Electro-optic Crystal Research Articles

Optical Scheme of Obtaining Highest Transmission Factor in Case of KDP Based Electro-Optic Crystal by the Adjustment of Suitable Biasing Voltage and Number of Feedback Passing

Abstract The electro-optic effect becomes prominent when an electric field is applied across the optic axis of a dielectric material inducing the changes in refractive indices of the medium. Kerr and Pockels cells are massively used as optical switches because of their nonlinear electro-optic character. Potassium di-hydrogen Phosphate (KDP) crystal is a very good electro-optic Pockels material as well as has a good switching behavior in optical and opto-electronic domain. The principle of this electro optic effect can be successfully used to develop optical switch based information processing devices. Transmission co-efficient of the polarized light beam have been modulated in a new way by introducing a multi passing scheme across a electro-optic material. Here transmission co-efficient plays a vital role to examine how much output intensity changes for developing feedback one by one in ascending order. In this paper a novel study for the sizable increase of transmission co-efficient of a KDP crystal for a fixed biasing voltage is reported adopting proper feedback mechanism of light. This paper we also observe an theoretical basis for achieving unit transmission coefficient which minimum amount of voltage require for adopting comparatively many large number of feedback.

Sharp variation of transmission factor of KDP crystal by super-parabolic biasing voltage

Electro-optic KDP crystal has already been established as a potential and high speed electro-optic switch due to its Pockels effect. Several electro-optic devices have been implemented by this material which is used for optical modulation. Here, in this paper the authors report a new scheme for getting a very sharp change in the transmission factor of the crystal with biasing signal. The electrical output obtained from the multiple number of integrator circuits in series is used here as a biasing signal to the crystal.

High-Precision Calibration Algorithm for Large Field-of-View Polarization-Modulated 3D Imaging

To improve the performance of 3D LiDAR imaging and reduce the negative effects of interference fringes on polarization modulation of electro-optical crystals, a calibration algorithm based on polarization modulation principle was proposed in this letter. The reflect light of targets is received by the system, modulated by an electro-optical crystal, then divided into two complementary light beams after passing through polarization beam splitting (PBS), and finally received by two electron-multiplying CCDs (EMCCDs). The depth map and intensity image can be reconstructed by adding the two modulated images. However, the cone interference effect of the electro-optic crystal affects the modulation of the polarized light, resulting in the distance error. By fitting the curve of grayscale ratio of each pixel acquired by two EMCCDs with the changing of voltage, the actual range can be calculated by the light intensity corresponding to the accurate polarization modulation. Ultimately, we found that the algorithm showed outstanding performance on high-precision imaging with an error less than 0.1m in a wide field-of-view of 0.9 mrad.

Unified description on principles of fourier transform infrared spectroscopy and terahertz time-domain spectroscopy

Fourier transform and terahertz time-domain (TD) spectroscopic techniques are widely used in infrared and terahertz frequency bands, and they are usually considered as two different techniques. Here, however, we show that the two techniques are based on the same basic principle. In a Fourier transform infrared spectroscopy (FTIR), the infrared radiation from a continuous incoherent light source passes through a Michelson interferometer, and the interferogram as a function of optical path difference is recorded. The spectral information is decoded from the interferogram by using the Fourier transform process. In a terahertz TD spectrometer, the terahertz transient electrical pulse train from a femtosecond-laser-pumped photoconductive antenna (PCA) is sampled by another PCA (or an electro-optical crystal) gated by a probe femtosecond laser beam from the same femtosecond laser. In this paper, the principle of terahertz TD spectroscopy is described in Fourier domain. We show that the recorded TD terahertz “electrical waveform” can be regarded as an interferogram also. The FTIR and the terahertz TD spectroscopic techniques can be described within the same framework. The asynchronous optical sampling (dual comb) terahertz time-domain technique, a variant form of the traditional terahertz TD spectroscopic technique, can also be described by the same principle. Such a unified description on principles of FTIR and terahertz TD spectroscopies is useful for improving the performance of terahertz emitters and detectors, and developing new terahertz spectroscopic techniques based on femtosecond lasers.

