Probe type humidity sensing multi-point measurement method and device based on optical fiber microspheres

Abstract

The invention belongs to the technical field of optical fiber sensing, and particularly relates to a probe type humidity sensing multi-point measurement method and device based on optical fiber microspheres. According to the core scheme, a single-mode optical fiber and a multi-mode optical fiber are welded to form a Fabry-Perot (FP) microsphere sensing head, and a high-sensitivity FP interference cavity is constructed by controlling the curvature of microspheres on the end face of the multi-mode optical fiber and the thickness of an agarose gel film; temperature cross interference is eliminated by welding fiber bragg gratings (FBG) with different wavelengths; a 1 * N beam splitter is used for realizing multi-point sensing, and a mixed spectrum is separated through a Fourier transform (FFT) frequency domain filtering technology, so that independent demodulation of N sensing heads is realized. The probe-type humidity sensor has the advantages of compact probe-type structure, high sensitivity, accurate temperature compensation and multi-point synchronous measurement, and is suitable for accurate humidity detection in the fields of environmental monitoring, industrial control and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing, and in particular relates to a probe-type humidity sensing multi-point measurement method and device based on optical fiber microspheres. Background Art

[0002] Humidity sensors are widely used in environmental monitoring, industrial process control, agriculture, meteorology and biomedicine. Traditional humidity sensors mainly include capacitive, impedance and thermal conductivity humidity sensors. Capacitive humidity sensors measure humidity by detecting the influence of water vapor content in the air on the capacitance value of capacitors. They have the advantages of high sensitivity, good linearity and fast response speed, but they are easily affected by temperature changes, have poor long-term stability and aging of dielectric materials. Impedance humidity sensors determine humidity by measuring the resistance change of hygroscopic materials (such as aluminum oxide and lithium chloride). They have simple structure and low cost, but are greatly affected by temperature and pollution, have poor long-term stability and slow response speed. Thermal conductivity humidity sensors determine humidity by measuring the thermal conductivity of air. They have fast response speed, but low accuracy, are greatly affected by airflow and have high cost. At the same time, these traditional humidity sensors often face problems such as poor stability, limited sensitivity and susceptibility to electromagnetic interference in use. Fiber optic humidity sensors are gradually becoming a promising alternative due to their inherent advantages such as high sensitivity, strong anti-electromagnetic interference ability and remote sensing. Currently, common fiber optic humidity sensors include fiber Bragg grating type, distributed sensing type, fiber surface plasmon resonance type and fiber interferometry type.

[0003] Fiber Bragg grating (FBG) sensors detect changes in the external environment by writing Bragg gratings in optical fibers and using changes in the grating reflection wavelength. They have the characteristics of strong multiplexing, easy multi-point distributed measurement, anti-electromagnetic interference and long-distance transmission, but their sensitivity is relatively low and they are not suitable for high-precision measurement. Distributed optical fiber sensors achieve distributed measurement of physical quantities (such as temperature and strain) along the entire length of the optical fiber by analyzing the changes in optical signals along the length of the optical fiber. They have the characteristics of full coverage measurement, high spatial resolution, long-distance monitoring and multi-parameter measurement. However, their system installation and operation are relatively complex and require complex optical fiber wiring and demodulation systems. Fiber surface plasmon resonance (SPR) sensors detect the resonance effect of light on the metal surface through the interaction between the optical fiber surface and the metal film. They have the advantage of high sensitivity, but their structure is complex and requires metal coating on the optical fiber surface. The manufacturing process requirements are high. They are generally transmission structures, which are inconvenient to use. In addition, the SPR resonance peak is wide and not easy to multiplex at multiple points. Fiber optic interferometry sensors use the interference effect of light waves in optical fibers to detect changes in external physical quantities, including Michelson sensors, Mach-Zehnder sensors (MZIs) and Fabry-Perot (FP) sensors. This structure has the advantages of high sensitivity and simple manufacturing methods.

