Continuously apodized fiber-to-chip surface grating coupler with refractive index engineered subwavelength structure

Broadband 2 × 2 multimode-interference coupler on the silicon-nitride platform

In this paper, we present the design, optimization, and implementation of a sub-wavelength grating (SWG) multi-mode interference coupler (MMI) on the silicon nitride photonic integrated circuit (PIC) platform with a significantly enhanced bandwidth compared to the conventional MMI. We extend the SWG MMI theory, previously presented for the silicon-on-insulator platform, to the Si3N4/SiO2 platform. Our approach involves an initial parameter optimization for a non-paired design, followed by a shift to a paired design that offers a smaller footprint and a broader bandwidth. The optimized SWG MMI exhibits a 1 dB bandwidth of 300 nm for both the insertion loss and power imbalance, making it a significant addition to silicon nitride photonics.

Highly efficient ultra-broad beam silicon nanophotonic antenna based on near-field phase engineering

Optical antennas are a fundamental element in optical phased arrays (OPA) and free-space optical interconnects. An outstanding challenge in optical antenna design lies in achieving high radiation efficiency, ultra-compact footprint and broad radiation angle simultaneously, as required for dense 2D OPAs with a broad steering range. Here, we demonstrate a fundamentally new concept of a nanophotonic antenna based on near-field phase-engineering. By introducing a specific near-field phase factor in the Fraunhofer transformation, the far-field beam is widened beyond the diffraction limit for a given aperture size. We use transversally interleaved subwavelength grating nanostructures to control the near-field phase. A Bragg reflector is used at the end of the grating to increase both the efficiency and the far-field beam width. The antenna has a compact footprint of 3.1 µm × 1.75 µm and an ultra-broad far-field beam width of 52° and 62° in the longitudinal and transversal direction, respectively, while the radiation efficiency reaches 82% after incorporating a bottom reflector to further improve the directionality. This unprecedented design performance is achieved with a single-etch grating nanostructure in a 300-nm SOI platform.

The Design, Fabrication and Characterization of Grating Couplers for SiGe Photonic Integration Employing a Reflective Back Mirror

We propose and demonstrate an efficient grating coupler for integrated SiGe photonic devices. A bottom metal layer is adopted to enhance the coupling efficiency on the wafer backside. A low coupling loss of -1.34 dB and -0.79 dB can be theoretically obtained with optimal parameters for uniform and apodized grating couplers, respectively. The fabrication process is CMOS compatible without need of wafer bonding. The influence of fabrication errors on the coupling efficiency is analyzed in terms of substrate thickness, grating dimension and material refractive index. The results indicate a large tolerance for the deviations in practical fabrication. The measured coupling loss of the uniform grating is -2.7 dB at approximately 1465 nm with a 3 dB bandwidth of more than 40 nm. The proposed grating coupler provides a promising approach to realize efficient chip-fiber coupling for the SiGe photonic integration.

High-Efficiency Grating Couplers for Pixel-Level Flat-Top Beam Generation

We demonstrate a kind of grating coupler that generates a high quality flat-top beam with a small beamwidth from photonic integrated circuits into free-space. The grating coupler is designed on a silicon-on-insulator wafer with a 220-nm-thick silicon layer and consists of a dual-etch grating (DG) and a distributed Bragg reflector (DBR). By adjusting the structural parameters of DG and DBR, a pixel-level (6.6 µm) flat-top beam with a vertical radiation of −0.5 dB and a mode match of 97% at 1550 nm is realized. Furthermore, a series of high-efficiency grating couplers are designed to create a flat-top beam with different scales.

Efficient 4.95 µm-8.5 µm dual-band grating coupler with crosstalk suppression capability

In many integrated optics systems, grating couplers are a key component of interfacing the external light source with in-plane photonic devices. Grating couplers with dual-band capability are often desired for expanding the operation spectrum of photonic systems. Here, we propose and theoretically investigate, for the first time, a 4.95 µm-8.5 µm dual-band grating coupler on a Ge-on-SOI platform. In addition to conventional structures, Bragg gratings are introduced to two wavelength division directions for crosstalk suppression. With this design, the simulated coupling efficiencies have respectively reached 59.93% and 46.38% for the 4.95 µm and 8.5 µm bands. This mid-infrared dual-band grating coupler may be useful for defense and environmental monitoring applications.

Ultra-broadband and ultra-compact polarization beam splitter based on a tapered subwavelength-grating waveguide and slot waveguide

In this work, we propose an ultra-broadband and ultra-compact polarization beam splitter (PBS) on a standard silicon-on-isolator platform. Assisted by a tapered subwavelength-grating waveguide and a slot waveguide, the working bandwidth of the directional-coupler-based PBS covers the entire O-, E-, S-, C-, L- and U-bands and the coupling length is only 4.6 µm. The insertion losses (ILs) of the device are simulated to be less than 0.8 dB and the extinction ratios (ERs) are larger than 10.9 dB at the wavelength range of 1260-1680 nm for both TE and TM polarizations. The experimental results show the average ILs are less than 1 dB for both polarizations at our measured wavelength ranges, which are consistent with the simulation results. It has the largest 1-dB bandwidth among all the reported broadband PBSs to the best of our knowledge.

