An on-line device for detecting absolute density of seawater and a method thereof

Abstract

The application belongs to the technical field of density detection, and particularly relates to a seawater absolute density online detection device and method. A waveform generator is used to control a tunable semiconductor laser diode, so that the wavelength of the output light signal is a periodic sawtooth signal. The sawtooth light signal is transmitted to a refractive index detection probe filled with seawater samples in a resonant cavity structure with a comb spectrum. In one sawtooth cycle, the reflected light intensity of the optical resonant cavity has multiple minimum values. The time interval of any two adjacent light intensity minimum value points is equal. The wavelength change range corresponding to the time interval is the free spectral range of the optical resonant cavity. The free spectral range of the optical resonant cavity is obtained by calculating the wavelength change range in the time interval. The seawater refractive index is calculated based on the relationship between the free spectral range and the refractive index of the medium in the optical resonant cavity. The absolute density value of the seawater is determined according to the numerical relationship between the seawater absolute density and the refractive index.

Description

Technical Field

[0001] This invention belongs to the field of density detection technology, specifically relating to an online detection device and method for the absolute density of seawater, which can perform online detection of the absolute density of seawater under complex environments. Background Technology

[0002] Seawater absolute density is the mass of seawater per unit volume. It is a fundamental parameter reflecting the physical and chemical processes of seawater, and is influenced by temperature, absolute salinity, and pressure. Decreasing temperature, increasing absolute salinity, and increasing pressure all lead to an increase in seawater absolute density. Online detection of seawater absolute density is of great significance to marine science and marine engineering, and is considered one of the most promising approaches to studying seawater absolute salinity. In recent years, technologies for detecting seawater absolute density have received increasing attention.

[0003] Chinese Patent 201611260339.0 discloses a vibrating tube type online density meter, comprising: a main pipe, a vibrating tube, a sampling device, and a driving device. The two ends of the main pipe are detachably connected to a delivery pipeline, and the main pipe and the delivery pipeline are coaxial. The two ends of the vibrating tube are interconnected with the main pipe. It also includes a sampler located at the end of the vibrating tube, which is fixedly connected to the inner wall of the main pipe to guide the medium in the main pipe into the vibrating tube. The sampling device is disposed on the vibrating tube and is used to detect the vibration frequency of the vibrating tube. The driving device is used to receive the vibration frequency collected by the sampling device and maintain the vibrating tube in a vibration state at its natural frequency according to the vibration frequency. Using a U-shaped vibrating tube as the measuring tube, the density value of the fluid is calculated from the natural frequency of the vibrating tube detected by the sampling device. This natural frequency depends on the mass of the vibration system. The mass of a certain volume of fluid flowing through the measuring tube is determined by its density. Its high-precision measurement target relies on a complex delivery pipe design and precise machining process, making the implementation process complex and unsuitable for online seawater monitoring.

[0004] Chinese Patent 202210350354.3 discloses a seawater density measuring device based on laser ranging technology, comprising a seawater container, a high-precision density meter, a liquid level correction device, a laser measurement module, and a display control system. The high-precision density meter includes a connected weight and a first float, and a first connecting rod located above and connected to the first float, with a first reflector fixedly connected to the top of the first connecting rod. The liquid level correction device includes a second float, and a second connecting rod located above and connected to the second float, with a second reflector fixedly connected to the top of the second connecting rod. The lower parts of the high-precision density meter and the liquid level correction device are located inside the seawater container, while the upper parts extend into... Inside the laser measurement module, there is a laser rangefinder and a laser sensor located at the top. The display and control system includes a power supply for the laser rangefinder and a data processing system, used to receive and process the data obtained by the laser measurement module. Based on laser ranging technology, the distances between the reflector of the densitometer and the reflector of the liquid level correction device and the laser rangefinder are measured respectively. The two sets of values ​​at the same moment are subtracted and the difference is compared with the calibration value to obtain the instantaneous density value. The density value of the seawater in the container is obtained by averaging multiple instantaneous density values. When the liquid surface of the densitometer is tilted due to factors such as vibration, inaccurate ranging results will be obtained, which is not suitable for online measurement in complex marine environments.

