Parallel Cascaded Refractive Index Sensor and Refractive Index Detection Method Based on Vernier Effect

Abstract

This invention discloses a parallel cascaded refractive index sensor and a refractive index detection method based on the vernier effect. The device includes: single-mode fiber L1 and single-mode fiber L2, connected in parallel; the polymer coating layer of single-mode fiber L1 is stripped; both single-mode fiber L1 and single-mode fiber L2 have two semi-circular bends, which do not satisfy the total internal reflection condition; the method includes: immersing the upstream bend of single-mode fiber L1 into the liquid to be tested, and acquiring the wavelength shift data of the trough of the spectral line; based on the wavelength shift data, determining the refractive index of the liquid to be tested from a preset wavelength shift-refractive index relationship curve. This invention uses the bending of single-mode fiber to create a refractive index sensor, utilizing the phenomenon that changes in external refractive index cause a shift in the transmission spectral line, and amplifying the shift using the vernier effect principle to construct a fiber optic interferometer sensing structure, thereby achieving the purpose of high-sensitivity measurement of refractive index.

Description

Technical Field

[0001] This invention relates to a parallel cascaded refractive index sensor and a refractive index detection method based on the vernier effect, belonging to the field of fiber optic sensing and detection technology. Background Technology

[0002] Fiber optic sensors offer numerous advantages, including compact structure, strong resistance to external interference, and high sensitivity of sensing signals. In recent years, fiber optic sensors have been extensively researched and applied in a wide range of fields, such as temperature measurement, pressure sensing, refractive index measurement, magnetic field detection, and strain monitoring.

[0003] Among a range of fiber optic sensors, those based on the vernier effect have seen rapid development due to their ability to significantly improve the sensitivity of fiber optic interferometers. In recent years, cascaded and parallel fiber optic interferometers based on the vernier effect have been proposed. For example, a vernier effect fiber optic sensor proposed in 2014 utilizes the large mode field characteristics of hollow photonic crystal fiber (HC-PCF) by embedding two HC-PCF segments into single-mode fiber (SMF) to form a cascaded Fabry-Perot interferometer (FPI) structure. When applied to stress and magnetic field measurements, this sensor showed approximately 29 times higher sensitivity compared to a single FPI. A vernier effect fiber optic temperature sensor based on a tandem Sagnac interferometer, proposed in 2015, showed approximately 9 times higher sensitivity compared to a single Sagnac interferometer. A strain sensing sensitivity proposed in 2019 using a parallel configuration of two Fabry-Perot interferometers was 4.6 times that of a single open-cavity FPI. However, to date, no research has been reported on parallel cascaded MZ fiber optic sensors based on the vernier effect, indicating a promising future for research and development. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide a parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect. The present invention makes a refractive index sensor by bending a single-mode fiber, takes advantage of the phenomenon that external refractive index changes will cause transmission spectral line shift, and uses the vernier effect principle to amplify the shift, constructs a fiber optic interferometer sensing structure, and achieves the purpose of high-sensitivity measurement of refractive index.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a parallel cascaded refractive index sensor based on the vernier effect, comprising:

[0006] A single-mode fiber L1, which serves as the sliding part of the scale for sensing, and a single-mode fiber L2, which serves as the fixed part of the scale for reference, are connected in parallel to the same light source. The polymer coating layer of the single-mode fiber L1 is stripped off.

[0007] Both the single-mode fiber L1 and the single-mode fiber L2 are provided with two semi-circular bends used as MZ interferometers. Each semi-circular bend does not meet the total internal reflection condition, and the radii of each semi-circular bend are different. The distance between two adjacent semi-circular bends on the same single-mode fiber is greater than the radius of any bend.

[0008] Along the propagation direction of the single-mode fiber L1, the upstream bend is used to immerse the liquid with the refractive index to be measured, while the downstream bend is placed in an environment with a constant refractive index.

[0009] The outlet ends of the single-mode optical fibers L1 and L2 are coupled and then connected to the spectrometer.

[0010] Furthermore, the single-mode fiber L1 and the single-mode fiber L2 are connected in parallel via a coupler.

[0011] Furthermore, the radii of each of the aforementioned semicircular bends satisfy:

[0012] Each semicircular bend in the transmission spectrum has at least two complete troughs.

