Spot-size Converter Research Articles

Vertically integrated spot-size converter in AlGaAs-GaAs.

We report on the demonstration of a spot size converter (SSC) for monolithic photonic integration at a wavelength of 850nm on a GaAs substrate. We designed and fabricated a dual-waveguide AlGaAs chip. The design consists of a lower waveguide layer for efficient end-fire coupling to a single-mode fiber, an upper waveguide layer for high refractive index contrast waveguides, and a vertical SSC to connect the two waveguide layers. We measured a SSC conversion efficiency of 91% (or -0.4 dB) between the upper and lower waveguide layers for the TE mode at a wavelength of 850nm.

Efficient spot size converter for higher-order mode fiber-chip coupling.

We propose and demonstrate a silicon-based spot size converter (SSC), composed of two identical tapered channel waveguides and a Y-junction. The SSC is designed for first-order mode fiber-to-chip coupling on the basis of mode petal separation and the recombination method. Compared with a traditional on-chip SSC, this method is superior with reduced coupling loss when dealing with a higher-order mode. To the best of our knowledge, we present the first experimental observations of a higher-order SSC which is fully compatible with a standard fabrication process. Average coupling losses of 3 and 5.5dB are predicted by simulation and demonstrated experimentally. A fully covered 3dB bandwidth over a 1515-1585nm wavelength range is experimentally observed.

Design of compact surface optical coupler based on vertically curved silicon waveguide for high-numerical-aperture single-mode optical fiber

A surface optical coupler based on a vertically curved Si waveguide was designed for coupling with high-numerical aperture single-mode optical fibers with a mode-field diameter of 5 µm. This coupler has a quite small device size, with a height of approximately 12 µm, achieved by introducing an effective spot-size converter configured with the combination of an extremely short Si exponential-inverse taper and a dome-structured SiO2 lens formed on the coupler top. The designed coupler shows high-efficiency optical coupling, with a loss of 0.8 dB for TE polarized light, as well as broad-band coupling with a 0.5-dB-loss band of 420 nm.

Mirror-based polarization-insensitive broadband vertical optical coupling for Si waveguide

To achieve an efficient, broadband, and polarization-insensitive vertical optical input/output for Si photonics, we demonstrated vertical optical coupling for a Si photonic wire waveguide with an integrated 45° mirror and a spot size converter. The output beam from the fabricated mirror was evaluated and a high-quality single mode beam was obtained. On the basis of coupling loss measurements, the coupling losses for the Si photonic wire waveguide were estimated to be 0.85 and 0.55 dB in TE and TM polarizations, respectively. The wavelength-dependent loss was ±0.1 dB over a wavelength range of 1500–1600 nm.

Double-stage guided-mode converter for pure TM-mode guiding in pillar photonic-crystal waveguide devices.

We propose a double-stage guided-mode converter for pillar photonic-crystal (PhC) waveguide devices. The converter consists of a pillar-to-wire waveguide coupler and a transverse-magnetic-mode-selective spot-size converter. The former secures high-efficiency wide-band optical coupling of a pillar-PhC waveguide to a wire waveguide. The latter improves the coupling efficiency of the wire waveguide and an outside waveguide such as an optical fiber and also the signal-to-noise ratio of light guided in the pillar-PhC waveguide. The transmission band of a fabricated pillar-PhC waveguide having the converters on both ends was 88 nm in wavelength. The cutoff at the band edge was steep and deep with an extinction ration of 40 dB in a 4-nm wavelength range.

Theoretical and experimental investigations of efficient light coupling with spatially varied all dielectric striped waveguides

Integrated photonic systems require efficient, compact, and broadband solutions for strong light coupling into and out of optical waveguides. The present work investigates an efficient optical power transferring the problem between optical waveguides having different widths of in/out terminals. We propose a considerably practical and feasible concept to implement and design an optical coupler by introducing gradually index modulation to the coupler section. The index profile of the coupler section is modulated with a Gaussian function by the help of striped waveguides. The effective medium theory is used to replace the original spatially varying index profile with dielectric stripes of a finite length/width having a constant effective refractive index. 2D and 3D finite-difference time-domain analyzes are utilized to investigate the sampling effect of the designed optical coupler and to determine the parameters that play a crucial role in enhancing the optical power transfer performance. Comparing the coupling performance of conventional benchmark adiabatic and butt couplers with the designed striped waveguide coupler, the corresponding coupling efficiency increases from approximately 30% to 95% over a wide frequency interval. In addition, to realize the realistic optical coupler appropriate to integrated photonic applications, the proposed structure is numerically designed on a silicon-on-insulator wafer. The implemented SOI platform based optical coupler operates in the telecom wavelength regime (λ = 1.55 μm), and the dimensions of the striped coupler are kept as 9.77 μm (along the transverse to propagation direction) and 7.69 μm (along the propagation direction) where the unit distance is fixed to be 465 nm. Finally, to demonstrate the operating design principle, the microwave experiments are conducted and the spot size conversion ratio as high as 7.1:1 is measured, whereas a coupling efficiency over 60% in the frequency range of 5.0–16.0 GHz has been also demonstrated.

