High-efficiency hybrid amorphous silicon grating couplers for sub-micron-sized lithium niobate waveguides

Low-loss grating coupler with a subwavelength structure on a thin-film lithium niobate substrate

We demonstrated a low-loss O-band grating coupler on an x-cut thin-film lithium niobate substrate by implementing subwavelength and apodized structures. The subwavelength gratings were used to mitigate the refractive index discontinuity between the input taper and grating region, which was the first application of such a structure for grating coupler optimization on a thin-film lithium niobate substrate. The coupling efficiency was measured to be -1.99 dB/coupler at a wavelength of 1312.8 nm, which was the lowest loss among the reported lithium niobate grating couplers that do not use metal mirrors. The proposed design does not require metal mirrors or any additional material layers and can be easily fabricated with a single-step lithography and etching processes.

High coupling efficiency waveguide grating couplers on lithium niobate

We propose and validate a new, to the best of our knowledge, approach for high coupling efficiency (CE) grating couplers (GCs) in the lithium niobate on insulator photonic integration platform. Enhanced CE is achieved by increasing the grating strength using a high refractive index polysilicon layer on the GC. Due to the high refractive index of the polysilicon layer, the light in the lithium niobate waveguide is pulled up to the grating region. The optical cavity formed in the vertical direction enhances the CE of the waveguide GC. With this novel structure, simulations predicted the CE to be -1.40 dB, while the experimentally measured CE was -2.20 dB with a 3-dB bandwidth of 81 nm from 1592 nm to 1673 nm. The high CE GC is achieved without using bottom metal reflectors or requiring the etching of the lithium niobate material.

Wide Bandwidth Silicon Nitride Strip-Loaded Grating Coupler on Lithium Niobate Thin Film

In this research, a vertical silicon nitride strip-loaded grating coupler on lithium niobate thin film was proposed, designed, and simulated. In order to improve the coupling efficiency and bandwidth, the parameters such as the SiO2 cladding layer thickness, grating period, duty cycle, fiber position, and fiber angle were optimized and analyzed. The alignment tolerances of the grating coupler parameters were also calculated. The maximum coupling efficiency and the −3 dB bandwidth were optimized to 33.5% and 113 nm, respectively. In addition, the grating coupler exhibited a high alignment tolerance.

Efficient self-imaging grating couplers on a lithium-niobate-on-insulator platform at near-visible and telecom wavelengths

Lithium-niobate-on-insulator (LNOI) has emerged as a promising platform in the field of integrated photonics. Nonlinear optical processes and fast electro-optic modulation have been reported with outstanding performance in ultra-low loss waveguides. In order to harness the advantages offered by the LNOI technology, suitable fiber-to-chip interconnects operating at different wavelength ranges are demanded. Here we present easily manufacturable, self-imaging apodized grating couplers, featuring a coupling efficiency of the TE₀ mode as high as ≃47.1% at λ=1550 nm and ≃44.9% at λ=775 nm. Our approach avoids the use of any metal back-reflector for an improved directivity or multi-layer structures for an enhanced grating strength.

Low-loss edge-coupling thin-film lithium niobate modulator with an efficient phase shifter

Thin-film lithium-niobate-on-insulator (LNOI) is a very attractive platform for optical interconnect and nonlinear optics. It is essential to enable lithium niobate photonic integrated circuits with low power consumption. Here we present an edge-coupling Mach-Zehnder modulator on the platform with low fiber-chip coupling loss of 0.5 dB/facet, half-wave voltage V_π of 2.36 V, electro-optic (EO) bandwidth of 60 GHz and an efficient thermal-optic phase shifter with half-wave power of 6.24 mW. In addition, we experimentally demonstrate single-lane 200 Gbit/s data transmission utilizing a discrete multi-tone signal. The LNOI modulator demonstrated here shows great potential in energy-efficient large-capacity optical interconnects.

