Structural design method of resonant optical voltage sensor
阅读说明：本技术 一种谐振式光学电压传感器的结构设计方法 (Structural design method of resonant optical voltage sensor ) 是由 李慧 郭飞 温琛 倪佩仁 冯丽爽 于 2020-05-29 设计创作，主要内容包括：本发明公开了一种谐振式光学电压传感器的结构设计方法,属于光学电压传感器领域；首先对多物理场进行理论仿真分析,当满足两束偏振光路径的热应力一致性时,两束线偏振光以BGO晶体中心对称的地方入射；然后根据谐振腔的光学参数,建立谐振式光学电压传感器的光路传输模型；并构建谐振式光学电压传感器的仿真平台,在结构体设计时在BGO晶体两侧通过反射镜形成法布里-珀罗谐振腔。然后通过优化设计,得到满足谐振腔的分辨率和清晰度条件下的腔反射率r<Sub>1</Sub>,r<Sub>2</Sub>,与谐振腔两反射面不严格平行的偏差角θ<Sub>2</Sub>以及满足谐振腔的清晰度条件下的对轴角度误差θ；最后,利用结构体的参数要求设计好谐振式光学电压传感器实际结构；本发明可提高谐振式光学电压传感器测量精度。(The invention discloses a structural design method of a resonant optical voltage sensor, belonging to the field of optical voltage sensors; firstly, carrying out theoretical simulation analysis on a plurality of physical fields, and when the thermal stress consistency of two polarized light paths is met, two polarized light beams are incident at a central symmetrical position of a BGO crystal; then, establishing a light path transmission model of the resonant optical voltage sensor according to the optical parameters of the resonant cavity; and constructing a simulation platform of the resonant optical voltage sensor, and forming a Fabry-Perot resonant cavity on two sides of the BGO crystal through reflectors during structural body design. Then, through optimization design, the cavity reflectivity r meeting the requirements of the resolution ratio and the definition of the resonant cavity is obtained 1 ,r 2 Deviation angle theta not strictly parallel to two reflecting surfaces of resonant cavity 2 And the axial angle error theta under the condition of satisfying the definition of the resonant cavity; finally, the resonant optical voltage transmitter is designed according to the parameter requirement of the structural bodyThe actual structure of the sensor; the invention can improve the measurement precision of the resonant optical voltage sensor.)
1. A structural design method of a resonant optical voltage sensor is characterized by comprising a structural body temperature field and stress field consistency design method and a resonant cavity parameter design method; the method comprises the following specific steps:
the method comprises the following steps that firstly, under two light paths of an external voltage and a non-external voltage, electric fields, temperature fields and stress fields which affect the precision of the resonant optical voltage sensor are subjected to theoretical and simulation analysis, and the conditions of two polarization light paths which are incident at the central symmetry place of a BGO crystal and meet the consistency of the temperature fields and the stress fields are obtained;
the specific analysis process is as follows:
comprehensively considering the influence of electric field, temperature field and stress field, the optical property of BGO crystal is the sum of the independent action results of each external field, and EUFor the path of the applied voltage, ETThe optical path is a light path without external voltage; eUBGO crystal refractive index variation delta n in optical pathU，ETBGO crystal refractive index variation delta n in optical pathT，ΔnUAnd Δ nTThe relationship between the electric field, the temperature field and the stress field physical field factors is as follows:
wherein n is0Refractive index of o light, Δ T is temperature change amount, b11Is the thermo-optic coefficient of the crystal; p is a radical of11,p12,p44Are all the coefficients of elastography, gamma41Is the linear electro-optic coefficient, σ, of the crystalUx,σUy,σUzAre respectively EUPositive stress component, σ, in the optical path in the x, y, z direction of a spatial coordinate systemTx,σTy,σTzAre respectively ETA positive stress component in the light path along the x, y, z directions of the spatial coordinate system, U being EUApplied voltage, gamma, of crystals in the optical path41(T) is a function of the change of the electro-optic coefficient with temperature;
the above relation gives the refractive index variation deltan of the BGO crystal of the two polarized light pathsUAnd Δ nTThe expression of (2) lays a foundation for establishing a light path transmission model of the resonant optical voltage sensor, and ensures the consistency of temperature fields and stress fields of the two polarized light paths except for the electric field as far as possible when the auxiliary structure body is designed;
analyzing the influence of the optical parameters of the resonant cavity on the voltage sensitivity precision, and combining the refractive index variation delta n of the BGO crystal under two light paths of external voltage and non-external voltageUAnd Δ nTEstablishing a light path transmission model of the resonant optical voltage sensor;
the optical parameter of the resonant cavity is mainly the cavity reflectivity r1,r2One-way loss coefficient mu of cavity and deviation angle theta of two reflection surfaces of resonant cavity not strictly parallel2And the principal axis x of refractive index of the fiber collimator and the BGO crystal1' an axis-to-axis angle error θ;
reflectivity of the cavity r1,r2Related to the wavelength of operation and the reflective facestock material;
the cavity single pass loss coefficient μ includes: single pass loss factor mu caused by light not incident perpendicular to the reflecting surface of the resonant cavity1The calculation is as follows:
one-way loss coefficient mu caused by two non-strictly parallel reflecting surfaces of resonant cavity2The calculation is as follows:
coefficient of diffraction loss mu3The calculation is as follows:
absorption and scattering loss coefficient mu4(ii) a And satisfies the condition that mu is mu1+μ2+μ3+μ4；
When electro-optical effect occurs, two beams of orthogonal polarized light are o light and e light respectively in BGO crystal, and the refractive indexes of the two beams of orthogonal polarized light are no＝n0-ΔnU,ne＝n0，ΔnUFor each physical field factor in EUThe amount of refractive index change induced in the optical path; the resonant peak frequency of the o-ray path is fUSatisfy the phase change of the o light back and forth oncem is an integer; and using analyzer to make polarization analysis at output end of resonant cavity to eliminate E-light component, adopting triangular wave phase modulation technique to demodulate E in digital circuitUResonant frequency of the circuit, resonant cavity demodulation outputComprises the following steps:
wherein
reference light path E without voltageTAlso performs light modulation and polarization analysis, where Δ nTFor each physical field factor in ETAmount of refractive index change, f, caused in optical pathTSatisfying the phase change of the o light back and forth once for the resonant peak frequency of the o light componentk is an integer, since Δ nUAnd Δ nTThe numerical value is smaller, and m is k; after the triangular wave phase modulation technique is adopted, E is demodulated in a digital circuitTResonant frequency of the circuit, resonant cavity demodulation outputComprises the following steps:
when, f-f is requiredT0, when the incident light frequency is locked at the o-ray resonance peak frequency f of the reference light pathT；
Detecting the difference between the resonant frequencies of the two optical paths as fU-fTObtaining a relational expression of the change of the optical refractive index of the BGO crystal o caused by the resonance frequency difference and various physical field factors, wherein the relational expression is as follows:
in the absence of stress errors
Thus, the above two fU-fTThe expression (2) is used for assisting in guiding the modulation of the resonant cavity parameters during the manufacturing of the resonant optical voltage sensor structure body, and the modulation should be kept consistent; when an optical voltage sensor voltage sensitive unit formed by the resonant cavity is manufactured, the reflectivity is improved, the parallel angle difference of two reflecting surfaces of the resonant cavity is reduced, and the definition F of the resonant cavity can be improved, so that the voltage detection sensitivity of the resonant optical voltage sensor is improved;
constructing a simulation platform of the resonant optical voltage sensor based on a simulation model of the influence of an electric field, a temperature field and a stress field on the resonant optical voltage sensor and a light path transmission model of the resonant optical voltage sensor, and forming a Fabry-Perot resonant cavity on two sides of the BGO crystal through reflectors during structural design;
the resonant optical voltage sensor comprises: the device comprises a sensing unit structure body, a semiconductor laser, a Y waveguide, a BGO (bismuth germanate) crystal, a PIN-FET detector, an optical fiber collimator, a reflector and a signal processing circuit;
one side of the BGO crystal is coated with a film after being polished, the other side is provided with a reflector, the reflectivity of the two sides is equal, and a Fabry-Perot resonant cavity is formed;
the sensing unit structure is used as a base, and the optical fiber collimator, the BGO crystal, the reflector and the PIN-FET detector are sequentially arranged from front to back;
