Input Waveguide Research Articles

Electroabsorption modulator based on inverted-rib-type silicon waveguide including double graphene layers

We investigate, theoretically, a compact graphene-based electroabsorption modulator (EAM). The compactness of the EAM arises from an inverted-rib-type (IRT) silicon waveguide including a graphene–oxide–graphene stack. The EAM consists of input and output waveguides, which are conventional silicon strip waveguides, and the IRT waveguide efficiently connected to them through tapering regions. The stack is located in the region where the fundamental transverse electric mode of the IRT waveguide is mainly confined. Hence, the IRT waveguide mode strongly interacts with the graphene layers. Moreover, the IRT waveguide can be realized without complex high-precision processes. The calculated modulation depth of the IRT waveguide is 0.41 dB μm−1 when the chemical potential of graphene is tuned between 0.2 and 0.6 eV. It is more than two times larger than those of previous graphene-covered silicon waveguides. The EAM, with a 3 dB extinction ratio, employs an IRT waveguide of length 7–8 μm. This EAM is analyzed and found to have an optical bandwidth of 100 nm, an electrical bandwidth of up to 46.4 GHz, and energy consumption smaller than 630 fJ bit−1. Such EAMs based on IRT waveguides may play an important role in off-chip optical interconnection.

Low power hybrid plasmonic microring-on-disks electro-optical modulators

Different electro-optical modulator designs based on electromagnetically induced transparency are proposed. A conductor–gap–silicon input waveguide is coupled to microrings-on-disks on each side. A low voltage modulating signal is applied to the modulator in a push-pull configuration, which changes the refractive index of the embedded layer of the electro-optical polymer. The proposed microrings-on-disks and cascaded microring modulators with submicron radii can efficiently modulate the light wave with moderate propagation losses. The microring-on-disk modulator achieved ultrasmall capacitance, 1.06 fF, and low power consumption, 2.12 fJ/bit. Both modulators have low insertion losses and high extinction ratios.

GaN directional couplers for on-chip optical interconnect

Here, we propose, fabricate and characterize GaN directional couplers for on-chip optical interconnect on a GaN-on-silicon platform. Suspended InGaN/GaN multiple-quantum-well diodes are adopted for both the transmitter and the receiver, and GaN directional couplers are used to achieve the light coupling between suspended waveguides that are directly connected to the transmitter and the receiver. The proposed on-chip optical interconnects are experimentally demonstrated by light propagation and in-plane data transmission using visible light. The light propagation images directly show that the emitted light can be laterally coupled into the suspended waveguide, and the guided light from the input waveguide then couples to the coupled waveguide due to the overlapped slab. The receiver detects the transmitted light from the coupled waveguide to complete the in-plane visible light communication process, as confirmed by pseudo-random binary sequence data and eye diagrams at the transmission rate of 30 Mbps.

Wideband turnstile junction coaxial waveguide orthomode transducer

A wideband turnstile junction coaxial waveguide orthomode transducer (OMT) is presented in this paper, featuring coaxial waveguide input and orthogonal rectangular waveguide outputs. It primarily comprises of a turnstile junction, bending stepped impedance transformers and power combiners. The symmetrical geometry helps achieve wide operating bandwidth and balanced output phases. The OMT covers the whole Ku band from 12 to 18 GHz, which aims at wideband dual-polarized signal combination and separation within coaxial-type multi-band antenna systems. An experimental prototype is manufactured and the measured results confirm that the reflection coefficient is lower than −15 dB within the whole band, and the port isolation is better than 35 dB. Turnstile junction coaxial waveguide OMTs can be adopted widely in circular/coaxial waveguide hybrid feeding networks for multi-band satellite communication/remote sensing antenna systems.

Inline waveguide to suspended stripline transition with DC/IF return path

ABSTRACTThis article presents an inline waveguide to suspended stripline (SSL) transition. The transition adopts unilateral finline to transform the input waveguide TE10 mode to SSL Quasi‐TEM mode. The module of the transition is split into two halves along the center of the waveguide E‐plane. The substrate is flip mounted on the grooves of the lower half of the module. The finline is extended to the mounting groove and bonded to the waveguide channel. The waveguide feed direction is inline with the SSL wave transmission direction, and DC/IF return path is also provided by the finline. A G‐band prototype is designed and fabricated. The measured results show that the insertion loss is 1.3 to 2 dB at frequencies from 140 to 213 GHz, with return loss more than 10 dB. © 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:729–732, 2017

Mid-infrared subwavelength modulator based on grating-assisted coupling of a hybrid plasmonic waveguide mode to a graphene plasmon.

