A diaphragm vibration-coupled waveguide microring ultrasonic sensor

Abstract

This invention discloses a diaphragm vibration-coupled waveguide microring ultrasonic sensor, belonging to the field of optical micro / nano sensor fabrication technology. It includes a single-mode optical fiber, a support integrated at the end of the single-mode fiber, a waveguide microring, and a vibrating diaphragm. The vibrating diaphragm is fixed to the axis of the single-mode fiber by the support. The waveguide microring is located between the single-mode fiber and the vibrating diaphragm, with its optical signal input / output end connected to one end of the single-mode fiber. The farthest point of the waveguide microring from the single-mode fiber is a horizontal segment. A waveguide parallel to the horizontal segment of the waveguide microring is provided on the end face of the vibrating diaphragm near the waveguide microring. External acoustic pressure acts on the vibrating diaphragm, causing it to deform and thus changing the distance between the diaphragm and the waveguide microring, resulting in a change in their coupling efficiency and consequently, the intensity of the returned light. The ultrasonic signal can be detected by detecting the change in light intensity. The sensor has the highest sensitivity at the resonant frequency of the vibrating diaphragm.

Description

Technical Field

[0001] This invention belongs to the field of optical micro-nano sensor fabrication technology, and particularly relates to an ultrasonic sensor based on diaphragm vibration coupling waveguide microring. Background Technology

[0002] With the development of modern industrial technology, ultrasonic sensors are used in aerospace, marine exploration, object imaging, seismic wave detection, biosignal detection, partial discharge detection, and non-destructive testing. Compared to electrical ultrasonic sensors, fiber optic ultrasonic sensors have advantages such as small size, high sensitivity, and strong resistance to electromagnetic interference. Among various fiber optic ultrasonic sensors, phase-modulated fiber optic sensors, while possessing high sensitivity and a large dynamic range, have relatively complex demodulation systems and are susceptible to environmental factors. In contrast, intensity-modulated fiber optic sensors offer simpler demodulation systems, lower costs, and are easier to implement.

[0003] Currently, intensity-modulated ultrasonic sensors mainly utilize the influence of acoustic wave vibrations on light loss, coupling, and reflection in optical fibers. In 1977, Nelson et al. first reported an optical fiber sensor for dynamic strain detection. The bent optical fiber was placed within a U-shaped device; acoustic wave vibrations caused the fiber to bend, and changes in light intensity were detected to achieve acoustic wave detection at a frequency of 1163 Hz. However, this sensor had low sensitivity and initial bending loss. Building on this, Chen et al. proposed a fused conical coupler for ultrasonic detection. The ultrasonic sensor based on the conical coupler exhibited high sensitivity, with a high response in the frequency range of tens to hundreds of kHz. The sensor's sensitivity for detecting signals at a frequency of 155 kHz was 5.6 V / kPa. Although using fused conical micro / nano structures can improve ultrasonic measurement sensitivity, the manufacturing process is relatively complex, making it difficult to achieve consistent sensor dimensions. For traditional fused conical couplers, their performance is often related to the fabrication process; they are large in size, have poor stability, and are difficult to guarantee high reliability.

[0004] In recent years, ultrasonic sensors based on the micro / nano waveguide coupling principle have attracted widespread attention from researchers. One of the main representatives is the waveguide microring resonator based on the whispering-gallery mode. Its sensing sensitivity is linearly related to the sensor's quality factor. Due to its extremely stringent requirements for surface roughness, high-sensitivity sensing often requires electron beam processing or high-precision photolithography, which limits the device's practicality to some extent. Wei et al. designed a reflective waveguide microring resonator. Although its quality factor is low, the sensor's shortcomings can be partially compensated for by using interferometric phase demodulation technology, achieving a sensitivity of 289.16 mV / MPa. While this increases the system demodulation complexity, the use of 3D laser direct writing technology reduces the device fabrication process requirements, providing new research ideas for device design.

