Wire Waveguides Research Articles

Numerical simulation of low loss silicon photonic wire waveguide with multiple cladding layers

A different silicon photonic wire waveguide is proposed, which uses multiple thin cladding layers in order to reduce the index contrast between core and cladding interface. The reduced index contrast in the proposed waveguide has led to reduction in the scattering losses by 37% as compared to silicon wire waveguide for 400 nm × 220 nm waveguide dimension. The proposed waveguide has shown significant reduction in bending losses. It offers the bending loss of 0.0118 dB at the radius of 1 μm and 0.0063 dB for a radius of 2 μm at 1.55 μm wavelength as compared to 0.086 and 0.013 dB at the radius of 1 and 2 μm, respectively, offered by silicon photonic wire waveguide at 1.5 μm wavelength. The use of polymer material as top cladding layer resulted in decreasing the sensitivity of effective index against temperature for the designed waveguide by a factor of 2 as compared to silicon wire waveguide.

Transmission of 2.86 Tb/s data stream in silicon subwavelength grating waveguides.

In this paper, we perform an investigation of terabit-scale data transmission in silicon subwavelength grating (SWG) waveguides for wavelength-division multiplexing (WDM) optical signals. Silicon SWG waveguide is capable of decreasing the light confinement in silicon core by engineering the geometry, leading to relatively lower optical nonlinearity compared to silicon wire waveguide. We demonstrate ultrahigh-bandwidth 2.86 Tb/s data transmission through the fabricated 2-mm-long silicon SWG waveguide over a wide range of launch powers. In the experiment, 75 WDM channels are utilized with each carrying 38.12 Gb/s orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. With the benefit of efficient reduction on optical nonlinearity, the optimum launch power is increased by 8 dB in SWG waveguide, indicating higher tolerance to the nonlinear impairments, compared to a silicon wire waveguide with identical length. With the optimum launch power, all 75 channels exhibit bit-error rate (BER) values less than 4e-5 after SWG waveguide transmission. We also evaluate the terabit-scale data transmission performance through four silicon SWG waveguides with different lengths (1 mm, 2 mm, 4 mm and 12 mm). The required optical signal-to-noise ratios (OSNRs) to achieve BER level of 1e-3 are around 15.27, 15.47, 16.66 and 20.38 dB, respectively.

Compact and low-loss liquid crystal loaded Mach-Zehnder optical switch based on Si wire waveguide

Compact and low-loss liquid crystal loaded Mach-Zehnder optical switch based on Si wire waveguide

Ultra-fine metal gate operated graphene optical intensity modulator

A graphene based top-gate optical modulator on a standard silicon photonic platform is proposed for the future optical telecommunication networks. On the basis of the device simulation, we proposed that an electro-absorption light modulation can be realized by an ultra-narrow metal top-gate electrode (width less than 400 nm) directly located on the top of a silicon wire waveguide. The designed structure also provides excellent features such as carrier doping and waveguide-planarization free fabrication processes. In terms of the fabrication, we established transferring of a CVD-grown mono-layer graphene sheet onto a CMOS compatible silicon photonic sample followed by a 25-nm thick ALD-grown Al2O3 deposition and Source-Gate-Drain electrodes formation. In addition, a pair of low-loss spot-size converter for the input and output area is integrated for the efficient light source coupling. The maximum modulation depth of over 30% (1.2 dB) is observed at a device length of 50 μm, and a metal width of 300 nm. The influence of the initial Fermi energy obtained by experiment on the modulation performance is discussed with simulation results.

Crystalline/Amorphous Si Integrated Optical Couplers for 2D/3D Interconnection

The in-plane and interlayer waveguide-type couplers between crystalline Si and amorphous-Si:H wire waveguides, for 2D/3D hybrid-material integration are presented in this paper. The in-plane-type coupler achieves stable coupling between two waveguides by using tapers located at the tips of the waveguides. The interlayer-type coupler can connect two waveguides, despite an interlayer distance of 1 μm, with a simple process flow, by introducing a trident structure. An experiment was conducted in which the in-plane and interlayer-type couplers realized low coupling losses (coupling efficiencies) of 0.16 dB (96%) and 0.49 dB (89%) per coupler, respectively.

