Wing-Han Wong

Zr4+ diffusion-doping effect on refractive index of LiNbO3: A comparison with bulk-doping case

Zr4+-doped LiNbO3 plates were prepared by diffusion of ZrO2 films coated onto congruent LiNbO3 substrates in wet O2. After diffusion, Zr4+-doping effect on refractive index of LiNbO3 and Li2O out diffusion were studied by prism coupling technique. The results show that the Li2O out diffusion is ignorable and Zr4+ doping has little effect on both the ordinary and extraordinary indices. The little effect of diffusion-doping is clearly different from the bulk doping case reported previously, in which both ordinary and extraordinary indices show definite Zr4+ doping concentration effect. The difference is attributed to the different ion arrangements from crystal growth to in-diffusion.

Electro-optic property of Ti 4+ -doped LiNbO 3 single crystal

Electro-optic property of Ti4+-doped LiNbO3 single crystal is investigated. The electro-optic coefficients γ33 and γ13 of a series of bulk Ti4+-doped congruent LiNbO3 crystals with different Ti4+ concentrations up to 12 mol% in crystal were measured by Mach-Zehnder interferometric method and correlated with the Ti4+-doping concentration. Both clamped and unclamped coefficients were measured. The results show that both γ33 and γ13 reveal a degradation tendency with the increase of Ti4+ concentration in both cases of clamped and unclamped measurements. Nevertheless, the degradation is no more than 15% for the considered Ti4+ concentration up to 12 mol%. The little effect is explained qualitatively and comprehensibly.

Electro-optically tunable super-broadband filter based on long period grating in Ti:LiNbO₃ waveguide.

We report an electro-optically tunable optical filter based on a parallel structure of two long period gratings in the two same Ti:LiNbO3 strip waveguides: one 675-μm-pitch grating in one waveguide and another 880-μm-pitch grating in the other waveguide. The stop-band is observed in the 1.1-1.3 (1.4-1.6) μm spectral region for the grating pitch 675 (880) μm. Its contrast increases linearly to ∼30u2009u2009dB as the voltage is increased to 300 V, and the linearity is similar for the two cases of 675 and 880 μm pitches. Higher than 300 V, the contrast decreases due to photorefractive (PR) effect and/or over-coupling. Accompanying the contrast modulation, the resonant wavelength is simultaneously linearly tuned by making use of the PR effect. For the 675 (880) μm pitch, the tuning range is 160 (200) nm for the 400 (300) V voltage change range. With the two gratings, one can realize >360u2009u2009nm super-broadband filtering.

A simple, compact, low-cost, highly efficient thermometer based on green fluorescence of Er3 +/Yb3 +-codoped NaYF4 microcrystals

We developed a highly efficient optical thermometer based on intensity ratio of upconversion green fluorescence of Er3+/Yb3+-codoped NaYF4 microcrystals. The sensor consists simply of a 980nm laser diode, one narrow-band interference filter, two lenses, one Si-photocell and one multimeter, while being without use of spectrometer and additional electronics. The device not only has a simple, compact structure (hence a low cost), but also displays highly efficient sensing performance, characterized by large signal-to-noise ratio due to strong fluorescence intensity, high thermal resolution and sensitivity, which have the values 1.3K and 1.24×10-2K-1, respectively, at the physiological temperature 310K. The excellent sensing performance of the device was further confirmed by the results of the measurements repeated using a spectrometer. The thermometer is highly generalized that can be applied to other luminescent materials, and shows great potential for the physiological temperature sensing in biological tissues and cells.

Diffusion control of an ion by another in LiNbO3 and LiTaO3 crystals

Diffusion-doping is an effective, practical method to improve material properties and widen material application. Here, we demonstrate a new physical phenomenon: diffusion control of an ion by another in LiNbO3 and LiTaO3 crystals. We exemplify Ti4+/Xn+ (Xn+u2009=u2009Sc3+, Zr4+, Er3+) co-diffusion in the widely studied LiNbO3 and LiTaO3 crystals. Some Ti4+/Xn+-co-doped LiNbO3 and LiTaO3 plates were prepared by co-diffusion of stacked Ti-metal and Er-metal (Sc2O3 or ZrO2) films coated onto LiNbO3 or LiTaO3 substrates. The Ti4+/Xn+-co-diffusion characteristics were studied by secondary ion mass spectrometry. In the Xn+-only diffusion case, the Xn+ diffuses considerably slower than the Ti4+. In the Ti4+/Xn+ co-diffusion case, the faster Ti4+ controls the diffusion of the slower Xn+. The Xn+ diffusivity increases linearly with the initial Ti-metal thickness and the increase depends on the Xn+ species. The phenomenon is ascribed to the generation of additional defects induced by the diffusion of faster Ti4+ ions, which favors and assists the subsequent diffusion of slower Xn+ ion. For the diffusion system studied here, it can be utilized to substantially shorten device fabrication period, improve device performance and produce new materials.

