A mode-insensitive polymer variable optical attenuator based on MZI-MMI structure

Abstract

A mode-insensitive polymer variable optical attenuator based on an MZI-MMI structure belongs to the field of polymer integrated optical technology. The variable optical attenuator can be used in a mode division multiplexing system. From bottom to top, it is composed of a silicon substrate, a polymer lower cladding, a polymer waveguide core layer, a polymer upper cladding, and a heating electrode. The device is based on an MZI optical waveguide structure, and the two modulation arms of the MZI optical waveguide structure adopt a 1×1 MMI structure. The heating electrode is modulated, and the multimode interference effect of the light changes after the signal light enters the multimode waveguide of the polymer waveguide core layer. At the output end of the multimode waveguide, not only signal light with the same mode as the input signal is generated, but also signal light of other higher-order modes is generated. The signal light of the higher-order mode is attenuated in the output bending waveguide of the polymer waveguide core layer, and the output signal light power is equal to the input signal light power minus the attenuated higher-order mode signal light power, thereby achieving the attenuation of the input signal light.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer integrated optics, and in particular relates to a mode-insensitive polymer variable optical attenuator based on an MZI-MMI structure for a mode division multiplexing system. Background Art

[0002] Optical fiber communication is developing rapidly. According to forecasts, there will be a capacity crisis in 2025. The demand for bandwidth has become the primary research issue for the future development of optical communication. At present, with the widespread application of new technologies such as wavelength division multiplexing, time division multiplexing and polarization multiplexing, the capacity of light to transmit information has been further improved. If the density of multiplexing and the order of modulation format are further increased in order to increase the transmission capacity, it will cause great signal damage. Therefore, in order to provide greater information capacity, people urgently need to find new solutions to find new multiplexing methods that fundamentally solve the contradiction between bandwidth demand.

[0003] Physically speaking, increasing and fully utilizing the spatial dimension is an effective means to further increase the capacity of optical fiber communications. The spatial dimension includes parameters that have yet to be developed in optical fiber transmission, such as modes and multi-core optical fibers. Mode division multiplexing is the use of different optical modes in the same optical fiber to transmit different information. These modes are orthogonal to each other and do not affect each other. They can be transmitted simultaneously in multimode optical fibers. Mode division multiplexing technology adds a new degree of freedom to optical transmission, thereby multiplying the transmission capacity of a single optical fiber and greatly improving the utilization rate of the optical spectrum. It is one of the effective methods to significantly increase the capacity of optical fiber communication channels. It has become a cutting-edge and hot research topic in the field of optical communications internationally. Just as the wavelength division multiplexing system has multiplied the capacity of the optical fiber communication system, the mode division multiplexing system makes it possible to match bandwidth supply and demand, which once again greatly improves the information capacity and frequency efficiency of optical fiber communications.

[0004] Variable optical attenuators are important devices in optical communication systems. They can be used for power balancing between multiple channels in optical networks, and can also perform gain control and gain flattening on optical amplifiers. Variable optical attenuators can be divided into traditional mechanical types, fused fiber types, and integrated types. Most of the current variable optical attenuators are traditional mechanical types. Although the optical performance is relatively good, this type of variable optical attenuator is large in size, difficult to integrate, has high energy consumption, poor mechanical reliability, and high cost. The planar optical waveguide variable optical attenuator overcomes these shortcomings and can achieve flexible waveguide structure design. It has low cost, low insertion loss, low mode-related loss, and can be mass-produced. However, there is a lack of mode-insensitive variable optical attenuators used in mode division multiplexing systems, and there is still a lot of room for research on mode-insensitive variable optical attenuators.

[0005] There are many materials used to prepare planar optical waveguide variable optical attenuators, among which polymer materials have a high thermo-optical coefficient and are very suitable for tunable planar optical waveguide devices. Moreover, the preparation of optical waveguides based on polymer materials only requires simple semiconductor processes, and the preparation process is simple and low-cost. Summary of the invention

[0006] In order to overcome the shortcomings of the conventional variable optical attenuator, the present invention aims to provide a LP-supported optical attenuator that can be applied to a mode division multiplexing system. 01 LP 11a LP 11b and LP 21 The invention provides a mode-insensitive polymer variable optical attenuator based on an MZI-MMI structure with four modes, and realizes the insensitive attenuation of the signal light of the four modes. The invention effectively expands the application of polymer-based integrated optoelectronic devices in mode division multiplexing systems, and has important practical application value.

[0007] The mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure described in the present invention is composed of a silicon substrate, a polymer lower cladding, a polymer waveguide core layer, a polymer upper cladding layer and a heating electrode from bottom to top, the polymer waveguide core layer and the polymer upper cladding layer are located on the polymer lower cladding layer, and the polymer waveguide core layer is coated in the polymer upper cladding layer and the polymer lower cladding layer. Figure 1 As shown. The polymer upper and lower claddings are both made of polymer material EpoClad, with a refractive index of 1.56; the polymer waveguide core layer is made of polymer material EpoCore, with a refractive index of 1.572; and the heating electrode is an aluminum electrode. The present invention adopts the traditional Mach-Zehnder interferometer (MZI) and Multimode interference (MMI structure) planar optical waveguide structures, both of which are the most basic structures in optical waveguide devices and are widely used in planar optical waveguide type variable optical attenuators and optical communication fields.

