Optical fiber sensor based on mixed structure optical fiber MZI

Abstract

The invention provides an optical fiber sensor design based on an optical fiber Mach-Zehnder interferometer (MZI), and particularly relates to an in-fiber ladder-shaped special structure, which is particularly suitable for online monitoring of a reaction process in a microfluidic system and can measure refractive indexes of three points in the reaction process at the same time. The optical fiber sensor is formed by welding a common single-mode optical fiber, a coreless optical fiber and a stepped single-mode optical fiber. The broadband light source emits an optical signal through the single-mode optical fiber, a high-order mode excited in the coreless optical fiber enters a cladding of the single-mode optical fiber to be transmitted, due to the effect of an evanescent field and the influence of the environment refractive index and the temperature, the signal excites multiple modes in the optical fiber through the cascaded coreless optical fiber, and the high-order mode is transmitted in the cladding of the single-mode optical fiber. And an optical path difference is introduced through a step structure positioned between the two coreless optical fibers. When a solution to be detected is immersed into the stepped groove, the structure can generate refractive index change, and the propagation path of light can be adjusted by finely adjusting different sizes of the stepped groove to generate different phase differences, so that the light is coupled back into a fiber core of the single-mode fiber at the other end in the coreless fiber cascaded at the other end; and due to the special step-shaped structure, the refractive index values at three positions of the central step can be measured at the same time. According to the design, the structure of a traditional Mach-Zehnder interferometer is optimized, the number of elements is reduced, the stability and reliability of the system are improved, and the Mach-Zehnder interferometer is suitable for the fields of precision measurement, environment sensing, optical fiber communication and the like.

Description

(1) Technical Field

[0001] The fiber optic sensor of the hybrid structure fiber optic MZI of the present invention can be widely applied to the monitoring field where the real-time change of the external multi-point refractive index exists. By accurately detecting the optical path difference or phase difference, this sensor can monitor the changes caused by the refractive index in real time, and has high sensitivity and high precision, belonging to the field of fiber optic sensing technology. (2) Background Art

[0002] With the rapid development of fiber optic sensing technology, fiber optic interferometers are increasingly widely used in multiple fields. Especially in high-precision sensing and environmental monitoring, fiber optic interferometers have become an important technical means due to their characteristics of high sensitivity, high precision, and anti-electromagnetic interference. Traditional fiber optic interferometers, such as Mach-Zehnder interferometers, are widely used in the monitoring of physical quantities such as temperature, pressure, and strain, and have achieved important application results in fields such as industrial monitoring, building structure health monitoring, sensor networks, and aerospace.

[0003] Existing fiber optic Mach-Zehnder interferometers generally consist of a light source, a fiber optic beam splitter, an interference region, and a detector. The light source transmits an optical signal through the fiber optic. After being split by the beam splitter and distributed to different paths, the coherent effect in the interference region forms interference fringes, and finally the changes of the interference fringes are monitored by devices such as photodetectors. The disadvantages of these technologies are that the design of the fiber optic path is complex, usually a long transmission distance is required to generate sufficient interference effects, and it is susceptible to environmental factors such as temperature and pressure fluctuations, which may lead to a decrease in system stability. For example, Patent CN104697624A discloses an acoustic wave detector based on a Michelson interferometer. This acoustic wave detector forms a Michelson interferometer by using the reflected light on both sides of a thin film arranged on the acoustic wave detection sensing head. The sound pressure acts on the thin film to cause the optical path of one arm of the Michelson interferometer to increase and the optical path of the other arm to decrease, and by increasing the optical path difference between the two arms of the Michelson interferometer, the sensitivity of the acoustic wave detector is improved. Another related technology is Patent CN106124478A, which proposes a design of a fiber optic interferometer based on multi-path modulation, and enhances the response ability to external environmental changes by changing the optical path difference of different paths. Although this design can effectively improve the sensitivity of the fiber optic interferometer under small environmental changes, its structure is relatively complex, requires multiple fiber optic paths, and is easily affected by temperature and pressure fluctuations during dynamic measurement.

[0004] In addition, patent CN202411618096 proposes an interferometer sensor array based on photonic crystal fiber and a preparation method thereof. The interferometer sensor array based on photonic crystal fiber is formed by cascading multiple photonic crystal fiber interferometers with different collapse lengths, and each of the photonic crystal fiber interferometers includes: a first single-mode fiber, a photonic crystal fiber, and a second single-mode fiber; the first single-mode fiber and one end of the photonic crystal fiber are fused to form a first collapse area, and the other end of the photonic crystal fiber is fused to the second single-mode fiber to form a second collapse area; wherein the sum of the collapse lengths of the first collapse area and the second collapse area is the collapse length of the photonic crystal fiber interferometer. The interferometer sensor array realizes the precise measurement of strain, and cascading the photonic crystal fiber interferometer can realize simultaneous real-time monitoring of multiple sets of parameters. Therefore, how to improve the sensitivity, stability and response speed of fiber interferometers in different applications has become a key issue in the current technological development.