step four, carrying out optimization design according to the optical parameters of the resonant cavity and the requirement of the structural body on the shaft angle by the simulation platform to obtain the cavity reflectivity r meeting the requirements of the resonant cavity on the resolution and definition1,r2(ii) a Deviation angle theta of two non-strictly parallel reflecting surfaces of resonant cavity2And the axial angle error theta under the condition of satisfying the definition of the resonant cavity;
according to the influence of the optical parameters of the resonant cavity on the voltage sensitivity precision and the analysis result of the simulation platform, the definition of the resonant cavity reaches more than 60 percent, and when the interference light intensity of the resonant optical voltage sensor fluctuates by +/-10 percent, the reflectivity of the resonant cavity with symmetrical structure is obtainedr1＝r2The conditions to be satisfied by the optical parameters are as follows: the axial angle error theta of the optical fiber collimator and the BGO crystal refractive index main shaft needs to be controlled within 0.3 rad; the single-pass loss coefficient mu of the cavity is 0.0219, the reflectivity is more than 0.967, and the parallel angle difference theta of two reflecting surfaces of the crystal resonant cavity2Not greater than 1';
step five, the structural body requires accurate axis alignment of the optical device and uniform electric field and temperature field inside the BGO crystal according to the simulation platform, and the following requirements are required to be met:
(1) control theta of parallel angle difference of two reflecting surfaces of crystal resonant cavity2Required to be no greater than 1';
(2) the requirement of controlling the angle error between the collimator and the crystal to be within 0.3rad is required;
(3) the requirements of electric field, thermal stress and temperature field uniformity in the structural unit are as follows:
selecting the optimal light through hole position to ensure that the distribution of the electric field basically meets the requirement;
keeping the stress variation caused by the heat source to be minimum when the heat flux flows along the same direction as the light propagation;
the symmetry of the structure and the position of the double light paths is kept, and the temperature fields in the double light paths are kept consistent;
step six, actually calculating the external voltage by using the designed resonant optical voltage sensor;
the method specifically comprises the following steps:
the semiconductor laser is arranged at the front side of the sensing unit structure, firstly, a laser beam emitted by the laser source passes through the polarizer to be changed into a polarized light beam polarized along the Y direction, and the light beam is divided into two linearly polarized light beams E through the Y waveguideUAnd ET(ii) a Followed by two linearly polarized light beams EUAnd ETRespectively enters the BGO crystal through the optical fiber collimator and keeps vibrating along the y direction; then, a voltage in the x direction is applied to the upper electrode above the BGO crystal, so that half of the applied voltage of the BGO crystal generates the variation of the refractive index in the y direction, namely linearly polarized light EUA Pockels effect is generated; with two linearly polarized light beams EUAnd ETThe Pockels effect is accumulated when the BGO crystal resonator continuously reciprocates;therefore, the two beams of light generate resonance frequency difference due to the existence of the Pockels effect; at this time, in the linearly polarized light ETIs modulated on half of the superimposed triangular wave phase phimTo make linearly polarized light ETOptical resonance, i.e. the phase of a triangular wave is equivalent to that of linearly polarized light EUSuperimposing a frequency fmThe resonant frequency difference delta f of the two beams of light is in direct proportion to the applied voltage and is independent of the temperature;
two beams of light with resonance frequency difference pass through BGO crystal and then enter PIN-FET detector to become two paths of linearly polarized light EUAnd ETMeasuring the frequency difference delta f of two resonant frequencies to detect voltage, namely calculating | fU-fTThe value of | is given by:thereby calculating an applied voltage;
|fU-fTl is two linearly polarized light beams EUAnd ETFrequency difference of the resonance frequency; n is0Is the original refractive index of the BGO crystal; u is the magnitude of the applied voltage.
2. The structural design method of a resonant optical voltage sensor according to claim 1, wherein the modeling of the electric field, the temperature field and the stress field in the first step is specifically analyzed as follows:
1) after an electric field is applied to the outside, two beams of orthogonal polarized light pass through the BGO crystal, and the calculation formula of the phase difference is as follows:
λ is the wavelength of the incident light; n isy'Is a refractive index in a direction perpendicular to the optical axis; n iseIs the refractive index of e light; n is0Is the refractive index of o light; gamma is the electro-optic coefficient tensor of the BGO crystal; eZElectric field strength in the z direction; l is the length of the BGO crystal, and h is the thickness of the BGO crystal in the electric field; u is an externally applied voltage;
the phase difference comprises two parts: the intrinsic birefringence of the BGO crystal and the birefringence caused by an external applied electric field; and the phase difference is in direct proportion to the external applied voltage U and is related to the geometrical shape of the crystal;
2) influence of externally applied temperature field:
the change of the external temperature generates temperature additional linear birefringence through the linear electro-optic effect and the thermo-optic effect of the BGO crystal, and the measurement precision and the stability of the optical voltage transformer are influenced;
the temperature error calculation formula of the linear electro-optic effect is as follows:
the calculation formula of the temperature error of the thermo-optic effect is as follows:
3) influence of the stress field:
under the action of external stress, the refractive index of the BGO crystal changes to generate an elasto-optic effect;
the linear relational expression is:
Δβ＝p·σ
wherein σ is the stress to which the BGO crystal is subjected and is expressed as (σ)j)＝[σ1σ2σ3σ4σ5σ6]TAnd p is the fourth-order tensor of the BGO crystal elastic optical coefficient.
3. The structural design method of a resonant optical voltage sensor according to claim 1, wherein the axial angle error in step two is: when linearly polarized light collimated by the optical fiber collimator enters the BGO crystal, the vibration direction deviates from the BGO crystal refractive index main axis x1' and has an included angle; setting the axial angle error of the optical fiber collimator and the main axis of the BGO crystal refractive index as theta; the incident light is divided into two beams of orthogonal polarized light, the vibration direction is respectively along x1',x3', the output of the resonant cavity is two beamsSuperposition of light output intensities, the total output intensity at this time being:
when no voltage is applied, the refractive index of the BGO crystal is n0。
4. The structural design method of a resonant optical voltage sensor according to claim 1, wherein the specific structure of the resonant optical voltage sensor in the third step is as follows:
the method specifically comprises the following steps: firstly, constructing a cuboid as a sensing unit structural body, and hollowing the interior of the cuboid to form a groove; two holes A for fixing optical fibers are formed in the front side wall of the cuboid, a boss M is placed in the groove of the cuboid, and an inverted triangular groove is dug in the boss M at the position corresponding to the two holes A respectively for placing two optical fiber collimators;
a platform L of a voltage sensing unit is placed behind the boss M in the cuboid groove, a groove D is formed below the platform L, and a lower electrode is installed in the center of the groove D in a drilling mode; a BGO crystal is placed on the upper part of the platform L, and an upper electrode packaged on the sealing cover is placed on the top of the BGO crystal;
a boss K is arranged on the rear side of the platform L, and two grooves corresponding to the two linearly polarized light beams are engraved in the center of the boss K and used for placing a PIN-FET detector; a light outlet O corresponding to the front side wall is formed in the center of the rear side wall of the cuboid, and light of the PIN-FET detector is connected to a subsequent signal processing circuit through the light outlet O;
the semiconductor laser is arranged in front of the base, light beams emitted by the semiconductor laser firstly pass through a single mode fiber, pass through a Y waveguide, enter an optical fiber A hole and become two beams of linearly polarized light, are collimated by an optical fiber collimator and then enter a BGO crystal, emergent light is received by a multimode fiber, and the multimode fiber band resonant sensing connector is connected with a front end light path part of the PIN-FET detector and then is transmitted to a subsequent signal processing circuit through a light outlet O for processing.