Change nonlinear-optical response of electro-optic crystal in conditions of contributions incoherent background illumination

Influence of incoherent background on diffraction divergence of narrow light beams of He-Ne laser with wavelength of 633 nm due to refractive index changes of Fe-doped lithium niobate sample is experimentally studied. Incoherent background is produced by light-emitting diodes (LED´s) of different average wavelengths and laser diode emitting in blue region of visible. It has been experimentally demonstrated that nonlinear diffraction of coherent red beams of He-Ne laser with diameters on full width of half maximum (FWHM) near to 15–20 μm may be totally compensated by means of assistance of incoherent background with shorter average wavelengths 455–465 nm and even much lower intensity compared with that for coherent beam.

Flat liquid jet as a highly efficient source of terahertz radiation.

Polar liquids are strong absorbers of electromagnetic waves in the terahertz range, therefore, historically such liquids have not been considered as good candidates for terahertz sources. However, flowing liquid medium has explicit advantages, such as a higher damage threshold compared to solid-state sources and more efficient ionization process compared to gases. Here we report systematic study of efficient generation of terahertz radiation in flat liquid jets under sub-picosecond single-color optical excitation. We demonstrate how medium parameters such as molecular density, ionization energy and linear absorption contribute to the terahertz emission from the flat liquid jets. Our simulation and experimental measurements reveal that the terahertz energy has quasi-quadratic dependence on the optical excitation pulse energy. Moreover, the optimal pump pulse duration, which depends on the thickness of the jet is theoretically predicted and experimentally confirmed. The obtained optical-to-terahertz energy conversion efficiency is more than 0.05%. It is comparable to the commonly used optical rectification in most of electro-optical crystals and two-color air filamentation. These results, significantly advancing prior research, can be successfully applied to create a new alternative source of terahertz radiation.

Reduced ionic effect and accelerated electro-optic response in a 2D hexagonal boron nitride planar-alignment agent based liquid crystal device

The presence of excess free-ion impurities in liquid crystals (LCs) gives rise to a number of problems in the electro-optical liquid crystal displays (LCDs), e.g., slow electro-optical responses and image sticking effects. Here we experimentally present that the two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheet can serve as a planar-alignment agent and as an ion-capturing agent at the same time in an electro-optic LC device. The 2D h-BN nanosheet is employed as a planar-alignment agent on one side of an LC cell, where the standard planar-aligning polyimide (PI) layer is used on the other side of the cell. The LC exhibits uniform planar-alignment in this h-BN/PI hybrid device. It is found that the free-ion impurities in the LC are significantly suppressed in this h-BN/PI hybrid cell compared to that in a standard PI/PI LC cell. The free-ion density is reduced in the hybrid cell due to the 2D h-BN nanosheet’s ion-capturing process. The reduction of ionic impurities results in an accelerated electro-optic response of the LC in the h-BN based hybrid cell—which may have potential application for faster electro-optic devices.

Residual amplitude modulation and its mitigation in wedged electro-optic modulator.

We theoretically analyze and experimentally investigate the dependence of residual amplitude modulation (RAM) on the beam radius within the electro-optic crystal (EOC), the wedge angle of the EOC and the overlap efficiency between the extraordinary and ordinary beams, and the overlap efficiency is determined by the distance from the wedge facet to the downstream polarizer. The results show that the RAM with the maximum optical path difference Δ at the edge of light spot presents a sinc-like curve,and the magnitude of Δ is directly proportional to the beam radius and the wedge angle. As a scaling factor, with the decrease of the overlap efficiency between the ordinary and extraordinary beams, the RAM can be further reduced.

Tilted-pulse-front excitation of strong quasistatic precursors.

