CN101957227B - Photonic crystal fiber optic liquid level sensor and sensing system formed by same - Google Patents

Abstract

The invention belongs to liquid level measurement, and in particular relates to a photonic crystal optical fiber liquid level sensor and a system composed thereof. A photonic crystal fiber liquid level sensor is characterized in that it comprises a single-mode fiber SMF-28 and a photonic crystal fiber SC-4.0, the beginning of the single-mode fiber SMF-28 and the photonic crystal fiber SC-4.0 are fused, and the photonic crystal fiber The beginning cladding air hole is closed at the fusion point to form a completely collapsed region at the beginning, and the length of the completely collapsed region at the beginning is equal to 65 μm; a fusion splicer is used to discharge the end of the photonic crystal fiber to form a collapsed region at the end, and the end of the photonic crystal fiber is coated with a reflective film. The entire sensor is completely optical fiber, no need to corrode the optical fiber, and it is easy to manufacture. The sensing system is not affected by stray light and temperature, the signal noise is small, the system has high sensitivity and good reliability.

Description

Photonic crystal fiber optic liquid level sensor and its sensing system

technical field

The invention belongs to liquid level measurement, and in particular relates to a photonic crystal optical fiber liquid level sensor and a system composed thereof.

Background technique

In industrial production, it is often necessary to measure the liquid level. Traditional detection methods include manual methods, float methods, and electrical methods, but these methods have inherent defects in automation, precision, and safety. The characteristics of intrinsic safety and high precision of optical fiber sensor just overcome the defects of traditional methods. Among the many optical fiber liquid level sensors, the light intensity modulation type is the main type, and there are leakage mode [1], total internal reflection based [2]-[5], liquid surface reflection type, etc. However, these optical fiber liquid level sensors also have obvious deficiencies. Some are severely limited by the physical and chemical properties of the liquid to be measured, some have a small range, some have low precision, and some have insufficient reliability. Interferometric fiber optic sensors have higher accuracy, while Fiber-optic Fabry-Perot (F-P) cavity interferometer has a simple structure and has been extensively studied in recent years. In addition, some liquid level sensors based on fiber gratings have appeared recently [6], [7].

Application (Patent) No. CN200420102303.6 "Optical Fiber Fabry-Perot Cavity Level Sensor" uses the influence of the measured liquid level change on the cavity length for sensing. However, the manufacturing process is complicated and not suitable for mass production.

In recent years, fiber optic interferometric sensing has been studied to monitor temperature, pressure, gas density or refractive index. One way to achieve interference in a single-mode fiber is to use a long-period grating [8], another way is to couple light from a single-hole fiber to a holey fiber [9], and another way is to use a fiber taper [10 , 11]. In 2008, Rajan Jha proposed a photonic crystal fiber Michelson interferometer for absolute refractive index measurement. He fused a large mode field photonic crystal fiber (LMA-8 Crystal Fiber) (Figure 1a) and a single-mode fiber (SMF-28) , and cut the end of the photonic crystal fiber with a fiber cleaver as a reflective mirror, the interference fringes will red shift with the increase of the external refractive index, the length of the collapsed region formed after welding is about 300 μm, and the loss is 5-9dB. The loss is related to the collapsed region The length of the collapse zone is related to the welding parameters, and the shorter the length of the collapse zone, the smaller the loss [12].

In the existing optical fiber liquid level measurement technology, for liquid level sensing in various environments, corroded fiber gratings or tapered optical fibers or optical fiber dislocation fusion splicing are used, which not only reduces the mechanical strength of the optical fiber itself, but also has temperature cross sensitivity. effects, limiting their use. Using photonic crystal fiber Michelson interferometer to measure liquid level can solve the above problems. The mode field strength ratio of the core mode and the cladding mode that needs to participate in the interference is greater than or equal to 1 when the liquid level is zero, so that as the liquid level increases, the contrast of the interference fringe will decrease monotonously, and at the same time, in order to get as large as possible The measurement range, the loss introduced during welding should be about 3dB. This patent uses SC-4.0 (Figure 1b) and single-mode fiber (SMF-28) for fusion splicing. The cladding part of SC-4.0 with small holes is smaller, and the collapse zone after fusion is shorter, equal to 65 μm. The loss introduced by the fusion process is 3dB. In order to enhance the interference signal, and to prevent silver from entering the cladding holes during coating, the end of the optical fiber fusion splicer is used to discharge in advance to close the air hole, and then a 50nm silver film is plated on the end of the photonic crystal fiber to enhance the reflectivity.

