X-cut Lithium Niobate Crystals Research Articles

Pressure sensor with large sensing dynamic range based on pressure-induced birefringence in LN crystal

In this paper, an optical pressure sensor based on pressure-induced birefringence in Lithium Niobate (LN) crystal is designed to achieve large sensing dynamic range. By using the photo-elastic effect of LN crystal, the optical phase change caused by the external pressure can be converted into the change of the output light intensity. Considerable sensor range can be obtained without special processing of the LN crystal, which makes it have high pressure tolerance. The sensing range of 24.85 MPa is demonstrated to be achievable on x-cut LN crystal with a length of 10 mm in the z-axis direction. The sensing sensitivity can reach 0.281 mW/MPa. By customizing the sensing range, it is expected to use the LN crystal based pressure sensor with the ease of fabrication to monitor high pressure scenarios, such as marine and large buildings monitoring.

Self-Injection Locking of a Distributed Feedback Laser Diode Using a High-Finesse Fabry-Perot Microcavity

Optical microcavities have been widely used in nonlinear optics, quantum optics, and laser technologies. Here we demonstrate the self-injection locking of a distributed feedback diode laser using home-made high-finesse Fabry-Perot microcavity. The Fabry-Perot microcavity is fabricated from an x-cut lithium niobate crystal with highly reflective coatings. Frequency pulling effect can be observed for a successful locking, and results in a single-longitudinal mode lasing with narrow linewidth. The lasing wavelength and output power are found robust to the laser-diode current and temperature variations, in comparison to the free-running case. We further characterize the laser linewidth with beat note measurement with a high-performance external cavity diode laser, with beat-note linewidth of 601.85 kHz. This results shows a new method for laser frequency stabilization in a simple setup, and may find applications in telecommunication and spectroscopy.

Key kinematic parameters in a low-loss power splitter written by femtosecond laser micromachining

In this work we design, fabricate and characterize a 1 × 2 Y-branch power splitter based on simplified coherent coupling. This device was constructed by type II waveguide structures inscribed by a direct femtosecond laser writing technique in x-cut lithium niobate crystal. First of all, a theoretical study that links the kinematic and writing fluence of the process is developed, which allows us to establish the design trade-off and justify the best geometry chosen. Then, the design was optimized and tested by using commercial software, resulting in a compact and low-loss photonic circuit. The efficiency of the proposed device is compared with two others: a curved and a straight splitter. Finally, the experimental results were compared with simulations and then a statistical analysis of multiple comparisons was also conducted, obtaining 3.7 dB ± 0.1 dB insertion losses and 4.5% of the unbalanced coupling ratio.

Tunable diffraction grating with transparent indium-tin oxide electrodes on a lithium niobate X-cut crystal

A tunable diffraction grating based on an electrooptic X-cut lithium niobate crystal has been manufactured and experimentally analyzed. The period of electrodes is 290 μm, the electrode width is 117.5 μm, and the thickness of an electrode is 150 – 160 nm. The electrodes are made of a transparent conducting indium-tin oxide that serves as an antireflection coating with the aim of increasing the optical transmission. In order to prevent crystal polarization switching and electrical breakdown an optimized electrode topology with end ellipticity 1:1 and increased interelectrode gap is used. The optical diagram of the tunable grating with alternating electrode potentials for various gap voltages is analyzed. The intensity of the zero order of diffraction is shown to decrease by 40 % at a voltage of 800 V. At the same time, the origination of new diffraction orders at angles ± λ / (2d) is noted. The measurement of the forward-bias and reverse-bias regions of the modulation characteristic reveals the absence of hysteresis, which confirms the correctness of the electrode topology design.

