Testing system for half-wave voltage of phase modulator

Abstract

The invention provides a phase modulator half-wave voltage test system, which belongs to the technical field of phase modulator performance test, and is characterized in that an optical signal enters a polarizer to be converted into linearly polarized light, and light energy is uniformly distributed to two orthogonal polarization modes of a fast axis and a slow axis through a polarization-maintaining tail fiber of the phase modulator; a preset four-state square wave modulation signal is loaded to a phase modulator, a modulated optical signal is converted into left-handed and right-handed circularly polarized light through a quarter-wave plate, the left-handed and right-handed circularly polarized light is transmitted to a Faraday rotating mirror and then is reflected, the rotation directions of the left-handed and right-handed circularly polarized light are interchanged, the left-handed and right-handed circularly polarized light is returned along an original path, interference occurs at a polarizer, and the polarization direction of the polarization is changed. And the interference light signal is finally transmitted to the signal processing module for signal acquisition and processing. According to the invention, in-situ accurate measurement of the half-wave voltage of the phase modulator is realized, an original optical path structure does not need to be disassembled or damaged, and the method is suitable for performance monitoring and intelligent diagnosis of phase modulators in various high-precision optical fiber sensing systems, and has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of phase modulator performance test, and more particularly relates to a test system for half-wave voltage of a phase modulator. BACKGROUND

[0002] As a new generation of power measurement equipment, the all-fiber current transformer (FOCT) has been widely used in the ultra-high voltage direct current transmission system due to its good insulation performance, wide dynamic range, no magnetic saturation and other advantages. Its core sensing principle is based on the Faraday magneto-optical effect, which measures the phase difference caused by the magnetic field generated by the current when the light wave propagates in the sensing optical fiber, thereby accurately inverting the measured current value. In this precise optical sensing system, the photoelectric phase modulator is a key device for realizing signal modulation, closed-loop control and precision compensation, and its performance directly determines the measurement accuracy and long-term stability of the transformer.

[0003] One of the core performance parameters of the phase modulator is its half-wave voltage (Vπ), which is the driving voltage required to produce a π phase shift. The value of the half-wave voltage is directly related to the modulation efficiency and scale factor of the system; once Vπ drifts, it will directly cause the distortion of the proportional coefficient of the current measurement, introducing a system error that is difficult to correct. This problem is particularly prominent in the converter station in high-altitude areas. The unique low air pressure, large temperature difference, strong radiation and other harsh environments in this area will exacerbate the performance degradation of the phase modulator (such as LiNbO2 or PZT modulator) through physical mechanisms such as thermal stress, material aging and partial discharge, causing the half-wave voltage to deviate from the initial calibrated value, thereby seriously threatening the reliable operation of the direct current transmission system.

[0004] In order to ensure measurement accuracy, it is essential to regularly or online monitor the half-wave voltage of the phase modulator. However, the existing precise measurement techniques for half-wave voltage (such as measurement schemes based on Mach-Zehnder interferometers or Sagnac interferometers) have the following shortcomings in actual engineering applications: (1) System destructive: The existing precise measurement method based on an interferometer (such as a Mach-Zehnder interferometer) usually requires the separation of the phase modulator to be measured from its working optical path, which will irreversibly destroy the structural integrity and optical consistency of the system for the all-fiber current transformer that realizes low loss and high stability through permanent fusion.

[0005] (2) Introducing secondary performance risks: Even if re-fusion can be performed after the measurement is completed, this process will introduce unpredictable additional insertion loss, end face reflection and polarization mode dispersion. These may permanently damage the long-term accuracy and stability of the transformer.

[0006] (3) Lack of online monitoring capability: the traditional method is an offline, laboratory-based calibration means, which cannot realize real-time or periodic monitoring of the internal modulator state of the on-network current transformer.

