Refractive index sensor based on on-chip integrated rectangular optical waveguide and design method thereof

Abstract

The invention discloses a refractive index sensor based on an on-chip integrated rectangular optical waveguide and a design method thereof, and relates to the technical field of optical sensor design. The sensor comprises a cladding, a first core layer waveguide, a second core layer waveguide and a hollow cavity, wherein the first core layer waveguide and the second core layer waveguide are arranged in the cladding; the hollow cavity is arranged between the first core layer waveguide and the second core layer waveguide; the cladding comprises a cladding front end face and a cladding rear end face, the cladding front end face is connected with the first end of the first core layer waveguide, the second end of the first core layer waveguide is connected with the first end of the hollow cavity, the second end of the hollow cavity is connected with the first end of the second core layer waveguide, and the second end of the second core layer waveguide is connected with the second end of the second core layer waveguide. And the second end of the second core layer waveguide is connected with the rear end surface of the cladding. According to the sensor provided by the invention, the structure of the sensor can be effectively simplified, and meanwhile, the refractive index measurement sensitivity and measurement precision of the sensor can be effectively improved in the optical refractive index measurement process.

Description

Technical Field

[0001] The present invention relates to the technical field of optical sensor design, and in particular to a refractive index sensor based on an on-chip integrated rectangular optical waveguide and a design method thereof. Background Art

[0002] Optical sensing technology is a technology that uses the properties of light to detect and measure physical quantities. At present, optical sensing technology has been applied to many application fields such as environmental monitoring, biomedical treatment, diagnosis, and biochemical sensing analysis of materials. The important advantages of optical sensing technology are that the sensor is small in size, light in weight, easy to integrate, high in precision, high in resolution, and strong in resistance to electromagnetic interference. It can solve measurement problems that conventional electrical sensors are not fully capable of.

[0003] Refractive index is an important property of optical materials. Refractive index measurement is widely used in various fields, such as biomedicine, chemical analysis, environmental monitoring, etc. Accurately measuring the refractive index of liquids is of vital importance. For example, in the biomedical field, changes in the refractive index of biological fluids (such as blood, urine, etc.) can reflect the physiological state and pathological changes in the body; in chemical analysis, the refractive index of the solution can provide information about the concentration and chemical composition of the solute; in environmental monitoring, changes in the refractive index of water bodies can help detect the content of pollutants in the water.

[0004] Existing refractive index sensor technologies mainly include those based on surface plasmon resonance (SPR), fiber interferometer, Mach-Zehnder interferometer, etc. However, these sensors have certain limitations, such as: although the SPR-based sensor has high sensitivity, it is sensitive to environmental factors, easily affected by temperature, electromagnetic fields, etc., and the preparation process is complex; in practical applications, fiber interferometer-type sensors have problems such as easy damage to the optical fiber and complex signal processing; Mach-Zehnder interferometer sensors are often large in size, which is not conducive to integration and miniaturization.

[0005] Fabry-Perot cavity (FP cavity) sensors are able to provide high-precision measurements due to their principle of light interference. However, the traditional FP cavity refractive index sensor is difficult to meet the demand for accurate measurement of small refractive index changes in terms of sensitivity improvement. At the same time, when measuring the refractive index of the liquid, the interaction between the FP cavity and the liquid is not sufficient, which affects the measurement accuracy and reliability. In addition, in order to improve the measurement sensitivity, the traditional FP cavity sensor adopts a complex structural design, which not only increases the manufacturing difficulty and cost, but also introduces more error sources, resulting in the problem of low refractive index measurement sensitivity and measurement accuracy of the sensor. Summary of the invention

[0006] In order to overcome the problems of complex sensor structure, high manufacturing difficulty and cost in the existing optical sensor technology, and low measurement sensitivity and measurement accuracy in the process of optical refractive index measurement, the present invention proposes a refractive index sensor based on on-chip integrated rectangular optical waveguide and a design method thereof, which can effectively simplify the structure of the sensor and effectively improve the refractive index measurement sensitivity and measurement accuracy of the sensor in the process of optical refractive index measurement.

