Portable optical fiber type refractive index sensor based on smart phone

Abstract

The invention discloses a portable optical fiber type refractive index sensor based on a smart phone in the technical field of optical fiber optics, which comprises a main optical fiber and a light input optical fiber, and the main optical fiber and the light input optical fiber are responsible for light input and transmission; the coupling optical fiber and the light output optical fiber are used for outputting and transmitting light; the smart phone is provided with a flashlight and a camera, and one end of the coupling optical fiber is arranged in front of the camera; the camera is used for capturing light, and when the refractive index of a surrounding medium changes, the light transmission mode between the main optical fiber and the coupling optical fiber also changes, so that the refractive index is measured. The invention provides a simple, cost-effective and robust refractive index sensor, integration with a smartphone not only ensures simple deployment, but also achieves seamless data transmission and storage, making it very suitable for laboratory and field applications.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber optics, and in particular relates to a portable optical fiber type refractive index sensor based on a smart phone. Background Art

[0002] The fiber optic refractive index sensor on the market is a sensor that measures the refractive index of liquids using optical fiber as a medium. It has the advantages of small size, light weight, anti-electromagnetic interference, good electrical insulation and high sensitivity. Optical fiber sensors (OFS) used for refractive index sensing can be divided into different categories according to their basic measurement principles. Notable examples include Fabry-Perot interferometers (FPIs), multimode interferometers (MMIs) and Mach-Zehnder interferometers (MZIs). In addition, long-period gratings (LPGs) and fiber Bragg gratings (FBGs) use wavelength modulation, in which changes in the refractive index cause the wavelength of the transmitted light to shift. This wavelength modulation method is usually more complex and requires advanced measurement equipment for precise detection. In contrast, sensors based on intensity modulation rely on changes in light intensity when the refractive index changes, and their implementation is simpler and the measurement technology is more direct. Among the sensors based on intensity modulation, polymer optical fibers (POFs) stand out as a cost-effective option, combining affordability with the inherent advantages of optical fibers, such as flexibility and easy integration with devices such as smartphones, to achieve portable, real-time refractive index sensing.

[0003] Previously, Ujihara et al. proposed a fluorinated graded-index tapered polymer optical fiber (POF) that creates a refractive index sensor by using strong illumination propagation within the fiber. Similarly, De-Jun et al. also used tapering technology for refractive index sensing. These sensors rely on the principle that the evanescent wave generated by the propagating light interacts with the surrounding medium, resulting in a reduction in light transmission. The authors took a different approach and fabricated a D-shaped fiber sensor using a side half-polishing method, which also works based on the interaction between the evanescent wave and the external environment. Yanjun Hu et al. introduced a refractive index sensor based on narrow grooves and gold-plated structures, but the process of manufacturing these structures increased the complexity and cost of the sensor. Another approach was proposed by Shin JD et al. and Ye et al., who explored drilling holes in POFs to develop refractive index sensors, including a portable multi-hole sensor. However, the drilling step introduces additional complexity and may limit the performance of the sensor. Another study investigated a multi-parameter sensor for liquid level and refractive index measurement using both a grooved polymer optical fiber (POF) and a U-shaped sensor structure.

[0004] Refractive index (RI) sensors play a key role in a variety of applications, including chemical analysis, environmental monitoring, and biomedical diagnostics. However, traditional RI sensors usually require bulky equipment, complex setups, and expensive materials, which limits their portability and wide application. There is still a significant research gap in a simple, cost-effective, and portable fiber-based RI sensor. Therefore, a portable fiber-optic RI sensor based on a smartphone is urgently needed to solve the above problems. Summary of the invention

[0005] In order to overcome the shortcomings of the prior art, the present invention provides a portable optical fiber refractive index sensor based on a smartphone, which integrates plastic optical fiber with a smartphone, provides a lightweight and portable solution, eliminates the need for bulky equipment, greatly reduces the overall cost of the sensing system compared to traditional methods, and is easy to manufacture. The system adopts a simple twisted macro-bend optical fiber configuration, which is very simple to manufacture and assemble. Moreover, the sensor can be adapted to a variety of applications, including chemical analysis, environmental monitoring and biomedical sensing.

