Liquid crystal modulation variable optical attenuator based on multimode interference coupling structure

Abstract

The invention discloses a liquid crystal modulation variable optical attenuator based on a multimode interference coupling structure, and belongs to the technical field of planar optical waveguide devices. The device is composed of a silicon dioxide substrate, a polymer optical waveguide core layer, a polymer cladding, a graphene lower electrode, a liquid crystal groove, a liquid crystal material, an ITO upper electrode and a glass substrate. The polymer optical waveguide core layer and the polymer cladding layer have the same thickness and are located on the silicon dioxide substrate together, the polymer optical waveguide core layer is wrapped in the polymer cladding layer, the graphene lower electrode is located on the polymer optical waveguide core layer and the polymer cladding layer, the liquid crystal groove is located on the graphene lower electrode, and the liquid crystal groove is located on the silicon dioxide substrate. The liquid crystal material is filled in the liquid crystal groove, and the glass substrate covers the liquid crystal material and the liquid crystal groove; and the ITO upper electrode is prepared on the glass substrate and is arranged in the liquid crystal material towards the graphene lower electrode. The variable optical attenuator combines the characteristics of large MMI process tolerance, small size and easy integration, and has practical application significance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of planar optical waveguide devices, and particularly relates to a liquid crystal modulation variable optical attenuator based on a multimode interference coupling structure and modulated by liquid crystal materials. TECHNICAL BACKGROUND

[0002] With the rapid development of the Internet, optical communication has gradually become an important part of people's daily life. With the increase of information, the wavelength division multiplexing technology plays an important role in the whole optical communication network, and a large amount of information is transmitted through the multiplexing and demultiplexing technology of different signals. The wavelength division multiplexing technology can multiplex and demultiplex multiple wavelengths of optical signals, so as to improve the transmission efficiency and system capacity of the optical communication system. In the dense wavelength division multiplexing system, the light field intensity needs to be adjusted during transmission, so a variable optical attenuator is usually introduced between single channels to realize accurate regulation of the light field intensity, effectively improve the system accuracy and flexibility of the optical network, and the variable optical attenuator is an indispensable part of the whole optical network.

[0003] The variable optical attenuator includes mechanical technology, adjustable diffraction grating technology, MEMS technology, liquid crystal technology, magneto-optic technology, planar optical waveguide technology, etc., and mainly relies on the characteristics of semiconductor materials to realize the adjustment of the attenuation of the optical signal. Among them, the combination of liquid crystal and optical waveguide technology has become the mainstream research direction.

[0004] The performance indicators of the variable optical attenuator mainly include insertion power consumption, loss, working bandwidth, channel crosstalk, maximum attenuation amount, etc. For the variable optical attenuator, a larger attenuation amount should be obtained under the premise of maintaining low power consumption and large bandwidth, so as to obtain ideal performance. The variable optical attenuator can use different materials to realize the above performance, and the liquid crystal material has the advantages of low manufacturing cost, large process tolerance and low power consumption, which can meet the research and development needs of the variable optical attenuator. SUMMARY

[0005] The purpose of the application is to provide a liquid crystal modulation variable optical attenuator based on a multimode interference coupling structure, which can be used in a dense wavelength division multiplexing system. The variable optical attenuator can adjust the intensity of the input optical signal and realize the attenuation of the signal. The device adopts an active structure, changes the crystal direction of the liquid crystal material through an external electric field, changes the effective refractive index of the optical signal in the waveguide, and realizes the attenuation of the optical signal intensity.

