Cavity-enhanced Fabry-Perot interference resonant cavity

Abstract

The invention discloses a cavity enhanced Fabry-Perot interference resonant cavity, and belongs to the technical field of sensing detection. The transduction film is used as a transduction medium, vibration is generated through induction, signals are converted into vibration of the film, optical signals are modulated, and information to be measured can be obtained. According to the invention, the cavity enhancement structure is introduced, the optical fiber collimator and the transduction film jointly form an interference resonant cavity, and the phase sensitization of the optical microcavity is realized by improving the reflectivity of the cavity and prolonging the time of capturing light. Through stable interference of the resonant cavity, strong optical constraint is generated, and interaction between a light beam and a transmission signal is enhanced. In addition to the characteristics of high sensitivity, small loss, small size and high integration, the device has high expandability in the design aspects of bandwidth and sensitivity, and good sensing performance and design flexibility of the device provide a new opportunity for propelling the military and civil fields such as monitoring technology and biomedicine.

Description

Technical Field

[0001] The present invention belongs to the field of sensor detection technology, and more specifically, relates to a cavity-enhanced Fabry-Perot interference resonant cavity. Background Art

[0002] Broadband sensitive detection is the basis of sonar, navigation, trace gas sensing, industrial process monitoring, photoacoustic spectroscopy, oil and gas pipeline leakage monitoring, photoacoustic imaging and other technologies. Although the traditional piezoelectric and capacitive devices currently used in sensing technology have advantages such as large dynamic range and good linearity, they have some inherent defects, such as susceptibility to electromagnetic interference and poor long-distance sensing capabilities, making them unsuitable for working in harsh environments. In addition, traditional sensors have the mutual constraints of broadband response and miniaturization, resulting in limited sensitivity, large size, poor consistency and other problems. Optical sensing technology, as an emerging technology formed by the cross-integration of advanced sensing technology and optics, can not only overcome the difficulties of the above-mentioned traditional sensors, but also has the advantages of anti-electromagnetic interference, high sensitivity, light weight, and small size. It is the latest research direction for the development of a new generation of sensing and detection equipment. Fiber optic sensing systems have many advantages such as resistance to electromagnetic interference, high sensitivity, large dynamic range, light weight, flexible structural design, long transmission distance, easy setup, large-scale array and simultaneous measurement of multiple parameters. They have become an important direction for the development of detection technology and provide a technical platform for sonar, navigation, trace gas sensing, industrial process monitoring, photoacoustic spectroscopy, oil and gas pipeline leakage monitoring, photoacoustic imaging and other fields.

[0003] At present, in order to detect effective information in advance at a sufficiently long distance, more and more attention is paid to the detection of broadband weak signals in the fields of people's livelihood and national defense and military security. High-sensitivity sensing technology has greatly broadened the detection range of signals, effectively improved the universality of special monitoring equipment, and has the potential for deep mining and great application prospects.

[0004] For Fabry-Perot (FP) optical sensors, it is usually considered to use transducer diaphragms of various materials to improve the detection sensitivity, such as metal diaphragms, silicon wafers, polymer diaphragms, two-dimensional material diaphragms, diaphragms with special structures, etc. However, the sensor needs to consider a series of issues such as component structure, sensitivity, frequency response, and packaging. In terms of sensitivity, a wider response frequency range and miniaturization constitute a mutually restrictive relationship, which restricts the improvement of sensitivity. Current technology has not yet effectively balanced these two factors. Secondly, the individual volume of the sensor is in the order of centimeters or even meters, and there are problems such as inability to miniaturize and poor consistency. Current research focuses on low-frequency signals, while there is little research on broadband and high-frequency signals. As the frequency increases, the required sensor aperture becomes smaller, and it needs to reach the micrometer or even nanometer level, which further causes a decrease in sensitivity and an increase in the difficulty of the manufacturing process. In actual engineering applications, large-size devices will make it difficult to achieve dense deployment, thereby limiting the enhancement of broadband reconnaissance and noise suppression capabilities. Micro-electromechanical system (MEMS) devices are mainly made of extremely pure silicon wafers, using processes such as photolithography and etching, coating, and chemical deposition. They have the advantages of small size, low cost, high reliability, light weight, and easy mass production. The combination of fiber optic sensing technology and MEMS technology can provide new solutions for miniaturization to micro-integrated fiber optic sensing systems, and at the same time provide a new platform for high-precision optical sensing technology. Summary of the invention

[0005] In view of the defects of the prior art and the need for improvement, the present invention provides a cavity-enhanced Fabry-Perot interferometer resonant cavity, which aims to improve the optical detection performance, break through the bottleneck of the mutual constraints of broadband and size, realize integrated integration and packaging, and at the same time provide powerful design flexibility to realize high-performance sensing with efficient light enhancement and micro-integration preparation.