[0005] In summary, existing seawater absolute density detection technologies suffer from several technical shortcomings, particularly unsuitability for long-term online monitoring at sea. Therefore, this study aims to develop and design an online seawater absolute density detection device and method with a measurement range that can cover the variation range of seawater absolute density, low power consumption, and small size. This method employs periodic sawtooth wave signal modulation of a tunable semiconductor laser diode to effectively avoid phase fading, enabling online detection of seawater absolute density under complex environments. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and to develop and design an online detection device and method for the absolute density of seawater. Based on the refractive index of seawater, the relationship between the refractive index and the absolute density of seawater is established according to the Lorentz relation, and the absolute density value of seawater is determined.

[0007] To achieve the above objectives, the main structure of the online absolute density detection device for seawater according to the present invention includes a tunable semiconductor laser diode, a fiber optic self-focusing lens, and a photodetector, which are respectively connected to an optical fiber circulator, and a tunable semiconductor laser diode connected to the photodetector via a data acquisition card, a signal processing unit, a control unit, a waveform generator, and a current driver; the fiber optic self-focusing lens is connected to a seawater sample cavity, and a first plane mirror and a second plane mirror are provided at both ends of the seawater sample cavity.

[0008] The invention relates to an optical fiber circulator, a tunable semiconductor laser diode, a photodetector, a data acquisition card, a signal processing unit, a control unit, a waveform generator, and a current driver, which together constitute a floating platform and require power to operate. An optical fiber self-focusing lens, a seawater sample cavity, a first plane mirror, and a second plane mirror 7 together constitute a seawater refractive index detection probe with a resonant cavity structure, which is placed in seawater and does not require power. The floating platform is connected to the seawater refractive index detection probe via an optical fiber. The measuring end is not energized, and the floating platform outputs a periodically changing optical signal. The wavelength of the optical signal changes in a sawtooth pattern within each cycle.

[0009] The online method for detecting the absolute density of seawater involved in this invention achieves the purpose of measuring the absolute density of seawater by measuring the refractive index of seawater. The specific process is as follows:

[0010] (1) Sawtooth wave generation: First, the control unit drives the waveform generator to output a periodic sawtooth wave voltage signal. , The calculation formula is:

[0011] ,in, for The initial value within each sawtooth wave signal period, i.e. The voltage value at that moment. , , The maximum voltage value within each sawtooth wave signal period. , The amplitude of the sawtooth wave signal. Let t be the period of the sawtooth wave voltage signal, and t be the time variable. The number of cycles in the sweep frequency signal;

[0012] (2) Wavelength tuning: The sawtooth wave voltage signal from step (1) is tuned. Input current driver, periodic current variation, tunable semiconductor laser diode output wavelength that varies periodically with a sawtooth voltage signal. , The calculation formula is:

[0013] ,in, for The initial value within each sawtooth wave signal period, i.e. The wavelength value corresponding to the time, and This is the minimum wavelength during the scanning process. , The maximum value within each period of the sawtooth wave signal. The sawtooth wave voltage signal changed Wavelength changes caused by time;

[0014] (3) Optical resonant cavity configuration: The optical signal emitted by the tunable semiconductor laser diode is input at port ① of the fiber optic circulator and output at port ②. After passing through the fiber self-focusing lens, it sequentially illuminates the optical resonant cavity composed of the first and second plane mirrors. The uniform spacing of the spectrum of the optical resonant cavity is called the free spectral range (FSR). The formula for calculating the FSR is: ,in, Vacuum wavelength, The refractive index of the medium filling the optical resonant cavity. Let be the cavity length of the optical resonant cavity, i.e., the distance between the first and second plane mirrors. Since the mode-hopping range of a tunable semiconductor laser diode is much smaller than the output wavelength, then... Therefore, the vacuum wavelength is considered as the average wavelength, i.e. The formula for calculating the free spectral range (FSR) in the frequency domain is: Where c is the speed of light in vacuum, for an optical resonator with a constant cavity length, the free spectral range (FSR) is only related to the refractive index of the medium inside the optical resonator. Within each cycle of the sawtooth wave voltage signal, the output wavelength of the tunable semiconductor laser diode varies with time in a sawtooth wave pattern. The uniform frequency interval (FSR) of the optical resonator's spectrum in the spectrum is manifested as a uniform time interval in the time domain. ,Right now ,in, The electrically tunable slope of a tunable semiconductor laser diode. ;

[0015] (4) Signal processing: After a portion of the light entering the optical resonant cavity is reflected, it returns to port ② of the fiber optic circulator and is output from port ③ of the fiber optic circulator. After being received by the photodetector, the analog signal is converted to a digital signal on the data acquisition card. The converted digital signal is then transmitted to the signal processing unit for processing: First, normalization is performed. Then, all minimum values ​​of the reflected signal intensity within a frequency sweep period are found, and the times when the minimum values ​​occur within that period are recorded. The time interval is obtained by subtracting the times of two adjacent minimum values. Finally, use Calculate the free spectral range of the optical resonator at this time. ,pass Inverse calculation of the refractive index of the medium inside the optical resonant cavity: Since the refractive index of seawater changes relatively slowly, the change in the refractive index of seawater in each measurement period is ignored.

[0016] (5) Cavity calibration: Before measuring the absolute density of seawater, the cavity length of the optical resonant cavity is first calibrated. When the optical resonant cavity is not filled with other media, it is assumed that the refractive index of the air inside the optical resonant cavity is 100%. Under normal circumstances At this point, a sawtooth wave voltage signal is used to modulate the current driver, and the time interval is calculated. To obtain the free spectral range of the optical resonator at this time. ,according to The cavity length of the optical resonant cavity is obtained. ;

[0017] (6) Establishing the density relationship: Based on the Lorentz relation, establish the relationship between the absolute density of seawater and its optical refractive index: ,in, These are constants related to the properties of seawater. , Let Avogadro's constant be 1. The average molecular polarizability, molar mass;

[0018] (7) Calculate the absolute density: Substitute the cavity length d from step (5) into the formula for calculating the refractive index of the medium inside the optical resonant cavity from step (4) to obtain the formula for calculating the refractive index of seawater: The absolute density of seawater is calculated based on the Lorentz relation as follows: .

[0019] The wavelength of the output optical signal of the tunable semiconductor laser diode involved in this invention varies linearly with the input voltage of the current driver. As the input voltage increases, the wavelength of the optical signal increases accordingly. The change in optical signal wavelength caused by a unit change in input voltage is called the electrical modulation slope of the tunable semiconductor laser diode. , The wavelength of the optical signal is measured using a wavelength meter.

[0020] The formula for calculating the reflected light intensity I(t) of the optical resonator involved in this invention is as follows: ,in, Let the reflectivity of the first plane mirror and the second plane mirror be denoted as . For phase difference, , Let be the angle between the laser beam and the normal to the first plane mirror. This indicates that the laser beam is incident perpendicularly to the first plane mirror. When incident perpendicularly... When the phase difference hour, It is a non-negative integer, that is... , When the intensity of the reflected light reaches a minimum, the wavelength corresponding to this minimum point is called the resonant wavelength, and the two adjacent resonant wavelengths are called the two resonant wavelengths. and The difference is the free spectral range of the optical resonator. , If we consider two adjacent resonant wavelengths to be approximately equal, that is... ,but The expression in the frequency domain can be further written as It can be seen that when an optical resonant cavity with a constant cavity length is filled with a certain medium, if its free spectral range is known, its optical refractive index can be uniquely determined.