Compact and highly-efficient broadband surface grating antenna on a silicon platform

We present a compact silicon-based surface grating antenna design with a high diffraction efficiency of 89% (-0.5 dB) and directionality of 0.94. The antenna is designed with subwavelength-based L-shaped radiating elements in a 300-nm silicon core, maintaining high efficiency with a compact footprint of 7.6 µm × 4.5 µm. The reflectivity remains below -10 dB over the S, C and L optical communication bands. A broad 1-dB bandwidth of 230 nm in diffraction efficiency is achieved with a central wavelength of 1550 nm.

Hybrid microwave-optical scanning probe for addressing solid-state spins in nanophotonic cavities

Spin-photon interfaces based on solid-state atomic defects have enabled a variety of key applications in quantum information processing. To maximize the light-matter coupling strength, defects are often placed inside nanoscale devices. Efficiently coupling light and microwave radiation into these structures is an experimental challenge, especially in cryogenic or high vacuum environments with limited sample access. In this work, we demonstrate a fiber-based scanning probe that simultaneously couples light into a planar photonic circuit and delivers high power microwaves for driving electron spin transitions. The optical portion achieves 46% one-way coupling efficiency, while the microwave portion supplies an AC magnetic field with strength up to 9 Gauss at 10 Watts of input microwave power. The entire probe can be scanned across a large number of devices inside a ³He cryostat without free-space optical access. We demonstrate this technique with silicon nanophotonic circuits coupled to single Er³⁺ ions.

Deterministic design of focusing apodized subwavelength grating coupler based on weak form and transformation optics

The focusing apodized subwavelength grating coupler (F-ASGC) has advantages of high coupling efficiency, small footprint and simple fabrication process, which make it a popular component for chip-scale coupling and testing of integrated optical circuit. However, the design of F-ASGC based on effective medium theory lacks accuracy, causing the drawbacks of peak wavelength deviation and performance degradation. In this work, we propose a deterministic design method of F-ASGC. Our grating coupler is formed by assembling various subwavelength grating units according to their complex effective indexes. The complex effective indexes of these grating units are accurately obtained by the weak form calculation. Then combining with transformation optics, we strictly analyze the F-ASGC for the first time. The simulation results show that the deterministically designed F-ASGC has high coupling efficiency of -2.51 dB, 3 dB bandwidth of 51 nm, and accurate central wavelength of 1553.1 nm. And we also fabricated it on the commercial SOI wafer. The measured maximum efficiency is -3.10 dB, the 3 dB bandwidth is 55 nm, and the central wavelength is 1551.5 nm.

Grating Couplers on Silicon Photonics: Design Principles, Emerging Trends and Practical Issues

Silicon photonics is an enabling technology that provides integrated photonic devices and systems with low-cost mass manufacturing capability. It has attracted increasing attention in both academia and industry in recent years, not only for its applications in communications, but also in sensing. One important issue of silicon photonics that comes with its high integration density is an interface between its high-performance integrated waveguide devices and optical fibers or free-space optics. Surface grating coupler is a preferred candidate that provides flexibility for circuit design and reduces effort for both fabrication and alignment. In the past decades, considerable research efforts have been made on in-plane grating couplers to address their insufficiency in coupling efficiency, wavelength sensitivity and polarization sensitivity compared with out-of-plane edge-coupling. Apart from improved performances, new functionalities are also on the horizon for grating couplers. In this paper, we review the current research progresses made on grating couplers, starting from their fundamental theories and concepts. Then, we conclude various methods to improve their performance, including coupling efficiency, polarization and wavelength sensitivity. Finally, we discuss some emerging research topics on grating couplers, as well as practical issues such as testing, packaging and promising applications.

Perfectly vertical surface grating couplers using subwavelength engineering for increased feature sizes

We present perfectly vertical grating couplers for the 220 nm silicon-on-insulator platform incorporating subwavelength metamaterials to increase the minimum feature sizes and achieve broadband low back-reflection. Our study reveals that devices with high coupling efficiencies are distributed over a wide region of the design space with varied back-reflections, while still maintaining minimum feature sizes larger than 100 nm and even 130 nm. Using 3D-finite-difference time-domain simulations, we demonstrate devices with broadband low back-reflection of less than -20dB over more than 100 nm bandwidth centered around the C-band. Coupling efficiencies of 72% and 67% are achieved for minimum feature sizes of 106 nm and 130 nm, respectively. These gratings are also more fabrication tolerant compared to similar designs not using metamaterials.

