System and method for measuring salt turbidity of seawater based on radio frequency over fiber

Abstract

The invention discloses a seawater salt turbidity measuring system and method based on radio frequency over fiber, and mainly relates to the technical field of marine environment sensors. According to the seawater salt turbidity measuring system, a tunable signal source transmits a double-channel radio frequency signal with a fixed frequency difference, and the radio frequency signal is modulated by an electro-optical modulator and a laser to generate an optical carrier radio frequency signal. An optical carrier radio frequency signal enters the seawater sensing cavity through the collimation emission light path, the transmitted optical carrier radio frequency is converted into a radio frequency signal through the coupling receiving light path and the photoelectric detector, the salt turbidity information of the seawater is reflected to the radio frequency characteristic of the radio frequency signal, the radio frequency signal is subjected to frequency mixing measurement based on a double-frequency mixing measurement method, and the salt turbidity information of the seawater is obtained. And a seawater measurement signal is obtained. The seawater salt turbidity measurement method comprises the following steps: extracting group delay data and phase noise data of seawater measurement signals, constructing a seawater turbidity calculation model and a system salt turbidity decoupling model, and calculating the seawater salt turbidity based on the group delay data and the phase noise data.

Description

Technical Field

[0001] The present invention relates to the technical field of marine environment sensors, mainly to the optical sensing technology field of seawater salinity and turbidity, and specifically refers to a seawater salinity and turbidity measurement system and method based on radio over fiber (RoF). Background Art

[0002] Existing methods for measuring seawater salinity based on conductivity and optical refractive index have good application effects in low-turbidity and stable environments. However, in complex sea areas, especially in high-turbidity sea areas, the conductivity and optical refractive index of seawater are unstable, and their measurement results are easily affected by impurities such as suspended particles and sediment in seawater, making it difficult to meet the high-precision measurement requirements of seawater salinity. At the same time, the stability of the seawater salinity measurement system cannot be guaranteed.

[0003] Therefore, there is an urgent need for a seawater salinity and turbidity measurement system and method that can effectively work in complex marine environments and has both high precision and high reliability. Summary of the Invention

[0004] The main purpose of the present invention is to provide a seawater salinity and turbidity measurement system and method based on radio over fiber (RoF) to solve the problems of insufficient measurement accuracy and stability in seawater salinity measurement in complex marine environments, especially under high turbidity and dynamic change conditions, and to improve the measurement accuracy and stability of seawater salinity.

[0005] To achieve the above object, the present invention provides a seawater salinity and turbidity measurement system based on radio over fiber (RoF), including: a tunable signal source, an electro-optic modulator, a laser, a collimated emission optical path, a seawater sensing chamber, a coupling and receiving optical path, a photodetector, and a radio frequency receiving module. The tunable signal source, the electro-optic modulator, the collimated emission optical path, the seawater sensing chamber, and the photodetector are sequentially connected to form a detection link;

[0006] The tunable signal source is used to emit a radio frequency signal with a fixed frequency difference in two channels;

[0007] The electro-optic modulator is connected to the laser and is used to modulate the radio frequency signal onto the optical signal emitted by the laser through the electro-optic effect to generate a radio over fiber (RoF) signal;

[0008] The collimated emission optical path is connected to the electro-optic modulator and the seawater sensing chamber and is used to irradiate the radio over fiber (RoF) signal into the seawater sensing chamber;

[0009] The seawater sensing chamber is used for optical measurement of seawater;

[0010] The coupling and receiving optical path is connected to the seawater sensing chamber and the photodetector and is used to combine the optical powers of the same wavelength;

[0011] The photodetector is used to convert the optically - carried RF signal into an RF signal;

[0012] The RF receiving module is used to perform mixing measurement on the RF signals emitted by the tunable signal source and the photodetector.

[0013] Further, the tunable signal source emits a first RF signal and a second RF signal with two channels, and the frequencies are f1 and f2 respectively. The frequency difference between the first RF signal and the second RF signal is fixed, and the fixed frequency difference is Δf = |f1 - f2|. The phase difference between the first RF signal and the second RF signal is

[0014] A 50:50 power splitter is provided in the tunable signal source. Through the 50:50 power splitter, the first RF signal is equally divided into a first - path first RF signal and a second - path first RF signal, and the second RF signal is equally divided into a first - path second RF signal and a second - path second RF signal. The second - path first RF signal enters the detection link to perform optical measurement on the seawater in the seawater sensing chamber, and a seawater - measured RF signal is obtained.