Static FT imaging spectrometer based on a modified waveguide MZI array.

We proposed a large aperture push-broom static Fourier transform (FT) imaging spectrometer for hyper-spectral imaging. The spectroscopic system was based on the silicon-on-insulator (SOI) platform, consisting of spot size converters, a Mach-Zehnder interferometer (MZI) array, and a detector array. The arm length differences of the 201 MZI arrays are linearly varied. It is one on-chip FT spectrometer operating in a large spectral range from 1100 to 1700nm with a resolution of 24 cm-1 and small foot print (chip size: 11 mm×15 mm). The waveguide-MZI-based large aperture push-broom FT spectrometer can provide a compact solution for high spectral resolution remote sensing in both the fore-optic lens and the spectroscopic system.

Tapered Spot Size Converter by Mask-Transfer Self-Written Technology for Optical Interconnection

A compact spot size converter (SSC) is the key to the future of optical interconnection. Hence, we proposed a tapered SSC using mask-transfer self-written (MTSW) technology with UV-curable resin. MTSW technology guarantees an easy and fast fabrication concept that makes it a promising technology for SSC devices. A prototype SSC adopting this technology was successfully fabricated. It was also confirmed that a tapered shape could be prepared from diffraction effect. A numerical calculation performed using the beam propagation method simulation shows that radiation loss decreases as SSC length increases. Comparison between the simulation and actual fabrication results shows acceptable value. Thus, this approach is reliable and has potential in the future of optical interconnection.

Compact InP spot‐size converter with vertically tapered core layer formed by micro‐loading effect

A compact (200 μm) spot-size converter (SSC) with a vertically tapered core layer is developed, which is fabricated with a simple wafer process using the micro-loading effect. It is confirmed that the vertical spot size of the SSC exceeded 2 μm, which was double that of the original waveguide. As a result, an InP waveguide integrated with the SSCs exhibited a coupling loss of <1 dB per facet for a lensed fibre with a 2.5 μm mode-field diameter. The fluctuation in the total of coupling and excess losses of the SSC was <0.5 dB.

In brief

PAGE 800 A relaxation oscillator with no comparator and no reference voltage is presented in work from China. By replacing a conventional comparator with a pseudo-inverter chain it avoids comparator offset effect, achieving low temperature coefficient and ultra-low power consumption. The resulting fully-integrated relaxation oscillator is aimed at energy harvesting applications requiring ultra-low power consumption. PAGE 789 Researchers in China present a terahertz transmit array made using PCB technology. Their proposed approach is low-cost with simple structures and they expect it to reduce the overall cost of THz systems. The device they have demonstrated provides 360° phase coverage with a transmission coefficient better than −3.52 dB at 250 GHz. The oscillator is designed to achieve ultra-low power consumption for use in energy harvesting systems PAGE 797 A team in Japan have developed a vertically-tapered spot-size converter (VT-SSC) using a simpler fabrication process than previously reported InP-based SSCs. The VT core layer was formed by dry etching using the micro-loading effect and the resulting device is very compact at just 200 µm. The team say the compactness and easy fabrication of the VT-SSC mean it can be used to improve the optical coupling loss that occurs during InP photonic integrated circuit assembly. Low-cost PCB techniques were used to manufacture the terahertz transmit array PAGE 814 Work from the UK explores the possibilities of using social media data to map service blackspots in cellular networks. In the example presented, natural language processing techniques have been applied to twitter posts to identify quality of experience complaints and this is combined with the geo-tagging data to identify blackspots and present small-cell deployment recommendations. The simpler manufacture and compact form of the converter will help reduce optical losses incurred during circuit assembly PAGE 810 Researchers in Korea have developed a process that allows dies to align spontaneously on a pad of substrate without the use of die-attach equipment. Their new process uses the under bump metallurgy (UBM) on the back of the chips to improve the die shift issue for fan-out wafer-level packaging, reducing maximum die shift to less than 1 µm. They believe this approach could also be applied to panel-level fan-out packages. Natural language processing of tweets is used to identify quality of experience complaints in mobile networks and map service blackspots The new approach uses UBM on the reverse of the chip to reduce die shift during manufacture

