Symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing

Abstract

The invention discloses a symmetric low-noise Sagnac interferometer based on wavelength division multiplexing, and is applied to the field of non-contact ultrasonic detection. The all-fiber non-contact ultrasonic detection light path comprises a communication waveband broadband light source, a first polarization controller, a non-polarization-maintaining circulator, a first wavelength division multiplexer, a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a second polarization controller, a non-polarization-maintaining long fiber ring, a fourth polarization beam splitter and a fifth polarization beam splitter. The device comprises a first polarization beam splitter, a sixth polarization beam splitter, a second wavelength division multiplexer, a polarization maintaining fiber focus lens, a 1 / 4 slide, a third polarization controller, a seventh polarization beam splitter and a balance detector. By combining wavelength division multiplexing, controlling multiple paths of different interference light to keep orthogonal high-symmetry design and using a balance detector to carry out differential detection, an extremely low detection noise level can be kept while pi / 2 phase bias and maximum fringe contrast are realized, and high-sensitivity non-contact ultrasonic detection is further realized.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber gyroscopes, and in particular relates to a symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing. Background Art

[0002] Sagnac interferometer is used for laser ultrasonic detection.

[0003] The photoacoustic effect was first discovered by Bell in 1880. When a beam of light is irradiated onto a light-absorbing medium, a thermoelastic effect or ablation effect occurs and ultrasound is generated. In recent years, with the continuous improvement of computer speed and data processing capabilities, laser ultrasonic imaging technology and photoacoustic imaging technology have developed rapidly. Both types of technologies require optical excitation and detection of ultrasound. Common ultrasonic detection methods include contact piezoelectric transducers, capacitive microsensors, electromagnetic ultrasonic transducers, and non-contact ultrasonic detection technology based on optical methods. In contrast, non-contact ultrasonic detection methods have some unique advantages: no coupling agent is required and will not be affected by the ultrasound transmitted in the sample to be tested; it has the potential for higher spatial resolution; it can still meet detection needs in some application scenarios where traditional methods cannot work, such as high temperature, complex surfaces, and strong electromagnetic interference.

[0004] Many studies have shown that Sagnac interferometer has great development potential in the field of non-contact ultrasonic detection. Despite this, the development of Sagnac interferometer in the field of ultrasonic detection has not been mature for a long time, and successful application cases are relatively rare. All-fiber is an important means to achieve compact, flexible, portable and compatible optical systems. However, fiber-optic Sagnac interferometers still face difficulties in polarization-induced signal fading and phase shift, which limits their further development.

[0005] The non-contact ultrasonic detection principle of a typical non-polarization-maintaining Sagnac interferometer can be found in Figure 1 , Figure 2 .

[0006] Figure 1 The most common fiber-optic Sagnac interferometer is shown in Figure 1. The most basic principle of ultrasonic detection by Sagnac interferometer is to detect the optical path difference between two beams of light that arrive at the sample surface at different times due to the ultrasonic vibration of the sample. Figure 1In the method, the light source can be a laser light source or a broadband light source. The light emitted by the light source first passes through a 2×2 3dB coupler (called the first coupler), and then outputs from the two ports on the right side of the first coupler. Since the length of one arm is usually much longer than the other arm in actual use, one arm is called the long arm and the other arm is called the short arm. The light emitted from the long and short arms enters the second 2×2 3dB coupler, and then outputs through any end on the right side of the second coupler. The light beam is then converged onto the sample through the sample probe. The light reflected or scattered by the sample surface is collected by the sample probe and enters the original light path again, and then passes through the second coupler and the first coupler in turn, and finally outputs from the lower left port of the first coupler. The output light intensity is detected by a photodetector. The change in the output light intensity can be used to measure the vibration of the sample surface.