5. A structural design method of a resonant optical voltage sensor according to claim 1, wherein in the fifth step, (1) the requirement for controlling the parallel angle difference between the two reflecting surfaces of the crystal resonator is as follows:
fixing the reflector: the reflector is very thin, and a corresponding auxiliary device is designed to be vertically placed and kept fixed; the structure for placing the BGO crystal and the reflector is required to keep certain flatness and smoothness, and is required to be convenient for adding and fixing electrodes, a multimode fiber bracket for placing the PIN-FET detector for receiving light is required to be kept horizontal, and the central position of the optical fiber is required to be consistent with the height of an optical path so as to receive the most light; for voltage resolution up to 0.1V and r1＝r2When the definition of the resonant cavity is required to be more than 60, the reflectivity is required to be more than 0.967 when the resonant cavity is manufactured and debugged, and the parallel angle difference theta between two reflecting surfaces of the crystal resonant cavity is required2Not greater than 1';
(2) the control requirement for the axial angle error between the collimator and the crystal is as follows:
the platform for placing the self-focusing lens of the collimator is stable, so that the polarized light entering the BGO crystal has accurate axial angle; one end of the collimator is an FC joint which is directly connected with the laser, and the other end of the collimator is a self-focusing lens which is in the shape of a cylinder, wherein the diameter of the thickest section of the cylinder is 1.4 mm; the base fixes the collimator, the BGO crystal and the detector, and the center of the collimator, the light through hole of the BGO crystal and the light receiving position of the multimode fiber are positioned at the same height; the collimator must be placed horizontally, no included angle exists, and a certain pressing sheet can be adopted for fixing when necessary; the polarized light incident into the BGO crystal has accurate axial angle, if deviation exists, the measurement precision is influenced, and according to theoretical analysis, the axial angle error theta of the main axes of the refractive indexes of the collimator and the crystal is required to be within 0.3 rad;
(3) the requirements of electric field, thermal stress and temperature field uniformity in the structural unit are as follows:
under the preset crystal size, the influence of the electric field can exist in the crystal part without the electrode due to the boundary effect of the electric field, and the optimal position of the light through hole is selected to ensure that the distribution of the electric field is basically satisfiedHowever, if a special insulating means is not adopted or the size of the crystal is further increased, the detection precision of the resonant optical voltage sensor is influenced by the existence of the electric field; the heat source induced stress variation is minimized while maintaining the heat flux circulating in the same direction as the light propagation; moreover, as can be seen from the simulation results of the temperature field and the stress field, the symmetry of the structure and the position of the double optical paths needs to be kept, so that the temperature fields in the double optical paths can be kept consistent, and the birefringence errors caused by the temperature are offset; therefore, the structure should be designed such that two linearly polarized light beams EUAnd ETThe BGO crystal is incident at a central symmetrical position, so that the consistency of the thermal stress of two light path paths can be ensured as much as possible, and the influence of a physical field borne by the crystal on two beams of incident light can be offset consistently, so that the designed optical sensor has better stability in actual work.
Technical Field
The invention belongs to the technical field of optical voltage sensors, and relates to a structural design method of a resonant optical voltage sensor.
Background
The electric power work comprises power generation, power transformation, power transmission, power distribution, user power utilization and the like, is the process of production and use of electric energy, and simultaneously, the electric power system plays a great role in national economic development. With the progress of society, the demand of power systems is increasing day by day, and because the power resources are unevenly distributed in various places and cannot be stored in large quantities, and the continuous supply and change of electric energy have randomness to restrict the operation of the power systems, the capacity of power transmission needs to be improved, and the voltage level of power operation needs to be improved, and the voltage sensor is a necessary component device of the power systems, and the development progress is also towards the novel intelligent and modernized technology.
Voltage sensors are classified into three categories according to the mechanism of voltage transformation: capacitive voltage sensors, electromagnetic voltage sensors, and optical voltage sensors. The capacitive voltage sensor and the electromagnetic voltage sensor cannot meet the increasing development requirements of the current power industry due to the defects of complex insulation, overlarge volume, inconvenience in transportation and the like. With the further development of voltage sensor technology, the optical voltage sensor has its advantages and is well studied by many students.
The optical voltage sensor has the characteristics that: firstly, the insulating property is good, and the high-voltage side and the low-voltage side can be separated to the maximum extent, so that the safety is enhanced; and secondly, the stability is high, because the magnetic core is not arranged, the influence of factors such as inherent magnetic saturation and magnet resonance can not be caused, and the problem of waveform distortion caused by sensitivity reduction is avoided. Moreover, the capacitive voltage sensor and the electromagnetic voltage sensor may generate a situation of a secondary circuit short circuit, and have high danger coefficients and hidden dangers, and the existence of the inductance can influence the detected high-frequency component, while the optical voltage sensor is easy to transport and use due to small volume, has high response speed, and can reliably and efficiently detect the voltage.
Although it has such many advantages, in the actual working process, the temperature drift faced by the optical voltage sensor is not yet solved, and the reliability of being able to work for a long time is also used for solving. Therefore, in order to solve these problems, it is necessary to explore a new structure for improving the temperature and the stability in long-term operation.
Disclosure of Invention
The invention provides a structural design method of a resonant optical voltage sensor aiming at the problems of temperature drift, unreliable long-term operation and the like of the conventional optical voltage sensor, and the resonant optical voltage sensor is used for analyzing multiple physical interference fields such as an electric field, a temperature field, a stress field and the like based on a resonant optical voltage sensing unit with a Pockels effect, guiding the subsequent model selection and analysis and structural design of each component of the resonant optical voltage sensor, improving the measurement precision of the resonant optical voltage sensor and promoting the miniaturization application of the resonant optical voltage sensor.
The structural design method of the resonant optical voltage sensor comprises the following specific steps:
the method comprises the following steps of firstly, carrying out theoretical and simulation analysis on a multi-physical field influencing the precision of the resonant optical voltage sensor, and obtaining that two beams of linearly polarized light are incident at a central symmetrical position of a BGO crystal under the condition that the thermal stress consistency of two beams of polarized light paths is met.
The method comprises the following steps of (1) considering the influence of physical fields such as an electric field, a temperature field and a stress field on a resonant optical voltage sensor to establish a simulation model;
the specific model analysis is as follows:
1) after an electric field is applied to the outside, two beams of orthogonal polarized light pass through the BGO crystal, and the calculation formula of the phase difference is as follows:
λ is the wavelength of the incident light; n isy'Is a refractive index in a direction perpendicular to the optical axis; n iseIs the refractive index of e light; n is0Is the refractive index of o light; gamma is the electro-optic coefficient tensor of the BGO crystal; eZElectric field strength in the z direction; l is the length of the BGO crystal, and h is the thickness of the BGO crystal in the electric field; u is an external forceApplying a voltage;
the phase difference comprises two parts: the intrinsic birefringence of the BGO crystal and the birefringence caused by an external applied electric field; and the phase difference is proportional to the externally applied voltage U and is related to the crystal geometry.
2) Influence of externally applied temperature field:
the change of the external temperature generates temperature additional linear birefringence through the linear electro-optic effect and the thermo-optic effect of the BGO crystal, and the measurement precision and the stability of the optical voltage transformer are influenced.
