Ocean salinity sensor based on on-chip integrated waveguide multimode interference and manufacturing method

Abstract

The invention discloses a marine salinity sensor based on on-chip integrated waveguide multimode interference and a manufacturing method. The marine salinity sensor comprises a silicon-based substrate, a waveguide lower cladding, a waveguide upper cladding, a single-mode waveguide core layer and a few-mode waveguide core layer, the single-mode waveguide core layer is connected with the introduction single-mode optical fiber; the few-mode waveguide core layer is connected with the leading-out single-mode optical fiber; the waveguide upper cladding is provided with a detection port for promoting external liquid to directly contact with the surface of the few-mode waveguide core layer; an optical signal is input from the leading-in single-mode optical fiber, is coupled with the at least-mode waveguide core layer through the single-mode waveguide core layer, and after entering the few-mode waveguide core layer, the optical signal excites a plurality of modes, generates inter-mode interference and then returns to the single-mode waveguide core layer to be output; the salinity change of the external liquid enables the refractive index of the liquid to change, and then the effective mode refractive index in the few-mode waveguide core layer is influenced, so that the output spectrum drifts, and the salinity information of the liquid can be inverted based on the spectrum drift amount.

Description

Technical Field

[0001] The invention belongs to the technical field of optical waveguide sensing and photonics application, and in particular relates to an ocean salinity sensor based on on-chip integrated waveguide multimode interference and a manufacturing method thereof. Background Art

[0002] With the continuous deepening of marine scientific research and environmental protection, ocean salinity, as an important parameter for evaluating the marine environment, has received widespread attention. Changes in salinity not only affect ocean physical processes (such as ocean circulation and water mass characteristics), but also have a profound impact on climate change prediction, ecosystem health assessment, and marine biological resource development. In addition, in the fields of marine disaster monitoring, fishery management, and military detection, salinity is a key environmental parameter, and its accurate measurement is also crucial. Therefore, the development of an accurate, stable and highly sensitive ocean salinity sensor that can meet the needs of real-time monitoring in complex marine environments has important scientific significance and application value.

[0003] Traditional salinity measurement methods are mainly based on the conductivity method, which calculates salinity by measuring the conductivity of seawater. However, this method has limitations, such as interference from temperature, pressure and other dissolved substances, resulting in low measurement accuracy and stability. In addition, traditional sensors are easily affected by the corrosive marine environment during long-term use, and their anti-interference ability and service life are difficult to meet the high standards of modern marine research.

[0004] In recent years, fiber optic sensing technology has shown great application potential in marine salinity monitoring due to its advantages of high sensitivity, corrosion resistance, small size and wide application range. Traditional fiber optic salinity sensors usually use fiber Bragg gratings (FBGs). Although they have improved the measurement accuracy and stability to a certain extent, the sensitivity of fiber Bragg grating sensors is limited and it is difficult to meet the precise measurement requirements of low salinity changes. The existing sensors have a low degree of integration and are difficult to achieve large-scale deployment and long-term reliable operation.

[0005] At the same time, the rapid development of integrated photonics has provided new opportunities for the design of new salinity sensors. Intermodal interference technology based on few-mode waveguides combined with waveguide-fiber coupling can not only significantly improve the sensitivity of the sensor, but also achieve flexible regulation of the sensing characteristics by optimizing the waveguide structure. Few-mode waveguides can obtain more spectral information by supporting multiple propagation modes to generate intermodal interference, thereby improving the accuracy and anti-interference ability of sensing. In addition, the integrated optical waveguide has a compact structure, excellent corrosion resistance and environmental adaptability, and is very suitable for harsh marine environment applications.

[0006] Traditional salinity sensors have obvious deficiencies in measurement accuracy, environmental adaptability and long-term stability, while integrated photon sensing technology based on few-mode waveguide intermodal interference provides a new technical path for realizing high-sensitivity, real-time monitoring salinity sensors. This application studies and develops a new type of ocean salinity sensor based on few-mode waveguide intermodal interference, which will have a profound impact on the technological progress in the field of marine environmental monitoring. Summary of the invention

[0007] In order to overcome the deficiencies of the prior art, the present invention provides an ocean salinity sensor based on on-chip integrated waveguide multimode interference, wherein the ocean salinity sensor can detect seawater salinity by outputting drift changes of interference spectra.