1. Giovanni Betta, Antonio Pietrosanto, and Antonio Scaglione, " A Digital Liquid Level Transducer Based on Optical Fiber, "IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, 45(2), 551-555(1996).

2. Pabitra Nath, Pranayee Datta, and Kanak Ch Sarma, "All fiber-optic sensor for liquid level measurement," MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, 50(7), 1982-1984(2008).

3. Pekka Raatikainen, Ivan Kassamakov, Roumen Kakanakov an Mauri Luukkala, "Fiber-optic liquid-level sensor," sensors and actuators A, 58 93-97(1997).

4. F. P??erez-Oc??on , M. Rubino, J.M. Abril, P. Casanova, J.A. Mart????nez, " Fiber-optic liquid-level continuous gauge," sensors and actuators A,125,124 -132(2006).

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6. Binfeng Yun, Na Chen, and Yiping Cui, "Highly Sensitive Liquid-Level Sensor Based on Etched Fiber Bragg Grating," IEEE PHOTONICS TECHNOLOGY LETTERS, 19(21), 1747-1749(2007).

7. Tuan Guo, Qida Zhao, Qingying Dou, Hao Zhang, Lifang Xue, Guiling Huang, and Xiaoyi Dong, "Temperature-insensitive fiber Bragg grating liquid-level sensor based on bending cantilever beam," IEEE TE LE 1 PHOTONICS ), 2400-2402(2005).

8. H. J. Patrick, A. D. Kersey, and F. Bucholtz, "Analysis of the response of long period fiber gratings to external index of refraction," J. Lightw. Technol., 16(9), 1606–1612(1998).

9. L. Yuan, J. Yang, Z. Liu, and J. Sun, "In-fiber integrated Michelson interferometer," Opt. Lett. , 31(18), 2692–2694(2006).

10. Z. Tian, S. S.-H. Yam, J. Barnes, W. Bock, P. Greig, J. M. Fraser, H. P. Loock, and R. D. Oleschuk, "Refractive index sensing with Mach–Zehnder interferometer based on concatenating twosingle-mode fiber tapers , " IEEE Photon. Technol. Lett. , 20(8), 626–628(2008).

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12. Rajan Jha, Joel Villatoro, "Ultrastable in reflection photonic crystal fiber modal interferometer for accurate refractive index sensing," APPLIED PHYSICS LETTERS, vol.93, 191106 (2008).

Contents of the invention

A photonic crystal fiber liquid level sensor is characterized in that it includes a single-mode fiber SMF-28 and a photonic crystal fiber SC-4.0,

The starting end of the single-mode fiber SMF-28 and the photonic crystal fiber SC-4.0 are fused, and the starting cladding air hole of the photonic crystal fiber is closed at the fusion point to form a completely collapsed region at the beginning; the length of the completely collapsed region at the beginning is equal to 65 μm; The end of the photonic crystal fiber is welded to form a collapsed area at the end; the end of the photonic crystal fiber is coated with a reflective film.

A fusion splicer can be used to discharge the end of the photonic crystal fiber to form an end collapsed region.

As an optimization method, the length of the photonic crystal fiber is 10-50mm.

As a further optimization method, the length of the photonic crystal fiber is 20mm.

The reflective film described as a further optimization method is a metallic silver film with a thickness of 50 nm.

As a further optimization method, a protective cover is provided outside the photonic crystal optical fiber liquid level sensor, and several small holes are provided on the protective cover away from the photonic crystal optical fiber part.

A transitional buffer sleeve is also provided between the single-mode optical fiber and the protective sleeve described as a further optimization method.

A sensing system formed by a photonic crystal fiber liquid level sensor is characterized in that: it includes a 1510-1590nm ASE wide light source, a single-mode fiber, a circulator, a photonic crystal fiber sensor, a spectrum analyzer and a computer; the ASE wide light source is connected to the circulator F Port, the G port of the circulator is connected with the above-mentioned photonic crystal fiber liquid level sensor, the H port of the circulator is connected with the spectrum analyzer, and the data of the spectrum analyzer is read into the computer; the photonic crystal fiber liquid sensor is placed vertically in liquid.