The temperature error calculation formula of the linear electro-optic effect is as follows:
wherein, γijkIs the linear electro-optic coefficient of BGO crystal, d gammaijkdT is the rate of change of the linear electro-optic coefficient of the BGO crystal with temperature (about 1.54 x 10-4) Δ T is the amount of temperature change, EkIs an applied electric field.
The calculation formula of the temperature error of the thermo-optic effect is as follows:
wherein b is a thermo-optic coefficient matrix of BGO crystal, b11Is the thermo-optic coefficient of the crystal.
3) Influence of the stress field:
under the action of external stress, the refractive index of the BGO crystal is changed to generate an elasto-optic effect.
The linear relational expression is:
Δβ＝p·σ
wherein σ is the stress to which the BGO crystal is subjected and is expressed as (σ)j)＝[σ1σ2σ3σ4σ5σ6]TWhere σ is1、σ2、σ3Is the positive stress, sigma, applied to the BGO crystal along the main axis4、σ5、σ6Is BGO crystalThe shear stress is applied under the axis coordinate. p is the fourth-order tensor of the elastic optical coefficient of the BGO crystal;
the influence of an electric field, a temperature field and a stress field is comprehensively considered, and the optical property of the BGO crystal is the sum of the independent action results of all external fields. The reverse dielectric tensor variation of the BGO crystal is then expressed as:
wherein p is11,p12,p44All are elasto-optic coefficients. Gamma ray41Is the linear electro-optic coefficient of the crystal, E3Is the third order component of the electric field; p is a radical ofijklIs the elasto-optic coefficient, σ, of the crystalklThe stress on the crystal is large, and the problem of thermal stress distribution is solved by using elasticity mechanics under a space coordinate system x, y and z. Therefore, when studying the influence of temperature-induced stress on the optical properties of a crystal by the elasto-optical effect, coordinate system transformation is required, and the relationship between the two coordinate systems is as follows:
wherein, theta0Is the principal axis x after crystal transformation1Around transformed x3The axis is rotated counter-clockwise to the angle traveled by the x-axis, which for a twin crystal sensing unit is 45. Order to
after transformation, σx,σy,σzIs the positive stress, sigma, applied to the BGO crystal along the x, y and z directions of a space coordinate systemxy,σyz,σxzThe BGO crystal is subjected to shear stress along the directions of a space coordinate system x, y and z.
EUFor the path of the applied voltage, ETThe light path is without applied voltage. EUBGO crystal refractive index variation delta n in optical pathU，ETBGO crystal refractive index variation delta n in optical pathT，ΔnUAnd Δ nTThe relationship between the electric field, the temperature field and the stress field physical field factors is as follows:
wherein sigmaUx,σUy,σUzAre respectively EUPositive stress component, σ, in the optical path in the x, y, z direction of a spatial coordinate systemTx,σTy,σTzAre respectively ETA positive stress component in the light path along the x, y, z directions of the spatial coordinate system, U being EUApplied voltage, gamma, of crystals in the optical path41(T) is a function of the change in the electro-optic coefficient with temperature.
In order to minimize the errors caused by multiple physical fields, two linearly polarized light beams EUAnd ETThe light path is incident from a central symmetrical place of the BGO crystal, and when the consistency of the thermal stress of two light path paths is ensured as much as possible, the refractive index variation delta n of the BGO crystal of two polarized light paths is deducedUAnd Δ nTThe expression of (2) is used for ensuring the consistency of the temperature field and the stress field of the two polarized light paths except for the electric field as much as possible when the auxiliary structural body is designed.
Analyzing the influence of the optical parameters of the resonant cavity on the voltage sensitivity precision, and establishing a light path transmission model of the resonant optical voltage sensor;
the optical parameter of the resonant cavity is mainly the cavity reflectivity r1,r2The one-way loss coefficient mu of the cavity and the deviation angle theta of the two reflecting surfaces of the resonant cavity which are not strictly parallel2And the principal axis x of refractive index of the fiber collimator and the BGO crystal1' an axis-to-axis angle error θ;
reflectivity of the cavity r1,r2And operating wavelength andthe reflective surface material is related.
The cavity single pass loss coefficient μ includes: single pass loss factor mu caused by light not incident perpendicular to the reflecting surface of the resonant cavity1The calculation is as follows:
wherein m is1The number of round trips within the cavity before light escapes; l is the cavity length of the resonant cavity, theta1D is the thickness of the position deviation of the reflection point reaching the BGO crystal voltage adding direction.
One-way loss coefficient mu caused by two non-strictly parallel reflecting surfaces of resonant cavity2The calculation is as follows:
wherein m is2The number of round trips within the cavity before light escapes; theta2The deviation angle is the deviation angle of the two reflecting surfaces of the resonant cavity which are not strictly parallel.
Coefficient of diffraction loss mu3The calculation is as follows:
wherein S' is the area of the rectangular hole diffraction main pole, and S is the area of the resonant cavity; theta3Diffraction angle for the main maximum edge;
absorption and scattering loss coefficient mu4. And satisfy
μ＝μ1+μ2+μ3+μ4(1.4)
The basic model of light propagation in a resonant cavity formed by a BGO crystal coating is as follows: light enters the resonant cavity from the crystal incident surface and reaches the exit surface port, a small part of light is directly transmitted through the high-reflection film, a large part of light is reflected by the high-reflection film and is continuously transmitted in the resonant cavity to reach the incident surface of the resonant cavity, a small part of light is transmitted through the incident surface of the resonant cavity, and a large part of light is transmitted by the high-reflection filmThe reflection continues to be transmitted in the cavity to the exit face of the cavity, thus completing a cycle. It can be seen from this process that the light exiting the exit port of the resonator is coherently superimposed. The collimator is supposed to be completely aligned with the crystal main axis, so that the light intensity of two beams of orthogonally polarized light emitted from the collimator is equal when the two beams of orthogonally polarized light enter a resonant cavity formed by a BGO crystal coating film, and the electric field of the light entering the cavity is supposed to be A0The initial phase is 0, which can be expressed as: a. the0＝AeiωtWhere ω is the angular frequency of the incident light, ω is 2 pi f, and f is the incident light frequency of the BGO crystal; tau is the degree time that the light makes one round trip inside the cavity,
then the electric field A0The electric field that first exits through the exit face of the resonator can be expressed as:
the light emitted from the exit port of the resonant cavity is formed by superposing the following series of lights:
…
where j is 2,3,4 ….
Summing the corresponding electric field components results in a light intensity that is output without regard to the transmission end of the axial error resonator. The electric field of the light exiting the exit port of the resonant cavity can be expressed as:
when the axial angle error of the optical fiber collimator and the main axis of the BGO crystal refractive index is not considered, the total output light intensity of the emergent end is as follows:
the width of the free spectral line of the resonator to represent the spacing between adjacent resonant peaks is:
further, the full width at half maximum of the resonant cavity is as follows:
cavity definition (also known as finesse), defined as the ratio of the free spectral range to the full width at half maximum, represents the finesse of the peaks or valleys of the cavity transfer function, denoted by F, by definition cavity definition F is:
setting principal axis x of refractive index of fiber collimator and BGO crystal1' the error of the axial angle is theta, the incident light is divided into two beams of orthogonal polarized light, the vibration direction is respectively along x1',x3' the resonator output is the superposition of two beams of light output intensity, and the total output intensity is:
wherein
Take place ofWhen the electro-optical effect is achieved, two beams of orthogonal polarized light are o light and e light respectively in the BGO crystal, and the refractive indexes of the two beams of orthogonal polarized light are no＝n0-ΔnU,ne＝n0，ΔnUFor each physical field factor in EUThe amount of refractive index change induced in the optical path. The resonant peak frequency of the o-ray path is fUSatisfy the phase change of the o light back and forth once
fmthe equivalent frequency of the triangular wave phase modulation imposed on the Y waveguide.When, f-f is requiredU0, when the incident light frequency is locked at the resonant peak frequency f of the o-ray pathU。
Reference light path E without voltageTThe path is also optically modulated and analyzed, where Δ nTFor each physical field factor in ETAmount of refractive index change, f, caused in optical pathTSatisfying the phase change of the o light back and forth once for the resonant peak frequency of the o light component
Detecting the difference between the resonant frequencies of the two optical paths as fU-fTObtaining a relational expression of the change of the optical refractive index of the BGO crystal o caused by the resonance frequency difference and various physical field factors, wherein the relational expression is as follows:
in the absence of stress errors
Therefore, the formula (1.13) can assist in guiding the modulation of the resonant cavity parameters during the manufacturing of the resonant optical voltage sensor structure, so as to be consistent with the condition of no stress error as much as possible.
