RU2539130C1 - Fibre-optic device for measurement of electric field intensity - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: optimisation of sensor structure, providing for occurrence of dissimilar modulation of a refraction index in supply of a difference of single-polarity voltages phases to a double-channel modulator, results in the possibility to use for modulation of the phase of any frequency of a control signal and in absence of the necessity to develop a delay line. Repeated passage of light reflected from the mirror through an integral-optical sensitive element and the second supplying optic fibre with double beam refraction, and also rotation of the light polarisation plane in a Faraday rotator by 90 degrees and usage of the second photodetector provide for doubling of modulation amplitude, reduction of optical noise of the source.

EFFECT: increased accuracy of electric field intensity measurement and reduced frequency of signal modulation.

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Description

The invention relates to the field of fiber-optic measuring devices based on fiber-optic phase polarimetric sensors of interferometric type and is intended to measure the intensity of the electric field, converted into the intensity of the optical signal.

The principle of operation of the device is based on the Pockels effect and consists in determining the optical phase difference as a result of interference between two orthogonal linearly polarized modes that occurs under the influence of the measured electric field strength on the sensitive element.

Known fiber-optic devices for measuring electric field strength (US patents No. 6952107 B2, 04/10/2005, G01R 31/308, US No. 6380725, 04/30/2002, G01R 31/00, US No. 62522388, 01/26/2001, G01R 31 / 00, US No. 6140810, 10.31.2000, G01R 15/24; G01R 29/12, US No. 5029273, 07/02/1991, G01R 31/00; G01R 27/02, US No. 4524322, 06/18/1985, G01R 31/02 ; G01R 31/00, EP No. 0682261, 11/15/1995, G01D 5/26; G01J 4/00; G01M 11/00; G01N 21/23; G01R 15/24, EP No. 0826970 A2, 04.03.1998, G02F 1 / 01, RU No. 2032181, dated 03/27/1995, G01R 13/40), which are manufactured according to the principle of sensors of a passing type, in which a sensitive element is placed between the light source and the detector. Birefringent optical fiber in such devices is used as a line for transmitting light, and in some devices also as a sensing element.

The disadvantage of such devices is the high degree of dependence of the phase change of the optical signal propagating through the optical channel from external conditions.

Deprived of this drawback and more promising for widespread practical use due to its greater sensitivity to electric field strength is a fiber-optic device with a sensitive element of a reflective type. In such a device, phase-modulated orthogonally polarized modes pass through the sensing element in two directions - first in the forward direction, and then in the opposite direction due to reflection from the mirror end, and then interfere. And due to the presence of a Faraday rotator of the plane of polarization installed up to the sensitive element, any external influences acting on the section of the optical path to the Faraday rotator are compensated. (Patents US No. 8233754, July 31, 2012, G02B 6/00; G02B 6/02), US No. 7147388, 12/12/2006, G02B 6/255, US No. 6122415, 09/19/2000, G02B 6/00; G01R 31/00)

A fiber-optic device for measuring electric field strength is known which uses the same interference measurement principle (K. Bohnert, S. Wildermuth, A. Frank, H. Brandle, Fiber-optic voltage sensor using fiber gyro technology. Procedia Engineering 5 ( 2010), 1091-1094), adopted as a prototype.

The prototype device consists of a depolarized low coherent light source, a birefringent optical fiber as a medium for transmitting an optical signal, a Y splitter, on one side of which there is an optical radiation source and receiver. On the other hand, a polarizer, a phase difference modulator (MRF) between two orthogonal modes of a channel waveguide made on the basis of an integrated optical channel waveguide on a substrate of a LiNbO ₃ crystal, a fiber delay line, a Faraday rotator, a sensitive element, and a compensation fiber with end-mounted mirror.

The sensitive element is a birefringent optical fiber that is wound on a piezoelectric cylinder, the diameter of which changes under the influence of an applied electric field and, as a result, an additional phase difference arises in the sensitive optical fiber.