It was recently predicted [M. I. Bakunov, A. V. Maslov, and M. V. Tsarev, Phys. Rev. A95, 063817 (2017)10.1103/PhysRevA.95.063817] that concurrent processes of optical rectification and multiphoton absorption of an ultrashort laser pulse in an electro-optic crystal can generate a quasistatic electromagnetic precursor propagating ahead of the laser pulse. The electric and magnetic fields in the precursor can exceed the fields in the ordinary terahertz wave generated behind the laser pulse. We propose a way to enhance the precursor's magnitude tremendously, by at least two orders of magnitude, by using tilted-pulse-front excitation technique and higher orders of multiphoton absorption. In particular, we show that a pulse of 500 fs duration and 70 GW/cm2 peak intensity from a Yb-doped laser amplifier can generate in a 5-mm-thick LiNbO3 crystal a 0.5-mm-long precursor with the strengths of the electric and magnetic fields as high as 0.4 MV/cm and 0.13 T, respectively. Strong quasistatic (subterahertz) fields can be a useful tool for particle acceleration, molecular orientation, ultrafast control of magnetic order in matter, and in terahertz streaking techniques.

Suppressing residual amplitude modulation to the 10−7 level in optical phase modulation

Residual amplitude modulation is one of the major stability-degrading factors in many precision measurements. Using an electro-optic (EO) crystal with wedged input and output surfaces is an effective way to suppress residual amplitude modulation. Here the mechanism of residual amplitude modulation in this approach is investigated. The residual amplitude modulations measured in standard and wedged EO crystals bear similarities in their temperature and polarization dependences, implying that a mixture of the two orthogonal polarizations in the extraordinary light is responsible for the residual amplitude modulation in the wedged EO crystal. Similar to a standard EO crystal, a non-uniform spatial distribution of residual amplitude modulation is also observed in the extraordinary light emerging from the wedged EO crystal. The optical isolator after the EO crystal is replaced by a Faraday rotator, and an improvement in the long-term stability is observed. With the wedged-crystal approach, residual amplitude modulation as low as 2×10-7 is observed, contributing a frequency instability of 8×10-18 (500s) in Pound-Drever-Hall frequency stabilization with a discrimination slope of 1×10-4 V/Hz.

Two-dimensional hexagonal boron nitride nanosheet as the planar-alignment agent in a liquid crystal-based electro-optic device

The planar-alignment agent in an electro-optic liquid crystal (LC) device plays an essential role for the LC's electro-optical characteristics. Rubbed polyimide (PI) layers are conventionally used as the planar-alignment agent in traditional liquid crystal displays (LCDs). Here we experimentally demonstrate that the 2D hexagonal boron nitride (h-BN) nanosheet can serve as the planar-alignment agent in an LC cell. This h-BN has higher chemical stability and more optical transparency than the PI layer. Two h-BN-covered indium tin oxide (ITO) glass slides (without any conventional PI layers) are placed together to fabricate an LC cell. A nematic LC inside this h-BN-based cell exhibits uniform planar-alignment-which is probed by a crossed polarized optical microscope. This planar-alignment at the molecular scale is achieved due to the coherent overlay of the benzene rings of the LC molecules on the hexagonal BN lattice. This h-BN-based LC cell shows the typical electro-optical effect when an electric field is applied via ITO electrodes. The dielectric measurement across this h-BN-based electro-optic cell shows a standard Fréedericksz transition of the LC, confirming that the 2D h-BN, as the planar-alignment agent, supplies adequate anchoring energy-which can be overcome by the Fréedericksz threshold voltage. Finally, we show that the h-BN-based LC cell exhibits more optical transparency than a regular PI alignment layer-based LC cell.