[0004] Fiber optic interferometric humidity sensors combine the advantages of fiber optic sensors and achieve high-precision, wide-range measurement of humidity through special design and structure. Among interferometric fiber optic sensors, FP interferometers have attracted much attention due to their simple structure, low cross-sensitivity and miniaturization; however, the sensitivity of traditional interferometric sensors is mostly in the pm range, which is not high enough to meet the needs of high-sensitivity detection. In addition, these interferometric sensors are mostly single-point measurements and cannot meet the requirements of multi-point detection and simultaneous measurement of multiple parameters.

[0005] In view of the problems of existing optical fiber humidity sensors, such as complex structure, high cost, great influence of temperature on measurement results, low sensitivity, potential danger, etc., the present invention proposes a probe-type humidity sensing multi-point measurement method and device based on optical fiber microspheres. The sensor has a simple structure, low cost, good stability, high sensitivity, can compensate for temperature effects, is safe and reliable, and is suitable for various scenarios requiring single-point or multi-point humidity monitoring. Summary of the invention

[0006] The present invention proposes a probe-type humidity sensing multi-point measurement method and device based on optical fiber microspheres to solve the problems of the existing FP sensor having a complex structure, low sensitivity and being unfavorable for multi-point sensing.

[0007] The method adopted by the present invention to solve the technical problem comprises the following steps:

[0008] Step 1: Device selection: select a spectrometer, a broadband light source operating in the range of 1450-1650nm, single-mode optical fiber, and N Fabry-Perot sensor heads; the Fabry-Perot sensor head is composed of an FBG temperature compensation unit and an FP microsphere sensor head, and the FP microsphere sensor head is composed of a single-mode optical fiber, a multi-mode optical fiber, an optical fiber microsphere, and an agarose gel film.

[0009] Step 2: The manufacturing process of the Fabry-Perot sensor head is as follows: select a section of single-mode optical fiber and a section of step-refractive-index multimode optical fiber, fuse one end of the multimode optical fiber with the single-mode optical fiber, cut the length of the multimode optical fiber to 200-300 μm, and use a fusion splicer to heat the other end of the multimode optical fiber to a molten state and then cool it, so that the end of the optical fiber forms a spherical structure due to surface tension; insert the sensor head into an agarose gel heated at 85°C to 95°C for 10 to 15 seconds using an optical fiber displacement platform, and obtain an agarose gel film with a thickness of 10-100 μm by controlling the number of coatings; then, The sensor head is placed on a heating table and heated at 50°C for 3-4 hours to completely solidify the agarose gel film to obtain a FP microsphere sensor head; according to the above method, N FP microsphere sensor heads with agarose gel films of different thicknesses are prepared, and the thickness difference of the agarose gel films is controlled to be ≥5μm; the FP microsphere sensor heads with agarose gel films of different thicknesses have different FP cavity lengths, and the curvature radius R of the optical fiber microsphere and the thickness L of the agarose gel film must satisfy R≥2L; finally, the FBG is fused with the single-mode optical fiber end of the FP microsphere humidity sensor head, and the entire Fabry-Perot sensor head is completed.

[0010] For the fabricated Fabry-Perot sensor head, after the signal light propagates through the single-mode optical fiber and enters the multi-mode optical fiber, a part of it is reflected at the interface between the multi-mode optical fiber and the agarose gel film, and the other part of the light will enter the agarose gel film through the interface. The light entering the agarose gel film will be reflected at the interface between the agarose gel film and the air, and then the two reflected lights will interfere coherently, and the reflected light intensity I satisfies the following relationship:

[0011]

[0012] Where I1 and I2 represent the light intensity reflected back to the single-mode fiber at the interface between the multimode fiber and agarose and the light intensity reflected back to the single-mode fiber at the interface between the agarose gel film layer and the air, n and L represent the refractive index and thickness of agarose, respectively, and φ is the phase of the light wave.

[0013] The phase φ of the Fabry-Perot sensor head can be expressed as:

[0014]

[0015] Where λ is the central wavelength of the incident light. From formula (2), we can see that when the phase ξ is equal to an even multiple of π, the reflected light intensity reaches a maximum value, corresponding to the peak in the reflected spectrum.

