A seesaw type optical fiber end face weak force sensor

Abstract

This invention discloses a weak force sensor with a seesaw-shaped fiber endface, comprising a seven-core fiber, a seesaw-shaped additive structure, and an interference beam. The seven-core fiber consists of a central main core and six symmetrically distributed secondary cores. The seesaw-shaped additive structure consists of a short vertical beam, a vertical beam on the central main core of the seven-core fiber, and a horizontal beam connecting it. The entire structure is fabricated using two-photon polymerization technology on a femtosecond laser processing platform. The interference beam consists of two beams within an interference cavity aligned with the lower surfaces of the left and right sides of the seesaw-shaped additive structure and the endfaces of the two secondary cores on the left and right sides of the seven-core fiber. This invention uses two-photon polymerization technology to build an additive structure on the endface of the seven-core fiber, resulting in a small overall size of the sensing structure, suitable for applications in limited spaces. The optical dynamic vernier structure formed by parallel Fabry-Perot interferometers on both sides of the sensing structure enhances sensitivity through differential amplification of weak mechanical disturbances and strengthens anti-interference capability through common-mode noise reduction.

Description

Technical Field

[0001] This invention relates to the field of weak force measurement, and specifically to a weak force sensor for a seesaw-type fiber optic end face. Background Technology

[0002] Currently, traditional methods utilize MEMS technology to measure weak forces, but these methods suffer from limitations such as low sensitivity in certain scenarios, susceptibility to temperature drift and electromagnetic interference, and difficulty in miniaturization. In contrast, fiber optic sensors offer significant advantages in weak force measurement, including small size, integrability, and resistance to electromagnetic interference. However, existing fiber optic weak force sensors still face challenges related to external temperature drift interference and low sensitivity. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies by providing a seesaw-type fiber optic end face micro-force sensor for measuring micro-forces.

[0004] The technical problem to be solved by the present invention is to provide a weak force sensor for the end face of a seesaw-type optical fiber, comprising a seven-core optical fiber, a seesaw-type additive structure, and an interference beam; the seven-core optical fiber consists of a central main fiber core and six symmetrically distributed secondary fiber cores; the seesaw-type additive structure consists of a short vertical beam, a vertical beam on the central main fiber core of the seven-core optical fiber, and a horizontal beam connected thereto; the interference beam consists of two beams within an interference cavity on the lower surfaces of the left and right sides of the seesaw-type additive structure, which are aligned with the end faces of the two secondary fiber cores on the left and right sides of the seven-core optical fiber.

[0005] According to the above scheme, the central main fiber core is located at the geometric center of the fiber cross-section, and six symmetrically distributed secondary fiber cores are arranged in a regular hexagon around the central main fiber core.

[0006] According to the above scheme, the vertical beam is located on the central main fiber core of the seven-core optical fiber, and the vertical beam intersects perpendicularly at the geometric center of the horizontal beam to form a "T" shaped beam structure. A short vertical beam is connected above the right edge of the horizontal beam.

[0007] According to the above scheme, the left and right interference optical paths of the seesaw-type additive structure are respectively composed of two beams in the interference cavity of the lower surfaces of the left and right sides of the seesaw-type additive structure and the end faces of the two sub-fiber cores in the seven-core fiber; the two sub-fiber cores are located in positions symmetrical about the central fiber core.

[0008] According to the above scheme, the weak force testing system includes a computer, a demodulator, an optical coupler, a fan-in connector, a seven-core optical fiber, and a nanoindenter; the 1st and 4th ports of the fan-in connector are connected to the two outputs of the optical fiber coupler, and the outputs are connected to the seven-core optical fiber, which is used to connect the 1st and 4th sub-cores of the seven-core optical fiber.

[0009] According to the above scheme, the nanoindentation instrument includes a sample stage, a sample clamping device, an optical microscope module, and an indenter. The sample stage enables precise movement of the sample in the horizontal direction (X and Y axes), thereby completing the positioning and testing of different indentation points. The sample clamping device serves to fix the seven-core optical fiber. The optical microscope is used to accurately locate the indentation position and observe the microstructure of the indentation in real time. The indenter applies a weak pressure to the seesaw-type additive structure.

