De-Long Zhang

Accurate measurement of 1.5 μm lifetime of Er3+ in LiNbO3 crystals and waveguides

Two optimized methods for accurately measuring the 1.5 μm lifetime of Er3+ in LiNbO3 crystals and waveguides are proposed. The first method, suitable for measurement of bulk material, was suggested by forming an annealed proton exchanged sandwich structure and fluorescence was collected through an adjustable aperture. This excitation collection scheme was used to investigate bulk Er-doped LiNbO3 crystals with eight different Er3+ doping levels, three different crystal thicknesses, and three different cut orientations. The 1.5 μm lifetime was investigated in three configurations: with both a sandwich and an aperture, with a sandwich but without an aperture, and without not only a sandwich but also without an aperture. In addition, the effect of the aperture was also studied. The second method, suitable for a waveguide structure, was proposed on the basis of transversely gathering the fluorescence aided by an outcoupling fiber. It was optimized by comparing three different fluorescence collection schemes. U...

Absorption characteristics of vapor transport equilibrated Er:LiNbO3 crystals

At room temperature, polarized visible and near infrared emission characteristics of 488 nm pumped vapor transport equilibration (VTE) treated Er:LiNbO3 crystals were investigated in comparison with the corresponding as-grown ones. The observed characteristics of the VTE crystal, whether it precipitates or not, include the retained polarization dependence, slight shift (less than 5 A) of peak position, narrowing of some individual emission peaks, the appearance of additional emission peak near 1536.5 nm and the about 10% lengthening of 1.5 μm lifetime. In addition, in the case of lower Er doping level, the spectral shape are well reserved for the VTE crystals and no obvious changes in both visible and IR emission intensity. On the other hand, for those highly doped crystals, the VTE treatment not only results in a definite change of relative intensity between some emission peaks in IR region, but also causes the drop of both visible and IR emission intensity. Combining with the related absorption characte...

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...

Characterization of near-stoichiometric Ti:LiNbO 3 strip waveguides with varied substrate refractive index in the guiding layer

We have demonstrated the possibility that near-stoichiometric Ti:LiNbO(3) strip waveguides are fabricated by carrying out vapor transport equilibration at 1060 degrees C for 12 h on a congruent LiNbO(3) substrate with photolithographically patterned 4-8 microm wide, 115 nm thick Ti strips. Optical characterizations show that these waveguides are single mode at 1.5 microm and show a waveguide loss of 1.3 dB/cm for TM mode and 1.1 dB/cm for TE mode. In the width/depth direction of the waveguide, the mode field follows the Gauss/Hermite-Gauss function. Secondary-ion-mass spectrometry (SIMS) was used to study Ti-concentration profiles in the depth direction and on the surface of the 6 microm wide waveguide. The result shows that the Ti profile follows a sum of two error functions along the width direction and a complementary error function in the depth direction. The surface Ti concentration, 1/e width and depth, and mean diffusivities along the width and depth directions of the guide are similar to 3.0 x 10(21) cm(-3), 3.8 microm, 2.6 microm, 0.30 and 0.14 microm(2)/h, respectively. Micro-Raman analysis was carried out on the waveguide endface to characterize the depth profile of Li composition in the guiding layer. The results show that the depth profile of Li composition also follows a complementary error function with a 1/e depth of 3.64 microm. The mean ([Li(Li)]+[Ti(Li)])/([Nb(Nb)]+[Ti(Nb)]) ratio in the waveguide layer is about 0.98. The inhomogeneous Li-composition profile results in a varied substrate index in the guiding layer. A two-dimensional refractive index profile model in the waveguide is proposed by taking into consideration the varied substrate index and assuming linearity between Ti-induced index change and Ti concentration. The net waveguide surface index increments at 1545 nm are 0.0114 and 0.0212 for ordinary and extraordinary rays, respectively. Based upon the constructed index model, the fundamental mode field profile was calculated using the beam propagation method, and the mode sizes and effective index versus the Ti-strip width were calculated for three lower TM and TE modes using the variational method. An agreement between theory and experiment is obtained.

