Grating-Assisted Fiber to Chip Coupling for SOI Photonic Circuits

A 2 μm Wavelength Band Low-Loss Spot Size Converter Based on Trident Structure on the SOI Platform

A 2 μm wavelength band spot size converter (SSC) based on a trident structure is proposed, which is coupled to a lensed fiber with a mode field diameter of 5 μm. The cross-section of the first segment of the tapered waveguide structure in the trident structure is designed as a right-angled trapezoidal shape, which can further improve the performance of the SSC. The coupling loss of the SSC is less than 0.9 dB in the wavelength range of 1.95~2.05 μm simulated by FDTD. According to the experimental results, the lowest coupling loss of the SSC is 1.425 dB/facet at 2 μm, which is close to the simulation result. The device is compatible with the CMOS process and can provide a good reference for the development of 2 μm wavelength band integrated photonics.

Ultralow-loss optical interconnect enabled by topological unidirectional guided resonance

Grating couplers that interconnect photonic chips to off-chip components are crucial for various optoelectronics applications. Despite numerous efforts in past decades, the existing grating couplers are still far from optimal in energy efficiency and thus hinder photonic integration toward a larger scale. Here, we propose a strategy to achieve ultralow-loss grating couplers by using unidirectional guided resonances (UGRs), suppressing the useless downward radiation with no mirror on the bottom. By engineering the dispersion and apodizing the geometry of grating, we experimentally realize a grating coupler with a record-low loss of -0.34 dB and 1-dB bandwidth exceeding 30 nm at the telecom wavelength of 1550 nm and further demonstrate an optic via with a loss of only -0.94 dB. Given that UGRs ubiquitously exist in a variety of grating geometries, our work sheds light on a systematic method to achieve energy-efficient optical interconnect and paves the way to large-scale photonic integration.

Integrated polarization-free Bragg filters with subwavelength gratings for photonic sensing

We present polarization-free Bragg filters having subwavelength gratings (SWGs) in the lateral cladding region. This Bragg design expands modal fields toward upper cladding, resulting in enhanced light interaction with sensing analytes. Two device configurations are proposed and examined, one with index-matched coupling between transverse electric (TE) and transverse magnetic (TM) modes and the other one with hybrid-mode (HM) coupling. Both configurations introduce a strong coupling between two orthogonal modes (either TE-TM or HM1-HM2) and rotate the polarization of the input wave through Bragg reflection. The arrangements of SWGs help to achieve two configurations with different orthogonal modes, while expanding modal profiles toward the upper cladding region. Our proposed SWG-assisted Bragg gratings with polarization independency eliminate the need for a polarization controller and effectively tailor the modal properties, enhancing the potential of integrated photonic sensing applications.

Polarization independent grating in a GaN-on-sapphire photonic integrated circuit

In this work, we report the realization of a polarization-insensitive grating coupler, single-mode waveguide, and ring resonator in the GaN-on-sapphire platform. We provide a detailed demonstration of the material characterization, device simulation, and experimental results. We achieve a grating coupler efficiency of -5.2 dB/coupler with a 1 dB and 3 dB bandwidth of 40 nm and 80 nm, respectively. We measure a single-mode waveguide loss of -6 dB/cm. The losses measured here are the lowest in a GaN-on-sapphire photonic circuit. This demonstration provides opportunities for the development of on-chip linear and non-linear optical processes using the GaN-on-sapphire platform. To the best of our knowledge, this is the first demonstration of an integrated photonic device using a GaN HEMT stack with 2D electron gas.

Advances in silicon-based, integrated tunable semiconductor lasers

Tunable semiconductor lasers have many important applications such as wavelength division multiplexing, light detection and ranging, and gas detection. The increased interest in silicon photonics has led to the rapid development of miniaturized on-chip tunable semiconductor lasers. However, silicon has poor light-emitting properties. Therefore, realizing high-performance tunable semiconductor lasers requires the integration of light sources with silicon. In this study, we review silicon-based light source integration methods and the development of silicon-based integrated tunable semiconductor lasers. Considering that narrow-linewidth performance greatly expands the applications of tunable semiconductor lasers, methods for reducing the linewidth of tunable lasers are summarized. Finally, the development trends and prospects for silicon-based integrated light sources and silicon-based integrated tunable lasers are analyzed and discussed.