[0005] In order to overcome these deficiencies in the prior art, the present invention introduces a stepped single-mode fiber structure in the fiber Mach-Zehnder interferometer and simplifies the system design. By accurately designing the geometric dimensions of these grooves (such as depth, width, spacing, etc.), the propagation characteristics of the optical signal can be adjusted. The deficiencies in the prior art are overcome, and not only the sensitivity and stability of the interferometer can be improved, but also the system complexity and manufacturing cost can be reduced, providing a more efficient and reliable solution for the fields of environmental monitoring, sensor networks, etc. Through this technology, the sensor can better adapt to different environmental conditions and improve the response speed and sensitivity to small changes. (III) Summary of the invention

[0005] The object of the present invention is to provide an optical fiber Mach-Zehnder interferometer based on a stepped single-mode optical fiber structure, specifically an interferometer system which improves the optical path difference adjustment accuracy and sensitivity by improving the optical fiber path design.

[0006] Based on the traditional Mach-Zehnder interferometer, the system optimizes the propagation characteristics of the optical fiber by introducing a single-mode optical fiber with a stepped structure, thereby improving the responsiveness to external refractive index changes. The following is the detailed technical content of the present invention.

[0007] The fiber-optic Mach-Zehnder interferometer uses the interference phenomenon of light to split the light beam emitted by the same light source into two parts, propagate along different paths respectively, and then recombine the two parts of the light beam. During the merging process, due to the existence of the optical path difference, the phase of the light wave changes, forming interference fringes. The intensity change of the interference fringes is closely related to the change of the external refractive index.

[0008] In traditional fiber optic interferometers, the optical path difference is mainly caused by the geometric length difference of the fiber or the change in the refractive index of the fiber. However, the traditional single-mode fiber structure has limitations in the adjustment accuracy of the optical path difference. Especially in high-precision sensing applications, it is difficult to cope with extremely small changes.

[0009] The core innovation of the present invention lies in precisely controlling the propagation characteristics of light by introducing a stepped single-mode fiber structure in the fiber optic path. The stepped structure realizes precise adjustment of the light beam by adjusting the refractive index distribution of the fiber core, thereby making the optical path difference more controllable. Specifically, the design of the stepped structure can make the propagation speed of light inside the fiber change in an adjustable manner by controlling the reflection and refraction of the fiber. Let the refractive index distribution of the stepped fiber be:

[0010] where n 1 is the refractive index of the core, n 2 is the refractive index of the fiber stepped structure, n 3 is the refractive index of the cladding, r 1 and r 2 are the radii of different layers respectively. In the stepped fiber, the existence of different refractive index regions causes the propagation speed of light to be different in different regions, thereby generating different optical path differences. Assuming that the path length of light propagating in the stepped fiber is L, the optical path difference ΔL can be expressed as:

[0011] where v(r) is the light speed in different regions, and can be specifically expressed as:

[0012] where c is the light speed and n(r) is the refractive index of different regions. The change in the optical path difference is related to the refractive index of the external environment, causing the phases of different modes in the optical path to change during transmission, thereby affecting the optical path difference and further causing changes in the interference fringes. (IV) Description of the Drawings

[0016] Figure 1 are the 2D and 3D schematic diagrams of the structure of the fiber optic sensor based on the hybrid structure fiber MZI. This structure consists of a single-mode fiber 1, a coreless fiber 2, and a stepped single-mode fiber 3, and is composed of a symmetric structure in this order.

[0017] Figure 2 is the schematic diagram of the measurement refractive index system of the fiber optic sensor based on the hybrid structure fiber MZI applied to droplet microfluidics technology. (V) Specific Embodiments

[0018] The fiber optic Mach-Zehnder interferometer of the present invention can be used for the measurement of multi-point refractive indices. The following is throughFigure 2 Describe the application of the present invention and explain how to use the technology of the present invention to detect the liquid to be tested.

[0019] The system consists of a Y-shaped microfluidic channel 1 and an optical fiber sensor 2 of a hybrid-structured fiber MZI.

[0020] When the system starts, a broadband light source (BBS) emits a stable optical signal. This light source is generally a laser diode (LD) or other types of light sources, and its wavelength and power are set to the values required by the system.

[0021] Take the liquid to be reacted 1 and the liquid to be reacted 2. First, place the Y-shaped microfluidic channel on the reaction table, inject the liquid 1 into one end of the Y-shaped microfluidic channel, and inject the liquid to be tested 2 into the other end of the Y-shaped microfluidic channel.

