Efficient erbium-doped thin-film lithium niobate waveguide amplifiers.

Lithium niobate on insulator (LNOI) is an emerging photonic platform with great promise for use in future optical communications, nonlinear optics, and microwave photonics. An important integrated photonic building block, active waveguide amplifiers, however, are still missing in the LNOI platform. Here, we report an efficient and compact waveguide amplifier based on erbium-doped LNOI waveguides, achieved using a sequence of erbium-doped crystal growth, ion slicing, and lithography-based waveguide fabrication. Using a compact 5 mm long waveguide, we demonstrate an on-chip net gain of >5dB for 1530 nm signal light with a relatively low pump power of 21 mW at 980 nm. The efficient LNOI waveguide amplifiers could become an important fundamental element in future lithium niobate photonic integrated circuits.

Judd-Ofelt spectroscopic properties of Er3+-doped NaLa(WO4)2 polycrystalline powder.

Er3+-doped NaLa(WO4)2 is a promising phosphor material for applications in many fields including the ratiometric thermometry based on thermal effect of fluorescence intensity ratio (FIR) of green fluorescence of Er3+, which is directly correlated with Judd-Ofelt parameters Ωi (i = 2, 4, 6). Present paper reports synthesis and Judd-Ofelt spectroscopic properties of Er3+-doped NaLa(WO4)2 µm-sized phosphor. The phosphor was synthesized by solid-state chemical reaction and characterized by X-ray diffraction and Raman scattering techniques. The results show that the phosphor is dominated by NaLa(WO4)2 crystalline phase and has a maximum phonon energy 927 cm-1. Judd-Ofelt analysis was done for the phosphor using a safe, reliable method based on diffuse reflection spectrum and Er3+ 1.5 µm fluorescence lifetime. First, diffuse reflection spectrum of the phosphor was measured and relative absorption spectrum was calibrated from it using Kubelka-Munk theory. Second, Er3+ 1.5 µm fluorescence lifetime of the phosphor was measured and absorption cross-section spectrum was obtained based on the assumption that the 1.5 µm emission has 100% quantum efficiency. Finally, based on the absorption cross-section spectrum, standard Judd-Ofelt analysis was carried out to extract the Ωi. Radiative rate, fluorescent branch ratio and radiative lifetime of some transitions have been obtained from the known Ωi values. In addition, FIR proportional factor was evaluated in terms of Ωi and compared with those values of other materials. The result shows that the phosphor has a better prospect for the application in ratiometric thermometry.

Error evaluation of Judd-Ofelt spectroscopic analysis.

Judd-Ofelt (J-O) spectroscopic knowledge of a rare-earth-doped luminescent material is crucial to its application. Although a large number of papers with regard to the J-O study of various rare-earth-doped luminescent materials have been reported each year, few papers presented the errors of the J-O intensity parameters Ωi (i = 2, 4, 6) and radiative probabilities evaluated from them. Present study focuses on the error evaluation of the J-O parameters and radiative probabilities. An error theory is established for the J-O analysis and radiative probability. Two error analysis methods based on root mean square of the difference either between measured and calculated oscillator strengths (δfrms) or between measured and calculated line strengths (δSrms) are studied. Explicit error expressions are presented for the J-O parameters and radiative probability. The validity of the theory is verified by applying it to widely studied Er3+, Tm3+, Ho3+ and Nd3+ ions that are doped into four single-crystals (LiNbO3, SrGdGa3O7, LiYF4 and YVO4) and a glass. The two methods are identical in nature and give similar results of errors of Ωi.

Sextuple ratiometric thermometry based on 980-nm-upconverted green fluorescence of Er3+ ions in submicron crystals.

980-nm-upconverted 530 and 550 nm Er3+ green fluorescence spectra of Er3+/Yb3+-codoped NaGd(WO4)2 submicron crystals were measured in the temperature range of 298-383 K. A sextuple ratiometric thermometry is proposed. It is established on the basis of six schemes of fluorescence intensity ratio (FIR) that considers three component peaks of the 530 nm emission band and two component peaks of the 550 nm emission band, which involve electronic transitions between two Stark sublevels of Er3+. The study shows that the phosphor shows strong green fluorescence, which is verified by measured quantum yield, and thermally stable spectral structure desired for the sextuple ratiometric thermometry. All of the six FIR schemes display highly efficient sensing performances with slightly different thermal sensitivities. Each scheme gives a temperature value and the six schemes give an averaged result. In parallel, we have also carried out an ex vivo experimental study on the temperature characteristics of the green fluorescence of the phosphor. Almost same results have been obtained, verifying biological applicability of the phosphor. The ex vivo experimental results also show that the sextuple thermometry increases considerably the accuracy and reliability of temperature measurement in comparison with the conventional intensity integration method.

