Refractive index sensor of cascaded polymer waveguide MMI coupler and preparation method

Abstract

The invention discloses a refractive index sensor of a cascaded polymer waveguide MM I coupler and a preparation method of the refractive index sensor. The refractive index sensor comprises a silicon wafer substrate, an upper cladding and a lower cladding which are arranged on the silicon wafer substrate, and an optical waveguide structure which is arranged between the upper cladding and the lower cladding and is based on a Mach-Zehnder interference structure and a multimode interference coupler, wherein the upper cladding is provided with a detection port; and a waveguide core layer in the optical waveguide structure is positioned in the detection port, so that a hole digging structure is formed. The refractive index sensor disclosed by the invention is higher in integration level, sensitivity and linearity, wider in working bandwidth and better in adaptability, and can meet diversified detection requirements.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer optical waveguide photonic integrated devices, and in particular relates to a refractive index sensor of a cascaded polymer waveguide MMI coupler and a preparation method thereof. Background Art

[0002] With the development of photonic technology, refractive index sensors have found widespread application in fields such as biomedicine, environmental monitoring, and chemical analysis. Currently, traditional refractive index sensors suffer from complex optical structures, difficult integration, and limited sensitivity. Therefore, optimizing the sensing structure, improving sensor sensitivity, and achieving integration are key areas of current refractive index sensor research.

[0003] Mach-Zehnder Interferometer (MZI) refractive index sensors, leveraging the interference effect of light, are able to respond sensitively to minute changes in refractive index. They exhibit significant advantages in terms of simple structure, high integration, sensitivity, and stability, and have garnered widespread attention in fields such as biomedicine and environmental monitoring. Multimode interferometer (MMI) couplers, as the core component of MZI refractive index sensors, utilize the multimode interference effect of light waves to achieve efficient beam splitting and combining within a compact space. This effectively reduces device size, improves optical power utilization, and enhances sensor stability and integration, giving MZI refractive index sensors unique advantages in on-chip integration and high-sensitivity applications.

[0004] However, although the existing MZI refractive index sensors have made certain progress in sensing performance, there is still room for further optimization in terms of sensitivity, response speed and working bandwidth. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, the present invention provides a refractive index sensor of a cascaded polymer waveguide MMI coupler. The refractive index sensor has higher integration, sensitivity and linearity, wider working bandwidth and better adaptability, and can meet diverse detection needs.

[0006] The second object of the present invention is to provide a method for preparing a refractive index sensor of a cascaded polymer waveguide MMI coupler.

[0007] The technical solution of the present invention to solve the above technical problems is:

[0008] A refractive index sensor using a cascaded polymer waveguide (MMI) coupler comprises a silicon wafer substrate, an upper cladding layer and a lower cladding layer arranged on the silicon wafer substrate, and an optical waveguide structure based on a Mach-Zehnder interference structure and a multimode interference coupler and arranged between the upper and lower cladding layers; wherein a detection port is provided on the upper cladding layer; and a waveguide core layer in the optical waveguide structure is located within the detection port, thereby forming a hole structure.

[0009] Preferably, the optical waveguide structure includes an input end portion, a sensing arm portion and an output end portion, wherein:

[0010] The input end portion includes an input single-mode waveguide, a first MMI input tapered waveguide, a first MMI multimode waveguide, a first MMI output tapered sensing arm connecting waveguide, and a first MMI output tapered reference arm connecting waveguide, wherein the output end of the input single-mode waveguide is connected to the input end of the first MMI input tapered waveguide; the output end of the first MMI input tapered waveguide is connected to the input end of the first MMI multimode waveguide; and the output end of the first MMI multimode waveguide is respectively connected to the first MMI output tapered sensing arm connecting waveguide and the first MMI output tapered reference arm connecting waveguide located on both sides thereof;

[0011] The sensing arm portion includes a first curved waveguide, a second curved waveguide, an MZI reference arm, a first section of the MZI sensing arm, a second section of the MZI sensing arm, a third curved waveguide, and a fourth curved waveguide, wherein the input end of the first curved waveguide is connected to the output end of the first MMI output tapered reference arm connecting waveguide, and the output end of the first curved waveguide is connected to the input end of the MZI reference arm; the output end of the MZI reference arm is connected to the input end of the third curved waveguide; the input end of the second curved waveguide is connected to the input end of the first MMI output tapered sensing arm connecting waveguide, and the output end of the second curved waveguide is connected to the first section of the MZI sensing arm; the output end of the second section of the MZI sensing arm is connected to the input end of the fourth curved waveguide; a gap is provided between the output end of the first section of the sensing arm and the input end of the second section of the sensing arm, thereby forming the excavated structure;

[0012] The output end portion includes a second MMI input sensing arm tapered connecting waveguide, a second MMI input reference arm tapered connecting waveguide, a second MMI multimode waveguide, a second MMI output tapered waveguide and an output few-mode waveguide, wherein the input end of the second MMI input sensing arm tapered connecting waveguide is connected to the output end of the third curved waveguide, and the output end of the second MMI input sensing arm tapered connecting waveguide is connected to the second MMI multimode waveguide; the input end of the second MMI input reference arm tapered connecting waveguide is connected to the output end of the fourth curved waveguide, and the output end of the second MMI input reference arm tapered connecting waveguide is connected to the second MMI multimode waveguide; the output end of the second MMI multimode waveguide is connected to the input end of the second MMI output tapered waveguide, and the output end of the second MMI output tapered waveguide is connected to the output few-mode waveguide.

