Q factor of lithium niobate piezoelectric transducers at high excitation levels

The variations of the electroacoustic parameters (Q factor, electromechanical coupling coefficient, and capacitance) of lithium niobate piezoelectric transducers with increasing high-frequency excitation voltage are studied experimentally. The relative acoustic strain is found to reach a maximum of about 10^?4 in the frequency range from 2 to 3 MHz. The Q factor of the transducers may increase by 100% in the range of acoustic strains studied. This increase is accompanied with acoustic emission. The reason for this effect is the block structure of the lithium niobate crystal.     

Structure of elastic and electric fields induced near the boundary of a LiNbO3 crystal in the course of recording of photorefractive gratings through the photovoltaic mechanism

This paper reports on the results of the investigation into the structure of elastic and electric fields induced near the boundary of the X cut of a lithium niobate crystal during the formation of a photorefractive grating with a wave vector K parallel to the threefold symmetry axis due to the photovoltaic effect. The elastic and electric fields induced by the photorefractive grating and the changes in the components of the dielectric impermeability tensor of the lithium niobate crystal at the frequency of the light wave are calculated using numerical analysis. It is demonstrated that light waves with orthogonal polarizations can effectively interact at a photorefractive grating formed near the electrically short-circuited boundary of a lithium niobate crystal.     

Broad-Band Integrated Optical Electric Field Sensor Using Reflection Mach-Zehnder Waveguide Modulator

A lithium niobate (LiNbO₃) broad-band photonic sensor using reflection-type Mach-Zehnder optical waveguide modulator has been designed, fabricated, and experimentally demonstrated. The bare chip size of the sensor is microminiaturized as small as 20×5×0.5 mm³. The sensor has a wide band frequency response from 10 kHz to 20 GHz with variation less than ± 5 dB. The sensor system shows better linear characteristic from 100 mV/m to 700 V/m, and the sensitivity is 33 mV/m. Besides, the nanosecond EMP with intensity of 30 kV/m has been measured in the time domain.     

High frequency shear horizontal plate acoustic wave devices

It has been shown recently that shear horizontal acoustic waves propagating in piezoelectric plates whose thickness h is much less than the acoustic wavelength λ possess a number of attractive properties for use in sensor and signal processing applications. In order to exploit the potential benefits of these waves, however, one needs to fabricate devices on very thin plates. We have developed a suitable fabrication method which can be used to realize devices on such thin plates. In this method, the device is first fabricated on a plate of normal thickness (approximately 500 μm) and the substrate is then lapped from the back side to reduce the thickness. The technique has been utilized to realize devices on plates of thickness less than 70 μm. A shear horizontal plate acoustic wave (SH-PAW) delay line of fundamental resonant frequency greater than 25 MHz and insertion loss less than 7 dB has been realized on a 60 μm thick Y – cut, X – propagation lithium niobate substrate. The device also shows strong response near the third harmonic frequency of 75 MHz.     

Surface acoustic wave device properties of (B, Al)N films on 128° Y-X LiNbO3 substrate

A c-axis orientated aluminium nitride (AlN) film on a 128° Y-X lithium niobate (LiNbO₃) surface acoustic wave (SAW) device which exhibit a large electromechanical coupling coefficient (k²) and a high SAW velocity property, is needed for future communication applications. In this study, a c-axis orientated (B, Al)N film (with 2.6 at.% boron) was deposited on a 128° Y-X LiNbO₃ substrate by a co-sputtering system to further boost SAW device properties. The XRD and TEM results show that the (B, Al)N films show highly aligned columns with the c-axis perpendicular to the substrate. The hardness and Young's modulus of (B, Al)N film on 128° Y-X LiNbO₃ substrates are at least 17% and 7% larger than AlN films, respectively. From the SAW device measurement, the operation frequency characteristic of (B, Al)N film on 128° Y-X LiNbO₃ is higher than pure AlN on it. The SAW velocity also increases as (B, Al)N film thickness increases (at fixed IDT wavelength). Furthermore, the k² of (B, Al)N on the IDT/128° Y-X LiNbO₃ SAW device shows a higher value than AlN on it.     