[0007] exist Figure 1 In the entire optical path, the light beam is divided into four paths according to the path the light passes through, as shown in the attached figure. Figure 2 As shown: the first path corresponds to the path of taking the long arm twice, the second path corresponds to the path of taking the short arm twice, the third path corresponds to the path of taking the long arm first and then the short arm, and the fourth path corresponds to the path of taking the short arm first and then the long arm. Since the optical path difference between the long arm and the short arm is much larger than the coherence length of the light source, the first and second paths of light will not interfere with any remaining light beams. Interference can only occur between the third and fourth paths. If the sample is stationary, that is, there is no vibration signal of the surface undulation brought by the ultrasonic wave, then the optical path of the third and fourth paths of light is exactly the same; if there is vibration on the sample surface, since the two paths of light first arrive at the sample surface at different times, this time difference will bring additional optical path difference to the interferometer, resulting in the movement of interference fringes and changes in light intensity. The specific situation depends on the delay corresponding to the long and short arms and the amplitude and frequency of the vibration signal.

[0008] First, consider the fluctuation displacement z(t) of the sample surface caused by ultrasound, as shown in formula (1).

[0009]

[0010] Where A is the ultrasonic amplitude, ω A is the ultrasonic frequency, is the ultrasonic phase.

[0011] The time delay Δτ corresponding to the long arm and short arm of the Sagnac interferometer is shown in formula (2).

[0012]

[0013] Where n is the refractive index of the fiber medium, ΔL is the difference in length between the long and short arms, and c is the speed of light in a vacuum.

[0014] The optical path difference corresponding to this delay can be obtained from formula (1), as shown in formula (3).

[0015]

[0016] In short, in the Sagnac interferometer, ultrasound causes a change in the optical path difference, which in turn causes a change in light intensity. Detecting the change in light intensity can detect ultrasound. Summary of the invention

[0017] The purpose of the present invention is to provide a symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing, which can be applied to the field of non-contact ultrasonic detection.

[0018] Technical solution:

[0019] A symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing is applied to the field of non-contact ultrasonic detection. It is characterized in that its all-fiber non-contact ultrasonic detection optical path includes a communication band broadband light source, a first polarization controller, a non-polarization maintaining circulator, a first wavelength division multiplexer, a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a second polarization controller, a non-polarization maintaining long fiber ring, a fourth polarization beam splitter, a fifth polarization beam splitter, a sixth polarization beam splitter, a second wavelength division multiplexer, a polarization maintaining fiber focusing mirror, a 1 / 4 glass plate, a third polarization controller, a seventh polarization beam splitter, and a balanced detector, and their connection relationship is as follows:

[0020] The broadband light source, the first polarization controller, and the port I of the non-polarization-maintaining circulator are connected in sequence;

[0021] Port II of the non-polarization-maintaining circulator is connected to the first wavelength division multiplexer;

[0022] The first wavelength division multiplexer is connected in two ways, respectively connected to the first polarization beam splitter and the second polarization beam splitter;

[0023] The first polarization beam splitter is divided into two paths, one of which is directly connected to the fifth polarization beam splitter, and the other is connected to the fifth polarization beam splitter after passing through the third polarization beam splitter, the second polarization controller, the non-polarization-maintaining long fiber ring, and the fourth polarization beam splitter in sequence:

[0024] The second polarization beam splitter is divided into two paths, one of which is directly connected to the sixth polarization beam splitter, and the other is connected to the sixth polarization beam splitter after passing through the third polarization beam splitter, the second polarization controller, the non-polarization-maintaining long fiber ring, and the fourth polarization beam splitter in sequence:

[0025] The fifth polarization beam splitter and the sixth polarization beam splitter are respectively connected to the second wavelength division multiplexer; the second wavelength division multiplexer is connected to the polarization-maintaining fiber focusing mirror, and the 1 / 4 glass slide is built into the polarization-maintaining fiber focusing mirror;

[0026] The port III of the non-polarization-maintaining circulator is connected to the third polarization controller, and the third polarization controller, the seventh polarization beam splitter, and the balanced detector are connected in sequence.