[0008] The second object of the present invention is to provide a method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multi-mode interference.

[0009] The technical solution of the present invention to solve the above technical problems is:

[0010] An ocean salinity sensor based on on-chip integrated waveguide multimode interference comprises a silicon-based substrate, a waveguide lower cladding layer and a waveguide upper cladding layer arranged on the silicon-based substrate, and a single-mode waveguide core layer and a few-mode waveguide core layer arranged between the waveguide lower cladding layer and the waveguide upper cladding layer, wherein:

[0011] The single-mode waveguide core layer is arranged on both sides of the few-mode waveguide core layer, wherein the single-mode waveguide core layer on one side is connected to the introduced single-mode optical fiber, and is the input single-mode waveguide core layer; the single-mode waveguide core layer on the other side is connected to the lead-out single-mode optical fiber, and is the output single-mode waveguide core layer;

[0012] The waveguide upper cladding is provided with a detection port, and the detection port is used to enable external liquid to directly contact the surface of the few-mode waveguide core layer;

[0013] The optical signal is input from the input single-mode optical fiber, coupled to the few-mode waveguide core layer through the input single-mode waveguide core layer, and after entering the few-mode waveguide core layer, the optical signal will excite multiple modes and generate inter-mode interference. After reaching the output single-mode waveguide core layer, the optical signal is output to the external spectrum analyzer through the output single-mode optical fiber;

[0014] The change of salinity of external liquid will change the refractive index of the liquid. When the external seawater is in direct contact with the few-mode waveguide core layer through the detection port, the effective refractive index of the mode in the few-mode waveguide core layer will change. The change of the effective refractive index of different modes will cause the output interference spectrum to change. The drift of the output spectrum is measured by an external spectrum analyzer to demodulate the refractive index of seawater, and then the salinity of seawater is obtained through the correspondence between the refractive index of seawater and salinity.

[0015] Preferably, the detection port is formed on the waveguide upper cladding by an etching process.

[0016] Preferably, the trough wavelength of the output spectrum is measured by an external spectrometer, the refractive index of the external liquid is obtained through the functional relationship between the refractive index and the trough wavelength, the difference between the refractive index of the external liquid and the refractive index of pure water is calculated, and the salinity value of the seawater is obtained by calculating the ratio between the difference and the proportionality coefficient.

[0017] Preferably, the functional relationship between the refractive index and the trough wavelength is obtained by linearly fitting the refractive index of the external liquid and the trough wavelength of the corresponding output spectrum.

[0018] A method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multimode interference comprises the following steps:

[0019] Step 1: Fabricate a waveguide lower cladding on a silicon substrate;

[0020] Step 2: Use mask lithography technology to make a single-mode waveguide core layer and a few-mode waveguide core layer, and then coat the single-mode waveguide core layer and the few-mode waveguide core layer with a waveguide upper cladding layer;

[0021] Step 3: Etch the upper cladding of the waveguide to create a detection port that allows the external liquid to directly contact the few-mode waveguide core layer, and a window that allows the introduction of single-mode optical fiber and the input single-mode waveguide core layer, as well as the lead-out single-mode optical fiber and the output single-mode waveguide core layer to couple.

[0022] Preferably, in step 1, a silicon wafer reaching a predetermined purity is selected as a substrate material, and then the silicon wafer is plasma cleaned to obtain a silicon-based substrate; the selected cladding material is poured onto the silicon-based substrate and evenly spread by a glue spreader to obtain a waveguide lower cladding that meets the thickness requirements; then the waveguide lower cladding is soft-baked to solidify it; after the soft baking is completed, the waveguide lower cladding is irradiated with ultraviolet rays, and after completion, the waveguide lower cladding is hard-baked.