[0005] Based on existing ultrasonic sensor technology, how to improve sensor sensitivity while reducing sensor size is a direction that the field is constantly exploring. Summary of the Invention

[0006] To address the problems of large structural size, complex fabrication process, and low sensitivity in existing optical ultrasonic sensors, this invention proposes a diaphragm vibration-coupled waveguide microring ultrasonic sensor. Using two-photon 3D printing technology, an optical ultrasonic sensor is fabricated on the flat end face of a single-mode optical fiber. This ultrasonic sensor features a waveguide microring and a vibrating diaphragm forming a waveguide coupling. Acoustic waves cause the diaphragm to vibrate, altering the waveguide coupling distance and thus changing the waveguide coupling efficiency. The coupling effect caused by the diaphragm vibration is far greater than the coupling effect produced by the direct action of acoustic waves on the waveguide. The coupling efficiency can be adjusted by changing the designed distance between the vibrating diaphragm and the waveguide microring.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] An ultrasonic sensor based on diaphragm vibration coupling waveguide microring, characterized in that it includes a single-mode optical fiber, a support integrated into the end of the single-mode optical fiber, a waveguide microring, and a vibrating diaphragm;

[0009] The vibrating diaphragm is fixed on the axis of the single-mode fiber by a bracket. The waveguide microring is located between the single-mode fiber and the vibrating diaphragm. The optical signal input / output end of the waveguide microring is connected to one end of the single-mode fiber. The farthest end of the waveguide microring from the single-mode fiber is a horizontal segment.

[0010] The vibrating diaphragm has a waveguide on its end face near the waveguide microring, which is parallel to the horizontal section of the waveguide microring.

[0011] As a preferred embodiment of the invention, the waveguide microring is fixed to one end of a single-mode fiber via a frustum waveguide, the large end face of the frustum waveguide is connected to one end of the single-mode fiber, and the small end face of the frustum waveguide is connected to the optical signal input / output end of the waveguide microring.

[0012] As a preferred embodiment of the invention, the waveguide located on the end face of the diaphragm is a cylindrical waveguide, which is fixed at the center of the diaphragm.

[0013] As a preferred embodiment of the invention, the cylindrical waveguide has a diameter of 2 to 5 micrometers and a length of 20 to 40 micrometers.

[0014] As a preferred embodiment of the invention, a gap is left between the waveguide and the horizontal segment of the waveguide microring located on the end face of the vibrating diaphragm, and the gap is less than 100 nanometers.

[0015] As a preferred embodiment of the invention, the vibrating diaphragm is circular and coaxial with the single-mode optical fiber.

[0016] As a preferred embodiment of the invention, the vibrating diaphragm has a thickness of 3 to 10 micrometers and a diameter of 100 to 150 micrometers.

[0017] As a preferred embodiment of the invention, the waveguide microring is symmetrical about the axis of the single-mode fiber, and the cross-section of the waveguide microring is a circle with a radius of 2 to 5 micrometers.

[0018] As a preferred embodiment of the invention, the ultrasonic sensor is fabricated using 3D printing technology based on two-photon polymerization, and the structural material integrated into the end of the single-mode optical fiber is a photocurable material.

[0019] As a preferred embodiment of the invention, the ultrasonic sensor has an open structure, and the waveguide microring and the waveguide located on the end face of the vibrating diaphragm are located inside the support.

[0020] Compared with the prior art, the advantages of the present invention are:

[0021] (1) This invention uses two-photon 3D printing technology to directly fabricate an integrated micro-nano ultrasonic sensor on the end face of an optical fiber. It has high integration, small structure size, and good stability.

[0022] (2) Compared with the traditional sensing mechanism of direct sound wave action on waveguide, this sensor utilizes the diaphragm vibration effect to highly sensitively control the coupling efficiency of waveguide microrings and realize highly sensitive measurement of ultrasonic signals. When the incident sound frequency is equal to the diaphragm resonant frequency, the sensor's sound pressure sensitivity can be maximized. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of a diaphragm vibration-coupled waveguide microring ultrasonic sensor device according to an embodiment of the present invention;

[0024] Figure 2 The image shown is a microscope image of the sensor structure illustrated in an embodiment of the present invention.

[0025] Figure 3 This is a schematic diagram of the sensor testing experimental apparatus in an embodiment of the present invention;

[0026] Figure 4 This is a time-domain image of the impulse response signal obtained after testing the sensor in an embodiment of the present invention;

[0027] Figure 5 In this embodiment of the invention, the ultrasonic frequency signal is demodulated by fast Fourier transform of the time-domain image of the impulse response signal obtained after sensor testing.

[0028] Figure 6This is a time-domain image of a 640kHz continuous sine wave signal obtained after testing the sensor in this embodiment of the invention.

[0029] In the diagram: 1-single-mode fiber, 2-support, 3-frustum waveguide, 4-waveguide microring, 5-vibrating diaphragm. Detailed Implementation

[0030] The following detailed implementation methods will further illustrate this.

