dB Coupling Loss Research Articles

Multilayer stacked crystalline silicon switch with nanosecond-order switching time.

To realize compact and denser photonic integrated circuits, three-dimensional integration has been widely accepted and researched. In this article, we demonstrate the operation of a 3D integrated silicon photonic platform fabricated through wafer bonding. Benefiting from the wafer bonding process, the material of all layers is c-Si, which ensures that the mobility is high enough to achieve a nanosecond response via the p-i-n diode shifter. Optical components, including multimode interferences (MMIs), waveguide crossing, and Mach-Zehnder interferometer (MZI)-based switch, are fabricated in different layers and exhibit great performance. The interlayer coupler and crossing achieve a 0.98 dB coupling loss and <-43.58 dB cross talk, while the crossing fabricated in the same layer shows <-36.00 dB cross talk. A nanosecond-order switch response is measured in different layers.

Detachable interface toward a low-loss reflow-compatible fiber coupling for co-packaged optics (CPO).

High-density reflow-compatible fiber I/O is one of the challenges for co-packaged optics (CPO). This paper developed a detachable coupling interface based on expanded beam edge coupling, which can be applied for optical coupling between lasers, PICs, and fibers, seamlessly supporting many channels with high efficiency. It comprises a removable fiber connector and a permanent chip/device connector, in which microlens/lens arrays are used for waveguide mode expansion and MT-like connectors are used for position registration. An effective alignment scheme based on beam detection was developed and implemented in an assembly station for building the removable fiber connectors, while the permanent chip/device connector was assembled by active alignment to a pre-made fiber connector mated with a registration connector. Promising results were obtained from the proof-of-concept demonstrations of the coupling from SiP PIC and III/V lasers to fibers using the off-the-shelf lenses and modified MT registration connectors. In both cases, less than 1 dB coupling loss was achieved with an expanded beam size of 160 µm in diameter. Even with a relatively large lens offset of ∼35 µm, the detachable fiber array connectors showed good interchangeability. Such a coupling interface is expected to be solder-reflow compatible by replacing the plastic registration connectors with ceramic ones, making it a promising candidate for the solution to CPO fiber I/O.

Low-loss three-dimensional printed ridge waveguides using stereolithography

Ridge waveguides were three-dimensional printed using a stereolithography printer and hydrogel resin formulation. The ridge waveguides were 13, 20, and 30 μm wide, 3 to 6 μm high, and 4.4 mm long. The loss of the waveguides was measured using the cutback method and ranged between 0.28 and 1.2 cm − 1 (or 1.2 and 5.2 dB / cm) with transmittances up to 0.94 (0.27 dB coupling loss) using 635 nm light. Our work demonstrates a quick and inexpensive method to fabricate integrated photonic chips with the promise to fabricate more complex photonic devices and systems.

Bridging the gap between resonance and adiabaticity: a compact and highly tolerant vertical coupling structure

We present a compact, highly tolerant vertical coupling structure, which can be a generic design that bridges the gap between conventional resonant couplers and adiabatic couplers for heterogeneously integrated devices. We show insights on relaxing the coupler alignment tolerance and provide a detailed design methodology. By the use of a multisegmented inverse taper structure, our design allows a certain proportion of the odd supermode to be excited during the coupling process, which simultaneously facilitates high tolerance and compactness. With a total length of 87 μm, our coupler is almost threefold shorter than the state-of-the-art alignment-tolerant adiabatic couplers and outperforms them by demonstrating a more than 94% coupling efficiency (for &lt; 0.3 dB coupling loss) with ± 1 μm misalignment tolerance, which, to our best knowledge, is a new record for III-V-on-silicon vertical couplers. Furthermore, our design has high tolerance to fabrication-induced structural deformation and ultrabroad bandwidth. These features make it particularly suitable for building densely integrated III-V-on-silicon photonic circuits with commercially available microtransfer printing assembly tools. The proposed design can be widely adopted in various integration platforms.

Numerical Demonstration of 800 Gbps WDM Silicon Photonic Transmitter with Sub-Decibel Surface-Normal Optical Interfaces.