[0023] Preferably, in step 2, a few-mode waveguide photoresist is poured on the surface of the waveguide lower cladding film, and the few-mode waveguide photoresist is spread on the surface of the waveguide lower cladding film to a preset thickness by a coating machine; then the silicon-based substrate on which the few-mode waveguide photoresist is spin-coated is placed in a photolithography machine, and the few-mode waveguide photoresist is masked and photolithographically processed according to the designed waveguide core layer graphic file. After completion, the obtained sample is placed in a developer at room temperature to remove the portion of the photoresist that is not exposed to ultraviolet light, and then developed; after the development is completed, a hard bake is performed; then a cladding material is poured on the prepared single-mode waveguide core layer and the few-mode waveguide core layer, and evenly applied by a coating machine to obtain a waveguide upper cladding that meets the thickness requirements, and then the waveguide upper cladding is soft-baked to solidify it, and after the soft bake is completed, the waveguide upper cladding is irradiated with ultraviolet light, and then hard-baked is performed after completion.

[0024] Preferably, in step 3, at the sensing region of the waveguide upper cladding, the waveguide upper cladding is removed by selective photolithography and etching techniques and corrosion processes to generate a detection port, thereby causing the few-mode waveguide core layer to be directly exposed to the external liquid environment.

[0025] Preferably, the corrosion process is:

[0026] (1) Spin-coating a layer of photoresist with a thickness of 2–5 μm on the surface of the coated and cured waveguide upper cladding layer; then using a mask to expose the photoresist to ultraviolet light, wherein the length of the designed detection port is 10–12 μm and the depth is 2–5 μm; after development, the photoresist in the unexposed area is removed to form a mask pattern in the detection port area;

[0027] (2) etching the exposed upper cladding layer of the waveguide using a reactive ion etching device, wherein a mixed gas of CF4 and O2 is selected as the etching gas;

[0028] (3) wet etching is then used to further smooth the surface of the detection port;

[0029] (4) The etched sample is placed in deionized water for cleaning to remove corrosion residues; then the sample is dried using nitrogen to avoid water stains on the surface.

[0030] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0031] 1. The ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention combines the efficient coupling technology of optical fiber and waveguide and the inter-mode interference effect, and realizes high-sensitivity measurement of salinity through the design of direct contact of external liquid with the few-mode waveguide core layer. Compared with the prior art, the ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention has the advantages of compact structure, high sensitivity, strong anti-interference ability and wide application range, and is suitable for real-time monitoring of salinity in marine environment.

[0032] 2. The ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention utilizes the high responsiveness of the inter-mode interference effect of the few-mode waveguide to the refractive index change, and can realize the accurate measurement of salinity; in addition, the ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention adopts an integrated on-chip waveguide design, which can significantly reduce the volume of the ocean salinity sensor of the present invention and is suitable for harsh marine environments; in addition, the ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention can optimize the sensitivity, working range and anti-interference ability of the sensor by adjusting the geometric size and refractive index distribution of the few-mode waveguide. It can be applied to many fields such as marine environment monitoring, climate change research, and marine disaster warning.

[0033] 3. The ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention significantly improves the sensitivity, stability and environmental adaptability of the salinity sensor through the innovative design of direct contact of the external liquid with the few-mode waveguide core layer and the unique advantage of the few-mode waveguide inter-mode interference, providing a new solution for real-time monitoring of ocean salinity. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of the ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention.

[0035] Figure 2 These are cross-sectional views of the ocean salinity sensor based on on-chip integrated waveguide multimode interference in the directions of (1)-(1), (2)-(2), and (3)-(3) of the present invention.

[0036] Figure 3 It is the output spectrum outputted to the external spectrum analyzer through the single-mode optical fiber.

[0037] Figure 4 This is a graph showing the functional relationship between the refractive index of the first trough (dip1) and the trough wavelength.

[0038] Figure 5 This is a graph showing the functional relationship between the refractive index of the second trough (dip2) and the trough wavelength.

[0039] Figure 6 The present invention is a flow chart for preparing an ocean salinity sensor based on on-chip integrated waveguide multimode interference.