[0027] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0028] The present invention is based on the application background of wavelength division multiplexing symmetrical low-noise Sagnac interferometer in non-contact ultrasonic detection. By combining wavelength division multiplexing, controlling the high symmetry design of multiple different interference lights to maintain orthogonality, and using a balanced detector for differential detection, it can achieve π / 2 phase bias and maximum fringe contrast while maintaining an extremely low noise level, thereby achieving high-sensitivity non-contact ultrasonic detection. Broaden the scope of application and application scenarios of non-polarization-maintaining Sagnac interferometers (i.e., the long fiber loop in the optical path is a non-polarization-maintaining fiber). Specifically:

[0029] First, the optical path design is highly symmetrical, which theoretically proves that the output signal of the Sagnac interferometer of the present invention has a stable π / 2 phase bias. By adjusting several polarization controllers in the optical path, the maximum interference fringe contrast can be achieved at the same time, thereby obtaining the most ideal interference signal in theory;

[0030] Second, the optical path combines the wavelength division multiplexing method and uses a symmetrical design of multiple polarization beam splitters in the optical path, so that the utilization rate of the interference light intensity can theoretically reach 100%;

[0031] Third, using a balanced detector to detect the differential light intensity signals in the two orthogonal polarization directions of the interferometer can not only amplify the signal by two times, but also suppress common mode noise including light intensity noise, achieve high signal and low noise output, and thus obtain high signal-to-noise ratio and sensitivity;

[0032] Fourth, the main part of the Sagnac interferometer (non-polarization-maintaining long fiber ring) uses single-mode optical fiber, which greatly reduces the application cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 : Schematic diagram of the traditional, simple and typical fiber-optic Sagnac optical path.

[0034] Figure 2 : Figure 1 There are four beam paths in the optical path.

[0035] Figure 3 : Schematic diagram of the structure of a symmetrical low-noise Sagnac interferometer system based on wavelength division multiplexing in Example 1 of the present invention (open loop). DETAILED DESCRIPTION

[0036] The present invention discloses a symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing. According to the order of the light beam passing through the optical path after starting from the light source, it includes a broadband light source of a polarized communication band, a first polarization controller, a non-polarization-maintaining circulator, a first wavelength division multiplexer, a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a second polarization controller, a non-polarization-maintaining long fiber ring, a fourth polarization beam splitter, a fifth polarization beam splitter, a sixth polarization beam splitter, a second wavelength division multiplexer, a polarization-maintaining fiber focusing lens, a 1 / 4 glass plate, a third polarization controller, a seventh polarization beam splitter, and a balanced detector. Their connection relationship is as follows:

[0037] The broadband light source, the first polarization controller, and the port I of the non-polarization-maintaining circulator are connected in sequence;

[0038] Port II of the non-polarization-maintaining circulator is connected to the first wavelength division multiplexer;

[0039] The first wavelength division multiplexer is connected in two ways, respectively connected to the first polarization beam splitter and the second polarization beam splitter;

[0040] The first polarization beam splitter is divided into two paths, one of which is directly connected to the fifth polarization beam splitter, and the other is connected to the fifth polarization beam splitter after passing through the third polarization beam splitter, the second polarization controller, the non-polarization-maintaining long fiber ring, and the fourth polarization beam splitter in sequence:

[0041] The second polarization beam splitter is divided into two paths, one of which is directly connected to the sixth polarization beam splitter, and the other is connected to the sixth polarization beam splitter after passing through the third polarization beam splitter, the second polarization controller, the non-polarization-maintaining long fiber ring, and the fourth polarization beam splitter in sequence:

[0042] The fifth polarization beam splitter and the sixth polarization beam splitter are respectively connected to the second wavelength division multiplexer; the second wavelength division multiplexer is connected to the polarization-maintaining fiber focusing mirror, and the 1 / 4 glass slide is built into the polarization-maintaining fiber focusing mirror to form a whole as a sample probe;

[0043] The port III of the non-polarization-maintaining circulator is connected to the third polarization controller, and the third polarization controller, the seventh polarization beam splitter, and the balanced detector are connected in sequence.