[0006] To achieve the above-mentioned purpose, the present invention provides a cavity-enhanced Fabry-Perot interference resonant cavity, comprising a fiber collimator and a transducer film, wherein the transducer film serves as a first end face, and the inner end face of the fiber collimator serves as a second end face, together constituting an interference resonant cavity; the transducer film is used to deform under the action of an acoustic signal to change the cavity length of the interference resonant cavity; the fiber collimator is used to serve as a transmission medium; when working, incident light enters the interference resonant cavity through the outer end face of the fiber collimator, reflects back and forth at the first end face and the second end face of the interference resonant cavity, undergoes back and forth reflection of a preset order, and is finally output from the second end face.

[0007] Preferably, the material of the energy conversion film is one or more of a composite material, a two-dimensional material, a metal material, an inorganic non-metallic material, and an organic polymer material.

[0008] Preferably, the transducer film includes a reflective layer and a deformation layer from the inside to the outside, wherein the reflective layer is used to reflect incident light, and the deformation layer is used to deform under the action of an acoustic signal to change the cavity length of the interference resonant cavity. The fineness F of the Fabry-Perot interferometer resonant cavity is determined by the reflectivities R1 and R2 of the first end face and the second end face: The reflection order is 2F / π. The material of the deformation layer is preferably silicon nitride, which has the characteristics of high heat resistance, high hardness, high chemical stability, ultra-low light loss, etc. The material of the reflection layer is a metal material with high reflectivity, such as gold.

[0009] Preferably, the reflectivity of the first end surface is 99%, and the reflectivity of the second end surface is 90%. From F=54, it can be known that the reflection order of the round-trip reflection is 10.

[0010] Preferably, the material of the deformation layer is silicon nitride, and the material of the reflection layer is gold.

[0011] Preferably, the thickness of the transducer film is 300-500 nm.

[0012] The present invention also provides a photoacoustic signal detection system, comprising: an excitation light source, a circulator, the above-mentioned Fabry-Perot interference resonant cavity and an interference demodulation module; the circulator comprises a first port, a second port and a third port, the incident light emitted by the excitation light source is incident through the first port, emitted from the second port to the fiber collimator, and after being collimated by the fiber collimator, is incident into the interference resonant cavity, and experiences a preset order of back and forth reflection at the first end face and the second end face of the interference resonant cavity, the interference resonant cavity is used to perform acoustic wave modulation on the incident light under the action of the sound signal to be measured, to form a FP reflection interference light signal with a spectral phase change, the FP reflection interference light signal is finally output from the second end face, returns to the second port along the original path, and then enters the interference demodulation module through the third port, the interference demodulation module is used to calculate the phase change of the incident light according to the acquired FP reflection interference light signal, and demodulate the sound signal. The demodulation principle is that when the information to be measured is applied to the transducer film, it is regarded as an external force on the transducer film. The mechanical vibration generated by the transducer film causes the cavity length of the interference resonance cavity to change, thereby realizing the analysis of large dynamic high-frequency optical signals and broadband high-fidelity signal demodulation.

[0013] The above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0014] 1. Compared with the traditional Fabry-Perot cavity structure, this device innovatively proposes a micro-resonant cavity enhancement technology. By using the transducer film as the first end face of the interference resonant cavity and the inner end face of the fiber collimator as the second end face, that is, introducing a cavity enhancement structure, the cavity reflectivity is improved, the light capture time is extended, and the sensitivity of the optical microcavity is enhanced. By stabilizing the interference resonant cavity, a strong optical constraint is generated, a high-order interference effect is achieved, the cavity fineness is effectively improved, and the interaction between the light beam and the transmitted acoustic signal is enhanced. The device has the characteristics of high sensitivity, no loss, small size, and high integration.

[0015] 2. The cavity-enhanced Fabry-Perot interferometer resonant cavity is based on the dependence between microcavity sensing performance and cavity gain. It reduces light transmission loss, maintains high beam quality, and forms a high-precision resonant cavity through multiple reflections in the cavity, meeting the requirements of high sensitivity and broadband response. In actual use, the silicon micromachining process is used to break through the repeatability bottleneck of optical microcavities, and high-performance sensing is achieved through efficient optical enhancement and micro-integration preparation, pushing its sensing detection performance to a new stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic diagram of a cavity-enhanced Fabry-Perot interferometer resonant cavity provided by the present invention.

[0017] Figure 2 Schematic diagram of a photoacoustic signal detection system in an embodiment of the present invention.

