Y-cut LiNbO3 Crystal Research Articles

A tailored 100%-efficient 532/1064-nm demultiplexer in y-cut LiNbO3 crystal.

In this Letter, we report a tailored 532/1064-nm demultiplexer based on a multimode interference (MMI) coupler with an efficiency of 100%. The device structure is designed according to the self-imaging principle, and the propagation and the wavelength division performance are simulated by the beam propagation method. The demultiplexer is fabricated in a y-cut LiNbO3 crystal by femtosecond laser direct writing (FLDW) combined with the ion implantation technique. The end-face coupling system is used to measure the near field intensity distribution, and the spectra collected from the output ports are obtained by spectrometers. The simulated and the experimental results indicate that the customized demultiplexer in the LiNbO3 crystal presents excellent wavelength division performance operating at 532 nm and 1064 nm. This work demonstrates the application potential of FLDW technology for developing miniaturized photonic components and provides a new strategy for fabricating high-efficiency integrated wavelength division devices on an optical monocrystalline platform.

Formation mechanism of femtosecond laser induced guiding structures in y-cut LiNbO3 crystal

In this work, we have used femtosecond laser direct writing to inscribe guiding structures in y-cut LiNbO3 crystal and explored the mechanism of laser modification. Experimental results reveal the fabrication-parameter windows for the modifications with refractive index increase and decrease. We investigate the guiding properties under different polarizations at 632.8 nm. The spectral shifts of Raman modes have also been analyzed to clarify the laser-induced modifications on the crystal lattice. This study demonstrates that the lattice expansion and compression, and the damage induced by the laser energy are the key factors affecting the structural vibration and refractive index modification types.

Measurement method of electro-optic coefficients using photoelastic modulation.

To realize high-speed, accurate, and sensitive measurement of electro-optic (EO) coefficients, a novel measurement method using photoelastic modulation is proposed. A simple polarimetry is constructed based on the photoelastic modulator, and the phase difference induced by the EO sample is loaded in the modulation signals. Employing the digital phase-locked technology, the DC, first, and second harmonic terms are extracted, respectively, and then the EO coefficients are demodulated. The experimental system is set up to analyze the principle of this scheme in detail. A Y-cut LiNbO3 crystal plate is selected as the sample, and the EO coefficient γ22 is studied under different applied DC voltages. The results demonstrate that the measurement accuracy and sensitivity of the system are 0.22% and 1.5×10-14 m/V, respectively, when the sampling time is set as 198.9ms. The measuring rate can be further enhanced, if the appropriate reduction of the measurement sensitivity is allowed.

Characteristics of microdomains and microdomain patterns recorded by electron beam irradiation on Y-cut LiNbO3 crystals

We present the results of investigations of planar domain patterns (isolated domains and domain gratings) fabricated by irradiation of the nonpolar Y-surface of LiNbO3 crystals by an electron beam (EB) incident normally onto the surface. The EB recorded domains were investigated using atomic force microscopy, confocal second harmonic generation microscopy, and chemical etching as an auxiliary method. The dependence of the domain characteristics on irradiation conditions (acceleration voltage U, EB current I, and irradiation time tirr) were determined. The length Ld of both isolated domains and domain gratings along the polar axis Z grows linearly with tirr (at U, I = const) with no tending to saturation. The plots Ld(tirr) obtained for U = 10 and 15 kV are practically identical, whereas the values of Ld for U = 5 kV are essentially lower. The domain thickness Td along the Y-direction, i.e., the depth of the switched layer grows with acceleration voltage U. These results are discussed in terms of space-charge fields formation arising under EB irradiation of insulators. The linearity of Ld(tirr) is accounted for by the frontal domain growth via the viscous friction law. The experimental dependence of Td on U supports the suggestion that the domain thickness is determined by the penetration depth Re of primary electrons, which in turn is governed by U. The difference in Ld(tirr) plots for different U is accounted for by different electron emission σ. Indirect evidences of a defect structure modification in a thin surface layer with respect to the crystal bulk are obtained.