[0008] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0009] As attached Figure 2 The figure is a schematic diagram of the polymer waveguide core layer and the heating electrode structure of the mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure of the present invention. The function of the present invention is to 01 LP 11a LP 11b and LP 21The four modes of optical signals are insensitively attenuated. The entire device is based on the MZI optical waveguide structure. The modulation arm of the MZI optical waveguide structure adopts the MMI structure. From left to right along the transmission direction of light, the polymer waveguide core layer is composed of the input few-mode straight waveguide 1 (which can transmit LP 01 LP 11a LP 11b and LP 21 four modes), the first input curved waveguide 2 (which can transmit LP 01 LP 11b two modes), the second input curved waveguide 3 (can transmit LP 01 LP 11b two modes), a first input tapered waveguide 4, a second input tapered waveguide 5, a first multimode waveguide 6, a second multimode waveguide 7, a first output tapered waveguide 8, a second output tapered waveguide 9, a first output curved waveguide 10 (capable of transmitting LP 01 LP 11b two modes), the second output curved waveguide 11 (can transmit LP 01 LP 11b two modes), output few-mode straight waveguide 12 (can transmit LP 01 LP 11a LP 11b and LP 21 The structure of the present invention is composed of four modes, a heating electrode 13 parallel to the multimode waveguide is arranged on the polymer upper cladding at the symmetric center of the first multimode waveguide 6 and the second multimode waveguide 7; the first input tapered waveguide 4, the first multimode waveguide 6 and the first output tapered waveguide 8 together constitute a 1×1 first MMI structure, and the second input tapered waveguide 5, the second multimode waveguide 7 and the second output tapered waveguide 9 together constitute a 1×1 second MMI structure; light is input from the input few-mode straight waveguide 1, and is divided into two beams of light with equal power (the power is half of the input signal light power) through the first input bent waveguide 2 and the second input bent waveguide 3, and then respectively input into the completely identical 1×1 first MMI structure and the second MMI structure, and then the signal light output from the first MMI structure and the second MMI structure is coupled to the output few-mode straight waveguide 12 through the first output bent waveguide 10 and the second output bent waveguide 11, and output from the output few-mode straight waveguide 12.

[0010] The thickness of each part of the polymer waveguide core layer is equal to h, the distance between the heating electrode and the projection of the first multimode waveguide 6 and the second multimode waveguide 7 on the upper surface of the polymer lower cladding is equal to x; the length of the input few-mode straight waveguide 1 and the output few-mode straight waveguide 12 is equal to L 1 , the width is equal to W 1 The lengths of the first input curved waveguide 2, the second input curved waveguide 3, the first output curved waveguide 10, and the second output curved waveguide 11 are equal to L 2, the width is equal to W 2 The first input tapered waveguide 4, the second input tapered waveguide 5, the first output tapered waveguide 8, and the second output tapered waveguide 9 are waveguides with gradually varying widths, and their lengths are equal to L 3 , the width of the junction between the input bending waveguide and the output bending waveguide is equal to W 2 , the width of the junction with the multimode waveguide is equal to W 3 The lengths of the first multimode waveguide 6 and the second multimode waveguide 7 are equal to L 4 , the width is equal to W 4 ; The length of the heating electrode 13 is L 5 , width W 5 .

[0011] The working principle of the mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure is as follows:

[0012] Since the present invention uses polymer materials, the refractive index difference between the cladding and the core of the waveguide is small, so the influence of mode polarization can be ignored. According to the refractive index of the polymer material, under the condition of a given waveguide core height h, the Helmholtz equation of rectangular waveguide (Ma Chunsheng, Design and Simulation of Optical Waveguide Devices, Higher Education Press) is used to calculate the LP in the waveguide 01 LP 11a LP 11b and LP 21 The effective refractive index of the mode varies with the waveguide width (e.g. Figure 3 ), according to the effective refractive index matching method and the beam propagation method, the width W of the waveguide core layer is determined 1 and W 2 , so that the waveguide width is W 1 Can transmit LP 01 LP 11a LP 11b and LP 21 Four modes, waveguide width W 2 Can transmit LP 01 and LP 11b Two modes, and W 1 =2*W 2 Then the size parameters of the MMI structure are determined. First, given the multimode waveguide width W 4 Determine the multimode waveguide length L under the condition of 4 , so that when the four modes are input, no attenuation occurs in the unmodulated state. Then, under the condition of a given tapered waveguide length (the given length is long so that the mode mismatch loss caused by the tapered waveguide can be ignored), the width W at the junction of the tapered waveguide and the multimode waveguide is 3 After the optimization is completed, the length L of the tapered waveguide is3 Optimize to minimize the size and loss of the device; after optimizing a single MMI structure, optimize the electrode structure parameters and reduce the heating electrode length L 5 Set to the same length as the multimode waveguide L 4 Similarly, the spacing x between the heating electrode and the multimode waveguide is set to 0, and the width W of the heating electrode is 5 Optimize to maximize the maximum attenuation while reducing power consumption; finally, adjust the length L of the curved waveguide 2 Optimize to keep both losses and device size as small as possible.