And thirdly, constructing a simulation platform of the resonant optical voltage sensor based on a simulation model of the influence of physical fields such as an electric field, a temperature field, a stress field and the like on the resonant optical voltage sensor and a light path transmission model of the resonant optical voltage sensor, and forming a Fabry-Perot resonant cavity on two sides of the BGO crystal through reflectors during structural design.
The resonant optical voltage sensor comprises: the optical fiber sensing device comprises a sensing unit structure body, a semiconductor laser, a Y waveguide, a BGO (bismuth germanate) crystal, a PIN-FET detector, an optical fiber collimator, a reflector and a signal processing circuit.
And after two sides of the BGO crystal are polished, one side is coated with a film, the other side is provided with a reflector, the reflectivity of the two sides is equal, and a Fabry-Perot resonant cavity is formed.
The sensing unit structure body is used as a base, and the optical fiber collimator, the BGO crystal, the reflecting mirror and the PIN-FET detector are sequentially arranged from front to back.
The method specifically comprises the following steps: firstly, constructing a cuboid as a sensing unit structural body, and hollowing the interior of the cuboid to form a groove; the optical fiber collimator comprises a cuboid, a boss and a plurality of optical fiber collimators, wherein the cuboid is provided with a plurality of holes A for fixing optical fibers, the boss is arranged in a cuboid groove, and the boss is respectively dug in positions corresponding to the holes A to place the optical fiber collimators.
A platform L of a voltage sensing unit is placed behind a boss M in a cuboid groove, a groove D is formed below the platform L, and a lower electrode is installed in the center of the groove D in a drilling mode. A BGO crystal is placed on the upper part of the platform L, and an upper electrode packaged on the sealing cover is placed on the top of the BGO crystal;
a boss K is arranged on the rear side of the platform L, and two grooves corresponding to two linearly polarized light beams are engraved in the center of the boss K and used for placing a PIN-FET detector. And a light outlet O corresponding to the front side wall is formed in the center of the rear side wall of the cuboid, and light of the PIN-FET detector is connected to a subsequent signal processing circuit through the light outlet O.
The semiconductor laser is arranged in front of the base, light beams emitted by the semiconductor laser firstly pass through a single mode fiber, pass through a Y waveguide, enter an optical fiber A hole and become two beams of linearly polarized light, are collimated by an optical fiber collimator and then enter a BGO crystal, emergent light is received by a multimode fiber, and the multimode fiber band resonant sensing connector is connected with a front end light path part of the PIN-FET detector and then is transmitted to a subsequent signal processing circuit through a light outlet O for processing.
Step four, carrying out optimization design according to the optical parameters of the resonant cavity and the requirement of the structural body on the shaft angle by the simulation platform to obtain the cavity reflectivity r meeting the requirements of the resonant cavity on the resolution and definition1,r2Deviation angle theta not strictly parallel to two reflecting surfaces of resonant cavity2(ii) a Obtaining an axis angle error theta meeting the definition condition of the resonant cavity;
the conditions are satisfied as follows: the definition of the resonant cavity reaches more than 60 percent, and when the interference light intensity of the resonant optical voltage sensor fluctuates by +/-10 percent, the axial angle error theta of the optical fiber collimator and the main shaft of the BGO crystal refractive index needs to be controlled within 0.3 rad.
Through analyzing the parameters, the definition reaches more than 60, and the reflectivity r of the resonant cavity structure is symmetrical1＝r2The requirement is that the single-pass loss coefficient mu of the cavity is less than or equal to 0.0219, the reflectivity reaches more than 0.967, and the parallel angle difference theta of the two reflecting surfaces of the crystal resonant cavity is equal to or greater than2Not greater than 1'.
Step five, the structural body requires accurate axis alignment of the optical device and uniform electric field and temperature field inside the BGO crystal according to the simulation platform, and the following requirements are required to be met:
(1) control requirement for parallel angle difference of two reflecting surfaces of crystal resonant cavity
Fixing the reflector: the mirrors are thin and cannot be independently placed and kept stable, and corresponding auxiliary devices must be designed for fixation. The structure in which the BGO crystal and mirrors are placed should maintain a certain flatness and finish and facilitate the addition and attachment of electrodes, the multimode fiber holder in which the receiving PIN-FET detector is placed should be kept horizontal, while the center of the fiber is positioned to coincide with the height of the optical path in order to maximize the light received. For voltage resolution up to 0.1V and r1＝r2When the definition of the resonant cavity is required to be more than 60, the reflectivity is required to be more than 0.967 when the resonant cavity is manufactured and debugged, and the parallel angle difference theta between two reflecting surfaces of the crystal resonant cavity is required2Not greater than 1'.
(2) Control requirement for axial angle error between collimator and crystal
The platform for placing the collimator self-focusing lens is stable, so that the polarized light entering the BGO crystal has accurate axial angle. One end of the collimator is an FC joint which is directly connected with the laser, and the other end of the collimator is a self-focusing lens which is in the shape of a cylinder, wherein the diameter of the thickest section of the cylinder is 1.4 mm; the base fixes the collimator, the BGO crystal and the detector, and the center of the collimator, the light through hole of the BGO crystal and the light receiving position of the multimode fiber are positioned at the same height; the collimator must be placed horizontally, no included angle exists, and a certain pressing sheet can be adopted for fixing when necessary; the polarized light incident into the BGO crystal has accurate axial angle, if deviation exists, the measurement precision is affected, and according to theoretical analysis, the axial angle error theta of the main axes of the refractive indexes of the collimator and the crystal is required to be within 0.3 rad.
(3) Requirements for electric field, thermal stress and temperature field uniformity within a structural unit
Under the preset crystal size, due to the boundary effect of an electric field, the influence of the electric field can exist on the crystal part without an electrode, the light passing positions of 1.5mm and 8.5mm are selected to enable the distribution of the electric field to basically meet the requirement, but if a special insulating means is not adopted or the size of the crystal is further increased, the detection precision of the resonant optical voltage sensor is influenced by the existence of the electric field. The heat source induced stress variation is minimized while maintaining the heat flux circulating in the same direction as the light propagation. Moreover, as can be seen from the simulation results of the temperature field and the stress field, the symmetry between the structure and the position of the dual optical path needs to be maintained, so that the temperature fields in the dual optical paths can be kept consistent, and birefringence errors caused by temperature are offset. Therefore, the structure should be designed such that two linearly polarized light beams EUAnd ETThe BGO crystal is incident at a central symmetrical position, so that the consistency of the thermal stress of two light path paths can be ensured as much as possible, and the influence of a physical field borne by the crystal on two beams of incident light can be offset consistently, so that the designed optical sensor has better stability in actual work.
