Abstract
Dielectrically loaded thin-film lithium niobate (LiNbO3) on insulator (LNOI) platforms have enabled a range of photonic integrated circuit components, such as high-speed optical modulators, switches, and nonlinear devices, while avoiding the direct etching of the LiNbO3 thin film. Silicon nitride (Si3N4) is one of the most attractive dielectric loading materials as it has a similar refractive index and transparency window to LiNbO3 and can be deposited and patterned by mature fabrication processes. The patterning of Si3N4 opens the opportunity to fabricate grating couplers in the same fabrication step, providing efficient optical interfaces for wafer-scale testing. In this paper, we investigate and demonstrate single-step etched grating couplers on a Si3N4-LNOI (X-cut) platform. The grating couplers (straight and curved) are designed and fabricated for TE-polarized modes along the Y and Z crystallographic directions, considering the LiNbO3 crystal’s birefringence. The experimentally demonstrated coupling losses are as low as 4.02 and 4.24 dB along the crystallographic Y and Z directions, respectively. The corresponding peak wavelengths are 1609 and 1615 nm, respectively. The measured 3-dB bandwidths are wider than 70 nm for both crystallographic directions. We also numerically investigated the influence of fabrication variations and the fiber angle on the transmission. To the best of our knowledge, this work is the first demonstration of grating couplers with different light propagation directions on the Si3N4 loaded LNOI platform.
Highlights
Lithium niobate (LiNbO3) is an indispensable material for fiber-optic communications due to its strong electro-optic coefficient (r33 = 30.8 pm/V) and high second-order optical nonlinearity (d33 = −33 pm/V).1,2 Traditional waveguide fabrication processes for bulk LiNbO3, such as titanium in-diffusion3 and proton exchange,4 provide only a small refractive index contrast, which limits the tightest waveguide bending radius to several millimeters, resulting in bulky and standalone optical components, such as commercial optical modulators
The refractive index of the loading material should be slightly lower than that of LiNbO3 to achieve a strong confinement of the mode in the thin-film LiNbO3, which is important for efficient electro-optic and nonlinear optical processes
Si3N4 is attractive as it has a similar refractive index and transparency window to LiNbO3, and more importantly, it can be deposited and patterned by the mature fabrication processes that have been developed in Complementary MetalOxide-Semiconductor (CMOS) foundries
Summary
Traditional waveguide fabrication processes for bulk LiNbO3, such as titanium in-diffusion and proton exchange, provide only a small refractive index contrast, which limits the tightest waveguide bending radius to several millimeters, resulting in bulky and standalone optical components, such as commercial optical modulators. This has dramatically changed in recent years, as the commercial availability of the thin-film LiNbO3 on insulator (LNOI) technology is making LiNbO3 available as a waveguide platform in photonic integrated circuits. The Si3N4-LNOI platform has the potential to combine the strong electro-optical coefficient and high second-order optical nonlinearity of LiNbO3 with the mature fabrication processes and low propagation loss of Si3N4, which enabled experimental demonstrations, such as electrooptic modulation and second-harmonic generation.
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