[0013] Preferably, the detection port is located between the output end of the second curved waveguide and the input end of the fourth curved waveguide; the first section of the sensing arm and the second section of the sensing arm are entirely located within the detection port; the length of the detection port is equal to the distance between the output end of the second curved waveguide and the input end of the fourth curved waveguide, and the width is 40 μm, and an extension groove extending toward the edge of the upper cladding is provided in the detection port; the length of the extension groove is less than the length of the detection port.

[0014] Preferably, the gap between the first section of the sensor arm and the second section of the sensor arm is 10 μm-100 μm.

[0015] Preferably, the waveguide material of the optical waveguide structure is EpoCore, with a refractive index of 1.569; the thickness of the core waveguide in the optical waveguide structure is equal and is 4-7 μm; the waveguide material of the upper cladding and the lower cladding is Epoclad, with a refractive index of 1.559; the thickness of the upper cladding and the lower cladding is 10-30 μm; the width of the input single-mode waveguide is 4-7 μm; the width of the input end of the first MMI input tapered waveguide is the same as the width of the input single-mode waveguide, and the first The width of the output end of the MMI input tapered waveguide is 15 μm; the width of the first MMI multimode waveguide is 30-50 μm; the width of the second MMI multimode waveguide is 20-40 μm; the width of the input end of the second MMI output tapered waveguide is 20-30 μm, and the width of the output end of the second MMI output tapered waveguide is the same as the width of the output few-mode waveguide, both of which are 9 μm; in addition, the width of the other core layer waveguides in the optical waveguide structure is the same as the width of the input single-mode waveguide.

[0016] A method for preparing a refractive index sensor of a cascaded polymer waveguide MMI coupler comprises the following steps:

[0017] Step S1: preparing a silicon wafer substrate;

[0018] Step S2: preparing a lower cladding layer on a silicon wafer substrate;

[0019] Step S3: fabricating an optical waveguide structure on the prepared lower cladding layer;

[0020] Step S4: After the optical waveguide structure is prepared, an upper cladding layer is prepared on the lower cladding layer, and a detection port and a hole structure are processed on the upper cladding layer.

[0021] Preferably, in step S1, the silicon wafer substrate is prepared as follows:

[0022] The silicon wafer is split into substrates that meet the size requirements using a diamond knife; then the substrate is cleaned twice using a plasma machine, each cleaning time is 80 seconds, to remove impurities on the surface of the silicon wafer substrate.

[0023] Preferably, in step S2, the process of preparing the lower cladding layer on the silicon wafer substrate is:

[0024] Step S201: Spin-coating the EpoClad material on the pre-treated silicon wafer substrate using a spin coater at a rotation speed of 3000 rpm to obtain sample 1;

[0025] Step S202: After the spin coating is completed, the sample is transferred to a heating table for pre-baking at a temperature of 95-110° C. for 10 minutes; after the pre-baking is completed, an annealing treatment is performed for 5 minutes;

[0026] Step S203: After the pre-baking is completed, the sample 1 is subjected to UV exposure curing using a photolithography machine, and the UV exposure curing time is 30-60 seconds;

[0027] Step S204: Transfer the sample 1 after exposure to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; and perform annealing after baking for 30 minutes.

[0028] Preferably, in step S3, the process of preparing the optical waveguide structure on the lower cladding layer is:

[0029] Step S301: using a plasma machine to clean sample 1 twice, each cleaning time is 80 seconds, to remove impurities on the surface of sample 1;

[0030] Step S302: After cleaning, a core waveguide material EpoCore with a thickness of 5 μm is spin-coated on the lower cladding layer of sample 1 using a spin coater at a speed of 4000 rpm to obtain sample 2;

[0031] Step S303: After the spin coating is completed, the sample 2 is transferred to a heating table for pre-baking at a temperature of 95-110° C. for 10 minutes; after the pre-baking is completed, an annealing treatment is performed for 5 minutes;

[0032] Step S304: After the pre-bake is completed, the core waveguide of the optical waveguide structure is subjected to a photolithography process using a mask and an ultraviolet lithography machine;

[0033] Step S305: Transfer sample 2 to a heating table and bake at 95°C for 10 minutes. After baking, perform annealing for 5 minutes.

