Concentration and refractive index coordinated regulation gain equalization optical waveguide amplifier and preparation method thereof

Abstract

The invention relates to a concentration and refractive index coordinated regulation gain equalization optical waveguide amplifier and a preparation method thereof, and belongs to the technical field of polymer optical waveguide amplifiers, the optical waveguide amplifier structurally comprises a substrate (1), an outer core layer (2), an inner core layer (3) and an upper cladding (4); the preparation method comprises the steps of cleaning the substrate, preparing inner and outer core layer solutions, preparing the inner core layer, preparing the outer core layer, preparing the upper cladding and the like. The few-mode optical waveguide amplifier designed by the invention can realize balanced amplification of five signal modes under the condition of single-mode pumping, can compensate for the loss problem caused by an on-chip high-capacity system, and assists in realizing expansion of a communication system.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer optical waveguide amplifiers, and in particular relates to a concentration and refractive index coordinated control gain balanced optical waveguide amplifier and a preparation method thereof. Background Art

[0002] At present, a new round of technological revolution represented by AI is sweeping the world. Against the background of massive growth and upgrading of computing power infrastructure, optical chips based on on-chip optical interconnection, as the core components of optical modules, are in increasing demand. Ultra-large capacity, ultra-high speed, and larger-scale functional integration have become the core trends of optical chips. Space division multiplexing technology is one of the main ways to increase the capacity of optical modules. It is considered to be a promising solution to alleviate the capacity limitations of transmission systems and has received widespread attention from the global academic community.

[0003] Mode Division Multiplexing (MDM) is an on-chip manifestation of space division multiplexing. It uses modes as multiplexed transmission channels, which can multiply the single-wavelength communication capacity in a mode-channel-dependent manner. However, the expansion of communication systems strongly depends on the development of optical amplifiers. When the transmission mode is expanded, the deployment density of the core components of the system will increase further. The insertion loss of additional multifunctional devices and the transmission loss of multi-channel modes will greatly limit the scale of on-chip integration, thereby restricting the realization of large-capacity on-chip systems. Therefore, facing the loss problem brought about by large-capacity on-chip systems, few-mode optical waveguide amplifiers with high mode gain and balance are the key technologies currently being studied.

[0004] At present, there have been some reports on few-mode optical waveguide amplifiers, which mainly balance the signal mode gain through reconfigurable pump configuration schemes, refractive index adjustment schemes and concentration layered doping schemes. The pump mode control scheme requires the use of multiple mode pumps when balancing the gain. This scheme makes the system complex and is even more unfavorable for on-chip integration. In the double-layer crown structure of the latter two schemes, since the core layer refractive index presents an incompletely symmetrical structure, distortion will occur when transmitting higher-order modes, such as the LP21 mode, and balanced amplification of multiple mode transmission cannot be achieved.

[0005] In the existing rectangular waveguide structure, the high-order mode light field is mainly distributed at the edge of the core layer area, while the fundamental mode light field is mainly distributed in the center of the core layer area, resulting in the gain of the high-order mode being lower than that of the fundamental mode, making it difficult to achieve gain balance under a single pump condition; however, in the double-layer crown waveguide structure prepared based on the existing overlay process, since the refractive index of the core layer is an incompletely symmetrical structure, mode distortion occurs when transmitting higher-order modes, resulting in the problem of being unable to transmit and amplify higher-order modes. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a gain-balanced few-mode optical waveguide amplifier with coordinated concentration and refractive index control and a preparation method thereof. The method is based on a double-layer annular structure few-mode waveguide, and by fine-tuning the refractive index of the inner and outer core waveguides, a completely symmetrical refractive index structure is achieved in the core layer to ensure distortion-free transmission of higher-order modes. Concentration doping control is introduced, the doping concentration of nanoparticles in the inner and outer core layers is designed, and the interaction between the light field distribution of each mode and the gain medium is adjusted to achieve the goal of balanced amplification of multiple signal modes by only pumping a single mode.

