Gain equalized few-mode optical waveguide amplifier with refractive index profile adjustment and preparation method

Abstract

The application discloses a gain equalization few-mode optical waveguide amplifier with refractive index distribution adjustment and a preparation method, and belongs to the technical field of polymer optical waveguide amplifiers; the few-mode optical waveguide amplifier sequentially comprises a silica substrate, a core layer and a polymethyl methacrylate upper cladding layer from bottom to top; the core layer comprises an inner core layer and an outer core layer; the refractive index of the inner core layer is greater than the refractive indexes of the upper cladding layer and the substrate; the refractive index of the outer core layer is slightly higher than the refractive index of the inner core layer; the inner and outer core layers are polymer materials with the same doping nanoparticle concentration. The preparation method comprises the steps of spin coating, heating and curing, photoetching and developing, overlaying and the like. The few-mode optical waveguide amplifier designed in the application can realize few-mode signal equalization amplification under single mode pumping condition, the test system is simple, the inter-mode gain difference is small, the on-chip mode division multiplexing system loss can be compensated, and the development of the mode division multiplexing technology is promoted.

Description

Technical Field

[0001] This invention belongs to the field of polymer optical waveguide amplifier technology, specifically relating to a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment and its fabrication method. Background Technology

[0002] In recent years, with the rapid development of network communication technology, optical communication systems based on single-mode fiber are gradually approaching their capacity limits. Under the immense pressure of current communication demands, Mode Division Multiplexing (MDM), utilizing orthogonal mode dimensions as parallel channels in few-mode fibers, can overcome the limitations of single-mode transmission capacity and is a novel multiplexing scheme that can exponentially increase the capabilities of existing optical communication. This scheme has gained widespread attention both domestically and internationally and is generally considered a key technology for addressing communication needs in the next stage.

[0003] During optical signal transmission, various mode devices, such as mode converters and multiplexers / demultiplexers, generate mode dispersion, coupling loss, and transmission loss, all of which reduce transmission distance, increase bit errors, and negatively impact the performance of MDM systems. Optical amplifiers can effectively address this issue. They directly amplify the optical signal without requiring photoelectric conversion, making them indispensable for compensating for attenuated optical transmission power. Few-mode waveguide amplifiers can simultaneously amplify multiple mode signals, effectively compensating for losses caused by mode devices and improving MDM system performance.

[0004] When amplifying multiple-mode signals, few-mode optical waveguide amplifiers inevitably generate inter-mode gain differences (DMGs). Larger DMGs lead to decreased system capacity and increased interruption probability. Therefore, the development of high-gain, balanced-amplification few-mode optical waveguide amplifiers has become a research hotspot and a challenge. Currently, gain equalization in few-mode optical waveguide amplifiers mainly employs pump mode modulation and rare-earth ion concentration-layered doping schemes. The pump mode modulation scheme relies on multiple pump modes, controlling the DMG by adjusting the overlap between different pump modes and the signal mode. This scheme requires multiple pump sources, resulting in a complex, large, and costly system that is difficult to integrate. Furthermore, introducing multiple pump modes enhances crosstalk, affecting signal transmission quality. The rare-earth ion concentration-layered doping scheme divides the active core layer into high- and low-rare-earth ion-doped regions, achieving gain equalization by adjusting the strength of the interaction between the signal mode and the gain medium. This often involves sacrificing the maximum mode gain to achieve a lower mode gain difference. Summary of the Invention

[0005] In existing rectangular waveguide structures, the fundamental mode optical field distribution is mainly concentrated in the core center, resulting in a high proportion of optical power. Meanwhile, higher-order mode optical fields are mainly distributed at the core edge, with fewer in the core center, leading to a lower proportion of optical power. This results in lower gain for higher-order modes compared to the fundamental mode. This invention provides a gain-equalized few-mode waveguide amplifier based on refractive index distribution adjustment and its fabrication method. This method is based on a double-layer crown-structured few-mode waveguide. Under optimal rare-earth ion doping concentration, the optical field distribution of each mode is adjusted by fine-tuning the refractive index of the inner and outer core waveguide layers. High gain and low mode gain difference can be achieved using single-mode pumping. Furthermore, the testing system is simple, easy to integrate, and can compensate for losses in on-chip mode division multiplexing systems. This provides a new approach for achieving high-fidelity and low-crosstalk multi-signal mode transmission, promoting the development of mode division multiplexing technology.