[0034] Step S306: developing the surface film of the core waveguide of the optical waveguide structure, and wet-etching the core waveguide pattern after photolithography using a developer corresponding to the core waveguide material EpoCore, with the wet etching time being 15-30s;

[0035] Step S307: Rinse the developer in the second sample with isopropyl alcohol, and then dry the second sample with a nitrogen gun;

[0036] Step S308: Transfer sample 2 to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; after baking, perform annealing for 30 minutes.

[0037] Preferably, in step S4, the steps of preparing an upper cladding layer on the lower cladding layer and processing a detection port and a hole structure on the upper cladding layer are:

[0038] Step S401: Spin-coating EpoClad material on Sample 2 using a spin coater at a speed of 3000 rpm to obtain Sample 3;

[0039] Step S402: After the spin coating is completed, the sample 2 is transferred to the heating table for pre-baking at a temperature of 95-110°C for 10 minutes; after the pre-baking is completed, annealing is performed for 5 minutes;

[0040] Step S403: Perform UV photolithography on the detection port in the upper cladding layer using the designed mask, with the UV exposure time being 5-20 seconds;

[0041] Step S404: Transfer sample 3 to a heating platform for baking at a temperature of 95-110° C. for 10 minutes, and then perform annealing for 5 minutes after baking.

[0042] Step S405: Developing the baked sample 3, wet-etching the core waveguide pattern after photolithography using a developer corresponding to the EpoClad material, with the wet etching time being 15-30 seconds;

[0043] Step S406: using an isopropyl alcohol solution to rinse away the residual developer in the sample 3, and then using nitrogen gas to blow dry the sample 3;

[0044] Step S407: Transfer sample 3 to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; after baking, perform annealing for 30 minutes.

[0045] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0046] 1. The refractive index sensor of the cascaded polymer waveguide MMI coupler of the present invention utilizes optical sensing technology to achieve refractive index detection in various application scenarios, such as water quality testing, blood testing, and fruit sugar content testing. Furthermore, because the waveguide core layer within the detection port of the optical waveguide structure is disconnected, forming a hole structure, the liquid to be detected enters and fills the detection port. This causes the disconnected waveguide core layer (i.e., the MZI sensing arm) in the optical waveguide structure to form three-dimensional contact with the liquid to be detected, increasing the magnitude of the refractive index change, compensating for the phase difference of the MZI interference, and achieving refractive index sensing by detecting changes in optical power at the output end of the multimode interference coupler. Consequently, the refractive index sensor of the present invention has the characteristics of high linearity, high sensitivity, strong stability, and ease of fabrication and integration.

[0047] 2. The method for preparing the refractive index sensor of the cascaded polymer waveguide MMI coupler of the present invention fully utilizes the diverse types of organic polymer materials, resulting in a fabricated refractive index sensor having a simple structure, high linearity, ease of processing and integration, and high stability. Furthermore, the accuracy and sensitivity of the refractive index sensor can be effectively improved. Furthermore, the refractive index sensor of the present invention utilizes the wavelength-insensitive nature of MMI to achieve sensing detection over an ultra-wide operating bandwidth. This makes it suitable for on-chip photonic integrated sensor devices and can be effectively applied to a variety of refractive index sensing environments.

[0048] 3. The use of polymer materials in the preparation method of the refractive index sensor of the cascaded polymer waveguide MMI coupler of the present invention makes the production process of the refractive index sensor relatively simple. Only some conventional preparation processes are required, and no expensive equipment and processes are required. The production cost is relatively low and the preparation efficiency is high. It has broad application prospects in large-scale integrated production and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a schematic structural diagram of a refractive index sensor of a cascaded polymer waveguide MMI coupler according to the present invention;

[0050] Figure 2 A schematic cross-sectional view of a refractive index sensor of a cascaded polymer waveguide MMI coupler according to the present invention;

[0051] Figure 3 This is a process flow chart for preparing a refractive index sensor of a cascaded polymer waveguide MMI coupler according to the present invention;

[0052] In the figure: 1. Input single-mode waveguide; 2. First MMI input tapered waveguide; 3. First MMI multimode waveguide; 4. First MMI output tapered sensing arm connecting waveguide; 5. First MMI output tapered reference arm connecting waveguide; 6. First curved waveguide; 7. Second curved waveguide; 8. MZI reference arm; 9. First section of MZI sensing arm; 10. Second section of MZI sensing arm; 11. Third curved waveguide; 12. Fourth curved waveguide; 13. Second MMI input tapered reference arm connecting waveguide; 14. Second MMI input tapered sensing arm connecting waveguide; 15. Second MMI multimode waveguide; 16. Second MMI output tapered waveguide; 17. Output few-mode waveguide; 18. Silicon wafer substrate; 19. Lower cladding; 20. Upper cladding; 21. Detection port; 22. Hole-punched structure. DETAILED DESCRIPTION

[0053] The present invention will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0054] Explanation of terms: MMI: multimode interference coupler; MZI: Mach-Zehnder interferometer.