[0015] Further, a mixing measurement module is integrated in the RF receiving module. Through the mixing measurement module, the RF signals emitted by the tunable signal source and the photodetector are subjected to mixing measurement based on the double - mixing measurement method to obtain a seawater - measured signal, including:

[0016] The first mixing: The left - hand mixer of the mixing measurement module mixes the first - path first RF signal and the first - path second RF signal to obtain a background mixing signal, which is equivalent to a reference mixing signal; the right - hand mixer of the mixing measurement module mixes the seawater - measured RF signal and the second - path second RF signal to obtain a measurement mixing signal. The measurement mixing signal carries the depth information of the group delay and phase noise caused by different salt turbidities of seawater.

[0017] The second mixing: The mixing measurement module mixes the background mixing signal and the measurement mixing signal to obtain a seawater - measured signal. The seawater - measured signal carries the depth information of the group delay and phase noise caused only by different salt turbidities of seawater.

[0018] The present invention also provides a method for measuring seawater salt turbidity based on optically - carried RF, including:

[0019] S1. Based on the seawater salt turbidity measurement system, a seawater - measured signal is obtained, and the group delay data and phase noise data caused only by different salt turbidities of seawater are extracted from the seawater - measured signal;

[0020] S2. Perform data processing on the group delay data and phase noise data, and calculate the integral area value of the phase noise spectrum;

[0021] S3. Construct a mathematical model for calculating seawater turbidity, and calculate the seawater turbidity based on the integrated area value of the phase noise spectrum;

[0022] S4. Construct a system salt-turbidity decoupling model and calculate seawater salinity based on the integrated area value of the phase noise spectrum and group delay data.

[0023] Furthermore, the phase-frequency characteristic signal to be demodulated of the seawater measurement signal is extracted based on the double-mixing method, and the phase noise data is extracted according to the phase-frequency characteristic signal to be demodulated. The phase noise extraction formula is:

[0024]

[0025] in, Phase noise data The value at frequency offset f, P phase (f) is the phase noise data The phase noise power spectral density at the frequency offset f, P carrier is the carrier power;

[0026] Under the condition of the same bandwidth of optical radio frequency signal, the phase noise spectrum corresponding to different turbidity is extracted, the phase noise spectrum is smoothed and denoised and integrated, and the integral area value of the phase noise spectrum is calculated, which is recorded as A. pn (T), the integration range is the single sideband frequency sweep range of the phase-frequency characteristic with the frequency of the optically carried RF signal as the starting frequency.

[0027] Furthermore, a seawater turbidity calculation model was constructed based on the integral area value A of the phase noise spectrum. pn (T), reverse calculation of seawater turbidity T:

[0028] A pn (T)=∝0+∝1T+∝2T 2 +···+∝ n T n ;

[0029] Among them, ∝0,∝1,···∝ n is the fitting coefficient under the Taylor polynomial expansion, which is equivalent to the calibration parameter of the measurement system under specific conditions;

[0030] According to the fact that the phase measurement step is less than π in single frequency modulation, the relationship between the phase measurement step and the background group delay is:

[0031]

[0032] in, Indicates the phase measurement step, f stepIt represents the single-frequency modulation step size, and τ0 represents the group delay of the optical carrier radio frequency signal passing through the vacuum sensing chamber, which is equivalent to the background group delay.

[0033] Furthermore, according to the relationship between the phase measurement step size and the background group delay during single-frequency modulation, the single-frequency modulation step size f set by the system is determined. step The constraint condition is:

[0034]

[0035] Based on the single-frequency modulation step size f step , the sweep frequency f of the seawater turbidity measurement system is determined. scan According to the relationship between the system group delay and the measured phase difference, the system group delay is calculated:

[0036]

[0037] where τ g is the system group delay, which is equivalent to the total group delay variable of the seawater to be measured after optical carrier transmission. is the measured phase difference.

[0038] Furthermore, the total differential of the system group delay is taken to calculate the measurement accuracy error Δτ of the system group delay:

[0039]

[0040] where represents the change accuracy of the measured phase difference, df scan represents the change accuracy of the system sweep frequency, and Δθ represents the initial and end phase difference.

