dB Coupling Efficiency Research Articles

High-efficient subwavelength structure engineered grating couplers for 2-µm waveband high-speed data transmission.

The 2-µm waveband is becoming an emerging window for next-generation high-speed optical communication. To enable on-chip high-speed data transmission, improving the signal-to-noise ratio (SNR) by suppressing the coupling loss of a silicon chip is critical. Here, we report grating couplers for TE and TM polarized light at the 2-µm waveband. With a single-step fully etched process on the 340 nm silicon-on-insulator (SOI) platform, the devices experimentally demonstrate high coupling efficiency of -4.0 dB and 1-dB bandwidth of 70 nm for the TE polarized light, while -4.5 dB coupling efficiency and 58 nm 1-dB bandwidth for the TM polarized light. For comprehensive performance, both of them are among the best grating couplers operating in the 2-µm waveband so far. We also demonstrate 81Gbps high-speed on-chip data transmission using pulse amplitude modulation 8-level (PAM-8) signals.

Ball Lens Embedded Through-Package Via To Enable Backside Coupling Between Silicon Photonics Interposer and Board-Level Interconnects

Development of an efficient and densely integrated optical coupling interface for silicon photonics based board-level optical interconnects is one of the key challenges in the domain of 2.5D/3D electro-optic integration. Enabling high-speed on-chip electro-optic conversion and efficient optical transmission across package/board-level short-reach interconnections can help overcome the limitations of a conventional electrical I/O in terms of bandwidth density and power consumption in a high-performance computing environment. In this context, we have demonstrated a novel optical coupling interface to integrate silicon photonics with board-level optical interconnects. We show that by integrating a ball lens in a via drilled in an organic package substrate, the optical beam diffracted from a downward directionality grating on a photonics chip can be coupled to a board-level polymer multimode waveguide with a good alignment tolerance. A key result from the experiment was a 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m chip-to-package 1-dB lateral alignment tolerance for coupling into a polymer waveguide with a cross-section of 20 × 25 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^2$</tex-math></inline-formula> . An in-depth analysis of loss distribution across several interfaces was done and a −3.4 dB coupling efficiency was measured between the optical interface comprising of output grating, ball lens and polymer waveguide. Furthermore, it is shown that an efficiency better than −2 dB can be achieved by tweaking few parameters in the coupling interface. The fabrication of the optical interfaces and related measurements are reported and verified with simulation results.

Femtosecond laser micro-machining enabled all-fiber mode selective converter.

We demonstrate a compact all-fiber mode selective converter enabled by femtosecond laser micro-machining on the few-mode fiber (FMF) facet. By introducing a micro-structure into the FMF core, we can achieve a π spatial phase difference to the fundamental mode of light to be converted. Theoretical optimization reveals that various high-order modes, including LP11, LP02, and LP21 modes, can be selectively converted by various micro-structures on the FMF facet, with a mode extinction ratio of more than 25 dB and mode coupling efficiency of better than 45% over the C-band. Finally, a proof-of-concept experiment is conducted by inscribing a micro-slot on the two-mode fiber facet for the LP01 to LP11 mode conversion. A micro-slot with a width of 6.7 µm and a depth of 5.4 µm is fabricated under the optimal femtosecond laser parameters. A LP01 to LP11 mode conversion with an average insertion loss of 2.7 dB is realized over the C-band, together with a mode intensity profile correlation efficient of more than 65%. Efficient higher-order mode conversion is feasible with a precise femtosecond laser micro-machining.

Efficient arsenic trisulfide vertical grating coupler on lithium niobate for integrated photonic applications

An efficient vertical grating coupler design for arsenic trisulfide (As2S3) on silicon dioxide (SiO2) on lithium niobate (LN) is proposed, fabricated, and experimentally verified. We report 4dB coupling efficiency per grating for vertical fiber coupling at a wavelength of 1550nm with a 3dB bandwidth of 40nm using an aluminum reflector mirror between the LN and SiO2 interface. This coupler is the first step towards the demonstration of high-performance integrated photonic devices, which would simultaneously benefit from the acousto-optic properties of As2S3 and electro-optic and acoustic properties of LN. This hybrid platform is deemed to impact a broad range of applications such as imaging, ranging, and inertial sensing.

