Optical frequency domain reflectometry

Abstract

This invention discloses an optical frequency domain reflectometer, comprising a beam splitting module, a main interference module, and an auxiliary interference module. The beam splitting module splits a swept laser provided by a laser into a first detection signal and a first reference signal. The auxiliary interference module generates a reference interference signal based on the first reference signal. The main interference module includes a free-space optical circulator and a first beam splitting unit and a first coupling unit, all of which are free-space optical devices. In this embodiment, the main interference module uses only free-space optical devices, fundamentally eliminating the inherent insertion loss accumulation and Fresnel reflection array of cascaded fiber optic devices. This significantly improves the coherent mixing signal-to-noise ratio and measurement repeatability while eliminating the need for pigtail interconnects and adhesive sealing nodes, thereby reducing the overall size and enhancing vibration resistance, ultimately meeting the deployment requirements of engineering and uncontrolled scenarios.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and in particular to an optical frequency domain reflectometer. Background Technology

[0002] Optical frequency domain reflectometers (OFDRs), based on linearly swept frequency light sources and coherent detection principles, can achieve millimeter-level resolution distributed fiber optic sensing. Existing OFDR systems mostly construct optical paths using discrete fiber components (splitters, circulators, couplers), introducing the following drawbacks in optical transmission: First, insertion loss accumulates, as the probe light and backscattered Rayleigh signal repeatedly pass through various devices, resulting in significant loss aggregation that severely weakens coherent mixing power, limiting sensing distance and dynamic range. Second, residual reflections occur at the pigtail ends, fusion splices, and connectors of various devices, forming parasitic interference cavities with extremely short optical path differences from the main signal. This creates dense parasitic peaks near the zero frequency of the beat frequency spectrum, compressing the near-end dead zone and raising the noise floor. Simultaneously, the reflected light coherently interacts with the local oscillator light, generating spurious correlation peaks that interfere with the demodulation of the true signal. These drawbacks force the system to rely on high-power light sources and complex calibration algorithms to maintain performance, resulting in large overall size and high cost. Furthermore, the system is severely constrained by laboratory environment debugging and polarization maintenance, failing to meet the requirements for engineering, compact design, and deployment in uncontrolled scenarios. Summary of the Invention

[0003] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes an optical frequency domain reflectometer that avoids the influence of fiber optic devices in optical frequency domain reflectometers and meets the requirements for engineering and compact deployment.

[0004] According to a first aspect of the present invention, an optical frequency domain reflectometer includes a beam splitting module, a main interference module, and an auxiliary interference module, wherein the beam splitting module is used to split a swept laser into a first detection signal and a first reference signal; The auxiliary interference module is used to generate a reference interference signal based on the first reference signal; The main interference module includes a first beam splitting unit, a free-space optical circulator, and a first coupling unit; the first beam splitting unit is used to split the first detection signal into a second detection signal and a second reference signal; the free-space optical circulator is used to transmit the second detection signal to the detection optical fiber and acquire the Rayleigh backscattered signal fed back by the detection optical fiber; the first coupling unit is used to generate a detection interference signal based on the second reference signal and the Rayleigh backscattered signal; both the first beam splitting unit and the first coupling unit are composed of free-space optical devices.

[0005] The embodiments of the present invention have at least the following beneficial effects: The optical frequency domain reflectometer provided in the embodiments of the present invention uses free-space optical devices for its first beam splitting unit, free-space optical circulator, and first coupling unit in the main interference module, forming a full-space optical path structure. This scheme splits the first probe signal according to a first power ratio through the first beam splitting unit, completes the probe light transmission and Rayleigh backscattered signal recovery through the free-space optical circulator, and achieves spatial coherent mixing of the reference light and the signal light through the first coupling unit. The main interference module of the embodiments of the present invention uses all free-space optical devices, fundamentally eliminating the inherent insertion loss accumulation and Fresnel reflection array of cascaded fiber optic devices. This significantly improves the coherent mixing signal-to-noise ratio and measurement repeatability, while eliminating the need for pigtail interconnects and adhesive sealing nodes, thereby achieving a reduction in overall size and enhanced vibration resistance, ultimately meeting the deployment requirements of engineering and uncontrolled scenarios.

[0006] According to some embodiments of the present invention, the first beam splitting unit includes a first waveplate and a first polarization beam splitter; the first detection signal is incident on the first polarization beam splitter through the first waveplate; the angle between the optical axis of the first waveplate and the projection of the polarization direction of the first detection signal onto the same plane is a first preset angle.

[0007] According to some embodiments of the present invention, the first coupling unit includes a first reflector, a second polarization beamsplitter, a first half-wave plate, and a third polarization beamsplitter; the first reflector is used to reflect the second reference signal into the second polarization beamsplitter; the second polarization beamsplitter is used to combine the second reference signal and the Rayleigh backscattered signal; the first half-wave plate is disposed between the output end of the second polarization beamsplitter and the input end of the third polarization beamsplitter; the first half-wave plate is used to deflect the polarization directions of the second reference signal and the Rayleigh backscattered signal, so that the second reference signal and the Rayleigh backscattered signal interfere in the third polarization beamsplitter to obtain the detection interference signal.

[0008] According to some embodiments of the present invention, the first coupling unit includes a second reflector, a second half-wave plate, a first unpolarized beam splitter, a fourth polarized beam splitter, and a fifth polarized beam splitter; the second reflector is used to reflect the second reference signal to the first unpolarized beam splitter, and the second reference signal passes through the second half-wave plate during the reflection transmission; the first unpolarized beam splitter is used to split the second reference signal and the Rayleigh backscattered signal incident on the first unpolarized beam splitter according to a fixed ratio; the fourth polarized beam splitter and the fifth polarized beam splitter are respectively disposed at the two output ends of the first unpolarized beam splitter, and both the fourth polarized beam splitter and the fifth polarized beam splitter are used to cause interference of the output signal of the first unpolarized beam splitter to obtain the detection interference signal.

