Waveguide Taper Research Articles

Efficient adiabatic silicon-on-insulator waveguide taper

A silicon-on-insulator-based adiabatic waveguide taper with a high coupling efficiency and small footprint is presented. The taper was designed to reduce the incidence of mode conversion to higher-order and radiation modes inside the waveguide. In connecting a 0.5-μm-wide output waveguide and a 12-μm-wide input waveguide of a grating coupler, a compact 120-μm-long taper was demonstrated, achieving a transmission of 98.3%. Previously, this transmission level could only be achieved using a conventional linear taper with a length of more than 300 μm.

A Hybrid Integrated Light Source on a Silicon Platform Using a Trident Spot-Size Converter

This paper reports a hybrid integrated light source fabricated on a Si platform using a spot-size converter (SSC) with a trident Si waveguide. Low-loss coupling for 1.55 μm and 1.3 μm wavelengths was achieved with merely the simple planar form of a Si waveguide with no use of complicated structures such as vertical tapers or an extra dielectric core overlaid on the waveguide. The coupling loss tolerance up to a 1 dB loss increase was larger than the accuracy of our passive alignment technology. The coupling efficiency was quite robust against manufacturing variations in the waveguide width compared with that of a conventional SSC with an inverse taper waveguide. A multi-channel light source with highly uniform output power and a high-temperature light source were fabricated with a 1.55 μm quantum well laser and a 1.3 μm quantum dot laser, respectively. The integration scheme we report can be used to fabricate light sources for high-density, multi-channel Si optical interposers.

Broadband absorption engineering of hyperbolic metafilm patterns

Perfect absorbers are important optical/thermal components required by a variety of applications, including photon/thermal-harvesting, thermal energy recycling, and vacuum heat liberation. While there is great interest in achieving highly absorptive materials exhibiting large broadband absorption using optically thick, micro-structured materials, it is still challenging to realize ultra-compact subwavelength absorber for on-chip optical/thermal energy applications. Here we report the experimental realization of an on-chip broadband super absorber structure based on hyperbolic metamaterial waveguide taper array with strong and tunable absorption profile from near-infrared to mid-infrared spectral region. The ability to efficiently produce broadband, highly confined and localized optical fields on a chip is expected to create new regimes of optical/thermal physics, which holds promise for impacting a broad range of energy technologies ranging from photovoltaics, to thin-film thermal absorbers/emitters, to optical-chemical energy harvesting.

Design and fabrication of a metal-based asymmetric and variable Y-branch plastic optical fibre coupler

A metal-based asymmetric and variable plastic optical fibre or POF coupler utilising two-optical designs: a Y-branch structure with a novel hollow waveguide taper and an attenuation technique based on lateral displacement of two fibres has been developed. The non-symmetrical coupling ratios are obtained by attenuation due to the lateral displacement of two adjoining fibres. The device has been fabricated on a metal block using high speed CNC machining tool. The fabricated device has an excess loss of 6.3 dB while the coupling ratios are 46.27% and 53.73% when operating as a 3 dB coupler. In the asymmetric coupler mode, the coupling ratio ranges from 14.23% to 85.77% with excess loss varying from 6.3 dB to 8.33 dB. In the variable coupler mode, the device provides coupling ratios of 17.32% to 82.68% and excess loss varying from 6.71 dB to 9.72 dB.

Waveguide tapering for beam-width control in a waveguide transducer

In a waveguide transducer that transmits an ultrasonic wave through a waveguide unit to a test structure, it is most preferred to send a non-dispersive ultrasonic wave of a narrow beam width. However, there is an unresolved conflict between the generation of the non- or less-dispersive wave and the transmission of a narrow-beam wave into a test structure. Among others, the thickness of the waveguide unit in a waveguide transducer is the key variable determining these two conflicting criteria, but the use of a uniformly-thick waveguide of any thickness cannot fulfill the two conflicting criteria simultaneously. In this study, we propose a specially-engineered tapered waveguide unit for the simultaneous satisfaction. An excitation unit is installed at the end of the thin region of the tapered waveguide and generates only the lowest non-dispersive shear-horizontal wave. Then the generated wave propagates through the tapered region of the waveguide unit and reaches the thick region of the waveguide with insignificant mode conversion to higher modes. If the tapered waveguide is used, the surviving lowest mode in the thick region of the waveguide is shown to carry most of the transmitted power and is finally propagated into a test structure. Because the beam size of the propagated wave and the thickness of the contacting waveguide region are inversely related, the thick contacting region of the tapered waveguide ensures narrow beam width. Numerical and experimental investigations were performed to check the effectiveness of the proposed waveguide-tapering approach.