[0018] Figure 3 4 is a structural diagram of the transducer film in an embodiment of the present invention.

[0019] In all the drawings, the same reference numerals are used to represent the same elements or structures, where: 1 - excitation light source, 2 - circulator, 3 - detection optical fiber, 4 - optical fiber collimator, 5 - transduction film, 6 - interference resonant cavity, 7 - interference demodulation module. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] The present invention provides a cavity-enhanced Fabry-Perot interference resonant cavity, comprising a fiber collimator 4 and a transducer film 5, wherein the transducer film 5 serves as a first end face, and the inner end face of the fiber collimator 4 serves as a second end face, together constituting an interference resonant cavity 6; the transducer film 5 is used to deform under the action of an acoustic signal to change the cavity length of the interference resonant cavity 6; the fiber collimator 4 is used as a transmission medium; when working, incident light enters the interference resonant cavity 6 through the outer end face of the fiber collimator 4, is reflected back and forth at the first end face and the second end face of the interference resonant cavity 6, undergoes back and forth reflection of a preset order, and is finally output from the second end face.

[0022] Specifically, the material of the energy conversion film 5 is one or more of a composite material, a two-dimensional material, a metal material, an inorganic non-metallic material, and an organic polymer material.

[0023] Specifically, the transducer film 5 includes a reflective layer and a deformation layer from the inside to the outside, wherein the reflective layer is used to reflect incident light, and the deformation layer is used to deform under the action of the acoustic signal to change the cavity length of the interference resonant cavity. The fineness F of the Fabry-Perot interferometer resonant cavity is determined by the reflectivity R1 and R2 of the first end face and the second end face: The reflection order is 2F / π. The material of the deformation layer is preferably silicon nitride, which has the characteristics of high heat resistance, high hardness, high chemical stability, ultra-low light loss, etc. The material of the reflection layer is a metal material with high reflectivity, such as gold.

[0024] Specifically, the reflectivity R1 of the first end surface is 99%, and the reflectivity R2 of the second end surface is 90%. It can be known from F=54 that the reflection order of the round-trip reflection is 10.

[0025] Specifically, the material of the deformation layer is silicon nitride, and the material of the reflection layer is gold.

[0026] Specifically, the thickness of the transducer film is 300-500 nm.

[0027] The present invention also provides a photoacoustic signal detection system, comprising: an excitation light source 1, a circulator 2, the above-mentioned interference resonant cavity 6 and an interference demodulation module 7; the circulator 2 comprises a first port, a second port and a third port, the incident light emitted by the excitation light source 1 is incident through the first port, emitted from the second port to the fiber collimator 4, and after being collimated by the fiber collimator 4, is incident into the interference resonant cavity 6, and experiences a preset order of back and forth reflection at the first end face and the second end face of the interference resonant cavity 6, the interference resonant cavity is used to perform acoustic wave modulation on the incident light under the action of the sound signal to be measured, to form a FP reflection interference light signal with a spectral phase change, the FP reflection interference light signal is finally output from the second end face, returns to the second port along the original path, and then enters the interference demodulation module 7 through the third port, the interference demodulation module 7 is used to calculate the phase change of the incident light according to the acquired FP reflection interference light signal, and demodulate the sound signal. The demodulation principle is that when the information to be measured is applied to the transducer film 5, it is regarded as an external force on the transducer film 5. The mechanical vibration generated by the transducer film 5 causes the cavity length of the interference resonance cavity 6 to change, thereby realizing the analysis of large dynamic high-frequency optical signals and broadband high-fidelity signal demodulation.

[0028] The WLI phase demodulation based on Fourier transform is adopted. The spatial spectrum is obtained by Fourier transforming the spectrum, and the interference phase information is demodulated at the characteristic frequency in the spatial spectrum. Combined with Fast Fourier Transform (FFT), efficient and fast acoustic signal demodulation can be achieved. Since the parameters such as reflectivity are fixed values ​​for the specific interference structure, the beam interference spectrum can be expressed as

[0029]

[0030] When the acoustic field acts on the interference resonant cavity and causes the film to vibrate, if the cavity length changes by ΔL, the wavelength at the trough will shift laterally. If Δλ = -λΔL / L0, and ΔL = L0, the FSR can be considered unchanged, and the shifted spectrum can be expressed as

[0031]

[0032] Therefore, the spatial frequency of the spectrum can be expressed as

[0033]

[0034] According to the properties of Fourier transform, after Fourier transforming the spectral function, the phase information of the spectral function can be obtained by solving the inverse tangent of the ratio of the real part to the imaginary part at the spatial frequency, that is,

[0035]