[0007] The present invention is achieved through the following technical solutions: A concentration and refractive index coordinated control gain balanced optical waveguide amplifier, the structure includes a substrate 1, an outer core layer 2, an inner core layer 3 and an upper cladding layer 4; wherein the outer core layer 2, the inner core layer 3 and the upper cladding layer 4 are jointly located on the substrate 1, the inner core layer 3 is annularly coated in the outer core layer 2, the outer core layer 2 is coated in the upper cladding layer 4, the refractive index of the outer core layer 2 is higher than the refractive index of the inner core layer 3, the inner and outer core layers are polymer materials doped with nanoparticles, wherein the doping concentration of the inner core layer 3 is lower than the doping concentration of the outer core layer 2, and together constitute a double-layer annular structure core layer with coordinated control of concentration and refractive index.

[0008] Furthermore, the substrate 1 is a silicon dioxide substrate with a refractive index of 1.444-1.451, the refractive index of the outer core layer 2 is 1.57-1.58, which is greater than the refractive index of the inner core layer 3, and the refractive index difference between the inner and outer core layers is 0.005-0.01, and the refractive index of the upper cladding layer 4 is 1.48-1.50.

[0009] Furthermore, the inner core layer 3 has a width of 3-4 μm and a thickness of 3-4 μm, the outer core layer 2 coated on the inner core layer 3 has a width of 1-2 μm, and the upper cladding layer 4 on the silicon dioxide substrate 1 has a thickness of 7-10 μm.

[0010] Furthermore, the doped nanoparticles are NaYF4:Er 3+ ,Yb 3+ Nanoparticles, BaYF4:Er 3+ ,Yb 3+ Nanoparticles, NaYF4:Er 3+ ,Yb 3+ ,Ce 3+ or NaYF4:Er 3+ ,Yb 3+ ,Gd 3+ Nanoparticles.

[0011] Further, the polymer material used in the inner core layer 3 is SU-8 2005, and the polymer material used in the outer core layer 2 is SU-8 2002, but it is not limited to these two SU-8 polymer materials, as long as the refractive index of the polymer material doped with nanoparticles meets the designed refractive index value. The material used for the upper cladding layer 4 is polymethyl methacrylate, but it is not limited to this material.

[0012] Preferably, the doping concentration of the inner core layer 3 is 2.67*1025cm-3, and the doping concentration of the outer core layer 2 is 6.06*10 25 cm -3 .

[0013] On the other hand, the present invention also provides a method for preparing a concentration and refractive index coordinated control gain balanced optical waveguide amplifier, comprising the following steps: S1. Cleaning the silicon dioxide substrate with acetone, ethanol and deionized water in sequence; S2, preparing inner and outer core layer polymer solutions doped with nanoparticles; S3, coating the bottom outer core layer solution on the silicon dioxide substrate by spin coating, heating and curing, photolithography and baking to form the bottom outer core layer; S4, after the core layer of step S3 is cooled to room temperature, the inner core layer solution is coated on the surface thereof by spin coating, and heated and cured to form the inner core layer; S5, aligning the photoresist plate I with the inner core layer, UV exposure, post-baking, and development, transferring the pattern on the photoresist plate I to the inner core layer, and heating to cure the hard film to form an inner core layer waveguide; S6 uses a spin coating method to coat the outer core layer solution on the inner core layer, and heats and solidifies it to form an upper outer core layer; S7, aligning the photoresist II with the inner core waveguide, overlaying by ultraviolet exposure, baking and developing, overlaying the outer core waveguide onto the inner core waveguide, and heating to cure the hard film to form the outer core waveguide; S8. Spin-coat a cyclopentanone solution of polymethyl methacrylate onto the waveguide, and heat and cure the solution to form an upper cladding layer.

[0014] Furthermore, the step S2 of preparing the inner and outer core layer polymer solutions doped with nanoparticles comprises the following specific steps: Take a 1 mg / ml NaYF4:5%Gd3+,20%Yb3+,2%Er3+@NaYF4 nanoparticle solution and mix it with SU-8 2005 at a mass ratio of 1:25, and mix it with SU-8 2002 at a mass ratio of 1:11, and ultrasonicate them at 40kHz frequency in an ultrasonic machine for 3 hours to fully mix them, and prepare them into inner and outer core layer materials.