[0006] As shown in Figure 1 (a) and Figure 1(b) As shown, the liquid crystal modulation variable optical attenuator of the multi-mode interference coupling structure of the present application is a planar optical waveguide structure, which is composed of a silicon dioxide substrate 9, a polymer optical waveguide core layer, a polymer cladding layer 4, a graphene lower electrode 5, a liquid crystal groove 6, a liquid crystal material 7, an ITO upper electrode 8, and a glass substrate 10; the polymer optical waveguide core layer and the polymer cladding layer 4 have the same thickness, and both are located on the silicon dioxide substrate 9, and the polymer optical waveguide core layer is covered in the polymer cladding layer 4, the graphene lower electrode 5 is located on the polymer optical waveguide core layer and the polymer cladding layer 4, the liquid crystal groove 6 is located on the graphene lower electrode 5, the liquid crystal material 7 is filled in the liquid crystal groove 6, and the glass substrate 10 covers the liquid crystal material 7 and the liquid crystal groove 6; the ITO upper electrode 8 is prepared on the glass substrate 10 and is placed in the liquid crystal material 7 towards the graphene lower electrode 5; the graphene lower electrode 5, the liquid crystal groove 6, the liquid crystal material 7, and the ITO upper electrode 8 constitute an adjusting area.

[0007] The structure of the polymer optical waveguide core layer is shown in the accompanying drawings Figure 1 (c) As shown, the symmetric structure along the light input direction is sequentially composed of an input waveguide 1, a multi-mode interference coupling waveguide 2, and an output waveguide 3; the input straight waveguide 1 has a width W1=3~6μm and a length L1=50~1000μm; the multi-mode interference coupling waveguide 2 has a length L2=200~1000μm and a width W2=10~40μm; the output straight waveguide 3 has a length L3=50~1000μm and a width W3=3~6μm; the polymer optical waveguide core layer has a thickness H1=2~10μm; the graphene lower electrode 5 has a length L4=300~1100μm, a width W4=20~50μm, and a thickness H4=0.34~1.4nm; the liquid crystal groove 6 has a length L5=250~1050μm, a width W5=15~45μm, a groove depth H2=3~5μm, and a groove sidewall thickness X1=1~6μm, and L4>L5>L2, W4>W5>W2; the ITO upper electrode 8 has a width W6=4~10μm, a thickness H3=50~200nm, and a length L6=240~1040μm, which is slightly smaller than the length L5 of the groove inside the liquid crystal groove 6, and W5-2X1=W2, L5-2X1=L2; along the light input direction, the left and right center positions of the ITO upper electrode 8 are offset from the left and right center positions of the multi-mode interference coupling waveguide 2 by 4~9μm.

[0008] The preparation method of the variable optical attenuator of the present application is shown in the accompanying drawings Figure 3 , and the specific steps are as follows:

[0009] A: Cleaning of the silicon dioxide substrate

[0010] Use the cotton ball soaked in acetone solution to wipe the silicon substrate in the same direction for several times: then use the cotton ball soaked in ethanol solution to wipe the silicon substrate in the same direction for several times; after the surface is cleaned, use deionized water to flush the surface of the silicon wafer in the wiping direction, and use the ear cleaning ball to dry the surface of the silicon substrate after cleaning, and then put it into a clean culture dish;

[0011] B: Preparation of the polymer optical waveguide core layer

[0012] The polymer optical waveguide core layer (the polymer optical waveguide core layer material is a series of wet-etchable ultraviolet negative photoresist materials including EpoCore, SU-82002, SU-8 2005, etc., and the refractive index of the polymer optical waveguide core layer material is higher than that of the polymer cladding layer material) is spin-coated on the silicon substrate by a spin coating process to form a polymer optical waveguide core layer film, and the rotation speed of the spin coater is set to 1000-5000 revolutions per minute to prepare the polymer optical waveguide core layer; then the prepared polymer optical waveguide core layer is pre-baked at 40-120°C for 10-30 minutes, and then naturally cooled to room temperature; the polymer optical waveguide core layer film is subjected to photolithography, the wavelength of the ultraviolet light is 350-400 nm, the waveguide mask is complementary to the polymer optical waveguide core layer structure to be prepared, and the waveguide mask is tightly attached to the polymer optical waveguide core layer film after exposure, the exposure time is 4-40 seconds, and the polymer optical waveguide core layer film in the polymer optical waveguide core layer structure to be prepared is exposed to ultraviolet light; the silicon substrate after photolithography is taken out of the photolithography machine and subjected to post-baking, i.e. baking at 40-100°C for 10-40 minutes, and then naturally cooling to room temperature; the prepared polymer optical waveguide core layer is developed, i.e. soaked in a developing solution for 2-15 seconds, and then the residual core layer material on the surface is cleaned with isopropyl alcohol, and the above operation is repeated multiple times until a clear mask corresponding to the polymer waveguide core layer is obtained; the surface of the obtained device is then washed with deionized water and dried with an ear cleaning ball in the same direction, and finally baked at 100-150°C for 20-70 minutes to harden the film, and the preparation of the polymer optical waveguide core layer is completed after natural cooling to room temperature;