[0013] The MZI structure in the present invention is used to split and couple optical signals. We design the MMI structure to 01 Mode and LP 11b The mode is insensitive to attenuation. The relative positions of the heating electrode and the first MMI structure and the second MMI structure are the same. When the heating electrode is modulated, the effective refractive index of the multimode waveguide in the area around the heating electrode decreases. After the signal light enters the multimode waveguide, the multimode interference effect of the light changes. At the output end, not only the signal light with the same mode as the input signal is generated, but also other higher-order mode signal lights are generated. Since the output bending waveguide only supports LP 01 and LP 11b mode, so the signal light of higher-order mode is attenuated here. Since the two MMI waveguide structures are exactly the same and the relative position of the heating electrode is the same, the two MMI waveguide structures are modulated in the same way, so that the signal light power in the two output curved waveguides is the same, and the phase difference is the same as the signal light phase difference in the input curved waveguide. Therefore, at the output few-mode straight waveguide 12, the two signal lights are coupled into the same mode as the input signal light, and the output signal light power is equal to the input signal light power minus the attenuated higher-order mode signal light power, thereby achieving the attenuation of the input signal light. The role of the tapered waveguide is to reduce the mode mismatch loss between the few-mode waveguide and the multimode waveguide.

[0014] Input is LP 01 LP 11a LP 11b and LP 21 The attenuation states of each modulation temperature of the mode are shown in the attached figure. Figure 4 , Attachment Figure 5 , Attachment Figure 6 and attached Figure 7 As shown in the attached figure, at the working wavelength of 1.55μm, the attenuation of each mode changes with the temperature of the heating electrode. Figure 8 It can be seen that the attenuation curves of the four modes almost completely overlap with the temperature, indicating that the attenuation of the four modes is insensitive. When the heating electrode temperature is 27K, the attenuation is the largest, LP 01The attenuation of the mode is 12.6dB, LP 11a The attenuation of the mode is 12.6dB, LP 11b The attenuation of the LP mode is 11.2dB. 21 The attenuation of the mode is 11.1dB. The multimode variable optical attenuator uses the principle of multimode interference to change the interference state of the signal light in the multimode waveguide through modulation, generating a higher-order mode at the output waveguide of the MMI structure, and then controlling the size of the output waveguide so that it cannot transmit a higher-order mode, thereby achieving the attenuation of the optical signal.

[0015] We simulated the effect of the operating wavelength on the device, as shown in the attached figure. Fig. 9 As shown, in the operating wavelength range of 1.52μm to 1.62μm, when ΔT=0K, the attenuation of the four modes is almost 0, and when ΔT=27K, the attenuation of the four modes is greater than 10.5dB. The attenuation of the device when the electrode heating temperature is 0K and 27K does not change significantly with wavelength, and the attenuation of each mode is almost equal at different operating wavelengths.

[0016] Furthermore, we further optimized the proposed mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure. Fig.11 , which is a mode-insensitive polymer variable optical attenuator based on the MZI-butterfly type MMI structure. First, we added a right-angled triangle structure waveguide B1, B2, B3 and B4 on both sides of the output end of the first multimode waveguide 6 and the second multimode waveguide 7, respectively. The hypotenuse of the right-angled triangle waveguide is set toward the output tapered waveguide; these right-angled triangle structure waveguides effectively reduce the reflection of light at the output end and reduce the crosstalk of the signal light. Then, we no longer fix the spacing x between the electrode and the multimode waveguide to 0, and we simulated the LP when x is 0μm, 1μm, 2μm and 3μm respectively. 01 LP 11a LP 11b and LP 21 The curve of the mode changing with temperature is shown in the attached figure. Fig.12 In order to make the maximum attenuation temperature as small as possible and the maximum attenuation as large as possible, we chose the value of x to be 2μm. At an operating wavelength of 1.55μm, the attenuation of each mode changes with the temperature of the heating electrode as shown in the attached figure. Fig.13 The attenuation of the four modes is not sensitive to temperature. When the heating electrode temperature is 31K, the attenuation is the largest. 01 The attenuation of the mode is 35.1dB, LP 11a The attenuation of the mode is 33.8dB, LP 11b The attenuation of the mode is 32.1dB, LP 21The attenuation of the mode is 32.9dB. We then simulated the wavelength sensitivity of the device, as shown in the attached figure. Fig.14 As shown, in the operating wavelength range of 1.52μm to 1.62μm, when ΔT=0K, the attenuation of the four modes is almost 0, and when ΔT=31K, the attenuation of the four modes is greater than 20dB. The attenuation of the device when the electrode heating temperature is 0K and 31K does not change significantly with wavelength, and the attenuation of each mode is almost equal at different operating wavelengths. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 : A cross-sectional schematic diagram of a mode-insensitive polymer variable optical attenuator based on an MZI-MMI structure according to the present invention;