Two-dimensional grating coupler on an X-cut lithium niobate thin-film

A two-dimensional grating coupler for coupling light between a standard single-mode fiber and ridge waveguides on an X-cut lithium niobate thin-film is designed and demonstrated. Using circular holes for grating cells, simulated coupling losses reach -3.88 dB at 1550 nm and -5.78 dB at 1563 nm with 1-dB bandwidths of 49 nm and 45 nm for P-polarized and S-polarized light inputs, respectively. Experimentally, peak coupling losses of -5.13 dB at 1561 nm and -7.6 dB at 1568 nm are obtained for P-polarized and S-polarized light inputs, respectively, and corresponding 1 dB bandwidths are about 30 nm. An approach to improve the coupling performance of the grating coupler is also proposed using two crossing ellipses as grating cells as well as a bottom metal reflector. The coupling loss and the polarization dependent loss are decreased to around -3.4 dB and 0.44 dB, respectively.

Efficient grating couplers on a thin film lithium niobate-silicon rich nitride hybrid platform

A grating coupler on a thin film x-cut lithium niobate-silicon rich nitride hybrid platform is proposed and demonstrated. An inverse taper is applied to suppress higher-order mode excitation. A coupling efficiency of -5.82dB and 3 dB bandwidth of 57 nm are obtained near the wavelength of 1550 nm between the standard single-mode fiber (SMF-28) and sub-micrometer waveguides.

High-efficiency chirped grating couplers on lithium niobate on insulator

High-efficiency chirped grating couplers (GCs) with coupling efficiencies (CE) approaching 90%/coupler were designed by using a particle swarm optimization algorithm. These GCs were fabricated on $z$-cut lithium niobate on insulator (LNOI) with an Au layer on the lithium niobate substrate. The maximum CEs for transverse electric and transverse magnetic polarization input were measured up to ${\sim}{72.0}\%$/coupler and ${\sim}{61.6}\%$/coupler, respectively, which are the state-of-the-art values for LNOI GCs as far as we know. These GCs contribute to the realization of high-efficiency LNOI on-chip integrated optics.

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.

Highly efficient thermo-optic tunable micro-ring resonator based on an LNOI platform

We demonstrate a high-efficiency thermo-optic (TO) tunable micro-ring resonator in thin-film lithium niobate. Thermal insulation trenches around the heated micro-ring resonator and the underlying silicon substrate significantly reduce the heating power consumption and improve the tuning efficiency. Compared to conventional TO devices without thermal insulation trenches, the proposed device achieves a full free spectral range wavelength shift with a 14.9 mW heating power, corresponding to a thermal tuning efficiency of 53.7 pm/mW, a more than 20-fold improvement of tuning efficiency. The approach enables energy-efficient high-performance TO devices such as optical switches, wavelength routers, and other reconfigurable photonic devices.

High-performance coherent optical modulators based on thin-film lithium niobate platform

The coherent transmission technology using digital signal processing and advanced modulation formats, is bringing networks closer to the theoretical capacity limit of optical fibres, the Shannon limit. The in-phase/quadrature electro-optic modulator that encodes information on both the amplitude and the phase of light, is one of the underpinning devices for the coherent transmission technology. Ideally, such modulator should feature a low loss, low drive voltage, large bandwidth, low chirp and compact footprint. However, these requirements have been only met on separate occasions. Here, we demonstrate integrated thin-film lithium niobate in-phase/quadrature modulators that fulfil these requirements simultaneously. The presented devices exhibit greatly improved overall performance (half-wave voltage, bandwidth and optical loss) over traditional lithium niobate counterparts, and support modulation data rate up to 320 Gbit s-1. Our devices pave new routes for future high-speed, energy-efficient, and cost-effective communication networks.

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.

