ISSN 1060-992X, Optical Memory and Neural Networks, 2017, Vol. 26, No. 2, pp. 145–149. © Allerton Press, Inc., 2017.
An Electric Field Sensor Based on Reflected Light Intensity Modulation from Electro-Optical Media A. V. Kniazkov* and S. N. Davydov Peter the Great St. Petersburg Polytechnic University, Institute of Physics, Nanotechnology and Telecommunications, St. Petersburg, Russia *e-mail:
[email protected] Received December 21, 2016; in final form, April 8, 2017
Abstract⎯An optical sensor of an alternating electric field is described. The sensing element is a crystal quartz plate which reflects incident light by its front surface. Reflected light intensity is modulated by the tested electric field by means of changing the refractive index of the plate. Modulation of the reflection coefficient of the quartz by the tested electric field occurs due to the electro-optic effect. It is noted that the main advantage of the sensor working on the modulation of the reflected light is the ability to use opaque electro-optical media and thin films. Keywords: electro-optic sensor, modulation of light reflection, nontransparent EO media DOI: 10.3103/S1060992X17020096
1. INTRODUCTION The work of well-known electro-optical (EO) sensors of the electric field is based, as a rule, on the volume EO effect of phase changes of the wave which passes the EO material. This change in the wave phase takes place because the refractive index alterations caused by the measured electric field are different in cases of the ordinary and extra-ordinary waves [1, 2]. To get a signal from the sensor, various interferometer schemes are used which convert the phase modulation of the transmitted wave into the amplitude modulation. The basic scheme of such conversion is the polarization-optical scheme [1–4]. Volume EO effect is also used in the waveguide sensors that transform EO phase modulation into amplitude one by means of the Mach-Zehnder interferometer [5–10]. In traditional EO sensors, transmitted waves are used. That is why transparent EO materials are required there. New synthesized EO materials can possess, on the one hand, enhanced electric field sensitivity, and thus they can be rather prospective in terms of measuring of electric field Е. But on the other hand, they are not always transparent. This fact makes it possible and prospective to elaborate field sensors based on the reflection of light but not on the transmission. There are some techniques of measuring EO coefficients of thin films by reflection [11–13]. These techniques imply using light reflection from the film rear surface, and thus the double passage of light through the bulk material is included there. Authors [14] have proposed an integrated EO waveguide sensor in which a light wave propagates through the waveguide and is reflected from the end of the waveguide. But there, again, the light double passing through the working substance is included into the measuring process. A review of EO sensors with fiber coupling, in which multiple transmission of light through the volume of EO material is used, is given in [15]. There, multiple reflections from the front and back surfaces of the EO material take place. Recently, a new electric field sensor has been proposed and developed in which total internal reflection in EO crystals is used [16]. In contrast to the sensors described, the work of the proposed sensor is based on the usual reflection of the light wave from the front surface of EO material. Direct conversion of EO phase modulation of the refractive index n into the amplitude modulation of the light intensity is used here. 2. THEORY AND EXPERIMENTS At normal incidence, light reflectance R of dielectrics is defined by the Fresnel expression:
( )
R = n −1 . n +1 145
2
(1)
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8
5
1 ext. face a
ε
2
6
3
4
Z
Fig. 1. Optical scheme of the experimental setup electric field sensor: (1) quartz plate; (2) light guide; (3) light reflecting prism; (4) semiconductor laser; (5) photo detector; (6) dual-channel oscilloscope; (7) cone-like electrode; (8) high voltage AC generator.
Because of the EO effect and the action of the external field, the refractive index modulation δn (induced birefringence) can take place. This, in turn, causes a change in the reflection coefficient δR which, in case of EO crystals with linear EO effect, can be written as:
δR =
δI R ∂R 2(n − 1)n3 = δn = ref E , I0 ∂n (n + 1)3
(2)
where δIR is reflected wave intensity change caused by the induced refractive index change, I0 is the incident wave intensity, and ref is effective EO coefficient, E is the value of a electric field. It can be seen from the expression given here that in case of materials with induced birefringence there exists sheer linkage between the change in the reflection coefficient, effective EO coefficient and field strength. The possibility was recently shown to measure EO coefficients of the induced birefringence of the EO materials by measuring the modulation of light reflection coefficient [16, 17]. In the present work, vice versa, a sensor is described in which unknown value of electric field E can be found by measuring δR, the value of ref to be known. To realize such a sensor using reflected light, a crystal quartz plate was chosen as the main component. For quartz, the known effective EO coefficient ref = 0.6 pm/V. In Fig. 1, the experimental setup is schematically shown which was made to test the sensor. The sensor itself consists of a quartz plate 1 which is joint to a light guide 2 which in turn is joint to a light-reflecting prism 3. A semiconductor laser 4 of a wavelength λ = 650 nm and power P = 10 mWt is the source of polarized light. The alternating electric field under test is formed via applying some definite potential V between the cone-like electrode 7 and a plane grounded electrode which was placed on the far right side and is not shown in the picture. The potential is produced by a high voltage AC source 8. Thus, the reflection coefficient of the quartz plate is modulated by the tested electric field. The light wave, having been transmitted by the light-reflecting prism 3 and having experienced volumetric EO phase modulation effect in quartz plate 1, is reflected from the front face of the quartz plate. Then it propagates in the same direction as the wave of registration does. Apart from this “useful” wave, there exists a transmitted wave coming up to the back (outer) boundary of the quartz plate. It as well experiences volumetric EO phase modulation effect, is reflected by the back quartz boundary and can propagate towards semiconductor photo-detector 5. To reduce the influence of this parasitic signal, the back surface of the quartz plate is made rough. As the result, the parasitic wave becomes scattered. It is shown in Fig. 1 as thin dashed arrowed lines. The photo-detector signal is proOPTICAL MEMORY AND NEURAL NETWORKS
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δR × 106 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
2
4
6
8
10
12
14
16 E, kV/cm
Fig. 2. Dependence of the deviation of the crystal quartz plate reflectivity on electric field intensity: z = 1 mm, a = 1.5 mm. Solid line is the fitting line.