[0013] Secondly, the present invention provides a parallel cascade refractive index detection method based on the vernier effect, comprising the following steps:

[0014] The upstream bend of single-mode fiber L1 is immersed in the liquid to be tested to obtain the wavelength shift data of the trough of the spectral line.

[0015] Based on wavelength offset data, the refractive index of the liquid under test is determined from a preset wavelength offset-refractive index relationship curve.

[0016] Furthermore, the preset wavelength offset-refractive index relationship curve includes:

[0017] The upstream bend of single-mode fiber L1 was sequentially placed into liquid samples with different refractive indices, and the trough wavelength shift data of the spectral lines were obtained under different refractive index liquids.

[0018] By mapping the wavelength offset to the refractive index, a wavelength offset-refractive index relationship curve is constructed.

[0019] Furthermore, the wavelength shift data of the troughs of the spectral lines are obtained, including:

[0020] Under different refractive index conditions, the transmission spectrum of the fiber L1 was obtained, and envelope fitting was performed to obtain the vernier fitted spectrum.

[0021] Given an original spectral plot, perform envelope fitting and solve for the trough wavelength offset data of the vernier fitted spectral lines with different refractive indices relative to the original spectral plot after envelope fitting in the vernier fitted spectral lines.

[0022] Furthermore, the wavelength shift data of the troughs of the spectral lines are obtained, including:

[0023] Under different refractive index conditions:

[0024] The transmission spectrum of the single-mode fiber L1 was obtained, and envelope fitting was performed to obtain the vernier fitted spectrum of the single-mode fiber L1.

[0025] The transmission spectrum of the single-mode fiber L2 was obtained, and envelope fitting was performed to obtain the vernier fitted spectrum of the single-mode fiber L2.

[0026] The vernier fitting curves of single-mode fiber L1 and single-mode fiber L2 are superimposed, and the superimposed curve is envelope-fitted to obtain the quadratic vernier fitting spectrum.

[0027] Specify an original spectrum and perform two envelope fittings. Solve for the trough wavelength offset data of the second vernier fitted spectrum with different refractive indices relative to the original spectrum after the two envelope fittings in the second vernier fitted spectrum.

[0028] Furthermore, during envelope fitting, the troughs are connected, and the connection formula includes:

[0029] I(envelope)=a0+a1·cos(λw)+b1·sin(λw)+a2·cos(2λw)+b2·sin(2λw)+a3·cos(3λw)+b3·sin(3λw);

[0030] Where I (envelope) is the intensity of the envelope; λ is the wavelength; w, a0, a1, a2, a3, b1, b2, and b3 are all constants and can be obtained from the known horizontal and vertical coordinates of the troughs.

[0031] Furthermore, the original spectral diagram is as follows:

[0032] The spectrum was obtained by bending the upstream of the single-mode fiber L1 in the refractive index fiber sensor and placing it in an environment with a refractive index of 1.

[0033] The beneficial effects achieved by this invention are as follows:

[0034] This invention provides a parallel cascaded refractive index sensor and a refractive index detection method based on the vernier effect. This invention only requires bending a single-mode optical fiber to construct an ultrasensitive refractive index fiber sensor by utilizing the shift of transmission spectral lines caused by different refractive indices. At the same time, by utilizing the principle of the vernier effect, the offset is amplified, which improves the detection accuracy of the refractive index sensor. The entire structure is simple and compact, and can be made into a micro-sized high-precision sensor. It is also low in manufacturing cost, uses abundant raw materials, and is conducive to product production. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the refractive index fiber sensor in the parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect provided in the embodiments of the present invention;

[0036] Figure 2 This is a schematic diagram of the single bent fiber interference structure in the parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect provided in the embodiments of the present invention;

[0037] Figure 3 This is a simulation result of the L1 envelope of a single-mode fiber in the parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect provided in the embodiments of the present invention;

[0038] Figure 4 This is a simulation result of the L2 envelope of a single-mode fiber in the parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect provided in the embodiments of the present invention;

[0039] Figure 5 These are the original spectral lines, the first envelope fitting spectral lines of the original spectral lines, and the second envelope fitting spectral lines of the original spectral lines in the parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect provided in the embodiments of the present invention.

