CN115507952B - Feedback optical isolator of Faraday polarizer and isolation method thereof - Google Patents

Abstract

The invention belongs to the technical field of nuclear fusion, and particularly relates to a feedback optical isolator of a Faraday polarizer and an isolation method thereof, wherein the feedback optical isolator of the Faraday polarizer comprises: wire grid, quarter wave plate, light absorbing cotton, metal protection cylinder, rotation mechanism and step motor; the invention can obtain Faraday rotation angle with obviously reduced oscillation amplitude on the phase analyzer, and greatly improves the measurement accuracy of Faraday rotation angle. The invention can effectively filter the feedback light from the end face of the detector in the Faraday polarization measurement, eliminate the oscillation interference signal in the Faraday rotation angle, realize the effective and accurate measurement of the Faraday rotation angle, and has good practical effect.

Description

A feedback optical isolator of a Faraday polarimeter and its isolation method

Technical Field

The invention belongs to the technical field of nuclear fusion, and in particular relates to a feedback light isolation method of a Faraday polarimeter.

Background technique

In the study of magnetic confinement nuclear fusion plasma, current density distribution is the most important plasma parameter for conducting magnetohydrodynamic instabilities, plasma transport and confinement studies. At present, Faraday polarimeter and dynamic Stark effect are the two most recognized diagnostic techniques. In many nuclear fusion experimental devices, Faraday polarimeter is usually integrated with far infrared (FIR) laser interferometer to form FIR laser polarization/interferometer, which has the function of measuring electron density and current density distribution (derived by measuring Faraday rotation angle).

The measurement accuracy of the Faraday polarimeter directly determines the reconstruction accuracy of the current density distribution and directly affects the physical analysis of the plasma. According to the Faraday rotation angle formula, its absolute value is determined by the electron density and the current value. In the current tokamak devices at home and abroad, the maximum value of the Faraday rotation angle is less than 20°, which increases the difficulty of diagnosis. Under the HL-2A discharge conditions, the Faraday rotation angle is smaller, with a maximum value of less than 5°. The stray light interference is particularly obvious, such as the reflected light from the diagnostic window, half-wave plate, and quarter-wave plate, especially the feedback light from the detector end face, which mixes with the original detection light to form a standing wave to interfere with the detection information, and ultimately causes the Faraday rotation angle measurement value to produce obvious oscillation interference during the rise/fall of the electron density, causing the Faraday rotation angle measurement to fail.

Therefore, how to use appropriate optical isolation technology to eliminate the oscillation interference signals encountered in Faraday angle measurement, especially the feedback light from the detector end face, is very important to improve the accuracy of Faraday rotation angle measurement and current density distribution reconstruction.

The formic acid laser polarization/interferometer system on the HL-2A device is large and complex, using hundreds of optical components. The optical path principle diagram of the diagnostic system is shown in Figure 2.

In the optical path, two beams of formic acid laser are adjusted to form orthogonal linear polarized light that is transmitted in the same line. After long waveguide transmission and a large number of reflective lenses, they enter the main optical path system. To achieve Faraday polarization measurement, a quarter-wave plate is required in the system to transform the orthogonal linear polarized light into left-handed/right-handed circularly polarized light (L-/R-). The left-handed/right-handed circularly polarized light passes through the plasma (due to the different refractive indices of the two, a phase difference will be generated after passing through the plasma), and the detection beam is received and mixed by a high-sensitivity Schottky detector. Finally, by comparing the phase difference between the detection channel and the reference channel, the phase angle (small angle) directly related to the Faraday rotation measurement can be obtained.

However, due to the Michelson optical path adopted by the HL-2A polarimeter (i.e., the optical path passes through the plasma twice) and the large spot of the formic acid laser beam, stray light or feedback light may be introduced during the multi-channel optical path transmission process, such as the feedback light generated by the diagnostic window and the wave plate surface, thereby interfering with the Faraday rotation measurement accuracy. This has become a common problem encountered in polarimeter measurements at home and abroad.

In recent years, a large number of studies have shown that the stray light that affects Faraday polarization measurement mainly comes from the feedback light at the end face of the detector. After reflecting back and forth between the reflector on the inner wall of the vacuum chamber and the surface of the detector, it forms a standing wave, which eventually leads to serious oscillation interference in the Faraday rotation angle measurement during the rise/fall of electron density. The above situation will introduce large errors to the measurement value of the Faraday rotation angle, or even cause it to fail.

