Probe for optical fiber acoustic sensor

Abstract

The utility model relates to the technical field of optics, discloses a probe for an optical fiber acoustic sensor, and aims to solve the problems of phase fading and polarization fading of an F-P cavity interference type optical fiber acoustic sensor. The probe comprises a relay end, a first optical fiber head and a second optical fiber head which are embedded into a first magnetic ring and arranged at an interval are arranged in the relay end, and a first Faraday piece in the first magnetic ring is located between the first optical fiber head and the second optical fiber head; a third optical fiber head and a second magnetic ring are embedded in the sound sensing end, a second Faraday piece in the second magnetic ring is located between a diaphragm and the third optical fiber head, and the diaphragm is arranged at the tail end of the hard tubular structure to couple external sound signals; the third optical fiber head and the second optical fiber head are directly connected through a single-mode optical fiber so that an F-P resonant cavity can be formed between the end face, perpendicular to the optical axis, of the first optical fiber head and the diaphragm. And the first Faraday piece and the second Faraday piece are used for respectively rotating the polarization states of the incident light and the return light by 45 degrees.

Description

Technical Field

[0001] This utility model relates to the field of optical technology, and in particular to a probe for a fiber optic acoustic sensor. Background Technology

[0002] Most commonly used acoustic sensors are based on piezoelectric or capacitive sensing principles, and after decades of development, the technology has become relatively stable. However, due to their electroacoustic conversion principles, traditional electroacoustic sensors cannot be used in certain application scenarios, such as those with strong electromagnetic interference or in flammable and explosive environments.

[0003] Fiber optic acoustic sensors have been developed as a new type of acoustic product. Compared with traditional electroacoustic sensors, they have many advantages such as simple and compact structure, high sensitivity, resistance to electromagnetic interference, corrosion resistance, and low power consumption. They have broad application prospects in high-risk environments such as flammable and explosive environments, strong electromagnetic interference, and strong corrosion. Currently, they have been applied in fields such as hydrophones, local gas monitoring, material property analysis, non-destructive testing of structures, and partial discharge detection of transformers.

[0004] Currently, fiber optic acoustic sensing technology mainly falls into three categories: intrinsic interferometry, intensity modulation (IMM), and intrinsic FP-cavity interferometry, each with its own characteristics. Intrinsic interferometry features relatively long interferometer arms and typically includes additional optical components for coupling multiple optical paths. Intensity modulation directly modulates the light intensity along the optical path, resulting in a complex structure and requiring high precision. Furthermore, intensity modulation leads to significant light loss, resulting in a lower signal-to-noise ratio. Intrinsic FP-cavity interferometry has a simpler principle and a smaller structure, but it requires the wavelength to be located at the orthogonal operating point. The orthogonal operating point fluctuates considerably with environmental changes, leading to phase attenuation in the output signal. Utility Model Content

[0005] The purpose of this invention is to disclose a probe for fiber optic acoustic sensors to solve the problems of phase fading and polarization fading in FP cavity interferometric fiber optic acoustic sensors.

[0006] To achieve the above objectives, the probe for the fiber optic acoustic sensor disclosed in this utility model includes:

[0007] The relay end has a rigid tubular shell and is equipped with a first fiber optic head and a second fiber optic head that are embedded with a first magnetic ring and spaced apart from each other. The first Faraday plate inside the first magnetic ring is located between the first fiber optic head and the second fiber optic head.

[0008] The housing is a rigid tubular structure for the acoustic sensing end; a third optical fiber head and a second magnetic ring are embedded inside. The second Faraday plate inside the second magnetic ring is located between the diaphragm and the third optical fiber head, and the diaphragm is deployed at the end of the rigid tubular structure to couple external acoustic signals.

[0009] The third fiber head and the second fiber head are directly connected by a single-mode fiber, so that the end face of the first fiber head perpendicular to the optical axis and the diaphragm form an FP resonant cavity; wherein the cavity length meets the requirements of the modulator for the frequency modulation range and modulation depth of the light source; and the first Faraday plate and the second Faraday plate are used to rotate the polarization state of the incident and returned light by 45° respectively.

[0010] Preferably, the present invention further includes a collimating lens disposed between the diaphragm and the third fiber optic head. Further, the collimating lens is disposed between the diaphragm and the second Faraday plate, and the collimating lens and the two adjacent end faces of the third fiber optic head are parallel and form an 8° angle with the vertical plane of the optical axis; the end face of the second fiber optic head adjacent to the first Faraday plate forms an 8° angle with the vertical plane of the optical axis.

[0011] Preferably, the present invention has holes in the housing between the diaphragm and the adjacent collimating lens or the second Faraday plate for deploying the dustproof and breathable membrane.