[0022] The optical signal is transmitted through a single-mode optical fiber or other types of optical fibers. When the optical fiber is exposed to the liquid, the refractive index of the liquid will affect the propagation characteristics of the optical signal in the optical fiber. Since the coreless optical fiber has no cladding, its core region is completely exposed to the surrounding medium (such as air or the liquid to be tested). When the refractive index of the surrounding medium changes, it will directly affect the characteristics of the light waves of different modes propagating in the coreless optical fiber. The change in the refractive index of the liquid causes the propagation speed of light in the optical fiber to change, thereby affecting the optical path difference.

[0023] Fix the sensing region of the in-fiber MZI interferometer in the fluid of the reaction zone or the outlet section of the Y-shaped microfluidic channel. When the reaction liquid 1 and the reaction liquid 2 enter the reaction zone, due to the target reaction, the refractive index of the resulting mixed liquid 3 is different from that of the liquid 1 and the liquid 2, causing the refractive index of the liquid to change.

[0024] Due to the change in the refractive index of the liquid, the propagation speed and propagation path of light in the optical fiber change, resulting in a change in the optical path difference. The change in the optical path difference causes a change in the phase of the light wave, and then interference fringes are formed in the interference region. The change in the intensity of the interference fringes is proportional to the change in the optical path difference, and the change in the refractive index causes a change in the position or intensity of the interference fringes.

[0024] Due to the described stepped structure, the optical path differences of the refractive indices at points A, B, and C of the sensor have certain differences, and the refractive index values at points A, B, and C can be obtained simultaneously, improving the accuracy of the measurement.

[0025] Real-time monitoring is carried out through a detector (usually a photodetector). The detector receives the combined optical signal and converts the change in the interference fringes into an electrical signal. The electrical signal output by the detector is further analyzed by a signal processing unit. By analyzing the spectrum and intensity change of the interference signal, the change value of the optical path difference is obtained.

[0026] By obtaining the change value of the optical path difference, the refractive index change degree and trend of the mixed liquid 3 are judged. When the obtained refractive index tends to be stable, it can be judged that the reaction is over. At the same time, according to the refractive index value of the liquid after stabilization, it can be compared whether the mixed liquid generated by the reaction is the target liquid.

[0027] The refractive index change calculated by the signal processing unit is displayed in real time through a display device or other output means, and the user can directly read the measurement results. These results can be used for further analysis or joint monitoring with other measurement systems.

[0028] The fiber optic Mach-Zehnder interferometer of the present invention can provide high-precision and high-sensitivity measurement results and is applied to multi-point refractive index measurement during the reaction of the object to be measured. By adopting a stepped single-mode fiber structure, the system can effectively cope with various environmental changes, has strong stability and adaptability, and is suitable for multiple fields such as industrial production and environmental monitoring.

[0029] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0030] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0031] The above are only the embodiments of the present invention and do not limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included within the scope of the claims of the present invention.

Claims

1. An optical fiber sensor based on a hybrid structure optical fiber MZI, characterized in that: It consists of the following main parts: 1 single-mode optical fiber, 2 coreless optical fiber and 3 ladder-structured single-mode optical fiber. In the sensor structure: the single-mode optical fiber 1 is responsible for ensuring that the light source is transmitted in a single mode after passing through, without mode mixing or multi-mode interference, and ensuring that the optical signal maintains a stable phase relationship during propagation. The coreless optical fiber 2 is used to transmit the optical signal from the single-mode optical fiber to the next part, providing a low-loss optical transmission path. Due to the difference in waveguide structure and refractive index distribution between the single-mode optical fiber and the coreless optical fiber, the incident fundamental mode light will no longer propagate in a single mode, but will excite multiple modes and transmit it to the next part. The main function of the ladder-shaped single-mode optical fiber 3 is to adjust the propagation path and optical path difference of the light. Because the coreless optical fiber 2 disperses the light into the cladding and core of the ladder-shaped single-mode optical fiber 3 during transmission, and through the design of the ladder-shaped groove, the change in the refractive index of the surrounding medium will affect the effective refractive index of different modes, so the accumulated optical path difference between different modes will also change accordingly. Leading to the generation of optical path difference. This optical path difference provides a phase difference for interference and is a key part in achieving the interference effect. The stepped single-mode fiber 3 is located between the coreless fibers 2. After passing through the stepped single-mode, the coreless fiber is used again to couple the light beam to the next portion of the single-mode fiber, ensuring that these optical signals are combined in an appropriate manner so that they can produce interference effects in the interference area. The hybrid structure is achieved by fusing 1 single-mode fiber, 2 coreless fibers, and 3 stepped single-mode fibers together using a fiber fusion splicer.

