CN109974814B

CN109974814B - Low-temperature response Michelson liquid level sensor based on multimode interference and measuring method

Info

Publication number: CN109974814B
Application number: CN201910292656.8A
Authority: CN
Inventors: 冯文林; 冯德玖; 杨晓占; 苏萍; 刘敏
Original assignee: Chongqing University of Technology
Current assignee: Chongqing University of Technology
Priority date: 2019-04-12
Filing date: 2019-04-12
Publication date: 2021-05-04
Anticipated expiration: 2039-04-12
Also published as: CN109974814A

Abstract

The invention discloses a novel Michelson optical fiber liquid level sensor based on an intermode interference principle; the sensor is formed by welding a light transmitting single-mode fiber SMF1, a coreless fiber NCF and a sensing single-mode fiber SMF 2. The input light excites a high-order mode at the SMF1-NCF interface; at the NCF-SMF2 interface, a portion of the light couples into the SMF2 core and a portion couples into the SMF2 cladding. The two portions of light re-enter the NCF via reflection from the silver mirror to form intermodal interference and are finally coupled into SMF1 via the SMF1-NCF interface. Simulations and experiments have confirmed that the mode energy distribution of the SMF2 cladding is similar to that in NCF, which results in a high level sensitivity of the two sensing area parts of the sensor. The sensor has the advantages of simple and compact structure, low cost, flexible operation and higher application prospect.

Description

Low-temperature response Michelson liquid level sensor based on multimode interference and measuring method

Technical Field

The invention relates to the technical field of liquid level sensors, in particular to a Michelson liquid level sensor based on multimode interference low-temperature response.

Background

In industrial and agricultural production and life, the monitoring of the liquid level has important significance. In flammable and explosive environments such as petroleum, chemical engineering and the like, capacitive, resistive, magnetostrictive and ultrasonic sensors have various defects and cannot meet the measurement requirements. The optical fiber sensor has the advantages of strong electromagnetic interference resistance, no working current, high sensitivity, capability of working in a high-temperature corrosion environment and the like. Optical fiber sensors are widely used for measurement in the fields of temperature, vibration, pressure, current, gas, liquid level, etc. When measuring flammable, explosive and corrosive liquids, optical fiber liquid level sensors have received extensive attention. At present, a great variety of optical fiber liquid level sensors are developed, such as a discrete point type, an optical fiber grating type, an optical power modulation type, a mechanical structure auxiliary type and the like. These sensors have the disadvantages of single measuring range, low linearity, poor stability or complex structure.

Compared with a Mach-Zehnder sensor and a Sagnac sensor, the optical fiber Michelson interference sensor has the advantages that the structure is more compact, the narrow space can be detected, the manufacturing procedure is simpler, and the processing difficulty is smaller compared with a Fabry-Perot sensor. These characteristics have led to the widespread interest and research of michelson fiber optic sensors. The Xiao Liang and the like adopt an elliptical dual-mode optical fiber as a coupler and a high-order mode filter to form a Michelson optical fiber liquid level sensor, and the sensitivity of the sensor is less than 48.93 pm/mm. Shao Min et al fusion splice a fine-core fiber directly to a single-mode fiber to yield a Michelson fiber level sensor of simple construction with sensitivity of-129 pm/mm and high temperature crosstalk (38 pm/deg.C). These structures are not sensitive and the problem of temperature cross talk is not well solved.

Disclosure of Invention

In view of this, the present invention provides a michelson liquid level sensor based on multimode interference low temperature response, which has the advantages of simple and compact structure, low cost, flexible operation, and high application prospect.

In order to achieve the purpose, the invention provides the following technical scheme:

the invention provides a multimode interference-based low-temperature response Michelson liquid level sensor which comprises a first single-mode fiber, a coreless fiber and a second single-mode fiber; one end of the first single-mode fiber is connected with one end of the coreless fiber, and the other end of the coreless fiber is connected with the second single-mode fiber; and a reflecting film is plated on the other end face of the second single-mode optical fiber.

Further, the reflecting film is a silver film, a gold film, a nickel film or an aluminum film.

Further, the mode energy distribution of the cladding of the second single mode fiber is the same or similar to the mode energy distribution in the coreless fiber.