A method of manufacturing a photonic crystal fiber optic liquid level sensor:

The photonic crystal fiber liquid level sensor is to weld the photonic crystal fiber SC-4.0 (Figure 1b) and the ordinary single-mode fiber SMF-28 with a fiber fusion splicer, and the cladding air hole of the photonic crystal fiber is completely collapsed at the fusion point to form a collapsed area. The length of the collapsed region is equal to 65 μm, and the core loss is 3dB. Only half of the energy propagates in the core of the photonic crystal fiber, and the other half of the energy that does not enter the core continues to propagate in the cladding. Therefore, the role of the collapsed area here is equivalent to a coupler with a coupling efficiency of 3dB, which divides the light transmitted in the core of the single-mode fiber into two beams with equal energy and transmits them in the core and cladding of the photonic crystal fiber respectively; The end of the optical fiber with a point of about 10-50mm is cut off with a fiber cleaver, and the end is pre-discharged with an optical fiber fusion splicer to close the air hole, and then the end surface is coated with a reflective film as a reflector. An optical fiber Michelson interferometer is formed by a single-mode optical fiber, a fusion point, a photonic crystal fiber cladding, a photonic crystal fiber core, and a reflection film. In this Michelson interferometer, the photonic crystal fiber only has the fiber core and the fiber cladding, and the length of the photonic crystal fiber is 10-50mm, which constitutes the photonic crystal fiber liquid level sensor, which uses the light energy transmitted in the fiber cladding The loss and optical interference fringe characteristics of the fiber cladding and the fiber core are used to measure the liquid level of the measured liquid. The refractive index of the liquid to be measured is greater than the refractive index of the cladding of the photonic crystal fiber.

The sensing principle of this sensor is to use the energy loss and light interference characteristics of the optical fiber cladding light in the optical fiber Michelson interferometer at the interface between the photonic crystal fiber and the external environment to measure the liquid level:

(1) The light transmitted in the core of the single-mode fiber is divided into two parts when it reaches the collapsed region of the fusion splicing point. One part of the light continues to travel forward in the core of the photonic crystal fiber to form a core mode, and the other part is coupled to the photonic crystal fiber package Transport in the layer forms cladding modes.

(2) When the photonic crystal fiber is placed in air or water or other liquids whose refractive index is lower than the refractive index of the photonic crystal fiber cladding, the cladding light is totally reflected at the interface between the cladding and the external environment, due to the photonic crystal The length of the optical fiber is only 10-50mm, and the light energy of the cladding can be considered as no loss.

(3) When part or all of the photonic crystal fiber is placed in a liquid (such as edible salad oil) whose refractive index is greater than the refractive index of the photonic crystal fiber cladding, the cladding light is reflected and refracted at the interface between the cladding and the external liquid , resulting in attenuation of light energy in the cladding.

(4) When the attenuated cladding light and the core light with constant energy reach the reflective film, they are respectively reflected back to the cladding and core of the photonic crystal fiber and continue to transmit in reverse. Optical fiber cladding continues to reflect and refract light. The higher the liquid level to be measured, the more energy will be refracted. Combining (3) and (4), the measurement object of the present invention is the liquid level whose refractive index is greater than the refractive index of the photonic crystal fiber cladding.

(5) When the reflected fiber cladding and fiber core light reaches the fusion point again, the photonic crystal fiber core light and photonic crystal fiber cladding light interfere, and the interference light is transmitted in the single-mode fiber.

(6) Among the two beams of light that interfere, the light in the core of the photonic crystal fiber is not affected by the external liquid level; but the light in the cladding of the photonic crystal fiber attenuates energy due to refraction, but its phase does not occur Variety. This affects the fringe contrast of the interference signal. The higher the liquid level, the more attenuation of light energy in the cladding, the smaller the fringe contrast, and the relationship between them is definite.

(7) The interference signal is read out by the spectrum analyzer through the circulator. The data of the spectrum analyzer is read into the computer, and the contrast of the interference fringes is calculated to measure the liquid level of the measured liquid.

The principle of optical coupling in the collapsed area of the welding point ( as shown in Figure 7):

Z is the light wave transmission distance. At Z=0, the light wave enters the collapse region of the photonic crystal fiber from the single-mode fiber, the fundamental mode of the single-mode fiber is diffracted, and the mode field becomes wider. The mode field diameter MFD can be approximated by a Gaussian beam.