This work reports a mid-infrared modulator based on a hybrid plasmonic waveguide with graphene on a grating in its slot region. The modulator utilizes a graphene plasmon for electro-optic tuning in a more practical and effective way than graphene-plasmon-based waveguide devices studied up to now. The hybrid plasmonic waveguide can be easily and efficiently integrated with input and output photonic waveguides. It supports a hybrid plasmonic waveguide mode and a graphene-plasmon-based waveguide mode. Grating-assisted coupling of the former to the latter in it is demonstrated to work successfully even though the two modes have significantly different propagation constants and losses. Theoretical investigation of the modulator shows that the coupling via the grating of length 5.92 μm generates a deep rejection band at a wavelength of 8.014 μm in the transmission spectrum of the output photonic waveguide of the modulator. With the graphene chemical potential tuned between 0.6 eV and 0.65 eV, the transmission at the wavelength is modulated between -27 dB and -1.8 dB. The subwavelength modulator, which may have a large bandwidth and small energy consumption, is expected to play a key role in free-space communications and sensing requiring mid-infrared integrated photonics.

Electronic-Photonic Integrated Circuit for 3D Microimaging

An integrated electronic-photonic phase-locked loop (PLL) modulates the frequency of a tunable laser for use in frequency-modulated continuous-wave (FMCW) lidar 3D imaging. The proposed lidar can perform 180k range measurements per second. The rms depth precision is 8 $\mu \text{m}$ at distances of ±5 cm from the range baseline. The range window is 1.4 m, with a precision of 4.2 mm at the edges of the window. Optical circuitry, including input light couplers, waveguides, and photodiodes, is realized on a 3 mm $\times $ 3 mm silicon-photonic chip. The 0.18- $\mu \text{m}$ CMOS ASIC of the same area comprises the front-end transimpedance amplifier, analog electro-optical PLL, and digital control circuitry consuming 1.7 mA from a 1.8 V supply and 14.1 mA from a 5-V supply. The latter includes 12.5-mA bias current for the distributed Bragg reflector section of the tunable laser. The two chips are integrated using through-silicon-vias implemented in the silicon-photonic chip.

Polarization insensitive arrayed-input spectrometer chip based on silicon-on-insulator echelle grating

We demonstrate a polarization insensitive arrayed-input spectrometer using echelle diffraction grating (EDG) for hyperspectral imaging. The EDG consists of 65 input waveguides and 129 output waveguides, allowing spectral measurements of 65 image pixels at a time when used in combination with a micro-electromechanical system micro mirror array. The spectral resolution reaches 7.8 nm for wavelengths ranging from 1250 to 1700 nm. The measured loss is −2 dB, and the crosstalk is lower than −20 dB. The 3 μm silicon-on-insulator platform provides the device with polarization insensitive characteristics. The chip size is only 6 mm×10 mm.

Extraordinary optical transmission inside a waveguide: spatial mode dependence.

We study the influence of the input spatial mode on the extraordinary optical transmission (EOT) effect. By placing a metal screen with a 1D array of subwavelength holes inside a terahertz (THz) parallel-plate waveguide (PPWG), we can directly compare the transmission spectra with different input waveguide modes. We observe that the transmitted spectrum depends strongly on the input mode. A conventional description of EOT based on the excitation of surface plasmons is not predictive in all cases. Instead, we utilize a formalism based on impedance matching, which accurately predicts the spectral resonances for both TEM and non-TEM input modes.

Investigation for ultra-shorten coupling length in woodpile structure

An off-plane directional coupler in woodpile structure is investigated theoretically by finite-difference time-domain method. Our study shows that the coupling length can be adjusted effectively by controlling the refractive index in central region between input and coupling waveguides. And if the distance between rods is more than 1.5a, a coupling length less 2a can be also obtained, where a is the lattice constant. Also, we investigate the effect of the width of rods on the coupling length. The results show that the coupling length can be shortened further by decreasing the width of rods. All results we simulated are very important for the high-density integrated circuit.

High-efficiency apodized-imaging chip-fiber grating coupler for silicon nitride waveguides.

