Pockels Crystal Research Articles

Investigation of internal electric fields in graphene/6H-SiC under illumination by the Pockels effect.

In this paper, we introduce a method for mapping profiles of internal electric fields in birefringent crystals based on the electro-optic Pockels effect and measuring phase differences of low-intensity polarized light. In the case of the studied 6H-SiC crystal with graphene electrodes, the experiment is significantly affected by birefringence at zero bias voltage applied to the crystal and a strong thermo-optical effect. We dealt with these phenomena by adding a Soleil-Babinet compensator and using considerations based on measurements of crystal heating under laser illumination. The method can be generalized and adapted to any Pockels crystal that can withstand sufficiently high voltages. We demonstrate the significant formation of space charge in semi-insulating 6H-SiC under illumination by above-bandgap light.

(Invited) Visualization of Charge Density Distribution in Parallel-Plate Dielectric Barrier Discharge Under Atmospheric-Pressure Air

Dielectric barrier discharge (DBD) is one of the atmospheric nonequilibrium plasma and is typically driven by sinusoidal or pulse voltage with electrodes that dielectric is inserted. DBD has been used in some industries such as plasma medicine and material processing. However, the fundamental process of DBD has not been elucidated yet. When a parallel plate DBD is driven by sinusoidal voltage under atmospheric pressure, numerous filamentary discharges called microdischarges (MDs) are observed. The surface charge deposited on the dielectric after the discharge plays a major role in the behavior of these MDs. As the MD develops during the gap and reaches the dielectric surface, a surface discharge occurs and deposits the charge on the dielectric surface.We performed dielectric surface charge measurements using Bi4Ge3O12, a Pockels crystal, in a short-gap, parallel-plate DBD of 0.5 and 0.3 mm driven by a sinusoidal high voltage. The high-speed camera enabled measurement of the spatio-temporal distribution of surface charge and estimation of the net electric field and the characteristics of microdischarges (MDs).The net electric field strength in the gap was estimated from the measurement results. The axial reduced electric field strength generated by the surface charge was found to be equivalent to 50-60% of the peak value of the applied electric field with all the voltage amplitude and gap length in this study, and the gap electric field after discharge was suppressed to less than 100 Td. Such a strong field keeps the MDs in a nonequilibrium plasma and also prevents re-discharge in the same position at the half cycle. On the other hand, the lateral electric field strength generated by the surface charge is largely characterized by the charge distribution—i.e., the voltage amplitude, gap length, and charge polarity—as mentioned above. Higher amplitude of applied voltage leads to an increase in the area of high lateral electric field strength above 100 Td. Compared to the gap length of 0.5 mm, the lateral electric field weakened, while the axial electric field generated by the surface charge was stronger in the gap length of 0.3 mm. As for the relationship between the charge polarity and the lateral electric field, the lateral field at the anodic dielectric, which the negative charge is deposited on, is dominant in the region of less than 100 Td. Meanwhile, on the cathodic dielectric, which the positive charge is deposited on, the area of above 100 Td is larger. That is, the strong lateral electric field can be observed near the cathodic dielectric, rather than the anodic dielectric. Thus, different chemical reactions may occur in the vicinity of the anodic and cathodic dielectrics, respectively, because of the difference in the drifting charged particles and the lateral field strength.

All-optical method of generation of Phase Shift Keying data using optical pockels crystal

All-optical method of generation of Phase Shift Keying data using optical pockels crystal

Phase Diversity Electro-optic Sampling: A new approach to single-shot terahertz waveform recording

