Spot-size Converter Research Articles

Mosquito method based polymer tapered waveguide as a spot size converter.

We create a compact low-loss spot-size converter (SSC) which utilizes a tapered core polymer optical waveguide with circular cross-sectional graded-index (GI) core using the Mosquito method we developed. First, we theoretically analyze the mutual diffusion between the core and cladding monomers, which is a feature unique to the Mosquito method when forming GI cores. The monomer diffusion effect depends on the initial core diameter that is dispensed by a microdispenser and the diffusion time before UV curing: in a small core the monomer diffuses more rapidly than in a large core. Using this diffusion dependence on the initially dispensed core diameter, it is theoretically found that a tapered polymer waveguide based SSC can adiabatically convert the mode-field diameter between 4.0 and 8.6 μm at a 1550-nm wavelength waveguide as short as 4 mm. Next, the tapered waveguide based SSC with the designed structure is experimentally fabricated using the Mosquito method, and we confirm an 8-mm long tapered waveguide with an insertion loss of 1.83dB functions as a SSC that converts the MFD from 4.7 μm to 7.5 μm at 1550-nm wavelength.

Vertical field enhancement of a spot-size converter using a nanopixel waveguide and window structure.

Spot-size converter (SSC) is an essential building block for integrated photonic circuits applied as a mode transformer between optical components. One typical issue for SSC is the difficulty of broadening the vertical field profile. Herein, we propose a nanopixel SSC (1 × 2 μm2) with changing hole size and density. Unlike a typical SSC, this configuration controls both the lateral and vertical field profiles relatively easily by enhancing the nanopixel density. A vertical field expansion of 1.21 μm was obtained by enhancing the nanopixel density. In addition, we designed the optical field in the lateral direction using deep neural network (DNN)-based learning to realize a perfect circular spot for high coupling efficiency that reached -3 dB at λ0 = 1.572 μm when the optical field aspect ratio was adjusted to 1 after training for 200 epochs. Furthermore, the vertical expansion was increased from 1.21 to 4.9 μm and the coupling efficiency from -3 to -0.41 dB by combining it a silicon dioxide window structure (5 × 15 × 10 μm3). The 1-dB operating bandwidth of the designed SSC structure is 100 nm (1.5-1.6 μm), while fabrication tolerance of the nanopixels and window structure length for the designed SSC structure are ±15 nm and ±250 nm when the coupling efficiency drops by 1 dB.

Nonparaxial Mode-Size Converter Using an Ultracompact Metamaterial Mikaelian Lens

Mode-division multiplexing (MDM) is a promising approach to increase data transmission capacity by using different modes as independent data channels. Intermodal crosstalk and coherent mode interference are detrimental to the performance of an MDM system and should be avoided in the mode converters and mode expanders for MDM. The Mikaelian lens is well known to be free from spherical aberrations and is therefore an excellent candidate for focusing high order waveguide modes which have main mode field distributions located off-axis. Here we proposed an ultracompact mode-independent mode-size converter using an integrated Mikaelian lens formed by refractive index engineering using a subwavelength grating. The Mikaelian lens is fabricated on the silicon-on-insulator (SOI) platform and can be used to convert the size of high order modes at the interface between the silicon waveguide and a multimode fiber in MDM systems. We present a proof-of-concept demonstration of a low loss spot size converter for three transverse electric (TE) modes between a 14 μm wide multimode waveguide grating coupler and a 1.56 μm wide multimode silicon waveguide. The measured insertion loss of the Mikaelian lens mode size converter is 0.21 dB for TE0 mode, 0.26 dB for TE1 mode and 0.28 dB for TE2 mode. 1-dB bandwidth of all the three modes are about 60 nm. The 15 μm long Mikaelian lens has comparable experimental performance as a 450 μm long adiabatic taper.