2. The optical fiber sensor of the hybrid structure optical fiber MZI according to claim 1, characterized in that: The refractive index change of the stepped single-mode optical fiber is achieved by introducing a stepped groove structure in the core area of ​​the optical fiber. The depth and position of the stepped groove are precisely designed to adjust the propagation speed of the optical signal in the optical fiber, thereby achieving the control of the optical path difference and the introduction of the phase difference. The key to step-shaped single-mode optical fiber lies in the spatial distribution of its refractive index, which changes gradually in the core area of ​​the optical fiber. This change is achieved through physical structural design. Specifically, the refractive index of the optical fiber is changed by introducing a step-shaped groove (or a step-shaped structure) in the core area of ​​the optical fiber. When the optical signal propagates, the change in the refractive index in the optical fiber changes the propagation speed and optical path of the light, thereby introducing a phase difference.

3. The optical fiber sensor of the hybrid structure optical fiber MZI according to claim 1, characterized in that: The outer diameter of single-mode fiber and coreless fiber is 125μm, and the core diameter of single-mode fiber is 9μm.

4. The optical fiber sensor of the hybrid structure optical fiber MZI according to claim 1, characterized in that: The length of the single-mode optical fiber is 400 μm, the length of the coreless optical fiber is 100 μm, and the length of the stepped single-mode optical fiber is 400 μm.

5. The optical fiber sensor of hybrid structure optical fiber MZI according to claim 2, characterized in that: The stepped groove structure refers to a series of stepped grooves in the core area of ​​the optical fiber. The depth and position of these grooves have a direct impact on the propagation path and speed of light. By accurately designing the geometric dimensions of these grooves (such as depth, width, spacing, etc.), the propagation characteristics of the optical signal can be adjusted.

6. The optical fiber sensor of the hybrid structure optical fiber MZI according to claim 1, characterized in that: An interference detector needs to be connected to the receiving end of the optical fiber. The interference detector is a photodetector or a phase detector. The interference detector is connected to the output end of the optical fiber and is used to detect the interference fringes of the combined optical signal. The interference fringes reflect the change in the optical path difference and analyze the change in the external multi-point refractive index by processing the output signal.

7. The optical fiber sensor of hybrid structure optical fiber MZI according to claim 1, characterized in that: The interference detector is connected through the output end of the optical fiber. The optical signal after the optical fiber is combined is transmitted to the detector, and the detector detects according to the interference fringes of the received optical signal. The interference fringes are generated by the phase difference caused by the optical path difference. The combined optical signal forms different interference patterns in the interference area according to the change of the optical path difference. The interferometer can monitor the changes in interference fringes in real time, use a signal processing unit to process the detection signal in real time, and then calculate the changes in optical path difference or phase difference according to the changes in interference fringes, and output measurement results related to the refractive index. The measurement results can be used for environmental monitoring, sensor networks or precision measurement systems.

8. The optical fiber sensor of the hybrid structure optical fiber MZI according to claims 1-7, characterized in that: The method is: S1. Before using the hybrid structure fiber MZI fiber sensor, you first need to prepare a suitable light source. Usually a laser source with stable wavelength and intensity is used, such as a semiconductor laser or a helium-neon laser. Make sure that the light source can provide sufficient optical power and stability so that the optical signal can be transmitted through the optical fiber and interfere effectively. S2. Connect the output of the light source to the optical fiber system through a fiber coupler. The light signal of the light source will enter the optical fiber system and be transmitted through the optical fiber. At this time, the light signal will propagate through the single-mode optical fiber and be guided to the interference area of ​​the optical fiber Mach-Zehnder interferometer. S3. The optical signal is transmitted along the optical fiber and passes through the structural parts of the stepped single-mode optical fiber. The design of these structural parts will change the propagation speed and optical path difference of the light, causing the phase difference of the optical signal to change inside the interferometer. The optical signal interferes in the interference area, and due to the optical path difference or phase difference of different paths, the combined optical signal will produce interference fringes of alternating light and dark. The formation of interference fringes is closely related to external environmental factors, which affect the interference pattern by affecting the optical path difference or phase difference. S4. The interference fringes of the optical signal are detected by an interference detector. The detector can be a spectrometer or a photodetector, which is used to capture the intensity and phase changes of the combined optical signal. The electrical signal transmitted by the detector to the signal processing unit will be processed in real time. The signal processing unit analyzes the interference signal and extracts the changes in the optical path difference or phase difference. Based on the changes in the interference fringes, the signal processing unit calculates the refractive index and temperature. S5. The signal processing unit outputs processed measurement results, which reflect the changes in external refractive index conditions.

Images