Molding free-space light with guided wave-driven metasurfaces

Metasurfaces with unparalleled controllability of light have shown great potential to revolutionize conventional optics. However, they mainly require external light excitation, which makes it difficult to fully integrate them on-chip. On the other hand, integrated photonics enables packing optical components densely on a chip, but it has limited free-space light controllability. Here, by dressing metasurfaces onto waveguides, we molded guided waves into any desired free-space modes to achieve complex free-space functions, such as out-of-plane beam deflection and focusing. This metasurface also breaks the degeneracy of clockwise- and counterclockwise-propagating whispering gallery modes in an active microring resonator, leading to on-chip direct orbital angular momentum lasing. Our study shows a viable route toward complete control of light across integrated photonics and free-space platforms and paves a way for creating multifunctional photonic integrated devices with agile access to free space, which enables a plethora of applications in communications, remote sensing, displays, etc.

Edge Couplers in Silicon Photonic Integrated Circuits: A Review

Silicon photonics has drawn increasing attention in the past few decades and is a promising key technology for future daily applications due to its various merits including ultra-low cost, high integration density owing to the high refractive index of silicon, and compatibility with current semiconductor fabrication process. Optical interconnects is an important issue in silicon photonic integrated circuits for transmitting light, and fiber-to-chip optical interconnects is vital in application scenarios such as data centers and optical transmission systems. There are mainly two categories of fiber-to-chip optical coupling: off-plane coupling and in-plane coupling. Grating couplers work under the former category, while edge couplers function as in-plane coupling. In this paper, we mainly focus on edge couplers in silicon photonic integrated circuits. We deliver an introduction to the research background, operation mechanisms, and design principles of silicon photonic edge couplers. The state-of-the-art of edge couplers is reviewed according to the different structural configurations of the device, while identifying the performance, fabrication feasibility, and applications. In addition, a brief comparison between edge couplers and grating couplers is conducted. Packaging issues are also discussed, and several prospective techniques for further improvements of edge couplers are proposed.

Sub-decibel silicon grating couplers based on L-shaped waveguides and engineered subwavelength metamaterials

The availability of low-loss optical interfaces to couple light between standard optical fibers and high-index-contrast silicon waveguides is essential for the development of chip-integrated nanophotonics. Input and output couplers based on diffraction gratings are attractive coupling solutions. Advanced grating coupler designs, with Bragg or metal mirror underneath, low- and high-index overlays, and multi-level or multi-layer layouts, have proven less useful due to customized or complex fabrication, however. In this work, we propose a rather simpler in design of efficient off-chip fiber couplers that provide a simulated efficiency up to 95% (-0.25 dB) at a wavelength of 1.55 µm. These grating couplers are formed with an L-shaped waveguide profile and synthesized subwavelength grating metamaterials. This concept jointly provides sufficient degrees of freedom to simultaneously control the grating directionality and out-radiated field profile of the grating mode. The proposed chip-to-fiber couplers promote robust sub-decibel coupling of light, yet contain device dimensions (> 120 nm) compatible with standard lithographic technologies presently available in silicon nanophotonic foundries. Fabrication imperfections are also investigated. Dimensional offsets of ± 15 nm in shallow-etch depth and ± 10 nm in linewidth's and mask misalignments are tolerated for a 1-dB loss penalty. The proposed concept is meant to be universal, which is an essential prerequisite for developing reliable and low-cost optical couplers. We foresee that the work on L-shaped grating couplers with sub-decibel coupling efficiencies could also be a valuable direction for silicon chip interfacing in integrated nanophotonics.

Backscattering design for a focusing grating coupler with fully etched slots for transverse magnetic modes

Grating couplers are a fundamental building block of integrated optics as they allow light to be coupled from free-space to on-chip components and vice versa. A challenging task in designing any grating coupler is represented by the need for reducing back reflections at the waveguide-grating interface, which introduce additional losses and undesirable interference fringes. Here, we present a design approach for focusing TM grating couplers that minimizes these unwanted reflections by introducing a modified slot that fulfills an anti-reflection condition. We show that this antireflection condition can be met only for the Bloch mode of the grating that concentrates in the dielectric. As a consequence the light is scattered from the grating coupler with a negative angle, referred to as "backscattering design". Our analytic model shows that the anti-reflection condition is transferrable to grating couplers on different waveguide platforms and that it applies for both TE and TM polarizations. Our experimentally realized focusing grating coupler for TM-modes on the silicon photonics platform has a coupling loss of (3.95 ± 0.15) dB at a wavelength of 1.55 µm. It has feature sizes above 200 nm and fully etched slots. The reflectivity between the grating coupler and the connected waveguide is suppressed to below 0.16%.

High positional freedom SOI subwavelength grating coupler (SWG) for 300 mm foundry fabrication

We present an apodized, single etch-step, subwavelength grating (SWG) high positional freedom (HPF) grating coupler based on the 220 nm silicon-on-insulator (SOI) with 2μm BOX substrate. The grating coupler was designed for 1550 nm light with transverse electric (TE) polarization. It has a measured maximum coupling efficiency of -7.49 dB (17.8%) and a -1 dB/-3 dB bandwidth of ~14 nm/29.5 nm respectively. It was fabricated in a 300mm state of the art CMOS foundry. This work presents an SOI-based grating coupler with the highest-to the best of our knowledge- -1 dB single mode fiber lateral alignment of 21.4 μm × 10.1 μm.