[0007] In summary, in order to meet the realistic needs of ensuring the accurate and reliable operation of the full-fiber current transformer in the high-altitude complex environment, a new type of half-wave voltage measurement technology is proposed for the above three problems, which must be able to conveniently, quickly and accurately monitor or measure the half-wave voltage of the built-in phase modulator without damaging or interrupting the integrity of the existing optical path system of the transformer, thereby providing key data support for the intelligent operation and maintenance and condition-based maintenance of power equipment. SUMMARY

[0008] In view of the above defects or improvement needs of the prior art, the present application provides a test system for the half-wave voltage of a phase modulator, thereby solving the technical problems of the existing half-wave voltage measurement technology which destroys the integrity of the optical path system, cannot be monitored online and is greatly disturbed by the environment.

[0009] To achieve the above-mentioned purpose, according to one aspect of the present application, a test system for the half-wave voltage of a phase modulator is provided, comprising: an SLD light source, a circulator, a polarizer, a phase modulator, a quarter-wave plate, a roundness sensing ring, a Faraday rotator mirror and a signal processing module. The SLD light source is connected to the first port of the circulator, the second port of the circulator is connected to the polarizer, the phase modulator, the quarter-wave plate, the roundness sensing ring and the Faraday rotator mirror (8) in sequence, and the signal input end of the signal processing module is connected to the third port of the circulator, and the signal output end is connected to the phase modulator. The optical signal output by the SLD light source enters the first port of the circulator, is output to the polarizer through the second port of the circulator and is converted to linearly polarized light by the polarizer; the linearly polarized light is fused to the fast axis and the slow axis of the polarization maintaining optical fiber of the phase modulator to form two orthogonal polarized lights, the signal processing module generates a four-state square wave modulation signal and loads it to the phase modulator to modulate the two orthogonal polarized lights; the modulated optical signal is converted to left-handed circularly polarized light and right-handed circularly polarized light after passing through the quarter-wave plate and enters the roundness sensing ring for transmission; the left-handed circularly polarized light and the right-handed circularly polarized light are exchanged in rotation after being transmitted to the Faraday rotator mirror and return to the quarter-wave plate along the original path to restore to linearly polarized light, and interference light is generated at the polarizer; the interference light enters through the second port of the circulator and is output by the third port of the circulator, and is finally collected by the signal processing module; the signal processing module extracts the harmonic component of the interference light signal and adjusts the amplitude of the four-state square wave modulation signal in real time in combination with the closed-loop feedback control algorithm, so that the system works in a balanced state.

[0010] Preferably, the SLD light source is a superluminescent diode, having wide spectrum and low coherence characteristics.

[0011] Preferably, a 45° axial fusion is adopted between the polarizer and the phase modulator, so as to uniformly distribute the energy of the linearly polarized light to the fast axis and the slow axis of the polarization maintaining optical fiber.

[0012] Preferably, the signal processing module comprises a photodetector and an FPGA processing unit; the input end of the signal processing module is connected to the third port of the circulator, and the output end thereof is connected to the AD acquisition end of the FPGA processing unit; and the DA output end of the FPGA processing unit is connected to the phase modulator.

[0013] Preferably, the four-state square wave modulation signal output by the FPGA processing unit is a staircase waveform with four different levels, and the period is 4τ, wherein τ is the transit time of light wave in the polarization maintaining sensing ring.

[0014] Preferably, the photodetector is used to collect the interference light, convert the interference light signal of the interference light into an interference electric signal, and transmit the interference electric signal to the AD acquisition end of the FPGA processing unit.

[0015] Preferably, the FPGA processing unit is used to collect the response amplitudes of the four modulation steps in the interference electric signal in real time, calculate the deviation, and dynamically adjust the square wave modulation amplitude based on a closed-loop feedback equation, so as to realize the tracking of the half-wave voltage.

[0016] Preferably, a polarization maintaining delay ring is arranged between the phase modulator and the quarter-wave plate, and the polarization maintaining delay ring is used to introduce a fixed time delay between the two orthogonal polarized lights of the fast axis and the slow axis, so as to avoid the instantaneous interference of the light signal in the transmission process.

