Sodium ion conductivity detection system and method based on taiji-shaped lithium niobate waveguide

Abstract

The application discloses a sodium ion conductivity detection system and method based on a taiji-shaped lithium niobate waveguide. The system comprises a lithium niobate film, a taiji-shaped Mach-Zehnder interference waveguide formed on the lithium niobate film, the taiji-shaped Mach-Zehnder interference waveguide comprising an MMI-type beam splitter, a reference arm and a sensing arm symmetrically curved in a taiji shape and an MMI-type beam combiner, and metal electrodes arranged on the sides of the reference arm and the sensing arm respectively. When a sodium ion solution is introduced into a sensing area, the change of the sodium ion solution conductivity changes the local electric field distribution at the electrode-liquid interface, thereby changing the effective modulation voltage of the waveguide of the sensing arm and changing the refractive index of the waveguide of the sensing arm through the Pockels effect to output an interference light intensity change signal, and then the sodium ion solution conductivity to be detected is detected. The application changes the local electric field at the spiral electrode interface and modulates the refractive index of the waveguide by the change of the sodium ion solution conductivity, realizes the output of the interference light intensity change signal, and detects the sodium ion solution conductivity to be detected.

Description

Technical Field

[0001] This invention relates to the field of optical sensing device technology, and more specifically to a sodium ion conductivity detection system and method based on a taiji-shaped lithium niobate waveguide. Background Technology

[0002] Sodium ions (Na) + Sodium ions, especially sodium chloride (NaCl), are widely used in electrochemical experiments, for regulating ionic strength and osmotic pressure, environmental / seawater simulation, and the preparation of physiological saline in biomedicine. In experiments involving cells or tissues, NaCl solutions are often used to control the external ionic environment or simulate osmotic stress conditions. Studies have shown that NaCl can influence cell membrane potential and the conductivity of ions through the exchange of electrolytes. The conductivity of sodium ions, particularly sodium chloride (NaCl), is approximately proportional to the ion concentration in low-concentration regions, but deviates from this proportionality due to ion interactions in high-concentration regions. Sodium ions are chemically stable, abundant, and have a well-defined effect on regulating ionic strength. + The altered ionic gradient of channels / transporters can serve as a biophysical signal regulating proliferation and division; by activating or inhibiting Na+... + / H + The exchanger (NHE1) can alter intracellular pH, thereby affecting the G2→M pathway and thus regulating the timing of mitosis; it can also alter Na+. + / K + -ATPase activity or destruction of Na+ + Homeostasis is also associated with cell proliferation and malignant transformation; in addition, the osmotic / ionic toxicity caused by exogenous sodium salts (such as NaCl) leads to mitotic abnormalities in plants and other models, such as chromosome bridging, nuclear fragmentation, and reduced mitotic index. Therefore, it is one of the basic reagents for constructing controllable conductivity and ionic environments in the fields of biology, materials, and engineering.

[0003] Traditional conductivity measurements typically rely on electrochemical electrode methods, requiring direct contact between metal electrodes and the solution, which suffers from problems such as electrode corrosion, low integration, and poor anti-interference capabilities. In contrast, integrated optical sensing devices offer advantages such as small size, fast response speed, and resistance to electromagnetic interference. However, most mainstream optical sensing devices are currently used for measurements of refractive index, concentration, or temperature, and there is still no mature solution for directly detecting liquid conductivity using optical methods. Among existing technologies, Mach-Zehnder interferometry (MZI) optical sensing devices exhibit high sensitivity for refractive index or chemical composition detection, achieving interference signal detection by modulating the optical path difference between the two arms. However, how to incorporate liquid conductivity into electro-optic modulation and convert it into an optical signal has always been a challenge. Therefore, developing an on-chip optical sensing device that combines conductivity sensing with a lithium niobate electro-optic modulator is of great significance. Summary of the Invention