Step six, actually calculating the external voltage by using the designed resonant optical voltage sensor;
the method specifically comprises the following steps:
the semiconductor laser is arranged at the front side of the sensing unit structure, firstly, a laser beam emitted by the laser source passes through the polarizer to be changed into a polarized light beam polarized along the Y direction, and the light beam is divided into two linearly polarized light beams E through the Y waveguideUAnd ET(ii) a Followed by two linearly polarized light beams EUAnd ETRespectively enters the BGO crystal through the optical fiber collimator and keeps vibrating along the y direction; then, a voltage in the x direction is applied to the upper electrode above the BGO crystal, so that half of the applied voltage of the BGO crystal is generated in the y directionGenerating a change in refractive index, i.e. linearly polarized light EUA Pockels effect is generated; with two linearly polarized light beams EUAnd ETThe Pockels effect is accumulated by the continuous round trip in the BGO crystal resonator. Therefore, the two beams of light generate resonance frequency difference due to the existence of the Pockels effect; at this time, in the linearly polarized light ETIs modulated on half of the superimposed triangular wave phase phimTo make linearly polarized light ETOptical resonance, i.e. the phase of a triangular wave is equivalent to that of linearly polarized light EUSuperimposing a frequency fmAnd the resonant frequency difference delta f of the two beams of light is in direct proportion to the applied voltage and is independent of the temperature.
Two beams of light with resonance frequency difference pass through BGO crystal and then enter PIN-FET detector to become two paths of linearly polarized light EUAnd ETMeasuring the frequency difference delta f of two resonant frequencies to detect voltage, namely calculating | fU-fTThe value of | is given by:
|fU-fTL is two linearly polarized light beams EUAnd ETFrequency difference of the resonance frequency; n is0Is the original refractive index of the BGO crystal; u is the magnitude of the applied voltage.
The invention has the advantages that:
1. compared with the prior art, the structural design method of the resonant optical voltage sensor has the advantages that most of the error elimination schemes of the conventional optical sensor are implemented by adding optical devices, and the optical path is complex and the connection difficulty is high. The invention plates the reflectivity film on the incidence surface of the BGO crystal, arranges the reflector with the same reflectivity on the emergence surface, and the film and the reflector form a Fabry-Perot resonant cavity with high reflectivity, the structure is simple and easy to operate, the light path is simplified, the measuring error of the sensor is reduced, the measuring precision is improved, the sensitivity of the resonant voltage sensor and the axial angle error theta, the cavity reflectivity r are quantized1，r2And the relation of the definition F.
2. A structural design method of a resonant optical voltage sensor puts forward the requirements of the parallel angle difference of two reflecting surfaces of a crystal resonant cavity and the voltage sensitivity of the resonant optical voltage sensor to guide the structural design of resonant cavity resonance so as to realize high sensitivity and miniaturization of the resonant voltage sensor.
3. A structural design method of a resonant optical voltage sensor is characterized in that whether the thermal stress consistency of two optical paths is a main factor causing temperature error is determined through simulation analysis of multiple physical fields such as a stress field, an electric field and a temperature field, and the designed structure is provided so that two beams of linearly polarized light E can be formedUAnd ETThe light is incident from the central symmetry place of the BGO crystal, so that the thermal stress symmetry of two light path paths can be ensured as much as possible, and the temperature error can be counteracted, thereby improving the temperature environment adaptability of the resonant optical voltage sensor.
Drawings
Fig. 1 is a flow chart illustrating steps of a method for designing a resonant optical voltage sensor according to the present invention;
FIG. 2 is a schematic diagram of a geometric loss analysis of a resonant cavity according to the present invention;
FIG. 3 is a schematic diagram of simulation analysis of the resonant cavity resolution of the resonant optical voltage sensor according to the present invention;
FIG. 4 is a schematic view of a crystal cutting method and an optical axis according to the present invention;
FIG. 5 is a schematic diagram of a structure of a sensing unit according to an embodiment of the present invention;
FIG. 6 is a schematic diagram of an optical path of a resonant optical voltage sensor according to the present invention;
FIG. 7 is a structural diagram of a sensing unit of the resonant optical voltage sensor according to the present invention;
FIG. 8 is a schematic view of a BGO crystal sensing unit of the invention;
FIG. 9 is a schematic view of the BGO electric field distribution of the present invention with electrodes;
FIG. 10 is a diagram showing the simulation results of electrostatic field of the single crystal sensor unit according to the present invention;
FIG. 11 is a graph showing the simulation result of the stress field of the single crystal sensor unit according to the present invention;
FIG. 12 is a diagram showing the simulation result of the temperature field of the single crystal sensor unit according to the present invention;
FIG. 13 is a graph showing the results of temperature field isothermal surface simulation of the single crystal sensing unit of the present invention;
FIG. 14 is a diagram of the relationship between the power and the frequency of the resonant optical voltage sensor according to the present invention.
Detailed Description
The following describes embodiments of the present invention in detail and clearly with reference to the examples and the accompanying drawings.
The structural design method of the resonant optical voltage sensor, as shown in fig. 1, comprises the following specific steps:
the method comprises the following steps of firstly, carrying out theoretical and simulation analysis on a multi-physical field influencing the precision of the resonant optical voltage sensor, and obtaining that two beams of linearly polarized light are incident at a central symmetrical position of a BGO crystal under the condition that the thermal stress consistency of two beams of polarized light paths is met.
The specific optimization is as follows:
1) influence of the additional electric field:
the externally applied electric field has influence on the measurement of the sensor, and the phase difference caused by the electric field is not only related to the applied voltage, but also related to the geometry of the crystal, thereby indicating the necessity of structure body research.
The axis of the crystal is caused by an electric field increased to two in accordance with the direction of the optical axis, the cross section of the spatial distribution of the refractive index in the plane of the optical axis is changed from the previous circle to an ellipse, the length of the principal axis is related to the external applied electric field, and the birefringence characteristics caused by the external applied electric field are influenced by the transmission direction of light. Because the transmission speeds are different, after the light passes through the crystal with the length of l, the phase difference of the two beams is calculated as follows:
λ is the wavelength of the incident light; n isy'Is a refractive index in a direction perpendicular to the optical axis; n iseIs the refractive index of e light; n is0Is the refractive index of o light; gamma is the electro-optic coefficient tensor of the BGO crystal; eZElectric field strength in the z direction; h isThe thickness of the BGO crystal in the electric field; u is an externally applied voltage;
the phase difference comprises two parts: the former formula is the effect of the intrinsic birefringence of the BGO crystal, and the latter is the birefringence caused by an externally applied electric field. From the latter, it can be seen that the phase difference caused by the electric field is proportional to the externally applied voltage U and is related to the crystal geometry.
2) Influence of externally applied temperature field:
under the actual working condition of the temperature error of the linear electro-optic effect, the linear electro-optic coefficient of the BGO crystal changes along with the change of temperature, and the optical property of the crystal is influenced; the calculation formula is as follows:
wherein, γijkIs the linear electro-optic coefficient of BGO crystal, d gammaijkThe dT is the change rate of the linear electro-optic coefficient of the BGO crystal along with the temperature (about 1.54 multiplied by 10)-4) Δ T is the amount of temperature change, EkIs an applied electric field. It follows that a change in temperature will necessarily have an effect on the linear electro-optic effect of the crystal.
The thermo-optic effect means: the change of the external temperature can cause the change of the refractive index of the BGO crystal. The temperature error calculation formula is as follows:
wherein b is a thermo-optic coefficient matrix of BGO crystal, b11Is the thermo-optic coefficient of the crystal.
In summary, the change of the external temperature can generate temperature-added linear birefringence through the linear electro-optic effect and the thermo-optic effect of the BGO crystal, which affects the measurement accuracy and stability of the optical voltage transformer system.