A TE01 mode generator for testing high power transmission devices

To verify the performance of high power waveguide transmission devices such as the miter bend, the waveguide taper, and the quasi-optical mode converter by the low power measurement, a TE01 mode generator, converts the rectangular waveguide TE10 mode into the cylindrical waveguide TE01 mode, for Ka-band is designed, fabricated, and measured. The proposed generator consists of a TE10 to TE20 rectangular waveguide mode converter and a rectangular TE20 to circular TE01 mode converter. The converting process in each section is analyzed and the working principles are discussed. Two prototypes are built and tested. The back-to-back transmission measured results agree well with the numerical calculations. The measured optimum transmissions are 97% with a 1-dB bandwidth from 29.2 GHz to 31.6 GHz. The angle-independent transmissions demonstrate high mode purity and the presence of TE01 mode confirmed by the far-field measurement. The proposed mode generator features high conversion efficiency, high mode purity, and moderate broad bandwidth.

Design and characterization of low loss 50 picoseconds delay line on SOI platform

We design and experimentally demonstrate 50 picoseconds (ps) low loss delay line on 300 nm SOI platform. The delay line unit consists of straight rib waveguide and strip bend section linked by a transition taper waveguide. Low propagation loss of ~0.1 dB/cm is achieved on the straight rib waveguide. With taking into account both low loss and desirable delay, a complete design and characterization process for passive delay line is presented. Our measurement results show that about 0.7 dB excess loss is achievable for 50 ps delay. The loss can be further reduced by adjusting the layout parameters.

Experimental demonstration and analysis of compact silicon-nanowire-based couplers

Compact 2×2 couplers based on silicon nanowires are fabricated and tested. They include a directional (X) coupler, a cross-gap coupler (CGC), and a multimode interference (MMI) coupler. The length of the X coupler’s parallel film waveguide is 1 μm. The theoretical minimum excess loss of the X coupler is 0.73 dB, whereas its experimental value is 1.0817 dB. CGC has a coupling region length of 24 μm. The minimum excess loss of CGC, which is 0.6 dB in theory, is experimentally determined to be 0.6737 dB. Taper waveguides are used as input/output waveguides for the MMI coupler. The footprint of the MMI region is only 6×57 μm 2 . The excess loss of the MMI coupler is theoretically 0.46 dB, but its experimental value is 0.5423 dB. The experimental nonuniformity of the MMI coupler is 0.0063 dB when the center wavelength is 1.55 μm. The maximum excess loss of the MMI coupler is 0.8233 dB in the wavelength range of 1.52 to 1.58 μm. The simulated and experimental results show that a small 2×2 MMI coupler that is suitable for optoelectronic integration exhibits lower excess loss, wider bandwidth, and better uniformity.

Optical coupling to nanoscale optomechanical cavities for near quantum-limited motion transduction

A significant challenge in the development of chip-scale cavity-optomechanical devices as testbeds for quantum experiments and classical metrology lies in the coupling of light from nanoscale optical mode volumes to conventional optical components such as lenses and fibers. In this work we demonstrate a high-efficiency, single-sided fiber-optic coupling platform for optomechanical cavities. By utilizing an adiabatic waveguide taper to transform a single optical mode between a photonic crystal zipper cavity and a permanently mounted fiber, we achieve a collection efficiency for intracavity photons of 52% at the cavity resonance wavelength of λ ≈ 1538 nm. An optical balanced homodyne measurement of the displacement fluctuations of the fundamental in-plane mechanical resonance at 3.3 MHz reveals that the imprecision noise floor lies a factor of 2.8 above the standard quantum limit (SQL) for continuous position measurement, with a predicted total added noise of 1.4 phonons at the optimal probe power. The combination of extremely low measurement noise and robust fiber alignment presents significant progress towards single-phonon sensitivity for these sorts of integrated micro-optomechanical cavities.