[0040] Figure 6 This is a graph showing the spectral changes and the movement of the vernier fitting envelope in the parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect provided in the embodiments of the present invention.

[0041] Figure 7 This is a structural diagram of the nested refractive index fiber optic sensor in the parallel cascaded refractive index sensor and refractive index detection method based on the vernier effect provided in the embodiments of the present invention. Detailed Implementation

[0042] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0043] Example 1:

[0044] Embodiment 1 of the present invention provides a parallel cascaded refractive index sensor based on the vernier effect, such as... Figures 1 to 7 As shown, this invention only requires bending a single-mode optical fiber to construct an ultrasensitive refractive index fiber sensor by utilizing the shift phenomenon of transmission spectral lines caused by different refractive indices. At the same time, the principle of vernier effect is used to amplify the shift, thereby improving the detection accuracy of the refractive index sensor. The entire structure is simple and compact, and can be made into a micro-sized high-precision sensor. It is also low in manufacturing cost, uses abundant raw materials, and is conducive to product production.

[0045] Specifically, such as Figure 1 As shown, in the embodiments of the present invention, two single-mode optical fibers are selected, namely single-mode fiber L1 and single-mode fiber L2. Furthermore, the present invention is based on the vernier effect. In the embodiments of the present invention, single-mode fiber L1 serves as the sliding part of the scale for sensing, and single-mode fiber L2 serves as the fixed part of the scale for reference. Single-mode fiber L1 and single-mode fiber L2 are connected in parallel via a 3dB coupler. The couplers at the input ends of single-mode fiber L1 and single-mode fiber L2 are connected to a light source, and the couplers at the output ends are connected to a spectrometer.

[0046] The embodiments of the present invention utilize the principle of total internal reflection, such as... Figure 2 As shown, when the optical fiber is bent to a certain extent, the total internal reflection condition is no longer met, and some light leaks from the fiber core to the cladding. This causes the excited cladding mode to propagate in the cladding of the bent fiber, while the residual core mode continues to propagate in the core. Finally, at the end of the bent fiber, the cladding mode is coupled back to the core and interferes with the residual core mode. If the refractive index of the external environment is different from that of the cladding during this process, the cladding mode will be affected, and the resonance peak will shift. During testing, different refractive indices are determined by different shift amounts.

[0047] Based on the above working principle, to enhance the interaction between the evanescent field and the external environment, this invention removes the polymer coating layer of the single-mode fiber L1, and then sets two semi-circular bends on the single-mode fiber L1. These two semi-circular bends have different radii and neither satisfies the total internal reflection condition. The distance between the two semi-circular bends is greater than the radius of either semi-circular bend. Along the direction of light propagation, the semi-circular bend located upstream of the single-mode fiber L1 is used to immerse in the solution to be tested. After the light leaks into the cladding, the resonance peak of the light in the cladding will shift because the cladding is immersed in a solution with a changing refractive index. The semi-circular bend downstream of the single-mode fiber is placed in a constant-temperature environment with a constant refractive index. Since the shift corresponding to each refractive index is uniquely determined, the refractive index can be determined by determining the shift.

[0048] The single-mode fiber L2 used for reference also has two semi-circular bends. The distance between the two semi-circular bends is greater than the radius of any one of the semi-circular bends, and the radii of these four semi-circular bends are different.

[0049] The introduction of single-mode fiber L2 is used to amplify the offset data and improve the sensitivity of the test:

[0050] like Figure 6 As shown in Figure a, if only single-mode fiber L1 is used for measurement, the shift of the refractive index relative to the original spectral line is small. To improve sensitivity, the two spectral lines in a need to be envelope-fitted to obtain a first-order vernier-fitted spectral line, as shown in Figure a. Figure 6 As shown in Figure b, the offset is significantly increased, and the sensitivity is significantly improved. To further improve the sensitivity, a single-mode fiber L2 is introduced in parallel. The spectral line passing through the single-mode fiber L2 is enveloped once to obtain the first vernier-fitted spectral line of the single-mode fiber L2. Then, the first vernier-fitted spectral lines of the single-mode fiber L1 and L2 are superimposed, and the superimposed spectral line is envelope-fitted to obtain the second vernier-fitted spectral line. Simultaneously, the original spectral line is envelope-fitted twice to obtain the second vernier-fitted spectral line of the original spectral line, as shown in Figure b. Figure 6 As shown in Figure c, the offset increases further, and the sensitivity is further improved.

