Thin Layer Design of X-Cut Lithium Niobate Electrooptic Modulator With Slotted SiO2 Substrate

In this letter, we present a novel lithium niobate (LN) electrooptic modulator. The proposed modulator incorporates a thin layer of LN and a slotted substrate structure that together facilitate velocity and impedance matching. A full-wave analysis of the proposed modulator is undertaken in order to establish the frequency dispersion effects with respect to the devices microwave effective index N_m, characteristic impedance Z_c, and half-wave voltage length product V_piL.     

基于X-LiNbO3/SiO2/Si低温漂、大机电耦合SH-SAW谐振器的设计

铌酸锂(LN)单晶薄膜具有较高的机电耦合系数(k² eff＞30%),其水平剪切(SH)声学模式常被应用于开发具有大机电耦合系数的薄膜声学谐振器和超宽带滤波器。但LN 的频率温度系数较大(TCF ＞ -50×10^-6/℃),这不仅会降低滤波器的可用有效带宽,同时也会限制器件的功率处理能力。采用3D周期有限元模型对基于X 切LN/SiO₂/Si结构SH 声表面波(SH-SAW)谐振器进行了优化研究。研究结果表明,当SH-SAW 传播角ψ=-10°~-20°、LN和SiO₂ 膜厚分别为hLN=0.1λ 和h_SiO2 =0.2λ(λ 为叉指换能器周期)、铝电极金属化率η=0.4、电极相对厚度h_Al/λ=5%~10%时,SH-SAW 谐振器的 k² eff 约为30%,且其TCF＜-20×10^-6/℃,有望用于开发新一代的低温漂、超宽带5G SAW 滤波器。     

Measurement of Frequency Shift Characteristics Based on LiNbO3 Waveguide Electro-Optic Intensity Modulator

High-speed and wide-band LiNbO3 waveguide electro-optic intensity modulator has drawn great attention in the field of optical fiber communication and sensor. This paper reports the research results on the measurement of frequency shift characteristics of Mach-Zehnder electro-optic intensity modulator. Two measurement methods of frequency shift characteristics for high and low frequency modulations are studied in theory and experiment and demonstrate different results. The realization of a multi-wavelength optical source based on Mach-Zehnder electro-optic intensity modulator has been introduced. The technique to reach the maximum intensity for interesting shift frequency, particularly for heterodyne detection of Brillouin distributed optical fiber sensing, has been given.     

Optical switching and power control in LiNbO3 coupledwaveguide arrays

A diversity of switching and power-distribution functions in coupled multi-waveguide arrays is discussed and demonstrated. Functionality is achieved by varying the effective index in each waveguide independently by means of the electro-optic effect. The splitting and switching functions are attained by the use of a multi-variable optimization procedure. Specifically we demonstrate [1×2] and [1×5] routing and power division and [2×2] cross-connection. Polarization diversity and polarization-independent switching are attained using a special crystal orientation and electrode configuration. Single-polarization routing was measured in most cases, with crosstalk figures of the order of - 15 dB. Polarization-independent switching was achieved with crosstalk of less than - 19 dB for a three-waveguide [1× ] switch. By addition of dummy guides, a polarization-independent [2×2] cross-connection function could be attained with crosstalk figures better than -30 dB     

A 10-GHz Resonant-Type LiNbO 3 Optical Modulator Array

A resonant-type optical modulator array connected with antennas can effectively convert a micro/millimeter-wave to a light wave for the uplink of a radio-on-fiber system. We demonstrated the operation of an array that contained four modulators using power simultaneously received by microstrip antennas connected to each modulator. We confirmed that the optical phase change induced by the received power was proportional to the number of connected antennas     

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     

Examination of LiNbO3/nitride heterostructures

Oxides such as LiNbO₃, PbTiO₃, etc. have large polarization effects arising from the ferroelectric and piezoelectric effects. Recent work on nitride heterostructures has shown that polar charge can be used to substitute for dopant charge and controlled to create a two-dimensional electron (and hole) gas, resulting in ohmic contacts and diode like current–voltage characteristics. In this paper, we examine the potential of using ferroelectrics with traditional semiconductors for applications in novel electronic devices. In particular, we examine their use in creating very high sheet charge densities of mobile charge and tailorable current–voltage characteristics. It is seen that owing to hysteresis based effects in the ferroelectric layer, the direction of variation of voltage has a marked effect on the charge induced as well as the variation of that charge with voltage.     

