Grating Coupler Research Articles

Thermo-optic tunable optical filters with GHz-bandwidth and flat-top passband on thin film lithium niobate platform.

Lithium niobate on insulator (LNOI) is a new photonic integrated platform that provides high optical confinement and retains the inherent excellent properties of lithium niobate (LN). Tunable filters are one of the indispensable devices for integrated optics. Here we design and fabricate a thermo-optic (TO) tunable optical filter using two cascaded racetrack microring resonators (MRRs) based on LNOI. The filter shows a narrow and flat top passband with intra band ripple less than 0.3 dB, 3 dB bandwidth of 4.8 GHz and out-of-band rejection of about 35 dB. The insertion loss of the filter is about -14 dB, including grating coupling loss about -6.5 dB and on-chip loss less than -1 dB. The heating power for center wavelength shift of the filter is about 89.4 mW per free spectral range (FSR). Relevant applications of such filters include optical information processing and microwave photonics.

A Novel Grating Coupling Coefficient Extraction Method for DFB Laser Diodes Based on Side Mode Spacing

A novel method to extract the grating coupling coefficient of distributed feedback (DFB) lasers by comparing the theoretical and experimental values of the side mode spacing is demonstrated. Compared with the traditional method, the proposed method in this paper transforms the solution process of the lasing model with multiple unknown parameters into that with only the side mode spacing and coupling coefficient, which significantly reduces the computational workload. Furthermore, the bias current of the measured spectrum can be much higher than the threshold current, which makes the method less affected by noise. This paper theoretically analyzes the changing relationship between multiple parameters by calculating the lasing mode distribution, and the results show that the side mode spacing is only sensitive to the coupling coefficient. In addition, the grating coupling coefficients (57–61 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) of the fabricated DFB laser diodes are experimentally extracted at 40-80 mA currents. The variation of the coupling coefficient with current is less than 2‰/mA, and the method exhibits pretty good stability. Meanwhile, the grating coupling coefficient extraction method for the complex grating types is also considered in this paper.

Independent Control of Mode Selection and Power Extraction in Terahertz Semiconductor Lasers

Mode selection and power extraction are the core of single-mode semiconductor lasers, but generally, they restrict each other. A large gain area with high radiation efficiency is necessary for high output power but will induce more competitive modes and increase the threshold gain of the desired mode. Here, we demonstrate a novel laser cavity─an active distributed Bragg reflector (ADBR) and a grating coupler (GC) are monolithically integrated into a laser ridge─enabling independent control of mode selection and power extraction. The ADBR features a reflection peak with an extremely narrow bandwidth that releases the constraint on the cavity length, while the GC provides a controllable high radiation efficiency. The concept is implemented onto terahertz quantum cascade lasers, exhibiting robust single-mode emission with high output power and high operating temperature. Given the universality of the ADBR and GC, our concept can be utilized in different material systems at different wavelengths.

High-efficiency grating coupler for an ultralow-loss Si3N4-based platform.

Integrated silicon nitride waveguides of 100 nm height can achieve ultralow propagation losses below 0.1 dB/cm at the 1550 nm wavelength band but lack the scattering strength to form efficient grating couplers. An enhanced grating coupler design based on an amorphous silicon layer on top of silicon nitride is proposed and demonstrated to improve the directionality of the coupler. The fabrication process is optimized for a self-alignment process between the amorphous silicon and silicon nitride layers without increasing waveguide losses. Experimental coupling losses of 5 dB and a 3 dB bandwidth of 75 nm are achieved with both regular and focusing designs.

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.

Optimizing two-dimensional polarization-diversity metagrating couplers for silicon photonics