We propose and numerically demonstrate an 800 Gbps silicon photonic transmitter with sub-decibel surface-normal optical interfaces. The silicon photonic transmitter is composed of eight silicon Mach–Zehnder optical modulators and an interleaved AMMI WDM device. This WDM device comprises two 1 × 4 angled MMI and a Mach–Zehnder interferometer (MZI) optical interleaver with an apodized bidirectional grating which has about −0.5 dB coupling loss. Both the Mach–Zehnder electro-optical modulators and MZI optical interleaver regard the bidirectional grating coupler as vertical optical coupler and 3-dB power splitter/combiner. By importing the S-parameter matrices of all the components which have been carefully designed in simulation software, the circuit-level model of the optical transmitter can be built up. On this basis, the static and dynamic performance characterization were carried out numerically. For NRZ modulation, the optical transmitter exhibits the overall optical loss of 4.86–6.72 dB for eight wavelength channels. For PAM4 modulation, the optical loss is about 0.5 dB larger than that of NRZ modulation, which varies between 5.38–7.27 dB. From the eye diagram test results, the WDM silicon photonic transmitter can achieve single channel data transmission at 100 Gb/s NRZ data or 50 GBaud/s PAM4 symbol rate with acceptable bit error rate.

Effect of wall thickness on broadband design of a Ka-band TE21-mode coupler

Commonly, the classical formulas for designing TE21-mode coupler are suitable for the operating frequency under Ka-band by virtue of ignoring wall thickness. However, in real application of TE21-mode coupler design beyond Ka-band, the new formulas need to consider the effect of wall thickness. Herein, we demonstrate a design measure of a TE21-mode coupler beyond Ka-band via computer simulation, giving attention to wall thickness. The experimental results show that a measure of linear second weighted sum for coupling hole size could improve coupler performance. Amazingly, such coupler shows 10% bandwidth with less than 0.5 ​dB coupling loss and more than 40 ​dB isolation is possible.

Alignment tolerant analysis of a compact 4 × 25 Gbps TOSA with a thin-film filter multiplexer

A 100 Gbps transmitter optical sub-assembly (TOSA) based on a thin-film filter (TFF) multiplexer (MUX) demonstrated high alignment tolerance in a small volume. The model was based on the transfer-matrix method to investigate the effect of collimation and focus lens structures on coupling efficiency. The tolerances of the focus lens, MUX, and collimation lens were analyzed by simulation and experiment, obtaining the main values in 1 dB coupling loss of ±25, ±450, and ±3.3 um, respectively. Test results indicate that the transmission in a single mode fiber over 10 km is achieved with the 100 Gbps TOSA extinction ratio over 40 dB.

Metal based grating coupler on a thin-film lithium niobate waveguide.

Thin-film lithium niobate (LN) has been proved to be an excellent platform for building compact active and nonlinear photonic components on a chip. The coupling of a sub-micron sized LN waveguide and a single-mode fiber remains as one challenging issue. An efficient grating coupler made of Au stripes on an LN ridge waveguide is demonstrated here. The fabrication of the grating is convenient, using just a standard lift-off process of metal films. The peak coupling efficiency of an optimized structure reaches 50.4%, i.e., -3 dB coupling loss, at 1.55 µm wavelength for the fundamental transverse-electrical mode, where the 1-dB coupling bandwidth is 58 nm. Experimentally, fabricated devices, with buried oxide layer thicknesses slightly off the optimal values, exhibit coupling efficiencies of 43.8% and 33.7% for 400 nm and 600 nm thick LN layers.

Chalcogenide glass photonic integration for improved 2 μm optical interconnection

In this work, on-chip chalcogenide glass photonic integrations with several fundamental photonic building blocks are designed and fabricated based on the As 2 S 3 platform for improved 2 μm optical interconnection, achieving a broadened wavelength bandwidth and improved fabrication tolerance. A 600 nm thick As 2 S 3 strip waveguide has low propagation loss of 1.447 dB/cm at 2 μm. Broadband vertical coupling is realized by a grating coupler with 4.3 dB coupling loss. A Bragg grating filter, power splitter, Mach–Zander interferometer, and mode converter for on-chip mode division multiplexing (MDM) are also reported at 2 μm with reliable performances. Finally, a record high MDM optical interconnection capacity of 3 × 80 Gbps at 2 μm is experimentally demonstrated based on the proposed As 2 S 3 chip, drawing promising prospects for future photonic integration and high-speed interconnection at the 2 μm waveband.

