Gamma-Ray Irradiation-Enhanced Wet-Etching of Proton-Exchanged Lithium Niobate

Gamma-ray irradiation-enhanced wet-etching of proton-exchanged Z-cut Y-propagating LiNbO₃ is presented. The etched ridge height of a sample with gamma-ray irradiation of intensity 650 krad/h on a LiNbO₃ substrate for 30 min is three times larger than that without. Moreover, experimental results show the propagation loss values of Ti-indiffused LiNbO₃ waveguides fabricated on the irradiated ridge structures are only slightly larger than those on the nonirradiated ones.     

Wet-etched ridge waveguides in y-cut lithium niobate

By the technique of nickel indiffusion proton exchange (NIPE) and the technique of buffered proton exchange (PE) melt, wet-etched ridge waveguides in y-cut LiNbO₃ are fabricated for the first time. The fabricated ridge waveguides have smooth surfaces and are good enough for low-loss waveguides. Moreover, a ridge waveguide Mach-Zehnder modulator in y-cut LiNbO₃ is fabricated. The measured half-wave voltage is about 30% lower than that of a conventional modulator. The wet-etched ridge waveguides in y-cut and z-cut substrates are also characterized for comparison     

Ferroelectric integrated optics: recent developments

Advances in rare-earth-doped waveguide lasers in LiNbO₃, waveguided second harmonic generation through quasi-phase-matching in LiNbO₃ and LiTaO₃, and pulsed laser epitaxial growth of LiNbO₃ on Sapphire are reported. Efficient lasers operating in CW, Q-switch, and mode-lock modes were fabricated in Nd:LiNbO₃. In order to assess the potential of bulk-doped Er:LiNbO₃ samples, spectroscopic measurements have been carried out, the main limiting mechanism identified, the corresponding up-conversion coefficient evaluated, and a comprehensive gain model developed. First lasing action in this material, when pumped at 1.48 μm, is described. Quasi-phase-matched generation of blue light with conversion efficiencies up to 150%/Wcm² is also demonstrated, and finally we present the fabrication of low-loss LiNbO₃ single-crystal waveguiding layers     

Mach-Zehnder modulators with lithium niobate ridge waveguidesfabricated by proton-exchange wet etch and nickel indiffusion

Mach-Zehnder ridge waveguide modulators on c⁺-LiNbO₃ by proton-exchange wet etch and nickel indiffusion are fabricated for the first time. Modulators operating at 0.633 μm and 1.31 μm are both produced, and the half-wave voltages are measured to be 4 V and 22.5 V, respectively, for an electrode length of 6 mm. The extinction ratios are 14 dB and 13.2 db, respectively, and can be improved by forming the ridge structure only at the electrode interaction region instead. These results make the wet-etched LiNbO₃ optical devices promising to take over the dry-etched ones in the near future     

Lithium Niobate Ridge Waveguides Fabricated by Wet Etching

The fabrication of LiNbO₃ ridge waveguides etched by a mixture of HF and HNO₃ using chromium (Cr) stripes as masks is reported. Smooth etched surfaces are obtained by adding some ethanol into the etchant. Under-etching is nearly avoided by annealing the sample with the Cr masks before the wet etching process. Low-loss monomode ridge guides with a height of up to 8 mum and a width between 4.5 and 7.0 mum are demonstrated. As an example, the propagation losses in a 6.5-mum-wide and 8-mum-high structure are 0.3 dB/cm for transverse-electric and 0.9 dB/cm for transverse-magnetic polarization, respectively, at 1.55-mum wavelength     

Effects of ridge depth on characteristics of shieldedvelocity-matched (SVM) Ti:LiNbO3 optical modulators withridge structures

The effects of ridge depth on the microwave effective index, characteristic impedance of the traveling-wave electrode, and modulation bandwidth of shielded velocity-matched Ti:LiNbO₃ optical modulators with ridges are investigated numerically. It is clarified that the driving voltage has an optimum ridge depth (3-4 μm) for various gaps of the electrode, and there are optimum overlaid layer thicknesses at the velocity matching point for given values of the ridge depth     

Fabrication of ridge waveguides in LiNbO3 thin film crystal by proton-exchange accelerated etching

A simple fabrication method of adhered LiNbO₃ ridge waveguides by accelerated etching of the proton-exchanged region is proposed and demonstrated. The waveguides were fabricated and tested at 1.55 mum wavelength. Strongly confined guided modes were obtained. The waveguides are suitable for efficient nonlinear-optic wavelength conversion devices.     