[0055] See also Figure 1 and Figure 2 The refractive index sensor of the cascaded polymer waveguide MMI coupler of the present invention comprises a silicon wafer substrate 18, an upper cladding layer 20 and a lower cladding layer 19 arranged on the silicon wafer substrate 18, and an optical waveguide structure based on a Mach-Zehnder interference structure and a multimode interference coupler arranged between the upper cladding layer 20 and the lower cladding layer 19; wherein,

[0056] The upper cladding layer 20 is provided with a detection port 21; the waveguide core layer of the optical waveguide structure is located in the detection port 21, thereby forming a hole structure 22;

[0057] The optical waveguide structure includes an input end portion, a sensing arm portion and an output end portion, wherein:

[0058] The input end portion includes an input single-mode waveguide 21, a first MMI input tapered waveguide, a first MMI multimode waveguide 3, a first MMI output tapered sensing arm connecting waveguide 4, and a first MMI output tapered reference arm connecting waveguide 5. The output end of the input single-mode waveguide 21 is connected to the input end of the first MMI input tapered waveguide; the output end of the first MMI input tapered waveguide is connected to the input end of the first MMI multimode waveguide 3; and the output end of the first MMI multimode waveguide 3 is respectively connected to the first MMI output tapered sensing arm connecting waveguide 4 and the first MMI output tapered reference arm connecting waveguide 5 located on both sides thereof. The above structure also constitutes the input-end 3-dB power splitter of the refractive index sensor of the present invention.

[0059] The sensing arm portion includes a first curved waveguide 6, a second curved waveguide 7, an MZI reference arm 8, a first section 9 of the MZI sensing arm, a second section 10 of the MZI sensing arm, a third curved waveguide 11, and a fourth curved waveguide 12, wherein the input end of the first curved waveguide 6 is connected to the output end of the first MMI output tapered reference arm connecting waveguide 5, and the output end of the first curved waveguide 6 is connected to the input end of the MZI reference arm 8; the output end of the MZI reference arm 8 is connected to the input end of the third curved waveguide 11; the input end of the second curved waveguide 7 is connected to the input end of the first MMI output tapered sensing arm connecting waveguide 4, and the output end of the second curved waveguide 7 is connected to the first section 9 of the MZI sensing arm; and the output end of the second section 10 of the MZI sensing arm is connected to the input end of the fourth curved waveguide 12;

[0060] In this embodiment, the middle of the MZI sensor arm is disconnected to form the first section 9 of the MZI sensor arm and the second section 10 of the MZI sensor arm. There is a gap between the output end of the first section 9 of the MZI sensor arm and the input end of the second section 10 of the MZI sensor arm, thereby forming the hole structure 22.

[0061] The output end portion includes a second MMI input sensing arm tapered connecting waveguide, a second MMI input reference arm tapered connecting waveguide, a second MMI multimode waveguide 15, a second MMI output tapered waveguide 16 and an output few-mode waveguide 17, wherein the input end of the second MMI input sensing arm tapered connecting waveguide is connected to the output end of the third curved waveguide 11, and the output end of the second MMI input sensing arm tapered connecting waveguide is connected to the second MMI multimode waveguide 15; the input end of the second MMI input reference arm tapered connecting waveguide is connected to the output end of the fourth curved waveguide 12, and the output end of the second MMI input reference arm tapered connecting waveguide is connected to the second MMI multimode waveguide 15; the output end of the second MMI multimode waveguide 15 is connected to the input end of the second MMI output tapered waveguide 16, and the output end of the second MMI output tapered waveguide 16 is connected to the output few-mode waveguide 17.

[0062] In this embodiment, an MMI multimode waveguide is introduced as the input and output of the refractive index sensor of the present invention, and a tapered waveguide is introduced. By utilizing the wavelength-insensitive characteristics of the MMI and the broadband effect of the tapered waveguide, the refractive index sensor of the present invention can operate over a wider wavelength range and simultaneously detect and analyze optical signals of multiple different wavelengths. This makes the refractive index sensor of the present invention more adaptable in different application scenarios and can meet diverse detection needs.

[0063] Through the above arrangement, the optical signal enters the first section of the MMI multimode waveguide through the input single-mode waveguide 21 and the first MMI input tapered waveguide. In the first section of the MMI multimode waveguide, the input signal optical power is split into two identical optical signals using the self-imaging effect of the multimode interference coupler. The first optical signal enters the second MMI multimode waveguide 15 from the first MMI output tapered reference arm connecting waveguide 5, the first curved waveguide 6, the MZI reference arm 8, the third curved waveguide 11, and the second MMI input tapered sensor arm connecting waveguide 14; the second optical signal enters the second MMI multimode waveguide 15 from the first MMI output tapered sensor arm connecting waveguide 4, the second curved waveguide 7, the first section of the MZI sensor arm 9, the second section of the MZI sensor arm 10, the fourth curved waveguide 12, and the second MMI input tapered sensor arm connecting waveguide. The reference arm connecting waveguide 13 enters the second MMI multimode waveguide 15; during this process, the liquid to be detected fills the detection port 21, so that the three surfaces of the first section 9 of the MZI sensor arm and the second section 10 of the MZI sensor arm are in contact with the liquid to be detected, thereby increasing the value of the refractive index change of the optical signal and increasing the phase difference between the first optical signal and the second optical signal, thereby improving the sensitivity of the refractive index sensor of the present invention; the first optical signal and the second optical signal are interfered and output in the second MMI multimode waveguide 15, and the intensity of the output optical signal changes with the change of the refractive index at the detection port 21 and due to the interference effect of the MZI. Finally, the change in the intensity of the optical signal can indirectly reflect the refractive index change of the liquid to be detected.

