A displacement measurement method based on optical microcavity

Abstract

The application discloses a displacement measurement method based on an optical microcavity, adopts two parallel placed optical fibers to form a SNAP microcavity, changes the overlapping length between the two optical fibers to realize the axial length change of the SNAP microcavity. Since the microcavity length change will cause the change of the intrinsic resonant wavelength of each order axial mode, different displacements correspond to different resonant wavelength distributions. By monitoring the resonant wavelength, the free spectral range and the transmittance change of each order resonant mode in the transmission spectrum of the SNAP microcavity in real time, and establishing the mapping relationship between the resonant spectrum and the displacement, high-precision displacement measurement is realized. The application fundamentally avoids the problem that different displacements produce similar resonant spectra in the traditional method, and significantly improves the resolution, stability, anti-interference ability and measurement accuracy of the displacement measurement.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing and displacement measurement technology. Specifically, this invention relates to a high-precision displacement measurement method based on a variable-length surface nanoscale axial photonics (SNAP) microcavity, which can be applied to fields such as micro-nano displacement detection, precision manufacturing, and microelectromechanical systems. Background Technology

[0002] With the increasing demands for precision in micro- and nano-scale displacement measurement in the precision manufacturing field, high-sensitivity and miniaturized displacement sensing technology has become a research hotspot. Compared with traditional electrical sensors, fiber optic probes have advantages such as small size, strong resistance to electromagnetic interference, high temperature and corrosion resistance, long-distance transmission capability, and ease of integration. They are particularly suitable for high-precision online detection in complex environments, and therefore have broad application prospects in the field of displacement sensing.

[0003] Currently, common fiber optic displacement sensing methods mainly include fiber Bragg gratings, Fabry-Perot covariance, and optical microcavity resonant sensing. Among these, optical microcavities based on whispering-gallery modes, due to their ultra-high Q value and strong optical field localization capability, can convert minute displacement changes into significant spectral responses, resulting in higher measurement accuracy. SNAP microcavities, by introducing nanoscale radius variations on the fiber surface to form axial optical potential wells and bottle-shaped resonant cavities, can support multiple high-order axial modes, making them a high-performance displacement sensing structure.

[0004] Existing displacement measurement methods based on SNAP microcavities primarily utilize the different field distribution characteristics of different axial modes in the axial direction. When the SNAP microcavity undergoes displacement, the coupling position changes, leading to different coupling strengths for each axial mode. This, in turn, causes changes in the resonance characteristics of different modes, specifically manifested as variations in parameters such as transmittance and linewidth of each mode in the transmission spectrum. By establishing a mapping relationship between the resonance spectrum characteristics and the displacement, inverse displacement measurement can be achieved.

[0005] However, traditional SNAP microcavity displacement measurement methods are essentially based on the difference in resonance spectra at different displacement positions for identification. Their measurement accuracy is highly dependent on the distinguishability of the resonance spectra corresponding to different displacements. In practical applications, it is easy for different displacements to correspond to similar or even nearly overlapping resonance spectra, leading to misjudgments during displacement identification and resulting in significant measurement errors. This severely impacts the stability and anti-interference capabilities of the sensing system. Summary of the Invention

[0006] This invention provides a displacement measurement method based on optical microcavities to solve the technical problems of lidar technology in the background art, such as expanding the ranging range, improving measurement accuracy and speed, and reducing system size.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a displacement measurement method based on optical microcavity, comprising a tunable laser, a polarization controller, a waveguide coupler, a photodetector, a first optical fiber, a second optical fiber, and a displacement device;

[0008] The output of the tunable laser is connected to the input of the polarization controller, the output of the polarization controller is connected to the input of the waveguide coupler, the output of the waveguide coupler is connected to the input of the photodetector, the first optical fiber is fixed, the second optical fiber is fixed on the displacement device, the waveguide coupler is in contact with the first optical fiber, the displacement device moves along the axis of the second optical fiber, and the overlapping part of the first optical fiber and the second optical fiber forms a SNAP microcavity;

[0009] The tunable laser is used to generate laser light with continuously and precisely scanned wavelengths, and the light is input into the polarization controller.

[0010] The polarization controller is used to adjust the polarization state of light in the waveguide coupler to achieve high-efficiency coupling.

[0011] The waveguide coupler couples optical waves into the SNAP microcavity and forms a resonant mode within the cavity;

[0012] The SNAP microcavity is the core device for displacement measurement and is used to generate resonant modes.

[0013] The photodetector is used to convert the received optical signal into an electrical signal to form a transmission spectrum.