Further, the first single-mode optical fiber is connected with the coreless optical fiber in a core-to-core mode; the second single-mode fiber is connected with the coreless fiber in a core-to-core mode.

Further, the connection of the first single-mode fiber and the coreless fiber is completed by welding through an optical fiber welding machine; and the connection between the coreless optical fiber and the second single-mode optical fiber is completed by welding by using an optical fiber welding machine.

Further, the length of the coreless fiber ranges from 14.8 mm to 15.2 mm.

Further, the length of the second single-mode fiber is 28-32 mm.

Further, the ratio of the length of the coreless fiber to the length of the second single-mode fiber is 0.3-0.6.

The invention also provides a liquid level measuring method by utilizing the Michelson liquid level sensor, which comprises the following steps:

connecting the Michelson liquid level sensor with an optical fiber circulator;

the circulator is connected with the light source and the spectrometer;

vertically arranging a Michelson liquid level sensor in a container to be measured;

acquiring a spectrogram of a Michelson liquid level sensor in a container to be detected;

acquiring a wave valley value in the spectrogram;

and acquiring the liquid level value in the detection container according to the trough value.

Further, the valley value is calculated according to the following formula:

wherein, λ'_cFor detecting the lowest wave length value of the trough, N is a positive integer, L is the sum of the lengths of NCF and SMF2, and L is_airLength of sensing part of sensor in air, L_liquidFor the length of the sensor immersed in water

The effective refractive index of the fiber in the air and in the liquid respectively is the effective refractive index of two high-order modes.

The invention has the beneficial effects that:

the novel Michelson optical fiber liquid level sensor based on the principle of intermode interference provided by the invention is characterized in that a section of coreless fiber (NCF) is welded between two sections of single-mode fibers. The front single-mode fiber (SMF1) is a conducting fiber for inputting light and outputting light, and the end face of the rear single-mode fiber (SMF2) is plated with a silver film. When light enters NCF from SMF1, a high-order mode is excited, the light is coupled into SMF2 at an NCF-SMF2 interface, a part of the light is coupled into an SMF2 fiber core, a part of the light is coupled into an SMF2 cladding, the fiber core mode and the cladding mode are reflected by a silver film and then pass through two coreless-single mode interfaces to complete coupling twice to form an inter-mode interference output, namely the two parts of light enter the NCF again through the reflection of the silver mirror to form inter-mode interference, and finally the light is coupled into SMF1 through an SMF1-NCF interface; the lengths of NCF and SMF2 were investigated by simulation and experiment and found to be 14.8-15.2mm, 30mm respectively for the optimum lengths of NCF and SMF 2. The NCF length is in the range of 14.8-15.2mm, the NCF and SMF2 have quite similar sensing characteristics in liquid level sensing experiments, and in the range of 15-90 ℃, when the ratio L of the NCF length to the SMF2 length is L_ncf:L_smf2And when the value is approximately equal to 0.5, the sensor shows the characteristic of being insensitive to the temperature. In the liquid level range of 0-44mm, the liquid level sensitivities in water, 5% NaCl, 10% NaCl and 15% NaCl aqueous solutions are 0.21616, 0.23242, 0.25341 and 0.27687nm/mm respectively, and the refractive index sensitivity is 2223.5658 pm/mm/RIU. The sensor has clear interference fringes, simple structure, high liquid level sensitivity, good linear fitting degree and low temperature sensitivity, and has certain application prospect in the flammable and explosive field.

Drawings

In order to make the object, technical scheme and beneficial effect of the invention more clear, the invention provides the following drawings for explanation:

fig. 1 is a schematic diagram of a sensor structure.

FIG. 2 is a diagram of a liquid level and temperature test apparatus.

Fig. 3 is a graph of different length SMF2 spectra.

FIG. 4 is a schematic diagram of the effect of the liquid level measurement test.

Fig. 5 is a schematic diagram illustrating the effect of the temperature sensitivity test of the sensor.

In the figure: 1 is a first single mode fiber, 2 is a second single mode fiber, 3 is a coreless fiber, and 4 is a reflecting film; 21 is a light source, 22 is a fiber circulator, 23 is a spectrometer, 24 is a glass sheet, 25 is a thermometer, 26 is a liquid level experiment, 27 is a temperature experiment, and 28 is a heat insulation layer.