[0007] To achieve the purpose of the present invention, the present invention adopts the following technical solutions:

[0008] A refractive index sensor based on an on-chip integrated rectangular optical waveguide, the sensor comprising: a cladding, a first core waveguide arranged in the cladding, a second core waveguide and a hollow cavity arranged between the first core waveguide and the second core waveguide; the cladding comprises a cladding front end face and a cladding rear end face, the cladding front end face is connected to the first end of the first core waveguide, the second end of the first core waveguide is connected to the first end of the hollow cavity, the second end of the hollow cavity is connected to the first end of the second core waveguide, and the second end of the second core waveguide is connected to the cladding rear end face.

[0009] In the above technical solution, the light beam enters the first core waveguide from the front end face of the cladding, and then enters the refractive index sensor, and under the restriction of the cladding, the light beam propagates from the first core waveguide to the hollow cavity, and the hollow cavity is filled with experimental liquid. The refractive index in the cavity is changed by the experimental liquid in the hollow cavity, and then the optical path is changed, so that the light beam enters the second core waveguide to realize the function of the sensor based on optical interference. Through the above-mentioned structural design, various optical elements can be integrated on a small chip or module, thereby effectively simplifying the structure of the sensor to reduce the manufacturing difficulty and cost of the sensor. At the same time, the contact surfaces of the first core waveguide, the second core waveguide and the hollow cavity are designed to be smooth surfaces, which can effectively improve the refractive index measurement sensitivity and measurement accuracy of the sensor during the optical refractive index measurement.

[0010] Preferably, it also includes a first reflection surface arranged between the first core waveguide and the hollow cavity, a second reflection surface arranged between the hollow cavity and the second core waveguide, and a third reflection surface arranged between the second core waveguide and the rear end face of the cladding.

[0011] Preferably, the first core waveguide includes a first core waveguide front end face and a first core waveguide rear end face, and the second core waveguide includes a second core waveguide front end face and a second core waveguide rear end face; the first core waveguide front end face is connected to the cladding front end face, and the first core waveguide rear end face is connected to the first reflection surface; the second core waveguide front end face is connected to the second reflection surface, and the second core waveguide rear end face is connected to the third reflection surface.

[0012] In the above technical solution, after the light beam passes through the first core layer waveguide, the first reflection surface reflects the incident light beam for the first time. The reflected light beam is incident on the hollow cavity filled with experimental liquid for refracting to change the optical path. Then the light beam enters the second reflection surface for light reflection, and the reflected light beam is transmitted to the second core layer waveguide, and finally transmitted to the third reflection surface for total reflection of the light beam, thereby realizing the function of the sensor based on optical interference, effectively improving the refractive index measurement sensitivity and measurement accuracy of the sensor.

[0013] Preferably, an opening is provided on the upper surface of the hollow cavity through the upper surface of the cladding, and the shape of the hollow cavity is rectangular.

[0014] Preferably, the material of the cladding layer is a low refractive index material.

[0015] Preferably, the materials of the first core waveguide and the second core waveguide are both high refractive index materials.

[0016] Preferably, the reflectivity of the first reflective surface is 30%, the reflectivity of the second reflective surface is 40%, and the third reflective surface is a total reflective surface.

[0017] Preferably, the first reflecting surface, the second reflecting surface and the third reflecting surface are all coated with a metal film.

[0018] In the above technical solution, the upper surface of the hollow cavity is hollowed out, so that the external experimental liquid can be injected into the hollow cavity and the liquid can be replaced, thereby improving the performance of the hollow cavity. At the same time, the sensor can directly contact the liquid sample and fully interact with the liquid, so that the present invention has good adaptability in liquid refractive index measurement and can accurately measure the refractive index of different types of liquids, whether biological liquids, chemical solutions or environmental water samples; the cladding is made of low refractive index material, and the first core layer waveguide and the second core layer waveguide are made of high refractive index material, which can effectively improve the refractive index measurement sensitivity and measurement accuracy of the sensor during the optical refractive index measurement; the reflectivity of the first reflection surface is 30%, the reflectivity of the second reflection surface is 40%, and the third reflection surface is a total reflection surface, which can better reflect the light beam during the optical refractive index measurement, and the reflection effect can achieve the expected effect; the first reflection surface, the second reflection surface and the third reflection surface are plated with a metal film, which can better protect the reflection surface, so as to reduce the manufacturing cost of the sensor and provide a better reflection effect.