The optical axis of the fiber polarizer is rotated at an angle of 45 degrees to the birefringence axes of the integrated waveguide to excite two orthogonal modes. In the phase difference modulator (MRF), the phases of the orthogonal modes are affected by the modulating voltage of the control signal. Due to the Faraday rotator in the opposite direction, the polarization planes of the two orthogonal modes propagating through the fiber rotate 90 degrees for each mode. The birefringence axes of the sensing element and the fiber compensation section are rotated 90 degrees relative to each other.

The principle of operation of the prototype is as follows. A linearly polarized mode passes through the fiber from the light source through the splitter, then the fiber polarizer, which then splits into two orthogonal modes at the junction with the phase difference modulator (MRF), which pass through the MRF and are introduced into the delay line. Two orthogonal modes, as they propagate in a birefringent medium (Birefringence), acquire a phase difference. Then, after the delay line, the light passes through the Faraday rotator and falls on the sensitive element, in which the induced phase difference arises under the influence of the electric field strength. After the sensing element, the light passes through the compensation section of the fiber with a mirror placed at the end and returns back, thereby doubling the effect of the measured physical quantity. And due to the mutual orthogonal orientation of the axes of the sensing element and the compensation section of the fiber, the phase delay in the sensing element and other external influences (for example, temperature, vibration) are compensated, resulting in a stray phase difference. Further, passing through the Faraday rotator, the plane of polarization rotates 90 degrees. Due to this, there is a complete compensation of the phase incursion arising in the MYFF and the delay line. In the opposite direction, the light passes through the MYRF, the polarizer and the splitter, after which the interference signal hits the photodetector. In the design described above, upon returning the light pulse to the joint with a polarizer of 45 degrees, the phase difference between the orthogonal modes, acquired due to the action of the electric field on the sensitive element, will remain. The resulting phase shift Δφ can be represented by the following expression (1)

, $$\Delta \varphi =2\cdot S\cdot k\cdot l\left(\text{one}\right)$$

,

where S is the parameter of the electric field strength value, l is the length of the sensitive element, k is the sensitivity coefficient of the phase change from the applied physical quantity.

The main disadvantage of the prototype device is the need to change the modulation polarity at a high frequency f _m , calculated by the formula (2), due to the short length of the optical path so that the applied modulation effect on the phase of the optical signal is not compensated.

, $$f_m=\frac{C}{2\cdot L\cdot n}\left(\text{2}\right)$$

,

where C is the speed of light in vacuum, L is the length of the optical path from the MRF to the mirror, n is the refractive index of light in the fiber.

This disadvantage leads to the need to use complex electrical circuits and expensive electronic components to modulate the optical signal, as well as to increase the length of the delay line of the optical path from expensive optical components. Another disadvantage is the influence of the instability of the power level of the light source, which leads to error and drift of the interference signal.

The technical problem to be solved by the claimed invention is directed is to increase the accuracy of measuring electric field strength and lowering the frequency of signal modulation

The problem is solved as follows.