[0013] C: Preparation of the polymer cladding layer

[0014] The polymer cladding layer material PMMA is spin-coated on the polymer optical waveguide core layer and the silicon substrate by a spin coating process, the rotation speed of the spin coater is set to 1000-3000 revolutions per minute, and the rotation is performed for 10-40 seconds to ensure that the cladding layer material completely wraps the polymer optical waveguide core layer; after the spin coating is completed, the polymer waveguide cladding layer is prepared by baking at 100-150°C for 10-40 minutes, the thickness of the cladding layer is the same as that of the polymer optical waveguide core layer, and thus the preparation of the passive part of the entire device is completed;

[0015] D: Preparation of the graphene lower electrode

[0016] The single-layer graphene film with PMMA support layer is placed in a culture dish containing deionized water, and then transferred to the surface of the polymer optical waveguide core layer and polymer cladding layer, so that the polymer optical waveguide core layer is in close contact with the graphene film; after air drying at room temperature, heating at a temperature of 70-90°C for 10-50 min; after natural cooling to room temperature, a few drops of acetone solution are added to the surface of the PMMA support layer to remove the PMMA support layer, and then deionized water is used to remove the residual acetone solution, and finally the obtained device is heated at 80-100°C for 50-70 min, thereby completing the preparation of the graphene lower electrode;

[0017] E: Liquid crystal transfer:

[0018] First, a commonly used liquid crystal groove material (including EpoClad, EpoCore, NOA73, NOA63, PMMA, etc.) is spin-coated on the graphene electrode to form a thin film, and the spin coater speed is set to 1000-5000 revolutions per minute to prepare a liquid crystal groove film; then the prepared liquid crystal groove film is pre-baked, i.e. baked at 60-150°C for 4-30 minutes, and after baking, it is naturally cooled to room temperature; the liquid crystal groove film is photoetched, the ultraviolet light wavelength is 350-400 nm, the waveguide mask plate is complementary to the liquid crystal groove film structure to be prepared, and after the waveguide mask plate is tightly attached to the liquid crystal groove film, it is exposed for 4-40 seconds, so that the polymer optical waveguide core layer film in the polymer optical waveguide core layer structure to be prepared is exposed to ultraviolet light; the photoetched device is taken out of the photoetching machine and post-baked at 55-155°C for 10-30 minutes, and then naturally cooled to room temperature; the prepared liquid crystal groove film is developed, i.e. soaked in a developing solution for 3-15 seconds, and then the surface residual core layer material is cleaned with isopropyl alcohol, and the above operation is repeated multiple times until a clear mask corresponding liquid crystal groove is obtained; the obtained device is then rinsed with deionized water and dried with an ear cleaning ball in the same direction, and finally baked at 100-150°C for 20-60 minutes to harden the film, and then naturally cooled to room temperature to complete the preparation of the liquid crystal groove;

[0019] Then, a micro pipette is used to add liquid crystal material (such as nematic liquid crystal E7, K15, 5CB, etc.) to the entrance of the liquid crystal groove, and under the action of capillary force, the liquid crystal material spontaneously penetrates and fills the liquid crystal groove, realizing the transfer and introduction of the liquid crystal functional layer to the planar optical waveguide modulation area; finally, static treatment is performed to ensure uniform distribution of the liquid crystal material in the liquid crystal groove;

[0020] F: Preparation of ITO upper electrode:

[0021] Adopt ITO (Indium Tin Oxide) coated glass as upper electrode, first, use isopropanol, deionized water along the same direction to flush ITO coated glass, to remove surface contaminants, then spin a layer of positive photoresist on the ITO film surface of ITO coated glass, the rotation speed is 1000~3000 revolutions / minute, spin 20~30 seconds, after spin coating, heat at 50~100℃ for 10~30 minutes, photoetch the positive photoresist film, the wavelength of ultraviolet light is 350~400nm, the waveguide mask is the same as the upper electrode structure to be prepared, after the waveguide mask and the positive photoresist film are tightly attached, expose, the exposure time is 4~40 seconds, so that the area outside the electrode to be prepared is exposed to ultraviolet light, then develop in the developing solution, so that the photoresist in the exposed area is dissolved, thereby exposing the ITO film in the corresponding area, after development, immediately rinse the sample with deionized water, and dry with nitrogen, to obtain a clear photoresist mask pattern;

[0022] The ITO film is selectively etched by using a hydrochloric acid-based wet etching solution, the etching solution is prepared by diluting hydrochloric acid and deionized water in a certain volume ratio (the volume ratio of HCl to H2O is 1:5~10), after the excess part of the ITO film electrode is washed away, the device is again immersed in isopropanol to remove the residual photoresist, then isopropanol and deionized water are used for cleaning to obtain the required ITO upper electrode, then sealant is applied to the interface connection area between the liquid crystal groove periphery and the ITO coated glass to form a packaging frame, and the ITO coated glass with ITO upper electrode is aligned and attached to the waveguide core layer under a microscope, so that the ITO upper electrode faces the waveguide core layer direction, during the attaching process, the ITO coated glass side is slowly pressed together to effectively remove the interface air and avoid the generation of bubbles, to realize reliable packaging of the liquid crystal layer and integration of the ITO lower electrode, finally, the ITO upper electrode and the lower electrode together constitute an electrode structure for applying an electric field, thereby preparing a liquid crystal modulation variable optical attenuator based on a multimode interference coupling structure.

[0023] Compared with the existing device structure and preparation technology, the beneficial effects of the present application are:

[0024] The variable optical attenuator of the present application combines the characteristics of large process tolerance, small size and easy integration of the MMI process, and also utilizes the durability, stability and low thermal crosstalk of liquid crystal materials, to realize an integrated low-power variable optical attenuator, in addition, the process for preparing the device using polymer materials is relatively simple, only conventional processes such as spin coating and photoetching are needed, without high-difficulty processes, and the production cost is low, the efficiency is high, and mass production is possible, which is a variable optical attenuator that can be applied to practical applications. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1(a) is a schematic diagram of the cross section of the variable optical attenuator at the center position of the ITO upper electrode;

[0026] Figure 1 (b) is a top view of the variable optical attenuator;

[0027] Figure 1 (c) is a schematic diagram of the polymer optical waveguide core layer and the silica substrate structure;

[0028] Figure 2 is a schematic diagram of the cross section of the variable optical attenuator at the center position of the polymer optical waveguide core layer;

[0029] Figure 3 is a process flow chart of the variable optical attenuator;

[0030] Figure 4 is a process flow chart of the ITO upper electrode;

[0031] Figure 5 (a) is a light field transmission diagram of the variable optical attenuator core layer in the passive state;

[0032] Figure 5 (b) is a light field transmission diagram of the variable optical attenuator core layer in the upper and lower electrode energized state;

[0033] Figure 6 is a simulation curve of the output power and voltage relationship of the variable optical attenuator. DETAILED DESCRIPTION

[0034] Example 1

[0035] Referring to Figure 1 (a), (b), the variable optical attenuator is composed of a silica substrate 9, a polymer optical waveguide core layer, a polymer cladding layer 4, a graphene lower electrode 5, a liquid crystal groove 6, a liquid crystal material 7, an ITO upper electrode 8, and a glass substrate 10. The polymer optical waveguide core layer is composed of a first input straight waveguide 1, a multimode interference waveguide 2, and an output straight waveguide 3. The material of the polymer optical waveguide core layer is SU-8 2005, the material of the liquid crystal groove 6 is EpoCore, the material of the polymer cladding layer is PMMA, and the liquid crystal material is E7 (Liquid crystal mixture E7, SYNTHON Chemicals GmbH & Co. KG company sells mixed liquid crystal material). K15 (Sigma-Aldrich / Merck company: product name is 4'-Pentyl-4-biphenylcarbonitrile), 5CB (TCI: product name 4-Cyano-4'-pentylbiphenyl).