Anisotropy of the magnetoelectric effect in tri-layered composites based on single-crystalline piezoelectrics

A numerical and experimental study of the anisotropy of the magnetoelectric (ME) response in tri-layered composites based on single-crystalline piezoelectrics has been carried out. Quasi-static and resonant direct ME measurements were performed on Metglas/Piezoelectric/Metglas tri-layered structures featuring thin foils of the magnetostrictive Metglas alloy and differently oriented piezocrystals of LiNbO3 (LNO) and GaPO4 (GPO). The transversal ME effects were found to be strongly dependent on the direction of the applied in-plane magnetic bias fields as well as the orientation of the crystals. Multiple peaks of the ME response were observed corresponding to different contour resonance modes of the piezocrystals. The largest ME coefficient of up to 249 V/(cm·Oe) was observed for a composite with a 41 °Y-cut LNO crystal at 323.1 kHz. In summary, we have shown that crystalline lead-free LNO and GPO can exhibit relatively large anisotropic ME effects in multilayers. The relations between the directional ME coupling and the material constants of the differently oriented piezocrystals were pinpointed. The precise control of this orientation should therefore allow one to engineer almost any desired quasi-static and resonant ME anisotropic properties for any specific application.

T-junction droplet generator realised in lithium niobate crystals by laser ablation

AbstractA femtosecond laser at 800 nm was used to create micro-fluidic circuits on lithium niobate (LiNbO3) substrates by means of laser ablation, using different scanning velocities (100-500 μm/s) and laser pulse energies (1-20 μJ). The T-junction geometry was exploited to create on y-cut LiNbO3 crystals a droplet generator, whose microfluidic performance was characterized in a wide range of droplet generation frequencies, from few Hz to about 1 kHz.

Investigation of the Piezoelectric Effect in LiNbO3 and LiTaO3 Crystals by X-Ray Diffraction

The behavior of LiNbO3 and LiTaO3 crystals with domain structures in an external electric field was studied using high-resolution X-ray diffractometry and topography. Domain structures in Z-cut LiTaO3 and Y-cut LiNbO3 crystals have been formed by the aftergrowth thermoelectricall treatment technique were investigated. Strong elastic distortions near different domain boundaries in the Y-cut LiNbO3 crystal were detected. The possibility of using X-ray diffraction to measure piezoelectric charge constants tensor components is demonstrated.

Fast and Simultaneous Measurement of Both Birefringence and Azimuth Angle Using a y-Cut LiNbO3 Phase Modulator

For fast and simultaneous measurement of both birefringence and azimuth angle of samples, a technique using transverse phase modulator is proposed. The phase modulator includes y-cut z-propagation LiNbO3 crystal. Several experiments using rotating quarter-wave plates and LiNbO3 crystal in presence of direct voltage have been performed, and the measurement results agree well with predicted theoretical data.

Three-dimensional electric-field mapping system using crystal principal axes electro-optic rotation

This article presents three-dimensional electric-field mapping measurement system using crystal principal axes electro-optic rotation. An experimental system using a probe tip made of y-cut LiNbO3 crystal is constructed. The mapping measurement results for three directional electric fields on a test device are reported.

New Optical-Waveguide Pockels Sensor for Measuring Electric Fields

An optical electric field sensor using a Pockels crystal which has the advantage of directly measuring the electric field in a space with limited disturbance of its distribution has been studied. We present an optical waveguide technique which has been developed to miniaturize the sensor. In a prototype sensor a light beam is propagated along a z-direction waveguide in a y-cut LiNbO3 crystal. The width of the waveguide is 7 µ m and its length is 4.39 mm. The sensor is not based on an interferometer but on a single waveguide. The sensor has a simple and optimum configuration, and its output has good linearity to applied electric fields. Its frequency and temperature characteristics are presented. A new and smaller sensor with a waveguide 1 mm in length has been developed. It has almost the same configuration as the prototype sensor except for a smaller size. Results of measurement of ac and step electric fields using this sensor are presented.

Generation of terahertz radiation from a poled polymer

We report the generation of terahertz radiation via optical rectification from a poled polymer using femtosecond optical pulses. We have measured the refractive index and power absorption in the terahertz frequency range. The corresponding difference frequency mixing coherence length in the poled polymer (1 mm at 1 THz) is ∼20 times greater than that of LiNbO3 (50 μm at 1 THz). The observed far-infrared electric field radiated from a 16 μm thick polymer sample is only 4 times smaller than that from a 1 mm thick y-cut LiNbO3 crystal. We discuss conversion efficiencies for thicker polymer samples.

Waveguide electrooptic modulator with a limiting modulation frequency higher than 10 GHz

A study was made of the main characteristics of a waveguide electrooptic modulator with an interrupted cumulative interaction proposed earlier by the present authors. A Y-cut LiNbO3 crystal was used in the form of a structure in which a bent optical waveguide passed out of the interelectrode zone and returned after a certain distance. Calculations were made of the frequency dependence of phase modulation taking into account attenuation of a microwave modulating wave in the modulator circuit. In the low-frequency range the measured half-wave modulator voltage was 4.5 V at the wavelength of 0.63 μm. The results of calculations and measurements in the low-frequency range showed that this structure can provide phase modulation slightly greater than π/2 at a control voltage of 4.5 V and modulation frequencies of 13–15 GHz.