Improved design for SOI based evanescently coupled multilayer spot-size converter

We report an improved version of a spot-size converter (SSC) consisting of a silicon nanowire evanescently coupled to a phase-matched Poly-Si multilayer structure. With wider transversal dimensions the multilayer structure expands the mode significantly thus increasing the coupling efficiency with the conventional single-mode fiber. Detailed optimization process of a 17-layer based SSC is discussed and its coupling efficiency with a high-NA fiber of radius 2 μm is obtained as 98% providing only 0.087 dB loss. Vertical alignment tolerance between the optimized SSC and a high-NA fiber of radius 2 μm is also shown. This novel design does not consist of a taper and can be fabricated by using CMOS compatible process. It has a short device length and more relaxed alignment tolerances with the fiber. Full-vectorial and computationally efficient finite element method and the least squares boundary residual method have been used for the analysis and optimization of the proposed structure.

Ultra-compact low-loss broadband waveguide taper in silicon-on-insulator

A novel design of large bandwidth, fabrication tolerant, CMOS-compatible compact tapers (15 um) have been proposed and experimentally demonstrated in silicon-on-insulator. The proposed taper along with linear grating couplers for spot-size conversion exhibits no degradation in the coupling efficiency compared to a standard focusing grating in 1550 nm band. A single taper design has a broadband operation over 600 nm that can be used in O, C and L-band. The proposed compact taper is highly tolerant to fabrication variations; 80 nm change in the taper width and 200 nm in end waveguide width varies the taper transmission by <0.4 dB. The footprint of the device i.e. taper along with the linear gratings is ~ 250 {\mu}m2; this is 20X smaller than the adiabatic taper and 2X smaller than the focusing grating coupler.

CMOS Fabricated Large Array of Free Standing Substrate-Less Photonic Crystal Cavities for Biosensing Applications

In this paper, we present a methodology to postprocess a large array of a few hundred nanometer thin photonic membranes that were fabricated using complementary metal-oxide-semiconductor (CMOS) technological platform. The postprocessing results in local removal of the silicon substrate and of the buried oxide, which provides a free access from both side of the photonic structures. The membranes are patterned with photonic crystal cavities by deep ultraviolet lithography. We show that the proposed process is compatible with the integration of micrometer-sized SU8-based polymer waveguides. These polymer waveguides, together with high index contrast adiabatic nanometer-sized silicon inverted tapers, act as spot size converters. In addition, although a significant number of processing steps is required to achieve such free-standing substrate-less cavities, the quality (Q) factors of the cavities that is of the order of a few thousand remains unaffected.

Compact Silicon Photonic Grating Coupler With Dual-Taper Partial Overlay Spot-Size Converter

A compact silicon photonic grating coupler enabling fiber-to-chip light coupling at a minimized footprint is proposed. The proposed coupler, which is partially overlaid at the tapered region, downsizes the effective length of the spot-size converter to 60 μ m while reducing the lateral height and width adiabatically to 220 nm and 320 nm, respectively. The incorporation of such a coupler in a photonic integrated circuit requires a total physical length and width of 75 μ m and 14 μ m, respectively, to enable effective interfacing with single mode fiber. The coupling efficiency and bandwidth of the structure at the light wavelength of 1550 nm are quantified as 68% and 67 nm, respectively, which outperform most recent demonstrations.

Design and fabrication of surface trimmed silicon-on-insulator waveguide with adiabatic spot-size converters.

