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Optical microring resonators in fluorine-implanted lithium niobate

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Abstract

We report on the production and characterisation of optical microring resonators and optical channel waveguides by using fluorine-ion implantation and planar structuring in lithium niobate. We demonstrate the production of single-mode planar waveguides by low fluence fluorine-ion implantation (ϕ=2.5·1014 ions/cm2) into lithium niobate wafers. The waveguides are strongly confined by the amorphous 2-µm wide optical barrier induced by the implantation process. A refractive index contrast of Δno=0.17 at the telecom wavelength λ=1.5 µm has been determined between the waveguide and the barrier. Planar structuring with ridge height of up to 1.2 µm has been achieved by laser lithography masking and Ar+ sputtering. For TE waves, the channel waveguides exhibit propagation losses lower than 8 dB/cm. First ring resonators with 80-µm radius have been fabricated by planar structuring in fluorine-ion implanted lithium niobate. The measured resonance curves show an extinction ratio of 14 dB, a free spectral range of 2.0 nm and a finesse of 4.

©2008 Optical Society of America

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Figures (8)

Fig. 1.
Fig. 1. (a). Depth position of the high-energy (closer to surface, open circles) and low-energy (closed circles) crystalline-amorphous boundaries as a function of implantation fluence. Measurements were performed on samples irradiated with 22 MeV F4+ ions (del =4.7 µm): our measurements (open circles), data from [13] (full circles). The barrier width dB for fluence ϕ=2.5·1014 ions/cm2 is shown. The lines connecting the points are guides to the eye. (b). Simulated depth position of the maximal electronic stopping power del in F-implanted LiNbO3 as a function of ion energy. The line is the polynomial approximation discussed in the text. dB and the estimated waveguide thickness for ϕ=2.5·1014 ions/cm2 and E=14.5 MeV are shown.
Fig. 2.
Fig. 2. Calculated combined bending and tunnelling losses (left scale) and effective mode index Neff at ring’s outer rim (right scale) as a function of the waveguide bend radius. Neff of the straight waveguide with the same cross-section is 2.159. Calculation is for the first TE optical mode and wavelength 1.55 µm. Waveguide cross-section dimensions as in Fig. 3.
Fig. 3.
Fig. 3. Simulation of light propagation in the waveguide of interest. (a) Vertical index profile of the waveguide structure (at X=0). (b) Simulated electric field profile of the first TE optical mode at λ=1.55 µm in the waveguide with 80 µm bend radius. Optical barrier width is 2 µm. Waveguide cross-section is trapezoidal with waveguide height 1.35 um, ridge height 1.2 µm, base width 3.7 µm and top width 2.7 µm.
Fig. 4.
Fig. 4. Scanning electron micrograph of a microring resonator and a bus waveguide, structured in LiNbO3. (a). The whole ring and the bus waveguide. Ring radius is 80 µm, ridge height is 1.2 µm. (b). Enlarged coupling region, the gap size is ~0.2 µm.
Fig. 5.
Fig. 5. SEM image of the fabricated ridge structure. The waveguide cross-section is trapezoidal with the base width 3.7 µm, top width 2.7 µm and ridge height 1.2 µm. The amorphised barrier layer beneath the ridge is visible.
Fig. 6.
Fig. 6. Measured TE wave transmission spectrum of the bus waveguide coupled to a microring resonator. Small oscillations seen at the regions outside of resonances stem from Fabry-Perot resonances of the 5.6 mm long waveguide.
Fig. 7.
Fig. 7. Simulated electric potential in the waveguide cross-section upon the application of a voltage V=100 V between the electrodes. Equipotential contours separate a 5 V potential drop. About 55% of the potential drop occurs in the SiO2 buffer layer.
Fig. 8.
Fig. 8. (red) Measured light transmission as a function of the roundtrip phase θ. The line connecting the points is a guide to the eye. (blue) Transmission curve (T=cos2(φ/2)) of an equivalent Mach-Zehnder modulator, for phase φ=θ - π. Points of maximum transmission slopes |dT/|max are marked with arrows.

Tables (1)

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Table 1. Properties of demonstrated microring resonators in LiNbO3

Equations (4)

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Δ λ FSR = λ 2 2 π R · N g = 2.03 n m .
δ ( N eff ( λ ) L λ ) = 0 gives ( 1 λ N eff λ N eff λ 2 ) δ λ + 1 λ δ N eff = 0 ,
hence δ λ = λ N g δ N eff .
V π eq = π 2 ( d T d V max ) 1 = π 2 ( d T d θ d θ d V max ) 1 = V π 0 2 d T d θ max ,
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