[0004] The purpose of this invention is to provide a sodium ion conductivity detection system and method based on a taiji-shaped lithium niobate waveguide. The system changes the local electric field at the interface of the spiral electrode by altering the conductivity of the sodium ion solution, and modulates the refractive index of the waveguide through the Pockels effect, thereby outputting an interference light intensity change signal to detect the sodium ion conductivity.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A sodium ion conductivity detection system based on a taiji-shaped lithium niobate waveguide includes: Lithium niobate thin film; A taiji-shaped Mach-Zehnder interferometer waveguide is formed on the lithium niobate film; wherein the taiji-shaped Mach-Zehnder interferometer waveguide includes: an MMI-type beam splitter, a reference arm and a sensing arm that are symmetrically curved in a taiji shape, and an MMI-type beam combiner; The output end of the MMI type beam splitter is connected to the input ends of the reference arm and the sensing arm, which are symmetrically curved in a Taiji shape, respectively. The output ends of the reference arm and the sensing arm are connected to the input end of the MMI type beam combiner. Metal electrodes are respectively arranged on the sides of the reference arm and the sensing arm, wherein the metal electrode located on the side of the sensing arm is a spiral electrode, which is used to contact the sodium ion solution to be tested to form a sensing area. When the sodium ion solution to be tested is introduced into the sensing area, the change in the conductivity of the sodium ion solution changes the local electric field distribution at the electrode-liquid interface, thereby changing the effective modulation voltage of the waveguide of the sensing arm. The Pockels effect changes the refractive index of the waveguide of the sensing arm to output an interference light intensity change signal, thereby detecting the conductivity of the sodium ion solution to be tested.

[0006] Furthermore, the metal electrode is made of gold, and in the taiji-shaped Mach-Zehnder interferometer waveguide, the transmission waveguides other than the MMI type beam splitter and MMI type beam combiner are single-mode.

[0007] Furthermore, the geometric parameters of the spiral electrode include: line width, spacing, and number of turns; The liquid equivalent current path length and liquid equivalent passage cross-sectional area at the electrode-liquid interface are determined by the geometric parameters of the spiral electrode to regulate the response sensitivity and linearity of the conductivity of the sodium ion solution to be tested.

[0008] Furthermore, the system also includes: a photodetector; The photodetector is connected to the output of the MMI type beam combiner and is used to receive and measure the interference light intensity change signal.

[0009] This invention also provides a method for detecting sodium ion conductivity in a lithium niobate waveguide based on a taiji-shaped waveguide, comprising the following steps: S1. An optical signal is input into a Mach-Zehnder interferometer waveguide based on a taiji-shaped lithium niobate waveguide. After beam splitting, optical signals for the reference arm and the sensing arm are formed respectively. A bias voltage is applied to the metal electrodes of the reference arm and the sensing arm. S2. The sodium ion solution to be tested is introduced to the spiral electrode on the side of the sensing arm, so that the sodium ion solution to be tested comes into contact with the spiral electrode to form an electrical contact sensing area. S3. The sodium ion solution to be tested in the electrical contact sensing area constitutes the liquid equivalent resistance between the electrodes. The change in the conductivity of the sodium ion solution to be tested adjusts the effective modulation voltage acting on the waveguide of the sensing arm by changing the liquid equivalent resistance, thereby changing the effective electric field strength at the waveguide of the sensing arm. Based on the change in the effective electric field strength, the refractive index of the inductive arm waveguide is changed by utilizing the Pockels effect of the lithium niobate crystal, thereby generating a phase difference relative to the reference arm. S4. The optical signals of the reference arm carrying the phase difference and the sensing arm are interfered and output. By detecting the change in the output light intensity, the conductivity of the sodium ion solution to be tested can be deduced.

[0010] Furthermore, in S3, the adjustment process of the effective modulation voltage of the inductive arm waveguide is specifically as follows: The change in conductivity of the sodium ion solution to be tested causes a change in the liquid equivalent resistance in the spiral electrode region of the sensing arm, which in turn changes the effective modulation voltage acting on the waveguide of the sensing arm through the voltage divider relationship.

[0011] Furthermore, in S3, the equivalent resistance and conductivity of the liquid satisfy the following relationship: R s = L s / ( σ A s ); In the formula, L s Let be the equivalent current path length in the liquid. A s The equivalent cross-sectional area of ​​the liquid path. R s The equivalent resistance of the liquid is... σ The conductivity of the sodium ion solution is given.

[0012] Furthermore, the method for calculating conductivity in S4 is as follows: Based on the Pockels effect of lithium niobate crystal, the relationship between the refractive index change of the inductive arm waveguide and the phase change of the reference arm is established. The change in light intensity is mapped to a conductivity value using a pre-calibrated curve or numerical solution.