Therefore, a feedback optical isolator of a Faraday polarimeter and an isolation method thereof are further designed to solve the above technical problems.

Summary of the invention

The present invention provides a feedback light isolator of a Faraday polarimeter and an isolation method thereof, which are used to solve the technical problem in the prior art that due to the large spot of the formic acid laser beam of the HL-2A polarimeter, stray light or feedback light may be introduced during the multi-channel optical path transmission, thereby interfering with the Faraday rotation measurement accuracy.

Technical solution of the present invention:

A feedback optical isolator for a Faraday polarimeter comprises: a wire grid, a quarter wave plate, light-absorbing cotton, a metal protective cylinder, a rotating mechanism and a stepping motor; the wire grid and the quarter wave plate are respectively mounted on the metal protective cylinder, the metal protective cylinder is a hollow cylindrical structure as a whole, the inner wall of the metal protective cylinder is provided with light-absorbing cotton, the rotating mechanism is arranged through the inside of the metal protective cylinder, the wire grid and the quarter wave plate are respectively arranged on the rotating mechanism, and the rotating mechanism is also connected to the stepping motor.

Light is passed through the two ends of the metal protective cylinder; the wire grid is a metal nickel wire grid element; the angle between the optical axis of the quarter wave plate and the direction of the wire grid metal wire is 45°; a detector is arranged opposite to the plane of the quarter wave plate arranged in the metal protective cylinder, and the detector is a Faraday polarizer Schottky detector.

A method for isolating the feedback optical isolator of the Faraday polarimeter as described above comprises the following steps:

Step 1: Assemble a feedback optical isolator in the optical path of the Faraday polarizer;

Step 2: Install a high-sensitivity Schottky detector on the detection light path of the formic acid laser polarimeter;

Step 3: Use a stepper motor to drive the rotating mechanism to control the orientation and angle of the wire grid and the quarter wave plate;

Step 4: Perform comparative measurements under the same plasma parameter discharge conditions, adjust the wire grid rotation mechanism at intervals of 1°, and monitor the oscillation amplitude of the Faraday signal on the phase analyzer during the electron density rise/fall stage in real time.

Step 5: Under the same plasma discharge conditions, adjust the quarter-wave plate rotation mechanism at intervals of 1° to monitor the oscillation amplitude of the Faraday signal on the phase analyzer during the electron density rise/fall stage in real time.

Step 6: Compare the minimum values A and B of the Faraday signal oscillation amplitude recorded in step 4 and step 5, and select the best position of the wire grid and the quarter-wave plate.

When assembling the feedback optical isolator in the step one, it also includes: during assembly, one end of the quarter-wave plate in the feedback optical isolator is facing the Schottky detector; at the same time, before assembling the feedback optical isolator, it is necessary to adjust the angle between the optical axis of the quarter-wave plate and the direction of the wire grid to 45°, and preset the light beam entering the feedback optical isolator as standard left-handed/right-handed circularly polarized light.

When installing the high-sensitivity Schottky detector in the step 2, it also includes: the detector output signal is connected to the phase analyzer after passing through a 50-fold preamplifier to monitor the Faraday rotation signal in real time and compare and analyze the changes in its oscillation amplitude during the increase/decrease of electron density.

The step 3 controls the orientation and angle of the wire grid and the quarter wave plate, and also includes: setting the center position of the metal protective cylinder of the feedback optical isolator and the center position of the end face of the detector to be at the same horizontal height, and at the same time, adjusting the inclined assembly angle of the wire grid and the quarter wave plate in the vertical direction to 1°-2° to prevent the reflected light introduced by the quarter wave plate itself from interfering with the Faraday rotation measurement;

The step 4 also includes: by monitoring the discharge data, recording the minimum value A of the Faraday signal oscillation amplitude, and the angle α1 between the optical axis of the quarter wave plate and the wire grid direction at this time.

The step five also includes: by measuring the discharge data, recording the minimum value B of the Faraday signal oscillation amplitude, and the angle α2 between the optical axis of the quarter wave plate and the wire grid direction at this time.

The step six also includes: when the amplitude of the oscillation interference signal reaches a minimum, the orientation of the wire grid and the quarter-wave plate is determined, the optimal matching position of the optical components inside the feedback optical isolator is obtained, and applied to subsequent discharge measurements; the plasma discharge measurement is tracked again, and the signal obtained in the phase analyzer at this time is the effective value of the Faraday rotation angle after filtering out the feedback light interference from the detector end face to the greatest extent.