[0010] According to the above scheme, the weak force testing system is characterized in that the weak force applied by the nanoindenter to the short vertical beam of the seesaw-type additive structure will cause the interference cavity lengths on the left and right sides of the seesaw-type structure to be stretched and compressed respectively, thereby causing a corresponding drift in the interference spectral envelope on the demodulated spectrum. The force information can be obtained by reading the amount of this spectral drift.

[0011] According to the above scheme, the seven-core optical fiber is characterized in that only the two secondary cores on the outer side of the central core are used, and the other cores are not used in the optical path.

[0012] According to the above scheme, the seesaw-type additive structure is characterized in that it is fabricated on the end face of a seven-core optical fiber using two-photon polymerization technology on a femtosecond laser processing platform. The central vertical beam supports the horizontal beam. Due to gravity, the cavity length of the right interferometer is slightly compressed, while the cavity length of the left interferometer is slightly elongated due to the seesaw effect, resulting in an initial cavity length difference between the two interferometers.

[0013] According to the above scheme, the structure and sensing principle of the sensor are as follows:

[0014] The left and right interference optical paths of the seesaw-type additive structure are respectively composed of a left reference Fabry-Perot interferometer and a right sensing Fabry-Perot interferometer, formed by the seesaw-type additive structure and two symmetrical fiber cores about the central fiber core in a seven-core optical fiber. The two Fabry-Perot interferometers are connected in parallel to form an optical vernier structure, and the cavity lengths of the two interferometers change in opposite directions, creating a "seesaw" effect. This seesaw structure is a push-pull configuration; while the cavity length of one side is compressed, the other side is stretched, thus forming a push-pull dynamic vernier effect. The specific sensing principle is analyzed as follows.

[0015] For a single interferometer, such as Figure 1 As shown, numbers 10 and 8 represent the left and right FP interferometers, respectively, and their phase differences are:

[0016] (1)

[0017] Where i=1,2 represent the left FP interferometer and the right sensing FP interferometer, respectively, and Li is the cavity length of the interferometer. Let represent the wavelength of the m-th order interference resonance. When the phase difference satisfies the condition of (2m+1)π (constructive interference), the wavelength at the resonance valley of the interference spectrum and the free spectral range (FSR) are respectively:

[0018] (2)

[0019] (3)

[0020] When a single interferometer is subjected to a weak external force, its wavelength shift can be expressed as:

[0021] (4)

[0022] Where k represents the elastic spring constant of the cantilever structure, representing the force-displacement sensitivity, and is related to the spring stiffness of the structure. The above equation demonstrates the linear relationship between wavelength drift and applied force; its sensing performance depends on the observed wavelength. The initial cavity length Li and the structural elastic coefficient k.

[0023] For two parallel interferometers, their overall output light intensity can be expressed as:

[0024] (5)

[0025] Here, A and B represent the DC and AC components of the spectrum, respectively, and their magnitudes are determined by the reflected light intensity of the FPI reflector. It's worth noting that the phase change of this interferometer is primarily determined by the cavity length, which remains constant throughout the sensing process, thus ensuring a stable carrier wave. Furthermore, the cavity length difference in the push-pull structure significantly enhances the sensitivity of the dynamic vernier compared to traditional vernier structures. More importantly, since external disturbances (such as temperature and vibration) elicit essentially the same response from both sides of the interferometer, their impact on the cavity length difference is minimal, greatly improving the sensor's anti-interference capability.

[0026] Considering the slight difference in FSR caused by the minute difference in cavity length between the two interferometers, the proposed sensing structure interference spectrum envelope resonance valley wavelength... , and Wavelength shifts are respectively

[0027] (6)

[0028] (7)

[0029] (8)

[0030] Since the envelope resonance valley wavelength is formed by the superposition of the spectra of two individual interferometers, therefore Furthermore, there is a relationship within the push-pull structure. Then the above formula can be expressed as:

[0031] (9)

[0032] In the above formula , These are the spectral drift amplification factors of the vernier structure relative to the left and right FPI interferometers, respectively. This is the equivalent magnification factor, which depends on the quotient of the sum and difference of the cavity lengths of the two interferometers.