Detailed Description

The present invention is further described with reference to the following drawings and specific examples so that those skilled in the art can better understand the present invention and can practice the present invention, but the examples are not intended to limit the present invention.

Example 1

As shown in fig. 1, fig. 1 is a schematic diagram of a sensor structure; the embodiment provides a novel Michelson optical fiber liquid level sensor based on the principle of intermode interference; the basic principle is as follows:

the sensor is composed of a section of single mode fiber (SMF1) which is welded with a section of coreless fiber (NCF) and then welded with a section of single mode fiber (SMF2), and the right end face of the SMF2 is plated with a silver film to increase the reflectivity. The single mode fiber (SMF1) provided in this embodiment is a first single mode fiber, the single mode fiber (SMF2) is a second single mode fiber, and the coreless fiber (NCF) is a coreless fiber.

When light enters the NCF from the SMF1, mode excitation is generated at the interface of the SMF1 and the NCF due to the mismatch of mode field diameters, and multiple high-order modes LP_lmPropagating in NCF, and exciting only linear circular polarization mode LP when single-mode fiber and coreless fiber are connected to core_0m(m is a positive integer). At the interface between the NCF and the SMF2, a part of light in the coreless fiber enters the core of the SMF2 to propagate to form a core mode, a part of light enters the cladding of the SMF2 to form a cladding mode, and the light of the two modes is reflected by the silver film at the right end face of the SMF2 and then enters the core of the SMF2 and the cladding mode to be coupled into the coreless fiber to generate interference, and the basic principle is as follows:

when light enters the NCF from SMF1, at the boundary, the initial light field can be seen as a superposition of a series of circularly polarized modes:

where E (r,0) is the initial optical field at the interface of SMF1 and the NCF interface. Phi is a_m(r) is the m-th order LP_0mMode(s). r is the radius of the coreless fiber, C_mFor the excitation coefficients of the higher modes of the respective orders, it can be expressed as:

in a coreless fiber, the optical field at the cross-section at the propagation distance z is:

wherein, beta_mIs the m-th order LP_0mThe propagation constant of the mode. Propagation constant beta of each higher order mode in the fiber_mThis will result in different modes of light, which, in turn, will produce a phase difference Δ Φ during propagation. The light reflected through the silver film twice through NCF and SMF2, co-propagating a distance Z of 2L (L being the sum of the lengths of NCF and SMF 2). The optical path difference that creates the interference condition at the SMF1-NCF interface can be expressed as:

wherein, N is a positive integer,

and

the effective refractive indices for the different modes. From this, the wavelengths at which the interference is constructive (N is an even number) and destructive (N is an odd number) can be obtained:

when the external refractive index environment changes, the longitudinal propagation constant and the effective refractive index of each transmission mode in the optical fiber are changed differently, so that the optical path difference between the modes is changed, and the formula (5) shows that lambda_cA movement is generated. Ambient refractive index environment and λ_cAre uniquely corresponding.

When the sensor is used to measure liquid level, a portion of the sensor is placed in the air and a portion of the sensor is placed in the liquid being measured. The sensor is modulated by two refractive index environments of air and liquid at the same time, and the modulated wavelength is lambda'_cCan be expressed as:

The effective refractive index difference between the high-order modes in the optical fiber is increased along with the increase of the refractive index of the external environment; λ 'as the sensor is gradually immersed in the liquid'_cWill move towards the long wavelength band. This theory is applicable to a coreless fiber sensing section, the sensing section of the sensor of this embodiment is composed of NCF and SMF2, and later simulations and experiments prove that the single mode fiber section (SMF2) and the coreless fiber show very similar characteristics in liquid level and refractive index measurements. Therefore, this theory can also be used as an approximation theory in this experiment.

The embodiment further provides a simulation experiment process, which is specifically as follows:

the simulation experiment adopts a Beam Propagation Method (BPM), the simulation center wavelength is 1550nm, and the background refractive index n is 1. In some studies, short length NCFs were used as simple mode exciters and couplers, and this example designed a sensor group (group A) with a length of 3mm, and a single NCF group (group B, L)_ncf45mm) and experimental group (group C, L)_ncf14.8-15.2mm) were run in control simulations or experiments. In group A, B, C, SMF1 were all 2.5mm in length and SMF2 were all 30mm in length. The simulation parameters are shown in table 1.