[0044] Specifically, the broadband light source and the first polarization controller, the first polarization controller and the non-polarization maintaining circulator, the third polarization beam splitter and the second polarization controller, the second polarization controller and the non-polarization maintaining long fiber ring, the non-polarization maintaining long fiber ring and the fourth polarization beam splitter, the non-polarization maintaining circulator and the third polarization controller, the third polarization controller and the seventh polarization beam splitter are connected using single-mode optical fiber;

[0045] Specifically, the first wavelength division multiplexer and the first polarization beam splitter, the first wavelength division multiplexer and the second polarization beam splitter, the first polarization beam splitter and the fifth polarization beam splitter, the second polarization beam splitter and the sixth polarization beam splitter, the fourth polarization beam splitter and the fifth polarization beam splitter, the fourth polarization beam splitter and the sixth polarization beam splitter, the third polarization beam splitter and the first polarization beam splitter, the third polarization beam splitter and the second polarization beam splitter, the second wavelength division multiplexer and the fifth polarization beam splitter, the second wavelength division multiplexer and the sixth polarization beam splitter, the second wavelength division multiplexer and the polarization-maintaining fiber focusing mirror, and the connections therebetween use polarization-maintaining fiber.

[0046] Furthermore, the first polarization beam splitter, the second polarization beam splitter, the third polarization beam splitter, the fourth polarization beam splitter, the fifth polarization beam splitter, and the sixth polarization beam splitter are polarization-maintaining polarization beam splitters, and the seventh polarization beam splitter is a polarization-maintaining or non-polarization-maintaining polarization beam splitter.

[0047] The polarized light beam starts from the broadband light source, passes through the first polarization controller and the circulator, and reaches the first wavelength division multiplexer to be divided into two beams of light λ1 and λ2 with different wavelengths. Then λ1 and λ2 reach the first polarization beam splitter and the second polarization beam splitter respectively. By adjusting the first polarization controller, λ1 and λ2 are again divided into two beams of light with orthogonal polarization states and equal light intensities after passing through the first polarization beam splitter and the second polarization beam splitter respectively. At this time, there are four beams of light in the optical path, namely, the p light and s light of λ1, and the p light and s light of λ2. After passing through their respective corresponding optical paths, these four beams of light are combined into two beams through the fifth polarization beam splitter or the sixth polarization beam splitter, and then combined into one beam through the second wavelength division multiplexer. The combined light is emitted through the sample probe containing a 1 / 4 glass slide and then focused to the surface of the sample to be tested. The sample probe collects the reflected light from the surface of the sample to be tested and returns it to the original optical fiber path again. Since the sample probe has a built-in 1 / 4 glass slide, the polarization state of the reflected light is interchanged with the polarization state of the incident light and remains orthogonal. Similar to the incident light, the reflected light whose polarization state is orthogonal to the incident light returns to the optical fiber path and passes through the second wavelength division multiplexer, the fifth polarization beam splitter, and the sixth polarization beam splitter in turn to be divided into four beams again. These four beams pass through the corresponding optical path and pass through the first polarization beam splitter, the second polarization beam splitter, and the first wavelength division multiplexer to synthesize one beam. Then it is emitted from port III of the non-polarization-maintaining circulator, and after passing through the third polarization controller, it is divided into two beams with orthogonal polarization states by the polarization-maintaining or non-polarization-maintaining seventh polarization beam splitter, and its differential light intensity is detected by a balanced detector.

[0048] The various optical devices described in the above technical solutions are existing technologies and products. Their functions are introduced as follows:

[0049] ① Broadband light source: In order to avoid interference of reflected light in optical devices, a light source with a certain spectral width is selected.

[0050] ② Polarization controller. The strain in the optical fiber will change the polarization state of the light transmitted in the optical fiber. The polarization controller can apply stress to the optical fiber to generate strain that can be maintained for a period of time.

[0051] ③Wavelength division multiplexer. Separates two or more frequency bands of light from the input light.

[0052] ④ Polarization beam splitter. It can split the input light into two perpendicularly polarized lights. It can also couple the two beams together along the perpendicular polarization directions.

[0053] ⑤ Non-polarization-maintaining long optical fiber ring. Made of single-mode optical fiber.

[0054] ⑥Fiber optic focusing lens. It focuses the light in the fiber to a certain point in space. It also collects light in space into the fiber.

[0055] ⑦Circulator. It has three ports: I, II, and III. Light entering port I will exit from port II, and light entering port II will exit from port III. Port III cannot be used for light input.

[0056] ⑧ Balanced detector. It can be regarded as composed of two identical optical probes. The balanced detector can output the differential signal of the two optical probes.