Activation energy of DC-drift of x-cut LiNbO3 opticalintensity modulators

The DC-bias-induced drift phenomenon in LiNbO₃ optical intensity modulators is a main cause of device wearout failure. In order to estimate the device lifetime, an activation energy value Ea of the drift is needed, and Ea=1.0 eV is already known for z-cut LiNbO₃ modulators. However, Ea of x-cut LiNbO₃ modulators is not known even though there is a possibility that the Ea depends on the crystal orientation. Here, Ea=1.4 eV is obtained experimentally for the x-cut LiNbO₃ modulator with a SiO₂ buffer layer from their drift measurements between 50°C-140°C     

Growth of low-scattering single-domain photorefractive LiNbO3 crystals

Low-scattering single-domain LiNbO₃ crystals can be grown from a 3 mol% LiVO₃-doped LiNbO₃ (50 mol% Li ₂O-50 mol% Nb₂O₅) quasi-flux system. Approximately one order of magnitude less scattered light was detected in this crystal than in the commercially available congruent reference LiNbO₃ sample. For quantitative scattering measurements an integrating sphere technique developed at Lasergenics Corp. was utilized. Although no direct evidence can prove the improvement of the Li-Nb-O stoichiometry, the “as grown” single-domain character, the higher refractive index, and the significantly lower scattering are indirect evidences revealing unusual changes and improvements in the quality of the LiNbO₃ single crystals. These crystals may be promising for high-density holographic data storage applications     

Highly efficient 40-GHz bandwidth Ti:LiNbO3 opticalmodulator employing ridge structure

The design and fabrication of a Ti:LiNbO₃ optical modulator employing a ridge structure with a shielding plane are described. The ridge structure offers a relatively low microwave propagation loss and large interaction between the microwave and optical wave under the condition of a 50 Ω characteristic impedance system, resulting in a large modulation bandwidth and low driving voltage. As a result, a 3-dB optical bandwidth of 40 GHz with a driving voltage of 3.6 V is achieved at 1.5 μm     

Improved electrooptic modulator with ridge structure in X-cut LiNbO3

A ridge-type Mach-Zehnder modulator on X-cut LiNbO₃ is fabricated by wet etching with proton-exchange pretreatment. Experimental results show that, with the assistance of the ridge structure, the drive voltage of the proposed modulator is reduced. The enhancement of overlap integral caused by ridge structure is also validated from numerical simulation. Moreover, an improved design to further reduce the drive voltage of the ridge-type Mach-Zehnder is proposed by a proper placement of waveguide relative to the electrode     

LiNbO3晶体纳秒级光开关的研究

LiNbO3晶体由于其透光范围广，响应速度快，不易潮解等特点而广泛用于高精度和高速光开关快门。为了将LiNbO3光开关曝光时间拓宽到纳秒领域从而弥补现有的高速光开关的不足，文章阐述了LiNbO3作为光开关理论原理，对多波段通光的晶体长宽比进行了优化设计，并对LiNbO3纳秒级光开关的可行性进行了实验验证。实验半波电压与理论符合很好，实验光信号和电信号在纳秒级范围内能实现同步响应。研究结果证实了LiNbO3光开关在纳秒级范围内的可行性，并为LiNbO3纳秒级光开关的制作提供了理论和实验依据。     

Evaluation of absorption and emission cross sections of Er-dopedLiNbO3 for application to integrated optic amplifiers

The polarization-dependent absorption and emission spectra of the ⁴I_13/2-⁴I_15/2 transition (λ~1.5 μm) in single crystal bulk Er:LiNbO₃ have been measured. Low-temperature (10 K) measurements of the Stark split energy levels of these two manifolds indicate at least two Er³⁺ sites. McCumber theory is applied to determine the Er:LiNbO₃ absorption and emission cross sections. These values are used to calculate the gain characteristics of Er:LiNbO₃ channel waveguides. Calculations indicate that a gain of 10 dB is achievable in a waveguide of several centimeters using ~20-mW pump power     

Ion exchange model for α phase proton exchange waveguides inLiNbO3

An H⁺/Li⁺ exchange model is found to be applicable to describe the diffusion of protons when optical waveguides are formed in LiNbO₃ by proton exchange methods where the proton doped crystal structure stays in the pure α phase. The H ⁺ and Li⁺ self-diffusion coefficients in the ion exchange model are determined as a function of the proton exchange temperature both for x-cut and z-cut LiNbO₃. In this way a very useful tool for predicting the proton concentration profiles and hence the refractive index profiles of α phase proton exchange LiNbO₃ waveguides is achieved     

Lithium–Niobate Channel Waveguide for the Realization of Long-Period Gratings

We propose a special lithium-niobate (LiNbO₃) single-mode waveguide for the realization of long-period gratings, which consists of a channel core embedded in a thin slab cladding. We fabricated the waveguide on a z-cut LiNbO₃ substrate with a two-step proton-exchange process and demonstrated its suitability for grating application with a number of removable photoresist long-period gratings deposited on the waveguide surface. The waveguide fabrication process and the LiNbO₃ waveguide structure could be further explored for the development of electrooptic gratings for high-speed applications.     