[0031] This invention provides a diaphragm-vibrating waveguide microring ultrasonic sensor device, which is fabricated by two-photon polymerization 3D printing technology. The printed ultrasonic sensor structure is located on the end face of an optical fiber. It has a compact structure with an overall volume size of less than 160 micrometers. Furthermore, by utilizing the strong coupling characteristics of the waveguide microring and the vibrating diaphragm, it can realize the detection of high-frequency ultrasonic waves.

[0032] like Figure 1 As shown, the structure of the diaphragm-vibrating waveguide microring ultrasonic sensor proposed in this invention includes a single-mode optical fiber 1, a hollow cylindrical support 2, a waveguide microring 3, a circular waveguide 4, and a vibrating diaphragm 5. The hollow cylindrical support 2, waveguide microring 3, circular waveguide 4, and vibrating diaphragm 5 are all made of photocurable material and are fabricated using 3D printing technology based on two-photon polymerization. The hollow cylindrical support 2 and the single-mode optical fiber serve as a connector to maintain structural stability, while the waveguide microring 3, circular waveguide 4, and vibrating diaphragm 5 constitute an intensity-modulated sensor.

[0033] In this embodiment, the single-mode optical fiber 1 has a diameter of 125 micrometers. The commercial two-photon 3D printer used is the Nanoscribe Professional GTII model, and the material is the photosensitive IP-DIP material. This material has a low Young's modulus and high acoustic pressure sensitivity. The hollow cylindrical support 2 for printing the ultrasonic sensor has a diameter of 125 micrometers and a length of 130-150 micrometers. The waveguide microring 3 has a bottom radius of 8-10 micrometers, a top radius of 2-5 micrometers, and a length of 20-40 micrometers. The circular waveguide 4 connected to the circular platform has a radius of 2-5 micrometers. The vibrating diaphragm 5 has a thickness of 3-10 micrometers and a diameter of 125 micrometers. The cylindrical waveguide is located at the center of the vibrating diaphragm and is parallel to the top horizontal part of the waveguide microring. It has a diameter of 2-5 micrometers and a length of 20-40 micrometers, which can be adjusted according to the required ultrasonic frequency. Figure 2 This is a microscope image of the sensor structure shown in an embodiment of the present invention.

[0034] The working principle of the ultrasonic sensor of the present invention is based on the waveguide coupling principle, wherein the circular waveguide 4 and the vibrating diaphragm 5 constitute waveguide coupling. When the input light is input from the single-mode optical fiber 1 into the waveguide micro-ring 3, the light entering the waveguide is split into two paths and propagates in the circular waveguide 4 in the clockwise and counterclockwise directions respectively. The horizontal section at the top of the micro-ring is parallel to the waveguide part at the center of the vibrating diaphragm and the spacing between the waveguides is less than 100 nanometers to achieve waveguide evanescent field coupling.

[0035] For two coupled waveguides A and B, when the two waveguides are close together and coupling occurs, the wave field can be approximately expressed as the sum of the wave fields under undisturbed conditions, as shown in the formula:

[0036]

[0037] The coupling equation is

[0038]

[0039] Where K is the coupling coefficient, β is the propagation constant, and E A (z), E B (z) represent the electric fields of waveguide A and waveguide B, respectively, E Ay (x), E By (x) represent the y-direction components of the two waveguide fields, E y It is the total wave field, i and j are the imaginary parts, and k is the imaginary part. A k B is the coupling coefficient, and z is the displacement in the z-direction.

[0040] The power of the coupled waveguide A and waveguide B is:

[0041]

[0042] Among them, P A0 P B0 These are the powers of waveguides A and B at z = 0, respectively, where Δk is the phase mismatch factor, and P... A (z), P B (z) represents the power of waveguide A and waveguide B, respectively;

[0043] From equation (3), it can be seen that when [K 2 +(Δk) 2 ] 1 / 2 When z = π / 2, P A (z) The power reaches its maximum value, meaning the maximum power conversion is achieved between the two guided modes. This distance z is defined as the coupling length, denoted by L. c The formula is as follows:

[0044]

[0045] When ka =k b When the propagation constants of the two waveguides are the same, at z = L c At this point, complete power conversion is achieved, i.e., Δk = 0, and the corresponding coupling length is:

[0046]

[0047] Therefore, the coupling efficiency of a directional coupler depends on the length of the coupling region, which in turn depends only on the coupling coefficient K. The larger the coupling coefficient, the smaller the coupling length required for complete energy transfer, and the smaller the device size.