Recording electric field evolution in single-shot with THz bandwidth is needed in science including spectroscopy, plasmas, biology, chemistry, Free-Electron Lasers, accelerators, and material inspection. However, the potential application range depends on the possibility to achieve sub-picosecond resolution over a long time window, which is a largely open problem for single-shot techniques. To solve this problem, we present a new conceptual approach for the so-called spectral decoding technique, where a chirped laser pulse interacts with a THz signal in a Pockels crystal, and is analyzed using a grating optical spectrum analyzer. By borrowing mathematical concepts from photonic time stretch theory and radio-frequency communication, we deduce a novel dual-output electro-optic sampling system, for which the input THz signal can be numerically retrieved—with unprecedented resolution—using the so-called phase diversity technique. We show numerically and experimentally that this approach enables the recording of THz waveforms in single-shot over much longer durations and/or higher bandwidth than previous spectral decoding techniques. We present and test the proposed DEOS (Diversity Electro-Optic Sampling) design for recording 1.5 THz bandwidth THz pulses, over 20 ps duration, in single-shot. Then we demonstrate the potential of DEOS in accelerator physics by recording, in two successive shots, the shape of 200 fs RMS relativistic electron bunches at European X-FEL, over 10 ps recording windows. The designs presented here can be used directly for accelerator diagnostics, characterization of THz sources, and single-shot Time-Domain Spectroscopy.

Electric-Field Induced Shift in the Plasmon Resonance Due to the Interfacial Pockels Effect of Water on a Silver Surface

In the research on application of an optical modulator used for information communication, materials having a large Pockels effect, a refractive index change proportional to the electric-field, are required. It is known that the interfacial water on the surface of transparent oxide electrodes has a giant Pockels coefficient, which is an order of magnitude larger than the solid Pockels crystals practically used. It is important to know the Pockels coefficient of water on not only an oxide surface, but also a metal surface to understand the mechanism of the interfacial Pockels effect of water. However, a method has not yet been established for evaluating the coefficient of water–metal interface. Here we present an evaluation method of the Pockels coefficient of interfacial water on a metal (silver) surface from the spectral shift of the surface plasmon polariton resonance due to electric-field induced refractive index change of the interfacial water. The Pockels coefficient of interfacial water on Ag was evaluated as |r33|=5±1 pm/V, without need for exact knowledge of the thickness of the interfacial layer (electric double layer of water) as long as the penetration depth of the plasmon is larger than the thickness.

The conceptual design of 1-ps time resolution neutron detector for fusion reaction history measurement at OMEGA and the National Ignition Facility.

The nuclear burn history provides critical information about the dynamics of the hot-spot formation and high-density fuel-shell assembly of an Inertial Confinement Fusion (ICF) implosion, as well as information on the impact of alpha heating, and a multitude of implosion failure mechanisms. Having this information is critical for assessing the energy-confinement time τE and performance of an implosion. As the confinement time of an ICF implosion is a few tens of picoseconds, less than 10-ps time resolution is required for an accurate measurement of the nuclear burn history. In this study, we propose a novel 1-ps time-resolution detection scheme based on the Pockels effect. In particular, a conceptual design for the experiment on the National Ignition Facility and OMEGA are elaborated upon herein. A small organic Pockels crystal "DAST" is designed to be positioned ∼5 mm from the ICF implosion, which is scanned by a chirped pulse generated by a femto-second laser transmitted through a polarization-maintained optical fiber. The originally linearly polarized laser is changed to an elliptically polarized laser by the Pockels crystal when exposed to neutrons, and the modulation of the polarization will be analyzed. Our study using 35-MeV electrons showed that the system impulse response is 0.6 ps. The response time is orders of magnitude shorter than current systems. Through measurements of the nuclear burn history with unprecedented time resolution, this system will help for a better understanding of the dynamics of the hot-spot formation, high-density fuel-shell assembly, and the physics of thermonuclear burn wave propagation.

Directly High-Voltage Measuring System Based on Pockels Effect

To measure high voltage up to ± 450 kV in the dc to GHz range, a new optical high-voltage measuring system using a Pockels crystal in a longitudinal modulation arrangement is developed. The maximum measurable voltage for conventional Pockels sensors has been limited by the half-wavelength voltage Vπ. In the new system, a two-wavelength dual laser system is introduced to expand measurable voltage up to the least common multiple of Vπ for each light. The measured results for dc, ac, lightning impulse, and step voltages by the system are presented and compared with the predicted values.