[0019] A design method for a refractive index sensor based on an on-chip integrated rectangular optical waveguide, the method being used for designing the reflectivity of the first reflection surface, the second reflection surface and the third reflection surface based on the on-chip integrated rectangular optical waveguide refractive index sensor, the process comprising:

[0020] For the first reflection surface and the second reflection surface, let the refractive index of the incident medium be n0, the refractive index of the metal film be n, the thickness be h, the wavelength of the light be λ, and the incident light angle be θ;

[0021] Reflection coefficient for vertical polarization:

[0022]

[0023] Vertical polarization reflectivity:

[0024] R s =|r s | 2 ;

[0025] Reflection coefficient for horizontal polarization:

[0026]

[0027] Horizontal polarization reflectivity:

[0028] R p =|r p | 2 ;

[0029] Where θ' is the refraction angle, and n0 sinθ=n sinθ', when the incident light is natural light, the total reflectivity is:

[0030]

[0031] According to the thin film interference principle, the optical path difference of light reflected on the upper and lower surfaces of the metal film is Δ=2nh. By controlling the metal film material, incident light parameters, metal film thickness h, and the relationship between the optical path difference and the wavelength, the reflectivity of the first reflection surface is set to 30%, and the reflectivity of the second reflection surface is set to 40%;

[0032] The third reflection surface is designed as a total reflection surface for total light reflection by using a metal material with high reflectivity.

[0033] Furthermore, an opening is provided on the upper surface of the hollow cavity through the upper surface of the cladding layer to contact the external air, and an experimental liquid is injected into the hollow cavity through the opening to change the refractive index of the hollow cavity;

[0034] After the experimental liquid is injected into the hollow cavity, the first core waveguide, the second core waveguide and the hollow cavity are resonantly coupled using the vernier effect method to capture the refractive index change characteristics generated by the spectrum.

[0035] In the above technical solution, the reflectivity of the first reflection surface is 30%, the reflectivity of the second reflection surface is 40%, and the third reflection surface is a total reflection surface, which can better reflect the light beam during the optical refractive index measurement process, and the reflection effect can achieve the expected effect, effectively improving the refractive index measurement sensitivity and measurement accuracy of the sensor; the vernier effect method is adopted to couple the first core layer waveguide, the second core layer waveguide and the hollow cavity to the resonant cavity to capture the refractive index change characteristics generated by the spectrum, and can make a significant response to tiny refractive index changes, so that when detecting the refractive index change, it can produce more obvious spectral changes, thereby achieving higher measurement sensitivity.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The present invention proposes a refractive index sensor based on an on-chip integrated rectangular optical waveguide and a design method thereof, wherein a light beam enters a first core waveguide from a front end surface of a cladding, and thus enters the refractive index sensor, and under the restriction of the cladding, the light beam propagates from the first core waveguide to a hollow cavity, wherein an experimental liquid is filled in the hollow cavity, and the refractive index in the cavity is changed by the experimental liquid in the hollow cavity, thereby changing the optical path, so that the light beam enters a second core waveguide, so as to realize the function of the sensor based on optical interference, and through the aforementioned structural design, various optical elements can be integrated on a small chip or module, thereby effectively simplifying the structure of the sensor, so as to reduce the manufacturing difficulty and cost of the sensor, and at the same time, the contact surfaces of the first core waveguide, the second core waveguide and the hollow cavity are designed to be smooth surfaces, so as to effectively improve the refractive index measurement sensitivity and measurement accuracy of the sensor during the optical refractive index measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of the structure of a refractive index sensor based on an on-chip integrated rectangular optical waveguide provided in an embodiment of the present application;

