Methane sensor based on micro-nano optical fiber coupler and methane concentration measuring method

Abstract

The invention discloses a methane sensor based on a micro-nano optical fiber coupler and a methane concentration measuring method. The methane sensor comprises a broadband light source, a sensing unit and an oscilloscope which are sequentially connected from front to back, the sensing unit comprises a micro-nano optical fiber coupler, an indium oxide coating and a silver coating, and the silver coating and the indium oxide coating are sequentially arranged in the middle of the micro-nano optical fiber coupler from inside to outside; one end of the micro-nano optical fiber coupler is provided with a P1 port and a P2 port which are used as input ends, and the other end of the micro-nano optical fiber coupler is provided with a P3 port and a P4 port which are used as output ends. Ports P1 and P2 of the micro-nano optical fiber coupler are connected with the output end of the broadband light source, and a port P3 of the micro-nano optical fiber coupler is connected with the oscilloscope. The methane concentration is calculated based on the optical interference principle of the micro-nano optical fiber coupler, and the influence of external factors such as volume, pressure and temperature is eliminated. The device has the advantages of being small in size, high in external interference resistance and good in corrosion resistance, can better adapt to various environments, and has wide development space and good practicability.

Description

Technical Field

[0001] The invention belongs to the technical field of methane measurement, and in particular relates to a methane sensor based on a micro-nano optical fiber coupler and a methane concentration measurement method. Background Art

[0002] Methane is a greenhouse gas and is flammable and explosive, so detecting its concentration is of great significance. Most of the existing methane sensors on the market are resistive sensors, which have problems such as high cost, poor anti-interference ability and low sensitivity. Fiber optic sensors, with their own characteristics, sense the measured value by measuring the changes in optical parameters. They have the advantages of low cost, safety, reliability, and small size. They are widely used in many fields such as electricity, energy, and aviation.

[0003] However, ordinary fiber optic sensors have poor sensing effects on methane, and how to improve their sensing effects is of great research value. At the same time, existing methane gas processing and analysis methods usually perform differently in terms of sensitivity, response time, measurement accuracy, equipment volume, and maintenance frequency, and have certain advantages and limitations in their respective application scenarios. At present, for fiber optic methane sensors, the sensing sensitivity is generally improved by improving the optical fiber structure. However, the material properties of the light sensing unit limit the final sensitivity. Secondly, traditional gas collection and analysis methods are easily affected by environmental parameters such as temperature, pressure, and collection volume. Therefore, there are obvious defects in gas collection efficiency, response time, measurement accuracy, and anti-interference ability. Summary of the invention

[0004] The purpose of the present invention is to provide a methane sensor based on a micro-nano optical fiber coupler and a methane concentration measurement method, aiming to solve the above-mentioned problems.

[0005] The present invention is mainly achieved through the following technical solutions:

[0006] The methane sensor based on micro-nano optical fiber coupler comprises a broadband light source, a sensing unit and an oscilloscope which are sequentially connected from front to back; the sensing unit comprises a micro-nano optical fiber coupler, an indium oxide coating and a silver coating, and the middle part of the micro-nano optical fiber coupler is sequentially provided with a silver coating and an indium oxide coating from the inside to the outside; one end of the micro-nano optical fiber coupler is provided with P1 and P2 ports as input ends, and the other end is provided with P3 and P4 ports as output ends; the P1 and P2 ports of the micro-nano optical fiber coupler are connected to the output end of the broadband light source, and the P3 port is connected to the oscilloscope.

[0007] In order to better realize the present invention, further, a silver coating is deposited on the middle part of the micro-nano optical fiber coupler based on physical vapor deposition technology, and an indium oxide coating is coated on the surface of the silver coating based on spin coating technology.

[0008] In order to better implement the present invention, further, the thickness of the silver coating is 12 nm, and the thickness of the indium oxide coating is 150 nm.

[0009] In order to better implement the present invention, further, the broadband light source is a modular SLED light source, and the central wavelength of the light emitted is 950nm; the splitting ratio of the P1 and P2 ports of the micro-nano optical fiber coupler is 1:9.

[0010] In order to better realize the present invention, further, the micro-nano optical fiber coupler is made of two single-mode optical fibers soaked in hydrofluoric acid and then melt-tapered by oxyhydrogen flame.

[0011] In order to better implement the present invention, further, the broadband light source, the sensor unit and the oscilloscope are respectively connected through arc discharge fused single-mode optical fibers.