Modelling, characterization, and applications of silicon on insulator loop terminated asymmetric Mach Zehnder interferometer

This work presents a loop terminated asymmetric Mach-Zehnder interferometer (LT-aMZI) structure based on the widespread silicon-on-insulator (SOI) platform. Four different path length differences of the LT-aMZI, which correspond to free spectral ranges (FSR) from 0.8 to 6.4 nm, are designed. These designs are compared to the common asymmetric Mach-Zehnder interferometer (C-aMZI) and are shown to be more compact. These devices are suitable for optical filtering as well as wavelength demultiplexing (WDM) applications. A compact analytical model is derived that accurately describe the operation of the LT-MZI devices. The designs are then fabricated using Electron Beam Lithography (EBL) and characterized. The experimental data show good agreement when compared to the simulation results. To our knowledge, this is the first time LT-aMZI fabrication and characterization. Moreover, the LT-MZI spectrum can be tuned not only by the interferometer arms phase difference like C-MZI, but also by using its directional couplers coefficients, forming a spectral tunable filter. Finally, we determine the performance parameters of optical sensors and modulators and show that our proposed LT-MZI structure will enhance the sensor figure of merit (FOM) and modulator speed, power consumption and V_π × L compared to C-MZI. A comparison between symmetric and asymmetric MZI sensors and the advantage of the latter is also mentioned.

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.

Subwavelength Grating Double Slot Waveguide Racetrack Ring Resonator for Refractive Index Sensing Application

In this paper, a racetrack ring resonator design based on a subwavelength grating double slot waveguide is presented. The proposed waveguide scheme is capable of confining the transverse electric field in the slots and the gaps between the grating segments. This configuration facilitates a large light-matter interaction which elevates the sensitivity of the device approximately 2.5 times higher than the one that can be obtained via a standard slot waveguide resonator. The best sensitivity of the design is obtained at 1000 nm/RIU by utilizing a subwavelength grating double slot waveguide of period 300 nm. The numerical study is conducted via 2D and 3D finite element methods. We believe that the proposed sensor design can play an important role in the realization of highly sensitive lab-on-chip sensors.

Efficiency Enhanced Grating Coupler for Perfectly Vertical Fiber-to-Chip Coupling

In this work, a bidirectional grating coupler for perfectly vertical coupling is proposed. The coupling efficiency is enhanced using a silicon nitride (Si₃N₄) layer above a uniform grating. In the presence of Si₃N₄ layer, the back-reflected optical power into the fiber is diminished and coupling into the waveguide is increased. Genetic algorithm (GA) is used to optimize the grating and Si₃N₄ layer simultaneously. The optimal design obtained from GA shows that the average in-plane coupling efficiency is enhanced from about 57.5% (−2.5 dB) to 68.5% (−1.65 dB), meanwhile the average back-reflection in the C band is reduced from 17.6% (−7.5 dB) to 7.4% (−11.3 dB). With the help of a backside metal mirror, the average coupling efficiency and peak coupling efficiency are further increased to 87% (−0.6 dB) and 89.4% (−0.49 dB). The minimum feature size of the designed device is 266 nm, which makes our design easy to fabricate through 193 nm deep-UV lithography and lowers the fabrication cost. In addition, the coupler proposed here shows a wide-band character with a 1-dB bandwidth of 64 nm and 3-dB bandwidth of 96 nm. Such a grating coupler design can provide an efficient and cost-effective solution for vertical fiber-to-chip optical coupling of a Wavelength Division Multiplexing (WDM) application.

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.

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.