[0064] See also Figure 1 and Figure 2 The detection port 21 is located between the output end of the second curved waveguide 7 and the input end of the fourth curved waveguide 12; the first section of the sensing arm and the second section of the sensing arm are entirely located within the detection port 21; the length of the detection port 21 is equal to the distance between the output end of the second curved waveguide 7 and the input end of the fourth curved waveguide 12, the length is greater than 100 μm, and the width is 40 μm. An extension groove extending toward the edge of the upper cladding layer 20 is provided in the detection port 21; the length of the extension groove is less than the length of the detection port 21.

[0065] See also Figure 1 and Figure 2 , the gap between the first section of the sensor arm and the second section of the sensor arm is 10 μm-100 μm.

[0066] See also Figure 1 and Figure 2 The waveguide material of the optical waveguide structure is EpoCore, and the refractive index is 1.569; the thickness of the core waveguides in the optical waveguide structure is equal, and is 4-7 μm; in this embodiment, the thickness of the core waveguides in the optical waveguide structure is 5 μm.

[0067] See also Figure 1 and Figure 2 The waveguide material of the upper cladding 20 and the lower cladding 19 is Epoclad, and the refractive index is 1.55; the thickness of the upper cladding 20 and the lower cladding 19 are both 10-30 μm; in this embodiment, the thickness of the upper cladding 20 and the lower cladding 19 are both 10 μm.

[0068] See also Figure 1 and Figure 2 , the thickness of the silicon wafer substrate 18 is 1 mm.

[0069] See also Figure 1 and Figure 2 , the width of the input single-mode waveguide 21 is 4-7 μm; the width of the input end of the first MMI input tapered waveguide is the same as the width of the input single-mode waveguide 21, and the width of the output end of the first MMI input tapered waveguide is 15 μm; the width of the first MMI multimode waveguide 3 is 30-50 μm; the width of the second MMI multimode waveguide 15 is 20-40 μm; the width of the input end of the second MMI output tapered waveguide 16 is 20-30 μm, and the width of the output end of the second MMI output tapered waveguide 16 is the same as the width of the output few-mode waveguide 17, and both are 9 μm; in addition, the widths of the other core waveguides in the optical waveguide structure are the same as the width of the input single-mode waveguide 21.

[0070] See also Figure 1 and Figure 2 The working mechanism of the refractive index sensor of the cascaded polymer waveguide MMI coupler of the present invention is based on the interference effect of the MZI and the self-imaging effect of the MMI. The interference phase is compensated by the hole structure 22 design of the upper cladding layer 20 and the MZI sensing arm to improve the sensitivity of the refractive index sensor of the present invention.

[0071] Phase difference of MZI The light intensity I of the interference output is expressed as:

[0072]

[0073] Where: λ is the wavelength, Δn and L are the refractive index change and the length of the MZI sensor arm (i.e., the distance between the input end of the first section 9 of the MZI sensor arm and the output end of the second section 10 of the MZI sensor arm), I0 is the input light intensity,

[0074] In the design of the sensing area (i.e., the detection port 21) of the present invention, a hole structure 22 of the MZI sensor arm is introduced, so that the liquid to be detected filled in the detection port 21 can contact three surfaces of the first section 9 and the second section 10 of the MZI sensor arm, thereby increasing the value of the refractive index change, thereby achieving high-sensitivity refractive index sensing with a shorter MZI sensor arm length;

[0075] In addition, by selecting an MMI multimode waveguide as the input and output ends of the refractive index sensor of the present invention and introducing a tapered waveguide connection, the wavelength-insensitive characteristics of the MMI multimode waveguide and the tapered waveguide are utilized, thereby further improving the operating bandwidth of the refractive index sensor of the present invention, so that the refractive index sensor of the present invention can simultaneously detect and analyze light signals of different wavelengths, thereby making the refractive index sensor of the present invention more adaptable in different application scenarios to meet diverse detection needs; the self-imaging effect of the MMI can be used to determine the length of the MMI, thereby achieving high-efficiency and low-loss transmission of a wide wavelength, wherein,

[0076] The self-imaging effect of MMI can be expressed as:

[0077]

[0078] Where: β0 and β1 are the propagation constants of the fundamental mode and the first-order mode in the multimode waveguide, n r is the equivalent refractive index of the multimode waveguide region, λ0 is the central wavelength of the input signal light;

[0079] Formula (3) is used to calculate that the self-imaging period lengths of the first MMI multimode waveguide 3 and the second MMI multimode waveguide 15 in the present invention are 740 μm and 1500 μm, respectively. This effectively reduces the size of the refractive index sensor of the present invention, making the structure of the refractive index sensor of the present invention more compact and facilitating the implementation of an on-chip integrated refractive index sensor.

[0080] The refractive index sensor of the present invention performs photolithography on the upper cladding layer 20 at set positions of the MZI sensor arm (i.e., the first section 9 of the MZI sensor arm and the second section 10 of the MZI sensor arm) by a multi-step photolithography method, thereby forming a detection port 21 of the refractive index sensor of the present invention and providing a gap in the MZI sensor arm, so that the MZI sensor arm is divided into the first section 9 of the MZI sensor arm and the second section 10 of the MZI sensor arm. During detection, the liquid to be detected is injected into the detection port 21, so that the effective refractive index of the light signal passing through the MZI sensor arm changes. By detecting the change in optical power of the output port of the output few-mode waveguide 17, the refractive index sensing detection of the solution to be detected is achieved.

[0081] See also Figure 3The method for preparing the refractive index sensor of the cascaded polymer waveguide MMI coupler of the present invention comprises the following steps:

[0082] Step S1: preparing a silicon wafer substrate 18, specifically the following steps:

[0083] The silicon wafer is split into substrates that meet the size requirements by using a diamond knife; then the substrate is cleaned twice by a plasma machine, each cleaning time is 80 seconds, to remove impurities on the surface of the silicon wafer substrate 18.

[0084] Step S2: preparing a lower cladding layer 19 on the silicon wafer substrate 18, specifically the following steps:

[0085] Step S201: using a spin coater to spin-coat EpoClad material on a pre-treated silicon wafer substrate 18 at a rotation speed of 3000 rpm to obtain sample 1;

[0086] Step S202: After the spin coating is completed, the sample is transferred to a heating table for pre-baking at a temperature of 95-110° C. for 10 minutes; after the pre-baking is completed, an annealing treatment is performed for 5 minutes;

[0087] Step S203: After the pre-baking is completed, the sample 1 is subjected to UV exposure curing using a photolithography machine, and the UV exposure curing time is 30-60 seconds;

[0088] Step S204: Transfer the sample 1 after exposure to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; and perform annealing after baking for 30 minutes.

[0089] Step S3: Fabricate an optical waveguide structure on the prepared lower cladding layer 19. The specific steps are as follows:

[0090] Step S301: using a plasma machine to clean sample 1 twice, each cleaning time is 80 seconds, to remove impurities on the surface of sample 1;

[0091] Step S302: After cleaning, a core waveguide material EpoCore with a thickness of 5 μm is spin-coated on the lower cladding layer 19 of sample 1 using a coating machine at a rotation speed of 4000 rpm to obtain sample 2;

[0092] Step S303: After the spin coating is completed, the sample 2 is transferred to a heating table for pre-baking at a temperature of 95-110° C. for 10 minutes; after the pre-baking is completed, an annealing treatment is performed for 5 minutes;

[0093] Step S304: After the pre-bake is completed, the core waveguide of the optical waveguide structure is subjected to a photolithography process using a mask and an ultraviolet lithography machine;

[0094] Step S305: Transfer sample 2 to a heating table and bake at 95°C for 10 minutes. After baking, perform annealing for 5 minutes.

[0095] Step S306: developing the surface film of the core waveguide of the optical waveguide structure, and wet-etching the core waveguide pattern after photolithography using a developer corresponding to the core waveguide material EpoCore, with the wet etching time being 15-30s;

[0096] Step S307: Rinse the developer in the second sample with isopropyl alcohol, and then dry the second sample with a nitrogen gun;

[0097] Step S308: Transfer sample 2 to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; after baking, perform annealing for 30 minutes.

[0098] Step S4: After the optical waveguide structure is prepared, an upper cladding layer 20 is prepared on the lower cladding layer 19, and a detection port 21 and a hole structure 22 are processed on the upper cladding layer 20. The specific steps are as follows:

[0099] Step S401: Spin-coating EpoClad material on Sample 2 using a spin coater at a speed of 3000 rpm to obtain Sample 3;

[0100] Step S402: After the spin coating is completed, the sample 2 is transferred to the heating table for pre-baking at a temperature of 95-110°C for 10 minutes; after the pre-baking is completed, annealing is performed for 5 minutes;

[0101] Step S403: performing UV photolithography on the detection port 21 in the upper cladding layer 20 using a designed mask, with the UV exposure time being 5-20 seconds;

[0102] Step S404: Transfer sample 3 to a heating platform for baking at a temperature of 95-110° C. for 10 minutes, and then perform annealing for 5 minutes after baking.

