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CN-116009292-B - Polarization independent electro-optic modulator based on thin film lithium niobate multimode waveguide

CN116009292BCN 116009292 BCN116009292 BCN 116009292BCN-116009292-B

Abstract

The invention discloses a polarization-independent electro-optic modulator based on a thin film lithium niobate multimode waveguide, which comprises a silicon substrate layer, an oxygen-buried layer, a lithium niobate layer and a metal electrode positioned on the lithium niobate layer. The X-cut thin film based lithium niobate layer comprises an input and output straight waveguide, an input and output end mode converter, a multimode beam splitter, a multimode waveguide phase shift arm and a multimode beam combiner. When the TE 0 mode is input from any input straight waveguide, the modulation principle is the same as that of the traditional Mach-Zehnder electro-optic modulator, the TE 0 mode is converted into a first-order transverse electric mode, namely TE 1 mode, through an input end mode converter when the TM 0 mode is input, the TE 1 mode 3dB beam splitting is realized through a multimode beam splitter, the TE 0 mode is modulated by a multimode waveguide phase shift arm, the TE 1 mode is output through interference of a multimode beam combiner, and the TE 1 mode is finally converted into the TM 0 mode by an output end mode converter. The invention effectively expands the application range of the electro-optical modulator and solves the modulation problem of light with different polarization modes.

Inventors

  • WANG ZONG
  • CHEN KAIXUAN
  • GUO CHANGJIAN

Assignees

  • 华南师范大学

Dates

Publication Date
20260512
Application Date
20221220

Claims (6)