[0036] In the formula The value range of is (-π / 2,π / 2). As the excess pressure, the sound pressure only cares about the relative change of pressure in the sound field detection, so the phase demodulation also only cares about the phase change information. Phase jump correction can be performed to obtain phase change

[0037] For multi-beam interference, consider the interference of the light that travels back and forth M times and reflected N times in the cavity, assuming that MN = D, D = 1, 2, 3, ..., the interference light intensity I MN It can be expressed as:

[0038]

[0039] Compared with double-beam interference, the optical path difference of the multi-reflected interference light is magnified by D times. For the same cavity length change ΔL, its phase change will also be magnified by D times. At this time, the phase sensitivity of the interference resonant cavity can be expressed as

[0040]

[0041] Therefore, after Fourier transform of the multi-beam interference spectrum, there are multiple characteristic peaks in the spatial spectrum. Phase demodulation of high-order peaks can improve the phase sensitivity of the fiber optic acoustic sensor.

[0042] Example 1

[0043] Figure 1 A schematic diagram of a cavity-enhanced Fabry-Perot interferometer resonant cavity provided in this embodiment, Figure 2 Schematic diagram of the photoacoustic signal detection system in this embodiment, including an excitation light source 1, a circulator 2, a detection optical fiber 3, a fiber collimator 4, a transducer film 5, an interference resonant cavity 6 and an interference demodulation module 7. Specifically, the transducer film 5 serves as a first end face and also as a transducer to detect and generate signals, and the fiber collimator 4 serves as an optical transmission medium and as a second end face, one end of which has an insertion hole and the other end is processed with a fixing groove adapted to the size of the transducer film 5. The fiber collimator 4 is inserted into the package and fixed, and the transducer film 5 and the fiber collimator 4 form an interference resonant cavity 6. Figure 3 4 is a structural diagram of the transducer film in an embodiment of the present invention.

[0044] In the embodiment of the present invention, the optical fiber collimator 4 shapes the light beam so that the incident light beam waist position matches the designed reflection order, thereby limiting transmission loss and improving transmission efficiency.

[0045] In an embodiment of the present invention, the transducer film 5 is manufactured using micro-electromechanical system technology. Compared with traditional silicon optical technology, silicon nitride material has outstanding advantages such as high hardness, low density, and good mechanical properties. The key difficulty is to ensure that the film properties are intact while etching the film at the bottom, and the film material can be manufactured in one piece through a sacrificial demolding process. The bottom wafer is a standard 4-inch silicon wafer. This embodiment also provides a method for preparing the above-mentioned cavity-enhanced Fabry-Perot interferometer resonant cavity, comprising the following steps:

[0046] First, a 400nm silicon nitride layer is added to the upper surface of the wafer as a deformation layer using a low-pressure chemical vapor deposition (LPCVD) process. A shallow cylinder with a depth of 200um and a radius of 1.5mm is etched in the center of the lower surface of the wafer using hard mask lithography and deep reactive ion etching (DRIE) technology, and the deformation layer is precisely controlled to be cut off to form a resonant cavity. Finally, a 60nm gold film is plated on the lower surface of the deformation layer as a reflective layer using electron beam evaporation. Then, a high-reflective dielectric film is plated on the inner end face of the fiber collimator; one end of the package is processed to fit the insertion hole of the fiber collimator, and the other end is processed to fit the fixed groove of the transducer film. The fiber collimator end is inserted into the insertion hole and fixed, and the transducer film is fixed to the package through the fixed groove, forming an interference resonant cavity as a whole. Through a series of processing steps, the optical path length is precisely defined to effectively match the waist position of the Gaussian beam. An integrated structure is formed. This method uses MEMS technology to prepare transducer film, realizes the integrated integration and packaging of transducer film-optical fiber-device, has the advantages of mature technology, high mechanical strength, good consistency, etc., etches transducer film on silicon substrate, integrates and slices silicon wafer to form a single device, can be thinned in the case of large size, while maintaining flatness, not easy to break, realize low-cost batch manufacturing, further reduce elastic coefficient, greatly improve sensitivity, and meet the requirements of miniaturization and high sensitivity. At the same time, by increasing reflectivity, improving cavity fineness, extending the time of capturing light, realizing the sensitivity of optical microcavity, and enhancing the interaction between light beam and signal.

[0047] In the embodiment of the present invention, the film parameters are set to a radius of 1.5 mm, a thickness of 460 nm, and an internal stress of 200 MPa. The resonant frequency of the interference resonant cavity 6 is about 20 kHz, and the length is only about 100 um, achieving small batch, integrated packaging and low-cost batch manufacturing.