[0016]

[0017] λ d is the wavelength corresponding to the maximum light intensity, and m is an arbitrary integer

[0018] The interval between two adjacent troughs, i.e. the free spectral range (FSR), is:

[0019]

[0020] As the humidity increases, after agarose absorbs moisture in the air, its refractive index n and the thickness L of the agarose gel film will change, causing the phase difference Δφ to change. According to formula (2), at this time, the resonance trough in the sensor reflection spectrum will drift accordingly. The humidity change is ΔRH, the change in the agarose refractive index is Δn, the change in the thickness of the agarose gel film is ΔL, the wavelength drift of the FP cavity is Δλ, the central wavelength of the FP cavity before the humidity change is λ0, and the central wavelength of the FP cavity after the humidity change is λ1, then:

[0021]

[0022] Δn=S n *ΔRH,ΔL=S L *ΔRH (6)

[0023] S in formula (6) n and S L are the coefficients of change of agarose refractive index and thickness caused by humidity, respectively. Their values ​​are determined by the humidity-sensitive material and are constant within a certain humidity range. The relationship between the wavelength drift of the FP cavity and the humidity change is:

[0024] Δλ=S·ΔRH (7)

[0025] Combining equations (5), (6) and (7), the humidity sensitivity S of the Fabry-Perot sensor head can be expressed as:

[0026]

[0027] The coefficient K is calibrated, and the relative humidity change value ΔRH can be obtained by measuring the change value Δλ of the peak wavelength of the FP spectrum.

[0028] The FBG temperature compensation unit in the Fabry-Perot sensor head is a periodically changing refractive index structure in the optical fiber. When the wavelength of the incident light meets the Bragg condition, reflection occurs. The fiber grating FBG reflected light λ FBG The wavelength can be expressed as:

[0029] λ FBG =2n eff Λ (9)

[0030] The FBG temperature compensation unit is not coated with humidity-sensitive materials and is not humidity-sensitive, and is only affected by the external temperature. The formula for the change of FBG reflection wavelength with temperature is:

[0031] Δλ FBG =λ FBG (α T +ξ T )ΔT (10)

[0032] N FBGs with different wavelengths are selected and welded to N FP microsphere sensor heads to form N Fabry-Perot sensor heads. The FBG temperature compensation unit of each Fabry-Perot sensor head is used to measure the ambient temperature at that location, compensate for the influence of the ambient temperature on the FP microsphere sensor head, and accurately obtain the relative humidity value of the environment. Step three: Device connection: A broadband light source is connected to one input end of a 2×1 coupler through a transmission optical fiber, the output end of the 2×1 coupler is connected to the light input end of a 1×N beam splitter, the N output ends of the 1×N beam splitter are respectively connected to N Fabry-Perot sensor heads, and the input end of the 1×N beam splitter is connected to a spectrometer through another input end of the 2×1 coupler.

[0033] Step 4: The broadband light source generates signal light, which is transmitted to the 1×N beam splitter by a single-mode optical fiber. The 1×N beam splitter divides the optical signal into N channels, each of which is connected to a Fabry-Perot sensor head. Each signal light is reflected at the Fabry-Perot sensor head. Multiple reflected lights pass through the 1×N beam splitter and are synthesized into one beam. After passing through a 3dB coupler, the light is output to the spectrometer. The optical signal is collected by the spectrometer and then filtered and demodulated.

[0034] The 1×N beam splitter is an optical fiber element with N channels, each channel is connected to a Fabry-Perot sensor head; each channel Fabry-Perot sensor head is composed of a FBG temperature compensation unit and a FP microsphere sensor head connected in series and fused; the N FP microsphere sensor heads all use agarose as a coating material, but with different thicknesses, and the corresponding FP cavity lengths L are different, so that the free spectral ranges of the N FP microsphere sensor heads are different, and the frequencies of periodic changes of the reflection spectra are different; the reflection spectra of the N channels are merged into a spectrometer through the 1×N beam splitter and the coupler, and the spectrometer obtains a mixed spectrum of the reflection spectra of the N Fabry-Perot sensor heads.