  1. 1. A polarization-independent electro-optical modulator based on a thin-film lithium niobate multimode waveguide is characterized by comprising a silicon substrate layer (22), an oxygen buried layer (21) and a lithium niobate layer (20) which are sequentially stacked from bottom to top, wherein a T-structure metal electrode (18), a metal traveling wave signal electrode (10) and a metal traveling wave grounding electrode (9) are arranged above the lithium niobate layer (20), a thin-film lithium niobate waveguide is formed on the X-cut lithium niobate layer (20) through an etching technology, the thin-film lithium niobate waveguide comprises an input straight waveguide (1), an input end mode converter (2), a multimode beam splitter (5), a first S-bend waveguide (6), an input gradual taper (7) of a multimode waveguide phase shift arm, an output gradual taper (11) of the multimode waveguide phase shift arm, a second S-bend waveguide (12), a multimode combiner (14), an output end mode converter (16) and an output straight waveguide (17) which are sequentially connected, the metal electrode is arranged around the multimode waveguide phase shift arm (8) and comprises a metal traveling wave signal electrode (18) which is periodically arranged on one metal traveling wave side of the T-structure metal traveling wave signal electrode (18) which is periodically arranged on two sides of the metal traveling wave electrode (18), the silicon substrate layer (22) is partially etched through small holes (19) at the lithium niobate layer (20) to form a cavity (23), specifically, metal traveling wave signal electrodes (10) which are respectively connected with a group of periodically arranged T-shaped structure metal electrodes (18) at two sides are positioned between two arms (8) of the multimode waveguide phase shift arm, metal traveling wave grounding electrodes (9) which are respectively connected with a group of periodically arranged T-shaped structure metal electrodes (18) at one side are positioned at the outer sides of two arms of the multimode waveguide phase shift arm (8), and the whole Mach-Zehnder modulation structure is symmetrically arranged up and down and left and right; The input end mode converter (2) and the output end mode converter (16) have the same structure and are used for realizing the mutual conversion of a narrower end TM 0 mode and a wider end TE 1 mode, while a TE 0 mode is kept unchanged, the narrower end waveguide width W 1 of the input end mode converter (2) is smaller than the mode hybridization width of a TM 0 mode and a TE 1 mode and supports the TE 0 ,TM 0 mode, the wider end waveguide width W 2 of the input end mode converter (2) is larger than the mode hybridization width of a TM 0 mode and a TE 1 mode, and the length L 1 of the input end mode converter (2) is determined by a mode evolution theory so as to realize the complete coupling of the narrower end TM 0 mode and the wider end TE 1 mode; The multimode beam splitter (5) and the multimode beam combiner (14) are completely identical in structure and comprise a2×2 multimode interference structure and two groups of input and output gradual change cones, the width W 4 and the length L 3 of the interference structure of the multimode beam splitter are determined by a multimode interference self-imaging theory, when TE 0 or TE 1 modes are input from one port, 3dB beam splitting of TE 0 or TE 1 modes is simultaneously realized, the tail end width W 2 of the input gradual change cone (3) of the multimode beam splitter and the tail end width W 5 of the output gradual change cone (4) of the multimode beam splitter are larger than the mode hybridization width of TM 0 and TE 1 modes, mode conversion of light is avoided, the structures of the output gradual change cone (15) of the multimode beam combiner and the multimode input gradual change cone (3) of the multimode beam splitter are completely identical and symmetrically arranged, and the input gradual change cone (13) of the multimode beam combiner and the output gradual change cone (4) of the multimode beam splitter are completely identical and symmetrically arranged.
  2. 2. The polarization independent electro-optic modulator based on thin film lithium niobate multimode waveguide of claim 1, wherein the modulator material is selected as X-cut thin film lithium niobate on insulator comprising a silicon substrate layer (22), a low refractive index buried oxide layer (21), a high refractive index ridge lithium niobate layer (20), and a lower refractive index air over cladding.
  3. 3. The polarization-independent electro-optic modulator based on the thin-film lithium niobate multimode waveguide according to claim 1, wherein the multimode waveguide phase shift arms (8) on two sides of the metal traveling wave signal electrode (10) are ridge multimode waveguides, two ends of the multimode waveguide phase shift arm (8) are respectively connected with the first S-bend waveguide (6) and the second S-bend waveguide (12) through the input gradual taper (7) of the multimode waveguide phase shift arm and the output gradual taper (11) of the multimode waveguide phase shift arm, so that TE 0 mode and TE 1 mode are respectively and adiabatically transmitted, the structures of the input gradual taper (7) of the multimode waveguide phase shift arm and the output gradual taper (11) of the multimode waveguide phase shift arm are completely the same and are symmetrically arranged, and lithium niobate below the metal electrodes on two sides of the multimode waveguide phase shift arm (8) is not etched for reducing metal absorption loss.
  4. 4. A polarization independent electro-optic modulator based on a thin film lithium niobate multimode waveguide according to any of claims 1-3, characterized in that the width W 6 of the multimode waveguide phase shift arm (8) is required to ensure that the half-wave voltages of the two modes are equal and the width is as small as possible when modulating light in TE 0 mode and TE 1 mode in the modulation region section to achieve simultaneous efficient modulation of light of different polarizations.
  5. 5. The polarization-independent electro-optic modulator based on the thin-film lithium niobate multimode waveguide according to claim 1, wherein in the modulation region part, the structural parameters of the metal traveling wave electrode containing the metal electrode (18) with the T structure and the multimode waveguide phase shift arm (8) are optimized by using full-wave three-dimensional electromagnetic simulation software based on the electromagnetic field finite element method for analyzing the microwave engineering problem, so that the TE 0 mode and the TE 1 mode realize lower microwave and light wave loss simultaneously, the silicon substrate part is etched with lower half-wave voltage, the refractive index of the microwave group is designed to be intermediate between the refractive indexes of the multimode waveguide TE 0 mode and the TE 1 mode group, and the refractive index mismatch of the microwave and the light wave is reduced, so that the two modes can reach larger electro-optic bandwidth modulation simultaneously.
  6. 6. The polarization-independent electro-optic modulator based on the thin-film lithium niobate multimode waveguide according to claim 1, wherein the thickness of the T-shaped metal electrode (18) is 200nm, the electrode material is gold, the thickness of the metal traveling wave grounding electrode (9) and the metal traveling wave signal electrode (10) is 1.1 μm, and the electrode material is gold.