Zirconium-diffusion-doped Ti:LiNbO3 strip waveguide for integrated optics

We report Zr4+-doped Ti:LiNbO3 strip waveguide fabricated by Zr4+-diffusion-doping followed by diffusion of 8 μm wide, 100 nm thick Ti-strips on a Z-cut congruent substrate. Optical study shows that the waveguide well supports both TE and TM, is single-mode at the 1.5 μm wavelength, and has a loss ≤ 1.3/1.5 dB/cm for the TE/TM mode. Secondary ion mass spectrometry study shows that the Zr4+-profile part having a Zr4+-concentration above the threshold of photorefractive damage covers 60% (70%) ordinary (extraordinary) index profile in the waveguide. We conclude that the waveguide is optical-damage-resistant.

Emission and Absorption Cross Sections of Er3+:LiNbO3 Crystal: Composition Effect

We report Er3+-doping, Mg2+-codoping and crystal composition effects on emission and absorption cross sections of Er3+-only doped and Er3+/Mg2+-codoped LiNbO3 crystals, in which Er3+ concentration ranges from 0.3 to 2.7 mol% and Mg2+ concentration from 1.2 to 7.4 mol%. The emission cross section spectra were calculated from the measured fluorescent spectra and the absorption cross section spectra were computed using the McCumber relation. The results show that increasing Er3+ concentration tends to decrease the cross section, while codoping with Mg2+ or increasing crystal composition tends to increase the cross section. As the composition is close to the stoichiometry, the cross section value tends to a constant. These effects are related to Er3+ site redistribution induced by doping or crystal composition alteration.

Optical damage resistant Ti-diffused Zr/Er-codoped lithium niobate strip waveguide for high-power 980 nm pumping

Ti⁴⁺-diffused Zr⁴⁺/Er³⁺-codoped LiNbO₃ strip waveguide was fabricated on an X-cut LiNbO₃ substrate by thermal diffusion in sequence of Er³⁺, Zr⁴⁺ and Ti⁴⁺. Secondary ion mass spectrometry study shows that the Ti⁴⁺ ions follow a sum of two error functions in the width direction and a Gauss function in the depth direction of the waveguide. Both Er³⁺ and Zr⁴⁺ profiles follow the desired Gauss function, and entirely cover the Ti⁴⁺ profile. Optical study shows that the waveguide is TE or TM single mode at 1.5 μm wavelength, and has a loss of 0.3 (0.5) dB/cm for the TM (TE) mode. In the case of 980 nm pumping, the waveguide shows stable 1547 nm signal output under high-power pumping without optical damage observed, and a net gain of 1.1 dB/cm is obtained for the available pump power of 120 mW.

Near-stoichiometric Ti:Sc:LiNbO 3 strip waveguide for integrated optics

We demonstrate near-stoichiometric Ti:Sc:LiNbO3 strip waveguide fabrication starting from a congruent LiNbO3 substrate with a technological process in sequence of Sc3+-diffusion-doping, Ti diffusion, and post Li-rich vapor transport equilibration. We show that the waveguide is in a near-stoichiometric composition environment, well supports single-mode propagation at 1.5 μm wavelength under both TE and TM polarizations, shows considerable polarization dependence, and has a loss ≤ 0.4/0.7 dB/cm for TE/TM mode. The Ti4+ surface profile can be fitted by a sum of two error functions, the depth profile can be fitted by a Gaussian function, and the Sc3+-profile part which has a concentration above the threshold of photorefractive damage entirely covers the waveguide, showing that the waveguide is expected to be optical-damage-resistant.

Near-stoichiometric Ti-diffused LiNbO(3) strip waveguide doped with Zr(4+).

We report a near-stoichiometric Ti:Zr:LiNbO(3) strip waveguide fabricated from a congruent substrate with a technological process in the following sequence: Zr4+-diffusion-doping, diffusion of 8-μm-wide, 100-nm-thick Ti strips, and post-Li-rich vapor transport equilibration. We show that Zr(4+)-doping has little effect on the LiNbO(3) refractive index, and the waveguide is in a near-stoichiometric environment. The waveguide well supports both the TE and TM modes, shows weak polarization dependence, is in single mode at the 1.5 μm wavelength, and has a loss of ≤0.6/0.8 dB/cm for the TE/TM modes. A secondary ion mass spectrometry analysis shows that the Zr(4+)-profile part with a concentration above the threshold of photorefractive damage entirely covers the waveguide, implying that the waveguide would be optical-damage resistant.