脉冲泵浦波长转换改善泵浦带宽容限的研究

基于准相位匹配铌酸锂波导的波长转换,分析了采用脉冲泵浦的方法提高泵浦带宽容限的机理.研究表明:窄脉冲的频带内包含有大量的频谱分量,这些谱分量可以通过倍频效应和和频效应两种机制产生0.8 μm波段的光场,使泵浦带宽容限得到了改善.理论计算和实验都证明了在40G Hz 的ps级脉冲泵浦的波长转换时,－3 dB泵浦带宽容限提高到大约3 nm.     

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A ₂ type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.     

Generation of very flat optical frequency combs from continuous-wave lasers using cascaded intensity and phase modulators driven by tailored radio frequency waveforms

We demonstrate a scheme based on a cascade of lithium niobate intensity and phase modulators driven by specially tailored RF waveforms to generate an optical frequency comb with very high spectral flatness. In this Letter, we demonstrate a 10 GHz comb with 38 comb lines within a spectral power variation below 1 dB. The number of comb lines that can be generated is limited by the power handling capability of the phase modulator, and this can be scaled without compromising the spectral flatness. Furthermore, the spectral phase of the generated combs in our scheme is almost purely quadratic, which, as we will demonstrate, allows for high-quality pulse compression using only single-mode fiber.     

Experimental and simulated performance of lithium niobate 1–3 piezocomposites for 2 MHz non-destructive testing applications

Lithium niobate piezocomposites have been investigated as the active element in high temperature resistant ultrasonic transducers for non-destructive testing applications up to 400 °C. Compared to a single piece of lithium niobate crystal they demonstrate shorter pulse length by 3×, elimination of lateral modes, and resistance to cracking. In a 1–3 connectivity piezocomposite for high temperature use (200–400 °C), lithium niobate pillars are embedded in a matrix of flexible high temperature sealant or high temperature cement.In order to better understand the design principles and constraints for use of lithium niobate in piezocomposites experiments and modelling have been carried out. For this work the lithium niobate piezocomposites were investigated at room temperature so epoxy filler was used. 1–3 connectivity piezocomposite samples were prepared with z-cut lithium niobate, pillar width 0.3–0.6 mm, sample thickness 1–4 mm, pillar aspect ratio (pillar height/width) 3–6, volume fraction 30 and 45%. Operating frequency was 1–2 MHz.Experimental measurements of impedance magnitude and resonance frequency were compared with 3-D finite element modelling using PZFlex. Resonance frequencies were predicted within 0.05 MHz and impedance magnitude within 2–5% for samples with pillar aspect ratio ⩾3 for 45% volume fraction and pillar aspect ratio ⩾6 for 30% volume fraction. Laser vibrometry of pulse excitation of piezocomposite samples in air showed that the lithium niobate pillars and the epoxy filler moved in phase. Experiment and simulation showed that the thickness mode coupling coefficient k_t of the piezocomposite was maintained at the lithium niobate bulk value of approximately 0.2 down to a volume fraction of 30%, consistent with calculations using the (Smith and Auld, 1991 [1]) model for piezocomposites.     

Interaction of light with acoustic waves excited by an inphase multielement transducer in the 1.0–2.5 GHz range

Interaction between a weakly divergent optical beam and an acoustic wave generated in the range 1.0–2.5 GHz by an inphase multielement electroacoustic piezoelectric transducer is analyzed. A piezoelectric (Y + 36°)-cut LiNbO₃ plate is fixed on the surface of an X-cut LiNbO₃ acoustic duct with the help of metallic sublayers (Cr, Cu, In, Cu, or Cr). The inphase structure of the transducer is formed by the upper electrodes inter-connected by short conductors. The signal is applied through a coaxial Chebyshev transformer. The efficiencies of electroacoustic conversion and acoustooptic interaction are calculated as functions of frequency. The experimental setup, method, and results are described.     