TABLE 1 simulation parameters

Simulation results show that most of energy of the A sensor at an NCF-SMF2 interface (Z is 3mm, and all distances Z start from the SMF1-NCF interface) is close to a fiber core, so that less energy enters an SMF2 cladding, and the energy distribution regularity in an SMF2 cladding is poor. In the process of experimental exploration, the sensor A is low in sensitivity when measuring liquid level, and the output spectrum shows irregular change due to the change of the refractive index or the liquid level, so that the sensor A cannot be used for measuring the liquid level and the refractive index. The high order modes in the NCF and SMF2 of the C sensor have a very similar distribution to group B. The results of the C-sensor cross-sectional mode field (XZ) distribution near the NCF-SMF2 interface (Z ═ 14.8-15.2mm) show that the optical field is concentrated at the fiber periphery for most of the energy, which causes more energy to be coupled into the SMF2 cladding, exhibiting a similar cladding mode energy distribution. Therefore, the present embodiment performs an experiment using the C sensor.

The experiments provided in this example were performed at room temperature (20 ℃) using a C sensor, the sensor level and temperature measurement device being as shown in FIG. 2. The fusion of each part of the sensor was completed by an optical fiber fusion splicer, and the end face of SMF2 was coated with a reflective film by silver mirror reaction. The light emitted by the laser source reaches the sensing probe through the optical fiber circulator, and the light reflected by the sensing probe reaches a spectrometer (OSA) through the circulator. The sensor is fixed on the graduated scale in parallel, and the tail end of the sensor coincides with the zero scale mark of the graduated scale, and the sensor and the graduated scale are isolated through a glass slide to prevent the optical fiber from contacting the graduated scale and influencing the spectrum result. The scale was fixed to an iron stand so that it was placed vertically down into the beaker.

As shown in fig. 2, fig. 2 is a diagram of a liquid level and temperature test device, in the simulation, the optimized length of the NCF should be selected to be in a range of 14.8-15.2mm, and in the actual operation, the NCF length is also in the range due to the processing error. The experimental spectra of 30mm and 50mm in air were compared for the length of SMF2 (NCF length about 15 mm).

As shown in fig. 3, fig. 3 is a spectrum diagram of SMF2 of different lengths, and a 50mm length results in a smaller free spectral range and a smaller trough contrast than a 30mm length, which will affect the range of applications of the sensor. Therefore we chose SMF2 of 30mm length as the sensing element and the C sensor spectrum has three major troughs, here dip2 (trough 2) was chosen as the monitoring trough.

As shown in fig. 4, fig. 4 is a schematic diagram of the effect of the liquid level measurement test, wherein a in fig. 4 represents that the dip2 wave trough of the C sensor varies with the liquid level; b in fig. 4 represents the level sensitivity of the C sensor in solutions with indices of refraction of 1.3333, 1.3424, 1.351, 1.3609, respectively; c in fig. 4 represents the liquid level/refractive index sensitivity of the C sensor; d in FIG. 4 represents the stability test of the C sensor in air for 0-75min, in which water, 5% NaCl, 10% NaCl and 15% NaCl aqueous solutions were prepared, and their corresponding refractive indices were 1.3333, 1.3424, 1.3510 and 1.3609, respectively. The solution was slowly added to the beaker and the spectral data was recorded every two millimeters with the surface of the solution just touching the top of the sensor as zero, with a measurement range of 0-44 mm. After the solution measurement is finished, the sensor and the beaker are cleaned by deionized water, dried and the next group of solution is measured. The experimental results show that the characteristic valleys of the sensor exhibit a red shift, as can be seen in a of fig. 4, respectively. The sensor level sensitivities are 0.21616, 0.23242, 0.25341, 0.27687nm/mm, respectively, as can be seen in b of fig. 4. From b in FIG. 4, the refractive index sensitivity was 2223.5658pm/mm/RIU, the degree of linear fit R²Is 0.99327. The sensor has high sensitivity, and the sensors show high linear fitting degree (R) at 0-15mm (NCF) and 15-44mm (SMF2) of the sensor²> 0.99) in combination with previous simulations, the NCF and SMF2 level sensor performance in this configuration was consistent.