[0017] Overall, compared with the prior art, the above technical scheme conceived by the present application can achieve the following beneficial effects: 1.The phase modulator half-wave voltage testing system provided by the present application, the light signal output by the SLD light source enters the first port of the circulator, is output to the polarizer through the second port of the circulator, and is converted into linearly polarized light by the polarizer; the linearly polarized light is fused to the fast axis and the slow axis of the polarization maintaining fiber of the phase modulator to form two routes of orthogonal polarized light, the signal processing module generates a four-state square wave modulation signal and loads it on the phase modulator to modulate the two routes of orthogonal polarized light; the modulated light signal is converted into left-handed circularly polarized light and right-handed circularly polarized light after passing through the quarter-wave plate and enters the circularly preserving sensing ring for transmission; the left-handed circularly polarized light and the right-handed circularly polarized light are transmitted to the Faraday rotator mirror, the rotation directions of the left-handed circularly polarized light and the right-handed circularly polarized light are exchanged, and the left-handed circularly polarized light and the right-handed circularly polarized light return to the quarter-wave plate along the original route to restore linearly polarized light, and interference light is generated at the polarizer; the interference light enters through the second port of the circulator and is output by the third port of the circulator, and finally is collected by the signal processing module; the signal processing module extracts the harmonic components of the interference light signal, and combines a closed-loop feedback control algorithm to adjust the amplitude of the four-state square wave modulation signal in real time, so that the system works in a balanced state, the system innovatively uses the inherent optical path of the sensing system as a measurement carrier, realizes in-situ accurate measurement of the half-wave voltage of the phase modulator, does not need to disassemble or damage the original optical path structure, effectively suppresses the inherent loss and environmental interference of the system through optimization of the signal processing algorithm, significantly improves the measurement accuracy and reliability, and provides a half-wave voltage measurement scheme with high precision, high stability and easy integration, which is suitable for performance monitoring and intelligent diagnosis of phase modulators in various high-precision optical fiber sensing systems, and has a wide application prospect in the fields of power system monitoring and industrial automation.

[0018] 2.The phase modulator half-wave voltage testing system provided by the present application has true in-situ non-destructive detection and high-precision measurement capability, and the core lies in improving the closed-loop feedback measurement principle: by utilizing the inherent optical path of the system and injecting a specific modulation signal, a high-stability measurement system is constructed. The system calculates the deviation of the interference output signal in real time, and dynamically adjusts the modulation voltage by using a digital closed-loop equation, so as to accurately lock the half-wave voltage. This process can effectively suppress low-frequency interference such as light intensity fluctuation, so that the measurement result has high precision and strong anti-interference performance, and provides a data basis for realizing reliable online calibration.

[0019] 3.The phase modulator half-wave voltage testing system provided by the present application has fast dynamic response and tracking capability, based on the optimized closed-loop control algorithm, the system can quickly and dynamically track the change of the half-wave voltage, and the locking time can reach sub-millisecond level. This fast response characteristic makes the present application not only suitable for periodic detection, but also capable of capturing the rapid drift of the half-wave voltage caused by environmental stress (such as instantaneous temperature change in high-altitude areas), which meets the urgent needs of real-time and dynamic monitoring of equipment status in intelligent operation and maintenance.

[0020] 4.The system integration scheme of the phase modulator half-wave voltage test system realizes an online monitoring function, the system highly integrates a closed-loop measurement, signal processing and data analysis module, and can automatically perform a measurement task regularly or in real time.Through tracking of a long-term drift rule of the half-wave voltage and big data analysis in combination with environmental parameters such as temperature and air pressure, the system realizes online evaluation and early fault warning of a health state of the phase modulator, and significantly improves an operation intelligent level and a predictive maintenance capability of the entire sensing system. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a schematic diagram of a phase modulator half-wave voltage test system in an embodiment of the present application.

