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     

Ti:LiNbO_3质子交换光波导

本文研究了在Ti扩散LiNbO_3衬底上用苯甲酸质子交换制备光波导的方法,以及单模条件、折射率分布、交换系数、表面状态和热稳定性等问题。     

Anomalous refractive index change in proton exchanged LiNbO3 waveguides after annealing

An anomalous refractive index change in proton-exchanged LiNbO₃ optical waveguides formed at the benzoic acid melt after annealing has been observed and explained     

Theoretical modeling of optical amplification in Er-doped Ti:LiNbO3 waveguides

An accurate theoretical analysis is presented describing optical amplification in Er-diffused Ti:LiNbO₃ channel waveguides. It follows as far as possible the theory already developed for Er-doped fibers. As optical pumping around λ_p≈1.48 μm is considered, a quasi-two-level model for the Er³⁺ ions is used with wavelength-dependent cross sections. The optical gain in the 1.53 μm<λ<1.64-μm wavelength range is evaluated. The characteristic parameters, as Er concentration profile, cross sections, pump, and signal mode distributions and waveguide (scattering) losses are taken from experiments. Examples of numerically calculated pump-, small-signal-gain-, and ASE-evolutions are presented. The model has been tested by comparing computed and experimentally observed gain characteristics for Xˆ- and Yˆ-cut LiNbO₃; an almost quantitative agreement has been obtained     

Integrated optical Ti:Er:LiNbO3 soliton source

Efficient diode-pumped and harmonically mode-locked operation of a fully packaged Ti:Er:LiNbO₃ waveguide laser at 1562- (TE) and 1575-nm (TM) wavelength has been demonstrated. The mode-locked pulses have been characterized in terms of time-bandwidth product, fine tuning, and stability. Noise suppression of 42 dB at the relaxation oscillation frequency has been achieved by feedback-controlled pumping. An external amplitude modulator has been used for encoding of the mode-locked pulse train with different bit sequences. Bit error rates of 10^-10 for a 1-0-1-0-bit sequence has been observed for more than half an hour     

一种新型的铌酸锂晶体调制信号源

本文介绍一种能产生可编程序的双脉冲序列或单脉冲序列的信号源(脉冲幅度:100V～300V,脉冲宽度:30ns～50μs,上升沿或下降沿:<10ns,重复频率:100Hz～1MHz).分析了其电原理,给出了其电路和数据.     

Optical and structural characterization of Z-cut LiNbO/sub 3/ optical waveguides formed in a mixed proton source

In this paper, the optical and structural characteristics of Z-cut LiNbO/sub 3/ optical waveguides are experimentally investigated. The waveguides have been fabricated by fully immersing the samples in a closed beaker containing a mixture of benzoic and adipic acid at different concentration ratio. The influence of the dilution ratio on crystallographic phases, wavelength dispersion, diffusion coefficient, and index profile has been determined by using infrared absorption spectroscopy and optical characterization. The control of the thickness of guiding layers corresponding to /spl beta//sub 1/ and /spl beta//sub 2/ phases can be realized by varying the dilution ratio of the melt.     

Fabrication of LiNbO3 channel waveguides using water

A channel waveguide formed in LiNbO₃ was fabricated by using distilled water instead of acids for proton exchange and a self-aligned SiO₂ cap for diffusion. It was demonstrated that the water reaction is an important reaction to cause the index change in proton exchange     

Fabrication tolerance of Ti:LiNbO3 waveguides

A simulation program based on the three-dimensional beam propagation method (BPM) is used to study the fabrication conditions of single-mode Ti:LiNbO₃ waveguides. The calculated cutoff wavelengths are in good agreement with experimental data. The thickness of titanium needed to support the fundamental and first-order modes for three Ti strip widths (6, 7, and 8 μm) is calculated. The two-dimensional BPM is used to calculate the coupling length from the fabrication conditions. Results are consistent with measured data. The fabrication tolerances of the coupling length of directional couplers are also presented     

Second-harmonic generation efficiency in periodically poled LiNbO3 waveguides

Quasi-phase-matched second-harmonic generation efficiency in periodically poled LiNbO₃ using titanium indiffusion, has been theoretically investigated. This permits clarifying how the involved guided modes convert the triangle-shaped form of the ferroelectric polarization in the crystal, via the overlap integral between this form and these modes, into an effective periodic nonlinear coefficient. As a consequence, the periodic function that really permits the quasi-phase-matching operation is the modulation of this effective nonlinear coefficient, the shape of which can be far from a triangular shape, as shown using different cases of mode profiles     

Ti扩散LiNbO_3单模波导参数的测量

对Ti扩散LiNbO_3波导分别采用双曲正割和抛物线分布来近似其折射率的高斯分布,在两个波长下由相应的两种近似本征值方程分别推得了单模波导参数的计算公式,并在4880(?)和6328(?)激光波长下利用棱镜耦合器测量了单模波导参数,对测量结果给出必要的理论分析。     

钛扩散铌酸锂平面波导的反应离子刻蚀

Reactive ion etching（RIE） of LiNbO₃（LN） in SF₆ plasma atmosphere was studied for optimizing the preparation conditions for LN ridge waveguides.The samples to be etched are Ti-diffused LN slab waveguides overlaid with a chromium film mask that has a Mach-Zehnder interferometer（MZI） array pattern.The experimental results indicate that the LN-etching rate(R_LN) and the Cr-etching rate(R_Cr) as well as the rate ratio R_LN/R_Cr increase with either increasing the radio-frequency（RF） power at a given SF₆ flow rate or increasing the SF₆ flow rate at a fixed RF power.The maximum values of R_LN = 43.2 nm/min and R_LN/R_Cr = 3.27 were achieved with 300 W RF power and 40 sccm SF₆ flow.When the SF₆ flow rate exceeds 40 sccm,an increase in the flow rate causes the etching rates and the rate ratio to decrease.The scanning electron microscope images of the LN ridge prepared after～20 min etching show that the ridge height is 680 nm and the sidewall slope angle is about 60°.     