[0015] Compared with the prior art, the advantages of the present invention are as follows: The present invention discloses a gain-balanced few-mode optical waveguide amplifier with coordinated concentration and refractive index regulation and a preparation method thereof. In order to realize distortion-free transmission of multiple signal modes and gain balance at the same time, a concentration-layered doped annular waveguide structure is proposed, and the traditional core area is divided into an inner core layer and an outer core layer. Through the design of a double-layer annular refractive index matching structure, the core layer can ensure distortion-free transmission of multiple signal light modes, and at the same time, the refractive index of the outer core layer is increased, so that the interaction between the high-order mode and the gain medium is increased. The overlapping integral factor is used to verify that the gain balance of multiple signal lights cannot be achieved using a single-mode pump. On this basis, concentration-layered doping regulation is introduced, and the degree of interaction between each signal mode and the gain medium is further balanced by calculating the optimal ratio of the doping concentration of the inner and outer core layer nanoparticles, so as to realize gain-balanced amplification of multiple signal modes under the condition of single-mode pumping.

[0016] The gain-balanced few-mode optical waveguide amplifier with coordinated regulation of concentration and refractive index described in the present invention can achieve few-mode gain balance of multiple signal modes using only a single-mode pump, and the test system is simple and easy to integrate. It can well solve the defect of the small number of amplification modes in the prior art, make up for the loss problem caused by large-scale system integration on a chip, and promote the expansion and development of communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. 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 according to the actual scale.

[0018] FIG1 is a schematic diagram of the structure of a gain-balanced few-mode optical waveguide amplifier with coordinated regulation of concentration and refractive index, and a distribution diagram of refractive index and doping concentration of the present invention; FIG2 is a schematic flow diagram of a method for preparing a gain-balanced few-mode optical waveguide amplifier with coordinated concentration and refractive index control according to the present invention; FIG3 is a simulation diagram of the light field distribution of five signal modes (LP01, LP11a, LP11b, LP21a, LP21b) and one pump light mode (LP01) of a gain-balanced few-mode optical waveguide amplifier with coordinated regulation of concentration and refractive index of the present invention using the beam propagation method in Comsol software; FIG. 4 is a curve showing the relationship between the few-mode gain and the pump power of a gain-balanced few-mode optical waveguide amplifier with coordinated regulation of concentration and refractive index according to the present invention, obtained by Matlab software programming simulation. DETAILED DESCRIPTION

[0019] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification: Example 1

[0020] As shown in FIG1 , the present embodiment provides a gain-balanced few-mode optical waveguide amplifier combining concentration-layered doping with refractive index regulation, wherein the few-mode optical waveguide amplifier is composed of a silicon dioxide substrate 1, an outer core layer 2, an inner core layer 3, and a polymethyl methacrylate (PMMA) upper cladding layer 4 from bottom to top; the outer core layer 2, the inner core layer 3, and the polymethyl methacrylate upper cladding layer 4 are located together on the silicon dioxide substrate 1, the outer core layer 2 is annularly coated on the inner core layer 3, and the outer core layer 2 is coated in the polymethyl methacrylate upper cladding layer 4, the outer core layer 2 and the inner core layer 3 are polymer materials doped with different concentrations of nanoparticles, wherein the doping concentration of the inner core layer 3 is lower than the doping concentration of the outer core layer 2, forming a double-layer annular structure core layer with concentration-layered doping.

[0021] In this embodiment, the outer core layer 2 is SU-8 2002 with a refractive index n1 of 1.572, the inner core layer 3 is SU-8 2005 with a refractive index n2 of 1.565, and the refractive index difference n1-n2 is 0.007, but it is not limited to these two SU-8 polymers. It only needs that the refractive index of the polymer material doped with nanoparticles meets the designed refractive index value.

[0022] In this embodiment, the inner core layer 3 has a width of about 4 μm and a thickness of about 4 μm, the outer core layer 2 coated on the inner core layer 3 has a width of about 1 μm, and the thickness of the polymethyl methacrylate upper cladding layer 4 on the silicon dioxide substrate 1 is 7 μm.

[0023] In this embodiment, the doped nanoparticles are NaYF4:5%Gd 3+ ,20% Yb 3+ ,2% Er 3+ @NaYF4 core-shell nanoparticles, but not limited to these nanoparticles (it can also be BaYF4:Er 3+ ,Yb 3+ 、NaYF4:Er 3+ ,Yb 3+ ,Ce 3+ wait); In this embodiment, a NaYF4:5%Gd3+,20%Yb3+,2%Er3+@NaYF4 nanoparticle solution with a concentration of 1mg / ml was mixed with SU-8 2005 at a mass ratio of 1:25 and with SU-8 2002 at a mass ratio of 1:11, and then ultrasonicated at a frequency of 40kHz for 3h in an ultrasonic machine to fully mix them, and prepared into inner and outer core layer materials.