[0022] 1-SLD light source; 2-circulator; 3-polarizer; 4-phase modulator; 5-polarization-maintaining delay ring; 6-quarter-wave plate; 7-polarization-maintaining sensing ring; 8-Faraday rotator mirror; 9-photodetector; 10-FPGA processing unit. DETAILED DESCRIPTION

[0023] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0024] As Figure 1As shown, the present application proposes a phase modulator half-wave voltage testing system, comprising: an SLD light source 1, a circulator 2, a polarizer 3, a phase modulator 4, a polarization maintaining delay ring 5, a quarter-wave plate 6, a polarization maintaining sensing ring 7, a Faraday rotator mirror 8, a photodetector 9 and a FPGA processing unit 10; the DA end of the FPGA processing unit 10 generates a radio frequency signal input to the phase modulator 4, the light from the SLD light source 1 is input from the first port of the circulator 2, the output end of the second port is connected to the polarizer 3 to become linearly polarized light, and then the linearly polarized light is 45° fused with the phase modulator 4. At this time, the linearly polarized light is distributed on the fast axis and the slow axis and is modulated into a square wave by the phase modulator 4. The polarization maintaining delay ring 5 separates the light on the fast axis and the slow axis without interference, and the quarter-wave plate 6 changes it into left-handed circularly polarized light and right-handed circularly polarized light. Finally, after passing through the polarization maintaining sensing ring 7, the Faraday rotator mirror 8 is reflected to the third port of the circulator 2, and the fast axis and the slow axis interfere. The interference light is detected by the photodetector 9, and the light information is converted into electrical information and input to the AD acquisition end of the FPGA processing unit 10. The FPGA processing unit 10 adjusts the square wave amplitude of the DA output in real time by analyzing the harmonic characteristics corresponding to the four-state square wave modulation in the interference signal and combining the closed-loop feedback algorithm, so that the system works in a specific balanced state, thereby accurately and in situ solving the half-wave voltage of the phase modulator to be measured without destroying the integrity of the optical path. The system has compact structure, strong anti-interference ability, and is suitable for online monitoring and intelligent diagnosis of the performance of the phase modulator in the high-precision optical fiber sensing system.

[0025] Further, the SLD light source 1 used in the system is a superluminescent diode (SLD), which is a light source made by using the directional radiation phenomenon in a strong excitation state. Compared with ordinary light-emitting diodes, it has the characteristics of high output power, small divergence angle and high coupling efficiency. The wavelength distribution range is between semiconductor lasers and ordinary light-emitting diodes, which can meet the requirements of high power and short coherence length.

[0026] Further, the function of the polarizer 3 is to ensure that the linearly polarized light is incident into the optical path. Further, the function of the 45° fusion of the polarizer 3 and the phase modulator 4 is to convert the linearly polarized light into two mutually orthogonal polarized lights distributed on the fast axis and the slow axis.

[0027] Further, the polarization maintaining delay ring 5 needs to be long enough to ensure that the two orthogonal linearly polarized lights can be separated without interference.

[0028] Further, the quarter-wave plate 6 can convert the two mutually orthogonal linearly polarized lights into left-handed circularly polarized light and right-handed circularly polarized light.

[0029] To further explain, the mirror used is a Faraday rotator mirror 8, which can convert left-handed circularly polarized light into right-handed circularly polarized light, and vice versa.

[0030] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0031] The light emitted by the SLD is represented by the Jones matrix as follows:

[0032] After passing through the polarizer, it becomes a single linearly polarized light:

[0033] The polarization-maintaining fiber pigtail of the polarizer and the polarization-maintaining fiber pigtail of the phase modulator are fused together at a 45° angle, so that the linearly polarized light output from the polarizer and propagating along the X-axis enters the modulator pigtail at a 45° angle. The linearly polarized light entering the modulator pigtail is then re-decomposed into X and Y axes.

[0034] After passing through the phase modulator, an additional phase difference is introduced between the fast and slow axes:

[0035] Linearly polarized light enters the sensing ring, passes through a 45° axial weld, and then enters the λ / 4 waveplate. The λ / 4 waveplate is a segment... A specific length of polarization-maintaining fiber is precisely fabricated into a λ / 4 waveplate. Its core function is to utilize the inherent refractive index difference between the fast and slow axes (X-axis and Y-axis) of the polarization-maintaining fiber to induce a fixed phase delay of π / 2 (i.e., a quarter wavelength) in light waves propagating along both axes. When two linearly polarized beams with equal amplitude and phase propagate along the X and Y axes of the polarization-maintaining fiber, respectively, the resulting light's vibration trajectory on the fiber cross-section is a straight line. After passing through the λ / 4 waveplate, due to the introduction of a 90° phase difference between the two beams, their combined vibration trajectory changes from linear polarization to circular polarization. Specifically, the linearly polarized light components that originally propagated along the X and Y axes are transformed into left-handed and right-handed circularly polarized light, respectively, after passing through the λ / 4 waveplate. This transformation process provides the necessary physical conditions for realizing current sensing or phase modulation detection based on circularly polarized light interference.

[0036]

[0037] The sensing fiber is drawn using a special process and is a circularly symmetrical fiber with the ideal circular symmetry structure. Its refractive indices along the X and Y axes are identical, and the light waves propagate at the same speed along the two orthogonal axes, without inherent linear birefringence. When an external magnetic field generated by a current-carrying conductor acts on this sensing fiber, based on the Faraday magneto-optical effect, the polarization planes of the linearly polarized light propagating along the X and Y axes in the fiber will rotate synchronously, with the rotation angle proportional to the magnetic field strength. To facilitate the analysis of the influence of the magnetic field on the light wave propagation characteristics, the incident linearly polarized light can be equivalently decomposed into a pair of circularly polarized lights with the same propagation direction but opposite rotation directions (i.e., left-handed and right-handed circularly polarized lights).

[0038] Under the influence of a magnetic field, the rotation direction of the polarization plane is uniquely determined by the direction of the magnetic field. When this rotation direction coincides with the inherent rotation of a circularly polarized light, it is equivalent to accelerating the phase propagation speed of that circularly polarized light; conversely, when it is opposite to the inherent rotation of the circularly polarized light, it is equivalent to slowing down its phase propagation speed. This non-reciprocal phase velocity difference allows two circularly polarized light beams to accumulate a phase difference proportional to the current after completing two-way propagation, thereby enabling accurate measurement of the current.

[0039]

[0040] When circularly polarized light propagating in the sensing fiber reaches the end reflector, the light wave will return along its original path. During the reflection process, the direction of light propagation is reversed, and its polarization rotation is also interchanged: that is, the incident left-handed circularly polarized light becomes right-handed, and right-handed becomes left-handed.

[0041] Meanwhile, due to the non-reciprocity of the Faraday magneto-optical effect, when the direction of light propagation and the direction of the magnetic field are simultaneously reversed, the resulting phase accumulation effect does not cancel out, but rather superimposes. Therefore, the polarization plane continues to rotate along its original direction on the return path, causing the total phase difference caused by the Faraday effect to double compared to one-way propagation.

[0042] When a light wave carrying a doubled Faraday phase difference passes through a λ / 4 waveplate again, its polarization mode changes: the polarized light originally on the fast axis (X-axis) couples to the slow axis (Y-axis) and returns, while the polarized light originally on the slow axis (Y-axis) couples to the fast axis (X-axis) and returns. This process achieves cross-transposition of the two beams on their principal polarization axes.

[0043] Finally, when the two beams of light arrive at the polarizer again, due to the different optical path lengths experienced in the polarization-maintaining delay ring and the sensing ring, the slower-moving light wave catches up with the faster-moving light wave. The two beams converge and interfere at the polarizer, and the resulting interference signal contains phase information proportional to the measured current.

[0044] in, Additional phase shift introduced for the modulator, It is the Faraday rotation angle caused by the electric current.

[0045] Assume the FPGA's DA outputs a four-state square wave with amplitudes of 3S, S, -3S, and -S, and the duty cycle is set to... The modulation phase of the four-state square wave is as follows:

[0046] Four-state square wave modulation can not only... When no current is applied, The output voltage in the four states is as follows:

[0047]

[0048]

[0049]

[0050] The output voltage of the square wave can only be changed when... At this time, S is the half-wave voltage of the phase modulator.