[0009] According to some embodiments of the present invention, the beam splitting module includes a second waveplate and a sixth polarization beam splitter; the second waveplate is disposed at the incident end of the sixth polarization beam splitter, and the angle between the optical axis of the second waveplate and the projection of the polarization direction of the swept laser onto the same plane is a second preset angle.

[0010] According to some embodiments of the present invention, the auxiliary interference module includes a second beam splitting unit, an optical fiber delay loop, and a second coupling unit; the second beam splitting unit is used to split the first reference signal into a first horizontal signal and a first vertical signal, and the second beam splitting unit includes a third waveplate and a seventh polarization beam splitter; the third waveplate is disposed at the incident end of the seventh polarization beam splitter, and the angle between the optical axis of the third waveplate and the projection of the polarization direction of the first reference signal onto the same flat plate is a third preset angle; the optical fiber delay loop is used to generate an optical path difference between the first horizontal signal and the first vertical signal; the second coupling unit is used to generate a reference interference signal based on the first horizontal signal and the first vertical signal having the optical path difference.

[0011] According to some embodiments of the present invention, the second coupling unit includes an eighth polarization beamsplitter, a third half-wave plate, and a ninth polarization beamsplitter; the eighth polarization beamsplitter is used to combine the first horizontal signal and the first vertical signal having an optical path difference; the third half-wave plate is disposed between the output end of the eighth polarization beamsplitter and the input end of the ninth polarization beamsplitter; the third half-wave plate is used to deflect the polarization directions of the first horizontal signal and the first vertical signal, so that the first horizontal signal and the first vertical signal having an optical path difference interfere in the ninth polarization beamsplitter to obtain the reference interference signal.

[0012] According to some embodiments of the present invention, the second coupling unit includes a fourth half-wave plate and a second unpolarized beam splitter; the first horizontal signal emitted from the seventh polarized beam splitter is incident on the second unpolarized beam splitter via the fourth half-wave plate; the second unpolarized beam splitter is used to cause the first vertical signal and the first horizontal signal to interfere to generate a reference interference signal.

[0013] According to some embodiments of the present invention, the optical frequency domain reflectometer further includes a first photoelectric conversion unit and a second photoelectric conversion unit; the first photoelectric conversion unit is used to convert the detection interference signal into a detection electrical signal; and the second photoelectric conversion unit is used to convert the reference interference signal into a reference electrical signal.

[0014] According to some embodiments of the present invention, the beam splitting module, the main interference module, the auxiliary interference module, the first photoelectric conversion unit and the second photoelectric conversion unit are all disposed inside the packaging shell; the packaging shell is also provided with a TEC temperature control module for regulating the temperature inside the packaging shell.

[0015] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0016] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a structural diagram of the first embodiment of the optical frequency domain reflectometer of the present invention; Figure 2 This is a schematic diagram showing the positional relationship between the first waveplate and the first polarizing beam splitter in an embodiment of the present invention; Figure 3 This is a structural diagram of a second embodiment of the optical frequency domain reflectometer of the present invention.

[0017] Reference numerals: 1. First waveplate; 2. First polarization beamsplitter; 3. Free-space optical circulator; 4. Second polarization beamsplitter; 5. First half-waveplate; 6. Third polarization beamsplitter; 7. Second reflector; 8. Second half-waveplate; 9. First unpolarization beamsplitter; 10. Fourth polarization beamsplitter; 11. Fifth polarization beamsplitter; 12. Second waveplate; 13. Sixth polarization beamsplitter; 14. Third waveplate; 15. Seventh polarization beamsplitter; 16. Eighth polarization beamsplitter; 17. Third half-waveplate; 18. Ninth polarization beamsplitter; 19. Fourth half-waveplate; 20. Second unpolarization beamsplitter; 21. Fiber delay ring; 22. First collimating lens; 23. Second collimating lens; 24. Third collimating lens; 25. Fourth collimating lens; 26. First photodetector; 27. Second photodetector; 28. Third photodetector; 29. ​​Fourth photodetector; 30. Fifth photodetector; 31. Encapsulation housing; 32. Detailed Implementation

[0018] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0019] In the description of this invention, "several" means one or more, "multiple" means two or more, "greater than," "less than," "exceeding," etc. are understood to exclude the stated number, and "above," "below," "within," etc. are understood to include the stated number. If "first," "second," etc. are used in the description, they are only for the purpose of distinguishing technical features and should not be construed as indicating or implying relative importance or implicitly indicating the number of indicated technical features or the order of the indicated technical features.

[0020] The concepts involved in this invention are explained below: OFDR: Optical Frequency Domain Reflectometry, is an optical diagnostic technique based on the principle of linear frequency modulated laser interferometry and coherent detection. It achieves high-resolution, distributed measurement of fiber optic links by analyzing the frequency domain signal of backscattered Rayleigh light. Free-space optical devices: These are devices that allow light beams to propagate in air or vacuum and control the light field through discrete optical elements, as opposed to devices that constrain propagation via fiber optic waveguides. PBS: Polarization beam splitter, which uses thin-film interference or birefringence to separate incident light according to polarization state. It usually allows p-polarized light to be fully transmitted and s-polarized light to be highly reflected, thereby achieving beam separation and beam combining. NPBS: Non-polarized beam splitter, used to split incident light into two beams, transmitted and reflected, in a fixed ratio. Its key characteristic is that the splitting ratio is not affected by the polarization state of the incident light and can maintain the polarization state of the output light. Birefringent crystal polarization beam splitter: Utilizes the birefringence effect of crystals (such as Glan-Taylor prisms, Glan-Thompson prisms, Wollaston prisms, etc.) to separate o-rays and e-rays in space; Free-space optical circulator: It is a non-reciprocal optical element that uses a combination of a Faraday rotator and a polarization beam splitter to enable the optical signal to circulate unidirectionally along the port sequence (e.g., 1→2→3). Moreover, this device does not involve optical fibers and the optical path propagates entirely in the air. Frequency-sweeping laser: refers to a single-frequency tunable laser source whose output light frequency is continuously and linearly tuned over time. It is mainly used for optical coherent detection and high-resolution distributed sensing. Rayleigh backscatter signal: refers to a portion of the optical field that is continuously weakly reflected when incident light travels in an optical fiber and encounters random fluctuations in the refractive index of the fiber core, and returns along the original path. In OFDR, it is used as a distributed probe signal to demodulate strain and temperature changes at various locations along the optical fiber. Beat frequency interference: In OFDR systems, beat frequency interference refers to the instantaneous frequency difference between the swept local oscillator light and the backscattered signal light at various positions in the optical fiber. The coherent superposition of the two generates a low-frequency beat frequency signal with a frequency proportional to the position of the scattering point, thereby achieving distance-intensity demodulation through spectrum analysis. Waveplate: An optical element made of birefringent crystal or liquid crystal polymer. It uses the difference in the propagation speed of o-ray and e-ray in the fast and slow axis directions to produce a fixed phase delay. Common types include λ / 2 waveplate, λ / 4 waveplate and full waveplate.

