Ping-Rang Hua

Er3+ diffusion in congruent LiNbO3 crystal doped with 4.5 mol % MgO

Standard thermal diffusion of Er3+ into X-cut congruent LiNbO3 crystal with the MgO doping level close to the antiphotorefractive threshold concentration, 4.5 mol%, was attempted at the temperature close to Curie point of the crystal. Single-crystal x-ray diffraction, photoluminescence spectroscopy and secondary ion mass spectrometry (SIMS) were used to study the crystalline phase with respect to Er3+ ions and the depth profile of the diffused Er3+ ions. The results show that the thickness of the Er metal film coating should not be thicker than 4.6 nm. In this case, the diffusion is complete, the diffused surface is clean, and the Er3+ ions in the diffused layer still retain the LiNbO3 phase. On the other hand, the diffusion will be incomplete if the initial thickness of the Er metal film is thicker than 4.6 nm. The residual Er3+ ions form ErNbO4 grains on the surface of the crystal. SIMS analysis shows that the diffused Er3+ ions follow a monoexponential decay profile. The experimental results also show ...

Judd-Ofelt analysis of spectroscopic property of Er3+ in congruent and near-stoichiometric Zn∕Er-codoped LiNbO3 crystals

Congruent LiNbO3 crystals doped with 5.5∕1, 6∕0.15, and 7∕0.8mol%∕mol% of Zn∕Er were thermally treated at 1120°C over 101h using vapor transport equilibration (VTE) technique. Unpolarized visible and infrared absorption spectra of the VTE-treated and corresponding as-grown crystals were recorded at room temperature. It is shown that VTE treatment has brought these crystals closer to stoichiometric composition. X-ray powder diffraction and optical absorption results show that the VTE treatment induces the crystalline phase transformation with respect to Er3+ from the original Er3+:LiNbO3 to the ErNbO4 in all crystals studied. The formation of the ErNbO4 phase results in substantial absorption changes that include the drop of the transparency in the visible region, the reduction of Er3+ absorption, and the Zn-doping-level-dependent changes in spectral shape and peaking position at 0.98 and 1.5μm regions. Based upon the measured absorption spectra, the influences of Zn-doping level and VTE treatment on the E...

Sellmeier equation for doubly Er/Mg-doped congruent LiNbO 3 crystals

Refractive indices of congruent LiNbO3 crystals codoped with 0.5 mol% Er2O3 and 0-5.0 mol% MgO were measured at room temperature in a wavelength range from 473 to 1536 nm using critical angle measurement and minimum deviation methods. For reference, measurements by the critical angle method were also performed on a congruent LiNbO3 and a congruent MgO (5 mol%):LiNbO3. The measured data are compared with previously reported theoretical and/or experimental results of pure and MgO-doped LiNbO3, especially with the results predicted by the MgO content-related Sellmeier equation (SE) reported by Schlarb and Betzler. It is shown that the Schlarb and Betzler SE consistently predicts a lower index, by as much as 4×10−3, than what is actually measured by the authors for both congruent LiNbO3 and for MgO:LiNbO3, whether it is codoped with Er or not. To check the Er doping effect, measurements by the critical angle method were also performed on some congruent LiNbO3 crystals singly doped with 0-1.0 mol% Er2O3. The results show that Er doping has no measurable effect. It is concluded that there may be a systematic error as one uses the Schlarb and Betzler SE to predict the index of the undoped, only MgO-doped, and Er/Mg-codoped crystals. The Schlarb and Betzler SE parameters are refined to fit our experimental data. The corrected version of the SE suitable for MgO:LiNbO3 with or without Er doping is given.

Photorefractive-Damage-Resistant Locally Er–Mg-Doped Near-Stoichiometric Ti : Mg : Er : LiNbO

We report photorefractive-damage-resistant near-stoichiometric (NS) Ti : Mg : Er : LiNbO3 strip waveguides fabricated on an initially congruent, undoped Z-cut substrate in sequence by local Er-doping in air, Mg-Ti prediffusion in wet O2 atmosphere, and post vapor-transport-equilibration. These waveguides with an initial Ti-strip width of 4-7 μm are single-mode at 1.3-1.5 μm, support only transverse-magnetic mode, and have a loss of 1.4 dB/cm. The waveguides are NS, still retain LiNbO3 phase, and show stable 1531-nm small-signal enhancement under the 980-nm pump power of at least 216 mW, implying that the photorefractive effect is effectively suppressed. The waveguides would open up a way towards new devices.

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A number of congruent LiNbO 3 crystals homogeneously doped with 5 mol% Mg in growth melt were subjected to Li-rich vapor transport equilibration (VTE) treatments at 1100 °C for different durations. Secondary ion mass spectrometry study shows that the VTE induces the Mg diffusion within the crystal and an inhomogeneous Mg depth profile. The surface Mg concentration, determined from measured ordinary refractive index, shows a strong VTE duration dependence. Neutron activation analysis shows that the amount of MgO diffusing out of the crystal is ignorable, allowing to conclude that the Mg ions counter diffuse to the crystal surface at the early stage of VTE and then come back toward equilibrium as the Li concentration comes to equilibrium. The VTE-induced Li 2 O content increase in crystal was determined by the gravimetric method. The crystalline phase, crystal composition, and site occupation of Mg and Li are discussed.