Oscillatory waveform caused by piezoelectric vibration of pockels crystal and its effective suppression

AbstractIn measurements of impulse voltages by Pockels sensors, oscillatory components have been observed in the tail parts of the optical signals. In this paper, oscillatory components of Bi4Ge3O12 (BGO) Pockels crystals of various shapes are investigated. The tested BGO crystals are rectangular solids, a cylindrical solid, and a tapered quadratic solid. The oscillatory components are derived from the light outputs and the frequency analysis is performed. The oscillation frequency of the rectangular crystals is lower than the theoretical value and that of a cylinder is the same as that of circumscribed rectangular solid. In the case of the tapered BGO crystal, it is possible to reduce the oscillatory components by distributing piezoelectric vibration over a broad frequency bandwidth. © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Quasi-reciprocal reflective optical voltage sensor based on Pockels effect with digital closed-loop detection technique

A novel Pockels effect based optical voltage sensor (OVS) consisting of quasi-reciprocal reflective optical circuit is proposed and demonstrated in this paper. The quasi-reciprocal reflective optical circuit is realized so that a digital closed-loop detection technique adopted from fiber gyroscopes can be introduced to obtain the voltage-induced Pockels phase. Due to the digital closed-loop detection scheme, the proposed OVS is insensitive to the fluctuation of the light intensity and the dynamic range is independent of the half-wave voltage of the Pockels crystal compared to the conventional crystal bulk-type with the light intensity based detection scheme. A prototype of the proposed OVS is designed and evaluated. The calculated results of the electric field distribution show that the maximal measured voltage of the sensing element is up to 15 kV. The dc voltage from 0 to 3000 V and 50 Hz ac voltage from 0 to 5000 V are measured with good linearity. The proposed OVS achieves accuracy within ± 1% and ± 0.44% with the measured dc voltage above 800 V and ac voltage above 500 V, respectively. The influences of the alignment error in the sensing element on the measurement accuracy are also theoretically analyzed and experimentally verified.

A Miniature Pockels Cell with Novel Electrode Geometry

This paper discusses important elements of the Pockels sensing cell design. A novel electrode geometry is analyzed in order to obtain maximum sensitivity response from Pockels crystals (Bi12GeO20). This neither transversal nor truly longitudinal geometry, results in electrical field distribution along the sensing beam path that provides high modulation depth. Demonstrated performance level is in agreement with theoretical studies. Delta-sigma polarization detection method allows high linearity of the detector transfer function and measurement independent on laser intensity variations. Channel gain equalization process necessary for accurate delta-sigma normalization is provided by a walk off prism.

Two-dimensional potential and charge distributions of positive surface streamer

Information on the potential and the field profile along a surface discharge is required for quantitatively discussing and clarifying the propagation mechanism. The sensing technique with a Pockels crystal has been developed for directly measuring the potential and electric field distribution on a dielectric material. In this paper, the Pockels sensing system consists of a pulse laser and a CCD camera for measuring the instantaneous two-dimensional potential distribution on a 25.4 mm square area with a 50 µm sampling pitch. The temporal resolution is 3.2 ns which is determined by the pulse width of the laser emission. The transient change in the potential distribution of a positive surface streamer propagating in atmospheric air is measured with this system. The electric field and the charge distributions are also calculated from the measured potential profile. The propagating direction component of the electric field near the tip of the propagating streamer reaches 3 kV mm−1. When the streamer stops, the potential distribution along a streamer forms an almost linear profile with the distance from the electrode, and its gradient is about 0.5 kV mm−1.

Optical Voltage Sensor Using a Pulse-Controlled Electrooptic Quarter Waveplate

An optical voltage sensor is presented which can be used for the measurement of dc voltage by use of a Pockels crystal associated with a pulse-controlled electrooptic quarter waveplate. The measurand dc voltage signal is converted into the amplitude of an ac pulse-modulated voltage-sensing signal. Therefore, the output sensing signal is immune to electromagnetic interference owing to the applications of pulse modulation and lock-in amplifying techniques. In addition, the voltage-sensing signal is obtained from a single light path and free from light intensity fluctuations. The dc voltage of /spl plusmn/0.4 to /spl plusmn/180 V has been measured with 1.8% nonlinear error.

Electrooptic Crystal Multiplier Based on Two Cascaded Pockels Crystals

A novel electrooptic crystal multiplier is proposed which can perform voltage multiplication by using two cascaded Pockels crystals with different polarization azimuths. Different from a conventional analog multiplier, the novel crystal-based multiplier allows us to modulate light wave by voltage product signal, and to obtain the voltage product signal from the measurement of light intensity. Thus, this multiplier possesses electrical insulation property and broad-band frequency response characteristics. The primary experimental result has demonstrated the feasibility of this crystal-based multiplier and its four-quadrant multiplication performance.