[0021] Optical frequency domain reflectometers (OFDRs), based on linearly swept frequency light sources and coherent detection principles, can achieve millimeter-level resolution distributed fiber optic sensing. Existing OFDR systems mostly construct optical paths using discrete fiber components (splitters, circulators, couplers), introducing the following drawbacks in optical transmission: First, insertion loss accumulates, as the probe light and backscattered Rayleigh signal repeatedly pass through various devices, resulting in significant loss aggregation that severely weakens coherent mixing power, limiting sensing distance and dynamic range. Second, residual reflections occur at the pigtail ends, fusion splices, and connectors of various devices, forming parasitic interference cavities with extremely short optical path differences from the main signal. This creates dense parasitic peaks near the zero frequency of the beat frequency spectrum, compressing the near-end dead zone and raising the noise floor. Simultaneously, the reflected light coherently interacts with the local oscillator light, generating spurious correlation peaks that interfere with the demodulation of the true signal. These drawbacks force the system to rely on high-power light sources and complex calibration algorithms to maintain performance, resulting in large overall size and high cost. Furthermore, the system is severely constrained by laboratory environment debugging and polarization maintenance, failing to meet the requirements for engineering, compact design, and deployment in uncontrolled scenarios.

[0022] Please refer to Figures 1-3 , Figure 1 This is a structural diagram of an optical frequency domain reflectometer according to an embodiment of the present invention. This embodiment discloses an optical frequency domain reflectometer, including a beam splitting module, a main interference module, and an auxiliary interference module. The beam splitter module is used to split the swept laser into a first detection signal and a first reference signal; The auxiliary interference module is used to generate a reference interference signal based on the first reference signal; The main interference module includes a first beam splitting unit, a free-space optical circulator 3, and a first coupling unit. The first beam splitting unit is used to split the first detection signal into a second detection signal and a second reference signal. The free-space optical circulator 3 is used to transmit the second detection signal to the detection fiber and acquire the Rayleigh backscattered signal fed back by the detection fiber. The first coupling unit is used to generate a detection interference signal based on the second reference signal and the Rayleigh backscattered signal. Both the first beam splitting unit and the first coupling unit are composed of free-space optical devices.

[0023] It should be noted that the optical frequency domain reflectometer provided in this embodiment of the invention includes a beam splitting module, a main interference module, and an auxiliary interference module. The devices in the main interference module are all constructed using free-space optical devices. Therefore, in the main interference module of this embodiment, the first beam splitting unit, the free-space optical circulator 3, and the first coupling unit all transmit signals through free-space light in the air. The complete signal transmission process of this invention is as follows: First, a swept-frequency laser is provided by a laser seed integrated within the optical frequency domain reflectometer or by an externally connected laser. Then, the beam splitting module divides the swept-frequency laser into a first detection signal and a first reference signal. Subsequently, based on the optical path design of this embodiment, the first detection signal is transmitted to the main interference module to generate a detection interference signal, and the first reference signal is transmitted to the auxiliary interference module to generate a reference interference signal. Further, the first beam splitting unit of the main interference module, through a free-space optical device, divides the first detection signal into a second reference signal and a second detection signal according to a first power ratio. In this process, the second detection signal is transmitted to the free-space optical circulator 3. The free-space optical circulator 3 then transmits the second detection signal to the detection fiber coupled to it in a fixed sequence, and then obtains the Rayleigh backscattered signal from the detection fiber and transmits it to the first coupling unit. Finally, the first coupling unit uses free-space optical devices, such as waveplates and polarization beam splitters, to make the second reference signal and the Rayleigh backscattered signal beat frequency interfere, thereby obtaining an interference beat frequency signal that can achieve distance-intensity demodulation.

[0024] In one embodiment of the present invention, the first end of the free-space optical circulator 3 is disposed at the reflected light output end of the first polarization beam splitter 2, that is, on the side of the first polarization beam splitter 2 from which the second detection signal is emitted. The second detection signal enters from the first end of the free-space optical circulator 3 and exits from the second end of the free-space optical circulator 3 according to the optical path sequence, into the first collimating lens 23 disposed at the second end of the free-space optical circulator 3. It is understood that during the assembly process of the optical frequency domain reflectometer, the first collimating lens 23 needs to be spatially aligned with the detection fiber to be connected so that the second detection signal can be focused into the detection fiber during the measurement process. During the propagation of the second detection signal in the detection fiber, a Rayleigh backscattered signal carrying deformation-related information of the detection fiber is generated. This signal will be transmitted back along the detection fiber to the first collimating lens 23. At this time, the first collimating lens 23 will transmit the Rayleigh backscattered signal fed back from the detection fiber to the second end of the free-space optical circulator. Due to the non-reciprocity of the free-space optical circulator, the Rayleigh backscattered signal will be transmitted to the third end of the free-space optical circulator and exit from that port.