[0057] ⑨Single-mode fiber and polarization-maintaining fiber. Single-mode fiber is an ordinary optical fiber that only allows one mode of light to be transmitted. However, the strain of single-mode fiber will affect the polarization state of the transmitted light. Polarization-maintaining fiber is a special single-mode fiber. Due to the special structural design inside the polarization-maintaining fiber, the polarization state of the light in it can be maintained. Due to the special structure of the polarization-maintaining fiber, its price is thousands of times that of single-mode fiber.

[0058] ⑩ 1 / 4 glass slide built into the sample probe: the polarization state of the reflected light is interchanged with the polarization state of the incident light and remains orthogonal.

[0059] ⑾ Light source: a polarized communication band broadband light source, including but not limited to common broadband light sources such as LED, SLD, and SLED.

[0060] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings.

[0061] The present invention proposes a symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing, the principle of which is:

[0062] The light source uses a broadband light source. After the light source is polarized, it passes through the first polarization controller, the circulator and the first wavelength division multiplexer in sequence, and the overall optical path is divided into an upper half (λ1) and a lower half (λ2).

[0063] First, let's look at the upper part of the optical path. After passing through the first polarization beam splitter, λ1 is divided into two beams of orthogonal polarized light with equal light intensity, which are recorded as p-light and s-light of λ1. The p-light of λ1 goes through the short arm first, and the s-light goes through the long arm first. Then, it passes through the fifth polarization beam splitter to combine the p-light and s-light, and then reaches the second wavelength division multiplexer to combine with λ2, and then reaches the sample probe. The light emitted by the sample probe hits the sample, and the backscattered light reflected by the surface of the sample probe is collected and returned to the original optical fiber path. Since the sample probe has a 1 / 4 glass slide placed at 45 degrees, one forward pass and one reverse pass through the 1 / 4 glass slide is equivalent to passing through a 1 / 2 glass slide, so the polarization state of the p-light and s-light will be interchanged here, that is, the original p-light becomes s-light, and the original s-light becomes p-light. After the polarization states are interchanged, the p-light of λ1 that originally traveled along the short arm will become s-light when returning and travel along the long arm, and the s-light of λ1 will become p-light when returning and travel along the long arm. The p-light and s-light of λ1 remain orthogonal along the way, and then pass through the first wavelength division multiplexer in turn to be combined with λ2, and then pass through the third polarization controller and the seventh polarization beam splitter for polarization splitting and use a balanced detector for differential detection.

[0064] Looking at the lower part of the optical path, in principle, the wavelengths selected for λ1 and λ2 should be very close, so the polarization state change process of λ2 is approximately considered to be synchronized with λ1. Therefore, after passing through the second polarization beam splitter, λ2 is divided into two beams of orthogonal polarized light with equal light intensity, which are recorded as p-light and s-light of λ2 respectively. The s-light of λ2 first goes through the short arm, and the p-light first goes through the long arm, and then passes through the sixth polarization beam splitter to combine the p-light and s-light and then reaches the second wavelength division multiplexer to combine with λ1, and then reaches the sample probe. The light emitted by the sample probe hits the sample, and the backscattered light reflected by the surface of the sample probe is collected and returned to the original optical fiber path. Similar to λ1, the p-light and s-light exchange occurs here in λ2. After the polarization state is exchanged, the s-light of λ2 that originally went through the short arm will become p-light and go through the long arm when it returns, and the p-light of λ2 will become s-light and go through the long arm when it returns. The p-light and s-light of λ2 remain orthogonal along the way, and then pass through the first wavelength division multiplexer in turn to be combined with λ1, undergo polarization splitting through the third polarization controller and the seventh polarization beam splitter, and are differentially detected using a balanced detector.

[0065] Figure 3 In the present invention, the design of the shared optical path of the polarizing and analyzing non-polarization maintaining long fiber ring composed of the third polarization beam splitter and the fourth polarization beam splitter allows the non-polarization maintaining long fiber ring to use only ordinary single-mode optical fiber with almost negligible cost instead of polarization maintaining optical fiber, thereby greatly reducing the cost and improving the compatibility of the system.