[0033] (10)

[0034] It is evident that the sensitivity of the final micro-force sensor structure is independent of the initial cavity length Li of the interferometer, and depends only on the difference in initial cavity length, the observation wavelength, and the structural elastic coefficient. Therefore, when an external disturbance such as a temperature fluctuation LT occurs, since the initial cavity lengths of the two interferometers are similar and their thermal expansion coefficients are consistent, the impact is comparable, and the L1-L2 term becomes (L1+LT)-(L1+LT)=L1-L2, thus effectively suppressing the external disturbance differentially.

[0035] The beneficial effects of this invention are:

[0036] (1) The present invention uses the two-photon polymerization technology of the femtosecond laser processing platform to build an additive structure on the end face of a seven-core fiber, which makes the overall size of the sensing structure of the present invention small and suitable for application scenarios with limited space.

[0037] (2) By connecting the Fabry-Perot interferometers on both sides of the sensing structure in parallel to form an optical dynamic vernier structure, the differential amplification of weak mechanical disturbances greatly improves the sensitivity.

[0038] (3) Since the response of external cross disturbances (such as temperature, vibration, etc.) to the left and right interferometers is basically the same, common mode noise reduction greatly improves the sensor’s anti-interference capability. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the weak force sensor at the end face of the seesaw-type optical fiber in this embodiment.

[0040] Figure 2 This is a schematic diagram of the processing system according to an embodiment of the present invention;

[0041] Figure 3 This is a schematic diagram of the structure of the fiber optic clamping module according to an embodiment of the present invention;

[0042] Figure 4 This is a schematic diagram of a weak force measurement system according to an embodiment of the present invention.

[0043] Among them: 1-7, fiber core; 8, right-side interference beam; 9, seesaw-type additive structure; 10, left-side interference beam; 11, seven-core fiber; 12, femtosecond laser; 13, beam expander; 14, acousto-optic modulator; 15, XY high-speed galvanometer; 16, CCD; 17, mirror; 18, 60X objective lens; 19, fiber clamping module; 20, support; 21, 3D displacement stage; 22, seven-core fiber; 23, fiber adapter; 24, fixture; 25, photoresist; 26, dropper; 27, computer; 28, demodulator; 29, fiber coupler; 30, fan-in; 31, seven-core fiber; 32, sample stage; 33, sample clamping device; 34, optical microscope module; 35, indenter; 36, nanoindenter. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0045] like Figure 1 The diagram shows a weak force sensor with a seesaw-type fiber end face, comprising a seven-core fiber 11, a seesaw-type additive structure 9, and an interference beam. The seven-core fiber 11 consists of a central main fiber core 7 and six symmetrically distributed secondary fiber cores 1-6. The seesaw-type additive structure 9 consists of a short vertical beam, a vertical beam on the central main fiber core of the seven-core fiber, and a horizontal beam connected to it. The entire structure is fabricated using two-photon polymerization technology on a femtosecond laser processing platform. The interference beam consists of a left interference beam 10 and a right interference beam 8 from the seesaw-type additive structure.

[0046] like Figure 2 and 3 As shown, the manufacturing process of the seesaw-type additive structure includes:

[0047] 1) First, place the end face of the seven-core optical fiber 22 to be processed into the fixture 24, cut it flat and clean it.

[0048] 2) Then, after placing the clamped optical fiber on the femtosecond laser platform 12, apply photoresist 25 to the end face using the dropper 26;

[0049] 3) Then, use the 60X objective lens 18 to focus on the end face of the seven-core fiber, and move it to the clamp 24 to ensure that the fiber core is aligned with the writing direction;

[0050] 4) Next, the T-beam structure was sliced ​​and then scanned and exposed layer by layer at 515 nm, 16 mW and 80 MHz to allow the liquid colloidal material to polymerize in situ into a solid T-beam structure.

[0051] 5) Finally, immerse the written sample in PGMEA solution for 7 minutes to wash away the unexposed liquid photoresist. After 7 minutes, immerse the sample in IPA for 3 minutes to wash away the residual PGMEA solution. Then, gently dry it with nitrogen gas and irradiate it with ultraviolet light for 10 minutes to enhance crosslinking.