[0012] Optionally, the housing of this utility model is made of glass tube, and the first optical fiber head, the second optical fiber head and the third optical fiber head are glued to the housing through an intermediate glass tube.

[0013] Optionally, in this invention, the light source wavelength is 1550nm, the frequency modulation range is 200MHz, the modulation depth is 2.63, and the cavity length is greater than 0.2 meters.

[0014] This utility model has the following beneficial effects:

[0015] 1. Based on the probe structure of this utility model, the demodulator collects the light intensity change after the interference of the reflected light from the diaphragm surface and the reflected light from the end face of the first optical fiber. It calculates the axial displacement of the diaphragm caused by the external sound based on the light intensity change, and then determines the magnitude of the external sound based on the axial displacement. The structure is simple and compact, with high sensitivity, anti-electromagnetic interference, corrosion resistance, and low power consumption.

[0016] 2. Single-mode fiber, as a resonant cavity, offers a wide range of selectable cavity lengths, making it compatible with various demodulation schemes.

[0017] 3. Under the combined action of the two Faraday plates, the polarization states of the two reflected lights forming the interference are 180° out of phase. They can be considered to be polarized on the same straight line, and there is no polarization interference fading. Only a constant phase difference of 1 / 2λ is introduced, where λ is the wavelength of the light source. External stress applied to the optical fiber has no effect on the polarization state of the transmitted light. Thus, the two Faraday plates eliminate the influence of random polarization drift on the visibility of the interference light output.

[0018] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0019] The accompanying drawings, which form part of this application, are used to provide a further understanding of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an undue limitation of the present invention. In the drawings:

[0020] Figure 1 This is a schematic diagram of the overall structure of the probe for the fiber optic acoustic sensor disclosed in this embodiment of the present invention.

[0021] Figure 2 yes Figure 1 The diagram shows the internal structure of the relay terminal.

[0022] Figure 3 yes Figure 1 The diagram shows the internal structure of the acoustic sensor. Detailed Implementation

[0023] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered by the claims.

[0024] Example 1

[0025] This embodiment discloses a probe for a long-cavity, long-polarization-fading-resistant fiber FP cavity acoustic sensor.

[0026] like Figure 1 As shown, the probe in this embodiment mainly includes: a sound-sensing end 01, a single-mode optical fiber 02, and a repeater end 03.

[0027] like Figure 2 As shown, the structure of repeater 03 is as follows: the first fiber optic head 031 has a 0° end face without coating (optionally coated with a semi-transparent, semi-reflective film, selected according to the reflectivity of the film), and is glued to the first glass tube 032. The second fiber optic head 036 has a 0° end face (optionally an 8° end face to further reduce reflected light), coated with an anti-reflection film according to the specific light source wavelength (if a 1550nm wavelength light source is used, a 1550nm anti-reflection film is applied), and similarly, it is glued to the first glass tube 032. During assembly, the first Faraday chip 035 can be attached to the end face of the second fiber optic head 036; in the first magnetic ring 034, when the two fiber optic heads 031 and 036 are adjusted to a distance of about 10µm, fine-tuning is made according to the magnitude of reflected light until the light output is at its maximum, and then the internal components are glued to the second glass tube 033.

[0028] The main function of the repeater end 03 is to generate the first reflected signal and the first transmitted signal at the end face 031 of the first optical fiber head, and to add a first Faraday plate between the adjacent end faces of the two optical fiber heads 031 and 036 so that the polarization state of the light reflected back by the diaphragm is rotated by 90°.

[0029] The tail end of the second fiber optic connector 036 is connected to the acoustic sensor 01 via a single-mode fiber optic cable 02.

[0030] The main function of the acoustic sensor 01 is to convert the sound signal into an optical phase signal through the diaphragm 017, and to eliminate polarization fading through the second Faraday plate 015. For example... Figure 3 As shown, in the structural components of the acoustic sensing end, a single-mode fiber optic cable 02, a third fiber optic connector 011, a third glass tube 013, and a collimating lens 012 constitute a single-mode fiber collimating lens; preferably, the end faces through which the light path passes are coated with a 1550nm anti-reflection film; the coupling surfaces of the third fiber optic connector 011 and the collimating lens 012 are both at an 8° angle to reduce interface reflection light. A second magnetic ring 018 is fixed at the front end of the collimating lens 012, and a second Faraday plate 015 is fixed inside it. A fourth glass tube 014 is sleeved on the outside of the third glass tube 013, and a diaphragm 017 is fixed at the front end of the fourth glass tube. The inner and outer diameters of the two glass tubes have a certain matching margin to facilitate angle adjustment and ensure that the final returned light can correctly enter the fiber core. Near the diaphragm end, the fourth glass tube 014 has two holes 016 that can be used to deploy a dustproof and breathable membrane to balance the internal and external pressure and reduce the impact of environmental stresses such as temperature on the diaphragm 017.

