Water quality multi-parameter detection optical fiber sensing system and method based on FMCW

Abstract

The invention provides a water quality multi-parameter detection optical fiber sensing system and method based on FMCW. Relates to the technical field of optical fiber sensing, and solves the problem that simultaneous detection of multiple parameters of water quality cannot be realized in the prior art, and the system is characterized in that a light beam output by a laser is divided into a first light beam and a second light beam through a first optical coupler; the light beams respectively pass through a reference arm and a sensing arm of the Mach-Zehnder interferometer, and the light beams of the sensing arm enter a parallel branch of the to-be-detected sample parameter detection device; a parallel branch of the to-be-detected sample parameter detection device returns a detection light beam; an interference phenomenon occurs between the reference light beam and the detection light beam to generate an interference signal, the balance detector receives the interference signal, and the data acquisition and processing device converts the interference signal into an interference spectrum, performs spectrum analysis on the interference spectrum and determines detection parameters of the sample to be detected. According to the invention, high-precision and high-sensitivity water quality multi-parameter synchronous measurement is realized.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to an optical fiber sensing system and method for detecting multi-parameters of water quality based on FMCW (Frequency Modulated Continuous Wave). Background Art

[0002] Fiber optic sensors have the advantages of compact structure, resistance to electromagnetic interference, low cost, small size, and easy reuse, and are widely used in various fields. Among them, fiber optic Fabry-Perot (FPI) sensors have the advantages of high sensitivity and resistance to external vibration interference. They can realize real-time, efficient, large-scale continuous monitoring of environmental parameters and can be applied to the field of marine environmental monitoring. FPI sensors are based on the principle of optical interference. They have the advantages of high measurement accuracy, fast response speed, simple preparation, and strong stability. They are the preferred choice for multi-parameter detection of water quality.

[0003] However, the sensitivity of a single FPI sensor is very limited, and it is difficult to achieve simultaneous high-precision measurement of multiple parameters. Summary of the invention

[0004] A brief overview of the present invention is provided below in order to provide a basic understanding of certain aspects of the present invention. It should be understood that this overview is not an exhaustive overview of the present invention. It is not intended to identify key or important parts of the present invention, nor is it intended to limit the scope of the present invention. Its purpose is merely to present certain concepts in a simplified form as a prelude to a more detailed description discussed later.

[0005] In view of this, the present invention provides a water quality multi-parameter detection optical fiber sensing system and method based on FMCW, so as to at least solve the problem that the prior art cannot realize the simultaneous detection of water quality multi-parameters.

[0006] According to one aspect of the present invention, there is provided a water quality multi-parameter detection optical fiber sensing system based on FMCW, comprising: a tunable laser, a Mach-Zehnder interferometer, a first optical coupler, a second optical coupler, a third optical coupler, a balanced detector, a parameter detection device for a sample to be tested, and a data acquisition and processing device;

[0007] The Mach-Zehnder interferometer comprises a reference arm and a sensor arm, wherein the sensor arm is provided with an optical fiber circulator;

[0008] The light beam output by the tunable laser is divided into a first light beam and a second light beam by the first optical coupler;

[0009] The first light beam passes through the reference arm of the Mach-Zehnder interferometer to form a reference light beam carrying the original wavelength information of the light source; the reference light beam is transmitted to the second optical coupler;

[0010] The second light beam is transmitted to the third optical coupler through the first branch of the optical fiber circulator, and then is divided into the first sensor light beam and the second sensor light beam through the third optical coupler and respectively incident on the parallel branches of the parameter detection device of the sample to be tested;

[0011] The two sensing light beams returned by the parallel branch of the parameter detection device for the sample to be tested and carrying the information of the wavelength change of the light source caused by the sample to be tested are coupled into detection light beams through a third optical coupler;

[0012] The detection beam is transmitted to the second optical coupler through the second branch of the optical fiber circulator;

[0013] The reference light beam and the detection light beam passing through the second optical coupler interfere with each other to generate an interference signal, and the balanced detector receives the interference signal;

[0014] The data acquisition and processing device converts the collected interference signal into an interference spectrum, performs spectrum analysis on the interference spectrum, and determines the detection parameters of the sample to be tested.

[0015] In a preferred embodiment of the present invention, the device for detecting the parameters of the sample to be tested includes a first interferometer, a second interferometer, a third interferometer and a fourth interferometer. The first interferometer, the second interferometer, the third interferometer and the fourth interferometer are connected in series in pairs and then in parallel to form two parallel branches of the device for detecting the parameters of the sample to be tested.

[0016] In a preferred embodiment of the present invention, the system further comprises: a polarization controller;

[0017] The polarization controller is arranged on the optical path of the reference arm of the Mach-Zehnder interferometer, and the reference light beam is transmitted to the second optical coupler after being polarized by the polarization controller.

[0018] In a preferred embodiment of the present invention, the first interferometer and the third interferometer are connected in series to form a parallel branch of the device for detecting the parameters of the sample to be tested, and the second interferometer and the fourth interferometer are connected in series to form another parallel branch of the device for detecting the parameters of the sample to be tested;

[0019] The first interferometer is formed by fusion splicing a single-mode optical fiber with a hollow-core optical fiber filled with polydimethylsiloxane at one end, and the interference cavity is a closed cavity;

[0020] The second interferometer is formed by staggered fusion of single-mode optical fibers, and the interferometer cavity is an open cavity;

[0021] The third interferometer is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with PVA material or PAA material;

[0022] The fourth interferometer is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with ammonia nitrogen sensitive material PCA.

[0023] In a second aspect, the present invention further provides a water quality multi-parameter detection optical fiber sensing method based on FMCW, comprising:

[0024] The light beam outputted by the tunable laser is divided into a first light beam and a second light beam through a first optical coupler;

[0025] The first light beam passes through the reference arm of the Mach-Zehnder interferometer to form a reference light beam carrying the original wavelength information of the light source, and the reference light beam is transmitted to the second optical coupler;

[0026] The second light beam is transmitted to the third optical coupler through the first branch of the optical fiber circulator, and then is divided into the first sensor light beam and the second sensor light beam through the third optical coupler to be incident on the parallel branches of the parameter detection device of the sample to be tested respectively;

[0027] The two sensing light beams returned by the parallel branch of the parameter detection device for the sample to be tested and carrying the information of the wavelength change of the light source caused by the sample to be tested are coupled into detection light beams through a third optical coupler;

[0028] The detection beam is transmitted to the second optical coupler through the second branch of the optical fiber circulator;

[0029] The reference beam and the detection beam passing through the second optical coupler interfere with each other to generate an interference signal, and a balanced detector receives the interference signal;

[0030] The data acquisition and processing device converts the collected interference signal into an interference spectrum, performs spectrum analysis on the interference spectrum, and determines the detection parameters of the sample to be tested.

[0031] In a preferred embodiment of the present invention, the parallel branches of the first sensor beam and the second sensor beam respectively incident on the parameter detection device of the sample to be tested include:

[0032] The first sensing light beam is incident on a parallel branch of the parameter detection device of the sample to be tested formed by the first interferometer and the third interferometer connected in series;

[0033] The second sensing light beam is incident on the second interferometer and connected in series with the fourth interferometer to form another parallel branch of the device for detecting the parameter of the sample to be measured.

[0034] In a preferred embodiment of the present invention, converting the interference signal into an interference spectrum by a data acquisition and processing device comprises:

[0035] The interference signal received by the balanced detector is subjected to Fourier transformation to obtain an interference spectrum corresponding to the interference signal.

[0036] In a preferred embodiment of the present invention, performing spectrum analysis on the interference spectrum to determine the detection parameters of the sample to be tested includes:

[0037] Performing spectrum analysis on the interference spectrum to obtain spectrum information corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer respectively;

[0038] The reflection peak corresponding to each interference cavity is intercepted respectively by a window function corresponding to the frequency range of each interference cavity;

[0039] Performing an inverse Fourier transform on each reflection peak to obtain wavelength information spectra of the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer respectively;

[0040] The frequency spectra corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer are respectively demodulated by using the root mean square envelope and the sine fitting;

[0041] Superimposing the frequency spectra corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer and the interference cavity of the fourth interferometer in pairs to obtain an interference spectrum envelope;

[0042] The interference spectrum envelope is used to determine the detection parameters of the sample to be tested.

[0043] In a preferred embodiment of the present invention, the detection parameters of the sample to be tested include:

[0044] Temperature, salinity, pH and ammonia nitrogen value of the sample to be tested.

[0045] In a preferred embodiment of the present invention, using the interference spectrum envelope to determine the detection parameters of the sample to be tested includes:

[0046] According to the translation of the interference spectrum envelope relative to the experimentally calibrated interference spectrum envelope, a relationship matrix between the translation and temperature, salinity, pH value, and ammonia nitrogen value is established, and the temperature, salinity, pH value, and ammonia nitrogen value of the sample to be tested are determined by solving the relationship matrix.

[0047] The present invention realizes multi-parameter measurement of water quality based on reflective sensing based on the interference principle, combines the interferometer serial-parallel structure with the optical frequency modulated continuous wave (FMCW) interferometric measurement technology, and combines the FMCW technology, the optical vernier effect and the optical fiber sensor, which can not only produce the vernier effect, but also realize high-precision and high-sensitivity multi-parameter synchronous measurement of water quality. It solves the problems of the existing water quality detection method, such as the difficulty of simultaneous high-precision measurement of multiple parameters, the narrow measurement range of itself, and the complexity of multi-parameter decoupling, and provides a higher-precision and higher-sensitivity measurement solution to improve the performance of multi-parameter measurement of seawater.

[0048] These and other advantages of the present invention will become more apparent through the following detailed description of the best embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The present invention may be better understood by referring to the following description given in conjunction with the accompanying drawings, wherein the same or similar reference numerals are used throughout the drawings to represent the same or similar components. The accompanying drawings, together with the following detailed description, are included in and form a part of this specification and are used to further illustrate the preferred embodiments of the present invention and to explain the principles and advantages of the present invention. In the drawings:

[0050] Figure 1 1 is a schematic diagram showing the structure of the FMCW-based water quality multi-parameter detection optical fiber sensing system of the present invention;

[0051] Figure 2 is a schematic diagram showing the structure of the interferometer in the device for detecting the parameters of the sample to be tested of the present invention; wherein, Figure 2 (a) is a schematic diagram of the structure of the first interferometer; Figure 2 (b) is a schematic diagram of the structure of the second interferometer; Figure 2 (c) is a schematic diagram of the structure of the third interferometer; Figure 2 (d) is a schematic structural diagram of the fourth interferometer;

[0052] Figure 3 is a spectrum diagram showing an interference signal received by the balanced detector of the present invention;

[0053] Figure 4 is a schematic diagram showing an envelope spectrum corresponding to an interferometer in a device for detecting parameters of a sample to be tested according to the present invention; Figure 4 (a) is a schematic diagram of the envelope spectrum corresponding to the first interferometer; Figure 4 (b) is a schematic diagram of the envelope spectrum corresponding to the second interferometer; Figure 4 (c) is a schematic diagram of the envelope spectrum corresponding to the third interferometer; Figure 4 (d) is a schematic diagram of the envelope spectrum corresponding to the fourth interferometer;

[0054] Figure 5 is an interference spectrum corresponding to the interferometer in the device for detecting the parameters of the sample to be tested of the present invention; Figure 5 (a) is the interference spectrum corresponding to the first interferometer; Figure 5 (b) is the interference spectrum corresponding to the second interferometer; Figure 5 (c) is the interference spectrum corresponding to the third interferometer; Figure 5 (d) is the interference spectrum corresponding to the fourth interferometer;

[0055] Figure 6 is a waveform diagram showing the interference spectrum envelope of the first interferometer and the second interferometer of the present invention;

[0056] Figure 7 is a waveform diagram showing the interference spectrum envelope of the first interferometer and the third interferometer of the present invention;

[0057] Figure 8 is a waveform diagram showing the interference spectrum envelope of the first interferometer and the fourth interferometer of the present invention;

[0058] Fig. 9 3 is a waveform diagram showing the interference spectrum envelopes of the third interferometer and the fourth interferometer of the present invention.

[0059] Those skilled in the art will appreciate that the elements in the drawings are shown only for simplicity and clarity and are not necessarily drawn to scale. For example, the size of some elements in the drawings may be enlarged relative to other elements to help improve the understanding of the embodiments of the present invention. DETAILED DESCRIPTION

[0060] Exemplary embodiments of the present invention will be described below in conjunction with the accompanying drawings. For the sake of clarity and conciseness, not all features of actual implementations are described in the specification. However, it should be understood that many implementation-specific decisions must be made in the process of developing any such actual implementation in order to achieve the developer's specific goals, such as meeting those constraints related to the system and business, and these constraints may vary from implementation to implementation. In addition, it should be understood that although the development work may be very complex and time-consuming, it is only a routine task for those skilled in the art who benefit from the present disclosure.

[0061] It is also necessary to explain here that, in order to avoid obscuring the present invention due to unnecessary details, only the device structure and / or processing steps closely related to the scheme according to the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0062] like Figure 1As shown, an embodiment of the present invention provides a water quality multi-parameter detection optical fiber sensing system based on FMCW, comprising: a tunable laser, a Mach-Zehnder interferometer, a first optical coupler, a second optical coupler, a third optical coupler, a balanced detector, a parameter detection device for a sample to be tested, and a data acquisition and processing device;

[0063] The Mach-Zehnder interferometer comprises a reference arm and a sensor arm, wherein the sensor arm is provided with an optical fiber circulator;

[0064] The light beam output by the tunable laser is divided into a first light beam and a second light beam by the first optical coupler;

[0065] The first light beam passes through the reference arm of the Mach-Zehnder interferometer to form a reference light beam carrying the original wavelength information of the light source; the reference light beam is transmitted to the second optical coupler;

[0066] The second light beam is transmitted to the third optical coupler through the first branch of the optical fiber circulator, and then is divided into the first sensor light beam and the second sensor light beam through the third optical coupler and respectively incident on the parallel branches of the parameter detection device of the sample to be tested;

[0067] The two sensing light beams returned by the parallel branch of the parameter detection device for the sample to be tested and carrying the information of the wavelength change of the light source caused by the sample to be tested are coupled into detection light beams through a third optical coupler;

[0068] The detection beam is transmitted to the second optical coupler through the second branch of the optical fiber circulator;

[0069] The reference light beam and the detection light beam passing through the second optical coupler interfere with each other to generate an interference signal, and the balanced detector receives the interference signal;

[0070] The data acquisition and processing device converts the collected interference signal into an interference spectrum, performs spectrum analysis on the interference spectrum, and determines the detection parameters of the sample to be tested.

[0071] In the embodiment of the present invention, in order to solve the problems of difficulty in simultaneous high-precision measurement of multiple parameters of water quality, narrow measurement range itself, and complex multi-parameter decoupling, FMCW technology, optical vernier effect and fiber optic sensor are combined, and the amplification effect of the vernier effect is used to improve the measurement accuracy and sensitivity, thereby enhancing the performance of multi-parameter measurement of seawater.

[0072] Among them, frequency modulated continuous wave (FMCW) technology is a measurement method widely used in the fields of radar, fiber optic sensing and lidar. The main principle of this technology is to transmit a continuous wave signal whose frequency is modulated linearly with time, and receive the echo signal reflected by the target. The frequency change law of the transmitted wave and the echo waveform is the same, but there is a time difference. This time difference reflects the time it takes for the electromagnetic wave to travel between the radar and the target, and can be used to measure the information of the target. When this technology is applied to the field of fiber optic sensing, it can also be called optical frequency domain reflectometry (OFDR).

[0073] As an effective method to improve sensitivity, the optical vernier effect utilizes the interaction of two interference spectra with similar but different free spectral ranges to achieve high-resolution measurement by amplifying tiny changes in external physical quantities.

[0074] In a preferred embodiment of the present invention, the device for detecting the parameters of the sample to be tested includes a first interferometer, a second interferometer, a third interferometer and a fourth interferometer. The first interferometer, the second interferometer, the third interferometer and the fourth interferometer are connected in series in pairs and then in parallel to form two parallel branches of the device for detecting the parameters of the sample to be tested.

[0075] In a preferred embodiment of the present invention, the system further comprises: a polarization controller;

[0076] The polarization controller is arranged on the optical path of the reference arm of the Mach-Zehnder interferometer, and the reference light beam is transmitted to the second optical coupler after being polarized by the polarization controller.

[0077] Since the existing fiber grating sensor is limited by its own measurement range and multi-parameter decoupling difficulty, it is difficult to measure water quality multi-parameters with high precision in a large range at the same time. In traditional light sources, the sensitivity of a single fiber optic sensor based on the interference principle is low, and there are problems such as low signal-to-noise ratio of reflected light signal, poor resolution and low detection accuracy in the system. In addition, although FMCW technology has been applied in optical radar (LiDAR) and optical communications, its application in multi-parameter measurement of seawater quality has not been fully studied and developed. In order to solve the above problems, an embodiment of the present invention proposes a water quality multi-parameter detection fiber optic sensing system based on FMCW. The water quality multi-parameter detection fiber optic sensing system based on FMCW in the embodiment of the present invention includes a tunable laser (TLS), a group of Mach-Zehnder interferometers (MZI), a balanced detector (BPD), a polarization controller (PC), four FP interferometers and a data acquisition and processing device (DAQ).

[0078] In a preferred embodiment of the present invention, the first interferometer and the third interferometer are connected in series to form a parallel branch of the device for detecting the parameters of the sample to be tested, and the second interferometer and the fourth interferometer are connected in series to form another parallel branch of the device for detecting the parameters of the sample to be tested;

[0079] The first interferometer is formed by fusion splicing a single-mode optical fiber with a hollow-core optical fiber filled with polydimethylsiloxane at one end, and the interference cavity is a closed cavity;

[0080] The second interferometer is formed by staggered fusion of single-mode optical fibers, and the interferometer cavity is an open cavity;

[0081] The third interferometer is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with PVA material or PAA material;

[0082] The fourth interferometer is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with ammonia nitrogen sensitive material PCA.

[0083] Polydimethylsiloxane (PDMS) is a material that is extremely sensitive to temperature. It will expand and contract when the temperature changes, and has good light transmittance and refractive index. By combining the high thermal expansion and high thermo-optical properties of PDMS with optical fiber, the length of the cavity will change when the environment in which the optical fiber is located changes, thereby improving the temperature measurement sensitivity.

[0084] The FMCW-based water quality multi-parameter detection optical fiber sensing system of the embodiment of the present invention can overcome the shortcomings of the prior art, realize multi-parameter measurement of water quality based on reflective sensing based on the interference principle, and utilize four Fabry-Perot interferometers in series-parallel structure combined with optical FMCW interferometry measurement technology. It can combine the advantages of FMCW technology and optical fiber sensing technology. This structure can not only produce a vernier effect, but also realize high-precision and high-sensitivity synchronous measurement of multi-parameters of water quality.

[0085] like Figure 1 As shown, an FMCW-based water quality multi-parameter detection optical fiber sensing system according to an embodiment of the present invention is mainly composed of three parts: the first part is a Mach-Zehnder interferometer (MZI). The upper branch is a reference arm, on which a polarization controller (PC) is placed to reduce the polarization effect; the lower branch is a sensing arm, which is connected to a sensing area (where a parameter detection device for a sample to be tested is set) through a fiber optic circulator. The second part is the sensing area, where the optical fiber led out from port 2 (corresponding to the first branch) of the fiber optic circulator is connected to the parameter detection device for the sample to be tested, and the parameter detection device for the sample to be tested includes four sets of interferometer series-parallel structures, such as Figure 2As shown, the first interferometer FP1 is formed by fusion of a single-mode optical fiber and a hollow-core optical fiber filled with PDMS at one end, and the interference cavity is a closed cavity, which is sensitive to temperature. The second interferometer FP2 is formed by staggered fusion of single-mode optical fibers, and the interference cavity is an open structure, which is sensitive to salinity. The third interferometer FP3 is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with pH-sensitive materials PVA / PAA, which is sensitive to pH. The fourth interferometer FP4 is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with ammonia nitrogen-sensitive material PCA, which is sensitive to ammonia nitrogen. The first interferometer FP1, the third interferometer FP3 and the second interferometer FP2, the fourth interferometer FP4 are in parallel relationship, the first interferometer FP1 and the third interferometer FP3 are in series relationship, and the second interferometer FP2 and the fourth interferometer FP4 are in series relationship. Interferometers FP1, FP2, FP3 and FP4 produce cursor effects in pairs. The third part is a data acquisition and processing device. All reflected light information in the sensing area is input into the balanced detector (BPD) via the optical fiber circulator port 3 (corresponding to the second branch), and is finally received by the data acquisition and processing device (DAQ).

[0086] The parameters of the interferometer used in the embodiment of the present invention are as follows:

[0087] The first interferometer (i.e., the sensing cavity FP1) is formed by sequentially fusion-joining a single-mode optical fiber and a hollow-core optical fiber filled with PDMS at one end (the single-mode optical fiber can have a diameter of 125 microns and a core diameter of 8-10 microns; the outer diameter of the hollow-core optical fiber can be 125 microns and the inner diameter can be 50 microns).

[0088] The second interferometer (i.e., sensing cavity FP2) is made of three single-mode optical fibers spliced ​​at an offset of 65.5-67.5 microns. The optical path of the second interferometer is about 0.90-0.99 times that of the first interferometer to ensure a vernier effect between the first interferometer and the second interferometer.

[0089] The third interferometer 3 (i.e., the sensing cavity FP3) is made of three single-mode optical fibers that are staggered and fused together with a staggered amount of 65.5-67.5 microns. A pH-sensitive material is injected into the middle hollow space to form a sensing cavity. The optical path of the sensing cavity is approximately 1.01-1.1 times that of the first interferometer and approximately 1.01-1.1 times that of the fourth interferometer, so as to ensure a vernier effect between the first interferometer and the third interferometer, and between the second interferometer and the fourth interferometer.

[0090] The fourth interferometer (i.e., the sensing cavity FP4) is made by staggered fusion of three single-mode optical fibers with a staggered amount of 65.5-67.5 microns, and an ammonia nitrogen sensitive material is injected into the middle hollow to form a sensing cavity. The optical path of the sensing cavity is approximately 1.0-1.05 times that of interferometer 1 to ensure a vernier effect between the first interferometer and the fourth interferometer.

[0091] In a second aspect, the present invention further provides a water quality multi-parameter detection optical fiber sensing method based on FMCW, comprising:

[0092] The light beam outputted by the tunable laser is divided into a first light beam and a second light beam through a first optical coupler;

[0093] The first light beam passes through the reference arm of the Mach-Zehnder interferometer to form a reference light beam carrying the original wavelength information of the light source, and the reference light beam is transmitted to the second optical coupler;

[0094] The second light beam is transmitted to the third optical coupler through the first branch of the optical fiber circulator, and then is divided into the first sensor light beam and the second sensor light beam through the third optical coupler to be incident on the parallel branches of the parameter detection device of the sample to be tested respectively;

[0095] The two sensing light beams returned by the parallel branch of the parameter detection device for the sample to be tested and carrying the information of the wavelength change of the light source caused by the sample to be tested are coupled into detection light beams through a third optical coupler;

[0096] The detection beam is transmitted to the second optical coupler through the second branch of the optical fiber circulator;

[0097] The reference beam and the detection beam passing through the second optical coupler interfere with each other to generate an interference signal, and a balanced detector receives the interference signal;

[0098] The data acquisition and processing device converts the collected interference signal into an interference spectrum, performs spectrum analysis on the interference spectrum, and determines the detection parameters of the sample to be tested.

[0099] In a preferred embodiment of the present invention, the parallel branches of the first sensor beam and the second sensor beam respectively incident on the parameter detection device of the sample to be tested include:

[0100] The first sensing light beam is incident on a parallel branch of the parameter detection device of the sample to be tested formed by the first interferometer and the third interferometer connected in series;

[0101] The second sensing light beam is incident on the second interferometer and connected in series with the fourth interferometer to form another parallel branch of the device for detecting the parameter of the sample to be measured.

[0102] In a preferred embodiment of the present invention, converting the interference signal into an interference spectrum by a data acquisition and processing device comprises:

[0103] The interference signal received by the balanced detector is subjected to Fourier transformation to obtain an interference spectrum corresponding to the interference signal.

[0104] In a preferred embodiment of the present invention, performing spectrum analysis on the interference spectrum to determine the detection parameters of the sample to be tested includes:

[0105] Performing spectrum analysis on the interference spectrum to obtain spectrum information corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer respectively;

[0106] The reflection peak corresponding to each interference cavity is intercepted respectively by a window function corresponding to the frequency range of each interference cavity;

[0107] Performing an inverse Fourier transform on each reflection peak to obtain wavelength information spectra of the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer respectively;

[0108] The frequency spectra corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer are respectively demodulated by using the root mean square envelope and the sine fitting;

[0109] Superimposing the frequency spectra corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer and the interference cavity of the fourth interferometer in pairs to obtain an interference spectrum envelope;

[0110] The interference spectrum envelope is used to determine the detection parameters of the sample to be tested.

[0111] In a preferred embodiment of the present invention, the detection parameters of the sample to be tested include:

[0112] Temperature, salinity, pH and ammonia nitrogen value of the sample to be tested.

[0113] In a preferred embodiment of the present invention, using the interference spectrum envelope to determine the detection parameters of the sample to be tested includes:

[0114] According to the translation of the interference spectrum envelope relative to the experimentally calibrated interference spectrum envelope, a relationship matrix between the translation and temperature, salinity, pH value, and ammonia nitrogen value is established, and the temperature, salinity, pH value, and ammonia nitrogen value of the sample to be tested are determined by solving the relationship matrix.

[0115] The system of the embodiment of the present invention is based on the FMCW water quality multi-parameter detection optical fiber sensing method, which is implemented based on the above-mentioned optical fiber sensing system, and forms four different interference spectrum envelopes by obtaining the cursor effect between the first interferometer FP1 and the second interferometer FP2, the third interferometer FP3, the fourth interferometer FP4, and the third interferometer FP3 and the fourth interferometer FP4, and then establishes a relationship matrix between the four interference spectrum envelope translations and the temperature, salinity, pH value and ammonia nitrogen value, and realizes the simultaneous measurement of temperature, salinity, pH value and ammonia nitrogen value by solving the relationship matrix. The optical fiber sensing method of the embodiment of the present invention combines the advantages of FMCW technology and optical fiber sensing technology, and at the same time utilizes the amplification effect of the optical cursor effect, which not only improves the detection accuracy and sensitivity of the optical fiber sensing system, but also realizes the simultaneous measurement of temperature, salinity, pH value and ammonia nitrogen value.

[0116] The signal light collected by the data acquisition and processing device is a beat frequency signal of four FP interferometers, so the interference spectrum is relatively complex. In order to obtain the interference spectrum envelopes between the four first interferometers FP1, the second interferometer FP2, the third interferometer FP3, and the fourth interferometer FP4, the present application first performs spectrum analysis on the complex beat frequency signal, selects the two end surface interference envelope spectra of the four FP interferometers through the window function, and uses the root mean square (RMS) envelope and sinusoidal signal fitting to demodulate the required spectrum waveform, and then uses the interference spectrum to superimpose the interference spectrum to obtain the interference spectrum envelope between the first interferometer FP1 and the second interferometer FP2, the interference spectrum envelope between the first interferometer FP1 and the third interferometer FP3, the interference spectrum envelope between the first interferometer FP1 and the fourth interferometer FP4, and the interference spectrum envelope between the third interferometer FP3 and the fourth interferometer FP4. According to the translation of the interference spectrum envelope obtained by detection relative to the experimentally calibrated interference spectrum envelope (corresponding to specific temperature, salinity, pH value, and ammonia nitrogen value), a relationship matrix between the interference spectrum translation and temperature, salinity, pH value, and ammonia nitrogen value is established. By solving the relationship matrix, simultaneous measurement of temperature, salinity, pH value, and ammonia nitrogen value can be achieved.

[0117] Example

[0118] Fiber optic sensing systems such as Figure 1As shown, it is composed of a tunable laser light source (1480nm-1640nm), a fiber circulator, a first interferometer FP1, a second interferometer FP2, a third interferometer FP3, a fourth interferometer FP4, a balanced detector, a polarization controller, a data acquisition and processing device, and three optical couplers. The light emitted from the tunable laser source enters the Mach-Zehnder interferometer (MZI) after passing through the first coupler (OFC), and a part of the light enters the upper reference arm, on which a polarization controller (PC) is placed to reduce the polarization effect. The other part of the light enters the lower sensing arm, and then enters the sensing area of ​​the series-parallel structure through the fiber circulator, the first interferometer FP1 (the interferometer is only sensitive to temperature), the second interferometer FP2 (the interferometer is only sensitive to salinity), the third interferometer FP3 (the interferometer is only sensitive to pH), and the fourth interferometer FP4 (the interferometer is only sensitive to ammonia nitrogen). The reflected light after being reflected by the four FP interferometers appears as delayed light relative to the reference light. Due to the different positions of each FP interferometer, the time delay τ of the light beam reaching each FP cavity is different. Each reflection end face will produce a sweep signal with a specific frequency difference. The reflected light from the four FP interferometer reflection end faces is then collected together through the fiber circulator, and forms a complex interference spectrum with the upper reference arm signal under the action of the second optical coupler, which is received by the balanced detector (BPD) and converted into an electrical signal, and finally received by the data acquisition and processing device (DAQ).

[0119] The data collected from the data acquisition and processing device (DAQ) is a complex interference signal. The complex interference signal is visualized (such as Figure 3 As shown), the complex interference signal is then Fourier transformed (FFT) to obtain the spectrum information of the four FP cavities (corresponding to the four reflection peaks), and then the four reflection peaks are individually cut out by setting a window function corresponding to the frequency range of each reflection peak, and then each reflection peak is inverse Fourier transformed (IFFT) to obtain the wavelength information spectrum of the four FP cavities (as shown Figure 4 Then, the root mean square envelope (RMS) and sine fitting are used to demodulate the spectrum of each FP cavity (as shown in Figure 5 As shown in Figure 2, since the free spectral ranges of the four FP cavities are similar but not equal, adding two of them together can produce a vernier effect (as shown in Figure 2). Figures 6 to 9 As shown in Figure 2, multi-parameter measurement of water quality is realized.

[0120] In the MZI interferometer, the incident light is divided into the reference arm and the sensor arm through the first coupler. The electric field expression of the reference arm can be expressed as:

[0121]

[0122] Where E0 is the initial amplitude of the electric field. Represents the electric field phase. For a linear frequency modulated light source, it can be expressed as follows:

[0123]

[0124] Where f0 represents the initial optical frequency of the laser light source, f0 = c / λ0. γ represents the frequency sweep rate of the incident light, t is time, and c is the speed of light in a vacuum. The detection light reflected from the sensor arm can be expressed as a frequency sweep signal with a fixed time delay difference from the reference light:

[0125]

[0126] Where r represents the end face reflectivity of the FP sensing cavity. τ represents the time delay of the reflection end face in the sensing arm, which is described by the formula τ = 2nl / c, n is the refractive index of the sensing cavity, and l is the difference in length between the reflection point and the reference arm. Therefore, the beat frequency signal after the reference light and the detection light meet and interfere is:

[0127] I=[E r (t)+E s (t)][E r (t)+E s (t)]* (4)

[0128] * indicates the conjugation operation.

[0129] Therefore, the above formula is expanded to calculate and the beat frequency signals of the eight reflection end faces of the four FP cavities are obtained. The following formula is obtained by simplifying and combining:

[0130]

[0131]

[0132] Among them, the subscripts represent the serial numbers of the reflection surfaces, 1 is the front reflection end surface of the first interferometer, 2 is the back reflection end surface of the first interferometer, 3 is the front reflection end surface of the second interferometer, 4 is the back reflection end surface of the second interferometer, 5 is the front reflection end surface of the third interferometer, 6 is the back reflection end surface of the third interferometer, 7 is the front reflection end surface of the fourth interferometer, 8 is the back reflection end surface of the fourth interferometer, and the beat frequency f b =γτ, which is proportional to the length of the sensor arm. The interference spectrum received by the data acquisition and processing device is a complex interference spectrum superimposed by 8 interference spectra, which can be approximately expressed as:

[0133] I total =I1+I2+I3+I4+I5+I6+I7+I8 (13)

[0134] The free spectral range of the interferometer based on the Mach-Zehnder principle can be expressed as:

[0135] FSR=λ 2 / nΔL (14)

[0136] Where λ is the central wavelength, and ΔL is the difference in length between the reference arm and the sensor arm. Therefore, the free spectral range of the first interferometer FP1, the second interferometer FP2, the third interferometer FP3 and the fourth interferometer FP4 can be expressed as:

[0137]

[0138] Since the reflectivity of the two surfaces of the FP cavity is weak, only the double-beam interference of one reflection is considered. The interference envelope spectrum of the reflected light of the four FP cavities can be expressed as follows:

[0139]

[0140] Among them, I0 is the incident light intensity value, η1, η2, η3, η4 are the transmission coefficients of the light in the first interferometer FP1, the second interferometer FP2, the third interferometer FP3, and the fourth interferometer FP4 cavity, r1 and r2 are the reflection coefficients of the two end faces of the first interferometer FP1 cavity, r3 and r4 are the reflection coefficients of the two end faces of the second interferometer FP2 cavity, r5 and r6 are the reflection coefficients of the two end faces of the third interferometer FP3 cavity, r7 and r8 are the reflection coefficients of the two end faces of the fourth interferometer FP4 cavity. n1 and L1 are the refractive index and cavity length of the first interferometer FP1 cavity, n2 and L2 are the refractive index and cavity length of the second interferometer FP2 cavity, n3 and L3 are the refractive index and cavity length of the third interferometer FP3 cavity, and n4 and L4 are the refractive index and cavity length of the fourth interferometer FP4 cavity.

[0141] Since the free spectral ranges of the envelope spectra of the first interferometer FP1 and the second interferometer FP2 are close but not equal, a cursor effect will be generated between the first interferometer FP1 and the second interferometer FP2, and the interference spectrum envelope I en-12 It can be expressed as:

[0142]

[0143] Among them, E 12 is the interference spectrum envelope amplitude; M 12 is the magnification of the vernier effect between the first interferometer FP1 and the second interferometer FP2, that is, the interference spectrum envelope I en-12 The temperature and salinity sensitivity of the first interferometer FP1 and the second interferometer FP2 is 12 times.

[0144] Since the free spectral ranges of the envelope spectra of the first interferometer FP1 and the third interferometer FP3 are close but not equal, a vernier effect will be generated between the first interferometer FP1 and the third interferometer FP3, and the interference spectrum envelope I en-13 It can be expressed as:

[0145]

[0146] Among them, E 13 is the interference spectrum envelope amplitude; M 13 is the magnification of the vernier effect between the first interferometer FP1 and the third interferometer FP3, that is, the interference spectrum envelope I en-13 The temperature and pH sensitivity of the first interferometer FP1 and the third interferometer FP3 are 13 times.

[0147] Since the free spectral ranges of the envelope spectra of the first interferometer FP1 and the fourth interferometer FP4 are close but not equal, a vernier effect will be generated between the first interferometer FP1 and the fourth interferometer FP4, and the interference spectrum envelope I en-14 It can be expressed as:

[0148]

[0149] Among them, E 14 is the interference spectrum envelope amplitude; M 14 is the magnification of the vernier effect between the first interferometer FP1 and the fourth interferometer FP4, that is, the interference spectrum envelope I en-14 The temperature and ammonia nitrogen value sensitivity of the first interferometer FP1 and the fourth interferometer FP4 is M 14 times.

[0150] Since the free spectral ranges of the envelope spectra of the third interferometer FP3 and the fourth interferometer FP4 are close but not equal, a vernier effect will be generated between the third interferometer FP3 and the fourth interferometer FP4, and the interference spectrum envelope I en-34 It can be expressed as:

[0151]

[0152] Among them, E 34 is the interference spectrum envelope amplitude; M 34 is the magnification of the vernier effect between the third interferometer FP3 and the fourth interferometer FP4, that is, the interference spectrum envelope I en-34 The pH value and ammonia nitrogen value sensitivity are M of the third interferometer FP3 and the fourth interferometer FP4. 34 times.

[0153] Since the data acquisition and processing device obtains the complex interference spectrum of the laser source after passing through the series-parallel structure of four FP interferometers, in order to obtain the interference spectrum envelope I en-12 ,I en-13 ,I en-14 ,I en-34, it is necessary to perform spectrum analysis on the complex interference spectrum to obtain the interference spectrum of each of the four FP interferometers, and then obtain the interference spectrum envelope I by superimposing the interference spectrum en-12 ,I en-13 ,I en-14 ,I en-34 .

[0154] When the temperature, salinity, pH value and ammonia nitrogen value change, the interference spectrum envelope I en-12 ,I en-13 ,I en-14 ,I en-34 The translation Δλ 12 , Δλ 13 , Δλ 14 , Δλ 34 They can be expressed as:

[0155]

[0156] The corresponding matrix equation can be expressed as:

[0157]

[0158] Among them, ΔT, ΔS, ΔP, and ΔN represent the changes of temperature and salinity relative to the calibration point (T0, S0), the changes of temperature and pH value relative to the calibration point (T0, P0), the changes of temperature and ammonia nitrogen value relative to the calibration point (T0, N0), and the changes of pH value and ammonia nitrogen value relative to the calibration point (P0, N0), respectively; Δλ 12 , Δλ 13 , Δλ 14 , Δλ 34 They are the interference spectrum envelope I en-12 ,I en-13 ,I en-14 ,I en-34 The translation of the interference spectrum envelope relative to the calibration point; α1 and β1 are the interference spectrum envelope I en-12 The temperature and salinity sensitivity of is a constant and can be measured experimentally; α2 and β2 are the interference spectrum envelope I en-13 The temperature and pH sensitivity of is a constant and can be measured experimentally; α3 and β3 are the interference spectrum envelope I en-14 The temperature and ammonia nitrogen value sensitivity are constants and can be measured experimentally; α4 and β4 are the interference spectrum envelope I en-34 The pH value and ammonia nitrogen value sensitivity are constants and can be measured experimentally. By solving the matrix equation, the measured temperature T, salinity S, pH value P, and ammonia nitrogen value N can be obtained, which are:

[0159]

[0160] Figure 3 is the original interference spectrum signal. Since the data acquisition and processing device receives the complex interference spectrum signal after the coupling of the four interferometric cavities, the complex interference spectrum signal is analyzed by Fourier transform to extract the four cavity interference envelope spectra I1(λ), ​​I2(λ), I3(λ), and I4(λ), as shown in Figure 4 Then, the four waveforms of the required spectrum are demodulated using the root mean square (RMS) envelope and the sinusoidal signal fitting, such as Figure 5 The free spectral ranges of the four interferometers are very close and unequal, and can be cascaded to produce a vernier effect, such as Figure 6-Figure 9 shown.

[0161] Although the present invention has been described according to a limited number of embodiments, it will be apparent to those skilled in the art, with the benefit of the above description, that other embodiments may be envisioned within the scope of the invention thus described. In addition, it should be noted that the language used in this specification is selected primarily for readability and teaching purposes, rather than for explaining or defining the subject matter of the present invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the present invention is illustrative, not restrictive, with respect to the scope of the present invention, which is defined by the appended claims.

Claims

1. Water quality multi-parameter detection optical fiber sensing system based on FMCW, It is characterized in that include: A tunable laser, a Mach-Zehnder interferometer, a first optical coupler, a second optical coupler, a third optical coupler, a balanced detector, a parameter detection device for a sample to be tested, and a data acquisition and processing device; The Mach-Zehnder interferometer comprises a reference arm and a sensor arm, wherein the sensor arm is provided with an optical fiber circulator; The light beam output by the tunable laser is divided into a first light beam and a second light beam by the first optical coupler; The first light beam passes through the reference arm of the Mach-Zehnder interferometer to form a reference light beam carrying the original wavelength information of the light source; the reference light beam is transmitted to the second optical coupler; The second light beam is transmitted to the third optical coupler through the first branch of the optical fiber circulator, and then is divided into the first sensor light beam and the second sensor light beam through the third optical coupler and respectively incident on the parallel branches of the parameter detection device of the sample to be tested; The two sensing light beams returned by the parallel branch of the parameter detection device for the sample to be tested and carrying the information of the wavelength change of the light source caused by the sample to be tested are coupled into detection light beams through a third optical coupler; The detection beam is transmitted to the second optical coupler through the second branch of the optical fiber circulator; The reference light beam and the detection light beam passing through the second optical coupler interfere with each other to generate an interference signal, and the balanced detector receives the interference signal; The data acquisition and processing device converts the collected interference signal into an interference spectrum, performs spectrum analysis on the interference spectrum, and determines the detection parameters of the sample to be tested.

2. The system according to claim 1, It is characterized in that The device for detecting the parameter of the sample to be tested comprises a first interferometer, a second interferometer, a third interferometer and a fourth interferometer. The first interferometer, the second interferometer, the third interferometer and the fourth interferometer are connected in series in pairs and then in parallel to form two parallel branches of the device for detecting the parameter of the sample to be tested.

3. The system according to claim 1 or 2, It is characterized in that Also includes: Polarization controller; The polarization controller is arranged on the optical path of the reference arm of the Mach-Zehnder interferometer, and the reference light beam is transmitted to the second optical coupler after being polarized by the polarization controller.

4. The system according to claim 2, It is characterized in that The first interferometer and the third interferometer are connected in series to form a parallel branch of the device for detecting the parameters of the sample to be tested, and the second interferometer and the fourth interferometer are connected in series to form another parallel branch of the device for detecting the parameters of the sample to be tested; The first interferometer is formed by fusion splicing a single-mode optical fiber with a hollow-core optical fiber filled with polydimethylsiloxane at one end, and the interference cavity is a closed cavity; The second interferometer is formed by staggered fusion of single-mode optical fibers, and the interferometer cavity is an open cavity; The third interferometer is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with PVA material or PAA material; The fourth interferometer is formed by staggered fusion of single-mode optical fibers, and the open cavity in the middle is filled with ammonia nitrogen sensitive material PCA.

5. Fiber optic sensing method for multi-parameter water quality detection based on FMCW, It is characterized in that include: The light beam outputted by the tunable laser is divided into a first light beam and a second light beam through a first optical coupler; The first light beam passes through the reference arm of the Mach-Zehnder interferometer to form a reference light beam carrying the original wavelength information of the light source, and the reference light beam is transmitted to the second optical coupler; The second light beam is transmitted to the third optical coupler through the first branch of the optical fiber circulator, and then is divided into the first sensor light beam and the second sensor light beam through the third optical coupler to be incident on the parallel branches of the parameter detection device of the sample to be tested respectively; The two sensing light beams returned by the parallel branch of the parameter detection device for the sample to be tested and carrying the information of the wavelength change of the light source caused by the sample to be tested are coupled into detection light beams through a third optical coupler; The detection beam is transmitted to the second optical coupler through the second branch of the optical fiber circulator; The reference beam and the detection beam passing through the second optical coupler interfere with each other to generate an interference signal, and a balanced detector receives the interference signal; The data acquisition and processing device converts the collected interference signal into an interference spectrum, performs spectrum analysis on the interference spectrum, and determines the detection parameters of the sample to be tested.

6. The method according to claim 5, It is characterized in that The parallel branches of the first sensor beam and the second sensor beam respectively incident on the parameter detection device of the sample to be tested include: The first sensing light beam is incident on a parallel branch of the parameter detection device of the sample to be tested formed by the first interferometer and the third interferometer connected in series; The second sensing light beam is incident on the second interferometer and connected in series with the fourth interferometer to form another parallel branch of the device for detecting the parameter of the sample to be measured.

7. The method according to claim 6, It is characterized in that Converting the interference signal into an interference spectrum by a data acquisition and processing device comprises: The interference signal received by the balanced detector is subjected to Fourier transformation to obtain an interference spectrum corresponding to the interference signal.

8. The method according to claim 6, It is characterized in that Performing spectrum analysis on the interference spectrum to determine the detection parameters of the sample to be tested includes: Performing spectrum analysis on the interference spectrum to obtain spectrum information corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer respectively; The reflection peak corresponding to each interference cavity is intercepted respectively by a window function corresponding to the frequency range of each interference cavity; Performing an inverse Fourier transform on each reflection peak to obtain wavelength information spectra of the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer respectively; The frequency spectra corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer, and the interference cavity of the fourth interferometer are respectively demodulated by using the root mean square envelope and the sine fitting; Superimposing the frequency spectra corresponding to the interference cavity of the first interferometer, the interference cavity of the second interferometer, the interference cavity of the third interferometer and the interference cavity of the fourth interferometer in pairs to obtain an interference spectrum envelope; The interference spectrum envelope is used to determine the detection parameters of the sample to be tested.

9. The method according to any one of claims 5 to 8, It is characterized in that The test parameters of the samples to be tested include: Temperature, salinity, pH value and ammonia nitrogen value of the sample to be tested.

10. The method according to claim 9, It is characterized in that Using the interference spectrum envelope, determining the detection parameters of the sample to be tested includes: According to the translation of the interference spectrum envelope relative to the experimentally calibrated interference spectrum envelope, a relationship matrix between the translation and temperature, salinity, pH value, and ammonia nitrogen value is established, and the temperature, salinity, pH value, and ammonia nitrogen value of the sample to be tested are determined by solving the relationship matrix.

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