[0006] In order to achieve the above purpose, the following technical scheme is adopted: The present invention provides a portable optical fiber refractive index sensor based on a smart phone, comprising:

[0007] A main optical fiber and an optical input optical fiber, wherein the main optical fiber and the optical input optical fiber are responsible for inputting and transmitting light;

[0008] A coupling optical fiber and a light output optical fiber, wherein the coupling optical fiber and the light output optical fiber are used for outputting and transmitting light;

[0009] A smart phone, wherein the smart phone is provided with a flashlight and a camera, and one end of the coupling optical fiber is placed in front of the camera;

[0010] A 3D printed housing and a glass sheet, wherein the 3D printed housing is provided with a hole for the optical fiber to pass through;

[0011] The flashlight is configured to generate light and the camera is configured to capture the light. When the refractive index of the surrounding medium changes, the light transmission mode between the main optical fiber and the coupling optical fiber will also change accordingly, thereby measuring the refractive index.

[0012] Furthermore, the main optical fiber and the coupling optical fiber are both uncoated polymer optical fibers, and the specifications thereof are Mitsubishi graded-index SK-40 optical fibers, with a diameter of 980 microns and a surrounding cladding thickness of more than 20 microns.

[0013] Furthermore, the light output optical fiber and the light input optical fiber are both made of a combination of polymethyl methacrylate resin and a fluorine-containing polymer compound.

[0014] Furthermore, the main optical fiber is wound around the coupling optical fiber and fixed by a nylon tape, the length of the twisted coupling portion exceeds 60 mm, the bending radius is set to 8 mm, and both the main optical fiber and the coupling optical fiber pass through the 3D printed housing.

[0015] Further, when measuring the refractive index value, the twisted coupling portion of the main optical fiber and the coupling optical fiber is placed on a glass sheet.

[0016] Further, the 3D printed housing is configured to house components of the polymer optical fiber and the smartphone.

[0017] Furthermore, the main fiber and the coupling fiber receive a double treatment, being twisted and undergoing macrobends.

[0018] Furthermore, the 3D printed housing is arranged on a smart phone, and the smart phone is fixed on a bracket.

[0019] Furthermore, the main optical fiber and the flashlight are connected without any gap between them.

[0020] Furthermore, the refractive index of the optical output fiber and the optical input fiber is measured to be 1.49, and the larger core diameter and relatively thinner cladding region lead to significant bending-induced optical loss through the side coupling effect.

[0021] The beneficial effects of the present invention are:

[0022] (1) The POF-based sensor of the present invention utilizes the principle of intensity modulation coupling loss in twisted and large-bend optical fibers. The change in the refractive index of the surrounding medium directly affects the optical power coupling, thereby achieving high sensitivity and fast response time;

[0023] (2) Unlike traditional electronic sensors, POF-based sensors are naturally immune to electromagnetic and radio frequency interference. This makes them suitable for use in environments with frequent electromagnetic activities, ensuring reliable performance without signal attenuation;

[0024] (3) Polymer optical fibers (POFs) are well known for their robustness in harsh environments. The proposed sensor can withstand extreme temperatures, humidity, and exposure to potentially corrosive substances, thereby extending its service life and improving reliability.

[0025] (4) The use of commercially available polymer optical fibers, combined with a simple fabrication process involving twisting and large bends, ensures a low-cost and scalable solution that integrates the sensor with readily available smartphones, further reducing the need for expensive specialized equipment;

[0026] (5) By integrating the POF sensor directly with a smartphone, using the phone flash as the light source and the camera as the detector, the system provides a compact and portable solution for real-time refractive index measurement. The custom 3D-printed housing ensures easy alignment and secure mounting, making it ideal for both field and laboratory applications.

[0027] (6) This sensor can be used in a variety of applications, including chemical analysis, food quality monitoring, biomedical diagnostics, and environmental sensing. Its adaptability to different liquids and media makes it a versatile tool for various industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic structural diagram of a portable optical fiber refractive index sensor of the present invention;

[0029] Figure 2 A schematic diagram of the structure of the TMBOC technology of the present invention for refractive index sensing and optical coupling;

[0030] Figure 3 for Figure 2 A schematic diagram of the local enlarged structure at point A in the middle;

[0031] Figure 4 It is a schematic diagram showing the comparison of the sensitivity of the sensor with and without the TCMB effect of the present invention;

[0032] Figure 5 Schematic diagram of the response of the refractive index sensor under different bending radii in the same liquid (refractive index is 1.333) in the present invention.