[0036] Referring to Figure 1(c), the polymer optical waveguide core layer is composed of input straight waveguide 1, multimode interference coupling waveguide 2 and output straight waveguide 3 along the left-right symmetrical structure of the light input direction, and the input straight waveguide 1 and the output straight waveguide 3 are front-back symmetrical structures about the multimode interference coupling waveguide 2; the width W1 of the input straight waveguide 1 is 4μm, and the length L1 is 60μm; the width W2 of the multimode interference coupling waveguide 2 is 25μm, and the length L2 is 680μm; the width W3 of the output straight waveguide 3 is 4μm, and the length L3 is 60μm; the thickness of the polymer optical waveguide core layer is H1=3μm. The graphene lower electrode 5 covers the entire multimode interference coupling waveguide 2, the length L4 is 700μm, the width W4 is 30μm, and the thickness H4 is 1nm; the width and length of the liquid crystal groove 6 inside the groove are slightly larger than the width and length of the multimode interference coupling waveguide 2, the length L5 is 684μm, the width W5 is 29μm, the groove depth H2 is 4μm, and the groove side wall thickness X1 is 2μm; the width W6 of the ITO upper electrode 8 is 8μm, the thickness H3 is 100nm, and the length L6 is 680μm, which is slightly smaller than the length of the liquid crystal groove 6 inside the groove; along the light input direction, the longitudinal center position of the ITO upper electrode 8 is offset from the longitudinal center position of the multimode interference coupling waveguide 2 by 6.8μm. Wherein, W5-2X1=W2, L5-2X1=L2.

[0037] Referring to Fig. 1, Figure 2 Fig. 2 is a schematic view of the cross section of the position of the multimode interference coupling waveguide 2 in the variable optical attenuator, from bottom to top in turn: silica substrate 9, SU-8 2005 polymer optical waveguide core layer 2, PMMA polymer cladding layer 4, graphene lower electrode 5, EpoCore liquid crystal groove 6, E7 liquid crystal material 7, ITO upper electrode 8, glass substrate 10; the thickness of the silica substrate 9 is 1.5mm, the thickness of the polymer optical waveguide core layer 2 is 3μm, and the thickness of the polymer cladding layer is 3μm.

[0038] Referring to Fig. 1, Figure 5 (a) is the waveguide core layer light field transmission diagram of the variable optical attenuator in the passive state, and the materials and waveguide sizes selected in embodiment 1 are selected; from the simulation diagram, it can be obviously obtained that when the input wavelength is 1550nm, due to the internal multimode interference effect, the imaging point length is the same as the multimode interference coupling waveguide length, and the cladding layer and the core layer have different refractive indexes, the light field is limited in the core layer, and the output power of the output straight waveguide 3 has almost no loss.

[0039] Referring to Fig. 1, Figure 5(b) is the light field transmission diagram of the waveguide core layer of the variable optical attenuator in the state of the applied voltage 1.87V, the materials and waveguide sizes selected in the selected embodiment 1 are selected, from the simulation diagram, it can be obviously obtained that when the input wavelength is 1550nm, the applied voltage changes the effective refractive index of the liquid crystal material, thereby causing the change of the internal light field of the multimode interference coupling waveguide, the imaging point shifts, and the output straight waveguide 3 has almost no light transmission.

[0040] Referring to Figure 6 , the output power of the output straight waveguide of the variable optical attenuator core layer changes with the applied voltage, and the maximum attenuation amount is reached when the applied voltage is 1.87V.