Polarization-diversity couplers are promising industrially scalable optical devices that can couple optical signals with unknown polarization states into silicon photonic chips. Here we propose a novel, to the best of our knowledge, two-dimensional (2D) metagrating coupler (MGC) for the polarization-diversity system, which improves its coupling efficiency for both polarizations. Compared with the previously reported 2D grating couplers (GCs) with identical grating cells, the proposed apodized design gradually changes the aspect ratio and orientation angles of the elliptical patterns simultaneously. This 2D array of the varied grating cells modulates the diffracted mode field patterns and phases locally, and achieves a better overlap with the Gaussian fiber modes for both polarizations. The peak coupling loss (PCL) for the S-polarized and P-polarized light are − 1.6 d B and − 2.7 d B , respectively. The calculated polarization dependent loss is lower than 0.2 dB from 1522 nm to 1540 nm. The device is robust to the fabrication tolerance and fiber misalignment. The simulated fabrication tolerance analysis show that the etch depth error of ±20 nm results in less than 0.3 dB and 0.1 dB PCL drop, while the grating cell feature size error of ± 40 n m results in less than 0.6 dB and 0.2 dB PCL drop for both types of polarization light, respectively. The fiber misalignment should be within the range from − 2 µ m to + 2 µ m in two perpendicular directions, and the coupling angle deviation should be within the range from − 3 ∘ to + 3 ∘ , in order to assure 0.5 dB penalty loss for both polarizations. The performance of the design is insensitive to the mask misalignments. To assure 0.5 dB penalty loss for both polarizations, Δ θ is suggested to be within the range from − 3 ∘ to + 3 ∘ .

Wavelength selection in focusing non-uniform grating couplers: A coupling efficiency curve asymmetry and bandwidth analysis

We report a combined experimental and computational investigation of the multi-wavelength response of a series of focusing non-uniform grating couplers designed on multiple SOI thicknesses interfaced with two different fiber coupling schemes. In particular, we demonstrate either the existence of an asymmetry of the coupling efficiency curve shape, with respect to the tuning wavelength (1550 nm), related to the metastructure features, and how the photonic band structure of such metastructures can be calculated and exploited to evaluate their multi-wavelength response, in terms of multi-BW analysis. These results can be a positive stimulus for a further comprehension of the electromagnetic response of non-uniform grating couplers, particularly interesting as a valid option for integrated photonics applications.

Foundry-fabricated grating coupler demultiplexer inverse-designed via fast integral methods

Silicon photonics is an emerging technology which, enabling nanoscale manipulation of light on chips, impacts areas as diverse as communications, computing, and sensing. Wavelength division multiplexing is commonly used to maximize throughput over a single optical channel by modulating multiple data streams on different wavelengths concurrently. Traditionally, wavelength (de)multiplexers are implemented as monolithic devices, separate from the grating coupler, used to couple light into the chip. This paper describes the design and measurement of a grating coupler demultiplexer—a single device which combines both light coupling and demultiplexing capabilities. The device was designed by means of a custom inverse design algorithm which leverages boundary integral Maxwell solvers of extremely rapid convergence as the mesh is refined. To the best of our knowledge, the fabricated device enjoys the lowest insertion loss reported for grating demultiplexers, small size, high splitting ratio, and low coupling-efficiency imbalance between ports, while meeting the fabricability constraints of a standard UV lithography process.

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.

High-efficiency grating coupler based on fast directional optimization and robust layout strategy in 130 nm CMOS process.

We experimentally demonstrated a high-efficiency grating coupler by combining an interleaved etch and apodized structure for fiber-to-chip coupling. The grating coupler was optimized using the fast directional optimization method to achieve apodization. The grating coupler utilized a layout strategy involving an extended mask to avoid alignment errors for a multi-etch structure. The coupling efficiency was measured to be -2.2 dB at a wavelength of 1549 nm with a 3 dB bandwidth of 47 nm. The grating coupler, having no gold reflector, subwavelength index matching structure, or additional material layers, was fabricated using a commercial silicon photonics process with a minimum feature size of 140 nm. This grating coupler design provides a robust and effective coupling scheme and the proposed method can be employed to adopt the design in accordance with standard foundry design rules.

Exciton-polariton dispersion in an A III B V total internal reflection planar waveguide with a quantum well

In this work we propose means to determine the dispersion of light in a semiconductor total internal reflection (TIR) planar waveguide featuring dielectric response of excitons from a quantum well (QW). The given theoretical apparatus is then used to reproduce the data experimentally acquired when probing the AlGaAs-based waveguide (with a GaAs QW in its waveguiding layer) with a continuous laser through a coupling grating and then gathering the light transmitted through the waveguiding layer.