[0033] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0034] legend:

[0035] 1. Flashlight; 2. 3D printed housing; 3. Main optical fiber; 4. Coupling optical fiber; 5. Glass sheet; 6. Smartphone; 7. Camera; 8. Pipette; 9. Light output optical fiber; 10. Light input optical fiber. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0037] Unless otherwise defined, all professional and scientific terms used herein have the same meanings as those familiar to those skilled in the art. In addition, any methods and materials similar or equivalent to those described herein can be applied to the present invention.

[0038] Example

[0039] like Figure 1 and Figure 2 As shown, a portable optical fiber refractive index sensor based on a smartphone includes

[0040] The main optical fiber 3 and the optical input optical fiber 10 are responsible for the input and transmission of light;

[0041] Coupling optical fiber 4 and light output optical fiber 9, coupling optical fiber 4 and light output optical fiber 9 are used for light output and transmission;

[0042] A smart phone 6, wherein the smart phone 6 is provided with a flashlight 1 and a camera 7, and one end of the coupling optical fiber 4 is placed in front of the camera 7;

[0043] A 3D printed housing 2 and a glass sheet 5, wherein the 3D printed housing 2 is provided with a hole for the optical fiber to pass through;

[0044] The flashlight 1 is configured to generate light and the camera 7 is used to capture the light. When the refractive index of the surrounding medium changes, the light transmission mode between the main optical fiber 3 and the coupling optical fiber 4 will also change accordingly, thereby measuring the refractive index.

[0045] Among them, the main optical fiber 3 and the coupling optical fiber 4 both use uncoated polymer optical fiber, whose specifications are Mitsubishi gradient refractive index SK-40 optical fiber, with a diameter of 980 microns and a surrounding cladding thickness of more than 20 microns. The optical output optical fiber 9 and the optical input optical fiber 10 are both composed of a combination of polymethyl methacrylate resin and a fluorine-containing polymer compound. The main optical fiber 3 is wound around the coupling optical fiber 4 and fixed by a nylon belt. The length of the twisted coupling part exceeds 60 mm, and the bending radius is set to 8 mm. The main optical fiber 3 and the coupling optical fiber 4 both pass through the 3D printed shell 2.

[0046] In addition, when measuring the refractive index value, the twisted coupling parts of the main optical fiber 3 and the coupling optical fiber 4 are placed on the glass sheet 5, the 3D printed housing 2 is configured to accommodate the components of the polymer optical fiber and the smart phone 6, the main optical fiber 3 and the coupling optical fiber 4 receive double treatment, being twisted and undergoing large bending, the 3D printed housing 2 is set on the smart phone 6, and the smart phone 6 is fixed on the bracket, the main optical fiber 3 and the flashlight 1 are connected without any gap between the two, the refractive index measurement value of the light output fiber 9 and the light input fiber 10 is 1.49, and the larger core diameter and relatively thin cladding area cause significant bending-induced optical loss through the side coupling effect.

[0047] The smartphone-based sensing setup involves transmitting light and recording video to monitor intensity changes. The recorded video is then carefully analyzed frame by frame using MATLAB, a powerful numerical computing tool. First, each frame is converted to grayscale using the “rgb2gray” function, which converts color information into a range of grayscale values ​​that is directly related to brightness. The total light intensity is then quantified by adding the grayscale values ​​of all pixels in each frame. This process can be expressed as the following formula:

[0048]

[0049] Where M and N are the frame sizes. Since the video resolution is 1920×1080, M and N are set to 1920 and 1080, respectively. This systematic approach helps calculate the absolute intensity of each frame, enabling seamless measurement through continuous video analysis. For analysis purposes, the point where the sensing area is fully immersed in the liquid is designated as the baseline or zero-order position to facilitate data normalization. The formula is: R=R0Rd, where R is the relative intensity, R0 represents the absolute intensity value at (RI≈1), and R_d represents the state of the sensor in the liquid. Then, the change in relative intensity ΔR is expressed as:

[0050]

[0051] The correction factor c is applied to the baseline intensity R0 and the sensitivity of the refractive index sensor can be derived as follows:

[0052]

[0053] Here, S_(RI) represents sensitivity, which is defined as the ratio of the relative intensity change ΔR to the corresponding refractive index change Δn. To ensure the stability of the measurement, the entire experimental setup, including the smartphone 6, remained stationary during the entire process. This precaution eliminated the need for compensation adjustments due to possible movement of the phone, simplified the measurement process, and improved reliability.