                            （1）

(1)

为单模光纤模场半径，约4.5μm ，n1是纯石英折射率1.46。通过65μm的塌陷区后，在波长为1550nm时MFD约扩大到Z=0处的1.5倍，约为13.5μm，而光子晶体光纤基阶芯模模场半径仅为3.5 μm。所以通过约65μm的塌陷区后会激发出基阶芯模和高阶包层模。两个受激模分别在光子晶体光纤纤芯和包层中传播，被反射模反射后又都回到熔接点，在这里两者发生干涉。干涉强度I由下式给出

The mode field radius of the single-mode fiber is about 4.5μm, and n1 is pure silica with a refractive index of 1.46. After passing through the collapse region of 65 μm, the MFD expands to about 1.5 times that at Z=0 at a wavelength of 1550 nm, which is about 13.5 μm, while the fundamental mode field radius of the photonic crystal fiber is only 3.5 μm. Therefore, after passing through the collapsed region of about 65 μm, the fundamental core mode and the higher-order cladding mode will be excited. The two excited modes propagate in the core and cladding of the photonic crystal fiber respectively, and after being reflected by the reflection mode, they both return to the splicing point, where the two interfere. The interference intensity I is given by

(2)

,

为光程差, I₁ 是纤芯中传播的光强,不受液位变化影响，I₂ 是包 in

,

is the optical path difference, I ₁ is the light intensity propagating in the fiber core, which is not affected by the liquid level change, I ₂ is the package

The light intensity propagating in the layer varies with the measured liquid level. The maximum and minimum light intensities are given by the following two equations

(3)

                                      (4)

(4)

        (5)

(5)

where ΔP is the logarithmic difference between the maximum and minimum light intensities. When the liquid level changes within a certain range, ΔP will decrease approximately linearly with the liquid level. It can also be seen from the above formula that this liquid level sensing system can eliminate errors caused by light source fluctuation and optical path interference.

related materials

1. PCF model SC-4.0-1040-46, specific parameters

Material: Pure Quartz

Refractive index: 1.46

Core diameter: 4.2 ± 0.5 μm

Cladding diameter: 125 ± 3 μm

Coating layer diameter: 245 ± 5μm

Mode Field Diameter (MDF) 1550nm: 3.4 ± 0.2 μm

Attenuation 1550 nm : < 2.2 dB/km

2. Specific parameters of SMF-28

Core diameter: 8.2 μm

Cladding diameter: 125 ± 1 μm

Coating layer diameter: 250 ± 1μm

Mode field diameter (MDF) 1550nm: 9.2 ± 0.8 μm

3. Refractive index of salad oil: 1.47

4. Optical circulator ( as shown in Figure 6)

An optical circulator is a three-port non-reciprocal magnetic device used for uploading and downloading signals in an optical network. Port F input, port G output; port G input, port H output

Beneficial effect

1. In the existing optical fiber liquid level measurement technology, for liquid level sensing in various environments, corroded optical fiber gratings or tapered optical fibers or optical fiber dislocation fusion are used, which not only reduces the mechanical strength of the optical fiber itself, but also has temperature Cross-sensitivity effects limit their use. Using photonic crystal fiber Michelson interferometer to measure liquid level can solve the above problems. It is required that the mode field strength ratio of the core mode and the cladding mode participating in the interference is greater than or equal to 1 when the liquid level is zero, so that as the liquid level increases, the contrast of the interference fringe will decrease monotonously, and at the same time, in order to obtain the largest possible Measurement range, the loss introduced when welding is required is about 3dB. This patent uses SC-4.0 (Figure 1b) and single-mode fiber (SMF-28) for fusion splicing. The cladding part of SC-4.0 with small holes is smaller, and the collapse zone after fusion is shorter, equal to 65 μm. The loss introduced by the fusion process is 3dB. In order to enhance the interference signal, and to prevent silver from entering the cladding holes during coating, the end of the optical fiber fusion splicer is used to discharge in advance to close the air hole, and then a 50nm silver film is plated on the end of the photonic crystal fiber to enhance the reflectivity.