3) Influence of the stress field:
under the action of external stress, the refractive index of the BGO crystal is changed to generate an elasto-optic effect. The linear relationship at its first approximation can be expressed as:
Δβ＝p·σ
wherein σ is the stress to which the BGO crystal is subjected and is expressed as (σ)j)＝[σ1σ2σ3σ4σ5σ6]TWhere σ is1、σ2、σ3Is the positive stress, sigma, applied to the BGO crystal along the main axis4、σ5、σ6Is the shearing stress borne by the BGO crystal under the main axis coordinate. p is the fourth-order tensor of the elastic optical coefficient of the BGO crystal; the influence of an electric field, a temperature field and a stress field is comprehensively considered, and the optical property of the BGO crystal is the sum of the independent action results of all external fields. The reverse dielectric tensor variation of the BGO crystal is then expressed as:
wherein p is11,p12,p44All are elasto-optic coefficients. Gamma ray41Is the linear electro-optic coefficient of the crystal, E3Is the third order component of the electric field;
further derivation yields EUBGO crystal refractive index variation delta n in optical pathU，ETBGO crystal refractive index variation delta n in optical pathT，ΔnUAnd Δ nTThe relationship between the electric field, the temperature field and the stress field physical field factors is as follows:
it can be seen that the refractive indexes of the two optical paths along the propagation direction are different due to the thermal stress, and the refractive index change is a main influence factor influencing the resonant frequency difference of the dual optical paths, thereby influencing the detection sensitivity of the resonant optical voltage sensor. Therefore, the refractive index variation Δ n of the BGO crystal of the two polarized light paths is deducedUAnd Δ nTExpression (2)When the auxiliary structure body is designed, the consistency of the temperature field and the stress field of the two polarized light paths except the electric field is ensured as much as possible.
Thereby guaranteeing the consistency of the thermal stress of the two optical paths as much as possible.
And step two, analyzing the influence of the optical parameters of the resonant cavity on the voltage sensitivity precision, and establishing a light path transmission model of the resonant optical voltage sensor. The optical parameter of the resonant cavity is mainly the cavity reflectivity r1,r2And the cavity single pass loss coefficient mu.
Reflectivity of the cavity r1,r2The influencing factors of (a) are mainly related to the operating wavelength and the reflective surface material. If the reflective surface is contaminated, the reflectance is reduced. The higher the reflectivity, the higher the clarity of the cavity.
The cavity single pass loss coefficient μ includes: single pass loss factor mu caused by light not incident perpendicular to the reflecting surface of the resonant cavity1One-way loss coefficient mu caused by two non-strictly parallel reflecting surfaces of resonant cavity2Diffraction loss coefficient [ mu ]3Absorption and scattering loss coefficient mu4And satisfies the condition that mu is mu1+μ2+μ3+μ4。
1) Light is not incident perpendicular to the reflective surface of the cavity, at which time the behavior of the light in the cavity is represented by fig. 2(a), and the light will eventually exit as it travels back and forth in the cavity multiple times. Number m of round trips in the cavity before escaping by light1The magnitude of the loss can be estimated. The deviation angle of light incident on the reflecting surface of the resonant cavity is assumed to be theta1The cavity length of the resonator is L, and as can be seen from fig. 2(a), when light makes the mth round trip in the cavity, the position deviation Δ L of the reflection point of the light on the reflection surface of the resonator is 2m θ1And L. When the position deviation of the reflection point reaches the thickness d of the BGO crystal in the voltage adding direction, the light escapes from the resonant cavity, thereby calculating the round-trip transmission times of the escaped light in the resonant cavityThe single-pass loss coefficient caused by the incidence of light not vertical to the reflecting surface of the resonant cavity can be known
2) The two reflecting surfaces of the cavity are not exactly parallel, at which time the behavior of light in the cavity is represented by FIG. 2(b), again by the number m of round trips in the cavity before light escapes2The magnitude of the loss can be estimated.
The deviation angle of two non-strictly parallel reflecting surfaces of the resonant cavity is assumed to be theta2The cavity length is L, and it can be seen from FIG. 2(b) that the position of the reflection point of light on the reflection surface of the cavity shifts when the light makes the mth round tripWhen the position deviation of the reflection point reaches the thickness d of the BGO crystal in the voltage adding direction, the light escapes from the resonant cavity, thereby calculating the round-trip transmission times of the light before escaping in the resonant cavity
3) Diffraction losses are caused by the rectangular hole diffraction of light through a resonant cavity of finite size. Assuming that the diffraction energy is all uniformly distributed in the main maximum light spot, the diffraction angle theta of the main maximum edge3The method meets the condition of the rectangular hole diffraction main maximum:
where λ is the wavelength of light incident on the BGO crystal. Calculating the difference between the rectangular hole diffraction main maximum area S' and the area S of the resonant cavity to obtain the diffraction loss of the cavity:
4) absorption and scattering losses are caused by the absorption and scattering of light by the BGO crystal. Calculated according to the physical properties of the BGO crystal, the absorption loss coefficient of the BGO crystal with the length of 1cm is mu4＝0.0035。
The analysis shows that the diffraction loss and the crystal absorption loss are relatively fixed. Let deviation angle theta1＝1'，θ2Calculated as
Calculating the resolution and the cavity reflectivity r1,r2The relation of the cavity one-way loss coefficient mu and the deviation angle theta of mu not strictly parallel to the two reflecting surfaces of the resonant cavity2The relationship (2) of (c). Performing simulation calculation to obtain definition, wherein the resonant cavity has symmetrical structure, and r is1＝r2The results are shown in FIG. 3.
The definition F is calculated as follows:
setting principal axis x of refractive index of fiber collimator and BGO crystal1' the error of the axial angle is theta, the incident light is divided into two beams of orthogonal polarized light, the vibration direction is respectively along x1',x3' as shown in fig. 4, the resonator output is a superposition of the intensities of the two light outputs. When electro-optical effect occurs, two beams of orthogonal polarized light are o light and E light respectively in BGO crystal, analyzer is used for analyzing polarization at output end of resonant cavity to eliminate E light component, and after triangular wave phase modulation technique is adopted, E light is demodulated in digital circuitUResonant frequency of the circuit, resonant cavity demodulation output
fmthe equivalent frequency of the triangular wave phase modulation imposed on the Y waveguide. EUoutWhen'' 0, f-f is requiredU0, when the incident light frequency is locked at the resonant peak frequency of the o-ray pathfU。
The reference light path without direct voltage is also modulated by light, and the analyzer is used to analyze the output end of resonant cavity to eliminate E light component, and after triangular wave phase modulation technique is adopted, the E light component is demodulated in digital circuitTResonant frequency of the circuit, resonant cavity demodulation outputComprises the following steps:
when, f-f is requiredT0, when the incident light frequency is locked at the o-ray resonance peak frequency f of the reference light pathT。
Detecting the difference between the resonant frequencies of the two optical paths as fU-fTObtaining a relational expression of the change of the optical refractive index of the BGO crystal o caused by the resonance frequency difference and various physical field factors, wherein the relational expression is as follows:
in the absence of stress errors
Therefore, the modulation of the resonant cavity parameters during the manufacturing of the resonant optical voltage sensor structure can be guided in an auxiliary way according to the theoretical analysis results, so that the modulation is consistent with the condition without stress errors as much as possible. By analyzing the main optical parameters influencing the output performance of the resonant cavity, the deviation angle theta of the two non-strictly parallel reflecting surfaces of the resonant cavity is obtained2And the cavity reflectivity r1，r2Is a key optical parameter of the resonant cavity. When the voltage sensitive unit of the optical voltage sensor formed by the resonant cavity is manufactured, the reflectivity is improved, and the parallel angle difference of two reflecting surfaces of the resonant cavity is reduced, namely the reflectivity is improvedThe definition F of the resonant cavity is obtained, so that the voltage detection sensitivity of the resonant optical voltage sensor is improved.