[0025] Furthermore, in the existing technology, the main interference module uses fiber optic devices such as fiber optic splitters, fiber optic circulators, and fiber optic couplers for beam splitting, guiding the second probe signal into the probe fiber and recovering the Rayleigh backscattered signal, and coupling the second reference signal and the Rayleigh backscattered signal. The drawbacks of using these devices are: first, the insertion loss accumulates, and the probe light and the backscattered Rayleigh signal pass through each device multiple times, and the superimposed loss severely weakens the coherent mixing power, limiting the sensing distance and dynamic range; second, residual reflections are generated at the pigtail ends, fusion splices, and connectors of each device, forming a parasitic interference cavity with a very short optical path difference from the main signal, forming dense parasitic peaks near the zero frequency of the beat frequency spectrum, compressing the near-end dead zone, raising the noise floor, and at the same time, the reflected light and the local oscillator light coherently generate false correlation peaks, interfering with the demodulation of the real signal. In this embodiment of the invention, by replacing the above-mentioned devices with free-space optical devices, such as replacing the fiber optic splitter and fiber optic coupler with a combination of polarization beam splitter and waveplate; and replacing the fiber optic circulator with a free-space optical circulator and optical path coupling of the first collimating lens 23 with the probe fiber, the optical frequency domain reflectometer can ignore the defects caused by fiber optic devices during the design process. This reduces the optical frequency domain reflectometer's dependence on high-power lasers, and eliminates the need to consider residual reflections generated by the pigtail ends, fusion splices, and connectors of additional devices. It also reduces interference during demodulation of the measurement signal.

[0026] Additionally, refer to Figure 1 The first beam splitting unit includes a first waveplate 1 and a first polarization beam splitter 2; the first detection signal is incident on the first polarization beam splitter 2 through the first waveplate 1; the angle between the optical axis of the first waveplate 1 and the projection of the polarization direction of the first detection signal onto the same plane is a first preset angle.

[0027] It should be noted that the free-space optical device of the first beam splitting unit includes a first waveplate 1 and a first polarization beam splitter 2. The first waveplate 1 can be a λ / 2 waveplate or a λ / 4 waveplate, and the first polarization beam splitter 2 can be a polarization beam splitter prism, a polarization beam splitter cube, a birefringent crystal polarization beam splitter, or other free-space optical devices that can achieve beam splitting, or combinations of free-space optical devices.

[0028] Specifically, the relative positions of the first waveplate 1 and the incident side of the first polarizing beam splitter 2 are referenced. Figure 2 The first waveplate 1 is fixedly attached to one side of the incident end of the first polarization beam splitter 2, so that the angle between the optical axis of the first waveplate 1 and the polarization direction of the swept laser incident on the first waveplate 1 is a first preset angle. Figure 2In the diagram, 'a' represents the initial position of the optical axis before the first waveplate 1 rotates, 'b' represents the position of the optical axis after the first waveplate 1 rotates, and 'c' represents the direction of linear polarization of the linearly polarized light incident on the first waveplate 1. According to Malus's law, when the angle requirement of the first preset angle is met, the linearly polarized light, after passing through the beam-splitting structure composed of the first waveplate 1 and the first polarizing beam splitter 2, will generate two orthogonal components: a second reference signal and a second detection signal, with powers of: ······①, ·····②, In equations ① and ②, The first preset angle is shown in the diagram below. Figure 2 As shown, This is transmitted light (in this embodiment of the invention, it is the second reference signal). The reflected light (in this embodiment of the invention, the second detection signal) is therefore, according to Malus's law, , The splitting ratio of the two optical signals is: ······③, As shown in ①, ②, and ③, the beam splitting structure composed of the first waveplate 1 and the first polarization beam splitter 2, two free-space optical devices, can achieve the same function as a fiber optic beam splitter, while avoiding the inherent defects of fiber optic beam splitters as fiber optic devices. Furthermore, the power ratio of the two optical signals can be changed by adjusting the optical axis of the first waveplate 1 and the polarization direction of the swept laser incident on the first waveplate 1, thus meeting the requirements of different beam splitting ratios during measurement. It is understood that the purpose of rotating the first waveplate 1 is to make its optical axis form a first preset angle θ with the incident linearly polarized light. Therefore, the specific rotation angle of the first waveplate 1 is usually not the same as the first preset angle, and needs to be determined according to the polarization direction of the incident linearly polarized light. It is understood that once the first preset angle θ is determined, those skilled in the art can determine the specific rotation angle of the first waveplate 1; the optical axis of the first waveplate 1 can be a fast axis or a slow axis; the first waveplate 1 can be a λ / 2 waveplate or a λ / 4 waveplate, and the type of waveplate can be adjusted accordingly. Adaptive adjustments are also necessary.

[0029] Additionally, refer to Figure 1The first coupling unit includes a first reflector 5, a second polarization beamsplitter 4, a first half-wave plate 6, and a third polarization beamsplitter 7. The first reflector 5 is used to reflect the second reference signal into the second polarization beamsplitter 4. The second polarization beamsplitter 4 is used to combine the second reference signal and the Rayleigh backscattered signal. The first half-wave plate 6 is disposed between the output end of the second polarization beamsplitter 4 and the input end of the third polarization beamsplitter 7. The first half-wave plate 6 is used to deflect the polarization direction of the second reference signal and the Rayleigh backscattered signal, so that the second reference signal and the Rayleigh backscattered signal interfere in the third polarization beamsplitter 7 to obtain a detection interference signal.