[0048] For a standard circular diaphragm, the resonant frequency can be expressed as:

[0049]

[0050] In the formula, ω is the resonant frequency; α is the frequency factor; D is the bending stiffness; E is Young's modulus; μ is Poisson's ratio; r is the effective radius of the sensitive diaphragm; h is the thickness of the sensitive diaphragm; and ρ is the density of the sensitive diaphragm material.

[0051] When the refractive index and incident light wavelength remain constant within the sensing area, external ultrasonic pressure acts on the vibrating diaphragm 5, causing the diaphragm to deform and change the distance between the diaphragm and the waveguide microring. Consequently, the intensity of the light returning from the waveguide microring changes. By demodulating the corresponding light intensity, the intensity and frequency information of the ultrasonic signal can be effectively demodulated.

[0052] The diaphragm vibration-coupled waveguide microring ultrasonic sensor of the present invention can effectively broaden the frequency response bandwidth of the sensor by adjusting the geometric dimensions of the sensor structure. Specifically, by adjusting the geometric parameters of the circular vibrating diaphragm, namely the diaphragm thickness and effective radius, the ultrasonic frequency response range and sound pressure response sensitivity of the waveguide microring ultrasonic sensor can be adjusted.

[0053] To test and analyze the frequency response characteristics of the ultrasonic sensor prepared in this embodiment, a [facility / system] was built. Figure 3 The experimental setup shown consists of three parts: an ultrasonic source section, an optical ultrasonic sensor section, and a data processing section. The ultrasonic source section comprises a signal generator and an ultrasonic transducer; in this embodiment, a piezoelectric transducer is used. The optical ultrasonic sensor section comprises a tunable exciter, a fiber optic circulator, a fiber optic end-diaphragm vibration waveguide microring ultrasonic sensor, a photodetector, and an oscilloscope. The data processing section includes a data acquisition unit and a computer.

[0054] A diaphragm-vibrating waveguide microring ultrasonic sensor and an ultrasonic transducer are placed in a water tank to test ultrasonic signals in the water. A signal generator is connected to a piezoelectric transducer in the water tank, which generates ultrasonic signals. The type of ultrasonic signal is determined by the input of the signal generator. A tunable laser outputs a single-wavelength laser beam, which passes through an optical fiber circulator to the diaphragm-vibrating waveguide microring ultrasonic sensor at the fiber optic end. After receiving the optical signal at one end of the single-mode fiber in the sensor, the optical signal enters the waveguide microring from the other end of the single-mode fiber, splits into two beams (forward and reverse), and returns after circling the waveguide microring once. The optical signal returning from the sensor passes through the optical fiber circulator and is received by a photodetector. The photodetector converts the returned optical signal into an electrical signal and displays it on an oscilloscope. Simultaneously, a data acquisition unit acquires and processes the data displayed on the oscilloscope. Meanwhile, during the transmission of optical signals, the vibrating diaphragm in the sensor deforms under the pressure of the ultrasonic signal generated by the piezoelectric transducer, changing the distance between the vibrating diaphragm and the waveguide microring. Consequently, the intensity of the light returning from the waveguide microring changes. By demodulating the returned optical signal received by the photodetector, the intensity and frequency information of the ultrasonic signal can be effectively demodulated.

[0055] In this embodiment, using Figure 3 The experimental setup shown tests the frequency response bandwidth of the sensor. A pulse signal is input to the signal generator. The sensor and piezoelectric transducer are placed facing each other, with a distance of 1-10 cm. The ultrasonic waves emitted by the piezoelectric transducer are transmitted through the water to the sensor, causing the diaphragm 5 to deform and vibrate. Due to the change in the distance between the diaphragm and the waveguide microring, the intensity of the signal light returning from the waveguide microring changes accordingly. After passing through the fiber optic circulator, it is transmitted to the photodetector, and the result is obtained through an oscilloscope. Figure 4 The image shown is a time-domain image after receiving an impact signal. Analysis of the frequency components of the received impact signal is performed. After performing a Fast Fourier Transform on the acquired impact signal, the frequency components are obtained as follows: Figure 5 As shown, the results indicate that the sensor has a wide frequency response bandwidth and a high-frequency characteristic frequency component (around 800kHz). The high-frequency characteristic frequency component is detected by vibration of the diaphragm structure, and the amplitude near the characteristic frequency, i.e. the corresponding sound pressure sensitivity, is much higher than that at other frequencies. It can also stably receive single-frequency continuous sine wave signals.