The repeatability of the sensor under a stable environment is also one of the performance standards for evaluating the sensor. And (3) placing the sensor C in the air, keeping the indoor temperature constant, enabling the workbench to be stable and free of vibration, recording data every five minutes, and enabling the measurement interval to be 0-75 min. Experimental results the maximum spectral drift of the sensor is about 10.61pm, as shown by d in figure 4. The cause of sensor drift may be from the influence of fluctuations within the light source and spectrometer.

In the temperature characteristic measurement test, the A, C sensor and the thermometer are respectively arranged in a beaker with a heat insulation layer, and the heat insulation layer has the functions of slowing down the falling speed of the temperature of hot water, reducing the convection strength of the water in the temperature reduction process of the hot water and enabling the sensor to work in a relatively stable liquid environment. Boiling water was added to the beaker until the sensor was completely immersed. Taking 90 ℃ as a starting point, recording the spectral data every 5 ℃, and measuring the interval between 90 ℃ and 15 ℃.

The measurement results are shown in fig. 5. Fig. 5 is a schematic diagram of the temperature sensitivity test effect of the sensor, wherein the test effect of the temperature sensitivity of the sensor is A, C respectively, and it can be seen from the diagram that the temperature sensitivity of the sensor a is high, 57.57 pm/deg.c, and the maximum variation amplitude of the wave trough value is about 4.45 nm. It can be used for temperature sensors, but is not suitable for refractive index measurements. The C sensor has the maximum variation of the wave trough value of only 0.1nm in the temperature range of 15-90 ℃, and the wave trough frequency spectrum drift has very low linearity, so that the C sensor is not sensitive to the temperature in the measuring interval relative to the liquid level sensitivity (more than 200pm/mm, as shown in C in figure 4) of the sensor. The reason that the C-sensor is not sensitive to temperature may be that NCF has the opposite effect of SMF2 on the modulation of the internal modes of the fiber as the temperature changes. The reason why the wave valley value of the sensor C fluctuates is probably that strong convection exists in the liquid in the hot water cooling process, the experimental result is influenced by the convection disturbance optical fiber of the water and the fluctuation of the light source and the spectrometer, and the influence of the factors is not large compared with the high temperature sensitivity of the sensor A.

For the liquid level and temperature tests of the type B sensor, there have been studies in the prior art, the temperature sensitivity of the type B sensor is < 38.7 pm/deg.c, which still requires temperature compensation when used for liquid level measurement, which increases the demodulation difficulty and the system cost, and thus the type C sensor has advantages over the type B sensor.

In summary, the lengths of the coreless fiber NCF and the second single-mode fiber SMF2 in the michelson optical fiber liquid level sensor provided by the present embodiment are about 15 ± 0.2mm and 30mm, respectively. When the length of the NCF is 15 +/-0.2 mm, the single-mode fiber (SMF2) of the sensing part and the NCF show consistent liquid level sensitivity, so that the linear fitting degree of final data is high. This property arises from the fact that a particular length range of NCFs can transmit more energy into the cladding of a single-mode fiber, making the mode profile of the cladding of a single-mode fiber very similar to that of an NCF, which can provide some reference for the selection of parameters for a fiber mode coupler. The sensor has high refractive index sensitivity of 2223.5658pm/mm/RIU (RIU represents refractive index unit), and the ratio L of the length of NCF to the length of SMF2_ncf:L_smf2And when the temperature is approximately equal to 0.5, the temperature sensitivity of the sensor is very low. The characteristic can simplify the structure of the fiber grating double-parameter measurement sensor which is common at present for eliminating the temperature influence, and reduce the system cost. The sensor has simple structure, low cost, easy processing, high liquid level/refractive index sensitivity and relatively low temperature sensitivity. It has potential application prospect in the petrochemical field with inflammability, explosiveness and toxicity.