Beneficial effects of the present invention:

The present invention enables a phase analyzer to obtain a Faraday rotation angle with a significantly reduced oscillation amplitude, thereby greatly improving the measurement accuracy of the Faraday rotation angle.

The feedback light isolation method was experimentally verified on the formic acid laser (HCOOH, wavelength 432.5μm) polarization/interferometer system of the China HL-2A Tokamak. The polarization/interferometer system uses two formic acid lasers, and the interferometer and polarimeter measurement functions are separated, that is, the electron density and Faraday rotation angle are measured separately. The feedback light isolation system is installed in the Faraday polarization optical path, successfully eliminating the feedback light signal on the Faraday rotation measurement branch, and improving the measurement accuracy of the Faraday rotation angle.

The method can effectively filter out the feedback light from the detector end face in Faraday polarization measurement, eliminate the oscillation interference signal in the Faraday rotation angle, and realize effective and accurate measurement of the Faraday rotation angle, with good practical effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a working principle diagram of the feedback optical isolation method designed by the present invention;

FIG2 is a schematic diagram of the optical path of a formic acid laser polarizer on an existing HL-2A device;

FIG3 is a plasma discharge current diagram;

FIG4 is a chord-integrated electron density diagram;

FIG5 is a diagram of the Faraday rotation angle of the polarization/interferometer before the application of the feedback optical isolator;

FIG6 is a diagram of the Faraday rotation angle of the polarization/interferometer after the feedback optical isolator is applied;

Detailed ways

The feedback optical isolator of a Faraday polarimeter and the isolation method thereof of the present invention are described in detail below in conjunction with the accompanying drawings and embodiments.

This feedback optical isolator was experimentally verified on the formic acid laser (HCOOH, wavelength 432.5μm) polarization/interferometer system of the China HL-2A Tokamak. The polarization/interferometer system uses two formic acid lasers, and the interferometer and polarimeter measurement functions are separated, that is, the electron density and Faraday rotation angle are measured separately. We installed this feedback optical isolation system into the Faraday polarization optical path, successfully eliminated the feedback optical signal on the Faraday rotation measurement branch, and improved the measurement accuracy of the Faraday rotation angle.

The formic acid laser polarization/interferometer system on the HL-2A device is large and complex, using hundreds of optical components. The optical path principle diagram of the diagnostic system is shown in Figure 2. In the optical path, two formic acid laser beams are adjusted to form orthogonal linear polarized light that is transmitted in the same line. After long waveguide transmission and a large number of reflective lenses, they enter the main optical path system; to achieve Faraday polarization measurement, a quarter-wave plate is required in the system to transform the orthogonal linear polarized light into left-handed/right-handed circularly polarized light (L-/R-); the left-handed/right-handed circularly polarized light passes through the plasma (due to the different refractive indices of the two, a phase difference will be generated after passing through the plasma), and the detection beam is received and mixed by a high-sensitivity Schottky detector. Finally, by comparing the phase difference between the detection channel and the reference channel, the phase angle (small angle) directly related to the Faraday rotation measurement can be obtained.

Since the HL-2A polarimeter uses a Michelson optical path (i.e., the optical path passes through the plasma twice), and the spot of the formic acid laser beam is relatively large, stray light or feedback light may be introduced during the multi-channel optical path transmission, such as the feedback light generated by the diagnostic window and the wave plate surface, thereby interfering with the Faraday rotation measurement accuracy, which has become a common problem encountered in polarimeter measurements at home and abroad. In recent years, a large number of studies have shown that the stray light that affects Faraday polarization measurement mainly comes from the feedback light on the detector end face, which forms a standing wave after reflecting back and forth between the inner wall reflector of the vacuum chamber and the detector surface, and ultimately causes serious oscillation interference in the Faraday rotation angle measurement value during the rise/fall of the electron density. The above situation will introduce a large error to the Faraday rotation angle measurement value, or even failure.

In order to solve the above oscillation interference problem, we have continuously summarized and analyzed in long-term experimental research and developed a feedback optical isolator that can filter out the feedback light from the detector end face to a large extent. The working principle of the feedback optical isolator is shown in Figure 1.