[0054] The performance of the refractive index sensor was carefully evaluated to understand the impact of introducing the TMBOC phenomenon, such as Figure 4 and Figure 5 As shown in the figure, the response of the sensor was analyzed, both when the TMBOC technology was used and when it was not used, focusing on its sensitivity to various refractive index liquids. The results showed that when the TMBOC technology was used, the sensitivity of the sensor was significantly improved. The sensitivity of the refractive index sensor was (SRI = 137.2 ± 5.3% / RIU). The sensor using the TMBOC technology showed a linear response, and the correlation coefficient (R 2) is 0.99144, indicating that the coupling power is lost with the change of the surrounding refractive index medium. The improvement in sensitivity can be attributed to the TMBOC effect, which induces additional coupling power through the macroscopic band structure and shows more coupling power loss when immersed in liquids with different refractive indices. This technology is particularly suitable for detecting small changes in refractive index because sensors enhanced with TMBOC are more responsive than sensors without TMBOC. In order to further optimize the performance of the sensor, the influence of different bending radii on plastic optical fiber sensors was explored, such as Figure 5 shown.

[0055] The dynamic response of the sensor to changes in the refractive index of the external medium has been thoroughly examined, e.g. Figure 5 As shown, in this experiment, the sensor experienced a transition from an air refractive index of approximately 1 to a liquid with a refractive index of 1.333. The results showed that the sensor was able to detect rapid changes in refractive index, as evidenced by the significant decrease in coupled power loss after the sensor was immersed in the liquid. It is worth noting that the sensor with a bending radius of 8 mm performed better than the other sensors, showing the largest change in coupled power loss during the transition, indicating that the sensor is extremely sensitive to refractive index changes and is suitable for real-time monitoring applications. The rapid stabilization of the sensor response further highlights its potential for use in environments where fast and accurate refractive index measurements are required.

[0056] When used specifically: a 3D printed housing 2 is used to accommodate the POFs. There is a hole with a radius of 0.6 mm on the housing for the optical fiber. The dimensions of the 3D printed housing 2 have been carefully determined, with a height of 33.850 mm, a width of 83.580 mm, a length of 53.814 mm, and a total weight of only 32.88 grams. These dimensions were selected on the basis of maintaining a lightweight and compact design while accommodating the optical fiber and smartphone components. The precisely designed 0.6 mm radius hole on the connector can fit tightly against the optical fiber and completely enclose the emitted light, minimizing potential sources of error and improving the accuracy of refractive index measurements. The selection of a 3D printed housing is particularly advantageous because it allows for rapid prototyping and customization, so that the design can be optimized according to the specific requirements of the refractive index sensor. The lightweight and compact design of the 3D printed housing helps to improve the overall portability and user-friendliness of the refractive index sensor.

[0057] The proposed portable refractive index sensor design adopts a sensor that utilizes a twisted structure of polymer optical fibers. In this configuration, one optical fiber designated as the main fiber 3 is responsible for transmitting light and is connected to an LED light source. The other fiber is called the coupling fiber 4. The TMBOC fibers undergo a dual treatment: they are both twisted and undergo large bends to achieve efficient light coupling. This specific operation enables the fiber to become sensitive to changes in the refractive index of the surrounding medium.

[0058] When the refractive index of the surrounding medium changes, the light transmission mode between the main optical fiber 3 and the coupling optical fiber 4 will also change, so that the refractive index can be measured. Figure 2 and Figure 3 Demonstrating the TMBOC method, where the light output fiber 9 and the light input fiber 10 are fiber-to-fiber optically coupled, and demonstrating the power loss due to external field induced radiation transmitted from the light output fiber 9 to the light input fiber 10, which is a key aspect of the sensor operation;

[0059] The smartphone 6-based refractive index sensing system utilizes the capabilities of smartphones 6, especially the iPhone 14 Pro Max, to capture and process light intensity data and monitor changes in light intensity, such as Figure 1 As shown, the smartphone has a dual function: generating light through its flashlight 1 function and simultaneously capturing this light through the camera 7. The custom 3D printed housing 2, designed using SolidWorks Pro software and printed using a Bambu Lab P1S 3D printer, plays a key role in ensuring the proper alignment and protection of the optical fibers and smartphone components. To achieve these functions, the housing is mounted on the smartphone, which is then fixed to the bracket 6. The main optical fiber 3 is firmly connected to the flashlight 1 without any gaps, while the coupling optical fiber 4 is placed about 0.5 mm away from the camera 7 to allow for optimal focus and light intensity measurement.