1.6 Tbps Silicon Photonics Integrated Circuit and 800 Gbps Photonic Engine for Switch Co-Packaging Demonstration

We describe the performance of high bandwidth-density silicon photonic based integrated circuits (SiPh ICs) that enable the first fully functional photonic engine (PE) module co-packaged with an Ethernet switch. We demonstrate the 1.6 Tbps SiPh transmitter (Tx) IC that integrates on-die all the lasers, micro ring modulators, monitor photodetectors, spot size converters, and V-grooves that are needed to support sixteen 106.25 Gbps PAM4 optical transmit channels. This SiPh Tx, together with discrete receiver (Rx) SiPh ICs, enabled an 800 Gbps PE. The PE is designed to allow up to sixteen modules to be co-packaged around a high-bandwidth switch ASIC. The PE test results described in this article were obtained using sixteen 53.125 Gbps electrical channels that were multiplexed to drive eight simultaneously operating 106.25 Gbps optical channels. We report DR4 IEEE standards compliant high-speed optical eye performance as well as full link operation. Post-FEC error-free operation over temperature and over extended time duration is demonstrated on all channels.

Polarization-Insensitive Vertically Curved Si Surface Optical Coupler Bent by Ion Implantation

For input coupling from single-mode optical fibers to photoreceivers made with silicon photonics integrated circuits (Si-PICs), we constructed a polarization-insensitive optical surface coupler with a vertically curved silicon waveguide. The polarization insensitivity was attained by the equalization of the radiative transmission losses of TE and TM polarizations in the vertically curved waveguide (VCW) structure while keeping the straight structure of the spot-size converter (SSC) connected above the curved structure. Numerical simulations revealed that the radius of curvature of 6 μm or larger nearly enabled the equalization, and thus we developed a bending technology using ion implantation to realize simultaneously a VCW radius of curvature of 6 μm and a straight SSC of length 7 μm by varying the implantation angles. The bent silicon core was subsequently coated with SiO2 by plasma-enhanced chemical vapor deposition to function as a collimation lens. The coupling properties of the fabricated coupler and the single-mode fiber with a mode-field diameter of 5 μm exhibited a polarization dependence loss of less than 0.25 dB, a 1-dB bandwidth of 180 nm, and coupling losses of about 1.6 dB at 1550 nm wavelength. A sharp inversely tapered Si waveguide that affects these characteristics significantly can be formed by applying ArF-immersion lithography on the 45-nm-node and hence fabrication is compatible with mass production processes.

High Efficiency Silicon Edge Coupler Based On Uniform Arrayed Waveguides With Un-Patterned Cladding

In this work, we demonstrate an edge coupler (EC) for the coupling between the single mode fiber (SMF) and the silicon photonic waveguide. High coupling efficiency can be achieved by using arrayed waveguides at 1550 nm with an un-patterned cladding. With such a design, the light will be “pulled” to the top cladding, avoid the using of a cantilever structure. Compared to the conventional inverse taper spot size converter, the widths for the arrayed waveguides are the same, thus it is not critical to determine the position of the end facet for dicing/polishing. We fabricated the proposed structure with only one-step lithography following by a dry etching process. The measured coupling loss is around 1.7 dB/facet, which agrees well with the simulations.

On-chip arbitrary-mode spot size conversion

Abstract Manipulating on-chip optical modes via components in analogy with free-space devices provides intuitional light control, and this concept has been adopted to implement single-lens–assisted spot size conversion using integrated device. However, the reported schemes have been demonstrated only for fundamental mode, while high-order or irregular modes are preferred in specific applications. The 4-f system is widely used in Fourier optics for optical information processing. Under the inspiration of the 4-f system and the beam expander in bulk optics, a spot size converter (SSC) with two metamaterial-based graded-index waveguides is proposed and demonstrated. The proposed device is capable of widening an arbitrary mode while preserving its profile shape. Compared with conventional SSC using adiabatic taper, the footprint can be reduced by 91.5% under a same intermode crosstalk. Experimentally, an expansion ratio of five is demonstrated for regular modes. Furthermore, for an irregular mode, the functionality is numerically verified without structure modification. This work offers a universal solution to on-chip spot size conversion and may broaden the on-chip application prospects of Fourier optics.

High-efficiency edge-coupling based on lithium niobate on an insulator wire waveguide.