[0040] In the figure: 1-1: silicon-based substrate; 1-2: waveguide lower cladding; 1-3: single-mode waveguide core layer; 1-4: few-mode waveguide core layer; 1-5: waveguide upper cladding; 1-6: single-mode optical fiber introduction; 1-7: single-mode optical fiber extraction; 1-8: detection port. DETAILED DESCRIPTION

[0041] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0042] Example 1

[0043] See also Figure 1-Figure 5 The ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention comprises a silicon-based substrate, a waveguide lower cladding layer and a waveguide upper cladding layer arranged on the silicon-based substrate, and a single-mode waveguide core layer and a few-mode waveguide core layer arranged between the waveguide lower cladding layer and the waveguide upper cladding layer, wherein:

[0044] The single-mode waveguide core layer is arranged on the few-mode waveguide core layer, wherein the single-mode waveguide core layer on one side is connected to the introduced single-mode optical fiber, and is the input single-mode waveguide core layer; the single-mode waveguide core layer on the other side is connected to the lead-out single-mode optical fiber, and is the output single-mode waveguide core layer;

[0045] The waveguide upper cladding is provided with a detection port, and the detection port is used to enable external liquid to directly contact the surface of the few-mode waveguide core layer. The waveguide upper cladding is formed by an etching process;

[0046] The optical signal is input from the input single-mode optical fiber, coupled to the few-mode waveguide core layer through the input single-mode waveguide core layer, and after entering the few-mode waveguide core layer, the optical signal will excite multiple modes and generate inter-mode interference. After reaching the output single-mode waveguide core layer, the optical signal is output to the external spectrum analyzer through the output single-mode optical fiber;

[0047] The change of salinity of the external liquid will cause the refractive index of the liquid to change. When the external seawater directly contacts the few-mode waveguide core through the detection port, it will cause the effective refractive index of the mode in the few-mode waveguide core to change. The change of the effective refractive index of different modes will cause the output interference spectrum to change. The output spectrum is measured by an external spectrum analyzer (such as Figure 3 As shown, the horizontal axis is wavelength (Wavelength) and the vertical axis is the drift of wave speed (transmission), which is used to demodulate the refractive index of seawater, and then the salinity of seawater is obtained through the corresponding relationship between the refractive index of seawater and salinity.

[0048] Specifically, according to the longitudinal coupling mechanism of the waveguide, when the optical signal is coupled from a single-mode waveguide to a few-mode waveguide, multiple modes will be excited in the few-mode waveguide. This is because there is a non-zero overlap between the input light field and the field distribution of each mode in the few-mode waveguide, which leads to energy coupling into multiple guided modes. This process can be described by expanding the input field on the waveguide mode basis function. Specifically, the input field E in It can be expressed as the guided mode field of the few-mode waveguide {E i} is a linear combination of:

[0049]

[0050] The coupling coefficient c i Determined by the overlap integral of the input light field and the i-th mode of the few-mode waveguide:

[0051]

[0052] The phase difference between different modes will cause interference.

[0053]

[0054] Where: Δn eff is the effective refractive difference between the modes; L is the length of the optical fiber sensing area; λ is the wavelength of the input optical signal.

[0055] Interference will make the intensity of light output different at each wavelength. When Δφ=(2m+1)π, the energy of the two light beams coupled has a minimum value and a trough appears. The wavelength of the interference trough is:

[0056]

[0057] When the salinity of the external liquid changes, the refractive index of the external liquid also changes. Since the few-mode waveguide core is in direct contact with the external liquid, the effective refractive index of each mode in the few-mode waveguide core will also change, and the effective refractive index difference will also change, which will cause the spectrum to drift:

[0058]

[0059] Where: Δn eff is the effective refractive difference between the modes; Δn c It is the difference in effective refractive index caused by the change of refractive index of seawater.