Chip-fiber grating couplers have been widely used in silicon-on-insulator platforms. In silicon nitride waveguides, the lower index contrast yields gratings with a weaker radiation strength and poor directionality, thereby limiting the coupling efficiency. Here we propose and design an apodized self-imaging grating coupler in silicon nitride that images an in-plane waveguide input to an output optical fiber placed at a specific distance above the chip. Both amplitude and phase apodizations are employed to engineer the transfer function of the self-imaging grating to produce an image field matching the fiber mode profile. Two-step etch staircase grating teeth are used to achieve directionality as high as 93%. Full three-dimensional finite-difference time-domain simulations show coupling from a 40 μm×40 μm grating to an SMF-28 single mode fiber with a record calculated efficiency of 86% (0.66dB loss) and a 3dB bandwidth of 40nm near the 1550nm wavelength.

Realization of large-scale photonic crystal cavity-based devices

This paper demonstrates an approach for fabricating large-scale photonic crystal (PhC)-based devices using a combination of optical and focused ion beam (FIB) lithography techniques. Optical lithography along with reactive ion etching parameters is optimized to realize the layout of device structure and thereafter FIB milling is optimized to realize the designed PhC structure at those identified locations. At first, with the help of a specially designed mask and using optical lithography along with reactive ion etching, a number of rectangular areas of dimension of 10 μm×20 μm along with input and output waveguides of width ∼700 nm and thickness of ∼250 nm have been fabricated. Subsequently, use of FIB milling, a periodic PhC structure of lattice constant of 600 nm, having a hole diameter of ∼480 nm along with a defect hole diameter of ∼250 nm have been realized successfully on the selected areas. This method shows a promising application in fabricating PhC structure with device size >1 cm2 at large scale, eliminating the problems of standard nanolithography techniques.

A new RF window designed for high-power operation in an S-band LINAC RF system

A new RF window is designed for high-power operation at the Pohang Light Source-II (PLSII) S-band linear accelerator (LINAC) RF system. In order to reduce the strength of the electric field component perpendicular to the ceramic disk, which is commonly known as the main cause of most discharge breakdowns in ceramic disk, we replace the pill-box type cavity in the conventional RF window with an overmoded cavity. The overmoded cavity is coupled with input and output waveguides through dual side-wall coupling irises to reduce the electric field strength at the iris and the number of possible mode competitions. The finite-difference time-domain (FDTD) simulation, CST MWS, was used in the design process. The simulated maximum electric field component perpendicular to the ceramic for the new RF window is reduced by an order of magnitude compared with taht for the conventional RF window, which holds promise for stable high-power operation.

A High-Power Single Rectangular Grating Sheet Electron Beam Traveling-Wave Tube

In this paper, a high-power Ka-band traveling-wave tube (TWT) driven by a sheet electron beam is proposed and demonstrated by means of the simulation and experimental methods. The single grating rectangular waveguide with large width-to-height ratio is used as the slow-wave structure (SWS). An $E$ -plane bend energy coupler is put forward to feed this wide SWS. An irregular transition structure between the input waveguide and the SWS is suggested and designed, which is beneficial to suppress the self-excited oscillation. For a practical electron beam (20 mm $\times \,\, 1$ mm, 150.1 kV, 850 A, and 20-ns), the output power of this single rectangular grating sheet beam TWT is larger than 750 kW from 34.5 to 35.4 GHz. The maximum power is 1.21 MW at 35 GHz, corresponding to a gain of 20.4 dB.

Investigation on Sheet Beam Folded V-Shape Groove Waveguide for Millimeter-Wave TWT

In this paper, a novel slow-wave structure (SWS) called folded V-shape groove waveguide SWS is proposed. The high frequency characteristics of this SWS is studied. An input and output waveguide transition structure, which is appropriate for this new kind of SWS is put forward. The transmission loss of the folded V-shape groove waveguide is lower than that of folded rectangular groove waveguide. The dimension of the folded V-shape groove waveguide is larger than that of the folded rectangular groove waveguide at the millimeter waveband, even at terahertz waveband. Based on this kind of SWS, a W-band sheet beam traveling-wave tube is studied, whose beam-wave interaction is calculated by means of the simulation software. The calculation results show that the output power can be over 260 W within 3-dB bandwidth from 84 to 98 GHz, the gain can reach up to 36 dB, and the electron efficiency is 12.33% at the center frequency.

Light localization in optically induced deterministic aperiodic Fibonacci lattices

As light localization becomes increasingly pronounced in photonic systems with less order, we investigate optically induced two-dimensional Fibonacci structures that are supposed to be among the most ordered realizations of deterministic aperiodic patterns. For the generation of corresponding refractive index structures, we implement a recently developed incremental induction method using nondiffracting Bessel beams as waveguide formation entities. Even though Fibonacci structures present slightly reduced order, we show that transverse light transport here is significantly hampered in comparison with discrete diffraction in a periodic lattice. Numerical simulations that support our experimental findings help to identify three cases of input waveguide configurations that significantly determine the initial propagation in a Fibonacci structure. These crucial starting conditions determine the character of light transport, yielding either localization or enhanced expansion. A diverse set of light transport scenarios is identified therein.