[0021] Compared with existing technologies, this invention uses a waveform generator to control a tunable semiconductor laser diode, making its output optical signal a periodic sawtooth wave signal. This sawtooth wave optical signal is transmitted to a refractive index detection probe filled with a seawater sample inside a resonant cavity structure with a comb-like spectrum. Within one sawtooth wave period, the reflected light intensity of the optical resonant cavity has multiple minimum values, and the time interval between any two adjacent minimum light intensity points is equal. The wavelength variation range corresponding to this time interval is the free spectral range of the optical resonant cavity. By calculating the wavelength variation range within this time interval, the free spectral range of the optical resonant cavity is obtained. Based on the relationship between the free spectral range and the refractive index of the medium inside the optical resonant cavity, the refractive index of the seawater is calculated. Then, based on the numerical relationship between the absolute density of seawater and its refractive index established by the Lorentz relation, the absolute density value of the seawater is determined, thereby realizing online detection of the absolute density of seawater. Its measuring end is not energized, with a simple structure, low power consumption, and small size. It has the characteristics of leakage prevention, corrosion prevention, simple installation, and convenient maintenance, making it suitable for online detection of the absolute density of seawater in complex environments. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the main structure of the online seawater absolute density detection device involved in this invention.

[0023] Figure 2 This is a schematic diagram of the output response of the current driver of the present invention under periodic sawtooth wave signal excitation.

[0024] Figure 3 This is a schematic diagram illustrating the periodic change in the output wavelength of the tunable laser diode when the drive current output by the current driver of the present invention changes periodically.

[0025] Figure 4 This is a schematic diagram illustrating the variation of the intensity of the reflected light signal from the normalized optical resonator as a function of wavelength, as per the present invention. Detailed Implementation

[0026] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0027] Example 1:

[0028] The main structure of the online absolute density detection device for seawater in this embodiment includes an optical fiber circulator 1, a tunable semiconductor laser diode 2, an optical fiber self-focusing lens 3, a photodetector 4, a seawater sample chamber 5, a first plane mirror 6, a second plane mirror 7, a data acquisition card 8, a signal processing unit 9, a control unit 10, a waveform generator 11, and a current driver 12. The optical fiber circulator 1 is connected to the tunable semiconductor laser diode 2, the optical fiber self-focusing lens 3, and the photodetector 4. The optical fiber self-focusing lens 3 is connected to the seawater sample chamber 5. The first plane mirror 6 and the second plane mirror 7 are arranged at the left and right ends of the seawater sample chamber 5. The photodetector 4 is connected to the first plane mirror 6 and the second plane mirror 7 in sequence. The data acquisition card 8, signal processing unit 9, control unit 10, waveform generator 11, and current driver 12 are connected to the tunable semiconductor laser diode 2 to form a loop. The fiber optic circulator 1 has a ① port, a ② port, and a ③ port. The tunable semiconductor laser diode 2 has a fiber optic interface and an electrical interface. The electrical interface of the tunable semiconductor laser diode 2 is connected to the current driver 12, and the fiber optic interface is connected to the ① port of the fiber optic circulator 1. The ② and ③ ports of the fiber optic circulator 1 are respectively connected to the fiber optic self-focusing lens 3 and the photodetector 4. The input and output terminals of the data acquisition card 8 are respectively connected to the photodetector 4 and the signal processing unit 9.

[0029] The tunable semiconductor laser diode 2 involved in this embodiment has a center wavelength of 633nm and an extremely narrow linewidth of 1MHz. Its output wavelength has a linear relationship with the control current. The first plane mirror 6 and the second plane mirror 7 are both made of ultra-low expansion glass with a low coefficient of thermal expansion, so that the cavity length of the optical resonant cavity does not change with temperature and is regarded as a constant during the measurement process. The first side 61 of the first plane mirror 6 is coated with a high transmittance film to allow more light to enter the optical resonant cavity. The second side 62 of the first plane mirror 6 and the first side 71 of the second plane mirror 7 are coated with a high reflectance film, and the second side 72 of the second plane mirror 7 is coated with a high transmittance film so that the light beam entering the optical resonant cavity forms multi-beam interference within it. When the temperature changes, the cavity length of the optical resonant cavity will not change, and temperature compensation is not required when measuring the optical path difference.

[0030] In this embodiment, a periodic sawtooth wave signal is used to modulate the tunable semiconductor laser diode 2, which can effectively avoid phase fading. Based on the refractive index detection probe being a resonant cavity structure, the optical resonant cavity is composed of a first plane mirror 6, a second plane mirror 7, and the seawater sample to be measured. The laser is reflected multiple times between the first plane mirror 6 and the second plane mirror 7, generating a multi-beam interference phenomenon. Finally, the reflected light is returned by the refractive index detection probe.

[0031] According to the multi-beam interference theory, the formula for calculating the intensity I(t) of the optical signal returned by the refractive index detection probe is: (1), where, The reflectivity of the first plane mirror 6 and the second plane mirror 7 in this embodiment is... , For phase difference, (2), where, Let be the refractive index of the medium filling the optical resonant cavity. When the filling medium is seawater, The range of variation is 1.33~1.35. The cavity length of the optical resonant cavity is given in this embodiment. , The output wavelength of the tunable semiconductor laser diode 2, The angle between the laser beam and the normal to the first plane mirror 6 is... This indicates that the laser beam is incident perpendicularly to the first plane mirror 6. When incident perpendicularly, (3) If the tunable semiconductor laser diode 2 outputs a light signal with a fixed wavelength, i.e. , Since is a constant, according to formulas (1) and (3), the intensity of the interference light I(t) returned by the refractive index detection probe is calculated as follows: (4) The derivative of optical signal intensity with respect to time The calculation formula is: (5), among which, Let be the derivative of the refractive index of seawater with respect to time. and Therefore Two functions, each with a variable, are calculated as follows: (6) and (7), as can be seen from calculation formulas (6) and (7), when or When m is a non-negative integer, we can obtain (8) The above analysis shows that when the output optical signal of the tunable semiconductor laser diode 2 is a fixed wavelength , and when or At that time, even if the refractive index of seawater Changes over time, i.e. optical signal intensity Differential with respect to time It is also zero. Completely independent of seawater refractive index When the refractive index detection probe changes, it operates in the least sensitive region, resulting in the so-called phase fading phenomenon. Therefore, the wavelength of the output light signal of the tunable semiconductor laser diode 2 must be adjusted to avoid the influence of the phase fading phenomenon and achieve accurate physical quantity sensing.

[0032] The range of this embodiment is limited by the mode-free adjustment range of the tunable semiconductor laser diode 2. For an optical resonator with a cavity length of 10 cm, its free spectral range is approximately 11 GHz; for an optical resonator with a cavity length of 40 cm, its free spectral range is approximately 3 GHz. The mode-free adjustment range of the tunable semiconductor laser diode 2 is greater than this range. Its size is very small, making it easy to integrate on a floating platform. Furthermore, a drive current in the hundreds of milliamperes range can obtain an optical signal with an optical power of over ten milliwatts, covering the range of seawater absolute density variation from 1020 to 1070 kg / m³. 3 .

Claims

1. A method for online detection of seawater absolute density, implemented using an online seawater absolute density detection device, characterized in that, The specific process is as follows: (1) Sawtooth wave generation: First, the control unit drives the waveform generator to output a periodic sawtooth wave voltage signal. ; (2) Wavelength tuning: The sawtooth wave voltage signal from step (1) is tuned. Input current driver, periodic current variation, tunable semiconductor laser diode output wavelength periodically varying in relation to a sawtooth wave voltage signal. ; (3) Optical resonant cavity configuration: The optical signal emitted by the tunable semiconductor laser diode is input at port ① of the fiber optic circulator and output at port ②. After passing through the fiber self-focusing lens, it sequentially illuminates the optical resonant cavity composed of the first plane mirror and the second plane mirror. The uniform spacing of the spectrum of the optical resonant cavity is called the free spectral range (FSR). (4) Signal processing: After a portion of the light entering the optical resonant cavity is reflected, it returns to port ② of the fiber optic circulator and is output from port ③ of the fiber optic circulator. After being received by the photodetector, the analog signal is converted to a digital signal on the data acquisition card. The converted digital signal is then transmitted to the signal processing unit for processing. (5) Cavity calibration: Before measuring the absolute density of seawater, the cavity length of the optical resonant cavity is calibrated; (6) Establish density relationship: Establish the relationship between the absolute density of seawater and its optical refractive index based on the Lorentz relation; (7) Calculate the absolute density: Substitute the cavity length in step (5) into the formula for calculating the refractive index of the medium in the optical resonant cavity in step (4) to obtain the formula for calculating the refractive index of seawater. Calculate the absolute density of seawater according to the Lorentz relation. The main structure of the online seawater absolute density detection device includes a tunable semiconductor laser diode, a fiber optic self-focusing lens, and a photodetector, all connected to an optical fiber circulator. The tunable semiconductor laser diode is connected to the photodetector via a data acquisition card, a signal processing unit, a control unit, a waveform generator, and a current driver. The absolute density of seawater is measured by measuring the refractive index of the seawater. The fiber optic self-focusing lens is connected to the seawater sample cavity, and a first plane mirror and a second plane mirror are provided at both ends of the seawater sample cavity.