[0051] To meet the sensitivity requirements of the study, such as Figure 7 As shown, multiple refractive index fiber optic sensors can be cascaded and nested. However, when testing the sensitivity, it is only necessary to place the liquid to be tested into any one of the refractive index fiber optic sensors in the first stage, which is close to the light source.

[0052] To ensure that the change data of the resonance peak can be clearly obtained, when setting the semi-circular bend, ensure that the transmission spectrum of each semi-circular bend has at least two complete troughs.

[0053] In the specific fabrication process: a light source, two 3dB couplers, the aforementioned single-mode fiber L1 and single-mode fiber L2, and a spectrometer are required. Connect the two ends of single-mode fiber L1 and single-mode fiber L2 to the two 3dB couplers respectively, and connect single-mode fiber L1 and single-mode fiber L2 in parallel to complete the construction of the test subject. Then, connect one 3dB coupler to the light source and the other 3dB coupler to the spectrometer.

[0054] During testing, the liquid to be tested is immersed in the semi-circular bend upstream of the single-mode fiber L1, and the refractive index of the liquid is determined by a spectrometer.

[0055] For example, in an embodiment of the present invention, the refractive index around the interference structure was changed by replacing NaCl solutions of different concentrations, and the refractive indices were recorded as 1.33, 1.34, 1.35, 1.36, 1.37 and 1.38, respectively.

[0056] like Figure 7 As shown, to improve detection sensitivity, this device can be cascaded and nested. Multiple test subjects are preset and cascaded and nested. Each test subject is connected to the others through its own 3dB coupler. After the multiple test subjects are connected, if there are multiple test subjects in the first and last stages, the test subjects at each stage are coupled. The input port of the coupled first-stage test subject is connected to the light source, and the output port of the coupled last-stage test subject is connected to the spectrometer.

[0057] When using a cascaded nested structure for refractive index testing, in the first-level nested structure, any test subject is selected, and the upstream bend of its single-mode fiber L1 is chosen and immersed in the solution to be tested to measure the refractive index.

[0058] Example 2:

[0059] Based on the parallel cascaded refractive index sensor based on the vernier effect in Embodiment 1, Embodiment 2 of the present invention provides a method for using the fiber optic sensor to detect refractive index, such as... Figures 1 to 7 As shown, it includes the following steps:

[0060] Before conducting the test, the wavelength offset-refractive index relationship curve is first preset, including the following steps:

[0061] First, prepare liquid samples with different refractive indices. Then, sequentially place the upstream bend of the single-mode fiber L1 into these liquid samples with different refractive indices. Obtain the transmission spectra through the single-mode fiber L1 at each refractive index. Simultaneously, obtain the transmission spectrum of the device under an environment with a refractive index of 1. (Example:) Figure 6 As shown in Figure a, the spectrum will shift after the refractive index changes, and the spectrum corresponding to each refractive index is uniquely determined. The offset of a specified valley is also uniquely determined. Therefore, this invention constructs a wavelength offset-refractive index relationship curve by associating different offsets with the refractive index.

[0062] like Figure 6 As shown in 'a', but due to its small offset, its sensitivity is low. To improve the sensitivity, this invention performs the following steps after obtaining the spectral lines after the refractive index change:

[0063] like Figure 6 As shown in Figure b, the spectral lines of the single-mode fiber L1 with refracted changes are envelope-fitted to obtain the first vernier-fitted spectral lines.

[0064] The original spectral lines are envelope-fitted to obtain a first-order vernier-fitted spectral line.

[0065] Obtain the wavelength offset data of the first vernier fitted spectrum of the refractive index change relative to the original spectrum. Correspond different offsets with the refractive index to construct the wavelength offset-refractive index relationship curve.

[0066] like Figure 6 As shown in Figure b, it can be observed that compared to Figure a, the offset increases and the sensitivity increases.

[0067] To further improve sensitivity, this invention introduces a single-mode fiber L2 connected in parallel with single-mode fiber L1, such as... Figure 6 As shown in Figure c:

[0068] Obtain the first vernier fitting spectrum of light as the refractive index changes after passing through single-mode fiber L1;

[0069] The transmission spectrum of the light passing through the single-mode ray L2 is obtained, and the envelope is fitted to obtain the first vernier fitted spectrum of the single-mode fiber L2.

[0070] The first vernier fitted spectrum of single-mode fiber L1 is superimposed with the first vernier fitted spectrum of single-mode fiber L2, and the envelope of the superimposed spectrum is fitted to obtain the second vernier fitted spectrum.

[0071] The envelope of the first vernier-fitted spectrum of the original spectrum is performed to obtain the second vernier-fitted spectrum of the original spectrum.

[0072] Obtain the wavelength shift data of the trough of the quadratic vernier fitted spectrum relative to the original spectrum, and construct a wavelength shift-refractive index relationship curve by mapping different shifts to refractive indices. For example... Figure 6 As shown in Figure c.

[0073] In specific implementations of this invention, such as Figure 6 As shown in Figure a, comparing the spectral line with the original spectral line after the refractive index change, the spectral line with the refractive index change exhibits a redshift, with a shift distance of approximately 5 nm; as shown in Figure a. Figure 6 As shown in Figure b, comparing the above spectral lines after one vernier fitting reveals a shift of approximately 48 nm; Figure 6 As shown in Figure c, after introducing a parallel single-mode fiber L2, the transmission spectrum after passing through the single-mode fiber L2 is envelope-fitted to obtain its first vernier fitted spectrum. The first vernier fitted spectrum of the transmission spectrum of the single-mode fiber L1 with refractive index change is superimposed with the first vernier fitted spectrum of the spectrum of the single-mode fiber L2. After superposition, a second envelope is performed to obtain the second vernier fitted spectrum of refractive index change. The original spectrum is envelope-fitted twice to obtain the second vernier fitted spectrum of the original spectrum. It is compared with the second vernier fitted spectrum of refractive index change and it is found that it has shifted by about 100 nm.

[0074] It can be seen that the present invention uses a vernier magnification method to amplify the offset by up to 20 times, which can effectively improve the sensitivity of refractive index detection.

[0075] When performing envelope fitting on the spectral lines, this invention employs fourth-order Fourier fitting. In each envelope fitting, all valleys are connected, and the connection formula is as follows:

[0076] I(envelope)=a0+a1·cos(λw)+b1·sin(λw)+a2·cos(2λw)+b2·sin(2λw)+α3·cos(3λw)+b3·sin(3λw);

[0077] Where I (envelope) is the intensity of the envelope; λ is the wavelength; w, a0, a1, a2, a3, b1, b2, and b3 are all constants and can be obtained from the known horizontal and vertical coordinates of the troughs.

[0078] In an embodiment of the present invention, the original spectrum is obtained by bending the upstream of the single-mode fiber L1 in the refractive index fiber sensor and placing it in an environment with a refractive index of 1. Specifically, for ease of operation, the present invention selects an environment with a refractive index close to 1 as an air environment.

[0079] After completing the wavelength offset-refractive index relationship curve through the above steps, specific tests are conducted, including the following steps:

[0080] First, place the solution with the desired refractive index into the bend upstream of the single-mode fiber L1, observe the spectral analysis instrument, and obtain the wavelength shift data of the trough of the spectral line of the solution. Based on the preset wavelength shift-refractive index relationship curve, find the refractive index corresponding to the wavelength shift data, which is the refractive index of the liquid to be tested.

[0081] In use such Figure 7 When testing the cascaded nested structure shown, simply select any one of the test subjects in the first stage closest to the light source, immerse the upstream bend of its single-mode fiber L1 into the liquid to be tested, and you can obtain a more sensitive test result.