[0031] During operation, light enters from the side of the repeater 03 away from the acoustic sensor 01, enters the first fiber optic head 031, and generates 4% first reflected light at the end face, along with 96% transmitted light (when not coated, different semi-permeable films can produce different splitting ratios). The transmitted light continues to propagate through the first Faraday plate 035, where its polarization state is rotated by 45°, and coupled into the second fiber optic head 036. It then propagates through the single-mode fiber 02 into the acoustic sensor 01, is expanded and collimated into a large spot by the collimating lens 012, and then passes through the second Faraday plate 015 where its polarization state is rotated by another 45°. The spot hits the center of the diaphragm, is reflected by the gold-plated polyimide diaphragm, and the returned light passes through the second Faraday plate 015 again, causing its polarization state to rotate by another 45°. It is then focused and coupled into the single-mode fiber 02, returns to the repeater 03, and passes through the first Faraday plate 035 again where its polarization state is rotated by another 45° before interfering with the first reflected light.

[0032] Using the Jones matrix to analyze the polarization state of light, the polarization state of the light reflected back from relay end 03 did not rotate during the process. The Jones matrix of the process is the same as the Jones matrix T1 of the reflection.

[0033] The Jones matrix in the optical path theory process for the light reflected by diaphragm 017 is: T2 = F ′ 1R2F2R1F1. Where F1 is the positive-oriented Jones matrix of the first Faraday slice 035, F... ′R1 is the Jones matrix of the first Faraday film 035 in reverse direction, R2 is the Jones matrix of the forward optical path of fiber 02, R2 is the Jones matrix of the reverse optical path of fiber 02, and F2 is the Jones matrix of the entire process from entering the second Faraday film 015 to being reflected out.

[0034] In this process, light passes through the second Faraday plate 015, is reflected, and then passes through the second Faraday plate 015 again. Given a rotation angle of θ for the Faraday plate, the Jones matrix F2 of the process can be expressed as: When θ is 45°:

[0035] The stress birefringence effect of a common single-mode fiber can be viewed as an elliptic retarder with a unitary matrix. The forward optical path can be written as: Among them, a and b depend on the external stress environment. Its reverse optical path is a reverse elliptical retarder, and the Jones matrix is:

[0036] Combining the calculation of fiber transmission with the optical rotation process of the second Faraday plate 015, the Jones matrix is: The Jones matrix is ​​independent of the external stress environment, and the stress birefringence of the optical fiber has no effect on the polarization state of the light reflected back from the diaphragm. However, relative to the Jones matrix T1 of the light reflected back from repeater 03, its polarization state is rotated by 90°, making interference difficult. Therefore, a first Faraday film 035 is also added to repeater 03. Substituting this into the Jones matrix yields: When θ is 45°:

[0037] It can be seen that after the adjustment of the first Faraday film 035, The two beams are 180° out of phase, meaning they can be considered to be polarized along the same straight line, eliminating polarization interference fading. The only difference is a constant phase difference of 1 / 2λ, where λ is the wavelength of the light source. External stress applied to the fiber has no effect on the polarization state of the transmitted light. The two Faraday plates eliminate the influence of random polarization drift on the visibility of the interference light output.

[0038] The intensity I of the interference light output can be expressed as: I = I1 + I2 + 2I1 I2cosφ(t); where I1 and I2 are the light intensities of the two paths, and φ(t) is the phase difference between the two paths.

[0039] Using the phase of the reflected light from the fiber end face as a reference value, we have: Where φ0 is the fixed initial phase difference caused by the cavity length plus the constant phase difference of 1 / 2λ introduced by the optical path design, n is the refractive index, d is the diaphragm displacement, and λ is the wavelength of the light source. The phase difference is calculated using the read light intensity signal to obtain the diaphragm displacement, and finally the sound signal can be restored.

[0040] Furthermore, to reduce random low-frequency phase fluctuation noise in the optical path, a high-frequency carrier can typically be introduced to modulate the phase difference. This is achieved by modulating the light source frequency, resulting in: Where f is the modulation frequency, t is the time, and C is the modulation depth, which is proportional to the cavity length l. Δν is the frequency modulation range of the light source, and c is the speed of light in the denominator.