[0018] Figure 2 : A schematic structural diagram of a mode-insensitive polymer variable optical attenuator based on an MZI-MMI structure according to the present invention;

[0019] Figure 3 : When the waveguide core layer height h of the mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure of the present invention is 9 μm, the relationship curve between the effective refractive index of each mode in the waveguide and the change of the waveguide width;

[0020] Figure 4 (a) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K (no modulation) 01 Simulation diagram of light field transmission of the mode;

[0021] Figure 4 (b): Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K 01 Simulation diagram of light field distribution at the output end of the mode;

[0022] Figure 4 (c) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 01 Simulation diagram of light field transmission of the mode;

[0023] Figure 4 (d) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 01 Simulation diagram of light field distribution at the output end of the mode;

[0024] Figure 4 (e) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 01 Simulation diagram of light field transmission of the mode;

[0025] Figure 4 (f) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 01 Simulation diagram of light field distribution at the output end of the mode;

[0026] Figure 5 (a) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K 11a Simulation diagram of light field transmission of the mode;

[0027] Figure 5 (b): Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K 11a Simulation diagram of light field distribution at the output end of the mode;

[0028] Figure 5 (c) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 11a Simulation diagram of light field transmission of the mode;

[0029] Figure 5 (d) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 11a Simulation diagram of light field distribution at the output end of the mode;

[0030] Figure 5 (e) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 11a Simulation diagram of light field transmission of the mode;

[0031] Figure 5 (f) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 11a Simulation diagram of light field distribution at the output end of the mode;

[0032] Figure 6 (a) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K 11b Simulation diagram of light field transmission of the mode;

[0033] Figure 6 (b): Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K 11b Simulation diagram of light field distribution at the output end of the mode;

[0034] Figure 6(c) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 11b Simulation diagram of light field transmission of the mode;

[0035] Figure 6 (d) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 11b Simulation diagram of light field distribution at the output end of the mode;

[0036] Figure 6 (e) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 11b Simulation diagram of light field transmission of the mode;

[0037] Figure 6 (f) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 11b Simulation diagram of light field distribution at the output end of the mode;

[0038] Figure 7 (a) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K 21 Simulation diagram of light field transmission of the mode;

[0039] Figure 7 (b): Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 0K 21 Simulation diagram of light field distribution at the output end of the mode;

[0040] Figure 7 (c) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 21 Simulation diagram of light field transmission of the mode;

[0041] Figure 7 (d) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 13K 21 Simulation diagram of light field distribution at the output end of the mode;

[0042] Figure 7 (e) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 21 Simulation diagram of light field transmission of the mode;

[0043] Figure 7(f) Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with LP input at ΔT = 27K 21 Simulation diagram of light field distribution at the output end of the mode;

[0044] Figure 8 :Mode-insensitive polymer variable optical attenuator based on MZI-MMI structure with input LP 01 LP 11a LP 11b and LP 21 The attenuation curve of the heating electrode in the mode changes with the temperature;

[0045] Fig. 9 :The mode-insensitive polymer variable optical attenuator based on MZI-MMI structure has an input of LP at ΔT = 0K and ΔT = 27K 01 LP 11a LP 11b and LP 21 The attenuation curve of the optical signal of the mode changes with the wavelength;

[0046] Fig.10 : A flow chart of the preparation of the mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure of the present invention;

[0047] Fig.11 : A schematic structural diagram of a mode-insensitive polymer variable optical attenuator based on an MZI-butterfly type MMI structure according to the present invention;

[0048] Fig.12 :Mode-insensitive polymer variable optical attenuator based on MZI-butterfly MMI structure with different multimode waveguide and heating electrode spacing x, input LP 01 LP 11a LP 11b and LP 21 The attenuation curve of the mode changes with temperature;

[0049] Fig.13 : The attenuation curves of the mode-insensitive polymer variable optical attenuator based on MZI-butterfly MMI structure with four input modes as a function of heating temperature;

[0050] Fig.14 : The attenuation curves of the input signal light in four modes vary with wavelength at ΔT = 0K and ΔT = 31K for the mode-insensitive polymer variable optical attenuator based on MZI-butterfly MMI structure; DETAILED DESCRIPTION