[0041] Embodiment 2

[0042] Referring to Figure 3 , Figure 4 , the liquid crystal modulation variable optical attenuator described in the application is prepared according to the following specific method:

[0043] A: Cleaning of the silicon dioxide substrate

[0044] A cotton ball soaked in an acetone solution is used to wipe the silicon dioxide substrate in the same direction for multiple times, and then a cotton ball soaked in an ethanol solution is used to wipe the silicon dioxide substrate in the same direction for 3 times; after the surface is cleaned, the surface of the silicon dioxide substrate is washed in the wiping direction with deionized water, and after cleaning is completed, the surface of the silicon dioxide substrate is blown dry with an ear cleaning ball and is placed in a clean culture dish;

[0045] B: Preparation of the polymer optical waveguide core layer

[0046] The polymer optical waveguide core layer material SU-8 2005 is spin-coated on the silica substrate to form a polymer optical waveguide core layer film by a spin coating process, the rotation speed of the glue spreader is set to 1200 revolutions per minute, and the rotation is 25 seconds, and a 3-micron-thick polymer optical waveguide core layer is prepared; then the prepared polymer optical waveguide core layer is pre-baked, that is, a stepwise heating is adopted on the heating plate, baking at 50°C for 2 minutes, then heating to 90°C for continuous baking for 15 minutes, and after baking, natural cooling to room temperature; the polymer optical waveguide core layer film is subjected to photoetching, the wavelength of ultraviolet light is 365 nm, the waveguide mask plate is complementary to the polymer optical waveguide core layer structure to be prepared, and after the waveguide mask plate is tightly attached to the polymer optical waveguide core layer film, exposure is performed, the exposure time is 8 seconds, and the polymer optical waveguide core layer film in the polymer optical waveguide core layer structure to be prepared is exposed to ultraviolet light; the photoetched silica substrate is taken out of the photoetching machine and subjected to post-baking, that is, a stepwise heating is adopted on the heating plate, baking at 50°C for 2 minutes, then heating to 87°C for 15 minutes, and after baking, natural cooling to room temperature; the prepared polymer optical waveguide core layer is developed, that is, soaked in a developing solution for 4 seconds, then the residual core layer material on the surface is cleaned with isopropyl alcohol, and the above operation is repeated multiple times until a clear mask plate corresponding polymer waveguide core layer is obtained; the surface of the obtained device is washed with deionized water and dried with an ear cleaning ball in the same direction, and finally the prepared polymer optical waveguide core layer is baked at 120°C for 30 minutes on the heating plate to harden the film, and the preparation of the polymer optical waveguide core layer is completed after natural cooling to room temperature;

[0047] C: Preparation of polymer cladding layer:

[0048] The PMMA material is spin-coated on the polymer optical waveguide core layer and the silica substrate by a spin coating process, the rotation speed of the glue spreader is set to 2500 revolutions per minute, and the spin coating is performed for 30 seconds to ensure that the cladding material completely wraps the polymer optical waveguide core layer; after the spin coating is completed, the polymer waveguide cladding layer is prepared by baking at 120°C for 30 minutes, the cladding layer thickness is 3μm, and thus the preparation of the passive part of the entire device is completed;

[0049] D: Preparation of graphene lower electrode:

[0050] A single-layer graphene film (1cm×1cm in size) with a PMMA support layer is placed in a culture dish containing deionized water, and then transferred to the surface of the polymer optical waveguide core layer and the polymer cladding layer to make the polymer optical waveguide core layer closely contact with the graphene film; after air drying at room temperature, heating is performed at a temperature of 80°C for 40 minutes; after natural cooling to room temperature, acetone solution is slightly dropped on the surface of the PMMA support layer using a dropper to remove the PMMA support layer, and then deionized water is used to remove the residual acetone solution, and finally the obtained device is heated at 90°C for 60 minutes, and thus the preparation of the graphene lower electrode is completed;

[0051] E: Liquid crystal transfer:

[0052] First, the commonly used liquid crystal groove material EpoCore is spin-coated on the graphene electrode to form a thin film by using a spin coating process, the rotation speed of the glue uniformizing machine is set to 2500 revolutions per minute, and a 4μm thick liquid crystal groove thin film is prepared; then the prepared liquid crystal groove thin film is pre-baked, that is, baked at 120℃ for 5 minutes, and after the baking is completed, it is naturally cooled to room temperature; the liquid crystal groove thin film is subjected to photolithography, the wavelength of the ultraviolet light is 365nm, the waveguide mask plate is complementary to the liquid crystal groove thin film structure to be prepared, and after the waveguide mask plate is tightly attached to the liquid crystal groove thin film, it is exposed, the exposure time is 12 seconds, and the polymer optical waveguide core layer thin film in the polymer optical waveguide core layer structure to be prepared is exposed to ultraviolet light; the device after photolithography is taken out from the photolithography machine, and is subjected to post-baking, that is, baked at 95℃ for 25 minutes, and after the baking is completed, it is naturally cooled to room temperature; the prepared liquid crystal groove thin film is developed, that is, soaked in a developing solution for 5 seconds, and then the surface residual core layer material is cleaned with isopropyl alcohol, and the above operation is repeated multiple times until a clear mask plate corresponding liquid crystal groove is obtained; then the surface of the obtained device is washed with deionized water and dried with an ear cleaning ball in the same direction, and finally baked at 120℃ for 30 minutes to harden the film, and after natural cooling to room temperature, the preparation of the liquid crystal groove is completed;

[0053] Then, the liquid crystal material E7 is added to the liquid crystal groove inlet by using a micropipette, and under the action of capillary force, the liquid crystal material spontaneously penetrates and fills the liquid crystal groove, realizing the transfer and introduction of the liquid crystal functional layer to the planar optical waveguide modulation area; finally, static treatment is performed to ensure the uniform distribution of the liquid crystal material in the liquid crystal groove;

[0054] F: Preparation of the upper electrode on ITO:

[0055] The ITO (Indium Tin Oxide) coated glass is used as the upper electrode, first, isopropyl alcohol and deionized water are used to flush the ITO coated glass in the same direction to remove surface contaminants; then a layer of BP-212 positive photoresist is spin-coated on the surface of the ITO thin film of the ITO coated glass, the rotation speed is 2000 revolutions per minute, and the rotation is 25 seconds, after the spin coating is completed, it is heated at 87℃ for 20 minutes; the positive photoresist thin film is subjected to photolithography, the wavelength of the ultraviolet light is 365nm, the waveguide mask plate is the same as the upper electrode structure to be prepared, after the waveguide mask plate is tightly attached to the positive photoresist thin film, it is exposed, the exposure time is 10 seconds, and the area outside the electrode to be prepared is exposed to ultraviolet light; then it is developed in a developing solution, the photoresist in the exposed area is dissolved, thereby exposing the ITO thin film in the corresponding area; after the development is completed, the sample is immediately flushed with deionized water, and dried with nitrogen, obtaining a clear photoresist mask pattern;

[0056] The ITO film is selectively etched by using a wet etching solution mainly containing hydrochloric acid. The etching solution is prepared by diluting hydrochloric acid and deionized water according to a certain volume ratio (the volume ratio of HCl to H2O is 1:8). After the excess part of the ITO film electrode is washed away, the device is immersed in isopropyl alcohol again to remove the residual photoresist, and then isopropyl alcohol and deionized water are used for cleaning to obtain the required ITO upper electrode. Then, sealant is applied to the interface connection area between the liquid crystal groove periphery and the ITO-coated glass to form a packaging frame, and the ITO-coated glass with the ITO upper electrode is aligned and attached to the waveguide core layer under a microscope, so that the ITO upper electrode faces the waveguide core layer. During the attachment process, the ITO-coated glass is slowly pressed on one side to effectively remove the air at the interface and avoid the generation of bubbles, so as to realize reliable packaging of the liquid crystal layer and integration of the ITO lower electrode. Finally, the ITO upper electrode and the lower electrode together form an electrode structure for applying an electric field, thereby preparing a liquid crystal modulation variable optical attenuator based on a multimode interference coupling structure.