Proton-exchanged optical waveguides in Z-cut LiNbO3 using phosphoric acid

Proton-exchanged Z-cut LiNbO₃ planar waveguides formed using phosphoric acid were characterized optically. The refractive index profile and the diffusion parameters were studied systematically. These waveguides have propagation losses of less than 1 dB/cm and exhibit properties that are different from those obtained using benzoic acid. The index profile is not a simple step function and can be modeled accurately by a polynomial expression. A maximum surface index increase of 0.145 was measured at a 0.633-μm wavelength. The diffusion constant D₀ and the activation energy Q for the proton-exchange process using this acid were found to be 6.43×10⁸ μm²/h and 82.91 kJ/mol, respectively. The annealing properties of these waveguides were also established, and the effects of annealing on surface index change and waveguide depth increase were found to follow a power-law relationship     

Millimeter-wave Ti:LiNbO3 optical modulators

The design, fabrication, and characteristics of ridged Ti:LiNbO ₃ optical modulators that work in the millimeter-wave region are presented. A new concept of design under velocity matching is demonstrated for the proposed modulator. It has been shown by calculation that impedance matching is achieved and conductor loss is greatly reduced under velocity matching with wider gaps and a thicker coplanar waveguide electrode in conjunction with a ridge structure. Two types of Mach-Zehnder optical intensity modulators for the wavelength of 1.5 μm are developed. A fully packaged module for 40 Gb/s transmission with a half-wave voltage of 3.5 V and a broadband modulator responsible up to 100 GHz with a half-wave voltage of 5.1 V     

Hybrid modes in proton exchanged waveguides realized in LiNbO3, and their dependence on fabrication parameters

In this paper, we present a relation between material structure, hybrid modes, and propagation losses in proton exchanged (PE) LiNbO₃ waveguides. Rutherford backscattering (RBS) and X-ray diffraction studies are used to show that there are two essential reasons for losses and complex behavior in high δn_e PE waveguides. RBS studies show that using benzoic acid melt temperatures below 300°C leads to distorted waveguide layers and propagation losses higher than several dB/cm. At high temperature, the X-ray studies prove that the crystalline order is preserved, but induced strain leads to biaxial exchanged layers, which results in hybrid modes that can have very high losses. Finally, fabrication parameters allowing the realization of high quality, high δn_e, PE waveguides in LiNbO₃ are identified     

Increased acceptance bandwidth for quasi-phasematched secondharmonic generation in LiNbO3 waveguides

The authors demonstrate waveguide second harmonic generation using a variable-spaced phase-reversed quasi-phasematched grating that has an acceptance bandwidth 15 times greater than a uniformly quasi-phasematched interaction of the same length     

Cutoff wavelength of periodically segmented waveguides in Ti:LiNbO3

The cutoff wavelength for Ti:LiNbO₃ periodic segmented waveguides (PSWs) with different duty-cycles and of continuous waveguides has been measured for both polarizations, PSWs are described by an equivalent continuous waveguide and, using this model, a universal relationship between the cutoff wavelength of PSWs with different duty-cycles is proposed and verified experimentally     

LiNbO3 optical waveguide fabrication by high-temperatureproton exchange

An efficient method, the high-temperature proton exchange (HTPE), to fabricate high-quality LiNbO₃ optical waveguides is studied. The new proton exchange source, the stearic acid diluted by lithium stearate, is proposed for HTPE process. The known soft proton exchange (SPE) process can be realized by HTPE. There are no phase transitions when the α-phase waveguides are fabricated by SPE. This phase presents the same crystalline structure as that of LiNbO₃ and, as expected, maintains the excellent nonlinear and electrooptical properties of the bulk material. The kinetics of HTPE is studied     

LiNbO3 optical intensity modulator packaged with monitorphotodiode

The assembly technique to simply install a monitor photodiode (PD) with a LiNbO₃ (LN) Mach-Zehnder modulator in the same package is proposed. A light radiated from the Mach-Zehnder Y-branch toward the LN output facet is lead into a glass bead, which is bonded to the LN facet as a fixture of the optical fiber. The end of the glass bead is cut to have an angled face, and the radiated light is reflected normal to the LN waveguide axis and toward the PD fixed on the side of package. The curved surface of the glass bead is effective to roughly focus the light onto the PD. We demonstrate that such monitor PD outputs an intensity modulation exactly reverse to the LN output modulation with a sensitivity higher than 0.03 A/W per optical output power of LN, indicating a usability of the PD output to monitor the drift of LN modulator     

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。