Fiber-to-chip grating couplers for Lithium Niobate on Sapphire

TL;DR

The paper addresses the challenge of efficient fiber-to-chip coupling for Lithium Niobate on Sapphire (LNOS) photonic chips. It introduces a self-imaging, apodized grating coupler with a fixed period Λ and linearly diminishing filling factor FF = d/\Lambda, governed by the Bragg condition $ \frac{2\pi}{\lambda} n_c \sin \theta_c = \frac{2\pi}{\lambda} n_{eff} - q \frac{2\pi}{\Lambda}$ and designed for a negative first-order angle $\theta_c<0$. 2D and 3D simulations predict up to 42% coupling at 1550 nm, and experiments demonstrate single-end coupling >20% with a bandwidth exceeding 25 nm, validating the approach on X-cut LN on sapphire. This work enables practical LNOS-based hybrid photonic-quanta devices and scalable chip-to-fiber interfaces for quantum information processing and sensing.

Abstract

Lithium Niobate on Sapphire (LNOS) is an emerging platform for photonic integrated circuits, offering unique properties such as a wide transparency window, high nonlinearity, and strong electro-optic, nonlinear, and acousto-optic effects. Efficient light coupling between optical fibers and on-chip waveguides is crucial for practical applications. We present the design, simulation, and experimental demonstration of high-efficiency fiber-to-chip grating couplers for LNOS. The grating coupler design employs a self-imaging approach with a fixed period and linearly diminished filling factor, enabling a negative diffracted angle to match the fiber array and suppress higher-order diffraction. Numerical simulations predict a coupling efficiency of 42% at 1550 nm wavelength. The grating couplers are fabricated on an X-cut, 400 nm thick LN film with a 220 nm etching depth using electron beam lithography and inductively coupled plasma etching. Experimental characterization using a fiber array and a 6-axis displacement stage reveals a single-end coupling efficiency exceeding 20%, confirming the effectiveness of the design. The demonstrated grating couplers pave the way for efficient light coupling in LN-on-Sapphire photonic circuits, enabling diverse applications in classical and quantum information processing, sensing, and nonlinear optics.

Figures (6)

• Figure 1: (a) The schematic illustration of the grating coupling structure on a LNOS platform. (b) The x-y cross-sectional view of the grating coupler with the geometry parameters labels, also as the schematic of our two-dimensional model. The axis written in uppercase is the intrinsic coordinate of LN, and the one in lowercase is the model coordinate.
• Figure 2: (a) Field distribution at the near field when light coupling out from the waveguide. The four white dashed lines represent the four detectors we set. (b)-(i) Parameter dependences (thickness of LN $h$, etching depth $h_e$ and period $\Lambda$), where blue curves, yellow curves, purple curves, and red curves represent energy coupling upwards, transmit, reflected, and downwards respectively. Here, the efficiency values incorporate directional information based on energy flux direction, with positive values indicating upward diffraction and transmission, and negative values indicating reflection and substrate leakage.
• Figure 3: (a) Field distribution at the far field when 1550 nm light coupling into the waveguide. The white dashed line represents the approximate position of the fiber. (b) Far-field electric field cross-section (at $y=150\,\mathrm{\mu m}$), fitted by Gaussian distribution ($\sigma = 5.03\,\mathrm{\mu m}$). (c) The wavelength dependences for infrared wavelength.
• Figure 4: The near-field 3D full-vectorial simulation results. (a) The wavelength dependences. (b)-(e) Compare with the three upward diffraction electrical vector components. Four figures show $|E_x|$, $|E_y|$, $|E_z|$, and $|E|$ respectively, at the height of $y=3\,\mathrm{\mu m}$.
• Figure 5: Three-dimensional field distribution at far-field when applying R-S diffraction. (a)-(c) Two-dimensional cross-sections in X-Z planes, respectively at $y= 60\,\mathrm{\mu m}$, $100\,\mathrm{\mu m}$, and $120\,\mathrm{\mu m}$. (d) Two-dimensional cross-section of diffraction light in X-Y plane.