Photonic waveguide to free-space Gaussian beam extreme mode converter

Integration of photonic chips with millimeter-scale atomic, micromechanical, chemical, and biological systems can advance science and enable new miniaturized hybrid devices and technology. Optical interaction via small evanescent volumes restricts performance in applications such as gas spectroscopy, and a general ability to photonically access optical fields in large free-space volumes is desired. However, conventional inverse tapers and grating couplers do not directly scale to create wide, high-quality collimated beams for low-loss diffraction-free propagation over many millimeters in free space, necessitating additional bulky collimating optics and expensive alignment. Here, we develop an extreme mode converter, which is a compact planar photonic structure that efficiently couples a 300 nm × 250 nm silicon nitride high-index single-mode waveguide to a well-collimated near surface-normal Gaussian beam with an ≈160 µm waist, which corresponds to an increase in the modal area by a factor of >105. The beam quality is thoroughly characterized, and propagation over 4 mm in free space and coupling back into a single-mode photonic waveguide with low loss via a separate identical mode converter is demonstrated. To achieve low phase error over a beam area that is >100× larger than that of a typical grating coupler, our approach separates the two-dimensional mode expansion into two sequential separately optimized stages, which create a fully expanded and well-collimated Gaussian slab mode before out-coupling it into free space. Developed at 780 nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication, or other wavelengths. The technique can be expanded to more arbitrary phase and intensity control of both large-diameter, free-space optical beams and wide photonic slab modes.

Efficient and broadband subwavelength grating coupler for 3.7 μm mid-infrared silicon photonics integration

A grating coupler is an essential building block for compact and flexible photonics integration. In order to meet the increasing demand of mid-infrared (MIR) integrated photonics for sensitive chemical/gas sensing, we report a silicon-on-insulator (SOI) based MIR subwavelength grating coupler (SWGC) operating in the 3.7 μm wavelength range. We provide the design guidelines of a uniform and apodized SWGC, followed by numerical simulations for design verification. We experimentally demonstrate both types of SWGC. The apodized SWGC enables high coupling efficiency of -6.477 dB/facet with 3 dB bandwidth of 199 nm, whereas the uniform SWGC shows larger 3dB bandwidth of 263.5 nm but slightly lower coupling efficiency of -7.371 dB/facet.

Wavelength-division multiplexing Si photonic crystal beam steering device for high-throughput parallel sensing

We proposed and demonstrated a wavelength-division multiplexing (WDM) optical beam-steering device consisting of a thermally controlled doubly periodic Si two-dimensional bulk photonic crystal waveguide and coupled microring multiplexers. Beam forming and steering while maintaining a sharp profile is much easier in this device than with optical phased arrays which need the fine phase control. By dividing the range of beam-steering angles into different wavelength channels, it is possible to cover a wide range of angles, even when each angle is small. In this study, we fabricated a device with four wavelength channels, each of which showed beam steering of 4°-5° as a result of heating, resulting in a total of 16°. Two-dimensional steering is also achieved by loading a collimator lens and selecting one waveguide from those arrayed. We evaluated 112 resolution points with four wavelengths and 448 points in total by switching four waveguides. If this WDM concept is introduced into light detection and ranging and the number of wavelengths is increased, it will be possible to increase the sensing throughput, which is usually constrained by the round-trip time of light, by simultaneous parallel operation.

A compact silicon grating coupler based on hollow tapered spot-size converter

A new compact silicon grating coupler enabling fibre-to-chip light coupling at a minimized taper length is proposed. The proposed coupler, which incorporates a hollow tapered waveguide, converts the spot-size of optical modes from micro- to nano-scales by reducing the lateral dimension from 15 µm to 300 nm at a length equals to 60 µm. The incorporation of such a coupler in photonic integrated circuit causes a physical footprint as small as 81 µm × 15 µm with coupling efficiency and 3-dB coupling bandwidth as high as 72% and 69 nm respectively.

Compact single-etched sub-wavelength grating couplers for O-band application

We demonstrate two single-etched sub-wavelength grating coupler (SWGC) designs for O-band application, one targeting at high coupling efficiency and the other targeting at broad operating bandwidth. The high-efficiency SWGC has a measured peak coupling efficiency of -3.8 dB and a 3-dB bandwidth of 40 nm, and the broadband SWGC has a measured peak coupling efficiency of -4.3 dB and a 3-dB bandwidth of 71 nm. Focusing gratings have been used in our SWGCs to reduce the design footprints and the dimensions of our SWGCs are smaller than 45 μm × 24 μm. The back reflections of our SWGCs are suppressed to be below -15 dB over the wavelength range from 1260 nm to 1360 nm.