Broadband high-efficiency triple-tip spot size converter for edge coupling with improved polarization insensitivity

Fiber-to-chip couplers are an essential part of the interfaces to integrated photonic devices, and high coupling efficiency over a broad wavelength range is preferred for most applications. Furthermore, couplers with low polarization dependence are also of great practical significance. While tapered waveguide couplers have been applied to overcome the mode mismatch between waveguides and fibers, the inherent geometric asymmetry of high-index-contrast waveguides, especially for those with a thick core layer, leads to polarization dependence in their performance. Here a triple-tip spot size converter as an edge coupler is proposed to improve the polarization insensitivity and coupling efficiency over a broadband range for either thick or thin waveguides. For both TE and TM polarization, it is shown that our design can have below 1 dB coupling loss over a 600 nm window around 1550 nm wavelength for Si3N4 waveguides of typical core thicknesses. Meanwhile, this kind of coupler also shows better fabrication and assembly tolerance than the traditional single-tip spot size converter (inverse taper coupler). Our proposed device can be easily fabricated in simple fabrication processes compared to other designs and have good compatibility with the standard complementary metal-oxide–semiconductor processes.

Two-dimensional grating coupler on silicon with a high coupling efficiency and a low polarization-dependent loss.

Two-dimensional grating couplers are important components for silicon photonic circuits to achieve light coupling from/to a fiber for both polarizations. A two-dimensional grating coupler structure with a high coupling efficiency and a low polarization dependent loss is demonstrated. Using two crossing ellipses as the grating scatter and a diamond-like grating lattice, the polarization dependent loss of the grating coupler can be reduced. The coupling loss is further decreased with a metal mirror, which reaches -1.73 dB theoretically at 1310 nm wavelength. Experimentally, -2.37 dB coupling loss is achieved with an 1 dB coupling bandwidth of 29 nm. The corresponding PDL was measured lower than 0.2 dB in a wavelength range of 78 nm. The proposed configuration for the metal mirror also facilitates a robust wafer-scale post-processing as well as an easy fiber alignment.

Design optimization and tolerance analysis of a spot-size converter for the taper-assisted vertical integration platform in InP.

We report on the design optimization and tolerance analysis of a multistep lateral-taper spot-size converter based on indium phosphide (InP), performed using the Monte Carlo method. Being a natural fit to (and a key building block of) the regrowth-free taper-assisted vertical integration platform, such a spot-size converter enables efficient and displacement-tolerant fiber coupling to InP-based photonic integrated circuits at a wavelength of 1.31μm. An exemplary four-step lateral-taper design featuring 0.35dB coupling loss at optimal alignment of a standard single-mode fiber; ≥7 μm 1dB displacement tolerance in any direction in a facet plane; and great stability against manufacturing variances is demonstrated.

General architectures for on-chip optical space and mode switching

The optical switches for single-mode operation cannot be directly utilized in optical communication and interconnect systems adopting mode-division multiplexing. In this paper, three general architectures for on-chip optical space and mode switching are proposed, which are optimized for optical space switching, optical space switching plus local optical mode switching, and global optical mode switching, respectively. A silicon thermo-optic 2×2 four-mode optical switch is demonstrated. The minimum and maximum optical link insertion losses are 16.0 and 20.9 dB (including ∼6 dB coupling loss), respectively, in the wavelength range of 1525–1565 nm, while the optical signal-to-noise ratios of the optical links are larger than 15.3 dB. The optical power penalty at a bit error rate of 10−9 varies from 1.0 to 5.6 dB for 40 Gbps data transmission through different optical links. This work provides a systematic solution to on-chip information switching for different physical and mode channels.

Demonstration of a compact bilayer inverse taper coupler for Si-photonics with enhanced polarization insensitivity.

We demonstrate a compact (30 μm long) broadband bilayer inverse taper edge-coupler for silicon photonics with a 1.7 dB coupling loss from a commercially available focused fiber with 5 μm mode diameter. We compare the performance of our bilayer taper with a conventional SOI inverse taper coupler and show that our bilayer coupler achieves between 1.5 dB to 2 dB improvement in the coupling efficiencies for both the TE and TM polarizations. The dimensions and fabrication steps for our bilayer taper are simple and compatible with standard foundry processes.

Air-Suspended SU-8 Polymer Waveguide Grating Couplers

We present SU-8 polymer waveguide grating couplers for TE mode with enhanced refractive index contrast by employing air as upper and lower cladding layer. Numerical simulations predict a coupling loss of 4.9 dB, indicating a coupling efficiency of 32% at a maximum spectral response of 1550 nm. Based on a polymer lamination method, air-suspended waveguide grating couplers were successfully fabricated and characterized. Due to current limitations in the fabrication process, the perturbation of the experimental grating couplers is weaker than the original simulation. However, transmission measurements have shown that a single grating coupler exhibits approximately 8 dB coupling loss at a center wavelength of 1557 nm, indicating a coupling efficiency of 16% with respect to a single-mode optical fiber. The demonstrated parallel fabrication method employing the widely used SU-8 photoresist makes the polymer waveguide grating couplers very attractive for a wide range of low-cost polymer photonic applications.