[0024] The few-mode optical waveguide amplifier of this embodiment can simultaneously transmit and amplify five LP modes at 1550nm, namely LP01, LP11a, LP11b, LP21a, and LP21b, and can make the gain difference between modes less than 1dB through single-mode pumping (LP01), thereby achieving few-mode gain balance.

[0025] Example 2 This embodiment provides a method for preparing a gain-balanced few-mode optical waveguide amplifier combining concentration-layered doping with refractive index regulation. The process flow is shown in FIG2 , and specifically includes the following steps: S1. Cleaning of silica substrate: Use acetone cotton to wipe off the oil on the surface of the substrate, then use ethanol cotton to wipe off inorganic salts, and finally rinse with deionized water, clean it and blow dry it for later use; S2. Core material preparation: Take NaYF4:5%Gd with a concentration of 1mg / ml 3+ ,20% Yb 3+ ,2% Er 3+ @NaYF4 nanoparticle solution was doped with SU-8 2005 at a mass ratio of 1:25 and with SU-8 2002 at a mass ratio of 1:11, and ultrasonicated at 40kHz frequency for 3h in an ultrasonic machine to fully mix them, and then provided with inner and outer core layer materials respectively; S3, upper cladding polymethyl methacrylate (PMMA) preparation: weigh 3g of polymethyl methacrylate powder with oil paper and put it into a conical flask, add 27g of cyclopentanone, then add a rotor, and then put it on a heated magnetic stirrer, set the parameters to 200rad / min, temperature to 45℃, and stir for about 8h; S4, spin coating the bottom outer core layer material: After the outer core layer material after ultrasound is filtered through a 0.45 μm molecular sieve, it is spin coated on the silicon dioxide substrate. The substrate is slightly tilted with pointed tweezers to spread the material all over the substrate. Then, the coating was performed by spin coating using a coating machine at a speed of 3500 rad / min and a time of 30 seconds. S5, pre-baking: use a hot plate to dry the sample, using a step-by-step heating method, first heating to 60°C and maintaining this temperature for 10 minutes, then continue to heat to 90°C and maintain for 20 minutes, turn off the hot plate, and naturally cool to room temperature to form a bottom outer core layer film with a thickness of 1 μm; S6, exposure: the film cooled to room temperature is subjected to ultraviolet exposure (parameters of ultraviolet exposure lamp: wavelength 365nm, 19-22mw / cm2), the exposure time is 6.5 seconds, to form a bottom outer core layer; S7, post-baking: Place the exposed sample on a hot plate for drying, also using the step-by-step heating method, first raise the temperature to 65°C and keep this temperature for 10 minutes, then continue to raise the temperature to 95°C and keep it for 20 minutes, turn off the hot plate, and naturally cool to room temperature; S8, spin coating the inner core layer material: the inner core layer material after ultrasound is filtered through a 0.45 μm molecular sieve and then spin coated on the bottom outer core layer, the substrate is slightly tilted with pointed tweezers, the material is spread all over the substrate, and the spin coating is performed using a coating machine, and the spin coating parameters are first 500 rad / min, time 10 seconds, and then 5000 rad / min, time 20 seconds; S9, pre-baking: use a heating plate to dry the sample, and use a step-by-step heating method to dry it. First, heat it to 60°C and keep this temperature for 10 minutes, then continue to heat it to 90°C and keep it for 20 minutes. Turn off the heating plate and naturally cool it to room temperature to form an inner core layer film with a thickness of 4μm on the bottom outer