Example 2

In this embodiment, a michelson liquid level sensor based on multimode interference low-temperature response is used to detect the liquid level, and the specific steps are as follows:

one end of a single mode fiber (SMF1) is welded with one end of a coreless fiber (NCF), the other end of the coreless fiber is welded with another single mode fiber (SMF2), and the other end of the SMF2 is plated with a metal reflecting layer (silver, gold, nickel, aluminum and the like). The SMF1 is a light-transmitting fiber, when the ratio of the length of NCF to the length of SMF2 is L_ncf:L_smf20.5, thisThe ratio ensures that the temperature cross talk of the sensor is low.

The light field transmitted by the sensor is simulated, and the light field distribution in the NCF shows periodic change according to the simulation result, the length of the NCF is selected at the position where the intensity of the internal light field is periodically distributed and is far away from the axis of the optical fiber, so that more energy is coupled into the SMF2 cladding, the output spectrum of the sensor is more stable, and the SMF2 and the NCF show high consistency in liquid level measurement.

Connecting a sensor with an optical fiber circulator, wherein the circulator is connected with a light source and a spectrometer;

the sensor is vertically arranged in a container to be detected, and the liquid level change in the container is detected according to the wavelength value of the lowest position of a certain interference wave trough in a sensor spectrogram. When the liquid level changes, the wavelength value of the interference wave trough shows linear change.

As shown in a in fig. 4, the change of the abscissa of the lowest position of the trough is in a linear relationship with the height of the liquid in the container, the height of the liquid changes, the trough value also changes, the two are in one-to-one correspondence, when the sensor is immersed by the liquid with different heights, the transmission spectrum changes, and the liquid level is detected through the one-to-one correspondence relationship between the transmission spectrum and the height of the liquid level in the container.

The detection wave valley value is calculated according to the following formula:

The above-mentioned embodiments are merely preferred embodiments for fully illustrating the present invention, and the scope of the present invention is not limited thereto. The equivalent substitution or change made by the technical personnel in the technical field on the basis of the invention is all within the protection scope of the invention. The protection scope of the invention is subject to the claims.

Claims

1. Based on multimode interference low temperature response michelson level sensor, its characterized in that: the optical fiber comprises a first single-mode optical fiber, a coreless optical fiber and a second single-mode optical fiber; one end of the first single-mode fiber is connected with one end of the coreless fiber, and the other end of the coreless fiber is connected with the second single-mode fiber; the other end face of the second single-mode optical fiber is plated with a reflecting film;

the ratio of the length of the NCF to the length of the SMF2 meets the following condition: l is_ncf:L_smf2≈0.5；

The first single-mode fiber is connected with the coreless fiber in a core-to-core mode; the second single-mode fiber is connected with the coreless fiber in a core-to-core mode;

the first single-mode optical fiber and the coreless optical fiber are connected by adopting an optical fiber welding machine for welding; the connection of the coreless optical fiber and the second single-mode optical fiber is completed by welding through an optical fiber welding machine;

the length range of the coreless optical fiber is 14.8-15.2 mm;

the length of the second single-mode fiber is 28-32 mm.

2. The multi-mode interference based low temperature response michelson liquid level sensor of claim 1 wherein: the reflecting film is a silver film, a gold film, a nickel film or an aluminum film.

3. The multi-mode interference based low temperature response michelson liquid level sensor of claim 1 wherein: the mode energy distribution of the cladding of the second single mode fiber is the same or similar to the mode energy distribution in the coreless fiber.

4. Method for level measurement with a michelson level sensor according to any one of claims 1 to 3, wherein: the method comprises the following steps:

connecting the Michelson liquid level sensor with an optical fiber circulator;

the circulator is connected with the light source and the spectrometer;

acquiring a wave valley value in the spectrogram;

acquiring a liquid level value in the detection container according to the trough value;

the Michelson liquid level sensor comprises a first single-mode fiber, a coreless fiber and a second single-mode fiber; one end of the first single-mode fiber is connected with one end of the coreless fiber, and the other end of the coreless fiber is connected with the second single-mode fiber; the other end face of the second single-mode optical fiber is plated with a reflecting film;

the length range of the coreless optical fiber is 14.8-15.2 mm;

the length of the second single-mode fiber is 28-32 mm.

5. The method of claim 4, wherein: the valley value is calculated according to the following formula: