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。     

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     

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。     

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     

Design parameters of Ti:LiNbO3 waveguides for optimumacoustooptic interaction efficiency

A computer model of optical and surface acoustic wave modes in titanium indiffused Y-cut LiNbO₃ guiding structures is used to produce several graphs for the design of guided-wave acoustooptic devices with optimum interaction characteristics. The design concentrates on waveguide fabrication parameters in order to achieve the optimum acoustic frequency and the acoustooptic interaction bandwidth. The optimum acoustic frequency for a given value of Ti film thickness and diffusion depth is defined as the frequency causing the maximum value of the overlap integral     

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     

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     

Numerical optimization procedure for low-loss sharp bends in MgOCo-doped Ti:LiNbO3 waveguides

Recent results provided experimental evidence that losses in bent Ti:LiNbO₃ waveguides can be drastically reduced by using additional titanium and magnesium oxide. We determine an optimization procedure for the index profile in curved sections. The beam propagation method indicates that in optimized bends, losses are orders of magnitude below those of conventional bends. Tolerances in mask alignment and in deposited film thickness are estimated to be well compatible with present technology     

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     

High-speed shielded velocity-matched Ti:LiNbO3 opticalmodulator

A shielded velocity-matched Ti:LiNbO₃ optical modulator is investigated, focusing on the characteristics of the traveling-wave (TW) electrode and the optical waveguide. The optical waveguide is analyzed and its parameters are determined using the newly developed modified-step-segment method (MSSM). The TW electrode is analyzed using the second-order triangular element quasi-TEM finite element method (FEM). By taking the thickness of the coplanar waveguide (CPW) traveling-wave (TW) electrode into consideration, it is confirmed that there is an optimum overlaid layer thickness for a given electrode thickness. It is also shown that very wide modulation bandwidth can be attained by using the optimum CPW TW electrode thickness and overlaid layer thickness     

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.     

Modeling of Ti:LiNbO3 waveguide directional couplers

A simple and accurate method is presented for studying the coupling characteristics of 3D diffused channel waveguide directional couplers. The method is based on the scalar variational principle and differs from other methods as it does not require any assumption for the functional form of the trial field. The authors also propose an equivalent 2D directional coupler that also gives accurate results and, hence, saves considerable computational time, Comparison with experimental and other theoretical results is also presented     

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°.     

Ti:LiNbO3 stripe waveguide Bragg reflector gratings

The first demonstration of Bragg reflector grating filters in Ti:LiNbO₃ single-mode channel guides is reported. Filter bandwidths as narrow as 0.14 nm centred at 1.476 μm have been achieved for TE polarisation in Y-cut (X-propagating) material     

Erbium-doped single- and double-pass Ti:LiNbO3 waveguideamplifiers

Single-pass and double-pass Er-diffused Z- and X-cut Ti:LiNbO₃ waveguide amplifiers, optically pumped at λ _p≈1484 nm, have been investigated. With a 48 mm long Z-cut amplifier device, Er-diffusion doped at 1100°C, 6.7 dB (coupled pump power P_p,c=170 mW) and 14.7 dB (P_p,c=90 mW) net small-signal gain have been achieved with a single-pass and a double-pass configuration, respectively, at the signal wavelength λ_s=1531 nm. A Z-cut sample doped at 1135°C showed a considerably improved behavior. 11.3 dB single-pass net small-signal gain has been obtained (P_p,c=170 mW; sample length 5.7 cm). Theoretical calculations predict gain figures up to 20 dB in single-pass and 40 dB in double-pass Er:Ti:LiNbO₃ amplifiers with increased (realistic) lengths of 10 cm     

Fluctuational crosstalk in Ti:LiNbO3 directional couplerswitches

It has been found that the fluctuations in the Ti concentration along waveguides of Ti:LiNbO₃ directional coupler switches degrade the switch extinction ratio and introduce an unwanted crosstalk between both channels. We discuss analytically the effect of effective optical index fluctuations on the transfer matrix of the optical switch. We demonstrate the sensitivity of the mismatch to the statistics of the fluctuations of the Ti concentration     

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     

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     

Suppression of crosstalk drift in Ti:LiNbO3 waveguideswitches

The authors report on DC drift characteristics with short relaxation times (typically 20-30 s) of LiNbO₃-based electrooptic switches for use at the wavelength of 1.3 μm. They evaluated the switch elements without oxide buffer layers to focus on the substrate. Sputter-etching of more than 20 nm of the substrate surface after waveguide fabrication results in a remarkable suppression of the drift: drift-induced crosstalk of below -25 dB has been achieved. The sputtering process also reduces the range of scatter of voltage drift among four switches fabricated on a single substrate. With data on the capacitance and resistance between a pair of electrodes, the above effects are attributed to the removal of the surface layer by sputtering     

LiNbO3 optical waveguides formed in a new proton source

Proton-exchanged planar waveguides have been fabricated on Z-cut and X-cut lithium niobate crystals by using a new proton source formed by a mixture of benzoic and adipic acids. Waveguide index profiles and optical characteristics have been obtained at different values of the adipic-benzoic acid concentration ratio. The samples have been structurally characterized by Raman and infrared (IR) absorption spectroscopy and double-crystal X-ray diffraction. Good quality samples have been fabricated by using 30 mol% ratio dilution, showing very low scattering levels (<0.1 dB/cm), relatively high electrooptic coefficient (r₃₃=0.88 pm/V), and low relative percentage of interstitial protons (26%). The main factor limiting the waveguide optical properties is the substitutional-interstitial proton ratio, which can be easily controlled to produce good quality waveguides. A demonstration of the repeatability of the exchange process in the acid mixture is also provided