The working principle of the feedback optical isolator in Figure 1 is as follows: the left-handed/right-handed circularly polarized detection light carries information related to the Faraday rotation angle. When the two pass through the feedback optical isolator, first, the wire grid element converts the left-handed/right-handed circularly polarized light into linearly polarized light; then, after passing through the quarter-wave plate, its polarization state is converted into left-handed/right-handed circularly polarized light and enters the detector, which has no effect on the original detection optical path. However, when the feedback light generated by the end face of the detector is reflected back to the optical path system, after passing through the quarter-wave plate, its polarization plane is rotated 90° relative to the linear polarization light in the incident direction. According to the optical characteristics of the wire grid element, the direction of the returned linearly polarized light is parallel to the wire grid direction, and it will not be able to pass through the wire grid and be directly isolated, so that it cannot return to the main optical path system, thus ensuring that the source of the mixed signal entering the detector is the detection light wave that directly passes through the plasma. Finally, the Faraday rotation angle with a significantly reduced oscillation amplitude can be obtained on the phase analyzer (the oscillation amplitude can even be eliminated), which greatly improves the measurement accuracy of the Faraday rotation angle.

The specific structure of the feedback optical isolator described in the present invention is shown in Figure 1, including: a wire grid, a quarter wave plate, light-absorbing cotton, a metal protective cylinder, a rotating mechanism and a stepper motor; the wire grid and the quarter wave plate are respectively installed on the metal protective cylinder, the metal protective cylinder is a hollow cylindrical structure as a whole, the inner wall of the metal protective cylinder is provided with light-absorbing cotton, the rotating mechanism is arranged through the inside of the metal protective cylinder, the wire grid and the quarter wave plate are respectively arranged on the rotating mechanism, and the rotating mechanism is also connected to the stepper motor.

Light is passed through the two ends of the metal protective cylinder; the wire grid is a metal nickel wire grid element; the angle between the optical axis of the quarter wave plate and the direction of the wire grid metal wire is 45°; a detector is arranged opposite to the plane of the quarter wave plate arranged in the metal protective cylinder, and the detector is a Faraday polarizer Schottky detector.

After the feedback light generated by the detector end face in Figure 1 passes through the quarter-wave plate, its polarization plane rotates 90 degrees relative to the linear polarization light in the incident direction. At this time, the returning linear polarized light will not be able to pass through the wire grid, and the feedback light is effectively isolated.

In Figure 2, the orthogonal linear polarized light is transformed into left-handed/right-handed circularly polarized light after passing through a quarter-wave plate for Faraday polarization measurement; the feedback optical isolator is installed at the front end of the detector.

A method for isolating the feedback optical isolator of the Faraday polarimeter as described above comprises the following steps:

Step 1: Assemble a feedback optical isolator in the optical path of the Faraday polarizer, including: during assembly, one end of the quarter-wave plate in the feedback optical isolator faces the Schottky detector; at the same time, before assembling the feedback optical isolator, the angle between the optical axis of the quarter-wave plate and the direction of the wire grating needs to be adjusted to 45°, and the light beam entering the feedback optical isolator is preset to be standard left-handed/right-handed circularly polarized light.

Step 2: Install a high-sensitivity Schottky detector on the detection light path of the formic acid laser polarimeter; the detector output signal is connected to a phase analyzer after passing through a 50-fold preamplifier to monitor the Faraday rotation signal in real time and compare and analyze the changes in its oscillation amplitude during the increase/decrease of electron density.

Step 3: drive the rotating mechanism by a stepper motor to control the orientation and angle of the wire grid and the quarter wave plate, including: setting the center position of the metal protective cylinder of the feedback optical isolator and the center position of the end face of the detector to be at the same horizontal height, and at the same time, adjusting the inclined assembly angle of the wire grid and the quarter wave plate in the vertical direction to 1°-2° to prevent the reflected light introduced by the quarter wave plate itself from interfering with the Faraday rotation measurement;

Step 4: Comparative measurement under the same plasma parameter discharge conditions, adjust the wire grid rotation mechanism at intervals of 1°, and monitor the oscillation amplitude of the Faraday signal on the phase analyzer in real time during the electron density rise/fall stage. Step 4 also includes: by monitoring the discharge data, recording the minimum value A of the Faraday signal oscillation amplitude, and the angle α1 between the optical axis of the quarter wave plate and the wire grid direction at this time.