Ultra-Efficient Silicon-on-Insulator Grating Couplers With Backside Metal Mirrors

Coupling of light from off-chip into highly complex silicon-based devices is currently focus of efforts and research. In this work, the experimental demonstration of a −0.50 dB (89%) coupling efficiency in the C-band with the help of an aperiodic grating coupler design in a 250 nm silicon-on-insulator (SOI) technology is shown. Further, this work reports about the latest results regarding focusing grating couplers with footprints of only 30 μ m × 30 μm and a measured coupling efficiency of up to −0.93 dB (81%) at 1549 nm. Efficient grating coupler arrays are realized by using an expanded backside metal mirror-strip for the application of fiber arrays. In addition, this work presents aperiodic silicon grating couplers with an enhanced bandwidth to overcome the limited usability of grating couplers in wavelength division multiplexing (WDM) systems.

2-D Grating Couplers for Vertical Fiber Coupling in Two Polarizations

We demonstrate highly efficient couplers in a two-dimensional grating configuration for a vertical fiber-coupling fabricated in the silicon-on-insulator platform. A combination of sub-wavelength reflectors and blazed gratings enables both high coupling efficiency and vertical grating coupling. We experimentally achieve coupling efficiencies of −2.6 dB at 1544 nm and −3.4 dB at 1536 nm for x- and y-polarized LP01 modes, respectively. The experimental results are in good agreement with the simulated −2.4 dB coupling efficiency for both polarizations at a wavelength of 1550 nm. Simulated and measured crosstalk between two polarizations were −19 dB and <−16 dB, respectively.

Optimising apodized grating couplers in a pure SOI platform to -0.5 dB coupling efficiency.

We present a theoretical optimisation of 1D apodized grating couplers in a "pure" Silicon-On-Insulator (SOI) architecture, i.e. without any bottom reflector element, by means of a general mutative method. We perform a comprehensive 2D Finite Difference Time Domain study of chirped and apodized grating couplers in 220 nm SOI, and demonstrate that the global maximum coupling efficiency in that platform is capped to 65% (-1.9 dB). Moving to designs with thicker Si-layers, we identify a new record design in 340 nm SOI, with a simulated coupling efficiency of 89% (-0.5 dB). Going to thicker Si layers does not further improve the efficiency, implying that -0.5 dB may be a global maximum for a grating coupler in SOI without a bottom-reflector. Even after allowing for 193 nm UV-lithographic fabrication constraints, the 340 nm design still offers -0.7 dB efficiency. These new apodized designs are the first pure SOI couplers compatible with deep-UV lithography to offer better than -1 dB insertion losses. With only very minor changes to existing deposition and lithography recipes, they are compatible with the multi-project wafer runs already offered by Si-Photonics foundries.

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.

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.

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 3dB bandwidth with -1.7 dB coupling efficiency for an apodized grating in near-IR. -3.5 dB maximum coupling efficiency and ∼90 nm 3dB bandwidth are realized experimentally. In mid-IR, -5.5 dB maximum coupling efficiency from a uniform focusing grating is measured at 2.75μm, and ∼500 nm 3dB bandwidth is predicted theoretically.

Apodized focusing subwavelength grating couplers for suspended membrane waveguides

We demonstrate an apodized focusing subwavelength grating (SWG) for suspended membrane waveguides on silicon-on-insulator. Finite-difference time-domain simulation predicts −1.7 dB coupling efficiency and a 3 dB bandwidth of ∼50 nm for the transverse-magnetic mode apodized SWG, which has 98% field overlap with propagation mode in the single mode fiber. A modified phase matching formula is proposed to design the focusing apodized SWG. Better than −3.0 dB coupling efficiency and a 3 dB optical bandwidth of ∼50 nm is demonstrated experimentally.