[0039] Figure 2 A schematic diagram of coating a metal film on the end face of a waveguide provided in an embodiment of the present application;

[0040] In the figure: 1-cladding, 11-cladding front end face, 12-cladding rear end face; 2-core waveguide, 21-core waveguide front end face, 22-core waveguide rear end face; 3-hollow cavity, 31-hollow cavity front end face, 32-hollow cavity rear end face; 4-core waveguide, 41-core waveguide front end face; 42-core waveguide rear end face; 5-first reflection surface, 6-second reflection surface, 7-third reflection surface; 8-arbitrary waveguide; 9-arbitrary reflection surface. DETAILED DESCRIPTION

[0041] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. Preferred embodiments of the present invention are provided in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0042] In the description of this application, it should be understood that the terms "first", "second", "third", etc. are only used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence, nor can they be understood as indicating or implying relative importance. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0043] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only.

[0044] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0045] Example:

[0046] This embodiment provides a refractive index sensor based on an on-chip integrated rectangular optical waveguide, see Figure 1The sensor includes: a cladding 1, a first core waveguide 2 arranged in the cladding 1, a second core waveguide 4 and a hollow cavity 3 arranged between the first core waveguide 2 and the second core waveguide 4. The cladding 1 includes a cladding front end face 11 and a cladding rear end face 12, the cladding front end face 11 is connected to the first end of the first core waveguide 2, the second end of the first core waveguide 2 is connected to the first end of the hollow cavity 3, the second end of the hollow cavity 3 is connected to the first end of the second core waveguide 4, and the second end of the second core waveguide 4 is connected to the cladding rear end face 12. The light beam enters the first core waveguide 2 from the front end face 11 of the cladding, and then enters the refractive index sensor. Under the restriction of the cladding 1, the light beam propagates from the first core waveguide 2 to the hollow cavity 3. The hollow cavity 3 is filled with experimental liquid. The refractive index in the cavity is changed by the experimental liquid in the hollow cavity 3, and then the optical path is changed, so that the light beam enters the second core waveguide 4 to realize the function of the sensor based on optical interference. Through the aforementioned structural design, various optical elements can be integrated on a small chip or module, thereby effectively simplifying the structure of the sensor to reduce the manufacturing difficulty and cost of the sensor. At the same time, the contact surfaces of the first core waveguide 2, the second core waveguide 4 and the hollow cavity 3 are designed to be smooth surfaces, which can effectively improve the refractive index measurement sensitivity and measurement accuracy of the sensor during the optical refractive index measurement.

[0047] As a preferred embodiment, see Figure 1 The sensor also includes a first reflection surface 5 arranged between the first core waveguide 2 and the hollow cavity 3, a second reflection surface 6 arranged between the hollow cavity 3 and the second core waveguide 4, and a third reflection surface 7 arranged between the second core waveguide 4 and the rear end face 12 of the cladding.

[0048] As a preferred embodiment, see Figure 1 , the first core waveguide 2 includes a first core waveguide front end face 21) and a first core waveguide rear end face 22, and the second core waveguide 4 includes a second core waveguide front end face 41 and a second core waveguide rear end face 42; the first core waveguide front end face 21 is connected to the cladding front end face 11, and the first core waveguide rear end face 22 is connected to the first reflection surface 5; the second core waveguide front end face 41 is connected to the second reflection surface 6, and the second core waveguide rear end face 42 is connected to the third reflection surface 7. After the light beam passes through the first core waveguide 2, the first reflection surface 5 reflects the incident light beam for the first time, and the reflected light beam is incident into the hollow cavity 3 filled with experimental liquid for refraction to change the optical path, and then the light beam enters the second reflection surface 6 for light reflection, and the reflected light beam is transmitted to the second core waveguide 4, and finally transmitted to the third reflection surface 7 for total reflection of the light beam, thereby realizing the function of the sensor based on optical interference, and effectively improving the refractive index measurement sensitivity and measurement accuracy of the sensor.