Fiber-Chip Bi-Wavelength Multiplexing With Subwavelength Single-Etch Grating Coupler and Diplexer

We propose a silicon photonicgrating coupling scheme for dual-band multiplexing in optical fibers. By utilizing subwavelength grating effective refractive index engineering, a dual-band operating grating coupler and a wavelength-polarization diplexer are designed. Both devices are fabricated on a 220-nm silicon-on-insulator and require only one 220-nm full etch step to facilitate cost-effective production. The subwavelength grating coupler guides S-band and O-band lights from a single-mode fiber into a waveguide as transverse-electric and transverse-magnetic modes, while the subwavelength diplexer separates the two signals into two waveguides. Experimentally, our proposed scheme for coupling and multiplexing can achieve a total insertion loss of –4.26 dB at near 1490 nm and –5.86 dB at near 1310 nm.

Direct optical wire bonding through open-to-air polymerization for silicon photonic chips.

We developed an inter-chip optical link using direct optical wire (DOW) bonding by open-to-air polymerization. An arch-shaped wire was drawn from a tip in a similar way to a metal wire, but the wire was formed from a polymer solution that solidified in the air during wiring. The DOW bonding was examined for silicon photonic chips where grating couplers are integrated for input/output coupling. Cone-shaped studs were formed at the ends of the wire, and their geometry was optimized using finite-difference time-domain simulation to give a mode conversion function. Although the polymer wire had a multimode scale of 7 µm, the wire bonding between the grating couplers showed a relatively low insertion loss of 5.8 dB at a wavelength of 1590 nm compared to a conventional connection using single-mode fiber blocks. It also showed a larger wavelength tolerance within the range of ∼1520-1590 nm. DOW bonding between a grating coupler and a single-mode fiber were also examined to verify the feasibility of out-of-plane connection with edge-coupling devices. The grating-to-fiber wire link also exhibited a large wavelength tolerance.

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.

Double-groove rectangular gratings for high-efficiency wideband vertical coupling in planar-integrated optical systems

Double-groove rectangular gratings for high-efficiency wideband vertical coupling in planar-integrated optical systems

­­­­ Ultra-Broadband and Highly Efficient Silicon Nitride Bi-Layer Grating Couplers

­­­­ Ultra-Broadband and Highly Efficient Silicon Nitride Bi-Layer Grating Couplers

On-Chip Terahertz Demultiplexer and Grating Coupler Based on Reverse Design

On-Chip Terahertz Demultiplexer and Grating Coupler Based on Reverse Design

Research on High-Efficiency Coupling Gratings Applied to Silicon Germanium Materials

Research on High-Efficiency Coupling Gratings Applied to Silicon Germanium Materials

Demonstration of in-depth analysis of silicon photonics circuits using OFDR: waveguides with grating couplers.

Optical frequency domain reflectometry (OFDR) is a powerful technique to investigate backscatter in waveguides. However, its use in Si photonics circuits has so far been limited to measuring the propagation loss and group index of a waveguide. We demonstrate that the transmittance (T) and reflectance (R) of elemental devices comprising a Si photonics circuit can be determined by OFDR. An analysis of Si wire waveguides with grating couplers (GCs) is described in detail. The wavelength dependence of T and R of the GCs were determined by using a backscatter model incorporating time-equivalent multiple-reflection paths and were well reproduced by a numerical simulation.

Design and fabrication of a high-performance binary blazed grating coupler for perfectly perpendicular coupling

A high-performance binary blazed grating coupler (BBGC) on a silicon-on-insulator (SOI) platform for perfectly vertical coupling has been proposed. The period and the etching depth of the grating and the fill factors of the sub-gratings are simulated optimally with manufacturable feature sizes, and the coupling efficiency (CE) is as high as −1.78 dB at 1550 nm with a broad 3-dB bandwidth of around 100 nm. Then, a BBGC with the CE of −3.69 dB at 1550.5 nm and a 3-dB bandwidth of about 70 nm was experimentally demonstrated. Moreover, a large process tolerance of about 20 nm on the narrower sub-grating width was proved, achieving the insertion loss lower than −4.64 dB at 1550 nm. The realization of the BBGC on a SOI platform is simple, repeatable, and compatible with standard complementary metal-oxide semiconductor (CMOS) technology.