[0066] Figure 3 The all-fiber optical path system shown needs to find its optimal working point by adjusting the three polarization controllers to achieve the optimal performance of the interferometer. The specific steps are as follows:

[0067] 1. Maintain equal light intensities of λ1 and λ2 by attenuating light intensity or taking other light intensity control measures, and then adjust the first polarization controller so that λ1 and λ2 can simultaneously achieve equal light intensity distribution in both the p and s polarization directions after passing through the first polarization beam splitter and the second polarization beam splitter.

[0068] 2. Adjust the second polarization controller so that the λ1 s-light and λ2 p-light in front of the third polarization beam splitter can re-obtain a new set of s-light and p-light after passing through the third polarization beam splitter, the second polarization controller 2 and the non-polarization-maintaining long fiber ring in sequence and then polarization splitting by the fourth polarization beam splitter, wherein all the s-light emitted from the fourth polarization beam splitter is λ1, and all the p-light emitted is λ2. That is, by adjusting the second polarization controller, λ1 and λ2 can still maintain non-crosstalk after re-split through the common path of the non-polarization-maintaining long fiber ring. For the return path propagating in the opposite direction, this non-crosstalk property still holds true.

[0069] 3. Figure 3 The interference light field emitted from port III of the circulator shown includes four beams of light, namely, the light along the clockwise path and the light along the counterclockwise path of λ1, and the light along the clockwise path and the light along the counterclockwise path of λ2. Since λ1 and λ2 will not interfere, interference only occurs between the lights along the clockwise path and the counterclockwise path of λ1 and λ2 respectively. Since the lights along the clockwise path and the counterclockwise path of λ1 and λ2 respectively maintain orthogonal polarization states and λ1 and λ2 also maintain orthogonal polarization states to each other, the third polarization controller in front of the seventh polarization beam splitter can be adjusted so that the two beams of orthogonal polarized light that have passed through the polarization beam splitting can simultaneously obtain 100% interference efficiency, that is, obtain the most ideal interference signal.

[0070] Based on the application background of Sagnac interferometer in non-contact ultrasonic detection, by combining wavelength division multiplexing, controlling the high symmetry design of multiple different interference lights to maintain orthogonality and using balanced detectors for differential detection, it is possible to achieve π / 2 phase bias and maximum fringe contrast while maintaining an extremely low noise level, thereby achieving high-sensitivity non-contact ultrasonic detection.

[0071] The above description is only a description of the preferred embodiments of the present application, and is not intended to limit the scope of the present application. Any changes or modifications made by any person skilled in the art based on the above disclosed technical contents shall be deemed as equivalent effective embodiments and shall fall within the scope of protection of the technical solution of the present application.

Claims

1. A symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing, applied in the field of non-contact ultrasonic detection, characterized in that: The all-fiber non-contact ultrasonic detection optical path includes: a communication band broadband light source, a first polarization controller, a non-polarization-maintaining circulator, a first wavelength division multiplexer, a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a second polarization controller, a non-polarization-maintaining long fiber ring, a fourth polarization beam splitter, a fifth polarization beam splitter, a sixth polarization beam splitter, a second wavelength division multiplexer, a polarization-maintaining fiber focusing mirror, a 1 / 4 glass plate, a third polarization controller, a seventh polarization beam splitter, and a balanced detector. The connection relationship between them is as follows: The broadband light source, the first polarization controller, and the port I of the non-polarization-maintaining circulator are connected in sequence; Port II of the non-polarization-maintaining circulator is connected to the first wavelength division multiplexer; The first wavelength division multiplexer is connected in two ways, respectively connected to the first polarization beam splitter and the second polarization beam splitter; The first polarization beam splitter is divided into two paths, one of which is directly connected to the fifth polarization beam splitter, and the other is connected to the fifth polarization beam splitter after passing through the third polarization beam splitter, the second polarization controller, the non-polarization-maintaining long fiber ring, and the fourth polarization beam splitter in sequence: The second polarization beam splitter is divided into two paths, one of which is directly connected to the sixth polarization beam splitter, and the other is connected to the sixth polarization beam splitter after passing through the third polarization beam splitter, the second polarization controller, the non-polarization-maintaining long fiber ring, and the fourth polarization beam splitter in sequence: The fifth polarization beam splitter and the sixth polarization beam splitter are respectively connected to the second wavelength division multiplexer; the second wavelength division multiplexer is connected to the polarization-maintaining fiber focusing mirror, and the 1 / 4 glass slide is built into the polarization-maintaining fiber focusing mirror; The port III of the non-polarization-maintaining circulator is connected to the third polarization controller, and the third polarization controller, the seventh polarization beam splitter, and the balanced detector are connected in sequence.