Single-axial-mode Nd:YAG laser with precisely controllable laser pulse output time

In recent years, high-energy single-axial-mode Q-switched lasers have been widely studied and applied because of their wide applications such as in nonlinear optics, laser spectroscopy and light detection and ranging (LIDAR). Many applications require a Q-switched pulse that has not only single axial mode but also can be synchronized with an external system. But two most commonly used methods (the build-up time reducing technique and ramp fire technique) are difficult to achieve single-axial mode operation. In this work, we apply the ramp-hold-fire technique to an injection-seeded Nd:YAG laser. The slave oscillator is a self-filtering unstable resonator (SFUR). The SFUR oscillator can achieve a smooth spatial profile TEM<sub>00</sub> transverse mode. An RTP electro-optical crystal is adopted for intracavity phase modulator to modify the effective optical path length of the slave oscillator cavity. The seed-injection locking is realized by the ramp-hold-fire technique. The laser driver generates a pumping pulse. After a suitable time delay the driver is fired, a linear ramp voltage is applied to the RTP crystal. A photodiode detector monitors the interference signal. As soon as the interference peak is detected, the controlling electronics produces a stop signal. The ramp voltage on the RTP crystal is stopped and held at a fixed value. Then the Q-switch is fired at a set time, and finally single axial mode laser is demonstrated. Combining the advantages of intracavity phase modulation and Q-switch exact synchronization of the ramp hold fire technique, we obtain a narrow linewidth single-axial-mode laser pulse with precisely controllable output time. The laser is capable of generating 1064 nm pulse energy large than 50 mJ. The pulse build-up time is reduced by 31 ns to 48 ns. The pulse firing time is precisely controlled with jitter less than 1%. Then the frequency spectrum of the 1064 nm laser is measured with a commercial Fizeau wavemeter HighFinesse WS7. The multi-beam interference patterns of the pulse are shown to be smooth in the wavemeter. The wavelength is measured to be 1064.40416 nm and the linewidth is less than 0.5 pm which is limited by the instrument resolution. Meanwhile, the frequency stability is measured to be less than 0.1 pm (V-V) over 1700 pulses with a working frequency of 0.1 Hz.

Defects and Improvements of Pockels Transducer Voltage Segmentation

The sensing range and sensitivity of optical voltage transducer are badly limited by the inherent half-wave voltage of the electro-optic crystals and a nonlinear polarization demodulation based on Malus’ law. A multi-segmented structure can meet the requirement of the half-wave voltage but is only suitable for 500 kV and above voltage levels with incident wavelength of 632.8 nm, but it is not suitable for all voltage levels if the incident wavelength is greater than 1330 nm. By the way in the multi-segmented structure its optical path and devices are deviated easily by vibrations and thermal expansion and contraction. In this paper, the multi-segmented structure is improved to be suitable for various high-voltage levels with the wavelength of 632.8 nm or greater, and moreover, a new approach is presented to use MgTiO3 ceramics to enwrap around BGO (Bi4Ge3O12) crystal to reduce the integral voltage errors from 0.260% to 0.012%.

Characterizations of a graphene-polyimide hybrid electro-optical liquid crystal device

A planar liquid crystal (LC) cell is designed by placing together a monolayer graphene coated glass slide and a planar-aligning polyimide layer on an indium tin oxide (ITO) coated glass slide. It is shown that the monolayer graphene film on one side of the cell can serve as the planar-alignment agent. At the same time, the monolayer graphene film also functions as the transparent electrode. The successful optical and electro-optical operations of this hybrid LC cell with monolayer graphene on one side and with ITO and associated planar-aligning polyimide on the other side are demonstrated. The measured optical transmission of the graphene electrode is found to be much better than that of the combined ITO-PI layers. The electro-optical effect and the dynamic electro-optic response of the LC in this graphene-polyimide-based hybrid cell reveal the typical director reorientation of the LC on the application of an electric field.

Electro-Optic Sampling of Terahertz Waves Under Brewster’s Angle

We investigate the angular dependence of electro-optic sampling of terahertz radiation. We show that a non-perpendicular incident of copropagating terahertz and optical waves onto the electro-optic crystal only weakly influences the resulting electro-optic sensing efficacity while strongly reduces disturbing multiple reflections within the crystal. We found that close to Brewster’s angle, the detectors response is most favorable. Experimental data support our theoretical discussion.