Measurement of refractive index of powder by prism coupler.

Refractive index of optical material of powder is measured not as easily as a bulk material. Here, the prism coupling technique in combination with the immersion method is proposed to measure the refractive index of an optical material of powder. First, the powder material to be measured was dispersed in α-bromonaphthalene (C10H7Br) liquid to form a suspension mixture. The refractive index of the mixture, together with that of pure C10H7Br, was then measured at the wavelengths of 632.8, 1311, and 1553 nm using a commercial prism coupler. From the measured index values of pure C10H7Br and powder-dispersed mixture, the refractive index of the powder material was obtained on the basis of the Maxwell-Garnett model. Microcrystal powder from a LiNbO3 single-crystal, which has the known refractive index values, has been exemplified to demonstrate the method. The results show that the method is feasible with an accuracy of ±0.05.

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.

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.

Note: Electro-optic coefficients of Li-deficient MgO-doped LiNbO3 crystal.

A number of Li-deficient MgO-doped LiNbO3 (LN) crystals with different Li2O contents ranging from 43.4 mol. % to 44.5 mol. % were prepared by carrying out the Li-poor vapor transport equilibration treatment on 5 mol. % (in growth melt) MgO-doped LN crystals. Unclamped electro-optic (EO) coefficients γ13 and γ33 of these crystals were measured by Mach-Zehnder interferometry. The results show that γ13 (γ33) increases linearly by ∼14% (11%) as the Li2O content decreases from 44.5 mol. % of the as-grown state to 43.4 mol. % of the Li-deficient state. This feature is desired for the EO application of the Li-deficient MgO:LN crystal.

Polarization-insensitive, shallow Ti-diffused near-stoichiometric LiTaO3 strip waveguide for integrated optics.

We report on a Ti-diffused near-stoichiometric (NS) LiTaO3 strip waveguide fabricated by diffusion of an 8 µm wide, 160 nm thick Ti-strip followed by Li-rich vapor transport equilibration. It is found that the waveguide surface caves in ∼60 nm below the crystal surface. X-ray single-crystal diffraction shows that the indentation is due to Ti-induced lattice contraction. Optical studies show that the waveguide is in an NS composition environment, supports TE and TM single-mode propagation at 1.5 µm wavelength, is polarization insensitive, and has a shallow mode field profile and a loss of 0.2/0.3 dB/cm for the TE/TM mode. Secondary ion mass spectrometry analysis shows that the Ti profile follows a sum of two error functions in the width direction and a Gaussian function in the depth direction of the waveguide. With the optimized fabrication condition, the waveguide is promising for developing an optical-damage-resistant device that requires a shallow mode field profile.

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.

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 ∼30 dB 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 >360 nm super-broadband filtering.

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 Ti(4+)/X(n+) (X(n+) = Sc(3+), Zr(4+), Er(3+)) co-diffusion in the widely studied LiNbO3 and LiTaO3 crystals. Some Ti(4+)/X(n+)-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 Ti(4+)/X(n+)-co-diffusion characteristics were studied by secondary ion mass spectrometry. In the X(n+)-only diffusion case, the X(n+) diffuses considerably slower than the Ti(4+). In the Ti(4+)/X(n+) co-diffusion case, the faster Ti(4+) controls the diffusion of the slower X(n+). The X(n+) diffusivity increases linearly with the initial Ti-metal thickness and the increase depends on the X(n+) species. The phenomenon is ascribed to the generation of additional defects induced by the diffusion of faster Ti(4+) ions, which favors and assists the subsequent diffusion of slower X(n+) ion. For the diffusion system studied here, it can be utilized to substantially shorten device fabrication period, improve device performance and produce new materials.

Sharp plasmonic resonance on gold gratings in amplitude and phase domains.

In this work, we experimentally studied sharp plasmonic resonance on gold gratings in both amplitude and phase domains using a spectroscopic ellipsometry technique. We used numerical and analytical models to analyze the phase delay of TM and TE waves under a surface plasmon excitation condition, and the calculated result fits well with the experimental observations. In addition, the ellipsometry method used here provides an important tool to characterize the phase information in plasmonic and metamaterial devices.