[0041] For high-frequency carrier signal processing, based on the extreme points of the Bessel function, the modulation depth C generally needs to be 2.37 or 2.63 to accurately demodulate the carried signal. For narrow-linewidth light sources, the frequency modulation range is generally in the MHz range. If the light source used in this embodiment has a wavelength of 1550nm, a frequency modulation range of 200MHz, and a modulation depth of 2.63, then the specific cavity length requirement is as follows: Therefore, l needs to be at least 0.2m to meet the requirements of signal demodulation.

[0042] In contrast, typical FP cavities are on the order of millimeters in length, making demodulation using a frequency-modulated carrier wave from a light source virtually impossible. This invention, however, uses optical fiber as the resonant cavity, expanding the range of FP cavity length options, significantly reducing light source requirements, and providing more choices for subsequent optical path design.

[0043] Furthermore, the main function of the demodulator in this embodiment is to collect the change in light intensity after interference between the reflected light from the diaphragm surface and the reflected light from the end face of the first optical fiber, calculate the axial displacement of the diaphragm caused by the external sound based on the change in light intensity, and then determine the magnitude of the external sound based on the axial displacement. This is prior art well known to those skilled in the art and will not be elaborated upon.

[0044] In summary, the fiber optic acoustic sensor probe disclosed in this embodiment has at least the following beneficial effects:

[0045] 1. Based on the probe structure of this utility model, the demodulator collects the light intensity change after the interference of the reflected light from the diaphragm surface and the reflected light from the end face of the first optical fiber. It calculates the axial displacement of the diaphragm caused by the external sound based on the light intensity change, and then determines the magnitude of the external sound based on the axial displacement. The structure is simple and compact, with high sensitivity, anti-electromagnetic interference, corrosion resistance, and low power consumption.

[0046] 2. Single-mode fiber, as a resonant cavity, offers a wide range of selectable cavity lengths, making it compatible with various demodulation schemes.

[0047] 3. Under the combined action of the two Faraday plates, the polarization states of the two reflected lights forming the interference are 180° out of phase. They can be considered to be polarized on the same straight line, and there is no polarization interference fading. Only a constant phase difference of 1 / 2λ is introduced, where λ is the wavelength of the light source. External stress applied to the optical fiber has no effect on the polarization state of the transmitted light. Thus, the two Faraday plates eliminate the influence of random polarization drift on the visibility of the interference light output.

[0048] The above description is merely a preferred embodiment of this utility model and is not intended to limit the utility model. Various modifications and variations can be made to this utility model by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this utility model should be included within the protection scope of this utility model.

Claims

1. A probe for an optical fiber acoustic sensor, characterized in that, include: The relay end has a rigid tubular shell and is equipped with a first fiber optic head and a second fiber optic head that are embedded with a first magnetic ring and spaced apart from each other. The first Faraday plate inside the first magnetic ring is located between the first fiber optic head and the second fiber optic head. The housing is a rigid tubular structure for the acoustic sensor. The device has a third fiber optic head and a second magnetic ring embedded inside. The second Faraday plate inside the second magnetic ring is located between the diaphragm and the third fiber optic head, and the diaphragm is deployed at the end of the rigid tubular structure to couple external acoustic signals. The third fiber head and the second fiber head are directly connected by a single-mode fiber, so that the end face of the first fiber head perpendicular to the optical axis and the diaphragm form an FP resonant cavity; wherein the cavity length meets the requirements of the modulator for the frequency modulation range and modulation depth of the light source; and the first Faraday plate and the second Faraday plate are used to rotate the polarization state of the incident and returned light by 45° respectively.

2. The probe for an optical fiber acoustic sensor according to claim 1, characterized in that, Also includes: A collimating lens is positioned between the diaphragm and the third fiber optic head.

3. The probe for an optical fiber acoustic sensor according to claim 2, characterized in that, The collimating lens is positioned between the diaphragm and the second Faraday plate, and the collimating lens is parallel to the two adjacent end faces of the third fiber head and forms an 8° angle with the vertical plane of the optical axis; the end face of the second fiber head adjacent to the first Faraday plate forms an 8° angle with the vertical plane of the optical axis.

4. The probe for an optical fiber acoustic sensor according to claim 2, characterized in that, Also includes: The housing between the diaphragm and the adjacent collimating lens or the second Faraday plate has holes for deploying the dustproof and breathable membrane.

5. The probe for an optical fiber acoustic sensor according to any one of claims 1 to 3, characterized in that, The housing is made of glass tube, and the first, second, and third fiber optic heads are glued to the housing via an intermediate glass tube.

6. The probe for an optical fiber acoustic sensor according to claim 5, characterized in that, The light source has a wavelength of 1550nm, a frequency modulation range of 200MHz, a modulation depth of 2.63, and a cavity length greater than 0.2m.

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