[0014] The displacement device drives the second optical fiber to move axially, thereby changing the length of the SNAP microcavity.

[0015] Furthermore, the first optical fiber and the second optical fiber are placed in parallel contact, forming a SNAP microcavity at their overlapping portion.

[0016] Furthermore, the length of the SNAP microcavity ranges from 20 μm to 1000 μm.

[0017] Furthermore, the surfaces of the first and second optical fibers are treated with oxyhydrogen flame or electric arc discharge to form an ultra-smooth surface.

[0018] Furthermore, the Q value of the SNAP microcavity is greater than 10. 5 .

[0019] Furthermore, the waveguide coupler is a strip waveguide, a tapered optical fiber, or an angled optical fiber.

[0020] Furthermore, the diameters of the first and second optical fibers range from 10 μm to 1000 μm, and their diameters may be the same or different.

[0021] Furthermore, the displacement device is a high-precision displacement platform consisting of a piezoelectric ceramic actuator or a MEMS actuator.

[0022] The displacement measurement method based on optical microcavity specifically includes the following steps:

[0023] Step S1: The tunable laser generates laser light with continuous and precise wavelength scanning, so that the scanning light passes sequentially through the polarization controller and the waveguide coupler and is coupled into the SNAP microcavity; the light wave that meets the resonance condition forms a resonant mode in the SNAP microcavity, and the remaining light waves are output in the form of transmission through the waveguide coupler; the photodetector collects the transmitted light signal in real time.

[0024] Step S2: Use the displacement device to move the second optical fiber relative to the first optical fiber along the axial direction, thereby changing the length of the SNAP microcavity;

[0025] Step S3: Acquire and process the resonance spectrum signals under different displacements, extract the resonance wavelength, free spectral range (FSR) and transmittance characteristic parameters, establish a displacement-spectral feature mapping model, and realize displacement measurement.

[0026] The beneficial effects of adopting the above technical solutions are:

[0027] 1. The present invention provides a displacement measurement method based on optical microcavity, which uses a variable-length SNAP microcavity to realize displacement measurement. Instead of relying solely on the change of coupling strength, it utilizes the change of intrinsic resonant wavelength caused by the change of microcavity length to realize displacement identification.

[0028] 2. The displacement measurement method based on optical microcavity provided by this invention has unique resonance spectra at different displacements, which significantly improves the system's anti-interference ability and measurement stability.

[0029] 3. The displacement measurement method based on optical microcavity provided by this invention has a simple structure, is easy to integrate with MEMS systems and fiber optic sensing systems, and is suitable for micro-nano displacement detection and high-precision online monitoring. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the displacement measurement principle based on a variable-length SNAP microcavity;

[0031] Figure 2 These are three-dimensional resonance spectra of SNAP microcavities of different lengths;

[0032] Figure 3These are the resonance spectra at two different displacements of a variable-length SNAP microcavity;

[0033] Figure 4 These are the resonance spectra at two different coupling positions of a fixed-length SNAP microcavity;

[0034] Figure 5 The curve shows the FSR of a variable-length SNAP microcavity as a function of the microcavity length.

[0035] in:

[0036] 1. Tunable laser; 2. Polarization controller; 3. Waveguide coupler; 4. Photodetector; 5. First optical fiber; 6. Second optical fiber; 7. Displacement device. Detailed Implementation

[0037] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, in order to help those skilled in the art to have a more complete, accurate and in-depth understanding of the concept and technical solutions of the present invention, and to facilitate its implementation.

[0038] like Figures 1 to 5 As shown, this invention is a displacement measurement method based on an optical microcavity. It employs two parallel optical fibers to form a SNAP microcavity. By changing the overlap length between the two fibers, the axial length of the SNAP microcavity is varied. Since changes in the microcavity length cause alterations in the intrinsic resonant wavelengths of each axial mode, different displacements correspond to different resonant wavelength distributions. By real-time monitoring of the resonant wavelengths, free spectral range, and transmittance changes of each resonant mode in the SNAP microcavity transmission spectrum, and by establishing a mapping relationship between the resonant spectrum and displacement, high-precision displacement measurement is achieved.

[0039] Unlike traditional SNAP displacement measurement methods based on differences in local mode field distribution, this invention utilizes the change in microcavity length to directly control the axial intrinsic mode and resonant wavelength, so that different displacements correspond to a unique resonant spectrum. This fundamentally avoids the problem of similar resonant spectra generated by different displacements in traditional methods, and significantly improves the resolution, stability, anti-interference ability and measurement accuracy of displacement measurement.