[0051] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0052] Example 1

[0053] As attached Figure 1 As shown in the figure, the mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure of the present invention uses silicon material as a substrate, organic polymer material as a lower cladding layer, a waveguide core layer and an upper cladding layer, and a metal electrode as a heating electrode. The thickness of the polymer waveguide core layer of the strip structure is equal to h, and the horizontal distance between the heating electrode and the two multimode waveguides is x; as shown in the figure Figure 2 As shown, the input few-mode straight waveguide 1 (supports LP 01 LP 11a LP 11b and LP 21 Four modes), the length of the output few-mode straight waveguide 12 is equal to L 1 , the width is equal to W 1 The lengths of the first input curved waveguide 2, the second input curved waveguide 3, the first output curved waveguide 10, and the second output curved waveguide 11 are equal to L 2 , the width is equal to W 2 The lengths of the first input tapered waveguide 4, the second input tapered waveguide 5, the first output tapered waveguide 8, and the second output tapered waveguide 9 are equal to L 3 , the width of the junction with the multimode waveguide is equal to W 3 The lengths of the first multimode waveguide 6 and the second multimode waveguide 7 are equal to L 4 , the width is equal to W 4 ; The length of the heating electrode 13 is L 5 , width W 5 .

[0054] Example 2

[0055] This embodiment further explains the embodiment 1 in more detail.

[0056] First, determine the size parameters of each waveguide in the core layer. Fix the thickness h of the waveguide to 9 μm. Combined with the description in Example 1, the waveguide needs to support LP 01 LP 11a LP 11b and LP 21 Four modes, select the width W of the input few-mode straight waveguide 1 and the output few-mode straight waveguide 12 1 is 10 μm, the distance x between the heating electrode and the multimode waveguide is 0; the length L of the input few-mode straight waveguide 1 and the output few-mode straight waveguide 12 1 The length L of the first input curved waveguide 2, the second input curved waveguide 3, the first output curved waveguide 10, and the second output curved waveguide 11 is 1000 μm; 2 2000μm, width W2 W 1 Half of that, 5μm (supports LP 01 and LP 11b two modes); the length L of the first input tapered waveguide 4, the second input tapered waveguide 5, the first output tapered waveguide 8, and the second output tapered waveguide 9 3 The width W at the junction with the multimode waveguide is 500 μm. 3 is 10 μm; the length L of the first multimode waveguide 6 and the second multimode waveguide 7 4 730μm, width W 4 25 μm; the length L of the heating electrode 13 5 730μm, width W 5 The thickness of the upper cladding layer is 6 μm, the thickness of the core layer is 9 μm, the thickness of the upper cladding layer directly above the core layer is 6 μm, and the thickness of the heating electrode is 100 nm.

[0057] Example 3

[0058] A multimode variable optical attenuator for a mode division multiplexing system, the working principle of which is as follows:

[0059] Input signal light is LP 01 mode, when ΔT = 0K, the input to the two input bending waveguides is two LPs with the same power and phase. 01 The signal light in the curved waveguide enters the multimode waveguide through the tapered waveguide and undergoes multimode interference. The length of the multimode waveguide is designed so that the interference at the output port becomes LP when there is no modulation. 01 The signal light of the mode is output from the tapered waveguide. Since only LP 01 mode without higher-order modes, so no attenuation occurs. Since the two MMI waveguide structures are exactly the same, the signal light power input to the two output bent waveguides is the same, and the phase difference is the same as the phase difference of the signal light input to the two MMI waveguide structures. Therefore, the signal light in the two output bent waveguides will be coupled into the same LP as the input signal in the output few-mode straight waveguide. 01 mode, power vs. input LP 01 The signal optical power of the modes is the same. The light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 4 (a) and 4(b); When ΔT = 13K, the interference state of light in the multimode waveguide changes, and not only LP 01 mode signal light, but also generates other higher-order mode signal light. Since the output bending waveguide only supports LP 01 and LP 11bSince the two MMI structures have the same size and the same relative position to the heating electrode, the signal light output from the two output curved waveguides has the same attenuation and phase change, and enters the output few-mode straight waveguide to be coupled into LP 01 mode, the power is less than the input LP 01 The signal optical power of the mode, the light field transmission simulation diagram and the output end light field distribution simulation diagram are as shown in the attached figure. Figure 4 (c) and 4(d); when the heating electrode is modulated to make the signal light in the multimode waveguide interfere as much as possible into high-order modes at the output waveguide, the corresponding attenuation will be greater. When ΔT = 27K, the light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 4 (e) and 4(f), where the attenuation is maximum.