[0048] In the embodiment of the present invention, a 1550nm laser beam is used as a detection light source, which is coupled to the interference resonant cavity 6 through a circulator. The reflected light signal from the interference resonant cavity 6 is directed to the interference demodulation module to process the spectral signal and extract the signal information. In order to evaluate the sensing detection characteristics, the sensitivity, frequency response and minimum detectable pressure (MDP) of the device were measured. The interference demodulation module consists of a high-speed spectrum monitor. The sampling rate of the high-speed spectrum monitor can reach up to 17kHz, the wavelength range is 1510-1595nm, 512 points are sampled within this range, the wavelength resolution is about 0.166nm, and the sensitivity is -75dBm. The spectrum monitor is controlled by the signal processing terminal LabVIEW program, which can obtain the spectrum of the interference resonant cavity in real time, and calculate the phase change of the interference resonant cavity in real time through the WLI demodulation algorithm. The resonant frequency is around 20kHz, the broadband response frequency is 50Hz-10kHz, and the equivalent noise pressure should be as low as 13.4μPa / √Hz@2.5kHz. Silicon micromachining technology breaks the reproducibility bottleneck of optical microcavities, further reduces the elastic coefficient, and greatly improves the acoustic sensitivity. The detection sensitivity reaches -120.23dBre rad / μPa@1kHz, and the minimum detection pressure (MDP) is 88.7μPa / √Hz@1kHz. The proposed high-sensitivity, broadband sensing detection technology has the characteristics of high detection sensitivity, low noise, high system stability, and miniature size. This device can quickly transform scientific research results into practical applications, and is expected to be widely used in various military and civilian sensing detection fields such as broadband and weak signal non-destructive detection based on cavity enhancement mechanism, sonar signal listening, nuclear explosion monitoring, anti-missile warning and deep-sea submersible exploration, medical diagnosis, and ultrasonic imaging.

[0049] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A cavity-enhanced Fabry-Perot interferometer resonant cavity, characterized in that: The invention comprises an optical fiber collimator (4) and an energy conversion film (5), wherein the energy conversion film (5) serves as a first end face, and the inner end face of the optical fiber collimator (4) serves as a second end face, and together they constitute an interference resonance cavity (6); the energy conversion film (5) is used to deform under the action of an acoustic signal so as to change the cavity length of the interference resonance cavity (6); the optical fiber collimator (4) is used as a transmission medium; when working, incident light enters the interference resonance cavity (6) through the outer end face of the optical fiber collimator (4), is reflected back and forth on the first end face and the second end face of the interference resonance cavity (6), undergoes back and forth reflection of a preset order, and is finally output from the second end face.

2. The Fabry-Perot interferometer resonant cavity according to claim 1, characterized in that: The material of the energy conversion film (5) is one or more of a composite material, a two-dimensional material, a metal material, an inorganic non-metallic material, and an organic polymer material.

3. The Fabry-Perot interferometer resonant cavity according to claim 1, characterized in that: The transducer film (5) comprises a reflective layer and a deformation layer from the inside to the outside, wherein the reflective layer is used to reflect incident light, and the deformation layer is used to deform under the action of an acoustic signal so as to change the cavity length of the interference resonance cavity (6).

4. The Fabry-Perot interferometer resonant cavity according to claim 3, characterized in that: The material of the deformation layer is silicon nitride, and the material of the reflection layer is gold.

5. The Fabry-Perot interferometer resonant cavity according to claim 1, characterized in that: The reflectivity of the first end surface is 99%, and the reflectivity of the second end surface is 90%.

6. The Fabry-Perot interferometer resonant cavity according to claim 1, characterized in that: The thickness of the energy conversion film (5) is 300-500 nm.

7. A photoacoustic signal detection system, characterized in that: include: An excitation light source (1), a circulator (2), a Fabry-Perot interference resonant cavity and an interference demodulation module (7) as claimed in any one of claims 1 to 6; the circulator (2) comprises a first port, a second port and a third port; the incident light emitted by the excitation light source (1) is incident through the first port, emitted from the second port to the optical fiber collimator (4), collimated by the optical fiber collimator (4), and then incident into the interference resonant cavity (6), and undergoes a preset order of back and forth reflection at the first end face and the second end face of the interference resonant cavity (6); the interference resonant cavity (6) is used to perform acoustic wave modulation on the incident light under the action of the sound signal to be measured, so as to form a FP reflection interference light signal with a spectral phase change; the FP reflection interference light signal is finally output from the second end face, returns to the second port along the original path, and then enters the interference demodulation module (7) through the third port; the interference demodulation module is used to calculate the phase change of the incident light according to the acquired FP reflection interference light signal, and demodulate the sound signal.

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