[0013] Furthermore, under the conditions of small-signal approximation and the operating point being in an orthogonal bias state, the sensitivity of the output light intensity change to conductivity is related to the liquid equivalent current path length of the spiral electrode. L s and the equivalent cross-sectional area of ​​the liquid path A s The ratios are positively correlated.

[0014] According to specific embodiments provided by the present invention, the present invention has the following technical effects compared to the prior art: This invention integrates a taiji-shaped Mach-Zehnder interference waveguide onto a lithium niobate thin film, deeply leveraging the excellent Pockels electro-optic effect of lithium niobate. This achieves extremely rapid and hysteresis-free conversion of changes in the conductivity of sodium ion solution into interference light intensity signals, fundamentally endowing the system with high response speed and high detection sensitivity. Simultaneously, the taiji-shaped symmetrical bending structure not only significantly compresses the physical size of the device and effectively suppresses waveguide bending loss, but also significantly enhances the system's immunity to temperature drift and common-mode noise due to its inherent symmetry. Combined with low loss and high tolerance... The MMI-type beam splitter and combiner further ensures the stability of optical path transmission. Crucially, the innovative spiral metal electrode on the side of the sensing arm significantly expands the contact area between the electrode and the sodium ion solution to be measured, multiplying the changes in local electric field distribution caused by minute fluctuations in the conductivity of the sodium ion solution. This significantly enhances the intensity of the effective modulation voltage on the waveguide, ultimately enabling the system to achieve chip-level miniaturization while possessing strong anti-electromagnetic interference capabilities and advanced detection performance with high precision and high resolution for the conductivity of ions to be measured in trace amounts of liquid. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0016] The sodium ion conductivity detection system and method based on the taiji-shaped lithium niobate waveguide of the present invention will be further described below with reference to the accompanying drawings. Figure 1 This is a schematic diagram of an on-chip integrated sodium ion conductivity sensing device based on a taiji-shaped lithium niobate waveguide in Embodiment 1 of the present invention. Figure 2This is a schematic diagram of the overall process of the sodium ion conductivity detection method based on the taiji-shaped lithium niobate waveguide in Embodiment 2 of the present invention.

[0017] In the figure: 1. Metal electrode of the sensing arm; 2. Metal electrode of the reference arm; 3. G electrode; 4. MMI beam splitter; 5. Spiral electrode; 6. Sensing area formed by contact with the sodium ion solution to be measured; 7. Second interference arm; 8. MMI beam combiner. Detailed Implementation

[0018] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0019] To better understand the purpose, structure, and function of this invention, the invention will be described in further detail below with reference to the accompanying drawings.

[0020] Example 1 like Figure 1 As shown, this invention provides an on-chip integrated sodium ion conductivity sensing system based on a taiji-shaped lithium niobate waveguide, comprising: Lithium niobate thin film; A taiji-shaped Mach-Zehnder interferometer waveguide is formed on the lithium niobate film; wherein the taiji-shaped Mach-Zehnder interferometer waveguide includes: an MMI-type beam splitter 4, a reference arm and a sensing arm that are symmetrically curved in a taiji shape, and an MMI-type beam combiner 8; The output end of the MMI type beam splitter 4 is connected to the input ends of the reference arm and the sensing arm, which are symmetrically curved in a Taiji shape, respectively. The output ends of the reference arm and the sensing arm are connected to the input end of the MMI type beam combiner 8. Metal electrodes are respectively arranged on the sides of the reference arm and the sensing arm, wherein the metal electrode located on the side of the sensing arm is a spiral electrode, which is used to contact the sodium ion solution to be tested to form a sensing area. When the sodium ion solution to be tested is introduced into the sensing area, the change in the conductivity of the sodium ion solution changes the local electric field distribution at the electrode-liquid interface, thereby changing the effective modulation voltage of the waveguide of the sensing arm. The Pockels effect changes the refractive index of the waveguide of the sensing arm to output an interference light intensity change signal, thereby detecting the conductivity of the sodium ion solution to be tested.

[0021] The metal electrode is made of gold, and in the taiji-shaped Mach-Zehnder interferometer waveguide, the transmission waveguides other than the MMI type beam splitter 4 and the MMI type beam combiner 8 are single-mode.

[0022] The geometric parameters of the spiral electrode include: line width, spacing, and number of turns; The liquid equivalent current path length and liquid equivalent passage cross-sectional area are determined by the geometric parameters of the spiral electrode to regulate the response sensitivity and linearity of the sodium ion solution to be tested.