2. A symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing according to claim 1, characterized in that: The broadband light source and the first polarization controller, the first polarization controller and the non-polarization maintaining circulator, the third polarization beam splitter and the second polarization controller, the second polarization controller and the non-polarization maintaining long fiber ring, the non-polarization maintaining long fiber ring and the fourth polarization beam splitter, the non-polarization maintaining circulator and the third polarization controller, the third polarization controller and the seventh polarization beam splitter are connected using single-mode optical fiber; The first wavelength division multiplexer and the first polarization beam splitter, the first wavelength division multiplexer and the second polarization beam splitter, the first polarization beam splitter and the fifth polarization beam splitter, the second polarization beam splitter and the sixth polarization beam splitter, the fourth polarization beam splitter and the fifth polarization beam splitter, the fourth polarization beam splitter and the sixth polarization beam splitter, the third polarization beam splitter and the first polarization beam splitter, the third polarization beam splitter and the second polarization beam splitter, the second wavelength division multiplexer and the fifth polarization beam splitter, the second wavelength division multiplexer and the sixth polarization beam splitter, the second wavelength division multiplexer and the polarization-maintaining fiber focusing mirror, and polarization-maintaining fiber is used for connection between them.

3. The symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing according to claim 1, characterized in that: The light source is a polarized communication band broadband light source, including but not limited to LED, SLD, and SLED broadband light source.

4. The symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing according to claim 1, characterized in that: A polarized light beam starts from a broadband light source, passes through the first polarization controller and the circulator, and reaches the first wavelength division multiplexer to be divided into two beams of light λ1 and λ2 with different wavelengths. Then λ1 and λ2 reach the first polarization beam splitter and the second polarization beam splitter respectively. By adjusting the first polarization controller, λ1 and λ2 are again divided into two beams of light with orthogonal polarization states and equal light intensities after passing through the first polarization beam splitter and the second polarization beam splitter respectively.

5. The symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing according to claim 1, characterized in that: There are four light beams in the optical path, namely, p-light and s-light of λ1, and p-light and s-light of λ2; these four light beams pass through their corresponding optical paths and are combined into two beams by the fifth polarization beam splitter or the sixth polarization beam splitter, and then are combined into one beam by the second wavelength division multiplexer. The combined light is emitted through a polarization-maintaining fiber focusing mirror with a built-in 1 / 4 glass slide, i.e., a sample probe, and then focused to the surface of the sample to be tested. The sample probe collects the reflected light from the surface of the sample to be tested and returns it to the original optical fiber path again.

6. The symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing according to claim 1, characterized in that: The 1 / 4 glass plate built into the polarization-maintaining fiber focusing mirror allows the polarization state of the reflected light to be interchanged with the polarization state of the incident light and remain orthogonal.

7. The symmetrical low-noise Sagnac interferometer based on wavelength division multiplexing according to claim 1, characterized in that: The reflected light whose polarization state is orthogonal to the incident light returns to the optical fiber optical path and passes through the second wavelength division multiplexer, the fifth polarization beam splitter, and the sixth polarization beam splitter in sequence to be divided into four beams again. These four beams pass through the corresponding optical path and are combined into one beam through the first polarization beam splitter, the second polarization beam splitter, and the first wavelength division multiplexer; then they are emitted from the III port of the non-polarization-maintaining circulator, and after passing through the third polarization controller, they are divided into two beams with orthogonal polarization states by the seventh polarization beam splitter, and their differential light intensity is detected by a balanced detector.

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