2. The method for online detection of absolute density of seawater according to claim 1, characterized in that, In step (1) The calculation formula is: ,in, for The initial value within each sawtooth wave signal period, i.e. The voltage value at that moment. , , The maximum voltage value within each sawtooth wave signal period. , The amplitude of the sawtooth wave signal. Let t be the period of the sawtooth wave voltage signal, and t be the time variable. This represents the number of cycles in the sweep frequency signal.

3. The method for online detection of absolute density of seawater according to claim 1, characterized in that, In step (2) The calculation formula is: ,in, for The initial value within each sawtooth wave signal period, i.e. The wavelength value corresponding to the time, and This is the minimum wavelength during the scanning process. , The maximum value within each period of the sawtooth wave signal. The sawtooth wave voltage signal changed The wavelength change caused by time.

4. The method for online detection of absolute density of seawater according to claim 1, characterized in that, The formula for calculating FSR in step (3) is: ,in, Vacuum wavelength, The refractive index of the medium filling the optical resonant cavity. Let be the cavity length of the optical resonant cavity, i.e., the distance between the first and second plane mirrors. Since the mode-hopping range of a tunable semiconductor laser diode is much smaller than the output wavelength, then... Therefore, the vacuum wavelength is considered as the average wavelength, i.e. The formula for calculating the free spectral range (FSR) in the frequency domain is: Where c is the speed of light in vacuum, for an optical resonator with a constant cavity length, the free spectral range (FSR) is only related to the refractive index of the medium inside the optical resonator. Within each cycle of the sawtooth wave voltage signal, the output wavelength of the tunable semiconductor laser diode varies with time in a sawtooth wave pattern. The uniform frequency interval (FSR) of the optical resonator's spectrum in the spectrum is manifested as a uniform time interval in the time domain. ,Right now ,in, The electrically tunable slope of a tunable semiconductor laser diode. .

5. The method for online detection of absolute density of seawater according to claim 1, characterized in that, The specific process of transmitting the converted digital signal in step (4) to the signal processing unit for processing is as follows: First, normalization is performed; then, all minimum values ​​of the reflected signal intensity within a frequency sweep period are found, and the times when the minimum values ​​occur within that period are recorded. The time interval is obtained by subtracting the times of two adjacent minimum values. Finally, use Calculate the free spectral range of the optical resonator at this time. ,pass Inverse calculation of the refractive index of the medium inside the optical resonant cavity: .

6. The method for online detection of absolute density of seawater according to claim 1, characterized in that, The process of calibrating the cavity length of the optical resonant cavity in step (5) is as follows: when the optical resonant cavity is not filled with other media, it is assumed that the refractive index of the air inside the optical resonant cavity is... , At this point, a sawtooth wave voltage signal is used to modulate the current driver, and the time interval is calculated. To obtain the free spectral range of the optical resonator at this time. ,according to The cavity length of the optical resonant cavity is obtained. .

7. The method for online detection of absolute density of seawater according to claim 1, characterized in that, The relationship between the absolute density of seawater and its optical refractive index in step (6) is as follows: ,in, These are constants related to the properties of seawater. , Let Avogadro's constant be 1. The average molecular polarizability, It is the molar mass.

8. The method for online detection of absolute density of seawater according to claim 1, characterized in that, The formula for calculating the refractive index of seawater in step (7) is: The absolute density of seawater is calculated based on the Lorentz relation as follows: .

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