[0030] It should be noted that the second polarization beamsplitter 4 and the third polarization beamsplitter 7 are free-space optical devices. (Refer to...) Figure 1 In one embodiment of the present invention, the first incident end of the second polarization beam splitter 4 is disposed at the third end of the free space optical circulator 3. The Rayleigh backscattered signal can be transmitted to the second polarization beam splitter 4 after exiting from the third end of the free space optical circulator 3. The first reflector 5 is disposed at the transmission light exit end of the first polarization beam splitter 2, that is, on the side of the first polarization beam splitter 2 from which the second reference signal is emitted, so that the first reflector 5 can reflect the second reference signal to the second incident end of the second polarization beam splitter 4. Since the original signals of the second reference signal and the Rayleigh backscattered signal (i.e., the second detection signal) are obtained by orthogonally decomposing the first detection signal through the first polarization beam splitter 2, and the polarization directions of the two signals are orthogonal, when the two signals are incident on the two mutually perpendicular polarization principal axes of the second polarization beam splitter 4, although the second polarization beam splitter 4 can combine the two beams into one and output them, the polarization directions of the two signal beams after beam combining are orthogonal, and are parallel or perpendicular to the transmission axis of the third polarization beam splitter 7 to be entered. Ultimately, the second reference signal and the Rayleigh backscattered signal are completely separated to different ports in the third polarization beam splitter 7, and cannot achieve spatial overlap, so no interference occurs. Based on this, in this embodiment of the invention, a first half-wave plate 6 is attached to the output end of the second polarization beam splitter 4. The first half-wave plate 6 can simultaneously rotate the polarization direction of the two orthogonally linearly polarized beams by 45°. After passing through the first half-wave plate 6, the two beams form a 45° angle with respect to the principal axis of the third polarization beam splitter 7, meaning that equal-amplitude projection components exist on both the transmission and reflection axes of the third polarization beam splitter 7. When the combined beam enters the third polarization beam splitter 7, the horizontal components of the two beams interfere, and the vertical components also interfere, ultimately resulting in two probe interference signals located in two directions. The first coupling unit also uses free-space optical devices instead of fiber optic couplers, thus eliminating the need for fiber optic jumpers and fusion splices, thereby suppressing insertion loss accumulation and reflection array generation from the optical path source and ensuring the stability of the probe interference signal.

[0031] Additionally, refer to Figure 3The first coupling unit includes a second reflector 8, a second half-wave plate 9, a first unpolarized beam splitter 10, a fourth polarized beam splitter 11, and a fifth polarized beam splitter 12. The second reflector 8 is used to reflect the second reference signal to the first unpolarized beam splitter 10, and the second reference signal passes through the second half-wave plate 9 during the reflection transmission. The first unpolarized beam splitter 10 is used to split the second reference signal and the Rayleigh backscattered signal incident on the first unpolarized beam splitter 10 according to a fixed ratio. The fourth polarized beam splitter 11 and the fifth polarized beam splitter 12 are respectively disposed at the two output ends of the first unpolarized beam splitter 10. The fourth polarized beam splitter 11 and the fifth polarized beam splitter 12 are both used to cause interference of the output signal of the first unpolarized beam splitter 10 to obtain a detection interference signal.

[0032] It should be noted that the first non-polarizing beam splitter 10, the fourth polarizing beam splitter 11, and the fifth polarizing beam splitter 12 are free-space optical devices. Figure 3 The first coupling unit in the optical frequency domain reflectometer shown is another embodiment of the present invention. It is coupled with... Figure 1 The core difference in the corresponding embodiments lies in the use of a first non-polarizing beam splitter 10 (NPBS) to interfere with the second reference signal and the Rayleigh backscattered signal, thereby obtaining the detection interference signal. Figure 1 In the corresponding embodiment, the first polarization beamsplitter 2 orthogonally decomposes the first detection signal into P-beam (second reference signal) and S-beam (second detection signal). After the S-beam enters the detection fiber through the optical circulator, its polarization state is disrupted due to the random refractive index fluctuations and birefringence effect of the fiber, and phase delay accumulates during transmission, causing the back-returning Rayleigh scattered light to evolve into elliptically polarized light, which simultaneously contains the original vertical component of the S-beam and an additional horizontal component. When this elliptically polarized light enters the second polarization beamsplitter 4, its vertical component is reflected to the first half-wave plate 6 and the third polarization beamsplitter 7, while the horizontal component is directly transmitted and lost, resulting in partial loss of Rayleigh signal energy and affecting the quality of the interference signal.

[0033] In comparison, Figure 3 The embodiment has the following improvements: After the second reference signal and the Rayleigh backscattered signal enter the first unpolarized beamsplitter 10 (NPBS), due to the polarization insensitivity of the NPBS, both the second reference signal and the Rayleigh backscattered signal are split into two beams at a fixed ratio (e.g., 50:50), and the polarization state remains unchanged. Specifically, the Rayleigh backscattered light is split into two elliptically polarized beams, which then enter the fourth polarization beamsplitter 11 and the fifth polarization beamsplitter 12, respectively, and are decomposed into mutually orthogonal horizontal and vertical components at the fourth polarization beamsplitter 11 and the fifth polarization beamsplitter 12, respectively. Further, Figure 3In this embodiment, a second half-wave plate 9 is disposed between the second reflector 8 and the first unpolarized beam splitter 10. Without the second half-wave plate 9, the second reference signal, after being split by the first unpolarized beam splitter 10, would still consist of two pure P-beams. Upon entering the fourth polarized beam splitter 11 and the fifth polarized beam splitter 12, these beams would only interfere with the horizontal component of the elliptically polarized light, resulting in wasted energy in the vertical component. With the addition of the second half-wave plate 9, the polarization direction of the second reference signal is deflected by 45° before entering the first unpolarized beam splitter 10, ensuring that both beams after splitting possess both horizontal and vertical components. Therefore, in the fourth polarized beam splitter 11 and the fifth polarized beam splitter 12, the two components of the second reference signal interfere with the corresponding horizontal and vertical components of the elliptically polarized light, respectively, achieving two-way detection signal output, thus fully utilizing the energy of the Rayleigh backscattered light.

[0034] Additionally, refer to Figure 1 , Figure 2 The beam splitting module includes a second waveplate 13 and a sixth polarization beam splitter 14; the second waveplate 13 is disposed at the incident end of the sixth polarization beam splitter 14, and the angle between the optical axis of the second waveplate 13 and the projection of the polarization direction of the swept laser onto the same plane is a second preset angle.