[0060] The trough wavelength of the output spectrum is measured by an external spectrum analyzer, and the refractive index of the external liquid is obtained through the functional relationship between the refractive index and the trough wavelength. The difference between the refractive index of the external liquid and the refractive index of pure water is calculated, and the salinity value of the seawater is obtained by calculating the ratio between the difference and the proportionality coefficient;

[0061] That is, the relationship between refractive index and salinity is given by the empirical formula:

[0062] n=n0+kS;

[0063] Where: n is the refractive index of seawater, n0 is the refractive index of pure water, which is approximately 1.333; k is the proportionality coefficient, which is approximately 0.00016 / PSU, and S is the salinity (unit: PSU).

[0064] In this embodiment, the refractive index of the external liquid and the corresponding trough wavelength of the output spectrum are linearly fitted to obtain the functional relationship between the refractive index and the trough wavelength; Figure 3 As shown, Figure 4 and Figure 5 As shown in the figure, the horizontal axis is the refractive index of the external liquid (Refractive Index), and the vertical axis is the wavelength (Wavelength); Figure 4 and Figure 5 Linear fitting is performed on multiple measurement points in the experiment to obtain the functional relationship between the corresponding refractive index and the trough wavelength. Based on the functional relationship and in combination with an external spectrum analyzer to measure the trough wavelength of the output spectrum, the refractive index of the external liquid is obtained, and the salinity value of the seawater is calculated by the empirical formula in this embodiment.

[0065] Example 2

[0066] See also Figure 4 The method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multimode interference of the present invention comprises the following steps:

[0067] Step 1: Fabricate a waveguide lower cladding on a silicon substrate;

[0068] In this embodiment, a silicon wafer reaching a predetermined purity is selected as a substrate material, and then the silicon wafer is plasma cleaned to obtain a silicon-based substrate; the selected cladding material is poured onto the silicon-based substrate and evenly spread by a glue spreader to obtain a waveguide lower cladding that meets the thickness requirements; then the waveguide lower cladding is soft-baked to solidify it; after the soft baking is completed, the waveguide lower cladding is irradiated with ultraviolet rays, and after completion, the waveguide lower cladding is hard-baked.

[0069] Step 2: Use mask lithography technology to make a single-mode waveguide core layer and a few-mode waveguide core layer, and then coat the single-mode waveguide core layer and the few-mode waveguide core layer with a waveguide upper cladding layer;

[0070] In this embodiment, a few-mode waveguide photoresist is poured on the surface of the waveguide lower cladding film, and the few-mode waveguide photoresist is spread on the surface of the waveguide lower cladding film to a preset thickness through a coating machine; then the silicon-based substrate on which the few-mode waveguide photoresist is spin-coated is placed in a photolithography machine, and the few-mode waveguide photoresist is masked and photolithographically processed according to the designed waveguide core layer graphic file. After completion, the obtained sample is placed in a developer at room temperature to remove the portion of the photoresist that is not exposed to ultraviolet light, and then developed; after development, hard baking is performed; then, a cladding material is poured on the prepared single-mode waveguide core layer and the few-mode waveguide core layer, and evenly applied through a coating machine to obtain a waveguide upper cladding that meets the thickness requirements, and then the waveguide upper cladding is soft-baked to solidify it, and after the soft baking is completed, the waveguide upper cladding is irradiated with ultraviolet light, and then hard baking is performed after completion.

[0071] Step 3: Etch the upper cladding of the waveguide to create a detection port that allows the external liquid to directly contact the few-mode waveguide core layer, and a window that allows the introduction of single-mode optical fiber and the input single-mode waveguide core layer, as well as the lead-out single-mode optical fiber and the output single-mode waveguide core layer to couple.