[0006] This invention is achieved through the following technical solution:

[0007] A gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment is composed of a silicon dioxide substrate 1, an inner core layer 2, an outer core layer 3, and a polymethyl methacrylate (PMMA) cladding layer 4 from bottom to top. The inner core layer 2 and the outer core layer 3 are both strip structures. The inner core layer 2, the outer core layer 3, and the PMMA cladding layer 4 are all located on the silicon dioxide substrate 1. The inner core layer 2 is encased in the outer core layer 3, and the outer core layer 3 is encased in the PMMA cladding layer 4. The inner core layer 2 and the outer core layer 3 are both polymer materials with the same concentration of doped nanoparticles, forming a double-layer crown structure core layer.

[0008] Furthermore, the refractive index of the silicon dioxide substrate 1 is 1.444, the refractive index of the outer core layer 3 is 1.57-1.58, which is greater than the refractive index of the inner core layer 2, the refractive index difference between the inner and outer core layers is 0.01-0.02, and the refractive index of the upper cladding layer 4 is 1.48-1.50.

[0009] Furthermore, the inner core layer 2 has a width of 3-4 μm and a thickness of 3-4 μm, the outer core layer 3 has a width of 6-7 μm and a thickness of 1-2 μm, and covers the inner core layer 2; the upper cladding layer 4 on the silicon dioxide substrate 1 has a thickness of 7-10 μm; the doping concentration is on the order of 10. 20 -10 21 cm -3 .

[0010] Furthermore, the doped nanoparticles are NaYF4:Er 3+ ,Yb 3+ Nanoparticles, BayF4:Er 3+ ,Yb 3+ Nanoparticles or NaYF4:Er 3+ ,Yb 3+ Ce3+ Nanoparticles.

[0011] Furthermore, the inner core layer uses SU-8 2005 polymer material, and the outer core layer uses SU-8 2002 polymer material, but is not limited to these two SU-8 polymer materials. It is sufficient that the refractive index of the polymer material doped with nanoparticles meets the designed refractive index value. The upper cladding material used is polymethyl methacrylate, but is not limited to this material.

[0012] On the other hand, the present invention also provides a method for fabricating a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment, specifically including the following steps:

[0013] S1. Clean the silicon dioxide substrate sequentially with acetone, ethanol, and deionized water.

[0014] S2. Prepare polymer solutions for the inner and outer core layers of doped nanoparticles;

[0015] S3. The core layer solution is coated onto a silicon dioxide substrate using a spin coating method and then heated to cure to form the core layer.

[0016] S4. Align photomask I with the core layer, expose with ultraviolet light, bake, and develop to transfer the pattern on photomask I onto the core layer, and heat to cure the hard film to form the inner core layer waveguide.

[0017] S5. The outer core layer solution is coated onto the inner core layer using a spin coating method, and then heated and cured to form the outer core layer.

[0018] S6. Align the photomask II with the inner core waveguide, perform ultraviolet exposure for overlay etching, then bake and develop to overlay the outer core waveguide onto the inner core waveguide, and heat to cure the hard film to form the outer core waveguide.

[0019] S7. A cyclopentanone solution of polymethyl methacrylate is coated onto the waveguide using a spin coating method, and then heated and cured to form the upper cladding.

[0020] Furthermore, in step S2, the preparation of the core layer polymer solution specifically includes the following:

[0021] Take a NaYF4:20%Yb solution with a concentration of 1 mg / ml. 3+ 2% Er 3+ @NaYF4 nanoparticle solutions were doped with SU-82005 and SU-8 2002 at a mass ratio of 1:11, and then sonicated at 40 kHz for 3 hours to ensure thorough mixing. Inner and outer core materials were then prepared accordingly, and Er was measured. 3+ The doping concentration is 2.8 × 10⁻⁶. 20 cm -3 Yb 3+ The doping concentration is 2.8 × 10⁻⁶.21 cm -3 .