In-plane switching mode-based liquid-crystal hybrid Si wired Mach–Zehnder optical switch

A Mach–Zehnder optical switch based on a Si wire waveguide embedded in a liquid crystal overcladding is developed. Switching operation at a wavelength of 1550 nm with a voltage-length product of 1.86 V·mm is obtained for a device with a 300-µm-long loop-back phase shifter. The switching speed is qualitatively evaluated under various alignment film conditions, and switch-off and switch-on times of 7.9 and 8.4 ms, respectively, are achieved when the alignment layer is appropriately positioned relative to the waveguides.

Suspended silicon mid-infrared waveguide devices with subwavelength grating metamaterial cladding.

We present several fundamental photonic building blocks based on suspended silicon waveguides supported by a lateral cladding comprising subwavelength grating metamaterial. We discuss the design, fabrication, and characterization of waveguide bends, multimode interference devices and Mach-Zehnder interferometers for the 3715 - 3800 nm wavelength range, demonstrated for the first time in this platform. The waveguide propagation loss of 0.82 dB/cm is reported, some of the lowest loss yet achieved in silicon waveguides for this wavelength range. These results establish a direct path to ultimately extending the operational wavelength range of silicon wire waveguides to the entire transparency window of silicon.

Manipulating frequency-dependent diffraction, the linewidth, center frequency and coupling efficiency using periodic corrugations

In this paper, we demonstrate an approach to couple freely propagating THz pulses onto a metal wire waveguide utilizing grating as various shaped grooves; rectangular, triangular and circular grooves fabricated directly into the metal. Using broadband THz pulses incident on the wire, we use metal segments containing number of uniformly, symmetric spaced grooves around subwavelength aperture to launch surface propagating multi-cycle pulses along the wire. THz radiation detected at the end of the waveguide with grooves showed overall improved signal amplitude and increased bandwidth as compared to that for the non-machined waveguide. We observe a one-to-one correspondence between the groove number and the number of oscillations in the THz wave form radiated from the end of the waveguide. We further demonstrate that this coupled radiation is radially polarized. Although the alteration of cross-sectional parameters of the grooves, the shape of the individual grooves, in a controlled manner should allow for arbitrarily shaped THz pulses to be launched on the waveguide, all of which compared to rectangular grooves’ output signal and the efficient shaped groove has been chosen.

On-chip broadband ultra-compact optical couplers and polarization splitters based on off-centered and non-symmetric slotted Si-wire waveguides

In this work, we propose novel schemes to design on-chip ultra-compact optical directional couplers (DC) and broadband polarization beam splitters (PBS) based on off-centered and asymmetric dielectric slot waveguides, respectively. Slot dimensions and positions are optimized to achieve maximum coupling coefficients between two symmetric and non-symmetric slotted Si wire waveguides through overlap integral method. We observe >88% of enhancement in the coupling coefficients when the size-optimized slots are placed in optimal positions, with respect to the same waveguides with no slot. When the waveguides are parallel, in that case, a coupling length as short as 1.73 μm is accomplished for TM mode with the off-centered and optimized slots. This scheme enables us to design optical DC with very small footprint, Lc ∼ 0.9 μm in the presence of S-bends. We also report a compact (Lc ∼ 1.1 μm) on-chip broadband PBS with hybrid slots. Extinction ratios of 13 dB and 22.3 dB are realized with very low insertion loss (0.055 dB and 0.008 dB) for TM and TE modes at 1.55 μm, respectively. The designed PBS exhibits a bandwidth of 78 nm for the TM mode (C-and partial L-bands) and >100 nm for the TE mode (S + C + L wavelength bands). Such on-chip devices can be used to design compact photonic interconnects and quantum information processing units efficiently. We have also investigated the fabrication tolerances of the proposed devices and described the fabrication steps to realize such hybrid devices. Our results are in good agreement with 3D FDTD simulations.