[0041] Furthermore, according to the measurement accuracy error Δτ of the system group delay and the change accuracy of the measured phase difference the sweep frequency of the system is calculated, and the number of sampling points n in the sweep interval of the seawater turbidity measurement system during the measurement of the system group delay τ g is determined:

[0042]

[0043]

[0044] The measurement accuracy of the group delay is improved by traversing cyclic scanning and the method of taking differences between frequency points, and the measurement of the system group delay of the seawater turbidity to be measured is carried out.

[0045] Furthermore, a system turbidity decoupling model is constructed. Based on the integral area value of the phase noise spectrum and the group delay data, combined with the decoupling algorithm, the influence of turbidity on the system group delay is excluded, and the component of the system group delay caused only by salinity change is retained. The salinity of seawater is calculated based on the system group delay caused only by salinity change.

[0046] y s =a·τ g +b·A pn (T)+c;

[0047] Among them, y s is the salinity of the seawater to be measured, a is the calibration coefficient of the system regarding group delay under the calibration environment, b is the calibration coefficient of the mathematical model of the system phase noise under the calibration environment, and c is the compensation coefficient of the entire system.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] This paper focuses on exploring the mechanism of the influence of suspended particles in seawater on salinity sensing, constructs a seawater salt turbidity measurement system, obtains seawater measurement signals based on a dual-mixing measurement scheme, and stores the seawater salt turbidity information in the radio frequency characteristics of the seawater measurement signals. By extracting the group delay data and phase noise data in the seawater measurement signals caused only by the difference in seawater salt turbidity, a seawater turbidity calculation model is established using the phase noise data to calculate the seawater turbidity.

[0050] The present invention constructs a system salt-turbidity decoupling model and uses group delay data and phase noise data to measure seawater salinity. While ensuring the accuracy and stability of the salinity measurement results, the turbidity component in the system group delay is eliminated, eliminating the influence of seawater turbidity on seawater salinity measurement, and significantly improving the environmental adaptability of the system and the measurement accuracy of seawater salinity. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is an overall schematic diagram of a seawater salinity turbidity measurement system provided by an embodiment of the present invention.

[0052] Figure 2 This is an overall flow chart of the seawater salinity turbidity measurement method provided by an embodiment of the present invention.

[0053] Figure 3 This is a flow chart of the system salt-turbidity decoupling model provided in an embodiment of the present invention.

[0054] Figure 4 This is a phase noise characteristic spectrum diagram corresponding to zero scattering turbidity provided by an embodiment of the present invention.

[0055] Figure 5 This is a phase noise characteristic spectrum diagram corresponding to the scattering turbidity of 2.0016 provided in an embodiment of the present invention.

[0056] Figure 6 This is a phase noise characteristic spectrum diagram corresponding to the scattering turbidity of 3.3355 provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0057] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the present invention will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without creative efforts fall within the scope of protection of the present invention.

[0058] As Figure 1 shown, a seawater salinity turbidity measurement system based on radio over fiber includes: a tunable signal source, an electro-optic modulator, a laser, a collimated emission optical path, a seawater sensing chamber, a coupling reception optical path, a photodetector, and a radio frequency reception module. The tunable signal source, the electro-optic modulator, the collimated emission optical path, the seawater sensing chamber, and the photodetector are connected in sequence to form a detection link; a seawater measurement radio frequency signal is obtained through the detection link.

[0059] The tunable signal source emits a first radio frequency signal and a second radio frequency signal with dual channels and frequencies f1 and f2 respectively. The frequency difference between the first radio frequency signal and the second radio frequency signal is fixed, and the fixed frequency difference is Δf = |f1 - f2|. The phase difference between the first radio frequency signal and the second radio frequency signal is equivalent to the initial phase difference between the first radio frequency signal and the second radio frequency signal. The initial phase of the tunable signal source is artificially controllable when configuring the output signal. Therefore it is an artificially controllable factor. Since the frequency difference between the first radio frequency signal and the second radio frequency signal is fixed, the initial phase difference between the first radio frequency signal and the second radio frequency signal is fixed.

[0060] The phase delay difference between the first radio frequency signal and the second radio frequency signal is τ, which is the value to be measured. According to the relationship between the phase delay difference, the phase difference, and the frequency difference calculate the phase delay difference τ.

[0061] The electro-optic modulator is connected to the laser. Under the action of an external electric field, the refractive index of the material is changed, thereby modulating the amplitude, phase, and polarization state of the light wave. Through the electro-optic effect, the radio frequency signal emitted by the tunable signal source is loaded onto the light wave emitted by the laser to generate a radio over fiber signal, and then the signal is transmitted through the optical fiber.

[0062] The collimated emission optical path is connected to the electro-optic modulator and the seawater sensing chamber, and is used to irradiate the radio over fiber signal into the seawater sensing chamber; the collimated emission optical path focuses, adjusts, and aligns the radio over fiber signal emitted by the electro-optic modulator through a collimator. By changing the shape and size of the light beam, the scattered light beam is aggregated into parallel light rays to ensure the correct alignment of the optical paths between the components of the optical system and meet specific experimental or application requirements.

[0063] The seawater sensing chamber is used for optical measurement of seawater. It can be adjusted according to the needs of experiments or applications to perform a specific measurement of certain information such as the salinity, turbidity, and temperature of the seawater to be measured, and complete the optical measurement of specific parameters of the seawater. After the optical carrier radio frequency signal enters the seawater sensing chamber, the propagation of the signal is affected by seawater with different salinity and turbidity, resulting in changes in the group delay data and phase noise data of the signal due to the influence of different salinity and turbidity.

[0064] There is a linear relationship between the salinity change of seawater salinity and the change of light refractive index. For every 1% change in salinity, the refractive index changes by 2×10 -4 . The change in light refractive index directly affects the transmission speed of the optical carrier radio frequency signal in seawater, thereby changing the group delay. At the same time, suspended particles in seawater increase light scattering and absorption, introducing phase noise. As the seawater turbidity increases, the transmitted light energy attenuates and the signal-to-noise ratio decreases.

[0065] The coupled receiving optical path connects the seawater sensing chamber and the photodetector, and is used for combining the optical powers of the same wavelength.

[0066] The photodetector is used to convert the optical carrier radio frequency signal into a radio frequency signal; the photodetector changes the conductivity of the irradiated material through radiation and converts the optical signal into an electrical signal.

[0067] The radio frequency receiving module is used for mixing measurement of the radio frequency signals emitted by the tunable signal source and the photodetector.

[0068] A 50:50 power splitter is set in the tunable signal source. Through the 50:50 power splitter, the first radio frequency signal is equally divided into the first radio frequency first path signal and the first radio frequency second path signal, and the second radio frequency signal is equally divided into the second radio frequency first path signal and the second radio frequency second path signal. The first radio frequency second path signal performs optical measurement on the seawater in the seawater sensing chamber through the detection link to obtain the seawater measurement radio frequency signal. The first radio frequency second path signal is converted into an optical carrier radio frequency signal through an electro-optic modulator and enters the seawater sensing chamber through the collimated emission optical path. In the seawater sensing chamber, the propagation of the optical carrier radio frequency signal is affected by seawater with different salinity and turbidity, and the phase noise data and group delay data of the signal also change due to the influence of seawater salinity and turbidity. The signal passes through the seawater sensing chamber to obtain the optical carrier transmitted signal. The optical carrier transmitted signal enters the photodetector through the coupled receiving optical path. The photodetector converts the optical carrier transmitted signal from the optical carrier radio frequency signal into a radio frequency signal and reflects the salinity and turbidity information of the seawater onto the radio frequency characteristics of the radio frequency signal to obtain the seawater measurement radio frequency signal.

[0069] Assume that the input signals of the seawater salinity and turbidity measurement system are: the first radio frequency signal and the second radio frequency signal where A and B are the initial amplitudes of the first radio frequency signal and the second radio frequency signal respectively. and the initial phase, t represents time, and the initial phases of the first RF signal and the second RF signal differ by

[0070] The first RF signal is evenly divided into a first-path first RF signal and a second-path first RF signal by a 50:50 power divider. Among them, the second-path first RF signal enters the detection link as an input signal to perform optical measurement on seawater, and a seawater measurement RF signal is obtained, denoted as

[0071] A mixing measurement module is integrated in the RF receiving module. A left mixer and a right mixer are provided in the mixing measurement module. Based on the double mixing measurement method, the mixing measurement module performs mixing measurement on the RF signals emitted by the tunable signal source and the photodetector to obtain a seawater measurement signal, including:

[0072] The first mixing:

[0073] The background mixing signal S IF11 :

[0074]

[0075] The sum frequency and difference frequency of the input signal are output by the mixer:

[0076]

[0077] After filtering, the mixer filters out the high-frequency components and only retains the low-frequency terms. Therefore, the background mixing signal S IF11 is:

[0078]

[0079] where Δf1 is the frequency of the background mixing signal, and Δf1 = |f1 - f2|.

[0080] The measurement mixing signal S IF12 :

[0081] Right mixer (S1` and S2)

[0082]

[0083] After filtering, only the low-frequency terms are retained. Therefore, the measurement mixing signal S IF12 is:

[0084]

[0085] The first mixing: The left mixer of the mixing measurement module mixes the first RF signal of the first path and the first RF signal of the second path to obtain a background mixing signal, which is equivalent to the reference mixing signal; the frequency of the background mixing signal is Δf1 = Δf = f1 - f2; the right mixer of the mixing measurement module mixes the seawater measurement RF signal and the second RF signal of the second path to obtain a measurement mixing signal, and the frequency of the measurement mixing signal is Δf2 = Δf = f1 - f2. The measurement mixing signal carries the depth information of the group delay and phase noise caused by different salt turbidities.

[0086] The second mixing:

[0087] Mix the background mixing signal S IF11 and the measurement mixing signal S IF12 again to obtain the seawater measurement signal S IF2 ,

[0088] S IF2 = S IF11 ·S IF12 ;

[0089]

[0090] Use the trigonometric identity to expand:

[0091]

[0092] After filtering, only the low-frequency terms are retained:

[0093]

[0094] Therefore, the phase of the final signal is cos(2πf1τ g ). Measure the group delay of the link according to the phase of the final signal, which is equivalent to the system group delay τ g :

[0095]

[0096] where θ is the measured final output phase, that is, the phase offset.

[0097] The second mixing: The mixing measurement module mixes the background mixing signal with a frequency of Δf1 and the measurement mixing signal with a frequency of Δf2 to obtain a seawater measurement signal. The seawater measurement signal carries the depth information of the group delay and phase noise caused only by different seawater salt turbidities. Based on the double mixing method, the influence of the background noise on the signal is excluded, and the group delay data and phase noise data caused only by different seawater salt turbidities are extracted.

[0098] According to the actual situation, the true solution of the group delay of the measurement link is obtained by using different de - ambiguity schemes. The applicable ranges, advantages and disadvantages of different de - ambiguity schemes are shown in Table 1 as follows:

[0099] Table 1 Applicable ranges, advantages and disadvantages of different de - ambiguity schemes

[0100] Method Scope of application Advantages Disadvantages Multi-frequency method Applicable to large group delays Applicable to wideband systems Requires multiple measurements Coherent scanning method Applicable to continuous measurements Applicable to high-precision systems Requires high-frequency resolution Reference path difference Applicable to long-distance measurements Avoids large-scale ambiguity Requires an additional reference path Higher-order harmonic method Applicable to low-frequency wide systems Simple calculation Requires harmonic processing High-time resolution sampling Applicable to short-delay measurements Improves resolution Requires a high-speed sampler

[0101] Such as Figure 2 shown in a method for measuring seawater salinity turbidity based on radio - over - fiber, which includes:

[0102] S1. Obtain the seawater measurement signal based on the seawater salinity turbidity measurement system, and extract the group delay data and phase noise data in the seawater measurement signal that are only caused by different seawater salinity turbidities; the phase noise data includes the phase noise spectrum data under different turbidities.

[0103] S2. Perform data pre - processing on the group delay data and phase noise data, and calculate the integral area value of the phase noise spectrum; the data pre - processing includes noise reduction and smoothing processing and integral processing.

[0104] S3. Construct a mathematical model for calculating seawater turbidity, and calculate the seawater turbidity based on the integral area value of the phase noise spectrum according to the mathematical relationship between the phase noise integral area and seawater turbidity;

[0105] S4. Construct a decoupling model for the system salinity turbidity, and calculate the seawater salinity based on the integral area value of the phase noise spectrum and the group delay data.

[0106] Extract the phase - frequency characteristic signal to be demodulated based on the double - mixing method, and extract the phase noise data according to the phase - frequency characteristic signal to be demodulated The extraction formula for phase noise is:

[0107]

[0108] where is the value of the phase noise data at the frequency offset f, P phase (f) is the phase noise power spectral density of the phase noise data at the frequency offset f, and P carrier is the carrier power;

[0109] Under the condition of an optical - carrier - radio - frequency signal with the same bandwidth, extract the phase noise spectra corresponding to different turbidities, perform smoothing and noise reduction processing and integral processing on the phase noise spectra, and calculate the integral area value of the phase noise spectra, denoted as A pn (T), and the integration range starts from the optical - carrier - radio - frequency signal frequency and is the single - sideband sweep range of the phase - frequency characteristic.

[0110] Construct a seawater turbidity calculation model based on the integral area value A of the phase noise spectrum pn (T) to inversely calculate the seawater turbidity T:

[0111] A pn (T) = ∝0 + ∝1T + ∝2T 2 + ··· + ∝ n T n ;

[0112] where ∝0, ∝1, ··· ∝ n are the fitting coefficients under the Taylor polynomial expansion, equivalent to the calibration parameters of the measurement system in a specific environment.

[0113] As Figure 3 shown in the extraction process of the system group delay, first, after the system is calculated and measured under vacuum sensing conditions, the background group delay τ0 is obtained, that is, the group delay of the optical carrier radio frequency signal passing through the vacuum sensing chamber, which is used as the normalized reference quantity of the system. Then, zero-salinity pure water is added to the seawater sensing chamber, and the light transmission measurement of zero-salinity seawater is carried out based on the seawater turbidity measurement system to extract the group delay of zero-salinity seawater. The system is normalized and calibrated based on the background group delay and the group delay of zero-salinity seawater. Using the calibrated seawater turbidity measurement system, the phase-frequency characteristic signal to be demodulated is extracted based on the double mixing frequency method to obtain the phase to be measured According to the condition that the phase measurement step size is less than n during single-frequency modulation, the relationship between the phase measurement step size and the background group delay is obtained as:

[0114]

[0115] where represents the phase measurement step size, f step represents the single-frequency modulation step size, and τ0 represents the group delay of the optical carrier radio frequency signal passing through the vacuum sensing chamber, equivalent to the background group delay.

[0116] According to the relationship between the phase measurement step size and the background group delay during single-frequency modulation, the constraint condition for the single-frequency modulation step size f step set by the system is:

[0117]

[0118] Based on the single-frequency modulation step size f step , determine the sweep frequency f scan of the seawater turbidity measurement system. According to the relationship between the group delay to be measured and the measured phase difference, calculate the system group delay:

[0119]

[0120] where τ is the system group delay, To measure the phase difference, f scan is the swept frequency of the system.

[0121] Take the total differential of the system group delay τ g to calculate the measurement accuracy error Δτ of the system group delay:

[0122]

[0123] where represents the measurement phase difference change accuracy, and Δθ represents the initial and end phase difference.

[0124] According to the measurement accuracy error Δτ of the system group delay and the measurement phase difference change accuracy calculate the swept frequency f of the system scan to determine the number of sampling points n in the swept frequency range for the measurement of the seawater salinity turbidity measurement system at the system group delay τ g :

[0125]

[0126]

[0127] Improve the group delay measurement accuracy by traversing cyclic scanning and the difference method between frequency points, and carry out the measurement of the system group delay of the seawater salinity turbidity to be measured.

[0128] Construct a system salinity decoupling model. Based on the integral area value of the phase noise spectrum and the group delay data, combine the decoupling algorithm to eliminate the influence of turbidity on the system group delay, retain the component of the system group delay caused only by salinity change, and calculate the salinity of seawater:

[0129] y s = a·τ g + b·A pn (T)+ c;

[0130] where y s is the salinity of the seawater to be measured, a is the calibration coefficient of the system regarding the group delay under the calibration environment, b is the calibration coefficient of the system phase noise mathematical model under the calibration environment, and c is the compensation coefficient of the entire system.

[0131] Figures 4 to 6 is the phase-frequency characteristic curve diagram extracted by the photodetector after the optical carrier radio frequency signal passes through the measurement link with the same optical carrier radio frequency signal at 120 MHz and with the scattering turbidity as a single variable, Figure 4 is 0 scattering turbidity (NTU), that is, the phase noise characteristic spectrum diagram corresponding to clear seawater, Figure 5 is the phase noise characteristic spectrum diagram corresponding to 2.0016 scattering turbidity, Figure 6The spectral diagram of the phase noise characteristics corresponding to the 3.3355 scattering turbidity provided by the embodiment of the present invention, through comparative analysis Figure 4 , Figure 5 and Figure 6 , according to the phase-frequency characteristic curve, the single-sideband phase noise spectrum is extracted, and it can be obtained that the integral area in the phase noise characteristic spectrum is positively correlated with the actual turbidity, and the phase noise at the same bandwidth frequency increases with the increase of the turbidity of seawater. Therefore, the higher the turbidity of seawater, the greater the impact on the measurement of seawater salinity.

[0132] The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can be made, and these improvements and retouches should also be regarded as the protection scope of the present invention.

Claims

1. A seawater salinity and turbidity measurement system based on radio over fiber, characterized in that Including: A tunable signal source, an electro-optic modulator, a laser, a collimated emission optical path, a seawater sensing chamber, a coupled reception optical path, a photodetector, and a radio frequency reception module. The tunable signal source, the electro-optic modulator, the collimated emission optical path, the seawater sensing chamber, and the photodetector are connected in sequence to form a detection link; The tunable signal source is used to emit a radio frequency signal with a fixed frequency difference in two channels; The electro-optic modulator is connected to the laser and is used to modulate the radio frequency signal onto the optical signal emitted by the laser through the electro-optic effect to generate an optical carrier radio frequency signal; The collimated emission optical path is connected to the electro-optic modulator and the seawater sensing chamber and is used to irradiate the optical carrier radio frequency signal into the seawater sensing chamber; The seawater sensing chamber is used for optical measurement of seawater; The coupled reception optical path is connected to the seawater sensing chamber and the photodetector and is used for combining the optical powers of the same wavelength; The photodetector is used to convert the optical carrier radio frequency signal into a radio frequency signal; The radio frequency reception module is used for mixing measurement of the radio frequency signals emitted by the tunable signal source and the photodetector.

2. The seawater salinity and turbidity measurement system based on radio over fiber according to claim 1, characterized in that The tunable signal source emits a first RF signal and a second RF signal with two channels, having frequencies f1 and f2 respectively. The frequency difference between the first RF signal and the second RF signal is fixed, and the fixed frequency difference is Δf = |f1 - f2|. The phase difference between the first RF signal and the second RF signal is A 50:50 power splitter is provided in the tunable signal source. The first radio frequency signal is equally divided into a first radio frequency first path signal and a first radio frequency second path signal through the 50:50 power splitter, and the second radio frequency signal is equally divided into a second radio frequency first path signal and a second radio frequency second path signal. The first radio frequency second path signal enters the detection link to perform optical measurement on the seawater in the seawater sensing chamber to obtain a seawater measurement radio frequency signal.

3. The seawater salinity and turbidity measurement system based on radio over fiber according to claim 2, wherein A mixing measurement module is integrated in the radio frequency reception module. The radio frequency signals emitted by the tunable signal source and the photodetector are subjected to mixing measurement based on the double mixing measurement method through the mixing measurement module to obtain a seawater measurement signal, including: The first mixing: The first radio frequency first path signal and the second radio frequency first path signal are mixed through the left mixer of the mixing measurement module to obtain a background mixing signal, which is equivalent to a reference mixing signal; the seawater measurement radio frequency signal and the second radio frequency second path signal are mixed through the right mixer of the mixing measurement module to obtain a measurement mixing signal. The measurement mixing signal carries the depth information of the group delay and phase noise caused by different salt turbidities of seawater; The second mixing: The background mixing signal and the measurement mixing signal are mixed through the mixing measurement module to obtain a seawater measurement signal. The seawater measurement signal carries the depth information of the group delay and phase noise caused only by different salt turbidities of seawater.