Excess Loss Reduction in Low Cost Wide Waveguide Gap Polarization Mode Converter

We propose and experimentally demonstrate a segmented waveguide taper (SWT) mode adapter for a polarization mode converter to reduce excess loss induced by a 100- $\mu{\rm m}$ -wide waveguide gap housing a 92- $\mu{\rm m}$ -thick quartz half waveplate. The SWT mode adapter is a combination of periodically segmented waveguide taper and traditional taper formed in a silica-based waveguide. It is proved that this SWT mode adapter requires no vertical tapering process but can enlarge the mode size in both vertical and horizontal directions. Our newly designed SWT mode adapter based on the 0.75%- $\Delta$ silica waveguide is able to reduce the excess loss significantly from 5 dB to less than 1.5 dB.

Silicon knife-edge taper waveguide for ultralow-loss spot-size converter fabricated by photolithography

For ultralow-loss and polarization-insensitive spot-size converters (SSCs) on a silicon platform, we propose and demonstrate a silicon knife-edge taper waveguide with a gradual decrease in height as well as width toward the taper end. The taper was fabricated using a double-patterning method involving i-line stepper photolithography and angled sidewall dry-etching. The SSC, with the knife-edge taper covered with a polymer secondary core, exhibited mode conversion losses of 0.35 and 0.21 dB for transverse electric-like and transverse magnetic-like modes, respectively.

3D display based on complete digital optical phase conjugation

Digital optical wavefront reconstruction for 3D display at tens or hundreds centimeters scale with viewing angle over 100° calls for resolutions at sub-wavelength scale. However the pixel pitches of spatial light modulators (SLM) are at micrometer or tens of micrometer scale. When people generated 3D images by digital holography using SLMs they had to make the interference patterns very sparse, resulting in small image size or narrow viewing angle. The paper employed an adiabatic waveguide taper made of quantities of single-mode waveguides to decompose complicated light fields into simpler forms so that complete digital optical phase conjugation (CDOPC) could be performed using low resolution SLMs. Then the paper introduced an optical transforming unit to project the reconstructed small wavefronts over a large volume while maintaining large viewing angle. The paper shows both theoretically and experimentally that 3D display systems based on CDOPC could create large size 3D images with large viewing angle and diffraction limited image quality. Besides, CDOPC might also be widely used for 3D measurement, optical tweezers, high energy laser beam forming, etc.

Way of designing a periodic grating coupler with fully etched slots

In order to couple into or out of a silicon photonic waveguide on silicon on insulator (SOI) substrate from optical fibers, we present a simple but practical method to design a grating coupler. The grating is periodic with fully etched slots; strong reflection between the fully etched grating and the waveguide is avoided by adding an antireflection interface. Theoretical coupling efficiency up to 43% is demonstrated. A taper waveguide used to link the grating and waveguide is also designed.

Terahertz Wave Confinement in Pillar Photonic Crystal with a Tapered Waveguide and a Point Defect

We demonstrate a photonic crystal cavity with a tapered waveguide and a point defect to highly confine terahertz waves. The terahertz wave is first guided into the tapered waveguide, gradually compressed to its end, and finally confined in the point defect cavity. Numerical simulations with the finite-difference time-domain method indicate that the narrow band terahertz wave is highly confined in the point defect cavity with a quality factor of 5323. The demonstrated device may be used as an antenna for enhancing light-matter interactions in the point defect cavity at terahertz frequencies and may improve the sensitivity of terahertz near-field microscopy.

Highly compact 2×2 multimode interference coupler in silicon photonic nanowires for array waveguide grating demodulation integration microsystem

A highly compact 2×2 multimode interference (MMI) coupler based on silicon-on-insulator that can be used in an array waveguide grating demodulation integration microsystem is designed. The coupler is simulated using the beam propagation method. Taper waveguides are used as input/output waveguides. The footprint of the MMI region is only 6μm×57μm. The excess loss is 0.46dB, and the uniformity is 0.06dB with transverse electric polarization when the center wavelength is 1.55μm. The maximum excess loss is 1.55dB in the range of 1.49μm–1.59μm. The simulation results show that a small 2×2MMI coupler exhibits low excess loss, wide bandwidth, and good uniformity suitable for the requirements of the system on chip.

