mode core that has a higher refractive index. Unlike a conventional directional coupler, where the two cores have the same refractive index, the hybrid-core structure allows the design of a directional coupler to couple only the fundamental mode of a few-mode core to a single-mode core without affecting the high-order modes of the few-mode core. To demonstrate the idea, we design and fabricate a vertical coupler formed with a three-mode core and a single-mode core with polymer material. Our experimental device has a length of 15 mm and a coupling ratio for the fundamental mode varying from 92.5% to 98.7% in the C-band (from 1530 to 1565 nm), which corresponds to a relative residual power of the fundamental mode in the three-mode core varying from -10.9 to -18.9 dB. The crosstalks to the single-mode core from the two high-order modes of the three-mode core are smaller than -13.8 dB in the C-band. The performance of the device is polarization insensitive. The proposed hybrid-core mode (de)multiplexer allows an effective control of the fundamental mode of a few-mode waveguide and thus opens up new possibilities for the design of mode-controlling devices, such as mode-group (de)multiplexers, mode-dependent-loss compensators, and mode add-drop multiplexers, for mode-division-multiplexing applications based on circular or elliptical few-mode fibers.

Thermo-Optically Controlled Vertical Waveguide Directional Couplers for Mode-Selective Switching

We propose a three-dimensional waveguide structure for the realization of mode-selective switches, where the high-order spatial modes of a few-mode waveguide can be switched to various single-mode waveguides with thermo-optically controlled vertical asymmetrical directional couplers. To demonstrate the idea, we design and fabricate two specific devices with a polymer material for a waveguide that supports three spatial modes. The two devices differ in the application of the thermo-optic effect to deactivate or activate the directional couplers. For one device, the switching powers measured at the wavelength 1550 nm are lower than 20.6 mW and the switching times are shorter than 4.4 ms. The extinction ratios are higher than 15.6 dB across the C-band measured at fixed switching powers. For the other device, the switching powers measured at 1550 nm are lower than 22.6 mW, the switching times are shorter than 2.9 ms, and the extinction ratios are higher than 14.1 dB in the C-band. The performances of the devices are weakly sensitive to the polarization state of light. Our proposed mode-selective switches, which have a scalable configuration and require low power consumption, could be developed into various active mode-controlling devices for applications in reconfigurable mode-division-multiplexing networks.

Refractive Index Sensing Using Silicon-on-Insulator Waveguide Based Directional Coupler

We propose a refractive index (RI) sensor based on a vertical directional coupling between two silicon-on-insulator (SOI) waveguides. It is shown that by optimizing the SOI waveguide parameters, high-bulk and adlayer sensitivities can be achieved. Using the optimized waveguide parameters for the aqueous cover, we report the bulk sensitivity 7000 (3400) nmRIU for RI1.33, and the adlayer sensitivity 9 (5) nmnm for an adsorbed bio layer of RI 1.45, for the transverse electric (TE) (transverse magnetic (TM)) mode propagation in the optical communication wavelength band. The proposed sensor has the potential for bio sensing applications due to high-bulk and adlayer sensitivities.

Mode Multiplexer With Cascaded Vertical Asymmetric Waveguide Directional Couplers

We report a mode (de)multiplexer based on the structure of cascaded vertical waveguide directional couplers, where the spatial modes of a few-mode core are coupled to various single-mode cores placed above the few-mode core. Using five cascaded directional couplers, we design and fabricate a six-mode (de)multiplexer with polymer material that can spatially combine or separate the E11, E21, E12, E22, E31, and E13 modes. A typical fabricated device, which has a length of 29 mm, provides coupling ratios varying from 62% to 90% and modal crosstalks from −28.2 to −11.6 dB in the wavelength range from 1530 to 1565 nm with weak polarization dependence. The proposed structure is simple and scalable and could be developed into a range of mode-controlling devices for mode-division-multiplexing applications based on few-mode fibers.

Polarization-independent and colorless switching operation of three-dimensional 4 × 4 polymer optical switch with two-layered waveguide structure

We fabricated a three-dimensional (3D) 4 × 4 polymer optical interconnection switch with a two-layered waveguide structure equipped with four Mach–Zehnder interferometer optical switches (MZ-OSs), two vertical directional couplers (VDCs), and a multimode interferometer (MMI) coupler. The experimental result for the device shows polarization-independent and colorless switching operation by the thermo-optic (TO) effect in the 1550-nm-wavelength range with a switching power of about 18 mW.

Three-Dimensional Analysis of an Ultrashort Optical Cross-Bar Switch Based on a Graphene Plasmonic Coupler

A high-performance electro-optical cross-bar switch based on a vertical graphene plasmonic directional coupler is proposed. The structure consists of two pattern-free graphene plasmonic waveguides which have major advantages over their previous counterparts. An ultrashort switching length of just 223 nm is obtained at the frequency of 37.5 THz. To the best of our knowledge, this switching length is remarkably shorter than those in the recently reported structures. Our designed structure is simulated using a three-dimensional finite-difference time-domain method. According to the illustrated results, extinction ratios of 10.5 and 9.52 dB are obtained at the cross and bar ports, respectively, and a wide spectral width of more than 1 μ m is presented. The required switching voltage is also calculated as 6 V and a very high 3 dB bandwidth of more than 60 GHz is obtained. An extremely low switching energy of 3.8 fJ/bit is also estimated.

Low-loss Electrically Controllable Vertical Directional Couplers

We propose a nearly lossless, compact, electrically modulated vertical directional coupler, which is based on the controllable evanescent coupling in a previously proposed graphene-assisted total internal reflection (GA-FTIR) scheme. In the proposed device, two single-mode waveguides are separate by graphene-SiO2-graphene layers. By changing the chemical potential of the graphene layers with a gate voltage, the coupling strength between the waveguides, and hence the coupling length of the directional coupler, is controlled. Therefore, for a properly chosen, fixed device length, when an input wave is launched into one of the waveguides, the ratio of their output powers can be controlled electrically. The operation of the proposed device is analyzed, with the dispersion relations calculated using a model of a one-dimensional slab waveguide. The supermodes in the coupled waveguide are calculated using the finite-element method to estimate the coupling length, realistic devices are designed, and their performance was confirmed using the finite-difference time-domain method. The designed 3 μm by 1 μm device achieves an insertion loss of less than 0.11 dB, and a 24-dB extinction ratio between bar and cross states. The proposed low-loss device could enable integrated modulation of a strong optical signal, without thermal buildup.

Analysis on vertical directional couplers with long range surface plasmons for multilayer optical routing

A structure featuring vertical directional coupling of long-range surface plasmon polaritons between strip waveguides at λ = 1.55 μm is investigated with the aim of producing efficient elements that enable optical multilayer routing for 3D photonics. We have introduced a practical computational method to calculate the interaction on the bent part. This method allows us both to assess the importance of the interaction in the bent part and to control it by a suitable choice of the fabrication parameters that helps also to restrain effects due to fabrication issues. The scheme adopted here allows to reduce the insertion losses compared with other planar and multilayer devices.