L-shaped fiber-chip grating couplers with high directionality and low reflectivity fabricated with deep-UV lithography

Grating couplers enable position-friendly interfacing of silicon chips by optical fibers. The conventional coupler designs call upon comparatively complex architectures to afford efficient light coupling to sub-micron silicon-on-insulator (SOI) waveguides. Conversely, the blazing effect in double-etched gratings provides high coupling efficiency with reduced fabrication intricacy. In this Letter, we demonstrate for the first time, to the best of our knowledge, the realization of an ultra-directional L-shaped grating coupler, seamlessly fabricated by using 193 nm deep-ultraviolet (deep-UV) lithography. We also include a subwavelength index engineered waveguide-to-grating transition that provides an eight-fold reduction of the grating reflectivity, down to 1% (-20  dB). A measured coupling efficiency of -2.7  dB (54%) is achieved, with a bandwidth of 62 nm. These results open promising prospects for the implementation of efficient, robust, and cost-effective coupling interfaces for sub-micrometric SOI waveguides, as desired for large-volume applications in silicon photonics.

Dual-etch apodised grating couplers for efficient fibre-chip coupling near 1310 nm wavelength

We present our recent work on fibre-chip grating couplers operating around 1310 nm. For the first time, we demonstrate the combination of dual-etch and apodization design approaches which may achieve a coupling efficiency of 85% (-0.7 dB). Subwavelength structures were employed to modify the coupling strength of the grating. -1.9 dB efficiency was measured from a first set of fabricated structures.

Single-etch subwavelength engineered fiber-chip grating couplers for 1.3 µm datacom wavelength band

We report, for the first time, on the design and experimental demonstration of fiber-chip surface grating couplers based on subwavelength grating engineered nanostructure operating in the low fiber chromatic dispersion window (around 1.3 μm wavelengths), which is of great interest for short-reach data communication applications. Our coupler designs meet the minimum feature size requirements of large-volume deep-ultraviolet stepper lithography processes. The fiber-chip couplers are implemented in a standard 220-nm-thick silicon-on-insulator (SOI) platform and are fabricated by using a single etch process. Several types of couplers are presented, specifically the uniform, the apodized, and the focusing designs. The measured peak coupling efficiency is -2.5 dB (56%) near the central wavelength of 1.3 μm. In addition, by utilizing the technique of the backside substrate metallization underneath the grating couplers, the coupling efficiency of up to -0.5 dB (89%) is predicted by Finite Difference Time Domain (FDTD) calculations.

Large-aperture subwavelength grating couplers

Subwavelength nanostructure grating couplers fabricated on silicon-on-insulator substrates are used to simplify the fabrication process while maintaining high coupling efficiency. The main obstacle for their application in photonic integrated circuits is the small aperture size of the nanostructure when TE polarization is involved, since they are difficult to achieve with 193 nm deep-ultraviolet lithography and cause problems in inductively coupled plasma etching. A larger lateral period has been used to increase the aperture size. Here, we propose that decreasing the effective index of the nanostructure can also enlarge the aperture size. We analyze the two methods in detail with a rectangle-hole nanostructure and 220 nm thick waveguide layer, aiming at TE polarization centered at 1560 nm. We find performance degenerations for large lateral periods, and this can be simply compensated by adjusting the width of the rectangle hole. The minimum linewidth of the nanostructure can reach 240 nm, while the coupling efficiency is just slightly decreased. The backreflections of a large-aperture grating increase but stay in the same order with ordinary ones, and we also show that this can be overcome by apodizing the grating structure. Finally, we experimentally demonstrate the designed large-aperture grating couplers and the coupling efficiencies are higher than 35%, and reach a rectangle-hole width.

Single step etched two dimensional grating coupler based on the SOI platform

We design and demonstrate a single step etched two dimensional grating coupler (2D GC) based on the silicon-on-insulator (SOI) platform using subwavelength cylinders. Taking the lag effect into consideration, the 2D GC is manufactured in the same lithography and etching processes with the strip waveguide. The measured coupling loss of the proposed 2D GC is -6.0 dB, which is comparable with conventional shallowly etched 2D GC, realizing a fabrication simplification without performance degradation.

Design of broadband subwavelength grating couplers with low back reflection

We present a methodology to design broadband grating couplers using one-dimensional subwavelength gratings. Using the presented method, we design subwavelength grating couplers (SWGCs) with 1-dB bandwidths ranging from 50 to 90 nm. Our designed SWGCs have competitive coupling efficiency, as high as -3.8  dB for the fundamental TE mode, and state-of-the-art back reflections, as low as -23  dB.

High-directionality fiber-chip grating coupler with interleaved trenches and subwavelength index-matching structure

We present the first experimental demonstration of a new fiber-chip grating coupler concept that exploits the blazing effect by interleaving the standard full (220 nm) and shallow etch (70 nm) trenches in a 220 nm thick silicon layer. The high directionality is obtained by controlling the separation between the deep and shallow trenches to achieve constructive interference in the upward direction and destructive interference toward the silicon substrate. Utilizing this concept, the grating directionality can be maximized independent of the bottom oxide thickness. The coupler also includes a subwavelength-engineered index-matching region, designed to reduce the reflectivity at the interface between the injection waveguide and the grating. We report a measured fiber-chip coupling efficiency of -1.3  dB, the highest coupling efficiency achieved to date for a surface grating coupler in a 220 nm silicon-on-insulator platform fabricated in a conventional dual-etch process without high-index overlays or bottom mirrors.