[0082] Principle analysis of the invention:

[0083] Single-bend fiber interference structure such as Figure 2 As shown, when the optical fiber bends to a certain extent, the total internal reflection condition is no longer satisfied, and some light leaks from the fiber core into the cladding. This causes the excited cladding modes to propagate in the cladding of the bent fiber, while the remaining core modes continue to propagate in the core. Finally, at the end of the bent fiber, the cladding modes are coupled back to the core and interfere with the remaining core modes. The interference formula is as follows:

[0084]

[0085] Where I1 is the intensity of the interference light, I co，1 It is the intensity of the light in the core mode after the first fiber bend, I cl，1 It is the intensity of the cladding mode light after the first fiber bend; It is the phase difference between the core mode and the cladding mode, and the specific calculation formula is as follows:

[0086]

[0087] Where, n co，eff and n cl，eff These are the effective refractive indices of the core mode and cladding mode, respectively; L1 is the interference length of the upstream semicircular bend; Δn eff =n co，eff -n cl，eff λ is the effective refractive index difference between the core mode and the cladding mode. D1 It is the wavelength of the transmission peak in the transmission spectrum. When the phase difference between the interfering modes is an odd multiple of π, the above equation can be expressed as:

[0088]

[0089] Where k is an integer;

[0090] The wavelength of the resonance peak in the transmission spectrum is:

[0091]

[0092] The FSR of this interferometer can be expressed as:

[0093]

[0094] Since the effective refractive index of the cladding mode depends on the refractive index of the external environment, the resonant peak will shift when the sensor is affected by the external environment. By measuring the wavelength shift of the resonant peak, the change in the refractive index of the external environment can be detected. Therefore, the change in the refractive index sensing resonant peak wavelength of a single MZI as a function of the external refractive index can be obtained as follows:

[0095]

[0096] Here, ncl refers to the external refractive index, and A refers to the rate of change of the relative refractive index difference with respect to the external refractive index. Similarly, interference formulas for other curved fiber regions, phase differences between core and cladding modes, and wavelength variations of resonance peaks in the transmission spectrum can be obtained.

[0097] Since the polymer coating is stripped from single-mode fiber L1, it enhances the interaction between the evanescent field and the external environment. Single-mode fiber L2, however, is not immersed in liquid and its interference is unaffected by the external environment. Therefore, L2 serves as the fixed part of the scale for reference, while single-mode fiber L1 serves as the sliding part for sensing. Similarly, the downstream interferometer in single-mode fiber L1, serving as the fixed part of the scale, cannot be submerged in water; we need to ensure it remains completely unaffected. The upstream interferometer in single-mode fiber L1, however, is immersed in liquid, serving as the sliding part of the scale.

[0098] In this system, the single-mode fiber L1 can be considered as two cascaded MZIs. Light emitted from the broadband light source (BBS) passes through the dual-MZI structure single-mode fiber L1, and the superimposed interference spectrum of the transmitted light is obtained in an OSA spectrometer. Because the optical path difference generated by interferometers 1 and 2 is different, their superimposed interference spectrum modulates the signal intensity. A 4th-order Fourier fitting curve is selected to connect all the troughs, such as... Figure 3 As shown, the formula used for connection is as follows:

[0099] I(envelope)=a0+a1·cos(λw)+b1·sin(λw)+α2·cos(2λw)+b2·sin(2λw)+a3·cos(3λw)+b3·sin(3λw);

[0100] Where I (envelope) refers to the intensity of the envelope, and λ refers to the wavelength on the horizontal axis. a0, a1, a2, a3, bl, b2, and b3 are all constants and can be obtained from the known horizontal and vertical coordinates of the troughs.

[0101] Similarly, the simulation results for the envelope of single-mode fiber L2 are as follows: Figure 4 As shown;

[0102] The light emitted from the light source (BBS) passes through a parallel double MZI structure. The light passing through L1 and L2 is then superimposed again, and the final superimposed spectrum of the transmitted light can be obtained in an OSA spectrometer. The simulation results of its envelope are as follows: Figure 5 As shown;

[0103] Figure 6 This shows the changes in spectral lines under varying refractive index and the shift of the vernier fitting envelope. Figure 6 The spectral lines in (a) have undergone a redshift, with a shift distance of approximately 5 nm. Figure 6 The first vernier-fitted spectral line in (b) shifted by approximately 48 nm, while the second vernier-fitted spectral line shifted by nearly 100 nm, resulting in a magnification of 20.