High-performance silicon-on-insulator grating coupler with completely vertical emission.

We study the physical concept of utilizing a critical coupling to obtain a high-performance grating coupler with completely vertical emission on a silicon-on-insulator substrate. Following our design strategy, we numerically show that when our grating coupler is coupled to a standard single-mode fiber operating at 1310 nm wavelength, a -1.46 dB coupling loss, a 20 nm spectral full-width-half-maximum, and a -24 dB back reflection can be achieved at the same time without full optimization. A practical design that largely relaxes the stringent lithography requirement is also proposed and presented.

Low-loss compact multilayer silicon nitride platform for 3D photonic integrated circuits.

We design, fabricate, and demonstrate a silicon nitride (Si(3)N(4)) multilayer platform optimized for low-loss and compact multilayer photonic integrated circuits. The designed platform, with 200 nm thick waveguide core and 700 nm interlayer gap, is compatible for active thermal tuning and applicable to realizing compact photonic devices such as arrayed waveguide gratings (AWGs). We achieve ultra-low loss vertical couplers with 0.01 dB coupling loss, multilayer crossing loss of 0.167 dB at 90° crossing angle, 50 μm bending radius, 100 × 2 μm(2) footprint, lateral misalignment tolerance up to 400 nm, and less than -52 dB interlayer crosstalk at 1550 nm wavelength. Based on the designed platform, we demonstrate a 27 × 32 × 2 multilayer star coupler.

Nonlinear properties of dispersion engineered InGaP photonic wire waveguides in the telecommunication wavelength range.

We propose high index contrast InGaP photonic wires as a platform for the integration of nonlinear optical functions in the telecom wavelength window. We characterize the linear and nonlinear properties of these waveguide structures. Waveguides with a linear loss of 12 dB/cm and which are coupled to a single mode fiber through gratings with a -7.5 dB coupling loss are realized. From four wave mixing experiments, we extract the real part of the nonlinear parameter γ to be 475 ± 50 W(-1)m(-1) and from nonlinear transmission measurements we infer the absence of two-photon absorption and measure a three-photon absorption coefficient of (2.5 ± 0.5) x 10(-2) cm(3)GW(-2).

Bidirectional grating coupler based optical modulator for low-loss Integration and low-cost fiber packaging

We proposed and demonstrated a novel optical modulator based on a bidirectional grating coupler designed for perfectly vertical fiber coupling. The grating functions as the fiber coupler and 3-dB splitter. To observe the interference, an arm difference of 30μm is introduced. As a result of the high coupling efficiency and near perfect split ratio of the grating coupler, this device exhibits a low on-chip insertion loss of 5.4dB (coupling loss included) and high on-off extinction ratio more than 20dB. The modulation efficiency is estimated to be within 3-3.84V•cm. In order to investigate the fiber misalignment tolerance of this modulator, misalignment influence of the static characteristics is analyzed. 10Gb/s Data transmission experiments of this device are performed with different fiber launch positions. The energy efficiency is estimated to be 8.1pJ/bit.

Integrated Optical Carrier for Optical/Electrical Interconnect

A simple and novel design, integrating discrete commercial micro-lens and vertical illuminated optoelectronic component in a substrate with high accuracy, is presented here. Without affecting the optical performances, this integrated optical carrier also allows high-frequency radio-frequency interconnects. This feature is critical for high-speed operation. The high accuracy integrated optical carrier improves the optical coupling efficiency and helps to relax the tight circuit assembly tolerance requirement. The integrated optical carrier can be used for various photonic applications which employ vertical illuminated optoelectronic components. An integrated optical carrier prototype is designed here for the optical electrical interconnect printed circuit board (OECB). For this OECB design, the simulated results show that the integrated optical carrier helps to give an assembly misalignment tolerance of more than ±20 μm with an increase of 1 dB coupling loss. In addition, the simulated optical insertion loss from the transmitter to the receiver is less than 1.4 dB. The optical performance of the prototype integrated optical carrier is measured and compared with the simulation results to ascertain the design concept. For different OECB waveguide designs, dimensions and positions, this integrated optical carrier design is amended to give a better performance and misalignment tolerance.