Subwavelength index engineered surface grating coupler with sub-decibel efficiency for 220-nm silicon-on-insulator waveguides

Surface grating couplers are fundamental components in chip-based photonic devices to couple light between photonic integrated circuits and optical fibers. In this work, we report on a grating coupler with sub-decibel experimental coupling efficiency using a single etch process in a standard 220-nm silicon-on-insulator (SOI) platform. We specifically demonstrate a subwavelength metamaterial refractive index engineered nanostructure with backside metal reflector, with the measured peak fiber-chip coupling efficiency of -0.69 dB (85.3%) and 3 dB bandwidth of 60 nm. This is the highest coupling efficiency hitherto experimentally achieved for a surface grating coupler implemented in 220-nm SOI platform.

Broadband polarization independent nanophotonic coupler for silicon waveguides with ultra-high efficiency

Coupling of light to and from integrated optical circuits has been recognized as a major practical challenge since the early years of photonics. The coupling is particularly difficult for high index contrast waveguides such as silicon-on-insulator, since the cross-sectional area of silicon wire waveguides is more than two orders of magnitude smaller than that of a standard single-mode fiber. Here, we experimentally demonstrate unprecedented control over the light coupling between the optical fiber and silicon chip by constructing the nanophotonic coupler with ultra-high coupling efficiency simultaneously for both transverse electric and transverse magnetic polarizations. We specifically demonstrate a subwavelength refractive index engineered nanostructure to mitigate loss and wavelength resonances by suppressing diffraction effects, enabling a coupling efficiency over 92% (0.32 dB) and polarization independent operation for a broad spectral range exceeding 100 nm.

Highly efficient and perfectly vertical chip-to-fiber dual-layer grating coupler

A novel high-efficiency silicon-chip-to-fiber grating coupler is investigated here. By introducing a dual layer grating structure with an inter-layer lateral shift to mimic 45° tilted mirror behavior, perfectly vertical coupling is successfully demonstrated. Our numerical results show that a peak silicon-chip-to-fiber coupling efficiency about 70% is possible near 1550 nm. Meanwhile, for the entire telecom C-band, i.e. wavelengths from 1530 nm to 1565 nm, the coupling efficiency is > 50% and the back reflection is less than < 1%. Our proposed high-performance silicon perfectly vertical coupling structure is suitable for interfacing with multi-core fiber platform, which may play an important role in the future CMOS photonic integration technology.

Suspended SOI waveguide with sub-wavelength grating cladding for mid-infrared

We present a new type of mid-infrared silicon-on-insulator (SOI) waveguide. The waveguide comprises a sub-wavelength lattice of holes acting as lateral cladding while at the same time allowing for the bottom oxide (BOX) removal by etching. The waveguide loss is determined at the wavelength of 3.8 μm for structures before and after being underetched using both vapor phase and liquid hydrofluoric acid (HF). A propagation loss of 3.4 dB/cm was measured for a design with a 300 nm grating period and 150 nm holes after partial removal (560 nm) of BOX by vapor phase HF etching. We also demonstrate an alternative design with 550 nm period and 450 nm holes, which allows a faster and complete removal of the BOX by liquid phase HF etching, yielding the waveguide propagation loss of 3.6 dB/cm.

Fiber-chip grating coupler based on interleaved trenches with directionality exceeding 95

We propose a fiber-chip grating coupler that interleaves the standard full and shallow etch trenches in a 220 nm thick silicon layer to provide a directionality upward exceeding 95%. By adjusting the separation between the two sets of trenches, constructive interference is achieved in the upward direction independent of the bottom oxide thickness and without any bottom reflectors, overlays, or customized etch depths. We implement a transverse subwavelength structure in the first two grating periods to minimize back-reflections. The grating coupler has a calculated coupling efficiency of CE~-1.05 dB with a 1 dB bandwidth of 30 nm and minimum feature size of 100 nm, compatible with deep-UV lithography.

Fully etched apodized grating coupler on the SOI platform with -0.58 dB coupling efficiency

We design and fabricate an ultrahigh coupling efficiency (CE) fully etched apodized grating coupler on the silicon-on-insulator (SOI) platform using subwavelength photonic crystals and bonded aluminum mirror. Fabrication error sensitivity and coupling angle dependence are experimentally investigated. A record ultrahigh CE of -0.58 dB with a 3 dB bandwidth of 71 nm and low back reflection are demonstrated.