[0035] It should be noted that the sixth polarization beam splitter 14 is a free-space optical device. To reduce the overall size of the optical frequency domain reflector, in some embodiments, the laser (not shown in the figure) is not integrated inside the optical frequency domain reflector, but the swept laser is transmitted to the optical frequency domain reflector through the second collimating lens 24. After the swept laser enters the optical frequency domain reflector through the second collimating lens 24 of the beam splitting module, it will pass through a beam splitting structure with the same structure as the first beam splitting unit, that is, through the sixth polarization beam splitter 14 and the second waveplate 13, which is set according to Malus's law and whose polarization direction is projected onto the same plane at a second preset angle, to achieve beam splitting of a certain proportion, dividing the swept laser provided by the laser into a first detection signal and a first reference signal. The positional relationship between the second waveplate 13 and the sixth polarization beam splitter 14 is as follows: Figure 2 The same applies as shown. Similarly, the beam splitting module also uses free-space optical devices, which avoids the defects involved in fiber optic beam splitters and can improve the stability of the first detection signal and the first reference signal.

[0036] Additionally, refer to Figure 1 , Figure 3The auxiliary interference module includes a second beam splitting unit, an optical fiber delay ring 22, and a second coupling unit. The second beam splitting unit is used to split the first reference signal into a first horizontal signal and a first vertical signal. The second beam splitting unit includes a third waveplate 15 and a seventh polarization beam splitter 16. The third waveplate 15 is disposed at the incident end of the seventh polarization beam splitter 16, and the angle between the optical axis of the third waveplate 15 and the projection of the polarization direction of the first reference signal onto the same flat plate is a third preset angle. The optical fiber delay ring 22 is used to generate an optical path difference between the first horizontal signal and the first vertical signal. The second coupling unit is used to generate a reference interference signal based on the first horizontal signal and the first vertical signal with the optical path difference.

[0037] It should be noted that the seventh polarization beamsplitter 16 is a free-space optical device. In the OFDR system, the reference interference signal generated by the auxiliary interferometer module is used to perform equal-frequency interval resampling correction on the nonlinear phase noise of the swept laser, thereby eliminating the influence of the light source frequency modulation nonlinearity on the measurement accuracy of the main interferometer. In this embodiment of the invention, the second beam splitting unit composed of the third waveplate 15 and the seventh polarization beamsplitter 16 divides the first reference signal and the third power ratio associated with the third preset angle into a first horizontal signal and a first vertical signal. The third waveplate 15 is attached to the incident end of the seventh polarization beamsplitter 16, and the beam splitting principle is the same as that of the first waveplate 1 and the first polarization beamsplitter 2. Further, in the auxiliary interferometer, an optical fiber delay loop 22 is set for the first horizontal signal and the first vertical signal to create an optical path difference between the first horizontal signal and the first vertical signal. The aim is to use the frequency tuning characteristics of the swept laser to convert the fixed optical path difference between the first horizontal signal and the first vertical signal into a stable beat frequency, thereby generating an equal-frequency interval trigger signal as the nonlinear correction signal of the main interferometer, i.e., the reference signal for detecting the interference signal. Finally, the first vertical signal and the first horizontal signal with optical path difference are introduced into the second coupling unit to achieve interference between the first horizontal signal and the first vertical signal, thus obtaining the reference interference signal.

[0038] Furthermore, in this embodiment of the invention, the third collimating lens 25 and the fourth collimating lens 26 realize the incident and emitted optical signals in the fiber delay ring 22. The fiber delay ring 22 is an optical device that introduces an adjustable time delay, which is achieved by coupling the optical signal into a closed fiber loop and controlling the number of optical signal loops. The third collimating lens 25 couples the first vertical signal from spatial light into the fiber delay ring 22; the fiber delay ring 22 introduces a defined optical path delay through its loop length, so that the first vertical signal has a fixed time delay relative to the first horizontal signal; the fourth collimating lens 26 converts the delayed first vertical signal back into spatial light from the fiber delay ring 22 and transmits it to the second coupling unit. The fiber delay ring 22 can adjust the delay by changing the fiber length or controlling the number of light loops in the ring to adapt to the auxiliary beat frequency requirements of different sweep ranges or measurement distances. In one embodiment of the invention, the fiber delay ring 22 can reduce its volume by using low-bending-loss fiber and tightly winding it into a miniaturized coil, significantly reducing the package size while maintaining a long optical path delay.

[0039] Additionally, refer to Figure 1 The second coupling unit includes an eighth polarization beamsplitter 17, a third half-wave plate 18, and a ninth polarization beamsplitter 19. The eighth polarization beamsplitter 17 is used to combine a first horizontal signal and a first vertical signal with an optical path difference. The third half-wave plate 18 is disposed between the output end of the eighth polarization beamsplitter 17 and the input end of the ninth polarization beamsplitter 19. The third half-wave plate 18 is used to deflect the polarization directions of the first horizontal signal and the first vertical signal, so that the first horizontal signal and the first vertical signal with an optical path difference interfere in the ninth polarization beamsplitter 19 to obtain a reference interference signal.

[0040] It should be noted that the eighth polarization beamsplitter 17 and the ninth polarization beamsplitter 19 are free-space optical devices. The second coupling unit uses the eighth polarization beamsplitter 17 to combine the first horizontal signal and the first vertical signal, which have an optical path difference. Since the first horizontal signal and the first vertical signal are orthogonal in their polarization directions, the combined first horizontal signal and the first vertical signal need to pass through the third half-wave plate 18, so that the polarization directions of the first horizontal signal and the first vertical signal are simultaneously deflected by 45°. This allows the first horizontal signal and the first vertical signal to interfere in the ninth polarization beamsplitter 19, obtaining a reference interference signal. It can be understood that the principle of interference between the first horizontal signal and the first vertical signal is similar to... Figure 1 In the corresponding embodiments, the interference principle achieved by the second reference signal and the Rayleigh backscattering signal is the same.

[0041] Additionally, refer to Figure 3The second coupling unit includes a fourth half-wave plate 20 and a second unpolarized beam splitter 21; the first horizontal signal emitted from the seventh polarized beam splitter 16 passes through the fourth half-wave plate 20 and is incident on the second unpolarized beam splitter 21; the second unpolarized beam splitter 21 is used to cause the first vertical signal and the first horizontal signal to interfere to generate a reference interference signal.