[0060] Input signal light is LP 11a mode, when ΔT = 0K, the input to the two input bending waveguides is two LPs with the same power and opposite phases. 01 The signal light in the curved waveguide enters the multimode waveguide through the tapered waveguide and undergoes multimode interference. The length of the multimode waveguide is designed so that the interference at the output port becomes LP when there is no modulation. 01 The signal light of the mode is output from the tapered waveguide. Since only LP 01 mode without higher-order modes, so no attenuation occurs; since the two MMI waveguide structures are exactly the same, the signal light power input to the two output bent waveguides is the same, and the phase difference is the same as the phase difference of the signal light input to the two MMI waveguide structures. Therefore, the signal light in the two output bent waveguides will be coupled into the same LP as the input signal in the output few-mode straight waveguide. 11b mode, power vs. input LP 11b The signal optical power of the modes is the same. The light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 5 (a) and 5(b); when ΔT = 13K, the interference state of light in the multimode waveguide changes, and not only LP 01 mode signal light, but also generates other higher-order mode signal light. Since the output bending waveguide only supports LP 01 and LP 11b Since the two MMI structures have the same size and the same relative position to the heating electrode, the signal light output from the two output curved waveguides has the same attenuation and phase change, and enters the output few-mode straight waveguide to be coupled into LP 11a mode, the power is less than the input LP 11aThe signal optical power of the mode, the light field transmission simulation diagram and the output end light field distribution simulation diagram are as shown in the attached figure. Figure 5 (c) and 5(d); when the heating electrode is modulated to make the signal light in the multimode waveguide interfere as much as possible into high-order modes at the output waveguide, the corresponding attenuation will be greater. When ΔT = 27K, the light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 5 (e) and 5(f), where the attenuation is maximum.

[0061] Input signal light is LP 11b mode, when ΔT = 0K, the input to the two input bending waveguides is two LPs with the same power and phase. 11b The signal light in the curved waveguide enters the multimode waveguide through the tapered waveguide and undergoes multimode interference. The length of the multimode waveguide is designed so that the interference at the output port becomes LP when there is no modulation. 11b The signal light of the mode is output from the tapered waveguide. Since only LP 11b mode without higher-order modes, so no attenuation occurs. Since the two MMI waveguide structures are exactly the same, the signal light power input to the two output bent waveguides is the same, and the phase difference is the same as the phase difference of the signal light input to the two MMI waveguide structures. Therefore, the signal light in the two output bent waveguides will be coupled into the same LP as the input signal in the output few-mode straight waveguide. 11b mode, power vs. input LP 11b The signal optical power of the modes is the same. The light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 6 (a) and 6(b); When ΔT = 13K, the interference state of light in the multimode waveguide changes, and not only LP 11b mode signal light, but also generates other higher-order mode signal light. Since the output bending waveguide only supports LP 01 and LP 11b Since the two MMI structures have the same size and the same relative position to the heating electrode, the signal light output from the two output curved waveguides has the same attenuation and phase change, and enters the output few-mode straight waveguide to be coupled into LP 11b mode, the power is less than the input LP 11b The signal optical power of the mode, the light field transmission simulation diagram and the output end light field distribution simulation diagram are as shown in the attached figure. Figure 6 (c) and 6(d); when the heating electrode is modulated to make the signal light in the multimode waveguide interfere as much as possible into high-order modes at the output waveguide, the corresponding attenuation will be greater. When ΔT = 27K, the light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 6(e) and 6(f), where the attenuation is maximum.

[0062] Input signal light is LP 21 mode, when ΔT = 0K, the input to the two input bending waveguides is two LPs with the same power and opposite phases. 11b The signal light in the curved waveguide enters the multimode waveguide through the tapered waveguide and undergoes multimode interference. The length of the multimode waveguide is designed so that the interference at the output port becomes LP when there is no modulation. 11b The signal light of the mode is output from the tapered waveguide. Since only LP 11b mode without higher-order modes, so no attenuation occurs. Since the two MMI waveguide structures are exactly the same, the signal light power input to the two output bent waveguides is the same, and the phase difference is the same as the phase difference of the signal light input to the two MMI waveguide structures. Therefore, the signal light in the two output bent waveguides will be coupled into the same LP as the input signal in the output few-mode straight waveguide. 21 mode, power vs. input LP 21 The signal optical power of the modes is the same. The light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 7 (a) and 7(b); when ΔT = 13K, the interference state of light in the multimode waveguide changes, and not only LP 11b mode signal light, but also generates other higher-order mode signal light. Since the output bending waveguide only supports LP 01 and LP 11b Since the two MMI structures have the same size and the same relative position to the heating electrode, the signal light output from the two output curved waveguides has the same attenuation and phase change, and enters the output few-mode straight waveguide to be coupled into LP 21 mode, the power is less than the input LP 21 The signal optical power of the mode, the light field transmission simulation diagram and the output end light field distribution simulation diagram are as shown in the attached figure. Figure 7 (c) and 7(d); when the heating electrode is modulated to make the signal light in the multimode waveguide interfere as much as possible into high-order modes at the output waveguide, the corresponding attenuation will be greater. When ΔT = 27K, the light field transmission simulation diagram and the output end light field distribution simulation diagram are shown in the attached figure. Figure 7 (e) and 7(f), where the attenuation is maximum.