Focusing sub-wavelength grating couplers with low back reflections for rapid prototyping of silicon photonic circuits

We demonstrate fully-etched fiber-waveguide grating couplers with sub-wavelength gratings showing high coupling efficiency as well as low back reflections for both transverse electric (TE) and transverse magnetic (TM) modes. The power reflection coefficients for the TE and TM modes have been significantly suppressed to -16.2 dB and -20.8 dB, respectively. Focusing grating lines have also been used to reduce the footprint of the design. Our sub-wavelength grating couplers for the TE and TM modes show respective measured insertion losses of 4.1 dB and 3.7 dB with 1-dB bandwidths of 30.6 nm (3-dB bandwidth of 52.3 nm) and 47.5 nm (3-dB bandwidth of 81.5 nm), respectively.

Focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces

We report on the design and characterization of focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces. With implementation of waveguide dispersion engineered subwavelength structures, an ultra-wide 1-dB bandwidth of over 100 nm (largest reported to date) near 1550 nm is experimentally achieved for transverse-electric polarized light. By tapering the subwavelength structures, back reflection is effectively suppressed and grating coupling efficiency is increased to -4.7 dB. A compact device footprint of 40 µm × 20 µm is realized by curving the gratings in a focusing scheme.

Subwavelength grating filtering devices

We propose and simulate the characteristics of optical filters based on subwavelength gratings. In particular, we demonstrate through numerical simulations the feasibility of implementing SWG Bragg gratings in silicon-on-insulator (SOI). We also propose SWG ring resonators in SOI and verify their operation using numerical simulations and experiments. The fabricated devices exhibit an extinction ratio as large as 30 dB and a Q-factor as high as ~20,000. These fundamental SWG filters can serve as building blocks for more complex devices.

Experimental demonstration of polarization-insensitive air-cladding grating couplers for silicon-on-insulator waveguides

We present an air-cladding apodized focusing subwavelength grating that can effectively couple two polarizations into a single waveguide. For the transverse magnetic mode, -3.2  dB maximum coupling efficiency with ∼28  nm 1 dB bandwidth is achieved. With the same grating, -4.3  dB maximum coupling efficiency with ∼58  nm 1 dB bandwidth is achieved for the transverse electric mode. The minimum difference between two polarizations' coupling peaks is demonstrated to be ∼32  nm. At the 1525 nm wavelength range, the polarization-insensitive grating is demonstrated with -6.5  dB coupling efficiency. The polarization-insensitive coupling wavelength can be controlled experimentally.

On-chip silicon optical phased array for two-dimensional beam steering

A 16-element optical phased array integrated on chip is presented for achieving two-dimensional (2D) optical beam steering. The device is fabricated on the silicon-on-insulator platform with a 250 nm silicon device layer. Steering is achieved via a combination of wavelength tuning and thermo-optic phase shifting with a switching power of P(π)=20  mW per channel. Using a silicon waveguide grating with a polycrystalline silicon overlay enables narrow far field beam widths while mitigating the precise etching needed for conventional shallow etch gratings. Using this system, 2D steering across a 20°×15° field of view is achieved with a sidelobe level better than 10 dB and with beam widths of 1.2°×0.5°.

Colorless grating couplers realized by interleaving dispersion engineered subwavelength structures

We investigate the waveguide dispersion of subwavelength structures, and propose that the waveguide dispersion can be reduced by reducing the period of subwavelength structures. A 3 dB bandwidth increment of 20% has been observed by introducing this concept into previously demonstrated grating couplers. To fully exploit the bandwidth merits of the structures, gratings with interleaved subwavelength structures were designed and fabricated. Two typical types of interleaving geometries have been investigated. Both demonstrated a 1 dB bandwidth ∼70  nm, a 3 dB bandwidth ∼117  nm, and a peak efficiency ∼-5.1  dB at 1570 nm for transverse-electric polarized light. The simulation confirms that the dispersion engineering adds an extra 12 nm to the 1 dB bandwidth.

Ultrahigh-efficiency apodized grating coupler using fully etched photonic crystals

We present an efficient method to design apodized grating couplers with Gaussian output profiles for efficient coupling between standard single mode fibers and silicon chips. An apodized grating coupler using fully etched photonic crystal holes on the silicon-on-insulator platform is designed, and fabricated in a single step of lithography and etching. An ultralow coupling loss of -1.74 dB (67% coupling efficiency) with a 3 dB bandwidth of 60 nm is experimentally measured.

Silicon nitride CMOS-compatible platform for integrated photonics applications at visible wavelengths

Silicon nitride is demonstrated as a high performance and cost-effective solution for dense integrated photonic circuits in the visible spectrum. Experimental results for nanophotonic waveguides fabricated in a standard CMOS pilot line with losses below 0.71dB/cm in an aqueous environment and 0.51dB/cm with silicon dioxide cladding are reported. Design and characterization of waveguide bends, grating couplers and multimode interference couplers (MMI) at a wavelength of 660 nm are presented. The index contrast of this technology enables high integration densities with insertion losses below 0.05 dB per 90° bend for radii as small as 35 µm. By a proper design of the buried oxide layer thickness, grating couplers with efficiencies above 38% for the TE polarization have been obtained.