Microwave absorption properties of rod-shaped Co-Ni-P shells prepared by metallizing Bacillus

The rod-shaped Co-Ni-P shells were prepared by metalling Bacillus. The microstructures and composition of the shells were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive analysis (EDS). The electromagnetic parameters were measured by the coaxial line method in the frequency of 2-18 GHz. It was found that the Bacillus were successfully coated with Co-Ni-P, and the inner structure of the shells are hollow in structure. The shells exhibit excellent microwave absorption properties in 5-17 GHz frequency. The microwave reflection loss is above −10 dB in 5.38-16.6 GHz frequency. The maximum microwave reflection loss reaches −35.83 dB at 9.12 GHz for samples thickness 2.4 mm, and the widest bandwidth for microwave reflection loss above −10 dB is about ∼5.32 GHz for samples thickness 2.0 mm. These results confirm the feasibility of applying Bacillus as biotemplates for fabrication of the metallic shells as lightweight microwave absorption materials are very promising for applications.     

Microwave dielectric properties of Ba(Zn<Subscript>1/3</Subscript>Ta<Subscript>2 / 3</Subscript>)O<Subscript>3</Subscript> for application in high power waveguide window

Higher dielectric constant, low dielectric loss and good transmission characteristicshave been the goal for developing the ceramic waveguide window for high power windowapplications. The choice of materials having high k with low dielectric lossand reduced window size is key parameters to achieve maximum microwave transmissionwithout unleashing microwave dissipation. The microwave dielectric properties ofsynthesized Ba(Zn_1/3Ta_{2 /3})O₃ (BZT) ceramics have been studied for high power windowapplications. The structural studies are correlated with microwave dielectric propertiesof BZT. The maximum values of dielectric constant ?_r =30, Q ×f₀ = 102 THz and near zero temperaturecoefficient of resonance frequency were obtained for BZT ceramics sintered at thetemperature of 1550 °Cfor 4 h. The measured results are used to design a tapered transition from air filledwaveguide to narrow (reduced width and height) dielectric filled waveguide using Heckenslinear taper at a specific frequency. The simulation result shows that the lowerreflection loss is obtained for the tapered transition of the narrow BZT window ascompared to the standard waveguide BZT window. The return loss of –34 dB is obtained forS-bandwaveguide window with a bandwidth of 675 MHz. The return loss observed in the narrow BZTwindow is –46 dB with a bandwidth of 570 MHz at a center frequency of 3.63 GHz. Most ofthe disadvantages in conventional windows will be rectified using the design of the tapertransion employing narrow waveguide window in high power applications.     

Improved 4H–SiC MESFET with double source field plate structures

An improved 4H–SiC power MESFET with double source field plates (DSFP) for high-power applications is proposed (DSFP-MESFET). The DSFP structure significantly modifies the electric field in the drift layer. The influence of the DSFP structure on saturation current, breakdown voltage (V_b), and small-signal characteristics of the DSFP-MESFET were studied by numerical device simulation. The V_b of 359 V is obtained for the DSFP-MESFET compared to 301 V of the conventional source field plate MESFET (LSFP-MESFET). Hence, the maximum output power density of 24.7 and 21.8 W/mm are achieved for the DSFP-MESFET and LSFP-MESFET, respectively, which means 13% improvement for the proposed device. Also, the cut-off frequency (f_T) of 24.5 and the maximum oscillation frequency (f_max) of 89.1 GHz for the 4H–SiC DSFP-MESFET are obtained compared to 23.1 and 85.3 GHz for that of the LSFP-MESFET structure, respectively. The DSFP-MESFET shows a superior maximum stable gain (MSG) exceeding 23.3 dB at 3.1 GHz, which is presenting the potential of the proposed device for high-power operations.     

Simulation of a flat optical frequency comb using a single-drive multi-RF Mach–Zehnder modulator in a cascaded intensity modulator chain

We proposed a scheme based on two cascaded lithium niobate intensity modulators to generate an optical frequency comb with very high flatness. Single-drive multi-RF waveforms are used for driving the first intensity modulator, and 9 lines within 1 dB power variation can be obtained. When cascading with another intensity modulator, by specially adjusting the DC bias and the drive amplitudes of the RF signals of the two intensity modulators, 27 or 45 comb lines with a spectral power variation about 1 dB are obtained. The scheme is relatively simple and adjustable, and the frequency interval of the OFC varies with microwave frequency applied on modulators.     