(Invited) SiOxNy Back-End Integration Technologies for Heterogeneously Integrated Si Platform

Network traffic is continuously increasing, and the global IP traffic reaches 2 zettabytes in 2019 [1]. Photonic integrated circuits on Si-based substrates are highly demanded for coping with increasing telecommunication network traffic and energy consumption in low-cost ways. Particularly, Si and Ge provide modulators and photodetectors in telecom wavelength with a full monolithic way on SOI substrates. In addition, InP provides lightsources based on low-temperature direct-bonding technique. To connect these dynamic and active devices, we propose back-end photonic wiring by using SiOxNy-based waveguides. In this paper, I review recent progress of our work. First, I briefly review our early work; Si-Ge-SiOx integration technology and application to the WDM receiver. To increase integration density, recently, we also have developed a silicon-nitride (SiN) waveguide[1]. SiN has moderately high refractive index and provides moderately small passive devices with tolerance for nonlinear effects. We confirmed 1-dB/cm propagation loss for 1.3-, 1.5-, and 1.6-um wavelength range and applied to a compact and low-loss 16-ch. arrayed-waveguide grating (AWG) filter with 200GHz spacing. In addition, for lightsource integration, a InP-wire waveguide was successfully integrated with the SiOx waveguide via a spot-size converter (SSC) on SiO2/Si substrates[2]. The fabricated InP waveguide provides 5.2-dB/cm propagation loss and connected to the SiOx waveguide with a 0.7-dB loss and less than -50-dB reflectance. The InP waveguide can be used as output waveguide for the membrane laser, which utilizes strong optical confinement within the active area to obtain high modulation efficiency and low power consumption[3, 4]. With introducing distributed-reflector (DR) laser structure, the membrane laser exhibits low threshold current of 0.6 mA, output power of 0.7 mW, and 25.8-Gbit/s NRZ direct modulation with 132-fJ/bit energy consumption. In addition, the output InP wire waveguide is successfully integrated with the SiOxwaveguide, which exhibits fiber coupling loss of 2.7 dB and low reflectance at the chip facet to obtain sufficient optical output power and stable single-mode operation. [1] K. Okazaki et al., Proc. GFP 2014, Vancouver, paper WP43. [2] H. Nishi et al., IEEE Photonics Journal, vol. 7, pp. 4900308, 2015. [3] S. Matuso et al., J. Lightwave Technol., vol. 33, pp. 1217, 2015. [4] H. Nishi et al., Proc. ECOC 2015, Valencia, paper We.2.5.3.

Comparative study between the results of effective index based matrix method and characterization of fabricated SU-8 waveguide

A study regarding the validity of effective-index based matrix method (EIMM) for the fabricated SU-8 channel waveguides is reported. The design method is extremely fast compared to other existing numerical techniques, such as, BPM and FDTD. In EIMM, the effective index method was applied in depth direction of the waveguide and the resulted lateral index profile was analyzed by a transfer matrix method. By EIMM one can compute the guided mode propagation constants and mode profiles for each mode for any dimensions of the waveguides. The technique may also be used to design single mode waveguide. SU-8 waveguide fabrication was carried out by continuous-wave direct laser writing process at 375nm wavelength. The measured propagation losses of these wire waveguides having air and PDMS as superstrates were 0.51dB/mm and 0.3dB/mm respectively. The number of guided modes, obtained theoretically as well as experimentally, for air-cladded waveguide was much more than that of PDMS-cladded waveguide. We were able to excite the isolated fundamental mode for the later by precise fiber positioning, and mode image was recorded. The mode profiles, mode indices, and refractive index profiles were extracted from this mode image of the fundamental mode which matched remarkably well with the theoretical predictions.