[0023] Specifically, this embodiment demonstrates that spiral electrodes offer significant advantages over simple parallel or straight electrodes in sodium ion conductivity sensors. Traditional parallel electrodes, due to their small contact area and uneven electric field distribution, exhibit sluggish response and low sensitivity to changes in the conductivity of sodium ion solutions. Spiral electrodes, by increasing the contact area with the sodium ion solution, optimize the electric field distribution path, thereby significantly improving the sensor's sensitivity to changes in sodium ion solution conductivity. Spiral electrodes can improve the uniformity of the electric field by altering the electrode geometry (including linewidth, spacing, and number of turns), allowing for more effective control of the electric field distribution and reducing errors caused by uneven electric field distribution. Furthermore, the spiral electrode design, by forming more contact points on the electrode surface, significantly increases the contact area between the electrode and the sodium ion solution, thereby enhancing the electric field strength in the sensing area.

[0024] The system also includes: a photodetector; The photodetector is connected to the output of the MMI type beam combiner and is used to receive and measure the interference light intensity change signal.

[0025] The conductivity of the sodium ion solution under test can be deduced by measuring the change in interference intensity of the interference light intensity signal.

[0026] The working principle of the device in this embodiment is as follows: The input optical signal is split into two paths by an MMI-type beamsplitter, which enter the lithium niobate waveguides of the reference arm and the sensing arm, respectively. A DC bias voltage is applied to the two waveguides, generating an electric field between the electrodes of the reference arm and the sensing arm. When the sodium ion solution to be measured is introduced to the spiral electrode 6 near the sensing arm, the conductivity of different concentrations of sodium chloride changes the local electric field distribution at the electrode-liquid interface, thereby changing the effective modulation voltage applied to the waveguide. According to the Pockels effect of lithium niobate crystal, this modulation voltage change causes a corresponding change in the refractive index of the sensing arm waveguide; while the refractive index of the reference arm remains unchanged or changes with the voltage within a known range. The optical signals of the reference arm and the sensing arm meet and interfere at the MMI-type beam combiner 8. The phase difference caused by the refractive index change of the spiral electrode 5 causes a measurable change in the output light intensity. Finally, by measuring the change in the interference intensity of the output optical signal, the conductivity of the sodium chloride liquid can be deduced. The entire process does not require consideration of other complex factors such as temperature and capacitance, and achieves a direct conversion from conductivity to electro-optic modulation to optical output.

[0027] The integrated sodium chloride conductivity sensor of this invention employs lithium niobate thin-film technology, achieving on-chip integration of optical components and the sensing device. All components are integrated on a lithium niobate thin film, forming an on-chip integrated optoelectronic device structure.

[0028] In practical applications, LiNbO3 waveguides can be designed as single-mode structures to improve transmission efficiency, and Au is preferred as the electrode material to ensure good conductivity and chemical stability. By rationally optimizing the geometry of the spiral electrodes (such as width, spacing, and number of coil turns), the sensitivity and detection range of the sensing device can be further improved. The entire device adopts an on-chip integrated design, allowing the light source, waveguide, electrodes, and detector to be integrated onto a single chip, thereby significantly reducing system size and improving anti-interference capabilities.

[0029] Example 2 like Figure 2 As shown, the present invention also provides a method for implementing the sodium ion conductivity detection system based on the taiji-shaped lithium niobate waveguide in Embodiment 1, comprising the following steps: S1. An optical signal is input into a Mach-Zehnder interferometer waveguide based on a taiji-shaped lithium niobate waveguide. After beam splitting, optical signals for the reference arm and the sensing arm are formed respectively. A bias voltage is applied to the metal electrodes of the reference arm and the sensing arm. Specifically, in this embodiment: A monochromatic light source is provided to couple the optical signal into the MMI-type beam splitter input of the Mach-Zehnder interferometer waveguide. Simultaneously, a stable bias voltage is applied to the waveguide arm.

[0030] S2. The sodium ion solution to be tested is introduced to the spiral electrode on the side of the sensing arm, so that the sodium ion solution to be tested comes into contact with the spiral electrode to form an electrical contact sensing area. The geometric parameters of the spiral electrode include: line width, spacing, and number of turns; The liquid equivalent current path length and the liquid equivalent passage cross-sectional area are determined by the geometric parameters of the spiral electrode to regulate the response sensitivity and linearity of the conductivity of the sodium ion solution to be tested.