[0035] Step 5: Filtering and processing of mixed spectrum: Mixed spectrum I is:

[0036]

[0037] The mixed spectrum obtained in formula (11) is subjected to Fourier transform to obtain the frequency domain spectrum, so as to perform filtering operation in the frequency domain to extract different frequency components. The mixed spectrum is subjected to Fourier transform to obtain the frequency domain spectrum, which is expressed as:

[0038]

[0039] The frequency domain spectrum obtained by Fourier transform is separated using a frequency domain filter to extract N frequency intervals corresponding to the N FP microsphere sensor heads used.

[0040] The frequency filtering formula is as follows:

[0041]

[0042] Where v1 and v2 are the frequency ranges of the filters.

[0043] Based on the N frequencies obtained by filtering, the signal of each FP microsphere sensor head is subjected to inverse Fourier transform (IFFT) to reconstruct the spectrum of each of the N FP microsphere sensor heads, where the spectrum s(λ) of the i-th FP microsphere sensor head is:

[0044]

[0045] For the wavelength shift of each interference spectrum peak separated and reconstructed by detection, after temperature compensation according to formula (10), the relative humidity change of the corresponding Fabry-Perot sensor head can be obtained by formula (7). To achieve the above steps, the specific device of the present invention includes: a broadband light source, a 2×1 coupler, a 1×N beam splitter, N Fabry-Perot sensor heads, and a spectrometer; the Fabry-Perot sensor head is formed by fusing a fiber Bragg grating FBG and a FP microsphere sensor head, the FP microsphere sensor head is formed by fusing a single-mode optical fiber and a multi-mode optical fiber, and then making an optical fiber microsphere on the end face of the multi-mode optical fiber, and coating an agarose gel film on the surface of the optical fiber microsphere; the FBG used in the N Fabry-Perot sensor heads has different reflection wavelengths; the agarose of the FP microsphere sensor head used in the N Fabry-Perot sensor heads has different coating material thicknesses; the broadband light source is connected to one port at one end of the dual port of the 2×1 coupler, one port of the 2×1 coupler is connected to the input end of the 1×N beam splitter through a transmission optical fiber, the output end of the 1×N beam splitter is connected to the N Fabry-Perot sensor heads, and the other port at one end of the dual port of the 2×1 coupler is connected to the spectrometer.

[0046] The beneficial effects of the present invention are:

[0047] 1. The present invention uses an agarose-coated FP cavity as a sensor head, and uses the hygroscopic properties of agarose: agarose is a highly hygroscopic material, and the humidity expansion coefficient of a 1% gel film is 0.15 μm / % RH. This property makes the sensor extremely sensitive to humidity changes and can detect small humidity changes. At the same time, agarose itself has good selectivity and is not interfered by other gases.

[0048] 2. The present invention uses a probe sensor, which is smaller in size and easier to package than a transmission sensor. It can be used as a probe in various narrow environments.

[0049] 3. The present invention adopts Fabry-Perot sensor heads with different cavity lengths as a multiplexing device. The sensor can be connected to multiple sensor heads with different FP cavity lengths. The sensor heads will not interfere with each other. The spectrum of each sensor head can be obtained by performing FFT transformation on the total spectrum, thereby realizing the function of simultaneous humidity measurement at multiple points. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a schematic diagram of the probe-type humidity sensing multi-point measurement method and device structure based on optical fiber microspheres.

[0051] Figure 2 This is a combined spectrum diagram of three-point multiplexing of a probe-type humidity sensing multi-point measurement method and device based on optical fiber microspheres.

[0052] Figure 3 This is a probe-type humidity sensing multi-point measurement method based on optical fiber microspheres and a demodulation spectrum diagram of a device with a cavity length of 15μm.

[0053] Figure 4 This is a probe-type humidity sensing multi-point measurement method based on optical fiber microspheres and a demodulation spectrum diagram of a device with a cavity length of 24μm.

[0054] Figure 5 This is a probe-type humidity sensing multi-point measurement method based on optical fiber microspheres and a demodulation spectrum diagram of a device with a cavity length of 60μm.

[0055] Figure 6 This is a diagram of the demodulation spectrum changes when the humidity of only two points of the probe-type humidity sensor multi-point measurement method and device based on optical fiber microspheres changes. Specific implementation method

[0056] The present invention will be further described below in conjunction with the accompanying drawings.