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] This invention discloses a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment and its fabrication method. To balance the overlap between different modes and the core gain medium, and to maximize the overlap between the pump mode and signal mode optical fields, a double-layer crown waveguide structure is proposed. The traditional core region is divided into an inner core layer and an outer core layer. By increasing the refractive index of the outer core layer, the proportion of high-order mode optical power is enhanced, increasing the interaction between high-order modes and the gain medium, thereby achieving few-mode gain equalization. Taking advantage of the ease of adjusting the refractive index of polymers, a suitable polymer is selected as the matrix for doped nanoparticles, which can effectively adjust the refractive indices of the inner and outer core layers. Simultaneously, the nanoparticles in both the inner and outer core layers are uniformly doped, with the optimal doping concentration being the highest doping concentration achievable. This achieves high mode gain without sacrificing the maximum mode gain value for gain balance. The rare-earth ion-doped organic polymer material used in this invention has significant advantages, including a wide variety of matrix materials, high rare-earth ion doping concentration, simple processing, easy integration, low cost, and easy refractive index adjustment.

[0024] The present invention discloses a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment, which can achieve high mode gain and few-mode gain equalization using only a single mode pump. The test system is simple and easy to integrate. It can effectively solve the defects of existing technologies, effectively compensate for the loss generated by various mode devices in on-chip mode division multiplexing systems, solve the problem of high-capacity transmission in optical communication systems, and promote the development of mode division multiplexing technology. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0026] Figure 1 The present invention provides a schematic diagram of a gain-equalized few-mode optical waveguide amplifier structure based on refractive index distribution adjustment and a refractive index distribution diagram.

[0027] Figure 2 In this embodiment of the invention, the gain material is NaYF4: 20% Yb 3+ 2% Er 3+ Transmission electron microscopy image of @NaYF4 nanoparticles, showing uniform particle distribution and an average particle size of approximately 32 nm.

[0028] Figure 3 In this embodiment of the invention, a 980nm infrared laser is used to excite NaYF4:20%Yb 3+ 2% Er 3+ Emission spectrum of @NaYF4 nanoparticles;

[0029] Figure 4 : A schematic diagram of the fabrication method of the gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment according to the present invention;

[0030] Figure 5 The present invention discloses a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment. The optical field distribution of three signal modes (LP01, LP11a, LP11b) and pump mode (LP21b) is simulated in Comsol software using the beam propagation method.

[0031] Figure 6 The curve showing the relationship between the gain of a gain-equalized optical waveguide amplifier based on refractive index distribution adjustment and pump power, obtained by simulation using Matlab software.

[0032] Figure 7 : A test system diagram for forward pumping test of a gain equalization few-mode optical waveguide amplifier based on refractive index distribution adjustment according to the present invention. Detailed Implementation

[0033] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0034] Example 1

[0035] like Figure 1 As shown, this embodiment provides a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment. The few-mode optical waveguide amplifier is composed of a silicon dioxide substrate 1, an inner core layer 2, an outer core layer 3, and a polymethyl methacrylate (PMMA) cladding layer 4 from bottom to top. The inner core layer 2 and the outer core layer 3 are both strip structures. The inner core layer 2, the outer core layer 3, and the PMMA cladding layer 4 are all located on the silicon dioxide substrate 1. The inner core layer 2 is encased in the outer core layer 3, and the outer core layer 3 is encased in the PMMA cladding layer 4. The inner core layer 2 and the outer core layer 3 are both polymer materials with the same concentration of doped nanoparticles, forming a double-layer crown structure core layer.

[0036] In this embodiment, the inner core layer 2 is SU-8 2005 with a refractive index n1 of 1.56, and the outer core layer 3 is SU-8 2002 with a refractive index n2 of 1.57. The refractive index difference n2-n1 is 0.01, but it is not limited to these two SU-8 polymers. As long as the polymer material doped with nanoparticles has a refractive index that meets the designed refractive index value, it is acceptable.

[0037] In this embodiment, the inner core layer 2 has a width of 4μm and a thickness of 4μm, the outer core layer 3 has a width of 6μm and a thickness of 1μm and is wrapped around the inner core layer 2, and the polymethyl methacrylate overlay 4 on the silicon dioxide substrate 1 has a thickness of 7μm.

[0038] In this embodiment, the nanoparticles doped with this invention are NaYF4:20%Yb 3+ 2% Er 3+ @NaYF4 core-shell structured nanoparticles, but not limited to these nanoparticles (BaYF4:Er) 3+ ,Yb 3+ NaYF4:Er 3+ ,Yb 3 + Ce 3+ (etc.), with a doping concentration on the order of 10 20 -10 21 cm -3 ;from Figure 2 As can be seen from the above, the NaYF4:20%Yb used in this embodiment 3+ 2% Er 3+ The @NaYF4 nanoparticles have a uniform particle distribution and an average particle size of approximately 32 nm.

[0039] The few-mode waveguide amplifier in this embodiment can simultaneously transmit and amplify three LP modes at 1550nm, namely LP01, LP11a, and LP11b. Furthermore, it can achieve few-mode gain equalization by using single-mode pumping (LP21b) to make the gain difference between modes less than 1dB.

[0040] Example 2

[0041] like Figure 4 As shown, this embodiment provides a method for fabricating a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment, specifically including the following steps:

[0042] S1. Cleaning of silicon dioxide substrate: Wipe the surface of the substrate with acetone cotton to remove oil stains, then wipe with ethanol cotton to remove inorganic salts, etc., and finally rinse with deionized water. After cleaning, blow dry and set aside for use.

[0043] S2. Core material preparation: Take NaYF4 at a concentration of 1 mg / ml: 20% Yb 3+ 2% Er 3+ @NaYF4 nanoparticle solution was doped with SU-8 2005 and SU-8 2002 at a mass ratio of 1:11 and ultrasonicated at 40 kHz for 3 hours to ensure thorough mixing. Inner and outer core materials were then prepared accordingly. Er was measured. 3+ The doping concentration is 2.8 × 10⁻⁶. 20 cm -3 Yb 3+ The doping concentration is 2.8 × 10⁻⁶. 21 cm -3 ;

[0044] S3. Preparation of upper cladding polymethyl methacrylate (PMMA): Weigh 3g of polymethyl methacrylate powder with oil paper and put it into an Erlenmeyer flask. Add 27g of cyclopentanone, then add the rotor. Place the flask on a heated magnetic stirrer, set the parameters to 200r, the temperature to 45℃, and stir for about 8 hours.

[0045] S4. Spin-coating the inner core material: After ultrasonication, the inner core material is filtered through a 0.45μm molecular sieve and then spin-coated onto a silicon dioxide substrate. The substrate is tilted slightly with pointed tweezers to spread the material evenly across the entire substrate. Then, a spin coater is used for spin coating. The spin coating parameters are 3000 rad / min and 20 seconds.

[0046] S5. Pre-baking: Dry the sample using a hot plate: Use a stepped heating method to dry the sample. First, heat the sample to 60°C and maintain this temperature for 10 minutes. Then, continue heating the sample to 90°C and maintain this temperature for 20 minutes. Turn off the hot plate and allow the sample to cool naturally to room temperature to form an inner core layer film with a thickness of 4μm.

[0047] S6. Photolithography: Using a photomask with a striped structure as a mask, ultraviolet lithography is performed (UV exposure lamp parameters: wavelength 365nm, 19-22mw / cm). 2 The exposure time is 7 seconds, and the inner core layer is made into a waveguide of a preset shape;

[0048] S7. Post-baking: Place the photolithographically etched sample on a hot plate for drying. The same step heating method is used. First, heat the sample to 65°C and maintain this temperature for 10 minutes. Then, continue heating to 95°C and maintain this temperature for 20 minutes. Turn off the hot plate and allow it to cool naturally to room temperature. The refractive index is measured to be 1.5662 using an ellipsometer.

[0049] S8. Development: Develop the sample with SU-8 developer, then clean off any remaining developer with isopropanol and rinse thoroughly with deionized water. Develop until the waveguide structure is clear; over-development or under-development will affect waveguide performance.

[0050] S9. Curing: Place the developed sample on a hot plate to heat and cure the film, removing residual solvent. The temperature is 120℃ and the time is 30 minutes. Then turn off the hot plate and let it cool to room temperature naturally.