High-Efficiency SOI Fiber-to-Chip Grating Couplers and Low-Loss Waveguides for the Short-Wave Infrared

We report on high-efficiency silicon-on-insulator (SOI) grating couplers and low-loss single-mode optical waveguides operating in a short-wave infrared. A -3.8 dB coupling efficiency from a standard single-mode fiber to an SOI waveguide at 2.1 μm is obtained experimentally. Single-mode waveguide losses in the short-wave infrared below 0.6 dB/cm are reported.

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.

CMOS compatible integration of Si/SiO_2 multilayer GRIN lens optical mode size converter to Si wire waveguide

We present a CMOS compatible mass manufacturable, compact Si/SiO(2) multilayer GRIN lens mode size converter from standard single mode fiber to 300 nm-thick Si waveguide. The fiber-to-GRIN lens coupling loss is 2.6 ± 0.3 dB (coupling efficiency: 51~60%) with optimized focal length of 11.6~11.8 μm and Si/SiO(2) multilayer thickness of 7.4 μm.

Single etch grating couplers for mass fabrication with DUV lithography

Surface grating couplers enable efficient coupling of light between optical fibers and planar waveguide circuits. While traditional grating designs require two etch steps for efficient coupling to silicon-on-insulator waveguides, recently proposed subwavelength structured gratings can achieve the same coupling efficiencies with a single etch step, thereby significantly reducing fabrication complexity. Here we demonstrate that such couplers can be fabricated on a large scale with ultra-violet lithography, achieving a 5 dB coupling efficiency at 1,550 nm. Through both simulations and experiments we give physical insight on how pattern fidelity impacts the performance of these couplers, and propose strategies to deal with inevitable process variations.

Optofluidic waveguides for reconfigurable photonic systems

We report the development of two liquid waveguide based photonic elements for use in reconfigurable photonic systems. This work demonstrates the ability to couple light from a conventional optical fiber to an adaptable liquid-core/liquid-cladding waveguide and back again to an optical fiber(s) enabling us to take advantage of both liquid- and solid-state photonic modalities. We demonstrate and characterize the use of this fiber-in and fiber-out system as either an optical switch or signal attenuator. Microscale flow control enables the adaptive morphology and tunable position of the liquid waveguide yielding an attenuation range of 3.1-10.7 dB, operability over a broad bandwidth spanning the range of wavelengths from visible to telecommunication, and a 1x2 sub-second switching system with a cross-talk as low as 20 dB and maximum coupling efficiency of 3.87 dB.

Technique of microball lens formation for efficient optical coupling

A batch-fabricated microball lens array not only provides accurate coupling distances but also replaces traditional components such as aspheric lenses and expensive graded-index lenses without sacrificing performance and reduces assembly cost. The results of extensive experiments show a critical aspect ratio of approximately 0.3. That is, when the aspect ratio is larger than 0.3, the final shape of a reflowed lens changes to that of a microball rather than of a mushroom. Using a laser with a 633-nm wavelength yields an optimum coupling distance of approximately 8 microm with an insertion loss below -1.3 dB (coupling efficiency, approximately 73%).

Semiconductor laser to single-mode fiber coupler

Promising results of various coupling experiments between laser diodes and single-mode fibers to determine the optimum coupling method are reported. The cylindrical lens method is shown to be most efficient, with a coupling loss of 1.8 dB under optimum alignment conditions. More than -2 dB coupling efficiency is attained with a cylindrical lens whose radius is less than 8 microm. The laser to fiber coupling characteristics are estimated by Gaussian beam approximation. The optimum radius of a cylindrical lens, which gives maximum coupling efficiency, is derived by theoretical calculation, using the ray matrix method. The cylindrical lens alignment tolerance is also shown theoretically and experimentally.