[0040] The specific working method is described below using specific embodiments:

[0041] Example 1:

[0042] This invention is a displacement measurement method based on optical microcavity. The working principle is as follows: the wavelength scanning laser emitted from the tunable laser 1 enters the SNAP microcavity through the waveguide coupler 3, and the light wave that meets the resonance condition forms a standing wave in the cavity.

[0043] When two optical fibers, specifically fiber 5 and fiber 6, are placed in parallel contact, their whispering-gallery modes interact in the overlapping region, causing a shift in the local cutoff wavelength and thus forming a SNAP microcavity along the fiber axis. This SNAP microcavity can axially confine the whispering-gallery modes and form multiple discrete axial eigenmodes.

[0044] When the displacement device 7 drives the second optical fiber 6 to move axially, the overlap area between the two optical fibers changes, which in turn causes a change in the effective axial length of the SNAP microcavity. Since the intrinsic resonant wavelengths and FSRs of each axial mode of the SNAP microcavity are related to the microcavity length, different displacement states will correspond to different resonant spectrum distributions.

[0045] The photodetector 4 acquires the transmission spectrum in real time and demodulates the spectrum into the corresponding displacement value through the pre-calibrated displacement-spectrum mapping relationship.

[0046] This invention achieves high-precision displacement measurement by real-time monitoring of the resonant wavelength, free spectral range, and transmittance changes of each resonant mode in the transmission spectrum of a SNAP microcavity, and by establishing a mapping relationship between the resonant spectrum and displacement. Unlike traditional SNAP displacement measurement methods based on differences in local mode field distribution, this invention directly modulates the axial intrinsic modes and resonant wavelengths using changes in the microcavity length, ensuring that different displacements correspond to unique resonant spectra. This fundamentally avoids the problem of similar resonant spectra generated by different displacements, significantly improving the resolution, stability, anti-interference capability, and measurement accuracy of displacement measurements.

[0047] In this embodiment, the tunable laser 1 operates at a wavelength around 1550nm and a linewidth of 300kHz.

[0048] In this embodiment, the waveguide coupler 3 is a tapered optical fiber with a waist diameter of 1.5 μm.

[0049] In this embodiment, the diameter of the first optical fiber 5 and the second optical fiber 6 are both 125 μm, the material is silicon oxide, and the surface of both is treated with oxyhydrogen flame heating.

[0050] In this embodiment, the waveguide coupler 3 is placed in perpendicular contact with the first optical fiber 5.

[0051] Figure 2 Three-dimensional resonance spectra are shown for SNAP microcavity lengths of 200 μm, 400 μm, and 600 μm. It can be seen that changes in cavity length alter the resonant wavelengths of each mode and cause changes in the FSR. As the cavity length increases, the FSR decreases, and more modes are detected within the same wavelength scanning range. Therefore, even with the waveguide coupler 3 remaining stationary relative to the first fiber 5, a resonant spectrum with clear distinction can still be obtained.

[0052] Figure 3The resonant spectra are shown for cavity lengths of 390 μm and 400 μm, corresponding to an axial displacement of 10 μm in the second fiber 6. In this embodiment, the waveguide coupler 3 is 210 μm away from the right end face of the first fiber 5 and remains constant. It can be more clearly seen that as the cavity length increases, the FSR of the resonant spectrum decreases, ensuring the distinction between the two spectra. Simultaneously, due to the change in the field distribution of each mode, the transmittance of the corresponding modes also changes, further enhancing the distinction between the resonant spectra.

[0053] In contrast. Figure 4 The resonance spectrum of a traditional fixed-length SNAP microcavity displacement sensor is shown, with the microcavity length fixed at 400 μm. Figure 2 As shown in (b), the coupling positions are 10 μm and 20 μm, respectively. It can be seen that the resonance spectra at the two different positions have the same resonance wavelength, and only the transmittance changes with the displacement. This can easily lead to similar resonance spectra at different positions, resulting in measurement errors and reducing the stability of the sensor.

[0054] Figure 5 The FSR of the SNAP microcavity as a function of the microcavity length is shown. Since the effective diameter variation profile of the SNAP microcavity formed in this scheme is rectangular, the wavelength spacing between different resonant modes varies; the higher the order, the larger the spacing between adjacent modes. Here, the average wavelength spacing of all adjacent modes is used as the FSR of the microcavity. It can be seen that the FSR of the SNAP microcavity decreases monotonically with increasing microcavity length, fundamentally eliminating the possibility of similar resonant spectra.