[0031] The mechanism by which spiral electrodes improve sensitivity to changes in solution resistance compared to parallel plate / straight bar electrodes revolves around the formula for the equivalent resistance of the solution:

[0032] Combined with electrode geometry (Equivalent current path length in liquid) By controlling the equivalent cross-sectional area of ​​the liquid path and optimizing the contact area and electric field distribution path, the conductivity is ultimately amplified through a chain reaction of electro-optic modulation. The changing detection signal. The specific mechanism and comparative analysis are as follows: The response sensitivity is the key to sensing the solution equivalent resistance. It is conductivity direct mapping, and Inversely proportional to, with Proportional to, and Inversely proportional. For sensing sensitivity, the requirement is... Able to follow Tiny changes produce significant and detectable alterations, which in turn change the effective modulation voltage through voltage divider relationships. Ultimately, this is translated into a significant change in optical phase / intensity via the Pockels effect.

[0033] Parallel plate / straight bar electrodes , To fix the geometric parameters, for The response slope is fixed and gentle; while the spiral electrode, through its structural design, can be flexibly adjusted. , The geometric characteristics of the electrode increase the electrode-liquid contact area, thereby improving the equivalent resistance. More sensitive to changes in σ becomes the core of improving overall sensing sensitivity. The spiral winding structure significantly increases the contact area between the electrode and the liquid, allowing the liquid to form multi-dimensional current paths in the gaps between the spiral coils and on the electrode surface, no longer limited to a single direction; at the same time, the larger contact area allows... , The spiral geometry (line width, spacing, number of turns) can be precisely controlled to avoid current path saturation, allowing the electrodes to capture [current]. A small change in Rs is transformed into a significant change in Rs.

[0034] The length of the liquid equivalent current path can be extended and made controllable. ,enlarge right The range of change. Parallel plate electrodes. The vertical distance between the two plates is a fixed short distance that cannot be extended; the straight electrode... The linear length of the electrode allows for very little adjustment. All are constant values. right The range of change is only determined by a fixed... / The decision is based on a gradual slope and low sensitivity.

[0035] Spiral electrode The current must form a loop between the electrodes along the spiral's winding path, determined by the number of turns of the spiral, the coil circumference, and the electrode spacing. Significantly extended (the more turns, the denser the coil), The longer); and Continuous controllability can be achieved by adjusting the helical geometry parameters. When it increases, and The inverse relationship has a steeper slope. A slight decrease would lead to A significant increase, A slight increase can lead to A significant reduction was achieved. right The amplified amplitude of the change allows subsequent electro-optic modulation to capture more obvious signal changes.

[0036] At the same time, the cross-sectional area of ​​the liquid equivalent pathway can be precisely controlled. ,optimization right The response linearity and detection range of the parallel plate electrodes. The area of ​​the electrode plates facing each other, the area of ​​the straight electrode. It is the product of the electrode linewidth and the liquid contact thickness. These are all single, fixed values ​​and cannot be adjusted according to testing requirements. When entering a high concentration region, it is easy for inter-ion interactions to lead to... and The linear relationship deviates, while the fixed one... This deviation cannot be compensated for, resulting in poor detection linearity and insufficient sensitivity in the low concentration region.

[0037] Spiral electrodes Determined by the electrode line width and the gap width of the spiral coil, each turn of the electrode in the spiral forms an independent micro-current path with the adjacent coil, resulting in an overall... This is the sum of the cross-sectional areas of all the tiny pathways; by adjusting the electrode linewidth (widening it results in a single pathway). Increasing the coil gap (increasing the number of channels) can achieve... Refined and tiered regulation.

[0038] Combination Controllability allows / The ratio matching is different Detection range (increase the low concentration range) / Improve sensitivity and reduce the high concentration range. / (Ensuring linearity) solves the problems of narrow detection range and poor linearity of parallel plate / straight bar electrodes, allowing... In the full concentration range The changes all exhibit stable and sensitive responses.