Design Optimization and Comparative Analysis of Silicon-Nanowire-Based Couplers

Three kinds of highly compact 2 × 2 couplers based on silicon nanowire are designed and optimized for the array waveguide grating (AWG) demodulation integration microsystem in this paper. These couplers are directional (X) coupler, cross gap coupler (CGC), and multimode interface (MMI) coupler. The couplers are simulated using the beam propagation method. The distance between the input/output waveguides is set to 10 μm considering the test of a single device in the following work. The total footprint of X coupler is 10 μm ×300 μm. The length of parallel film waveguide is 1 μm. After optimization, the minimum excess loss is 0.73 dB. CGC has a footprint of 10 μm × 300 μm , a coupling region length of 24 μm, and a minimum excess loss of 0.6 dB. Taper waveguides are used as input/output waveguides for MMI coupler. The footprint of MMI region is only 6 μm × 57 μm. The excess loss is 0.46 dB after optimization. Uniformity is 0.06 dB with transverse electric polarization when the center wavelength is 1.55 μm. The maximum excess loss is 1.55 dB in the range of 1.49 μm to 1.59 μm. The simulation results show that a small 2 × 2 MMI coupler exhibits lower excess loss, wider bandwidth, and better uniformity than X coupler and CGC. MMI coupler is suitable for the requirements of optoelectronic integration.

Two-dimensional plasmonic eigenmode nanolocalization in an inhomogeneous metal-dielectric-metal slot waveguide

We show that transverse, with respect to the propagation direction, local narrowing of a metal-dielectricmetal plasmonic slot waveguide leads to a two-dimensional surface plasmon nanolocalization and can squeeze the plasmon eigenmode into a spot with a characteristic size of about several tens of nanometers. We demonstrate that the simultaneous waveguide tapering and decreasing transverse narrowing scale make possible an enhancement of the plasmon propagation distance in comparison with the homogeneous waveguide. We also find the fundamental limit of 2D plasmon nanolocalization, which is of the same order as the depth of penetration of the electromagnetic field into a metal which is actually independent of the field frequency in the near infrared domain.

High-Power Terahertz-Range Planar Gyrotrons with Transverse Energy Extraction

To increase the output power of terahertz gyrotrons to several hundred kilowatts, we suggest using a planar geometry of interaction space with a sheet electron beam and transverse energy extraction. An advantage of this scheme in comparison with conventional cylindrical geometry is the possibility to ensure effective mode selection over the open transverse coordinate in combination with radiation outcoupling that leads to a substantial reduction of Ohmic losses. Similar to unstable resonators in optics for further growth of the radiation power it is beneficial to introduce waveguide tapering.

Research of Coupling Structure between Photonic Crystal Waveguide and Dielectric Taper Waveguide

There are two types of coupling structures for the photonic crystal of triangular array of air holes. The two types of coupling structure between dielectric taper waveguide(DTW) and photonic crystal waveguide(PCW) is investigated. The coupling transmission efficiency is calculated for different coupling structure of the input and output, we found that the coupling transmission efficiency is changed with different coupling width and distance. The highest transmission efficiency is 92.6% and 99.6% for the input and output port respectively.

Machining of acrylic-based Y-branch plastic optical fiber coupler with suspended waveguide taper

An acrylic-based Y-branch plastic optical fiber (POF) with a middle suspended waveguide taper has been developed. The suspended high index contrast waveguide taper structure has been designed in such a way that it is surrounded by air-cladding. Non-sequential ray tracing has been performed on the device giving an insertion loss of 4.68 dB and coupling ratio of 50: 50. The middle waveguide taper is constructed on the acrylic block itself without using any additional optical waveguiding medium injected into the engraved taper region. Fabrication of the devices is done by producing the device structures on an acrylic block using high speed computer numerical control (CNC) machining tool. Input and output POF fibers are inserted into this device structure in such a way that they are passively aligned to the middle waveguide taper structure. The device shows an insertion loss of 5.9 dB, excess loss of 2.9 dB and a splitting ratio of 50: 50.