Step 5: Under the same plasma discharge conditions, adjust the quarter-wave plate rotation mechanism at intervals of 1° to monitor the oscillation amplitude of the Faraday signal on the phase analyzer in the electron density rise/fall stage in real time. Step 5 also includes: by measuring the discharge data, recording the minimum value B of the Faraday signal oscillation amplitude, and the angle α2 between the optical axis of the quarter-wave plate and the wire grid direction at this time.

Step 6: Compare the minimum values A and B of the Faraday signal oscillation amplitude recorded in step 4 and step 5, and select the optimal position of the wire grid and the quarter wave plate. When the amplitude of the oscillation interference signal reaches the minimum, the orientation of the wire grid and the quarter wave plate is determined, and the optimal matching position of the optical components inside the feedback optical isolator is obtained, and applied to the subsequent discharge measurement; track the plasma discharge measurement again, and the signal obtained in the phase analyzer at this time is the effective value of the Faraday rotation angle after filtering out the feedback light interference from the detector end face to the greatest extent.

From Figures 3 to 6, it can be seen that the oscillation amplitude of the Faraday signal is completely different from the string-integrated electron density during the electron density rise/fall stage, and a large interference value appears. Subsequently, we installed the feedback optical isolator into the Faraday polarimeter detection channel, and the Faraday rotation angle measurement results are shown in Figure 6, and the measurement results are significantly improved.

Example

(1) A feedback optical isolator is installed in the optical path of the Faraday polarizer in FIG1 (on the optical path in front of the detector), wherein the placement direction is such that one end of the quarter-wave plate of the feedback optical isolator faces the Schottky detector. At the same time, before the operation of this method, the angle between the optical axis of the quarter-wave plate and the direction of the wire grid needs to be adjusted to 45°, that is, it is assumed that the light beam entering the feedback optical isolator is a standard left-handed/right-handed circularly polarized light.

(2) A high-sensitivity Schottky detector is installed on the detection light path of the formic acid laser polarimeter. The detector output signal is connected to the phase analyzer after passing through a 50-fold preamplifier to monitor the Faraday rotation signal in real time and compare and analyze the changes in its oscillation amplitude during the increase/decrease of electron density.

(3) Ensure that the center position of the feedback optical isolator opening is at the same horizontal height as the center position of the detector end face. At the same time, adjust the vertical tilt angle of the wire grid and the quarter-wave plate to 1°-2° to prevent the reflected light introduced by the quarter-wave plate itself from interfering with the Faraday rotation measurement.

(4) The rotating mechanisms of the assembled wire grid and quarter wave plate are connected to two stepper motors respectively, which can realize fine rotation control. Under the same plasma discharge conditions, the wire grid rotation mechanism is adjusted at intervals of 1°, and the oscillation amplitude of the Faraday signal on the phase analyzer during the electron density rise/fall stage is monitored in real time. By monitoring the 6-shot discharge data (rotation range: ±5°), the minimum value A of the Faraday signal oscillation amplitude and the angle α1 between the optical axis of the quarter wave plate and the wire grid direction at this time are recorded.

(5) Similar to step (4), under the same plasma discharge conditions, adjust the quarter-wave plate rotation mechanism at intervals of 1°, and monitor the oscillation amplitude of the Faraday signal on the phase analyzer in real time during the electron density rise/fall stage. By monitoring the 6-shot discharge data (rotation range: ±5°), record the minimum value B of the Faraday signal oscillation amplitude and the angle α2 between the optical axis of the quarter-wave plate and the wire grid direction at this time.

(6) Compare the minimum values A and B of the Faraday signal oscillation amplitude recorded in step (4) and step (5), and select the optimal position of the wire grid and the quarter-wave plate. Track the plasma discharge measurement again. At this time, the signal obtained in the phase analyzer is the effective value of the Faraday rotation angle after filtering out the feedback light interference from the detector end face to the greatest extent. Figures 3 to 6 show the application results of this feedback light isolation system in the HL-2A plasma discharge experiment.

The embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge scope of ordinary technicians in the field without departing from the purpose of the present invention.