[0049] As a preferred embodiment, see Figure 1 The upper surface of the hollow cavity 3 is in contact with the external air through an opening on the upper surface of the cladding 1, and the shape of the hollow cavity 3 is rectangular. The upper surface of the hollow cavity 3 is hollowed out, so that the external experimental liquid can be injected into the hollow cavity 3, and the liquid can be replaced, thereby improving the performance of the hollow cavity. At the same time, the sensor can directly contact the liquid sample and fully interact with the liquid, so that the present invention has good adaptability in the measurement of liquid refractive index, and can accurately measure the refractive index of different types of liquids, whether biological liquids, chemical solutions or environmental water samples.

[0050] As a preferred embodiment, see Figure 1 , the material of the cladding 1 is a low refractive index material.

[0051] As a preferred embodiment, see Figure 1 The materials of the first core waveguide 2 and the second core waveguide 4 are both high refractive index materials. The cladding 1 is made of low refractive index material, and the first core waveguide 2 and the second core waveguide 4 are made of high refractive index material, which can effectively improve the refractive index measurement sensitivity and measurement accuracy of the sensor during the optical refractive index measurement.

[0052] As a preferred embodiment, see Figure 1 The reflectivity of the first reflection surface 5 is 30%, the reflectivity of the second reflection surface 6 is 40%, and the third reflection surface 7 is a total reflection surface.

[0053] As a preferred embodiment, see Figure 1 The first reflection surface 5, the second reflection surface 6 and the third reflection surface 7 are all coated with metal films. The reflectivity of the first reflection surface 5 is 30%, the reflectivity of the second reflection surface 6 is 40%, and the third reflection surface 7 is a total reflection surface, which can better reflect the light beam during the optical refractive index measurement process, and the reflection effect can achieve the expected effect. The metal films are coated on the first reflection surface 5, the second reflection surface 6 and the third reflection surface 7, which can better protect the reflection surface, so as to reduce the manufacturing cost of the sensor and provide a better reflection effect.

[0054] This embodiment also provides a design method for a refractive index sensor based on an on-chip integrated rectangular optical waveguide. The method is used to design a refractive index sensor based on an on-chip integrated rectangular optical waveguide. The reflectivity of the first reflection surface 5, the second reflection surface 6 and the third reflection surface 7 is designed. The process includes:

[0055] See also Figure 2, according to the arbitrary core waveguide 8 and the arbitrary reflection surface 9, the first reflection surface 5 and the second reflection surface 6 are designed: the refractive index of the incident medium is n0, the refractive index of the metal film is n, the thickness is h, the wavelength of the light is λ, and the angle of the incident light is θ;

[0056] Reflection coefficient for vertical polarization:

[0057]

[0058] Vertical polarization reflectivity:

[0059] R s =|r s | 2 ;

[0060] Reflection coefficient for horizontal polarization:

[0061]

[0062] Horizontal polarization reflectivity:

[0063] R p =|r p | 2 ;

[0064] Where θ' is the refraction angle, and n0 sinθ = n sinθ', when the incident light is natural light, the total reflectivity is:

[0065]

[0066] According to the thin film interference principle, the optical path difference of light reflected on the upper and lower surfaces of the metal film is Δ=2nh. By controlling the metal film material, incident light parameters, metal film thickness h, and the relationship between the optical path difference and the wavelength, the reflectivity of the first reflection surface 5 is set to 30%, and the reflectivity of the second reflection surface 6 is set to 40%;

[0067] The third reflection surface 7 is designed to be a total reflection surface for total reflection of light by using a metal material with high reflectivity.

[0068] Furthermore, an opening is provided on the upper surface of the hollow cavity 3 to contact with the outside air through the upper surface of the cladding 1, and the experimental liquid is injected into the hollow cavity 3 through the opening to change the refractive index of the hollow cavity 3;

[0069] After the experimental liquid is injected into the hollow cavity 3, the first core waveguide 2, the second core waveguide 4 and the hollow cavity 3 are resonantly coupled using the Vernier effect method to capture the refractive index variation characteristics generated by the spectrum.