[0063] The attenuation curves of the four modes as the heating electrode temperature changes are shown in the attached figure. Figure 8 As shown, it can be seen that under the same temperature change conditions, the attenuation of the four modes is almost the same, and the maximum attenuation is reached when ΔT = 27K. Fig. 9Input LP for the variable optical attenuator at no attenuation and maximum attenuation 01 LP 11a LP 11b and LP 21 The attenuation curve of the mode changes with wavelength. It can be seen that within the wavelength range of 1.52μm-1.62μm, the variable optical attenuator is insensitive to wavelength changes.

[0064] Example 4

[0065] The following is combined with Fig.10 The preparation process of the present invention is described in detail, and the specific steps are as follows:

[0066] 1. Clean the silicon substrate: Cut the silicon substrate into appropriate sizes, put it into a beaker filled with acetone and ultrasonically clean it for 10 minutes, then put it into a beaker filled with ethanol and ultrasonically clean it for 10 minutes, then rinse it with deionized water and blow it dry with nitrogen, and then put it into a clean culture dish and seal it;

[0067] 2. Spin coating polymer lower cladding: Spin coating the polymer material EpoClad on the cleaned silicon substrate at a rotation speed of 2500 rpm, heat at 120°C for 5 minutes, expose the whole for 20 seconds, and heat at 120°C for 3 minutes, thus completing the preparation of the polymer lower cladding with a thickness of 6 μm;

[0068] 3. Spin coating the polymer core layer: Spin coating the polymer material EpoCore on the polymer lower cladding layer to form a polymer core layer film at a rotation speed of 1400 rpm, heat at 90°C for 5 minutes, and then cool to room temperature;

[0069] 4. Photolithography: The polymer core film is photolithographically processed. The wavelength of ultraviolet light is 365nm. The structure of the optical waveguide mask is complementary to the structure of the polymer waveguide core layer of the variable optical attenuator to be prepared. The mask is placed close to the polymer core film and exposed for 26 seconds to expose the few-mode straight waveguide, curved waveguide, and MMI structure core films of the polymer waveguide core layer to ultraviolet light. The film is heated at 85°C for 5 minutes and then naturally cooled to room temperature.

[0070] 5. Development: wet-etch the optical waveguide core layer structure after photolithography. First, wet-etch for 70 seconds with the developer corresponding to the EpoCore material to remove the unexposed core layer material, and then wet-etch for 20 seconds in an isopropanol solution to wash away the developer and the residual core layer film. Rinse with deionized water and blow dry with nitrogen, and then heat at 130°C for 30 minutes. In this way, the preparation of the polymer waveguide core layer with a strip structure is completed. The thickness h of the polymer waveguide core layer is 9 μm.

[0071] 6. Spin coating the polymer upper cladding: Spin coating the polymer material EpoClad on the polymer waveguide core layer and the polymer lower cladding layer using a spin coating process at a rotation speed of 1100 rpm, heating at 120°C for 5 minutes, exposing the entire layer for 34 seconds, and heating at 120°C for 3 minutes, thereby completing the preparation of an upper cladding layer with a thickness of 6 μm (the thickness of the polymer upper cladding layer on the polymer waveguide core layer);

[0072] 7. Aluminum evaporation: A 100nm thick metal Al film is deposited on the polymer upper cladding layer using an evaporation process;

[0073] 8. Spin coating BP212 photoresist: a positive photoresist BP212 film was prepared by spin coating process at a rotation speed of 2500 rpm, and heated at 95° C. for 20 minutes to obtain a BP212 film with a thickness of 2 μm;

[0074] 9. Photolithography: Perform photolithography on the BP212 film. The mask is the heating electrode structure to be prepared (the location of the heating electrode is as shown in the attached figure). Figure 2 As shown), exposure for 2 seconds, the photoresist in the area other than the heating electrode structure is exposed;

[0075] 10. Development: Place the sample in a NaOH solution with a mass concentration of 5‰ for 20 seconds to wash away the exposed photoresist BP212, then rinse it repeatedly with deionized water and blow it dry with nitrogen, heat it at 85℃ for 20 minutes, and then naturally cool it to room temperature to develop the Al electrode, place it in a NaOH solution with a mass concentration of 5‰ for 15 minutes, wash away the Al film part outside the heating electrode, and finally rinse it repeatedly with deionized water and blow it dry with nitrogen;

[0076] 11. Removing BP212 photoresist: Expose the sample for 2 seconds, then place it in ethanol for 5 seconds to remove the residual photoresist BP212 on the Al electrode, and finally rinse it with deionized water and blow it dry with nitrogen. In this way, a mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure that meets the requirements is prepared.