[0072] In this embodiment, at the sensing area of ​​the waveguide upper cladding, the waveguide upper cladding is removed by selective photolithography and etching technology and corrosion process to generate a detection port, thereby causing the few-mode waveguide core layer to be directly exposed to the external liquid environment; wherein,

[0073] The corrosion process is:

[0074] (1) Spin-coating a layer of photoresist with a thickness of 2–5 μm on the surface of the coated and cured waveguide upper cladding layer; then using a mask to expose the photoresist to ultraviolet light, wherein the length of the designed detection port is 10–12 μm and the depth is 2–5 μm; after development, the photoresist in the unexposed area is removed to form a mask pattern in the detection port area;

[0075] (2) using a reactive ion etching device to etch the exposed upper cladding layer of the waveguide, wherein a mixed gas of CF4 and O2 is selected as the etching gas to achieve highly selective etching of the optical material;]

[0076] In this embodiment, the etching parameters used are:

[0077] RF power: 100–200W;

[0078] Gas flow ratio: CF4:O2=3:1;

[0079] Chamber pressure: 50–80 mTorr;

[0080] Etch time: approximately 3–5 minutes to reach the target window depth (2–5 μm).

[0081] (3) wet etching is then used to further smooth the surface of the detection port;

[0082] In this embodiment, the etching liquid is buffered hydrofluoric acid (BHF, concentration 6:1); the etching time is 10-30 seconds, and the specific time is adjusted according to the actual etching rate of the material; the temperature is room temperature (about 22°C).

[0083] (4) The etched sample is placed in deionized water for cleaning to remove corrosion residues; then nitrogen is used to quickly dry the sample to avoid water stains on the surface;

[0084] Through the above-mentioned etching process, the detection port area can be accurately manufactured so that the few-mode waveguide core layer is directly exposed to the external liquid environment.

[0085] The etching process in this embodiment has the following advantages:

[0086] (1) High precision and high selectivity:

[0087] Through the combination of mask lithography and dry etching, the size and shape of the detection port can be accurately controlled to ensure the optimal connection between the detection port and the few-mode waveguide core layer. In addition, wet etching is used for post-processing, which can significantly reduce the surface roughness, improve the optical quality of the detection port area, and reduce optical scattering loss.

[0088] (2) Sensitivity optimization:

[0089] The etching process allows adjustable control of the depth, length and width of the detection port, thereby optimizing the contact area between the waveguide and the liquid and improving the sensitivity of the sensor.

[0090] (3) Repeatability and stability:

[0091] Standardized etching parameters and process steps are used to ensure the consistency of the detection port structure in mass production and to ensure the stability and reliability of the sensor.

[0092] The above is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited to the above content. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. An ocean salinity sensor based on on-chip integrated waveguide multimode interferometry, characterized in that: The invention comprises a silicon-based substrate, a waveguide lower cladding layer and a waveguide upper cladding layer arranged on the silicon-based substrate, and a single-mode waveguide core layer and a few-mode waveguide core layer arranged between the waveguide lower cladding layer and the waveguide upper cladding layer, wherein: The single-mode waveguide core layer is arranged on both sides of the few-mode waveguide core layer, wherein the single-mode waveguide core layer on one side is connected to the introduced single-mode optical fiber, and is the input single-mode waveguide core layer; the single-mode waveguide core layer on the other side is connected to the lead-out single-mode optical fiber, and is the output single-mode waveguide core layer; The waveguide upper cladding is provided with a detection port, and the detection port is used to enable external liquid to directly contact the surface of the few-mode waveguide core layer; The optical signal is input from the input single-mode optical fiber, coupled to the few-mode waveguide core layer through the input single-mode waveguide core layer, and after entering the few-mode waveguide core layer, the optical signal will excite multiple modes and generate inter-mode interference. After reaching the output single-mode waveguide core layer, the optical signal is output to the external spectrum analyzer through the output single-mode optical fiber; The change of salinity of external liquid will change the refractive index of the liquid. When the external seawater is in direct contact with the few-mode waveguide core layer through the detection port, the effective refractive index of the mode in the few-mode waveguide core layer will change. The change of the effective refractive index of different modes will cause the output interference spectrum to change. The drift of the output spectrum is measured by an external spectrum analyzer to demodulate the refractive index of seawater, and then the salinity of seawater is obtained through the correspondence between the refractive index of seawater and salinity.

2. The ocean salinity sensor based on on-chip integrated waveguide multimode interference according to claim 1, characterized in that: The detection port is formed on the waveguide upper cladding by an etching process.