Experimental demonstration of a two-mode (de)multiplexer based on a taper-etched directional coupler.

We experimentally demonstrate a compact, low-cross talk and fabrication-tolerant two-mode (de)multiplexer on the silicon-on-insulator platform. The device consists of a silicon wire waveguide coupled to a taper-etched waveguide. The partially etched taper structure is used to relax fabrication tolerance and thus to ensure high mode-conversion efficiency. The device is 68 μm in length, with a TE0-to-TE1 mode conversion loss of better than -0.8 dB demonstrated over the C-band wavelengths. In addition, the device demonstrates a low TE0-to-TE0 through waveguide insertion loss of better than -1.3 dB with modal cross talk lower than -26 dB, over a 65 nm wavelength range. Finally, we have experimentally demonstrated that the device is tolerant of fabrication errors of up to 20 nm. Better than -6 dB TE0-TE1 conversion loss with a cross talk lower than -23 dB over a 55 nm bandwidth has been obtained with a fabrication tolerance as large as 40 nm.

Membrane distributed-reflector laser integrated with SiOx-based spot-size converter on Si substrate.

We demonstrate monolithic integration of a 50-μm-long-cavity membrane distributed-reflector laser with a spot-size converter, consisting of a tapered InP wire waveguide and an SiOx waveguide, on SiO2/Si substrate. The device exhibits 9.4-GHz/mA0.5 modulation efficiency with a 2.2-dB fiber coupling loss. We demonstrate 25.8-Gbit/s direct modulation with a bias current of 2.5 mA, resulting in a low energy cost of 132 fJ/bit.

Topology optimized mode multiplexing in silicon-on-insulator photonic wire waveguides.

We design and experimentally verify a topology optimized low-loss and broadband two-mode (de-)multiplexer, which is (de-)multiplexing the fundamental and the first-order transverse-electric modes in a silicon photonic wire. The device has a footprint of 2.6 µm x 4.22 µm and exhibits a loss <1.2 dB in a 100 nm bandwidth measured around 1570 nm. The measured cross talk is <-12 dB and the extinction ratio is >14 dB in the C-band. Furthermore, we demonstrate that the design method can be expanded to include more modes, in this case including also the second order transverse-electric mode, while maintaining functionality.

Completely CMOS compatible SiN-waveguide-based fiber coupling structure for Si wire waveguides.

For Si wire waveguides, we designed a highly efficient fiber coupling structure consisting of a Si inverted taper waveguide and a CMOS-compatible thin SiN waveguide with an SiO2 spacer inserted between them. By using a small SiN waveguide with a 310 nm-square core, the optical field can be expanded to correspond to a fiber with a 4.0-μm mode field diameter. A coupled waveguide system with the SiN waveguide and Si taper waveguide can provide low-loss and low-polarization-dependent mode conversion. Both losses in fiber-SiN waveguide coupling and SiN-Si waveguide mode conversion are no more than 1 dB in a wide wavelength bandwidth from 1.36 μm to 1.65 μm. Through a detailed analysis of the effective refractive indices in the coupled waveguide system, we can understand mode conversion accurately and also derive guidelines for reducing the polarization dependence and for shortening device length.

Interlayer Polarization Beam Splitter Based on Asymmetrical Si Wire Directional Coupler

We proposed and designed a compact Si interlayer polarization beam splitter (PBS) that is based on a crystalline Si and hydrogenated amorphous Si (a-Si:H) vertical asymmetrical directional coupler toward 3-D Si photonics integrated circuits. The changeable thickness of the upper a-Si:H waveguide enables the realization of the TM-mode-coupled PBS with simple-structured parallel Si wire waveguides. The device with a 260- $\text {nm}\times 220$ -nm first-layer waveguide and a 340- $\text {nm} \times 200$ -nm second-layer waveguide operates in a broadband width of 77 nm, with a crosstalk of less than −20 dB and an insertion loss of $C$ -band. The device is robust in terms of alignment error between the two waveguides.