[0057] like Figure 1As shown, the specific device of the probe-type humidity sensing multi-point measurement method and device based on optical fiber microspheres of the present invention comprises a broadband light source 1, a 2×1 coupler 2, a 1×N beam splitter 3, a Fabry-Perot sensor head 4, and a spectrometer 5; the Fabry-Perot sensor head 4 is composed of FBG and FP microsphere sensor heads, and the FP microsphere sensor head is made by fusing a single-mode optical fiber 6 and a multi-mode optical fiber 8, and then making an optical fiber microsphere 9 on the end face of the multi-mode optical fiber, and coating an agarose gel film 10 on the surface of the microsphere. The broadband light source 1 is connected to the 201 port of the 2×1 coupler 2, the 202 port of the 2×1 coupler 2 is connected to N Fabry-Perot sensor heads 4 through the 1×N beam splitter 3, and the 203 port of the 2×1 coupler 2 is connected to the spectrometer 5.

[0058] The working mode of the present invention is as follows: light enters the 2×1 coupler 2 from the broadband light source 1 through the port 201, enters the single-mode optical fiber 6 through the port 202 of the 2×1 coupler 2 through the 1×N beam splitter 3, and after passing through the FBG7, a part of the light close to the central wavelength of the grating is reflected back, and the other light continues to be transmitted to the optical fiber microsphere 9 through the multimode optical fiber 8. At the interface between the optical fiber microsphere 9 and the agarose gel film 10, the light is divided into two beams, one beam of light is reflected back to the multimode optical fiber 8, and the other beam of light reaches the gel surface through the agarose gel film 10 and is reflected back at the interface between the gel and the air, and the two beams of light form FP interference in the agarose gel film 10. The formed interference spectrum passes through the optical fiber microsphere 9, the multimode optical fiber 8 and the single-mode optical fiber 6, and then enters the port 202 of the 2×1 coupler 2 through the 1×N beam splitter 3, and enters the spectrometer 5 through the port 203 after passing through the 2×1 coupler 2. When the ambient humidity changes, the hygroscopic properties of agarose cause its refractive index to change, thereby changing the spectral characteristics of the FP cavity. The drift of the spectral peak wavelength is detected by the spectrometer 5, and the relationship between the humidity change and the spectral peak wavelength drift can be calibrated to achieve humidity measurement. The change in external temperature can be compensated by the wavelength drift of the FBG. When using multi-point measurement, the light reflected back to the spectrometer by the N FP cavities will be transformed by FFT to obtain the frequency domain characteristic diagram of the spectrum. In the frequency domain characteristic diagram, the frequency domain characteristics corresponding to each FP cavity can be obtained by frequency domain filtering. The frequency domain characteristics corresponding to each FP cavity can be changed by IFFT to obtain the reflection spectrum diagram corresponding to each FP cavity. By observing the phase change of each reflection spectrum diagram, the humidity change of each sensor head can be inferred.

[0059] The present invention can realize the key technologies of humidity measurement of the probe-type humidity sensing multi-point measurement method and device based on optical fiber microspheres:

[0060] 1. Precision structural design of Fabry-Perot sensor head. To ensure seamless fusion of single-mode fiber and multi-mode fiber and the precision of fiber microsphere production on the end face of multi-mode fiber, the fiber microsphere produced by multi-mode fiber needs a certain curvature. At the same time, in order to reflect more light back into the fiber, the length of multi-mode fiber must be limited to a certain range.

[0061] 2. Application of the excellent properties of agarose. Evenly coating agarose on the end face of the optical fiber and forming an agarose gel film with a suitable concentration can increase the sensitivity and stability of the agarose gel film. This enables the sensor to maintain high-performance humidity measurement under different environmental conditions. At the same time, agarose has high mechanical strength, low temperature cross sensitivity within a certain range, and is easy to detect for a long time.

[0062] 3. Efficient optical signal transmission and demodulation system. Multi-point sensing can be achieved by analyzing agarose FP cavities of different thicknesses with a spectrum analyzer, in which the thickness control of agarose is the key, but the high-precision spectrometer is the basis for demodulation signals.

[0063] 4. FBG temperature compensation: In multi-point sensing, fiber Bragg gratings with a certain interval of central wavelengths need to be selected to avoid interference. Since the temperature cross sensitivity of the present invention is low, fiber Bragg gratings may not be used in an environment that does not require high precision.

[0064] In a specific implementation of the present invention, the specific model of the broadband light source 1 is IPSDS1506, and the output center wavelength of the broadband light source is 1550nm; the single-mode transmission optical fiber and the single-mode optical fiber used to make the Fabry-Perot sensor head are both conventional single-mode optical fibers (G625), and the core diameter of the single-mode optical fiber is 9μm, and the core diameter of the multi-mode optical fiber is 105μm, and the length is 200-300μm; the thickness of the agarose gel film on the Fabry-Perot sensor head is 15, 24, and 60μm, respectively; the 1×3 beam splitter (Thorlabs TN532R5F1A) used has a splitting ratio of 1:1:1 and an insertion loss of <0.5dB, and its three ports are respectively connected to three Fabry-Perot sensor heads; the experimental results show that the sensor has a linear fitting R in the humidity range of 30% to 55%. 2 =0.989; three agarose film sensors with different thicknesses are used in the example. When the ambient humidity of the humidity sensing units corresponding to 15μm and 24μm is changed by 5%, the ambient humidity of the humidity sensing unit corresponding to 60μm does not change. The reconstructed spectra of the former two sensor heads are moved by 28.28nm and 15.24nm, respectively, and the corresponding humidity sensitivities are 5.656nm / %RH and 3.048nm / %RH, respectively. The reconstructed humidity spectrum of the 60μm Fabry-Perot sensor head does not change, which shows that the present invention can achieve multi-point multiplexing without interfering with each other.

[0065] The above content shows and describes the basic principles and main features of the present invention. Without departing from the principles of the present invention, the present invention may also undergo various changes and improvements, and these changes and improvements all fall within the protection scope of the present invention claimed.

Claims

1. A probe-type humidity sensing multi-point measurement method based on optical fiber microspheres, characterized in that The steps include: Step 1: Select a spectrometer, a broadband light source operating in the range of 1450-1650nm; a single-mode optical fiber, and N Fabry-Perot sensor heads; the Fabry-Perot sensor head is composed of a fiber Bragg grating FBG and a FP microsphere sensor head, and the FP microsphere sensor head is composed of a single-mode optical fiber, a multi-mode optical fiber, an optical fiber microsphere, and an agarose gel film; Step 2: The manufacturing process of the Fabry-Perot sensor head is as follows: select a section of single-mode optical fiber and a section of step-refractive-index multimode optical fiber, fuse one end of the multimode optical fiber with the single-mode optical fiber, cut the length of the multimode optical fiber to 200-300 μm, and use a fusion splicer to heat the end face of the multimode optical fiber to a molten state at the other end of the multimode optical fiber and then cool it so that the end face of the optical fiber forms a spherical structure due to surface tension; insert the spherical structure into an agarose gel heated at 85°C to 95°C for 10 to 15 seconds using an optical fiber displacement platform, and obtain an agarose gel film with a thickness of 10-100 μm by controlling the number of coatings; then place the sensor head on the heating The agarose gel film is heated at 50° C. for 3-4 hours on a hot stage to completely solidify the agarose gel film to obtain a FP microsphere sensor head; according to the above method, N FP microsphere sensor heads with agarose gel films of different thicknesses are prepared, and the thickness difference of the agarose gel films is controlled to be ≥5 μm; the FP microsphere sensor heads with agarose gel films of different thicknesses have different FP cavity lengths, and the curvature radius R of the optical fiber microsphere and the thickness L of the agarose gel film must satisfy R≥2L; finally, the FBGs with different reflection wavelengths are respectively fused with the single-mode optical fiber ends of the N FP microsphere humidity sensor heads, and the N Fabry-Perot sensor heads are completed; Step 3: The broadband light source is connected to an input end of a 2×1 coupler through a transmission optical fiber, the output end of the 2×1 coupler is connected to the optical input end of a 1×N beam splitter, the N output ends of the 1×N beam splitter are respectively connected to N Fabry-Perot sensor heads, and the input end of the 1×N beam splitter is connected to a spectrometer through another input end of the 2×1 coupler; Step 4: A broadband light source generates signal light, which is transmitted to a 1×N beam splitter via a single-mode optical fiber. The 1×N beam splitter divides the optical signal into N channels, each of which is connected to a Fabry-Perot sensor head. For the i-th channel, the incident light first enters the FBG, and the FBG reflection wavelength is λ iFBG The reflected light returns along the original optical path and enters the spectrometer. By monitoring the change in the wavelength of λiFBG, the temperature change at the i-th sensor head can be obtained to compensate for the influence of the ambient temperature on the i-th FP microsphere sensor head and accurately obtain the relative humidity value of the environment at that location. The light beam passing through the FBG enters the FP microsphere sensor head. Part of the light is reflected at the interface between the multimode optical fiber and the agarose gel film, and the other part of the light enters the agarose gel film through the interface. The light entering the agarose gel film will be reflected at the interface between the agarose gel film and the air, and then the two reflected lights will interfere with each other. As the humidity increases, the agarose absorbs moisture in the air and its refractive index n i and the thickness of the agarose gel film L i will change, causing the phase Φ i Change, the phase Φ of the i-th FP microsphere sensor head i It can be expressed as: Where λ is the central wavelength of the incident light, n i , L i represent the refractive index and thickness of agarose on the i-th sensor head, Φ i is the phase of the light wave; At this time, the resonance peak in the sensor reflection spectrum drifts accordingly, and the peak position λ in the reflection spectrum of the i-th FP microsphere sensor head d for: By measuring the peak wavelength change of the spectrum of the FP microsphere sensor head, the relative humidity value at the i-th sensing position can be obtained; Humidity sensitivity S can be expressed as: S in the above formula n and S L They are the coefficients of variation of agarose refractive index and thickness caused by humidity, respectively, and their values ​​are determined by the humidity-sensitive material and are constant within a certain humidity range; The N FP microsphere sensor heads all use agarose as a coating material, but the thickness is different, the corresponding FP cavity length L is different, the free spectrum range of the N FP microsphere sensor heads is different, and the frequency of the periodic change of the reflection spectrum is different; the reflection spectrum of the N channels is merged into the spectrometer through a 1×N beam splitter and a coupler, and the spectrometer obtains a mixed spectrum of the reflection spectra of the N Fabry-Perot sensor heads; Step 5: Perform Fourier transform on the mixed spectrum to obtain its frequency domain spectrum, use a frequency domain filter in the frequency domain to separate the frequency domain spectrum obtained by Fourier transform, and extract N frequency intervals, which correspond to the N FP microsphere sensor heads used; based on the N frequencies obtained by filtering, perform inverse Fourier transform (IFFT) on the signal of each FP microsphere sensor head, reconstruct the spectrum of each N FP microsphere sensor head, and after temperature compensation, obtain the relative humidity change of the corresponding Fabry-Perot sensor head, so as to realize simultaneous sensing of multiple measurement points.

2. A probe-type humidity sensing multi-point measurement device based on optical fiber microspheres, which is applied to the multi-point measurement method of claim 1, characterized in that: The invention comprises a broadband light source, a 2×1 coupler, a 1×N beam splitter, N Fabry-Perot sensor heads, and a spectrometer; the Fabry-Perot sensor head is formed by fusing a fiber Bragg grating FBG and a FP microsphere sensor head; the FP microsphere sensor head is formed by fusing a single-mode optical fiber and a multi-mode optical fiber, and then making a fiber microsphere on the end face of the multi-mode optical fiber, and coating an agarose gel film on the surface of the fiber microsphere; the FBG used in the N Fabry-Perot sensor heads has different reflection wavelengths; the agarose of the FP microsphere sensor heads used in the N Fabry-Perot sensor heads has different coating material thicknesses; the broadband light source is connected to a port at one end of the dual port of the 2×1 coupler, one end of the 2×1 coupler is connected to the input end of the 1×N beam splitter through a transmission optical fiber, the output end of the 1×N beam splitter is connected to the N Fabry-Perot sensor heads, and the other port at one end of the dual port of the 2×1 coupler is connected to the spectrometer.

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