core layer; S10, photolithography: using photolithography plate I to perform ultraviolet photolithography on the inner core layer film after cooling to room temperature (parameters of ultraviolet exposure lamp: wavelength 365nm, 19-22mw / cm2), the exposure time is 8.6 seconds, to form an inner core layer waveguide; S11, post-baking: Place the photolithographic sample on a hot plate for drying, also using the step-by-step heating method, first raise the temperature to 65°C and keep this temperature for 10 minutes, then continue to raise the temperature to 95°C and keep it for 20 minutes, turn off the hot plate, and naturally cool to room temperature; S12, Development: Develop the sample with SU-8 special developer, then clean the residual developer with isopropyl alcohol and rinse with deionized water. Develop until the waveguide structure is clear. Over-development or under-development will affect the waveguide performance. S13, curing: put the developed sample on a hot plate to heat and cure the hard film to remove the residual solvent. The temperature is 120°C for 30 minutes, then turn off the hot plate and cool to room temperature naturally. S14, spin coating the outer core layer material: the outer core layer material after ultrasound is filtered through a 0.45 μm molecular sieve and then spin coated on the inner core layer, the substrate is slightly tilted with pointed tweezers, the material is spread all over the substrate, and the coating is performed using a coating machine, the spin coating parameters are 3500 rad / min, and the time is 30 seconds; S15, pre-baking: Use a hot plate to dry the sample. Use a step-by-step heating method to dry, first raise the temperature to 60°C and keep this temperature for 10 minutes, then continue to raise the temperature to 90°C and keep it for 20 minutes, turn off the hot plate, and naturally cool to room temperature to form an outer core layer film with a thickness of about 1μm; S16, photolithography: Use photolithography plate II as a mask, align the center of the outer core layer waveguide with the center of the inner core layer waveguide according to the plate alignment mark, perform ultraviolet photolithography (UV exposure lamp parameters: wavelength 365nm, 19-22mw / cm2), exposure time is 6.5 seconds, and engrave the outer core layer on the inner core layer waveguide; S17, post-baking: Place the photolithographic sample on a hot plate for drying, using a step-by-step heating method, first raise the temperature to 65°C and maintain this temperature for 10 minutes, then continue to raise the temperature to 95°C and maintain it for 20 minutes, turn off the hot plate, and naturally cool to room temperature; S18, Development: Develop the sample with SU-8 special developer, then clean the residual developer with isopropyl alcohol and rinse with deionized water. Develop until the waveguide structure is clear. Over-development or under-development will affect the waveguide performance. S19, curing: put the developed sample on a hot plate to heat and cure the hard film and remove the residual solvent. The temperature is 120°C for 30 minutes, then turn off the hot plate and let it cool to room temperature naturally; S20, spin coating PMMA upper cladding: Use PMMA prepared in the laboratory as the upper cladding, place it on a heated magnetic stirrer for a period of time before use to make the material not too viscous, and use a glue spreader to spin coat the PMMA upper cladding. The spin coating parameters are 3000rad / min, time 20 seconds; S21. Curing: Place the spin-coated sample on a hot plate to heat and cure the film at 120°C for 30 minutes, turn off the hot plate, and allow to cool naturally to room temperature.

