Abstract
The ridge waveguide integrated grating couplers (GCs) in lithium niobate on insulator (LiNbO3, LNOI) were designed, fabricated and characterized. Two ends of the tapered GCs were connected by the subwavelength gratings (SWG) waveguide of a sub-micrometric-diameter, the photonic-wire SWG structure was featured with the profile of side-walls corrugations, and the effect of geometrical dimensions on the output optical response was investigated. All the devices structure patterns for the integrated LNOI GCs could be simultaneously defined by one step of electron-beam lithography, and then easily fabricated by the optimized dry-etching processes, followed by samples surface cleaning. After the fabrication, a low coupling loss of − 5.1 dB/coupler at the telecommunication wavelength of 1561 nm was measured in the best thin-film LiNbO3 (TFLN) surface grating coupler for quasi-transverse-electric (quasi-TE) polarized signals, and a broad 3-dB optical bandwidth of wider than 95 nm was also obtained. The compact components exhibited magnificent performance, and might show the potential functionalities for the TFLN-based integrated optical waveguide devices.
Highlights
Single crystal thin film lithium niobate (TF-LiNbO3, thin film LiNbO3 (TFLN)) and LN-on-insulator (LNOI) wafer, have been emerging as an attractive platform for the applications of high-performance photonic integrated components (PICs), due to LN excellent properties of electro-optic, non-linear optics, high refractive index contrast between photonic waveguide core and its cladding, as well as the available wide optical transparency ranges from 0.4 to 5.2 m [1,2,3]
The tapered LNOI grating couplers (GCs) connected via a few hundreds of micro-meters-length subwavelength gratings (SWG) waveguide was theoretically reported, and the integrated GCs with sidewall corrugated SWG geometry could be regarded as the band-rejection filter [15]
It was reported that the GCs performance experience a dependence on the devices design and parameters selections as well as actual fabrication [25, 27], to obtain a maximum coupling efficiency (CE) product for the ridge-waveguide integrated LNOI GCs, the geometry and structure parameters of the proposed devices were fully considered such as the set of deeper etching depth of h for a better light mode confinement, large diffraction grating period, and the corresponding duty cycle
Summary
Single crystal thin film lithium niobate (TF-LiNbO3, TFLN) and LN-on-insulator (LNOI) wafer, have been emerging as an attractive platform for the applications of high-performance photonic integrated components (PICs), due to LN excellent properties of electro-optic, non-linear optics, high refractive index contrast between photonic waveguide core and its cladding, as well as the available wide optical transparency ranges from 0.4 to 5.2 m [1,2,3]. The SWG structure embedded in the middle region of photonic-wire waveguide and used as a low-loss optical waveguide, has not been systematically investigated for the applications of light-coupling and propagation in LNOI, and the traditional TFLN GCs usually have a high insertion loss (IL). Noticed that there ever reported a relatively low loss in LNOI GCs, but these improved GCs based on traditional structure were realized by conducting sufficient optimizations, including adding the oxide upper cladding or refractive index matching materials [18, 20], setting the highly-cost and complicated bottom reflector [21, 22], employing the curved focused chirped gratings with fully optimized geometrical parameters [23], requiring more complex structure design and fabrication processes [24], and simultaneously adopting multiple optimizations [25, 26]. The light coupling and transmission characteristics of LNOI GCs for TE mode have been systematically investigated, and the performance in terms of device structural and optical properties for the fabricated GCs embedded with SWG have been characterized
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