In a fiber-optic device for measuring electric field strength, containing optically coupled depolarized low-coherent light source, a fiber polarizer, a splitter, a phase difference modulator (MRF) is made integrally-optical and is located on a substrate of lithium niobate LiNbO ₃ , a Faraday rotator, sensitive element, compensation fiber, mirror, photodetector, and the output of the depolarized low coherent light source is connected to the input of the wave a window polarizer, the output of which is connected to the input of the MPF, and optically coupled fiber insulator, a second photodetector, a second fiber polarizer, a first birefringent optical fiber, a second birefringent optical fiber, and a phase difference modulator are introduced into the device (МРФ) is made of two-channel and consists of two single-channel phase difference modulators (МРФ), the splitter is made in the form of a channel integrated optical fiber waveguide X-shaped topology with two I input and two outputs, produced by titanium diffusion technology and disposed on said substrate of a crystal of lithium niobate LiNbO ₃ and forms with said dual phase difference modulator (MYFF) single multifunction integrated optic circuit (MIOS), wherein the single channel MYFF electrodes arranged parallel to each other on the two input channels of the splitter and are the first and second input port of the MIOS, and the central and external electrodes of the single-channel MRF are electrically interconnected ootvetstvenno sensitive element is a channel integrated optical waveguide produced by titanium diffusion technology, and is placed on the second substrate from the crystal niobate LiNbO _3, lithium output light radiation source connected to the input of the first fiber polarizer, whose output is connected with the channel integrated-optic the waveguide of the first input port of MIOS, the angle between the optical axes of the output fiber of the first fiber polarizer and the channel integrated optical waveguide the input MIOS input port is 45 degrees, the compensation fiber input is connected to the second MIOS input port, the output of which is connected to the input of the second fiber polarizer, the angle between the optical axes of the fibers being 45 degrees, and the output of the second fiber polarizer is connected to the input of the first photodetector, the first output the MIOS port is connected to the input of the first supplying optical fiber with DLP, the second MIOS output port is connected to the input of the fiber isolator, and its output is connected to the input of the second photodet ctor, the output of the first optical fiber with DLP is connected to the input of the Faraday rotator, the output of which is connected to the input of the second optical fiber with DLP, the output of which is connected to the input of the sensor, the mirror being located at the output end of the sensor, and the length of the compensation fiber is calculated by formula (3)

, $$L_c=2\frac{\Delta d\cdot \Lambda }{{\lambda }_c}\left(3\right)$$

,

where Δd is the difference in the optical path between the orthogonal modes, acquired due to the propagation of light in the second supply optical fiber with DLP and a sensitive element, Λ is the beat length of the optical elements, λ _c is the central wavelength of the depolarized low coherent light source.

The advantages achieved by the invention are that any frequency of the control signal can be used for phase modulation due to the fact that when a phase difference of a voltage of the same polarity is applied to a two-channel modulator on two adjacent channels of the splitter, opposite modulation of the refractive index occurs, without the need for creating a delay line. In addition, since the polarization is rotated 90 degrees on the Faraday rotator, then for the light transmitted to the mirror and vice versa, the modulation amplitude is doubled for the same modulation period, and the use of a second photodetector can reduce the optical noise of the light source, all this leads to increase the accuracy of the measured physical quantity.

The essence of the invention is illustrated in figure 1. figure 2, figure 3, where figure 1 shows a diagram of the device, figure 2 is a diagram of a sensitive element with a mirror, figure 3 is a multifunctional integrated optical circuit (MIOS).

The device (Fig. 1) contains optically coupled depolarized low coherent light source 1, a first fiber polarizer 2, a multifunctional integrated optical optical circuit (MIOS) 3, consisting of a two-channel phase difference modulator (MRF) 4 and a splitter 5, the first optical fiber with birefringence (DLP) 6, a Faraday rotator 7, a second optical fiber with DLP 8, a sensing element 9, a mirror 10, a compensation fiber 11, a second fiber polarizer 12, the first photodetector 13, in a curl insulator 14, a second photodetector 15. The output of the light source 1 is connected to the input of the first fiber polarizer 2, the output of which is connected to the first input port of the MIOS, and the angle between the optical axes of the output fiber of the first fiber polarizer 2 and the integrated optical channel waveguide of the first input port MIOS is 45 degrees, the first MIOS output port is connected to the input of the first birefringent optical fiber (DLF) 6, the output of which is connected to the input of the Faraday the carrier 7, and its output is connected to the input of the second birefringent optical fiber (DLF) 8, the output of which is connected to the input of the sensing element 9, a mirror 10 is located at the end of the sensitive element, the second MIOS input port is connected to the input of the compensation fiber 11, the output of which connected to the input of the second fiber polarizer 12, and the angle between the optical axes of the fibers is 45 degrees, and the output of the second fiber polarizer is connected to the input of the first photodetector 13, and the second output MIOS port coupled to an input fiber of the insulator 14, the output of which is connected to the input of the second photodetector 15.

In Fig. 2, the sensing element 9 is made in the form of a channel integrated optical waveguide 16 made by titanium diffusion technology and placed on the second substrate 17 from an electro-optical lithium niobate crystal LiNbO ₃ , a mirror 10 is located at the end of the waveguide.