[0042] It should be noted that the second unpolarized beam splitter 21 is a free-space optical device. The positional relationship between the seventh polarized beam splitter 16 and the second unpolarized beam splitter 21 in the second beam splitting unit is as follows: Figure 3 As shown in the corresponding embodiment, the first horizontal signal obtained by the seventh polarization beamsplitter 16 is directly transmitted to the second unpolarization beamsplitter 21. The first vertical signal is transmitted to the second unpolarization beamsplitter 21 after the optical signal is incident and emitted in the fiber delay ring 22, so that the first vertical signal and the first horizontal signal interfere to obtain a reference interference signal. It can be understood that due to the random refractive index fluctuations and birefringence effect in the fiber, the first vertical signal will be converted into elliptically polarized light after exiting the fiber delay ring 22, while the first horizontal signal transmitted by the seventh polarization beamsplitter 16 is P-light. Figure 3 In the corresponding embodiment, the first horizontal signal is deflected by the fourth half-wave plate 20, so that it has components in both the horizontal and vertical directions after entering the second unpolarized beam splitter 21, and then interferes with the horizontal and vertical components of the elliptically polarized light to obtain two reference interference signals.

[0043] Additionally, refer to Figure 1 The optical frequency domain reflectometer also includes a first photoelectric conversion unit and a second photoelectric conversion unit; the first photoelectric conversion unit is used to convert the detection interference signal into a detection electrical signal; the second photoelectric conversion unit is used to convert the reference interference signal into a reference electrical signal.

[0044] It should be noted that, referring to Figure 1 , Figure 3 The first photoelectric conversion unit in Figure 1 The corresponding embodiment includes a first photodetector 27 disposed on both sides of the third polarization beam splitter 7. Figure 3 The corresponding embodiment includes a second photodetector 28 disposed on both sides of the fourth polarization beamsplitter 11 and a third photodetector 29 disposed on both sides of the fifth polarization beamsplitter 12; the second photoelectric conversion unit is in Figure 1 The corresponding embodiment includes a fourth photodetector 30 disposed on both sides of the ninth polarization beam splitter 19. Figure 3The corresponding embodiment includes a fifth photodetector 31 disposed on both sides of the second non-polarizing beam splitter 21. The purpose of splitting the power of the interference signal obtained by the main interference module and the auxiliary interference module into two outputs is that: after the two signals are received by photodetectors with highly matched performance, they are amplified and differentially processed by a differential drive circuit. This structure can effectively amplify the AC component of the beat frequency signal while automatically canceling the DC component and common-mode noise (such as light source intensity fluctuations and environmental electromagnetic interference) shared by the two channels, thereby significantly improving the signal-to-noise ratio and amplitude of the output signal, making the weak interference signal easier for the acquisition system to identify and process, and ensuring the sensitivity and dynamic range of the measurement.

[0045] In addition, the beam splitting module, the main interference module, the auxiliary interference module, the first photoelectric conversion unit and the second photoelectric conversion unit are all located inside the packaging shell 32; the packaging shell 32 is also equipped with a TEC temperature control module for regulating the temperature inside the packaging shell 32.

[0046] It should be noted that the beam splitting module, main interference module, auxiliary interference module, first photoelectric conversion unit, and second photoelectric conversion unit are integrated into a single unit. Figure 1 , Figure 3 Within the same encapsulation housing 32 shown, the following beneficial effects can be achieved: First, the entire optical path uses free-space optical devices and is compactly arranged inside the housing, eliminating the bending radius limitations of fiber optic pigtails and the redundant space occupied by discrete device interconnections, significantly reducing the overall size of the device; Second, the encapsulation housing 32 is light-shielding, effectively blocking ambient light from entering the free-space optical path and preventing stray light from entering the detector and introducing additional noise or interfering with the beat frequency signal; Furthermore, a TEC temperature control module (not shown in the figure) is provided inside the housing. In one embodiment, the optical components in the above-mentioned embodiments are located on one side of the mounting plane, and the TEC temperature control module is located on the other side of the mounting plane; the TEC temperature control module can precisely regulate the internal temperature, suppress the disturbance of ambient temperature changes on the optical path, waveplate phase delay, and crystal birefringence characteristics of the free-space optical devices, and ensure the long-term measurement stability of the system.

[0047] The following is an overall embodiment of the present invention: This invention provides an optical frequency domain reflectometer based on free-space light integration, whose main components are all free-space light devices, integrated into a metal tube shell smaller than 70*70*13mm, including: 1. Spectrometer module: The external tuning light source (TSL) of the instrument receives wavelength-tuned linearly polarized light through a collimating lens (C-lens1).

[0048] 2. Reference optical path module: a) Waveplate (WP1): By adjusting the beam splitting ratio through the installation angle, the input light is sent into the polarization beam splitter (PBS1) and split into P-polarized light and S-polarized light; b) After the polarization angle of the S-polarized light is rotated by the waveplate (WP2), the S-light is split off by the polarization beam splitter (PBS2), and coupled into the optical delay ring (ODL) through the collimating lenses (C-lens2, C-lens3), and then input to the polarization beam splitter (PBS3); the P-light split off by PBS2 directly enters PBS3. c) PBS3 rotates the P-component light input from PBS2 and the S-component light input from C-Lens3 by 45° polarization angle through a waveplate (WP3) and then sends them into a polarization beam splitter (PBS4) to achieve 50:50 beam splitting and beat frequency interference. d) The photodetectors (PD1, PD2) receive the output optical signal of PBS4, convert it into an electrical signal, and output it to the acquisition card as a reference interference signal.

[0049] 3. Detection optical path module: a) The P-polarized light split from PBS1 is rotated by the waveplate (WP4) and then the S-beam is split by the polarization beam splitter (PBS5). It is input to the FR1 port of the free space optical circulator (FSC1) and output from the FR2 port. It is then connected to the external measurement link through the collimating lens (C-lens4) and the FC / APC interface. b) The reflected light from the measurement link returns to the FR2 port of FSC1 via C-lens4, and the S-polarized light is output from the FR3 port and input to the polarization beam splitter (PBS6). c) At the same time, the P-polarized light separated from PBS5 is reflected by the mirror (RARP1) and also input into PBS6; d) PBS6 rotates the output light from FR3 of circulator FSC1 and the reflected light from RARP1 into waveplate (WP5) by 45° polarization angle, and then sends them into polarization beam splitter (PBS7) to achieve 50:50 beam splitting and beat frequency interference; e) The photodetectors (PD3, PD4) receive the output optical signal from PBS7, convert it into an electrical signal, and output it to the acquisition card as a detection interference signal.