Distribution of the intensity of surface optical waves in the focal region of a geodesic lens made of LiNbO3:Ti

An experimental investigation was made of a redistribution of the intensity in the transverse cross section of a surface optical wave focused by a spherical geodesic lens in a Ti-diffused waveguide formed in a Y-cut LiNbO3 crystal. A wave theory of geodesic waveguide lenses was used to calculate the optical field in the focal and several other transverse cross sections for a spherical lens with parameters found experimentally. The results of these calculations were in good agreement with the experimental data.

Optimal crystal cuts and propagation directions for excitation of acoustic waves in LiNbO3

Acoustic power flow in a LiNbO3 single crystal was calculated to find the optimal crystal cutting orientation and the wave propagation direction for the efficient excitation of acoustic waves in the crystal. By solving the stiffened Christoffel equations and calculating the acoustic Poynting vectors with piezoelectric effect included, maximum longitudinal acoustic power flow in a 30° Z-rotated, Y-cut LiNbO3 crystal was found.

Aberration distortions of a spherical geodesic waveguide lens in LiNbO3:Ti

Aberration distortions were determined at the focus of a spherical geodesic lens made from a waveguide formed by the diffusion of Ti in a Y-cut LiNbO3 crystal. The experimental results were compared with calculations carried out in the geometric-optics and diffraction approximations. For a lens with a focal length f = 8.6 mm and a relative aperture of the incident beam amounting to 0.67, the size of the spot at the focus was ~ 4 μ when the relative amplitude of the side lobes was 9dB.

Fabrication of corrugated gratings with a high diffraction efficiency on the surface of LiNbO3

A technology was developed and optimal conditions were found for the fabrication of corrugated gratings with a high diffraction efficiency (up to 4%). These gratings had a period of 0.3 μ and they were formed on the surfaces of Y-cut LiNbO3 crystals.

ON THE BRAGG-DIFFRACTION OF SURFACE ACOUSTIC WAVES BY ACOUSTIC GRATING

The surface acoustic wave diffraction phenomena on acoustic gratings in Bragg regime have been explored.The modal approach has been used to treat this problem under strong Bragg condition. Some results, including the eigen-modes in the grating, the equation of the deflected angles, and the analytical expression of deflection efficiency which depends on the operation frequency and the grating parameters, have been obtained.Experimentally, the image of the diffraction acoustic fields on acoustic grating which was composed of some aluminium film stripes on y-cut LiNbO3 crystals have been visualized by means of Schlieren method. Some excellent photographs of diffraction fields have been obtained, and the deflection angles and deflection efficiency at different frequency have been measured by using optical method. The comparison between the theoretical and the experimental results shows that the agreement is satisfactory.

Investigation of an interferometric modulator based on channel waveguides in LiNbO3

A calculation is reported of the characteristics of an interferometric modulator based on single-mode channel waveguides in a Y-cut LiNbO3 crystal, carried out for different waveguide parameters and different electrode systems. An analysis is made of the influence of displacement of the electrode system relative to the waveguide system on the control voltage of the modulator. A description is given of an interferometric modulator with a control voltage 1.6 V and a modulation depth 90%. Measurements of the frequency characteristic of this modulator are reported.

Electric field intensity distributions inside a LiNbO3 crystal with different interdigital electrode configurations

The variations of the electric field intensity inside Z-cut and Y-cut LiNbO3 crystals with interdigital electrode structures are computed on the AC and DC modes. Experimental observations show that with an internal reflection modulator constructed from a Z-cut crystal, the voltage required for a given modulation is less when operated in the DC mode than when operated in the AC mode. The nature of the field intensity distribution, however, suggests that when the parallel transmission arrangement is used the reverse condition is achieved when the distance between the optical beam and the electrode plane is smaller than a quarter of the electrode pitch.

Light scattering on internal reflection from surface acoustic waves

The intensity of light scattered by surface acoustic waves (SAW) on internal reflection has been calculated for the geometry where the incident and scattered light are in a plane perpendicular to the SAW wave vector. The calculation was carried out for an arbitrary piezoelectric crystal having a free or metallized surface. Measurements on a y-cut LiNbO3 crystal and a z-propagating SAW give good agreement with the theory.