Quasi-Elastic Light Scattering in Congruent Lithium Niobate Crystals

Raman scattering spectra in the frequency range of 5–75 cm^–1 in lithium niobate samples with various structural imperfections were obtained. It is shown that the intensity of quasi-elastic light scattering on the wing of the Rayleigh line demonstrates an exponential dependence on the concentration of large-scale defects in the samples. Calibration curves for the intensity of quasi-elastic scattering dependence at low frequencies on the acoustic quality factor are constructed.     

Structuring of material parameters in lithium niobate crystals with low-mass, high-energy ion radiation

Ferroelectric lithium niobate crystals offer a great potential for applications in modern optics. To provide powerful optical components, tailoring of key material parameters, especially of the refractive index n and the ferroelectric domain landscape, is required. Irradiation of lithium niobate crystals with accelerated ions causes strong structured modifications in the material. The effects induced by low-mass, high-energy ions (such as ³He with 41?MeV, which are not implanted, but transmit through the entire crystal volume) are reviewed. Irradiation yields large changes of the refractive index ??n, improved domain engineering capability within the material along the ion track, and waveguiding structures. The periodic modification of ??n as well as the formation of periodically poled lithium niobate (PPLN) (supported by radiation damage) is described. Two-step knock-on displacement processes, ³He??Nb and ³He??O causing thermal spikes, are identified as origin for the material modifications.     

分布损耗加载回旋行波管多模稳态注波互作用理论与比较证实

基于目前国际上实验研究的均匀介质加载和周期介质加载结构,建立了一种分布式损耗加载回旋行波管(gyro-TWT)多模稳态注波互作用理论.利用这一理论,以TE01模式基波gyro-TWT注波互作用为例,将Ka和W波段的理论结果与实验和软件仿真进行比较,以证实理论的合理性.     

Investigation of acoustic waves of higher order propagating in plates of lithium niobate

This paper presents theoretical investigation of higher order acoustic plate waves propagating in single crystals of lithium niobate. The dependencies of wave velocity and electromechanical coupling coefficient of antisymmetric, symmetric, and shear horizontal modes on the parameter hf (h=plate thickness, f=operating frequency) are calculated as a function of propagation direction on X-, Y-, and Z-cut lithium niobate plates. It is found that several modes can provide values of K2 that are much greater than can be obtained with surface acoustic waves (SAWs). For example, K2 as high as 0.26 and 0.38 can be obtained from SH1 and A2 modes, respectively. This compares with a maximum value of K2=0.055 for SAWs. It is further shown that there are several crystal cut and propagation directions that can allow efficient excitation and detection of a single mode with minimal interference due to other modes.     

Ablation induced by single-and multiple-femtosecond laser pulses in lithium niobate

We present results on the surface-damage threshold of lithium niobate after single-and multiple-femtosecond laser pulse irradiation at the pulse duration of 80 fs, with a 800-nm wavelength, and a repetition rate of 1 kHz. The surface-ablation threshold was found to decrease significantly with an increase in the pulse number applied to the surface until reaching an almost constant level due to an incubation effect, which is attributed to the laser-induced defect formation. The threshold of lithium niobate under a single shot is found to be 2.82 J/cm², and the threshold fluence for an infinite number of pulses F _th(∞) converges to a common value of 0.52 ± 0.06 J/cm² for N > 80. The results have the potential for application in laser micromachining and the fabrication of related optical devices and applications in frequency conversion by a femtosecond laser in lithium niobate.     

Surface acoustic wave studies of subsurface damage

We have measured the attenuation of surface acoustic waves on polished substrates and correlated the frequency-dependent attenuation with sub-surface damage. The damage was produced in single crystal lithium niobate samples by lapping and polishing with commercially available diamond compound. Surface acoustic wave attenuation on these samples was measured from 100 MHz to 1.5 GHz using interdigital transducers, thus covering depths ranging from 35 μm to 2 μm below the lithium niobate surface