Splitter 5 and two-channel MPF 4 form a single multifunctional integrated optical-optical circuit (MIOS) 3.

Figure 3 splitter 5 is made in the form of a channel integrated optical waveguide on a substrate of an electro-optical crystal of lithium niobate LiNbO ₃ and has an X-shaped topology with two input and two output channels, two-channel MRF 4 is made integral optical and consists of two single-channel MRF 4a and 4b, the electrodes of which are parallel to each other on the two input channels of the coupler, which are simultaneously the first and second input port of the MIOS, with the central and external electrodes of the single-channel MPF electrically interconnected respectively.

The operation of the device shown in Fig. 1 is as follows: the light from the depolarized low coherent radiation source 1 passes through the first fiber polarizer 2, which converts the depolarized light to linearly polarized, and the plane of polarization is introduced at an angle of 45 degrees to the birefringence axes of the channel integral the optical waveguide of the MIOS input port 3. Two orthogonal modes are excited in the MIOS, guided along the “fast” and “slow” optical axis of the integrated optical waveguide. Next, the “fast” and “slow” mode passes through the first single-channel MRF 4a, splitter 5, the first optical fiber with DLP 6, and falls on the Faraday rotator 7, in which the plane of polarization of the “fast” and “slow” modes rotate 45 degrees and introduces a second supply optical fiber with DLP 8 in the same optical axis. Then the light enters the channel integrated optical-optical waveguide 16 of the sensing element 9 located on the substrate 17 and enters the mirror 10, after which it is reflected and again passed through the channel integrated optical-optical waveguide 16 of the sensing element 9, the second optical fiber with DLP 8 and returns to Faraday rotator 7, where the plane of polarization of the modes again rotate 45 degrees.

In the sum of the turns on the Faraday rotator 7, the planes of polarization of the reflected modes are introduced at an angle of 90 degrees to the axes of the first optical fiber with DLP 6. Thus, the light from the "fast" axis is introduced into the "slow" and vice versa. Due to the change of axes, the phase difference between the orthogonal modes, acquired due to the propagation of light in MIOS 3 and the first optical fiber with DLP 6, is compensated for. Next, the light passes through a splitter 5, the second single-channel MRF 46 and enters the compensation fiber 11, where compensation occurs the phase difference between the orthogonal modes, acquired due to the propagation of light in the channel integrated optical waveguide 16 and the second supply optical fiber with DLP 8. After that, two phase-compensated e orthogonal fashion are introduced at 45 degrees to the optical fiber axis of the second polarizer 12, the interference signal occurs resulting in a polarizer. Next, the interference signal enters the photodetector 13. To reduce the influence of optical noise of the light source when measuring the interference signal at the first photodetector 13, the second MIOS 3 output port is used, from which the radiation through the fiber insulator 14 enters the second photodetector 15.

As a specific embodiment of the device, it is proposed to use a depolarized low coherent light source in the form of a superluminescent diode with a central wavelength of 1550 nm and a spectrum width of 30 nm, a fiber polarizer made on the basis of a single-mode birefringent optical fiber like PANDA, and a second fiber polarizer made on the basis of a single-mode optical fiber with birefringence type PANDA, the splitter is made in the form of a channel integrated optical waveguide X- shaped topology with two input and two output channels, made using titanium diffusion technology and placed on a substrate of lithium niobate crystal LiNbO ₃ , and forms with a two-channel phase difference modulator (MRF), consisting of two single-channel phase difference modulators, each of which is made in in the form of two electrodes parallel to each other, which are two strips of gold located parallel to each other and placed on both sides of the integrated optical channel waveguide at some distance From it, a single multifunctional integrated optical-optical circuit (MIOS), the electrodes of single-channel MRFs being parallel to each other on the two input channels of the splitter and are the first and second input port of MIOS, the central and outer electrodes of single-channel MRFs being electrically connected to each other, respectively, Faraday a rotator made on the basis of a single-mode birefringent optical fiber of the PANDA type and a Faraday magneto-optical cell, a photodetector in the form of an InGaAs-based photodiode, Ora photodetector a photodiode based on InGaAs, the sensor element formed as a channel of an integrated optical waveguide made of titanium diffusion techniques on a substrate of a crystal of lithium niobate LiNbO ₃ and plated on the face of the crystal mirrors, fiber isolator based single-mode optical fiber birefringent type PANDA, compensation fiber in the form of a segment of an optical fiber type PANDA, the first supply optical fiber with birefringence (DLP) in the form of a segment of a single-mode optical PANDA birefringent fibers, a second birefringent optical fiber (DLP) as a segment of a single-mode birefringent PANDA fiber.

