Abstract
Tunable photonic circuits were demonstrated in the mid-Infrared (mid-IR) regime using integrated TiO2-on-LiNbO3 (ToL) waveguides. The upper waveguide ridge was made by a sputtered TiO2 thin film with broad transparency at λ = 0.4–8 µm and an optimized refractive index n = 2.39. The waveguide substrate is a z-cut single crystalline LiNbO3 (LN) wafer that has strong Pockels effect, thus enabling the tunability of the device through electro-optical (E-O) modulation. A sharp waveguide mode was obtained at λ = 2.5 µm without scattering or mode distortion found. The measured E-O coefficient γeff was 5.9 pm/V approaching γ31 of 8.6 pm/V of LN. The ToL waveguide showed a hybrid mode profile where its optical field can be modified by adjusting the TiO2 ridge height. Our monolithically integrated ToL modulator is an efficient and small footprint optical switch critical for the development of reconfigurable photonic chips.
Highlights
Another material option to achieve reconfigurable mid-IR photonics is lithium niobate, a ferroelectric and high nonlinear optical crystal[10,11]
From finite difference method (FDM) modeling, the ToL photonic circuits had a hybrid waveguide mode, where its optical field confined in the LN and the TiO2 layers can be optimized by the TiO2 thickness
A ToL waveguide with this configuration is capable of efficient E-O modulation, since the optical field was confined in the ferroelectric LN layer and its overlapping with the electric field significantly increased
Summary
Another material option to achieve reconfigurable mid-IR photonics is lithium niobate, a ferroelectric and high nonlinear optical crystal[10,11]. Previous studies have shown a LN switch in Near-Infrared (NIR) with a modulation speed exceeding 100 GHz17 and a low Vπ∙L of ∼10 Vcm[18], where Vπ∙L is the product of the voltage and the device length to create a π phase difference Both phase and intensity LN modulators can achieve a high extinction ratio >15 dB, which is significantly better than present Si based photonic devices[19]. ToL device was able to confine majority of the lightwave in the ferroelectric LN layer, which is critical to achieve efficient E-O modulation This is because TiO2 and LN have a lower refractive index contrast, ∆n = 0.21, compared to a ∆n = 1.3 between Si and LN. The developed ToL platform enables reconfigurable mid-IR photonic circuits desired for broadband optical communication
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