Claims (8)

1. A feedback optical isolator for a faraday polarizer, comprising: wire grid, quarter wave plate, light absorbing cotton, metal protection cylinder, rotation mechanism and step motor; the wire grid and the quarter wave plate are respectively arranged on the metal protection cylinder, the whole metal protection cylinder is of a hollow cylinder structure, light absorption cotton is arranged on the inner wall of the metal protection cylinder, the rotating mechanism is arranged inside the metal protection cylinder in a penetrating manner, the wire grid and the quarter wave plate are respectively arranged on the rotating mechanism, and the rotating mechanism is also connected with the stepping motor;

The two ends of the metal protection cylinder are communicated with light; the wire grid is a metal nickel wire grid element; the included angle between the optical axis of the quarter wave plate and the wire grid wire direction is 45 degrees; the plane of the quarter wave plate arranged in the metal protection cylinder is opposite to the plane of the quarter wave plate, and the detector is a Faraday polarimeter Schottky detector.

2. A method of isolating a feedback optical isolator for a faraday polarimeter as claimed in claim 1, comprising the steps of:

step one, a feedback optical isolator is assembled in a Faraday polarizer light path;

Step two, installing a high-sensitivity Schottky detector on a detection light path of the formic acid laser polarimeter;

step three, driving a rotating mechanism through a stepping motor, and controlling the directions and the included angles of the wire grid and the quarter wave plate;

step four, comparing and measuring under the same plasma parameter discharge condition, adjusting a wire grid rotating mechanism according to 1 degree interval, and monitoring the oscillation amplitude of a Faraday signal on the electron density rising/falling stage phase analyzer in real time;

Step five, under the same plasma discharge condition, adjusting a quarter wave plate rotating mechanism according to the interval of 1 DEG, and monitoring the oscillation amplitude of a Faraday signal on the electron density rising/falling stage phase analyzer in real time;

and step six, comparing the minimum values A and B of the Faraday signal oscillation amplitude recorded in the step four and the step five, and selecting the optimal positions of the wire grid and the quarter wave plate.

3. A method of isolating a feedback optical isolator of a faraday polarizer according to claim 2, wherein the step of assembling the feedback optical isolator further comprises: one end of a quarter wave plate in the feedback optical isolator is opposite to the Schottky detector during assembly; meanwhile, before the feedback optical isolator is assembled, the included angle between the optical axis of the quarter wave plate and the wire grid direction needs to be adjusted to be 45 degrees, and the light beam entering the feedback optical isolator is preset to be standard left-handed/right-handed circularly polarized light.

4. The isolation method of a feedback optical isolator of a faraday polarizer according to claim 3, wherein the step two of installing a high sensitivity schottky detector further comprises: the output signal of the detector is connected to a phase analyzer after passing through a 50-time preamplifier for monitoring the Faraday rotation signal in real time and comparing and analyzing the change of the oscillation amplitude of the Faraday rotation signal during the rising/falling period of the electron density.

5. A method of isolating a feedback optical isolator for a faraday polarizer according to claim 4, wherein the step three controls the orientation and angle of the wire grid and the quarter wave plate, further comprising: the central position of the metal protection cylinder of the feedback optical isolator and the central position of the end face of the detector are positioned at the same horizontal height, and meanwhile, the inclined assembly angle of the wire grid and the quarter wave plate along the vertical direction is adjusted to be 1-2 degrees, so that reflected light introduced by the quarter wave plate is prevented from interfering Faraday rotation measurement.

6. A method of isolating a feedback optical isolator for a faraday polarizer according to claim 5, wherein the fourth step further comprises: by monitoring the discharge data, the minimum value A of the Faraday signal oscillation amplitude and the included angle alpha 1 between the optical axis of the quarter wave plate and the wire grid direction at this time are recorded.

7. A method of isolating a feedback optical isolator for a faraday polarizer according to claim 6, wherein the fifth step further comprises: by measuring the discharge data, the minimum value B of the Faraday signal oscillation amplitude and the included angle alpha 2 between the optical axis of the quarter wave plate and the wire grid direction are recorded.

8. A method of isolating a feedback optical isolator for a faraday polarizer according to claim 7, wherein the sixth step further comprises: when the amplitude of the oscillation interference signal reaches the minimum, determining the orientations of the wire grid and the quarter wave plate, obtaining the optimal matching position of the optical component in the feedback optical isolator, and applying the optimal matching position to subsequent discharge measurement; and tracking the plasma discharge measurement again, wherein the signal obtained in the phase analyzer is the Faraday rotation angle effective value obtained by filtering the optical interference fed back by the end face of the detector to the greatest extent.