Description

Polarization independent electro-optic modulator based on thin film lithium niobate multimode waveguide Technical Field The invention relates to the field of integrated optoelectronic devices, in particular to a polarization independent electro-optic modulator based on a thin film lithium niobate multimode waveguide. Background Electro-optic modulator technology is a modulation technique that superimposes an electrical signal carrying information on a carrier light wave. The light modulation can change certain parameters of the light wave, such as amplitude, frequency, phase, polarization state, duration and the like, according to a certain rule. Electro-optic modulators are realized by the electro-optic Effect of materials, wherein the refractive index change of a material based on the Pockels Effect (Pockels Effect) is proportional to the electric field, the coefficient of which is related to the electro-optic coefficient of the material. The lithium niobate material has the advantages of high electro-optic coefficient and low loss in C wave band, and is the preferred material for electro-optic modulator. Meanwhile, the problems of small waveguide refractive index difference of the traditional lithium niobate material are solved by the occurrence of the lithium niobate thin film and the breakthrough of the etching technology, stronger mode restriction can be realized, and the method is a solution of a next generation photon integrated circuit. The modulator based on the thin film lithium niobate material has great application value in the future optical communication field as a next generation modulator with low modulation voltage, high modulation bandwidth and low insertion loss. The current electro-optical modulator basically modulates the TE fundamental mode, and the polarization state of the light wave needs to be strictly controlled from the beginning of the light wave passing through the grating input device, so that the interference of light in other polarization modes on the required TE polarized light is prevented, the modulation efficiency and the modulation signal quality are affected, and the light wave in other polarization states cannot be effectively utilized. Disclosure of Invention In order to overcome the defects of the prior art, the invention discloses a polarization independent electro-optic modulator based on a thin film lithium niobate multimode waveguide. Through analysis from the optical electromagnetic wave theory level, the modulator structure is designed and optimized, so that light waves in TM or TE polarization states can be effectively utilized and modulated, the problem of polarization control is solved, and the application range of the electro-optical modulator is widened. In order to achieve the above object, the present invention adopts the following structure: A polarization independent electro-optical modulator based on a thin film lithium niobate multimode waveguide comprises a silicon substrate layer, an oxygen buried layer, a lithium niobate layer and a metal layer which are sequentially stacked from bottom to top, wherein the thin film lithium niobate waveguide is formed on an X-cut lithium niobate layer through an etching technology, comprises an input straight waveguide, an input end mode converter, a multimode beam splitter, a first S-bend waveguide, a multimode waveguide phase shift arm, a second S-bend waveguide, a multimode beam combiner, an output end mode converter and an output straight waveguide which are sequentially connected, a metal traveling wave electrode is arranged at the multimode waveguide phase shift arm, the metal traveling wave electrode comprises a metal traveling wave signal electrode, two sides of which are respectively connected with a group of periodically arranged T-structure metal electrodes, and a metal traveling wave grounding electrode, one side of which is connected with a group of periodically arranged T-structure metal electrodes, and the silicon substrate layer is partially etched to form a cavity. Specifically, the metal traveling wave signal electrodes, of which two sides are respectively connected with a group of periodically arranged T-shaped structure metal electrodes, are positioned between two arms of the multimode waveguide phase shift arm, and the two metal traveling wave grounding electrodes, of which one side is connected with a group of periodically arranged T-shaped structure metal electrodes, are positioned outside the two arms of the multimode waveguide phase shift arm, so that the whole Mach-Zehnder modulation structure is symmetrically arranged. In a further embodiment, the input and output mode converters are identical in structure, and the interconversion of the narrower end TM 0 mode and the wider end TE 1 mode can be achieved while the TE 0 mode remains unchanged. The narrower end waveguide width W 1 of the mode converter is less than the mode hybridization width of the TM 0 mode and TE 1 mode and supports t