Bending waveguides made in x-cut lithium niobate crystals for technological applications

In this paper we analyse the performance of several designs of integrated optical deviators made in x-cut lithium niobate crystals by means of femtosecond laser writing using the double line approach. Straight and bent guiding structures have been designed and implemented using this technique. Well-confined propagation modes at communication wavelengths (1.55 μm) were conducted in these structures with acceptable overall losses (less than 2 dB cm−1). Further, a discussion about the optical propagation losses for curved and straight deviators devices is included in this work. At a low aperture angle (less than 0.2°), as expected, low losses were determined for both structures; however, a weak output light was observed for large angles (greater than 0.2°) in the straight optical circuits. In contrast, a smooth variation of the output was measured for the bent structures. The results presented in this paper support the possibility of the technological implementation of integrated optical circuits for optical communications fabricated with ultrashort laser writing in lithium niobate crystals. In addition, some hypotheses of loss mechanisms that are normally not considered are discussed in order to explain the differences between the measured values and predictions obtained by calculating with the usual models.

Polarization behaviour of femtosecond laser written waveguides in lithium niobate

In this work, we analysed the polarization of guided light in femtosecond laser written waveguides. The studied waveguides were performed with different laser pulse energies in an x-cut lithium niobate crystal. The guided intensities were experimentally measured and compared with numerical simulations reaching a qualitatively good accordance. This comparison allowed a verification of the “mechanical expansion theory” which is useful to compute the refractive index field. Also, information related to the modelling of waveguides generated with different laser pulse energies was obtained. Both of these facts are keys to design and manufacture optical circuits by using this technological approach.

A 2D μ-Raman analysis of low repetition rate femto-waveguides in lithium niobate by using a finite element model

We present a μ-Raman two-dimensional analysis of 1kHz femtosecond laser written waveguides in an x-cut lithium niobate crystal. At first, the fitting between a two-dimensional elastic model and experimental micro Raman spectroscopy data (MR) were examined to validate the numerical model. In this, the Raman Potential Deformation Theory (RPDT) was employed to fit the wavenumber shift of the A1(TO)4 phonon. Then, the adequate values for geometrical parameters of the numerical model (horizontal and vertical ellipse radius) were estimated. Finally, using these parameter values, the guided modes were computed for different ellipse expansions and compared with experimental guided modes.

Electronic and Ionic Transport Mechanisms of Stoichiometric Lithium Niobate at High-Temperatures

ABSTRACTThe electrical and electromechanical properties of lithium niobate single crystals are investigated at high-temperatures. The total electrical conductivity is determined as a function of temperature by impedance spectroscopy for Z-cut crystals with different lithium content. For stoichiometric lithium niobate (sLN) the activation energy is found to be (1.49 ± 0.03) eV in the temperature range from 500 to 900 °C.Further, the piezoelectric properties (resonance frequency, Q-factor) of X-cut lithium niobate crystals are determined at high temperatures for samples with compositions ranging from congruent to stoichiometric and, subsequently, compared to the conductivity data in order to identify loss contributions.In this context, the high-temperature stability is examined for X- and Z-cut samples with compositions ranging from congruent to stoichiometric. Series of samples with and without additional alumina protection layers are annealed in air at 900 °C for approximately 50 h. Subsequently, depth profiles are measured by SNMS. In all cases, no lithium loss is observed and, therefore, a high-temperature stability of sLN for at least 50 h at 900 °C can be assumed in ambient air.Further, the influence of protective layers with different thicknesses and compositions is investigated for X- and Z-cut samples. A lithium loss in the first 300 nm is observed for the Z-cut samples, while the X-cut samples show a behavior dependent on the type of protecting layer.

Diffusion of iron in lithium niobate: a secondary ion mass spectrometry study

Iron-doped X-cut lithium niobate crystals were prepared by means of thermal diffusion from thin film varying in a systematic way the process parameters such as temperature and diffusion duration. Secondary Ion Mass Spectrometry was exploited to characterize the iron in-depth profiles. The evolution of the composition of the Fe thin film in the range between 600°C and 800°C was studied, and the diffusion coefficient at different temperatures in the range between 900°C and 1050°C and the activation energy of the diffusion process were estimated.