[0051] S10, Spin-coating the outer core layer material: After ultrasonication, the outer core layer material is filtered through a 0.45μm molecular sieve and then spin-coated onto the inner core layer. The substrate is slightly tilted with pointed tweezers to spread the material evenly over the entire substrate. Spin-coating is performed using a spin coater with parameters of 3000 rad / min and a time of 20 seconds.

[0052] S11. Pre-drying: Dry the sample using a hot plate. Use a stepped heating method for drying. First, heat to 60°C and maintain this temperature for 10 minutes. Then, continue heating to 90°C and maintain for 20 minutes. Turn off the hot plate and allow it to cool naturally to room temperature to form an outer core layer film with a thickness of approximately 1 μm.

[0053] S12. Photolithography: Using a special photomask for overlay lithography, align the outer core waveguide center with the inner core waveguide center using alignment marks, and perform ultraviolet photolithography (UV exposure lamp parameters: wavelength 365nm, 19-22mw / cm²). 2 The exposure time is 7 seconds, and the outer core layer is made into a waveguide of a preset shape;

[0054] S13. Post-baking: The photolithographically etched sample is placed on a hot plate for drying. A stepped heating method is used. First, the temperature is raised to 65°C and held for 10 minutes. Then, the temperature is raised to 95°C and held for 20 minutes. The hot plate is then turned off and the sample is allowed to cool naturally to room temperature. The refractive index is measured to be 1.5763 using an ellipsometer.

[0055] S14. Development: Develop the sample with SU-8 special developer, then clean off any remaining developer with isopropanol and rinse thoroughly with deionized water. Develop until the waveguide structure is clear; over-development or under-development will affect waveguide performance.

[0056] S15. Curing: Place the developed sample on a hot plate to heat and cure the film, removing residual solvent. The temperature is 120℃ and the time is 30 minutes. Then turn off the hot plate and let it cool to room temperature naturally.

[0057] S16. Spin-coating PMMA top coating: Laboratory-prepared PMMA was used as the top coating. Before use, the material was placed on a heated magnetic stirrer for a period of time to prevent it from becoming too viscous. The PMMA top coating was then spin-coated using a spin coater. The spin-coating parameters were 3000 rad / min and 20 seconds. The refractive index was measured to be 1.485 using an ellipsometer.

[0058] S17. Curing: Place the spin-coated sample on a hot plate and heat it to cure the film at 120°C for 30 minutes. Then turn off the hot plate and allow it to cool naturally to room temperature.

[0059] The optical field of the fabricated waveguide was simulated, such as... Figure 5 The image shows the optical field simulation of three signal modes (LP01, LP11a, LP11b) and one pump mode (LP21b) using the beam propagation method in Comsol software. It can be observed that the optical field is well confined to the waveguide core region. The overlap integral factor between the three signal modes and the pump mode can be calculated using the overlap integral factor calculation formula. The differences between the three overlap integral factors are small. Substituting these values ​​into Matlab code allows for the simulation of the gain results. Figure 6 The figure shows the simulation curve of the gain of the optical waveguide amplifier fabricated in this embodiment as a function of pump power, calculated using Matlab software. The simulation results show that under the conditions of a 980nm pump light with a pump power of 400mW and pump mode LP21b, the gain of LP11b is 13.33dB, the gain of LP11a is 13.16dB, and the gain of LP01 is 12.79dB. The maximum gain difference between modes is 0.54dB. This demonstrates that the gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment in this embodiment can achieve high mode gain and low mode gain difference under single-mode pumping conditions, and has advantages such as simple testing system, low cost, and easy integration.

[0060] A test system for gain-equalized few-mode optical waveguide amplifiers based on refractive index distribution adjustment, such as... Figure 7 As shown, the output signal light from the tunable laser signal source (TLS) is split into three equal beams by a beam splitter (BS), and injected into the LP01, LP11a, and LP11b single-mode fiber ports of mode-selective photonic lantern 1 (MSPL1) to excite three signal modes. A 980nm pump light is injected into the LP21b single-mode fiber port of MSPL1. The LP01, LP11a, and LP11b signal lights are multiplexed with the LP21b pump light at the few-mode fiber output end of the photonic lantern and injected into a few-mode waveguide amplifier. The amplified signal light is then input to the few-mode fiber port of photonic lantern 2 (MSPL2) through end-face coupling and demultiplexed. The LP01, LP11a, and LP11b single-mode fiber output ports of photonic lantern 2 (MSPL2) are connected to a spectrometer (OSA: ANDOAQ-6315A) to test the gain characteristics of each signal mode. Simultaneously, a CCD camera is used to observe the light spots of each mode before and after amplification.