Polarization-independent one-dimensional grating coupler design on hybrid silicon/LNOI platform

We report here on one-dimensional (1D) grating couplers based on hybrid silicon/LNOI platform for polarization-independent and high-efficient single-polarization coupling efficiencies. A low index oxide buffer layer was introduced in between the top silicon high index grating coupler and bottom LNOI waveguide. With optimal design of the buffer layer thicknesses, modal and index matches can be tuned for either single polarization or both TE/TM polarization coupling applications. Over 70% coupling efficiency can be achieved for single polarization based on the basic uniform 1D grating coupler design without any bottom reflectors incorporated. Polarization independent coupling efficiency of 51% was also achieved. The spectral bandwidth is over 50 nm with polarization dependent loss of 0.1 dB. The proposed structure is simple to fabricate. Detailed modal and loss analysis suggest different dominant loss mechanisms in the proposed hybrid structure, where the introduction of the bottom mirror may not result in significant improvement in coupling efficiency, as the dominant loss mechanism arises from the top reflection loss.

High performance fully etched isotropic microring resonators in thin-film lithium niobate on insulator platform

We present our design, fabrication, and experimental results for very high-performance isotropic microring resonators with small radii (∼ 30 µm) based on single-mode strip waveguides and transverse magnetic (TM) polarization in a fully etched lithium niobate (Z-cut) thin-film on insulator. The loss of the devices is predicted to be < 10 dB/cm, and is measured to be ∼ 7 dB/cm. The measured optical responses of microring resonators exhibit an extinction of ∼ 25 dB (close to critical coupling), a 3 dB optical bandwidth of 49 pm (∼ 6 GHz) for all-pass structures, an extinction of ∼ 10 dB for add-drop structures, and a free spectral range of ∼ 5.26 nm, all of which are in excellent agreement with the design. This work is the first step towards ultra-compact and fully isotropic optical modulators in thin-film lithium niobate on insulator.

High modulation efficiency lithium niobate Michelson interferometer modulator

We demonstrate highly efficient lithium niobate thin film Michelson interferometer modulators with half-wave voltage length product of 1.4 V∙cm. Amorphous silicon grating couplers have been incorporated to achieve a 3.8-dB/port waveguide-fiber coupling loss. Devices with 1-mm phase shifter arms have a footprint of 2.5 mm × 1.7 mm. The demonstrated modulation data rates is up to 35 Gb/s.

High coupling efficiency grating couplers on lithium niobate on insulator

We demonstrate monolithically defined grating couplers in Z-cut lithium niobate on insulator for efficient vertical coupling between an optical fiber and a single mode waveguide. The grating couplers exhibit ∼ 44.6%/coupler and ∼ 19.4%/coupler coupling efficiency for TE and TM polarized light respectively. Taperless grating couplers are investigated to realize a more compact design.

Nanostructuring of LNOI for efficient edge coupling

We present the design, fabrication and characterization of LNOI fiber-to-chip inverse tapers for efficient monolithic edge coupling. The etching characteristics of various LNOI crystal cuts are investigated for the realization of butt-coupling devices. We experimentally demonstrate that the crystal cut limits the performance of mode matching tapers studied in this work. We report a butt-coupling loss of 2.5±0.5 dB/facet across the C/L-band and 6 dB/facet (at 1550 nm) by implementing 200 nm tip mode matching tapers in +Z-cut LNOI and X-cut MgO:LNOI, respectively. We anticipate that these results will provide insight into the nanostructuring of LNOI and into the further development of efficient butt-coupling in this platform.

Realization of alignment-tolerant grating couplers for z-cut thin-film lithium niobate

We present the design, modeling, fabrication, and characterization of grating coupler devices for z-cut lithium niobate near 1550 nm. We first experimentally measure the sensitivity of the insertion loss of a conventional grating coupler to translational misalignment through a three-factor full factorial design of experiment. Next, we design grating couplers that are significantly less sensitive to misalignment. The fabricated devices experienced less than 7 dB of excess insertion loss for combined misalignments of up to ± 5 μm in plane and up to -2 μm or + 10 μm out of plane.