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

Loss analysis of a grating coupler for single-mode fiber coupling into an SOI waveguide

Silicon-on-insulator (SOI) technology is widely used in silicon photonic integrated circuits. How to improve the coupling efficiency of the light coupling in free space and optical fibers into waveguides on SOI must be discussed. Grating coupling is a commonly used and highly efficient coupling method. This article discusses the causes of loss in grating couplers from three aspects: transmission, reflection, and mode mismatch, and proposes corresponding loss reduction solutions. The coupling efficiency of Si and SiN grating couplers optimized according to the loss reduction scheme has been improved by 25% and 45%, respectively.

Compact integrated mode-size converter using a broadband ultralow-loss parabolic-mirror collimator

In this Letter, we propose and demonstrate an integrated mode-size converter (MSC) with a compact footprint, low losses, and a broad bandwidth. By exploiting a parabolic mirror, the divergent light from a narrow waveguide (450 nm) is collimated to match the mode size of a wide waveguide (10 µm). The measured insertion loss (IL) is ≈ 0.15 dB over a 100-nm bandwidth. The mode-size conversion is achieved with a footprint as small as ≈ 20 × 32 µm², which is much shorter than the linear taper length required to attain the same level of losses.

Reconfigurable fiber-to-waveguide coupling module enabled by phase-change material incorporated switchable directional couplers

In silicon photonics, grating-assisted fiber-to-waveguide couplers provide out-of-plane coupling to facilitate wafer-level testing; however, their limited bandwidth and efficiency restrict their use in broadband applications. Alternatively, end-fire couplers overcome these constraints but require a dicing process prior usage, which makes them unsuitable for wafer-level testing. To address this trade-off, a reconfigurable fiber-to-waveguide coupling module is proposed and designed to allow for both grating-assisted and end-fire coupling in the same photonic circuit. The proposed module deploys a switchable directional coupler incorporating a thin layer of phase-change material, whose state is initially amorphous to render the coupler activated and hence facilitate grating-assisted coupling for wafer-level testing. The state can be altered into crystalline through a low-temperature annealing process to deactivate the directional coupler, thus facilitating broadband chip-level coupling through end-fire couplers. All the components encompassing conjoined switchable directional couplers as well as the grating and end-fire couplers were individually designed through rigorous simulations. They were subsequently assembled to establish the proposed reconfigurable coupling module, which was simulated and analyzed to validate the selective coupling operation. The proposed module gives rise to a low excess loss below 1.2 dB and a high extinction ratio over 13 dB throughout the C-band, when operating either under grating-assisted or end-fire input. The proposed reconfigurable coupling module is anticipated to be a practical solution for flexibly expediting the inspection of integrated photonic circuits on a wafer scale.

Metamaterial-based ultrashort multimode waveguide taper with low intermodal crosstalk

We propose and theoretically demonstrate an ultrashort multimode waveguide taper based on the all-dielectric metamaterial. Attributed to the gradient index distribution of the metamaterial, the spot sizes of the four lowest-order transverse magnetic (TM) modes can be expanded in a short distance of 6 μm with negligible mode conversions. Numerical results prove that the insertion losses of the taper are lower than 1 dB, 1.12 dB, 1.26 dB and 1.66 dB for the TM₀ - TM₃ modes, respectively, and the intermodal crosstalk values are below -15 dB for the four modes, both in the wavelength range of 1.5 μm - 1.6 μm. To the best of our knowledge, this is the first multimode waveguide taper that has low intermodal crosstalk of < -15 dB over a 100-nm bandwidth.

Silicon nitride waveguide devices based on gradient-index lenses implemented by subwavelength silicon grating metamaterials

The rapid development of photonic integrated circuits demands the design of efficient and compact waveguide devices such as waveguide tapers and crossings. Some components in the silicon nitride (SiN) waveguide platform are superior to their counterparts in the silicon waveguide platform. Designing a compact SiN waveguide taper and crossing is crucial to reduce the size of SiN photonic components. In this paper, we utilize the focusing property of the Luneburg lens to design an SiN taper connecting a 10-µm-wide waveguide to a 1-µm-wide waveguide. Three-dimensional full-wave simulations indicate that the designed 13-µm-long taper has an average transmission efficiency of 92% in the wavelength range of 1500-1600 nm. We also present an in-plane SiN waveguide crossing based on the imaging property of the square Maxwell's fisheye lens designed with quasi-conformal transformation optics. The designed waveguide crossing occupies a compact footprint of 5.65µm×5.65µm, while its average insertion loss is 0.46 dB in the bandwidth of 1500-1600 nm. To the best our knowledge, the designed SiN waveguide taper and crossing have the smallest footprints to date.

Crosstalk reduction of integrated optical waveguides with nonuniform subwavelength silicon strips

Suppression of the crosstalk between adjacent waveguides is important yet challenging in the development of compact and dense photonic integrated circuits (PICs). During the past few years, a few of excellent approaches have been proposed to achieve this goal. Here, we propose a novel strategy by introducing nonuniform subwavelength strips between adjacent waveguides. In order to determine the widths and positions of nonuniform subwavelength strips, the particle swarm optimization (PSO) algorithm is utilized. Numerical results demonstrate that the coupling length between adjacent waveguides is increased by three (five) orders of magnitude in comparison with the case of uniform (no) subwavelength strips. Our method greatly reduces crosstalk and is expected to achieve a highly compact integrated density of PICs.

A high efficiency silicon nitride waveguide grating coupler with a multilayer bottom reflector

We propose a high efficiency apodized grating coupler with a bottom reflector for silicon nitride photonic integrated circuits. The reflector consists of a stack of alternate silicon nitride and silicon dioxide quarter-wave films. The design, fabrication and optical characterization of the couplers has been presented. The measured fiber to detector insertion loss was −3.5 dB which corresponds to a peak coupling efficiency of −1.75 dB. A 3 dB wavelength bandwidth of 76.34 nm was demonstrated for the grating coupler with a 20-layer reflector. The fabrication process is CMOS-compatible and requires only a single etching step.

Alignment-tolerant broadband compact taper for low-loss coupling to a silicon-on-insulator photonic wire waveguide

We experimentally demonstrate a broadband, fabrication-tolerant compact silicon waveguide taper (34.2 μm) in a silicon-on-insulator wire waveguide. The taper works on multimode interference along the length of the taper. A single taper design has broadband operation with coupling efficiency >70% over 700 nm that can be used in O-, C-, and L-bands. The compact taper is highly tolerant to fabrication variations; ±100  nm change in the taper and end waveguide width varies the taper transmission by <5%. The footprint of the device, i.e., the taper along with linear gratings, is ≈442  μm², 11.5× smaller than the adiabatic taper. The taper with linear gratings provides coupling efficiency comparable to standard focusing gratings. We have also experimentally compared the translational and rotational alignment tolerance of the focusing grating with linear grating couplers.