[0050] Signal processing module: The acquisition card receives the reference signal from PD1 / PD2 and the detection signal from PD3 / PD4. It calibrates the detection signal using the reference signal to achieve accurate distance sensing of the detection link.

[0051] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. An optical frequency domain reflectometer, comprising a beam splitting module, a main interference module, and an auxiliary interference module, characterized in that, The beam splitting module is used to split the frequency-sweeping laser into a first detection signal and a first reference signal; The auxiliary interference module is used to generate a reference interference signal based on the first reference signal; The main interference module includes a first beam splitting unit, a free-space optical circulator, and a first coupling unit; the first beam splitting unit is used to split the first detection signal into a second detection signal and a second reference signal; the free-space optical circulator is used to transmit the second detection signal to the detection optical fiber and acquire the Rayleigh backscattered signal fed back by the detection optical fiber; The first coupling unit is used to generate a detection interference signal based on the second reference signal and the Rayleigh backscattering signal; both the first beam splitting unit and the first coupling unit are composed of free-space optical devices.

2. The optical frequency domain reflectometer of claim 1, wherein, The first beam splitting unit includes a first waveplate and a first polarization beam splitter; the first detection signal is incident on the first polarization beam splitter through the first waveplate; the angle between the optical axis of the first waveplate and the projection of the polarization direction of the first detection signal onto the same plane is a first preset angle.

3. The optical frequency domain reflectometer according to claim 2, characterized in that, The first coupling unit includes a first reflector, a second polarization beam splitter, a first half-wave plate, and a third polarization beam splitter; the first reflector is used to reflect the second reference signal into the second polarization beam splitter; the second polarization beam splitter is used to combine the second reference signal and the Rayleigh backscattered signal. The first half-wave plate is disposed between the output end of the second polarization beam splitter and the input end of the third polarization beam splitter; the first half-wave plate is used to deflect the polarization direction of the second reference signal and the Rayleigh backscattered signal, so that the second reference signal and the Rayleigh backscattered signal interfere in the third polarization beam splitter to obtain the detection interference signal.

4. The optical frequency domain reflectometer according to claim 2, characterized in that, The first coupling unit includes a second reflector, a second half-wave plate, a first unpolarized beam splitter, a fourth polarized beam splitter, and a fifth polarized beam splitter; the second reflector is used to reflect the second reference signal to the first unpolarized beam splitter, and the second reference signal passes through the second half-wave plate during the reflection transmission; The first non-polarized beam splitter is used to split the second reference signal and the Rayleigh backscattered signal incident on the first non-polarized beam splitter according to a fixed ratio; the fourth polarized beam splitter and the fifth polarized beam splitter are respectively disposed at the two output ends of the first non-polarized beam splitter, and both the fourth polarized beam splitter and the fifth polarized beam splitter are used to cause interference of the output signal of the first non-polarized beam splitter to obtain the detection interference signal.

5. The optical frequency domain reflectometer of any of claims 1 to 4, wherein, The beam splitting module includes a second waveplate and a sixth polarization beam splitter; the second waveplate is disposed at the incident end of the sixth polarization beam splitter, and the angle between the optical axis of the second waveplate and the projection of the polarization direction of the swept laser onto the same plane is a second preset angle.

6. The optical frequency domain reflectometer according to any one of claims 1 to 4, characterized in that, The auxiliary interference module includes a second beam splitting unit, an optical fiber delay loop, and a second coupling unit. The second beam splitting unit is used to split the first reference signal into a first horizontal signal and a first vertical signal. The second beam splitting unit includes a third waveplate and a seventh polarization beam splitter. The third waveplate is disposed at the incident end of the seventh polarization beam splitter, and the angle between the optical axis of the third waveplate and the projection of the polarization direction of the first reference signal onto the same flat plate is a third preset angle. The optical fiber delay loop is used to generate an optical path difference between the first horizontal signal and the first vertical signal. The second coupling unit is used to generate a reference interference signal based on the first horizontal signal and the first vertical signal having the optical path difference.

7. The optical frequency domain reflectometer of claim 6, wherein, The second coupling unit includes an eighth polarization beamsplitter, a third half-wave plate, and a ninth polarization beamsplitter; the eighth polarization beamsplitter is used to combine the first horizontal signal and the first vertical signal having an optical path difference; the third half-wave plate is disposed between the output end of the eighth polarization beamsplitter and the input end of the ninth polarization beamsplitter; the third half-wave plate is used to deflect the polarization directions of the first horizontal signal and the first vertical signal, so that the first horizontal signal and the first vertical signal having an optical path difference interfere in the ninth polarization beamsplitter to obtain the reference interference signal.

8. The optical frequency domain reflectometer of claim 6, wherein, The second coupling unit includes a fourth half-wave plate and a second unpolarized beam splitter; the first horizontal signal emitted from the seventh polarized beam splitter is incident on the second unpolarized beam splitter after passing through the fourth half-wave plate; the second unpolarized beam splitter is used to cause the first vertical signal and the first horizontal signal to interfere, so as to generate a reference interference signal.

9. The optical frequency domain reflectometer according to any one of claims 1 to 4, 7, and 8, characterized in that, The optical frequency domain reflectometer further includes a first photoelectric conversion unit and a second photoelectric conversion unit; the first photoelectric conversion unit is used to convert the detection interference signal into a detection electrical signal; the second photoelectric conversion unit is used to convert the reference interference signal into a reference electrical signal.

10. The optical frequency domain reflectometer according to claim 8, characterized in that, The beam splitting module, the main interference module, the auxiliary interference module, the first photoelectric conversion unit, and the second photoelectric conversion unit are all disposed inside the packaging shell; a TEC temperature control module for regulating the temperature inside the packaging shell is also disposed inside the packaging shell.

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