[0051] This invention not only overcomes the inherent defects of traditional half-wave voltage measurement schemes, which require the destruction of the optical path and cannot be applied online, but also comprehensively improves the system's measurement accuracy, anti-interference ability, and intelligence level by introducing advanced closed-loop measurement principles.

[0052] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A testing system for the half-wave voltage of a phase modulator, characterized in that, include: SLD light source (1), circulator (2), polarizer (3), phase modulator (4), quarter-wave plate (6), circular sensing ring (7), Faraday rotator (8) and signal processing module; The SLD light source (1) is connected to the first port of the circulator (2), and the second port of the circulator (2) is connected in sequence to the polarizer (3), the phase modulator (4), the quarter-wave plate (6), the circularity-preserving sensing ring (7) and the Faraday rotator (8); the signal input terminal of the signal processing module is connected to the third port of the circulator (2), and the signal output terminal is connected to the phase modulator (4). The optical signal output from the SLD light source (1) enters the first port of the circulator (2), and is output to the polarizer (3) through the second port of the circulator (2) and converted into linearly polarized light by the polarizer (3). The linearly polarized light is fused to the fast axis and slow axis of the polarization-maintaining fiber of the phase modulator (4) to form two orthogonally polarized lights. The signal processing module generates a four-state square wave modulation signal and loads it onto the phase modulator (4) to perform phase modulation on the two orthogonally polarized lights. The modulated optical signal is converted into left-hand circularly polarized light and right-hand circularly polarized light after passing through a quarter-wave plate (6) and enters the circularly polarized sensing ring (7). Transmission; the left-handed and right-handed circularly polarized light are transmitted to the Faraday rotator (8), where their rotation directions are interchanged. After passing through the circularity-preserving sensing ring (7), they return along the original path to the quarter-wave plate (6) and are restored to linearly polarized light. They then interfere at the polarizer (3) to generate interference light. The interference light enters through the second port of the circulator (2) and is output through the third port of the circulator. Finally, it is collected by the signal processing module. The signal processing module extracts the harmonic components of the interference light signal and, in conjunction with the closed-loop feedback control algorithm, adjusts the amplitude of the four-state square wave modulation signal in real time to keep the system in a balanced state.

2. The testing system for half-wave voltage of a phase modulator according to claim 1, characterized in that, The SLD light source (1) is a superluminescent diode.

3. The testing system for half-wave voltage of a phase modulator according to claim 1, characterized in that, The polarizer (3) and the phase modulator (4) are fused together at 45° axially so that the energy of the linearly polarized light is evenly distributed to the fast axis and slow axis of the polarization-maintaining fiber.

4. A test system for half-wave voltage of a phase modulator according to any one of claims 1-3, characterized in that, The signal processing module includes a photodetector (9) and an FPGA processing unit (10); the input terminal of the signal processing module (9) is connected to the third port of the circulator (2), and its output terminal is connected to the AD acquisition terminal of the FPGA processing unit (10); the DA output terminal of the FPGA processing unit (10) is connected to the phase modulator (4).

5. The test system for half-wave voltage of a phase modulator according to claim 4, characterized in that, The four-state square wave modulation signal output by the FPGA processing unit (10) is a stepped waveform with four different levels and a period of 4τ, where τ is the transit time of the light wave in the circular sensing ring.

6. The test system for half-wave voltage of a phase modulator according to claim 4, characterized in that, The photodetector (9) is used to collect the interference light, convert the interference light signal into an interference electrical signal and transmit it to the AD acquisition terminal of the FPGA processing unit (10).

7. A test system for half-wave voltage of a phase modulator according to claim 6, characterized in that, The FPGA processing unit (10) is used to collect the response amplitude of the four modulation steps in the interference electrical signal in real time and calculate the deviation, and dynamically adjust the square wave modulation amplitude based on the closed-loop feedback equation to realize the tracking of the half-wave voltage.

8. The test system for half-wave voltage of a phase modulator according to claim 1, characterized in that, A polarization-maintaining delay ring (5) is provided between the phase modulator (4) and the quarter-wave plate (6), which is used to introduce a fixed time delay between the two orthogonally polarized beams on the fast axis and the slow axis.

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