Strip Waveguides: A Way Towards New Devices

A special structure of a strip Ti : LiNbO3 waveguide embedded in a planar Ti : LiNbO3 waveguide is proposed to fabricate a long-period waveguide grating (LPWG). Its applicability is demonstrated by fabricating a long-period grating on the waveguide surface using usual photoresist. We show that an LPWG device based on such a special structure is thermally stable.

Postgrown Li-rich vapor transport equilibration-induced Mg diffusion within MgO:LiNbO 3 crystal

Diffusivity of Er3+ into LiNbO3 crystal has been studied as a function of Li2O content and crystal orientation. The Li2O content in single-crystal LiNbO3 plates was adjusted by Li-rich vapor transport equilibration (VTE) technique. Five X-cut and five Z-cut congruent substrates were VTE treated at first at 1100 °C for different durations ranging from 6 to 74 h, and then were subjected to a special post-VTE anneal procedure at the same temperature for 60 h to obtain homogeneous Li2O content. The Li2O content was determined from the measured fundamental optical absorption edge. Its homogeneity is verified by measuring the optical absorption edge as a function of the crystal thickness. After the composition characterization, the substrates were coated with 45 nm thick Er film and annealed at 1130 °C for 40 h in argon atmosphere. The Er3+ profile was analyzed by secondary ion mass spectrometry. The results show that the Er3+ diffusivity decreases monoexponentially with the increase in the Li2O content in both...

Long-Period Grating on Strip Ti : LiNbO

Multi-mode near-stoichiometric (NS) Ti:LiNbO3 planar waveguide was fabricated by co-work of Li-rich vapor transport equilibration and in-diffusion of Ti-film on an initially congruent LiNbO3 substrate. The Ti-concentration is profiled by secondary ion mass spectrometry. The refractive index profile is constructed from measured mode indices and correlated with the Ti-concentration profile. The results show that the index change and Ti-concentration follow an exponential relationship with a power index 0.75/0.49 for the ordinary/extraordinary ray. The relationship is different from that of either conventional congruent waveguide or homogeneously Ti-doped NS bulk material or NS waveguide fabricated by direct Ti-diffusion in an NS substrate.

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We have measured the polarized visible and near-infrared, and unpolarized mid-infrared (2.7 μm) emission spectra of photorefractive-damage-resistant locally Er-Mg-doped near-stoichiometric (NS) Ti:Mg:Er:LiNbO3 strip waveguide, fabricated on an X-cut initially congruent LiNbO3 substrate in sequence by local Er doping in air, Mg-Ti pre-diffusion in wet O2 and post Li-rich vapor transport equilibration treatment. From the measured emission spectra, the emission and absorption cross section spectra were calculated based upon McCumber theory. The spectroscopic features are discussed in comparison with the spectra recorded from the area outside the waveguide, and with the previously reported results of bulk-doped NS Er:Mg:LiNbO3 crystals and congruent Er:LiNbO3 bulk material or Ti:Er:LiNbO3 waveguide structure. The results show that the spectra of the NS waveguide are traditional and reveal small differences from those spectra outside the waveguide in spectral shape, polarization dependence, as well as cross section values. In contrast, the cross section values of the NS waveguide show considerable differences from those of bulk-doped NS material and congruent bulk material or waveguide structure. The 552, 673, 996, and 1531 nm emission lifetimes of the Er3+ ions outside the waveguide were also measured, and found to be comparable to the results of the bulk-doped NS crystal and the congruent bulk material or waveguide structure.

Waveguide Embedded in Planar Ti : LiNbO

A model is proposed for describing two-dimensional diffusion of Ti into an initially congruent LiNbO3 crystal under a Li-enriched atmosphere created by a mixed two-phase (Li3NbO4 and LiNbO3) powder at elevated temperature [vapor transport equilibration (VTE)]. The influence of VTE treatment on Ti diffusivity is considered in the model. To solve the model, four key input parameters including Li-concentration-dependent Li and Ti diffusivities and two switching times t1 and t2 were determined. Prior to solve the Ti-diffusion model, a one-dimensional VTE model is solved at first to obtain the dynamic Li2O concentration depth profile. Both the Li-diffusion and Ti-diffusion models were solved by using finite difference method. Based on secondary-ion-mass spectrometry analysis, the validity of the VTE and Ti-diffusion models as well as the numerical method employed are confirmed. After that, diffusion at 1100°C of an 8-μm-wide, 100-nm-thick Ti strip defined on the surface of a Z-cut or an X-cut substrate was sim...