Edge-coupling on single-crystal thin-film lithium niobate on insulator (LNOI) was systematically studied in this paper. An inverse taper-shaped spot-size converter (SSC) to convert the mode field from the laser chip to a nanoscale LNOI waveguide was adopted to improve the coupling efficiency. Structure of the edge coupler was fully investigated and optimized by using the eigenmode expansion method. The single-mode conditions of the LNOI waveguide for three common communication bands were taken into consideration. Further, the length and tip width of the inverse taper, the cross-section dimensions of SiON waveguide, and the sidewall angle were investigated with respect to coupling efficiency. As a result, the maximum coupling efficiency from an edge coupler to laser chip can reach 54%, 48%, and 58% at 1550, 1310, and 850 nm in Z-cut LNOI for quasi-TM mode, respectively. This proposed work gives a better understanding of the function of the edge coupler based on LNOI material and provides an appropriate method for the design of an edge coupler with high efficiency, which could benefit the further application of high-density monolithic integrated optical components.

Vertically tapered waveguide spot size converters fabricated via a linewidth controlled grey tone lithography for InP-based photonic integrated circuits.

We report a novel and simple fabrication process to realize vertically tapered spot size converters (SSC) on InP photonic integrated circuits. The vertical tapering was achieved via a linewidth controlled local optical dose variation, leading to a grey tone photoresist profile. The fabricated SSCs are compact, polarization insensitive and demonstrate a very high mode conversion efficiency of 95%. Integrated SSCs improved the overall loss by 5 dB giving a coupling loss as low as 1.3 dB/facet, for a lensed fibre with a mode field diameter of 3.0 µm. A good agreement was found between the fibre-to-fibre optical loss measurements and those predicted from simulations.

Broadband high-efficiency triple-tip spot size converter for edge coupling with improved polarization insensitivity

Fiber-to-chip couplers are an essential part of the interfaces to integrated photonic devices, and high coupling efficiency over a broad wavelength range is preferred for most applications. Furthermore, couplers with low polarization dependence are also of great practical significance. While tapered waveguide couplers have been applied to overcome the mode mismatch between waveguides and fibers, the inherent geometric asymmetry of high-index-contrast waveguides, especially for those with a thick core layer, leads to polarization dependence in their performance. Here a triple-tip spot size converter as an edge coupler is proposed to improve the polarization insensitivity and coupling efficiency over a broadband range for either thick or thin waveguides. For both TE and TM polarization, it is shown that our design can have below 1 dB coupling loss over a 600 nm window around 1550 nm wavelength for Si3N4 waveguides of typical core thicknesses. Meanwhile, this kind of coupler also shows better fabrication and assembly tolerance than the traditional single-tip spot size converter (inverse taper coupler). Our proposed device can be easily fabricated in simple fabrication processes compared to other designs and have good compatibility with the standard complementary metal-oxide–semiconductor processes.

Hybrid III-V-on-SOI optical spot size converter by self-aligned selective undercut dry etching of Si.

A technology called self-aligned selective undercut dry etching processing has been demonstrated for fabricating a highly efficient hybrid optical spot size converter (SSC) on a Si-on-insulator (SOI) template. The process was based on a bonded wafer between the upper InP-based multiple quantum well heterostructure and the lower SOI substrate. After defining the mask on the upper InP-based ridge waveguide, CF4/O2 dry reactive ion etching was then used for selective undercut etching of the Si material from the surrounding materials, forming a vertical waveguide coupler of the optical SSC. The lower waveguide, whose dimension is even smaller than the upper one, can thus be vertically self-aligned to the top ridge via an independent processing step. A laterally tapered waveguide ranging from 0.3 to 3 µm in width on the upper InP waveguide was fabricated. The phase-matching condition of the vertical coupler leads to a length of 45 µm and extracts 88% conversion efficiency. The selective undercut etching processing in III-V/SOI material provides a vertical self-alignment scheme for realizing compact and submicron scale heterogeneous integration in a Si photonics template.

Optical waveguide crossing based on SOI slab waveguides

We propose and numerically analyze a unique optical waveguide crossing based on a silicon-on-insulator (SOI) slab waveguide structure. The optical waveguide crossing consisting of four curved mirrors made of silica walls and beam spot-size converters formed in a silicon core layer of the SOI slab waveguide. Based on our design with a device footprint of 15 × 15 μm2, we obtained excellent device performances such as insertion loss of −0.3 dB, crosstalk of less than −35 dB in the wavelength range from 1.25 μm to 1.6 μm with the 3D-FDTD simulation.