2. This sensor has the advantages of ordinary optical fiber sensors, it is not susceptible to electromagnetic interference, the sensor structure is simple, the size is small, and it is suitable for harsh environments such as flammable and explosive. In addition, this sensor has many unique advantages. (1) The all-fiber Michelson interferometer is realized, and the functions of light splitting, light transmission, coupling light, reflection, beam combining and interference are realized on the optical fiber, and the sensor structure is miniaturized. (2) The sensing system is not affected by stray light. Because the sensing system measures interference spectrum signals, stray light and signal light do not satisfy the coherence condition. Therefore, stray light does not affect the measurement results. (3) The sensing system is not affected by temperature. Because the temperature change will affect the phase difference between the core light and the cladding light, the interference fringes will be shifted, but the fringe contrast will not be changed. However, this sensor system measures the contrast of interference fringes, so temperature changes do not affect the measurement results. (3) The sensor is easy to manufacture, does not need to corrode the optical fiber, and has low signal noise. In a word, the sensor and its system are simple in structure, miniaturized and fully optical. The liquid level is measured by using the light reflection, refraction and light interference characteristics of the fiber cladding, and a complete microstructure Michelson interferometer and liquid level sensor are realized by using single-mode fiber and photonic crystal fiber. The system has high sensitivity and good reliability. The refractive index of the liquid measured by the sensor is greater than the refractive index of the photonic crystal fiber cladding.

Description of drawings

Figure 1a is the end view of photonic crystal fiber LMA-8 Crystal Fiber;

Figure 1b is the end view of the photonic crystal fiber SC-4.0 used in this sensor;

Figure 1c is the end view of the photonic crystal fiber SC-4.0 hole completely collapsed;

Fig. 2 is a structural diagram of a photonic crystal fiber optic liquid level sensor;

Among them: optical fiber coating 1, single-mode optical fiber cladding 2, single-mode optical fiber core 3, splicing point 4, photonic crystal optical fiber cladding 5, photonic crystal optical fiber core 6, reflective film 7, protective sleeve 8, end Curing glue 9, curing glue 10, transition buffer sleeve 11;

Fig. 3 is a structural schematic diagram of a photonic crystal fiber optic liquid level sensing system;

Fig. 4 is the interference signal spectrogram obtained by the computer in the photonic crystal fiber liquid level sensing system;

Fig. 5 is a relationship diagram between measuring salad oil liquid level and interference fringe contrast with photonic crystal fiber optic liquid level sensor;

Fig. 6 is a schematic diagram of an optical circulator, wherein port F is input and port G is output; port G is input and port H is output;

Fig. 7 is a schematic diagram of the optical coupling principle in the collapsed area of the fusion point;

Detailed ways

Example 1

The implementation of the above-mentioned sensor and system will be described below by taking the measurement of the liquid level of salad oil as an example in conjunction with the accompanying drawings. The liquid level measurement method of other liquids whose refractive index is larger than that of the photonic crystal fiber cladding is the same, the difference is that the sensitivity of liquid level sensing of different refractive indices is different.

A photonic crystal fiber liquid level sensor is characterized in that it comprises a single-mode fiber SMF-28 and a photonic crystal fiber SC-4.0, the beginning of the single-mode fiber SMF-28 and the photonic crystal fiber SC-4.0 are fused, and the photonic crystal fiber The cladding air hole at the beginning of the fusion splicing point is closed to form a completely collapsed region of the fusion point; the end of the photonic crystal fiber is discharged by a fusion splicer to form a terminal collapsed region, and the end is coated with a reflective film; the length of the fusion point collapsed region is equal to 65 μm. in,

The photonic crystal fiber length is 20mm.

The reflective film is a metallic silver film with a thickness of 50nm.

A protective cover is also provided outside the photonic crystal fiber liquid level sensor, and several small holes are arranged on the protective cover away from the photonic crystal fiber part.

A transitional buffer sleeve is also provided between the single-mode optical fiber and the protective sleeve.

Its production method is:

(1) Connect a single-mode fiber (SMF-28, silica fiber, with a core diameter of about 8.2 μm and a fiber cladding diameter of 125 μm) with a photonic crystal fiber (SC-4.0, with a fiber core diameter of about 4.2 μm, Fiber cladding diameter 125μm) is welded with a fiber optic fusion splicer. The air hole of the photonic crystal fiber cladding 5 is closed at the fusion point 4 to form a completely collapsed area. The length of the collapsed area is equal to 65 μm. The fusion point acts as a coupler, and the The fundamental mode is divided into the core mode and the cladding mode of the photonic crystal fiber, and the energy of the two is equal.

(2) Use an optical fiber coating machine to coat the optical fiber coating layer 1 around the fusion point.

(3) Cut off the photonic crystal fiber with a fiber cutter at a distance of 20mm from the fusion point, and use a fiber fusion splicer to discharge the end to close the air hole (Figure 1c) and coat a layer of 50nm metallic silver film on the cut surface as a reflective film7 .