[0039] According to the formula for the sensitivity of output light intensity to conductivity:

[0040] The spiral electrode allows, through the above method, R s The change in σ is more significant, resulting in:

[0041] The larger the absolute value of this core term, the greater the output light intensity. For conductivity rate of change Larger - that is Small changes can bring This significant change directly reflects the mathematical improvement in sensitivity achieved by spiral electrodes compared to parallel plate / straight electrode designs. Simulation data shows that sensors using spiral electrode designs exhibit approximately 15-30% higher conductivity detection sensitivity compared to traditional parallel electrode structures.

[0042] Compared to traditional parallel electrode designs, this invention employs a spiral electrode structure, which optimizes the contact area and electric field distribution between the electrode and the sodium ion solution being tested, thereby significantly improving the sensitivity to the conductivity of the sodium ion solution. By comparing the effects of different electrode designs, the sensor using the spiral electrode demonstrates significant advantages in several key performance indicators.

[0043] In this embodiment, several parameters of the spiral electrode significantly affect its performance: linewidth, which refers to the width of each ring in the spiral electrode, directly affects the contact area between the electrode and the sodium ion solution being tested, as well as the uniformity of the electric field distribution. A smaller linewidth results in a smaller contact area between the electrode and the liquid, leading to uneven current density and electric field distribution, and decreased sensitivity. Conversely, a larger linewidth results in a larger contact area, a more uniform electric field distribution, and improved sensitivity.

[0044] The formulas affecting the electric field are as follows: ; In the formula, For electric field strength, To apply voltage, This represents the distance between the electrodes. As the linewidth increases, The smaller the electric field, the stronger the electric field becomes, which in turn increases the sensitivity.

[0045] For response speed, a larger linewidth results in a longer current path, leading to increased charging time. This relationship can be expressed using the RC time constant: ; In the formula, For resistance, For capacitors. Increasing the linewidth will affect the capacitance. This affects the response time. The spacing refers to the center-to-center distance between the spiral electrodes, which determines the distribution of the electric field between the electrodes. Increasing the spacing results in a more uniform electric field distribution, increased current path, and enhanced sensitivity, but may slow down the response speed.

[0046] The relationship between the electric field distribution and the spacing can be expressed by the following formula: ; spacing The larger the electric field strength, the greater the electric field strength. The smaller the value, the lower the sensitivity.

[0047] For response time, a larger spacing design increases the current transport path, leading to a delay in the application of the electric field. Therefore, increasing the spacing results in a longer response time, manifested as a longer charging and discharging process. The delay can be estimated using the following formula: ; In the formula, To respond to the delay, Let be the velocity of the ion in the electric field.

[0048] The number of turns, or the number of loops in the spiral electrode, directly determines the strength and range of the electric field. More turns result in a stronger electric field, allowing for more effective control of conductivity changes and thus improved sensitivity. The relationship between electric field strength and the number of turns can be expressed by the following formula: ; Increasing the number of turns increases the electric field strength, thereby enhancing the sensitivity of the sensing region, especially in measurements of highly conductive liquids. The relationship between response time and the number of turns is similar to the effect of linewidth; increasing the number of turns generally increases the time the electric field is applied, thus affecting the response speed.

[0049] A larger number of turns may imply a higher electric field strength and current transport time, leading to an increase in response time. This can be described by the following relationship: ; In the formula, Total response time This refers to the response time of a single turn.

[0050] S3. The sodium ion solution to be tested in the electrical contact sensing area constitutes the liquid equivalent resistance between the electrodes. The change in the conductivity of the sodium ion solution to be tested adjusts the effective modulation voltage acting on the waveguide of the sensing arm by changing the liquid equivalent resistance, thereby changing the effective electric field strength at the waveguide of the sensing arm. Based on the change in the effective electric field strength, the refractive index of the inductive arm waveguide is changed by utilizing the Pockels effect of the lithium niobate crystal, thereby generating a phase difference relative to the reference arm. In this embodiment, the specific process is as follows: Interference detection—the light waves from the two interference arms interfere at the beam combiner, and the output light signal is received by a photodetector. Due to the change in the local electric field of the sensing arm, its optical path is different from that of the reference arm, resulting in a change in the phase difference between the two beams, which manifests as a change in the intensity of the light intensity interference fringes at the output end.

[0051] In step S3, after the solution comes into contact with the spiral electrode, the change in the conductivity of the sodium ion solution causes a change in the liquid equivalent resistance in the spiral electrode region of the sensing arm, which in turn changes the effective modulation voltage acting on the waveguide of the sensing arm through the voltage divider relationship.