InP Monolithically Integrated Transmitters Based on High Speed Directly Modulated DFB Lasers

In this paper we summarize results concerning high-speed integrated multi-wavelength transmitters fabricated using a generic InP integration platform. The photonic integrated circuits (PICs) include tunable directly modulated DFB lasers emitting in the C-band, monitoring photodetectors, MMI couplers and output spot size converters. The directly modulated lasers, designed in order to fit within a 100 GHz grid, show a maximum 3-dB small signal bandwidth of 21 GHz and a tunability range of about 4 nm. In particular, we report an 8-channel transmitter showing operation up to 30 Gb/s per channel (for 200+ Gb/s applications) and a 4-channel transmitter capable of operating up to 34 Gb/s per channel (for 100+ Gb/s applications). Error free operation (bit error rates <10−9) is achieved for all the channels in all the cases.

Polymer Microlens on Pillar Grown From Single-Mode Fiber Core for Silicon Photonics

A novel coupling device composed of a combination of microlens and pillar having an approximately $8~\mu \text{m}$ diameter and $20~\mu \text{m}$ height and grown from single-mode fiber (SMF) core end is proposed. This device was successfully fabricated by using a self-written waveguide and dipping method with UV-curable resin. Theoretical and experimental studies were done. A high coupling efficiency of more than −0.97dB was obtained for coupling to an ultrahigh numerical aperture (0.41) SMF with a spot diameter of $3.7~\mu \text{m}$ at a $1.55~\mu \text{m}$ wavelength in place of a silicon chip with an integrated spot-size converter. In contrast, it is only −4.0dB by a conventional SMF.

Low-cost and high-efficiency single-mode-fiber interfaces to silicon photonic circuits

A high-efficiency and large-alignment-tolerance fiber interface to silicon-on-insulator wire waveguides is introduced. A spot-size converter is built through a full three-dimensional polymer waveguide and a silicon inverse taper is employed to expand the optical mode field from a silicon sub-micro waveguide to a normal cleaved single-mode fiber with a mode size of about 10μm. The fabrication for the three-dimensional structure relies on simple spin-coating processes. The coupling loss from the fiber to the silicon waveguide through the proposed structure reaches -1.0dB for both transverse electrical and transverse magnetic modes, respectively. The polarization-dependent loss for the best coupling case is kept within -0.6dB in the whole measured wavelength range. The coupling facet of the proposed fiber interface structure is polishable and facilitates an easy package procedure with standard fiber arrays.

Polymer micro-lenses as an long-coupling-distance interfacing layer in low-cost optical coupling solution between optical fibers and photonic integrated waveguide circuits

We present a low-cost scheme for non-permanent optical signal coupling for prospective application in single use photonic integrated chips. The proposed scheme exploits the use of polymer kinoform microlenses. The feasibility of the proposed solution is demonstrated by the experimental investigation of the optical signal coupling from single mode optical fiber (SMF) to the test structure of SixNy integrated waveguide. Full Text: PDF ReferencesM. Smit et al., "An introduction to InP-based generic integration technology," Semiconductor Science and Technology, 29 (8), 083001, 2014 CrossRef R. Baets et al., "Silicon Photonics: silicon nitride versus silicon-on-insulator," in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2016), paper Th3J.1. CrossRef K. Shiraishi et al., "A silicon-based spot-size converter between single-mode fibers and Si-wire waveguides using cascaded tapers," Appl. Phys. Lett. 91, 141120 (2007) CrossRef Y. Sobu et al., "GaInAsP/InP waveguide dual core spot size converter for optical fiber,"IEEE Photonic Society 24th Annual Meeting, 469-470, (2011). CrossRef F. Van Laere et al., "Compact and Highly Efficient Grating Couplers Between Optical Fiber and Nanophotonic Waveguides," Journal of Lightwave Technology, vol. 25, no. 1, pp. 151-156, Jan. 2007. CrossRef A. Kaźmierczak et al., "Light coupling and distribution or Si3N4/SiO2 integrated multichannel single mode sensing system," Opt. Eng. 48, 2009, pp. 014401 CrossRef M. Rossi et al., "Arrays of anamorphic phase-matched Fresnel elements for diode-to-fiber coupling," Appl. Opt. 34, 2483-2488 (1995) CrossRef M. Prasciolu et al, "Fabrication of Diffractive Optical Elements On-Fiber for Photonic Applications by Nanolitography," Japanese Journal of Applied Physics, Volume 42, (2003) CrossRef F.Schiappelli et al., "Efficient fiber-to-waveguide coupling by a lens on the end of the optical fiber fabricated by focused ion beam milling" Microelectronic Engineering Volumes 73-74, pp.397-404 (2004) CrossRef