Emission characteristics of near-stoichiometric Er∕Yb-codoped LiNbO3 crystals

Polarized visible, near-infrared, and 980-nm up-conversion emission characteristics in near-stoichiometric Z-cut Er(0.6mol%)∕Yb(0.3mol%)-codoped LiNbO3 crystals, prepared by vapor-transport-equilibration (VTE) technique, were investigated and discussed in comparison with corresponding as-grown crystal, thermally calcined ErNbO4 powder, as well as Z-cut only Er(2.0mol%)-doped LiNbO3 crystal that was subjected to the same VTE procedure as one of the Er∕Yb-codoped crystals. The VTE-induced crystalline phase transformation is discussed in combination with optical absorption results reported previously. The percentage of Er3+ ions present in the crystals as the ErNbO4 phase is evaluated based upon the optical absorption and the emission data at 1.5μm. An empirical expression for evaluating the Li composition in a near-stoichiometric Er(/Yb)-doped LiNbO3 crystal from the narrowing percentage of the σ- or α-polarized 1530nm emission peak is established based upon the emission spectra of singly Er-doped VTE LiNbO...

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.

Influence of vapor transport equilibration on spectroscopic properties of Er:LiNbO3 crystal heavily codoped with MgO

Z-cut MgO heavily codoped (6mol%) Er(0.4mol%):LiNbO3 crystals were thermally treated at 1110 and 1120°C over 100, 120, and 220h using vapor transport equilibration (VTE) technique. Visible and infrared-absorption and x-ray powder diffraction have been used to characterize their spectral, spectroscopic, and crystalline phase characteristics in comparison with the as-grown crystal. The experimental results show that VTE treatment results in the reduction of background absorption and 4–5‐nm blueshift of the optical-absorption edge, qualitatively showing that these crystals have been brought closer to stoichiometric composition. The VTE procedure also results in remarkable Er3+ absorption changes. These spectroscopic changes imply that the VTE treatment may result in the formation of ErNbO4 precipitates in all VTE crystals. In addition, the VTE treatment also results in substantial spectral changes with respect to OH− absorption. These OH− spectral changes are explained from the viewpoint of VTE-induced site-...

Origin of vapor transport equilibration induced formation of nanocrystalline ErNbO4 in Er:LiNbO3 crystal

The origin of the formation of nanoprecipitates of ErNbO4 induced by vapor transport equilibration (VTE) in Er:LiNbO3 crystals was investigated by comparing x-ray powder diffraction patterns of VTE-treated, annealed and as-grown Er:LiNbO3 crystals. The results indicated that annealing an Er:LiNbO3 crystal in ambient atmosphere, whether the crystal is x or z cut, and whether it is highly or lower doped, does not influence its crystalline structure, and hence cannot cause the formation of the precipitate. The elevated temperature is not a sufficient requirement for the formation of this additional phase, but is only an essential prerequisite, together with the high-doping level of erbium. The incorporation of a certain amount of additional Li+ ions accompanying the VTE procedure is considered as the direct reason for the formation of the precipitate. The mechanism of the formation of the precipitates of ErNbO4 was tentatively suggested from both microscopic and macroscopic points of view. In addition, the o...

Theoretical analysis of guided mode and effective refractive index at 1.53 /spl mu/m in Ti:LiNbO/sub 3/ strip waveguides

An analysis of guided modes and effective refractive indexes in c-cut Ti-diffused LiNbO/sub 3/ strip waveguides has been performed at the wavelength 1.53 /spl mu/m by the variational method. Dependences of mode size and effective refractive index on initial Ti-strip width W, diffusion temperature T and initial Ti-strip thickness H have been calculated for several lower order modes. As compared the plots of the mode size with the previous published experimental results, an agreement in general behavior has been obtained. The single-mode and cutoff conditions were discussed numerically, which are very useful in optimizing and designing the Er:Ti:LiNbO/sub 3/ waveguide devices. In addition, several advantages of 1.48 /spl mu/m pumping were summarized.