[0103] Step S405: Developing the baked sample 3, wet-etching the core waveguide pattern after photolithography using a developer corresponding to the EpoClad material, with the wet etching time being 15-30 seconds;

[0104] Step S406: using an isopropyl alcohol solution to rinse away the residual developer in the sample 3, and then using nitrogen gas to blow dry the sample 3;

[0105] Step S407: Transfer sample 3 to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; after baking, perform annealing for 30 minutes.

[0106] 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 scope of protection of the present invention.

Claims

1. A refractive index sensor of a cascaded polymer waveguide MMI coupler, characterized in that: The invention comprises a silicon wafer substrate, an upper cladding layer and a lower cladding layer arranged on the silicon wafer substrate, and an optical waveguide structure based on a Mach-Zehnder interference structure and a multimode interference coupler arranged between the upper cladding layer and the lower cladding layer; wherein, a detection port is provided on the upper cladding layer; and a waveguide core layer in the optical waveguide structure is located within the detection port, thereby forming a hole structure.

2. The refractive index sensor of the cascaded polymer waveguide MMI coupler according to claim 1, characterized in that: The optical waveguide structure includes an input end portion, a sensing arm portion, and an output end portion, wherein: The input end portion includes an input single-mode waveguide, a first MMI input tapered waveguide, a first MMI multimode waveguide, a first MMI output tapered sensing arm connecting waveguide, and a first MMI output tapered reference arm connecting waveguide, wherein the output end of the input single-mode waveguide is connected to the input end of the first MMI input tapered waveguide; the output end of the first MMI input tapered waveguide is connected to the input end of the first MMI multimode waveguide; and the output end of the first MMI multimode waveguide is respectively connected to the first MMI output tapered sensing arm connecting waveguide and the first MMI output tapered reference arm connecting waveguide located on both sides thereof; The sensing arm portion includes a first curved waveguide, a second curved waveguide, an MZI reference arm, a first section of the MZI sensing arm, a second section of the MZI sensing arm, a third curved waveguide, and a fourth curved waveguide, wherein the input end of the first curved waveguide is connected to the output end of the first MMI output tapered reference arm connecting waveguide, and the output end of the first curved waveguide is connected to the input end of the MZI reference arm; the output end of the MZI reference arm is connected to the input end of the third curved waveguide; the input end of the second curved waveguide is connected to the input end of the first MMI output tapered sensing arm connecting waveguide, and the output end of the second curved waveguide is connected to the first section of the MZI sensing arm; the output end of the second section of the MZI sensing arm is connected to the input end of the fourth curved waveguide; a gap is provided between the output end of the first section of the sensing arm and the input end of the second section of the sensing arm, thereby forming the excavated structure; The output end portion includes a second MMI input sensing arm tapered connecting waveguide, a second MMI input reference arm tapered connecting waveguide, a second MMI multimode waveguide, a second MMI output tapered waveguide and an output few-mode waveguide, wherein the input end of the second MMI input sensing arm tapered connecting waveguide is connected to the output end of the third curved waveguide, and the output end of the second MMI input sensing arm tapered connecting waveguide is connected to the second MMI multimode waveguide; the input end of the second MMI input reference arm tapered connecting waveguide is connected to the output end of the fourth curved waveguide, and the output end of the second MMI input reference arm tapered connecting waveguide is connected to the second MMI multimode waveguide; the output end of the second MMI multimode waveguide is connected to the input end of the second MMI output tapered waveguide, and the output end of the second MMI output tapered waveguide is connected to the output few-mode waveguide.

3. The refractive index sensor of the cascaded polymer waveguide MMI coupler according to claim 2, characterized in that: The detection port is located between the output end of the second curved waveguide and the input end of the fourth curved waveguide; the first section of the sensing arm and the second section of the sensing arm are entirely located within the detection port; the length of the detection port is equal to the distance between the output end of the second curved waveguide and the input end of the fourth curved waveguide, and the width is 40 μm. An extension slot extending toward an edge of the upper cladding is provided in the detection port; the length of the extension slot is less than the length of the detection port.

4. The refractive index sensor of the cascaded polymer waveguide MMI coupler according to claim 2, characterized in that: The gap between the first section of the sensor arm and the second section of the sensor arm is 10 μm-100 μm.

5. The refractive index sensor of the cascaded polymer waveguide MMI coupler according to claim 2, characterized in that: The waveguide material of the optical waveguide structure is EpoCore, with a refractive index of 1.569; the thickness of the core waveguide in the optical waveguide structure is equal and is 4-7 μm; the waveguide material of the upper cladding and the lower cladding is Epoclad, with a refractive index of 1.559; the thickness of the upper cladding and the lower cladding is 10-30 μm; the width of the input single-mode waveguide is 4-7 μm; the width of the input end of the first MMI input tapered waveguide is the same as the width of the input single-mode waveguide, and the first MM The width of the output end of the I input tapered waveguide is 15 μm; the width of the first MMI multimode waveguide is 30-50 μm; the width of the second MMI multimode waveguide is 20-40 μm; the width of the input end of the second MMI output tapered waveguide is 20-30 μm, and the width of the output end of the second MMI output tapered waveguide is the same as the width of the output few-mode waveguide, both of which are 9 μm; in addition, the width of the other core waveguides in the optical waveguide structure is the same as the width of the input single-mode waveguide.