Photonic integrated circuit components based on amorphous silicon-on-insulator technology

We present integrated-optic building blocks and functional photonic devices based on amorphous silicon-on-insulator technology. Efficient deep-etched fiber-to-chip grating couplers, low-loss single-mode photonic wire waveguides, and compact power splitters are presented. Based on the sub-μm photonic wires, 2×2 Mach–Zehnder interferometers and add/drop microring resonators (MRRs) with low device footprints and high finesse up to 200 were realized and studied. Compact polarization rotators and splitters with ≥10 dB polarization extinction ratio were fabricated for the polarization management on-chip. The tuning and trimming capabilities of the material platform are demonstrated with efficient microheaters and a permanent device trimming method, which enabled the realization of energy-efficient photonic circuits. Wavelength multiplexers in the form of cascaded filter banks and 4×4 routers based on MRR switches are presented. Fabrication imperfections were analyzed and permanently corrected by an accurate laser-trimming method, thus enabling eight-channel multiplexers with record low metrics of sub-mW static power consumption and ≤1°C temperature overhead. The high quality of the functional devices, the high tuning efficiency, and the excellent trimming capabilities demonstrate the potential to realize low-cost, densely integrated, and ultralow-power 3D-stacked photonic circuits on top of CMOS microelectronics.

Re-configurable multi-level temperature sensing by ultrasonic “spring-like” helical waveguide

This paper introduces a novel technique for multi-level temperature measurement using a single reconfigurable ultrasonic wire waveguide that is configured in the form of a helical spring. In this embodiment, the multiple sensing levels located along the length of the helical waveguide wire can be repositioned by stretching or collapsing the spring to provide measurements at different desired spacing in a given area/volume. This method can measure over a wide range of temperatures. The transduction is performed using Piezo-electric crystals that are attached to one end of the waveguide which act as transmitter as well as receiver. The wire will have multiple reflector embodiments (notches was used here) that allow reflections of input L(0,1) mode guided ultrasonic wave, in pulse echo mode, back to the crystal. Using the time of fight measurement at multiple predefined reflector locations, the local average temperatures are measured and compared with co-located thermocouples. The finite element modeling simulation was used to study the effect of excitation frequency and the mean coil diameter of the “spring-like” waveguide. This technique improves on the limitations of a straight waveguide technique earlier reported.

Low-loss silicon wire waveguides for optical integrated circuits

Abstract

Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides.

Fiber-chip edge couplers are extensively used in integrated optics for coupling of light between planar waveguide circuits and optical fibers. In this work, we report on a new fiber-chip edge coupler concept with large mode size for silicon photonic wire waveguides. The coupler allows direct coupling with conventional cleaved optical fibers with large mode size while circumventing the need for lensed fibers. The coupler is designed for 220 nm silicon-on-insulator (SOI) platform. It exhibits an overall coupling efficiency exceeding 90%, as independently confirmed by 3D Finite-Difference Time-Domain (FDTD) and fully vectorial 3D Eigenmode Expansion (EME) calculations. We present two specific coupler designs, namely for a high numerical aperture single mode optical fiber with 6 µm mode field diameter (MFD) and a standard SMF-28 fiber with 10.4 µm MFD. An important advantage of our coupler concept is the ability to expand the mode at the chip edge without leading to high substrate leakage losses through buried oxide (BOX), which in our design is set to 3 µm. This remarkable feature is achieved by implementing in the SiO2 upper cladding thin high-index Si3N4 layers. The Si3N4 layers increase the effective refractive index of the upper cladding near the facet. The index is controlled along the taper by subwavelength refractive index engineering to facilitate adiabatic mode transformation to the silicon wire waveguide while the Si-wire waveguide is inversely tapered along the coupler. The mode overlap optimization at the chip facet is carried out with a full vectorial mode solver. The mode transformation along the coupler is studied using 3D-FDTD simulations and with fully-vectorial 3D-EME calculations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 µm.