Wavelength independent multimode interference coupler

We propose an ultra-broadband multimode interference (MMI) coupler with a wavelength range exceeding the O, E, S, C, L and U optical communication bands. For the first time, the dispersion property of the MMI section is engineered using a subwavelength grating structure to mitigate wavelength dependence of the device. We present a 2 × 2 MMI design with a bandwidth of 450nm, an almost fivefold enhancement compared to conventional designs, maintaining insertion loss, power imbalance and MMI phase deviation below 1dB, 0.6dB and 3°, respectively. The design is performed using an in-house tool based on the 2D Fourier Eigenmode Expansion Method (F-EEM) and verified with a 3D Finite Difference Time Domain (FDTD) simulator.

Photonic wire biosensor microarray chip and instrumentation with application to serotyping of Escherichia coli isolates

A complete photonic wire molecular biosensor microarray chip architecture and supporting instrumentation is described. Chip layouts with 16 and 128 independent sensors have been fabricated and tested, where each sensor can provide an independent molecular binding curve. Each sensor is 50 μm in diameter, and consists of a millimeter long silicon photonic wire waveguide folded into a spiral ring resonator. An array of 128 sensors occupies a 2 × 2 mm2 area on a 6 × 9 mm2 chip. Microfluidic sample delivery channels are fabricated monolithically on the chip. The size and layout of the sensor array is fully compatible with commercial spotting tools designed to independently functionalize fluorescence based biochips. The sensor chips are interrogated using an instrument that delivers sample fluid to the chip and is capable of acquiring up to 128 optical sensor outputs simultaneously and in real time. Coupling light from the sensor chip is accomplished through arrays of sub-wavelength surface grating couplers, and the signals are collected by a fixed two-dimensional detector array. The chip and instrument are designed so that connection of the fluid delivery system and optical alignment are automated, and can be completed in a few seconds with no active user input. This microarray system is used to demonstrate a multiplexed assay for serotyping E. coli bacteria using serospecific polyclonal antibody probe molecules.

Broadband focusing grating couplers for suspended-membrane waveguides

We propose and demonstrate broadband focusing grating couplers for suspended-membrane waveguides on silicon-on-insulator both in near-infrared (near-IR) and in mid-IR wavelength range. Finite-difference time-domain simulation predicts ∼100  nm 3 dB bandwidth with -1.7  dB coupling efficiency for an apodized grating in near-IR. -3.5  dB maximum coupling efficiency and ∼90  nm 3 dB bandwidth are realized experimentally. In mid-IR, -5.5  dB maximum coupling efficiency from a uniform focusing grating is measured at 2.75 μm, and ∼500  nm 3 dB bandwidth is predicted theoretically.

Polarization-independent grating coupler for micrometric silicon rib waveguides

Grating couplers are a promising approach to implement efficient fiber-chip coupling. However, their strong polarization dependence makes dual-polarization operation challenging. In this Letter we propose, for the first time, a polarization-independent grating coupler for thick rib silicon-on-insulator (SOI) waveguides. Coupling efficiency is optimized by designing the grating pitch and duty cycle, without varying the bottom oxide thickness, which significantly simplifies practical implementation. Directivity of the grating coupler is enhanced by a high reflectivity layer under the bottom oxide after the selective removal of the Si substrate. Dual-polarization coupling efficiency of -2.8  dB is shown.

Wideband subwavelength gratings for coupling between silicon-on-insulator waveguides and optical fibers

We propose and experimentally demonstrate a novel subwavelength grating coupler on silicon-on-insulator, for coupling to optical fibers with a wide optical bandwidth. Theoretical analysis and design optimization of the coupler are described. About 73 nm 1 dB bandwidth was experimentally demonstrated with -5.6  dB coupling efficiency. Better than -3.4  dB efficiency with 86 nm 1 dB bandwidth is predicted for these structures with optimized buried oxide thickness.

Demonstration of a curved sidewall grating demultiplexer on silicon

We experimentally demonstrate a new type of waveguide multiplexer device designed for silicon photonics, with a crosstalk level as low as -35 dB and an operational wavelength range of 300 nm. A compact device footprint of only 100 × 160 µm2 offers an excellent potential for integration with other silicon nanophotonic circuits.

Efficient light coupling into in-plane semiconductor nanomembrane photonic devices utilizing a sub-wavelength grating coupler

We report a subwavelength grating (SWG) coupler for coupling light efficiently into in-plane semiconductor nanomembrane photonic devices for the first time. The SWG coupler consists of a periodic array of rectangular trenches fabricated on a silicon nanomembrane (SiNM) transferred onto a glass substrate. At a wavelength of 1555.56 nm, the coupling efficiency of the fabricated 10 µm wide, 17.1 µm long SWG is 39.17% (-4.07 dB), with 1 dB and 3 dB bandwidths of 29 nm and 57 nm, respectively. Peak efficiency varies by 0.26 dB when measuring 5 fabricated grating pairs. Coupling efficiency can further be improved with an improved SiNM transfer process. Such high efficiency couplers allow for the successful realization of a plethora of hybrid photonic devices utilizing nanomembrane technology.