[0057] It should be pointed out that the above specific embodiments are only representative examples of the present application, and the technical solutions of the present application include but are not limited to the above embodiments, and there are more preparation schemes and use scenarios. For example, each variable optical attenuator can be cascaded as a unit structure to form an optical communication system in combination with a WDM system to realize further expansion and application. Moreover, the materials of the design are not limited to this, and waveguide materials such as silicon, silicon nitride, lithium niobate, etc. can also be used. Those skilled in the art, based on the explicit disclosure of the present application or the description in the document without any doubt, all belong to the scope to be protected by the present patent.

Claims

1. A liquid crystal modulated variable optical attenuator of a multimode interference coupling structure, characterized by: For the planar optical waveguide structure, it is composed of a silica substrate (9), a polymer optical waveguide core layer, a polymer cladding layer (4), a graphene lower electrode (5), a liquid crystal groove (6), a liquid crystal material (7), an ITO upper electrode (8), and a glass substrate (10); the polymer optical waveguide core layer and the polymer cladding layer (4) have the same thickness, and are located on the silica substrate (9) together, and the polymer optical waveguide core layer is covered in the polymer cladding layer (4), the graphene lower electrode (5) is located on the polymer optical waveguide core layer and the polymer cladding layer (4), the liquid crystal groove (6) is located on the graphene lower electrode (5), the liquid crystal material (7) is filled in the liquid crystal groove (6), and the glass substrate (10) covers the liquid crystal material (7) and the liquid crystal groove (6); the ITO upper electrode (8) is prepared on the glass substrate (10) and is placed in the liquid crystal material (7) towards the graphene lower electrode (5); the polymer optical waveguide core layer is composed of an input waveguide (1), a multimode interference coupling waveguide (2), and an output waveguide (3) in sequence along the light input direction, and has a symmetrical structure; the input straight waveguide (1) has a width W1 = 3-6 μm and a length L1 = 50-1000 μm; the multimode interference coupling waveguide (2) has a length L2 = 200-1000 μm and a width W2 = 10-40 μm; the output straight waveguide (3) has a length L3 = 50-1000 μm and a width W3 = 3-6 μm; the polymer optical waveguide core layer has a thickness H1 = 2-10 μm; the graphene lower electrode (5) has a length L4 = 300-1100 μm, a width W4 = 20-50 μm, and a thickness H4 = 0.34-1.4 nm; the liquid crystal groove (6) has a length L5 = 250-1050 μm, a width W5 = 15-45 μm, a groove depth H2 = 3-5 μm, a groove sidewall thickness X1 = 1-6 μm, and L4 > L5 > L2, W4 > W5 > W2; the ITO upper electrode (8) has a width W6 = 4-10 μm, a thickness H3 = 50-200 nm, and a length L6 = 240-1040 μm, which is slightly smaller than the length L5 of the groove inside the liquid crystal groove (6), and W5-2X1 = W2, L5-2X1 = L2; along the light input direction, the left and right center positions of the ITO upper electrode (8) are offset from the left and right center positions of the multimode interference coupling waveguide (2) by 4-9 μm.

2. A liquid crystal modulating variable optical attenuator of a multi-mode interference coupling structure as set forth in claim 1, wherein: The material of the polymer optical waveguide core layer is one of EpoCore, SU-8 2002, and SU-8 2005, and the refractive index of the polymer optical waveguide core layer material is higher than that of the polymer cladding layer material.

3. A liquid crystal modulating variable optical attenuator of a multi-mode interference coupling structure as set forth in claim 1, wherein: The material of the polymer cladding layer is PMMA.

4. A liquid crystal modulating variable optical attenuator of a multi-mode interference coupling structure as set forth in claim 1, wherein: The material of the liquid crystal groove (6) is one of EpoClad, EpoCore, NOA73, NOA63, and PMMA.

5. A liquid crystal modulating variable optical attenuator of a multi-mode interference coupling structure as set forth in claim 1, wherein: The liquid crystal material (7) is one of nematic liquid crystals E7, K15, and 5CB.

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