Electric field measurement just beneath a surface discharge by optical‐waveguide pockels sensors

AbstractAlthough surface discharges are one of the principal kinds of phenomena associated with faults in electrical apparatus, they have not yet been fully elucidated. Electric field measurements in and around a corona streamer during the creeping advance of a surface discharge are very helpful in analyzing discharges. Pockels crystals have been used in the optical measurement of electric fields, and an optical waveguide technique has been successfully used to miniaturize the sensors. Electric field measurements are made just beneath a surface discharge with an optical waveguide Pockels sensor installed on a PMMA plate and output signals expressing the development of the discharge are obtained. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(2): 28–34, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10185

Pockels surface potential probe and surface charge density measurement

A surface potential probe based on Pockels sensing technique is developed and applied to surface potential distribution measurement on insulating material. The probe consists of a super luminescent diode, a Pockels crystal, a polarized beam splitter, a 1 8 wave plate and optical fibers. As the detecting part of the probe is isolated from a grounded electrode, this probe can be placed closer to the measured object. The resolving power is 2 mm which is equivalent to the twice size of the detecting part. The modulation technique is introduced to this probe and the minimum sensitivity reaches 10 V . This probe is applied to the measurement of the potential distribution on an insulating material just after occurrence of a surface discharge. From this measured potential profile, the distribution of surface charge density is computed through a surface charge method.

Pockels high-voltage measurement system

The practical use of optical sensors to measure direct high voltages can be limited by their high sensitivity. A new optical high voltage measurement technique for applying to high voltage levels up to 400 kV in a wide bandwidth between DC to 30 MHz is developed. A prototype of the optical voltage transformer is assembled using Pockels crystals in a multisegmented arrangement, a super luminescent diode (SLD) light source and a special optical fiber link. A good performance of linearity and accuracy is obtained for DC, AC, lightning impulse and step voltage measurements, In addition, an excellent agreement is observed between the measured and predicted values.

Potential Distribution Measurement of Surface Discharge Developing on Insulating Material

A new method for measuring the transient potential distribution of surface discharge is developed. The measuring system consists of a BGO Pockels crystal, an Ar ion laser, a beam expander, a streak camera and a CCD camera. The potential profile of a surface discharge along a linear path of 50 mm in length is measured with high space and time resolution: 0.1 mm and 2 ns in minimum resolution. Electric field near the tip of a stepwise negative surface discharge is around 24 kV/cm. When the stepwise surface streamer stops its propagation, the potential distribution around the tip of it forms similar profile regardless of the order of occurrence.

Potential distribution measurement of surface discharge by Pockels sensing technique

A new technique for measuring the transient potential distribution of electrical discharge is proposed. The measuring system consists of a Bi4Ge3O12 Pockels crystal, an Ar ion laser, a beam expander, a streak camera and a charge-coupled device camera. The potential profile of the discharge along a linear path of 50 mm in length is measured in the range of 0∼±36 kV. The temporal and spatial resolutions of this system are estimated to reach 2 ns and 0.1 mm, respectively. The potential distribution of a surface discharge creeping on a dielectric material is measured to clarify its developing mechanism. The tip of the propagating surface discharge has a high potential close to an applied potential. The maximum value of electric fields of a negative surface streamer exceeds 16 kV/cm in the direction of the streamer propagation.

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.

Improved device for measuring electrostatic fields by means of Pockels effect and optical phase compensation

Abstract In this paper, a device for measuring electrostatic field by the use of Pockels crystals with optical phase compensation is described. The arrangement of this device differs from those of previous devices [1–6] in that two electro-optic crystal samples are used: one is the probe and the other is the compensation crystal. Using the method of optical phase compensation, the ratio of the electric field in the compensation crystal sample to the measured field has been determined by experiment. In accordance with this ratio, the scale of a voltmeter can be recalibrated and the measuring result can be read directly by introducing a reflection method. The sensitivity of this device is two times higher than that of an earlier device [7].