4. A method for measuring seawater salinity and turbidity based on radio over fiber, which uses an optical measurement system for seawater salinity and turbidity as described in any one of claims 1-3, is characterized in that Including: S1. Based on the seawater salt turbidity measurement system, obtain a seawater measurement signal, and extract the group delay data and phase noise data caused only by different salt turbidities of seawater in the seawater measurement signal; S2. Perform data processing on the group delay data and the phase noise data, and calculate the integral area value of the phase noise spectrum; S3. Construct a seawater turbidity calculation mathematical model, and calculate the seawater turbidity based on the integral area value of the phase noise spectrum; S4. Construct a system salt turbidity decoupling model, and calculate the seawater salinity based on the integral area value of the phase noise spectrum and the group delay data.

5. The method for measuring seawater salinity turbidity based on radio over fiber according to claim 4, wherein Extract the phase-frequency characteristic signal to be demodulated of the seawater measurement signal based on the double mixing method, and extract the phase noise data according to the phase-frequency characteristic signal to be demodulated The extraction formula of phase noise is as follows: Among them, is the phase noise data at the frequency offset f, P phase (f) is the phase noise data at the frequency offset f, the phase noise power spectral density, P carrier is the carrier power; Under the condition of optical carrier radio frequency signals with the same bandwidth, extract the phase noise spectra corresponding to different turbidities, perform smoothing noise reduction processing and integration processing on the phase noise spectra, calculate the integrated area value of the phase noise spectra, denoted as A pn (T), and the integration range is the start frequency of the optical carrier radio frequency signal and the single-sideband sweep range of the phase-frequency characteristic.

6. The method for measuring seawater salinity turbidity based on radio over fiber according to claim 5, characterized in that, Construct a seawater turbidity calculation model, based on the integrated area value A of the phase noise spectrum pn (T), and inversely calculate the seawater turbidity T: A pn (T) = ∝0 + ∝1T + ∝2T 2 + ··· + ∝ n T n ; where ∝0, ∝1, ···, ∝ n are the fitting coefficients under the Taylor polynomial expansion, equivalent to the calibration parameters of the measurement system in a specific environment; According to when performing single-frequency modulation, the phase measurement step size is less than π, the relationship between the phase measurement step size and the background group delay is obtained as: Among them, represents the phase measurement step size, and f step represents the single-frequency modulation step size, and τ0 represents the group delay of the optical carrier radio frequency signal passing through the vacuum sensing chamber, which is equivalent to the background group delay.

7. A method for measuring seawater salinity turbidity based on radio over fiber according to claim 6, characterized in that, Determine the single-frequency modulation step f set by the system according to the relationship between the phase measurement step during single-frequency modulation and the background group delay step The constraint condition is: Based on the single frequency modulation step f step , determine the sweep frequency f of the seawater turbidity measurement system scan , calculate the system group delay according to the relationship between the system group delay and the measured phase difference: Among them, τ g is the system group delay, which is equivalent to the total group delay variable after the seawater to be measured is transmitted through the optical carrier, is the measured phase difference.

8. A method for measuring seawater salinity turbidity based on radio over fiber according to claim 7, characterized in that, Perform a total differential on the system group delay and calculate the measurement accuracy error Δτ of the system group delay: Among them, represents the measurement precision of the phase difference change, df scan represents the precision of the system sweep frequency change, and Δθ represents the initial end phase difference.

9. A method for measuring seawater salinity turbidity based on radio over fiber according to claim 8, wherein According to the system group delay measurement accuracy error Δτ and the measurement phase difference change accuracy Calculate the swept frequency of the system, and determine the number of sampling points n in the swept frequency range in the measurement of the system group delay of the seawater turbidity measurement system: Improve the measurement accuracy of the group delay by traversing cyclic scanning and the difference method between frequency points, and carry out the measurement of the system group delay of the seawater with the turbidity to be measured.

10. A method for measuring seawater salinity turbidity based on radio over fiber according to claim 9, characterized in that, Construct a decoupling model for the system turbidity. Based on the integral area value of the phase noise spectrum and the group delay data, combine the decoupling algorithm to eliminate the influence of turbidity on the system group delay, retain the component of the system group delay caused only by the change in salinity, and calculate the salinity of seawater based on the system group delay caused only by the change in salinity: y s = a·τ g + b·A pn (T) + c; Among them, y s is the salinity of seawater to be measured, a is the calibration coefficient of the system regarding group delay under the calibration environment, b is the calibration coefficient of the mathematical model of the system phase noise under the calibration environment, and c is the compensation coefficient of the entire system.

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