[0052] In S3, the equivalent resistance of the liquid and the conductivity of the sodium ion solution satisfy the following relationship: R s = L s / ( σ A s ); In the formula, L s Let be the equivalent current path length in the liquid. A s The equivalent cross-sectional area of ​​the liquid pathway; the change in the conductivity of the sodium ion solution causes R s change, R s The equivalent resistance of the liquid; the conductivity of the sodium ion solution. σ Changes cause liquid equivalent resistance R s Changes in liquid equivalent resistance R s The effective modulation voltage is changed by voltage division with the equivalent impedance inside the device.

[0053] Voltage division relationship of equivalent modulation voltage:

[0054] In the formula, The internal equivalent impedance of the device (Ω); External bias voltage (V); : The equivalent voltage (V) that acts on the active region of the waveguide and generates electro-optic modulation; Pockels The change in refractive index caused by the Polkell effect is expressed as:

[0055] In the formula, The effective distance or electric field characteristic scale between electrodes (m); The effective electric field strength at the waveguide (V·m) -1 ), ; The intrinsic refractive index of the waveguide at the operating wavelength is dimensionless. The electro-optic coefficient (m·V) along the direction of the induced electric field -1 ); This refers to the change in refractive index caused by the electric field; The expression for phase change is:

[0056] In the formula, : Effective phase cumulative length of the sensing arm (m); The operating wavelength (m); For the reason The resulting phase change (rad).

[0057] Under the conditions of small-signal approximation and the operating point being in an orthogonal bias state, The sensitivity of the output light intensity to conductivity is related to the liquid equivalent current path length of the spiral electrode. L s and the equivalent cross-sectional area of ​​the liquid path A s The ratios are positively correlated.

[0058] Among them, under the small signal approximation ( And select the working point At that time, the sensitivity of the output light intensity to the conductivity satisfies the following approximate relationship: ; in .

[0059] S4. The optical signals of the reference arm carrying the phase difference and the sensing arm are interfered and output. By detecting the change in the output light intensity, the conductivity of the sodium ion solution to be tested can be deduced.

[0060] The formula for calculating the phase difference is as follows:

[0061] In step S4, the method for calculating the conductivity of the sodium ion solution to be tested is as follows: Based on the relationship between refractive index change and phase change caused by the Pockels effect of lithium niobate crystal, the light intensity change value is mapped to the conductivity value using a pre-calibrated curve or numerical solution.

[0062] Numerically solve the conductivity based on the following algebraic equations. :

[0063] The expression for the output intensity relationship of the Mach-Zehnder interferometry is:

[0064] In the formula, Input light intensity (W or a relative amount); To achieve the output light intensity after beam combining, and Related; This is the static reference phase (rad).

[0065] By detecting the change in output light intensity, the expression for the conductivity of the sodium ion solution to be tested can be derived as follows: ; .

[0066] Specifically, in this embodiment: signal processing—measuring the change in output light intensity and comparing it with a pre-calibrated curve, and converting the change in interference light intensity into the conductivity value of the sodium chloride solution to be tested according to the Pockels effect model.

[0067] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A sodium ion conductivity detection system based on a taiji-shaped lithium niobate waveguide, characterized in that, include: Lithium niobate thin film; A taiji-shaped Mach-Zehnder interferometer waveguide is formed on the lithium niobate film; The Taiji-shaped Mach-Zehnder interferometer waveguide includes: an MMI-type beam splitter (4), a reference arm and a sensing arm that are symmetrically curved in a Taiji shape, and an MMI-type beam combiner (8). The output end of the MMI type beam splitter (4) is connected to the input ends of the reference arm and the sensing arm, which are symmetrically curved in a Taiji shape, respectively. The output ends of the reference arm and the sensing arm are connected to the input end of the MMI type beam combiner (8). Metal electrodes are respectively arranged on the sides of the reference arm and the sensing arm, wherein the metal electrode located on the side of the sensing arm is a spiral electrode, which is used to contact the sodium ion solution to be tested to form a sensing area. When the sodium ion solution to be tested is introduced into the sensing area, the change in the conductivity of the sodium ion solution changes the local electric field distribution at the electrode-liquid interface, thereby changing the effective modulation voltage of the waveguide of the sensing arm. The Pockels effect changes the refractive index of the waveguide of the sensing arm to output an interference light intensity change signal, thereby detecting the conductivity of the sodium ion solution to be tested.