Electro-optic effect in proton exchanged LiNbO3 andLiTaO3 waveguides

Electro-optic properties of proton-exchanged (PE) waveguide layers in LiTaO₃ and LiNbO₃ are studied and related to their optical characteristics. The proton-exchange process induces a degradation of the electro-optic activity in both types of waveguides, PE LiNbO₃ and PE LiTaO₃. The measured electro-optic effect is close to the detection sensitivity even when the exchange regime is performed at low temperatures for short periods of time. The PE samples have been annealed (APE waveguides) and the changes of their r₃₃ electro-optic coefficient has been followed at successively higher temperatures and periods of time. Subjected to annealing at temperatures between 265-420°C, the LiTaO₃ layers show a partially recovered r₃₃ coefficient, the recovering being different for quick and slow cooling of the samples. In thin APE LiNbO₃ waveguiding layers a restoration of r₃₃ up to 75% of the bulk value is observed due to the annealing at temperatures between 200-340°C     

Finite element analysis of LiNbO3 waveguides with Si orSi/SiO2 overlay

LiNbO₃ waveguides with Si overlays are emerging as a basic building block for a variety of integrated-optic components, including modulators, high-efficiency gratings, and narrowband WDM filters. However, the development and optimization of these devices are, in large part, hindered by the lack of understanding of the specifics of the Si-on-LiNbO₃ structure which appear to differ dramatically from those of the Si and LiNbO₃ waveguides, considered separately. In this work, we provide a specific insight into the waveguiding properties of vertically stacked Si-on-LiNbO₃ waveguides. In particular, we present a detailed theoretical analysis of the effect of the Si film on the modal characteristics (propagation constant and field distribution) of the structure. The vectorial finite element method (VFEM) is used to numerically investigate a step-index and graded-index single-mode channel waveguide in LiNbO₃, with a Si or Si/SiO₂ multimode overlay. We show that for ~70% of all Si thicknesses, in the range from 0 to 1.6 μm, the highest order normal mode of the entire structure has more than 99.9% of the total energy confined in the LiNbO₃ region, i.e., beneath the Si overlay. This fact is quite intriguing given the fact a planar Si layer of submicron thickness on bulk LiNbO₃ is already multimoded. Furthermore, we show that the effective mode index of the structure is considerably modified compared to that of the LiNbO₃ waveguide while the propagation loss is, on the other hand, practically unaffected (~0.3 dB/cm) even in the presence of the lossy Si film, as confirmed by our previous experimental results. Evidently, large modulation of the effective index and low-loss propagation provide an ideal combination of properties suitable for the fabrication of high-reflectance corrugated waveguide gratings, essential for a number of practical devices, in particular, WDM filters     

Optical and physical characterization of SiO2-x-Althin-film polarizer on x-cut LiNbO3 substrate

For the purpose of a mechanical evaluation of a metal-cladding polarizer, a precise characterization of SiO_2-x-Al thin-film succession fabricated on a LiNbO₃ substrate was made as well as an experimental optimization of the SiO_2-x-Al polarizer for the Ti:LiNbO₃ waveguide. A 10-nm-thick SiO_2-x was selected as the optimized underlay of a SiO_2-x-Al polarizer for the Ti:LiNbO₃ waveguide using a wavelength of λ=1.55 μm. Results of scratch testing show that the adhesive strength of SiO_2-x-Al films was almost the same level as that of Ti-Au films on a thick SiO₂ layer, commonly used for metallic underlay of Au-plated electrodes. From observing SiO_2-x -Al film using a transmission electron microscope, it was confirmed that the 10-nm-thick SiO_2-x underlay stratified well without serious thickness fluctuation     

4.6 °m-band difference frequency generation and CO isotopologue detection

There are many strong absorption lines of environmental gases in the mid-infrared region in atmospheric air, including those of greenhouse effect gases. Reported is difference frequency generation in a direct-bonded periodically poled LiNbO₃ ridge waveguide in the 4.6 m band, which is the most appropriate wavelength range for observing carbon monoxide absorption spectra. It is also shown that isotopologue absorption lines can be detected using the output light generated in a waveguide with mature telecommunication band laser diode light sources. The direct-bonded ridge waveguide consisted of a 50 mm-long chip and its wavelength conversion efficiency was approximately 4% /W.     

Low drive voltage, broad-band LiNbO3 modulators with andwithout etched ridges

Frequency dependent drive voltage is reported in velocity matched, packaged, Mach-Zehnder interferometers, made on Z-cut, Y-propagating LiNbO₃, with and without etched ridges. Drive voltages of ~7 V at 40 GHz are demonstrated, with the etched ridge device showing a 1/2-1 V advantage. Microwave modeling allowed the extraction of the effects of dispersion in material properties, occurring near the acoustic resonance, on bandwidth and drive voltage