[0012] The present invention is mainly achieved through the following technical solutions:

[0013] A methane concentration measurement method is performed using the above-mentioned methane sensor based on micro-nano optical fiber coupler. The calculation formula is:

[0014]

[0015] Where: S is the output signal of the methane sensor;

[0016] k is the sensitivity of the sensor;

[0017] k0 is the basic sensitivity constant;

[0018] n1 and n2 are the influence indexes of the coating thickness of indium oxide coating and silver coating on sensitivity, respectively;

[0019] and t Ag are the coating thicknesses of indium oxide coating and silver coating, respectively.

[0020] In order to better realize the present invention, further, in the experiment, the coating thickness is gradually changed. and t Ag , and record the corresponding sensor output signal S and the known methane concentration The experimental data were fitted based on the nonlinear regression model to optimize the fitting effect of formula (2), and finally the optimal n1 and n2 were obtained.

[0021] In order to better implement the present invention, further, the influence index n1 of the coating thickness of the indium oxide coating on the sensitivity is 0.9, and the influence index n2 of the coating thickness of the silver coating on the sensitivity is 1.2.

[0022] The beneficial effects of the present invention are as follows:

[0023] (1) The present invention uses indium oxide and silver as coating materials and combines micro-nano optical fiber coupling technology to construct a methane gas concentration sensor, which has a small size, strong anti-interference ability, good corrosion resistance, can meet the monitoring needs of narrow spaces, and is used for gas monitoring in substations, with broad development space. The present invention calculates methane concentration based on the optical interference principle of micro-nano optical fiber couplers, eliminates the influence of external factors such as volume, pressure and temperature, can better adapt to a variety of environments, has strong anti-interference ability, and has good practicality.

[0024] (2) The present invention improves the existing methane sensors, such as high cost, large size, low sensitivity, and susceptibility to environmental interference. Two single-mode optical fibers soaked in hydrofluoric acid are melted by hydrogen-oxygen flame to obtain coupled optical fibers, that is, micro-nano optical fiber couplers. The surface of the micro-nano optical fiber coupler is coated with a silver coating, which can enhance the evanescent field on the surface of the optical fiber and improve the sensitivity of the sensing system to methane under the action of the evanescent wave. At the same time, the indium oxide coating coated on the outer layer of the silver coating, which is a methane-sensitive material, further enhances the sensing effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the structure of a methane sensor based on micro-nano optical fiber coupler.

[0026] Among them: 1-indium oxide coating, 2-silver coating. DETAILED DESCRIPTION

[0027] Embodiment 1:

[0028] A methane sensor based on a micro-nano optical fiber coupler comprises a broadband light source, a sensing unit, and an oscilloscope. The sensing unit comprises an indium oxide coating 1, a silver coating 2, and a micro-nano optical fiber coupler. The middle part of the micro-nano optical fiber coupler is provided with a silver coating 2 and an indium coating in sequence from the inside to the outside. The present invention uses the evanescent field enhancement effect of the silver coating 2 to make the refractive index of the optical fiber change more significantly when it comes into contact with methane gas, thereby making the central wavelength offset of the output signal more obvious, thereby improving the sensitivity of the sensor. The methane sensor improves the existing methane sensing devices, such as low sensitivity, high cost, large size, and susceptibility to interference from the external environment.

[0029] The combination of the indium oxide coating 1 and the silver coating 2 can improve sensitivity while maintaining good stability. The high sensitivity of indium oxide complements the excellent optical and electrical properties of silver, allowing the sensor to work stably under a variety of environmental conditions. Secondly, the presence of the silver coating 2 can accelerate the response time of the signal and improve the overall detection efficiency.

[0030] like Figure 1As shown, the micro-nano optical fiber coupler includes four ports, P1, P2, P3, and P4, of which the P1 and P2 ports are used as input ports, and the P3 port is used as an output port; the P1 and P2 ports of the micro-nano optical fiber coupler are connected to the output port of the broadband light source, and the P3 output port is connected to the oscilloscope. First, the silver coating 2 is coated on the surface of the micro-nano optical fiber coupler, and then the indium oxide coating 1 is coated on the surface of the silver coating 2.

[0031] Preferably, the broadband light source is a modular SLED light source, emitting light with a central wavelength of 950 nm. Furthermore, the splitting ratio of the P1 and P2 ports of the micro-nano optical fiber coupler is 1:9.

[0032] Preferably, the micro-nano optical fiber coupler is made of two single-mode optical fibers soaked in hydrofluoric acid and then melt-tapered by oxyhydrogen flame. Specifically, the length of the micro-nano optical fiber coupler is 35 mm.

[0033] The oscilloscope is a sampling oscilloscope, which detects the output interference signal to obtain the central wavelength data.

[0034] The broadband light source, the sensor unit and the oscilloscope are all connected by arc discharge melting through single-mode optical fiber.