Ultracompact and high efficient silicon-based polarization splitter-rotator using a partially-etched subwavelength grating coupler.

On-chip polarization manipulation is pivotal for silicon-on-insulator material platform to realize polarization-transparent circuits and polarization-division-multiplexing transmissions, where polarization splitters and rotators are fundamental components. In this work, we propose an ultracompact and high efficient silicon-based polarization splitter-rotator (PSR) using a partially-etched subwavelength grating (SWG) coupler. The proposed PSR consists of a taper-integrated SWG coupler combined with a partially-etched waveguide between the input and output strip waveguides to make the input transverse-electric (TE) mode couple and convert to the output transverse-magnetic (TM) mode at the cross port while the input TM mode confine well in the strip waveguide during propagation and directly output from the bar port with nearly neglected coupling. Moreover, to better separate input polarizations, an additional tapered waveguide extended from the partially-etched waveguide is also added. From results, an ultracompact PSR of only 8.2 μm in length is achieved, which is so far the reported shortest one. The polarization conversion loss and efficiency are 0.12 dB and 98.52%, respectively, together with the crosstalk and reflection loss of −31.41/−22.43 dB and −34.74/−33.13 dB for input TE/TM mode at wavelength of 1.55 μm. These attributes make the present device suitable for constructing on-chip compact photonic integrated circuits with polarization-independence.

Bi-Directional Coupler as a Mode-Division Multiplexer/Demultiplexer

A bi-directional coupler supported with Bragg grating is proposed as a mode-division multiplexer/demultiplexer. The structure can demultiplex 3 modes with only two waveguides and a Bragg grating. The input waveguide is multimode, while the output waveguide is single mode. Both first- and second-order modes of the input waveguide are coupled to the output waveguide, propagating at opposite directions, while the fundamental mode is kept in the input waveguide. A theoretical analysis of the proposed demultiplexer is developed based on the perturbative mode-coupled theory. The insertion losses of both first- and second-order modes are derived under suitable phase-matched conditions. Expressions of the insertion losses are obtained for simple slab-waveguides coupler. Numerical results of the proposed demultiplexer are presented for the slab-waveguides coupler under different design parameters. Furthermore, the wavelength dependence of the device is addressed under mismatching conditions and a Lorentzian model of the Silicon material. In addition, FDTD simulation of proposed demultiplexer is performed to prove the validity of the proposed concept. Return loss due to reflection to the source is taken into consideration in the simulation in addition to the device wavelength dependence.

Ultrafast surface plasmon-polariton interference and switching in multiple crossing dielectric waveguides

In this paper, we investigate propagation effects and interference switching of surface plasmon-polaritons (SPPs) in a junction of multiple crossed waveguides. These waveguides are produced on a thin gold layer by a simple photolithographic procedure. The waveguide dimensions are optimized for SPP excitation and propagation along two crossed input waveguides. At the waveguide intersection, different possibilities for SPP propagation into multiple output waveguides are offered. Using leakage radiation microscopy, we find that the SPPs preferably propagate into only one specific direction different from the direction of the input waveguides with avoidance of signal backscattering into the input direction. Furthermore, it is demonstrated that the SPP intensity at the output waveguide can be tuned by interference effects induced by a phase shift of the excitation laser beams. Additionally, we study the influence of different angles between the two input and the one specific output waveguides of the junction structure on the propagation properties of SPP modes in order to demonstrate a highest possible energy flux into the output waveguide. The experimental investigations are supported by finite-difference time-domain simulations. Good agreement between experimental results and numerical simulations is obtained. Applications of this effect are discussed for realization of ultrafast optical/plasmonic switches and optical logic gate structures with potential for integration and cascading.

A Tunable 240–290 GHz Waveguide Enclosed 2-D Grid HBV Frequency Tripler

This paper presents a high-power 240–290 GHz waveguide enclosed two-dimensional (2-D) grid heterostructure barrier varactor (HBV) frequency multiplier. A 35 mW of output power is produced at 247 GHz with an input power of 900 mW. The operational bandwidth is tunable within a 50 GHz span by the use of an input tuner able to adjust the input matching of the 2-D grid HBV frequency multiplier. Tuning is achieved by moving a suspended dielectric slab in the input waveguide.