[0060] Sodium chloride solutions (NaCl) of different concentrations were prepared and their refractive index values ​​were tested by pipetting8 and using an Abbe refractometer. At room temperature of 25°C, the refractive index values ​​of the corresponding liquids were 1.333, 1.340, 1.348, 1.353, and 1.361, corresponding to concentrations of 0%, 3.6%, 7.2%, 9.0%, and 12.6%, respectively. The StaCam application simplifies the video recording process by allowing users to customize video recording settings such as ISO and shutter speed. These parameters are crucial for fine-tuning the camera's response to light, thereby ensuring the accuracy of the data. Adjusting the ISO sensitivity is critical, however, settings that are too high may introduce unwanted noise that may compromise the integrity of the measurement. Shutter speed calibration is equally important as it controls the light exposure and prevents intensity readings from becoming too low at a noise-free ISO configuration. It should be noted that these settings are not set in stone and need to be customized to different environmental conditions to ensure reliable and consistent performance.

[0061] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

[0062] The present invention and its implementation methods are described above, which is not restrictive. The drawings are only one of the implementation methods of the present invention, and the actual application is not limited thereto. In short, if ordinary technicians in the field are inspired by it and design methods and embodiments similar to the technical solution without creativity without departing from the purpose of the invention, they should all fall within the protection scope of the present invention.

Claims

1. A portable optical fiber refractive index sensor based on a smartphone, characterized in that: include A main optical fiber (3) and an optical input optical fiber (10), wherein the main optical fiber (3) and the optical input optical fiber (10) are responsible for inputting and transmitting light; A coupling optical fiber (4) and a light output optical fiber (9), wherein the coupling optical fiber (4) and the light output optical fiber (9) are used for outputting and transmitting light; A smart phone (6), wherein the smart phone (6) is provided with a flashlight (1) and a camera (7), and one end of the coupling optical fiber (4) is placed in front of the camera (7); A 3D printed housing (2) and a glass sheet (5), wherein the 3D printed housing (2) is provided with a hole for the optical fiber to pass through; The flashlight (1) is configured to generate light, and the camera (7) is used to capture the light. When the refractive index of the surrounding medium changes, the light transmission mode between the main optical fiber (3) and the coupling optical fiber (4) will also change accordingly, thereby measuring the refractive index.

2. A portable optical fiber refractive index sensor based on a smartphone according to claim 1, characterized in that: The main optical fiber (3) and the coupling optical fiber (4) are both uncoated polymer optical fibers, and the specifications thereof are Mitsubishi graded-refractive-index SK-40 optical fibers, with a diameter of 980 microns and a surrounding cladding thickness of more than 20 microns.

3. A portable optical fiber refractive index sensor based on a smartphone according to claim 2, characterized in that: The light output optical fiber (9) and the light input optical fiber (10) are both made of a combination of polymethyl methacrylate resin and a fluorine-containing polymer compound.

4. The portable optical fiber refractive index sensor based on a smart phone according to claim 3, characterized in that: The main optical fiber (3) is wound around the coupling optical fiber (4) and fixed by a nylon belt. The length of the twisted coupling portion exceeds 60 mm, and the bending radius is set to 8 mm. The main optical fiber (3) and the coupling optical fiber (4) both pass through the 3D printed housing (2).

5. The portable optical fiber refractive index sensor based on a smart phone according to claim 4, characterized in that: When measuring the refractive index value, the twisted coupling portion of the main optical fiber (3) and the coupling optical fiber (4) is placed on a glass plate (5).

6. The portable optical fiber refractive index sensor based on a smart phone according to claim 5, characterized in that: The 3D printed housing (2) is configured to house components of a polymer optical fiber and a smartphone (6).

7. The portable optical fiber refractive index sensor based on a smart phone according to claim 6, characterized in that: The main optical fiber (3) and the coupling optical fiber (4) receive a double treatment, being twisted and undergoing macrobends.

8. The portable optical fiber refractive index sensor based on a smart phone according to claim 7, characterized in that: The 3D printed housing (2) is arranged on a smart phone (6), and the smart phone (6) is fixed on a bracket.

9. The portable optical fiber refractive index sensor based on a smart phone according to claim 8, characterized in that: The main optical fiber (3) and the flashlight (1) are connected, and there is no gap between the two.

10. The portable optical fiber refractive index sensor based on a smart phone according to claim 9, characterized in that: The measured refractive index of the optical output fiber (9) and the optical input fiber (10) is 1.49, and the larger core diameter and relatively thin cladding region cause significant bending-induced optical loss through the side coupling effect.

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