cessed by a resonance amplifier which works at the frequency of the AC field. Both the amplified sensor signal and electrode high voltage are recorded by a dual-channel oscilloscope 6. The electric field E generated in the area of testing depends on the radius of the electrode a and the distance z, where the sensor is (see Fig. 1). It can be estimated by the formula [18]:
E ≈
2V
( )
ε(a + 2z)ln 2b a
,
(3)
where ε is dielectric permeability of EO medium, z is the distance between the electrode and the smooth quartz plane, a is the radius of the electrode top part, and b is the distance between this top and the grounded electrode. 3. RESULTS AND DISCUSSION Experimental study of the sensor was carried out via two series of measurements. During the first series of studies, the sensor was located at a fixed distance z = 1 mm. The amplitude E of low-frequency (80 Hz) periodic electric field grew slowly and linearly up to the maximal value, and then it linearly decreased. A typical dependence of quartz sensor reflectivity deviation δR on the value of the field E is shown in Fig. 2. The observed scatter of the experimental data received during the measurement of the reflectance is associated with the low value of the EO coefficient of quartz. The best fitting line shows nice linearity which means that our sensor has linear response over a wide electrical field range. In the second series of measurements, sinusoidal potential of fixed amplitude of 3000 V was applied to the electrode while the distance z was varied slowly from 1mm to 15 mm. Corresponding experimentally achieved and theoretically calculated dependencies of normalized sensor signal U/Umax on z are shown in Fig. 3. As it can be seen from the graph, the experimental data fit well with the theoretical curve, indicating unambiguity and reliability of the measured data in terms of determining the value of the electric field by means of the proposed sensor. OPTICAL MEMORY AND NEURAL NETWORKS Vol. 26 No. 2 2017
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U/Umax 1.0
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 z, cm
Fig. 3. Experimental dependence of normalized sensor signal U/Umax on the distance z between the electrode and front surface of the quartz plate (a = 1.5 mm, b = 0.1 m; dots) and corresponding theoretical dependence (a = 10 mm, b = 0.1 m; solid curve).
4. CONCLUSIONS Thus, this study has demonstrated the action of an EO alternating electric field sensor based on the modulation of the reflected light by the electric field under test. It has been shown that modulation of the light reflected from EO medium can be used to measure the strength of the electric field. The proposed sensor can be used for remote measurements of the electric field in high-voltage circuits. The sensor is based on the reflection geometry without the use of a volume EO effect in the system. Using the reflection geometry may greatly reduce the sensor’s size, improve its stability and significantly extend the spectrum of the sensor materials. The construction of the sensor is simple because it does not include a polarizationoptical scheme for transforming phase modulation into the amplitude one. The main advantage of the new sensor which operates with reflected light modulation is the opportunity to use new non-transparent EO media and thin EO films which are sensitive towards electric field. It has been shown that the reflected light modulation sensors can work in the field strength range from at least 100 to 1600 V/mm, the accuracy to be not worse than 10%. REFERENCES 1. Yariv, A. and Yeh, P., Optical Waves in Crystals, Wiley-Interscience, 2002, p. 604. 2. Shinagawa, M., Kobayashi, J., Yagi, S., and Sakai, Y., Sensitive electro-optic sensor using KTa(1 – x)NbxO3 crystal, Sensors Actuators A, 2013, vol. 192, pp. 42–48. 3. Qing, Y., Shangpeng, S., Rui, H., Wenxia, S., and Tong, L., Intense transient electric field sensor based on the electro-optic effect of LiNbO3, AIP Adv., 2015, vol. 5, pp. 1071301–1071310. 4. Garzarella, A.S.B., Qadri, D., and Wu, Ho, Optimal electro-optic sensor configuration for phase noise limited, remote field sensing applications, Appl. Phys. Lett., 2009, vol. 94, pp. 2211131–2211133. 5. Jung, H., Ti:LiNbO3 integrated optic electric-field sensors based on electro-optic effect, Fiber Integrated Opt., 2016, vol. 35, no. 4, pp. 161–180. 6. Lee, T.-H., Hwang, F.-T. Shay, W.-T., and Lee, C.-T., Electromagnetic field sensor using mach-zehnder waveguide modulator, Microwave Opt. Tech. Lett., 2006, vol. 48, no. 9, pp. 1897–1899. 7. Meier, T., Kostrzewa, C., Petermann, K., and Schuppert, B., Integrated optical E-field probes with segmented modulator electrodes, J. Light Wave Tech., 1994, vol. 12, no. 8, pp. 1497–1503. 8. Xiaolong, W., Chunrong, P., Dongming, F., Pengfei, Y., Bo, C., Fengjie, Z., and Shanhong, X., High performance electric field micro sensor with combined differential structure, J. Electron. (China), 2014, vol. 31, no. 2, pp. 143–150. 9. Wang, W.C., Lotem, H., and Forber, R., Optical electric-field sensors, Opt. Eng., 2006, vol. 45, no 12, pp. 1244021–1244028. OPTICAL MEMORY AND NEURAL NETWORKS
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