[0026] The light field of the prepared waveguide is simulated. As shown in FIG3, the light field simulation diagram of five signal modes (LP01, LP11a, LP11b, LP21a, LP21b) and one pump mode (LP01) is simulated using the beam propagation method in Comsol software. It can be found that the light field is well confined in the waveguide core area; by calculating the optimal doping concentration ratio of the inner and outer core layers, the gain balance of the five signal modes can be achieved when the LP01 mode is pumped, and the calculated results can be substituted into the Matlab code to simulate the gain results. As shown in FIG4, the gain of the optical waveguide amplifier prepared in the embodiment is simulated by using Matlab software programming to change the relationship between the pump power. The simulation results show that under the conditions of 980nm pump light pump power of 350mW and pump mode LP01, the average gain of the five signal modes is 10.604dB, and the maximum gain difference between modes is 0.48dB. This proves that the gain-balanced few-mode optical waveguide amplifier with concentration layered doping combined with refractive index regulation in this embodiment can achieve high mode gain and low mode gain difference under single mode pumping conditions, and has the advantages of simple test system, It has the advantages of low cost and easy integration.

[0027] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0028] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0029] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A concentration and refractive index coordinated gain-balanced optical waveguide amplifier, comprising a substrate (1), an outer core layer (2), an inner core layer (3) and an upper cladding layer (4); characterized in that: The outer core layer (2), the inner core layer (3) and the upper cladding layer (4) are located together on the substrate (1); the inner core layer (3) is annularly coated in the outer core layer (2); the outer core layer (2) is coated in the upper cladding layer (4); the refractive index of the outer core layer (2) is higher than the refractive index of the inner core layer (3); the inner and outer core layers are polymer materials doped with nanoparticles; the doping concentration of the inner core layer (3) is lower than the doping concentration of the outer core layer (2); and together they constitute a double-layer annular structure core layer in which concentration and refractive index are coordinated and regulated.

2. The concentration and refractive index coordinated control gain balanced optical waveguide amplifier according to claim 1, characterized in that: The substrate (1) is a silicon dioxide substrate with a refractive index of 1.444-1.451, the refractive index of the outer core layer (2) is 1.57-1.58, which is greater than the refractive index of the inner core layer (3), and the refractive index difference between the inner and outer core layers is 0.005-0.01, and the refractive index of the upper cladding layer (4) is 1.48-1.

50.

3. The concentration and refractive index coordinated gain balanced optical waveguide amplifier according to claim 1, characterized in that: The inner core layer (3) has a width of 3-4 μm and a thickness of 3-4 μm, the outer core layer (2) coated on the inner core layer (3) has a width of 1-2 μm, and the upper cladding layer (4) on the silicon dioxide substrate (1) has a thickness of 7-10 μm.

4. The concentration and refractive index coordinated gain balanced optical waveguide amplifier according to claim 1, characterized in that: The doped nanoparticles are NaYF4:Er 3+ ,Yb 3+ Nanoparticles, BaYF4:Er 3+ ,Yb 3+ Nanoparticles, NaYF4:Er 3+ ,Yb 3+ ,Ce 3+ or NaYF4:Er 3+ ,Yb 3+ ,Gd 3+ Nanoparticles.

5. The concentration and refractive index coordinated control gain balanced optical waveguide amplifier according to claim 1, characterized in that: The polymer material used in the inner core layer (3) is SU-8 2005, the polymer material used in the outer core layer (3) is SU-8 2002, and the material used in the upper cladding layer (4) is polymethyl methacrylate.

6. The concentration and refractive index coordinated control gain balanced optical waveguide amplifier according to claim 1, characterized in that: The doping concentration of the inner core layer (3) is 2.67*10 25 cm -3 The doping concentration of the outer core layer (2) is 6.06*10 25 cm -3 .

7. A method for preparing the concentration and refractive index coordinated control gain balanced optical waveguide amplifier as claimed in claim 1, comprising the following steps: S1. Cleaning the silicon dioxide substrate with acetone, ethanol and deionized water in sequence; S2, preparing inner and outer core layer polymer solutions doped with nanoparticles; S3, coating the bottom outer core layer solution on the silicon dioxide substrate by spin coating, heating and curing, photolithography and baking to form the bottom outer core layer; S4, after the core layer of step S3 is cooled to room temperature, the inner core layer solution is coated on the surface thereof by spin coating, and heated and cured to form the inner core layer; S5, aligning the photoresist plate I with the inner core layer, UV exposure, post-baking, and development, transferring the pattern on the photoresist plate I to the inner core layer, and heating to cure the hard film to form an inner core layer waveguide; S6 uses a spin coating method to coat the outer core layer solution on the inner core layer, and heats and solidifies it to form an upper outer core layer; S7, aligning the photoresist II with the inner core waveguide, ultraviolet exposure for overlay, post-baking and development, overlaying the outer core waveguide onto the inner core waveguide, and heating to cure the hard film to form the outer core waveguide; S8. Spin-coat a cyclopentanone solution of polymethyl methacrylate onto the waveguide, and heat and cure the solution to form an upper cladding layer.

8. The method for preparing a gain-balanced few-mode optical waveguide amplifier with coordinated concentration and refractive index control according to claim 7, characterized in that: The step S2 is to prepare the inner and outer core layer polymer solutions doped with nanoparticles, and the specific steps are: Take a 1 mg / ml NaYF4:5%Gd3+,20%Yb3+,2%Er3+@NaYF4 nanoparticle solution and mix it with SU-8 2005 at a mass ratio of 1:25, and mix it with SU-8 2002 at a mass ratio of 1:11, and ultrasonicate them at 40kHz frequency in an ultrasonic machine for 3 hours to fully mix them, and prepare them into inner and outer core layer materials.

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