[0061] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0062] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable way without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0063] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment, characterized in that, From bottom to top, it consists of a silica substrate (1), an inner core layer (2), an outer core layer (3), and a polymethyl methacrylate (PMMA) top cladding layer (4). The inner core layer (2) and the outer core layer (3) are both strip structures. The inner core layer (2), the outer core layer (3), and the PMMA top cladding layer (4) are all located on the silica substrate (1). The inner core layer (2) is encased in the outer core layer (3), and the outer core layer (3) is encased in the PMMA top cladding layer (4). The inner core layer (2) and the outer core layer (3) are both polymer materials with the same concentration of doped nanoparticles, forming a double-layer crown structure core layer. The refractive index of the silicon dioxide substrate (1) is 1.444, the refractive index of the outer core layer (3) is 1.57-1.58, which is greater than the refractive index of the inner core layer (2), the refractive index difference between the inner and outer core layers is 0.01-0.02, and the refractive index of the upper cladding layer (4) is 1.48-1.

50.

2. The gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment as described in claim 1, characterized in that, The inner core layer (2) has a width of 3-4 μm and a thickness of 3-4 μm, the outer core layer (3) has a width of 6-7 μm and a thickness of 1-2 μm, and is clad on the inner core layer (2). The upper cladding layer (4) on the silicon dioxide substrate (1) has a thickness of 7-10 μm; the doping concentration is on the order of 10. 20 -10 21 cm -3 .

3. The gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment as described in claim 1, characterized in that, The doped nanoparticles are NaYF4:Er 3+ ,Yb 3+ Nanoparticles, BayF4:Er 3+ ,Yb 3+ Nanoparticles or NaYF4:Er 3 + ,Yb 3+ Ce 3+ Nanoparticles.

4. The gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment as described in claim 1, characterized in that, The inner core layer uses SU-8 2005 polymer material, and the outer core layer uses SU-8 2002 polymer material.

5. The fabrication method of a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment as described in claim 1, characterized in that, Specifically, the steps include the following: S1. Clean the silicon dioxide substrate sequentially with acetone, ethanol, and deionized water. S2. Prepare polymer solutions for the inner and outer core layers of doped nanoparticles; S3. The core layer solution is coated onto a silicon dioxide substrate using a spin coating method and then heated to cure, forming the core layer. S4. Align photomask I with the core layer, expose with ultraviolet light, bake, and develop to transfer the pattern on photomask I onto the core layer, and heat to cure the hard film to form the inner core layer waveguide. S5. The outer core layer solution is coated onto the inner core layer using a spin coating method, and then heated and cured to form the outer core layer. S6. Align the photomask II with the inner core waveguide, perform ultraviolet exposure for overlay etching, then bake and develop to overlay the outer core waveguide onto the inner core waveguide, and heat to cure the hard film to form the outer core waveguide. S7. A cyclopentanone solution of polymethyl methacrylate is coated onto the waveguide using a spin coating method, and then heated and cured to form the upper cladding.

6. The fabrication method of a gain-equalized few-mode optical waveguide amplifier based on refractive index distribution adjustment as described in claim 5, characterized in that, Step S2, the preparation of the core layer polymer solution, specifically includes the following: Take a NaYF4:20%Yb solution with a concentration of 1 mg / ml. 3+ ,2%Er 3+ @NaYF4 nanoparticle solutions were doped with SU-8 2005 and SU-82002 at a mass ratio of 1:11, and then sonicated at 40 kHz for 3 hours to ensure thorough mixing. Inner and outer core materials were then prepared accordingly. Er was measured. 3+ The doping concentration is 2.8 × 10⁻⁶. 20 cm -3 Yb 3+ The doping concentration is 2.8 × 10⁻⁶. 21 cm -3 .

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