[0052] like Figure 4 As shown, the process of micro-force measurement includes:

[0053] 1) First, the demodulator 28 transmits an optical signal, which is split by the fiber coupler 29 and enters the seven-core fiber in the form of fan-in 30.

[0054] 2) Then fix the seven-core optical fiber on the sample operation stage 33 on the sample stage 32, and use the optical microscope 34 to adjust the position of the seven-core optical fiber so that it is aligned with the pressure head 35.

[0055] 3) Finally, a small force is applied downward by the pressure head, the sensor modulates the light signal, the reflected light is demodulated by the demodulator 28, and then the computer 27 processes the data.

Claims

1. A weak force sensor for a seesaw-type optical fiber end face, characterized in that, It includes a seven-core optical fiber, a seesaw-type additive structure, and an interference beam; the seven-core optical fiber consists of a central main fiber core and six symmetrically distributed secondary fiber cores; the seesaw-type additive structure consists of a short vertical beam, a vertical beam on the central main fiber core of the seven-core optical fiber, and a horizontal beam connected to it; the interference beam consists of two beams within the interference cavity on the lower surfaces of the left and right sides of the seesaw-type additive structure, which are aligned with the end faces of the two secondary fiber cores on the left and right sides of the seven-core optical fiber. The vertical beam is located on the central main fiber core of the seven-core optical fiber, and the vertical beam intersects perpendicularly at the geometric center of the horizontal beam to form a "T" shaped beam structure. A short vertical beam is connected above the right edge of the horizontal beam. The left and right interference optical paths of the seesaw-type additive structure are respectively composed of two beams in the interference cavity of the lower surface of the left and right sides of the seesaw-type additive structure and the end faces of the two sub-fiber cores in the seven-core fiber; the two sub-fiber cores are located in a position symmetrical about the central main fiber core. It is fabricated on the end face of a seven-core optical fiber using two-photon polymerization technology on a femtosecond laser processing platform. The vertical beam supports the horizontal beam. Due to gravity, the short vertical beam causes a slight compression of the cavity length of the right interferometer, while the cavity length of the left interferometer is slightly elongated due to the seesaw effect, resulting in an initial cavity length difference between the two interferometers.

2. The weak force sensor of the seesaw-type optical fiber end face according to claim 1, characterized in that, The central main fiber core is located at the geometric center of the fiber cross-section, and six symmetrically distributed secondary fiber cores are arranged in a regular hexagon around the central main fiber core.

3. A weak force sensor for a seesaw-type fiber optic end face according to claim 1, characterized in that, The weak force testing system includes a computer, a demodulator, an optical fiber coupler, a fan-in connector, a seven-core optical fiber, and a nanoindenter. The 1st and 4th ports of the fan-in connector are connected to the two outputs of the optical fiber coupler, and the outputs are connected to the seven-core optical fiber. Its function is to connect the 1st and 4th sub-cores of the seven-core optical fiber.

4. A weak force sensor for a seesaw-type fiber optic end face according to claim 3, characterized in that, The nanoindenter includes a sample stage, a sample clamping device, an optical microscope module, and an indenter. The sample stage enables precise horizontal movement of the sample along the X and Y axes, thereby locating and testing different indentation points. The sample clamping device secures the seven-core optical fiber. The optical microscope precisely locates the indentation position and observes the microstructure of the indentation in real time. The indenter applies a weak pressure to the seesaw-shaped additive structure.

5. A weak force sensor for a seesaw-type fiber optic end face according to claim 3, characterized in that, The weak force applied by the nanoindenter to the short vertical beam of the seesaw-shaped additive structure will cause the interference cavity lengths on the left and right sides of the seesaw-shaped structure to be stretched and compressed respectively, resulting in a corresponding drift in the interference spectral envelope in the demodulated spectrum. The force information can be obtained by reading the amount of this spectral drift.

6. A weak force sensor for a seesaw-type fiber optic end face according to claim 2, characterized in that, Only the two secondary fiber cores on the outer side of the central main fiber core are used; the other fiber cores are not used in the optical path.

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