[0070] It can be understood that setting the reflectivity of the first reflection surface 5 to 30%, the reflectivity of the second reflection surface 6 to 40%, and the third reflection surface 7 to a total reflection surface can better reflect the light beam during the optical refractive index measurement, and the reflection effect can achieve the expected effect, effectively improving the refractive index measurement sensitivity and measurement accuracy of the sensor; using the vernier effect method, the first core layer waveguide 2, the second core layer waveguide 4 and the hollow cavity 3 are resonant cavity coupled to capture the refractive index change characteristics generated by the spectrum, and can make a significant response to tiny refractive index changes, so that when detecting the refractive index change, it can produce more obvious spectral changes, thereby achieving higher measurement sensitivity.

[0071] Specifically, the present invention relates to the principle of Fabry-Perot cavity, which in turn relates to the principle of interference of light, that is, when two or more light waves meet in space, they are superimposed on each other, strengthened in some areas (constructive interference), and weakened in some areas (destructive interference). The interference condition depends on the optical path difference, which is the difference in the lengths of the propagation paths of the two beams of light. When the optical path difference is an integer multiple of the wavelength (ΔL=mλ, m=0,1,2,...), constructive interference occurs and the light intensity is enhanced; when the optical path difference is an odd multiple of half the wavelength, the light intensity is enhanced. , destructive interference occurs and the light intensity is weakened; and the Fabry-Perot cavity is composed of two parallel high-reflectivity reflection surfaces (in the present invention, that is, between the first reflection surface 5 and the second reflection surface 6, and between the second reflection surface 6 and the third reflection surface 7). The light is reflected multiple times between the two reflection surfaces. When a beam of light enters the FP cavity, it will reflect back and forth between the two reflectors. Each reflected light will be superimposed with the subsequent reflected light. If the spacing between the two reflection surfaces of the Fabry-Perot cavity is d, and the refractive index of the medium in the cavity is n, the optical path difference of the light in the cavity for one round trip is ΔL=2nd.

[0072] Specifically, the present invention also relates to the principle of vernier effect, that is, there are two Fabry-Perot cavities, whose free spectral ranges FSR are FSR1 and FSR2 respectively, and FSR1≈FSR2. When the two FP cavities are coupled with each other, their spectra are superimposed on each other, and the coupled spectrum ΔFSR=|FSR1-FSR2|, which is like the main scale and secondary scale scales on a vernier caliper. The main scale scale interval is slightly larger, and the secondary scale scale interval is slightly smaller. When the two are coordinated, the length can be read more accurately.

[0073] In the present invention, one is a FP cavity formed by a hollow cavity 3 (between the first reflection surface 5 and the second reflection surface 6), and the other is an equivalent FP cavity formed by a second core waveguide 4 (between the second reflection surface 6 and the third reflection surface 7). When external liquid is injected into the hollow cavity 3 to change its refractive index, it will cause the optical path difference to change, thereby causing the resonance peak of the FP cavity to move. Since the two coupled FP cavities produce a vernier effect, the movement of the resonance peak is amplified, and by detecting the movement of the resonance peak in the transmission spectrum or reflection spectrum, the change in the refractive index of the liquid can be measured more accurately.

[0074] In this embodiment, the light beam enters the sensor from the core waveguide 2 and propagates under the confinement of the cladding 1;

[0075] When the light beam enters the FP cavity formed by the hollow cavity 3, since the cavity has no upper cladding, the injected liquid can change the refractive index in the cavity, thereby changing the optical path;

[0076] The light beam is reflected multiple times between the reflection surfaces formed by the first reflection surface 5, the second reflection surface 6 and the third reflection surface 7. Since the reflectivity of the first reflection surface 5 is 30%, the reflectivity of the second reflection surface 6 is 40%, and the third reflection surface 7 is a total reflection surface, the reflection surface is used for the reflection of light in the FP cavity to realize the function of the sensor based on light interference. The above description is only an embodiment of the present invention, and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the present invention specification and drawings, or directly or indirectly used in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A refractive index sensor based on an on-chip integrated rectangular optical waveguide, characterized in that: The sensor comprises: a cladding (1), a first core waveguide (2) arranged in the cladding (1), a second core waveguide (4), and a hollow cavity (3) arranged between the first core waveguide (2) and the second core waveguide (4); the cladding (1) comprises a cladding front end face (11) and a cladding rear end face (12); the cladding front end face (11) is connected to the first end of the first core waveguide (2); the second end of the first core waveguide (2) is connected to the first end of the hollow cavity (3); the second end of the hollow cavity (3) is connected to the first end of the second core waveguide (4); and the second end of the second core waveguide (4) is connected to the cladding rear end face (12).