(4) Remove the optical fiber coating layer 1 between the reflective film and the fusion point.

(5) Paste the entire photonic crystal fiber including the single-mode fiber near the fusion point to the inner wall of a protective sheath 8 with many small holes with curing glue 10, and the position where the curing glue 10 is pasted is close to the end of the protective sheath.

(6) Install a plastic transitional buffer sleeve 11 at the optical fiber lead-out end of the protective sleeve to prevent the optical fiber from being broken.

(7) The salad oil can contact the photonic crystal fiber cladding through the small hole, the liquid level in the protective cover and the liquid level of the salad oil.

(8) When the sensor is taken out from the salad oil, the salad oil in the protective cover will flow out from the small hole at the end.

(9) The protective layer can also be omitted, and the sensor can also be detected directly by placing the sensor in the salad oil solution.

The composition of the photonic crystal fiber liquid level sensing system is shown in Figure 3. The ASE light source A with a wavelength of 1510nm-1590nm enters the F port of the circulator B through the optical fiber F1, passes through the G port of the circulator B and a section of optical fiber F2, and reaches the photonic crystal optical fiber liquid level sensor C. In sensor C, half of the light energy is transmitted in the core of the photonic crystal fiber without being affected by changes in the external liquid level, and the other half of the light energy is coupled into the cladding of the photonic crystal fiber. When the light in the cladding and core of the photonic crystal fiber reaches the reflective film, it is reflected back to the cladding and core respectively. The light in the cladding produces reflection and refraction at the interface between the fiber cladding and salad oil, and only total reflection at the interface between the fiber cladding and air, and the refracted light energy is affected by the external liquid level. When the reflected light reaches the splicing point again, the cladding light of the photonic crystal fiber is coupled into the fiber core of the single-mode fiber, and the core light of the photonic crystal fiber is also coupled into the fiber core of the single-mode fiber, and the core of the single-mode fiber The two beams of light interfere. The returned interference signal light is transmitted to the spectrum analyzer D through the H port of the circulator B and the optical fiber F3. The interference signal spectrum data obtained by the spectrum analyzer is sent to the computer E, and the interference signal spectrum data is processed by the computer to obtain and display the salad oil level. The interference signal spectrum data obtained by the computer is shown in Figure 4 as the interference spectrum when measuring different liquid levels. The measurement and calculation process realizes the calibration of the sensitivity coefficient of the sensor to the salad oil level, and then tests the salad oil level. The calibration diagram is shown in Figure 5. During the test, the sensor is placed vertically in the tested salad oil, and the computer in the sensing system acquires the spectral data of the interference signal, and calculates the measured salad oil level according to the calibrated sensitivity coefficient, so as to realize the transmission of the salad oil level. feel. In this example, the sensitivity of measuring the salad oil level is 0.84dB/mm, and the resolution can reach 0.05mm.

Claims (5)

1. A photonic crystal optical fiber liquid level sensor is characterized in that comprising single-mode optical fiber SMF-28 and photonic crystal optical fiber SC-4.0, the beginning end fusion of described single-mode optical fiber SMF-28 and photonic crystal optical fiber SC-4.0, photonic crystal optical fiber SC-4.0 The cladding air hole at the beginning of the crystal fiber is closed at the fusion point to form a completely collapsed region at the beginning; the length of the completely collapsed region at the beginning is equal to 65 μm; the end of the photonic crystal fiber is fused to form a collapsed region at the end, and the end of the photonic crystal fiber is coated with a reflective film. 2. A photonic crystal fiber liquid level sensor according to claim 1, characterized in that the length of the photonic crystal fiber is 10-50 mm. 3. A photonic crystal fiber liquid level sensor according to claim 2, characterized in that the length of the photonic crystal fiber is 20mm. 4. A photonic crystal fiber liquid level sensor according to claim 1, characterized in that said reflective film is a metallic silver film with a thickness of 50nm. 5. The sensing system formed by a photonic crystal optical fiber liquid level sensor according to claim 1, characterized in that: it includes a 1510nm-1590nm ASE wide light source, a circulator, the photonic crystal optical fiber liquid level sensor, and a spectrum analyzer and a computer; the ASE wide light source is connected to the F port of the circulator, the G port of the circulator is connected to the photonic crystal optical fiber liquid level sensor, the H port of the circulator is connected to the spectrum analyzer, and the data of the spectrum analyzer is read into the computer; the photon The crystal fiber liquid sensor is placed vertically in the liquid.

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