[0056] Similarly, use Figure 3The experimental setup shown tests the sensor's response at a single-frequency ultrasonic wave. In the experiment, a fixed-frequency continuous sinusoidal signal is input to the signal generator. The sensor and piezoelectric transducer are placed directly opposite each other, with a distance of 1-10 cm. The signal generator produces a continuous sinusoidal signal, which is input to the piezoelectric transducer. The ultrasonic wave generated by the piezoelectric transducer travels through the water to the ultrasonic sensor at the fiber optic end face, causing the diaphragm to deform and vibrate. Due to the change in the distance between the diaphragm and the waveguide micro-ring, the intensity of the returned signal changes accordingly. After passing through the fiber optic circulator, the signal is transmitted to a photodetector. The acquired ultrasonic signal is recorded using an oscilloscope. Figure 6 The waveform in the time domain corresponds to a 640kHz sine wave signal, which indicates that the sensor can accurately detect the frequency information corresponding to a single frequency ultrasonic wave.

[0057] For the purposes of illustration and description, the foregoing illustrative examples relating to the invention are provided. This is not intended to be an exhaustive description of the invention or to limit it to the precise forms described; modifications and variations can be made based on the foregoing description. The embodiments were chosen and described to explain the principles of the invention and as practical applications thereof, enabling those skilled in the art to use the invention in various embodiments and to make various modifications for specific purposes. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of this invention should be considered within the scope of protection of this invention.

Claims

1. A waveguide microring ultrasonic sensor based on diaphragm vibration coupling, characterized in that, It includes a single-mode fiber (1), a support (2) integrated into the end of the single-mode fiber (1), a waveguide microring (4), and a vibrating diaphragm (5). The vibrating diaphragm (5) is fixed on the axis of the single-mode fiber (1) by the bracket (2). The waveguide micro-ring (4) is located between the single-mode fiber (1) and the vibrating diaphragm (5). The optical signal input / output end of the waveguide micro-ring (4) is connected to one end of the single-mode fiber (1). The farthest end of the waveguide micro-ring (4) from the single-mode fiber (1) is the horizontal segment. The vibrating diaphragm (5) has a waveguide on its end face near the waveguide microring (4) that is parallel to the horizontal section of the waveguide microring (4). There is a gap between the waveguide on the end face of the vibrating diaphragm (5) and the horizontal section of the waveguide microring (4). External sound pressure acts on the vibrating diaphragm to deform it, thereby changing the distance between the vibrating diaphragm and the waveguide microring, which causes the coupling efficiency of the waveguide microring to change, thereby changing the intensity of the returned light. The ultrasonic signal is detected by detecting the change in light intensity.

2. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The waveguide micro-ring (4) is fixed to one end of the single-mode fiber (1) through a frustum waveguide (3). The large end face of the frustum waveguide (3) is connected to one end of the single-mode fiber (1), and the small end face of the frustum waveguide (3) is connected to the optical signal input / output end of the waveguide micro-ring (4).

3. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The waveguide located on the end face of the vibrating diaphragm (5) is a cylindrical waveguide, which is fixed at the center of the vibrating diaphragm (5).

4. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 3, characterized in that, The cylindrical waveguide has a diameter of 2-5 micrometers and a length of 20-40 micrometers.

5. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The gap is less than 100 nanometers.

6. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The vibrating diaphragm (5) is circular and coaxial with the single-mode optical fiber (1).

7. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The vibrating diaphragm (5) has a thickness of 3 to 10 micrometers and a diameter of 100 to 150 micrometers.

8. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The waveguide microring (4) is symmetrical about the axis of the single-mode fiber (1), and the cross-section of the waveguide microring (4) is a circle with a radius of 2 to 5 micrometers.

9. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The ultrasonic sensor is fabricated using 3D printing technology based on two-photon polymerization, and the structural material integrated into the end of the single-mode optical fiber (1) is a photocurable material.

10. The ultrasonic sensor based on diaphragm vibration coupling waveguide microring according to claim 1, characterized in that, The ultrasonic sensor has an open structure, and the waveguide microring (4) and the waveguide located on the end face of the vibrating diaphragm (5) are located inside the support.

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