And thirdly, constructing a simulation platform of the resonant optical voltage sensor based on a simulation model of the influence of physical fields such as an electric field, a temperature field, a stress field and the like on the resonant optical voltage sensor and a multi-beam interference principle, and forming a Fabry-Perot resonant cavity on two sides of the BGO crystal through reflectors during structural body design.
The resonant optical voltage sensor comprises: the optical fiber sensing device comprises a sensing unit structure body, a semiconductor laser, a Y waveguide, a BGO (bismuth germanate) crystal, a PIN-FET detector, an optical fiber collimator, a reflector and a signal processing circuit.
As shown in FIG. 5, the sensing unit structure is used as a base, and the fiber collimator, the BGO crystal, the reflector and the PIN-FET detector are arranged in sequence from front to back.
The method specifically comprises the following steps: firstly, constructing a cuboid as a sensing unit structural body, and hollowing the interior of the cuboid to form a groove; the optical fiber collimator comprises a cuboid, a boss and a plurality of optical fiber collimators, wherein the cuboid is provided with a plurality of holes A for fixing optical fibers, the boss is arranged in a cuboid groove, and the boss is respectively dug in positions corresponding to the holes A to place the optical fiber collimators.
A platform L of a voltage sensing unit is placed behind a boss M in a cuboid groove, a groove D is formed below the platform L, and a lower electrode is installed in the center of the groove D in a drilling mode. A BGO crystal is placed on the upper part of the platform L, and an upper electrode packaged on the sealing cover is placed on the top of the BGO crystal;
in view of the fact that the parallelism of the coated surface can be guaranteed to be higher, improvement is made under the condition that reflectors are arranged on two sides of a crystal, the method that a film is coated on one side of the crystal, and the reflectors are arranged on the other side of the crystal is adopted, and the parallelism of the reflectors is adjusted to be parallel to the coated surface, so that the parallelism problem caused by a processing technology is solved. The method specifically comprises the following steps: the incidence surface of the BGO crystal is plated with a 96.7% high-reflectivity film, the exit surface is provided with a reflector with the same reflectivity, and the film and the reflector form a high-reflectivity Fabry-Perot resonant cavity. The parallelism of the two faces enables the cavity to be resonant to achieve high sensitivity of the resonant voltage sensor.
The size of the light passing surface of the selected reflector is 5mm multiplied by 10mm multiplied by 2mm, and the size of the light passing surface of the selected reflector is matched with that of the crystal.
The semiconductor laser is arranged in front of the base and consists of a gain area, a phase area and a Bragg grating reflection area, and the equivalent refractive index is changed by adjusting the driving current of the gain area through the tuning of the driving current, so that the tuning of the laser wavelength is realized.
The PIN-FET detector can convert an optical signal into an electrical signal.
The selected fiber collimator is selected to be a C-LENS collimator.
Step four, carrying out optimization design according to the optical parameters of the resonant cavity and the requirement of the structural body on the shaft angle by the simulation platform to obtain the cavity reflectivity r meeting the requirements of the resonant cavity on the resolution and definition1,r2Deviation angle theta not strictly parallel to two reflecting surfaces of resonant cavity2(ii) a Obtaining an axis angle error theta meeting the definition condition of the resonant cavity;
through analyzing the parameters, the definition reaches more than 60, and the reflectivity r of the resonant cavity structure is symmetrical1＝r2When the crystal resonant cavity is used, the requirement is met that the cavity single-pass loss coefficient mu is less than or equal to 0.0219, the reflectivity reaches more than 0.967, and the parallel angle difference theta of two reflecting surfaces of the crystal resonant cavity2Not greater than 1'.
According to the simulation results of the element electric field, the temperature field, the stress field and the electromagnetic field of the resonant sensing unit based on finite element analysis and the comprehensive consideration of factors influencing the temperature stability of the resonant sensing unit, the corresponding structural body design guiding principle is provided, and an optimal structure is designed to bear various components so as to inhibit the influence of multiple physical fields as far as possible.
The method specifically comprises the following steps:
1. when the voltage sensitive unit of the optical voltage sensor formed by the resonant cavity is manufactured, the reflectivity is improved (the reflectivity should reach more than 0.967), and the parallel angle difference (theta) of two reflecting surfaces of the resonant cavity is reduced2Not more than 1'), namely the definition of the resonant cavity can be improved, so that the voltage detection sensitivity of the resonant optical voltage sensor is improved, and theoretical derivation gives newThe sensitivity of the resonant voltage sensor and the error of the axis angle are controlled within 0.3 rad; the cavity reflectivity should satisfy r1＝r2And the quantitative relation of the definition F, the incident light, the collimator, the BGO crystal light-transmitting position and the light-receiving point of the detector need to be strictly aligned, so that the definition can reach the expected target.
2. The invention provides a quantitative relation between the axial angle error of the collimator and the crystal refractive index main shaft and the sensitivity of a novel resonant type voltage sensor, and ensures the closed-loop error sensitivity of the resonant type optical voltage sensor.
3. And (4) controlling a heat source. According to theoretical analysis of the influence of various physical fields on the precision of the resonant optical voltage sensor, when heat flux is kept to flow in the same direction as light propagation, the stress change caused by a heat source is minimum. Moreover, as can be seen from the simulation results of the temperature field and the stress field, the symmetry between the structure and the position of the dual optical path needs to be maintained, so that the temperature fields in the dual optical paths can be kept consistent, and birefringence errors caused by temperature are offset. Therefore, the structure should be designed such that two linearly polarized light beams EUAnd ETThe light enters from the central symmetry position of the BGO crystal, so that the consistency of the thermal stress of the two light path paths can be ensured as much as possible.
4. Electric field distribution. Because of the boundary effect of the electric field, the crystal part without an electrode has the influence of the electric field, and the detection precision of the resonant optical voltage sensor is influenced by the existence of the electric field. Therefore, the structure should be designed such that two linearly polarized light beams EUAnd ETThe symmetrical 1.5mm and 8.5mm positions of the BGO crystal are used as light through holes for incidence, so that the electric field of the two paths of light can be uniformly distributed as much as possible.
Step five, actually calculating the applied voltage by using the designed resonant optical voltage sensor;
the method specifically comprises the following steps:
as shown in fig. 6, the semiconductor laser is disposed on the front side of the sensor unit structure, and first, a laser beam emitted from the laser light source passes through the polarizer to be converted into a polarized beam polarized in the Y direction, which is then split into two linearly polarized beams E by the Y waveguideUAnd ET(ii) a Followed by two linearly polarized light beams EUAnd ETRespectively enters the BGO crystal through the optical fiber collimator and keeps vibrating along the y direction; then, a voltage in the x direction is applied to the upper electrode above the BGO crystal, so that half of the applied voltage of the BGO crystal generates the variation of the refractive index in the y direction, namely linearly polarized light EUA Pockels effect is generated; with two linearly polarized light beams EUAnd ETThe Pockels effect is accumulated by the continuous round trip in the BGO crystal resonator. Therefore, the two beams of light generate resonance frequency difference due to the existence of the Pockels effect; at this time, in the linearly polarized light ETIs modulated on half of the superimposed triangular wave phase phimTo make linearly polarized light ETOptical resonance, i.e. phase of triangular wave phimEquivalent to linearly polarized light EUSuperimposing a frequency fmAnd the resonant frequency difference delta f of the two beams of light is in direct proportion to the applied voltage and is independent of the temperature.
Two beams of light with resonance frequency difference pass through BGO crystal and then enter PIN-FET detector to become two paths of linearly polarized light EUAnd ETMeasuring the frequency difference delta f of two resonant frequencies to detect voltage, namely calculating | fU-fTThe value of | is given by:
|fU-fTL is two linearly polarized light beams EUAnd ETFrequency difference of the resonance frequency; n is0Is the original refractive index of the BGO crystal; u is the magnitude of the applied voltage.
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