[0104] Theoretically, the amplification factor of the sensor can be derived by inferring the FSR. Assuming the FSRs of the fabricated interferometer are FSR1, FSR2, FSR3, and FSR4, after one vernier effect amplification, the FSR of the sensor is:

[0105]

[0106] The FSR amplified by the two vernier effects is:

[0107]

[0108] The magnification of this sensor is:

[0109]

[0110] Therefore, the sensitivity of this sensor is:

[0111]

[0112] The simulation results show that the sensitivity of the refractive index sensor has been significantly improved.

[0113] The embodiments of the present invention provide a parallel cascaded balloon-shaped ultrasensitive refractive index fiber based on the vernier effect. The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A parallel concatenated refractive index sensor based on Vernier effect, characterized in that: The single-mode optical fiber L1 as a slide for sensing and the single-mode optical fiber L2 as a fixed part for reference are connected in parallel to the same light source, and the single-mode optical fiber L1 is stripped of the polymer coating layer; Each of the single-mode optical fiber L1 and the single-mode optical fiber L2 is provided with two semicircular bends serving as a MZ interferometer, each of the semicircular bends does not satisfy the total reflection condition, the radius of each of the semicircular bends is different, and the distance between two adjacent semicircular bends on the same single-mode optical fiber is greater than the radius of the semicircular bend; In the direction of light propagation along the single-mode optical fiber L1, the semicircular bend upstream of the single-mode optical fiber L1 is used to immerse in the liquid with the refractive index to be measured, and the semicircular bend downstream of the single-mode optical fiber L1 is placed in an environment with a constant refractive index; The outlet ends of the single-mode optical fiber L1 and the single-mode optical fiber L2 are coupled and connected to a spectrum analyzer; The transmission spectrum of each semicircular bend has at least two complete troughs.

2. The parallel cascade refractive index sensor based on the vernier effect according to claim 1, wherein: The single-mode optical fiber L1 and the single-mode optical fiber L2 are connected in parallel through a coupler.

3. The method of claim 1 or 2, wherein the method is a method of detecting the refractive index of the parallel-cascaded refractive index sensor based on the Vernier effect. The package includes the following steps: Immerse the semicircular bend upstream of the single-mode optical fiber L1 in the liquid to be measured to obtain the wavelength shift data of the trough of the spectrum line; Determine the refractive index of the liquid to be measured based on the wavelength shift data from the preset wavelength shift-refractive index relationship curve; The preset wavelength shift-refractive index relationship curve includes: sequentially placing the semicircular bend upstream of the single-mode optical fiber L1 in different liquid samples with different refractive indexes, obtaining the wavelength shift data of the trough of the spectrum line under different refractive index liquids, and corresponding the wavelength shift to the refractive index to construct a wavelength shift-refractive index relationship curve; The wavelength shift data of the trough of the spectrum line includes: obtaining the transmission spectrum after the single-mode optical fiber L1, performing envelope fitting to obtain the vernier fitting spectrum line of the single-mode optical fiber L1, obtaining the transmission spectrum after the single-mode optical fiber L2, performing envelope fitting to obtain the vernier fitting spectrum line of the single-mode optical fiber L2, superimposing the vernier fitting spectrum line of the single-mode optical fiber L1 and the vernier fitting spectrum line of the single-mode optical fiber L2, and performing envelope fitting on the superimposed spectrum line to obtain a secondary vernier fitting spectrum line, specifying an original spectrum line, and performing envelope fitting twice to obtain the secondary vernier fitting spectrum line of the original spectrum line, and obtaining the wavelength shift data of the trough of the secondary vernier fitting spectrum line relative to the secondary vernier fitting spectrum line of the original spectrum line; During envelope fitting, the troughs are connected, and the connection formula includes: I(envelope)=a0+a1·cos(λw)+b1·sin(λw)+a2·cos(2λw)+b2·sin(2λw)+a3·cos(3λw)+b3·sin(3λw); wherein I(envelope) is the intensity of the envelope line; λ is the wavelength; w, a0, a1, a2, a3, b1, b2 and b3 are all constants, and can be obtained by the known horizontal and vertical coordinates of the troughs.

4. The parallel concatenated refractive index detection method based on vernier effect according to claim 3, characterized in that: The original spectrum diagram is: The spectrum diagram obtained by placing the semicircular bend upstream of the single-mode optical fiber L1 in the refractive index fiber sensor in an environment with a refractive index of 1.

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