2. The refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 1, characterized in that: It also includes a first reflection surface (5) arranged between the first core waveguide (2) and the hollow cavity (3), a second reflection surface (6) arranged between the hollow cavity (3) and the second core waveguide (4), and a third reflection surface (7) arranged between the second core waveguide (4) and the rear end face (12) of the cladding.

3. The refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 2, characterized in that: The first core waveguide (2) comprises a first core waveguide front end face (21) and a first core waveguide rear end face (22), and the second core waveguide (4) comprises a second core waveguide front end face (41) and a second core waveguide rear end face (42); the first core waveguide front end face (21) is connected to the cladding front end face (11), and the first core waveguide rear end face (22) is connected to the first reflection surface (5); the second core waveguide front end face (41) is connected to the second reflection surface (6), and the second core waveguide rear end face (42) is connected to the third reflection surface (7).

4. The refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 1, characterized in that: An opening is provided on the upper surface of the hollow cavity (3) through the upper surface of the cladding (1), and the shape of the hollow cavity (3) is rectangular.

5. The refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 1, characterized in that: The material of the cladding (1) is a low refractive index material.

6. The refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 1, characterized in that: The materials of the first core layer waveguide (2) and the second core layer waveguide (4) are both high refractive index materials.

7. The refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 2, characterized in that: The reflectivity of the first reflection surface (5) is 30%, the reflectivity of the second reflection surface (6) is 40%, and the third reflection surface (7) is a total reflection surface.

8. The refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 2, characterized in that: The first reflecting surface (5), the second reflecting surface (6) and the third reflecting surface (7) are all plated with a metal film.

9. A method for designing a refractive index sensor based on an on-chip integrated rectangular optical waveguide, the method being used for the refractive index sensor according to any one of claims 1 to 7, characterized in that: The reflectivity of the first reflecting surface (5), the second reflecting surface (6) and the third reflecting surface (7) is designed, and the process includes: For the first reflection surface (5) and the second reflection surface (6), the refractive index of the incident medium is n0, the refractive index of the metal film is n, the thickness is h, the wavelength of the light is λ, and the angle of the incident light is θ; Reflection coefficient for vertical polarization: Vertical polarization reflectivity: R s ＝|r s | 2 ； Reflection coefficient for horizontal polarization: Horizontal polarization reflectivity: R p ＝|r p | 2 ； Where θ' is the refraction angle, and n0 sinθ=n sinθ', when the incident light is natural light, the total reflectivity is: According to the thin film interference principle, the optical path difference Δ=2nh of light reflected from the upper and lower surfaces of the metal film is set, by controlling the metal film material, incident light parameters, metal film thickness h, and the relationship between the optical path difference and the wavelength, the reflectivity of the first reflection surface (5) is set to 30%, and the reflectivity of the second reflection surface (6) is set to 40%; The third reflection surface (7) is designed to be a total reflection surface for total light reflection by using a metal material with high reflectivity.

10. The design method of the refractive index sensor based on on-chip integrated rectangular optical waveguide according to claim 9, characterized in that: The upper surface of the hollow cavity (3) is exposed to the outside air through an opening provided on the upper surface of the cladding (1), and the experimental liquid is injected into the hollow cavity (3) through the opening to change the refractive index of the hollow cavity (3); After the experimental liquid is injected into the hollow cavity (3), the first core waveguide (2), the second core waveguide (4) and the hollow cavity (3) are resonantly coupled using the vernier effect method to capture the refractive index change characteristics generated by the spectrum.

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