[0035] Preferably, the silver coating 2 is coated on the surface of the micro-nano optical fiber coupler by physical vapor deposition (PVD) technology, and the thickness and quality of the silver coating 2 are controlled to optimize the enhancement effect of the evanescent field. The indium oxide coating 1 is coated on the outer surface of the silver coating 2 by spin coating. The silver coating 2 can enhance the evanescent field on the surface of the optical fiber, and under the action of the evanescent wave, improve the sensitivity of the sensing system to methane. At the same time, the outermost coated indium oxide coating 1 is a methane-sensitive material, which further enhances the sensing effect. Specifically, the thickness of the indium oxide coating 1 is 150nm, and the thickness of the silver coating 2 is 12nm.

[0036] When in use, first build the optical path according to the optical path diagram, place it in the environment to be detected, and wait for a period of time for the system to stabilize before sensing methane gas.

[0037] Working principle: The light emitted by the broadband light source enters the micro-nano fiber coupler through a single-mode optical fiber and is split, with a splitting ratio of 1:9. When the light passes through the taper area, phase interference occurs. At the same time, the silver and indium oxide coatings 1 coated on the surface of the micro-nano fiber coupler make the refractive index change with the change of the external methane gas concentration, and the central wavelength of the output signal shifts. At the same time, the evanescent field on the surface of the optical fiber is enhanced in this process, and the sensing effect is improved. Finally, the oscilloscope realizes the sensing of methane gas by analyzing the offset of the central wavelength of the output signal. As shown in Table 1, by changing the thickness of the coating, the present invention prepares samples 1 and 2 of methane sensors. Compared with the existing optical fiber methane sensors, the sensitivity of the methane sensors prepared by the present invention is significantly improved, the response time is shorter, and the anti-interference ability is better. Moreover, when the thickness of the indium oxide coating 1 is 150nm and the thickness of the silver coating 2 is 12nm, the performance of the methane sensor is better.

[0038] Table 1

[0039] Performance parameters Ordinary fiber optic methane sensor Sample 1 Sample 2 Sensitivity 50ppm 35ppm 25ppm Response time 30 s 20s 10s cost 5000 4000 3500 volume 150mm×100mm 70mm×30mm 70mm×30mm Anti-interference ability medium powerful powerful Operating temperature range -10℃ to 50℃ -20℃ to 60℃ -20℃ to 60℃ Coating thickness none Silver 20nm, Indium Oxide 200nm Silver 12nm, Indium Oxide 150nm Environmental adaptability Poor excellent excellent Reliability (MTBF) 1000 hours 3000 hours 3000 hours Average failure rate 5% / year 2% / year 2% / year Production time 4 weeks 2 weeks 2 weeks Material usage 50 g 20g 20g Supply voltage range 5V±10％ 4.5V±10％ 3V±5％ Signal processing accuracy ±5nm ±3nm ±1nm Operating humidity range 10% to 80% 5% to 95% 5% to 95% Housing Material plastic Stainless steel Stainless steel Protection level IP54 IP68 IP68 Measuring range 0-10000ppm 0-20000ppm 0-20000ppm Measuring linearity ±10％ ±2％ ±2％ Service life 2 years 5 years 5 years

[0040] Embodiment 2:

[0041] A methane concentration measurement method is performed using the above-mentioned methane sensor based on micro-nano optical fiber coupler. The specific calculation method is as follows:

[0042] (1) Effect of coating thickness on sensitivity:

[0043] The coating thickness of indium oxide and silver is and t Ag It means that the sensitivity k can be expressed as a function of coating thickness:

[0044]

[0045] Where: k0 is the basic sensitivity constant, for example: 0.1-1V / ppm.

[0046] n1 and n2 are the influence index of coating thickness on sensitivity (depending on material properties and structural design).

[0047] (2) Relationship between sensor output signal and concentration:

[0048] Combined with the coating thickness, the output signal S of the sensor can be expressed as:

[0049]

[0050] (3) Final concentration calculation formula:

[0051] Combining formula (1) and formula (2), the concentration of methane is calculated for:

[0052]

[0053] Preferably, the coating thickness is varied stepwise during the experiment. and t Ag , and record the corresponding sensor output signal S and the known methane concentration The nonlinear regression model was used to fit the experimental data to find the best n1 and n2, so that the fitting effect of formula (2) was the best. Finally, n1 was 0.9 and n2 was 1.2.

[0054] The traditional methane gas calculation formula is obtained based on the ideal gas state equation:

[0055]

[0056] in: is the amount of methane,

[0057] V is the volume of methane.