Theoretical and experimental studies reveal that a predefined single-mode rib waveguide fabricated in silicon-on-insulator (SOI) substrate with a device layer thickness of 2μm can be adiabatically trimmed down to submicron waveguide dimensions (<1 μm), resulting in regional modification of waveguide properties. The fabrication process involves physical trimming/removal of a waveguide surface by plasma etchants that is spatially filtered by a shadow mask with a rectangular aperture inside a reactive ion etching system. The exact position of a shadow mask above a sample surface has been optimized (∼500 μm) to obtain the desired adiabatic spot-size converters of length up to 1mm at both ends of the trimmed waveguides. For experimental demonstration, three different sets of 15-mm-long single-mode waveguides fabricated in 2-μm SOI were adiabatically trimmed in the middle for three different lengths of 3, 5, and 7mm, respectively. Excess propagation loss and group index of a trimmed submicron waveguide section were extracted by analyzing the wavelength-dependent Fabry-Perot transmission characteristics of the device with polished input/output end facets. The insertion loss of a typical spot-size converter designed for the guidance of TE-like polarization has been recorded to be ∼0.25 dB for a wide range of wavelengths (1500 nm≤λ≤1600 nm). As predicted by numerical simulation, no polarization rotation has been observed in all the trimmed submicron waveguides. The proposed surface trimming technique can be potentially used to tune the waveguide cross-section/geometry for phase error correction and/or to avail stronger light-matter interactions at a desired location of an integrated optical circuit.

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.

Simple production of membrane-based LiNbO3 micro-modulators with integrated tapers.

We report on free-standing electro-optical LiNbO3 waveguides with integrated tapers made by optical grade dicing. Membranes with a calibrated thickness are produced simultaneously with tapers acting as spot-size converters. Thereby, thicknesses from 450 to 500 μm can simply be achieved together with integrated tapers guaranteeing low insertion losses. These developments open the way to the low-cost production of compact and low-power-consuming electro-optical components. As an example, a 200 μm-long free-standing electro-optical Fabry-Perot is demonstrated with a figure of merit of only 0.19 V·cm in a 4.5 μm-thick membrane.

Optical Switches Based on Silicon Photonics for ROADM Application

We demonstrate compact and low-loss 8 × 8 silicon photonic switch modules, which are applicable to transponder aggregators (TPAs) in colorless, directionless, and contentionless reconfigurable optical add-drop multiplexers. Newly designed silicon optical switch chips incorporating spot size converters with polarization insensitive and wavelength insensitive properties over C/L bands are packaged. The developed module shows about 6-dB average excess optical loss, including optical coupling loss, on all 64 optical paths with low-polarization-dependent loss and low crosstalk. Using these compact optical switch modules, we construct a TPA prototype featuring over 100-port optical switch subsystem densely mounted on one board and confirm its feasibility.

Cost Effective Silica-Based 100 G DP-QPSK Coherent Receiver

We present a cost-effective dual polarization quadrature phase-shift coherent receiver module using a silica planar lightwave circuit (PLC) hybrid assembly. Two polarization beam splitters and two 90° optical hybrids are monolithically integrated in one silica PLC chip with an index contrast of 2%-Δ. Two four-channel spot-size converter integrated waveguide-photodetector (PD) arrays are bonded on PD carriers for transverse-electric/transverse-magnetic polarization, and butt-coupled to a polished facet of the PLC using a simple chip-to-chip bonding method. Instead of a ceramic sub-mount, a low-cost printed circuit board is applied in the module. A stepped CuW block is used to dissipate the heat generated from trans-impedance amplifiers and to vertically align RF transmission lines. The fabricated coherent receiver shows a 3-dB bandwidth of 26 GHz and a common mode rejection ratio of 16 dB at 22 GHz for a local oscillator optical input. A bit error rate of is achieved at a 112-Gbps back-to-back transmission with off-line digital signal processing.

2.3 µm range InP-based type-II quantum well Fabry-Perot lasers heterogeneously integrated on a silicon photonic integrated circuit.

Heterogeneously integrated InP-based type-II quantum well Fabry-Perot lasers on a silicon waveguide circuit emitting in the 2.3 µm wavelength range are demonstrated. The devices consist of a "W"-shaped InGaAs/GaAsSb multi-quantum-well gain section, III-V/silicon spot size converters and two silicon Bragg grating reflectors to form the laser cavity. In continuous-wave (CW) operation, we obtain a threshold current density of 2.7 kA/cm2 and output power of 1.3 mW at 5 °C for 2.35 μm lasers. The lasers emit over 3.7 mW of peak power with a threshold current density of 1.6 kA/cm2 in pulsed regime at room temperature. This demonstration of heterogeneously integrated lasers indicates that the material system and heterogeneous integration method are promising to realize fully integrated III-V/silicon photonics spectroscopic sensors in the 2 µm wavelength range.