3. The ocean salinity sensor based on on-chip integrated waveguide multimode interference according to claim 1, characterized in that: The trough wavelength of the output spectrum is measured by an external spectrum analyzer, and the refractive index of the external liquid is obtained through the functional relationship between the external refractive index and the interference trough wavelength. The difference between the refractive index of the external liquid and the refractive index of pure water is calculated, and the salinity value of seawater is obtained by calculating the ratio between the difference and the proportional coefficient.

4. The ocean salinity sensor based on on-chip integrated waveguide multimode interference according to claim 3, characterized in that: The functional relationship between the refractive index and the trough wavelength is obtained by linearly fitting the refractive index of the external liquid and the trough wavelength of the corresponding output spectrum.

5. A method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multimode interference, characterized in that: The following steps are involved: Step 1: Fabricate a waveguide lower cladding on a silicon substrate; Step 2: Use mask lithography technology to make a single-mode waveguide core layer and a few-mode waveguide core layer, and then coat the single-mode waveguide core layer and the few-mode waveguide core layer with a waveguide upper cladding layer; Step 3: Etch the upper cladding of the waveguide to create a detection port that allows the external liquid to directly contact the few-mode waveguide core layer, and a window that allows the introduction of single-mode optical fiber and the input single-mode waveguide core layer, as well as the lead-out single-mode optical fiber and the output single-mode waveguide core layer to couple.

6. The method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multimode interference according to claim 5, characterized in that: In step 1, a silicon wafer with a predetermined purity is selected as a substrate material, and then the silicon wafer is plasma cleaned to obtain a silicon-based substrate; the selected cladding material is poured onto the silicon-based substrate and evenly spread by a glue spreader to obtain a waveguide lower cladding that meets the thickness requirements; then the waveguide lower cladding is soft-baked to solidify it; after the soft baking is completed, the waveguide lower cladding is irradiated with ultraviolet rays, and after completion, the waveguide lower cladding is hard-baked.

7. The method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multimode interference according to claim 5, characterized in that: In step 2, pour the few-mode waveguide photoresist on the surface of the waveguide lower cladding film, and spread the few-mode waveguide photoresist on the surface of the waveguide lower cladding film to a preset thickness through a coating machine; then put the silicon-based substrate with the few-mode waveguide photoresist spun on it into a photolithography machine, and perform mask photolithography on the few-mode waveguide photoresist according to the designed waveguide core layer graphic file. After completion, put the obtained sample into a developer at room temperature to remove the part of the photoresist not exposed to ultraviolet light, and perform development; after the development is completed, hard baking is performed; then pour the cladding material on the prepared single-mode waveguide core layer and the few-mode waveguide core layer, and spread it evenly through a coating machine to obtain a waveguide upper cladding that meets the thickness requirements, and then soft bake the waveguide upper cladding to solidify it, and after the soft baking is completed, ultraviolet irradiation is performed on the waveguide upper cladding, and hard baking is performed after completion.

8. The method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multimode interference according to claim 5, characterized in that: In step 3, at the sensing area of ​​the waveguide upper cladding, the waveguide upper cladding is removed by selective photolithography and etching technology and corrosion process to generate a detection port, thereby causing the few-mode waveguide core layer to be directly exposed to the external liquid environment.

9. The method for manufacturing an ocean salinity sensor based on on-chip integrated waveguide multimode interference according to claim 8, characterized in that: The corrosion process is: (1) Spin-coating a layer of photoresist with a thickness of 2–5 μm on the surface of the coated and cured waveguide upper cladding layer; then using a mask to expose the photoresist to ultraviolet light, wherein the length of the designed detection port is 10–12 μm and the depth is 2–5 μm; after development, the photoresist in the unexposed area is removed to form a mask pattern in the detection port area; (2) etching the exposed upper cladding layer of the waveguide using a reactive ion etching device, wherein a mixed gas of CF4 and O2 is selected as the etching gas; (3) wet etching is then used to further smooth the surface of the detection port; (4) The etched sample is placed in deionized water for cleaning to remove corrosion residues; then the sample is dried using nitrogen to avoid water stains on the surface.

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