[0058] The traditional methane gas calculation method mainly relies on volume, pressure and temperature, and can only be applied under specific pressure and temperature. The method adopted by the present invention is based on the optical interference principle of micro-nano fiber couplers, eliminating the influence of external factors such as volume, pressure and temperature, and can adapt to a variety of environments with strong anti-interference ability. As shown in Table 2, the concentration error measured by the present invention is kept within ±1%, showing its superior measurement accuracy and reliability. As shown in Table 3, the measurement accuracy, environmental adaptability and anti-interference of the present invention are better.

[0059] Table 2

[0060] Test samples Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Measured concentration (ppm) 50 200 500 1000 150 300 700 1200 Concentration calculated by traditional method (ppm) 55 220 550 1100 160 330 770 1320 Concentration calculated by the present invention (ppm) 50 198 502 1005 149 298 703 1205 Error (traditional vs measured) 10％ 10％ 10％ 10％ 10％ 10％ 10％ 10％ Error (invention vs. actual measurement) 0％ -1％ 0.40％ 0.50％ -0.70％ -0.70％ 0.40％ 0.40％

[0061] Table 3

[0062] parameter Traditional methods Example 2 Measurement accuracy ±10％ ±2％ Applicable concentration range 100-5000ppm 10-20000ppm Temperature adaptability -10℃ to 50℃ -20℃ to 60℃ External interference high Low Equipment size big Small Maintenance frequency Once a year Once every 2 years Response time 30s 8s Signal-to-Noise Ratio (SNR) 15dB 30dB Diffusion coefficient (D) <![CDATA[0.15m 2 / s]]> <![CDATA[0.25m 2 / s]]> Maximum signal output (S_max) 100mV 300mV

[0063] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A methane sensor based on micro-nano optical fiber coupler, characterized in that: The invention comprises a broadband light source, a sensing unit and an oscilloscope which are connected in sequence from front to back; the sensing unit comprises a micro-nano optical fiber coupler, an indium oxide coating (1) and a silver coating (2); the middle part of the micro-nano optical fiber coupler is provided with the silver coating (2) and the indium oxide coating (1) in sequence from the inside to the outside; one end of the micro-nano optical fiber coupler is provided with ports P1 and P2 as input ends, and the other end is provided with ports P3 and P4 as output ends; the P1 and P2 ports of the micro-nano optical fiber coupler are connected to the output end of the broadband light source, and the P3 port is connected to the oscilloscope.

2. The methane sensor based on micro-nano optical fiber coupler according to claim 1 is characterized in that: A silver coating (2) is deposited on the middle part of the micro-nano optical fiber coupler based on physical vapor deposition technology, and an indium oxide coating (1) is coated on the surface of the silver coating (2) based on spin coating technology.

3. The methane sensor based on micro-nano optical fiber coupler according to claim 2 is characterized in that: The thickness of the silver coating (2) is 12 nm, and the thickness of the indium oxide coating (1) is 150 nm.

4. The methane sensor based on micro-nano optical fiber coupler according to claim 1, characterized in that: The broadband light source is a modular SLED light source, and the central wavelength of the light emitted is 950nm; the splitting ratio of the P1 and P2 ports of the micro-nano optical fiber coupler is 1:

9.

5. The methane sensor based on micro-nano optical fiber coupler according to claim 1, characterized in that: The micro-nano optical fiber coupler is made of two single-mode optical fibers soaked in hydrofluoric acid and then melted and tapered by oxyhydrogen flame.

6. The methane sensor based on micro-nano optical fiber coupler according to claim 5, characterized in that: The broadband light source, the sensor unit and the oscilloscope are respectively connected via arc discharge fused single-mode optical fibers.

7. A method for measuring methane concentration, using the methane sensor based on micro-nano optical fiber coupler according to any one of claims 1 to 6, characterized in that: Methane concentration The calculation formula is: Where: S is the output signal of the methane sensor; k is the sensitivity of the sensor; k0 is the basic sensitivity constant; n1 and n2 are the influence indexes of the coating thickness of the indium oxide coating (1) and the silver coating (2) on the sensitivity, respectively; and t Ag are the coating thicknesses of the indium oxide coating (1) and the silver coating (2), respectively.

8. A methane concentration measurement method according to claim 7, characterized in that: In the experiment, the coating thickness was gradually changed and t Ag , and record the corresponding sensor output signal S and the known methane concentration The experimental data were fitted based on the nonlinear regression model to optimize the fitting effect of formula (2), and finally the optimal n1 and n2 were obtained.

9. A methane concentration measurement method according to claim 8, characterized in that: The influence index n1 of the coating thickness of the indium oxide coating (1) on the sensitivity is 0.9, and the influence index n2 of the coating thickness of the silver coating (2) on the sensitivity is 1.2.

Images