2. The sodium ion conductivity detection system based on a taiji-shaped lithium niobate waveguide according to claim 1, characterized in that, The metal electrode is made of gold, and the transmission waveguides in the taiji-shaped Mach-Zehnder interferometer waveguide, except for the MMI type beam splitter (4) and the MMI type beam combiner (8), are single-mode.

3. The sodium ion conductivity detection system based on a taiji-shaped lithium niobate waveguide according to claim 1, characterized in that, The geometric parameters of the spiral electrode include: line width, spacing, and number of turns; The liquid equivalent current path length and liquid equivalent passage cross-sectional area at the electrode-liquid interface are determined by the geometric parameters of the spiral electrode to regulate the response sensitivity and linearity of the conductivity of the sodium ion solution to be tested.

4. The sodium ion conductivity detection system based on a taiji-shaped lithium niobate waveguide according to claim 1, characterized in that, The system also includes: a photodetector; The photodetector is connected to the output of the MMI type beam combiner and is used to receive and measure the interference light intensity change signal.

5. A method for detecting sodium ion conductivity based on a taiji-shaped lithium niobate waveguide, applied to the sodium ion conductivity detection system based on a taiji-shaped lithium niobate waveguide as described in any one of claims 1-4, characterized in that, Includes the following steps: S1. An optical signal is input into a Mach-Zehnder interferometer waveguide based on a taiji-shaped lithium niobate waveguide. After beam splitting, optical signals for the reference arm and the sensing arm are formed respectively. A bias voltage is applied to the metal electrodes of the reference arm and the sensing arm. S2. The sodium ion solution to be tested is introduced to the spiral electrode on the side of the sensing arm, so that the sodium ion solution to be tested comes into contact with the spiral electrode to form an electrical contact sensing area. S3. The sodium ion solution in the electrical contact sensing area constitutes the liquid equivalent resistance between the electrodes. The change in the conductivity of the sodium ion solution to be tested adjusts the effective modulation voltage acting on the waveguide of the sensing arm by changing the liquid equivalent resistance, thereby changing the effective electric field strength at the waveguide of the sensing arm. Based on the change in the effective electric field strength, the refractive index of the inductive arm waveguide is changed by utilizing the Pockels effect of the lithium niobate crystal, thereby generating a phase difference relative to the reference arm. S4. The optical signals of the reference arm carrying the phase difference and the sensing arm are interfered and output. By detecting the change in the output light intensity, the conductivity of the sodium ion solution to be tested can be deduced.

6. The method for detecting sodium ion conductivity based on a taiji-shaped lithium niobate waveguide according to claim 5, characterized in that, In S3, the adjustment process of the effective modulation voltage of the inductive arm waveguide is as follows: The change in conductivity of the sodium ion solution to be tested causes a change in the liquid equivalent resistance in the spiral electrode region of the sensing arm, which in turn changes the effective modulation voltage acting on the waveguide of the sensing arm through the voltage divider relationship.

7. The method for detecting sodium ion conductivity based on a taiji-shaped lithium niobate waveguide according to claim 5, characterized in that, In S3, the liquid equivalent resistance and conductivity satisfy the following relationship: R s = L s / ( σ A s )； In the formula, L s Let be the equivalent current path length in the liquid. A s The equivalent cross-sectional area of ​​the liquid pathway; R s The equivalent resistance of the liquid; σ The conductivity of the sodium ion solution is given.

8. The method for detecting sodium ion conductivity based on a taiji-shaped lithium niobate waveguide according to claim 5, characterized in that, The method for calculating conductivity in S4 is as follows: Based on the Pockels effect of lithium niobate crystal, the relationship between the refractive index change of the inductive arm waveguide and the phase change of the reference arm is established. The change in light intensity is mapped to a conductivity value using a pre-calibrated curve or numerical solution.

9. The method for detecting sodium ion conductivity based on a taiji-shaped lithium niobate waveguide according to claim 5, characterized in that, Under the conditions of small-signal approximation and the operating point being in an orthogonal bias state, the sensitivity of the output light intensity change to conductivity is related to the liquid equivalent current path length of the spiral electrode. L s and the equivalent cross-sectional area of ​​the liquid path A s The ratios are positively correlated.

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