Coherence of cold atoms trapped by partially coherent light

Abstract The coherence of cold atom trapped and cooled by the partially coherent light (PCL) is investigated experimentally through Rabi oscillations. We control the degree of the spatial coherence of the laser beams by the modulation of electro-optic crystal. The experimental results show that the cold atoms captured by the different coherent degree of light have little different coherence properties and almost the same capacity of resisting disturbance, which indicates that the coherence of cold atom is affected weakly by the coherence of cooling field.

RTP voltage-increased electro-optic Q-switched Ho:YAG laser with double anti-misalignment corner cubes

We describe a pulsed Ho:YAG laser with double anti-misalignment corner cubes that uses an RbTiOPO4 crystal as the electro-optic Q-switch. Two polarizers coated with high reflectivity coatings for the s-polarized light at the laser wavelength were employed to achieve linearly polarized laser output. The maximum s-polarized output power of 2.95 W at 2091.3 nm was obtained under the absorbed pump power of 12.9 W by increasing the voltage applied on RbTiOPO4 electro-optic crystal to 1000 V. The corresponding optical-to-optical conversion efficiency was 22.9% and the slope efficiency was 67.9%. For the Q-switched operation, the pulse energy of Ho:YAG laser was 7.8 mJ with a pulse width of 57 ns at the repetition rate of 100 Hz, resulting in a peak power of 136.8 kW. The beam quality factor M2 of Ho:YAG laser was 1.33.

High-precision determination of the cut angle of an electro-optic crystal by conoscopic interference.

To precisely determine the cut angle of an electro-optic crystal, an improved conoscopic interference measurement method, developed to address the large influence of crystal alignment and melatope positioning errors on measurement results, is presented in this paper. Ray-tracing formulas are derived to calculate the cut angle and evaluate the measurement errors. A Twyman-Green interferometric partial system is employed to make auxiliary adjustments to the orientation of the crystal sample to ensure that the front surface of the crystal is completely perpendicular to the optical axis during the test. In this way, the alignment error can be controlled within 0.050mrad. An interferogram processing method, by which the centers of gravity of each independent conoscopic speckle are connected, is put forward to position the crystal melatope. The average result is 3.734mrad when the cut angle of a 6-mm-thick KDP electro-optic crystal is measured 12 times, with the measurement repeatability being approximately 5 times less than that of a sophisticated x-ray diffractometer. The experiment reveals that by using the method, the melatope positioning error can be reduced to less than 0.60μm. The improved method avoids the shortcomings of traditional conoscopic measurement methods, with the measurement error being reduced from 1.750 to 0.100mrad, and provides technical support for future applications of high-precision and low-cost crystal orientometers.

Quasistatic precursors of ultrashort laser pulses in electro-optic crystals

It was recently predicted [M. I. Bakunov, A. V. Maslov, and M. V. Tsarev, Phys. Rev. A 95, 063817 ( 2017)] that simultaneous optical rectification and multiphoton absorption of an ultrashort laser pulse in an electro-optic crystal can generate a quasistatic electromagnetic precursor propagating ahead of the laser pulse. We explore this phenomenon by finite-difference-time-domain modeling of Maxwell's equations supplemented with the constitutive relations of the optically ionized crystal. In particular, we elucidate the effects of the laser beam width and the pump depletion on the precursor generation. We show that focusing of the laser beam does not destroy the formation of the precursor, and the detrimental effect of the pump depletion can be eliminated by chirping the pump laser pulse.

Two-photon absorption in THz electro-optical sampling crystals

In this paper we present results of experimental study of two-photon absorption (2PA) induced in electro-optical crystals ZnTe and ZnSe by intensive optical radiation in 450– 950 nm wavelength range. These crystals are of interest due to common usage in THz systems. Femtosecond pump-probe spectroscopy with the supercontinuum source helps to induce and detect different types of non-linear processes such as 2PA and to estimate nonlinear part of refractive index.