6. A method for preparing a refractive index sensor for the cascaded polymer waveguide MMI coupler according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step S1: preparing a silicon wafer substrate; Step S2: preparing a lower cladding layer on a silicon wafer substrate; Step S3: fabricating an optical waveguide structure on the prepared lower cladding layer; Step S4: After the optical waveguide structure is prepared, an upper cladding layer is prepared on the lower cladding layer, and a detection port and a hole structure are processed on the upper cladding layer.

7. The method for preparing a refractive index sensor of a cascaded polymer waveguide MMI coupler according to claim 6, characterized in that: In step S1, the silicon wafer substrate is prepared as follows: The silicon wafer is split into substrates that meet the size requirements using a diamond knife; then the substrate is cleaned twice using a plasma machine, each cleaning time is 80 seconds, to remove impurities on the surface of the silicon wafer substrate.

8. The method for preparing a refractive index sensor of a cascaded polymer waveguide MMI coupler according to claim 7, characterized in that: In step S2, the process of preparing the lower cladding layer on the silicon wafer substrate is as follows: Step S201: Spin-coating the EpoClad material on the pre-treated silicon wafer substrate using a spin coater at a rotation speed of 3000 rpm to obtain sample 1; Step S202: After the spin coating is completed, the sample is transferred to a heating table for pre-baking at a temperature of 95-110° C. for 10 minutes; after the pre-baking is completed, an annealing treatment is performed for 5 minutes; Step S203: After the pre-baking is completed, the sample 1 is subjected to UV exposure curing using a photolithography machine, and the UV exposure curing time is 30-60 seconds; Step S204: Transfer the sample 1 after exposure to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; and perform annealing after baking for 30 minutes.

9. The method for preparing a refractive index sensor of a cascaded polymer waveguide MMI coupler according to claim 8, characterized in that: In step S3, the process of preparing the optical waveguide structure on the lower cladding layer is as follows: Step S301: using a plasma machine to clean sample 1 twice, each cleaning time is 80 seconds, to remove impurities on the surface of sample 1; Step S302: After cleaning, a core waveguide material EpoCore with a thickness of 5 μm is spin-coated on the lower cladding layer of sample 1 using a spin coater at a speed of 4000 rpm to obtain sample 2; Step S303: After the spin coating is completed, the sample 2 is transferred to a heating table for pre-baking at a temperature of 95-110° C. for 10 minutes; after the pre-baking is completed, an annealing treatment is performed for 5 minutes; Step S304: After the pre-bake is completed, the core waveguide of the optical waveguide structure is subjected to a photolithography process using a mask and an ultraviolet lithography machine; Step S305: Transfer sample 2 to a heating table and bake at 95°C for 10 minutes. After baking, perform annealing for 5 minutes. Step S306: developing the surface film of the core waveguide of the optical waveguide structure, and wet-etching the core waveguide pattern after photolithography using a developer corresponding to the core waveguide material EpoCore, with the wet etching time being 15-30s; Step S307: Rinse the developer in the second sample with isopropyl alcohol, and then dry the second sample with a nitrogen gun; Step S308: Transfer sample 2 to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; after baking, perform annealing for 30 minutes.

10. The method for preparing a refractive index sensor of a cascaded polymer waveguide MMI coupler according to claim 9, characterized in that: In step S4, the steps of preparing an upper cladding layer on the lower cladding layer and processing a detection port and a hole structure on the upper cladding layer are as follows: Step S401: Spin-coating EpoClad material on Sample 2 using a spin coater at a speed of 3000 rpm to obtain Sample 3; Step S402: After the spin coating is completed, the sample 2 is transferred to the heating table for pre-baking at a temperature of 95-110°C for 10 minutes; after the pre-baking is completed, annealing is performed for 5 minutes; Step S403: Perform UV photolithography on the detection port in the upper cladding layer using the designed mask, with the UV exposure time being 5-20 seconds; Step S404: Transfer sample 3 to a heating platform for baking at a temperature of 95-110° C. for 10 minutes, and then perform annealing for 5 minutes after baking. Step S405: Developing the baked sample 3, wet-etching the core waveguide pattern after photolithography using a developer corresponding to the EpoClad material, with the wet etching time being 15-30 seconds; Step S406: using an isopropyl alcohol solution to rinse away the residual developer in the sample 3, and then using nitrogen gas to blow dry the sample 3; Step S407: Transfer sample 3 to a heating platform for baking at a temperature of 120-140° C. for more than 60 minutes; after baking, perform annealing for 30 minutes.

Images