[0055] In summary, the displacement measurement method based on an optical microcavity of this invention forms a variable-length SNAP microcavity using two parallel contacting optical fibers. The effective axial length of the microcavity changes due to displacement variations, thereby achieving synchronous changes in the intrinsic resonant wavelengths and free spectral ranges of each axial mode. This method overcomes the limitation of traditional SNAP displacement sensing, which relies solely on changes in the local mode field distribution. This invention fundamentally avoids the problem of similar resonance spectra generated by different displacements in traditional methods, ensuring that different displacement states correspond to unique resonance spectrum distributions. This significantly improves the resolution, stability, anti-interference capability, and measurement accuracy of displacement measurements.

[0056] This invention has the advantages of simple structure, high sensitivity, easy integration and applicability to micro-nano scale precision measurement, and has good application prospects in high-precision fiber optic sensing, MEMS systems, biological detection and precision manufacturing.

[0057] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other situations without modification, are all within the protection scope of the present invention.

Claims

1. A displacement measurement method based on an optical microcavity, characterized in that: It includes a tunable laser (1), a polarization controller (2), a waveguide coupler (3), a photodetector (4), a first optical fiber (5), a second optical fiber (6), and a displacement device (7). The output end of the tunable laser (1) is connected to the input end of the polarization controller (2), the output end of the polarization controller (2) is connected to the input end of the waveguide coupler (3), the output end of the waveguide coupler (3) is connected to the input end of the photodetector (4), the first optical fiber (5) is fixed, the second optical fiber (6) is fixed on the displacement device (7), the waveguide coupler (3) and the first optical fiber (5) are in contact, the displacement device (7) moves along the axial direction of the second optical fiber (6), and the overlapping part of the first optical fiber (5) and the second optical fiber (6) forms a SNAP microcavity; The tunable laser (1) is used to generate laser with continuous and precise wavelength scanning and input it into the polarization controller (2); The polarization controller (2) is used to adjust the polarization state of light in the waveguide coupler (3) to achieve high-efficiency coupling. The waveguide coupler (3) couples optical waves into the SNAP microcavity and forms a resonant mode within the cavity; The SNAP microcavity is the core device for displacement measurement and is used to generate resonant modes. The photodetector (4) is used to convert the received optical signal into an electrical signal to form a transmission spectrum. The displacement device (7) drives the second optical fiber (6) to move axially to change the length of the SNAP microcavity.

2. The displacement measurement method based on an optical microcavity according to claim 1, characterized in that: The first optical fiber (5) and the second optical fiber (6) are placed in parallel contact, and a SNAP microcavity is formed at their overlapping part.

3. The displacement measurement method based on an optical microcavity according to claim 1, characterized in that: The length of the SNAP microcavity ranges from 20 μm to 1000 μm.

4. The displacement measurement method based on an optical microcavity according to claim 1, characterized in that: The surfaces of the first optical fiber (5) and the second optical fiber (6) are treated with hydrogen-oxygen flame or electric arc discharge to form an ultra-smooth surface.

5. The displacement measurement method based on an optical microcavity according to claim 1, characterized in that: The Q value of the SNAP microcavity is greater than 10. 5 .

6. The displacement measurement method based on an optical microcavity according to claim 1, characterized in that: The waveguide coupler (3) is a strip waveguide, a tapered fiber, or an inclined fiber.

7. The displacement measurement method based on an optical microcavity according to claim 1, characterized in that: The diameters of the first optical fiber (5) and the second optical fiber (6) range from 10 μm to 1000 μm, and their diameters may be the same or different.

8. The displacement measurement method based on an optical microcavity according to claim 1, characterized in that: The displacement device (7) is a high-precision displacement platform of piezoelectric ceramic actuator or MEMS actuator.

9. The displacement measurement method based on an optical microcavity according to any one of claims 1-8, characterized in that: Specifically, the following steps are included: Step S1: The tunable laser (1) generates laser light with continuous and precise wavelength scanning, so that the scanning light passes through the polarization controller (2) and the waveguide coupler (3) in sequence and is coupled into the SNAP microcavity; the light wave that meets the resonance condition forms a resonant mode in the SNAP microcavity, and the remaining light wave is output in the form of transmission through the waveguide coupler (3); the photodetector (4) collects the transmitted light signal in real time; Step S2: Use the displacement device (7) to drive the second optical fiber (6) to move axially relative to the first optical fiber (5), thereby changing the length of the SNAP microcavity; Step S3: Acquire and process the resonance spectrum signals under different displacements, extract the resonance wavelength, free spectral range (FSR) and transmittance characteristic parameters, establish a displacement-spectral feature mapping model, and realize displacement measurement.

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