Thus, the above shows that the claimed invention is feasible and ensures the achievement of the technical result, namely, that for modulating the phase there is no need to use a high frequency of the control signal and use the delay line due to the fact that when applying voltage of the same polarity to two adjacent channels of the splitter the opposite modulation of the refractive index arises for the “fast” axis of the waveguide. In this case, phase compensation does not occur for the reflected signal passed back and forth. In addition, since the polarization is rotated 90 degrees on the Faraday rotator, then for the light transmitted to the mirror and back, the modulation amplitude is doubled, which allows the use of lower voltage levels to modulate the light.

Claims (1)

Fiber-optic device for measuring electric field strength, containing optically coupled depolarized low-coherent light source, fiber polarizer, splitter, phase difference modulator, made in the form of an integrated optical channel waveguide placed on a substrate of lithium niobate LiNbO ₃ crystal, Faraday rotator, sensitive element, compensation fiber, mirror, photodetector, and the output of a depolarized low coherent light source is connected to the input of a fiber polarizer, the output of which is connected to the input of a phase difference modulator, characterized in that the device is equipped with optically coupled fiber insulator, a second photodetector, a second fiber polarizer, a first birefringent optical fiber, a second birefringent optical fiber, the phase difference modulator is made two-channel and consists of two single-channel phase difference modulators, the splitter is made in the form of a channel integrated optical Skog waveguide X-shaped topology with two input and two output channels, made according to the technology diffusion of titanium and disposed on said substrate of a crystal of lithium niobate LiNbO ₃ and forms a two-channel phase difference modulator single multifunction integrated optic circuit, wherein the electrodes are single-channel phase difference modulators located parallel to each other on the two input channels of the splitter and are the first and second input port of a multifunctional integrated optical circuit, m the central and external electrodes of single-channel phase difference modulators are electrically interconnected, respectively, the sensitive element is made in the form of a channel integrated optical fiber waveguide made by titanium diffusion technology and placed on a second substrate of lithium niobate LiNbO ₃ , the output of the light source is connected to the input the first fiber polarizer, the output of which is connected to the channel integrated optical waveguide of the first input port a non-optical circuit, the angle between the optical axes of the output fiber of the first fiber polarizer and the channel integrated optical waveguide of the first input port of the multifunctional integrated optical circuit is 45 degrees, the compensation fiber input is connected to the second input port of the multifunctional integrated optical circuit, the output of which is connected with the entrance of the second fiber polarizer, and the angle between the optical axes of the fibers is 45 degrees, and the output of the second fiber polar the isator is connected to the input of the first photodetector, the first output port of the multifunctional integrated optical fiber circuit is connected to the input of the first supplying optical fiber with birefringence, the second output port of the multifunctional integrated optical circuit is connected to the input of the fiber insulator, and its output is connected to the input of the second photodetector, the output of the first birefringent optical fiber connected to the input of the Faraday rotator, the output of which is connected to the input of the second optical fiber birefringent fiber, the output of which is connected to the input of the sensitive element, and the mirror is located on the output end of the sensitive element, and the length of the compensation fiber is calculated by the formula:

,
where Δd is the difference in the optical path between the orthogonal modes, acquired due to the propagation of light in the second birefringent optical fiber with a sensitive element, Λ is the beat length of the optical elements, λ _c is the central wavelength of the depolarized low coherent light source.

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