Proton exchange in Y-cut LiNbO3

In the present work, planar waveguides in Y-cut LiNbO₃ were obtained using modified proton exchange (PE) conditions of: PE and subsequent annealing (APE), PE in buffered melts (BMPE), APE followed by PE (APE+PE), and PE in vapours (VPE). Benzoic acid was used as the proton source in the PE, BMPE, and (APE+PE) experiments. Cinnamic acid was used for obtaining VPE-waveguides. The main aim was to prevent surface damage of Y-cut crystals due to the strains introduced by proton exchange. The investigations performed showed that the surface etching is probably due to lattice deformation anisotropy leading to higher strains in PE Y-cut samples. Most encouraging results were observed, when an optimized (APE+PE)-procedure was used for waveguide formation. This method is very attractive for the fast preparation of deep high-index and low-loss waveguides in Y-cut LiNbO₃. This procedure allows passive and active elements to be produced in one and the same Y-cut substrate of LiNbO₃. Similar preliminary results were obtained under VPE conditions.     

Broad-band reflection traveling-wave LiNbO3 modulator

A velocity matched traveling-wave Mach-Zehnder interferometer (MZI) in LiNbO₃ operating in reflection, is reported and drive voltages are compared to those for single-pass devices of the same length. The device achieves double-pass operation by simultaneously reflecting the optical and RF waves, Broad-band operation to 20 GHz with drive voltages from 0.5 to 1 V (<0.5 GHz) to ~4.5 V (at 20 GHz) is observed with the reflection device. This performance is superior to that of the single-pass device, which has drive voltages of 2.1 V (dc) to ~4.5 V (at 20 GHz)     

An LiNbO3 integrated coherence modulator

The authors describe the principle of operation and the performance of a newly integrated LiNbO₃ modulator specially designed to generate optical delays of several millimeters. This is achieved by using the simultaneous propagation of the TE and TM modes in a waveguide and by taking benefit of the high natural birefringence of LiNbO₃. This modulator is well-suited to coherence modulation of light in fiber-optic transmissions. When powered by a short coherence source, the large optical delays induced by a cascade of such modulators can be used as information carriers to transmit several signals simultaneously. A demonstration is reported using a single source and two modulators in cascade to transmit signals simultaneously     

钛扩散LiNbO_3波导干涉仪调制器

研制成Ti扩散LiNbO_3波导干涉仪调制器。在0.6328微米的光波长下加1千赫方波信号。50千赫的脉冲信号和100千赫～30兆赫的正弦波信号进行了调制实验。实验样品的最大调制深度87％,半波电压V_π=45伏,电容8微微法,3分贝带宽可达800兆赫。     

近红外掺镁周期极化铌酸锂光参变振荡器

研究了掺镁周期性极化铌酸锂光参变振荡器(PPMgLN-OPO)在近红外波段的调谐特性和输出特性。计算了PPMgLN-OPO的调谐曲线,并采用Nd:YVO4激光器产生的1.064μm激光作为抽运源,验证了其温度调谐特性,实现了1.8～2.6μm的可调谐红外激光输出。此外,比较了长度为2cm和3cm的PPMgLN晶体的OPO输出特性,并在抽运功率为6.7W时,获得了最高功率为3.2W的2μm激光输出,转换效率达47.8%。     

Polarization selective phase modulator in LiNbO3

An integrated optical phase modulator which selectively modulates only one mode of polarization is discussed. Selectivity is accomplished by controlling the ratio between the voltages applied to two different electrode sections. A method for determining the correct ratio and evaluating the polarization selectivity of the phase modulation Γ has been developed. Experimentally, a Γ of 46 dB for selective modulation in the TM direction and a corresponding value of 40 dB for the TE direction have been obtained. It is also shown that the concept is a possible alternative or complement to polarizers and polarization preserving components in optical sensors     

UV-induced surface-relief gratings on LiNbO3 channelwaveguides

We report a new technique for fabricating Bragg gratings in LiNbO ₃ channel waveguides. This technique involves the irradiation of the waveguide by means of an ultraviolet beam. We also demonstrate the applications of these gratings as tunable input couplers and as Bragg reflectors     

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     

Reflective waveguide array demultiplexer in LiNbO3

Wavelength demultiplexer fabricated on electrooptic LiNbO₃ is demonstrated. Reflection structure was employed to halve the device length. The full width at half maximum of the transmission peak was around 1 nm. The crosstalk was -12 to -25 dB. Polarization independence was attained using a reflective quarter-wave plate     

Some optical properties of KTP, LiIO3, and LiNbO3

Measurements of the absorption coefficient for KTP, LiIO₃ , and LiNbO₃ are discussed. The variation of the refractive index with temperature has been measured for KTP and LiIO₃. It is necessary to know both the absorption coefficient β and the variation in the indexes of refraction with temperature change dn/dT to determine the average power limit of a nonlinear interaction. With the dn/dT information, it is also possible to estimate the temperature half width of any nonlinear interaction by calculating the variation of the phase-matching condition with temperature     

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 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     

Thin layer design of X-cut LiNbO3 modulators

Microwave-optical velocity matching and 50 Ω impedance matching are difficult to achieve with LiNbO₃ traveling wave modulators. We perform a detailed study (simulations) of the microwave and optical performance characteristics for modulators using thin layer (few micrometers), X-cut LiNbO₃ and find significant improvements in velocity and impedance matching together with a lower V _πL