Heavy ion beam micromachining on LiNbO 3

In this work 3D micromachining of x-cut lithium niobate crystals was performed using the high energy heavy ion microbeam (HIM) at the Tandar Laboratory, Buenos Aires. The samples were machined using 35Cl beams at 70 MeV bombarding energy combined with wet etching with hydrofluoric acid solutions at room temperature. As the ion beam penetrates the sample, it induces lattice damage increasing dramatically the local etching rate of the material. This technique was applied to the fabrication of 3D waveguides with long control electrodes. The resulting structures indicate that well defined contours with nearly vertical sidewalls can be made. The results also show that with fluences of only 5 × 10 12 ions/cm 2, this technique is suitable for the fabrication of different shapes of LiNbO 3 control-waveguides that can be used in different optical devices and matched with the existing optical fibers.

Modification of the etching properties of x-cut Lithium Niobate by ion implantation

In this work x-cut Lithium Niobate crystals were implanted with 0.5MeV O ions (nuclear stopping regime), 5MeV O ions (sub-threshold electronic stopping regime) and 12.5MeV Ti ions (ion track regime) at the fluences required for the formation of a surface fully disordered layer. The damage depth profiles were determined by RBS-channeling. Wet etching was performed at room temperature in 50% HF:H2O solution. The data indicated an exponential dependence of the etching rate on the damage concentration. Independently of the damage regime, once random level in the RBS-channeling spectra was attained we measured the same etching rate (50–100nm/s) and the same volume expansion (∼10%) in all samples. These results indicate that the fully disordered layers obtained by electronic damage accumulation have the same chemical properties of those obtained by conventional nuclear damage accumulation and therefore they can be defined “amorphous”. The impressive etching selectivity of ion implanted regions makes this process suitable for sub-micro machining of Lithium Niobate.

Compositional and structural analysis of iron doped X-cut lithium niobate crystals

We present a systematic study on the correlation between the compositional and structural properties of iron doped X-cut lithium niobate crystals doped by the thermal diffusion of thin iron film deposited via radio frequency (RF) sputtering varying the preparation conditions. The iron concentration profiles measured by using secondary ion mass spectrometry were properly fitted to determine the iron diffusion coefficients and the activation energy of the diffusion process. High resolution X-ray diffraction technique was used to study the crystalline quality of the doped region and to estimate the strain induced by the dopant incorporation. Depending on the preparation condition compressive and tensile strains were found in the diffused region.

LiNbO/sub 3/ 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 (r/sub 33/=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.

Design of broad-band transducer for high-temperature application to nuclear reactors

Ultrasonic transducers are usually damped with metal powder-loaded plastic or lossy rubber materials, which are effective only over a narrow temperature region. A shear-wave transducer was designed using only materials capable of withstanding high temperature (650 °C, 1200 °F). The design employs a laminated stainless steel coupling block, a piezoelectric X-cut lithium niobate crystal, a laminated backing plate, and a clamp to force intimate mechanical contact between the crystal and the coupling block. Transmission of ultrasonic energy in the desired direction parallel to the interfaces of the coupling laminac is not affected by the laminated structure. However, subsequent reverberation in the coupling block is damped by Coulomb friction between interfaces after reflection at the third oblique surface of the coupling block. Radiation into the pressure pad forcing the crystal into contact with the coupling is strongly attenuated by laminating the pressure pad normal to the propagation direction. Uniformity of pressure is assured by applying pressure via a single ball bearing and thick solid blocks. The transducers were used to monitor flow in pipes by measuring sound velocity difference in the upstream and downstream directions. A time interval difference of 110 nsec for pulses transmitted simultaneously in both directions across a 4-in. Schedule 10 stainless steel pipe (108.2 mm ID) was readily measured to four significant figures, using a HP model 5345 Time Interval Averager for water flow at 500 gpm (31.5 l/sec).