Claims

1. A mode-insensitive polymer variable optical attenuator based on MZI-MMI structure, Features: From bottom to top, it is composed of a silicon substrate, a polymer lower cladding, a polymer waveguide core layer, a polymer upper cladding layer and a heating electrode. The polymer waveguide core layer and the polymer upper cladding layer are located on the polymer lower cladding layer, and the polymer waveguide core layer is coated in the polymer upper cladding layer and the polymer lower cladding layer. The entire device is based on an MZI optical waveguide structure, and the two modulation arms in the MZI optical waveguide structure adopt a 1×1 MMI structure. Along the transmission direction of light, the polymer waveguide core layer is composed of an input few-mode straight waveguide (1), a first input bent waveguide (2), and a second input bent waveguide (3). , a first input tapered waveguide (4), a second input tapered waveguide (5), a first multimode waveguide (6), a second multimode waveguide (7), a first output tapered waveguide (8), a second output tapered waveguide (9), a first output curved waveguide (10), a second output curved waveguide (11), and an output few-mode straight waveguide (12); a heating electrode (13) parallel to the multimode waveguide is arranged on a polymer upper cladding at the symmetric center of the first multimode waveguide (6) and the second multimode waveguide (7); the first input tapered waveguide (4), the first multimode waveguide (6), and the first The output tapered waveguide (8) together forms a 1×1 first MMI structure, and the second input tapered waveguide (5), the second multimode waveguide (7) and the second output tapered waveguide (9) together form a 1×1 second MMI structure; light is input from the input few-mode straight waveguide (1), and is divided into two beams of light with equal power after passing through the first input bent waveguide (2) and the second input bent waveguide (3), and then respectively input into the first MMI structure and the second MMI structure; then, the signal light output from the first MMI structure and the second MMI structure is respectively transmitted through the first output bent waveguide (1 0) and the second output bent waveguide (11) are coupled to the output few-mode straight waveguide (12), and then output from the output few-mode straight waveguide (12); the heating electrode is modulated, and the multi-mode interference effect of the light changes after the signal light enters the multi-mode waveguide, and not only the signal light with the same mode as the input signal is generated at the output end, but also other higher-order mode signal lights are generated, and the higher-order mode signal lights are attenuated in the output bent waveguide, and the output signal light power is equal to the input signal light power minus the attenuated higher-order mode signal light power, thereby achieving attenuation of the input signal light; The thickness of each part of the polymer waveguide core layer is equal to h, the distance between the heating electrode and the projection of the first multimode waveguide (6) and the second multimode waveguide (7) on the upper surface of the polymer lower cladding is equal to x; the length of the input few-mode straight waveguide (1) and the output few-mode straight waveguide (12) is equal to L 1 , the width is equal to W 1 The lengths of the first input curved waveguide (2), the second input curved waveguide (3), the first output curved waveguide (10), and the second output curved waveguide (11) are equal to L. 2 , the width is equal to W 2 The first input tapered waveguide (4), the second input tapered waveguide (5), the first output tapered waveguide (8), and the second output tapered waveguide (9) are waveguides with gradually changing widths, and their lengths are equal to L. 3 , the width of the junction between the input bending waveguide and the output bending waveguide is equal to W 2 , the width of the junction with the multimode waveguide is equal to W 3 ; The lengths of the first multimode waveguide (6) and the second multimode waveguide (7) are equal to L 4 , the width is equal to W 4 ; The length of the heating electrode (13) is L 5 , width W 5 ; W 1 is 10 μm, x is 0, L 1 1000 μm, L 2 2000 μm, W 2 5 μm, L 3 500 μm, W 3 10μm, W 4 25 μm, L 5 730 μm, W 5 is 10 μm; the thickness of the polymer lower cladding is 6 μm, the thickness of the polymer waveguide core layer is 9 μm, the thickness of the polymer upper cladding directly above the polymer waveguide core layer is 6 μm, and the thickness of the heating electrode is 100 nm.

2. A mode-insensitive polymer variable optical attenuator based on an MZI-MMI structure as claimed in claim 1, Features: The polymer upper and lower claddings are both made of polymer material EpoClad with a refractive index of 1.56; the polymer waveguide core layer is made of polymer material EpoCore with a refractive index of 1.572; and the heating electrode is an aluminum electrode.

3. A mode-insensitive polymer variable optical attenuator based on an MZI-MMI structure as claimed in claim 1, Features: The input few-mode straight waveguide (1) and the output few-mode straight waveguide (12) can transmit LP 01 LP 11a LP 11b and LP 21 Four modes: a first input curved waveguide (2), a second input curved waveguide (3), a first output curved waveguide (10) and a second output curved waveguide (11) can transmit LP 01 and LP 11b Two modes.

4. The mode-insensitive polymer variable optical attenuator based on the MZI-MMI structure according to claim 1 is a mode-insensitive polymer variable optical attenuator based on the MZI-butterfly MMI structure, Features: A right-angled triangle structure waveguide B1, B2, B3 and B4 are respectively added on both sides of the output ends of the first multimode waveguide (6) and the second multimode waveguide (7), and the hypotenuse of the right-angled triangle waveguide is arranged toward the output tapered waveguide; the distance between the heating electrode and the projection of the first multimode waveguide 6 and the second multimode waveguide 7 on the upper surface of the polymer lower cladding is equal to x = 2 μm.

Images