Photonic integration based on a ferroelectric thin-film platform

Photonic-integrated circuits (PICs) using ferroelectric materials are expected to be used in many applications because of its unique optical properties such as large electro-optic coefficients. In this study, a novel PIC based on a ferroelectric thin-film platform was designed and fabricated, where high-speed optical modulator, spot-size converters (SSCs), and a variable optical attenuator (VOA) were successfully integrated. A ferroelectric lanthanum-modified lead zirconate titanate (PLZT) thin film was epitaxially-grown by using a modified sol-gel method, and it exhibits large electro-optic coefficients (>120 pm/V) and low propagation loss (1.1 dB/cm). The optical modulator, a Mach-Zehnder type, exhibited a half-wave voltage (Vπ) of 6.0 V (VπL = 4.5 Vcm) and optical modulation up to 56 Gb/s. Also, the VOA (with attenuation range of more than 26 dB) was successfully integrated with the modulator. As a result, it is concluded that the developed ferroelectric platform can pave the way for photonic integration.

Simulation of structural design with high coupling efficiency in external cavity semiconductor laser

For external cavity semiconductor lasers (ECSLs), high coupling efficiency is critical to reducing the linewidth. In this paper, the coupling efficiency between the laser diode and the waveguide grating has been improved, with proposals for its improvement presented, including adding spot-size conversion (SSC) and using a silicon-on-insulator (SOI) waveguide. The results indicate an increase of coupling efficiency from 41.5% to 93.1%, which exhibits an improvement of approximately 51.6% over conventional schemes. The relationship between coupling efficiency and SOI waveguide structures is mainly concerned in this article. These findings provide a new way for the future research of the narrow linewidth of ECSL.

An Ultracompact GRIN‐Lens‐Based Spot Size Converter using Subwavelength Grating Metamaterials

AbstractGraded‐index materials offer virtually complete control over light propagation in integrated photonic chips but can be challenging to implement. Here, an anisotropic graded‐index metamaterial, synthesized with fully etched silicon subwavelength structures, is proposed. Based on this material, a spot size converter that expands the transverse electric (TE) mode field profile from a 0.5 µm wide silicon wire waveguide to a 15 µm wide waveguide within a length of only 14 µm is designed. Measured insertion losses are below 1 dB in an unprecedented 130 nm bandwidth, limited by the measurement setup, with full 3D finite‐difference time‐domain (FDTD) simulations predicting a bandwidth in excess of 300 nm. Furthermore, the device is well suited to feed fiber‐to‐chip grating couplers, while requiring a footprint ten times smaller than conventional adiabatic tapers.

Phase-matched multi-layer based polarisation-independent spot-size converter for silicon nanowire

The efficient coupling of optical power from a silicon nanowire (NW) to an optical fibre is challenging for both the quasi-TE and quasi-TM polarisations. Here, we propose a polarisation-independent spot-size converter (PI-SSC) based on phase-matched multi-layer waveguides for efficient coupling between a silicon NW and an optical fibre for both the polarisations. The fabrication process of the proposed PI-SSC is compatible with the complementary metal-oxide-semiconductor (CMOS) process. The optimisation for the proposed PI-SSC is studied by using a numerically efficient algorithm, combining a rigorous H-field based full-vectorial finite element method (VFEM) and the least squares boundary residual (LSBR) method. The simulation results show that using an eleven-layer based PI-SSC, the coupling losses between a silicon NW and a lensed fibre of radius 2 μm can be reduced to only 0.34 dB and 0.25 dB for the quasi-TE and quasi-TM polarisations, respectively. Furthermore, the output multi-layer is horizontally tapered, which further reduces the coupling loss for both the polarisations and the end face is easy to be polished.